Interplay of bias-driven charging and the vibrational Stark effect in molecular junctions

TL;DR

This study investigates how bias-driven charging and the vibrational Stark effect influence vibrational energies in molecular junctions, revealing significant reversible shifts in PCBM and C60 junctions through combined experimental and theoretical approaches.

Contribution

It introduces a combined experimental and DFT-based theoretical analysis of vibrational energy shifts caused by bias and charging effects in PCBM and C60 molecular junctions.

Findings

Large reversible vibrational energy shifts observed in PCBM junctions.

Vibrational Stark effect dominates in PCBM due to its permanent dipole.

Charging effects are more significant in C60 junctions.

Abstract

We observe large, reversible, bias driven changes in the vibrational energies of PCBM, based on simultaneous transport and surface-enhanced Raman spectroscopy (SERS) measurements on PCBM-gold junctions. A combination of linear and quadratic shifts in vibrational energies with voltage is analyzed and compared with similar measurements involving C60-gold junctions. A theoretical model based on density functional theory (DFT) calculations suggests that both a vibrational Stark effect and bias-induced charging of the junction contribute to the shifts in vibrational energies. In the PCBM case, a linear vibrational Stark effect is observed due to the permanent electric dipole moment of PCBM. The vibrational Stark shifts shown here for PCBM junctions are comparable to or larger than the charging effects that dominate in C60 junctions.