Mechanisms of Molecular Manipulation with the Scanning Tunneling Microscope at Room Temperature: Chlorobenzene$/\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$

We report a systematic experimental investigation of the mechanism of desorption of chlorobenzene molecules from the $\mathrm{S}\mathrm{i}(111)-(7\times 7)$ surface induced by the STM at room temperature. We measure the desorption probability as a function of both tunneling current and a wide range of sample bias voltages between $-3\mathrm{V}$ and $+4\mathrm{V}$. The results exclude field desorption, thermally induced desorption, and mechanical tip-surface effects. They indicate that desorption is driven by the population of negative (or positive) ion resonances of the chemisorbed molecule by the tunneling electrons (or holes). Density functional calculations suggest that these resonant states are associated with the $\pi$ orbitals of the benzene ring.

Figure 1

STM images ($100\times 100\AA$) of ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\mathrm{l}$ molecules on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ before (a) and after (b) a desorption scan. Image (desorption) scan parameters; sample bias $+1\mathrm{V}$ ($+2.2\mathrm{V}$), tunneling current 50 pA (50 pA). The white circles mark the locations of chlorobenzene molecules which desorb in this case.

Figure 2

(a) Rate of desorption of ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\mathrm{l}$ molecules from the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface as a function of the tunneling current in the STM (log-log plot with straight line fit); sample bias voltage $+3\mathrm{V}$ (filled circles) and $-2\mathrm{V}$ (filled squares). For ease of viewing the $-2\mathrm{V}$ data have been shifted up by an order of magnitude. (b) The same basic data plotted in terms of the change in tip-surface distance (positive value, tip moves towards the surface) which occurs when the current is altered. The reference set points are $+3\mathrm{V}$, 5 pA (filled circles) and $-2\mathrm{V}$, 10 pA (filled squares). Again, for viewing clarity the $-2\mathrm{V}$ data have been shifted up by a factor of 2.

Figure 3

Experimental desorption yield per electron of ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\mathrm{l}$ molecules from the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface as a function of the sample bias voltage in the STM (black squares), compared with (solid lines) calculated partial density of $p$ states at (a) the carbon atom in the ring and (b) the chlorine atom (for details see text).