Microstructure correlated optical, electrical and mechanical characterizations of (Bi1-xSbx)2Te3: Effect of compositional variations on different properties

Abstract

Nanocrystalline rhombohedral (Bi1-xSbx)2Te3 (with x = 0, 0.25, 0.5, 0.75, 1) powders are synthesized by a facile one-pot mechanical milling technique using elemental Bi, Sb, and Te powders, taken in different proportions within 3 h of milling. The Rietveld refined X-ray diffraction (XRD) patterns of the fabricated samples reveal the compositional variations of different microstructural parameters like crystallite size, lattice strain, and lattice parameters. The included impurity percentages of Bi and Sb explain the variation of lattice parameter a, whereas, the antisite defects are well interpreted with the change in lattice parameter c. The r.m.s. lattice strain is consistent with the formulation of different antisite defects acting as either n-type or p-type doping within the materials (Bi1-xSbx)2Te3. The microstructure is also revealed from the field-emission scanning electron microscope (FESEM) image and energy dispersive X-ray (EDX) spectrum and compared with the results obtained from the XRD pattern analysis. The narrow bandgaps of the materials are determined from the Fourier transform infrared (FTIR) spectra, and the correlation with the microstructure establishes their changes with the compositional x value. Here, both the blue and red shifts in bandgap with particle size variations are observed and explained. The positions of the conduction and valence bands are determined with composition variations (x). The non-Debye type, thermally activated conduction process of the semiconducting materials is confirmed from the ac electrical study of the samples at room and high temperatures. The variation of electrical conductivity with different alloying concentrations and temperatures is explained in terms of their particle size and density of antisite defects. The conduction mechanism is found to be a small polaron hopping model. The Vickers microhardnesses of the samples are measured and explained by correlating with the microstructural parameters. The Meyer's law establishes the normal indentation size effect (ISE). The microhardness of the materials partially supports the conductivity results.