Abstract
A thorough investigation of the structural parameters of micromechanically exfoliated multilayer WSe2 flakes was undertaken between 400 K to 110 K. Crystal structure of WSe2 remains in the trigonal prismatic structure in this temperature range, however, with a clear difference in the temperature dependence of the in-plane a, and the out-of-plane c, lattice parameters. The linear coefficients of thermal expansion of a and c are 5.132 × 10−6/K and 8.105 × 10−6/K, respectively. The temperature dependence of the unit-cell volume is analyzed using zero-pressure equation-of-state which yielded the Debye temperature of the WSe2 to be 160 K. Following the temperature dependence of the W-Se and W-W bond distances, a nonlinear behavior is observed in the former in contrast to a rather regular behavior of the later. This significant difference in the temperature dependence of the a and c lattice parameters can have consequences in the macroscopic physical properties of the system. A good correlation between the temperature dependence of the W-Se bond distance and Raman E2g1 mode has been observed.
Highlights
Transition-metal dichalcogenides (TMDC) are materials formed by stacking a layer of transition metal atom in between chalcogenide atoms
TMDC have a layered structure with MX2 formula where M is metal atom and X is chalcogenide atom; the hexagonally packed metal atoms M are in a trigonal prismatic coordination with X atoms [1]
The M-X bonds within a layer are strongly covalent in nature and the interlayer interaction is of van der Waals type
Summary
Transition-metal dichalcogenides (TMDC) are materials formed by stacking a layer of transition metal atom in between chalcogenide atoms. The study of temperature-dependent structural evolution as manifested in the thermal expansion coefficients and phonon properties is of vital importance for the optical and electrical devices based on WSe2 [8]. Thermal expansion properties of WSe2 were investigated by Murray and Evans in 1979, the temperature values, the quality of the data available and the number of temperature data points made those authors to quantify the structural evolution was limited [10].
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