Stark Effect and Nonlinear Impedance of the Asymmetric Ag-CO-Ag Junction: An Optical Rectenna

We present first-principles analysis of the Stark effect of CO adsorbed on an atomically sharp silver asperity, and current versus potential (I–V) characteristics of the Ag-CO-Ag junction. The analysis supports the suggestion that CO-bridged plasmonic junctions represent rectifying nanoantennas at optical frequencies and that the CO vibrational spectrum serves as a molecular voltmeter [M. Banik et al. ACS Nano2012, 6, 10343]. The Stark effect is principally controlled by the field-induced charge redistribution between the antibonding 2π*-orbitals of CO and the s-electrons of Ag. The Stark tuning rate of the CO stretch, 1.5 × 10–6 cm–1/V cm–1, is ∼25% larger on atomically sharp asperities than on flat Ag, and remains constant over a large window of applied fields (±0.8 V/Å). As such, both sign and strength of local electric field can be quantitatively determined by the vibrational shift of CO. The I–V curve of the Ag-CO-Ag junction is nonlinear, rendering it an effective rectifier with responsivity S = (∂2I/∂V2)/(∂I/∂V) = −2.8 μA/V at zero bias. A more explicit treatment of rectification at optical frequencies is presented through time-dependent density functional simulations of the coupled electronic and nuclear degrees of freedom of the junction, to include dynamical impedance in the confirmation of the optical rectenna. The computed impedance correctly predicts the experimentally observed sign and magnitude of the rectified optical field, as measured by the Stark effect.