MoO₃ and MoO₂ systems have attracted particular attention for many widespread applications thanks to their electronic and optical peculiarities; from the crystallographic point of view, MoO₃ adopts a thermodynamically stable orthorhombic phase (α-MoO₃) belonging to the space group Pbmn, while MoO₂ assumes a monoclinic arrangement characterized by space group P2₁/c. In the present paper, we investigated the electronic and optical properties of both MoO₃ and MoO₂ by using Density Functional Theory calculations, in particular, the Meta Generalized Gradient Approximation (MGGA) SCAN functional together with the PseudoDojo pseudopotential, which were used for the first time to obtain a deeper insight into the nature of different Mo–O bonds in these materials. The calculated density of states, the band gap, and the band structure were confirmed and validated by comparison with already available experimental results, while the optical properties were validated by recording optical spectra. Furthermore, the calculated band-gap energy value for the orthorhombic MoO₃ showed the best match to the experimental value reported in the literature. All these findings suggest that the newly proposed theoretical techniques reproduce the experimental evidence of both MoO₂ and MoO₃ systems with high accuracy.

HfO₂ can assume different crystalline structures, such as monoclinic, orthorhombic, and cubic polymorphs, each one characterized by unical properties. The peculiarities of this material are also strongly related to the presence of doping elements in the unit cell. Thus, the present paper has the main purpose of studying and comparing twelve different systems characterized by diverse polymorphs and doping percentages. In particular, three different crystalline structures were considered: the monoclinic P2₁/c, the orthorhombic Pca2₁, and the cubic Fm$\overline{3}$m phases of HfO₂. Each one has been studied by using Y as a doping agent with three different contents: 0% Y:HfO₂, 8% Y:HfO₂, 12% Y:HfO₂, and 16% Y:HfO₂. For all the systems, density functional theory (DFT) methods based on PBE/GGA, and on the HSE hybrid functionals were used to optimize the geometry as well as to study their optical properties. Depending on the polymorphs, Y affects the formation energy in different ways and causes changes in the optical properties. When the percentage of Y did not exceed 12%, a stabilization of the cubic phase fraction and an increase of the dielectric constant was observed. Additionally, the calculated optical bandgap energies and the refractive index are examined to provide an overview of the systems and are compared with experimental data. The bandgaps obtained are in perfect agreement with the experimental values and show a slight increase as the doping percentage grows, while only minor differences are found between the three polymorphs in terms of both refractive index and optical band gap. The adopted first principles study generates a reasonable prediction of the physical-chemical properties of all the systems, thus identifying the effects of doping phenomena.

We have studied the structural, electronic, magnetic, and optical properties of the VO ₂ (B) polymorph using first-principles calculations based on density functional theory (DFT).

In this paper we report on the use of an Ullmann-like aryl halide homocoupling reaction to obtain long Graphyne Molecular Wires (GY MWs) organized in dense, ordered arrays. Instead of using highly reactive terminal alkynes, we resort to a precursor wherein the acetylenic functional group is internal, namely protected by two phenyl rings, each bearing a Br atom in the para position to allow for linear homocoupling. In addition, two further factors concur with the production of dense and highly ordered arrays of very long GY MWs, namely the geometric compatibility between the substrate and both the organometallic intermediates and the final polymeric products of the synthesis, coupled with the presence of surface-adsorbed bromine atoms separating the MWs, which minimize inter-wire cross-linking secondary reactions.

HfO₂ shows different polymorphs, including monoclinic and orthorhombic ones, that exhibit singular properties. Moreover, the character of HfO₂ is also influenced by the Zr atoms as a doping agent. Here, an extensive study of the monoclinic P2₁/c and the orthorhombic Pca2₁ polymorphs of HfO₂, Hf_0.75Zr_0.25O₂, and Hf_0.5Zr_0.5O₂ is reported. For all six systems, density functional theory (DFT) methods based on generalized gradient approximations (GGAs) were first used; then the GGA + U method was settled and calibrated to describe the electrical and optical properties of polymorphs and the responses to the oxygen vacancies. Zr had different effects in relation to the polymorph; moreover, the amount of Zr led to important differences in the optical properties of the Pca2₁ polymorph. Finally, oxygen vacancies were investigated, showing an important modulation of the properties of Hf_xZr_yO₂ nanostructures. The combined GGA and GGA + U methods adopted in this work generate a reasonable prediction of the physicochemical properties of o- and m-Hf_xZr_yO₂, identifying the effects of doping phenomena.

A VO₂(B) polymorph has been thoroughly investigated by means of density functional theory (DFT) calculations to evaluate the structure, Raman spectrum, cohesive energy, phonon band structure, an delectronic and optical properties.