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 substra te was imaged both in the constant-current mode and in the current ima ging tunneling spectroscopy (CITS) mode of the scanning tunneling micr oscope. The constant-current images showed the well-known banded smect iclike structure. CITS images were collected over a range of CITS volt ages (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 separat ion. CITS images at higher constant currents showed little variation i n contrast over the range of voltages selected. They exhibited higher resolution than CITS images collected using lower constant current. Ho wever, the contrast in these CITS images collected at lower constant c urrent showed a strong dependence on voltage. The general trends obser ved in these images are discussed in terms of the tip-specimen separat ion and the relationship between the ''uncertainty principle'' and ene rgy resolution in scanning tunneling spectroscopy. The chemical-group specific contrast is discussed in terms of different di/dv for the gro ups. (C) 1995 American Vacuum Society.