STUDIES ON ZnS - CuS NANOPARTICLE SYSTEM.

CHAPTER - VI STUDIES ON ZnS - CuS NANOPARTICLE SYSTEM. 6.1 INTRODUCTION ZnS is an important direct band gap semiconductor. It has a band gap energy of 3.6 ev[1], displays a high refractive index (2.37) [2] and a high transmittance in the visible range [3,4] making this material a strong candidate for use in optoelectronic devices. CuS is also an important semiconductor that has a direct band gap of 2.5eV for bulk hexagonal wurtzite structure [5]. Zn x Cu 1-x S nanoparticle system can have the applications of both ZnS and CuS. In the present study Zn x Cu 1-x S, x=1,.8,.6,.4 and.2 nanoparticles were synthesized by solvothermal method. The as-synthesized nanoparticles were characterized by X ray diffraction to study the crystal structure and size. Their optical properties were studied using UV-VIS spectra and PL spectra and electrical properties using resistance measurements. The band gap energies obtained from resistance measurements are compared with the band gap energies from UV-VIS spectra. 6.2 SYNTHESIS Analytical Reagent (A.R) grade zinc acetate, copper (11) acetate and thiourea along with ethylene glycol were used for the preparation of nanocrystals Zn x Cu 1-x S. The amount of precursor materials taken are given in Table 6.1. 122

Table 6.1 Expected composition Amount of precursors(in gram) Zinc acetate copper(11) acetate Thiourea ZnS 10.98 --- 11.42 Zn.8 Cu.2 S 8.78 2 11.42 Zn.6 Cu.4 S 6.59 3.99 11.42 Zn.4 Cu.6 S 4.39 5.99 11.42 Zn.2 Cu.8 S 2.2 7.99 11.42 The samples were finally annealed at about 50 0 C for one hour to get phase pure nanoparticles. The colour of the above samples starts from pure white for ZnS and ends with black for Zn..2 Cu.8 S. 6.3 STRUCTURAL STUDIES X-ray diffraction (XRD) patterns of the samples were recorded on a Philips model PW-1830 X- 1.788A 0 in the range of scanning angles 10 0-80 0. Fig 6.1a shows the XRD pattern of ZnS nanoparticles synthesized by solvothermal method. 123

Fig 6.1a: XRD pattern of zinc sulfide nanoparticles Broad and diffused pattern of XRD lines are indicative of the small size of the ZnS nanoparticles and this pattern confirmed the formation of cubic ZnS, which matches with the standard JCPDS file (5-0566) [6-8]. Fig 6.1b shows the Gaussian fitted (1 1 1) peak of the zinc sulfide nanoparticles. Fig 6.1b: Gaussian fitted (1 1 1) peak of the ZnS nanoparticles. 124

The grain size calculated is 2nm. The SEM image of the powdered sample (fig 6.1c) shows the uniform distribution of ZnS nanoparticles. AFM image (fig 6.1d) confirms the nano size of the particles. Fig 6.1c: SEM image of zinc sulfide nanoparticles. Fig 6.1d: AFM image of zinc sulfide nanoparticles. Fig 6.2a shows the XRD pattern of Zn.8 Cu.2 S nanoparticles synthesized by solvothermal method. The XRD pattern exhibits prominent broad peaks at 33.4 0, 57.4 0 and 68 0 which could be indexed to scattering from (1 1 1), (2 2 0), (3 1 1) planes of cubic phase. Broadening of the peaks indicates the nano crystalline 125

nature of the material and the crystalline size is 2.6nm. Fig 6.2b shows the SEM image of the Zn.8 Cu.2 S nanoparticles Fig 6.2a: XRD pattern of Zn.8 Cu.2 S nanoparticles Fig 6.2b: SEM image of Zn.8 Cu.2 S nanoparticles Fig 6.3a shows the XRD pattern of Zn.6 Cu.4 S nanoparticles synthesized by solvothermal method. It can be indexed with hexagonal structure with lattice parameters a=3.8 Å and c=17 Å.The grain size calculated is 10.8 nm. The SEM 126

image of the powdered sample (fig 6.3b) shows the uniform distribution of Zn.6 Cu.4 S nanoparticles. Fig 6.3a: XRD pattern of Zn.6 Cu.4 S nanoparticles Fig 6.3b: SEM image of Zn.6 Cu.4 S nanoparticles Fig 6.4a shows the XRD pattern of Zn.4 Cu.6 S nanoparticles synthesized by solvothermal method. It can be indexed with hexagonal structure with lattice parameters a=3.85å and c=16å. The grain size calculated is 20nm. Fig 6.4b shows the SEM image of the Zn.4 Cu.6 S nanoparticles. 127

Fig 6.4a: XRD pattern of Zn.4 Cu.6 S nanoparticles] Fig 6.4b: SEM image of Zn.4 Cu.6 S nanoparticles Fig 6.5a shows the XRD pattern of Zn. 2 Cu.8 S nanoparticles synthesized by solvothermal method. It can be indexed with hexagonal structure with lattice parameters a=3.9 Å and c=16.5 Å. The grain size calculated is 21 nm. Fig 6.5b shows the SEM image of the Zn.2 Cu.8 S nanoparticles. 128

Fig 6.5a: XRD pattern of Zn.2 Cu.8 S nanoparticles Fig 6.5b: SEM image of Zn.2 Cu.8 S nanoparticles. SEM image of all the above samples shows the uniform distribution of nanoparticles and also confirm the nano size of the particles. The structure, lattice parameters and grain size for different composition are tabulated in table 6.2. 129

Table 6.2 Composition Structure Lattice Parameters Å Size Nm ZnS Cubic a=5.36 2 Zn.8 Cu.2 S Cubic a=5.26 2.6 Zn.6 Cu.4 S Hexagonal a=3.8, c=17 10.8 Zn.4 Cu.6 S Hexagonal a=3.85, c=16 20 Zn.2 Cu.8 S Hexagonal a=3.9, c=16.5 21 CuS(reported) Hexagonal a=3.8, c=16.4 From table 6.2, it is observed that the grain size increases as the content of copper sulfide is increased. 6.4 OPTICAL STUDIES Fig 6.6a shows the optical absorption spectra of ZnS nanoparticles. There is only one absorption band with edge around 350nm (3.5eV) which is in close agreement with the value of the bulk [1]. Optical absorption spectrum with the same absorption edge was also reported by J.F Xu and W.Ji for ZnS nanoparticles synthesized by ultrasonic radiation method [9]. PL spectrum of the zinc sulfide nanoparticles (fig 6.6b) consists of two emission bands. One band (460nm-490nm) with peak around 475nm and another one having peak around 515 nm exist. The emission peak observed at 475 nm is known as green emission peak. The emission peak at 515 nm is known as yellow green peak. Similar results have been reported by earlier workers [10, 11]. 130

Fig 6.6a: UV absorption spectrum of ZnS nanoparticles. Fig 6.6b: PL emission spectrum of ZnS nanoparticles.. 131

So ZnS nanoparticles synthesized by solvothermal method starts absorption at 350nm (3.5 ev) and starts emission from 460nm( 2.7eV). So there exists an energy level with energy around 0.8 ev above the valance band. Fig 6.6c shows the emission absorption energy level diagram. Fig 6.6c: Energy level diagram for absorption and emission of ZnS nanoparticles Fig 6.7a shows the optical absorption spectrum of Zn.8 Cu.2 S nanoparticles. There is only one absorption band with edge around 420nm. Fig 6.7b shows the PL emission spectra of the zinc sulfide nanoparticles. Peaks are observed around 475nm and 515nm. This observation is slightly different from that reported by Xu et al. [12]. In their work, the two emission bands are at 460 nm and 507 nm, respectively, in ZnS : CuS nanocrystals. 132

Fig 6.7a: UV absorption spectrum of Zn.8 Cu.2 S nanoparticles. Fig 6.7b: PL emission spectrum of Zn.8 Cu.2 S nanoparticles. Fig 6.8a shows the optical absorption spectra of Zn.6 Cu.4 S nanoparticles. There is one absorption band with edge around 560nm (2.3eV) and 133

another with edge around 900 nm (1.3eV). Fig 6.8b shows the PL emission spectra of the Zn.6 Cu.4 S nanoparticles. Peaks are observed around 480nm and 515nm. Intensity of the first peak is reduced and shifted as the composition of copper sulfide increased. Fig 6.8a: UV absorption spectrum of Zn.6 Cu.4 S nanoparticles. Fig 6.8b: PL emission spectrum of Zn.6 Cu.4 S nanoparticles. 134

Fig 6.9a shows the optical absorption spectra of Zn.4 Cu.6 S nanoparticles. There is one absorption band with edge around 590nm (2.0eV) and another with edge around 900 nm (1.3eV). Fig 6.9b shows the PL emission spectra of the Zn.4 Cu.6 S nanoparticles. The peaks are observed around 490nm and 520nm. Intensity of the first peak is further reduced and the second peak is slightly shifted as reported by W.Q.Peng and G.W.Cong [13]. Fig 6.9a: UV absorption spectrum of Zn.4 Cu.6 S nanoparticles. Fig 6.9b: PL emission spectrum of Zn.4 Cu.6 S nanoparticles. 135

Fig 6.10a shows the optical absorption spectra of Zn.2 Cu.8 S nanoparticles. There exists an absorption band from 380 nm to 900 nm. Absorption spectrum with sharp absorption edge disappeared as CuS content increases. Similar plots were also reported by Jun Zhang et al. [14]. PL emission spectra of Zn..2 Cu.8 S nanoparticles (fig 6.10b) exhibits only one peak around 520 nm similar to CuS nanoparticles [15]. Fig 6.10a: UV absorption spectrum of Zn.2 Cu.8 S nanoparticles Fig 6.10b: PL mission spectrum of Zn.2 Cu.8 S nanoparticles. 136

6.5 ELECTRICAL STUDIES The D.C electrical resistance of the pellet form of the samples were measured in the temperature range 310 K-485K. Fig 6.11a: Variation of resistance with temperature of ZnS nanoparticles Fig 6.11a shows the variation of resistance with temperature of zinc sulfide nanoparticles synthesized by solvothermal method. It consists of two regions. In the first region from 300K-420K it behaves as an insulator.there is no variation of resistance with temperature. Above 420K it has the semiconducting behavior. Zinc sulfide behaves as like it needs some thermal energy to push it from the insulating state to the semiconducting state, may be because of its high band gap energy. 137

Fig 6.11b: Experimental points and fitted curve for the semiconducting region of ZnS nanoparticles fitted to equation The measured resistances at various temperatures were R=R 0 T -3/2 exp ( E g /2kT)----------------(1) The best fit for the semiconducting region (430 K-485 K) was obtained for R 0 =1.42*10 9 and Eg= 0.73eV. The measured resistances and the curve for R=1.42*10 9 *T -3/2 exp(0.73/2kt) for 430K-485K is shown in fig 6.11b. Band gap energy 0.73 is in close agreement with the energy Ea (fig 6. 6c). 138

Fig 6.12: Variation of resistance with temperature of Zn.8 Cu.2 S nanoparticles. Fig 6.12 shows the variation of resistance with temperature of Zn.8 Cu.2 S nanoparticles synthesized by solvothermal method. There is no variation of resistance with temperature may be due to the insulating behaviour of zinc sulfide at lower temperature and at higher temperature due to the metallic behaviour of copper sulfide[16] and semi conducting behaviour of zinc sulfide. Fig 6.13a shows the variation of resistance with temperature of Zn.6 Cu.4 S nanoparticles synthesized by solvothermal method. It shows insulating behaviour up to 430K and then up to 470K due to large fluctuations resistance measurement becomes difficult, indicating that the sample undergoes some phase transition. 139

Fig 6.13a: Variation of resistance with temperature of Zn. 6 Cu.4 S nanoparticles A heavy jump in resistance from the order of 10 8 ohm to 10 3 ohm was also observed between 430K and 470K. Above 470K the resistance decreases with increase in temperature showing the semiconducting behaviour. The measured resistances at various temperatures in the semiconducting region were fitted to equation(1).the best fit for the semiconducting region (470 K-490 K) was obtained for R 0 = 7.8*10-5 and Eg=2.3eV. The measured resistances and the curve for R = 7.8 * 10-5 * T -3/2 exp (2.3/2kT) for 470K-490K is shown in fig 6.13b. The band gap energy 2.3 ev agrees with the first absorption edge (550nm) of the UV absorption spectrum of the same sample (fig 6.8a). 140

Fig 6.13b: Experimental points and fitted curve for semiconducting region of Zn.6 Cu.4 S nanoparticles Fig 6.14a: Variation of resistance with temperature of Zn. 4 Cu.6 S nanoparticles 141

Fig 6.14a shows the variation of resistance with temperature of Zn.4 Cu.6 S nanoparticles synthesized by solvothermal method. Up to 383K there is non uniform variation of resistance with temperature indicating that it is a region in the vicinity of phase transition. From 393K it shows the behaviour of a semiconductor. The semiconducting region can be fitted to band gap energy of 1.9 ev (fig 6.14b) which agrees with the first absorption edge of the UV absorption spectrum of the same sample(fig 6.9a). Fig 6.14b: Experimental points and fitted curve for semiconducting region of Zn.4 Cu.6 S nanoparticles Fig 6.15a shows the variation of resistance with temperature of Zn.2 Cu.8 S nanoparticles synthesized by solvothermal method. Up to 353K the sample behaves as a semiconductor. From 353K to 383K it shows the metallic behaviour of CuS [16] and above this temperature it again behaves as a semiconductor. The first semiconducting region can be fitted to band gap energy of 0.07eV (fig 6.15b) and the second semiconducting region can be fitted to band gap energy of 0.6eV 142

(fig 6.15c). So due to phase transition there is a jump in band gap energy from.07 ev to.6 ev. Fig 6.15a: Variation of resistance with temperature of Zn.2 Cu.8 S nanoparticles. Fig 6.15b: Variation of resistance with temperature. Experimental points and fitted curve for the first semiconducting region of Zn.2 Cu.8 S nanoparticles 143

Fig 6.15 c: Variation of resistance with temperature. Experimental points and fitted curve for the second semiconducting region of Zn.2 Cu.8 S nanoparticles The measured resistance at 313K for various composition using pellets of almost same size (circular pellet diameter = 13mm, distance between probes= 2mm ) are tabulated in table 6.3. Table 6.3 Composition Resistance(ohm) Zn.8 Cu.2 S 9,00,00,00,000 Zn.6 Cu.4 S 70,00,00,000 Zn.4 Cu.6 S 6,30,000 Zn.2 Cu.8 S 440 Starting from Zn. 8 Cu.2 S nanoparticles resistances of the above samples rapidly decreases with the increase in the content of CuS. 144

6.6 CONCLUSION 1. Zn x Cu 1-x S, x=1,.8,.6,.4 and.2 nanoparticles were synthesized by solvothermal method. 2. The structure lattice parameter and size of the nanoparticles obtained from XRD pattern depend on the value of x. 3. Very small size (2nm) of ZnS nanoparticles is also confirmed by SEM and AFM images. 4. It's noticed here from resistance measurements and from UV spectra that nanoparticles with UV spectrum having single sharp absorption edge do not undergo phase transition. 5. D.C electrical resistance measurement shows that up to 420K ZnS nanoparticles behave like an insulator and at higher temperatures it behaves as a semiconductor. 6. PL emission spectrum of the ZnS nanoparticles shows one high intense peak at 475 nm and a low intense peak at 515nm. As the content of CuS increases the intensity of the high intense peak decreases and disappeared. 7. Resistance measurement on Zn.6 Cu.4 S nanoparticles shows a phase transition above 450K, Zn.4 Cu.6 S above 380K and Zn.2 Cu.8 S above 380K. 8. After heavy oscillation Zn.6 Cu.4 S nanoparticles had a heavy jump from 10 8 ohm to 10 3 ohm at 470K. 9. The band gap energies obtained from resistance measurements agrees with that obtained from UV -VIS spectra. 145

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