Manipulation of polyatomic molecules with the scanning tunnelling microscope at room temperature:chlorobenzene adsorption and desorption from Si(111)-(7x7)

We report the imaging of chlorobenzene molecules chemisorbed on the Si(111)-(7 x 7) surface at room temperature with the scanning tunnelling microscope, and the desorption of the molecules by the tunnelling current. Detailed voltage-dependent imaging ( at positive bias) allows the elucidation of the number and orientation of all the adsorbate configurations in the 7 x 7 unit cell. At negative bias the adsorbate was observed to affect the imaging properties of neighbouring half unit cells. The threshold voltage required for desorption of the chlorobenzene molecules was invariant to small changes in the tip-state, the adsorption site ( corner adatom, middle adatom, faulted or unfaulted half of the unit cell) and the kind of doping of the substrate ( n or p type).