Adsorption of alkali, alkaline-earth, and 3$d$ transition metal atoms on silicene

The adsorption characteristics of alkali, alkaline-earth, and transition metal adatoms on silicene, a graphene-like monolayer structure of silicon are analyzed by means of first-principles calculations. In contrast to graphene, interaction between the metal atoms and the silicene surface is quite strong due to its highly reactive buckled hexagonal structure. In addition to structural properties, we also calculate the electronic band dispersion, net magnetic moment, charge transfer, work function, and dipole moment of the metal adsorbed silicene sheets. Alkali metals, Li, Na, and K, adsorb to hollow sites without any lattice distortion. As a consequence of the significant charge transfer from alkalis to silicene, metalization of silicene takes place. Trends directly related to atomic size, adsorption height, work function, and dipole moment of the silicene/alkali adatom system are also revealed. We found that the adsorption of alkaline-earth metals on silicene is entirely different from their adsorption on graphene. The adsorption of Be, Mg, and Ca turns silicene into a narrow gap semiconductor. Adsorption characteristics of eight transition metals Ti, V, Cr, Mn, Fe, Co, Mo, and W are also investigated. As a result of their partially occupied $d$ orbital, transition metals show diverse structural, electronic, and magnetic properties. Upon the adsorption of transition metals, depending on the adatom type and atomic radius, the system can exhibit metal, half-metal, and semiconducting behavior. For all metal adsorbates, the direction of the charge transfer is from adsorbate to silicene, because of its high surface reactivity. Our results indicate that the reactive crystal structure of silicene provides a rich playground for functionalization at nanoscale.

Figure 1

(a) Top view of the honeycomb lattice of silicene and its unit cell shown by a black parallelogram. (b) Tilted view of the 2$\times$2 supercell of silicene. Structural parameters: buckling $\delta$, Si-Si-Si angle, and Si-Si distance are indicated. (c) Electronic band dispersion of silicene (with and without SOC) and band decomposed charge densities of VB and CB at $\Gamma$, $M$, and $K$ symmetry points.

Figure 2

Preferable adsorption sites, hollow, top, hill, and bridge, on a silicene lattice.

Figure 3

Side and top view for characteristic adsorption geometries for alkali, alkaline-earth, and transition metal atoms.

Figure 4

Diffusion pathways and barriers of (a) alkali atoms and (b) $3d$, $4d$, and $5d$ transition metals through top, bridge, valley, and hollow sites.

Figure 5

Electronic band structures for perfect, Li, Na, and K adsorbed silicene. Fermi level is set to zero. Occupied bands are filled with yellow color.

Figure 6

Partial DOS for Be, Mg, and Ca adsorbates and nearest silicon atoms. Fermi level is set to zero. DOS is broadened by Gaussian smearing with 0.2 eV.

Figure 7

Partial DOS for Ti, V, Cr, and Mn adsorbates and nearest silicon atoms. Fermi level is set to zero. DOS is broadened by Gaussian smearing with 0.2 eV.

Figure 8

(a) Plot of the adsorption energy $E_{\text{Ads}}$ vs the work function $\Phi$ and (b) the work function $\Phi$ vs dipole moment p.