Lithium adsorption by $\mathrm{Ti}{\mathrm{Se}}_2$ of varying concentration via density functional theory

Alkali adsorption on a transition metal dichalcogenide substrate has been the subject of many investigations. However, a theoretical description of its coverage dependence is still missing. In a first attempt we considered Li as an alkali prototype at a specified selection of coverages, $\Theta =0.11$, 0.25, 0.33, 0.5, and 1 ML, adsorbed on $\mathrm{Ti}{\mathrm{Se}}_2$ (0001). By means of the density functional theory we obtained through structural optimization the coverage dependence of the physical properties as adsorption energy, work function, and valence electron distribution. From the knowledge of the preferred adsorption sites, an optimum coverage for the most stable structure could be inferred. The work function curve shows a behavior similar to that found for alkali adsorption on metals. The redistribution of the charge density at adsorption is presented.

Figure 1

Investigated supercells. Se (open circles), Ti (grey), and Li (dark grey with smaller radius).

Figure 2

Structure model of the geometrical changes induced by Li adsorption on $\mathrm{Ti}{\mathrm{Se}}_2$ (0001) (refer to Table ).

Figure 3

Change of work function and adsorption energy with Li adsorbed on $\mathrm{Ti}{\mathrm{Se}}_2$ at hcp sites.

Figure 4

Change of surface dipole moment and $\mathrm{Li}\mathrm{Se}$ distance for Li adsorbed on $\mathrm{Ti}{\mathrm{Se}}_2$ at hcp sites.

Figure 5

Charge density difference, $n^{\Delta }\left(\mathbf{r}\right)$, of Li adsorbed on $\mathrm{Ti}{\mathrm{Se}}_2$ system, with $\Theta =0.5$ and 1.0 for Li in the hcp site. $n^{\mathit{Li}}$ is taken from a free, neutral atom; an enlarged picture of the frame in the left figure is given on its right. Circles indicate projected position of Se atoms in first atomic plane, and square Ti atoms. Arrows in left figure indicate the position of Li adatoms; contours are displayed in a plane perpendicular to the (0001) surface; units are ${10}^{-3}{\mathrm{\AA }}^{-3}$.

Figure 6

PDOS for the $\mathrm{Li}\mathrm{Ti}{\mathrm{Se}}_2$ (0001) system with Li adsorbed at the hcp sites. PDOS is decomposed in $s$, $p$, $d$ components for Li, Se, and Ti.

Figure 7

Structure model of $\mathrm{Ti}{\mathrm{Se}}_2$, showing occupation sites in van der Waals gap. Black Li spheres show three intersandwich sites (one octahedral and two tetrahedral, $T_1$ and $T_2$).

Figure 8

Geometric structure of ${\mathrm{Li}}_x\mathrm{Ti}{\mathrm{Se}}_2$, showing Li in octahedral position and Se neighbors along its diffusion path across bridge to tetrahedral position.

Figure 9

Total energy of Li diffusion within van der Waals gap of bulk $\mathrm{Ti}{\mathrm{Se}}_2$.

Figure 10

Adsorption energy per Li atom and change of work function with Li in octahedral (van der Waals) sites.

Figure 11

Contour plot of electron difference density for Li at octahedral site localized in van der Waals gap. Contour units are ${10}^{-3}{\mathrm{\AA }}^{-3}$.