Tuning Electrical Properties through Control of TiSe2 Thickness in (BiSe)1+δ(TiSe2)n Compounds

Abstract

A series of (BiSe)1+δ(TiSe2)n compounds where n was varied from two to four were synthesized and electrically characterized to explore the extent of charge transfer from the BiSe layer to the TiSe2 layers. These kinetically stable heterostructures were prepared using the modulated elemental reactants (MER) method, in which thin amorphous elemental layers are deposited in an order that mimics the nanostructure of the desired product. X-ray diffraction (XRD), X-ray area diffraction, and scanning transmission electron microscopy (STEM) data show that the precursors formed the desired products. Specular diffraction scans contain only 00l reflections, indicating that the compounds are crystallographically aligned with the c-axis perpendicular to the substrate. The c-axis lattice parameter increases by 0.604(3) nm with each additional TiSe2 layer. In-plane diffraction scans contain reflections that can be indexed as the (hk0) of the BiSe and TiSe2 constituents. Area diffraction scans are also consistent with the samples containing only BiSe and TiSe2 constituents. Rietveld refinement of the 00l XRD data was used to determine the positions of atomic planes along the c-axis. STEM data supports the structures suggested by the diffraction data and associated refinements but also shows that antiphase boundaries occur approximately 1/3 of the time in the BiSe layers. All samples showed metallic behavior for the temperature-dependent electrical resistivity between 20 K and room temperature. Electrical measurements indicated that charge is transferred from the BiSe layer to the TiSe2 layer. The measured Hall coefficients were all negative indicating that electrons are the majority carrier and are systematically decreased as n was increased. Assuming a single parabolic band model, carrier concentration decreased when the number of TiSe2 layers is increased, suggesting that the amount of charge donated by the BiSe layer to the TiSe2 layers is constant. Seebeck coefficients were negative for all of the (BiSe)1+δ(TiSe2)n compounds studied, indicating that electrons are the majority carrier, and decreased as n increased. The effective mass of the carriers was calculated to be 5–6 me for the series of compounds.