Tuning the surface metal work function by deposition of ultrathin oxide films: Density functional calculations

Changes in the work function $\Phi$ of metal surfaces upon deposition of ultrathin oxide films have been studied by means of band structure density functional theory calculations. Four systems have been considered: $\mathrm{MgO}∕\mathrm{Ag}\left(100\right)$, $\mathrm{MgO}∕\mathrm{Mo}\left(100\right)$, ${\mathrm{TiO}}_2∕\mathrm{Mo}\left(100\right)$, and ${\mathrm{SiO}}_2∕\mathrm{Mo}\left(112\right)$. $\mathrm{MgO}$ films induce a decrease of $\Phi$ of 1 to $2\mathrm{eV}$ compared to the clean metal substrate; ${\mathrm{SiO}}_2$ and ${\mathrm{TiO}}_2$ induce an increase of $\Phi$ of about $0.5–1\mathrm{eV}$. The reasons for this behavior are different: for ${\mathrm{TiO}}_2$ and ${\mathrm{SiO}}_2$ the work function increase can be explained with the classical model of surface dipole due to metal-to-oxide charge transfer at the interface. On $\mathrm{MgO}∕\mathrm{metal}$ interfaces, where the charge transfer is negligible, the shift is due to the compression of the metal electron density enforced by the oxide layer, with consequent change in surface dipole. The results suggest that by appropriately choosing the metal support and the oxide film one can design nanostructured materials with new properties.

Figure 1

Projected density of states of $\mathrm{MgO}\left(3\mathrm{L}\right)∕\mathrm{Ag}\left(100\right)$ and $\mathrm{MgO}\left(3\mathrm{L}\right)∕\mathrm{Mo}\left(100\right)$ supported films. Dotted line: top two metal layers; solid line: three $\mathrm{MgO}$ layers. The vertical solid line indicates the position of the Fermi level (taken here as a measure of the surface work function) in the metal/oxide system; the vertical dotted line indicates the position of the Fermi level in the pure metal substrate.

Figure 2

Dependence of the work function (left) and of the surface dipole per surface unit area (right) on the $\mathrm{MgO}∕\mathrm{metal}$ interface distance in $\mathrm{MgO}\left(1\mathrm{L}\right)∕\mathrm{Ag}\left(100\right)$ and $\mathrm{MgO}\left(1\mathrm{L}\right)∕\mathrm{Mo}\left(100\right)$ interfaces. The curves have been determined for flat $\mathrm{MgO}$ films (no rumpling).

Figure 3

Projected density of states of ${\mathrm{TiO}}_2\left(2\mathrm{L}\right)∕\mathrm{Mo}\left(100\right)$ supported film. Dotted line: top two metal layers; solid line: two ${\mathrm{TiO}}_2$ layers. The vertical solid line indicates the position of the Fermi level (taken here as a measure of the surface work function) in the metal/oxide system; the vertical dotted line indicates the position of the Fermi level in the pure metal substrate.

Figure 4

Projected density of states of ${\mathrm{SiO}}_2\left(1\mathrm{L}\right)∕\mathrm{Mo}\left(112\right)$ supported film. Dotted line: top two metal layers; solid line: ${\mathrm{SiO}}_2$ layer. The vertical solid line indicates the position of the Fermi level (taken here as a measure of the surface work function) in the metal/oxide system; the vertical dotted line indicates the position of the Fermi level in the pure metal substrate.