The effect of four-phonon interaction on phonon thermal conductivity of hexagonal VTe2 and puckered pentagonal VTe2.

The traditional view is that complex structures have lower lattice thermal conductivity. However, it is observed that complex structures have higher lattice thermal conductivity than simple atomic structures in VTe2 systems after considering the four-phonon scattering effect. In this work, we calculate the lattice thermal conductivity of an H-VTe2 monolayer with a simple atomic structure and that of a PP-VTe2 monolayer with a complex atomic arrangement using first-principles calculations combined with the Boltzmann transport theory under the conditions of with and without the four-phonon scattering process. Our findings reveal that the lattice thermal conductivity of the PP-VTe2 monolayer along the x or y direction is 3-4 times lower than that of the H-VTe2 monolayer when only considering the three-phonon scattering process. After taking into account the four-phonon scattering process, the lattice thermal conductivity of both monolayers decreases. For the H-VTe2 monolayer, the lattice thermal conductivity decreases by 88.7% (from 1.33 to 0.15 W m-1 K-1) compared to only considering the three-phonon scattering process, mainly due to strong four-phonon scattering. In addition, the PP-VTe2 monolayer experiences a lower decrease in lattice thermal conductivity, with reductions of 12.5% (from 0.4 to 0.35 W m-1 K-1) and 11.7% (from 0.34 to 0.3 W m-1 K-1) in the x and y directions, respectively, because of the weak four-phonon scattering. Notably, the lattice thermal conductivity with the four-phonon scattering process of the H-VTe2 monolayer is twice as low as that of the PP-VTe2 monolayer. Hence, our findings suggest that even simple atomic structures can exhibit lower lattice thermal conductivity than complex structures when considering four-phonon interaction.

Strain-controlled electronic and magnetic properties of tVS2/hVS2 van der Waals heterostructures.

The structural, electronic and magnetic properties of the T-phase and H-phase of the VS2 monolayer and their heterobilayers are studied by means of first-principles calculations. We find that the two phases of the VS2 monolayer are both ferromagnetic (FM) semiconductors and that these two monolayers form a typical van der Waals (vdW) heterostructure with a weak interlayer interaction. By comparing the energy of different magnetic configurations, the FM state of the tVS2/hVS2 heterostructure is found to be in the ground state under normal conditions or biaxial strains. Under compressive strains, the anti-FM (AFM) and FM states degenerate. Based on the band structure obtained and the work function, it is found that the T-phase and H-phase are capable of forming an efficient p-n heterostructure. Due to spontaneous charge transfer at the interface, a gapless semiconductor is formed in our HSE06 calculations. We also find that the twist angle between the monolayers has a negligible impact on the band structure of the heterostructure in its spin-down channel. Moreover, the tVS2/hVS2 heterostructure is found to switch from a gapless semiconductor to a metal or a half-metal under some given biaxial or uniaxial strains. Therefore, the heterostructure could be a half-metallic property with strains, realizing 100% polarization at the Fermi level. Our study provides the possibility of realizing 100% spin-polarization at the Fermi level in these FM vdW heterostructures, which is significant for further spin transport exploration.

Density Functional Theory plus Hubbard U Study of the Segregation of Pt to the CeO2- x Grain Boundary.

Grain boundaries (GBs) can be used as traps for solute atoms and defects, and the interaction between segregants and GBs is crucial for understanding the properties of nanocrystalline materials. In this study, we have systematically investigated the Pt segregation and Pt-oxygen vacancies interaction at the ∑3 (111) GB in ceria (CeO2). The Pt atom has a stronger tendency to segregate to the ∑3 (111) GB than to the (111) and (110) free surfaces, but the tendency is weaker than to (112) and (100). Lattice distortion plays a dominant role in Pt segregation. At the Pt-segregated-GB (Pt@GB), oxygen vacancies prefer to form spontaneously near Pt in the GB region. However, at the pristine GB, oxygen vacancies can only form under O-poor conditions. Thus, Pt segregation to the GB promotes the formation of oxygen vacancies, and their strong interactions enhance the interfacial cohesion. We propose that GBs fabricated close to the surfaces of nanocrystalline ceria can trap Pt from inside the grains or other types of surface, resulting in the suppression of the accumulation of Pt on the surface under redox reactions, especially under O-poor conditions.

Lithium boride sheet and nanotubes: structure and hydrogen storage.

A new class of Li-B sheets, along with the related nanotubes, with a Li2B5 primitive cell has been designed using first-principles density functional theory. The dynamical stability of the proposed structures was confirmed by calculation of the soft phonon modes, and the calculated electronic structures show that all are metallic. The application of both the sheets and nanotubes for hydrogen storage has been investigated and it has been found that both of them can adsorb two H2 molecules around each Li atom, with an average binding energy of 0.152-0.194 eV per H2, leading to a gravimetric density of 10.6 wt%.

Two-dimensional monolayer BiSnO3: a novel wide-band-gap semiconductor with high stability and strong ultraviolet absorption.

The exploration of two-dimensional (2D) wide-band-gap semiconductors (WBGSs) holds significant scientific and technological importance in the field of condensed matter physics and is actively being pursued in optoelectronic research. In this study, we present the discovery of a novel WBGS, namely monolayer BiSnO3, using first-principles calculations in conjunction with the quasi-particle G0W0approximation. Our calculations confirm that monolayer BiSnO3exhibits moderate cleavage energy, positive phonon modes, mechanical resilience, and high temperature resistance (up to 1000 K), which demonstrate its structural stability, flexibility, and potential for experimental realization. Furthermore, band-structure calculations reveal that monolayer BiSnO3is a typical WBGS material with a band-gap energy (Eg) of 3.61 eV and possesses a unique quasi-direct electronic feature due to its quasi-flat valence band. The highest occupied valence flat-band originates from the electronic hybridization between Bi-6pand O-2pstates, which are in close proximity to the Fermi level. Remarkably, monolayer BiSnO3exhibits a high absorption capacity for ultraviolet light spanning the UVA to UVC regions, displaying optical isotropy absorption and an unusual excitonic effect. These intriguing structural and electronic properties establish monolayer BiSnO3as a promising candidate for the development of new multi-function-integrated electronic and optoelectronic devices in the emerging field of 2D WBGSs.

Two-dimensional ferromagnetic semiconductors of monolayer BiXO3 (X = Ru, Os) with direct band gaps, high Curie temperatures, and large magnetic anisotropy.

Two-dimensional (2D) ferromagnetic semiconductors are highly promising candidates for spintronics, but are rarely reported with direct band gaps, high Curie temperatures (Tc), and large magnetic anisotropy. Using first-principles calculations, we predict that two ferromagnetic monolayers, BiXO3 (X = Ru, Os), are such materials with a direct band gap of 2.64 and 1.69 eV, respectively. Monte Carlo simulations reveal that the monolayers show high Tc beyond 400 K. Interestingly, both BiXO3 monolayers exhibit out-of-plane magnetic anisotropy, with magnetic anisotropy energy (MAE) of 1.07 meV per Ru for BiRuO3 and 5.79 meV per Os for BiOsO3. The estimated MAE for the BiOsO3 sheet is one order of magnitude larger than that for the CrI3 monolayer (685 µeV per Cr). Based on the second-order perturbation theory, it is revealed that the large MAE of the monolayers BiRuO3 and BiOsO3 is mainly contributed by the matrix element differences between dxy and dx2-y2 and dyz and dz2 orbitals. Importantly, the ferromagnetism remains robust in 2D BiXO3 under compressive strain, while undergoing a ferromagnetic to antiferromagnetic transition under tensile strain. The intriguing electronic and magnetic properties make BiXO3 monolayers promising candidates for nanoscale electronics and spintronics.

A van der Waals Density Functional Investigation on the Improved Adsorption Properties of NO on the Rhn/MgO (100) Interface.

Using spin-polarized density functional theory calculations, we have studied the interaction and morphology of small Rhn clusters (n≤5) deposited on a MgO (100) surface, as well as the ability of Rhn/MgO systems to trap NO molecules. We show that Rhn clusters can tightly adhere to a MgO (100) surface because of hybridization of O 2p and Rh 4d orbitals. The most energetically preferred structure for each adsorbed Rhn cluster looks similar to the structure of the isolated cluster, except for an increment in the length of base edge(s) close to the substrate. We find that the Rhn/MgO system exhibits a much stronger ability to trap NO, especially for Rh2 and Rh4 clusters, than do the bare Rhn clusters. The hybridization of N 2p and Rh 4d orbitals is responsible for the strong adsorption of NO on Rhn/MgO. This can be explained by the increased Rh-Rh bond length that results in a decrease in the effective coordination number, and the presence of extra electrons obtained from the substrate, causing the N and O atoms to bond strongly to the Rh atoms, which in turn weakens the N-O molecular interaction. Additionally, binding to NO induces a reduction in the total magnetic moment of the Rhn/MgO system. Thus, putting Rhn on the MgO (100) surface can decrease competition between bonding and magnetism. However, whether it can be related to improve trapping of NO deserves further investigations.

First-principles investigation on the segregation of Pd at LaFe1-xPdxO3-y surfaces.

First-principles calculations were performed to investigate the effect of Pd concentration and oxygen vacancies on the stability of Pd at LaFeO3 surfaces. We found a much stronger tendency of Pd to segregate by taking the aggregation of Pd at LaFe1-xPdxO3-y surfaces into consideration, resulting in a pair of Pd-Pd around a vacancy. Moreover, we predicted that one oxygen-vacancy-containing FeO2-terminated surfaces would be stable at high temperatures by comparing the stability of LaFe1-xPdxO3-y surfaces, which further supports our previous conclusion that a Pd-containing perovskite catalyst should be calcined at 1,073 K or higher temperatures in air to enhance the segregation of Pd in the vicinity of surfaces to rapidly transform the Pd catalyst from oxidized to reduced states on the perovskite support.