Elsevier

Materials Chemistry and Physics

Volume 171, 1 March 2016, Pages 328-335
Materials Chemistry and Physics

Effect of Cu-doping on optical, electrical and magnetic properties of chemically synthesized MnS nanocrystals

https://doi.org/10.1016/j.matchemphys.2016.01.024Get rights and content

Highlights

  • •

    Pure and Cu-doped MnS nanocrystals were synthesized by the wet chemical technique.

  • •

    Optical band gap value is raised by the Burstein-Moss effect due to the introduction of Cu-dopant.

  • •

    It is a potential material for violet-blue light emission.

  • •

    Cu-doped MnS showed good electrical conductivity compared to pure γ-MnS.

  • •

    Doping of Cu induces superparamagnetic behavior in the system.

Abstract

Manganese sulfide and Cu-doped manganese sulfide nanocrystals have been synthesized by wet chemical technique. The structural, optical, electrical and magnetic properties of as-synthesized nanocrystals have been investigated. The average crystallite size and lattice strain of the samples have been calculated from powder X-ray diffraction patterns using the Williamson–Hall analysis. The results show that the average crystallite size decreased while both the lattice strain and the dislocation density values increased in the Cu-doped MnS nanocrystals. The surface morphology of Cu-doped MnS nanocrystals has lesser particle size than undoped sample and it shows spherical like structures with little agglomeration. The chemical composition of the prepared samples has been obtained from EDAX. It clearly indicates the presence of Cu ions in the MnS lattice. UV–visible spectroscopy shows a blue shift in the optical band gap with doping. The photoluminescence spectra on the doped sample show a quenching of the PL intensity due to strain induced by doping. The electrical conduction, dielectric and impedance properties of as-synthesized nanocrystals have been investigated in the frequency range 50 Hz–5 MHz and temperature range 323–473 K which are greatly affected by doping with Cu. The vibrating sample magnetometer measurement revealed that the undoped MnS has paramagnetic behavior while the Cu-doped MnS has superparamagnetic behavior. On Cu-doping, the saturation magnetization and remanence increases while the coercivity decreases.

Introduction

Semiconductor nanocrystals have drawn tremendous attention due to their interesting properties and potential applications in mesoscopic research and development of nanodevices. The manganese chalcogenides are magnetic materials that have unique physical, morphological and chemical properties [1]. The manganese sulfide is a wide band gap (Eg ≈ 3.8 eV) diluted magnetic p-type semiconductors. It has outstanding magneto-optical properties. The α-MnS exhibits anti-ferromagnetic behavior with a transition temperature TN = 130 K due to the correlations between the Mn2+ spins, and a paramagnetic moment peff = 5.6 μB [2]. In the paramagnetic phase, the α-MnS is a p-type semiconductor with an activation energy E = 0.3 eV [3]. Doping is one of the methods to adjust the energy levels in bandgap of chalcogenide nanostructured materials. Furthermore, transition metal (TM) ions doped into these nanostructures can influence the electronic structure. Among TMs, Cu is particularly interested as a doped because Cu has high ionization energy and low formation energy, which accelerate the incorporation of Cu into the MnS lattice. Copper is a prominent luminescence activator, which can create localized states in the bandgap of MnS (Two peaks were observed in PL). Apart from the change in the luminescence properties, magnetic properties also get altered which are governed by defects states in the MnS matrix. Moreover, Cu compound has a magnetic moment at room temperature which expected to modify magnetic and other properties of MnS.

MnS nanocrytals have been synthesized by using various methods such as solvothermal synthesis [4], successive ionic layer adsorption and reaction (SILAR) [5] and Hydrothermal method [6], [7]. In this work, metastable MnS and Cu-doped MnS nanocrystals have been synthesized by wet chemical synthesis technique. The effect of Cu-doping on the structural, electrical and magnetic properties has been investigated. The synthesized MnS and Cu-doped MnS nanocrystals having reduced dimensions allow the possibility of realizing controlled quantum confinement as a step towards quantum engineering. Owing to their attractive properties, these materials can be used in anode material for Li-ion batteries [8], biomedicine and optoelectronic devices [9], buffer material in solar cell [10] and magneto-optical devices [11]. The Cu-doped MnS with high electrical conductivity and superparamagnetism extends their application in the blue green light emitters [12], sensors, and electro-magnetic resonance applications [13]. To the best of our knowledge, only few investigations in electrical and magnetic studies of Cu-doped MnS are reported.

Section snippets

Synthesis

All reagents were used as received without any further purification from the Merck. The MnS nanocrystals were prepared as follows: 20 mL of 1 mol/L manganese acetate aqueous solution was mixed with 2 mL of triethanolamine and 20 mL of 1 mol/L ammonium chloride in 100 mL two necked round bottom flask. Under vigorous stirring, 0.4 mL of 0.7 mol/L trisodium citrate and 20 mL of 1 mol/L thioacetamide were slowly added one by one. After mechanical agitation for about 1 h, the solution with a pH of

Results and discussion

Fig. 1(a) shows the XRD pattern of pure MnS nanocrystals, the peaks at 2θ of 26.0°, 27.8°, 29.6° and 45.6° corresponding to the (100), (002), (101), and (110) diffraction planes. The lattice constants have been found to be a = 3.95 A° and c = 6.41 A° and are in agreement with the standard diffraction data of γ-MnS (JCPDS: 40–1289) [14]. The diffraction pattern of Cu-doped MnS sample is shown in Fig. 1(b). It is observed that the diffraction peaks of Cu-doped MnS is shifted to the higher angle

Conclusion

Wet chemical synthesis route has been successfully used to synthesize manganese sulfide and copper-doped manganese sulfide nanocrystals. The X-ray diffraction patterns have confirmed that the as-synthesized nanocrystals are of γ-wurtzite phase. Williamson–Hall plot method is used to estimate the crystallite size and lattice strain of the nanocrystals. The average crystallite size is found to be 11.2 nm and 3.37 nm for the undoped and Cu-doped MnS nanocrystals respectively. It is clear that the

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