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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C4NR07114J
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Improved UV photoresponse of ZnO nanorod arrays by resonant coupling with surface plasmons of Al nanoparticles
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this study, localized surface plasmon resonance mediated by aluminium nanoparticles (Al NPs) was employed to enhance ultraviolet (UV) response of the ZnO nanorod arrays (NRAs) photodetectors grown vertically on the Quartz substrate by a simple vapor transport method. The responsivity of the ZnO NRAs photodetector decorated with Al NPs was enhanced from 0.12 to 1.59 A/W and the sensitivity and response rate has been improved greatly compared with the bare one. The measurement results of the transmittance spectra and time-resolved photoluminescence spectra suggest that the improved photoresponse and the enhanced spontaneous emission of the ZnO NRAs photodetector with Al NPs decoration are both attributed to the resonant coupling between the excitons in ZnO and the LSPs in Al NPs. Our results demonstrated that the plasmon-enhanced ZnO NRAs photodetector have great applications in building sensors with fast response and reset time, high sensitivity, and good signal-tonoise ratio for photoelectric sensing.
Introduction
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Due to its wide application in many areas, such as missile launching detection, space and astronomical research, environmental monitoring, UV radiation calibration and monitoring, and optical communication, UV detectors have attracted a considerable amount of research interests 1-3. Many kinds of wide bandgap semiconductors, including GaN, ZnS, SiC, and ZnO etc., 4-6 have been developed and applied on UV photodetectors. Among these wide bandgap semiconductor, ZnO, with a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, has been a potential candidates for short-wavelength optoelectronics applications such as microlasers 7, 8 , light-emitting diodes 9, 10, and UV photodetectors 11, 12. Over the past decade, ZnO-based UV photodetectors have been fabricated from single crystals 13, thin films 14, 15 and nanostructures 16-18. Compared with ZnO UV photodetector based on traditional thin-film and bulk materials, the low-dimensional nanostructures UV detector usually have an advantage of higher responsivity and photoconductivity gain because of their high surface-to-volume ratios and the reduced dimension of the effective conductive channel. Since the first report on UV photodetector from single ZnO nanowire by kind et al. 19, many efforts have been made on one-dimensional ZnO, including NRAs to improve photodetection and photoresponse performance. As is well-known that photodetection and photoresponse are the key parameters to determine the capability of a photodetector, which is relative to the surface condition, structural quality and rate of oxygen adsorption and desorption. In addition, defect states in the as-grown ZnO nanostructures have an important effect on the photoresponse characteristics. The high defect concentration will reduce the photocarriers This journal is © The Royal Society of Chemistry [year]
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mobility due to the persistent carriers trapping and detrapping. 20 Therefore, it is for this reason leading to the large variation in photoresponse times previously reported by various groups. 19, 2124 It is still a challenge to achieve photodetectors with both high sensitivity and fast temporal response up to date. Surface plasmons (SPs) 25, 26, excited by the interaction between light and electron plasma waves at the metal surface, have attracted much attention in studies of light-emitting devices due to their fundamental scientific importance and promising practical application. For example, Okamoto et al. reported a method to improve light-emission efficiency through the energy transfer between quantum wells (QWs) of InGaN and surface plasmons (SPs) of metal 27. Liu et al. obtained 7-fold enhancement of electroluminescence of ZnO nanorod arrays LEDs by decorating with Ag NPs. 28 On the other hand, surface plasmons also provide a novel idea of enhancing the performance of photoelectric detector, because the metal surface plasmons and the photon absorption both occurred at the metal-dielectric interface. So, it is expected to contribute to improve the photodetection performance by this effective coupling. Tian et al. reported that the responsivity of ZnO film based UV photodetector decorated with Pt NPs is enhanced by up to 56% compared with that of the bare one. 29 In addition, a highperformance NIR light photodetector fabricated by coating the methyl-group terminated Si nanowire array with plasmonic gold nanoparticles (AuNPs) decorated graphene film was obtained by Luo et al. 30 As a kind of abundant and low cost metal in the world, the Aluminum (Al) element has been an important candidate of plasmonic material. 31, 32 Besides, the negative real part and relatively low imaginary part of Al dielectric function in UV range make it act as a better plasmonic material than either [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
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Junfeng Lu, Chunxiang Xu*, Jun Dai, Jitao Li, Yueyue Wang, Yi Lin, Panlin Li
Nanoscale
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Au or Ag in the blue and UV range 33. In this paper, the metal-semiconductor-metal (MSM) structured photodetector decorated with Al NPs has been fabricated. An obvious enhancement of the response sensitivity and good reproducibility for the Al-decorated photodetector can be observed compared with the bare one. In addition, the response rate of the ZnO UV detector decorated with Al NPs has been improved greatly, which is attributed to the coupling between the excitons of ZnO and LSPs of Al NPs. The photoluminescence (PL) and time-resolved photoluminescence (TRPL) are employed to confirm the surface plasmon resonance in the ZnO nanorods with Al NPs decoration.
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ZnO nanorods were produced through a simple vapor-phase transport process. A mixture of high purity ZnO and graphite powders (1:1 in mass ratio) was used as a source material, which was placed in the sealed end of a quartz test tube 30 mm in diameter and 300 mm in length. In order to compare the photoelectric properties of the ZnO UV detector before and after Al NPs decoration, two pieces of quartz substrate were put collaterally at the open end of the tube 25 cm away from the source material. Then, the whole test tube was transferred into a tube furnace, which had been previously heated to 1050 °C. After that, the pressure was exhausted to 7.5×10-2 Torr during reaction. Argon and oxygen (150:15 sccm) were introduced into the furnace as the carrier gases. The reaction lasted 20 min. In the initial growth stage, a ZnO film was deposited on the substrate because of the high zinc vapor concentration. And then, the ZnO nanorods were grown vertically on the ZnO film. The Al NPs were sputtered onto the ZnO nanorods by a radio frequency magnetic sputtering system. The chamber pressure is fixed at 2.0 Pa, the Ar flow is 50 sccm and the sputtering power is 100 W. To fabricate the MSM structure ZnO UV detector, the silver paste was coated on the top surface of ZnO nanorod arrays to form two Ag electrodes with internal of 200 µm and enough thickness. In order to ensure the uniformity of the distance between the electrodes for the two samples, a shelter was used to assist the fabrication of the electrodes. The morphology and structure of the as-synthesized products were characterized by X-ray diffraction (XRD-7000, Shimadzu) using Cu Kα radiation (λ = 0.15406 nm) and a field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra Plus) equipped with an X-ray energy dispersive spectrometer (EDS) (Oxford X-Max 50). The PL spectrum was measured by a fluorescence spectrophotometer (F-4600, Hitachi) with a Xe lamp at 325nm as the excitation source (Power density = 24 mW/cm2). Time-resolved photoluminescence (TRPL) experiments were performed by an optically triggered streak camera system (C10910, Hamamatsu) at 295 nm from frequency doubling of the fundamental 35 fs pulses at 590 nm with a repetition rate of 1KHz (OperA Solo, Coherent). The I-V characteristics and photocurrent of the photodetector are measured by Keithley 4200. The photocurrent is measured when the device is illuminated by a 325 nm laser.
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Figure 1a and 1b present the top view and 45°tilted scanning electron microscopy (SEM) images of the as-grown ZnO nanorod arrays. It is seen that the diameter is about 200nm from the enlarged top of the ZnO nanorods inserted in Figure 1a. The length of the ZnO nanorods is similar. Thus, it is beneficial to fabricate the ZnO nanorod arrays UV detector for us. The elemental mapping profiles of Al-decorated ZnO nanorods confirm the existence of Al element, as shown in Figure 1c. The elemental mapping images collected from the rectangle region in Figure 1b reveals that the Zn element and O element distribute uniformly corresponding distinctly to the profile of the ZnO nanorods, while the Al element disperses on the top surface of ZnO nanorods, which will be further characterized in Figure 1f. Typical XRD pattern for the bare ZnO nanorods are shown in Figure 1d. The high intensity of diffraction peaks reveals the growth direction along the [001] direction in Figure 1d. The two diffraction peaks at 34.46º and 72.59º are peculiar to (002) and (004) planes of the wurtzite ZnO. Furthermore, there is no other impurity XRD peaks in the ZnO nanorods. From the XRD pattern of ZnO nanorods, the strongest diffraction peak corresponds to the (002) diffraction plane means that the microrods mainly grow along the preferred [001] direction.
Fig. 1 (a) Top view and (b) 45°tilted SEM images of the as-grown ZnO nanorods. (c) Zn, O, Al element mapping images for the ZnO nanorod. (d) X-ray diffraction patterns of the ZnO nanorod arrays. HRTEM images of ZnO nanorod (e) without and (f) with Al NPs decoration the inset is TEM images of the two samples, respectively.
The inset of Figure 1e and 1f shows the TEM image of the bare ZnO nanorod and Al-decorated one. It can be seen that the diameters of Al NPs are in the range of 30-60 nm. For the bare ZnO nanorod, the high-resolution transmission electron microscope (HRTEM) image in Figure 1e exhibits the clear lattice fringes of a ZnO nanorod, the interplanar distance is 0.26 nm, which is in good agreement with the d spacing of the (0001) lattice plane of a hexagonal ZnO wurtzite structure. It reveals that the growth direction is mainly along the [0001], which is corresponding to the result of XRD pattern. For the Al-decorated ZnO nanorod, the HRTEM image reveals two kinds of lattice
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR07114J
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fringes from ZnO and Al NPs. On the ZnO side, the lattice constant is similar to the bare one, while on the nanoparticle side, the interplanar distance of 0.23 nm corresponds to the d spacing of the (111) lattice plane of an Al zinc blende structure.
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attributed to the direct resonant coupling between the excitons of ZnO and localized surface plasmons arising from Al NPs. Figure 2c illustrates the transmittance spectra of quartz substrate with and without Al NPs, respectively. The absorption edge of Al NPs is located at around 380 nm, while the spontaneous emission intensity of ZnO nanorods with Al NPs decoration is stronger than that of the bare one. Combined with the transmittance spectra of Al NPs, the result suggests that the enhancement of spontaneous emission is due to the absorption of Al NPs, and then the corresponding light energy may be transferred into the excitons of ZnO by the resonant coupling with the LSPs modes of Al NPs. TRPL experiments further confirmed that the enhancement of UV emission was due to the metal surface plasmon resonance coupling with ZnO, as shown in Figure 2b. It can be observed that the lifetime of ZnO nanorods decorated with Al NPs (~0.19 ns) has been shortened compared with the bare one (~0.93 ns).
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Fig. 2 (a) PL and (b) TRPL spectra at 380 nm of the ZnO nanorods before and after Al nanoparticles (NPs) decoration. (c) the transmittance spectra of quartz substrate with and without Al NPs under the same sputtering condition with the Al-decorated sample.
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In order to investigate the effect of Al nanoparticles on the optical properties of ZnO nanorods, the PL spectra measurements have been carried out at room temperature on the aforementioned two samples. Figure 2a shows the PL spectrum of the ZnO nanorods before and after Al NPs decoration excited by a Xe lamp at 325 nm, which generally consist of a near-band-edge (NBE) excitonic emission at ~380nm 34, 35 and a broad defect-related emission centered at ~500 nm. 36, 37 More than 8 folds enhancement of the UV emission can be observed by decorated Al NPs, while the defect-related emission of the Al-decorated ZnO nanorods is similar to the bare one. According to our previous work 31, the enhancement can be This journal is © The Royal Society of Chemistry [year]
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Fig. 3 (a) Schematic of ZnO nanorod arrays photodetector. I-V characteristics of ZnO nanorod arrays photodetector (b) without Al NPs decoration and (c) with Al NPs decoration both in dark and under 325 nm UV light illumination.
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Fig. 4 (a, b) The enlarged portions after Al NPs decoration of a 97-112 s range and a 173-188 s range corresponding to light-off to light-on and light-on to light-off transitions, respectively. (c) The reproducible on/off switching of the device upon 325 nm light illumination with a 30 s cycle at a bias of 5.0 V. (d, e) The enlarged portions before Al NPs decoration of a 130-145 s range and a 205-220 s range corresponding to light-off to light-on and light-on to light-off transitions, respectively.
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To examine the repeatability and response speed of the ZnO UV detector, the time-resolved photocurrent at 5 V bias with multiple UV on/off cycles has been measured, in which both turn-on time and turn-off time of UV light are 15 s. Eleven cycles of photocurrent switching demonstrate the repeatability and sensitivity of the ZnO nanorod arrays photodetector. As shown in Figure 4c, with the light irradiation on and off, the current of the 4 | Journal Name, [year], [vol], 00–00
device mostly exhibits two distinct states, a low-current state in dark and a high-current state under 325 nm UV light illumination. The current increases very sharply from one state to another state, indicating a very fast response speed of the two samples. At the rising edge, the process of the up current is further enlarged at the portions of a 97-112 s range and a 130-145 s range corresponding to the ZnO UV photodetector with and without Al NPs decoration, respectively, as shown in Figure 4a and 4d. At the failling edge, Figure 4b and 4e shows that the process of the down current has been enlarged at the portions of a 173-188 s range and a 205-220 s range corresponding to the two samples aforementioned, respectively. The results of the time-resolved photocurrent revealed that the response speed of the ZnO UV detector decorated with Al NPs is faster than the bare one. In the view of the photocurrent rise process, the photocurrent reaches a steady value after ~2 s and ~4 s corresponding to the Aldecorated ZnO UV detector and the bare one, respectively. The rise process can be fitted by an exponential function 39 I = I 0 (1 − e − t /τ ) (1) Where I 0 is the steady state photocurrent and τ
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Schematic of ZnO NRAs-based photodetector with Al NPs is shown in Figure 3a. Figure 3b and 3c demonstrate the typical I-V characteristics of the ZnO UV detectors with and without Al NPs both in dark and under UV illumination (λ = 325 nm, Power density = 300 mW/cm2), respectively. The current increases linearly with the applied voltage at dark, which may indicate that an Ohmic contact exists at the interfaces between the ZnO nanorod arrays and the Ag electrodes. Meanwhile, the dark current increases slightly with Al NPs decoration. Due to the lower work function of Al (4.29 eV) compared to ZnO, 38 an Ohmic contact between the ZnO nanorods and Al NPs is formed with downward band bending. In this case, more numbers of electrons can easily transfer from Al to the conduction band of ZnO at the interface. Then under 5V bias these electrons contributes to the current conduction process results in higher dark current. On the other hand, the photocurrent of the ZnO UV detector decorated with Al NPs is enlarged under UV light illumination, which can be attributed to the localized surface plasmons induced by the resonant coupling between Al NPs and ZnO nanorods. Thereby, the increased energy absorption results in more electron-hole generation.
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r is time constant for photocurrent rise (less than 0.03 s and ~0.8s corresponding to the Al-decorated ZnO UV detector and the bare one). On the other hand, the failling edge of the photocurrents can be well fitted by the exponential equation I = I 0 exp( − t / τ d ) , and the decay time τ d is the recovery time constant when the photocurrent decreases to its 1/e. It can be estimated to ~0.035 s and ~0.85 s corresponding to the Al-decorated ZnO UV detector and the bare one, which indicates that the reset time of ZnO NRAs UV photodetector can be decreased largely by Al NPs decoration. Here, the improvement of the response rate including rising and failling edge is attributed to the LSPR coupling of Al NPs with ZnO nanorods. At the moment of UV light illumination, the concentration of excitons generated by the transition of electrons from valence band to conduction band increases drastically. Simultaneously, due to the closed response of Al SPR to the ZnO interband electron transition 24, a resonant energy coupling from Al to ZnO occurred in the hybrid system. Thus, the response rate of the ZnO UV decorated with Al NPs at rising edge is faster than that of the bare one. On the other hand, the excitons of Al-decorated ZnO couple with LSPs of Al NPs rapidly at the moment of UV light off, which reduces the probability of the free electrons captured by Oxygen (O2) molecules [O2 + e- = O2-]. 40, 41 So, the concentration of excitons in the ZnO nanorods decreases quickly, which lead to the faster decreasing of the current compared with that of the bare one. In a word, both the faster growth and decay can be ascribed to the efficient excitation for ZnO by the effectively energy coupling with LSPs of Al. Table 1 compared the photoresponse of different ZnO-based UV photodetectors in previous reports.13-15,
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Table 1. Comparison of the photoresponse for ZnO-based photodetectors fabricated by different methods in previous reports Method Low Temp.
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350 nm 365 nm 325 nm 325 nm 325 nm 365 nm 325 nm 325 nm
Rise time 40 s 5s 3.7 s 48 s 6.28 s
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Lu, C. Xu, J. Dai, J. Li, Y. Wang, Y. Lin and P. Li, Nanoscale, 2015, DOI: 10.1039/C4NR07114J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Journal Name
Nanoscale
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DOI: 10.1039/C4NR07114J
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
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Improved UV photoresponse of ZnO nanorod arrays by resonant coupling with surface plasmons of Al nanoparticles
5
10
15
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this study, localized surface plasmon resonance mediated by aluminium nanoparticles (Al NPs) was employed to enhance ultraviolet (UV) response of the ZnO nanorod arrays (NRAs) photodetectors grown vertically on the Quartz substrate by a simple vapor transport method. The responsivity of the ZnO NRAs photodetector decorated with Al NPs was enhanced from 0.12 to 1.59 A/W and the sensitivity and response rate has been improved greatly compared with the bare one. The measurement results of the transmittance spectra and time-resolved photoluminescence spectra suggest that the improved photoresponse and the enhanced spontaneous emission of the ZnO NRAs photodetector with Al NPs decoration are both attributed to the resonant coupling between the excitons in ZnO and the LSPs in Al NPs. Our results demonstrated that the plasmon-enhanced ZnO NRAs photodetector have great applications in building sensors with fast response and reset time, high sensitivity, and good signal-tonoise ratio for photoelectric sensing.
Introduction
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Due to its wide application in many areas, such as missile launching detection, space and astronomical research, environmental monitoring, UV radiation calibration and monitoring, and optical communication, UV detectors have attracted a considerable amount of research interests 1-3. Many kinds of wide bandgap semiconductors, including GaN, ZnS, SiC, and ZnO etc., 4-6 have been developed and applied on UV photodetectors. Among these wide bandgap semiconductor, ZnO, with a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, has been a potential candidates for short-wavelength optoelectronics applications such as microlasers 7, 8 , light-emitting diodes 9, 10, and UV photodetectors 11, 12. Over the past decade, ZnO-based UV photodetectors have been fabricated from single crystals 13, thin films 14, 15 and nanostructures 16-18. Compared with ZnO UV photodetector based on traditional thin-film and bulk materials, the low-dimensional nanostructures UV detector usually have an advantage of higher responsivity and photoconductivity gain because of their high surface-to-volume ratios and the reduced dimension of the effective conductive channel. Since the first report on UV photodetector from single ZnO nanowire by kind et al. 19, many efforts have been made on one-dimensional ZnO, including NRAs to improve photodetection and photoresponse performance. As is well-known that photodetection and photoresponse are the key parameters to determine the capability of a photodetector, which is relative to the surface condition, structural quality and rate of oxygen adsorption and desorption. In addition, defect states in the as-grown ZnO nanostructures have an important effect on the photoresponse characteristics. The high defect concentration will reduce the photocarriers This journal is © The Royal Society of Chemistry [year]
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mobility due to the persistent carriers trapping and detrapping. 20 Therefore, it is for this reason leading to the large variation in photoresponse times previously reported by various groups. 19, 2124 It is still a challenge to achieve photodetectors with both high sensitivity and fast temporal response up to date. Surface plasmons (SPs) 25, 26, excited by the interaction between light and electron plasma waves at the metal surface, have attracted much attention in studies of light-emitting devices due to their fundamental scientific importance and promising practical application. For example, Okamoto et al. reported a method to improve light-emission efficiency through the energy transfer between quantum wells (QWs) of InGaN and surface plasmons (SPs) of metal 27. Liu et al. obtained 7-fold enhancement of electroluminescence of ZnO nanorod arrays LEDs by decorating with Ag NPs. 28 On the other hand, surface plasmons also provide a novel idea of enhancing the performance of photoelectric detector, because the metal surface plasmons and the photon absorption both occurred at the metal-dielectric interface. So, it is expected to contribute to improve the photodetection performance by this effective coupling. Tian et al. reported that the responsivity of ZnO film based UV photodetector decorated with Pt NPs is enhanced by up to 56% compared with that of the bare one. 29 In addition, a highperformance NIR light photodetector fabricated by coating the methyl-group terminated Si nanowire array with plasmonic gold nanoparticles (AuNPs) decorated graphene film was obtained by Luo et al. 30 As a kind of abundant and low cost metal in the world, the Aluminum (Al) element has been an important candidate of plasmonic material. 31, 32 Besides, the negative real part and relatively low imaginary part of Al dielectric function in UV range make it act as a better plasmonic material than either [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 13 January 2015. Downloaded by Purdue University on 15/01/2015 18:17:28.
Junfeng Lu, Chunxiang Xu*, Jun Dai, Jitao Li, Yueyue Wang, Yi Lin, Panlin Li
Nanoscale
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5
Published on 13 January 2015. Downloaded by Purdue University on 15/01/2015 18:17:28.
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Au or Ag in the blue and UV range 33. In this paper, the metal-semiconductor-metal (MSM) structured photodetector decorated with Al NPs has been fabricated. An obvious enhancement of the response sensitivity and good reproducibility for the Al-decorated photodetector can be observed compared with the bare one. In addition, the response rate of the ZnO UV detector decorated with Al NPs has been improved greatly, which is attributed to the coupling between the excitons of ZnO and LSPs of Al NPs. The photoluminescence (PL) and time-resolved photoluminescence (TRPL) are employed to confirm the surface plasmon resonance in the ZnO nanorods with Al NPs decoration.
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ZnO nanorods were produced through a simple vapor-phase transport process. A mixture of high purity ZnO and graphite powders (1:1 in mass ratio) was used as a source material, which was placed in the sealed end of a quartz test tube 30 mm in diameter and 300 mm in length. In order to compare the photoelectric properties of the ZnO UV detector before and after Al NPs decoration, two pieces of quartz substrate were put collaterally at the open end of the tube 25 cm away from the source material. Then, the whole test tube was transferred into a tube furnace, which had been previously heated to 1050 °C. After that, the pressure was exhausted to 7.5×10-2 Torr during reaction. Argon and oxygen (150:15 sccm) were introduced into the furnace as the carrier gases. The reaction lasted 20 min. In the initial growth stage, a ZnO film was deposited on the substrate because of the high zinc vapor concentration. And then, the ZnO nanorods were grown vertically on the ZnO film. The Al NPs were sputtered onto the ZnO nanorods by a radio frequency magnetic sputtering system. The chamber pressure is fixed at 2.0 Pa, the Ar flow is 50 sccm and the sputtering power is 100 W. To fabricate the MSM structure ZnO UV detector, the silver paste was coated on the top surface of ZnO nanorod arrays to form two Ag electrodes with internal of 200 µm and enough thickness. In order to ensure the uniformity of the distance between the electrodes for the two samples, a shelter was used to assist the fabrication of the electrodes. The morphology and structure of the as-synthesized products were characterized by X-ray diffraction (XRD-7000, Shimadzu) using Cu Kα radiation (λ = 0.15406 nm) and a field emission scanning electron microscopy (FESEM, Carl Zeiss Ultra Plus) equipped with an X-ray energy dispersive spectrometer (EDS) (Oxford X-Max 50). The PL spectrum was measured by a fluorescence spectrophotometer (F-4600, Hitachi) with a Xe lamp at 325nm as the excitation source (Power density = 24 mW/cm2). Time-resolved photoluminescence (TRPL) experiments were performed by an optically triggered streak camera system (C10910, Hamamatsu) at 295 nm from frequency doubling of the fundamental 35 fs pulses at 590 nm with a repetition rate of 1KHz (OperA Solo, Coherent). The I-V characteristics and photocurrent of the photodetector are measured by Keithley 4200. The photocurrent is measured when the device is illuminated by a 325 nm laser.
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Figure 1a and 1b present the top view and 45°tilted scanning electron microscopy (SEM) images of the as-grown ZnO nanorod arrays. It is seen that the diameter is about 200nm from the enlarged top of the ZnO nanorods inserted in Figure 1a. The length of the ZnO nanorods is similar. Thus, it is beneficial to fabricate the ZnO nanorod arrays UV detector for us. The elemental mapping profiles of Al-decorated ZnO nanorods confirm the existence of Al element, as shown in Figure 1c. The elemental mapping images collected from the rectangle region in Figure 1b reveals that the Zn element and O element distribute uniformly corresponding distinctly to the profile of the ZnO nanorods, while the Al element disperses on the top surface of ZnO nanorods, which will be further characterized in Figure 1f. Typical XRD pattern for the bare ZnO nanorods are shown in Figure 1d. The high intensity of diffraction peaks reveals the growth direction along the [001] direction in Figure 1d. The two diffraction peaks at 34.46º and 72.59º are peculiar to (002) and (004) planes of the wurtzite ZnO. Furthermore, there is no other impurity XRD peaks in the ZnO nanorods. From the XRD pattern of ZnO nanorods, the strongest diffraction peak corresponds to the (002) diffraction plane means that the microrods mainly grow along the preferred [001] direction.
Fig. 1 (a) Top view and (b) 45°tilted SEM images of the as-grown ZnO nanorods. (c) Zn, O, Al element mapping images for the ZnO nanorod. (d) X-ray diffraction patterns of the ZnO nanorod arrays. HRTEM images of ZnO nanorod (e) without and (f) with Al NPs decoration the inset is TEM images of the two samples, respectively.
The inset of Figure 1e and 1f shows the TEM image of the bare ZnO nanorod and Al-decorated one. It can be seen that the diameters of Al NPs are in the range of 30-60 nm. For the bare ZnO nanorod, the high-resolution transmission electron microscope (HRTEM) image in Figure 1e exhibits the clear lattice fringes of a ZnO nanorod, the interplanar distance is 0.26 nm, which is in good agreement with the d spacing of the (0001) lattice plane of a hexagonal ZnO wurtzite structure. It reveals that the growth direction is mainly along the [0001], which is corresponding to the result of XRD pattern. For the Al-decorated ZnO nanorod, the HRTEM image reveals two kinds of lattice
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR07114J
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fringes from ZnO and Al NPs. On the ZnO side, the lattice constant is similar to the bare one, while on the nanoparticle side, the interplanar distance of 0.23 nm corresponds to the d spacing of the (111) lattice plane of an Al zinc blende structure.
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attributed to the direct resonant coupling between the excitons of ZnO and localized surface plasmons arising from Al NPs. Figure 2c illustrates the transmittance spectra of quartz substrate with and without Al NPs, respectively. The absorption edge of Al NPs is located at around 380 nm, while the spontaneous emission intensity of ZnO nanorods with Al NPs decoration is stronger than that of the bare one. Combined with the transmittance spectra of Al NPs, the result suggests that the enhancement of spontaneous emission is due to the absorption of Al NPs, and then the corresponding light energy may be transferred into the excitons of ZnO by the resonant coupling with the LSPs modes of Al NPs. TRPL experiments further confirmed that the enhancement of UV emission was due to the metal surface plasmon resonance coupling with ZnO, as shown in Figure 2b. It can be observed that the lifetime of ZnO nanorods decorated with Al NPs (~0.19 ns) has been shortened compared with the bare one (~0.93 ns).
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Fig. 2 (a) PL and (b) TRPL spectra at 380 nm of the ZnO nanorods before and after Al nanoparticles (NPs) decoration. (c) the transmittance spectra of quartz substrate with and without Al NPs under the same sputtering condition with the Al-decorated sample.
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In order to investigate the effect of Al nanoparticles on the optical properties of ZnO nanorods, the PL spectra measurements have been carried out at room temperature on the aforementioned two samples. Figure 2a shows the PL spectrum of the ZnO nanorods before and after Al NPs decoration excited by a Xe lamp at 325 nm, which generally consist of a near-band-edge (NBE) excitonic emission at ~380nm 34, 35 and a broad defect-related emission centered at ~500 nm. 36, 37 More than 8 folds enhancement of the UV emission can be observed by decorated Al NPs, while the defect-related emission of the Al-decorated ZnO nanorods is similar to the bare one. According to our previous work 31, the enhancement can be This journal is © The Royal Society of Chemistry [year]
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Fig. 3 (a) Schematic of ZnO nanorod arrays photodetector. I-V characteristics of ZnO nanorod arrays photodetector (b) without Al NPs decoration and (c) with Al NPs decoration both in dark and under 325 nm UV light illumination.
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Journal Name, [year], [vol], 00–00 | 3
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DOI: 10.1039/C4NR07114J
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Fig. 4 (a, b) The enlarged portions after Al NPs decoration of a 97-112 s range and a 173-188 s range corresponding to light-off to light-on and light-on to light-off transitions, respectively. (c) The reproducible on/off switching of the device upon 325 nm light illumination with a 30 s cycle at a bias of 5.0 V. (d, e) The enlarged portions before Al NPs decoration of a 130-145 s range and a 205-220 s range corresponding to light-off to light-on and light-on to light-off transitions, respectively.
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To examine the repeatability and response speed of the ZnO UV detector, the time-resolved photocurrent at 5 V bias with multiple UV on/off cycles has been measured, in which both turn-on time and turn-off time of UV light are 15 s. Eleven cycles of photocurrent switching demonstrate the repeatability and sensitivity of the ZnO nanorod arrays photodetector. As shown in Figure 4c, with the light irradiation on and off, the current of the 4 | Journal Name, [year], [vol], 00–00
device mostly exhibits two distinct states, a low-current state in dark and a high-current state under 325 nm UV light illumination. The current increases very sharply from one state to another state, indicating a very fast response speed of the two samples. At the rising edge, the process of the up current is further enlarged at the portions of a 97-112 s range and a 130-145 s range corresponding to the ZnO UV photodetector with and without Al NPs decoration, respectively, as shown in Figure 4a and 4d. At the failling edge, Figure 4b and 4e shows that the process of the down current has been enlarged at the portions of a 173-188 s range and a 205-220 s range corresponding to the two samples aforementioned, respectively. The results of the time-resolved photocurrent revealed that the response speed of the ZnO UV detector decorated with Al NPs is faster than the bare one. In the view of the photocurrent rise process, the photocurrent reaches a steady value after ~2 s and ~4 s corresponding to the Aldecorated ZnO UV detector and the bare one, respectively. The rise process can be fitted by an exponential function 39 I = I 0 (1 − e − t /τ ) (1) Where I 0 is the steady state photocurrent and τ
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Schematic of ZnO NRAs-based photodetector with Al NPs is shown in Figure 3a. Figure 3b and 3c demonstrate the typical I-V characteristics of the ZnO UV detectors with and without Al NPs both in dark and under UV illumination (λ = 325 nm, Power density = 300 mW/cm2), respectively. The current increases linearly with the applied voltage at dark, which may indicate that an Ohmic contact exists at the interfaces between the ZnO nanorod arrays and the Ag electrodes. Meanwhile, the dark current increases slightly with Al NPs decoration. Due to the lower work function of Al (4.29 eV) compared to ZnO, 38 an Ohmic contact between the ZnO nanorods and Al NPs is formed with downward band bending. In this case, more numbers of electrons can easily transfer from Al to the conduction band of ZnO at the interface. Then under 5V bias these electrons contributes to the current conduction process results in higher dark current. On the other hand, the photocurrent of the ZnO UV detector decorated with Al NPs is enlarged under UV light illumination, which can be attributed to the localized surface plasmons induced by the resonant coupling between Al NPs and ZnO nanorods. Thereby, the increased energy absorption results in more electron-hole generation.
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r is time constant for photocurrent rise (less than 0.03 s and ~0.8s corresponding to the Al-decorated ZnO UV detector and the bare one). On the other hand, the failling edge of the photocurrents can be well fitted by the exponential equation I = I 0 exp( − t / τ d ) , and the decay time τ d is the recovery time constant when the photocurrent decreases to its 1/e. It can be estimated to ~0.035 s and ~0.85 s corresponding to the Al-decorated ZnO UV detector and the bare one, which indicates that the reset time of ZnO NRAs UV photodetector can be decreased largely by Al NPs decoration. Here, the improvement of the response rate including rising and failling edge is attributed to the LSPR coupling of Al NPs with ZnO nanorods. At the moment of UV light illumination, the concentration of excitons generated by the transition of electrons from valence band to conduction band increases drastically. Simultaneously, due to the closed response of Al SPR to the ZnO interband electron transition 24, a resonant energy coupling from Al to ZnO occurred in the hybrid system. Thus, the response rate of the ZnO UV decorated with Al NPs at rising edge is faster than that of the bare one. On the other hand, the excitons of Al-decorated ZnO couple with LSPs of Al NPs rapidly at the moment of UV light off, which reduces the probability of the free electrons captured by Oxygen (O2) molecules [O2 + e- = O2-]. 40, 41 So, the concentration of excitons in the ZnO nanorods decreases quickly, which lead to the faster decreasing of the current compared with that of the bare one. In a word, both the faster growth and decay can be ascribed to the efficient excitation for ZnO by the effectively energy coupling with LSPs of Al. Table 1 compared the photoresponse of different ZnO-based UV photodetectors in previous reports.13-15,
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Table 1. Comparison of the photoresponse for ZnO-based photodetectors fabricated by different methods in previous reports Method Low Temp.
High Temp.
Photodetector
UV light
ZnO film ZnO film ZnO nanorods ZnO nanoparticles ZnO microrod ZnO nanowire ZnO nanowire arrays ZnO nanorod arrays
350 nm 365 nm 325 nm 325 nm 325 nm 365 nm 325 nm 325 nm
Rise time 40 s 5s 3.7 s 48 s 6.28 s
