![]() ![]() 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. In this work, we have pursued detailed first-principles calculations to investigate the atomic structures, electronic and vibrational properties of few-layer TaSe 2 in the undistorted normal states. Many theoretical and experimental efforts have been devoted to semiconducting TMDs, but only a few theoretical calculations have been performed on metallic few-layer TaSe 2 36, 37. Similar effect has also been reported for ultrathin nanosheets of VSe 2 (4–8 trilayers): the CDW transition temperature increases from 100 K 34 in crystalline bulk to 135 K 35. Recent experiments have already demonstrated that the CDW transition temperature of mechanically exfoliated TiSe 2 films increases from ~200 K 32 to ~240 K 33 after the thickness is reduced to a few nanometers. The recently reported mechanical exfoliation of few-layer TaSe 2 (down to four trilayers) leads to a major question: what is the role of dimensionality and interlayer interactions in the evolution of structural and electronic properties including the CDW transitions 25, 26, 27, 28, 29, 30, 31. Besides being an importan CDW material 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, TaSe 2 has interesting applications as interconnect in devices 3, 4. In contrast, the 2H polytype does not transition into the incommensurate CDW state until ~120 K and the commensurate CDW state starts below 90 K 7. The bulk 1T-TaSe 2 transitions from normal state to incommensurate CDW state below 600 K and into commensurate CDW state below 473 K 6, 7, 9. TaSe 2 is a typical TMD that exhibits rich CDW phase transitions. Previous studies have mainly discussed the importance of Fermi-surface nesting 6, van Hove singularities 10 and electron-phonon coupling 11, 12, 13. The driving mechanisms for the CDW transition are still not completely understood. 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.Įarly 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 electronic properties of these 2D nanocrystals range from insulating to semiconducting, metallic and even superconducting and can differ dramatically from bulk crystals 1. Two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs) with chemical formula MX 2 (where M = Mo, W, Nb, Ta, or Ti and X = S, Se, or Te) have received remarkable attention due to their diverse properties and potential applications 1. Our results indicate strong electronic interlayer coupling, detail energetically possible stacking geometries and provide a basis for interpretation of Raman spectra. 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. 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. However, a description of the structural, electronic and vibrational properties for different crystal phases and stacking configurations, essential for interpretation of experiments, is lacking. Few-layer tantalum diselenides (TaSe 2) are typical metallic TMDs exhibiting rich CDW phase transitions. Two-dimensional metallic transition metal dichalcogenides (TMDs) are of interest for studying phenomena such as charge-density wave (CDW) and superconductivity. ![]()
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