Current imaging tunneling spectroscopy of an alkyl cyanobiphenyl liquid crystal

An adsorbed ordered monolayer of the liquid crystal molecule 4-n-octyl-4'-cyanobiphenyl (8CB) on a highly ordered pyrolitic graphite substrate was imaged both in the constant-current mode and in the current imaging tunneling spectroscopy (CITS) mode of the scanning tunneling microscope. The constant-current images showed the well-known banded smecticlike structure. CITS images were collected over a range of CITS voltages (350-1200 mV) and also a range of constant currents (0.008-0.100 nA) and voltages (500-900 mV) used to control the tip-specimen separation. CITS images at higher constant currents showed little variation in contrast over the range of voltages selected. They exhibited higher resolution than CITS images collected using lower constant current. However, the contrast in these CITS images collected at lower constant current showed a strong dependence on voltage. The general trends observed in these images are discussed in terms of the tip-specimen separation and the relationship between the ''uncertainty principle'' and energy resolution in scanning tunneling spectroscopy. The chemical-group specific contrast is discussed in terms of different di/dv for the groups. (C) 1995 American Vacuum Society.