Elsevier

Materials Research Bulletin

Volume 81, September 2016, Pages 127-133
Materials Research Bulletin

Effect of structure, size and copper doping on the luminescence properties of ZnS

https://doi.org/10.1016/j.materresbull.2016.05.010Get rights and content

Highlights

  • Blue and green emission intensity form ZnS is sensitive to crystallographic form.

  • For ZnS nanoparticles, emission characteristics are not affected by copper doping.

  • Cu solubility poor in ZnS nanoparticles compared to corresponding bulk.

Abstract

Luminescence properties of wurtzite and cubic forms of bulk ZnS have been investigated in detail and compared with that of ZnS nanoparticles. Blue emission observed in both hexagonal and cubic forms of undoped bulk ZnS is explained based on electron–hole recombination involving electron in conduction band and hole trapped in Zn2+ vacancies where as green emission arises due to electron hole recombination from Zn2+ and S2− vacancies. Conversion of wurtzite form to cubic form is associated with relative increase in intensity of green emission due to increased defect concentration brought about by high temperature heat treatment. Copper doping in ZnS, initially leads to formation of both CuZn and Cui (interstitial copper) centers, and latter to mainly CuZn centers as revealed by variation in relative intensities of blue and green emission from the samples.

Introduction

Zinc sulfide (ZnS) belongs to the category of II–VI semiconductors and has been extensively investigated, ever since electro-luminescence (EL) was observed from it by application of alternating current (AC) electric fields [1]. It has been widely used as a semiconductor material for different types of display applications which include light emitting diodes, cathode ray tubes, liquid crystal display (LCD), backlight displays for cellular phones, personal digital assistant (PDA) and palmtop computers [2], [3]. ZnS exists in two crystallographic forms namely the cubic zinc blend structure and hexagonal wurtzite structure with corresponding band gaps of 3.7 and 3.8 eV respectively at room temperature [4]. Earlier reports suggest that, cubic form of ZnS is the most stable crystallographic form at room temperature and it gets converted to hexagonal form upon heat treatment at temperatures above 1020 °C [5], [6]. Optical properties of both forms of ZnS can be modified by doping them with different transition metal ions. For example, cubic form of bulk ZnS samples doped with Cu and/or Cl ions gives blue and green emission depending upon doping concentration [7], [8]. Multicolor emission from ZnS can also be achieved by suitably modifying size of ZnS particles as well as dopant ions. For example number of ZnS based nano-materials have been developed, which can give emission in different wavelength region, by doping ions such as Mn2+, Ag+, Cu2+ or Cu+, etc [9], [10], [11], [12], [13]. There are also many reports on ZnS nanoparticles doped with different ions and their incorporation in polymer matrices for developing polymer based luminescent devices [14], [15], [16].

Among the large number of investigations on doped ZnS nanoparticles/nano-crystals, majority of them deals with the copper doped samples. In copper doped ZnS samples, upon UV excitation, emission peaks around 450 nm (blue emission) and around 525 nm (green emission) are observed. It is widely accepted that green emission or the G-band is arising from electron–hole recombination involving conduction band (levels close to conduction band) and CuZn acceptor levels [17]. The origin of blue emission is still under investigation and it is thought to be arising from the donor-acceptor recombination involving CuZn acceptors and Cui donors (copper ions at the interstitial site) [17]. It is worth mentioning here that most of the above mentioned studies are on cubic form of ZnS lattice.

As ZnS exists in both cubic and hexagonal forms, it will be interesting to study how this structural transition from cubic to hexagonal form (wurtzite structure) is affecting luminescence from samples. Luminescence properties namely emission spectra and excited state lifetime are also expected to change when ZnS is stabilized into cubic phase by reducing particle size to nanometers. In such cases copper doping can change significantly the luminescent properties. Evaluation of electro-luminescent properties of ZnS:Cu nanoparticles and its comparison with respect to that of bulk is an important aspect to be considered while exploring the possibility of developing improved ZnS based devices. It is always desirable to have powder electro-luminescence from Cu doped ZnS nanoparticles as it will be helpful for the development of flexible, bright display devices. However, getting powder electro-luminescence from nanoparticles appears to be more difficult than the corresponding bulk. Keeping the above aspects in mind, in the present study, luminescence properties of cubic and hexagonal forms of undoped bulk ZnS samples have been evaluated and the results have been compared with cubic form ZnS doped with Copper ions as well as ZnS nanoparticles with and without copper doping. To the best of authors’ knowledge this is the first time that a comparative study is being made on the photo and electro-luminescence properties of bulk and nanoparticles of ZnS with and without doping copper ions.

Section snippets

Experimental

ZnS nanoparticles, undoped and copper (0.5 at% and 1 at%) doped were synthesized by polyol method using zinc acetate, copper acetate and thiourea as the starting materials. Required amounts of zinc acetate and copper acetate were taken and dissolved in minimum amount of H2O. To this solution, ethylene glycol (20 ml) was added and transferred into a two necked RB flask. Ethylene glycol acts both as solvent and as stabilizing agent. The solution was heated up to 100 °C followed by addition of 2 g of

Results and discussions

Fig. 1 shows the XRD patterns of commercially obtained ZnS, before and after heating at 800 °C in vacuum along with ZnS nanoparticles prepared based on polyol method. As received commercial sample (bulk sample) corresponds to ZnS phase having hexagonal structure (wurtzite from) with lattice parameter “a” = 3.223(2) Å and “c” = 5.132(1) Å. Upon heating the sample in vacuum at 800 °C, wurtzite form of ZnS get transformed into the cubic zinc blende structure with lattice parameter “a” = 5.407(1) Å as can be

Conclusions

Based on the relative intensities of blue and green emission and the corresponding lifetime values form both undoped and Cu doped samples, it is confirmed that initially copper ions occupy both Zn site (CuZn) and interstitial site (Cui) in the lattice and latter to Zn2+ site. Blue and green emissions are observed even in the undoped sample and this is explained based on electron hole recombination involving conduction band, S2− and Zn2+ vacancies. Conversion of wurtzite to cubic form of ZnS is

Acknowledgements

Authors would like to acknowledge Dr. V. K. Jain, Head, Chemistry Division and Dr. B. N. Jagatap, Director, Chemistry Group, Bhabha Atomic Research Centre for their encouragements during the work. One of the authors, Dr. K. R. Rao, gratefully acknowledge DAE-BRNS for providing the financial support and Prof. Ch. Mastanaiah, Principal, Government College (A), Rajahmundry, Andhra Pradesh, for making available the research facilities.

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