High luminescent yield from Mn doped ZnS at yellow–orange region and 367 nm
Introduction
Semiconductor based white light sources attained much attention because of its high luminous efficiency, brightness, low power consumption [1] and environmental safety [2]. Warm white light emitting sources are a new generation product, fabricated by using ultraviolet light emitting diode (UV–LED) chip in the region 350–420 nm coated with blue–green and orange–yellow phosphors. The rare earth based phosphors used for the fabrication of white LEDs (WLEDs) has week red spectral emission. This week emission in turn contribute to high correlated colour temperature (Tc>4500 K) and low colour rendering indexes (Ra<80) which are undesirable [3], [4]. On the other hand most of the warm white light emitting phosphors are nitride or oxynitride compounds. It requires inert atmosphere for its production and are costly. Presently WLEDs are commercially fabricated using InGaN blue chips and yellow–orange emitting phosphor combination. Alternatively, ZnS based phosphors like ZnCdS:Ag, Cl (red) [5], ZnS:Cu, Al (green) [6], and ZnS:Ag (blue) [7], etc. with different colours are also used for making warm white light sources and WLEDs. High luminescence efficiency of the material is a pre-requisite in order to get better performing white light sources. ZnS:Mn shows emission in the yellow–orange region [8], [9], [10], [11] and its energy bands, luminescence centres can be tuned by changing the doping concentration [12]. Hydrothermally synthesised Mn doped (0.5, 1, 3, 10 and 20 wt%) ZnS shows a 495 nm (blue) and orange emission at 587 nm attributed to the 4T1−6A1 transition of Mn [10]. There is a shift in the absorption band edge to longer wavelength and a broad emission at 580 nm is also reported in ZnS doped with Mn [11].
Highly luminescent, non-toxic yellow–orange emitting sources are also important especially for biological labelling, which can replace the presently using labelling materials like ethidium bromide (EtBr). Synthesis condition of the materials is important in relation to its luminescence properties. Recently our group have reported intensity enhancement in the UV emission from ZnS microstructures attributed to the activation of whispering gallery modes (WGMs) when water–acetonitrile combination is used as solvent for the synthesis of ZnS [13]. In the present work, we are reporting the high luminescence yield from the solvo-hydrothermally (water–acetonitrile combination) grown Mn doped ZnS at yellow–orange region and at 367 nm. Further an attempt is made to understand the role of week hydrophobic polar solvent like acetonitrile on luminescence properties of solvo-hydrothermally (S-H) grown Mn doped ZnS nanocrystals.
Section snippets
Materials preparation
Manganese (Mn) doped ZnS were synthesised by the reaction of analytical reagent grade (AR grade) zinc acetate (0.4 M), manganese acetate and sodium sulphide (1 M). 3.4065 and 0.1054 g zinc acetate and manganese acetate respectively were used for the synthesis of Mn 3 weight percentage (wt%) doped ZnS. But 3.3713 g zinc acetate and 0.1405 g of manganese acetate were used for the synthesis of Mn 4 wt% doped ZnS. The Mn 5 wt% doped ZnS were made by using 3.3360 g zinc acetate and 0.1756 g of manganese
XRD analysis of hydrothermally and S-H synthesised Mn doped ZnS
The X-ray diffraction patterns (Fig. 1) of hydrothermally and S-H grown Mn 3, 4 and 5 wt% doped ZnS can be indexed to that of cubic sphalerite ZnS phase with JCPDS 05-0566. The XRD of S-H grown ZnS:Mn is almost identical to that of S-H grown ZnS reported previously [13]. The calculated lattice constants are found to be matching with that of the standard value 5.4060[14]. The lattice parameters (Table 1) of all the samples increases slightly with increase in the Mn doping concentration (wt%).
SEM analysis of hydrothermally and S-H synthesised Mn doped ZnS
From
Conclusions
The PL emissions of the ZnS doped with 3 and 5 wt% of Mn, synthesised by S-H method are having intense dual wavelength emission at UV and yellow–orange region. ZnS with Mn 3 wt% exhibit higher luminescence yield at 367 nm and yellow–orange region. The increase in luminescence yield of S-H grown sample with Mn 3 wt% doping is attributed to the increase in population of frozen sulphur vacancies formed due to the higher solubility of the sulphur in acetonitrile and the increase in specific area of
Acknowledgements
P. Sajan thanks University Grants Commission, Government of India, for providing Rajiv Gandhi National fellowship. The author acknowledges Professor M. K. Jayaraj, Nanophotonic and Optoelectronic Devices Laboratory, Department of Physics, CUSAT, for providing photoluminescence measurement facility under Department of Science and Technology nano mission initiative programme.
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