Vibrational and electronic heating in nanoscale junctions

TL;DR

This paper demonstrates that surface-enhanced Raman emission can be used to measure the effective vibrational and electronic temperatures in biased nanoscale junctions, revealing mode-specific heating effects under electrical and optical excitation.

Contribution

It introduces a novel in situ method using Raman emission to determine effective temperatures of vibrational and electronic states in nanoscale junctions, providing insights into heat flow at the molecular level.

Findings

Vibrational modes exhibit mode-specific heating with effective temperatures over 300 K.

Electronic effective temperature increases up to three times under bias voltages of a few hundred millivolts.

The observed temperature trends are consistent across different bias conditions, supporting theoretical models.

Abstract

Understanding and controlling the flow of heat is a major challenge in nanoelectronics. When a junction is driven out of equilibrium by light or the flow of electric charge, the vibrational and electronic degrees of freedom are, in general, no longer described by a single temperature[1-6]. Moreover, characterizing the steady-state vibrational and electronic distributions {\it in situ} is extremely challenging. Here we show that surface-enhanced Raman emission may be used to determine the effective temperatures for both the vibrational modes and the flowing electrons in a biased metallic nanoscale junction decorated with molecules[7]. Molecular vibrations show mode-specific pumping by both optical excitation[8] and dc current[9], with effective temperatures exceeding several hundred Kelvin. AntiStokes electronic Raman emission\cite[10,11] indicates electronic effective temperature also increases to as much as three times its no-current values at bias voltages of a few hundred mV. While the precise effective temperatures are model-dependent, the trends as a function of bias conditions are robust, and allow direct comparisons with theories of nanoscale heating.