The STM experiments were performed at room temperature in an ultra-high-vacuum system consisting of an analysis chamber (with a base pressure of 2 × 10-11 mbar) and a preparation chamber (5 × 10-11 mbar). An electrochemically etched polycrystalline tungsten tip was used to record STM images in constant current mode. The voltage V sample corresponds to the sample bias with respect to the tip. No drift corrections have been applied to any of the STM images presented herein.
The Si(111) substrate was p-type boron-doped with a resistivity in the range
0.1–1.0 ℧ cm. The Si(111)-(7 × 7) surface was prepared by in situ direct current
heating to 1520 K after the sample was first degassed at 870 K for 12 hours. The
Ag/Si(111)-
×
R30° surface was prepared by e-beam evaporation of
silver (Goodfellow Metals, 5 N) from a molybdenum crucible onto the Si
substrate, which was maintained at 770 K during the deposition. The Ag(111)
crystal (Surface Preparation Laboratory) was cleaned in situ by repeated
cycles of argon ion sputtering (Ek = 0.8keV) and annealing at 820 K. The
substrate cleanliness was verified by STM and LEED before deposition of the
NiDPP.
NiDPP was synthesized according to a published procedure [169] by Dr Natalia Sergeeva in the School of Chemistry, TCD. The molecules were evaporated in a preparation chamber isolated from the STM chamber at a rate of about 0.2 ML per minute from a tantalum crucible in a homemade deposition cell operated at a temperature of approximately 600 K. The total pressure during porphyrin deposition was in the 10-10 mbar range. Before evaporation, the NiDPP powder was degassed for about 10 hours to remove water vapour.