Scanning Tunneling Microscopy Contrast Mechanisms for ${\mathrm{TiO}}_2$

Controlled dual mode scanning tunneling microscopy (STM) experiments and first-principles simulations show that the tunneling conditions can significantly alter the positive-bias topographic contrast of geometrically corrugated titania surfaces such as rutile ${\mathrm{TiO}}_2(011)\mathrm{\text{−}}(2\times 1)$. Depending on the tip-surface distance, two different contrasts can be reversibly imaged. STM simulations which either include or neglect the tip-electronic structure, carried out at three density functional theory levels of increasing accuracy, allow assignment of both contrasts on the basis of the ${\mathrm{TiO}}_2(011)\mathrm{\text{−}}(2\times 1)$ structure proposed by Torrelles et al. [Phys. Rev. Lett. 101, 185501 (2008)]. Finally, the mechanisms of contrast formation are elucidated in terms of the subtle balance between the surface geometry and the different vacuum decay lengths of the topmost $\mathrm{Ti}(3d)$ and $\mathrm{O}(2p)$ states probed by the STM-tip apex.

Figure 1

Forward and backward scans of the same region recorded simultaneously during dual mode STM. The zigzag and beanlike images were obtained at large (1.7 V, 0.5 nA) and small (1.1 V, 1.5 nA) tip-sample distances, respectively. Images are rotated with arrows indicating the tip scan direction. Data were filtered using a box pass in the two-dimensional fast Fourier transform to remove high frequency noise only.

Figure 2

Side (a) and top (b) view of the ${\mathrm{TiO}}_2(011)\mathrm{\text{−}}(2\times 1)$ surface and adopted labeling. The O and Ti atoms are displayed as large and small balls, respectively. For clarity, only the two (three) topmost Ti and (O) layers have been shown in (b). The ($2\times 1$) unit cell is highlighted in (b), as are the in-plane geometries of the topmost $\mathrm{Ti}(1/1^{*})$, $\mathrm{O}(1/1^{*})$, and $\mathrm{O}(2/2^{*})$ atoms (colored triangles) for comparison with STM. The unfilled triangle shows the in-plane distances between the O(1) and O(2) atoms, which are in close agreement with the experimental geometry of the beanlike mode. The azimuths are as in Fig. 1 and distances shown are in angstrom.

Figure 3

The simulated HSE06 TH STM images at different heights (Å) above the surface [(a) 1.1 V, $5\times {10}^{-5}e{\AA }^{-3}$, (b) 1.1 V, ${10}^{-7}e{\AA }^{-3}$]. (c) The simulated W-tip HSE06 Bardeen image at 1.1 V and 0.05 nA. The bipyramid W-tip model is displayed in the inset. The top- and bottommost atoms form the two tip apexes. The atomic sites are marked by circles and labeled as in Fig. 2. The arrows in (c) mark the in-plane distance (Å) between the modeled brightest features.