Determination of rutile transition metal oxide (110) surface terminations by scanning tunneling microscopy contrast reversal

The surfaces of rutile transition-metal oxides $\left({\mathrm{TMO}}_2\right)$ are widely investigated for catalysis, photoelectrochemical solar cells, memristors, and supercapacitors, but their structures have remained controversial. Here we employ density functional theory to predict that a universal behavior of metallic ${\mathrm{TMO}}_2$ surfaces, i.e., the stoichiometric ${\mathrm{TMO}}_2$ surfaces, exhibit a contrast reversal in simulated scanning tunneling microscopy (STM) images at different scanning biases. The predictions are verified by experimental STM imaging of ${\mathrm{RuO}}_2\left(110\right)$ surfaces and this feature is shown to enable accurate determinations of the ${\mathrm{TMO}}_2\left(110\right)$ surface structures under various conditions. This work provides different insights into the electronic properties of ${\mathrm{TMO}}_2\left(110\right)$ surfaces and offers an effective method to directly map the surface structure and point defects using bias-dependent STM.

Figure 1

First-principles predicted bias-dependent STM images, charge densities, and density of states of two possible ${\mathrm{RuO}}_2\left(110\right)$ surfaces. (a) ${\mathrm{RuO}}_2\text{−}\left(1\times 1\right)$ surface. (b) ${\mathrm{RuO}}_2\text{−}\left(1\times 1\right)\text{−}{\mathrm{O}}_{\mathrm{cus}}$ surface. (c), (d), Contours showing the geometric progression of charge density for the ${\mathrm{RuO}}_2\text{−}\left(1\times 1\right)$ surface, with a factor of 3.16 separating neighboring contours. (c) Charge densities associated with electron states from ${\mathrm{E}}_{\mathrm{F}}\text{–}2\mathrm{eV}$ to $E_{\mathrm{F}}$. (d) Charge densities associated with electron states from ${\mathrm{E}}_{\mathrm{F}}$ to ${\mathrm{E}}_{\mathrm{F}}+2\mathrm{eV}$. (e) Site-projected density of states (pDOS) on the ${\mathrm{RuO}}_2\text{−}\left(1\times 1\right)$ surface atoms.

Figure 2

First-principles predicted bias-dependent STM images of ${\mathrm{TMO}}_2\left(110\right)$ surfaces. +1 and −1 represent the relative bright and dark contrasts of the ${\mathrm{O}}_{\mathrm{br}}$ rows in STM images, respectively. Red solid and blue dashed curves represent stoichiometric ${\mathrm{TMO}}_2\text{−}\left(1\times 1\right)$ and oxygen-rich $\left(1\times 1\right)\text{−}{\mathrm{O}}_{\mathrm{cus}}$ surface structures respectively. ${\mathrm{TiO}}_2$ is a semiconductor, and the other ${\mathrm{TMO}}_2$ are metallic. According to Fig. 2, ${\mathrm{TiO}}_2$ is already contrast reversed in the range of bias from −2 to 8 V. (k) summarizes the onset bias of the contrast reversal of ${\mathrm{TMO}}_2\text{−}\left(1\times 1\right)$ surfaces as a function of atomic number of TM. Since ${\mathrm{CrO}}_2\text{−}\left(1\times 1\right)$ does not show contrast reversal, the bias is labeled as an upper arrow in (k). As seen in panels (c)–(j), the contrast reversals are not vertical jumps. Instead, they have finite slopes, i.e., it takes some bias range for stripes to change from fully bright (dark) to fully dark (bright). Therefore, we use error bars in (k) to account for the bias ranges for contrast reversals to complete. The work functions of the stoichiometric ${\mathrm{TMO}}_2\text{−}\left(1\times 1\right)$ and oxygen-rich $\left(1\times 1\right)\text{−}{\mathrm{O}}_{\mathrm{cus}}$ surface structures calculated by DFT are indicated as the dashed and dotted vertical lines in each panel, respectively. The value of ${\mathrm{TiO}}_2$ is taken from Ref. [39]. Usually, the oxygen-rich surfaces have higher work functions.

Figure 3

Measured bias-dependent STM images of ${\mathrm{RuO}}_2\left(110\right)$ films from −2 to +2 V. (a)– (f) As-grown ${\mathrm{RuO}}_2\left(110\right)$ films cooled under vacuum from growth conditions. Red crosses are shown as reference points to guide eyes. The cyan arrows highlight defects which appear as bright spots for all biases and can be used as positional markers. The cyan circles highlight some defects which change contrast with changing biases. (g) Line profiles recorded perpendicular to the stripes at different bias voltages. (h),(i) ${\mathrm{RuO}}_2\left(110\right)$ film grown and cooled in ${\mathrm{O}}_2$ atmosphere. (j),(k), ${\mathrm{O}}_2$-grown ${\mathrm{RuO}}_2\left(110\right)$ film annealed (reduced) in UHV.

Figure 4

First-principles calculated STM images of other ${\mathrm{RuO}}_2$ surfaces. (a) STM image of the twofold ${\mathrm{Ru}}_8{\mathrm{O}}_{17}-\left(1\times 2\right)$ surface. (b) STM image of a reconstructed, ${\mathrm{RuO}}_4-\left(2\times 1\right)$-tetrahedra surface, in which ${\mathrm{RuO}}_2$ clusters are absorbed on top of the ${\mathrm{O}}_{\mathrm{cus}}$ atoms. (c) The bias-dependent STM images of a bridging oxygen vacancy on the stoichiometric ${\mathrm{RuO}}_2\text{−}\left(1\times 1\right)$ surface.