STM topograph of a reduced TiO2(110) surface, recorded at positive sample bias (+1.54 V, 10 pA, 60 Å × 60 Å). The color palette represents a height difference of 0.90 Å. The overlaid model illustrates the electronically governed STM contrast: rows of bridging oxygen atoms Ob (dark red spheres) appear topographically lower (dark) in STM images despite their geometrical elevation. The rows of fivefold coordinated Ti5c atoms (gray spheres) appear topographically higher (brighter) due to an electronic effect, i.e., the conduction band mainly consists of Ti 3d states. Bridging oxygen vacancies Ob-vac appear as rectangular protrusions centered on dark rows (marked by the arrow in the upper left corner). The white rectangle indicates the primitive unit cell of the surface, 6.50 Å × 2.95 Å in size.
Controlled desorption of single hydrogen atoms, induced by the STM tip. (Left) Series of constant current STM topographs (+1.56 V, 20 pA, 62 Å × 62 Å). The color palette represents a height difference of 0.45 Å for images 1 - 3 and 0.26 Å for image 4. The hydrogen atoms appear slightly off the center of the dark (depressed) Ob rows due to a tip artifact. (Right) Corresponding I(V) curves acquired at the positions indicated by the crosses in the STM images (the colors of the crosses and curves correspond) The feedback loop was turned off during spectral acquisition, after the tunneling gap had been established at +1.56 V and 20 pA. The I(V) curves were measured with increasing voltage and the sharp drop in current indicates the desorption, yielding threshold voltages of: (1) +2.19 V (green); (2) +2.07 V (red); (3) +2.30 V (blue); the black curve is representative for all I(V) spectra measured with decreasing voltage after desorption has occurred. The inset shows the distribution of the threshold voltages for 28 independent desorption events with an average value of +2.22 V and a standard deviation of +0.090 V; the inhomogeneities of the surface, apparent in STM topograph 4, are presumably caused by subsurface defects, which could only be resolved for certain tip conditions.
(a) Stick and ball model of the ZnEtio structure (gray: C, blue: N, red: Zn, hydrogen atoms are not shown); (b) STM constant current topographic image of rutile TiO2(110) with adsorbed ZnEtio molecules (+1.5 V, 0.10 nA, 96 Å × 96 Å). The color palette represents a height difference of 4.6 Å. The dark and light gray vertical rows correspond to those of Ti5c and Ob along the  direction of the TiO2(110) surface, respectively. ZnEtio molecules appear either as two-lobed (M1, M2) or four-lobed (N1, N2) structures. The dark protrusions on top of Ti5c rows are unidentified impurities. (c) dI/dV spectra recorded over the center of molecule M2 (red line), N1 (green line), and over the bare TiO2 surface (black line). (d) d2I/dV2 spectrum obtained from a two-lobed molecule. Three equidistant plateaus are resolved with spacing of ∼130 mV (marked by black vertical lines). The spectrum was averaged over 80 passes. All dI/dV and d2I/dV2 spectra were recorded using the lock-in technique with a bias modulation frequency of 264 Hz and amplitude of 10 mV (rms). The feedback loop was turned off during spectra acquisition, after the tunneling gap had been established at 2.0 V and 0.10 nA.
STM topographs (a) before and (b) after lateral manipulation of a single ZnEtio molecule on TiO2(110). The molecule was manipulated parallel to the row direction, i.e., along  away from the step-edge. The topographic images were obtained with a sample bias of +1.33 V and a tunneling current of 100 pA. The color palette represents a height difference of 5.2 Å. For lateral manipulation the tip was moved along the trajectory indicated by the black dashed line with active feedback loop and a sample bias of +0.60 V and a tunneling current of 460 pA. The overlaid red curve represents the tip height profile recorded during the manipulation, where relative tip height changes are depicted on a horizontal axis as a function of tip position along the manipulation path.
Series of STM and STS measurements of a four-lobed ZnEtio conformer with different distances to Ob-vac. The molecule was manipulated along the  direction to 13 positions; both STM topographs and dI/dV spectra were acquired at each position. (a) Selected STM topographs (+1.50 V, 40 pA, 53 Å × 53 Å) for four different positions of the ZnEtio molecule. The color palette represents a height difference of 3.3 Å. The overlaid grid represents the primitive rectangular unit cell of the substrate (6.50 Å × 2.95 Å), with corners at Ob positions; one such unit cell is shown in Fig. 1. Ob-vac appear as protrusions (bright rectangles) on dark rows and are marked by open circles. The arrow in image (1) indicates the manipulation path to the position depicted in (2) [the numbering of the Ob sites refers to the coordinate system used in (c)]. The sequence of ZnEtio coordinates changed during the manipulation series as follows: 4 → 6 → 8 [image (1)] → 7 → 5 [image (2)] → 3 → 1 [image (3)] → 4 image [(4)] → 6 → 8 → 10 → 8 (coordinates of the selected STM topographs are indicated) (b) Corresponding dI/dV spectra obtained with the lock-in technique above the molecule manipulated to different locations with respect to the Ob-vac. The numbers in open circles refer to the topographs depicted in (a). The inset depicts the top view of a Ob-vac, where dark red spheres represent Ob atoms, gray and black spheres Ti5c atoms. (c) Summary of peak onset (red squares) and voltage at peak maximum (blue circles) of the HOMO derived state vs. coordinate of ZnEtio position. Full symbols and the numbering highlight the four selected measurements, where corresponding topographs are depicted in (a) and dI/dV spectra in (b). The black lines serve as guide to the eye.
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