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

Abstract

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 Nano 2012, 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 = (∂2 I/∂V 2)/(∂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.

Microscopy with a single-molecule scanning electrometer

“…S2), as expected when molecular orbitals are not aligned with the Fermi level ( 26 ). Converting the applied bias to local field E L = V b /( g + d ) generates the calibration curve of the molecular field meter, which is cast in more transparent notation The magnitude of the linear coefficient Δμ = 〈1|μ|0〉 = a ( g + d ) = 160 cm −1 V −1 Å −1 (0.095 D) is in excellent agreement with the prediction of the density functional theory (DFT) calculations for CO adsorbed on tipped silver ( 15 ). The sign of effective dipole flips upon adsorption ( 15 ), and the linear STR is nearly three times that of the free molecule (see fig.…”

“…The displacement current, dD / dt , is common to the series capacitors consisting of the vibrating CO, vacuum gap, and targeted molecule. In the hardwired CO, the CM arises from the charge transfer photocurrent through the projected density of <CO(2π*)|Ag(5s)> states ( 15 ). Modeling shows the light field to be confined to ~3 Å in the vacuum capacitor by the CO-terminated tip ( 28 ).…”

“…Detection of Raman scattering on single CO molecules was demonstrated previously at the plasmonic junction of fusing silver nanospheres, with large enhancement ascribed to tunneling charge transfer plasmons ( 13 ). A formal treatment of the scattering process in this regime ( 14 ) and explicit time-dependent density functional simulations of the photofield-driven tunneling current at the CO-STM junction ( 15 ) provide the background for the present development.…”

Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

“…In a recent paper, calculations suggest that the dipole in the adsorbates dictates the direction of electron transfer across the gap junction. 43 The time dependent Stark shifts to lower frequency are consistent with the CN dipole controlling the direction of electron tunneling. The time dependent changes appear to result from capacitive charging across the smallest nanogaps.…”

Photonics and spectroscopy in nanojunctions: a theoretical insight

“…Alternatively, external illumination was utilized as means to control electron transfer and transport. [52][53][54][55][56][57][58][59][60][61][62] Measurements of light induced magnetization in chiral molecules were also reported in the literature. [63][64][65][66][67][68] For molecules chemisorbed on metallic surfaces or encapsulated in nanocavities molecular excitations are coupled with plasmons.…”