Chemical Synthesis of ZnS:Cu Nanosheets

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1 Chemical Synthesis of ZnS:Cu Nanosheets Bhaskarjyoti Bodo and P. K. Kalita Nanoscience Research Laboratory, Department of Physics, Guwahati College, Guwahati, Assam, Pin , India Abstract. ZnS thin films are synthesized through chemical bath deposition (CBD) technique from aqueous solution of ZnSO 4 and thiourea mixing in equal volume and equimolar ratio. A 1% CuSO 4 solution is mixed with the ZnSO 4 solution for doping before the final chemical reaction. SEM image shows the formation of mainly nanosheets, teeth and comb like structures. Absorption studies show red shift of enhanced band gap on Cu doping. Photoluminescence of ZnS:Cu reveals the enhancement of blue luminescence at 468nm and low intensity green emission at 493nm which is attributed to more Cu 2+ lying in the interstices. XRD shows that the prepared ZnS nanophosphors possess cubic zinc blende structures. Keywords: ZnS, Nanosheet, Chemical Synthesis, Doping. PACS: Kn INTRODUCTION: ZnS is a promising luminescent material among II-VI group which is used widely in photoluminescence (PL), electroluminescence (EL) devices and optical sensor because of its better chemical stability compared to other chalcogenides such as ZnSe. Doping on ZnS with transitional metal ions like Mn 2+ and Cu 2+ is an important aspects to yield different nanostructures.[1-6] Formation of nanostructures depend not only the metal ions but also on its concentration [ 3 ]. Doping type as well as on the sulfur containing ligands play the key role for particular luminescence emission with enhanced efficiency [2-3]. ZnS:Cu have the ability to produce multicolor emission such as blue, green even red depending upon the Cu ions lying in the interstices or substitute Zn 2+ ions in the lattice. Since the density of state (DOS) is greatly influenced corresponding to the shape of nanoparticles it will inherently effects the transition probabilities in ZnS:Cu and may cause different interionic emission. Hence more research on the copper doped ZnS nanoparticle is desirable to understand its luminescence characteristics. In this paper the chemical synthesis of ZnS:Cu nanosheet and its effect on optical properties has been presented. EXPERIMENTAL: Synthesis: Zinc sulphide nanoparticles were grown on glass substrate as their films as well as colloidal particles through chemical bath deposition (CBD) techniques at room 31

2 temperature. Equivolume and equimolar (0.5M) solution of Zinc sulphate dehydrate and thiourea was prepared by dissolving in deionised water. For copper doping similarly a cupric sulphate solution of molar concentration 0.005M was prepared. The mixture of two salts was the mixed with 3% solution of poly-vinyl Alcohol (PVA) and stirred. Ammonia was added to it until formation of clears metallic complex. The p H was kept at Then thiourea solution was allowed to mix with the neutral complex solution which yield pure ZnS and ZnS:Cu nanoparticles due to the chemical reaction between Zn metallic complex and sulfur ions. The glass substrate were cleaned and dried with a procedure of synthesis prior to deposition and dipped into the final matrix solution to cast films. Characterization: The structures of the undoped ZnS and ZnS:Cu were investigated using X-ray powder diffractometer (Model: Seifert XRD 3003 T/T) with CuK radiation ( = nm) scanning 2 in the range Microstructure was studied with Scanning Electron Microscope (SEM) [Model: LEO 1430VP] operated with an acceleration potential of 15kV to 20 kv. The UV-Visible absorption of the samples was recorded using an automated spectrometer (Model: HITACI ) in the wavelength range 200nm -800nm. PL emission spectra were measured with excitation wavelength 325nm using AMINC BOWMAN series-2 luminescence spectrometer [Model: A 357]. RESULTS AND DISCUSSION: Structural Studies: The structural analyses of the synthesized materials were done from the XRD and SEM measurement. The XRD traces of undoped and Cu doped ZnS thin films are shown in Fig.1. XRD traces show the samples possess mainly cubic zinc blende type structure. The most prominent planes are oriented along (200) and (222) for both samples. The others diffraction planes are (111) and (400) for undoped ZnS whereas the additional (220) reflection becomes noticeable for the Cu doped ZnS films. This was confirmed with the standard JCPDS data [File No: ]. However hexagonal trace of the peak (102) is also visible in both samples. On close observation it is found that the peak (222) is not symmetrical and but is broaden sufficiently. This broadening can be attributed to the overlapping of (201) plane of hexagonal phase. The existence of wurtzite and zinc blende structure plays an important role in controlling the shapes of ZnS nanostructure [2-4]. This is reflected in the Cu doped ZnS films. However hexagonal trace of (102) peak is also visible in both samples. The SEM image of ZnS:Cu as shown in the Fig.2.show the formation of mainly nanosheets along with some other viz : nanobelts, teeth and comb like structures. The growth of nanosheets are likely in the (200) direction of zinc blende structure whereas other structures are attributed to the growth competition in different directions of hexagonal phase. Formation of nanoflakes, dots and nanosheets on copper doping in 32

3 FIGURE 1. XRD PATTERN OF (a) UNDOPED ZnS AND (b) ZnS:Cu FIGURE 2. SEM IMAGE OF ZnS:Cu ZnS are also reported [1,2,4 ]. G H Yue et al synthesized single crystalline ZnS nanosheet prepared through single solvothermal method [ 7 ]. A relatively broad peak at (2) shows the presence of CuS in the doped ZnS. The broad hump around is attributed to the oxidation i.e. the formation of ZnO phase. The possibility of formation of ZnO is often occurred in CBD technique because of abundance of dissolved oxygen in the precursor solution and atmospheric oxygen. The effect of oxidation is also supported by the absorption study. The lattice constant calculated for doped and undoped samples are and respectively. A slight decrease in different lattice parameters is owing to the difference of ionic radius of copper and zinc. The peak broadening obviously exhibits the characteristics of nanosized materials. Crystal size is estimated using Scherrer formula which is found to be 5nm and 7nm for undoped ZnS and Cu doped ZnS films respectively. The SEM micrographs show a well distribution of sheet like structure. Sheets are not uniform in size. Most of long rectangular types of several micrometers have the average width 4m. Larger sheet of average area 20 m x30m is also formed. A closed observation on selected areas reveals that the edges of some sheets grow to form teeth and comb like shape. As the sheets are not flat, parallel to substrate, its thickness can be measured and found to be nearly 1m.The width of the teeth is estimated in the range 1-5 m. The formation mechanism of nanosheets is dependent upon the ZnS lattice cell stacked together. The tetrahedral in the crystal fells stacks according to the bonding force of atom in the interface. The corner atoms of a tetrahedron have the strongest force compared with those atoms at other positions. Hence each tetrahedron grows corresponding to that favourable growth direction (001) axis that usually follow in wurtzite type ZnS crystal. In zinc blende, the growth direction may be in (200) direction as in the present work. By Xiang Wu et al reported in TEM studies that cubic ZnS nanobelts may show hexagonal domain [8]. Optical Properties: The UV_Visible absorption spectra of undoped ZnS and Cu doped ZnS are shown in Fig.3. The as-prepared ZnS exhibits a sharp absorption peak at 290 nm whereas the 33

4 Cu doped ZnS shows the absorption at 296nm. Bulk ZnS has the absorption peak at 350nm corresponding to band gap 3.6eV. Hence the synthesized material shows a considerable blue shift because of quantum confinement of nanoparticles. The band gaps increases due to the quantum size effect. It is observed that absorption edge for copper doped ZnS is red shifted with respect to undoped ZnS. Red shift causes a decrease of band gap in ZnS nanophosphor. Similar result is also found by other workers [4, 6]. Broad absorption in edge peak in the UV region suggests that the incorporation of oxygen in the samples. The room temperature PL spectra are recorded with 325 nm excitation wavelength. Fig. 4 shows the respective PL curves for undoped ZnS and ZnS:Cu nanoparticles. Luminescence peaks arise in semiconductor nanostructures due to excitonic and trapped emissions. Although surface defects reduce excitonic emission via nonradiative surface recombination in nanoparticles, there are few reports of excitonic emission in ZnS nanoparticles due to the prevalence of trap-state emission [3, 5, 9]. In the present work slight increase in particle size is not noticeable but the incorporation of copper atoms in ZnS may act as killing centers in the form of CuS for excitonic emission. Another possibility is the band gap luminescence of nanosized ZnO whose presence is confirmed from the XRD and UV_Visible study. The prominent luminescence peak is found at 398nm for both ZnS and ZnS:Cu. This is generally found owing to the self activated Zn interstitials or sulfur vacancies. The luminescence peak at 421 nm & 432 nm for undoped ZnS are also attributed to the inherent defects of sulfur vacancies [2,5,9 ]. On doping these emissions are disappeared compared to other luminescence peaks. It is observed that there exists a series of four peaks in blue green emission region in both ZnS and ZnS: Cu nanoparticles. The luminescence peaks at 451nm and 455nm of undoped are due to the stoichimetric defects of sulfur vacancies. On Cu doping the luminescent at 451nm becomes prominent when the 455nm peak diminished. Another blue emission at 468 nm along with a small side peak at 472nm is also observed in undoped sample. This peak at 468nm becomes prominent blue luminescence center in ZnS: Cu while the other two side peaks at 482nm and 493 nm also appeared. The peak at 493 nm is assigned to the near green emission due to Cu + dopant. FIGURE 3. UV-Vis. SPECTRA OF (a) UNDOPED ZnS AND (b) ZnS:Cu FIGURE 4. PL EMISSION SPECTRA OF (a)undoped ZNS AND (b)zns:cu 34

5 Luminescence at 468nm, 472nm and 482nm also arise due to the recombination of carriers between Zn 2+ vacancy acceptors and conduction band. The green emission arises from the recombination between the shallow donor level centered by sulfur vacancy and t 2 energy level (d 6 ) of Cu 2+ ions that are embedded in ZnS matrix and substrate Zn 2+ ions.the overall PL intensity is found to increase for ZnS:Cu over undoped ZnS and enhance the blue luminescence. Formation of ZnS: Cu is quite complex as CuS nucleus may exist because of their less solubility than ZnS. During the chemical synthesis of ZnS: Cu some of core shell type structure particles CuS covered with ZnS will be also formed. However when the reaction of the system is stabilized CuS is expected to diffuse slowly and recognized by the formation of ZnS:Cu nanoparticles in ends [ 3-7 ]. It is reported that in the nanosized ZnS:Cu system, the copper core forms blue centre before green one especially for low copper concentration. Blue emission is corresponding to the Cu 2+ ions lying in the interstices of ZnS lattice where a green centers appears when it substitutes Zn ions in the lattice [5, 6]. The Cu 2+ ions may lie in the interstices of ZnS lattice and thereby introduce more defect states. Therefore the defect related blue emissions are enhanced after doping. Appearing of relatively low intensity green luminescence are owing to the lesser copper ions substituting Zn ions than the CuS adequacy of nuclei. CONCLUSION: Chemical bath deposited ZnS and copper doped ZnS possess predominantly cubic zinc-blende type structure. However weak wurtzite phase also exists. SEM image shows of growth of nanosheets preferably along (200) of cubic structure. A close observation on selected areas reveals the co-existence of other type viz: nanobelt, teeth and comb like structures. This is attributed to the secondary growth competition along different favourable directions of cubic and hexagonal domains. The absorption spectra show the sufficient blue shift for both undoped and doped ZnS. The absorption edge is red shifted on Cu doping. The PL spectra of ZnS show an enhancement of blue emissions on copper doping with an additional weak green emission at 493nm. The overall PL intensity increases after doping and the blue emission at 468nm become prominent. This may be due to the emission from large surface states formed in nanosheets along with Cu + ions in the interstices. The weak green emission is because of less Cu + ions incorporated in the lattice site. ACKNOWLEDGMENTS The authors sincerely thank the CIF and Department of Physics, IIT, Guwahati -39 and Department of Chemistry, Gauhati University, Guwahati-13 for providing the facilities for characterization. REFERENCES 1. Daniel Moore and Hong L Wang, J. Mater.Chem 16, (2007). 2. Alvaro A. A. de Queroz, Mayler, Martin Demetrio, A.W. Soares, and Ecio J Franca, J. Molecular Structure 873, (2008). 35

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