Abstract
Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. Few-layer tantalum diselenides (TaSe2) are typical metallic TMDs exhibiting rich CDW phase transitions. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. We present first- principles calculations of structural phase energetics, band dispersion near the Fermi level, phonon properties and vibrational modes at the Brillouin zone center for different layer numbers, crystal phases and stacking geometries. Evolution of the Fermi surfaces as well as the phonon dispersions as a function of layer number reveals dramatic dimensionality effects in this CDW material. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries, and provide a basis for interpretation of Raman spectra.
Highlights
Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity
Research into metallic TMDs was done with bulk materials and focused on charge-density wave (CDW), a structural distortion which results in a further electronic stabilization of the system, similar to the Peierls distortion in one-dimensional atomic chains[6,7,8,9]
The 2H polytype does not transition into the incommensurate CDW state until ~120 K, and the commensurate CDW state starts below 90 K7
Summary
Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. The electronic properties of these 2D nanocrystals range from insulating to semiconducting, metallic and even superconducting, and can differ dramatically from bulk crystals[1] The versatility of these materials is shown by the wide range of reported applications including: electrocatalysts for hydrogen evolution[1], opto- and spintronics[2], electrodes and interconnects[3,4], and electro-optical switch and data storage devices[5]. Our calculations reveal a strong dependence of the electronic and phonon properties on the layer number and on the the stacking geometry, and we discuss the implications of the effect of dimensionality on the CDW transition in this material
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