OPTO−ELECTRONICS REVIEW 18(4), 467–473

DOI: 10.2478/s11772−010−0037−4

Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO

S.K. MISHRA1*, R.K. SRIVASTAVA1, S.G. PRAKASH1, R.S. YADAV2, and A.C. PANDAY2

1Department of Electronics & Communication, University of Allahabad, Allahabad−211002, India 2Nanophosphor Application Centre, Physics Department, University of Allahabad, Allahabad−211002, India

In the present paper, ZnO nanoparticles (NPs) with particle size of 20–50 nm have been synthesized by hydrothermal method. UV−visible absorption spectra of ZnO nanoparticles show absorption edge at 372 nm, which is blue−shifted as com− pared to bulk ZnO. Photoluminescence (PL) and photoconductive device characteristics, including field response, light in− tensity response, rise and decay time response, and spectral response have been studied systematically. The photolumines− cence spectra of these ZnO nanoparticles exhibited different emission peaks at 396 nm, 416 nm, 445 nm, 481 nm, and 524 nm. The photoconductivity spectra of ZnO nanoparticles are studied in the UV−visible spectral region (366–691 nm). In spectral response curve of ZnO NPs, the wavelength dependence of the photocurrent is very close to the absorption and photoluminescence spectra. The photo generated current, Ipc =(Itotal –Idark) and dark current Idc varies according to the r r power law with the applied field IpcaV and with the intensity of illumination IpcaIL , due to the defect related mechanism in− cluding both recombination centers and traps. The ZnO NPs is found to have deep trap of 0.96 eV, very close to green band emission. The photo and dark conductivities of ZnO NPs have been measured using thick film of powder without any binder.

Keywords: photoconductivity, photoluminescence, .

1. Introduction are quite interesting because of their application in electropho− tography, detectors of oxidizing and reducing gases, etc. [14]. Recently, the optical and electronic properties of semicon− In this paper, we report photoluminescence and trans− ductor nanoparticles (NPs) have attracted tremendous inter− port measurements viz. photoconductivity and dark conduc− est [1–4]. Their advantages include tunable , en− tivity behaviour of ZnO nanoparticles in UV−visible spec− hanced quantum efficiency due to discrete energy density of tral region (366–691 nm). The dependence of photocurrent states, etc. Zinc oxide (ZnO) has attracted a great attention and dark current as a function of various parameters such as as a very promising semiconductor material because of its varying field, intensity of illumination, wavelength and rise wide band gap (~3.37 eV) and high exciton binding energy and decay time response has been studied. In semiconductor (~60 meV). It has attractive and novel optical and electronic materials, the photoconductivity arises due to photo−genera− properties such as superior UV emission characteristics, tion of electron−hole pairs in the material after absorption of high stability and room temperature luminescence [5–7]. photons which increases the carrier density and thereby con− Zinc−oxide nanoparticles have been synthesized by several ductivity of the material. The response time is an important methods such as chemical vapour deposition [8], sol−gel property of photoconductive materials. A good photosensi− method [9], spray pyrolysis [10], hydrothermal method, etc. tive material shows large change in conductivity and fast [11–13]. Here, we have synthesized ZnO nanoparticles by response. If trapping centres are in abundance, the response a hydrothermal method. It is widely used method that can con− time is slow. Trapping centres also increase the decay time trol the shape and size of ZnO NPs among all−solution based as they slowly release trapped carriers after removal of the approaches. Unlike other conventional methods, the hydro− excitation source. It is well known that, as−synthesized ZnO thermal method can produce ZnO NPs at a relatively low tem− NPs are usually n−type semiconductor due to oxygen vacan− ° perature (below 200 C). Hydrothermal method is a promising cies and other native defects such as interstitial Zn ions alternative synthesis method for preparation of ZnO NPs under which act as donors in ZnO lattice. It may also arise due to ambient conditions because it is convenient, economical, and H−atoms in ZnO matrix. Due to large surface to volume environment friendly. Photoconductive materials exhibit in− ratio, surface related phenomenon is primarily governed by crease in electrical conductivity upon illumination and have the adsorption and desorption of the chemisorbed oxygen at different applications. Photoconductivity studies of ZnO NPs the surface of ZnO which may contribute an important role in the photoconductivity of nanostructures (thin films, nano− rod, and nanoparticles) [15–20]. These structures *e−mail: [email protected]

Opto−Electron. Rev., 18, no. 4, 2010 S.K. Mishra Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO nanoparticles show high photoconductivity with slow dynamics. In the ble absorption spectrum of ZnO nanoparticles was recorded on study of photoconductivity, the dependence of photocurrent a Perkin Elmer LS−35 spectrometer. The room temperature as a function of various parameters such as light intensity, photoluminescence (PL) spectrum was obtained using Perkin applied field, energy of illumination, temperature and other Elmer LS−55 fluorescence spectrometer. The excitation wave− parameters also gives us important information regarding length used in the PL measurement was 325 nm. the material. The variation of photocurrent with light inten− The photo and dark conductivities of ZnO NPs have sity response provides an idea about the charge trapping and been measured using thick film of powder without any recombination process occurring inside the material. The binder. In photoconductivity and dark conductivity mea− rise and decay of photocurrent with time response can be surements, a cell was formed by putting a thick layer of useful to determine the nature of distribution of traps and powdered samples in between two Cu electrodes etched on recombination centres. a Cu plate (PCB), having a spacing of 1 mm. The powdered Many authors have reported the measurement of photo− layer was pressed with a transparent glass plate. This glass conductivity in ZnO thin films [15–17], nanorods [18], nano− plate has a slit for providing illumination area of 0.25 cm2. wires [19], nanoparticles [20] for different parameters. There In this cell type device, the direction of illumination is nor− are few reports on the study of photoconductivity [21,22] us− mal to field across the electrodes. The cell was mounted in ing thick films of bulk powder. Srivastava et al. [22] have a dark chamber with a slit where from the light is allowed to characterized dark and photoconductivity of CdS−Se mixed fall over the cell. The photo−response was measured using lattice where the direction of illumination and that of applied 300−W mercury lamp and visible photo−response was mea− field are normal to each other and studied photoconductive sured using a commercial bulb of 200 W as a photo−excita− characteristics with various parameters such as light intensity, tion source. Spectral photo−response was performed by Hg applied field, energy of illumination, and rise and decay of filters using mercury lamp for fixed incident photo−flux. photocurrent. Perhaps no work has reported for ZnO NPs us− A stabilized dc field (50 V/cm to 500 V/cm) was applied ing thick films and studied photoconductive characteristics across the cell to which a dc nano−ammeter, NM−122 (Scien− with above various parameters and calculated trap depth in tific Equipment, Roorkee) for the measurement of current one single paper. and RISH Multi 15S with adapter RISH Multi SI 232 were connected in series. The light intensity over the cell surface 2. Experimental section was changed by varying the distance between slit and light source. Before measuring photoconductivity of the sample, 2.1. Chemicals the cell is first kept in dark till it attains equilibrium.

The zinc acetate dehydrate Zn(CH3COOH)2 2H2O, sodium 3. Results and discussion hydroxide NaOH were from E. Merk Ltd. Mumbai, 400018, India. These chemicals were of high purity (99.999%) with 3.1. Structural study analytical grade and they were used directly without special treatment. Figure 1 shows the X−ray diffraction (XRD) pattern of ZnO nanoparticles synthesized by a hydrothermal method. X−ray diffraction pattern of ZnO nanoparticles synthesized by 2.2. Sample preparation The ZnO nanoparticles were prepared by a hydrothermal method. First, zinc acetate dihydrate was put into 105 mL of distilled water under vigorous stirring. After 10−min stir− ring, 10 mL of 2M NaOH aqueous solution was introduced into the above aqueous solution, resulting in a white aque− ous solution (pH value was equal to 11). The white solution was then transferred into stainless steel autoclaves, sealed and maintained at temperature of 80°C for 4 h. The precipi− tate in the autoclave was taken out and washed repeatedly with distilled water and ethanol to remove the ions possibly remaining in the final products. Then, white powder was obtained after drying at 60°C in air for 2 h.

2.3. Instrumentation The crystal structure of ZnO nanoparticles was characterized by X−ray diffraction (XRD), Rigaku D/MAX− 2200H/ PC, Cu Ka radiation. The scanning electron microscopy (SEM) ima− Fig. 1. XRD pattern of ZnO nanoparticles synthesized by a hydro− ges were taken on Quanta 200 FEG (FEI Company). UV−visi− thermal method.

468 Opto−Electron. Rev., 18, no. 4, 2010 © 2010 SEP, Warsaw a hydrothermal method revealed that ZnO has hexagonal wurtzite structure, and the peaks could be indexed accord− ing to JCPDS card No 79−2205 with a = 0.3249 nm and c = 0.5205 nm [23]. Furthermore, it can be seen that the diffrac− tion peaks have broadening which implies that the ZnO nanoparticles have nanocrystalline nature. No characteristic peak of impurity such as Zn(OH)2 was observed.

3.2. Morphology study Figure 2 shows the FE−SEM image of ZnO nanoparticles synthesized by a hydrothermal method. It is clear from the FE−SEM image of ZnO, that the particles are spherical with little agglomeration. The size of these spherical ZnO nano− particles is of 20–50 nm.

Fig. 3. UV−visible absorption spectrum of ZnO nanoparticles syn− thesized by a hydrothermal method at room temperature.

Oi, zinc interstitials Zni and oxygen antisites OZn. The origin of violet emission centered at 2.987 eV (~416 nm) is as− cribed to an electron transition from a shallow donor level of neutral Zni to the top level of the valence band [25]. There− fore, the shallow donor level of the Zni is suggested to locate at ~0.34 eV below the conduction band in this study because the violet photoluminescence appears at ~2.987 eV. A blue emission centered at ~2.56 eV (481 nm) is due to a radiative transition of an electron from the shallow donor level of Zni to an acceptor level of neutral VZn [26]. In this study, it can be estimated that the acceptor level of VZn locates at ~0.42 eV above the valence band. Another blue emission was reported to appear at around 2.77 eV (445 nm) [27]. Fig. 2. SEM image of ZnO nanoparticles synthesized by a hydro− This emission may be related to surface defects of ZnO thermal method. nanostructure or may be due to singly ionized located at ~0.21 eV above the valence band, although the detailed 3.3. UV-visible absorption study mechanism for blue emission at 445 nm has not been clari− Figure 3 shows room−temperature UV−visible absorption spectrum of ZnO nanoparticles synthesized by a hydrother− mal method. The ZnO nanoparticles have band edge absorp− tion at 372 nm which shows blue shift as compared to bulk ZnO.

3.4. Photoluminescence study Figure 4 illustrates the room−temperature photoluminescen− ce spectrum of ZnO nanoparticles synthesized by a hydro− thermal method at excitation wavelength of 325 nm. Several emission bands, including band edge emission at 396 nm (~3.12 eV) and defect related emission at 416 nm (~2.98 eV), blue emission at 445 nm (~2.77 eV), blue−green emission 481 nm (~2.56 eV) and green emission at 524 nm (~2.35 eV) were observed. It is commonly considered that band edge emission at ~396 nm should be attributed to the recombination of excitons [24]. In general, visible emission in ZnO is attributed to different intrinsic defects such as oxy− Fig. 4. Photoluminescence spectrum of ZnO nanoparticles synthe− gen vacancies Vo, zinc vacancies VZn, oxygen interstitials sized by a hydrothermal method at room temperature.

Opto−Electron. Rev., 18, no. 4, 2010 S.K. Mishra 469 Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO nanoparticles

Fig. 5. Schematic representation of emission from ZnO nanoparticles. fied. The green emission is observed at 524 nm (~2.35 eV). 3.5. Photoconductivity study The green emission may be due to Cu levels [28] or do− nor−acceptor recombination [29] or transition from conduc− 3.5.1. Effect of field tion band to oxygen antisites [30,31]. Dingle et al. [28] have Figure 6 shows current−voltage characteristics of ZnO NPs reported the green emission in ZnO, which was associated under illumination and in dark on an ln−ln scale. The ln(I) with the presence of Cu traces. As the chemicals used in the vs. ln(V) curves are straight lines having different slopes present work are of high purity, the presence of Cu in ZnO with respect to varying voltage according to the power law lattice is very unlikely therefore in the present study green a r Ip V , where ‘r’ represents the slope of different straight emission cannot be attributed to Cu levels. Heo et al. [29] line segments which is found to be different for low and have reported that green emission is commonly observed in high voltages. The dark current Idc varies sub−linearly (r= ZnO samples prepared in oxygen deficient environments 0.57) at the voltage below 10 V. Above 10 V, it varies resulting in vacancy of oxygen and they have attributed it to super−linearly (r = 1.21). Similar to dark current, the photo donor Vo – acceptor VZn recombination. Wen et al. [32] current Ipc also varies sub−linearly (r = 0.43) at low voltages reported yellow emission of ZnO crystal prepared with and super−linearly (r = 1.81) at high voltages. This sub−lin− NaOH by a hydrothermal method and they explained that ear (r < 1) to super−linear (r > 1) variation is attributed to this emission is due to the defects of Oi in the ZnO lattice. flow of trap limited as well as space charge limited current However, this group further found green emission in ZnO inside the material [33,34]. The current at higher voltages crystal synthesized by a hydrothermal method with hydra− arise from a space charge of excess carriers injected from zine. They explained that the green emission is due to one of electrodes. This mechanism is known as space charge excess Zn which comes from the reduction of Zn2+ ions in the reaction process. In the present work, ZnO NPs are syn− thesized by hydrothermal method without any reducing atmosphere means the environment is not oxygen deficient so the pre− sence of oxygen vacancies is very unlikely and the green emission found in our work cannot be attributed to donor Vo – acceptor VZn recombination. The possible rea− son for observed green emission in ZnO NPs is due to the radiative transition from conduction band to the edge of the acceptor levels of Ozn caused by oxygen antisites OZn as reported by Murphy et al. [30] and Lin et al. [31]. As the peak position of the green emission is 2.35 eV, the acceptor level of the oxygen antisites OZn may be located at ~0.97 eV above the valence band. The schematic representation of these emission peaks at ~396 nm, ~416 nm, ~481, and ~524 nm in ZnO NPs is shown in Fig. 5. The green band emission level observed in photoluminescence spectra is in good agreement with calcu− lated trap depth of 0.96 eV from the photoconductivity rise Fig. 6. Variation of photocurrent and dark current as a function of and decay time spectra (Fig. 8). applied voltage for ZnO nanoparticles.

470 Opto−Electron. Rev., 18, no. 4, 2010 © 2010 SEP, Warsaw limited current. If the material has traps, dark current will 3.5.3. Rise and decay time response photoconductivity also be determined by traps and trap−limited, space charge− measurements limited current [21,35]. The rise and decay time transient photoconductivity re− sponse measurements help to study the photoconductivity 3.5.2. Effect of intensity dynamics of ZnO NPs during which the light was abrup− Figure 7 illustrates the variation of photocurrent Ipc with the tly switched on and off at room temperature. Figure 8 light intensity IL for ZnO NPs on an ln−ln scale. The photo shows photoconductivity rise and decay time spectra of generated current, was obtained by subtracting the dark cur− ZnO NPs under UV and visible illumination with fixed rent from the total current, i.e., Ipc =(Itotal –Idark). Maximum photo−flux and bias voltage. UV excitation showed fast photocurrent is found for 30 V under experimental condi− rise and decay behaviour. The photocurrent rises to more tions. The lnIpc versus lnIL curves are straight lines with dif− than 80% within 5 min and falls to 80% of its maximum ferent slopes in two segments at lower and higher light value within 3 min. The rise and decay time response due intensities of illumination. Thus, the variation can be ex− to visible excitation showed slow dynamics. Further, a r pressed by the power law Ipc IL . The value of r=0.77 ZnO NPs are found to be approximately three times more shows, sub−linear light intensity dependence of photocur− photosensitive for UV illumination as compared to rent due to the defect states mechanism including both re− visible illumination. combination centers and traps. At higher intensity, the value ZnO NPs exhibit fast rise and decay time for UV illu− of r = 1.55 shows super linear nature of the photocurrent. mination as compared to visible illumination. When UV il− The sub−linear (r < 1) and superlinear (r > 1) nature of lumination is switched on, the photocurrent rises very photocurrent curves can be explained on the basis of class quickly. Fast rise time response may be attributed to the I and class II centres, respectively [36,37]. The sub−linear dominant fast process of photo generation of electron−hole behaviour of first segment indicates the continuous and pairs. When UV illumination is off, electron−hole recombi− exponential distribution of class−I centres between the con− nation process dominates, so the conductivity decreases duction band and electron Fermi−level. Further increase in quickly. intensity of illumination, electron Fermi−levels are sepa− The photoconductive response due to visible illumina− rated more and more, converting more of the class−I centres tion is found to be significantly low as compared to that to recombination centres. This gradually decreases the elec− for UV illumination. As visible illumination corresponds tron life time and the slow increase in photocurrent gives to lower band gap energy for ZnO NPs, electron−hole rise to the observed sublinearity in the photocurrent. As the pairs are generated corresponding to optical transitions light intensity increases, the electron Fermi−level and hole between defect states and the band edge whereas UV illu− Fermi−level are shifted towards conduction and valence mination generates electron−hole pairs corresponding to bands, respectively. Thereby converting some of the class−II optical transitions between valence band and conduction states into recombination centres and these recombination band. Slow photoconductive rise and decay response may centres participate in the recombination process, which ulti− be attributed to a large amount of recombination centres mately sensitizes the ZnO NPs, resulting in super−linearity and presence of trap levels and defect states within the in photocurrent vs. illumination intensity curves. band gap [30,35].

Fig. 8. Photoconductivity rise and decay time spectra: photoconduc− Fig. 7. Variation of photocurrent as a function of incident illumina− tive response due to UV and visible light excitation for ZnO nano− tion intensity (voltage 30 V and room temperature 25°C). particles.

Opto−Electron. Rev., 18, no. 4, 2010 S.K. Mishra 471 Photoluminescence and photoconductive characteristics of hydrothermally synthesized ZnO nanoparticles

3.5.4. Trap depth determination of the incident radiation by ZnO NPs changes with the The rise and decay time response curve for the visible exci− change of incident wavelength depending on its energy gap. tation can fit well with exponential curve as follows, the rise Different wavelengths were obtained from an Hg−lamp using filters and measurements made for a fixed incident time response Ipc =I0[1 – exp(–t/t)], the decay time re− photo flux. Photocurrent spectra of ZnO nanoparticles show sponse Idc =I0exp(–t/t) where Ipc and Idc are the pho− apparently two peaks, the larger one in the UV region below tocurrent transient and dark current transient, I0 is the steady current for rise and decay time response, t is the time, and t 350 nm and the smaller one between 400–550 nm. The is the time constant. wavelength dependence of photocurrent is found very close In rise and decay time response curve of current, the trap to that of the absorption spectrum (Fig. 3). UV peak may be depths are calculated by peeling off the decay portion of the attributed to ZnO band gap energy transition [40]. At ener− curves into the possible number of exponentials. The expo− gies, lower than the band gap, the photosensitivity of ZnO r nentials are given by the equation I = I0exp(–pt) according NPs may be attributed to electronic transitions between to Bube model [38] where I0 is the current at the time when deep levels and conduction band which is evidenced by PL light is switched off, I is the photocurrent at any instant of spectra (Fig. 4). In this higher wavelength region, both exci− time, r is the exponent which is equal to 1 for rise and decay tation and quenching may take place and magnitude of the function and the p=Sexp(–E/kT), probability of escape of photocurrent would be the resultant value from both an electron from trap per second discussed in the theory of mechanisms. Randall and Wilkins. The trap depth E for different expo− nentials can be calculated by using the following equation 4. Conclusions é loge (II0 )ù EkT=-ê log S log ú We have studied photoluminescence and photoconductivity ë eet û characteristics of ZnO nanoparticles. ZnO nanoparticles where E denotes the trap depth, k is the Boltzman constant with particle size of 20–50 nm and hexagonal wurtzite (1.381×10–23 J/K), T is the absolute temperature, and S is the structure were synthesized by a hydrothermal method. The frequency factor or attempt to escape frequency that is of the photoconductivity measurements of ZnO NPs were per− order of 1013 Hz at room temperature [35,39]. The varia− formed under UV−visible illumination. It shows that photo− tions in S give significant results of trap depth energy as response under UV illumination is three times better as indicated in PL emission spectra (Fig. 5) and found from compared to visible illumination. The wavelength depend− calculated photoconductivity rise and decay time spectra ence of the photocurrent is found very close to that of (Fig. 8), where assumed S =1013 Hz would result in a trap absorption spectrum. The photocurrent with voltage and depth for ZnO NPs located at 0.96 eV vs. the value of 0.72 intensity of illumination follows the power law. Slow pho− eV calculated for S = 109 Hz and 0.83 eV for 1011 Hz. toconductive response under visible illumination may be attributed to the presence of traps. The trap depth is found to 3.5.5. Spectral response studies be of 0.96 eV. The defect level responsible for the green Figure 9 shows the spectral response of ZnO nanoparticles band emission is located at 2.35 eV below the conduction on excitation energy (366–691 nm) of light. The absorption band in ZnO which is close to calculated trap depth from photoconductivity time spectra. Further investigations are needed to establish the accurate model for the trap levels responsible for slow photoconductive response and their relationship to defect levels responsible for the green band emission in ZnO. The present work suggests very promising device applications of ZnO NPs in devices, such as UV radiation detector, solar cells, photoconductor and sensors, etc.

References

1. Y. Wang and N. Herron, “Quantum size effects on the exciton energy of CdS clusters”, Phys. Rev. B42, 7253–7255 (1990). 2. J. Nanda, B.A. Kuruvilla, and D.D. Sarma, “Photoelectron spectroscopic study of CdS nanocrystallites”, Phys. Rev. B59, 7473–7479 (1999). 3. L.E. Brus, “Electron−electron and electron−hole interactions in small semiconductor crystallites−the size dependence of Fig. 9. Variation of photocurrent as a function of wavelength for the lowest excited electronic state”, J. Chem. Phys. 80, 30 V at room temperature. 4403–4409 (1984).

472 Opto−Electron. Rev., 18, no. 4, 2010 © 2010 SEP, Warsaw 4. S. Sapra and D.D. Sarma, “Evolution of the electronic struc− 23. J. Zhang, L.D. Sun, J.L. Yin, H.L. Su, C.S. Liao, and C.H. ture with size in II−VI semiconductor nanocrystals”, Phys. Yan, “Control of ZnO morphology via a simple solution Rev. B69, 125304 (2004). route”, Chem. Mater. 14, 4172−4177 (2002). 5. R.H. Bube, Photoconductivity of Solid, Wiley, Newyork, 24. K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, 1960. J.A. Voigt, and B.E. Gnade, “Mechanisms behind green pho− 6. T.K. Gupta, “Application of zinc oxide varistors”, J. Am. toluminescence in ZnO phosphor powders”, J. Appl. Phys. Ceram. Soc. 73, 1817–1839 (1990). 79, 7983 (1996). 7. D.C. Look, “Recent advances in ZnO materials and devices”, 25. X.M. Fan, J.S. Lian, L. Zhao, and Y. Liu, “Single violet lu− Mater. Sci. Eng. B80, 383–387 (2001). minescence emitted from ZnO films obtained by oxidation of 8. Y. Natsume, H. Sakata, T. Hirayama, and H. Yanagida, Zn film on quartz glass”, Appl. Surf. Sci. 252, 420–424 “Low−temperature conductivity of ZnO films prepared by (2005). 72 chemical vapour deposition”, J. Appl. Phys. , 4203–4207 26. T. Tatsumi, M. Fujita, N. Kawamoto, M. Sasajima, and Y. (1992). Horikoshi, “Intrinsic defects in ZnO films grown by molecu− 9. T. Okamura, Y. Seki, S. Nagakary, and H. Okushi, “Prepara− lar beam epitaxy”, Jpn. J. Appl. Phys. 43, 2602–2606 (2004). tion of n−ZnO/p−Si heterojunction by sol−gel process”, Jpn. 27. J. Wang and L. Gao, “Synthesis of uniform rod−like, multi− J. Appl. Phys. 31, L762–L764 (1992). −pod−like ZnO whiskers and their photoluminescence prop− 10. J. Aranovich, A. Ortiz, and R.H. Bube, “Optical and electri− erties”, J. Cryst. Growth 262, 290–294 (2004). cal properties of ZnO prepared by spray pyrolysis for solar cell applications”, J. Vac. Sci. Technol. 16, 994 (1979). 28. R. Dingle, “Luminescent transitions associated with divalent 11. R.S. Yadav and A.C. Pandey, “Small angle X−ray scattering copper impurities and the green emission from semiconduct− and photoluminescence study of ZnO nanoparticles synthe− ing zinc−oxide”, Phys. Rev. Lett. 23, 579–581 (1969). sized by hydrothermal process”, J. Exp. Nanosci. 2, 177–182 29. Y.W. Heo, D.P. Norton, and S.J. Pearton, “Origin of green (2007). luminescence in ZnO thin film grown by molecular−beam 12. R.S. Yadav, A.C. Pandey, and S.S. Sanjay, “ZnO porous epitaxy”, J. Appl. Phys. 98, 073502 (2005). structures synthesized by CTAB−assisted hydrothermal pro− 30. T.E. Murphy, K. Moazzami, and J.D. Phillips, “Trap−related cess”, Struct. Chem. 18, 1001 (2007). photoconductivity in ZnO epilayers”, J. Electron. Mater. 35, 13. R.S. Yadav, R. Mishra, and A.C. Pandey, “Particle size dis− 543–549 (2006). tribution study by small−angle X−ray scattering technique 31. B. Lin, Z. Fu, and Y. Jia, “Green luminescent center in and photoluminescence property of ZnO nanoparticles”, J. undoped zinc oxide films deposited on silicon substrates”, Exp. Nanosci. 4, 139 (2009). Appl. Phys. Lett. 79, 943–945 (2001). 14. F.H. Nicoll, “Ultraviolet ZnO laser pumped by an electron 32. F. Wen, W. Li, J. Moon, and J. Kima, “Hydrothermal synthe− beam”, Appl. Phys. Lett. 9, 13 (1966). sis of ZnO:Zn with green emission at low temperature with 15. S. Mridha and D. Basak, “Thickness dependent photocon− reduction process”, Solid State Commun. 135, 34–37 (2005). ducting properties of ZnO films”, Chem. Phys. Lett. 427, 33. R.W. Smith and A. Rose, “Space−charge−limited currents in 62–66 (2006). single crystals of cadmium sulfide”, Phys. Rev. 97, 1531–1537 16. M.J.H. Henseler, W.C.T. Lee, P. Miller, S.M. Durbin, and (1955). R.J. Reeves, “Optical and photoelectrical properties of ZnO 34. A. Rose, R.G.A Review 12, 362 (1951). thin films and the effects of annealing”, J. Cryst. Growth 35. S. Bhushan and D. Diwan, “Photoconductivity of ZnO phos− 287, 48–53 (2006). phors”, Natl. Acad. Sci. Lett. 7, 12 (1984). 17. P. Sharma, K. Sreenivas, and K.V. Rao, “Analysis of ultravi− 36. H. Meier, Organic : Dark and Photoconduc− olet photoconductivity in ZnO films prepared by unbalanced tivity of Organic Solids, Weinheim, Chemie GmbH, 1974. magnetron sputtering”, J. Appl. Phys. 93, 3963–3970 (2003). 37. P.K.C. Pillai, N. Schroff, N.N. Kumar, and A.K. Tripathi, 18. O. Harnack, C. Pacholski, H. Weller, A. Yasuda, and J.M. “Photoconductivity and dark−conductivity studies of Wessels, “Rectifying behavior of electrically aligned ZnO CdS Se (Cu) sintered layers”, Phys. Rev. B32, 8228–8233 nanorods”, Nanoletters 3, 1097–1101 (2003). 1–x x (1985). 19. H. Kind, H. Yan, B. Messer, M. Law, and P. Yang, “Nano− Photoconductivity of Solids wire ultraviolet and optical switches”, Adv. 38. R.H. Bube, , John Wiley, New Mater. 14, 158–160 (2002). York, 404, 1967. 20. J.B. Baxter and C.A. Schmuttenmaer, “Conductivity of ZnO 39. K. Moazzami, T.E. Murphy, S.P. Phillips, M.C.K. Cheung, nanowires, nanoparticles, and thin films using time−resolved and A.N. Cartwright, “Sub−bandgap photoconductivity in terahertz spectroscopy”, J. Phys. Chem. B110, 25229–25239 ZnO epilayers and extraction of trap density spectra”, Semi− (2006). cond. Sci. Tech. 21, 717–723 (2006). 21. S. Devi and S.G. Prakash, “Photoconductivity of (ZnO−CdO) 40. J. Carry, H. Carrere, M.L. Kahn, B. Chaudret, X. Marie, and mixed lattices”, Natl. Acad. Sci. Lett. (India) 13, 35 (1990). M. Respaud, “Photoconductivity of self−assembled ZnO na− 22. R.K. Srivastava and S.G. Prakash, “Photoconductivity and noparticles synthesized by organometallic chemistry”, Semi− dark conductivity of CdS−Se mixed lattice”, Natl. Acad. Sci. cond. Sci. Tech. 23, 025003 (2008). Lett. 30, 11–12 (2007).

Opto−Electron. Rev., 18, no. 4, 2010 S.K. Mishra 473