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SEM was used to characterize the morphology of the products. Figure 1 shows the SEM of the MgZnO NWs. Figure 1(a) and Figure 1(b) showed the top-view SEM images of the MgZnO NWs and EDS, respectively. From the Figure 1(b), it was clear that only the Zn, Mg and O peaks were observed while no other impurities were detected. Figure 1(c) showed the image of sample used for TEM. Figure 1(d) revealed that structure of the MgZnOnanowire was hexagonal structure(PDF#36-1451), the lattice parameter was 2.81 Å(1 Å=0.1 nm), 5.06 Å, respectively, which meaned the Mg ions started to replace the Zn ions. Meanwhile, the corresponding chemical composition of the grown MgZnO NWs was determined through the EDS.
Figure 1. (a) Top-view SEM images of the as-grown MgZnO NWs; (b) EDS image of MgZnO NWs; (c) Single morphology of MgZnO NW sample for TEM; (d) High resolution TEM of the MgZnO nanowire, insert images show the corresponding SAED pattern, respectively
In order to further confirm the formation of MgZnO NWs, the X-ray diffraction(XRD) and Photo-luminescence(PL) of ZnO NWs and MgZnO NWs were checked, showed in Figure 2. XRD patterns of NWs samples were taken to study the crystallographic information on the nanowires. From the diffraction peaks shown in Figure 2(a), it could deduce that the structure of MgZnO NWs was hexagonal, just like ZnO. Meanwhile, the normalized PL spectra of the MgZnO samples are shown in Figure 2(b), the PL spectra showed an obvious peak at the 378 nm. Comparing with the ZnO PL, the MgZnO showed an apparent hypsochromic shift, which was caused by the change of the band-gap with Mg substitution. Owing to Mg doping the ZnO nanowires, the Burstein-Moss effect is deemed to be the reason of the hypsochromic-shift.
Figure 2. (a) XRD of the ZnO nanowires and MgZnO nanowire; (b) PL spectrum of the ZnO nanowires and MgZnO nanowire. The PL shows the peak of MgZnO blue shift
Current-voltage(I-V) measurements were carried out at ambient condition to investigate the electrical properties of the fabricated UV photosensor in dark and under UV illumination. Metal electrodes could be directly fabricated through screen printing or shadow-mask assisted deposition method onto the as-obtained MgZnO nanowires.
In our experiment, silver paint was directly applied onto MgZnO nanowire arrys to form the electrods. The photoresponse tests were conducted in a dark environment with UV illumination (254 nm, 0.03 mW/cm2). The "on" and "off" of illumination were controlled by a metal chopper. The photoresponse of UV illumination are shown in Figure 3. Figure 3(b) shows the I-T characteristic curves of MgZnO nanowires photodetector with and without light illumination.
Figure 3. I-V characteristics of the ZnO NWs photodetectors (a) and ZnO-GQDs NWs photodetectors (b); I-T characteristics of the ZnO NWs photodetectors (c) and ZnO-GQDs NWs photodetectors (d)
To further analyze the photoconductive properties, the chemical states of NWs surface with and without UV radiation can be clarified based on the previous findings. Under dark conditions, the surface of MgZnO NWs absorbed oxygen from the atmosphere and formed a depletion layer, thereby producing negatively charged ions. As a result, the absorbed oxygen molecules on the surface of the MgZnO NWs trapped some of the free electrons, whereas the mobility of the remaining electrons decreased because of the depletion layers created on the surface.
$${\rm O_2} + 2{e^ - } \to \rm O_2^ - $$ (1) The water vapor molecules absorbed by the surface of MgZnO NWs also enhanced the depletion layer. The water vapor molecules captured not only free electrons but also free holes, thereby further lowering the conductivity NWs. Consequently, H2O molecules significantly affected the conductivity more than O2 molecules in MgZnO NWs.
$$ {\rm{H_2}O} + 4{h^ + } \to \frac{1}{2}{\rm O_2} + 2{\rm H^{\rm{ + }}} $$ (2) $$ 2{\rm {H_2}O} + 4{e^ - } \to {\rm H_2} + 2{\rm O{H^ - }} $$ (3) Electron-hole pairs were generated (
$hv \to {h^ + } + {e^ - }$ ) after applying UV light with 254 nm. The photogenerated holes were separated from the electrons by strong local electric fields[31-32], which reduced the electron-hole recombination process, thereby increasing the carrier lifetime. Consequently, the conductivity increased because of the increase in carrier density. The holes then relocated to the surface and suppressed the depletion region by discharging the adsorbed oxygen ions, thereby forming photo-desorbed oxygen.The response time is another important indicator of merit of a photodetector. To examine the response time of the MgZnO nanowires UV photodetector, the time-dependent photo-current at 3 V bias with multiple UV on/off cycles was measured, in which both the "on" and "off" times of the UV illumination were 20 s. It was well-known oxygen molecules absorbed at surface of MgZnO acting as electron acceptors to form O2 by capturing free electrons from the surface of MgZnO in dark and created a low conductive depletion layer near the surface of nanowires.
Upon UV illumination, the photogenerated holes in MgZnO migrated to the surface and neutralized the O2 ions, while the unpaired electrons significantly enhanced the conductivity of the sample. As shown in Figure 3(b), upon UV illumination, the current would first rapidly ramp from the dark current, followed by a slow increase; and as UV illumination was off, the current would first promptly fall, and then slowly decay to around the original level. These observed time-resolved photocurrent course could be described by a fast photoresponse process followed by a slow one, and the latter one was governed by the low rate of the turnover of oxygen chemisorption/desorption on the NWs surface. The dependence of both rose and decay of photocurrent on time could be well described by second-order decay functions as follows:
$$ I = {I_0} + A{{\rm e}^{ - t/{t_1}}} + B{{\rm e}^{ - t/{t_2}}} $$ (4) Where, I0, A, and B are constants, t1 and t2 are time constants.
Figure 3(c) and Figure 3(d) showed a typical rise and decay stage of the time-resolved photocurrent variation curve, respectively. By fitting the photocurrent data with the time, it was estimated the time constants for rise stage are tr1 = 0.47 s, tr2 = 3.99 s, with relative weight factors of 64% and 36% respectively; while the time constants for decay stage are td1 = 0.60 s, td2 = 2.01 s, with relative weight factors of 40% and 60%, respectively.
It was expected that the performance of our MgZnO nanowire photodetectors could be further improved by employing thermal annealing, plasmonic nanoparticles modifica-tion, heterojunction creation, and so on. For the rise stage, the fast process was a result of photocarriers generation excited by UV illumination, however, the slow one was governed by readsorption of oxygen molecules on MgZnO surface; when the UV illumination was off, the fast decay process was related to photocarrier recombination, and the slow one was controlled by the slow physisorption of oxygen molecules.
Fabrication of low Mg content MgxZn1-xO nanowires ultraviolet photosensors via chemical vapour deposition method
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摘要: 针对氧化锌紫外光电传感器对深紫外光探测能力弱的问题,通过镁掺杂氧化锌纳米线的方法调整氧化锌能带,从而提高氧化锌紫外光电传感器对深紫外光的探测灵敏度。通过扫描电子显微镜(SEM)、能谱分析(EDS)、透射电子显微镜(TEM)等表征手段对镁锌氧纳米线进行表征,结果表明成功制备镁锌氧纳米线。通过氧化锌纳米线探测器和镁掺杂氧化锌纳米线探测器对254 nm的深紫外进行对比测试,测试结果显示镁掺杂氧化锌探测器对254 nm波长的深紫外光的光响应性能增强,光电流从0.02 μA提高至0.57 μA。通过镁掺杂氧化锌纳米线的方法制备探测器,有效解决氧化锌纳米线探测器对深紫外光探测能力弱的问题,将为深紫外探测器的设计及制备方法提供有益的参考。
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关键词:
- doped /
- nanowires /
- MgZnO /
- ultraviolet photosensors
Abstract: To solve the problem that ZnO ultraviolet photosensors is poor in detecting deep ultraviolet light, a method of fabricating Mg doped ZnO nanowires was proposed to adjust the ZnO energy band, so as to improve the sensitivity of ZnO ultraviolet photosensors in detecting deep ultraviolet light. The MgZnO nanowires were characterized by scanning electron microscope(SEM), energy spectrum analysis(EDS), transmission electron microscope(TEM) and other characterization methods. The results show that MgZnO nanowires were successfully prepared. The ZnO nanowire detector and the Mg doped ZnO nanowire detector were test with the 254 nm deep ultraviolet light, and the test results show that the photoresponse of the Mg doped ZnO detector to the 254 nm deep ultraviolet light was enhanced, the photocurrent increased from 0.02 μA to 0.57 μA. The detector is prepared by Mg doped ZnO nanowires, which can effectively improve the ZnO nanowires detectability in detecting deep ultraviolet light. It will provide beneficial reference for the design and preparation of deep ultraviolet detectors.-
Key words:
- doped /
- nanowires /
- MgZnO /
- ultraviolet photosensors
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