Contrast Reversal and Shape Changes of Atomic Adsorbates Measured with Scanning Tunneling Microscopy

Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of $\mathrm{O}(2\times 2)/\mathrm{Ru}(0001)$ have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.

Figure 1

(a) $10\mathrm{nm}\times 5\mathrm{nm}$ STM image of a clean Ru(0001) surface recorded at a sample voltage of $-23\mathrm{mV}$ with a gap resistance of $2\mathrm{M}\Omega$. (b) $10\mathrm{nm}\times 5\mathrm{nm}$ STM image of the $\mathrm{O}(2\times 2)$ superstructure on Ru(0001) recorded with a sample voltage of $-0.1\mathrm{V}$ and a gap resistance of $200\mathrm{M}\Omega$. The oxygen atoms appear as round holes in these conditions.

Figure 2

A series of $2.2\mathrm{nm}\times 3.2\mathrm{nm}$ dual-mode STM images. The upper panels show images recorded at a constant sample voltage of $-30\mathrm{mV}$ and decreasing gap resistances (from left to right, 100, 10, and $2.5\mathrm{M}\Omega$). The reference images in the lower panels were all taken at a sample voltage of $-30\mathrm{mV}$ and a constant gap resistance of $30\mathrm{M}\Omega$.

Figure 3

Comparison of experimental (left panels) and simulated (right panels) STM images of $\mathrm{O}(2\times 2)/\mathrm{Ru}(0001)$. In both cases the sample voltage was $-30\mathrm{mV}$. The simulations have been performed with tunneling currents of 0.03 nA (above) and 0.3 nA (below) and agree with the experimental ones for $300\mathrm{M}\Omega$ (above) and $30\mathrm{M}\Omega$ (below) gap resistances. The maximum corrugation is $0.5\mathrm{\AA }$ in all cases.

Figure 4

Calculated LDOS for a W- (left panel) and an O- (right panel) terminated tip in an area of $6.8\times 4.8\mathrm{\AA }$. The apex atom is located in the center of the panels, but the color code differs by 1 order of magnitude in both panels.

Figure 5

Upper panel: $4\mathrm{nm}\times 5.2\mathrm{nm}$ STM image recorded with an oxygen atom at the tip apex. The sample voltage was $-0.35\mathrm{V}$ and the gap resistance $1.15\mathrm{G}\Omega$. The inset shows the image simulated with an O-terminated tip, calculated at $-0.1\mathrm{V}$ and $1\mathrm{G}\Omega$ and displaying the inversion of contrast. In both cases the maximum corrugation is $0.2\mathrm{\AA }$. Lower panel: $4\mathrm{nm}\times 5.2\mathrm{nm}$ STM image taken with a W tip at $-0.1\mathrm{V}$ and $200\mathrm{M}\Omega$. The experimental corrugation is $0.4\mathrm{\AA }$ while it is $0.5\mathrm{\AA }$ in the image calculated in the same conditions (see inset).