Properties of nanocrystalline ZnS:Mn

https://doi.org/10.1016/j.jcrysgro.2004.04.096Get rights and content

Abstract

In this paper, the effect of the Mn incorporation and its concentration variation over its whole solid solution range in nanocrystalline ZnS (0% to about 40%) has been studied. Such materials having a significant Mn content are also called diluted magnetic semiconductors (DMS). Till date, they have been studied only in the bulk or thin film form. Here the uniqueness of such a DMS in nanocrystallite form is presented. The samples were prepared by a chemical capping method. Zinc acetate, manganese acetate and sodium sulfide were used as reactants. Poly vinyl pyroledone was used as a capping agent for controlling particle size. The samples were observed to have cubic structure over the Mn concentration range studied. Unlike bulk samples, there was no crystalline phase change with Mn concentration variation. The material had a direct band gap of about 5.6 eV. It did not show any systematic change with Mn concentration. Photoluminescence intensity in ZnS:Mn showed significant enhancement upon Mn incorporation. There were two extra PL peaks after Mn incorporation in the nanocrystalline ZnS:Mn. PL also suggested Mn incorporation only up to about 40%. Lastly, the observed magnetic and electrical properties of nanocrystalline ZnS:Mn samples are stated.

Introduction

Traditionally, zinc sulfide has been studied both in bulk and thin film form for a very long time. It has been used all along for its wide and direct band gap by doping it with various elements. Such materials have applications in luminescent devices, phosphors, light emitters, optical sensors etc. The properties of ZnS have been changed and studied over various alloying concentrations by stoichiometric substitution and using various alloying and doping elements. Magnetic element based doping and alloying of ZnS have certain unique properties due to the magnetic ion content. Over the last 10 years or so, it has been realized that nanomaterials have some unique properties, which have been attributed to their size dependent band gap energy, e.g. much higher luminescent efficiency w.r.t. the corresponding bulk form, etc. There have been literature reports on the high photoluminescence efficiency of manganese-doped nanocrystaline ZnS [1], [2], [3], [4], [5], [6], [7]. However, all of these reports discuss Mn doping in very low concentrations. Here in contrast, we report on the properties of the effect of Mn alloying in nanocrystaline ZnS. Here we report on some properties in general of Mn alloyed ZnS nanoparticles.

Section snippets

Experimental procedure

Appropriate amounts of zinc acetate, magnesium acetate solutions in 2-propanol medium were taken and sodium sulfide solution was added and stirred continuously for a few hours at room temperature. As a capping agent, an appropriate amount of poly-vinyl pyroledone (PVP) was also added to the reaction medium i.e. for control of the particle size. Zinc sulfide precipitated out slowly after the reaction was stopped. The precipitates were cleaned repeatedly by de-ionized water and ultrasonic

Results and Discussions

In bulk ZnS:Mn samples, the alloying range possible is for 0⩽Mn ⩽43% [8]. Here the manganese concentration for doping/alloying has been varied over the whole alloying range. A representative XRD pattern of the prepared ZnS samples is shown in Fig. 1. The samples were seen to be cubic over the whole range (0%–40% Mn concentration, Table 1). It was seen that XRD patterns were very broad with three peaks corresponding to the (1 1 1), (2 2 0) and (3 1 1) planes. The average crystallite size calculated

Conclusions

The preparation of nanocrystalline ZnS:Mn by a chemical method over the whole alloying range is discussed. They were cubic with average particle size of 2 nm or so. Mn incorporation significantly increases the PL intensity. Convoluted PL spectra suggested that in addition to the two normal PL peaks of pure ZnS, two extra peaks at 635 and 680 nm were created. Using PL it was also possible to suggest incorporation of Mn into ZnS only up to about 40%. Optical band gap pf the sample was 5.6 eV—it did

Acknowledgements

The authors acknowledge the help of B. Satpathi of IOP Bhubaneswar for the TEM related work. One of the authors’ (N. Karar) acknowledges the associateship provided to him by the CSIR.

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