First-principles modeling of 3d-transition-metal-atom adsorption on silicene: a linear-response DFT + U approach.

By employing DFT + U calculations with the linear response method, we investigate the interactions between various 3d transition-metal atoms (Cr, Mn, Fe, Co) and silicene. In the cases of two-dimensional (2D) FeSi2 and CoSi2, the metal atoms tend to penetrate into the silicene layer. While CoSi2 is non-magnetic, FeSi2 exhibits a total magnetic moment of 2.21 µ(B)/cell. Upon the examination of 2D MSi6, a trend in anti-ferromagnetic (AFM) favorability in the z-direction is observed according to our DFT + U calculations. In the ferromagnetic (FM) states (less stable), each primary unit cell of CrSi6, MnSi6, and FeSi6 possesses different levels of total magnetization (4.01, 5.18, and 2.00 µ B/cell, respectively). The absolute magnetization given by AFM MSi6 structures varies in the range of 5.33-5.84 µ(B)/cell. A direct band gap in AFM MnSi6 (0.2 eV) is predicted, while the metastable FM FeSi6 structure has a wider band gap (0.85 eV). Interestingly, there are superexchange interactions between metal atoms in the MSi6 systems, which result in the AFM alignments.

A first-principles investigation of various gas (CO, H2O, NO, and O2) absorptions on a WS2 monolayer: stability and electronic properties.

Using first-principles calculations, we investigate the interactions between a WS2 monolayer and several gas molecules (CO, H2O, NO, and O2). Different sets of calculations are performed based on generalized-gradient approximations (GGAs) and GGA + U ([Formula: see text] eV) calculations with D2 dispersion corrections. In general, GGA and GGA + U establish good consistency with each other in terms of absorption stability and band gap estimations. Van der Waals density functional (vdW-DF) calculations are also performed to validate long-range gas molecule-WS2 monolayer interactions, and the resultant absorption energies of four gas-absorption cases (from 0.21 to 0.25 eV) are significantly larger than those obtained from calculations using empirical D2 corrections (from 0.11 to 0.19 eV). The reported absorption energies clearly indicate van der Waals interactions between the WS2 monolayer and gas molecules. The NO and O2 absorptions are shown to narrow the band gaps of the WS2 material to 0.75-0.95 eV and produce small magnetic moments (0.71 µB and 1.62 µB, respectively). Moreover, these two gas molecules also possess good charge transferability to WS2. This observation is important for NO- and O2-sensing applications on the WS2 surface. Interestingly, WS2 can also activate the dissociation of O2 with an estimated barrier of 2.23 eV.