The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have beenexplored for a series of molecules to address the relation between the behavior of a molecule and itsstructure. We probed interfacial electron transfer processes, particularly those involving unoccupied states,of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM)combined with current-voltage (
i-V) and current-distance (
i-d) measurements. The
i-V curves ofhexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentiallywith
V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped
i-V characteristicswere obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating thatpart of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked
i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be usedto define a threshold tip bias,
VTH, for resonant conduction. We also found that for all of the SAMs studiedhere, the current decreased with increasing distance,
d, between tip and substrate. The attenuation factor
mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> of hexadecanethiol was high, ranging from 1.3 to 1.4 Å
-1, and was nearly independent of the tip bias.The
mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15Å
-1 for tetranitro oligo(phenylene ethynylene) thiol,
VII, to 0.50 Å
-1 for dinitro oligo(phenylene) thiol,
VI, ata -3.0 V tip bias. Both the
VTH and
mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle"> values of these nitro-based SAMs were also strongly dependent onthe structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene,the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storageon nitro-based molecules. For a SAM of the dintro compound,
V, ~25% of charge collected in the negativescan is stored in the molecules and can be collected at positive voltages. A possible mechanism involvinglateral electron hopping is proposed to explain this phenomenon.
