A comparative investigation of thickness measurements of ultra-thin water films by scanning probe techniques

TL;DR

This study compares scanning probe techniques for measuring ultra-thin water film thickness, finding dynamic force spectroscopy to be more accurate than tunnelling spectroscopy, especially in surface science applications.

Contribution

It provides a direct comparison of distance tunnelling and dynamic force spectroscopy methods for water film thickness measurement, highlighting the superior accuracy of the latter.

Findings

Dynamic force spectroscopy yields more accurate water film thickness measurements.

Tunnelling spectroscopy tends to overestimate thickness by about 25 times.

Both methods can be directly compared through their results.

Abstract

The reliable operation of micro and nanomechanical devices necessitates a thorough knowledge of the water film thickness present on the surfaces of these devices with an accuracy in the nm range. In this work, the thickness of an ultra-thin water layer was measured by distance tunnelling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 1E-8 mbar. The tunnelling current as well as the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct conversion of the results of both methods is possible. From the standpoint of surface science, taking the state-of-the-art concerning adsorbates on surfaces into consideration, dynamic force spectroscopy provides the most accurate values. The previously reported tunnelling spectroscopy, requiring the application of significantly high voltages, generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy.