The tip-sample water bridge and light emission from scanning tunnelling microscopy

TL;DR

This study uses light emission from a scanning tunnelling microscope to monitor water meniscus formation at the nanoscale, revealing humidity-dependent growth and properties with high sensitivity, and providing insights into localized surface plasmon behavior.

Contribution

It introduces an empirical model linking water bridge growth to light emission spectra, enabling continuous, sub-nanometer monitoring of nanoscale water structures in STM.

Findings

Light emission decreases with increasing humidity.

Water filling correlates with spectral changes and plasmon volume.

Polarity influences water structuring and emission characteristics.

Abstract

Light emission spectrum from a scanning tunnelling microscope (LESTM) is investigated as a function of relative humidity and shown to be a novel and sensitive means for probing the growth and properties of a water meniscus in the nm-scale. An empirical model of the light emission process is formulated and applied successfully to replicate the decay in light intensity and spectral changes observed with increasing relative humidity. The modelling indicates a progressive water filling of the tip-sample junction with increasing humidity or, more pertinently, of the volume of the localized surface plasmons responsible for light emission; it also accounts for the effect of asymmetry in structuring of the water molecules with respect to polarity of the applied bias. This is juxtaposed with the case of a non-polar liquid in the tip-sample nano cavity where no polarity dependence of the light emission is observed. In contrast to the discrete detection of the presence/absence of water bridge in other scanning probe experiments by measurement of the feedback parameter for instrument control LESTM offers a means of continuously monitoring the development of the water bridge with sub-nm sensitivity. The results are relevant to applications such as dip-pen nanolithography and electrochemical scanning probe microscopy.