Frequency Dependent Silica Dissolution Rate Enhancement under Oscillating Pressure via an Electrochemical Pressure Solution-like, Surface Resonance Mechanism

Abstract

From atomic force microscopy (AFM) experiments, we report a new phenomenon in which the dissolution rate of fused silica is enhanced by more than 5 orders of magnitude by simply pressing a second, dissimilar surface against it and oscillating the contact pressure at low kHz frequencies in deionized water. The silica dissolution rate enhancement was found to exhibit a strong dependence on the pressure oscillation frequency consistent with a resonance effect. This harmonic enhancement of the silica dissolution rate was only observed at asymmetric material interfaces (e.g., diamond on silica) with no evidence of dissolution rate enhancement observed at symmetric material interfaces (i.e., silica on silica) within the experimental time scales. The apparent requirement for interface dissimilarity, the results of analogous experiments performed in anhydrous dodecane, and the observation that the silica “dissolution pits” continue to grow in size under contact stresses well below the silica yield stress refute a mechanical deformation or chemo-mechanical origin to the observed phenomenon. Instead, the silica dissolution rate enhancement exhibits characteristics consistent with a previously described ‘electrochemical pressure solution’ mechanism, albeit, with greatly amplified kinetics. Using a framework of electrochemical pressure solution, an electrochemical model of mineral dissolution, and a recently proposed “surface resonance” theory, we present an electro-chemo-mechanical mechanism that explains how oscillating the contact pressure between dissimilar surfaces in water can amplify surface dissolution rates by many orders of magnitude. This reaction rate enhancement mechanism has implications not only for dissolution but also for potentially other reactions occurring at the solid–liquid interface, e.g. catalysis.

A New Method of AFM‐Based Nanolithography Using Frequency Enhanced Electrochemical Pressure Solution Etching

“…Our tapping mode (TM) under water AFM experiments demonstrate the feasibility of using the (FEEPS etching to carry out nanolithography work on fused silica surfaces. Experiments investigating the effect of varying the AFM tip velocity upon the depth of the etched features showed that the process obeyed the same power law relationship between "contact time" and etching depth as was previously observed in hole etching experiments, [8] thus demonstrating the etching observed herein is dominated by the FEEPS etching mechanism. By varying the velocity of the AFM tip and hence, the dissolution time, gray-scale lithography that has proven challenging to obtain by others means became thus not only possible but also easy to design, giving features as deep as 60 nm in a single AFM tip pass (trace and retrace), comparable to what can be obtained on thermally sensitive polymers using a heated AFM probe.…”

“…What is immediately noticeable is the high inverse dependence of the etched trench depth with the AFM tip velocity; the depth decreases from 65 nm at a velocity of 0.2 µm s −1 down to 0.76 nm at 600 µm s −1 . It was found that the obtained data were well fitted (correlation coefficient of 0.97) using the power law, characteristic of FEEPS processes [8] (see Equation ( 3))…”

“…Although not studied in detail, preliminary etching results were obtained utilizing FEEPS onto GaN and Si substrates. [8] This material can be found in the Supporting Information). [8] Last, the machining dimensional accuracy of the method is closely related to the chord theorem, where the width of a feature on the surface is given by Equation ( 5), as a function of its depth (d) and the AFM tip radius R…”

“…[6][7] More recent atomic force microscopy experiments highlighted that pressure solution was functionally equivalent to mixed potential corrosion (differing only in the mechanism of charge transfer) where the kinetics were driven by the independent yet electrically coupled (via the Stern potential gradient between the two surfaces) removal rates of charged species from the surface. [8] The electric coupling of the removal rates accounts for the slower pressure solution kinetics observed in symmetric (silica-silica) compared to dissymmetric systems as the Stern potential in the symmetric case is zero (or at least has zero gradient when in close proximity). Ardagh et al, [9] proposed that catalysis reactions could be accelerated through the creation of surface potential resonances to break equilibrium in the dissolution rates, this process was also shown to accelerate pressure solution kinetics well above the theoretical Sabatier maximum, [8] in a process called frequency enhanced electrochemical pressure solution (FEEPS).…”

“…Ardagh et al, [9] proposed that catalysis reactions could be accelerated through the creation of surface potential resonances to break equilibrium in the dissolution rates, this process was also shown to accelerate pressure solution kinetics well above the theoretical Sabatier maximum, [8] in a process called frequency enhanced electrochemical pressure solution (FEEPS). The acceleration of pressure solution by more than 5 orders of magnitude due to the application of pressure oscillations of kHz frequencies [8] makes the process very suitable for lithography work. Consequently, FEEPS has the potential to improve upon many of the problems associated with leading atomic force microscopy (AFM)-based lithography techniques like nanoscratching and tribochemistry.A new method of direct-write nanolithography that is able to rapidly etch silica surfaces under a scanning atomic force microscopy (AFM) probe in tapping mode (TM) is reported.…”