Non-Contact AFM Imaging in Water Using Electrically Driven Cantilever Vibration

Abstract

An atomic force microscopy (AFM) imaging mode is presented that can simultaneously record surface topography and local electrical properties in aqueous solutions without mechanical contact between the AFM tip and the sample. The interaction between the electrically biased tip and the grounded sample in aqueous medium causes the AFM cantilever to vibrate. This operation mode is based on the previously developed SPFM technique, though using water as the medium instead of air introduces some important practical and theoretical differences, and also greatly extends the applicability of this technique. There are two vibration modes, one at the frequency of the applied voltage (ω) and one at twice this frequency (2ω). The surface topography can be imaged using feedback control of the 2ω vibration amplitude, which is very sensitive to the tip-sample separation distance in the range of 1-10 nm. The amplitude and phase of the 1ω vibration can be recorded simultaneously during imaging to obtain information on local surface charge or potential differences. Similar techniques exist for imaging in air or vacuum, but the addition of a polarizable medium such as water adds significant theoretical and practical complexities. This paper addresses those complexities and demonstrates the effectiveness of the technique for surface imaging and analysis in aqueous environments.

Probing charge screening dynamics and electrochemical processes at the solid–liquid interface with electrochemical force microscopy

“…Recent reports of charge density 26 or electric potential 21,22 mapping have been based on the assumption of the presence of a lossless dielectric, and hence a time-invariant electric field in the tip-sample junction. To date, little consideration has been given to the role ion dynamics play on the forces experienced by a VM-SPM probe in liquid.…”

“…To date, KPFM has been implemented in liquid in the absence of ions (nonpolar liquid) 20 and limited progress has been obtained in low-molarity electrolytes (o10 mM) using KPFM modes, which do not require bias feedback (for example, open-loop KPFM) 21,22 . While electrostatic forces are increasingly being used to actuate atomic force microscope (AFM) cantilever motion in liquids to measure surface topography [23][24][25][26] , surface charge density 26 and dielectric constants [27][28][29] , voltage-modulated (VM) SPM imaging in conductive electrolytes presents an unresolved challenge.…”

Dual harmonic Kelvin probe force microscopy at the graphene–liquid interface

“…18 It is widely accepted that electrochemical processes inhibit quantitative CPD measurements, limiting the application of open loop KPFM to date to high frequency and low molarity measurement conditions. 15,16 Here, using the theory of diffuse ion dynamics, we describe why KPFM approaches, 12,15,16 and voltage-modulated scanning probe microscopy in general, [19][20][21] breakdown in liquids containing mobile ions.…”

Nanoscale dielectric measurements from electrostatic force microscopy

“…As discussed below, various studies have recently been conducted to measure the charge, surface potential, dielectric constant, Stern potential, Debye length and other electrical surface properties in solution. 39,42,[61][62][63] Key to recent advances are the studies carried out by Sounart et al and Panchawagh et al 41,64 on the electromechanical stability of passivated parallel plate electrostatic actuators in conductive dielectric media. They show that by appropriately choosing the frequency of the AC signal one can control the forces between the various components of the actuator.…”