Tapping-mode AFM - Force Measurement Capabilities on Compliant Surfaces
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Tapping-mode AFM – Force Measurement Capabilities on Compliant Surfaces Ijeoma M. Nnebe1 and James W. Schneider1,2 Departments of Chemical Engineering1 and Biomedical Engineering2, Carnegie Mellon University Pittsburgh, PA 15213, U.S.A. ABSTRACT We investigate the feasibility of tapping-mode atomic force microscopy (TM-AFM) as a force measurement tool for compliant surfaces. For quantitative extraction of the tip-sample interactions, numerical modeling of the cantilever dynamics is required using a defined form for the interaction, with the results compared to experiment. Through TM force measurements on silicon, we illustrate that a forced damped harmonic oscillator model sufficiently represents the motion of the cantilever. Particularly for liquid operation, distance-dependent dissipation must be included in the model for accurate quantification of the tip-sample interactions and for successful reproduction of experimental force curves. This dissipation is not due to damping from the bulk viscous medium, but is likely frictional in origin. This investigation shows that TM force measurement in liquid is feasible and could be particularly advantageous for the measurement of intermolecular interactions from soft and easily deformed molecular layers. INTRODUCTION Direct force measurement techniques are now established and valuable tools in the measurement of intermolecular and inter-surface forces [1], [2], [3]. Atomic force microscopy (AFM) has high lateral resolution and a diversity of surfaces can be studied with the technique. Commonly, force measurements are conducted using dc atomic force microscopy (AFM), in which a static cantilever with attached probe is approached/retracted from a surface while its deflection is monitored as a function of the separation between the probe and surface. To maintain high lateral resolution, the probes used are integrated sharp tips. However, the compression pressures associated with these sharp tips commonly results in indentation of soft adsorbed layers before a measurable cantilever deflection is achieved. Additionally, the loads applied during this prolonged contact with the sample can be large enough to denature biomolecules [4]. An alternate mode of AFM force measurement designed to minimize the tipsample contact time with the purpose of providing a gentler and less-destructive mode of force measurement, is tapping-mode AFM (TM-AFM). TM-AFM is widely used for the imaging of biological and other easily deformable molecules [5]. In this mode, the cantilever is forced to oscillate at frequencies in the kilohertz range and large amplitudes resulting in intermittent contact between the tip and surface. Changes in the dynamic properties of its oscillation such as its amplitude of oscillation, mean deflection, and phase lag from the driving signal are then monitored with changes in tip-sample separation for force measurement. Tip-sample interactions cannot be directly calculated from the measured dynamic parameters in the TM force measurement, thereby making it a more comp
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