Photosensitive graphene transistors.

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Photosensitive Graphene Transistors Jinhua Li, Liyong Niu, Zijian Zheng, and Feng Yan*

which renders graphene a promising material for photodetectors in a broad wavelength/frequency range, as shown in Figure 1a.[6] In the digital era, high performance photodetectors are critical to the development of many technologies for imaging, sensing and communications, etc. For example, UV detectors have been used in many commercial and military applications,[7] including space communications, ozone layer monitoring, UV radiation monitoring and flame detection; Infrared (IR) detectors are highly desired for various demanding applications,[8] including telecommunication, night imaging, biological imaging and remote sensing; Terahertz detection has very important applications in medical diagnosis, industrial process control and homeland security.[9] A photodetector is a device that produces an output signal depending on the light intensity illuminated in its active region. The output is usually an electrical signal, but it can be a mechanical deflection of some element or, in the case of detection by vision, a physiological response.[10] High-performance photodetectors are currently dominated by inorganic semiconductor technologies, especially silicon-based devices.[11] However, the devices are mostly fabricated on rigid substrates with strictly controlled fabrication conditions. Consequently, low-cost highperformance photodetectors based on nanomaterials, graphene or organic semiconductors, which can be fabricated by convenient techniques (e.g., printing or coating) at relatively low temperatures have attracted much attention recently.[12–16] Such devices can be easily integrated into various systems, such as flexible, wearable, or portable electronics.[17] Moreover, an excellent performance being comparable to or even better than that of silicon-based photodetectors has been realized in some devices.[17–19] Considering the mature silicon-based manufacturing processes in semiconductor industry, these devices lie in complementing rather than replacing the standard silicon technologies. In general, photodetectors fall into two categories: photon (or quantum) detectors and thermal detectors.[10] Photon detectors respond to incident light radiation without first thermalizing its energy. The detection process involves a change in the characteristics of the detector caused by the absorption of individual photons. Thermal detectors convert incident light into heat, thereby raising the temperature of the active region

High performance photodetectors play important roles in the development of innovative technologies in many fields, including medicine, display and imaging, military, optical communication, environment monitoring, security check, scientific research and industrial processing control. Graphene, the most fascinating two-dimensional material, has demonstrated promising applications in various types of photodetectors from terahertz to ultraviolet, due to its ultrahigh carrier mobility and light absorption in broad wavelength range. Graphene field effect transistors are recognized as a type of excellent transducers for photodetection thanks to the inherent amplification function of the transistors, the feasibility of miniaturization and the unique properties of graphene. In this review, we will introduce the applications of graphene transistors as photodetectors in different wavelength ranges including terahertz, infrared, visible, and ultraviolet, focusing on the device design, physics and photosensitive performance. Since the device properties are closely related to the quality of graphene, the devices based on graphene prepared with different methods will be addressed separately with a view to demonstrating more clearly their advantages and shortcomings in practical applications. It is expected that highly sensitive photodetectors based on graphene transistors will find important applications in many emerging areas especially flexible, wearable, printable or transparent electronics and high frequency communications.

1. Introduction Graphene, the most famous two-dimensional (2D) material with a honeycomb lattice of carbon atoms, has received much research interest for its fascinating electronic, mechanical, and thermal properties and has shown many advantages in practical applications.[1–4] The gapless energy dispersion of graphene allows electron hole pairs to be generated by light over a broad bandwidth from ultraviolet (UV) to mid-infrared (MIR) and the intraband optical transitions in graphene extend the light absorption range to far-infrared (FIR) and terahertz,[5]

J. H. Li, Prof. F. Yan Department of Applied Physics The Hong Kong Polytechnic University Hong Kong, China E-mail: [email protected] L. Y. Niu, Prof. Z. J. Zheng Nanotechnology Center Institute of Textiles and Clothing The Hong Kong Polytechnic University Hong Kong, China

DOI: 10.1002/adma.201400349

Adv. Mater. 2014, DOI: 10.1002/adma.201400349

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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of the detector and lead to an electrical (or mechanical) signal. Graphene has been successfully used in both types of light detectors, depending on device designs and sensing mechanisms.[13,20,21] Transistors have shown unprecedented impacts on the electronic industry. Both the bipolar and unipolar transistors are good candidates for photodetectors because of the expected high gains induced by the amplification function of the transistors.[11,14–16,22,23] It is notable that the first graphene device reported by Novoselov and Geim in 2004 is a graphene field effect transistor (GFET),[24] which is also the major type of graphene-based photodetectors that have been developed thereafter. The photosensitive GFETs have been successfully used as terahertz, IR, visible and UV light detectors thanks to the broad absorption wavelength range of graphene and the high carrier mobilities. For example, GFETs have been used as ultra-fast photodetectors with an electrical bandwidth of 16 GHz and the maximum operation frequency is expected to be up to 500 GHz at optimum conditions.[13] Recently, IR phototransistors based on PbS QDs modified GFETs were reported to show the ultrahigh responsivities up to 107A/W.[18] In the GFETs, the photoresponse is mainly due to the light absorption of PbS QDs while graphene just provides a fast channel for charge transfer, which leads to the optical responsivity/gain several orders of magnitude higher than that of a device with PbS QDs or graphene only.[12] Therefore, GFET-based photodetectors have shown many advantages and are thus promising for lots of applications. Although graphene has been successfully used in many types of optoelectronics devices and sensors, this paper will give a review specifically on GFET-based photodetectors. For other optoelectronic applications of graphene, interested readers can refer to other related review papers. [25–34] In this review, we will introduce the basic working principles of GFETs and the sensing mechanisms

Prof. Feng Yan has research interests on thin film transistors, graphene, organic electronics, biosensors, solar cells and smart materials. He received his PhD degree in physics from Nanjing University in China. Then he joined the Engineering Department of Cambridge University in Feb 2001 as a Research Associate and joined National Physical Laboratory in UK in April 2006 as a Higher Research Scientist. He became an Assistant Professor at the Department of Applied Physics of the Hong Kong Polytechnic University in September 2006 and was promoted to Associate Professor in July 2012.

of GFET-based photodetectors at the beginning. Then we will review the different types of photodetectors (see Table 1 ), including terahertz, IR, visible and UV detectors based on GFETs, focusing on the device design, physics and performance. Because graphene quality is critical to the device performance, the transistors based on graphene prepared with different methods, including mechanical exfoliation, chemical vapor deposition (CVD) and reduction of graphene oxide (GO), etc. will be addressed separately. These methods have different advantages and shortcomings, which are important to practical applications of the devices. Finally, conclusions and outlook for this field will be addressed.

Figure 1. (a) Optical spectrum from terahertz to UV. (b) UV-visible spectra of CVD graphene films with 1 to 4 layers. Inset: the comparison of transmittance of a 10nm-thick graphene film (red), ITO (black) and fluorine tin oxide (FTO, blue). Reproduced with permission.[105] (c) Schematic diagrams of two typical GFET structures: bottom gate top contact structure and top gate top contact structure. (d) Typical transfer curve (IDS ∼ VG) of a GFET that shows ambipolar behavior.

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Table 1. Performance of photosensitive graphene transistors. Detector Type Terahertz

IR

Active layer

Spectral window

Responsivity

NEP

Mechanically exfoliated graphene

0.3 THz

150 mV/W

∼200 nW/Hz1/2 for SLG ∼300 nW/Hz1/2 for BLG

Mechanically exfoliated SLG

1.36∼10 THz

5 ∼10 mV/W

Mechanically exfoliated SLG or FLG

1.55 µm

∼0.5 mA/W

Mechanically exfoliated BLG

300 nm–6 µm

∼6.1 mA/W


0.658 µm/1.03 µm/2 µm /10.6 µm

∼2 × 105 V/W

Mechanically exfoliated FLG

2.75 µm

0.13 A/W


1.45–1.59 µm

0.1 A/W

Mechanically exfoliated SLG or BLG

1.3–17 µm

0.05 A/W

CVD-grown graphene

700–980 nm

6.11 µA/W

33fW/ Hz1/2 (at 5 K)

Bandwidth

Year

Ref.

2012

[9]

2013

[163]

40 GHz

2009

[98]

16 GHz

2010

[13]

>1 GHz at 10 K

2012

[20]

2013

[196]

2013

[197]

∼18 GHz

2013

[198]

2011

[218]

CVD-grown graphene

10.6 µm

∼8 µA/W

2013

[219]

Mechanically exfoliated SLG or BLG

850 nm

21 mA/W

2012

[199]


532–10 µm

8.61 A/W

2013

[186]

2012

[18]

(532 nm) 0.2A/W (NIR) 0.4A/W (MIR) Mechanically exfoliated graphene/PbS QDs

500 nm–1600 nm

∼107 A/W

CVD-grown graphene/PbS QDs

895 nm

107 A/W

2012

[17]

rGO and GNR

1550 nm

4 mA/W for rGO

2011

[226]

rGO

895 nm

∼0.7 A/W

2013

[230]

Ag-functionalized rGO

NIR

13.7 mA/W

2010

[231]

GO

455 nm

95. 8 mA/W

2013

[229]

655nm

20.8 mA/W

2013

[221]

10 Hz

1 A/W for GNR

Visible

980 nm

17.5 mA/W

CVD-grown graphene

400 nm-900 nm

435 mA/W


632.8 nm

1 mA/W

2009

[112]


532 nm

∼10 mA/W

2009

[115]


480 nm–750 nm

∼1.5 mA/W

2011

[111]


690 nm

0.25 mA/W

2013

[118]

Mechanically exfoliated Trilayer graphene

476.5 nm

∼10 mA/W

2013

[242]

[243]

1 pW/Hz1/2

514.5 Epitaxial 10-layer graphene

AM 1.5 light

∼4.5 mA/W

2013

CVD graphene/N-doped mosaic SLG

633 nm

∼0.1 mA/W

2012

[264]

Mechanically exfoliated graphene/ FeCl3- FLG

532 nm

∼0.1 V/W

2013

[247]

Eletro-oxidized eptaxial multilayer graphene

470 nm

2.5 A/W

2011

[263]

350 nm

200 A/W

Mechanically exfoliated SLG/ metal nanostructures

457–785 nm

∼10 mA/W

2011

[248]

CVD-grown SLG/Au nanparticles

458–633 nm

6.1 mA/W

2011

[249]

CVD-grown SLG/PbS QDs

400–750 nm

∼2.8 × 10 A/W

2012

[250]

1.1 ×

2013

[252]

Mechanically exfoliated SLG/chlorophyll


683 nm

3

106

A/W



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www.MaterialsViews.com Table 1. Continued. Detector Type

UV

Active layer

Spectral window

Mechanically exfoliated SLG/ Mechanically exfoliated MoS2

635 nm

rGO/CdSe composite

532 nm

Responsivity 5×

108

NEP

A/W

∼4 mA/W 4

Bandwidth

Year

Ref.

2013

[106]

2012

[260] [272]

Mechanically exfoliated SLG/ZnO QDs

325 nm

10 A/W

2013

CVD-grown graphene/ ZnO QDs

365 nm

∼1.3 A/W

2013

[273]

rGO

370 nm

∼0.86 A/W

2010

[257]

rGO

360 nm

∼0.12 A/W

2011

[275]

ZnO nano rod/rGO

370 nm

22.7 A/W

2011

[276]

SLG: Single-layer graphene: BLG: Bilayer graphene: FLG: Few-layer graphene

2. Device Fabrication and Physics 2.1. Preparation of Graphene The production of high-quality graphene in a scalable and economical way is of great importance in the fabrication of photosensitive GFET. Currently, various preparation methods have been developed and are generally classified by two types of approaches. One type is called top-down approach, i.e., using graphite as source material to break apart the stacked layers, which contains techniques such as mechanical exfoliation, chemical synthesis (graphene oxide and reduced graphene oxide) and direct exfoliation. Another type is bottom-up approach, namely using carbon precursors as source materials, which includes epitaxial growth on electrically insulating substrates and CVD growth on metal substrates. Each specific technique has its own pro and con in terms of cost, processability and material properties. The main preparation techniques are as follows.

2.1.1. Mechanically Exfoliated Graphene A. Geim and K. Novoselov were rewarded the 2010 Nobel Prize in physics for their discovery of graphene, which was acquired via a simple way known as Scotch tape or peeling-off method.[24] Namely, the tape is pressed down against highly oriented pyrolytic graphite and the van der Waals attraction in between can facilitate the delamination of a single carbon sheet when the tape is lifted away. The carbon sheet can be a pristine monolayer or multilayer graphene with high quality. With obvious color contrast, graphene sheets of monolayer or few layers can be readily identified under an optical microscopy.[35,36] Apart from that, typical manners for graphene characterization include Atomic Force Microscopy (AFM), Raman microscopy and Transmission Electron Microscopy (TEM). The pristine high quality graphene sheet exhibits extraordinary electronic and optical properties such as high carrier mobilities (∼2 × 105 cm2/Vs) at room temperature and high transmittance (97.7% for monolayer graphene) in a broad wavelength range (Figure 1b).[37–39] To date, GFET photodetectors based on mechanically exfoliated graphene with various device designs and sensing principles have been widely reported. Though of great importance in fundamental studies such as the investi-

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gation of intrinsic properties of graphene and related devices, the mechanical exfoliation method cannot meet the requirements for practical applications in view of the hard control of graphene shape, size and location as well as the low throughput unavailable for industrial scalability.

2.1.2. Reduced Graphene Oxide (rGO) Because of the above drawbacks of mechanical exfoliation method, challenges of developing other alternative routes to produce graphene with large amount have become the focus of research. Beyond the scalability, the processes should on one hand produce high-quality graphene without compromising high mobility, on the other hand provide controllable manners over the graphene sheet thickness with a view to building devices with uniform performance, ease of integration into peripheral systems as well as compatibility with various substrates. Among the follow-up strategies, graphite oxide, the oxidized form of graphite derived from the chemical oxidation of graphite, offers an important and frequently used method. The preparation of graphite oxide has been evolved for centuries and two alternative routes were developed by Staudenmaier in 1898[40] and Hummers 1958.[41] However, due to time-consuming and hazardous of the former, the latter (Hummers method) is a more widely used approach to produce graphite oxide by using graphite and an anhydrous mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate. Graphite oxide can be easily exfoliated to produce single-layer graphene oxide (GO) through ultrasonication[42] or mechanical stirring in aqueous solutions for long time.[36] GO, an important graphene derivative, comprises of plenty of oxygen-containing functional groups on its edges and basal plane such as carboxyl, epoxy and hydroxyl, the presence of which results in a larger interlayer spacing (6–12 Å) than graphite (3.4 Å) and offer active sites for graphene functionalization via covalent or noncovalent methods, such as the integration of biorecognition molecules for biosensors,[43–47] the modification for buffer layers in polymer solar cells and so on.[48,49] Graphene can be obtained by thermal or chemical reduction of GO to restore its π-conjugated structure, i.e. reduced graphene oxide (rGO).[50–55] Graphene production in ton scale is available in such a way, however, the chemical exfoliation way is restricted by large amount of residue defects




subject to the reaction temperature (usually 800∼1000 °C) and vacuum degree.[73–75] CVD graphene can be transferred to arbitrary substrates after the removal of metal substrates by chemical etching. There are various suitable metal substrates, such as Cu, Co, Pt, Ru, Ni and so on,[76–80] among which Cu and Ni are mostly used. Compared to Ni substrates, Cu can lead to higher percentage of single layer graphene with larger grain size after annealing process. Recently, great effort has been paid to the development of CVD method, which on one hand can readily control the growth thickness of graphene sheets and on the other hand can produce large size graphene sheets ranging from tens of microns up to 30 inches.[81] Moreover CVD method can facilitate the substitutional doping process via introducing heteroatoms such as nitrogen and boron, which will favor the functionalization or fine tune of graphene transistors. Moreover, CVD grown graphene could be patterned by microfabrication techniques and is thus suitable for applications in miniaturized devices and high density electric circuits.

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caused by incomplete reduction process. Compared to the mechanically exfoliated pristine graphene, rGO is of lower quality because the conductivity and charge carrier mobilities are jeopardized. Therefore, some other solution-based methods are developed to directly exfoliate graphite with the assistance of organic solvents,[56–58] surfactants,[59,60] salts,[61] ionic liquids,[62,63] lithium ions,[64] and the like or to exfoliate intercalated graphite compounds without oxidation process.[65–67] Graphene flakes or nanoribbons of higher quality with fewer defects can be obtained, though with a lower production yield in comparison with exfoliation of graphite oxide. Photosensitive GFETs based on rGO or graphene flakes have been studied for several years and have shown sensitivities even higher than the devices based on pristine graphene due to the higher light absorbance and longer lifetime of carriers in the former. However, the devices have a drawback of long response time and are thus suitable for low frequency applications only.

2.1.3. Epitaxial Graphene 2.2. GFETs Among bottom-up approaches, epitaxial growth of graphene via the sublimation of silicon from the surface of silicon carbide is a method to obtain relatively high-quality graphene. This thermal deposition process usually requires high temperature (>1000 °C) as well as ultrahigh vacuum conditions, both of which are key to the formation of monolayer and few layer graphene on SiC surfaces.[68,69] After the sublimation of Si and subsequent graphitization of the excess carbon left behind, graphene can form nearly perfect structure on both silicon-rich face (0001) and carbon-rich face of the hexagonal phase SiC (4H-SiC or 6H-SiC). More recently, graphene growth on cubic phase silicon carbide has been demonstrated.[70] Graphene grows with a single orientation on a Si-rich face, exhibiting a regular Bernal stacking and rather poor uniformity due to surface pits. On the contrary, graphene shows rotational stacking and higher conductivity on a C-rich face. The advantage of this method lies in that high-quality graphene can be directly synthesized on a insulating substrate, which is suitable for wafer-based applications. For example, various kinds of electronic components or devices can be constructed atop without involving transfer process. However, this method also has its limitations. Unlike metals used in CVD method that will be introduced below, SiC is hard to be removed, which renders the transfer process to other substrates difficult to be accomplished, although some attempts for the transfer of epitaxial graphene onto arbitrary substrates have been demonstrated, such as using thermal release tape.[71,72] 2.1.4. CVD-Grown Graphene The CVD technique is actually not very new, which had been attempted by Blakely and his group to study the thermodynamics of “monolayer graphite and bilayer graphite” growth on Ni (111) crystals in 1970s. In the CVD growth, carbon sources such as CH4 gas, methanol, or poly(methyl methacrylate) (PMMA) can be catalyzed by metals to deposit carbon atoms on metal surfaces and form large area graphene with the quality


Typically, a field effect transistor (FET) consists of three electrodes, including source, drain and gate, a channel region connecting the source and the drain, and a dielectric layer separating the gate from the channel. There are two geometries of FETs that have been used in studies, including bottom gate and top gate structures, as shown in Figure 1c. The conductivity of graphene channel can be controlled by the gate voltage (VG) between the gate and the source. Sometimes, devices with dual gates (top gate and bottom gate in one device) were prepared with a view to studying device physics. Because graphene is a semimetal with zero bandgap, GFETs normally show ambipolar behavior in transfer characteristics (channel current IDS vs. gate voltage VG), as shown in Figure 1d. The carrier density and the type of carriers in the channel can be tuned by the gate voltage. The conductivity of graphene can reach its minimum value near the Dirac point where the carrier density is close to zero, which is also called charge neutral point. On each side of the Dirac point, n-channel or p-channel will be formed, depending on the applied gate voltage. The position of Dirac point in a transfer curve is correlative with many factors, such as the difference between the work function of gate and graphene, the type and density of charges at the interface between graphene and substrate, and the impurity-induced doping level in graphene.[82] So the channel currents of the devices are very sensitive to the changes of potential and charge densities at graphene channels or interfaces, which makes GFETs promising transducers for various types of sensors, such as photodetectors, biosensors and chemical sensors.[13,17,18,83–90] The most attractive feature in GFETs is the ultrahigh carrier mobilities, which is over 1 × 106 cm2/Vs at low temperatures.[91,92] More importantly, the mobilities above 105 cm2/Vs have been achieved at room temperature,[93] which are ten times higher than that observed in InP high electron mobility transistors (HEMTs) and two orders of magnitude higher than those of silicon transistors. The mobilities of graphene are strongly dependent on its quality, gate insulator and supporting substrates



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because the interaction between graphene and insulator or substrate and the charge traps at or near interfaces would drastically reduce the carrier mobility in graphene.[94,95] So the mobilities of GFETs can be improved by optimizing the interface properties. The high mobility is important for most of the graphene-based photodetectors, such as highly IR sensitive hybrid GFETs and high frequency GFET-based photodetectors.[13,17,18] GFEFs can be operated at very high frequencies due to the high carrier mobilities, which outperform Si metal-oxide-semiconductor field-effect transistors (MOSFETs) and compete well with InP or GaAs high electron mobility transistors in terms of cut-off frequency.[96] The highest cut-off frequency of about 427 GHz have been observed in self-aligned GFETs with 67-nm gate, which is about three times higher than those of Si MOSFETs (∼150 GHz).[97] More importantly, GFET-based photodetectors have demonstrated ultrafast photoresponse up to 40 GHz with no degradation and the RC limited bandwidth of the devices up to ∼640 GHz was theoretically estimated, which may find important applications in high-speed optical communications.[98] Due to zero bandgap in graphene, on/off ratios of GFETs are usually lower than 20 that are the main obstacle in the applications of logic devices. The bandgap of graphene can be opened by constraining graphene in one dimension, such as graphene nanoribbons or quantum dots, biasing bilayer graphene, chemically modifying graphene, and so on.[82,99,100] However, the mobilities of graphene would suffer a serious decrease when the bandgap was opened. So, it is necessary to balance the bandgap and the mobility of graphene in improving the on/off ratio of GFETs.

2.3. Sensing Mechanisms

VPTE = (S1 − S2 )ΔT

The light absorption of graphene is dominated by two types of processes, including intraband optical transitions at low photon energies (in FIR and terahertz spectra range) and interband transitions at higher energies (from MIR to UV).[101] The intraband transition is the transition of carriers between quantized levels within the conduction or valence band while the interband transition is the transition between conduction and valence bands. In FIR and terahertz range, the intraband optical transition of graphene can be described well by a Drude model.[101,102] Plasmonic excitations associated with carriers in graphene in this frequency range can be induced in some devices with periodic grating structures, which can dramatically enhance the light absorption of the devices.[103] As shown in Figure 1b, from MIR to visible range, the light absorption is primarily due to interband transitions and the absorbance is _ approximately equal to an frequency independent value πe2/hc [ 104,105 ] ≈ 2.29% in pristine graphene. However, the light absorption of graphene in a GFET can be tuned by electrostatic gating because of the change of Fermi energy in graphene, which induces Pauli blocking of the optical transitions.[106] In UV range, there is an absorption peak at a photon energy of about 4.62 eV due to the excitonic effects.[105] It also can be found that graphene is an excellent material for transparent electrode for the high transmittance in broad wavelength range.[107] A typical photosensitive GFET is an optoelectronic device that can absorb optical energy and convert it into photocurrent

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or photovoltage across the channel of the device. There are many different sensing mechanisms that are related to the device design, wavelength range and the used materials. Here, we will introduce several main mechanisms, including photovoltaic effect, photothermoelectric effect, bolometric effect and field-effect doping. Many others effects will be addressed in details in the description of specific devices. Photovoltaic effect: This effect was firstly proposed to explain the origin of photoresponse observed in GFETs.[98,108–112] Many studies have demonstrated that photocurrents in GFETs were generated near graphene-metal contacts or graphene p-n junctions.[108,110–112] It was concluded that excitons are generated in graphene under light illumination and are separated to electrons and holes near the contacts or junctions due to the builtin electric fields. So it is reasonable to find that devices with asymmetric source/drain electrodes will show higher photocurrents than those with symmetric electrodes. But the photovoltaic effect is not the only one that can be found at the graphene contacts or junctions because hot carriers generated in graphene by light illumination can lead to pronounced photothermoelectric effect and bolometric effect as introduced below. Photothermoelectric effect: This effect arises from a lightinduced temperature difference that can lead to a thermoelectric voltage and plays an important role in the generation of photoresponse in many photosensitive graphene devices.[20,113–115] Under light illumination, hot carriers are generated and form a hot Fermion distribution due to slow energy transfer to crystal lattice in graphene. The photothermoelectric voltage in a graphene device can be derived from different Seebeck coefficient of different regions that form a junction.[113,114] The photothermoelectric voltage VPTE can be written as: (1)

where S1, S2 are the Seebeck coefficients at the two sides of the junction, ΔT is the electron temperature difference between the area of optical excitation and its surroundings. Bolometric effect: A bolometer is a thermal detector that produces an electrical signal by the temperature-dependent change in electrical resistance of its active element.[10] So the bolometric effect refers to the resistance change under light illumination because of the heating of the active material. Bolometric effect has been observed in many graphene photodetectors recently.[21,116–118] Due to the weak carrier-phonon coupling, carriers can be easily heated when light is absorbed by graphene, which causes a resistance change in a device because charge transport in graphene is sensitive to carrier temperature. Field-effect doping: In a GFET, the channel current can be modulated by gate voltages as well as charges trapped at certain interfaces due to the change of carrier density in the graphene channel, which is regarded as field-effect doping. Similarly, under light illumination, some photo-generated carriers will be trapped at interfaces or even graphene channel and lead to more or less free carriers in graphene due to electrostatic interactions. The field-effect doping will lead to the horizontal shift of the transfer curve of the GFET due to the change of the effective gate voltage, which has been observed in some GFETs especially hybrid devices like the IR sensitive GFETs based on PbS QDs-modified graphene.[17]




Being similar to typical photodetectors, light sensitive GFETs have several important parameters commonly used to describe the device performance, including responsivity, quantum efficiency, signal to noise ratio, electric bandwidth and detectivity, etc., which are explained in details as follows: Responsivity (R). It is also called spectral responsivity or radiant sensitivity. R can be defined by R=

Photocurrent (A) or photovoltage (V) I ph or Vph = Incident optical power(W) Po

(2)

where Iph is photocurrent, Vph photovoltage and Po incident optical power. It is used to characterize the generated photocurrent or photovoltage per incident optical power at certain wavelength. An important characteristic of a photodetector is the frequency response under rapidly modulated radiation. So the responsivity of the device under the modulation frequency f is normally presented by R(f). External quantum efficiency (EQE) ηe: This parameter is very important for photovoltaic devices.[119] In photosensitive GFETs dominated by photovoltaic effect, not all of the incident photons can be absorbed to create free electron and hole pairs to generate photocurrent or photovoltage. On the other hand, some photogenerated electron-hole pairs cannot be collected due to the recombination or trapping processes. Therefore, the EQE can be defined as

ηe =

Number of collected carriers Number of incident photons

(3)

ηe can be written in some cases as:

ηe =

I ph /e Po /hv

(4)

where e is electron charge, h Planck’s constant, and v frequency of light. EQE can be increased by increasing the optical absorption of active layer and preventing the recombination or trapping of carriers before they are collected. v = c/λ, where c is the speed of light and λ the wavelength of light, so ηe can be rewritten as:

ηe =

(5)

ηe ηe = total absorption 1 − refraction − transmission

(6)

In general, ηi is always larger than ηe because refraction and transmission can’t be eliminated completely. Electrical bandwidth: For a detector responded uniformly to light illumination with the modulation frequencies in the range between f1 and f2 (f1 < f2), and had no response outside of this


Δf =

∫

∞

0

2

R( f ) df R max

(7)

where Rmax is the maximum value of the function R(f). Signal to noise ratio (SNR): The detection of small amount of radiation energy is inhibited by the presence of noise in the detection process because noise produces a random fluctuation in the output of the detector. SNR can be defined by: SNR =

Signal power Noise power

(8)

So signal power only can be detected when it is above the noise level, which means SNR > 1. Noise equivalent power (NEP): The NEP is an important parameter of a photodetector. NEP is defined as the required optical input power to achieve a SNR of 1 within a bandwidth of 1 Hz, which can expressed as NEP =

Input power for SNR = 1 P = 1 Bandwidth Δf

(9)

where P1 is the incident power that results in SNR = 1 and Δf is the bandwidth. It is obvious that lowering the NEP is in need for better photodetectors. Detectivity D*: It can be defined as the reciprocal of NEP. It is a measure of detector’s sensitivity under the consideration of noise contributions. In order to better compare detector’s sensitivity, the same photosensitive area should be considered. Specific detectivity D* is defined by D* =

A 1/2 NEP

(10)

where A is the photosensitive area. The unit of D* is m Hz/W. Usually, cm Hz /W is more frequently used, which is called Jones.

3. Terahertz Sensitive GFETs

I ph hc ⋅ Po eλ

Internal quantum efficiency (IQE) ηi: In contrast, ηi denotes the number of photogenerated carriers per absorbed photon. ηi can be expressed as:

ηi =

range, the electric bandwidth should be Δ f = f2 – f1. In case the response dependent on frequency, the electric bandwidth of the detector is defined by:[10]

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2.4. Performance Parameters

Terahertz is typically referred to the frequencies from 100 GHz to 10 THz (wavelengths from 3 mm to 30µm), which lies in the frequency gap between IR and microwaves as shown in Figure 1. Terahertz technology is expected to be increasingly important in broad range of applications, including information and communication technologies, biology and medical science, homeland security, quality control of food and agricultural products, global environmental monitoring and ultrafast computers.[120,121] Being different from visible light, terahertz radiation can penetrate much more media related to our normal life. For example, terahertz rays can be used to test human body, cloth, and many plastic materials non-destructively.[32] Milestones of achievements of terahertz technologies in the past decades have been the development of terahertz



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time domain spectroscopy, terahertz imaging and high-power terahertz generation by means of nonlinear effects.[120,122,123] Existing commercially available detectors of terahertz radiation have some limitations, including cryogenic working temperature, narrow modulating bandwidth, low operating frequency and incompatible integration with other systems due to the special design and principle of operation.[32] It is therefore necessary to explore new types of terahertz detectors to overcome these disadvantages.

3.1. Principle of Terahertz Photodetectors Based on Field Effect Transistors Field effect transistors (FETs) as detectors of terahertz radiation were firstly proposed in theory by M. Dyakono and M. Shur in 1993.[124] Ideally, in a short channel high mobility FET, electrons can travel across the active region without collision during the transit time, which is called ballistic transport. However, high electron concentration in the channel can result in many electron-electron collisions during the transit time. So the individual electrons cannot be considered as ballistic particles but twodimensional electron gas as a whole that exhibits hydrodynamic behavior and can generate plasma wave. It is notable that the velocity of plasma waves is typically on the order of 108 cm/s, which is much larger than the drift velocity of electrons in the channel.[124] The plasma waves propagating in a FET channel should obey a linear dispersion law: ω = sk, where ω is frequency, s = (eU/m)1/2 is the velocity of plasma wave, e is the electronic charge, m is the electron effective mass, U is the gate-to-source voltage swing, and k is the wave vector.[125] The upper bound frequency f = ω/2π of this resonator depends on the velocity of plasma wave and gate length (L) and is given by: f ∼ s/L.[32] Assuming L ≤ 1 µm and s ∼ 108 cm/s, f can reach the terahertz range. So the plasma wave excitations in the short channels of FETs enable their response at frequencies much higher than the device cut-off frequency and the devices thus can be operated in terahertz range.[126] The FET-based technology offers several advantages, including compact design, high integration capability, broadband operation and fast response. To use a FET as a terahertz detector, an antenna structure should be integrated with the transistor to couple electromagnetic wave to plasma wave. Normally a device is designed with the source electrode connected to an antenna and the drain side being an open circuit. When electromagnetic waves incident on the device, an ac voltage at the source side of the channel will be generated, which will excite the plasma waves. So the channel of FET can be regard as a resonator for plasma waves and, consequently, dc voltage or current between source and drain will be created, which is proportional to the radiation power. According to the coupling methods of the plasma wave and terahertz radiation, FET-based terahertz detectors can be classified with resonant detectors and broadband detectors (nonresonant detectors).[32,82,127] Terahertz or subterahertz radiation detectors with FET structures were normally based on III-V materials, such as GaAs,[91] GaAs/AlGaAs,[92] InGaP/InGaAs/GaAs,[93] and InGaAs/ AlInAs[94] due to their high carrier mobilities. In 2002, W. Knap et al. reported the resonant detection of subterahertz radiation

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by two-dimensional electron plasma confined in a submicron gate GaAs/AlGaAs FET.[92] The resonant response of the device to 0.6 terahertz radiation was observed at the temperature of 8 K. Most of the FET-based terahertz detectors developed at the beginning were performed at cryogenic temperature and not convenient for practical applications. In 2004, T. Otsuji et al. reported the resonant terahertz detectors based on short-gate InGaP/InGaAs/GaAs pseudomorphic high-electron-mobility transistors at room temperature.[93] Moreover, terahertz detectors based InAs[128,129] and InAs/InSb[130] nanowires have been reported to exhibit high performance at room temperature. In 2012, M. S. Vitiello et al. demonstrated InAs nanowire-based FETs for highly sensitive room-temperature detectors operated at 0.3 terahertz.[128] The responsivity and NEP of the device were ∼1.5 V/W and ∼2.5 × 10−9 W/Hz1/2, respectively, being comparable to those of commercial terahertz detectors. However, the use of III-V materials is not compatible with Si semiconductor industry. In 2004, W. Knap et al. experimentally demonstrated the subterathertz and terahertz detection of Si MOSFETs operated at room temperature,[96] which aroused great interest for commercial applications. In 2006, the same group reported nonresonant terahertz detector of Si MOSFETs that showed responsivities up to 200 V/W and NEPs of about 10−10 W/Hz1/2 at 0.7 THz. The NEP is comparable to those of commercial room-temperature terahertz detectors.[100] In both pioneering works, terahertz detectors of Si MOSFETs exhibited many advantages, such as room temperature operation, fast response time, easy on-chip integration with read-out electronics and high reproducibility, leading to straightforward array fabrication.[32] So far, the performance of the Si MOSFETbased terahertz detectors is comparable to that of the commercial detectors based on Schottky barrier diodes.[131] It is notable that no commercially available terahertz devices can generate, detect, or manipulate electromagnetic waves over the entire terahertz range.[120] The unique characteristics of graphene, such as the ultrahigh carrier mobilities and gapless band structure, make it a promising material for detectors and modulators operating in the whole terahertz range.[9,132] Electrons and holes in graphene hold a linear dispersion relation with zero bandgap which results in unique features such as massless relativistic Fermions with back-scattering-free ultrafast transport[133,134] and negative dynamic conductivity in terahertz spectral range under the electrical pumping.[135–137] Very recently, the highest cut-off frequency of GFETs approaches to 427 GHz, which is observed in self-aligned graphene transistors with transferred gate stacks.[97] So the operation frequencies of graphene transistors can be much higher than those of the Si MOSFET (∼50 GHz) and reach terahertz range.[138] Furthermore, high-quality graphene can support plasma waves with little damp.[139] Therefore, plasma-based GFET photodetectors are expected to outperform many other terahertz detection technologies. Many other advantages of graphene in terahertz applications have been realized. The electrical properties of graphene can be tuned by raising or lowering the Femi level, which allows to unprecedentedly tune its electromagnetic structure by gate voltages in GFETs.[132] In addition, the fast energy relaxation of photoexcited electrons and holes through optical-phonon emission in graphene and their relatively slow recombination would lead




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to population inversion over a wide energy range under sufficiently high pumping intensity.[117,140] These unique features of graphene can be exploited to develop many high-performance terahertz devices including terahertz sensitive GFETs.

3.2. Theoretical Modeling of GFETs for Terahertz Detection Prior to experimental studies, some theoretical models were designed to develop novel graphene-based terahertz devices, such as resonant or nonresonant terahertz detectors, frequency multipliers, sources of terahertz radiation, terahertz plasmon oscillators and so on. GFETs can be used as transducers based on plasma wave excitation under terahertz irradiation. On the other hand, graphene can absorb terahertz waves due to interband transition and generate electron and holes in the channel of GFETs. Both processes could be utilized to develop terahertz sensitive GFETs according to the theoretical simulations reviewed below.

3.2.1. Plasma Waves in GFETs In 2006, V. Ryzhii reported the simulation of terahertz plasma waves in gated graphene heterostructures.[141] As shown in Figure 2a, the GFET has the standard bottom-gate FET structure with n+-Si substrate and SiO2 layer as the gate and the gate dielectric, respectively. The collective behavior of the degenerate 2Delectron or hole gas system in the GFET was regarded as plasma waves in terahertz range and carrier scattering was ignored in this model. Plasma waves with sufficiently long wavelength exhibited a linear dispersion ω = ks and the plasma wave velocity s ∝ vFWG1/4VG1/4 was related to three factors, including the Femi velocity of electrons vF, the thickness of the gate dielectric layer WG and the gate voltage VG. The plasma waves in the GFET could fall within the terahertz range even when the wave length is relatively long because the plasma wave velocity (s >> 108 cm/s) in graphene significantly exceeds that of 2Delectron gas in normal heterostructures.[142] More importantly, the plasma wave frequency could be tuned by the gate voltage in a wide range (Figure 2b), which is useful for realizing voltagetunable terahertz devices. The plasma wave velocities in GFETs are related to not only the gate voltage but also the temperature. So V. Ryzhii et al. then further generalized the study of plasma waves for the variations of gate voltages and temperatures in wide ranges.[143] These works indicated that GFETs can be used in different voltage tunable terahertz devices utilizing the plasma waves in graphene channel. It is noteworthy that some terahertz detectors were successfully fabricated based on this working principle very recently, which will be addressed in session 3.3.

3.2.2. Terahertz Absorption of Graphene in GFETs Graphene nanoribbon phototransistors were first proposed for FIR and terahertz detectors by V. Ryzhii et al. in 2008.[144] The performance of the devices was estimated by an analytical device model. The energy gap between the valence and conduction bands in the graphene nanoribbons as well as the intraband


Figure 2. (a) Schematic view of a GFET. (b) Dependences of plasma wave frequency on the gate voltage in GFETs with different gate dielectric thicknesses. Reproduced with permission.[141] Copyright 2006, American Institute of Physics.

subbands could be engineered by changing the shape, in particular, the width of the graphene nanoribbons.[144–149] The electron and hole confinement in one of the lateral directions resulted in the band gap between valence and conduction bands. The schematic architecture of a graphene nanoribbon phototransistor was shown in Figure 3a and 3b. The GFET had a dual-gate structure with optical input from the bottom, assuming that the substrate and the dielectric layer sandwiching the graphene nanoribbon array are transparent. It was estimated that the maximum responsivity of graphene nanoribbon phototransistors in the infrared and terahertz range could reach 50–250 A/W at room temperature, which will exceed the responsivity of the customary terahertz detectors made of narrow gap semiconductors like PbSnTe and CdHgTe.[150] Similar to graphene nanoribbons, the energy gap of bilayer graphene between the valence and conduction bands could also be tuned by the transverse electric field in GFETs.[151–153] Bilayer graphene field-effect phototransistors for terahertz and IR detection were also proposed and analyzed theoretically by V. Ryzhii and M. Ryzhii in 2009.[154] The device has the dual gate structure shown in Figure 3a. The positive bias of the back gate create the conducting source and drain section in the channel and the negative bias of the top gate create the potential barrier which could be controlled by the charge of photogenerated holes (Figure 3c). Both the potential distribution and the energy



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Figure 3. (a) Schematic view of a dual-gate GFET based on graphene nanoribbons. (b) Top view of graphene nanoribbons. Reproduced with permission.[144] Copyright 2008, Japan Society of Applied Physics. (c) Band diagram of a dual-gate GFET based on bilayer graphene under gate bias. Dark arrows indicate propagation directions of electron (opaque circles) and holes (open circles). Reproduced with permission.[154] Copyright 2009, American Physical Society. (d) Schematic view of a GFET with back-gate and top-gate structure. Reproduced with permission.[156] Copyright 2009, Elsevier.

gap in different section of the channel could be changed by gate voltages, which were closely related to the detection features of bilayer graphene phototransistors. The spectral characteristics, dark current, responsivity and photoelectric gain were calculated as functions of the applied voltages. It was estimated that the maximum responsivity, photoelectric gain and detectivity could reach 200 A/W, 360, and 109 cm Hz/W (Jones) at room temperature, respectively. It was also predicted that graphene bilayer phototransistors with optimized structure at the properly chosen applied voltages could surpass the other types of photodetectors such as HgCdTe and InSb -based detectors due to relatively high quantum efficiency. Theoretical models of terahertz and IR photodetectors based on multiple-layer graphene transistor structure with the reverse biased p-i-n junctions were also proposed by V. Ryzhii et al.[155,156] The schematic structure of the device is shown in Figure 3d. The two split gates are both separated from the multiple graphene layers by an insulating layer and provide the formation of the electrically induced p- and n-sections under applied gate voltages. The operation of this device is associated with the interband photogeneration of electron and hole pairs in the intrinsic section (i) of graphene layers by incoming terahertz or IR radiation. Under a biased voltage, the photogenerated electrons and holes propagate through the channel in the directions toward the n-section and the p-section, respectively. The device exhibited high responsivity and detectivity in terahertz and IR range at room temperature due to the relatively high quantum efficiency and low thermal generation rate. Based on this model, the authors predicted that the responsivity and detectivity of this phototransistor could substantially exceed those of the devices with other narrow-gap semiconductors. Although terahertz detectors relying on the light absorption of graphene channel in GFETs were simulated with the above models, real devices have never been reported until now, which is probably due to the difficulties in device fabrication and the

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low photon energy in terahertz region (only ∼41 meV at 10THz) that is too low to induce interband transitions in real devices.[101]

3.2.3. Plasmonic Enhancement in Terahertz Range Plasmons can effectively confine optical excitations in nanoscale volumes and lead to strongly enhanced absorption and further improvement in the performance of phototransistors.[12,157,158] Moreover, a plasmonic device requires a channel with low scattering for deeper modulation depth of resonant modes. It has been reported that graphene is an excellent candidate for plasmonic detectors at room temperature.[159,160] Periodic metallic grating gates in GFETs were theoretically proposed to enhance optical absorption in terahertz range by A. Abbas et al.[159] in 2012 and M. Karabiyik et al.[161] in 2013. The structure consists of a graphene layer on a sapphire substrate, a SiO2 layer deposited on graphene and periodic titanium grating gates on the top. A commercial simulation package of finite-difference-time domain (FDTD) method was used to investigate the plasmonic absorption characteristics of the device at terahertz frequency. According to FDTD simulations, the devices can even present pronounced resonant absorption peaks up to 6th harmonic at the room temperature in the frequency range from 1 to 8 THz. Moreover, the resonant frequency could be tuned by the gate voltage due to the modulation of channel carrier concentration. So the theoretical studies of the plasmonic enhancement in graphene will provide guidelines for the fabrication of GFETbased terahertz detectors with higher sensitivities.[162]

3.3. Characterization of Terahertz Sensitive GFETs Although numerous theoretical works on terahertz detectors based on GFETs have been carried out, only a few experimental




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studies have been reported until now. As explained in the above session, terahertz sensitive GFETs should have very high carrier mobilities to ensure the generation of plasma waves in terahertz range.[124] Therefore, only pristine graphene prepared by mechanical exfoliation has been successfully used in terahertz detectors. Room-temperature terahertz detectors based on GFETs were firstly realized by L. Vicarelii et al. in 2012.[9] The device structure of antenna-coupled terahertz GFETs was shown Figure 4a. A single- or double-layer graphene flake was mechanically exfoliated and attached on a Si/SiO2 substrate as the active layer of a top-gate GFET. A single lobe of a logarithmic-periodic circulartoothed antenna with an outer radius of 322 µm patterned by e-beam lithography was defined as a source contact for coupling terahertz radiation. The drain of the GFET was a metal line connected to an operational amplifier and could be regarded as open circuit, similar to the case in the theoretical model of FET-based terahertz detectors proposed by V. Ryzhii.[141] HfO2 insulating layer with the thickness of ∼35 nm was deposited

on the graphene layer by atom layer deposition method, followed by the deposition of top gate with the same antenna structure of source contact. The channel length was 7 to 10 µm and the gate width was 200 to 300 nm. The nonlinear response to the oscillating radiation field at the gate electrode was attributed to the contributions of thermoelectric and photoconductive origin. A DC signal between the source and the drain was induced by the received electromagnetic waves and was proportional to the received power. The asymmetry of the source and the drain in the coupling with radiation was key to the generation of DC signal. The selective responsivity to both the spatial mode and the polarization of incoming radiation was determined by the geometry of coupling antennas and the gate voltage. The responsivity of the GFET detector under 0.3 THz radiation could reach 70–150 mV/W at room temperature, as shown in Figure 4b and c, which however was much lower than those of terahertz FET detectors based on other semiconductors, such as the Si MOSFETs (R ∼ 200 V/W).[154]

Figure 4. (a) Antenna-coupled GFET-based terahertz detector. The antenna has resonant frequencies of ∼0.4, 0.7, 1, 1.4 THz. (b–c) The responsivity as a function of VG for detectors based on (b) SLG (with data for different angles between the beam polarization axis and the antenna axis) and (c) BLG. Reproduced with permission.[9] Copyright 2012, Nature Publishing Group. (d) Inner part of an antenna with interdigitated structure; Output signals of devices on (e) high-resistive substrate and (f) low-resistive substrate measured at a wavelength of 68 µm. Reproduced with permission.[163] Copyright 2013, American Institute of Physics.




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The devices exhibited the minimum NEP of ∼200 nW/Hz1/2 for single-layer graphene and ∼30 nW/Hz1/2 for bilayer graphene at room temperature, which were higher than those of previously reported FET based on InAs nanowires (NEP ∼1 nW/Hz1/2)[128] and Si (NEP ∼0.1 nW/Hz1/2).[100] The possible reason was due to the low on/off ratio of the GFETs compared with normal transistors. The GFETs were then successfully used for large-area terahertz imaging and showed objects clearly in a packaged cardboard box, indicating that the GFET terahertz photodetectors are beyond proof-of-concept and suitable for further development for practical applications. Recently, a room-temperature ultrafast GFET terahertz photodetector with the similar antennas as electrodes was reported by M. Mittendorff et al.[163] A graphene flake with a size of ∼10 µm × 10 µm was prepared by mechanical exfoliation and attached on a Si/SiO2 substrate as the active layer of a GFET. Source and drain contacts with the structure of logarithmicperiodic antenna were patterned on the graphene flake by e-beam lithography. The source contact was made of a 60 nmthick Pd layer while the drain was a bilayer consisting of Ti and Au layers with the thicknesses of 20 nm and 40 nm, respectively. The different source and drain electrodes would break the symmetry of the device and improve the photoresponse. The device at the center was structured into an interdigitated comb shown in Figure 4d and the outer diameter of the antennas was 1mm that determined the maximum wavelength of detectable radiation. Moreover, the wavelengths that were smaller than the shortest element of the antenna (∼10 µm) could not be effectively coupled to the antenna. The devices were characterized by using free-electron laser (FEL) that provided IR and terahertz pulses in the wavelength range from 5 µm to 250 µm with the repetition rate of 13 MHz. One device showed the signal rise-time of 50–100 ps, which was related to the resistance of the Si substrate. Another device on a higher resistive substrate showed shorter response time probably due to the shorter RC time of the circuit, as shown in Figure 4e and 4f. The responsivities of the detectors were only 510 nA/W at all characterized wavelengths from 31 to 151 µm (2–10 THz), which however are lower than those of many other FET-based terahertz detectors. So the graphene detector is one of the fast detectors in terahertz range but only suitable for applications under intense sources because the responsivity is relatively low. Moreover, compared with many other detectors, the GFET terahertz detectors can be prepared more conveniently. The above terahertz detectors have special antenna structures that can absorb terahertz light because the light absorption of graphene in this range is very low. Plasmonic technology can largely enhance the interaction between graphene and terahertz radiation,[103,132,137,139,164,165] which is expected to lead to better performance of the photodetectors. The study of tunable plasmonic excitations and light-plasmon coupling at terahertz frequencies in graphene micro-ribbon arrays was firstly reported by Ju et al. in 2011.[103] The structure of graphene-nanoribbonarray FET was shown in Figure 5. For incident light with the polarization direction perpendicular to the ribbon, an absorption peak due to plasmon oscillation was observed, as shown in Figure 5c. The strong light-plasmon coupling in graphene induces over 13% absorption at a resonant peak, which is one order of magnitude larger than that achieved in 2Delectron gas

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Figure 5. (a) Top-view illustration of a GFET based on graphene microribbon array. (b) Side view of the GFET on a Si/SiO2 substrate with ion-gel top gate. (c) Absorption peak of the graphene micro-ribbon array at 3 THz (1 THz = 33.3 cm−1) due to plasmon excitation. Solid line: experimental results; Dash lines: Plasmon resonance is characterized by a Lorentzian lineshape (blue dashed line). A small free carrier contribution described by Drude absorption (magenta dashed line) is also present as a result of graphene absorption outside the fabricated micro-ribbon array area. Reproduced with permission.[103] Copyright 2011, Nature Publishing Group.

in conventional semiconductors. Moreover, the terahertz resonance from 1–10 THz could be straightforwardly engineered by varying the ribbon width and the gate voltage. Therefore, the plasmonic technology is a feasible way for realizing highperformance terahertz graphene phototransistors in the future.

4. IR Sensitive GFETs IR radiation is typically referred to the electromagnetic radiation in the frequency range from about 1013 Hz to 4 × 1014 Hz (wavelengths from about 30 µm to 750 nm). Generally, there is no clear definition to distinguish the boundary between terahertz and IR radiation. The wavelength range from 0.75 µm to 3 µm is often called near-infrared (NIR), 3 µm to 12 µm middle infrared (MIR), and 12 µm to 30 µm far-infrared (FIR).[166] Although IR radiation is not visible to the naked eye, it has better transmission through various media than visible light because the relatively long wavelength of IR radiation leads to less scattering. IR detectors have very broad applications, such as biological/medical imaging (transparent tissue windows at




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800 nm and 1100 nm), thermal imaging (>1500 nm), telecommunication (1300–1600 nm), and remote sensing, etc.[167–170] Similar to other photodetectors, IR detectors can be divided into two main types including thermal detectors and photon detectors based on their operational principles.[171] For a thermal detector, incident radiation is absorbed to change the temperature of an active material and lead to changes of some physical properties that will generate an electrical output. The light responses of thermal detectors are generally wavelength independent, relatively slow and poorly sensitive, but most of the devices are cheap and can be conveniently operated at room temperature. For a photon detector, radiation is absorbed within the material (normally a semiconductor) by interaction with carriers (electrons or holes) and the change of carrier energy distribution can result in an electrical output signal. The IR response of a photon detector is related to the energy band structure of the semiconductor used in the device and is thus wavelength dependent. There are several types of IR photon detectors including photodiodes, photoconductors and phototransistors.[12,172] Compared with thermal detectors, photon detectors exhibit faster responses and higher sensitivities. However, many IR photon detectors require cryogenic cooling which makes them expensive and inconvenient. There are many different types of IR sensitive GFET that have been reported and the sensing mechanisms are quite different as addressed in Section 2.3, depending on device designs and operation conditions.

responsivity for devices based on pure QDs. Due to the relatively low carrier mobility in PbS QDs (1 × 10−3 to 1 cm2/Vs), it is rather difficult to improve the responsivity to a higher value. In 2012, Z. H. Sun et al. reported NIR hybrid phototransistors based on PbS QDs and semiconducting polymer poly(3-hexylthiophene) (P3HT) films prepared by a solution process.[183] Devices showed high responsivities of up to 2 × 104 A/W under NIR illumination (wavelength λ = 895 nm), which is one order of magnitude higher than those of photoconductors based on pure PbS QDs. Although great advances have been made in high-performance IR photodetectors, the detectable wavelengths are still limited in NIR and MIR range below ∼5 µm due to the limited bandgap of semiconductors.

4.2. IR Sensitive GFETs with Mechanically Exfoliated Graphene Graphene is a promising material for IR detectors[5,21,186] for its broadband absorption from IR to UV range.[6,28,154–156] Lots of IR sensitive GFETs reported before are based on mechanically exfoliated graphene because the coupling of carriers to acoustic phonons is weak in high-quality graphene. Consequently, nonequilibrium distribution of hot carriers within the energy range lower than the optical phonon energy can be excited in the devices and lead to strong photoresponse to IR light.[187]

4.2.1. IR Response of GFETs 4.1. Recent Development of IR Photodetectors IR detectors, especially uncooled detectors, have been developed rapidly in the past decades.[171,173–175] The first IR photoconductor was developed by T. W. Case in 1917.[176] He found that some materials, such as PbS, exhibited light response in IR range. Today, two types of materials, including InSb- and HgCdTe-based systems,[175] are commonly used in commercial IR photon detectors. In academic research, the applications of some novel functional materials, such as quantum wells (QWs),[177,178] QDs,[179–181] carbon nanotubes,[182] organic semiconductor devices[183] and graphene, in IR photodetectors have been extensively investigated because some of the devices are highly sensitive, low cost and more convenient to use. Optoelectronic devices based on colloidal QDs benefit from lowcost solution preparation. Many types of QDs have been successfully used in high-performance IR photon detectors.[12,179,183,184] In 2009, J.-M. Shieh et al. reported a Si QD-based three-terminal MOSFET which exhibits improved NIR optoelectronic response.[184] The high photo responsivity of 2.8 A/W obtained under 1550 nm radiation could be attributed to an improved current gain due to the amplification function of the transistor. Very recently, high-performance MIR HgTe/As2S3-QD phototransistors were reported by E. Lhuillier et al.[179] A responsivity greater than 100 mA/W and a detectivity of 3.5 × 1010 Jones had been obtained at the wavelength of 3.5 µm. E. H. Sargent et al. reported the ultrasensitive solution-cast PbS-QD photodetectors, which showed large responsivities greater than 103 A/W and normalized detectivities up to 1.8 × 1013 Jones at the wavelength of 1300 nm at room temperature.[185] This is the record


Besides the fundamental physical properties of graphene, the design and the operating conditions of GFETs are also critical to the photosensitive performance. The IR sensitive GFETs reported by many research groups actually had different device structures and were characterized at different conditions (e.g., temperature and bias voltage, etc.). Therefore, the related sensing mechanisms concluded from the different experimental results were diversified and even somewhat controversial. Nevertheless, the photoresponses of the devices were normally related to hot electrons excited by photons,[187] which can induce photovoltaic, photothermoelectric or bolometric effects.[98,108,115,188] In 2009, ultrafast IR sensitive GFETs were reported by F. N. Xia et al. from IBM.[98] The device structure is shown in Figure 6a. Mechanically exfoliated graphene was deposited on a Si/SiO2 substrate. Two types of high-frequency coplanar waveguide wirings made of Ti/Pd/Au (0.5/20/20 nm) metal films were deposited by electron-beam evaporation and patterned by electron-beam lithography. The maximum external DC responsivity under the irradiation of 1550 nm IR light could reach up to ∼5 mA/W at a gate bias of 80 V in devices with two to three layers of graphene. Internal quantum efficiencies at high gate voltages were estimated to reach 6–16%. More importantly, the photoresponse of the GFETs did not degrade with optical modulations up to 40 GHz, as shown in Figure 6c. The bandwidth of such a GFET photodetector is actually limited by the RC time of the transistor because the transit time limited bandwidth of the GFET is estimated to be about 1.5THz due to the ultrahigh carrier mobility in the channel. The photocurrent was attributed to the photovoltaic effect near graphene-metal contacts due to built-in electric fields, shown in Figure 6d.[108,115,188]



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Figure 6. (a) SEM and optical (inset) images of a broad-bandwidth IR sensitive GFET. Scale bars: main panel, 2 µm; inset, 80 µm. (b) Typical I–V curves of a GFET without and with light excitation. Inset: schematic of the photocurrent measurement. (c) Relative a.c. photoresponse S21 as a function of light intensity modulation frequency up to 40 GHz at a gate bias of 80 V. Inset: peak d.c. and high-frequency (a.c.) photoresponsivity as a function of gate bias. (d) Potential profile and photocurrent generation mechanism in a GFET operated under a short-circuit condition. Reproduced with permission.[98] Copyright 2009, Nature Publishing Group.

Then the same IBM research group reported the application of the GFETs for high-speed IR optical communication in 2010.[13] The GFET photodetectors were firstly used in a 10 Gbit/s optical data link. The GFET photodetector has an interdigitated source and drain electrodes, similar to that of traditional metal-semiconductor-metal (MSM) detector, as shown in Figure 7. A mechanically exfoliated bilyer graphene flake was deposited on a Si/SiO2 substrate. The source and drain electrodes were made of Pd/Au and Ti/Au, respectively. Due to the different work functions of Pd and Ti, the asymmetric design of the electrodes break the mirror symmetry of the builtin potential profile within the channel and lead to more efficient photodetection. The maximum external photoresponsivity of 6.1 mA/W was achieved under the irradiation of the wavelength of 1550 nm at room temperature, which was ∼15 times higher than that of the previously reported device with symmetric source and drain electrodes.[98] Because graphene has very similar light absorbance in the wavelength range from 300 nm to 6 µm, it is reasonable to expect that the GFET photodetectors will have similar responsivities in a broad wavelength range. Although photothermoelectric effect in the device could contribute to photocurrent generation, the high responsivity is primarily attributed to the charge separation of excited electron and hole pairs under internal E-field near graphene/metal contacts, in other words, the photovoltaic effect. Very recently, the same group reported that the photoresponse of biased GFET under light irradiation was dominated by the combination of photovoltaic and photo-induced bolometric effects.[118] They found that the operating conditions

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determine the photocurrent generation mechanism in graphene as shown in Figure 8. At low electrostatic doping conditions near the Dirac point, the photocurrent mechanism was dominated by photovoltaic effect. At highly n-type or p-type doping conditions, the photocurrent was dominated by the photo-induced bolometric effect. This work provides a possible approach of engineering the hot carrier photoresponse, which will be useful in future applications. It was found that the bolometric effect in GFET photodetectors is pronounced at low temperature. J. Yan et al. reported that the photoresponse in dual-gated GFETs under IR illumination (wavelength: 10.6 ∼ 0.658 µm) below 20K was dominated by a hot-electron bolometric mechanism.[21] As shown in Figure 9, the resistance of the graphene channel sandwiched between two metal gates was measured with a four-probe method. Due to small electron heat capacity and weak electron-phonon coupling in graphene,[189,190] IR irradiation could easily change the electron temperature and thus lead to a resistance variation in graphene. The GFET-based bolometer has many advantages, including fast response time, high sensitivity and low noiseequivalent power (NEP). The GFET device has the intrinsic response speed three to five orders of magnitude higher than commercial Si bolometers and superconducting sensors. The voltage responsivity of the device is ∼2 × 105 V/W, being comparable to that of commercial Si bolometers (104 ∼ 107 V/W), and the NEP could reach 33 fWHz1/2 that is several times lower than those of commercial ones. Moreover, the sensitivity of the device can be optimized to much higher values at lower temperatures (less than 1K) because of the strong temperature




REVIEW Figure 7. (a) Three-dimensional schematic of a GFET with asymmetric metal contacts. Inset: SEM image of a GFET. Scale bar: 5 µm. The spacing between the metal fingers is 1 µm and the finger width is 250 nm. (b) Photocurrent generated at zero source-drain bias versus gate voltage. Spot size of the excitation light is ∼5 µm in diameter. Total incident power is 5 mW. (c) Relative photoresponse versus modulation frequency of light intensity. The 3-dB bandwidth of this photodetector is ∼16 GHz. Inset: receiver eye-diagram obtained using this GFET photodetector, showing a completely open eye. Scale bar, 20 ps. (d) Current versus source-drain bias (VB) with and without light illumination, at an excitation wavelength of 1.55 µm. Inset: measured external photoresponsivity of the photodetector as a function of source-drain bias. Reproduced with permission.[13] Copyright 2010, Nature Publishing Group.

dependent heat resistance of graphene. Therefore, the technique could be a feasible way for achieving single-photon detection that is extremely important in contemporary astronomy and other fields. Further measurements of electrical transport and photoresponse were recently done by the same research group to study the bolometric and photovoltaic effects of dual-gated GFETs.[117] They found that the two effects have the identical response times at different gate voltages and temperatures, indicating that the two effects were governed by the same hot electronphonon thermal relaxation process. The thermal relaxation (cooling) of hot electrons by optical phonon in graphene occurs in picosecond time scale while the acoustic phonon assisted cooling process normally takes place in subnanosecond to nanoseconds. As shown in Figure 9c, the response times of the device were characterized to be 20–100ps for temperatures from 3–100 K, indicating that hot electron relaxation occurs through acoustic phonon emission. The response time was decreased to 25 ± 5ps at the temperatures above 10K, which will limit the cutoff frequency of the GFET-based IR detectors. The photothermoelectric effect was also proposed to explain photocurrent generation in graphene-based devices by X. Xu et al.[114] and M. C. Lemme et al.[111] The hot carriers were regarded to play an important role in the photothermoelectric effect in photosensitive GFETs.[20,21,113,117,191] In 2011, J. C. W. Song et al. reported that a novel type of photoresponse in graphene was


dominated by photothermoelectric effect and the hot carriers can result in efficient carrier multiplication to enhance the photocurrent response due to the weak electron phonon interactions.[113] More clear photocurrent generation mechanism about photothermoelectric effect were clarified experimentally by N. M. Gabor et al. in 2012.[20] The device geometry and band structure were shown in Figure 10a and b. The six-fold symmetry of photovoltage patterns as functions of bottom- and top-gate voltage were directly observed, as shown in Figure 10c. Therefore, it is clear that the intrinsic photoresponse in graphene is dominated by nonlocal hot carrier transport, rather than the photovoltaic effect. The measurement of photocurrent response time by timeresolved scanning photocurrent microscopy also demonstrated that hot carriers instead of phonons dominated energy transport across a tunable graphene p-n junction excited by ultrafast laser pulses.[191] The ultrafast response time for the graphene devices was predicted to reach a fundamental bandwidth of ∼500 GHz, which means the possibility of using graphene phototransistors to higher operating speeds.

4.2.2. GFETs Integrated with Optical Waveguides or Microcavities Due to relative low optical absorption of normal incident light by monolayer graphene, the responsivities of graphene based photodetectors were usually limited to several mA/W. The



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Figure 8. (a) Schematic of the photoconductivity measurement set-up (b) Photocurrent (a.c.) at VD = 0 V drain bias (short-circuit photocurrent) and VG = 5 V gate bias (n-type regime). Left: the laser-scanning image shows the source and drain electrodes. The outline of the graphene sheet is indicated. Middle: amplitude R of the photocurrent. Right: phase φ of the photocurrent. (c) Photocurrent (a.c.) under a drain bias of VD = –1 V and gate voltage of VG = 5 V. Laser scanning, amplitude R and phase images are shown. Reproduced with permission.[118] Copyright 2013, Nature Publishing Group.

utilization of in-plane evanescent wave is regarded as a effective technology to enhance the interaction between light in an optical waveguide and graphene.[192–195] This technology was successfully used to realize IR photodetectors with high responsivities by X. Wang et al.[196] The geometry of a waveguide-based photodetector with graphene/Si-heterostructure is shown in Figure 11a. The device could be regarded as a transistor with the gate voltage of zero. The responsivity as high as 0.13 A/W was achieved for 2.75 µm IR light at room temperature when 1.5V bias voltage was applied on the two Au electrodes. They found that the responsivities of the device in different wavelength ranges (visible, NIR and MIR) are three orders of magnitude different due to different working mechanisms. The high responsivity in MIR range could be attributed to strong absorption of evanescent light that propagates parallel to the graphene layer in the in-plane optical waveguide. A similar device was reported by X. Gan et al., which can be operated at high frequencies exceeding 20 GHz with the responsivity higher than 0.1 A/W, as shown in Figure 11b and c.[197]

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At the same time, A. Pospischil et al. reported a Si waveguide-integrated graphene photodetector with wideband.[198] The device is compatible with Si complementary metal-oxide semiconductor (CMOS) technology, which is key to realizing cost-effective integration of the devices and optics on a single chip. The schematic structure of the waveguide-integrated graphene photodetector and the photocurrent under different illumination power are shown in Figure 11d and e. A flat response was obtained across all optical telecommunication windows covering the wavelength range from 1300 nm to 1700 nm, as shown in Figure 11 f, limited only by the cut-off properties of the silicon waveguide. The maximum responsivity could reach ∼0.05 A/W, an order of magnitude larger than that achieved with normal-incidence graphene photodetectors.[13] The fraction of light absorbed in the bilayer graphene sheet was estimated to exceed 40% that is much higher than that of normal incident light. The internal quantum efficiency of the device was estimated up to ∼10%. Ultra-wideband operation and compatibility with CMOS technology would make the




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Fabry-Pérot microcavity-integrated graphene photodetector had been studied by M. Furchi et al. for IR detection, as shown in Figure 12b.[199] The optical absorption was 26-fold enhanced and could reach > 60% in the devices for 850 nm NIR light, which led to a responsivity up to 21mA/W. Moreover, the devices were only sensitive to designed wavelengths, making them promising for wavelength division multiplexing. The enhanced optical absorption and the wavelength-response selectivity make the planar microcavity controlled graphene devices good candidates for not only photodetectors but also many other applications, like electroabsorption modulators, optical attenuators, and light emitters.

4.2.3. GFETs with IR-Excited Plasmons

Figure 9. (a) Schematic of device geometry and electric-field-effect gating. (b) Optical microscopy image of a bilayer graphene device. Scale bar: 5 µm. Reproduced with permission.[21] Copyright 2012, Nature Publishing Group. (c) Temperature dependence of thermal response time for several different dual-gate voltage settings. Reproduced with permission.[117] Copyright 2013, American Physical Society.

silicon waveguide-integrated graphene photodetectors good candidates for future applications in IR detection. Integration with an optical microcavity is another approach to enhance light-matter interaction in a graphene photodetector.[199,200] The cavity-induced confinement can enhance light absorption rate of intra-cavity graphene and improve the efficiency and the spectral selection of photocurrent generation in devices.[200,201] M. Engel et al. reported the microcavity-integrated GFETs with the device architecture shown in Figure 12a.[200] It was found the enhancement of photocurrent could reach a factor of 20 in devices. Another


IR-radiation-excited plasmons in graphene have attracted great attention in recent years due to their potential applications in photoelectronics.[202–210] Z. Fei et al. reported 2Dplasmons of Dirac femions in mechanically exfoliated graphene on Si/SiO2 substrates excited by MIR irradiation.[203] It was found that the Dirac plasmons could dramatically enhance the near-field interaction with MIR surface phonon of SiO2 substrate at high inplane wavevectors and the plasmonic effect could be controlled by gate voltage. The absorption enhancement of MIR light based on graphene plasmonic technology was simulated by T. R. Zhan et al.[204] The plasmons were formed by a graphene monolayer on subwavelength dielectric grating (SWDGs) using scattering-matrix method, as shown in Figure 13a, b. It was predicted that the peak absorbance could reach 92% in 1DSWDG at normal incidence and 91% in 2DSWDG, which are much higher than the intrinsic absorption of a monolayer graphene. Therefore, the introduction of SWDGs could offer an excellent approach to enhance optical absorption in graphene and improve the sensitivity of graphene-based photodetectors. N. K. Emani et al. developed a hybrid structure of metal plasmonic fabricated on CVD grown graphene to enhance plasmonic resonances,[206] as show in Figure 13c,d. The plasmonic resonance in IR range could be controlled by the bowtie antenna period as well as the gate voltage. The interaction enhancement of incident IR light with the graphene sheet implies potential applications in realizing highly sensitive IR detectors based on this technique.

4.3. IR Sensitive GFETs with Graphene Nanoribbons Graphene nanoribbons were theoretically proposed for IR photodetectors due to their tunable bandgap in IR range.[211,212] The bandgaps of graphene nanoribbons can be easily tailored by engineering their widths.[147,148,213–215] J. Cai et al. reported that the maximum bandgap of graphene nanoribbons can reach 1.6 eV corresponding to a wavelength of ∼770 nm.[215] In other word, the bandgap absorption of graphene nanoribbons can cover all IR spectrum. The first graphene nanoribbon phototransistor for FIR and terahertz photodetection was proposed by V. Ryzhii et al. in 2008,[149] which is discussed in Section 3.2.2 for terahertz phototransistors. The IR photodetector based on single-layer armchair graphene nanoribbons with p-i-n structure



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Figure 10. (a) Optical microscope image of the dual-gated device incorporating boron nitride top-gate dielectric. Dashed white lines mark the boundaries of multilayer graphene. VTG, top-gate voltage. (b) Resistance versus bottom-gate voltage VBG and top-gate voltage VTG at VDS = 1.4 mV and T = 175 K. Four regions are labeled according to carrier doping, p-type or n-type. CNP is charge neutrality point. laser wavelength λ = 850 nm. (c) Experimental schematic (top) and schematic of monolayer graphene’s band structure of a p-n junction (bottom), showing electron bands (blue) and hole bands (red). The dashed line represents the Fermi energy EF. Reproduced with permission.[20] Copyright 2011, Science Publishing Group.

was proposed by E. Ahmadi et al.[212] The phototransistor has a dual top-gate structure, similar to that shown in Figure 3d. When the energies of incident photons were smaller than the energy gap of graphene nanoribbons, the photodetectors had

no response. It was estimated that the largest responsivity could reach ∼1.22 A/W at 77 K for single-layer graphene nanoribbons with the width of 2 nm. The IR photodetectors based on multi-layer armchair graphene nanoribbons were also modeled

Figure 11. (a) Schematic of a graphene photodetector integrated with silicon waveguide. Reproduced with permission.[196] Copyright 2013, Nature Publishing Group (b) Schematic of a waveguide-integrated GFET fabricated on an SOI wafer. (c) Dynamic optoelectrical response of the device. The relative a.c. photoresponse as a function of light intensity modulation frequency shows ∼1 dB degradation of the signal at a frequency of 20 GHz. Inset: 12 Gbit s−1 optical data link test of the device, showing a clear eye opening. Reproduced with permission.[197] Copyright 2013, Nature Publishing Group. (d) Colored SEM image of a waveguide-integrated graphene photodetector. (e) Photocurrent as a function of the incident optical power. (f) Wavelength dependence of the photocurrent. A flat response is obtained across all optical telecommunication windows. Reproduced with permission.[198] Copyright 2013, Nature Publishing Group.

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by E. Ahmadi et al.[216] The band structure and the energy gap of armchair graphene nanoribbons were calculated by using tight-binding model, including the edge deformation. It was found that the dark current limited detectivity increased with the increase of the number of graphene nanoribbon layers. The detectivity also increased with the increase of gate voltage (beyond threshold voltage) and the decrease of temperature. For a single layer graphene nanoribbon with the width of 5 nm, the detectivities were estimated to be ∼2.2 × 108 Jone at 300 K and ∼2.2 × 1011 Jone at 77 K. A quantum mechanical simulation was carried out to model Schottky-contacted single-layer-graphene photodetectors by Q. Gao et al.[217] The phototransistor had the normal bottom-gate structure. The largest internal quantum efficiency of 10.2% and the responsivity of 3.4 mA/W


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Figure 12. (a) Schematic representation and electrical interconnection of a GFET integrated with a microcavity. Inset: cross-sectional view of the device. The graphene sheet is embedded between two Ag mirrors and separated by two dielectric layers (Si3N4; Al2O3). The thickness L of the dielectric stack between the cavity mirrors determines the resonance wavelength λ of the optical microcavity. Also shown is a visualization of the intensity profile of the fundamental λ/2 cavity mode. Reproduced with permission.[200] Copyright 2012, Nature Publishing Group. (b) Schematic drawing of a graphene microcavity photodetector. Distributed Bragg mirrors form a high-finesse optical cavity. The incident light is trapped in the cavity and passes multiple times through the graphene. The graphene sheet is shown in red, and the metal contacts are in yellow. right: Electric field amplitude inside the cavity. Reproduced with permission.[199] Copyright 2012, American Chemical Society.

were estimated when the incident light only covered one-third of the channel area adjacent to source contact in the device with the same source and drain electrodes. The photocurrent was near constant in the photon energy range from 0.4 eV to 1.2 eV, which implies the operational wavelength range of the photodetector spanning across 1 to 3 µm. A periodic nanostructure of graphene nanoribbons was prepared by H. Yan et al. for MIR plasmonic resonance, as shown in Figure 14a.[207] CVD-grown graphene was transferred to Si/ SiO2 or Si/diamond-like carbon substrates and then nanoribbon arrays were formed by e-beam lithography and oxygen plasma etching. They observed that the plasmon resonance frequency was dependent on the width of nanoribbons, as shown in Figure 14b. The damping of plasmons might be resulted from graphene intrinsic optical phonons and scattering for the edges of graphene nanoribbons. Furthermore, dispersion and damping of plasmons could be affected by the substrates under graphene nanoribbons. Therefore, both the substrate and the optical phonons of graphene may be important to plasmon dissipation and lifetimes, which provides a guideline for the design of graphene-based plasmonic devices. More details of surface plasmon propagation in graphene had been investigated experimentally by Z. Fei et al.[209] and J. Chen et al.[208] The real-space images of plasmons were provided by near-field scattering microscopy with IR excitation light, as shown in Figure 14c. The plasmon wavelength in graphene exhibits the subwavelength feature, which is more than 40 times smaller than the wavelength of illumination. Both the amplitude and the wavelength of these plasmons could be tuned by varying the gate voltage, as shown in Figure 14d. Localized graphene plasmon resonance occurs for specific ribbon widths, which are slightly smaller than plasmon wavelengths. The photodetectors of graphene nanoconstrictions with individual gold nanogap antennas had been achieved by S. F. Shi et al.[218] The graphene nanoconstrictions between the individual nanogap gold electrodes acted as the channel of plasmon resonance, as shown Figure 15a and b. The plasmon-induced enhancement of the photocurrent could reach as high as 104 times at 770 nm and the maximum photoresponsivity of 6.11 μA/W could be achieved. The photodetectors also exhibited the strong dependence of photocurrents on the polarization of the incoming light and the polarization sensitivity could be up to 99%. The plasmon-resonant IR photodetectors based on graphene nanoribbon arrays were recently achieved by M. Freitag et al.[219] A single layer graphene was grown by CVD method on Cu foil and was then etched into nanoribbons by e-beam lithography with oxygen plasma. The widths of nanoribbons between 80 nm and 200 nm were designed to match plasmon resonance in MIR wavelength range, as shown in Figure 15c–d. The plasmon could interact with surface polar phonon of the SiO2 substrate, which would result in long-lived hybrid plasmon-phonon modes. The plasmonic enhancement factor could exceed an order of magnitude for s-polarization (polarization perpendicular to the nanoribbon axis) compared to excitations of electron-hole pairs alone for p-polarization (polarization parallel to the nanoribbon axis), as shown in Figure 15e. The enhanced light absorption due to localized plasmon and their subsequent decay primarily into phonons would result in photocurrent enhancement. However, the responsivities of the



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Figure 13. Schematic view of a graphene monolayer supported on (a) a 1D and (b) 2D subwavelength dielectric grating (SWDG) with thickness h, period Λ and strip/cylinder size Δ. In the SWDG (region II), blank and gray regions represent dielectrics of dielectric constants ε1 and ε2 respectively. The whole structure is on a dielectric-coated substrate (region IV) with dielectric constant ε3. The dielectric coating layer (region III) has dielectric constant ε2 and thickness d. Reproduced with permission.[204] Copyright 2012, American Physical Society. (c) SEM image of bowtie antenna array on top of a graphene sheet. (d) Measured optical transmission spectra for bowtie antenna arrays with different periodicities fabricated on graphene. Reproduced with permission.[206] Copyright 2012, American Chemical Society.

graphene plasmon-resonance photodetectors are still very low in IR range (only ∼10−5A/W) and further work is thus needed to improve the performance by optimizing the device design and fabrication conditions.

4.4. IR Sensitive Hybrid GFETs Although GFET photodetectors have shown fast response, being critical to high frequency applications, the responsivities of the devices are relatively low because the light absorption of a single layer graphene in IR range is about 2.3%. Therefore, various hybrid nanostructures have been proposed to enhance the performance of pure graphene photodetectors.[186,206,220] IR sensitive hybrid GFETs can be classified into two types according to their working principles. One type is the devices with graphene used as both light absorber and transport medium. Metal nanostructures on graphene for the enhancement of plasmonic resonance introduced in Section 4.2.3 can be regarded as this type of devices.[206] The second type of hybrid photodetectors have the active materials consisting of graphene and narrow-bandgap semiconductors. In this case, graphene is for transporting photo carriers due to its high carrier mobilities and the semiconductors are used to absorb IR light and generate carriers. G. Konstantatos et al. recently reported a hybrid GFET (Figure 16) that consisted of mechanically exfoliated monolayer

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or bilayer graphene covered with a thin film of PbS colloidal QDs, which showed a very high responsivity up to ∼107 A/W under IR illumination.[18] The device also demonstrated a high gain of ∼108 electrons per photon and a high detectivity of 7 × 1013 Jone. The high performance of the device is attributed to the following processes. Under light illumination, electron and hole pairs are generated in PbS QDs and holes tend to transfer to the graphene layer due to decreased energy. These holes have the main contribution to the photocurrent in the GFETs before they recombine with electrons trapped in the PbS QDs, as shown in Figure 16c. So the high gain could be attributed to the high hole mobility in graphene, which makes holes recirculate for many times before recombination. The photocurrent gain can be expressed as G = τlifetime/τtransfer, where τlifetime is the lifetime of photo-generated holes and τtransfer is the time of holes transferring across the channel from source to drain electrodes. Highly IR sensitive GFETs based on CVD-grown monolayer graphene and pyridine-capped PbS QDs with the similar sensing mechanism were recently reported by our group.[17] Compared with mechanically exfoliated graphene, CVD graphene can be more conveniently fabricated with larger area and is thus more suitable for practical applications. The structure of the GFET fabricated on a Si/SiO2 substrate is shown in Figure 17a. The maximum responsivity of the device under IR illumination, as shown in Figure 17b, can reach about 1 × 107A/W that is several orders of magnitude




REVIEW Figure 14. (a) Schematic for mid-infrared transmission measurement of graphene nanoribbons. The average width of the ribbons is ∼100 nm. (b) Extinction spectra (1 – Tper/Tpar) of graphene ribbons on diamond-like-carbon substrate with different ribbon widths. Tper and Tpar are the transmission of the light through the ribbon array with the electric field perpendicular or parallel to the ribbon, respectively. Inset: plasmon resonance frequency as a function of wave vector q = π/W, where W is the width of the nanoribbons. Reproduced with permission.[207] Copyright 2013, Nature Publishing Group. (c) Scattering-type scanning near-field optical microscopy (SNOM) for imaging propagating and localized graphene plasmons. Top: diagram of the experimental configuration used to launch and detect propagating surface waves in graphene (represented as blue rings). Middle: near-field amplitude image acquired for a tapered graphene ribbon on top of 6H-SiC. The imaging wavelength is λ0 = 9.7 µm. The tapered ribbon is 12 µm long and up to 1 µm wide. Bottom: colour scale image of the calculated local density of optical states (LDOS) at a distance of 60 nm from the graphene surface. (d) Near-field amplitude images for tapered CVD-grown graphene ribbons on a Si/SiO2 (300 nm) substrate, acquired under gate voltages VB ranging from –15 to +11 V. The illumination wavelength is λ0 = 11.06 µm. Localized modes are indicated by white and red arrows. Reproduced with permission.[208] Copyright 2012, Nature Publishing Group.

higher than those of PbS QDs-based IR photodetectors reported before.[185] The response of the GFET to light illumination was attributed to the change of effective gate voltage applied on the transistor, as shown in Figure 17c and d.[14,16] The ligand capped on the surface of PbS QDs is critical to the photocurrent and the response time of the device because the charge transfer process from the QDs to graphene layer was influenced by the ligand. It is interesting to note that the devices prepared on flexible substrates without gate electrodes showed similar photo responsivity and excellent bending stability (Figure 17e), indicating that the devices are suitable for high performance flexible IR photodetectors and will find broad potential applications in some emerging areas, like stretchable or wearable electronics. However, the response times of the devices are relatively long, so the devices are more suitable for some special applications that are not necessarily to be very fast.


Recent works on the graphene/Si heterojunction photodetectors also showed IR sensitive behavior. X. An et al. reported the performance of graphene/Si heterojunctions both in photocurrent and photovoltage modes.[221] In the photovoltage mode, the devices were extremely sensitive to weak NIR light with the photovoltage responsivities exceeding 107 V/W and the NEPs of ∼1 pW/Hz1/2. In the photocurrent mode, the devices were found to have tunable responsivities up to 435 mA/W and the corresponding incident photon conversion efficiency (IPCE) higher than 65%.

4.5. IR Sensitive GFETs Based on GO or rGO Because GO has low conductivity, the evolution of GO to rGO from insulator to semiconductor and then to semimetal by reduction have been systematically studied.[222] The bandgap



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Figure 15. (a) Schematic diagram of a graphene nanoconstriction in a nanogap between gold electrodes. (b) Polarization dependence of the photocurrent of the device at the wavelength of 780 nm, plotted relative to the long axis of the gold wire. Reproduced with permission.[218] Copyright 2011, American Chemical Society. (c) SEM image of a graphene nanoribbon array photodetector with 100 nm ribbon width and 100 nm spacing between ribbons (Scale bar is 2 µm). and schematic of the photoconductivity setup. (d) Scanning photocurrent image of the device. Scale bar is 30 µm. (e) Polarization dependence of the peak photocurrent under the same conditions. (f) Comparison of the lattice temperature increase of 140 nm GNR arrays (red spheres) and 2D graphene (black squares) under these conditions. Reproduced with permission.[219] Copyright 2013, Nature Publishing Group.

of rGO can be tuned by controlling the concentration of oxygen atoms, which is favorable to developing photodetectors with arbitrary target wavelengths.[223,224] GO and rGO have been successfully used in many solution processable IR photodetectors.[225–231] S. Ghosh. et al. firstly reported IR sensitive photoresistors based on large area rGO thin films,[225] in which the rGO sheets were reduced from GO deposited on glass substrates by solution process. The maximum photocurrent was obtained when IR illuminated was close to the electrode/rGO interface. The position dependent photoresponse was attributed to exciton dissociation at the Schottky barrier of the metal/rGO interface. Similar results of the photoresistors based on rGO were reported by B. Chitara et al.[226] However, the devices demonstrated relatively low responsivity of about 4 mA/W and external quantum efficiency of only 0.3% under the illumination of IR light (λ = 1550 nm). On the other hand, rGO is sensitive to ambient environment like moisture and oxygen, so the device performance is unstable in air. To solve this problem, T. Q. Trung et al. prepared the devices encapsulated with a hydrophobic tetratetracontane layer, which effectively improved the stability and reproducibility of devices.[227] Very recently, much more IR sensitive GFETs based on rGO were reported by H. Chang et al.[230] It was found that the IR photocurrent was significantly influenced by the careful control of oxygen defects and microstructure in rGO. The devices showed the responsivities of ∼0.7 A/W and the external quantum efficiencies of ∼97% under IR illumination (wavelength: 895 nm), as shown in Figure 17f. Other materials including GO,[229,232] Ag-functionalized hybrid rGO[231] and rGO/PbS nanocomposites[233] were also used in IR photoconductors or GFETs that however showed relatively poor performance compared with the aforementioned ones.

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4.6. IR Sensitive GFETs Based on Graphene Prepared with Other Methods Besides the types of graphene introduced above, graphene prepared by other methods was also tentatively used in IR sensitive GFETs.[102,234,235] N. Kurra et al. reported the preparation of transfer-free nanocrystalline graphene by using an electron beam induced carbonaceous deposition (EBICD) process.[234] Carbonaceous deposits were prepared and patterned on SiO2/Si substrates under focused electron beam and then transformed in nanocrystalline graphene pattern in the presence of Ni catalyst by thermal treatment in vacuum. The GFETs based on the nanocrystalline graphene exhibited p-type behavior with a mobility of ∼90 cm2/Vs. The devices were sensitive to IR laser (1064 nm) and showed photocurrents increasing linearly with the increase of laser power, which was attributed to the thermal effects as well as the photoexcitation under IR illumination. The maximum photocurrent was about 23% of the channel current under IR illumination. Recently, the same group reported IR photodetectors based on few layer graphene prepared with the similar methods.[235] Few layer graphene films were grown on SiO2/ Si substrates covered with sacrificial Ni thin films by thermal annealing with the aid of residual hydrocarbons or polymethylmethacrylate carbon sources in a vacuum chamber and then Ni thin films were etched to leave FLG on the substrates. The maximum relative current change of 73% could be obtained in this type of photodetectors. The photoresponse of the devices is attributed to the generation of photoexcited charge carriers. However, the photodetectors were not successfully characterized with standard parameters, like responsivity and detectivity, which makes it difficult to compare them with other GFET-based photodetectors.




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Another type is the device with graphene quantum dots (QDs) that has the band structure being different from that of pure graphene. Y. Zhang et al. reported the GFET based on graphene QDs that showed higher responsivity than that of pristine graphene FET under IR illumination.[186] The graphene QDs with different sizes were successfully prepared by using a Ti sacrificial layer. The trap states in graphene QDs can trap carriers and result in electrons and holes with longer lifetimes. Consequently, the carriers can recirculate for many times in the channel within their lifetime and a higher photocurrent could be achieved. The responsivities of 0.2 A/W for NIR light and 0.4 A/W for MIR light had been achieved.

5. Visible Sensitive GFETs Visible light is typically referred to the electromagnetic radiation in the wavelength range from 400 nm to ∼750 nm (frequency from ∼4 × 1014 Hz to 7.5 × 1014 THz), related to the sensitivity of human eyes. Visible photodetectors can be applied widely in many fields such as optical communications, remote sensing, spectrum analysis, surveillance, fluorescent biomedical imaging and so on.[236,237] In particular, visible photodetectors are the mainstream optical detectors for all imaging and video applications. The commercial visible photodetectors are mainly based on Si, Ge or other inorganic semiconductors.[238–241] Thin film visible photodetectors have also been developed by using QDs[185,236] and organic semiconductors.[172] Si-based photodetectors are popularly used for imaging applications in visible and NIR ranges due to their high sensitivity, low noise and good compatibility with current microelectronic technology. However, the Si-based photodetectors have suffered from crosstalk between the neighboring pixels and high temperature processing. In comparison, visible sensitive GFETs have many advantages including high sensitivity, easy fabrication, low cost and mechanical flexibility due to the unique electrical properties of graphene and the feasibility for patterning and transferring graphene by various methods.[241–243] Visible photodetectors based on GFETs with different device designs have been investigated by many groups from 2008.[108–110,115,188] In general, the optical absorption of graphene in visible range could induce the generation of photo-excited hot carriers in graphene and lead to photocurrents in GFETs,[35,244–246] being similar to the mechanism of IR sensitive GFETs.

5.1. Visible Light Sensitive GFETs Based on Mechanically Exfoliated Graphene 5.1.1. Device Performance and Sensing Mechanisms

Figure 16. (a) Schematic of a GFET based on graphene and PbS QDs. (b) Spectral responsivity R of a GFET with single layer graphene and PbS QDs. Exciton peak of the QDs is at 950 nm. (c) Energy level diagram of graphene/QD heterojunction and the doping effect of photo excited carriers. Reproduced with permission.[18] Copyright 2012, Nature Publishing Group.


The first visible sensitive GFET was prepared with mechanically exfoliated graphene by E. J. H. Lee et al.[108] Strong photoresponse was observed near two metal contacts under scanning photocurrent microscopy (SPCM), as shown in Figure 18a and 18b. Therefore the photoresponse of the device under visible light is dominated by contacts due to the potential drops at the metal/graphene interfaces. In addition, the photoresponse also



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Figure 17. (a) Schematic diagram of a GFET modified with PbS QDs under IR light. (b) Responsivity of a PbS QDs-modified graphene photoconductor as functions of applied voltage characterized under different light irradiance at the wavelength of 895 nm. (c) Transfer characteristics (IDS ∼ VG, VDS = 0.5 V) of a PbS QDs-modified GFET for different light irradiation at the wavelength of 895 nm. (d) Horizontal shift of transfer curves as a function of light irradiation (Ee). (e) Light irradiance dependent responsivity of a flexible PbS QDs-modified graphene photoconductor before and after a bending test. Reproduced with permission.[17] (f) Responsivity versus bias for a flexible IR sensitive photoconductor based on rGO with different reduction times (0, 20, 40, 90, 260 min). Incident radiation power is ∼14 mW/cm2. Black lines are fitted curves. Reproduced with permission.[230] Copyright 2013, American Chemical Society.

exhibited a gate-dependent modulation due to the changes of potential drops at the metal contacts. F. Xia et al. then revealed that the impact of the metal contacts on the channel potential profile can extend into the channel for more than 0.5 µm from both source and drain sides.[109,112] However, the maximum responsivity of the GFET based on single layer graphene was only about 1 mA/W when the light (wavelength λ = 632.8 nm) was focused on a region near the contacts, which is rather low compared with that of a commercially available photodetector.

J. Park et al. reported that both the photovoltaic current and photothermoelectric current can be observed in single-layer graphene FETs under SPCM depending on the operation conditions.[115] The strong electric field near the metal/graphene contacts can lead to exciton dissociation and the generation of photocurrent. They observed that more than 30% of the absorbed photon contributed to photocurrent near the contacts and the maximum responsivity of their devices to visible light (wavelength λ = 532 nm) could reach ∼10 mA/W. On the other

Figure 18. (a) AFM image of monolayer graphene with four gold electrodes. (b) Photocurrent image taken at VDS = VG = 0 V. Black and blue diamonds are positions for further measurements. Reproduced with permission.[108] Copyright 2008, Nature Publishing Group. (c) Schematics of the experimental setup and device geometry based on an single-layer and double-layer graphene junction. (d) Generated photocurrent (PC) at the single-layer/doublelayer graphene junction as a function of gate voltage Vg. Reproduced with permission.[114] Copyright 2010, American Chemical Society. (e) Schematic of the GFET with few-layer graphene (FLG) and FeCl3-intercalated few-layer graphene (FeCl3-FLG) and the experimental setup for photovoltage measurement. (f) Gate voltage dependence of the photovoltage when the laser position is located at the FLG/Au (blue), FLG/FeCl3-FLG (red), and FeCl3-FLG/ Au (black) interfaces. Reproduced with permission.[247] Copyright 2013, American Chemical Society.

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hand, the devices may generate thermoelectric currents under high intensity laser (wavelength λ = 410 nm) focused on or near the electrodes. The laser could be absorbed by the metal electrodes and result in local temperature increase, which will generate thermoelectric current in the graphene. Similar work was reported by M. C. Lemme et al,[111] who observed photoresponses in graphene p-n junctions actively controlled by gate voltages. Both photovoltaic and thermoelectric effects could induce photocurrents at the p-n junction while the thermoelectric effect was estimated to play a major role in the photocurrent generation. The responsivities of bilayer graphene FETs under the illumination of visible light (wavelength λ = 532 nm) could reach about 1.5 mA/W at optimum conditions. The generation mechanism of photothermoelectric current in GFETs under visible light was also investigated by X. D. Xu et al.[114] The interface junction of single-bilayer graphene was designed to distinguish the generation mechanisms of builtin electric field induced photovoltaic effect and thermoelectric dominated photocurrent. The schematic setup and device geometry were shown in Figure 18c. The photocurrent origin could be identified because the generated currents for the two different mechanisms have opposite directions. If the photocurrent generated at the single-bilayer junction was dominated by built-in electric field, the photocurrent would flow from the bilayer to the single layer. For the photothermoelectric effect, the current would flow from the single to the double layer for the higher density of states in the double layer graphene. Therefore, they identified that the photocurrent generated near the junction of single-bilayer graphene was mainly dominated by photothermoelectric effect. A similar work on the photoresponse of a junction of two types of graphene was recently reported by F. Wither et al.[247] They prepared a device with a heterostructure of graphene and FeCl3-intercalated few-layer graphene for visible light detection, as shown in Figure 18e. The ferric chloride molecules could penetrate between the layers of few-layer graphene and induce high p-type doping in the graphene layers. Large photovoltages up to 0.1V/W were generated at the interfaces of graphene/FeCl3-doped graphene, which was higher than that of the single-bilayer graphene junction reported by X. D. Xu et al.[114] The photovoltage was originated from the photothermoelectric effect due to the different Seebeck coefficient of graphene and FeCl3-doped graphene. In 2012, M. Fritag et al. compared the photovoltaic, photothermoelectric and bolometric effects in a drain biased GFET under visible light (wavelength: 690 nm).[118] The responsivity of the GFET was on the order of 2.5 × 10−4 A/W, which was dominated by the photovoltaic and the photo-induced bolometric effects while the photothermoelectric effect was insignificant. The photocurrent generation mechanism was dominated by the photovoltaic effect at low doping conditions and the photo-induced bolometric effect at n-type and p-type doping conditions. Recently, the same group reported that the substrate played an important role on the generation of photocurrent in GFETs, as shown in Figure 19.[242] They prepared GFETs with suspended graphene and characterized the photoresponse under the illumination of laser (wavelength: 514.5 nm or 476.5 nm). They estimated that the hot carrier temperature in suspended graphene could be an order of magnification higher than that of fully supported graphene due to the absence of

scattering between carriers and substrate surface polar phonons in the former. So the photothermoelectric effect was found to be the dominant mechanism for the photoresponse in the suspended GFETs. The responsivity of the suspended GFET is dependent on gate voltage and could reach up to 10 mA/W at room temperature, as shown in Figure 19d. Therefore, the energy exchange between graphene and its ambient can insignificantly affect the device performance.

5.1.2. Enhancement with Plasmonic Effect In order to enhance the interaction of graphene with incident light, microcavity[199,200] and plasmonic[33,248] techniques were adopted in graphene photodetectors. The enhancement of visible photoresponse by using microcavity-induced optical confinement is similar to the case in Section 4.2.2 for IR sensitive GFETs.[200] Hence, we just introduce some metallic plasmonic enhancement in visible range.[248,249] Three types of metallic plasmonic nanostructures were designed to improve the photoresponse of GFETs by T. J. Echtermeyer et al., as shown in Figure 20.[248] They observed that the photovoltages in the devices with plasmonic nanostructures are much higher than that in a non-structured device for all testing wavelengths from 457 nm to 785 nm. The enhancement of the photovoltage in their device is strongly inhomogeneous, related to the position and the polarization of light spot, and has the maximum value of ∼20 times at optimum operation conditions, as shown in Figure 20e and f. Y. Liu et al. reported the modification of graphene with Au nanoparticles for plasmonic enhancement in GFETs, as shown in Figure 20g and h.[249] The absorption of Au nanoparticle/graphene film showed a peak at 515 nm with the peak value of about 21% due to plasmonic resonance. The photocurrent of the device under the illumination of 530 nm light was enhanced for up to 1500% with the maximum responsivity of 6.1 mA/W.

5.2. Visible Sensitive Hybrid GFETs Hybrid graphene photodetectors are promising for visible light detection due to the combination of the materials with high light absorption and graphene with ultrahigh carrier mobilties.[250–254] Visible photodetectors based on PbS-QD-decorated graphene was reported by D. Zhang et al.[250] The device had the typical bottom-gate-top-contact structure. High-quality CVD-grown single layer graphene was transferred on the gate dielectric (SiO2) and then 5 nm-diameter PbS QDs were deposited on graphene by electron beam evaporation. It was found that the electrical properties of graphene after PbS decoration became very sensitive to visible light with wavelengths from 400 nm ∼ 750 nm. The responsivities of ∼2.8 × 103 A/W at a negative gate bias and ∼1.7 × 103 A/W at a positive gate bias were achieved, respectively. The sensing mechanism is very similar to that of the IR sensitive hybrid GFETs reported before.[17,18] The photoinduced holes in the PbS QDs can inject efficiently into the conductive graphene channel, which results in the increase of hole carrier density in p-doped region and the decrease of electron density in n-doped region. So the change of carrier densities in the channel leads to the photoresponse of channel current. It is



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Figure 19. (a) Image of a GFET with the source and drain contacts, the SiO2 support, and the trench. A three-layer exfoliated graphene flake was placed on top of the structure as indicated by dashed lines. (b) Spatially resolved responsivity of the GFET measured at wavelength λ = 514.5 nm and backgate voltage VG of 10 V. (c) Schematic of the device structure with suspended graphene. The trench depth is 300 nm. (d) Modeling of the photothermoelectric component of the photocurrent (blue) in comparison with experimental result (red). Reproduced with permission.[242] Copyright 2013, American Chemical Society.

obvious that the high carrier mobilities of graphene are the key factor for the high responsivities of the devices. K. Zheng et al. reported the visible photoresponse of GFETs based on single layer graphene decorated with TiO2 nanoparticles.[251] Graphene on a SiO2/Si substrate was immersed into well dispersed TiO2 ethanol solution and formed a TiO2-graphene hybrid film as the active layer of a GFET. Photoresponse of the hybrid device under visible light (wavelength: 500 nm) only can be observed in air, indicating that the response behavior was assisted by air. Surface defects on TiO2 nanoparticles could increase the density of oxygen anions adsorbed on the surfaces from the air. As shown in Figure 21, under visible light illumination, electron and hole pairs will be generated in TiO2 nanoparticles. Consequently, the adsorbed oxygen anions can combine with the light-induced holes and desorb from the surfaces of TiO2 nanoparticles while the electrons can transfer to the graphene channel and lead to the increase of channel current in n-channel region. Because TiO2 has a big bandgap (∼3.2eV), the TiO2 nanopartciles only have very weak light absorbance in visible range and thus lead to relatively low photoresponse of the devices to visible light. Similarly, the device exhibited photoresponse to UV light in air ambient. Much higher responsivity was obtained in UV range due to the much higher light absorbance of TiO2. Many organic materials can absorb light even more efficiently than inorganic semiconductors. S. Y. Chen et al.

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reported the hybrid GEFT based on chlorophyll modified graphene for visible photodetection.[252] Chlorophyll molecule was an organic semiconductor and was very efficient in visible light absorption. The graphene-chlorophyll hybrid FET demonstrated a high responsivity of 106 A/W and a high gain of 106 electrons per photon at the wavelength of 683 nm. The holes in chlorophyll are energetically favorable to transfer to graphene and led to efficient charge separation in the hybrid film. The electrons remained in the chlorophyll layer could be regarded as negative gate voltage that can result in field-effect p-type doping in graphene. These organic/graphene hybrid FETs are very promising for visible detection due to their high responsivities and the convenient fabrication process. MoS2-modified GFETs were reported recently by K. Roy et al.[106] MoS2 has strong light absorption (∼1 × 107 m−1) in visible range since the bandgaps are 1.9 eV and 1.2 eV for single- and multilayer MoS2, respectively. GFET based on MoS2/graphene bilayer film demonstrated the responsivities of 1 × 1010A/W at 130 K and 5 × 108A/W at room temperature under visible light (wavelength: 635 nm), making it one of the most sensitive graphene-based photodetectors. Other hybrid GFETs, like graphene/Si devices,[221,255] also showed the visible sensitive performance with the responsivities of hundreds of mA/W, which were however much lower than those of the above devices due to the different working principles.




REVIEW Figure 20. (a) Image of a graphene device with plasmonic nanostructures (in false colours). Blue, graphene; purple, SiO2 (300 nm); yellow, Ti/Au electrodes. Scale bar, 20 µm. (b) Image of a tested plasmonic nanostructures (in false colours). L and TR incident light polarizations are indicated. Scale bar: 1 µm. (c) Photovoltage maps of one of the nanostructured contacts (514 nm, TR polarization). Colour scale, from 0 µV (blue) to 20 µV (red). The signal (normalized to laser power) measured on a contact with finger structure (finger width 110 nm, pitch 300 nm) as a function of the position of the illumination spot (spot size ∼1.5 µm, Gate voltage 90 V). (d) Polarization-dependent enhancement at λ = 514 nm. Black squares: measured data; red line: cos2θ fit. Reproduced with permission.[248] Copyright 2011, Nature Publishing Group. (e) Schematics of a plasmon resonance enhanced photo sensitive GFET. channel length: ∼8 µm and channel width: ∼8 µm. (f) The enhancement of photocurrent of the GFET modified with Au nanoparticles prepared by annealing Au film of variable initial thickness (0, 4, 8 and 12 nm). Plasmon resonance intensity increases with increasing Au film thickness, leading to an increased enhancement effect up to ∼1,500%. The error bars show the variation of device performance from five devices. Reproduced with permission.[249] Copyright 2011, Nature Publishing Group.

5.3. Visible Sensitive GFETs Based on GO or rGO The solution processability of GO and rGO films leads to an efficient way for low cost and large area fabrication of devices. rGOand rGO/nanoparticle hybrid composites were widely used for visible photodetectors.[231,238,256–262] But most of the devices are actually photoresistors with two contacts, which can be regarded as GFETs with zero gate voltage. X. Geng et al. reported the visible photoresponse in the devices made of rGO/CdSe composites.[259] The rGO/CdSe

Figure 21. Schematic diagram of TiO2/graphene hybrid film and the generation of charges under UV and visible light. Reproduced with permission.[251]


solution was prepared by mixing rGO aqueous solution and pyridine-capped CdSe QDs aqueous solution and then spincoated on substrates to form rGO/CdSe composite films. The conductivity of the composite film with the thickness of ∼126.7 nm could be increased by ∼10.5% under the irradiation of 473 nm laser with the power of ∼94.5 mW. Similar work for the devices with rGO/CdSe nanoparticle composites were reported by Y. Lin et al.[260] The rGo/CdSe nanocomposites were prepared by adding rGO into the reaction solution during the process of synthesizing CdSe nanoparticles. The ratio of photocurrent and dark current could be as large as ∼1700%, which was dramatically enhanced in comparison with the aforementioned device reported by X. Geng et al.[259] The high photoresponse was primarily attributed to the robust combination and good interface between rGO and CdSe nanoparticles that lead to efficient and fast charge transfer from CdSe nanoparticles to rGO under light illumination. Moreover, the device showed a photoresponse time as short as ∼250 µs, which is much faster than most of the photosensitive hybrid GFETs.[17,18] Visible sensitive phototransistors with rGO/ZnO hybrid nanostructures were reported by Z. Zhan et al.[238] The asprepared rGO/ZnO nanostructures were dispersed into isopropanol and then spin-coated on SiO2/Si with patterned Ti/Au electrodes. The rGO/ZnO hybrid films were annealed at 700 °C for 30 minutes in Ar gas ambient, which resulted in the reduction of GO and carbon doping into ZnO nanoparticles. Under white light illumination, the ratio of photocurrent to dark current could reach up to 430, which was much higher than those of the aforementioned rGO/CdSe devices.[259,260] Because



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ZnO nanoparticles were efficiently doped by carbon atoms, the absorption of ZnO could be extended to visible light. The photocarrier transfer process in rGO/ZnO was similar to that in graphene/TiO2 hybrid films.[235] The adsorbed oxygen anions on the surfaces of ZnO nanoparticles can trap photogenerated holes and the photogenerated free electrons will transfer to rGO and lead to photocurrents. However,the device has a symmetric design and the observed photocurrent at zero bias is unexpected. So we assume that asymmetric structure was probably introduced indeliberately during the fabrication of the device.

5.4. Visible Sensitive GFETs Based on Other Types of Graphene Photodetectors based on epitaxial graphene on SiC with the asymmetric contacts of Ti and Pd were reported by R. Sun et al.[243] The photoresponse strongly depended on the thickness of epitaxial graphene, channel length and bias voltage. The maximum responsivity of 10-layer graphene devices could reach 1.11 mA/W at zero bias and ∼4.5 mA/W at a bias of ∼0.7 V. Enhanced photoresponse was realized by using electrochemically oxidized epitaxial multilayer graphene in nitric acid.[263] The responsivities of devices could reach 2.5 A/W at a wavelength of 470 nm and 200 A/W at a wavelength of 350 nm. The improved responsivities were attributed to the formation of deep traps at the electro-oxidized graphene interface, which prolong the lifetime of photocarriers. K. Yan et al. reported a phototransistor based on mosaic graphene with a single-crystalline p-n junction.[264] The discrete intrinsic graphene grain was first grown on polycrystalline copper substrate and then the laterally grafted growth of nitrogen-doped graphene was induced by introducing precursor gas of acetonitrile vapor, forming a continuous monolayer mosaic graphene. The mosaic graphene was transferred onto a SiO2/Si substrate as the active layer of a GFET. The carrier mobilities of the intrinsic and the nitrogen-doped graphene are ∼5000 cm2/Vs and ∼2500 cm2/Vs, respectively. A p-n junction was formed between the intrinsic region and the nitrogen-doped region in the same monolayer graphene. However, the responsivity of the device to focused laser (wavelength: 632.8nm) was only about 0.1 mA/W because of its large channel resistance and substrate-induced energy dissipation.

6. UV Sensitive GFETs UV light is typically referred to the electromagnetic radiation in the wavelength range from ∼400 nm to 10 nm (frequency range from ∼7.50 × 1014 Hz to 3 × 1016 Hz).[7,265] The UV range is commonly divided into four subdivisions. The wavelength range of 300 nm to 400 nm is often called the near ultraviolet (NUV), the wavelength range of 200 nm to 300 nm the middle ultraviolet (MUV), the wavelength range of 100 nm to 200 nm the far ultraviolet (FUV), and the wavelength range of 10 nm to 100 nm the extreme ultraviolet (EUV).[7] UV detectors can be widely used in chemical sensing, flame detection, ozone-hole sensing, short-range

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communication, and missile plume sensing in harsh and severe radiation environment, etc.[265,266]

6.1. Recent Development of UV Photodetectors In general, UV photodetectors can be divided into two categories: photon detectors and thermal detectors. Many UV photon detectors are based on semiconductors that can absorb UV light to generate electron-hole pairs and form photocurrents after efficient charge separation. Commercial UV photodetectors based on Si, GaP, diamond, GaN and AlGaN normally show responsivities in the range of 100–200 mA/W.[265] Visibleblind UV detections are important for many specific applications,[267–269] which however cannot be realized with narrow bandgap semiconductors like Si. Therefore, many widebandgap semiconductors (>3.0eV) such as silicon carbide (SiC), diamond, and aluminum gallium nitride (AlGaN) alloys have been used in UV photodetectors and are suitable for commercial applications.[265] Moreover, wide-bandgap oxides, such as nanocrystalline TiO2,[270] colloidal ZnO nanoparticles,[271] and Nb2O5 nanocrystals have shown advantages in the applications of visible-blind UV photodetectors due to the high responsivities (10 ∼ 100 A/W) of the devices. Graphene-based UV photodetectors have also aroused much research interest recently for the unique properties of graphene. Most of the reported graphene UV detectors are based on graphene hybrid or rGO due to the low light absorption of pristine graphene.

6.2. UV Sensitive Hybrid GFETs Hybrid GFETs based on wide bandgap oxide semiconductors (e.g. TiO2, ZnO) and pure graphene have been successfully used as UV photodetectors due to the combination of high UV absorption of oxides and high carrier mobilities in graphene. W. Guo et al. reported UV detectors based on ZnO QDs/graphene hybrid GFETs.[272] The devices demonstrated responsivities up to 104 A/W and gains of about 104. The sensing mechanism of the ZnO QDs/graphene phototransistor is similar to the visible photodetectors based on TiO2/graphene hybrid described in Section 5.2.[272,273] Oxygen molecules in air could be adsorbed on the surface of ZnO QDs and attract electrons in ZnO QDs to form negatively charged anions. Under UV illumination, electron and hole pairs are generated in ZnO QDs and the holes will migrate to the surface and discharge the adsorbed oxygen anions to turn into oxygen molecules. In the mean time, the electrons will transfer to graphene channel and result in n-type doping in graphene. However, the UV detectors only can be used in oxygen or air ambient because oxygen plays an important role in the operation of the devices. Q. Wang et al. reported a UV sensitive GFET with a TiO2 thin film decorated on single layer graphene.[274] The device shows increasing photocurrent with the increase of oxygen concentration under UV irradiation and it thus can be used as oxygen sensor. Similar work was recently reported by K. Zheng et al.[251] who fabricated the GFETs decorated with TiO2 nanoparticles. The devices showed photoresponse under UV illumination (wavelength: 254 nm) due to the same




REVIEW Figure 22. (a) Wavelength dependent transmission of rGO films with various reduction times at 150 °C. Inset: light transmission at 550 nm as a function of reduction time. (b) Photon energy dependent absorbance of rGO with various reduction time. (c) Energy bandgap of rGO obtained from optical measurements as a function of reduction time. (d) Transfer curves (IDS–VG) of a GFET based on 90 min reduced rGO under UV light (370 nm) with different intensity. Reproduced with permission.[257]

mechanism of oxygen-assisted charge transfer process, as shown in Figure 21.

6.3. UV Sensitive GFETs Based on rGO or rGO Hybrids rGO[257,275] and the hybrids of oxide semiconductor nanomaterials and rGO[261,276,277] have been used in UV photodetectors due to the tunable bandgap of rGO, good optoelectronic properties and solution processability. H. X. Chang et al. reported the solution-processed, few-layer rGO based GFETs for UV detection, as shown in Figure 22.[257] The bandgap of rGO could be well controlled by thermal reduction process ranging from 2.2 eV to 0.5 eV. Both the light absorbance and channel current increase with the increase of heat treatment time due to the decreased bandgap of rGO. The devices demonstrated the maximum responsivities of ∼0.86 A/W under the illumination of 370 nm UV light, which is comparable to that of the broadband phototransistor based on rGO described before.[230] K. K. Manga et al. reported high gain UV photodetectors based on TiO2/rGO composites prepared by inkjet printing.[261] TiO2/GO composite films were prepared by inkjet printing and then thermally reduced in hydrazine to form big-sized rGO. The printed photodetector with graphene films as electrodes and a TiO2/rGO hybrid thin film as active channel is shown in Figure 23a. The photodetector demonstrated the photoconducting gain of 85% (Figure 23b) and the detectivity of


2.3 × 1012 Jones in UV range. The high detectivity is comparable to that of commercial photodetectors. More importantly, the hybrid devices exhibited rise and fall response times of about 100 ms, which are much faster than many pure ZnO[271] and TiO2[278] photodetectors reported before. H. X. Chang et al. also reported a visible-blind UV photodetector based on solution-processed ZnO nanorod/rGO nanostructure,[276] as shown in Figure 23c and d. The detectors exhibited the maximum responsivities of 22.7A/W. In comparison, ZnO QD/rGO film showed the relatively low responsivities of about 0.35 A/W. The reason for the higher responsivity of ZnO nanorod/rGO film was possibly due to the good connection between ZnO nanorods and rGO and the further reduction of rGO induced by the growth of ZnO nanorods in solution. Similarly, Z. Wang et al. found that the photocurrent of 5% rGO decorated ZnO UV sensor was enhanced for 700 times compared with that of pure ZnO device under the same testing conditions.[277] The photoresponse current increased with the increase of rGO content until 5% and then decreased with more addition of rGO. They considered that rGO on ZnO nanostructures can help the separation of electron-hole pairs in the composite films and thus enhance the photoresponse of the devices. M. E. Itkis et al. reported that the responsivity of an UV photodetector based on HNO3-oxidized epitaxial graphene on SiC was more than 200 A/W under the illumination of UV light (wavelength: 350 nm),[263] which is comparable to that of some commercial UV photodetectors. It is notable that the study of graphene-based UV photodetectors was just started from 2010, many properties



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Figure 23. (a) Current voltage curves of a TiO2/graphene photoconductor measured in dark and under white-light (100 mW/cm−2) in vacuum or ambient conditions. Inset: schematic diagram of a printed graphene-based photoconductor. (b) Photoconducting gain for TiO2/graphene devices with different compositions characterized in ambient. Reproduced with permission.[261] (c) SEM image of ZnO nanorods. (d) electrons and holes generated in ZnO nanorods under UV illumination. (e) Time-resolved photocurrent of pure graphene, ZnO QD/graphene hybrid and the ZnO nanorod/graphene heterostructure under UV radiation (370 nm) with the intensity of 1.084 mW/cm2. Vbias = 5 V. Inset: an enlarged view of the photocurrents at the starting point. Reproduced with permission.[276] Copyright 2011, Royal Society of Chemistry.

of the devices have been rarely investigated until now and further works are needed for developing the practical applications.

7. Conclusions and Outlook In summary, graphene can absorb light over a broad bandwidth from UV to THz and is thus an excellent material for photodetectors. Various types of GFET-based photodetectors have been developed based on different device design and mechanisms and have shown excellent performance in many aspects. For terahertz detectors, GFETs integrated with antennas have been successfully used for terahertz detection due to extremely high plasma wave velocities (>>108 cm/s) in graphene. The

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sensing mechanism is based on the coupling of plasma wave in graphene with terahertz signal received by the antennas. Moreover, the performance of the terahertz detectors can be tuned by gate voltages. However, the device performance is much worse than that of theoretical predictions and commercially available devices. Further work is thus needed to optimize the device design and fabrication conditions. Possible approaches that can be adopted to improve the device performance include (a) To enhance the light absorption of devices by integrating plasmonic technologies; (b) To design new structures of antennas to improve the coupling between terahertz radiation and plasma waves in GFETs; (c) To fabricate the devices based on light absorption of graphene channel in terahertz range, which have never been realized in practical devices until now,




Acknowledgements This work is financially supported by the Research Grants Council (RGC) of Hong Kong, China (project number: T23–713/11) and the Hong Kong Polytechnic University (project number: G-YM45, 1-ZV8N, 1-ZVAW, A-PK92 and A-SA78). L.Y.N. acknowledges a “C.C. Lee Scholarship”. Received: January 23, 2014 Revised: February 11, 2014 Published online:

[1] A. K. Geim, Science 2009, 324, 1530. [2] X. Huang, Z. Zeng, Z. Fan, J. Liu, H. Zhang, Adv. Mater. 2012, 24, 5979. [3] Q. He, S. Wu, Z. Yin, H. Zhang, Chem. Sci. 2012, 3, 1764. [4] X. Qi, C. Tan, J. Wei, H. Zhang, Nanoscale 2013, 5, 1440. [5] K. F. Mak, L. Ju, F. Wang, T. F. Heinz, Sol. St. Commun. 2012, 152, 1341.


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although high sensitivities of such devices have been theoretical predicted. The working principles of GFET-based photodetectors from IR to UV range are very similar. The possible sensing mechanisms include photovoltaic, photothermoelectric, bolometric and field-effect doping effects, which are dependent on the device design and operation conditions. IR sensitive GFETs have attracted much attention in the past several years because of excellent performance of the devices. For example, ultrahigh operation frequencies up to 40 GHz of the devices based on high-quality graphene have been observed, which is potentially useful for high-speed optical communication. High responsivities up to 107 A/W have been observed in the devices based on hybrid or nanostructured graphene, which are much better than most of the IR photodetectors reported before. Similar cases can be found in GFET-based visible and UV photodetectors. For example, ultrahigh responsivities up to 5 × 108 A/W were observed in MoS2-modified GFETs under visible light. The performance of the GFET-based photodetectors is sensitive to the properties of metal/graphene contacts, graphene/ semiconductor heterojunctions and graphene/substrate interfaces. So the devices can be further optimized by adopting rational designs and choosing suitable materials including high-quality graphene, metals for contacts and semiconductor materials modified on graphene. Many approaches, such as plasmonic technologies, integration of microcavities and optical waveguides, etc. have been proved to be very useful in improving the device performance. Considering the practical applications of the devices, further work is also needed to improve the performance of the devices in terms of uniformity and stability. Compared to conventional photodetectors, GFET have shown many other advantages, including the broadband photoresponse from UV to IR range, mechanical flexibility, low cost and convenient fabrication by printing or roll-to-roll techniques. Robust and flexible GFET photodetectors have been realized by many research groups,[17,119,230] implying that the devices are excellent candidates for flexible and wearable electronics in the future.

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Supramolecular chemistry on graphene field-effect transistors.

Gate-tunable photoemission from graphene transistors.

Graphene-based lateral heterostructure transistors exhibit better intrinsic performance than graphene-based vertical transistors as post-CMOS devices.

Graphene transistors with multifunctional polymer brushes for biosensing applications.

Dynamic Wavelength-Tunable Photodetector Using Subwavelength Graphene Field-Effect Transistors.

Improved performance of graphene transistors by strain engineering.

High-mobility ambipolar ZnO-graphene hybrid thin film transistors.

Apparent pH sensitivity of solution-gated graphene transistors.

Ag Nanoparticle Electrode.

Graphene Klein tunnel transistors for high speed analog RF applications.

Advances in the fabrication of graphene transistors on flexible substrates.

Photosensitive and Flexible Organic Field-Effect Transistors Based on Interface Trapping Effect and Their Application in 2D Imaging Array.

photosensitive-nanowire composite thin-film transistors for transparent multicolor photodetectors array.

Physical Modeling of Gate-Controlled Schottky Barrier Lowering of Metal-Graphene Contacts in Top-Gated Graphene Field-Effect Transistors.

Water-free transfer method for CVD-grown graphene and its application to flexible air-stable graphene transistors.

Bilayer insulator tunnel barriers for graphene-based vertical hot-electron transistors.

Room-temperature negative differential resistance in graphene field effect transistors: experiments and theory.

Low-voltage graphene field-effect transistors based on octadecylphosphonic acid modified solution-processed high-k dielectrics.

High-Gain Graphene Transistors with a Thin AlOx Top-Gate Oxide.

Abrupt current switching in graphene bilayer tunnel transistors enabled by van Hove singularities.

Improved Drain Current Saturation and Voltage Gain in Graphene-on-Silicon Field Effect Transistors.

Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons.

Recording Spikes Activity in Cultured Hippocampal Neurons Using Flexible or Transparent Graphene Transistors.

Highly Stable and Tunable n-Type Graphene Field-Effect Transistors with Poly(vinyl alcohol) Films.