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Differential tunneling conductance of the interlayer van der Waals surface (0001) is investigated in n-Bi2 – xSbxTe3 – y – zSeySz multicomponent solid solutions at x = 0.2, y = z = 0.09, optimized for near room temperatures with a high power factor and, at x = 0.4, y = 0, z = 0.06, with optimal thermoelectric properties for low temperatures and a high Seebeck coefficient. It is shown that the intensity of the fluctuations ΔED of the Dirac point energy, the shift of the valence band top, and the presence of surface levels in the energy gap formed by impurity defects are determined by the composition and thermoelectric properties of solid solutions. The contribution of the surface states of Dirac fermions increases in n-Bi1.8Sb0.2Te2.82Se0.09S0.09 solid solution with a high power factor due to a significant decrease in the surface concentration near the charge-neutrality point and an increase in fermion mobility. [ABSTRACT FROM AUTHOR]

The morphology of the (0001) interlayer surface has been investigated by scanning tunneling microscopy in n-Bi1.6Sb0.4Te2.94Se0.06, n-Bi1.8Sb0.2Te2.82Se0.09S0.09, p-Bi0.8Sb1.2Te2.91Se0.09, and p-Bi0.7Sb1.3Te2.91Se0.09 solid solutions. Impurity and intrinsic defects (tellurium vacancies, antisite defects, and adatoms) were found on the (0001) surface, which are formed in these compositions due to the substitution of atoms in Bi2Te3 sublattices. The mean values of HM and the root-mean-square deviations HS with respect to height in the distribution of atoms on the surface have been determined depending on the composition of the solid solution. The effect of detected defects on the thermoelectric properties of solid solutions has been established. [ABSTRACT FROM AUTHOR]

Nanocomposite thermoelectrics based on Bi0.45Sb1.55Te2.985 solid solution of p-type conductivity are fabricated by the hot pressing of nanopowders of this solid solution with the addition of SiO2 microparticles. Investigations of the thermoelectric properties show that the thermoelectric power of the nanocomposites increases in a wide temperature range of 80–420 K, while the thermal conductivity considerably decreases at 80–320 K, which, despite a decrease in the electrical conductivity, leads to an increase in the thermoelectric efficiency in the nanostructured material without the SiO2 addition by almost 50% (at 300 K). When adding SiO2, the efficiency decreases. The initial thermoelectric fabricated without nanostructuring, in which the maximal thermoelectric figure of merit ZT = 1 at 390 K, is most efficient at temperatures above 350 K. [ABSTRACT FROM AUTHOR]

The thermoelectric properties in the temperature range 4.2–300 K and magnetoresistance oscillations in strong magnetic fields at low temperatures are studied in nanostructured layered films of topological thermoelectric materials n-Bi2 –xSbxTe3 –ySey. It is shown that the thermoelectric figure of merit in layered n-Bi2 –xSbxTe3 –ySey films is larger than that in the bulk material due both to an increase in the Seebeck coefficient below room temperature and to a decrease in the thermal conductivity and its weaker temperature dependence. Analysis of the magnetoresistance oscillations is used to determine the parameters of the topological surface states of Dirac fermions and estimate their influence on the thermoelectric properties. [ABSTRACT FROM AUTHOR]

In n-Bi2Te3 and n-Bi2Te3 –ySey thermoelectrics, the surface states of Dirac fermions of the interlayer Van der Waals surface (0001) are studied by scanning tunneling microscopy and spectroscopy. The surface morphology and modulated line profiles of the tunneling microscopy images are determined by local distortions of the surface density of electronic states and depend on the composition. The Dirac point ED in the studied compositions is localized in the band gap and shifts to the top of the valence band with increasing Se content in n-Bi2Te3 –ySey alloys. The dependence between the parameters of the surface states of Dirac fermions (Dirac-point position, Fermi velocity, surface fermion concentration) and thermoelectric properties (Seebeck coefficient and power parameter) in the thermoelectrics under study is determined. [ABSTRACT FROM AUTHOR]

Samples of the topological insulator n-Bi2Te3 (0001) have been studied in air by means of atomic force microscopy. The possibility to increase the sample conductivity by five or more orders of magnitude has been found. Bulk single crystalline films (few microns thick) and microflakes (∼100 nm thick) are compared. Unlike the local effect on the bulk film, the entire surface of the microflake can be switched to the conductive state. Besides, when switched, the conductivity of the microflake is usually higher by orders of magnitude than that of the bulk film. A possible mechanism for the effect is discussed. [ABSTRACT FROM AUTHOR]

In topological n-type thermoelectrics based on Bi 2 Te 3 with atomic substitutions Bi → In, Te → Se, S, the morphology and the surface states of Dirac fermions on the interlayer (0001) surface of van der Waals were studied by scanning tunneling microscopy and spectroscopy (STM/STS) techniques. By the STM method, the dark and light spots on the surface were found, which intensities depend on the composition and thermoelectric properties of solid solutions such as the Seebeck coefficient and thermoelectric power factor. The observed surface morphology features in the solid solutions are explained by distortions of surface electronic states originated from atomic substitutions, the influence of doping impurity, and formation mainly of substitutional impurity defects in thermoelectrics. The dips associated with substitutional impurities and antisite defects were found from the analysis of the height profiles obtained on the (0001) surface. Fast Fourier transform of the morphology STM images of the (0001) surface were used to obtain the interference patterns of the quasiparticles excitation caused by surface electrons scattering by defects. The Dirac point energy and its fluctuations, peak energies of surface defects, the positions of the valence and conduction band edges, and the energy gap were determined from an analysis of tunneling spectra. A correlation between the parameters of surface states of Dirac fermions and thermoelectric properties was found. Thus, a contribution of the fermions surface states increases with rise of the surface concentration in solid solutions with high power factor, and the largest concentration value was observed in the Bi 1.98 In 0.02 Te 2.85 Se 0.15 composition. The dependences of Fermi energy on the wave vector for different solid solutions are described by a set of Dirac cone sections located within the limits of the fluctuations of the Dirac point energy that explained by weak changes of the Fermi velocity for studied atomic substitutions in sublattices of bismuth telluride., (© 2020 IOP Publishing Ltd.)