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

Abstract

The light emission spectrum from a scanning tunnelling microscope (LESTM) is investigated as a function of relative humidity and shown to provide a novel and sensitive means for probing the growth and properties of a water meniscus on the nanometre 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 the polarity of the applied bias. This is juxtaposed with the case of a non-polar liquid in the tip-sample nanocavity where no polarity dependence of the light emission is observed. In contrast to the discrete detection of the presence/absence of a water bridge in other scanning probe experiments through measurement of the feedback parameter for instrument control, LESTM offers a means of continuously monitoring the development of the water bridge with sub-nanometre sensitivity. The results are relevant to applications such as dip-pen nanolithography and electrochemical scanning probe microscopy.

Indirect Detection of the Light Emission in the Local Tunnel Junction

“…Considering that, the spectral measurements were conducted at ambient conditions with the use of more sophisticated optical scheme described in the study by Shkoldin et al We used the same Au sample and Pt/Ir probes for measurements. As shown in the study by Shkoldin et al, relative humidity influences on the spectral position of emitted light. However, the peak shift is less than 0.1 eV, even if the relative humidity changes in a wide range .…”

“…As shown in the study by Shkoldin et al, relative humidity influences on the spectral position of emitted light. However, the peak shift is less than 0.1 eV, even if the relative humidity changes in a wide range . Thus, the optical emission spectrum peak position measured at ambient condition should be closed to be obtained in vacuum condition.…”

“…However, the peak shift is less than 0.1 eV, even if the relative humidity changes in a wide range. [36] Thus, the optical emission spectrum peak position measured at ambient condition should be closed to be obtained in vacuum condition. In our experiments, the spectrum peak is broadened, and its position is located near the 1.7 eV, which is in coincidence with I(V ) and other optical measurements.…”

The role of gap plasmons in light emission from tunnel junctions

“…In order to stimulate a photoemission-rate variation event, the bias voltage was abruptly increased during scanning to a value of 4.5 V and then reduced back to 2.25 V. A tunneling bias of 4.5 Volts is unstable under ambient conditions and can lead to electrical breakdown of the air over the applicable distance scale of ∼8 Angstroms. Accordingly, an instability is likely to lead to an event that affects the shape and/or the chemistry of the probe surface, which is typically enveloped in a water meniscus under ambient conditions [14] and may operate in the presence of surface adsorbates [6]. Such an event is viewable in each part of Figs.…”

Local low-energy electrical excitation of localized and propagating surface plasmons with a scanning tunneling microscope

“…STM tunnel currents of up to values of I tunnel = 6 nA are used and typically V sample = 2.8 V. It should be noted that as the STM operates in normal, laboratory air (25-45% relative humidity), a water bridge forms between the tip and sample. 11 This water bridge is expected to change the refractive index and tip-sample separation as compared to that for an STM operating in vacuum with the same tunneling parameters. tip (located in the center of the figure) excites plasmons on a 50 nm thick Au film.…”