  The transport properties of unsubstituted and amino-substituted butanethiol
molecules sandwiched between Au electrodes are investigated by using
first-principles approaches. New states are observed around the Fermi levels
when -NH2 is substituted for -H in the bridging butanethiol. The
amino-substituted states lead to a sharp increase of the current which is
credited to the resonant tunneling in the junction. We observe a novel
conductance peak at VSD = 0.1 V and negative differential resistance (NDR) in a
certain range of source-drain bias. In addition to the I-VSD characteristics,
we also investigate the current as a function of gate voltage (I-VG) and find
that, for a fixed source-drain bias (VSD = 0.01 V), the gate voltage can
modulate the conductance by up to 30 times in the amino-substituted butanethiol
junction. These I-VG characteristics suggest that the amino-substituted
butanethiol molecular junction may be a promising candidate for field-effect
transistors.
