Assembling Molecular Electronic Junctions One Molecule at a Time

Diffusion of metal atoms onto a molecular monolayer attached to a conducting surface
permits electronic contact to the molecules with minimal heat transfer or structural disturbance.
Surface-mediated metal deposition (SDMD) involves contact between “cold” diffusing metal atoms
and molecules, due to shielding of the molecules from direct exposure to metal vapor. Measurement of
the current through the molecular layer during metal diffusion permits observation of molecular
conductance for junctions containing as few as one molecule. Discrete conductance steps were
observed for 1
10 molecules within a monolayer during a single deposition run, corresponding to
“recruitment” of additional molecules as the contact area between the diffusing Au layer and molecules
increases. For alkane monolayers, the molecular conductance measured with SDMD exhibited an
exponential dependence on molecular length with a decay constant (β) of 0.90 per CH2 group,
comparable to that observed by other techniques. Molecular conductance values were determined for three azobenzene molecules,
and correlated with the offset between the molecular HOMO and the contact Fermi level, as expected for hole-mediated tunneling.
Current
voltage curves were obtained during metal deposition showed no change in shape for junctions containing 1, 2, and 10
molecules, implying minimal intermolecular interactions as single molecule devices transitioned into several molecules devices.
SDMD represents a “soft” metal deposition method capable of providing single molecule conductance values, then providing
quantitative comparisons to molecular junctions containing 106 to 1010 molecules.