Structural and photoluminescence properties of Eu-doped ZnS nanoparticles

Abstract

Pure and Eu-doped ZnS nanoparticles have been synthesized by chemical precipitation technique. Morphological study reveals the spherical nature of the synthesized nanoparticles as well as its average size to be 29–37 nm. The effect of Eu-doping on structural and optical properties of the synthesized nanoparticles has been investigated. Energy dispersive X-ray spectroscopy confirms the stoichiometry of the synthesized nanoparticles. X-ray diffraction study reveals that the synthesized nanoparticles possess the cubic crystal structure. XRD confirms that higher doping concentration of Eu from 10 to 20% results in the formation of Eu₂O₃. UV–visible spectroscopy reveals that the band gap of the pure ZnS nanoparticles is 4.2 eV, which further decreases with addition of Eu, to 3.9 eV. Fourier transforms Infra-red spectroscopy shows the presence of surface passivating agent, mercaptoethanol, on the surface of the synthesized nanoparticles. Photoluminescence spectrum for pure ZnS nanoparticles has been found to be broad and asymmetric, whereas sharp peaks are observed when it was doped with Eu. There are five strong emission peaks at 574, 591, 617, 700, 754 nm have been observed in the PL spectra of the Eu-doped ZnS nanoparticles, respectively associated with ⁵D₀→⁷F₆, ⁷F₄, ⁷F₂, ⁷F₁, ⁷F₀ transitions. The possible mechanism of observed structural and photoluminescence has been explained on the basis of Eu-doping.

Introduction

Nanocrystalline semiconductors have been the subject of numerous investigations in the past two decades due to their unique size-dependent properties, quantum confinement, and large surface to volume ration of atoms [1], [2], [3], [4], [5], [6], [7]. This results in tuning of band of semiconducting nanoparticles, and, therefore, highly luminescent materials can be obtained [1], [2], [3], [4], [5], [6], [7]. The surface states, dangling bonds, surface imperfection on the surface of nanoparticles play a crucial role in designing their optical properties. The radiative or non-radiative recombination of electrons/holes, i.e., an exciton at the surface of nanoparticles dominates their optical properties. These size-dependent optical properties have potential applications in the areas of solar energy conversion, light emitting devices, chemical/biological sensors, photo catalysis and optoelectronic devices [2], [3], [4], [5], [6], [7]. Zinc sulphide (ZnS) is a typical direct band gap II–VI semiconductor having a bulk band gap of 3.67 eV at room temperature. ZnS nanoparticles, used as a phosphor in thin film electroluminescent devices, light-emitting diode (LED), thin film solar cells, IR windows, flat-panel displays, photodiodes, photo catalytic degradation of organic pollutants, sensors, lasers, bio-medical imaging and as dilute magnetic semiconductors [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. ZnS has been used widely as phosphor for photoluminescence (PL), electroluminescence (EL) and cathodoluminescence (CL), triboluminescence devices due to its chemical stability as compared to other chalcogenides [8], [12]. Nanoparticles doped with optically active luminescence centers create new opportunities for luminescent materials as well as nano-scale devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. The rare earth elements are known as effective luminescent centers. A number of reports on the optical properties of doped-ZnS nanostructures are available [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. It is known that the rare-earth elements as dopants should be more interesting in modifying the photoluminescence properties of ZnS by considering their special 4f–4f intra-shell transitions [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38].

In the present communication, the syntheses of pure and Eu-doped ZnS nanoparticles by chemical precipitation method as well as the effect of Eu-doping on the structural and optical properties of doped-ZnS have been reported.