Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots

Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots

Accepted Manuscript Title: Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots Author: Jumi Kakati ...

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Accepted Manuscript Title: Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots Author: Jumi Kakati Pranayee Datta PII: DOI: Reference:

S0030-4026(15)00360-5 http://dx.doi.org/doi:10.1016/j.ijleo.2015.05.054 IJLEO 55543

To appear in: Received date: Accepted date:

29-3-2014 13-5-2015

Please cite this article as: Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.05.054 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Schottky junction UV photodetector based on CdS and visible photodetector based on CdS:Cu quantum dots a a,b

Jumi Kakati,bPranayee Datta

Department of Electronics and Communication Technology, Gauhati University,Guwahati-14,Assam.

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Abstract: We report the synthesis of cadmium sulphide(CdS) quantum dots(QD),copper doped cadmium sulphide (CdS:Cu) quantum dots and silver(Ag) nanoparticles embedded in polyvinyl alcohol (PVA) and fabrication of nano CdS(undoped and doped)-Ag schottky barrier. Chemical route is followed for CdS,CdS:Cu and Ag nanoparticles synthesis. Sizes and shapes of CdS and Ag nanoparticles are obtained from UV-VIS,PL, XRD, SEM,TEM,HRTEM,SAED and composition from EDS. Nano scale devices are fabricated with ITO/TPD/CdS(undoped/doped) nanocomposites(NCs)/Ag nanoparticles/Al to study the schottky barrier characteristics. Current-voltage (I-V) characteristics of the as-fabricated devices are performed under dark and light environment which show the schottky barrier characteristic of the devices. The schottky junctions exhibit rectifying behavior in the dark. The as-fabricated nano scale schottky devices are found to be sensitive to light sources differing in wavelength. We have seen that the responsivity as well as external quantum efficiency (EQE) of the devices increase after doping. The photo response and EQE of the devices are found to increase with decrease in QD size. For CdS-Ag (undoped and doped) schottky devices EQE is higher in the UV region, whereas responsivity is higher for CdS-Ag schottky devices (undoped) in the UV region and for CdS-Ag schottky devices (doped) in the visible region. Keywords: Schottky barrier; CdS quantum dots; CdS: Cu quantum dots; Ag nanoparticles; Photodetector.

particle at a given frequency range when the excitonic frequencies lie near the band edge [6]. We have fabricated Ag NCs/CdS (CdS: Cu) quantum dots schottky junction and characterized for electrical and optical properties. We report the synthesis of CdS, CdS: Cu, silver (Ag) quantum dots and nanoparticles and the synthesized quantum dots and nanoparticles are used to fabricate nano schottky barrier device. The devices consist of a sandwich structure of glass, Indium tin oxide (ITO), N,N´-Bis(3 Methylphenyl)-N,N´-bis(phenyl) benzidine (TPD), PVA/CdS (and PVA/CdS: Cu) quantum dots, silver nanoparticle, and an upper Al contact. The main task of this study is to search for photo response of the fabricated device for light source, having different wavelengths.

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1. Introduction

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The increasing interest in solar absorption has created a demand for the characterization of the films of absorbing semiconducting materials in the visible range for their application in photovoltaic devices. The band gap Eg is the most important parameter in semiconductor physics. Cadmium chalcogenide materials have band gaps 1.4 eV < Eg < 2.4 eV and a reasonable overlap with the solar spectrum. The optical, electrical and structural studies are very much helpful in characterizing the material for its applications in PV devices. Schottky junction plays an important role in improving the electrical and optical properties of the PV devices. In 2000 S. Kumar et al, in 2008 S. Mishra, in 2011 Y. Liu et al, in 2012 Z. Z. Li et al, in 2012 P. Datta et al demonstrated the schottky junction of Ag with CdTe, CdS, ZnO; its characteristics and PV device application [1-5]. M. R. Singh et al demonstrated the optical switching nano-devices of metallic nanoparticle – quantum dot hybrid system ,where the photonic crystal allows to switch the absorption of the metal

2. Experimental details To prepare polyvinyl alcohol (PVA)/CdS nanocrystals, PVA solutions are prepared taking 3gms of PVA in 100ml of distilled water and stirred at different rate.CdCl2 and Na2S solutions are prepared

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XRD (using Bruker AXS D8 Advanced powder X-Ray Difractometer), TEM, HRTEM and SAED (JEOL JEM 2100),SEM (LEO 1430VF and GEOL GSM 6390 lg). Photocurrent is measured by KEITHLY 2612A System Source meter using 150W Xe lamp as white light source.

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3. Result and discussion

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Figure 1 shows the absorption spectra of CdS nanocrystal (a), CdS: Cu nanocrystal (b) and silver nanoparticles (c). It is clear that there is blue shift in case of as prepared CdS, absorption starts at ~ 330 nm and decreases with increasing wavelength. For the Cu doped CdS sample red shift is clear as absorption starts at ~350 nm. The red shift in the absorption peak is strongly associated with the doping levels of Cu ions in the lattice of CdS [7]. Band gap has been assessed by using the Tauc’s relation [8] hνα= A (hν-Ea) ½ (1) Where A is a constant, Ea is band gap of the nanoparticles. α is absorption coefficient is Planck’s constant, ν is the frequency of the incident radiation. For CdS samples band gap obtained is in the range of 2.46eV to 2.62eV and for CdS: Cu it is 2.43eV to 2.58eV. Fig 1(c) reveals surface Plasmon resonance of the fabricated silver sample. Occurence of single peak at ~500 nm indicates nanostructure of the assynthesized silver particles. Similar observation was reported by other workers for Ag nanoparticles [9, 10].

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taking 2gms of each in 100ml of distilled water and the solutions are stirred at 600c for half an hour. The as-prepared PVA and CdCl2 solutions are taken in the 2:1 ratio and the as-prepared Na2S solution is added drop-wise till the mixture turns orange, and stirred till it becomes yellow. For the fabrication of CdS:Cu , 2gms of CuCl2 is taken in 100ml of water and stirred at 600C. Taking PVA and CdCl2 solution in 2:1 ratio, 2 drops of as-prepared CuCl2 solution is added. To this solution Na2S solution is added drop-wise till the solution turns black and then stirred at 600C for certain time. Samples are prepared by varying rotation per minute (RPM), Concentration, Temperature and stirring rate Silver nanoparticles are fabricated following thermal reduction process. Silver nitrate (AgNO3, MERCK Specialties Pvt. LTD., Mumbai) and PVA of analytical grade purity are used without further purification.AgNO3 solution was prepared by dissolving AgNO3 powder in distilled water. The mixture is stirred at room temperature.3wt % PVA solutions are prepared in distilled water. The mixture is stirred at 700 C for 3 hrs. in magnetic stirrer. AgNO3 solution and PVA solution is then mixed in 1:1 ratio and the reaction mixture is heated to 9000C. The solution is then kept overnight for stabilization. The samples have been characterized by UV-VIS spectrophotometer (using HITACHI U3210 Double beam), photoluminescence spectrometer (F-2500 FL Spectrometer),

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Figure 1 : UV-VIS spectra for(a) CdS(S4) ,(b) CdS:Cu(S4D) and (c)silver nanoparticles

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Figure 2: Photoluminescence spectra for (a) CdS (S4) and (b) CdS: Cu (S4D)

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region in case of doped sample when excited by the wavelength of 390 nm, is due to band edge luminescence. Band edge emission may be due to low-lying dark states of the nanocrystal interior.

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Figure 2 (a) and (b) shows the PL curve of the undoped and doped CdS samples respectively. The PL peak occur at ~450 nm for the undoped sample where as the peak position shifts (~500nm) to red

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Figure 3: (a) XRD pattern of CdS and CdS: Cu (b) XRD pattern of silver nanoparticle

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In order to reveal the crystalline phase structure, CdS and CdS: Cu samples are characterized by XRD. XRD patterns taken are given in figure 3(a). The XRD pattern of the CdS nanocrystals show the (100), (101) and (102) planes at 2θ values 24.80, 28.10 and 36.80 respectively, which match those of the hexagonal wurtzite(JCPDS Card No. 41-1049) structure of CdS crystallite. CdS: Cu nanocrystals show (100) and (101) planes. The peak broadening in the XRD pattern clearly indicates that very small nanocrystals are present in the samples. From the width of the XRD peak

broadening, the mean crystalline size can be calculated using Debye-Scherrer formula [11]. It is found to be 9-15 nm for undoped samples and 6-13nm for doped samples. Figure 3(b) shows the XRD pattern of Ag nanoparticles. The XRD pattern of the Ag nanocrystals show the (111) and (200) planes at 2θ values 37.50 and 43.80 respectively. The peak position shows the samples to be Ag and the structure to be cubic (JCPDS Card No. 040783). Debye Scherrer formula [11] is used to get the size of the particles from full width at half maximum. The estimated size is 49.5nm.

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Figure 4: TEM of the (a) CdS (S4), (b) CdS: Cu (S4D) quantum dots (c) Ag nanocrystals

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Figure 4(a), (b) and (c) shows TEM images of CdS undoped, doped and Ag samples respectively. It is difficult to resolve the individual character of the nanocrystals in samples. It is seen that most of the nanocrystals are of hexagonal shapes for CdS quantum dots and cubic for Ag nanocrystals and start to self-assemble into ordered structures which is seen from the HRTEM results shown in figure 5(a) and 5(b) for CdS undoped and doped

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samples respectively. Most CdS quantum dots are ~10nm in size and Ag nanoparticle are ~60nm, as can be seen in figure. Results obtained from XRD are found to be almost in agreement with those from TEM and HRTEM. The CdS nanoparticle in Figure 5 is of hexagonal structure and can be appreciated from the observed fringe spacing of (100), (010) (3.60 Å) and (002) (3.36 Å).

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Figure 5.HRTEM images of (a) CdS and (b) CdS: Cu

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fabricated CdS and CdS: Cu nanocrystals and cubic phase of Ag nanocrystals.

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The SAED pattern (figure 6 (a), (b) and (c)) reveal hexagonal phase of the as

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Figure 6: SAED image of (a) CdS (S4), (b) CdS: Cu (S4), (c) Ag nanocrystals

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Figure 7: SEM image of (a) CdS, (b) CdS: Cu

used to fabricate nano device which is shown in figure 8. The device architecture is based on a photodiode principle of operation where photo generated carriers are separated in

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SEM image of the CdS nanocrystals (Figure 7(a),(b)) shows the nearly spherical shape of the particles. The size depicted by this picture is larger than that obtained from other characterization techniques. This is because SEM reveals only the topographical observations. The synthesized CdS, CdS: Cu quantum dots and silver nanoparticles are

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Figure 8. Fabricated nano device (a)with CdS and CdS: Cu NCs, (b) SEM image

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electrons are extracted through the Al contact and holes through ITO contact. The dark current and the photocurrent are shown as a function of bias applied at the electrodes. The data were taken using KEITHLEY 2612A System Source meter LXI. Figure 9 shows the current–voltage (I-V) curve of such devices with undoped and doped CdS NC in the dark and sunlight.

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to host media and transported to the respective conctacts. The active layer consists of a nanocomposite film of CdS (and CdS: Cu) nanocrystals. Upon photo excitation electron hole pairs created in the nanocrystals get separated and electrons are transported via silver nanocrystals towards the anode and holes are transported via hole transport layer towards the cathode. Under forward bias,

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Figure 9. I-V characteristics of (a) undoped NCs in light, (b) undoped NCs in dark, (c) doped NCs in light and (d) doped NCs in dark

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The I-V characteristics are measured in air immediately after the external leads are attached to the electrodes. Dark current measured is <10-7 A for both undoped. When the devices were exposed to sun light current increases in both the cases showing the diode characteristics. Figure 9(c) shows that the Cu doped CdS has higher current than the undoped CdS, though the doping concentration is very low. The difference is because the electron traps are filled by the electrochemical doping. Recombination rates in case of the doped nanocrystals are slow in room temperature compared to the undoped one for which the ratio of the photocurrent to the dark current increases [12-19]. Also,

electrons are localized close to the Fermi level in case of doped nanocrystals for which Fermi level moves near the conduction band compare to the undoped one [17]. For the application of same electric field in both undoped and doped devices, photocurrent is more in the doped device. In the device a Schottky junction is formed because the electron density of CdS nanocomposites used is moderate than that of a metal with higher work function (herein the Ag).Work function of Au (Φm) is 4.55 eV for [111] plane [13] and work function of n-type CdS (Φs) material is 4.2eV [13]. Electron affinity is determined using the relation [14]

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measured is < 10-10 Amp for both undoped and doped NSDs in the voltage range -15V to 15V. An on/off current ratio, when the voltage changes from +10 to -10V, is found to be ~1.5 for all the undoped devices and ~1.3 for all the doped devices. The turn-on voltage is around 0.2V. By fitting equation (7) to the measured I-V curve of figure 10, we obtain η which are given in table 1. ln(I)=qV/nkT+ln(Io) (7) These findings confirm that schottky barrier is formed. The reverse saturation current of a schottky junction is given by equation Io=A*T2exp(-qΦb/kT) (8) Ideality factor η calculated for undoped and doped devices are given in table 1.

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χ= Φm- Φb (3) Depth of the Fermi level below the CdS conduction band edge is given by Φn = (kT) ln(Nc/ne) (4) where Nc is the effective density of state in the conduction band (Nc~2.44x1018cm3)[15], ne is the electron density of CdS nanocomposites.The electron density of CdS nanocomposites has been calculated using the formula (5) ne= Nc exp (-eΦb/kT) A space charge region (i.e. a junction) accompanied by a built-in field gets formed in the semiconductor near the Ag/CdS (CdS: Cu) interface. The built in potential Vi of the Schottky junction can be obtained using the equation Vi= (Φm- Φs)/e= (Φm –χ- Φn)/e (6) The photovoltaic effect is based on an externally measurable potential across a metal- semiconductor junction in response to incident optical radiation. The junction provides a built-in electric field for separating excess electrons and holes generated through the absorption of light with photon energies higher than the bandgap energy of the semiconductor. When the devices are exposed to sun light, current increases showing the diode characteristics with photovoltaic property given in figure 9. The difference is because the electron traps are filled by the electrochemical doping. Recombination rates in case of the doped nanocrystals are slow at room temperature compared to the undoped one for which the ratio of the photocurrent to the dark current increases [17]. Also, electrons are localized close to the Fermi level in case of doped nanocrystals for which Fermi level moves near the conduction band compared to the undoped one[20]. For the application of same electric field in both undoped and doped devices, photocurrent is more in the doped device. The photoconductivity of the devices are found to decreases with increase in average QD size. The I-V curve in the dark shows rectification characteristics. Dark current

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(b) Figure 10. Fitted straight line for (a)NSD10 and (b)NSD10:Cu

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Table 1: Schottky barrier height and built in potential for the Ag/CdS(CdS:Cu) NSDs

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Average ILight/IDark 16.9 15.5 15 20 20 18.8 17 18 20 24

local electric field, which may reduce the electron-hole recombination rates, increase the free carrier density, and lower the barrier height [13,14,16,17,18,19]. The average dark current (light-off) and photocurrent (light-on) results in an Ilight/Idark ratio which is given in table 1.

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We have fabricated a sizeable number of this kind of Schottky junction photovoltaic devices using samples of different sizes and illuminated them with different wavelengths of light. We can see the shift in the photocurrent when the light is switched from off state to on state. It is due to the photon generated electrons and holes at the schottky junction separated by the

Built in potential in volts 0.09 0.004 0.00625 0.0063 0.0062 0.0062 0.0063 0.0062 0.006 0.006

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Schottky barrier height in volts 0.94 0.95 0.92 0.922 0.92 0.91 0.92 0.91 0.9 0.9

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Nano Schottky Devices NSD6 NSD7 NSD8 NSD9 NSD10 NSD6D NSD7D NSD8D NSD9D NSD10D

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Figure 10. Device characteristics for different wavelength of same intensity at voltage v=5V (a) CdS/Ag NSDs (b) CdS:Cu/Ag NSDs

Figure 11 gives EQE vs λ [13, 14,16,17,18,19] of the undoped and doped devices. We get the maximum EQE of

39.8 10-9 % for CdS/Ag NSD and 63 109 % for CdS:Cu/Ag NSD for the available range of wavelength of the xenon source.

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We get the maximum EQE for CdS/Ag NSD is 39.8 10-9 % and 63 10-9 % for CdS: Cu/Ag NSD at λ=250nm. These results show that Cu doping has broadened the spectral range of the photocurrent response and improved the photoconductivity of CdS nancomposites. The device responsivity [13, 14] is a function of wavelength and it is higher for the doped samples compared to their undoped counterparts (figure 5.18). Responsivity increases with increase in λ for lower λ and then decreases with λ. Internal photoemission which is usually described by an escape cone model is more efficient at lower wavelengths, with the free-carrier absorption expected to be proportional to λ2 causing the responsivity to increase as wavelength increases. At long wavelengths, the free-carrier absorption is independent of the wavelength and the responsivity decreases with increasing wavelength due to the decreasing internal photoemission [6,12,13,17].

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Figure 11. Comparisons of external quantum efficiencies of the CdS/Ag and CdS:Cu/Ag NSDs with different size of quantum dots.

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Figure 11. Responsivity of the devices versus wavelengths of light (a)NSD6-10 and (b) NSD610:Cu

Figure 12(a) and (b) shows the visible photoresponse characteristics of undoped and doped devices respectively. The maximum photocurrent of the undoped device is 17 nAmp , while that of the doped device reaches 35 nAmp. The doped device enhances the visible photosensitivity.

Figure 12. Visible photoresponse characteristics for (a) NSD6 , (b) NSD6:Cu

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The characteristics of the photoconductive CdS quantum devices suggest that they are good candidates for optoelectronic

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O 2(g) + e− → O2 (ad)

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Acknowledgement: We would like to acknowledge the department of Chemistry, G.U, for providing PL and UV-VIS facilities and also the facilities for synthesis of Au nanoparticles and IIT, Guwahati for providing XRD, TEM, SEM, HRTEM, TEM and SAED facilities.

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Since the thickness of the film is small, usually a few tens of nm, the quantum dots are probably almost depleted of carriers [26], leading to a high resistance in the dark state. In the illuminated state, the absorption of light with energy greater than the band gap will generate not only electrons in the conduction band that increase free carrier densities and consequent conductivity of CdS quantum dots, but also holes with an equal density that can recombine with adsorbed/ chemisorbed oxygen ions at the dot surface and discharge O2 molecules:

to exhibit Schottky barrier characteristics and can be used as photodetector in the UV-VIS range. For CdS-Ag (undoped and doped) schottky devices EQE is higher in the UV region, whereas responsivity is higher for CdS-Ag schottky devices (undoped) in the UV region and for CdSAg schottky devices (doped) in the visible region. By choosing a suitable size of CdS quantum dots as well as Ag nanoparticles, we can enhance the performance of the device. Also use of conducting matrix may be expected to enhance the photodetector efficiency.

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switches, with the dark insulatingstate as “OFF” and the light-exposed conducting state as “ON”. The photoconductivity mechanism of polycrystalline CdS films has been studied previously by many researchers [20, 21], of which a complex process of electronhole generation, trapping, and recombination within CdS is involved[22]. Here we suggest that a similar mechanism would be applicable in this work. First of all, as a common feature of metal chalcogenide materials, the presence of oxygen on quantum dot surface is notable. The oxygen molecules adsorb on the dot surface as negatively charged ions by capturing free electrons from the n-type CdS, thereby creating a depletion layer with low conductivity near the dot surface [23,24,25]:

O2(ad) + h+ → O2(g).

Thus, the trapped electrons are released into the conduction band and also increase the conductivity of the quantum dots. This photoinduced conductivity changes allow us to reversibly switch the quantum dots between OFF and ON states.

References: [1] S. Kumara, S.K. Sharmaa, T.P. Sharmaa, M. Husain, Journal of Physics and Chemistry of Solids, 61, 1809-1813 (2000). [2] Shounak Mishra, Master's University of Kentucky, 2008.

Theses,

[3] Ying Liu, Qing Yang, Yan Zhang, Zongyin Yang, and Zhong Lin Wang, Adv. Mater. (2012). [4] Zhi Z Li, Guanzhong G Wang, Qianhui Q Yang, Zhibin Z Shao, Yang Y Wang, Nanoscale Research Letters, 7, 316(2012) . [5] Rhituraj Saikia, P.K.Kalita, P Datta, International Journal of Chemical Science and Technology (2012). [6] Ali Hatef, Seyed M Sadeghi and Mahi R Singh, Nanotechnology 23, 065701 (2012). [7] Aiwei Tang, Luoxin Yi,Wei Han, Feng

4. Conclusion In summary, both CdS/Ag undoped and doped nanocomposite NSDs are found

Teng,Yongsheng Wang,Yanbing Hou,and Mingyuan Gao, Appl.Phys.Lett.97 , 033112 (2010), .

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[14] Sandhya Gupta, Dinesh Patidar, N.S.Saxena, Kananbala Sharma, Chalcogenide Letters, 12 ,723 (2009) .

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[13] O. M. Osiele and O. Olubosede, Journal of Nigerian Association of Mathematical Physics., 11, 445 (2007).

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