Scanning tunneling microscopy of single dye molecules on GaAs(1Â 1Â 0) surfaces

Scanning tunneling microscopy and surface photovoltage images are reported for isolated dye molecules on the GaAs(1 1 0) surface. Profound differences in the molecular images are observed for different experimental conditions. Specifically, contrast variations with changing bias polarity and magnitude, and for different substrate doping type are examined. Several mechanisms are considered to describe the observations including direct and resonant tunneling, and switching of charges between molecular states in the band gap and the energy bands of the GaAs. This treatment enables both assignment of the molecular charge state polarity, and clarification of the mechanism for current flow.