Investigations on the defect structures for Mn2+ in CdSe nanocrystals and bulk materials and the criterion of occupation for Mn2+ in CdX (X = S, Se, Te) nanocrystals.

The spin Hamiltonian parameters and defect structures are theoretically studied for the substitutional Mn2+ at the core of CdSe nanocrystals and in the bulk materials from the perturbation calculations of spin Hamiltonian parameters for trigonal tetrahedral 3d5 clusters. Both the crystal-field and charge transfer contributions are taken into account in the calculations from the cluster approach. The impurity-ligand bond angles are found to be about 1.84° larger and 0.10° smaller in the CdSe:Mn2+ nanocrystals and bulk materials, respectively, than those (≈109.37°) of the host Cd2+ sites. The quantitative criterion of occupation (at the core or surface) for Mn2+ in CdX (X = S, Se, Te) nanocrystals is presented for the first time based on the inequations of hyperfine structure constants (HSCs). This criterion is well supported by the experimental HSCs data of Mn2+ in CdX nanocrystals. The previous assignments of signals SI as Mn2+ at the core of CdS nanocrystals are renewed as Mn2+ at the surface based on the above criterion. The present studies would be helpful to achieve convenient determination of occupation for Mn2+ impurities in CdX semiconductor nanocrystals by means of spectral (e.g., HSCs) analysis.

Adsorption and diffusion of magnesium on nitrogen-doped Mo2C monolayer.

The Mg adsorption and diffusion behaviors on nitrogen-doped (N-doped) Mo2C monolayer have been investigated by the first principles based on density functional theory (DFT). To investigate the effect of nitrogen concentration on adsorption energies, Mo2C1-xNx (x=0.0625, 0.125, 0.1875, and 0.25) with four different nitrogen doping concentrations have been considered in the present work. The results show that N-doped Mo2C is benefit for Mg adsorption. In particular, when the doping concentration reaches to 14.29%, the adsorption energies of Mg on Mo2C0.875N0.125 are in the region between -1.639 and -1.517 eV, e.g., the adsorption energies of Mg on TC1 and H2 sites are -1.639 eV and -1.625 eV, which are decreased by 16.49% and 18.43% as compared with the pristine Mo2C. The calculations on diffusion behaviors show that the Mg diffusing between two adjacent favored sites via a high-symmetry site along H3-B-H4 and H1-B-H1 paths possesses the barriers of 0.021 eV and 0.028 eV. Additionally, the partial density of states (PDOS) reveals the interaction between Mg and Mo2C0.875N0.125, and indicates that nitrogen doping causes the PDOS peaks transfer to a lower energy level, which is benefit for the bonding between Mg and Mo2C0.875N0.125. These results suggest that the adsorption and diffusion behaviors of Mg have been enhanced by nitrogen doping.

Monolayer Mo2C as anodes for magnesium-ion batteries.

The adsorption and diffusion behaviors of magnesium (Mg) on monolayer Mo2C have been investigated by the first principles method based on density functional theory (DFT). The structural stability and theoretical capacity of monolayer Mo2C as anodes for magnesium-ion batteries (MIBs) have also been investigated. The results show that Mg prefer to occupy the H and TC sites with the adsorption energies of - 1.439 and - 1.430, respectively, followed by B and TMo sites on Mo2C monolayer. The Mg prefers to diffuse along the H-TC-H path, furthermore, the other two possible paths (along H-B-H and H-TMo-H) also possess quite low energy barrier with the value of about 0.039 eV. The present results demonstrate that the adsorption energy per Mg atom and the volume expansion change mildly. The volume expansions change slightly from 0.7 to 7.08% with the variety of x, ranging from 0.167 to 2.0. The theoretical gravimetric capacity reaches to 469.791 mAhg-1 with relatively small deformation and expansion as x = 2.0. The results mentioned above suggest that Mo2C monolayer is one of the promising candidates for anode material of MIBs.

Catalytic activity for the hydrogen evolution reaction of edges in Janus monolayer MoXY (X/Y = S, Se, and Te).

Monolayer transition metal dichalcogenides (TMDs) have been regarded as the most promising low-cost alternatives to noble metals as catalysts for the hydrogen evolution reaction (HER). However, their limited catalytically active sites for the HER hinder their practical application. In this paper, the catalytic performances of the edge sites of Janus monolayer MoXY (X/Y = S, Se and Te) were investigated using density functional theory. The results show that both the Mo-edge and chalcogen atomic edges of Janus monolayer MoXY are catalytically active for the HER; thus Janus monolayer MoXY exhibits better catalytic performance than monolayer MoS2. These results are useful for the improvement of the catalytic performance of TMDs by the formation of the Janus monolayer MoXY.

Adsorption and diffusion of lithium in a graphene/blue-phosphorus heterostructure and the effect of an external electric field.

The adsorption and diffusion behaviors of lithium (Li) in a graphene/blue-phosphorus (G/BP) heterostructure have been investigated using a first principles method based on density functional theory (DFT). The effect of an external electric field on the adsorption and diffusion behaviors has also been investigated. The results show that the adsorption energy of Li on the graphene side of the G/BP heterostructure is higher than that on monolayer graphene, and Li adsorption on the BP side of the G/BP/Li system is slightly stronger than that on monolayer BP (BP/Li). The adsorption energy of Li reaches 2.47 eV, however, the energy barriers of Li diffusion decrease in the interlayer of the G/BP heterostructure. The results mentioned above suggest that the rate performance of the G/BP heterostructure is better than that of monolayer graphene. Furthermore, the adsorption energies of Li atoms in the three different most stable sites, i.e., HG, TP and H1 sites, increase by about 0.49 eV, 0.26 eV, and 0.13 eV, respectively, as the electric field intensity reaches 0.6 V Å-1. The diffusion energy barrier is significantly decreased by an external electric field. It is demonstrated that the external electric field can not only enhance the adsorption but can also modulate the diffusion barriers of Li atoms in the G/BP heterostructure.