Tunable properties of cadmium substituted ZnS nanocrystals

Abstract

Tailored Zn_1−xCd_xS (x = 0, 0.25, 0.5, 0.75 and 1) nanoparticles, synthesized by co-precipitation method under ultrasonic irradiation, were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), UV–Vis and photoluminescence (PL) spectroscopy measurements. According to the XRD results, substitution of Zn²⁺ by Cd²⁺ ion leads to an increase in the lattice parameters and the average size of zinc blended nanoparticles are in the range of 3–4 nm. Transmission electron microscopy image revealed the formation of nano-sized particles with dimension of 5 nm confirming that the samples are quantum dots. The shift observed in the absorption edges by increasing Cd²⁺ ion substitution is ascribed by the alloying effect but the enhancement of band gap energy compared to that of the corresponding bulk value is attributed to the nanometric grain size and quantum confinement effects. The position and intensity of PL emission peaks are tuned with Cd²⁺/Zn²⁺ ion content.

Introduction

Nano-sized II-VI wide band gap semiconducting materials have gained considerable amount of attention due to their tunable physical and chemical properties and variety of applications [1], [2], [3], [4], [5], [6], [7]. The properties of nano-sized semiconducting materials are significantly improved and varied from their bulk counterparts due to the large surface to volume ratio and quantum confinement effects. As the size of nanomaterials gets as small as the excitation Bohr radius, a significant increase can be observed in the band gap owing to the quantum confinement of electron and holes in a small region which consequently causes a raise in the blue shift in the absorption spectra [1]. Various methods are used to prepare quantum dots, and nano-structured materials, including precipitation method, solvothermal and hydrothermal process, wet-chemical method, and etc [8], [9], [10], [11], [12], [13]. Among them, co-precipitation method is a simple and low cost process for synthesis of II-VI semiconducting nanoparticles.

Doping and alloying are two of the common routes to tailor and tune the properties of semiconducting materials by varying the composition and introducing traps and discrete energy states in the band gap [14], [15], [16], [17], [18], [19], [20], [21], [22]. The ternary nanostructures such as ZnCdS, CdZnS, make a choice to improve and control the materials toward desired properties by changing the band gap, lattice parameters, size and morphology. Wide band gap ZnS and CdS materials crystallize in a same cubic zinc blende or wurtzite hexagonal structure. However, since the band gap of ZnS is higher than CdS, substituting or alloying these two with each other leads to a promising material with different applications. Jindal and Verma used solvothermal method to synthesize CdS/ZnS nanoslabs with ethylenediamine as the solvent and the chelating ligand with modified properties [17]. Sreejith et al. employed a simple method to synthesize the hexagonal modification of Cd_1−xZn_xS material without using any harmful chemicals [18]. Panda et al. prepared CdS/ZnS nanorods by a three-steps solvothermal method. They examined the synthesized nanorods’ applications for optoelectronic devices [19]. Pandey and co-workers synthesized ZnS/CdS quantum dots by using solution reaction method with controlled pH [20]. Peng et al. reported a new approach to synthesis of CdS/ZnS nanocrystals [21]. Liu et al. obtained ZnS-CdS and CdS-ZnS alloyed nanoparticles by a simple low-temperature solid state synthetic method and showed that the optical properties of the prepared nanoparticles were dependent on the synthetic conditions [22]. O’Brien and co-workers prepared nanoparticles of ZnS, CdS and Zn_xCd_1−xS from zinc and cadmium thiobiuret complexes by thermal decomposition to investigate the effect of different reaction parameters on size, morphology and optical properties [23].

In this study, ZnS nanoparticles substituted by Cd were prepared using facile co-precipitation method under ultrasonic irradiation to obtain homogenous nanoparticles. The effect of Cd²⁺ ions content on the structure and optical properties of Zn_1−xCd_xS nanocrystals was investigated. The control of optical properties of Zn_1−xCd_xS nanostructure was carried out by regulating the content of Zn²⁺/Cd²⁺ ions.