Abstract
We report high-pressure Raman-scattering measurements on the transition-metal dichalcogenide (TMDC) compound HfS2. The aim of this work is twofold: (i) to investigate the high-pressure behavior of the zone-center optical phonon modes of HfS2 and experimentally determine the linear pressure coefficients and mode Grüneisen parameters of this material; (ii) to test the validity of different density functional theory (DFT) approaches in order to predict the lattice-dynamical properties of HfS2 under pressure. For this purpose, the experimental results are compared with the results of DFT calculations performed with different functionals, with and without Van der Waals (vdW) interaction. We find that DFT calculations within the generalized gradient approximation (GGA) properly describe the high-pressure lattice dynamics of HfS2 when vdW interactions are taken into account. In contrast, we show that DFT within the local density approximation (LDA), which is widely used to predict structural and vibrational properties at ambient conditions in 2D compounds, fails to reproduce the behavior of HfS2 under compression. Similar conclusions are reached in the case of MoS2. This suggests that large errors may be introduced if the compressibility and Grüneisen parameters of bulk TMDCs are calculated with bare DFT-LDA. Therefore, the validity of different approaches to calculate the structural and vibrational properties of bulk and few-layered vdW materials under compression should be carefully assessed.
Highlights
Transition metal dichalcogenides (TMDCs) have attracted a great deal of attention during the last few years due to their remarkable properties and great potential for photonics and optoelectronics applications[1]
Three different types of calculations were performed with ABINIT in order to find the best agreement between theoretical and experimental lattice constants of HfS2 at ambient pressure: (i) calculations within the generalized gradient approximation (GGA), using fully-relativistic projector augmented-wave (PAW) potentials with Perdew-Burke-Ernzerhof (PBE) exchange-correlation functionals[20]; (ii) PAW-PBE calculation including Grimme’s D3 dispersion correction[21] to take into account long-range van der Waals interactions; (iii) calculations within the Perdew-Wang local density approximation (LDA); density functional theory (DFT)-LDA usually provides successful structural results in layered compounds because of a compensating effect between the overestimated covalent part of the interlayer bonding and the neglected Van der Waals (vdW) forces[14]
Similar bulk modulus values are found with GGA-vdW and LDA, the latter yields a somewhat lower compressibility
Summary
Transition metal dichalcogenides (TMDCs) have attracted a great deal of attention during the last few years due to their remarkable properties and great potential for photonics and optoelectronics applications[1]. Despite a great deal of research has been devoted to the study of Mo and W-based TMDCs, relatively little attention has been paid to Hf-containing layered compounds. The latter are semiconductor materials with band gaps in the visible or infrared spectral range (the band gap decreases with increasing the chalcogen atomic number)[3], which makes them attractive for a wide variety of optoelectronic devices like third-generation solar cells[4]. Recent advances on Hf-based TMDCs include the investigation of their promising thermoelectric properties[5,6,7], their epitaxial growth on graphite or MoS2 substrates[8], or the realization of different devices such as tunnel field-effect transistors (TFETs)[9] or few-layer transistors[10]. In order to fully understand the vibrational properties of monolayer and few-layered TMDCs, it is essential to throroughly investigate the lattice dynamics of the buk materials
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