Vibrational and dielectric properties of the bulk transition metal dichalcogenides

Interest in the bulk transition metal dichalcogenides for their electronic, photovoltaic, and optical properties has grown and led to their use in many technological applications. We present a systematic investigation of their interlinked vibrational and dielectric properties, using density functional theory and density functional perturbation theory, studying the effects of the spin-orbit interaction and of the long-range e$^-$- e$^-$ correlation as part of our investigation. This study confirms that the spin-orbit interaction plays a small role in these physical properties, while the direct contribution of dispersion corrections is of crucial importance in the description of the interatomic force constants. Here, our analysis of the structural and vibrational properties, including the Raman spectra, compare well to experimental measurement. Three materials with different point groups are showcased and data trends on the full set of fifteen existing hexagonal, trigonal, and triclinic materials are demonstrated. This overall picture will enable the modeling of devices composed of these materials for novel applications.Comment: 11 pages, 6 figure

Vibrational and dielectric properties of monolayer transition metal dichalcogenides

First-principles studies of two-dimensional transition metal dichalcogenides have contributed considerably to the understanding of their dielectric, optical, elastic, and vibrational properties. The majority of works to date focus on a single material or physical property. Here we use a single first-principles methodology on the whole family of systems, to investigate in depth the relationships between different physical properties, the underlying symmetry and the composition of these materials, and observe trends. We compare to bulk counterparts to show strong interlayer effects in triclinic compounds. Previously unobserved relationships between these monolayer compounds become apparent. These trends can then be exploited by the materials science, nanoscience, and chemistry communities to better design devices and heterostructures for specific functionalities.Comment: 4 figures, 11 page

Two-Step Phase Transition In Snse And The Origins Of Its High Power Factor From First Principles

The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure. In this Letter, we present a complete temperature evolution of the Seebeck coefficient as the material undergoes a soft crystal transformation and its consequences on other properties within SnSe by means of first-principles calculations. Our results are able to explain the full range of considered experimental temperatures

Spontaneous interlayer compression in commensurately stacked van der Waals heterostructures

Interest in layered two-dimensional materials, particularly stacked heterostructures of transition-metal dichalcogenides, has led to the need for a better understanding of the structural and electronic changes induced by stacking. Here, we investigate the effects of idealized heterostructuring, with periodic commensurate stacking, on the structural, electronic, and vibrational properties, when compared to the counterpart bulk transition-metal dichalcogenide. We find that in heterostructures with dissimilar chalcogen species there is a strong compression of the interlayer spacing, compared to the bulk compounds. This compression of the heterostructure is caused by an increase in the strength of the induced polarization interaction between the layers, but not a full charge transfer. We argue that this effect is real, not due to the imposed commensurability, and should be observable in heterostructures combining different chalcogens. Interestingly, we find that incommensurate stacking of Ti-based dichalcogenides can lead to the stabilization of the charge-density wave phonon mode, which is unstable in the 1T phase at low temperature. Mixed Ti- and Zr-based heterostructures are still dynamically unstable, but TiS2/ZrS2 becomes ferroelectric. © 2021 American Physical Society

Theoretical investigation of the electronic, vibrational and transport properties of layered transition metal chalcogenides and their stacked heterostructures

Chalcogenides exhibit a wide range of physical properties making them very at- tractive for future electronic and thermoelectric applications. Their layered nature allows them to be easily isolated in layers just few atoms thick. The new exciting properties arising when reducing the dimensionality of materials have motivated the scientific community to investigate these materials. In this thesis, we theoret- ically investigate the structural, electronic and vibrational properties of two fam- ilies of chalcogenides : mono-chalcogenides and Transition Metal Dichalcogenides (TMDs). This theoretical investigation was conducted with the powerful predictive capacities of Density Functional Theory and the Temperature Dependent Effec- tive Potential method. We investigate the excellent thermoelectric properties of SnSe, the nature of the phase transition occurring with temperature and highlight the link between the two. We study the structural transformation in few-layer SnSe and SnS, and its effect on vibrational properties. The finite temperature behavior of the vibrational properties of a selection of TMDs in their bulk form is presented, and their thermal conductivity is compared. We study the effects of heterostructuring by alternatively stacking different TMDs. We show a contrac- tion of the Van der Waals gap when TMDs with different chalcogens are combined. We also explain the suppression of the charge density wave instability in certain heterostructures. Finally we investigate the transfer of charge and the electronic properties of heterostructures of mono and di-chalcogenides called ferecrystals

Ab-initio Study of Ferecrystals

Ferecrystals are a new family of compounds first synthesized in 2007 by the group of D. Johnson at the University of Oregon. These materials consist of inter-growths of dichalcogenide and chalcogenide layers, and can be written as [(M X)_{1+δ} ]_m [T X_2 ]_n where M = Sn, Pb, Sb, Ni and some rare earths; T = Ti, V, Cr, Nb and Ta; X= S and Se. The integers m and n denote the numbers of consecutive formula unit layers in the different components of the inter-growth. The δ parameter reflects the difference of the in-plane cell constants between components of the inter-growth. This family of nanostructured materials shows promising properties for thermoelectric devices. The compounds studied here are [(SnSe)_1.29 ]_{234} [M oSe_2 ]_1 . We performed structural characterisation and examined the transfer of charge at the interface between the two materials. We show that there is a depletion of charge at the interface between the two compo- nents of the heterostructures and that structural distortions of the SnSe layers in the supercell are similar to those observed in slab calculations

Coupled Boltzmann Equation Solver: Effects of the Electron-Phonon Interaction on the Transport Coefficients

Recent experimental and theoretical calculations point to a complex interplay between the electron and phonon baths in a wide variety of materials [1,2]. We propose a method of coupling the Boltzmann equations for the electron and phonon baths within the relaxation time approximation which we use to calculate the thermoelectric transport coefficients. Our model for the coupled Boltzmann Equation solver includes analytic models, including ${\bf k \cdot p}$ Hamiltonians and tight-binding Hamiltonians, for both the electron and phonon energies and analytic models for the electron and phonon relaxation mechanisms. From these calculations we hope to better understand the role and interplay of electron-phonon and phonon-phonon interactions on the thermoelectric transport coefficients. [1] - Phys. Rev. Lett. ${\bf 114}$, 115901 (2015). [2] - PNAS ${\bf 112}$, 14777-14782 (2015)

Spectroscopic properties of few-layer tin chalcogenides

Stable structures of layered SnS and SnSe and their associated electronic and vibrational spectra are predicted using first-principles DFT calculations. The calculations show that both materials undergo a phase transformation upon thinning whereby the in-plane lattice parameters ratio a/b converges towards 1, similar to the high-temperature behaviour observed for their bulk counterparts. The electronic properties of layered SnS and SnSe evolve to an almost symmetric dispersion whilst the gap changes from indirect to direct. Characteristic signatures in the phonon dispersion curves and surface phonon states where only atoms belonging to surface layers vibrate should be observable experimentally

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

Despite numerous studies of transition metal dichalcogenides, the diversity of their chemical bonding characteristics and charge transfer is not well understood. Based on density functional theory we investigate their static and dynamic charges. The dynamic charge of the transition metal dichalcogenides with trigonal symmetry are anomalously large, while in their hexagonally symmetric counterparts, we even observe a counterintuitive sign, i.e., the transition metal takes a negative charge, opposite to its static charge. This phenomenon, so far never remarked on or analyzed, is understood by investigating the perturbative response of the system and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, a link is established between the sign of the Born effective charge and the process of π backbonding from organic chemistry. Experiments are proposed to verify the calculated sign of the dynamical charge in these materials. Employing a high-throughput search we also identify other materials that present counterintuitive dynamic charges

Boltzmann Transport Calculations in Systems with Electron-phonon Coupling

Recent experimental and theoretical calculations point to a complex interplay between the electron and phonon baths in a wide variety of materials [1,2]. We propose a method of coupling the Boltzmann equations for the electron and phonon baths within the relaxation time approximation to describe the changes in the electron and phonon distributions and thus calculate the thermoelectric transport coefficients. Our model for the coupled system will include tight-binding and ${\bf k \cdot p}$ Hamiltonians for both the electron and phonon energies and analytic calculations for the electron and phonon relaxation mechanisms. From these calculations we hope to better understand the role and interplay of electron-phonon and phonon-phonon interactions on the thermoelectric transport coefficients. [1] - Phys. Rev. Lett. ${\bf 114}$, 115901 (2015). [2] - PNAS ${\bf 112}$, 14777-14782 (2015)