Linear measurements of nanomechanical phenomena using small-amplitude AFM
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Linear measurements of nanomechanical phenomena using small-amplitude AFM Peter M. Hoffmann1, Shivprasad Patil1, George Matei1, Atay Tanulku1, Ralph Grimble2, Özgur Özer3, Steve Jeffery2, Ahmet Oral4, John Pethica3 1
Department of Physics, Wayne State University, Detroit MI 48201 2 Department of Materials, University of Oxford, Oxford, UK 3 Department of Physics, Trinity College, Dublin, Ireland 4 Department of Physics, Bilkent University, Ankara, Turkey
ABSTRACT: Dynamic Atomic Force Microscopy (AFM) is typically performed at amplitudes that are quite large compared to the measured interaction range. This complicates the data interpretation as measurements become highly non-linear. A new dynamic AFM technique in which ultra-small amplitudes are used (as low as 0.15 Angstrom) is able to linearize measurements of nanomechanical phenomena in ultra-high vacuum (UHV) and in liquids. Using this new technique we have measured single atom bonding, atomic-scale dissipation and molecular ordering in liquid layers, including water.
INTRODUCTION: Since its invention in 1986 [1], Atomic Force Microscopy (AFM) has rapidly become a major research tool for nanoscale imaging of general surfaces and performing measurements of forces down to the molecular and atomic scale. There are a number of possible ways to operate an AFM, the simplest being contact AFM, where the static deflection of the cantilever is used as feedback parameter for imaging. However, very soon after the invention of AFM, it was found that dynamic AFM modes have several advantages over the static mode, especially the possibility of reduced wear and achieving higher resolution. The key to true atomic resolution imaging is to avoid full contact between tip and surface. This can be achieved by using stiff cantilevers which can withstand the snap-in instability close to the surface. This necessitates much higher signal-to-noise than conventional AFM. This was obtained by using frequency detection instead of amplitude detection and true atomic resolution with a frequency-shift AFM was first achieved in 1995 [2]. A problem with frequency-shift AFM, however, has been the interpretation of force and dissipation data. Since the AFM is operated at resonance, lever amplitudes tend to be large compared to the range of typical molecular and atomic interactions. The measured frequency shift then becomes equal to an integral of the interaction force over the path of the tip. Thus an integral equation must be solved to extract the measured force. This can be done if only conservative forces are considered [3]. However, if dissipative forces are included as well, interpretation becomes difficult. In particular, measurements of interaction curves at roomtemperature using large amplitude AFM have shown abnormally large interaction ranges[4].
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These issues are still not satisfactorily resolved. Another problem with large-amplitude, resonance techniques is that they rely on a high Q factor of the lever and are therefore very difficult to use in liquid environment,
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