We report single molecule laser absorption by carbon nanotubes stamped under ultrahigh vacuum onto Si(100)2x1:H surfaces. Absorption is detected by scanning tunneling microscopy. Images are obtained with and without modulated laser excitation using lock-in amplification and a rear-illumination geometry to reduce thermal effects. Noise appears at topographic edges and is analyzed by a quantitative model in terms of scan speed, mechanical instabilities, and feedback current fluctuations at the edge of the nanotubes. Noise due to mechanical instabilities is shown to persist even in the limit of slow scan speed.

We image frequency-modulated single-molecule optical absorption using a scanning tunneling microscope as the detector (SMA−STM). As a first example of the technique, a semiconducting carbon nanotube adsorbed on a silicon surface is studied. Excitation is achieved using laser amplitude as well as frequency modulation, and these two complementary approaches are compared. Detection is achieved via the resulting change in tunneling current through the excited molecule. We distinguish three mechanisms, direct, relaxed, and bolometric, for detecting single-molecule absorption spectra. Kinetic models for these mechanisms as well as for surface heating are presented. The latter effect can be eliminated by frequency modulation, keeping the laser power density on the surface constant.

We investigate the effect of temperature tuning on the deposition of gold films onto sapphire. Transparent conductive films with roughness as low as 0.4 Å rms at 10 nm thickness allow STM imaging of aerosoldeposited carbon nanotubes. A Monte Carlo lattice model explains the experimentally observed surfaceroughening trends and shows that the interplay of deposition-induced kinetic roughening and thermal annealing and roughening can be optimized. Intrinsic surface roughness (e.g., stepping), previously postulated to play a role in film roughness, is not found to be important.

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