Practical aspects of Kelvin-probe force microscopy at solid/liquid interfaces in various liquid media

Abstract

The distributions of surface charges or surface potentials on biological molecules and electrodes are directly related to various biological functions and ionic adsorptions, respectively. Electrostatic force microscopy and Kelvin-probe force microscopy (KFM) are useful scanning probe techniques that can map local surface charges and potentials. Here, we report the measurement and analysis of the electrostatic and capacitive forces on the cantilever tip induced by application of an alternating voltage in order to discuss the feasibility of measuring the surface charge or potential distribution at solid/liquid interfaces in various liquid media. The results presented here suggest that a nanometer-scale surface charge or potential measurement by the conventional voltage modulation techniques is only possible under ambient conditions and in a non-polar medium and is difficult in an aqueous solution. Practically, the electrostatic force versus dc voltage curve in water does not include the minimum, which is used for the surface potential compensation. This is because the cantilever oscillation induced by the electrostatic force acting on the tip apex is overwhelmed by the parasitic oscillation induced by the electrostatic force acting on the entire cantilever as well as the surface stress effect. We both experimentally and theoretically discuss the factors which cause difficulties in application of the voltage modulation techniques in the aqueous solutions and present some criteria for local surface charge and potential measurements by circumventing these problems.

Simulating Solid-Liquid Interfaces in Atomic Force Microscopy

“…Other directions will perhaps find it more challenging for systematic combined studies. Applying electric fields across solid-liquid interfaces [116] in Kelvin probe force microscopy touches the realms of electrochemistry [117], an area of intense theoretical development itself, without introducing the complexities of AFM. In selfassembly, research is now demonstrating the extra control offered by assembly in liquids [118], with solvent, impurity concentration and pH, all parameters that can dramatically change the molecular structure that assembles on a surface, and that is subsequently imaged in AFM.…”

Quantitative comparison of closed-loop and dual harmonic Kelvin probe force microscopy techniques

“…8 Open-loop KPFM (OL-KPFM) techniques are increasingly being adopted as feedback-free methods, 7,9,[18][19][20][10][11][12][13][14][15][16][17] eliminating the need for the application of a DC bias 7 and enabling the mapping of voltage-sensitive materials 9,15,21,22 and surface potentials in liquid environments. [10][11][12]16,17,[23][24][25] These techniques are also of interest for their reduced sensitivity to electronic offsets and electronic cross talk instrumentation a) Authors to whom correspondence should be addressed: jason.kilpatrick@ ucd.ie and brian.rodriguez@ucd.ie issues, 4,7,[32][33][34][35]13,22,[26][27][28][29][30][31] which can affect both OL-and CL-KPFM operation to varying degrees. 35 Dual harmonic KPFM (DH-KPFM) is one example of OL-KPFM which has been utilized to measure surface potential 7,[9][10]…”

Analysis of capacitive force acting on a cantilever tip at solid/liquid interfaces

“…Recently, we discussed the feasibility to measure local surface charge or potential distribution at solid/liquid interfaces by the voltage modulation techniques in aqueous solutions. 27 In this paper, we report measurements and analysis of the capacitive force acting on a cantilever with a tip induced by the alternating voltage application in various liquid media. We measured the dependence of the capacitive force on the frequency of the alternating voltage and on the tip-sample distance.…”