Structural and AC conductivity study of CdTe nanomaterials

https://doi.org/10.1016/j.physe.2015.11.031Get rights and content

Highlights

  • CdTe nanomaterials have been synthesized by soft chemical route.

  • XRD and HRTEM indicate the cubic structure of the synthesized material.

  • Z-plots are modelled by 2 R-CPE units for grain and grain boundary effects of CdTe.

  • Dielectric relaxation of the sample is modelled by modified Cole–Cole equation.

  • The frequency dependence ac conductivity is analyzed by power law.

Abstract

Cadmium telluride (CdTe) nanomaterials have been synthesized by soft chemical route using mercapto ethanol as a capping agent. Crystallization temperature of the sample is investigated using differential scanning calorimeter. X-ray diffraction and transmission electron microscope measurements show that the prepared sample belongs to cubic structure with the average particle size of 20 nm. Impedance spectroscopy is applied to investigate the dielectric relaxation of the sample in a temperature range from 313 to 593 K and in a frequency range from 42 Hz to 1.1 MHz. The complex impedance plane plot has been analyzed by an equivalent circuit consisting of two serially connected R-CPE units, each containing a resistance (R) and a constant phase element (CPE). Dielectric relaxation peaks are observed in the imaginary parts of the spectra. The frequency dependence of real and imaginary parts of dielectric permittivity is analyzed using modified Cole–Cole equation. The temperature dependence relaxation time is found to obey the Arrhenius law having activation energy ~0.704 eV. The frequency dependent conductivity spectra are found to follow the power law. The frequency dependence ac conductivity is analyzed by power law.

Introduction

Semiconductor nanocrystals are at the centre of an active and interdisciplinary research area. The great interest stems from the unique changes in properties that occur when particle sizes are reduced to the nanometre scale [1], [2]. In particular, in semiconductor particles that are smaller than their Bohr sizes, quantum confinement leads to relatively large band gaps and consequently to different optical, electronic, and magnetic properties as compared to the corresponding bulk materials; such spherical nanoparticles are often referred to as quantum dots. Cadmium telluride is a stable crystalline compound formed from cadmium and tellurium. Among the most common II–VI semiconductor nanoparticles, CdTe is a famous one which has very advantageous properties as compared to the other chalcogenides of cadmium due to its great mobility and photosensitivity. It is also known that cadmium telluride is photosensitive to many kinds of radiation, for example to infrared and visible radiation and X-radiation, so that it may be used as photosensitive devices. Many researchers have studied the different properties of CdTe nanostructure. Joly et al. [3] have studied upconversion luminescence of CdTe nanoparticles. Semaltions et al. [4] have synthesized CdTe nanoparticles by laser ablation. Tang et al. [5] have studied on spontaneous organization of single CdTe nanoparticles into luminescent nanowires. Mohammed [6] has studied the structural and electrical properties of CdTe:Ag thin flims. Parfenyuk et al. [7] have investigated the electrical properties and low-temperature photoluminescence of Si-doped CdTe crystals. Terahertz detection has also been performed by Wu et al. [8] via electro-optical sampling using a similar II–VI semiconductor ZnTe. Hui et al. [9] have synthesized large-scale preparation of highly conductive three dimensional graphene and its applications in CdTe solar cells. Douri et al. [10] have studied the role of dopant concentration on conductivity and mobility of CdTe thin films. However, there is no report on the systematic study of the dielectric properties of CdTe to the best of our knowledge though it is well known that when we study the ac conductivity of nanomaterials making its pellet the boundary between the particles plays an important role in determining the conductivity. According to grain boundary trapping theory, free carriers are trapped by trapping states at the boundary, causing a depletion of charges in the grain region nearest to the boundary. Therefore the region near the surface of the particle becomes depleted of charges causing a space charge which should establish an energy barrier between adjacent particles. If thermo ionic emission of carriers over the barrier is the predominant transport mechanism, the conductivity should exhibit Arrhenius behaviour. In this study the impedance spectroscopy has been used to investigate such behaviour of the CdTe nanomaterials. Recently, we have investigated the dielectric relaxation of other II–VI semiconductor nanoparticles such as CdSe [11] and CdO [12]. In the present work the frequency dependent dielectric properties and its effect on the conductivity of CdTe nanomaterials, synthesized by the soft chemical route have been investigated over a temperature range from 313 to 593 K and frequency range from 42 Hz to 1.1 MHz using impedance spectroscopy.

Section snippets

Experimental

CdTe nanomaterials are prepared using cadmium acetate (((CH3COO)2Cd, 2H2O), Merck) and sodium tellurite ((Na2TeO3), Loba chemie). Mercapto ethanol ((C2H6OS), SRL) was used as the capping agent. The cadmium acetate solution was prepared by dissolving 3.331 g of cadmium acetate in 25.008 cc of distilled water with stirring by a magnetic stirrer. Sodium tellurite solution was prepared by dissolving 2.763 g of sodium tellurite in 25 cc of distilled water. Mercapto ethanol solution was prepared by

DSC analysis

Fig. 1 shows the heat flow curves as a function of temperature rates for CdTe nanomaterials. From these curves, we have obtained the glass transition temperature (Tg), the onset of crystallization temperature (Tc) which is the temperature that corresponds to the point of intersection between the tangent and the baseline of the exothermic peak in the curves, and the characteristic temperature (Tp) which is the temperature corresponding to the exothermal peak at which the reaction is fastest. In

Conclusions

The frequency dependent dielectric dispersion of CdTe nanomaterials synthesized by the soft chemical route is investigated in the temperature range from 313 K to 593 K and in the frequency range from 42 Hz to 1.1 MHz. X-ray diffraction pattern shows that the synthesized nanomaterials have cubic (sphalerite) structure. The nanostructures of the prepared material have been confirmed by high resolution transmission electron microscopy. The average particle size is found to be of 20 nm. The frequency

Acknowledgements

Sayantani Das acknowledges the financial support provided by UGC (Grant no. UGC/56/RFSMS/Physics dated 25/01/2012) in the form of RFSMS.

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