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Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145601 (http://iopscience.iop.org/0957-4484/26/14/145601) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 193.140.216.7 This content was downloaded on 23/04/2017 at 12:06 Please note that terms and conditions apply.
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Nanotechnology Nanotechnology 26 (2015) 145601 (12pp)
doi:10.1088/0957-4484/26/14/145601
Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse Ravi K Biroju1, Nikhil Tilak2, Gone Rajender2, S Dhara3 and P K Giri1,2 1
Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India 3 Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2
E-mail: [email protected] Received 18 December 2014, revised 9 February 2015 Accepted for publication 17 February 2015 Published 16 March 2015 Abstract
We demonstrate the graphene assisted catalyst free growth of ZnO nanowires (NWs) on chemical vapor deposited (CVD) and chemically processed graphene buffer layers at a relatively low growth temperature (580 °C) in the presence and absence of ZnO seed layers. In the case of CVD graphene covered with rapid thermal annealed ZnO buffer layer, the growth of vertically aligned ZnO NWs takes place, while the direct growth on CVD graphene, chemically derived graphene (graphene oxide and graphene quantum dots) without ZnO seed layer resulted in randomly oriented sparse ZnO NWs. Growth mechanism was studied from high resolution transmission electron microscopy and Raman spectroscopy of the hybrid structure. Further, we demonstrate strong UV, visible photoluminescence (PL) and enhanced photoconductivity (PC) from the CVD graphene–ZnO NWs hybrids as compared to the ZnO NWs grown without the graphene buffer layer. The evolution of crystalinity in ZnO NWs grown with ZnO seed layer and graphene buffer layer is correlated with the Gaussian line shape of UV and visible PL. This is further supported by the strong Raman mode at 438 cm−1 significant for the wurtzite phase of the ZnO NWs grown on different graphene substrates. The effect of the thickness of ZnO seed layers and the role of graphene buffer layers on the aligned growth of ZnO NWs and its enhanced PC are investigated systematically. Our results demonstrate the catalyst free growth and superior performance of graphene–ZnO NW hybrid UV photodetectors as compared to the bare ZnO NW based photodetectors. S Online supplementary data available from stacks.iop.org/NANO/26/145601/mmedia Keywords: graphene–ZnO NW hybrid, photoluminescence, rapid thermal annealing, photoconductivity (Some figures may appear in colour only in the online journal) 1. Introduction
conversion and storage devices [5] etc. However, the catalyst free growth of vertically aligned semiconductor nanowires (NWs) directly on different forms of graphene buffer layers is a challenging task. In this type of hetero structure, it could be possible to integrate the 2D graphene and 1D semiconducting NWs for Schottky junction devices with
In recent years, graphene–semiconductor hybrid nanostructures have drawn enormous attention due to their potential applications in photovoltaics [1, 2] nanogenerators [3], field emission devices [4], and efficient energy 0957-4484/15/145601+12$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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2. Experimental
enhanced functionalities for ensuing optoelectronic applications. Among all, the graphene mediated growth of vertically aligned ZnO hybrid nanostructures has received much attention [6], since ZnO is a wide direct band gap (3.3 eV) semiconductor and has large exciton binding energy (60 meV) which can possess fast photo response upon illumination of UV light in the graphene based photodetectors (PDs) [7] and gas sensing capability [8]. Recently, vertically aligned growth of ZnO nanorods (NRs)/NWs by vapor liquid solid (VLS) and vapor solid (VS) mechanisms has been reported on single (SLG) and few layer (FLG) graphene in the presence and absence of gold (Au) catalyst by vapor phase growth [3, 6, 9]. Fabrication of vertically aligned ZnO NRs/NWs with high aspect ratio and extremely large surface-to-volume ratio, specifically on graphene substrates without the aid of metal catalyst is still challenging. With the excellent electrical, mechanical and thermal characteristics of graphene layers, growing ZnO nanostructures and thin films on graphene would enable their novel physical properties to be exploited in the diverse range of sophisticated device applications [10]. Therefore, several graphene–semiconductor nanostructure hybrids have been successfully synthesized that show elegant combinations of properties not found in the individual components [11]. Recently, Xu et al developed a metal–semiconductor–metal PD using hydrothermally grown ZnO NWs and observed the existence of surface plasmon resonance arising from the underlying graphene layer, which exhibited a UV to visible rejection ratio of ∼ 4 [12]. To the best of our knowledge, there are limited studies on the graphene mediated growth of vertically aligned, high density and high aspect ratio ZnO NWs on graphene buffer layers by thermal vapor deposition technique and the role of graphene layer in the enhanced photoluminescence (PL) and photoconductivity (PC) are least understood [13]. Further, there are very few reports on the growth of ZnO NWs on chemical vapor deposited (CVD) and chemically derived graphene functional materials, such as graphene oxide (GO) and graphene quantum dots (GQDs), and the optical properties of such hybrid nanostructures for the hybrid photodetector applications are little explored [14]. In the present work, we demonstrate the catalyst free growth of aligned ZnO NWs on various substrates consisting of CVD graphene (GR), GO and GQDs with/without a thin ZnO seed layer on graphene. The structural quality of the as-grown graphene layer as well as the ZnO NWs grown on it is characterized by Raman spectroscopy along with the Raman mappings with 514.5 nm laser excitation. The effect of ZnO seed layer and different graphene buffer layers on the structure of ZnO NWs was explored by high resolution transmission microscopy (HRTEM) and its optical properties were studied systematically by UV–visible absorption and PL spectroscopy. Further, enhanced UV PC was observed in the case of ZnO NWs grown on graphene/SiO2 substrate as compared to that grown without the graphene layer.
2.1. Synthesis of CVD graphene and chemically derived graphene
Single and few layer graphene samples were synthesized by a thermal CVD method and transferred on to the Si/SiO2 and quartz substrate by standard wet chemical method. The chemically derived graphene such as GO and GQDs are obtained from simple chemical exfoliation techniques. The full details of the experiment and characterization of these samples are described in the supporting information (see sections: SI1–SI4).
2.2. Deposition of high quality ZnO thin films on graphene
High quality ZnO films with three different thicknesses: ∼300 nm (code-Z1), ∼100 nm (code-Z2) and ∼10 nm (codeZ3), were deposited by RF magnetron sputtering on various graphene substrates for the growth of ZnO NWs. High purity ZnO sputter target (99.999%, Kurt J Lesker, USA) was used as a source for the ZnO grain growth. Initially the chamber was evacuated to a base pressure of 6.7 × 10−6 mbar and during the sputtering it was maintained at 1 × 10−2 mbar. The RF power was kept at 100 W. The substrates were heated to 200 °C for better crystalinity and uniformity of the ZnO grains. Further, these ZnO thin films were subjected to rapid thermal annealing (RTA) treatment at 600 °C in Ar gas ambient (flow rate of 200 standard cubic centimeters (SCCM)) atmosphere for 3 min using a commercial RTA system (MILA3000, Ulvac, Japan) in order to further improve the crystalline quality as well as for the removal of excess oxygen traps on the ZnO thin films on various substrates. Improvements in the crystalinity of ZnO grains and their phases were confirmed from XRD and HRTEM analyses (discussed later).
2.3. Growth of ZnO NWs on graphene substrates
Commercial nano sized activated Zn powder (purity ∼ 99%, Aldrich) was taken as a source material in an alumina boat and placed at the center of a horizontal quartz tube kept inside a muffle furnace. The above prepared substrates were placed downstream ∼ 5 cm away from the source material. Initially the quartz tube was pumped down to a pressure of ∼10−3 mbar. In order to prevent the oxidation of the graphene layer, 300 SCCM of Ar gas was flushed into the chamber until it reached the set point with a rate of 18 °C min−1. When the furnace reached the desired temperature, 20 SCCM of O2 gas was introduced and gas pressure inside the chamber was maintained at 1.6 mbar for the growth time of 50 min. The source material temperature was maintained at ∼600 °C and the substrate temperature at ∼580 °C. After the completion of the reaction, the furnace was cooled down to room temperature. After the growth, the RTA is performed at 600 °C in Ar atmosphere (300 SCCM) for 3 min to improve the crystalline quality of the as-grown ZnO NWs. 2
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2.4. Characterization techniques
The morphology and crystal structure of the as-grown samples were studied using electron microscopy tools such as field emission scanning electron microscope (FESEM, Sigma, Zeiss) and HRTEM (HRTEM, JEM2100 operated at 200 kV, JEOL). Micro Raman measurements were performed using a high resolution Raman spectrometer (inVia, Renishaw), with the excitation source 514.5 nm (Ar+ laser) and monochromator using 1800 gr mm−1 lines with a thermoelectric cooled CCD detector in the backscattering configuration to examine the crystalline quality and number of layers in the graphene. Raman mapping was carried out with 10 × 10 μm2 frame size on the samples at 514.5 nm laser excitation with a spatial resolution of 100 × 600 nm using a Streamline imaging facility for covering large area. Additionally, PL measurement was carried out at room temperature using a 325 nm He–Cd laser excitation and a monochromator with 2400 gr mm−1 grating. Some of the PL measurements were performed using a 355 nm diode laser excitation in a commercial fluorimeter (AB2, Thermo Spectronic). The UV–vis–NIR absorption spectroscopy measurements were recorded using a commercial spectrophotometer (PerkinElmer UV Win Lab, UV3101PC). PC measurements were performed with a micro probe station (ECOPIA EPS-500) connected to a source– measure unit (Keithely 2400, USA) for current–voltage (I–V) characteristics and 300 W xenon lamp as a source to excite the sample. Note that the excitation wavelength was selected using a monochromator (Oriel Instruments, USA). The I–V setup was interfaced with a computer to collect the data using Lab Tracer 2.0 software.
3. Results and discussion 3.1. Raman studies of different graphene substrates
Prior to the growth of ZnO NWs, the qualities of the GR, GO and GQDs coated on SiO2 substrates were first characterized by Raman spectroscopy. The characteristic Raman spectra of as-grown GR, GO and GQDs are shown in figure S1 (supporting information SI 5). In addition, Raman mapping was performed on GR sample for the well-known D, G and 2D bands for the surface coverage and uniformity of graphene layer. Figures 1(a)–(c) represent the Raman mapping images scanned in an area of 10 × 10 μm2, which shows a full coverage of SLG as evident from a sharp and prominent 2D peak at ∼2700 cm−1. The graphitic G band at ∼1595 cm−1 signifies the sp2 hybridization of carbon atoms and assigned for the Eg2 (high) mode of in-plane C–C stretching vibration [15]. The ratio of intensities of 2D and G bands I(2D)/I(G) is ∼1.00, which indicates the presence of SLG and FLG. The high intensity of the defect band D at ∼1350 cm−1 implies the presence of point and line defects in graphene. Some of the defects might have been introduced during the wet transfer process. The crystalline quality of different graphene materials was estimated from HRTEM analysis to support the Raman data. HRTEM images of GR, GO and GQDs are
Figure 1. Spatial Raman mappings of CVD graphene: (a) D, (b) G
and (c) 2D bands, which indicate the presence of BLG and FLG. Note that all the mappings are recorded with 10 × 10 μm2 area at 514.5 nm laser excitation.
shown in the supporting information, figure S2. Corresponding SAED patterns are shown in the inset, which clearly show a hexagonal lattice pattern. Note that the multiple SAED spots in GO sample (see figure S2(b)) are a signature of few layer GO. These graphene substrates are named according to the pretreatment conditions, as described in table 1. The surface morphology of ZnO NWs grown in each case is presented in table 1. As discussed before, the GR, GO and GQDs ultrathin films were prepared on SiO2 substrates and ZnO films of 3
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Table 1. The sample descriptions for the various graphene and ZnO coated buffer layer substrates and morphology of the ZnO NWs in each case.
4
Sl. No.
Substrate code
Base substrate
Sample
Substrate description
Morphology of the ZnO NWs
1 2 3 4 5 6 8
GR GRZ3 Z1Z3 Z2Z3 Z3 GOZ3 GQD
Si/SiO2, Quartz Si/SiO2, Quartz Si/SiO2, Quartz Si/SiO2 Si/SiO2 Si/SiO2 Si/SiO2
GRNW GRZ3NW Z1Z3NW Z2Z3NW Z3NW GOZ3NW GQDNW
Graphene layer only 10 nm ZnO on graphene, RTA 10 nm ZnO on 300 nm ZnO layer, RTA 10 nm ZnO on 100 nm ZnO layer, RTA 10 nm ZnO film, RTA 10 nm ZnO on GO, RTA Graphene quantum dots
Randomly orientated sparse NWs Vertical NWs Dense and randomly orientated NWs Dense and randomly orientated NWs Randomly orientated NWs Randomly orientated NWs Randomly orientated NWs
Note: Z1: 300 nm ZnO film, Z2: 100 nm ZnO film and Z3: 10 nm ZnO film.
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Figure 2. FESEM images of various ZnO nanostructures grown on different graphene–ZnO and ZnO buffer layer substrates: (a) GRZ3NW, (b) Z1Z3NW, (c) GOZ3NW, and (d) GQDNW.
thickness ∼300 nm (code-Z1), ∼200 nm (code-Z2) and ∼10 nm (code-Z3) were deposited on various graphene buffer layers followed by RTA in an inert gas environment. The systematic study on the individual effects of graphene buffer layer, ZnO seed layer and ZnO buffer layers on the growth of ZnO NWs have been elucidated in the following sections. Further the growth mechanism, strong UV-visible PL and PC characteristics of various ZnO nanostructures on different graphene derivative materials as substrates are discussed below.
aligned NWs, while the GO and GQD layers do not promote such kind of growth. Interestingly, the ZnO NWs grown on GRZ3 substrate show very strong UV PL as compared to the NWs grown on Z1Z3 substrate. Note that the visible PL is prominent in all the as-grown ZnO NWs on graphene samples (discussed later). The Z1Z3 and Z2Z3 layers show similar crystalline features after RTA treatment, which are consistent with the XRD results. Further, HRTEM measurements were conducted on the GRZ3NW sample to understand the role of ZnO seed layer in the vertical growth of ZnO NWs. It was found that the growth of the NWs is initiated from the ZnO seed layer and no NW growth takes place directly on the GR layer. This may be due to fact that graphene has very low surface energy and is a very inert material to nucleate the ZnO on its surface, assuming a low defect density. However, GR layer helps to grow the ZnO NWs vertically on the ZnO seed. It is likely that due to the presence of graphene, no planar growth takes place for the ZnO NWs. In the present case, the vertical growth of NWs can be attributed to the presence of ZnO seed on graphene layer, based on the VS growth mechanism [13]. However, here we did not observe the growth of ZnO NWs with hexagonal facets, in contrast to our earlier work [6] where ZnO NWs were grown using gold as a catalyst and graphene as a substrate layer. Thus, epitaxial growth is less efficient in the present case. Note that ZnO NWs growth on Z1Z3 is very dense without any vertical alignment (see figure 2(b)). The possible explanation for this observation is that the thick ZnO layer covered with a thin ZnO layer, both have the same surface energy and the density of oxygen atoms is very high
3.2. Effect of ZnO seed layer
The RTA of the ZnO thin film leads to formation of bigger grains of ZnO with improved crystallinity and the partial removal of defects in the ZnO. These ZnO grains grown on the graphene layer play a crucial role in the vertical growth and alignment of the ZnO NWs. Due to similarity in hexagonal crystalline structure of graphene and ZnO, it leads to the epitaxial like growth of the ZnO NWs. Figure 2 represents the FESEM image of ZnO nanostructures grown on various graphene substrates coated with ZnO seed layers, as mentioned in table 1. Figure 2(a) illustrates the case of GRZ3, while figures 2(b)–(d) illustrate the case of Z1Z3, GOZ3 and GQD, respectively. In the inset of figure 2(a), the vertical growth of ZnO NWs on ultrathin ZnO buffer layer (Z3) coated GR substrate is clearly visible. On the other hand, dense bundles of randomly oriented ZnO NWs can be seen on GO and GQD substrates. This clearly indicates that GR layer below the Z3 film, which has some kind of epitaxial relation with the ZnO seed layer, promotes the growth of vertically 5
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Figure 3. (a) FESEM image of ZnO NW grown directly on GR showing sparse and randomly oriented ZnO NWs. (b) TEM image of the ZnO
grains on SiO2 with nearly uniform size (∼40 nm), and the inset shows the higher magnification TEM image of a spherical ZnO seed. (c) TEM image of a ZnO NW along with the ZnO seed layer in sample GRZ3NW, and the inset shows the corresponding SAED pattern showing (002) plane for the combined layer. (d) Magnified TEM image of region I in (c), which depicts the nucleation of the ZnO NW on ZnO seed and its corresponding lattice fringes showing (002) planes of the ZnO NW with lattice spacing of 2.6 Å.
represents the magnified view of region I in figure 3(c), which clearly depicts the nucleation of NW on ZnO seed and vertical alignment onto (002) planes. The IFFT image in the inset (magnified view of region II) shows the lattice fringes for (002) planes with d-spacing 2.6 Å that strongly supports the Raman data (discussed later). The crystalline quality and the crystalline phases of RTA treated ZnO seed layers are assessed from the XRD pattern. Figure 4(a) represents the XRD pattern of Z1Z3 substrate after RTA treatment. The XRD pattern shows strong peaks at 2θ = 31.96°, 34.66° and 36.44° corresponding to (100), (002) and (101) planes of ZnO. These ZnO grains act as the seeds for the growth of ZnO nanostructures during the vapor deposition process. This is evident from the TEM image of figure 3(c) where the ZnO NW is grown directly on a ZnO grain. Thus, the ZnO buffer layer plays a crucial role in the growth of ZnO NWs. This can be confirmed by looking at the FESEM images of the ZnO NWs formed on the GR (figure 3(a)) and GRZ3 (figure 2(a)) substrates. GR substrate
on these layers due to the presence of oxygen interstitials [6]. These excess oxygen in the substrate help in the planar growth of the NWs [6] resulting into randomly oriented NWs. Interestingly, the direct growth of NWs on the GR substrate without a ZnO layer (Z3) results in a growth of sparse ZnO NWs, as evidenced from the FESEM image of figure 3(a). Thus, in a catalyst free growth, self catalytic seed layer is essential for the aligned growth of the NWs. Self-catalyzed growth of vertical ZnO NWs on various dielectric substrates have been reported earlier. Herein, self-catalyzed growth of ZnO NWs on graphene substrate has been demonstrated. Figure 3(b) represents the HRTEM image of the ZnO grains with an average diameter ∼ 50 nm. Inset shows the lattice image of a spherical ZnO grain showing the polycrystalline lattice patterns of ZnO seeds. Further, HRTEM results obtained for the ZnO grains are in close agreement with the XRD results. Figure 3(c) shows the ZnO NW grown vertically on the ZnO seed and its corresponding SAED pattern showing the single crystal (002) planes. Figure 3(d) 6
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aligned GaAs and InAs NWs on graphene substrates has been reported by Munshi et al [16] and Hong et al [17], respectively and it was suggested to be assisted by the strong van der Waals interactions. Our results are consistent with the above reports. 3.3.2. Z1Z3. In this case, a 10 nm ZnO seed layer is present
on the 300 nm ZnO layer, without the graphene layer. A dense and randomly oriented ZnO NWs growth takes place on Z1Z3 (see figure 2(b)). This can be attributed to the large number of overlapping grains on the ZnO thin film. The buffer layer Z3 seems to have negligible effect on the NW alignment. Once again, it implies that absence of graphene layer leads to random orientation of the ZnO NWs. 3.3.3. GOZ3 and GQDs. Our results show that the GO and
GQDs do not play any significant role in the growth/ alignment of the ZnO NWs as compared to the case of GR buffer layer. We believe that there are primarily two factors affecting the growth and orientation of the ZnO NWs: (i) nonuniform coverage of the GO layer on the substrate, (ii) high density of defects on the GO layer. This is clearly shown by the FESEM image in the supporting information, figure S3. During the growth, ZnO NWs may nucleate at the defect sites in GO and random growth takes place due to the absence of any lattice matching between GO and ZnO. Similar results were also observed for the case of GQDs substrate; due to the absence of the seed layer on the GQD, random growth takes place. 3.4. Raman studies of ZnO NWs
Crystalline quality of ZnO NWs was probed by Raman spectroscopy on different graphene substrates based on the peak position, relative intensities and full width at half maxima (FWHM-Δω) of the Raman spectra. Figure 4(b) illustrates the comparative Raman spectra of all the ZnO NWs samples. The Raman spectrum shows a peak at ∼ 438 cm−1, which corresponds to the Eg2 (high) mode of ZnO NW significant for the wurtzite phase. The peak position of Eg2 (high) mode and FWHM values have been calculated from the Gaussian peak fit that are labeled in figure 4(b). Note that the FWHM of Eg2 (high) mode in GRZ3NW is relatively lower than that of the NWs grown on other graphene substrates. This is consistent with the strong UV peak in the PL spectrum (discussed later) indicating a better crystallinity of the ZnO NWs grown in the GRZ3NW sample. Note that Z1Z3NW shows relatively lower FWHM of Eg2 (high) mode than that of the GRZ3NW indicating better crystalinity in Z1Z3NW. This may be due to the contribution of thick ZnO buffer layer on the Raman spectrum. Thus, it can be concluded that the ZnO seed layer with GR buffer layer promotes better crystalinity. However, the Raman signal of graphene is not detected in the GRZ3NW samples, perhaps due to the dense coverage and long length of the NWs. To confirm the presence of graphene in these samples, we performed the Raman measurements on GRZ3 substrate (see supporting information, figure S4). Eg2 (high) in GRZ3 reveals the presence of ZnO on the GRZ3
Figure 4. (a) XRD pattern of Z1Z3 substrate after RTA treatment.
The prominent peaks correspond to the (100), (101) and (002) planes of wurtzite ZnO. (b) Typical Raman spectra of ZnO NWs showing strong Eg2 (438 cm−1) mode, which indicates the growth of crystalline ZnO NWs with wurtzite phase. Note that the peak positions and FWHM (Δω) are denoted in cm−1 unit.
without a ZnO seed layer shows very sparse growth of ZnO NWs, whereas GRZ3 shows aligned growth of ZnO NWs. The diameters of the NWs are in the range ∼ 20–30 nm (see figure 2(a)), have dense coverage and is due to the huge number of overlapping ZnO grains. 3.3. Effect of various buffer layers 3.3.1. GRZ3. In this case, a 10 nm ZnO seed layer is present over the graphene buffer layer for the growth of the ZnO NWs. The NWs grown on GRZ3 are much better aligned than those grown on Z1Z3 (figure 2(b)), GOZ3 (figure 2(c)) and GQDs (figure 2(d)). This implies that the graphene buffer layer assists the ZnO grains in the vertical alignment of the NWs. Interestingly, the lattice mismatch between the hexagonal ZnO crystal and graphene bond centered sites is very low [16]. This could lead to the epitaxial growth of ZnO NRs on graphene. Self-catalyzed VLS growth of vertically 7
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Figure 5. (a) A schematic shows the growth mechanism of ZnO NWs on GRZ3 and Z1Z3 substrates. (b) The epitaxial relationship between
ZnO in its hexagonal wurtzite phase at the bond centered sites of sp2 hybridized graphene layer.
substrate as shown in figure S3(a). Further, The Raman signatures of graphene (D, G and 2D bands) with reduced intensity of 2D band and redshift of the D and 2D bands (by ∼ 10 cm−1) reveal that there is a strong interaction between graphene and ZnO grains. The Raman peak located at ∼ 331 cm−1 in GOZ3NW corresponds to the 2E2 phonon [18]. A weak lower frequency mode at 236 cm−1 may be arising from the structural defects on the surface of the ZnO NWs.
quartz substrates, we have performed the UV–VIS absorption spectroscopy measurements. Figure 6(a) represents the absorption spectra of ZnO NWs grown on Z1Z3 and GRZ3 deposited on quartz substrates. The absorption spectra of GR and Z1Z3 substrates are also shown for comparison. After ZnO vapor deposition on graphene, a very strong absorption peak at ∼ 365 nm is clearly visible, which implies the growth of crystalline ZnO NWs. On the other hand, GR and Z1Z3 substrates show no significant UV absorption and a very weak absorption band at 373 nm, respectively. The UV absorption band intensity is significantly higher by a factor of 6 and 10 in GRZ3NW and Z1Z3NW, as compared to that of Z1Z3. The high UV absorption may be due to the large surface area and dense ZnO NWs array. The absorption data is consistent with the enhanced UV PL emission and PC from the graphene– ZnO NWs hybrids (discussed later). Note that there is an additional weak and broad absorption band identified in the visible region peaked at ∼480 nm in Z1Z3NW sample, which is attributed to the oxygenated defect either oxygen vacancy (Vo) or oxygen interstitials (Oi)) states that were formed due to the physical vapor deposition (PVD) growth at relatively lower temperature [6].
3.5. Growth mechanism
Figure 5(a) shows a schematic of the growth process and the morphology of the ZnO NWs grown on GRZ3 and Z1Z3 substrates, which is based on the experimental observations. When VS growth of ZnO NWs is performed simultaneously on both substrates, well aligned ZnO NWs were formed in the case of GRZ3 substrate, while dense and randomly oriented ZnO NWs were formed in the case of Z1Z3 substrate as noted earlier. FESEM image in each case is also included for comparison. Our results suggest that the presence of graphene promotes the vertical alignment of the ZnO NWs. The mechanism behind this can be understood from figure 5(b) which shows the epitaxial relationship between ZnO in its hexagonal wurtzite phase and bond centered sites of C–C bonds present in the sp2 hybridized graphene layer. The red shift (10 cm−1) of 2D band in the Raman spectra of GRZ3 substrate supports our assertion on the interaction between the GR and ZnO layers and the artificial lattice matching mentioned above (see supporting information; figure S4).
3.7. PL studies
UV and visible PL studies were conducted for the as-grown and RTA treated ZnO NWs samples by PL measurements with 325 and 355 nm excitations (supporting information, figure S5). Note that we have not observed any significant change in the UV and visible PL spectra after the RTA treatment of the ZnO NWs in comparison to the untreated NWs. Hence, for further discussion, the PL data are presented for the RTA treated samples. For the reference, the PL spectra of the as-grown ZnO NWs are described in the supporting information, figure S5. Figure 6(b) illustrates the PL evolution
3.6. Optical absorption studies
In order to assess the optical absorption in the ZnO nanostructures grown on graphene and ZnO buffer layer coated on 8
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Figure 6. (a) Comparison of absorption spectra for ZnO NWs on graphene and ZnO coated quartz substrates. For comparison, spectra of GR
and Z1Z3 prior to the growth of NWs are shown. The absorption peaks are denoted in nm unit. Note that the Z1Z3NW shows substantial absorption in the visible region, besides the strong UV absorption. (b) Comparative PL characteristics of GRNW, GRZ3NW, GQDNW, GOZ3NW and Z1Z3NW after RTA treatment, measured with 325 nm laser excitation.
oxygen is increased and supersaturated to plenty of oxygenated defects in the ZnO NWs during the PVD growth. Further, a comparative analysis was made on the PL data to understand the effect of ZnO seed layer (Z3), thickness of the ZnO buffer layer (Z1), graphene (GR) layer on the PL efficiency of the ZnO NWs. With two different laser excitations, the visible PL features are found to be similar. Presence of Z3 layer leads to a sharper and intense UV peak with a lower intensity visible band. Thus, the Z3 layer plays a crucial role in the catalyst free growth of ZnO NWs. Strong and broad band visible PL in all the samples (see figure 6(b)) is beneficial for displays and other light emitting applications. Figure 7 shows the Gaussian fitting for the PL spectra of ZnO NWs grown on graphene and ZnO thin film substrates. Symbols represent the experimental data and solid lines correspond to the fitted data. Due to the dissimilar peak positions and asymmetric line shape, three PL peaks were fitted for the broad visible emission band in each case and single peak for the UV emission peaked at 382 nm. As compared to the GRNW, the intensity of UV PL in Z1Z3NW and GRZ3NW is higher by a factor of 17 and 3, respectively (see figures 7(a) and (b)). Higher intensity UV PL in Z1Z3NW is most likely due to the contribution of the thick ZnO seed layer in the UV PL. On the other hand, in GRNW the integrated intensity of the fitted visible peaks at ∼ 494, 532 and 573 nm are higher than those Z1Z3NW and GRZ3NW, which signifies more number of oxygenated defects (Vo/Oi) in the ZnO NWs grown in absence of the Z3 layer (see figure 7(c)). Note that, the two visible PL peaks at ∼ 530 and 580 nm are common in all the samples, which signifies the formation of neutral Oi defects [19]. Nevertheless, Z1Z3NW and GRZ3NW samples have strong UV emission and relatively lower intensity of
in ZnO NWs grown on all the substrates with 325 nm excitation. The PL spectra show a sharp UV emission peak (∼380 nm) attributed to the near band edge emission and a broad visible emission band centered at ∼ 500 nm attributed to the transition between various surface defect energy levels in ZnO NWs. The most commonly observed defects that occur in the low temperature vapor phase growth of ZnO NWs are Vo, Oi and Zn interstitials (Zni) defects due to their low formation energies [6, 19]. It is interesting to note that the ratio of integrated intensity of UV to visible PL is relatively high in both Z1Z3NW and GRZ3NW as compared to the other samples. The integrated intensities and the ratios of UV and visible PL for all the samples are shown in table T1 in the supporting information. This implies that the crystalline quality of the NWs grown on GRZ3 substrate is significantly high. Note that the density of the NWs is high in case of Z1Z3NW and the substrate ZnO thick layer may contribute to the UV and visible PL. Thus, randomly oriented ZnO NWs in Z1Z3NW may not possess high crystalline quality, though the observed PL intensity is comparable to that of the aligned NWs grown on the graphene layer. On the other hand, visible PL intensity is very high in GOZ3NW sample as compared to all other samples. This strong visible PL from GOZ3NW might be partly due to the GO, which may emit a blue light which can be considered as a second source of excitation. Note that our result is in contrast to that of Zeng et al who reported the quenching of the visible PL due to the electron transfer between the excited ZnO and GO sheets [14]. In the present case, enhancement might be partly due to the visible PL from GO layers itself that contains oxygen functional groups and after the growth of ZnO NWs, concentration of 9
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3.8. Photoconductivity (PC) studies
In order to evaluate the photoresponse of the GR–ZnO hybrids, we have performed the PC measurements at a fixed bias voltage (2 V). For the PC measurements, Ag contact was deposited by thermal evaporation under high vacuum on top of the ZnO NWs surface by keeping a shadow mask with a channel width and length ∼0.1 × 0.1 mm2. The thickness of the Ag layer is ∼100 nm. A schematic of the device configuration is shown in figure 8(a). Figures 8(b)–(d) represent the steady state dark current and photocurrent as a function of voltage in the GRZ3NW, Z1Z3NW and GOZ3NW, respectively, with UV (∼365 nm) excitation. The corresponding time response of the photocurrent in each case is shown in the figures 8(e)–(g), respectively. Note that the PC of the ZnO NWs grown on GOZ3 and Z1Z3 layer is very less as compared to that of the NWs grown on the GRZ3 layer. At a bias voltage of 2 V, the PC in GRZ3NW is about 65 μA as compared to the PC of about 0.75 and 1.0 μA in Z1Z3NW and GOZ3NW, respectively. The high photocurrent in graphene case is probably resulting from the Schottky barrier formation between the bunch of vertically aligned ZnO NWs and graphene layer. In case of ZnO on GO films, it forms a heterojunction. Marginally higher PC in GOZ3NW as compared to Z1Z3NW might be due to the increased number of electron (e)–hole (h) pair generation and separation at the junction of GO and ZnO NWs interface, i.e. formation of excitons during the PC generation. Above certain bias voltage, the dark I–V curve is found to be nonlinear due to the presence of the traps/ defects in each case. Due to the semiconductor–semiconductor heterojunction, the magnitude of the PC is about two orders of the magnitude lower in Z1Z3NW and GOZ3NW as compared to the case of GRZ3NW (see figure 8(c)) under the same bias condition. Thus, graphene layer enhances the sensitivity of the UV PC of the ZnO layer on it. This is an interesting observation and it can be exploited in future optoelectronic devices. Note that the dark current is relatively high in GRZ3NW, which needs to be optimized for practical application. Less PC in case of GOZ3NW may be partly due to the disorder and more oxygenated functional groups present on GO, which is consistent with the PL data discussed earlier. Further, we have investigated the PC growth and decay behavior of ZnO NWs by fitting with bi-exponential function, following the report by Dhara et al [7]. PC growth can be expressed as:
(
Iph (t ) = I1 + A1 1 − e−t Figure 7. Gaussian line shape fitting of the PL spectra: (a) Z1Z3NW,
) − A2 e−t τ .
τ1
2
(1)
Here I1, A1 and A2 are the constants. τ1 and τ2 are the time constants, which are calculated from the fitting of the experimental data. For GRZ3NW, Z1Z3NW and GOZ3NW, the time constants of PC growth are found to be τ1 = 45.0 s, τ2 = 3.7 s, τ1 = τ2 = 1.6 s and τ1 = 15.8 s, τ2 = 0.1 s respectively. PC decay is expressed as:
(b) GRZ3NW, and (c) GRNW. The symbols represent the experimental data and solid lines correspond to the fitted data. The peak centers are denoted in nm unit.
visible emission. Note that in case of GO and GQDs coated substrates, the visible PL features are almost identical (see figure 6(b). The detailed peak parameters and possible assignments of the defect emissions are presented in the supporting information (table T2).
Iph (t ) = IPh (∞) + A 3 e−t
τ1
+ A 4 e−t τ2 .
(2)
Here A3 and A4 are the constants. τ1 and τ2 are calculated as 2.5, 88.5 s in the case of GRZ3NW, 1.4, 20.8 s in Z1Z3NW 10
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Figure 8. (a) A schematic of the Ag contact made on the ZnO NWs array for the photoconductivity (PC) measurement, illuminated with UV
light (365 nm). (b)–(d) Dark current and photo current as a function of voltage in GRZ3NW, Z1Z3NW and GOZ3NW respectively. (e)–(g) The time response of the respective photocurrent in GRZ3NW, Z1Z3NW and GOZ3NW at a fixed bias voltage (2 V). The symbols represent the experimental data and the solid lines correspond to the fitted data in each case. 11
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References
and 9.8, 37.9 s in GOZ3NW, respectively. Note that the IPh (∞) represents the photocurrent after long time, which is equal to the dark current. Thus, the time response of the photocurrent is relatively slow in GRZ3NW and GOZ3NW as compared to that of Z1Z3NW, though GRZ3NW possesses higher sensitivity. The time response is mainly controlled by the intrinsic defects in the ZnO layer. Further optimization of the growth conditions are required for a faster response ZnO PD made on the graphene layer. Besides the surface defects, graphene–ZnO hetero-interface may be partly responsible for the slow response of the PD. More studies are underway to pinpoint the mechanism.
[1] Park W I, Lee C-H, Lee J M, Kim N-J and Yi G-C 2011 Inorganic nanostructures grown on graphene layers Nanoscale 3 3522 [2] Fu X-W, Liao Z-M, Zhou Y-B, Wu H-C, Bie Y-Q, Xu J and Yu D-P 2012 Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector Appl. Phys. Lett. 100 223114 [3] Kumar B, Lee K Y, Park H K, Chae S J, Lee Y H and Kim S-W 2011 Controlled growth of semiconducting nanowire, nanowall, and hybrid nanostructures on graphene for piezoelectric nanogenerators ACS Nano 5 4197 [4] Zengcai S, Helin W, Yuhao L, Jing W, Hao L, Haoning W, Pingli Q, Wei Z and Guojia F 2014 Enhanced field emission from aligned ZnO nanowires grown on a graphene layer with hydrothermal method IEEE Trans. Nanotechnology 13 167 [5] Park H et al 2013 Graphene cathode-based ZnO nanowire hybrid solar cells Nano Lett. 13 233 [6] Biroju R K, Giri P K, Dhara S, Imakita K and Fujii M 2013 Graphene-assisted controlled growth of highly aligned ZnO nanorods and nanoribbons: growth mechanism and photoluminescence properties ACS Appl. Mater. Interfaces 6 377 [7] Dhara S and Giri P K 2011 Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires Nanoscale Res. Lett. 6 504 [8] Lee J M, Yi J, Lee W W, Jeong H Y, Jung T, Kim Y and Park W I 2012 ZnO nanorods–graphene hybrid structures for enhanced current spreading and light extraction in GaNbased light emitting diodes Appl. Phys. Lett. 100 061107 [9] Biroju R K and Giri P K 2013 Controlled fabrication of graphene ZnO nanorod, nanowire and nanoribbon hybrid nanostructures J. Nanosci. Lett. 4 34 [10] Geim A K and Novoselov K S 2007 The rise of graphene Nat. Mater. 6 183 [11] Son D I, Kwon B W, Park D H, Seo W-S, Yi Y, Angadi B, Lee C-L and Choi W K 2012 Emissive ZnO–graphene quantum dots for white-light-emitting diodes Nat. Nano 7 465 [12] Qiang X, Qijin C, Jinxiang Z, Weiwei C, Zifeng Z, Zhengyun W and Fengyan Z 2014 A metal–semiconductor– metal detector based on ZnO nanowires grown on a graphene layer Nanotechnology 25 055501 [13] Kim Y, Lee J-H and Yi G 2010 Vertically aligned ZnO nanostructures grown on graphene layers Appl. Phys. Lett. 95 213101 [14] Huiden Zeng Y C, Xie S, Yang J, Tang Z, Wang X and Sun L 2013 Synthesis, optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures Nanoscale Res. Lett. 8 133 [15] Ferrari A C and Basko D M 2013 Raman spectroscopy as a versatile tool for studying the properties of graphene Nat. Nanotechnology 8 235 [16] Munshi A M, Dheeraj D L, Fauske V T, Kim D-C, A T Helvoort J V, Fimland B and Weman H 2012 Vertically aligned GaAs nanowires on graphite and few-layer graphene: generic model and epitaxial growth Nano Lett. 12 4570 [17] Hong Y J, Lee W H, Wu Y, Ruoff R S and Fukui T 2012 Van der Waals epitaxy of InAs nanowires vertically aligned on single-layer graphene Nano Lett. 12 1431 [18] Dhara S and Giri P K 2011 Rapid thermal annealing induced enhanced band-edge emission from ZnO nanowires, nanorods and nanoribbons Funct. Mater. Lett. 4 25 [19] Janotti A and Van de Walle C G 2006 New insights into the role of native point defects in ZnO J. Cryst. Growth 287 58
4. Conclusion Catalyst free growth of ZnO NWs on different graphene buffer layers such as CVD graphene, GO and GQDs substrates in the presence/absence of ZnO seed layer was successfully demonstrated. Dense array of aligned ZnO NWs was formed in the case of RTA treated ZnO ultra-thin film on graphene, while the GO and GQDs substrates yield randomly oriented sparse ZnO NWs. HRTEM and Raman studies reveal the good crystalline quality of the ZnO NWs grown on graphene–ZnO buffer layer substrate in comparison with the ZnO NWs grown on ZnO seed layer and GO–ZnO buffer layer substrates. A growth mechanism was proposed based on the artificial epitaxial relationship between ZnO and graphene as compared to that of the NWs grown without the graphene layer. The evolution of the UV and visible PL was studied and correlated with effects of ZnO buffer layers on graphene substrates in the aligned growth of ZnO NWs with high crystalline quality. Highly enhanced PC was achieved in the case of ZnO NWs grown on graphene, which is consistent with the PL results. These results demonstrate the successful fabrication and superior performance of ZnO NW–graphene hybrid UV PDs as compared to the bare ZnO NW based PDs. These hybrid nanostructures will be the building block for the next generation optoelectronic devices.
Acknowledgments We thank Central Instruments Facility (CIF) for providing FESEM and micro Raman facilities. We acknowledge the financial support from CSIR (03(1270)/13/EMR-II) to carry out part of this work. We acknowledge DST (No Sr/55/NM– 01/2005) for providing the HRTEM facility at IIT Guwahati. We thank A K Sivadasan, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, for his help in the Raman mapping measurements. We also acknowledge Albert V Tamashausky, Asbury Graphite Mills, USA, for providing high purity graphite flakes to synthesize GO and GQDs. Dr Shilpa Sharma and N V V Subbarao are also duly acknowledged. 12
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Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145601 (http://iopscience.iop.org/0957-4484/26/14/145601) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 193.140.216.7 This content was downloaded on 23/04/2017 at 12:06 Please note that terms and conditions apply.
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Nanotechnology Nanotechnology 26 (2015) 145601 (12pp)
doi:10.1088/0957-4484/26/14/145601
Catalyst free growth of ZnO nanowires on graphene and graphene oxide and its enhanced photoluminescence and photoresponse Ravi K Biroju1, Nikhil Tilak2, Gone Rajender2, S Dhara3 and P K Giri1,2 1
Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India 3 Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2
E-mail: [email protected] Received 18 December 2014, revised 9 February 2015 Accepted for publication 17 February 2015 Published 16 March 2015 Abstract
We demonstrate the graphene assisted catalyst free growth of ZnO nanowires (NWs) on chemical vapor deposited (CVD) and chemically processed graphene buffer layers at a relatively low growth temperature (580 °C) in the presence and absence of ZnO seed layers. In the case of CVD graphene covered with rapid thermal annealed ZnO buffer layer, the growth of vertically aligned ZnO NWs takes place, while the direct growth on CVD graphene, chemically derived graphene (graphene oxide and graphene quantum dots) without ZnO seed layer resulted in randomly oriented sparse ZnO NWs. Growth mechanism was studied from high resolution transmission electron microscopy and Raman spectroscopy of the hybrid structure. Further, we demonstrate strong UV, visible photoluminescence (PL) and enhanced photoconductivity (PC) from the CVD graphene–ZnO NWs hybrids as compared to the ZnO NWs grown without the graphene buffer layer. The evolution of crystalinity in ZnO NWs grown with ZnO seed layer and graphene buffer layer is correlated with the Gaussian line shape of UV and visible PL. This is further supported by the strong Raman mode at 438 cm−1 significant for the wurtzite phase of the ZnO NWs grown on different graphene substrates. The effect of the thickness of ZnO seed layers and the role of graphene buffer layers on the aligned growth of ZnO NWs and its enhanced PC are investigated systematically. Our results demonstrate the catalyst free growth and superior performance of graphene–ZnO NW hybrid UV photodetectors as compared to the bare ZnO NW based photodetectors. S Online supplementary data available from stacks.iop.org/NANO/26/145601/mmedia Keywords: graphene–ZnO NW hybrid, photoluminescence, rapid thermal annealing, photoconductivity (Some figures may appear in colour only in the online journal) 1. Introduction
conversion and storage devices [5] etc. However, the catalyst free growth of vertically aligned semiconductor nanowires (NWs) directly on different forms of graphene buffer layers is a challenging task. In this type of hetero structure, it could be possible to integrate the 2D graphene and 1D semiconducting NWs for Schottky junction devices with
In recent years, graphene–semiconductor hybrid nanostructures have drawn enormous attention due to their potential applications in photovoltaics [1, 2] nanogenerators [3], field emission devices [4], and efficient energy 0957-4484/15/145601+12$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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2. Experimental
enhanced functionalities for ensuing optoelectronic applications. Among all, the graphene mediated growth of vertically aligned ZnO hybrid nanostructures has received much attention [6], since ZnO is a wide direct band gap (3.3 eV) semiconductor and has large exciton binding energy (60 meV) which can possess fast photo response upon illumination of UV light in the graphene based photodetectors (PDs) [7] and gas sensing capability [8]. Recently, vertically aligned growth of ZnO nanorods (NRs)/NWs by vapor liquid solid (VLS) and vapor solid (VS) mechanisms has been reported on single (SLG) and few layer (FLG) graphene in the presence and absence of gold (Au) catalyst by vapor phase growth [3, 6, 9]. Fabrication of vertically aligned ZnO NRs/NWs with high aspect ratio and extremely large surface-to-volume ratio, specifically on graphene substrates without the aid of metal catalyst is still challenging. With the excellent electrical, mechanical and thermal characteristics of graphene layers, growing ZnO nanostructures and thin films on graphene would enable their novel physical properties to be exploited in the diverse range of sophisticated device applications [10]. Therefore, several graphene–semiconductor nanostructure hybrids have been successfully synthesized that show elegant combinations of properties not found in the individual components [11]. Recently, Xu et al developed a metal–semiconductor–metal PD using hydrothermally grown ZnO NWs and observed the existence of surface plasmon resonance arising from the underlying graphene layer, which exhibited a UV to visible rejection ratio of ∼ 4 [12]. To the best of our knowledge, there are limited studies on the graphene mediated growth of vertically aligned, high density and high aspect ratio ZnO NWs on graphene buffer layers by thermal vapor deposition technique and the role of graphene layer in the enhanced photoluminescence (PL) and photoconductivity (PC) are least understood [13]. Further, there are very few reports on the growth of ZnO NWs on chemical vapor deposited (CVD) and chemically derived graphene functional materials, such as graphene oxide (GO) and graphene quantum dots (GQDs), and the optical properties of such hybrid nanostructures for the hybrid photodetector applications are little explored [14]. In the present work, we demonstrate the catalyst free growth of aligned ZnO NWs on various substrates consisting of CVD graphene (GR), GO and GQDs with/without a thin ZnO seed layer on graphene. The structural quality of the as-grown graphene layer as well as the ZnO NWs grown on it is characterized by Raman spectroscopy along with the Raman mappings with 514.5 nm laser excitation. The effect of ZnO seed layer and different graphene buffer layers on the structure of ZnO NWs was explored by high resolution transmission microscopy (HRTEM) and its optical properties were studied systematically by UV–visible absorption and PL spectroscopy. Further, enhanced UV PC was observed in the case of ZnO NWs grown on graphene/SiO2 substrate as compared to that grown without the graphene layer.
2.1. Synthesis of CVD graphene and chemically derived graphene
Single and few layer graphene samples were synthesized by a thermal CVD method and transferred on to the Si/SiO2 and quartz substrate by standard wet chemical method. The chemically derived graphene such as GO and GQDs are obtained from simple chemical exfoliation techniques. The full details of the experiment and characterization of these samples are described in the supporting information (see sections: SI1–SI4).
2.2. Deposition of high quality ZnO thin films on graphene
High quality ZnO films with three different thicknesses: ∼300 nm (code-Z1), ∼100 nm (code-Z2) and ∼10 nm (codeZ3), were deposited by RF magnetron sputtering on various graphene substrates for the growth of ZnO NWs. High purity ZnO sputter target (99.999%, Kurt J Lesker, USA) was used as a source for the ZnO grain growth. Initially the chamber was evacuated to a base pressure of 6.7 × 10−6 mbar and during the sputtering it was maintained at 1 × 10−2 mbar. The RF power was kept at 100 W. The substrates were heated to 200 °C for better crystalinity and uniformity of the ZnO grains. Further, these ZnO thin films were subjected to rapid thermal annealing (RTA) treatment at 600 °C in Ar gas ambient (flow rate of 200 standard cubic centimeters (SCCM)) atmosphere for 3 min using a commercial RTA system (MILA3000, Ulvac, Japan) in order to further improve the crystalline quality as well as for the removal of excess oxygen traps on the ZnO thin films on various substrates. Improvements in the crystalinity of ZnO grains and their phases were confirmed from XRD and HRTEM analyses (discussed later).
2.3. Growth of ZnO NWs on graphene substrates
Commercial nano sized activated Zn powder (purity ∼ 99%, Aldrich) was taken as a source material in an alumina boat and placed at the center of a horizontal quartz tube kept inside a muffle furnace. The above prepared substrates were placed downstream ∼ 5 cm away from the source material. Initially the quartz tube was pumped down to a pressure of ∼10−3 mbar. In order to prevent the oxidation of the graphene layer, 300 SCCM of Ar gas was flushed into the chamber until it reached the set point with a rate of 18 °C min−1. When the furnace reached the desired temperature, 20 SCCM of O2 gas was introduced and gas pressure inside the chamber was maintained at 1.6 mbar for the growth time of 50 min. The source material temperature was maintained at ∼600 °C and the substrate temperature at ∼580 °C. After the completion of the reaction, the furnace was cooled down to room temperature. After the growth, the RTA is performed at 600 °C in Ar atmosphere (300 SCCM) for 3 min to improve the crystalline quality of the as-grown ZnO NWs. 2
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2.4. Characterization techniques
The morphology and crystal structure of the as-grown samples were studied using electron microscopy tools such as field emission scanning electron microscope (FESEM, Sigma, Zeiss) and HRTEM (HRTEM, JEM2100 operated at 200 kV, JEOL). Micro Raman measurements were performed using a high resolution Raman spectrometer (inVia, Renishaw), with the excitation source 514.5 nm (Ar+ laser) and monochromator using 1800 gr mm−1 lines with a thermoelectric cooled CCD detector in the backscattering configuration to examine the crystalline quality and number of layers in the graphene. Raman mapping was carried out with 10 × 10 μm2 frame size on the samples at 514.5 nm laser excitation with a spatial resolution of 100 × 600 nm using a Streamline imaging facility for covering large area. Additionally, PL measurement was carried out at room temperature using a 325 nm He–Cd laser excitation and a monochromator with 2400 gr mm−1 grating. Some of the PL measurements were performed using a 355 nm diode laser excitation in a commercial fluorimeter (AB2, Thermo Spectronic). The UV–vis–NIR absorption spectroscopy measurements were recorded using a commercial spectrophotometer (PerkinElmer UV Win Lab, UV3101PC). PC measurements were performed with a micro probe station (ECOPIA EPS-500) connected to a source– measure unit (Keithely 2400, USA) for current–voltage (I–V) characteristics and 300 W xenon lamp as a source to excite the sample. Note that the excitation wavelength was selected using a monochromator (Oriel Instruments, USA). The I–V setup was interfaced with a computer to collect the data using Lab Tracer 2.0 software.
3. Results and discussion 3.1. Raman studies of different graphene substrates
Prior to the growth of ZnO NWs, the qualities of the GR, GO and GQDs coated on SiO2 substrates were first characterized by Raman spectroscopy. The characteristic Raman spectra of as-grown GR, GO and GQDs are shown in figure S1 (supporting information SI 5). In addition, Raman mapping was performed on GR sample for the well-known D, G and 2D bands for the surface coverage and uniformity of graphene layer. Figures 1(a)–(c) represent the Raman mapping images scanned in an area of 10 × 10 μm2, which shows a full coverage of SLG as evident from a sharp and prominent 2D peak at ∼2700 cm−1. The graphitic G band at ∼1595 cm−1 signifies the sp2 hybridization of carbon atoms and assigned for the Eg2 (high) mode of in-plane C–C stretching vibration [15]. The ratio of intensities of 2D and G bands I(2D)/I(G) is ∼1.00, which indicates the presence of SLG and FLG. The high intensity of the defect band D at ∼1350 cm−1 implies the presence of point and line defects in graphene. Some of the defects might have been introduced during the wet transfer process. The crystalline quality of different graphene materials was estimated from HRTEM analysis to support the Raman data. HRTEM images of GR, GO and GQDs are
Figure 1. Spatial Raman mappings of CVD graphene: (a) D, (b) G
and (c) 2D bands, which indicate the presence of BLG and FLG. Note that all the mappings are recorded with 10 × 10 μm2 area at 514.5 nm laser excitation.
shown in the supporting information, figure S2. Corresponding SAED patterns are shown in the inset, which clearly show a hexagonal lattice pattern. Note that the multiple SAED spots in GO sample (see figure S2(b)) are a signature of few layer GO. These graphene substrates are named according to the pretreatment conditions, as described in table 1. The surface morphology of ZnO NWs grown in each case is presented in table 1. As discussed before, the GR, GO and GQDs ultrathin films were prepared on SiO2 substrates and ZnO films of 3
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Table 1. The sample descriptions for the various graphene and ZnO coated buffer layer substrates and morphology of the ZnO NWs in each case.
4
Sl. No.
Substrate code
Base substrate
Sample
Substrate description
Morphology of the ZnO NWs
1 2 3 4 5 6 8
GR GRZ3 Z1Z3 Z2Z3 Z3 GOZ3 GQD
Si/SiO2, Quartz Si/SiO2, Quartz Si/SiO2, Quartz Si/SiO2 Si/SiO2 Si/SiO2 Si/SiO2
GRNW GRZ3NW Z1Z3NW Z2Z3NW Z3NW GOZ3NW GQDNW
Graphene layer only 10 nm ZnO on graphene, RTA 10 nm ZnO on 300 nm ZnO layer, RTA 10 nm ZnO on 100 nm ZnO layer, RTA 10 nm ZnO film, RTA 10 nm ZnO on GO, RTA Graphene quantum dots
Randomly orientated sparse NWs Vertical NWs Dense and randomly orientated NWs Dense and randomly orientated NWs Randomly orientated NWs Randomly orientated NWs Randomly orientated NWs
Note: Z1: 300 nm ZnO film, Z2: 100 nm ZnO film and Z3: 10 nm ZnO film.
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Figure 2. FESEM images of various ZnO nanostructures grown on different graphene–ZnO and ZnO buffer layer substrates: (a) GRZ3NW, (b) Z1Z3NW, (c) GOZ3NW, and (d) GQDNW.
thickness ∼300 nm (code-Z1), ∼200 nm (code-Z2) and ∼10 nm (code-Z3) were deposited on various graphene buffer layers followed by RTA in an inert gas environment. The systematic study on the individual effects of graphene buffer layer, ZnO seed layer and ZnO buffer layers on the growth of ZnO NWs have been elucidated in the following sections. Further the growth mechanism, strong UV-visible PL and PC characteristics of various ZnO nanostructures on different graphene derivative materials as substrates are discussed below.
aligned NWs, while the GO and GQD layers do not promote such kind of growth. Interestingly, the ZnO NWs grown on GRZ3 substrate show very strong UV PL as compared to the NWs grown on Z1Z3 substrate. Note that the visible PL is prominent in all the as-grown ZnO NWs on graphene samples (discussed later). The Z1Z3 and Z2Z3 layers show similar crystalline features after RTA treatment, which are consistent with the XRD results. Further, HRTEM measurements were conducted on the GRZ3NW sample to understand the role of ZnO seed layer in the vertical growth of ZnO NWs. It was found that the growth of the NWs is initiated from the ZnO seed layer and no NW growth takes place directly on the GR layer. This may be due to fact that graphene has very low surface energy and is a very inert material to nucleate the ZnO on its surface, assuming a low defect density. However, GR layer helps to grow the ZnO NWs vertically on the ZnO seed. It is likely that due to the presence of graphene, no planar growth takes place for the ZnO NWs. In the present case, the vertical growth of NWs can be attributed to the presence of ZnO seed on graphene layer, based on the VS growth mechanism [13]. However, here we did not observe the growth of ZnO NWs with hexagonal facets, in contrast to our earlier work [6] where ZnO NWs were grown using gold as a catalyst and graphene as a substrate layer. Thus, epitaxial growth is less efficient in the present case. Note that ZnO NWs growth on Z1Z3 is very dense without any vertical alignment (see figure 2(b)). The possible explanation for this observation is that the thick ZnO layer covered with a thin ZnO layer, both have the same surface energy and the density of oxygen atoms is very high
3.2. Effect of ZnO seed layer
The RTA of the ZnO thin film leads to formation of bigger grains of ZnO with improved crystallinity and the partial removal of defects in the ZnO. These ZnO grains grown on the graphene layer play a crucial role in the vertical growth and alignment of the ZnO NWs. Due to similarity in hexagonal crystalline structure of graphene and ZnO, it leads to the epitaxial like growth of the ZnO NWs. Figure 2 represents the FESEM image of ZnO nanostructures grown on various graphene substrates coated with ZnO seed layers, as mentioned in table 1. Figure 2(a) illustrates the case of GRZ3, while figures 2(b)–(d) illustrate the case of Z1Z3, GOZ3 and GQD, respectively. In the inset of figure 2(a), the vertical growth of ZnO NWs on ultrathin ZnO buffer layer (Z3) coated GR substrate is clearly visible. On the other hand, dense bundles of randomly oriented ZnO NWs can be seen on GO and GQD substrates. This clearly indicates that GR layer below the Z3 film, which has some kind of epitaxial relation with the ZnO seed layer, promotes the growth of vertically 5
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Figure 3. (a) FESEM image of ZnO NW grown directly on GR showing sparse and randomly oriented ZnO NWs. (b) TEM image of the ZnO
grains on SiO2 with nearly uniform size (∼40 nm), and the inset shows the higher magnification TEM image of a spherical ZnO seed. (c) TEM image of a ZnO NW along with the ZnO seed layer in sample GRZ3NW, and the inset shows the corresponding SAED pattern showing (002) plane for the combined layer. (d) Magnified TEM image of region I in (c), which depicts the nucleation of the ZnO NW on ZnO seed and its corresponding lattice fringes showing (002) planes of the ZnO NW with lattice spacing of 2.6 Å.
represents the magnified view of region I in figure 3(c), which clearly depicts the nucleation of NW on ZnO seed and vertical alignment onto (002) planes. The IFFT image in the inset (magnified view of region II) shows the lattice fringes for (002) planes with d-spacing 2.6 Å that strongly supports the Raman data (discussed later). The crystalline quality and the crystalline phases of RTA treated ZnO seed layers are assessed from the XRD pattern. Figure 4(a) represents the XRD pattern of Z1Z3 substrate after RTA treatment. The XRD pattern shows strong peaks at 2θ = 31.96°, 34.66° and 36.44° corresponding to (100), (002) and (101) planes of ZnO. These ZnO grains act as the seeds for the growth of ZnO nanostructures during the vapor deposition process. This is evident from the TEM image of figure 3(c) where the ZnO NW is grown directly on a ZnO grain. Thus, the ZnO buffer layer plays a crucial role in the growth of ZnO NWs. This can be confirmed by looking at the FESEM images of the ZnO NWs formed on the GR (figure 3(a)) and GRZ3 (figure 2(a)) substrates. GR substrate
on these layers due to the presence of oxygen interstitials [6]. These excess oxygen in the substrate help in the planar growth of the NWs [6] resulting into randomly oriented NWs. Interestingly, the direct growth of NWs on the GR substrate without a ZnO layer (Z3) results in a growth of sparse ZnO NWs, as evidenced from the FESEM image of figure 3(a). Thus, in a catalyst free growth, self catalytic seed layer is essential for the aligned growth of the NWs. Self-catalyzed growth of vertical ZnO NWs on various dielectric substrates have been reported earlier. Herein, self-catalyzed growth of ZnO NWs on graphene substrate has been demonstrated. Figure 3(b) represents the HRTEM image of the ZnO grains with an average diameter ∼ 50 nm. Inset shows the lattice image of a spherical ZnO grain showing the polycrystalline lattice patterns of ZnO seeds. Further, HRTEM results obtained for the ZnO grains are in close agreement with the XRD results. Figure 3(c) shows the ZnO NW grown vertically on the ZnO seed and its corresponding SAED pattern showing the single crystal (002) planes. Figure 3(d) 6
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aligned GaAs and InAs NWs on graphene substrates has been reported by Munshi et al [16] and Hong et al [17], respectively and it was suggested to be assisted by the strong van der Waals interactions. Our results are consistent with the above reports. 3.3.2. Z1Z3. In this case, a 10 nm ZnO seed layer is present
on the 300 nm ZnO layer, without the graphene layer. A dense and randomly oriented ZnO NWs growth takes place on Z1Z3 (see figure 2(b)). This can be attributed to the large number of overlapping grains on the ZnO thin film. The buffer layer Z3 seems to have negligible effect on the NW alignment. Once again, it implies that absence of graphene layer leads to random orientation of the ZnO NWs. 3.3.3. GOZ3 and GQDs. Our results show that the GO and
GQDs do not play any significant role in the growth/ alignment of the ZnO NWs as compared to the case of GR buffer layer. We believe that there are primarily two factors affecting the growth and orientation of the ZnO NWs: (i) nonuniform coverage of the GO layer on the substrate, (ii) high density of defects on the GO layer. This is clearly shown by the FESEM image in the supporting information, figure S3. During the growth, ZnO NWs may nucleate at the defect sites in GO and random growth takes place due to the absence of any lattice matching between GO and ZnO. Similar results were also observed for the case of GQDs substrate; due to the absence of the seed layer on the GQD, random growth takes place. 3.4. Raman studies of ZnO NWs
Crystalline quality of ZnO NWs was probed by Raman spectroscopy on different graphene substrates based on the peak position, relative intensities and full width at half maxima (FWHM-Δω) of the Raman spectra. Figure 4(b) illustrates the comparative Raman spectra of all the ZnO NWs samples. The Raman spectrum shows a peak at ∼ 438 cm−1, which corresponds to the Eg2 (high) mode of ZnO NW significant for the wurtzite phase. The peak position of Eg2 (high) mode and FWHM values have been calculated from the Gaussian peak fit that are labeled in figure 4(b). Note that the FWHM of Eg2 (high) mode in GRZ3NW is relatively lower than that of the NWs grown on other graphene substrates. This is consistent with the strong UV peak in the PL spectrum (discussed later) indicating a better crystallinity of the ZnO NWs grown in the GRZ3NW sample. Note that Z1Z3NW shows relatively lower FWHM of Eg2 (high) mode than that of the GRZ3NW indicating better crystalinity in Z1Z3NW. This may be due to the contribution of thick ZnO buffer layer on the Raman spectrum. Thus, it can be concluded that the ZnO seed layer with GR buffer layer promotes better crystalinity. However, the Raman signal of graphene is not detected in the GRZ3NW samples, perhaps due to the dense coverage and long length of the NWs. To confirm the presence of graphene in these samples, we performed the Raman measurements on GRZ3 substrate (see supporting information, figure S4). Eg2 (high) in GRZ3 reveals the presence of ZnO on the GRZ3
Figure 4. (a) XRD pattern of Z1Z3 substrate after RTA treatment.
The prominent peaks correspond to the (100), (101) and (002) planes of wurtzite ZnO. (b) Typical Raman spectra of ZnO NWs showing strong Eg2 (438 cm−1) mode, which indicates the growth of crystalline ZnO NWs with wurtzite phase. Note that the peak positions and FWHM (Δω) are denoted in cm−1 unit.
without a ZnO seed layer shows very sparse growth of ZnO NWs, whereas GRZ3 shows aligned growth of ZnO NWs. The diameters of the NWs are in the range ∼ 20–30 nm (see figure 2(a)), have dense coverage and is due to the huge number of overlapping ZnO grains. 3.3. Effect of various buffer layers 3.3.1. GRZ3. In this case, a 10 nm ZnO seed layer is present over the graphene buffer layer for the growth of the ZnO NWs. The NWs grown on GRZ3 are much better aligned than those grown on Z1Z3 (figure 2(b)), GOZ3 (figure 2(c)) and GQDs (figure 2(d)). This implies that the graphene buffer layer assists the ZnO grains in the vertical alignment of the NWs. Interestingly, the lattice mismatch between the hexagonal ZnO crystal and graphene bond centered sites is very low [16]. This could lead to the epitaxial growth of ZnO NRs on graphene. Self-catalyzed VLS growth of vertically 7
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Figure 5. (a) A schematic shows the growth mechanism of ZnO NWs on GRZ3 and Z1Z3 substrates. (b) The epitaxial relationship between
ZnO in its hexagonal wurtzite phase at the bond centered sites of sp2 hybridized graphene layer.
substrate as shown in figure S3(a). Further, The Raman signatures of graphene (D, G and 2D bands) with reduced intensity of 2D band and redshift of the D and 2D bands (by ∼ 10 cm−1) reveal that there is a strong interaction between graphene and ZnO grains. The Raman peak located at ∼ 331 cm−1 in GOZ3NW corresponds to the 2E2 phonon [18]. A weak lower frequency mode at 236 cm−1 may be arising from the structural defects on the surface of the ZnO NWs.
quartz substrates, we have performed the UV–VIS absorption spectroscopy measurements. Figure 6(a) represents the absorption spectra of ZnO NWs grown on Z1Z3 and GRZ3 deposited on quartz substrates. The absorption spectra of GR and Z1Z3 substrates are also shown for comparison. After ZnO vapor deposition on graphene, a very strong absorption peak at ∼ 365 nm is clearly visible, which implies the growth of crystalline ZnO NWs. On the other hand, GR and Z1Z3 substrates show no significant UV absorption and a very weak absorption band at 373 nm, respectively. The UV absorption band intensity is significantly higher by a factor of 6 and 10 in GRZ3NW and Z1Z3NW, as compared to that of Z1Z3. The high UV absorption may be due to the large surface area and dense ZnO NWs array. The absorption data is consistent with the enhanced UV PL emission and PC from the graphene– ZnO NWs hybrids (discussed later). Note that there is an additional weak and broad absorption band identified in the visible region peaked at ∼480 nm in Z1Z3NW sample, which is attributed to the oxygenated defect either oxygen vacancy (Vo) or oxygen interstitials (Oi)) states that were formed due to the physical vapor deposition (PVD) growth at relatively lower temperature [6].
3.5. Growth mechanism
Figure 5(a) shows a schematic of the growth process and the morphology of the ZnO NWs grown on GRZ3 and Z1Z3 substrates, which is based on the experimental observations. When VS growth of ZnO NWs is performed simultaneously on both substrates, well aligned ZnO NWs were formed in the case of GRZ3 substrate, while dense and randomly oriented ZnO NWs were formed in the case of Z1Z3 substrate as noted earlier. FESEM image in each case is also included for comparison. Our results suggest that the presence of graphene promotes the vertical alignment of the ZnO NWs. The mechanism behind this can be understood from figure 5(b) which shows the epitaxial relationship between ZnO in its hexagonal wurtzite phase and bond centered sites of C–C bonds present in the sp2 hybridized graphene layer. The red shift (10 cm−1) of 2D band in the Raman spectra of GRZ3 substrate supports our assertion on the interaction between the GR and ZnO layers and the artificial lattice matching mentioned above (see supporting information; figure S4).
3.7. PL studies
UV and visible PL studies were conducted for the as-grown and RTA treated ZnO NWs samples by PL measurements with 325 and 355 nm excitations (supporting information, figure S5). Note that we have not observed any significant change in the UV and visible PL spectra after the RTA treatment of the ZnO NWs in comparison to the untreated NWs. Hence, for further discussion, the PL data are presented for the RTA treated samples. For the reference, the PL spectra of the as-grown ZnO NWs are described in the supporting information, figure S5. Figure 6(b) illustrates the PL evolution
3.6. Optical absorption studies
In order to assess the optical absorption in the ZnO nanostructures grown on graphene and ZnO buffer layer coated on 8
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Figure 6. (a) Comparison of absorption spectra for ZnO NWs on graphene and ZnO coated quartz substrates. For comparison, spectra of GR
and Z1Z3 prior to the growth of NWs are shown. The absorption peaks are denoted in nm unit. Note that the Z1Z3NW shows substantial absorption in the visible region, besides the strong UV absorption. (b) Comparative PL characteristics of GRNW, GRZ3NW, GQDNW, GOZ3NW and Z1Z3NW after RTA treatment, measured with 325 nm laser excitation.
oxygen is increased and supersaturated to plenty of oxygenated defects in the ZnO NWs during the PVD growth. Further, a comparative analysis was made on the PL data to understand the effect of ZnO seed layer (Z3), thickness of the ZnO buffer layer (Z1), graphene (GR) layer on the PL efficiency of the ZnO NWs. With two different laser excitations, the visible PL features are found to be similar. Presence of Z3 layer leads to a sharper and intense UV peak with a lower intensity visible band. Thus, the Z3 layer plays a crucial role in the catalyst free growth of ZnO NWs. Strong and broad band visible PL in all the samples (see figure 6(b)) is beneficial for displays and other light emitting applications. Figure 7 shows the Gaussian fitting for the PL spectra of ZnO NWs grown on graphene and ZnO thin film substrates. Symbols represent the experimental data and solid lines correspond to the fitted data. Due to the dissimilar peak positions and asymmetric line shape, three PL peaks were fitted for the broad visible emission band in each case and single peak for the UV emission peaked at 382 nm. As compared to the GRNW, the intensity of UV PL in Z1Z3NW and GRZ3NW is higher by a factor of 17 and 3, respectively (see figures 7(a) and (b)). Higher intensity UV PL in Z1Z3NW is most likely due to the contribution of the thick ZnO seed layer in the UV PL. On the other hand, in GRNW the integrated intensity of the fitted visible peaks at ∼ 494, 532 and 573 nm are higher than those Z1Z3NW and GRZ3NW, which signifies more number of oxygenated defects (Vo/Oi) in the ZnO NWs grown in absence of the Z3 layer (see figure 7(c)). Note that, the two visible PL peaks at ∼ 530 and 580 nm are common in all the samples, which signifies the formation of neutral Oi defects [19]. Nevertheless, Z1Z3NW and GRZ3NW samples have strong UV emission and relatively lower intensity of
in ZnO NWs grown on all the substrates with 325 nm excitation. The PL spectra show a sharp UV emission peak (∼380 nm) attributed to the near band edge emission and a broad visible emission band centered at ∼ 500 nm attributed to the transition between various surface defect energy levels in ZnO NWs. The most commonly observed defects that occur in the low temperature vapor phase growth of ZnO NWs are Vo, Oi and Zn interstitials (Zni) defects due to their low formation energies [6, 19]. It is interesting to note that the ratio of integrated intensity of UV to visible PL is relatively high in both Z1Z3NW and GRZ3NW as compared to the other samples. The integrated intensities and the ratios of UV and visible PL for all the samples are shown in table T1 in the supporting information. This implies that the crystalline quality of the NWs grown on GRZ3 substrate is significantly high. Note that the density of the NWs is high in case of Z1Z3NW and the substrate ZnO thick layer may contribute to the UV and visible PL. Thus, randomly oriented ZnO NWs in Z1Z3NW may not possess high crystalline quality, though the observed PL intensity is comparable to that of the aligned NWs grown on the graphene layer. On the other hand, visible PL intensity is very high in GOZ3NW sample as compared to all other samples. This strong visible PL from GOZ3NW might be partly due to the GO, which may emit a blue light which can be considered as a second source of excitation. Note that our result is in contrast to that of Zeng et al who reported the quenching of the visible PL due to the electron transfer between the excited ZnO and GO sheets [14]. In the present case, enhancement might be partly due to the visible PL from GO layers itself that contains oxygen functional groups and after the growth of ZnO NWs, concentration of 9
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3.8. Photoconductivity (PC) studies
In order to evaluate the photoresponse of the GR–ZnO hybrids, we have performed the PC measurements at a fixed bias voltage (2 V). For the PC measurements, Ag contact was deposited by thermal evaporation under high vacuum on top of the ZnO NWs surface by keeping a shadow mask with a channel width and length ∼0.1 × 0.1 mm2. The thickness of the Ag layer is ∼100 nm. A schematic of the device configuration is shown in figure 8(a). Figures 8(b)–(d) represent the steady state dark current and photocurrent as a function of voltage in the GRZ3NW, Z1Z3NW and GOZ3NW, respectively, with UV (∼365 nm) excitation. The corresponding time response of the photocurrent in each case is shown in the figures 8(e)–(g), respectively. Note that the PC of the ZnO NWs grown on GOZ3 and Z1Z3 layer is very less as compared to that of the NWs grown on the GRZ3 layer. At a bias voltage of 2 V, the PC in GRZ3NW is about 65 μA as compared to the PC of about 0.75 and 1.0 μA in Z1Z3NW and GOZ3NW, respectively. The high photocurrent in graphene case is probably resulting from the Schottky barrier formation between the bunch of vertically aligned ZnO NWs and graphene layer. In case of ZnO on GO films, it forms a heterojunction. Marginally higher PC in GOZ3NW as compared to Z1Z3NW might be due to the increased number of electron (e)–hole (h) pair generation and separation at the junction of GO and ZnO NWs interface, i.e. formation of excitons during the PC generation. Above certain bias voltage, the dark I–V curve is found to be nonlinear due to the presence of the traps/ defects in each case. Due to the semiconductor–semiconductor heterojunction, the magnitude of the PC is about two orders of the magnitude lower in Z1Z3NW and GOZ3NW as compared to the case of GRZ3NW (see figure 8(c)) under the same bias condition. Thus, graphene layer enhances the sensitivity of the UV PC of the ZnO layer on it. This is an interesting observation and it can be exploited in future optoelectronic devices. Note that the dark current is relatively high in GRZ3NW, which needs to be optimized for practical application. Less PC in case of GOZ3NW may be partly due to the disorder and more oxygenated functional groups present on GO, which is consistent with the PL data discussed earlier. Further, we have investigated the PC growth and decay behavior of ZnO NWs by fitting with bi-exponential function, following the report by Dhara et al [7]. PC growth can be expressed as:
(
Iph (t ) = I1 + A1 1 − e−t Figure 7. Gaussian line shape fitting of the PL spectra: (a) Z1Z3NW,
) − A2 e−t τ .
τ1
2
(1)
Here I1, A1 and A2 are the constants. τ1 and τ2 are the time constants, which are calculated from the fitting of the experimental data. For GRZ3NW, Z1Z3NW and GOZ3NW, the time constants of PC growth are found to be τ1 = 45.0 s, τ2 = 3.7 s, τ1 = τ2 = 1.6 s and τ1 = 15.8 s, τ2 = 0.1 s respectively. PC decay is expressed as:
(b) GRZ3NW, and (c) GRNW. The symbols represent the experimental data and solid lines correspond to the fitted data. The peak centers are denoted in nm unit.
visible emission. Note that in case of GO and GQDs coated substrates, the visible PL features are almost identical (see figure 6(b). The detailed peak parameters and possible assignments of the defect emissions are presented in the supporting information (table T2).
Iph (t ) = IPh (∞) + A 3 e−t
τ1
+ A 4 e−t τ2 .
(2)
Here A3 and A4 are the constants. τ1 and τ2 are calculated as 2.5, 88.5 s in the case of GRZ3NW, 1.4, 20.8 s in Z1Z3NW 10
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Figure 8. (a) A schematic of the Ag contact made on the ZnO NWs array for the photoconductivity (PC) measurement, illuminated with UV
light (365 nm). (b)–(d) Dark current and photo current as a function of voltage in GRZ3NW, Z1Z3NW and GOZ3NW respectively. (e)–(g) The time response of the respective photocurrent in GRZ3NW, Z1Z3NW and GOZ3NW at a fixed bias voltage (2 V). The symbols represent the experimental data and the solid lines correspond to the fitted data in each case. 11
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References
and 9.8, 37.9 s in GOZ3NW, respectively. Note that the IPh (∞) represents the photocurrent after long time, which is equal to the dark current. Thus, the time response of the photocurrent is relatively slow in GRZ3NW and GOZ3NW as compared to that of Z1Z3NW, though GRZ3NW possesses higher sensitivity. The time response is mainly controlled by the intrinsic defects in the ZnO layer. Further optimization of the growth conditions are required for a faster response ZnO PD made on the graphene layer. Besides the surface defects, graphene–ZnO hetero-interface may be partly responsible for the slow response of the PD. More studies are underway to pinpoint the mechanism.
[1] Park W I, Lee C-H, Lee J M, Kim N-J and Yi G-C 2011 Inorganic nanostructures grown on graphene layers Nanoscale 3 3522 [2] Fu X-W, Liao Z-M, Zhou Y-B, Wu H-C, Bie Y-Q, Xu J and Yu D-P 2012 Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector Appl. Phys. Lett. 100 223114 [3] Kumar B, Lee K Y, Park H K, Chae S J, Lee Y H and Kim S-W 2011 Controlled growth of semiconducting nanowire, nanowall, and hybrid nanostructures on graphene for piezoelectric nanogenerators ACS Nano 5 4197 [4] Zengcai S, Helin W, Yuhao L, Jing W, Hao L, Haoning W, Pingli Q, Wei Z and Guojia F 2014 Enhanced field emission from aligned ZnO nanowires grown on a graphene layer with hydrothermal method IEEE Trans. Nanotechnology 13 167 [5] Park H et al 2013 Graphene cathode-based ZnO nanowire hybrid solar cells Nano Lett. 13 233 [6] Biroju R K, Giri P K, Dhara S, Imakita K and Fujii M 2013 Graphene-assisted controlled growth of highly aligned ZnO nanorods and nanoribbons: growth mechanism and photoluminescence properties ACS Appl. Mater. Interfaces 6 377 [7] Dhara S and Giri P K 2011 Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires Nanoscale Res. Lett. 6 504 [8] Lee J M, Yi J, Lee W W, Jeong H Y, Jung T, Kim Y and Park W I 2012 ZnO nanorods–graphene hybrid structures for enhanced current spreading and light extraction in GaNbased light emitting diodes Appl. Phys. Lett. 100 061107 [9] Biroju R K and Giri P K 2013 Controlled fabrication of graphene ZnO nanorod, nanowire and nanoribbon hybrid nanostructures J. Nanosci. Lett. 4 34 [10] Geim A K and Novoselov K S 2007 The rise of graphene Nat. Mater. 6 183 [11] Son D I, Kwon B W, Park D H, Seo W-S, Yi Y, Angadi B, Lee C-L and Choi W K 2012 Emissive ZnO–graphene quantum dots for white-light-emitting diodes Nat. Nano 7 465 [12] Qiang X, Qijin C, Jinxiang Z, Weiwei C, Zifeng Z, Zhengyun W and Fengyan Z 2014 A metal–semiconductor– metal detector based on ZnO nanowires grown on a graphene layer Nanotechnology 25 055501 [13] Kim Y, Lee J-H and Yi G 2010 Vertically aligned ZnO nanostructures grown on graphene layers Appl. Phys. Lett. 95 213101 [14] Huiden Zeng Y C, Xie S, Yang J, Tang Z, Wang X and Sun L 2013 Synthesis, optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures Nanoscale Res. Lett. 8 133 [15] Ferrari A C and Basko D M 2013 Raman spectroscopy as a versatile tool for studying the properties of graphene Nat. Nanotechnology 8 235 [16] Munshi A M, Dheeraj D L, Fauske V T, Kim D-C, A T Helvoort J V, Fimland B and Weman H 2012 Vertically aligned GaAs nanowires on graphite and few-layer graphene: generic model and epitaxial growth Nano Lett. 12 4570 [17] Hong Y J, Lee W H, Wu Y, Ruoff R S and Fukui T 2012 Van der Waals epitaxy of InAs nanowires vertically aligned on single-layer graphene Nano Lett. 12 1431 [18] Dhara S and Giri P K 2011 Rapid thermal annealing induced enhanced band-edge emission from ZnO nanowires, nanorods and nanoribbons Funct. Mater. Lett. 4 25 [19] Janotti A and Van de Walle C G 2006 New insights into the role of native point defects in ZnO J. Cryst. Growth 287 58
4. Conclusion Catalyst free growth of ZnO NWs on different graphene buffer layers such as CVD graphene, GO and GQDs substrates in the presence/absence of ZnO seed layer was successfully demonstrated. Dense array of aligned ZnO NWs was formed in the case of RTA treated ZnO ultra-thin film on graphene, while the GO and GQDs substrates yield randomly oriented sparse ZnO NWs. HRTEM and Raman studies reveal the good crystalline quality of the ZnO NWs grown on graphene–ZnO buffer layer substrate in comparison with the ZnO NWs grown on ZnO seed layer and GO–ZnO buffer layer substrates. A growth mechanism was proposed based on the artificial epitaxial relationship between ZnO and graphene as compared to that of the NWs grown without the graphene layer. The evolution of the UV and visible PL was studied and correlated with effects of ZnO buffer layers on graphene substrates in the aligned growth of ZnO NWs with high crystalline quality. Highly enhanced PC was achieved in the case of ZnO NWs grown on graphene, which is consistent with the PL results. These results demonstrate the successful fabrication and superior performance of ZnO NW–graphene hybrid UV PDs as compared to the bare ZnO NW based PDs. These hybrid nanostructures will be the building block for the next generation optoelectronic devices.
Acknowledgments We thank Central Instruments Facility (CIF) for providing FESEM and micro Raman facilities. We acknowledge the financial support from CSIR (03(1270)/13/EMR-II) to carry out part of this work. We acknowledge DST (No Sr/55/NM– 01/2005) for providing the HRTEM facility at IIT Guwahati. We thank A K Sivadasan, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, for his help in the Raman mapping measurements. We also acknowledge Albert V Tamashausky, Asbury Graphite Mills, USA, for providing high purity graphite flakes to synthesize GO and GQDs. Dr Shilpa Sharma and N V V Subbarao are also duly acknowledged. 12
