No abstract

Cu 2 MnSnS 4 shares several promising properties with the widely investigated Cu 2 ZnSnS 4 for photovoltaic applications such as containing only earth abundant and non-toxic elements, and suitable absorption characteristics for absorber materials. Thin film Cu 2 MnSnS 4 samples with various cation compositions are co-sputtered reactively followed by a high temperature anneal. Formation of Cu 2 MnSnS 4 and co-existence of several secondary phases is verified by XRD and Raman. Our investigation of the crystal structure based on first-principles DFT confirms that stannite crystal structure is preferred over kesterite, although, further verification considering cation disorder is needed. The direct band gap of Cu 2 MnSnS 4 is calculated as 1.52 eV (1.62 eV) for stannite (kesterite), which coincides with the range of the measured band gaps from spectrophotometry of 1.42-1.59 eV. After further annealing treatments below 240 C, the absorption shows reversible changes: the band gap blue-shifts and the Urbach tail energy is reduced. It is concluded that, just like Cu 2 ZnSnS 4 , disorder also occurs in Cu 2 MnSnS 4 . The implications of our findings are discussed and related to the current understanding of cation disorder in Cu 2 ZnSnS 4 and related compounds. Furthermore, for the first time first-principles DFT investigations are presented for the thiospinel Cu 2 MnSn 3 S 8 which is observed experimentally as a secondary phase in Sn-rich Cu 2 MnSnS 4 thin films.

A new structural phase is discovered for M2CO2 MXenes with M = Sc, Y, La, Lu, Tm, and Ho. The hexagonal carbon layer sandwiched between M atoms, typical for MXenes, is transformed into C3 trimers with anionic electrons localized in quasi zero‐dimensional lattice spaces in‐between the C3 units, so the systems can be described as [M6 C3 O6]+II : 2e− electrides. The systems are readily ionized into [M6 C3 O6]+II with very low ionization energy via an anti‐doping mechanism. It is shown that this new structure of Sc2CO2 can bind multiple lithium atoms, with low migration barriers. The findings indicate that these M2CO2 MXenes with unusual carbon trimers are a new family of 2D electride insulators with the potential for charge storage applications, thermal field emission, and as anode material in lithium batteries.

No abstract

First-principles many-body theory and time-dependent density functional theory were used to study the dimension effects on the band alignment and optical properties of s-triazine and graphitic C3N4.

First-principles quasi-particle theory has been employed to assess catalytic power of graphitic carbon nitride, g-C3N4, for solar fuel production. A comparative study between g-h-triazine and g-h-heptazine has been carried out taking also into account van der Waals dispersive forces. The band edge potentials have been calculated using a recently developed approach where quasi-particle effects are taken into account through the GW approximation. First, it was found that the description of ground state properties such as cohesive and surface formation energies requires the proper treatment of dispersive interaction. Furthermore, through the analysis of calculated band-edge potentials, it is shown that g-h-triazine has high reductive power reaching the potential to reduce CO2 to formic acid, coplanar g-h-heptazine displays the highest thermodynamics force toward H2O/O2 oxidation reaction, and corrugated g-h-heptazine exhibits a good capacity for both reactions. This rigorous theoretical study shows a route to further improve the catalytic performance of g-C3N4.

We have studied, by means of first-principles calculations, the electronic and optical properties of the sulvanite family: CuMX (M  =  V, Nb, Ta and X  =  S, Se), which, due to its broad range of gaps and chemical stability, have emerged as promising materials for technological applications such as photovoltaics and transparent conductivity. To address the reliability of those properties we have used semi-local and hybrid functionals (PBEsol, HSE06), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and time-dependent density functional theory (revised bootstrap kernel) to calculate the quasi-particle dispersion relation, band gaps, the imaginary part of the macroscopic dielectric function and the absorption coefficient. The calculated valence band maximum and the conduction band minimum are located at the R and X-points, respectively. The calculated gaps using PBEsol are between 0.81 and 1.88 eV, with HSE06 into 1.73 and 2.94 eV, whereas the GW values fall into the 1.91-3.19 eV range. The calculated dielectric functions and absorption coefficients show that all these compounds present continuous excitonic features when the Bethe-Salpeter equation is used. Contrarily, the revised bootstrap kernel is incapable of describing the excitonic spectra. The calculated optical spectra show that CuVS and CuMSe have good absorption in the visible, whereas CuNbS and CuTaS have it on the near ultraviolet.

No abstract