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Ternary metal oxide, Zn2GeO4, In2Ge2O7, have potential applications in many research areas. Using a single chemical vapor deposition method, high-quality single crystalline Zn2GeO4 nanowire (NW) mats and In2Ge2O7 NW mats were synthesized on a large scale. Nanowires mats based ultraviolet photodetectors were fabricated on rigid silicon substrates. By simply transferring the nanowire mats to a transparent adhesive PET tape, flexible photodetectors were also fabricated. Both the rigid and flexible photodetectors exhibited excellent photoconductive performance in terms of high sensitivity to the UV light, excellent stability and reproducibility, and fast response and recovery time.
©2012 Optical Society of America
Introduction
Ultraviolet (UV) photodetectors have been a hot technology topic in recent years because of their wide applications in fields like stratospheric ozone layer monitoring, flame safeguard, fire control areas and optical communications [1, 2]. Photoconductivity is a well-known property of semiconductors which implies the electrical conductivity changes due to the incident radiation. With large surface area, good crystallinity and excellent stability, binary metal oxide nanowires, such as ZnO [3], SnO2 [4, 5], and In2O3 [6] nanowires, have been widely studied for the fabrication of high performance UV photodetectors. Compared with binary oxide, ternary oxides have the advantages of tuning their physical properties by varying the proportion of each component. They usually possess more superior performance in UV detection. Therefore, it is a promising task to develop high efficient UV photodetectors based on new kinds of ternary oxides.
As an important group of semiconductors, germanate has attracted great research interest in recent years due to their promising applications as catalyst materials, humidity sensors, UV detectors, and high energy laser system materials. For example, zinc germanate (Zn2GeO4) is an active photocatalyst for the decomposition of organic pollutants [7] and for reduction of CO2 into renewable hydrocarbon fuel (CH4) [8]. It also exhibits bright white-bluish luminescence [9] and has been proved to be an excellent candidate as high-capacity anode for lithium batteries [10]. Indium germanate (In2Ge2O7) has also been found to be an important ternary oxide with excellent photoluminescence properties [11]. Very recently, with large surface-to-volume ratio and low dimensionality, one-dimensional germanate nanostructures were studied as high performance photodetectors with high sensitivity and fast response time. For instance, Yan et al. synthesized Zn2GeO4 nanowire networks and fabricated efficient ultraviolet photodetectors by using photolithography technique on SiO2/Si substrate, which showed fast response and recovery time within 1 s, and the on/off switching ratio was about 10 at a 20 V bias voltage with UV light intensity at ~0.2 mW/cm2. Li et al. fabricated single In2GeO7 nanowire based photodetectors with significantly shortened response by a standard microfabrication process in a clean room. The photodetector showed fast response and decay time (~3 ms), high responsivity (3.9 × 105 AW−1) and high quantum efficiency (2.0 × 108%). Li et al. fabricated visible-blind deep-ultraviolet Schottky photodetectors using individual Zn2GeO4 nanowire via a photolithography technique on SiO2/Si substrate. At an 8 V bias voltage, their device showed an extremely low dark current (<0.1 pA), a responsivity of 38.3 AW−1 (corresponding gain ~200), a high UV-to-visible discrimination ratio up to ~104, and a relatively fast response time upon 245 nm UV illumination [12–14]. However, in all these reports, expensive and tedious photolithography techniques were required, which might be one of the main hurdles for their practical application in UV detection. It remains great challenge to fabricate germanate nanowire based high performance photodetectors using an easy, low-cost and lithography-free process.
In this paper, using a simple chemical vapor deposition (CVD) method, single crystalline Zn2GeO4 and In2Ge2O7 nanowire mats were prepared on a large scale on SiO2/Si substrate. Nanowire mats based photodetectors were directly fabricated on the SiO2/Si substrate by printing silver electrodes. Flexible nanowire mats based photodetectors were also fabricated by directly transferring the nanowire mats onto a transparent adhesive PET tape. Both the rigid and the flexible UV photodetectors exhibit excellent device performance such as high sensitivity, high reversibility and fast response and recovery time. Our results imply that both the Zn2GeO4 NW mats and the In2Ge2O7 NW mats are great building blocks for new types of high performance ultraviolet photodetectors and optical switches.
2. Experimental setup
Germanate nanowires mats, Zn2GeO4 and In2Ge2O7 in this case, were synthesized by a simple thermal evaporation process in a horizontal furnace. GeO2, Zn, carbon powders (for Zn2GeO4), or GeO2, In2O3, carbon powders (for In2Ge2O7) were used as the raw materials. In a typical synthesis of Zn2GeO4 NW mats, mixed GeO2 (purity 99.99%, 0.04 g) and carbon powder (0.0138 g) was located into the center of an alumina boat, and a small amount of zinc powder (purity 99.99%, 0.03 g) was located 5 mm away upstream. Si/SiO2 wafer was used as the collecting substrate, which was placed at the downstream end of the alumina boat. A thin layer of gold nanoparticles with diameters of ~20 nm was coated on the substrate as the catalysts for nanowire growth. Before heating, pure N2 was flow through the system at a rate of 500 sccm for several minutes to eliminate oxygen in the reaction system. Then the furnace was heated to 1000 °C at a rate of 30 °C/min and maintained at this temperature for 30 min under a constant 〉ow of N2 at a rate of 100 sccm. After the furnace was cooled down naturally to room temperature, a layer of white product was found deposited on the substrate.
In2Ge2O7 NW mats were prepared via a similar process. The mixture of GeO2, In2O3 and activated carbon powders with a molar ratio of about 1:1:4 were loaded in the middle of an alumina boat. Si/SiO2 substrates covered with Au catalyst were also placed at the downstream position with ~1 cm away from the raw materials. After reacted at 1000 °C for 30 min, a layer of white product was found deposited on the substrate.
The as-prepared nanowires was investigated by using X-ray diffraction (XRD, X'Pert PRO, PANalytical B.V., Netherlands), scanning electron microscopy (SEM, JSM-6701F) and transmission electron microscope (HRTEM, JEOL 4000EX).
To fabricate the NW mats based devices on rigid substrate, two silver wires with an interval of 1.2 mm were fixed onto the as-grown NW mats on Si/SiO2 substrate with the aid of silver paste. After that, the prepared device was heated in a 60 °C oven for about 2 hours to solidify the silver paste. To fabricate the flexible NW mats based devices, a transparent adhesive PET tape (TAT) substrate was first attached to the surface of the as-grown NW mats under a certain pressure. By peeling the TAT off the Si/SiO2 substrates, the NW mats were successfully transferred to the TAT substrate. After parallel printing silver paste, as the electrodes, onto the TAT substrate, flexible devices were obtained for subsequent photoconductive measurements.
3. Results and discussion
Figure 1(a) shows the XRD pattern of the as-grown Zn2GeO4 nanowire mats. All peaks in this pattern can be readily indexed to pure rhombohedral Zn2GeO4 crystal phase (JCPDS: 11-687). No characteristic peaks from other crystalline impurities were found in this pattern, indicating the formation of high purity Zn2GeO4 product. Typical SEM image of the product shown in Fig. 1(b) reveals the formation of uniform nanowires on a large scale under current conditions. A high-magnification SEM in Fig. 1(c) depicts that the typical length and diameter of the Zn2GeO4 NWs are several hundred micrometers and 80-250 nm, respectively. After conducted a series of control experiments, we deduced that the growth rate of Zn2GeO4 NWs is about 10μm/min. Figures 1(d)-(f) are the representative TEM images of the synthesized Zn2GeO4 NWs, where microstructures of synthesized individual nanowire can be readily seen. Figure 1d depicts a single Zn2GeO4 NW with a uniform diameter of about 100 nm. Its corresponding HRTEM images were shown in Fig. 1(e)-(f). The corresponding selected-area electron diffraction (SAED) pattern shown in Fig. 1(f) inset confirms the formation of single crystalline nanowire. In Fig. 1(e) and 1(f), the measured d spacings was calculated to be around 0.41 nm and 0.30 nm, corresponding to the (300) and (113) planes of rhombohedral Zn2GeO4, respectively. Combined the SAED and HRTEM data, we could deduce the formation of single crystalline Zn2GeO4 nanowire with the preferred growth direction along the [001] orientation.
Fig. 1 (a) XRD pattern, (b), (c) SEM images, (d) TEM images and (e), (f) HRTEM images of the as-grown Zn2GeO4 NW mats. Inset in (f) is the corresponding selected area electron diffraction pattern.
The X-ray diffraction peaks in Fig. 2(a) can be indexed to a pure monoclinic In2Ge2O7 crystal phase (JCPDS: 82-0846). Figure 2(b) is a general SEM image of the as-deposited In2Ge2O7 product on the silicon substrates, in the form of uniform nanowire mats. Inset in Fig. 2(b) is a high-magnification SEM image of an individual In2Ge2O7 NW. Typical nanowires was found to have diameters of around 80-100 nm and length of hundreds micrometers.
Fig. 2 (a) XRD pattern of the In2Ge2O7 NW mats. (b) SEM images of as-grown In2Ge2O7 NW mats on silicon substrate.
To investigate the photoconductive properties of the as-grown Zn2GeO4 NW mats, photodetectors were directly fabricated on the as-deposited rigid SiO2/Si substrates. A schematic diagram of the rigid substrate photodetector based on Zn2GeO4 NW mats is shown in Fig. 3(a) , consisting of the SiO2/Si substrate, the NW mats and two Ag electrodes. The channel width is around 1.2 mm. Both the current-voltage (I-V) characteristics and the current as a function of time (I-T) of the device were recorded with a Keithley 2400 semiconductor parameter analyzer. The I-V curves were measured with bias from −2 V to 2 V at room temperature in ambient condition. Figure 3(b) is typical I-V curves of the device before and after irradiated with UV lights with wavelengths of 254 nm and 365 nm, respectively. From the curves, we can clearly see that, upon UV illumination, the photodetector exhibited a remarkable increase in the current. At a bias voltage of 2 V, the current was measured to be around 79.56 nA under 254 nm UV light irradiation, 15 nA under 365 nm light irradiation, and 0.39 nA in dark. The time-dependent photoresponse of the device was measured by periodically turning the UV light on and off (Fig. 3(c)). The ratio of the photocurrent to the dark current (Ion/Ioff) is about 17 at forward bias ( + 2 V) when illuminated under 254 nm UV light, while it is about 3.8 under 365 nm UV light illumination. The response speed is another important parameter of nanowire photodetector. Figure 3(d) gives the enlarged view of a single on/off cycle in which the 10% and 90% points to the peak value of the photocurrent are ticked for calculating the response and recovery time, which was found to be around 2.5 s and 4 s, respectively.
Fig. 3 (a) Schematic illustration of the rigid substrate photodetector based on Zn2GeO4 NW mats. (b) Typical I-V characters of the device measured in dark (black curve), upon 0.85 mWcm−2 365 nm (red curve) and 254 nm (blue curve) UV light illumination, respectively. (c) Temporal response of photocurrent of the device under 365 nm (red curve) and 254 nm (blue curve) UV light illumination. (d) Enlarged view of a single on/off cycle.
Considering the intensive interest of flexible electronics, which have emerged as promising next-generation candidates for electronics [15, 16], optoelectronic [17, 18], photovoltaic applications [19, 20], as well as chemical sensors [21], it is an interesting and a valuable task to develop a new kind of flexible, sensitive UV photodetector. We further fabricated flexible photodetector by using the Zn2GeO4 NW mats on a mechanically flexible substrate (TAT), as illustrated in Fig. 4(a) and 4(b). Figure 4(c) shows the corresponding I-V curves of the flexible ultraviolet photodetector measured using a two-probe configuration in the dark (black line in Fig. 4(c)) and under 254nm UV light illumination (blue line in Fig. 4(c)). From the curves, it can be seen that the dark current is close to zero with bias ranging from −20V to 20V. At a fixed bias of 15 V, the photocurrent was measured to increase from 0.029 nA in the dark to 0.873 nA upon 254 nm UV light illumination, giving an on/off switching ratio of about 30. The decline of photocurrent in the flexible photodetector comparing to that of the rigid photodetector can be ascribed to the inevitable re-arrangement of nanowires and the contracts between nanowires, since a transfer process was required to transfer NW mats to the flexible substrates. Besides, the contacts between nanowires, the flexible substrates and the electrodes also affect the performance of flexible device. We also investigated the mechanical stability of the device by bending repetitiously the device on the TAT substrate. After bending for 100 times, the I-V curve was nearly unchanged (red line in Fig. 4(c)) when compared with the corresponding I-V curve without bending. We checked the photocurrent through the device upon different bending conditions, namely lateral bending, longitudinal bending, and diagonal bending. There were no obvious changes in the device characteristics, which can be attributed to the compact NW mats and good Ohmic contact of the structure. These results indicate that our flexible ultraviolet photodetector is able to bear certain external mechanical force. Figure 4(d) shows the normalized time response of the flexible device under 254 nm UV light illumination at a bias voltage of 20 V. The response characteristic of the flexible device is proven to be fairly stable and reversible. Figure 4(e) is an I-T curve of a single on/off cycle of the flexible device. From the curve, it can be seen that the typical response time is about 2.5 s and it has a steep fall edge, giving a recovery time of <1 s. This flexible device shows superior photoconductive behavior comparing to the rigid substrate photodetector and we believe the performance can be further optimized by developing better close-packed cross-linked nanowire mats.
Fig. 4 (a) Schematic diagram of Zn2GeO4 NW mats transfer and a device structure. (b) Photograph of flexible ultraviolet photodetector based on Zn2GeO4 NW mats. (c) Typical I-V characters of the device measured in dark (black curve), upon 0.85 mWcm−2 254 nm (blue curve) UV light illumination and after 100 cycles of bending (red curve), respectively. (d) Temporal response of photocurrent of the device under 254 nm UV light illumination. (e) Enlarged view of a single on/off cycle.
The photoconductive properties of the as-synthesized In2Ge2O7 NW mats were also measured by fabricating photodetectors on rigid and flexible substrates, respectively. Figure 5(a) shows the schematic diagram of the rigid photodetector based on In2Ge2O7 NW mats, consisting of the SiO2/Si substrate, the NW mats and two Ag electrodes. The typical I-V characteristics of the photodetector before and after illuminated with 254 nm and 365 nm UV lights were depicted in Fig. 5(b). When the device was illuminated with 254 nm UV light, a high photocurrent of ~460 nA was recorded at a low bias of 3.0V. The photocurrent was found to be around 72 nA when illuminated with 365 nm UV light. Both values exhibited remarkable increase when compared with the value in the dark state (1.6 nA). Figure 5(c) shows the I-T curves of the device with the UV light periodically turned on and off. The photodetector also shows highly stable and reproducible characteristics, similar with the device built on Zn2GeO4 NW mats. Measured from a single on/off cycle, the Ion/Ioff is calculated to be about 245 at forward bias ( + 3 V) when illuminated under 254 nm UV light, while it is about 12 under 365 nm UV light illumination. Figure 5(d) gives a single time-dependent on/off cycle of the device, also indicating fast response and recovery time with the corresponding values of 10.5 s and 3.5 s, respectively.
Fig. 5 (a) Schematic illustration of the rigid substrate photodetector based on In2Ge2O7 NW mats. (b) Typical I-V characters of the device measured in dark (black curve), upon 0.85 mWcm−2 365 nm (red curve) and 254 nm (blue curve) UV light illumination, respectively. (c) Temporal response of photocurrent of the device under 365 nm (red curve) and 254 nm (blue curve) UV light illumination. (d) Enlarged view of a single on/off cycle.
Flexible photodetectors were also fabricated by using In2Ge2O7 NW mats as the active channels. Figure 6(a) shows the photograph of the corresponding device, where the nanowires and the two Ag electrodes can be clearly seen. Figure 6(b) shows the I-V curves of the flexible device before and after illuminated with a 254 nm UV light. Similar increase in current was also found for the flexible device. The current recorded at a bias of 15 V was found to increase from 0.05 nA in dark to 11.2 nA upon UV light illumination. The flexible device was quite stable and the current almost remained unchanged even after 100 cycles of bending, as can be seen from the curves in Fig. 6(b). Nonlinear behavior was observed for the I-V curves, indicating non-Ohmic contact between the NW mats and the Ag electrodes. Figure 6(c) gives the reproducible on/off switching curves of the flexible photodetector upon 254 nm light illumination measured at a bias voltage of 20 V. We can see that the Ion/Ioff reaches ~227, which is much higher than that of the flexible photodetector based on Zn2GeO4 NW mats. Similar to the rigid In2Ge2O7 NW mats device, the flexible also exhibited fast response and recovery characteristics with a response time of about 15 s and a recovery time shorter than 1 s (blue line in Fig. 6(d)).
Fig. 6 (a) Photograph of In2Ge2O7 NW mats on a flexible ultraviolet photodetector. Inset is the corresponding SEM images of a selected area. (b) Typical I-V characters of the device measured in dark (black curve), upon 0.85 mWcm−2 254 nm (blue curve) UV light illumination and after 100 cycles of bending (red curve), respectively. (c) Temporal response of photocurrent of the device under 254 nm UV light illumination. (d) Enlarged view of a single on/off cycle.
Figure 7 shows the schematic diagram depicting the generation of free carriers and the electrical transport through the NW-NW interface. Oxygen-adsorption process in the dark and oxygen-desorption process upon UV illumination are generally thought to be associated with the generation of free carriers [22]. In ambient conditions, oxygen molecules are absorbed onto the NW surface and capture free electrons from the NW through [O2(g) + e-→O2-(ad)], creating a low-conductivity depletion layer near the surface. When illuminated with UV light, electron-hole pairs are generated. The holes migrate to the surface to desorb the oxygen adsorbates through [h+ + O2-(ad) →O2(g)], resulting in a reduction in the depletion barrier thickness and an increase in the free-carrier concentration. Hence, photocurrent increases dramatically upon the UV light illumination. After turning off the illumination, oxygen molecules readsorb on the NW surfaces, returning the NWs to their initial low-conductivity state. As shown in Fig. 7(b), the electrons have to overcome the NW-NW junction barrier when tunneling from one NW to another. The NW-NW junction barrier, which is analogous to two back-to-back barriers, could be much lower in the effective barrier height upon illumination due to the greatly increased carrier density. Consequently, electrical transport through the NW-NW becomes much easier and photocurrent becomes much larger under UV irradiation [22].
Fig. 7 Schematic illustration of (a) oxygen-adsorption process in the dark and oxygen-desorption process upon UV illumination of the nanowire mats, (b) the NW-NW junction barrier for electron transfer in the nanowire mats, showing a decrease in NW-NW junction barrier height from the light-off state to light-on state.
4. Conclusion
In conclusion, single crystalline Zn2GeO4 and In2Ge2O7 Nanowire mats were prepared on a large scale on SiO2/Si substrate via a simple chemical vapor deposition method. By using a facile and low-cost lithography-free fabrication process, Zn2GeO4 NW mats and In2Ge2O7 NW mats based photodetectors have been successfully constructed on rigid and flexible substrates, respectively. Both the rigid device and flexible device exhibited quite good performance such as high Ion/Ioff values, fast response and recovery time, and high reversibility. Besides, the flexible devices showed excellent mechanical stability and the current remains almost unchanged even after bended for 100 cycles. We believe that the Zn2GeO4 NW mats and In2Ge2O7 NW mats as well as this facile method may find promising photoelectronic applications for future practical device design.
Acknowledgments
This work was supported by the National Natural Science Foundation (51002059, 21001046), the 973 Program of China (No.2011CB933300), the Basic Scientific Research Funds for Central Colleges (2010QN045), the Research Fund for the Doctoral Program of Higher Education (20090142120059, 20100142120053) and the Director Fund of WNLO. We thank Analytical and Testing Center of Huazhong University Science & Technology and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the samples measurements.
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