Chemical thermodynamics of selenium

Early transition metal dichalcogenides (TMDC), characterized by their quasi-two-dimensional layered structure, have attracted intensive interest due to their versatile chemical and physical properties, but a comprehensive understanding of their structural and electronic properties from a first-principles point of view is still lacking. In this work, four simple TMDC materials, MX2 (M = Zr and Hf, X = S and Se), are investigated by the Kohn-Sham density functional theory (KS-DFT) with different local or semilocal exchange-correlation (xc) functionals and many-body perturbation theory in the GW approximation. Although the widely used Perdew-Burke-Ernzelhof (PBE) generalized gradient approximation (GGA) xc functional overestimates the interlayer distance dramatically, two newly developed GGA functionals, PBE-for-solids (PBEsol) and Wu-Cohen 2006 (WC06), can reproduce experimental crystal structures of these TMDC materials very well. The GW method, currently the most accurate first-principles approach for electronic band structures of extended systems, gives the fundamental band gaps of all these materials in good agreement with the experimental values obtained from optical absorption. The minimal direct gaps from GW are systematically larger than those measured from thermoreflectance by about 0.1–0.3 eV, implying that excitonic effects may be stronger than previously estimated. The calculated density of states from GW quasi-particle band energies agrees very well with photo-emission spectroscopy data. Ionization potentials of these materials are also computed by combining PBE calculations based on the slab model and GW quasi-particle corrections. The calculated absolute band energies with respect to the vacuum level indicate that that ZrS2 and HfS2, although having suitable band gaps for visible light absorption, cannot be used for overall water splitting as a result of mismatch of the conduction band minimum with the redox potential of H+/H2.

Structural and electronic properties of ZrX2 and HfX2 (X = S and Se) from first principles calculations

Prediction of amorphous alloy formation by ion beam mixing

In recent years, the transition metal trichalcogenides (TX3) of Group IVB, VB and VIB have received much more attention because of the considerable diversity in their physical properties. The most striking feature of these compounds is that the structure here may be classified into three types depending on the number of different TX3 chains present in the unit cell. Thus ZrSe3, TaSe3 and NbSe3 are the representative compounds having one, two and three types of chain based on the different bond lengths for the (X2)2− pairs in the base of the TX3 trigonal prismatic framework. A similar model is also applicable in the case of NbS3, with the addition of a 2b-superstructure associated with the formation of niobium pairs. The chain structure also facilitates the process of intercalation which has been most effectively used in secondary batteries. These compounds exhibit the superconductivity phenomena and charge density wave, etc. and also find application in photoelectrochemical cells. An attempt has been made here to review the up-to-date chemistry of transition metal trichalcogenides related with their preparation, structure and properties such as physical and chemical, thermodynamic, electrical, magnetic and optical properties, intercalation and use in the photoelectrochemical cells. 93

Preparation, structure and properties of transition metal trichalcogenides

The strong in-plane anisotropy and quasi-1D electronic structures of transition-metal trichalcogenides (MX3; M = group IV or V transition metal; X = S, Se, or Te) have pronounced influence on moulding the properties of MX3 materials. In particular, the infinite trigonal MX6 prismatic chains running parallel to the b-axis are responsible for the manifestation of anisotropy in these materials. Several marvellous properties, such as inherent electronic, optical, electrical, magnetic, superconductivity, and charge density wave (CDW) transport properties, make transition-metal trichalcogenides (TMTCs) stand out from other 2D materials in the fields of nanoscience and materials science. In addition, with the assistance of pressure, temperature, and tensile strain, these materials and their exceptional properties can be tuned to a superior extent. The robust anisotropy and incommensurable properties make the MX3 family fit for accomplishing quite a lot of compelling applications in the areas of field effect transistors (FETs), solar and fuel cells, lithium-ion batteries, thermoelectricity, etc. In this review article, a precise audit of the distinctive crystal structures, static and dynamic properties, efficacious synthesis schemes, and enthralling applications of quasi-1D MX3 materials is made.

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Anisotropic quasi-one-dimensional layered transition-metal trichalcogenides: synthesis, properties and applications

Electrical properties of vapour grown ZrSe3 single crystals

Thermoreflectance spectra of ZrS3-xSex single crystals have been measured at 50 K The thermoreflectance spectra of ZrS3 and ZrSe3 are discussed through direct and indirect exciton transitions The observation of indirect transitions is due to a reflection effect on the back surface of the sample A calculation that accounts for multiple reflections and agrees with the known values of the absorption coefficient at the indirect fundamental gap is performed The direct exciton structures are described by a classical oscillator model which includes a dead layer at the surface of the crystal Both direct and indirect exciton energies of the solid solution ZrS3-xSex are observed as a function of x

https://iopscience.iop.org/article/10.1088/0022-3719/21/3/016/pdf

Optical transitions in ZrS3-xSex single crystals through temperature-modulated reflectivity

Thermoreflectance spectra of single-crystal ZrS2 and ZrSe2 were obtained at 60K. A Kramers-Kronig analysis was carried out and fits of experimental curves calculated. The resulting parameters are interpreted on the basis of existing band-structure and spin-orbit splitting calculations. A simplified band picture is presented.

Optical transitions in ZrS2 and ZrSe2 from thermoreflectance measurements

Spontaneous and stimulated photoluminescence of GaSe in the energy range 2.075–2.125 eV

The authors report the first-order Raman spectra of one-dimensional solid solution ZrS2.5Se0.5. The Raman active modes have been correlated with those of ZrS3 and ZrSe3.

https://iopscience.iop.org/article/10.1088/0022-3719/14/16/002/pdf

R. Provencher

Papers

Electrical properties of vapour grown ZrSe3 single crystals

Optical transitions in ZrS3-xSex single crystals through temperature-modulated reflectivity

Optical transitions in ZrS2 and ZrSe2 from thermoreflectance measurements

Spontaneous and stimulated photoluminescence of GaSe in the energy range 2.075–2.125 eV

Raman spectra of ZrS2.5Se0.5