Open Access
Add to BibSonomyAbstract
ZnTe is an important p-type semiconductor with great applications as field-effect transistors and photodetectors. In this paper, individual ZnTe nanowires based field-effect transistors was fabricated, showing evident p-type conductivity with an effect mobility of 11.3 cm2/Vs. Single ZnTe nanowire based photodetectors on rigid silicon substrate exhibited high sensitivity and excellent stability to visible incident light with responstivity and quantum efficiency as high as 1.87 × 105 A/W and 4.36 × 107% respectively and are stable in a wide temperature range (25-250 °C). The polarization-sensitivity of the ZnTe nanowires was studied for the first time. The results revealed a periodic oscillation with the continuous variation of polarization angles. Besides, flexible photodetectors were also fabricated with the features of excellent flexibility, stability and sensitivity to visible incident light. Our work would enable application opportunities in using ZnTe nanowires for ultrahigh-performance photodetectors in scientific, commercial and industrial applications.
©2013 Optical Society of America
1. Introduction
Successful implementation of “bottom-up” approaches for the design of functional one-dimensional (1-D) nanodevices requires that the electronic properties of the 1-D nanostructured building blocks could be well defined and controlled. Especially, the p-type semiconducting nanowires (NWs) have been demonstrated to be one of the key building blocks for many complex crossed-bridge nanodevices such as p-n diodes, bipolar junction transistors, complementary inverters and logic gates [1–3]. Tremendous efforts have also been devoted, very recently, to exploit the potential applications of various kinds of p-type NWs in the field of chemical sensors [4, 5], field effect transistors [6–9], photovoltaic devices [10, 11], piezoelectric nanogenerator [12], as well as photodetectors which range in wavelength from infrared rays through visible light to ultraviolet rays [13–16].
With a proper band gap of ~2.26 eV and a p-type conductivity characteristic, intrinsic ZnTe nanowires has been synthesized by many methods, such as hydrothermal method [17], metal-organic chemical vapor deposition [18], and vapor transport and deposition method [19]. Optical and electric transport properties have been systematically studied before [20]. Besides, ZnTe NWs has also been investigated as the building blocks of the FETs as well as the green/ultraviolet photodetector. Although several kinds of intrinsic ZnTe nanowires have been synthesized to fabricate FETs and photodetectors, the performances of the fabricated devices are relatively poor, especially the slow response and recover spends (~1.3s) and low quantum efficiency (2.17 × 103), which are still far from the practical demands [21]. Therefore, the synthesis of high-quality 1-D ZnTe nanostructures to fit for high performance devices with a fast response time, good reproducibility and high quantum efficiency is still a great task.
In this paper, we report on the synthesis of high quality single-crystalline ZnTe nanowires and demonstrate their applications as high performance FETs with a significantly improved device mobility of as high as 11.3 cm2/Vs. Besides, photodetector on both rigid and flexible substrates are fabricated. The quantum efficiency of the ZnTe photodetector is as high as 4.36 × 107%, two orders of magnitude higher the best record on ZnTe based nanostructures. Besides, the polarization-sensitivity of the ZnTe nanowire was studied for the first time.
2. Experimental setup
ZnTe nanowires were synthesized by a vapor transport process in a horizontal tube furnace. A small quartz tube containing mixed Zn, Te and carbon powder (molar ratio 1:1.5:2) was located into the furnace. Silicon chip (111) was used as the deposition substrate. Before catalysts coating, the substrate was soaked in acetone, ethanol and deionized water accompanied by supersonic treatment for 15 minutes successively. A thin layer of gold nanoparticles was coated on the substrate as the catalysts for the NW growth. Highly purity Ar gas was flown through the system at a rate of 500 sccm for 30 min to eliminate the air in the system and then maintained at a rate of 60 sccm during the following reactions. After purged for 30 min, the furnace was heated to 900 °C and maintained at that temperature for another 2 hours. After synthesis, a layer of red-brown product was found deposited on the substrate.
The X-ray diffraction patterns (XRD) were obtained from a X-ray diffractometer (X’Pert PRO, PANalytical B.V., the Netherlands) with radiation of a Cu target (Ka, λ = 0.15406 nm). The field emission scanning electron microscopy (FESEM) images (FEI Sirion 200, 10KV; JEOL JSM-6700F, 5 kV; FEI Quanta 200, 10KV). Room-temperature photoluminescence (PL) spectra were recorded with an FP-6500 spectrometer with the excitation wavelength of 325 nm. The lamp-house's power is 150 W.
Individual ZnTe NW based FETs and photodetectors were assembled by a standard micro-fabrication process in a clean room [22]. The as-grown NWs were first dispersed in ethanol and then dropped on the Si substrate coated with a 500 nm SiO2 layer. And then a standard photolithography and metal deposition process was carried out to obtain a Cr/Au (25 nm/80 nm) film on the surface of the substrate as the electrodes followed with a lift-off process. The flexible photodetector was fabricated by the same processes on a polyimide substrate.
The output/transfer characteristics of the FET and the current-voltage (I-V) as well as the current-time (I-T) characteristics of the photodetectors were measured using a probe station connected to a Keithley 4200-SCS semiconductor characterization system. The results of the electronic and optoelectronic properties measurement were an average over a number of similar devices. The illumination source for photoresponse measurements was a power adjustable monochromatic light source.
3. Results and discussion
Figure 1(a) represents the XRD pattern of the as-deposited products. All the diffraction peaks can be assigned to zinc blende ZnTe (JCPDS No. 15-746) and no characteristic peaks from other crystalline impurities were found in this pattern, indicating the purity of our sample. Figure 1(b) shows the photoluminescence (PL) spectrum of the as-grown ZnTe products at room temperature. A strong yellowish-green band edge emission at 552 nm was observed, which agreed well with that in previous report [21]. The representative morphologies of the obtained ZnTe products are shown in Figs. 1(c)-1(d). The low-magnification SEM image in Fig. 1(c) reveals the formation of uniform nanowires on a large scale, which are several tens to hundreds micrometers in length. A high-magnification SEM image in Fig. 1(d) confirms the good uniformity of the nanowire with diameters in the range of ca. 120-500 nm.
Transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) spectroscope was used to investigate the microstructure and composition of the as-grown NWs. Figure 2(a) is a TEM image of a single ZnTe NW, further confirming the uniformity of the nanowire. No particles or other structures were found on it. An EDS spectrum acquired from the NW was shown in Fig. 2(b), which exhibits strong Zn and Te peaks with an atomic ratio close to 1: 1, indicating the formation of high purity ZnTe product. In this spectrum, the C and Cu signals come from the carbon coated copper TEM grid [23, 24]. High resolution TEM (HRTEM) image and the corresponding selected area electron diffraction (SEAD) pattern are shown in Figs. 2(c)-2(d), indicating the superb crystal quality and single crystalline nature of the NWs. The clearly observed lattice spacings of 0.35 nm and 0.3 nm correspond well to the (1-11) and (200) planes of zinc blende ZnTe phase, respectively. Combining the HRTEM image and the SAED pattern, we can get the conclusion that the as-grown ZnTe NWs are single crystals with the preferred growth directions along the [1–11] plane.
Fig. 2 (a) TEM image, (b) EDS spectrum, (c) HRTEM image and (d) SAED pattern of the as-synthesized ZnTe NWs.
To investigate the electrical transport properties of the obtained ZnTe NWs, back-gated FETs based on single ZnTe NWs were fabricated via a conventional photolithograph and lift-off process. Figure 3(a) shows the typical source-drain (IDS vs. VDS) characteristics of the FET with a channel length of 10 μm (Inset in Fig. 3(a)). The device showed typical p-type transistor behavior as the conductance was gradually suppressed with increased gate voltage ranging from −20 V to 20 V [25]. Figure 2(b) gives a family of current-voltage curves that depicts the transfer characteristics of the FET. At a certain drain-source voltage (VDS), the drain-source currents (IDS) decrease gradually when the gate voltage increases from −30 V to 30 V, confirming the p-type nature of the ZnTe nanowire.
Fig. 3 Typical electrical transport properties of the single ZnTe NW FETs. (a) IDS-VDS curves at different VGS. Inset is a SEM image of the device. (b) IDS-VGS curves at different VDS. Inset is the linear (magenta) and logarithmic (purple) plots of the IDS-VGS curves at VDS = 40 V.
Inset in Fig. 3(b) gives a linear and logarithmic IDS-VGS plot of the same device at a drain voltage of 40 V. From this plot, we can find that the threshold voltage (Vth) and the current on/off ratio are 11 V and 3.3 × 103, respectively. In addition, the peak transconductance (gm), the gate capacitance (Ci), the device mobility (μeff) and the carrier density are calculated to be 488.5 nS, 9.66 × 10−16 F, 11.3 cm2/Vs and 1.55 × 1017 cm−3 following the Eqs. (1-4) [26–28]:
where ε0 and εs are the dielectric constant of vacuum and the relative dielectric constant of gate insulator material (SiO2 = 3.9), respectively. L, h and r are the nanowire channel length (10 μm), dielectric thickness of SiO2 (500 nm) and the radius of the nanowire (about 120 nm as confirmed by the SEM and TEM), respectively. Compared with previous reported parameters on ZnTe based nanostructures, the transconductance, device mobility and especially the on/off ratio are significantly improved, as summarized in Table 1. Such excellent performances are mainly attributed to a large aspect-ratio, high crystallinity, and perfect 1D geometry that facilitate the carrier transport.
Table 1. Comparison of FET Performances between This Work and the Previous Reports on Intrinsic and Doping ZnTe Nanostructures [16]
Intrinsic ZnTe NWs as well as Sb doped ZnTe nanoribbons have been proved to have good response to visible light [16, 21]. Here, the photoresponse of the current ZnTe NWs was also studied by configuring individual ZnTe NW as photodetector on Si/SiO2 substrate. The NW across the electrodes has the diameter about 280 nm and the length of 9.45 μm. Figure 4(a) shows the I-V curves of the device measured in dark condition as well as under 532 nm light irradiations with various light intensities ranging from 0.2 to 1.4 mW/cm2. From the curves, we can see that, as the incident light intensity increases from zero to 1.4 mW/cm2, the photocurrents increase steadily and finally reach 10.5 μA at the bias of 6.5 V. Figure 4(b) is the light intensity dependence of the photocurrent measured at the bias voltage of 6 V. By fitting the measured data (solid squares) with the exponential equation, we obtained the intensity law, ΔI = 5.75P0.91, where ΔI is the photocurrent and P is the light intensity [29]. In addition, the spectral responsivity (Rλ) and the photoconductive gain (G) of the present photodetector can be calculated from the following equations [30–32]:
where S is the effective illuminated area, h is the Planck’s constant, hν is the energy of an incident photon and q is the electron charge. The corresponding values are calculated to be Rλ = 1.87 × 105 A/W and G = 4.36 × 105, respectively. Compared with previous reported data on ZnTe based nanostructures, both of the current values are much higher, as summarized in Table 2. These high current values indicate the high efficiency of the photodetector in converting photon flux into an electoral current. As the gain is also defined by: G = τ/Tr, where τ is the majority carrier lifetime and Tr is the transit time between electrodes. The one-dimensionality of the nanowires can confine the active area of the charge carriers and, thus, hinder the diffusion process, which results in a short Tr. Besides, high-quality single crystal of the nanowire also facilitates the transport of the carrier, which also contributes to the high current gain.Fig. 4 (a) I-V curves of the detector exposed to light of various intensities. (b) Light intensity dependence of the photocurrent measured at the voltage of 6 V (solid squares). The blue line is the fitting results with the equation I = 5.75 × P0.91. (c, d) Time dependent photocurrent response and (e, f) Spectral response of the ZnTe NW based photodetedctor. The operation voltage and the light intensity of Figs. 4 (c) and 4(d) are 2.5 V, 1.2 mW/cm2. For Fig. (f), they are 10 Vand 0.6 mW/cm2, respectively.
Table 2. Comparison of the Photoconductive Parameters between This Work and the Previous Reports on Intrinsic and Doping ZnTe Nanostructures
The response and recover speed is a critical parameter of a photodetedctor. Time-dependent photoresponse of our device was also measured under a pulsed incident light with the wavelength of 532 nm created by a manual chopper. From the plot depicted in Fig. 4(c), we can see that the photocurrent changes very rapidly following the incident light’ switching between ON and OFF states, indicating the high response/recover speed of the device. Besides, the stability and the reversibility of the device are also proved to be excellent as the on/off currents here almost remain the same within all the measured cycles. Figure 4(d) shows a single on/off cycle, where the rise and decay time are calculated to be about 86 ms and 50 ms, respectively, which are much faster than previously reported intrinsic ZnTe NWs with the values of ~1.3 s [21]. Figure 4(e) is the typical I-V curves obtained when the photodetector is exposed to light of different wavelengths ranging from 300 to 800 nm with the fixed light intensity of 0.6 mW/cm2. Strong wavelength selectivity was found for the current ZnTe NW photodetector. Photon-induced currents increased gradually as the incident light wavelength changed from 300 to 500 nm and then they decreased rapidly with the increased light wavelength larger than 500 nm. The spectrum-dependent curves of the device are shown in Fig. 4(f). From which, we can conclude that the sensitivity is not obvious when the device is expose to the light with wavelength above 650 nm. It is caused by the fact that the incident photons with lower energy can’t excite enough hole-electron pairs, in coincident with the band theory [34]. While on the short wavelength side, there is also a sensitivity drop observed, which is believed to be caused by the enhanced absorption of high-energy photons at or near the surface region of the NW, since the surface carrier life time (τ) is much shorter than that of bulk carrier as reported before [35]. The photocurrent (ΔI) dropped due to the decrease of the carrier life time (τ) as expressed by the following equation [36]:
where Tr is the transit time of free electrons, μ is the mobility of free electrons, f is the intensity of excitation, V and L are the applied voltage and interelectrode spacing (10 μm here). At shorter wavelengths, there are fewer photons incident on the nanowire at the same pumping power, and this may be another reason for the decline of detector current.To investigate the polarization-sensitivity of the photodetectors, magnitude of the photocurrent as a function of the incident light polarization angle was recorded and the corresponding results were depicted in Fig. 5. For the measurements, a 532 nm lamp was used in conjunction with a linear polarizer as shown in Fig. 5(b) inset [37]. The polarizer can be rotated continually to provide variable angles between the ZnTe NW and the polarization of incident light. Figure 5(a) shows the specific relations between the photocurrent response and the angle θ, which is defined as the angle between the incident polarization and the long axis of the ZnTe NW. Photocurrent reaches its maximum value when polarization is parallel to the long axis of the nanowire (θ = 0°, 180°), in contrast to the minimal photocurrent when polarization is perpendicular to the long axis of the NW (θ = 90°, 270°) [38]. In Fig. 5(b), continual photocurrent response is explicitly seen as the incident light polarization angle is continuously rotated. The photocurrent was found to closely follow the variation of the polarization angle, and a cosine wave could be deduced by fitting the data. On average, the polarization ratio, ρ = (IP-IV)/(IP + IV) = 0.5, in which IP and IV are the photocurrent under parallel and perpendicular conditions, respectively [39]. The observed polarization-sensitivity is most likely due to the anisotropic light electric field confinement [40–42]: When the electric field is parallel to the NWs, it can be readily adsorbed since no confinement exists along this longitudinal direction. But when it is perpendicular to the NW, electric field component of light is effectively attenuated inside the NW due to confinement. As a result, the conductance of the NW was continuously modulated and showed a periodic oscillation with the continuous variation of polarization angles. From the above results, it is concludes that the current ZnTe NW devices are excellent polarization-sensitive light detectors with nanoscale spatial resolution.
Fig. 5 (a) Photocurrent as a function of the light polarization angle. (b) Photocurrent versus time as the polarization was manually rotated. Inset shows the measurement configuration for the polarization-sensitive measurements.
Flexible electronics has been very hot as they can be easily carried in a folded or rolled form or attached to clothing [43]. We have also developed a ZnTe NW photodetedctor based on the flexible polyimide substrate here. The NW across the electrodes has the diameter about 120 nm and the length of 10 μm. Figure 6(a) gives the plots of the photoresponse of the flexible device in dark and under different light irradiance conditions. Figure 6(b) illustrates the photoresponse switching behavior of the flexible ZnTe photodetector at different bias voltages, namely 3 V, 6 V and 9 V, respectively. It can be observed that the photocurrent can be rapidly switched from the “ON” state to the “OFF” state by periodically turning the incident light on and off with a power density of 1.2 mW/cm2 at different bias voltages, indicating the excellent stability of our flexible device.
Fig. 6 Performance of the flexible ZnTe NWs photodetector. (a) Photoresponses of the detector under an improved incident light level. (b) Photoresponses of the detector at different bias under the same illumination (532nm, 1.2mW/cm2). (c) The magnitude of photocurrents (red) and the corresponding on/off ratio (blue) under different bending radii. Inset is the optical micrograph of the experimental setup. (d) I-V curves measured before and after different bending cysles. The operation voltage of Figs. 6 (a) and 6(c) are 12.5 V and 9 V, respectively. The light intensity of (c, d) is 1.4 mW/cm2.
To investigate the flexibility and stability of the flexible ZnTe NW device, the device was bent to different curvature radius to precisely assess the optoelectronic performance of the device (as shown in Fig. 6(c) inset). Figure 6(c) shows the photocurrents measured under six bending conditions and their corresponding on/off ratio. The photocurrents maintained at about 30 ± 1.2 nA and the on/off ratio stayed at the level of 98 ± 4 under the condition with 532 nm light illumination of 1.4 mW/cm2 and at the bias of 9 V. To evaluate the mechanical stability of the device, the flexible device was bended repetitiously. Figure 6(d) records the I-V curves of the flexible ZnTe photodetectors after 50, 100, 150, 200 and 250 cycles of bending. It can be observed that the photocurrent of the device was nearly unchanged, revealing that the performance of the device is hardly affected by repeated bending stress.
Furthermore, the relationship between the photoresponses and the temperature are investigated. Photodetector fabricated on the Si/SiO2 substrate was heated to the target temperature with a rapid heating plate and the photocurrent measurement was performed as above. Figures 7(a)-7(b) present the typical temperature-dependent I-V curves of the device measured in the temperature range of 25-250 °C in the dark and under the 532 nm light illumination states (with the light intensity of 1.4 mW/cm2), respectively. For the given illumination (532 nm, 1.4 mW/cm2), both the dark current and the photocurrent increase almost linearly with an increased temperature from room temperature to 250 °C. It is easy to note that at every temperature point, the photocurrent is significantly greater than the darkcurrent, giving a photosensitivity of about 0.05-0.45. The dark current, increased with increased temperature, can be expressed as: I = Kexp(-B/T) [44]. In the equation, T denotes the absolute temperature; K represents a constant relative to materials and structures and B refers to the thermal index. The corresponding spectral responsivity (Rλ) and eternal quantum efficiency (EQE) are calculated to be 700-2000 A/W and 1.5 × 105-4.5 × 105%, respectively. The photoelectrical behaviors can be recovered well after returning the measurement conditions to room temperature again, and the detector still demonstrates a fast, reversible, and stable response in the large temperature range. This will enable access to new and interesting photodetectors that could work in the harsh high temperature conditions.
Fig. 7 Temperature-dependent photoresponses of the ZnTe photodetector (a) in the dark and (b) under 1.4 mW/cm2 illumination. (c) Photocurrent at a bias voltage of 10 V in the dark (black) as well as under the 1.4 mW/cm2 illumination conditions (blue), and the dependence of the corresponding photosensitivity on the temperature (red). (d) The dependence of responsivity (Rλ, red) and the external quantum efficiency (EQE, blue) on the temperature.
4. Conclusion
In a conclusion, we reported the synthesis of high quality single-crystal ZnTe NWs via a vapor transport process, which can be used as the active material for field-effect transistors and high performance rigid and flexible photodetectors. The as-fabricated FETs showed quite good operating characteristics with a carrier mobility of 11.3 cm2/Vs. The rigid photodetector on silicon substrate demonstrated a fast (~86 ms) and stable response, high spectral responsivity (1.87 × 105 A/W) and high quantum efficiency (4.36 × 107%). It exhibited an interesting polarization-dependent photoconductivity. Besides, the flexible photodetectors have the features of good mechanical flexibility down to 4 mm bending radius and electrical stability for repeated bending. The temperature-dependent photoconductive properties was also investigated to characterize the visible light detecting behavior of ZnTe NWs and the results demonstrated the stable performance of the device in a large temperature range (25 °C ~250 °C). The present work may open up the unique possibilities of using ZnTe nanowires for the ultrahigh-performance photodetectors in scientific, commercial and industrial applications.
Acknowledgments
This work was supported by the National Natural Science Foundation (51002059, 21001046, 91123008), the 973 Program of China (No.2011CB933300), the Natural Science Foundation of Hubei Province (2009CDB326), and the Basic Scientific Research Funds for Central Colleges (2010QN045). We thank the Analytical and Testing Center of Huazhong University Science & Technology for the samples measurements.
References and links
1. X. F. Duan, Y. Huang, Y. Cui, J. F. Wang, and C. M. Lieber, “Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices,” Nature 409(6816), 66–69 (2001). [CrossRef] [PubMed]
2. Y. Cui and C. M. Lieber, “Functional nanoscale electronic devices assembled using silicon nanowire building blocks,” Science 291(5505), 851–853 (2001). [CrossRef] [PubMed]
3. Y. Huang, X. F. Duan, Y. Cui, L. J. Lauhon, K. H. Kim, and C. M. Lieber, “Logic gates and computation from assembled nanowire building blocks,” Science 294(5545), 1313–1317 (2001). [CrossRef] [PubMed]
4. J. J. Chen, K. Wang, L. Hartman, and W. L. Zhou, “H2S detection by vertically aligned CuO nanowire array sensors,” J. Phys. Chem. C 112(41), 16017–16021 (2008). [CrossRef]
5. B. J. Hansen, N. Kouklin, G. H. Lu, I. K. Lin, J. H. Chen, and X. Zhang, “Transport, analyte detection, and opto-electronic response of p-type CuO nanowires,” J. Phys. Chem. C 114(6), 2440–2447 (2010). [CrossRef]
6. Y. Cui, Z. H. Zhong, D. L. Wang, W. U. Wang, and C. M. Lieber, “High performance silicon nanowire field effect transistors,” Nano Lett. 3(2), 149–152 (2003). [CrossRef]
7. Z. H. Zhong, F. Qian, D. L. Wang, and C. M. Lieber, “Synthesis of p-type gallium nitride nanowires for electronic and photonic nanodevices,” Nano Lett. 3(3), 343–346 (2003). [CrossRef]
8. J. S. Tang, C. Y. Wang, F. X. Xiu, M. R. Lang, L. W. Chu, C. J. Tsai, Y. L. Chueh, L. J. Chen, and K. L. Wang, “Oxide-confined formation of germanium nanowire heterostructures for high-performance transistors,” ACS Nano 5(7), 6008–6015 (2011). [CrossRef] [PubMed]
9. G. Z. Shen, P. C. Chen, Y. Bando, D. Golberg, and C. W. Zhou, “Bicrystalline Zn3P2 and Cd3P2 nanobelts and their electronic transport properties,” Chem. Mater. 20(23), 7319–7323 (2008). [CrossRef]
10. C. Liu, J. W. Sun, J. Y. Tang, and P. D. Yang, “Zn-doped p- Type gallium phosphide nanowire photocathodes from a surfactant-free solution synthesis,” Nano Lett. 12(10), 5407–5411 (2012). [CrossRef] [PubMed]
11. E. C. Garnett and P. D. Yang, “Silicon nanowire radial p-n junction solar cells,” J. Am. Chem. Soc. 130, 9224–9225 (2008). [PubMed]
12. M. P. Lu, J. H. Song, M. Y. Lu, M. T. Chen, Y. F. Gao, L. J. Chen, and Z. L. Wang, “Piezoelectric nanogenerator using p-type ZnO nanowire arrays,” Nano Lett. 9(3), 1223–1227 (2009). [CrossRef] [PubMed]
13. A. Zhang, H. K. Kim, J. Cheng, and Y. H. Lo, “Ultrahigh responsivity visible and infrared detection using silicon nanowire phototransistors,” Nano Lett. 10(6), 2117–2120 (2010). [CrossRef] [PubMed]
14. X. W. Zhang, J. S. Jie, Z. Wang, C. Y. Wu, L. Wang, Q. Peng, Y. Q. Yu, P. Jiang, and C. Xie, “Surface induced negative photoconductivity in p-type ZnSe: Bi nanowires and their nano-optoelectronic applications,” J. Mater. Chem. 21(18), 6736–6741 (2011). [CrossRef]
15. G. Z. Shen, P. C. Chen, Y. Bando, D. Golberg, and C. W. Zhou, “Pearl-like ZnS-decorated InP nanowire heterostructures and their electric behaviors,” Chem. Mater. 20(21), 6779–6783 (2008). [CrossRef]
16. D. Wu, Y. Jiang, Y. G. Zhang, J. W. Li, Y. Q. Yu, Z. F. Zhu, L. Wang, C. Y. Wu, L. B. Luo, and J. S. Jie, “Device structure-dependent field-effect and photoresponse performances of p-type ZnTe:Sb nanoribbons,” J. Mater. Chem. 22(13), 6206–6212 (2012). [CrossRef]
17. J. Zhang, P. C. Chen, G. Z. Shen, J. B. He, A. Kumbhar, C. W. Zhou, and J. Y. Fang, “P-type field-effect transistors of single-crystal zinc telluride nanobelts,” Angew. Chem. Int. Ed. Engl. 47(49), 9469–9471 (2008). [CrossRef] [PubMed]
18. Z. Li, J. Salfi, C. D. Souza, P. Sun, S. V. Nair, and H. E. Ruda, “Room temperature single nanowire ZnTe photoconductors grown by metal-organic chemical vapor deposition,” Appl. Phys. Lett. 97, 063510 (2010).
19. S. Y. Li, Y. Jiang, D. Wu, B. B. Wang, Y. G. Zhang, J. W. Li, X. M. Liu, H. H. Zhong, L. Chen, and J. S. Jie, “Structure and electrical properties of p-type twin ZnTe nanowires,” Appl. Phys., A Mater. Sci. Process. 102(2), 469–475 (2011). [CrossRef]
20. Q. F. Meng, C. B. Jiang, and S. X. Mao, “Ohmic contacts and photoconductivity of individual ZnTe nanowires,” Appl. Phys. Lett. 4, 043111 (2009).
21. Y. L. Cao, Z. T. Liu, L. M. Chen, Y. B. Tang, L. B. Luo, J. S. Jie, W. J. Zhang, S. T. Lee, and C. S. Lee, “Single-crystalline ZnTe nanowires for application as high-performance green/ultraviolet photodetector,” Opt. Express 19(7), 6100–6108 (2011). [CrossRef] [PubMed]
22. Y. C. Che, C. Wang, J. Liu, B. L. Liu, X. Lin, J. Parker, C. Beasley, H. S. Wong, and C. Zhou, “Selective synthesis and device applications of semiconducting single-walled carbon nanotubes using isopropyl alcohol as feedstock,” ACS Nano 6(8), 7454–7462 (2012). [CrossRef] [PubMed]
23. G. Z. Shen, B. Liang, X. F. Wang, P. C. Chen, and C. W. Zhou, “Indium oxide nanospirals made of kinked nanowires,” ACS Nano 5(3), 2155–2161 (2011). [CrossRef] [PubMed]
24. G. Z. Shen, B. Liang, X. F. Wang, H. T. Huang, D. Chen, and Z. L. Wang, “Ultrathin In2O3 nanowires with diameters below 4 nm: synthesis, reversible wettability switching behavior, and transparent thin-film transistor applications,” ACS Nano 5(8), 6148–6155 (2011). [CrossRef] [PubMed]
25. F. N. Ishikawa, H. K. Chang, K. M. Ryu, P. C. Chen, A. Badmaev, L. Gomez De Arco, G. Z. Shen, and C. W. Zhou, “Transparent electronics based on transfer printed aligned carbon nanotubes on rigid and flexible substrates,” ACS Nano 3(1), 73–79 (2009). [CrossRef] [PubMed]
26. J. L. Zhang, C. Wang, and C. W. Zhou, “Rigid/flexible transparent electronics based on separated carbon nanotube thin-film transistors and their application in display electronics,” ACS Nano 6(8), 7412–7419 (2012). [CrossRef] [PubMed]
27. G. Z. Shen, J. Xu, X. F. Wang, H. T. Huang, and D. Chen, “Growth of directly transferable In2O3 nanowire mats for transparent thin-film transistor applications,” Adv. Mater. 23(6), 771–775 (2011). [CrossRef] [PubMed]
28. H. T. Huang, B. Liang, Z. Liu, X. F. Wang, D. Chen, and G. Z. Shen, “Metal oxide nanowire transistors,” J. Mater. Chem. 22(27), 13428–13445 (2012). [CrossRef]
29. P. C. Wu, Y. Dai, Y. Ye, Y. Yin, and L. Dai, “Fast-speed and high-gain photodetectors of individual single crystalline Zn3P2 nanowires,” J. Mater. Chem. 21(8), 2563–2567 (2011). [CrossRef]
30. L. Li, P. S. Lee, C. Y. Yan, T. Y. Zhai, X. S. Fang, M. Y. Liao, Y. Koide, Y. Bando, and D. Golberg, “Ultrahigh-performance solar-blind photodetectors based on individual single-crystalline In₂Ge₂O₇ nanobelts,” Adv. Mater. 22(45), 5145–5149 (2010). [CrossRef] [PubMed]
31. L. Li, P. C. Wu, X. S. Fang, T. Y. Zhai, L. Dai, M. Y. Liao, Y. Koide, H. Q. Wang, Y. Bando, and D. Golberg, “Single-crystalline CdS nanobelts for excellent field-emitters and ultrahigh quantum-efficiency photodetectors,” Adv. Mater. 22(29), 3161–3165 (2010). [CrossRef] [PubMed]
32. T. Y. Zhai, Y. Ma, L. Li, X. S. Fang, M. Y. Liao, Y. Koide, J. N. Yao, Y. Bando, and D. Golberg, “Morphology-tunable In2Se3 nanostructures with enhanced electrical and photoelectrical performances via sulfur doping,” J. Mater. Chem. 20(32), 6630–6637 (2010). [CrossRef]
33. Z. Li, J. Salfi, C. D. Souza, P. Sun, S. V. Nair, and H. E. Ruda, “Room temperature single nanowire ZnTe photoconductors grown by metal-organic chemical vapor deposition,” Appl. Phys. Lett. 97, 063510 (2010).
34. J. S. Jie, W. J. Zhang, Y. Jiang, X. M. Meng, Y. Q. Li, and S. T. Lee, “Photoconductive characteristics of single-crystal CdS nanoribbons,” Nano Lett. 6(9), 1887–1892 (2006). [CrossRef] [PubMed]
35. Y. Jiang, W. J. Zhang, J. S. Jie, X. M. Meng, X. Fan, and S.-T. Lee, “Photoresponse properties of CdSe single-nanoribbon photodetectors,” Adv. Funct. Mater. 17(11), 1795–1800 (2007). [CrossRef]
36. D. P. Amalnerkar, “Photoconducting and allied properties of CdS thick film,” Mater. Chem. Phys. 60(1), 1–21 (1999). [CrossRef]
37. A. Singh, X. Y. Li, V. Protasenko, G. Galantai, M. Kuno, H. G. Xing, and D. Jena, “Polarization-sensitive nanowire photodetectors based on solution-synthesized CdSe quantum-wire solids,” Nano Lett. 7(10), 2999–3006 (2007). [CrossRef] [PubMed]
38. C. Y. Wu, J. S. Jie, L Wang, Y. Q. Yu, Q. Peng, X. W. Zhang, J. J. Cai, H. E. Guo, D. Wu, and Y. Jiang, “Chlorine-doped n-type CdS nanowires with enhanced photoconductivity,” Nanotechnology 21, 505203 (2010).
39. J. F. Wang, M. S. Gudiksen, X. F. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and photodetection from single indium phosphide nanowires,” Science 293(5534), 1455–1457 (2001). [CrossRef] [PubMed]
40. S. Han, W. Jin, D. H. Zhang, T. Tang, C. Li, X. L. Liu, Z. Q. Liu, B. Lei, and C. W. Zhou, “Photoconduction studies on GaN nanowire transistors under UV and polarized UV illumination,” Chem. Phys. Lett. 389(1-3), 176–180 (2004). [CrossRef]
41. A. Singh, X. Y. Li, V. Protasenko, G. Galantai, M. Kuno, H. G. Xing, and D. Jena, “Polarization-sensitive nanowire photodetectors based on solution-synthesized CdSe quantum-wire solids,” Nano Lett. 7(10), 2999–3006 (2007). [CrossRef] [PubMed]
42. Z. Y. Fan, P. C. Chang, J. G. Lu, E. C. Walter, R. M. Penner, C.- Lin, and H. P. Lee, “Photoluminescence and polarized photodetection of single ZnO nanowires,” Appl. Phys. Lett. 85(25), 6128–6130 (2004). [CrossRef]
43. Z. Liu, H. T. Huang, B. Liang, X. F. Wang, Z. R. Wang, D. Chen, and G. Z. Shen, “Zn2GeO4 and In2Ge2O7 nanowire mats based ultraviolet photodetectors on rigid and flexible substrates,” Opt. Express 20(3), 2982–2991 (2012). [CrossRef] [PubMed]
44. X. F. Wang, Z. Xie, H. T. Huang, Z. Liu, D. Chen, and G. Z. Shen, “Gas sensors, thermistor and photodetector based on ZnS nanowires,” J. Mater. Chem. 22(14), 6845–6850 (2012). [CrossRef]
