Nanomechanics Using an Ultra-Small Amplitude AFM
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Nanomechanics using an ultra-small amplitude AFM Peter M. Hoffmann, Steve Jeffery, Ahmet Oral1, Ralph A. Grimble, H. Özgür Özer1, and John B. Pethica Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom 1 Department of Physics, Bilkent University, Ankara, Turkey ABSTRACT A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable of atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface. INTRODUCTION Atomic Force Microscopy (AFM) holds tremendous promise as a tool to explore and map the mechanics of matter at the nano- and atomic scale. However, current limitations in the techniques has slowed progress in advancing this important technique. Most commonly used AFM techniques suffer from a variety of problems which either limit resolution or make quantitative interpretation of the obtained data very difficult. These problems include snapto-contact (contact AFM, intermittent contact AFM) or inherent non-linearities (most dynamic AFM modes) [1,2]. Here we present a new AFM technique which avoids these problems by using ultra-small amplitudes (linearization of the measurement), stiff levers (avoids snap-to-contact), and off-resonance operation (easy interpretation of the results, no need for high Q and thus useful in liquids). Using this new AFM technique we were able to map atomic bonding curves, measure atomic scale dissipation, and look at the mechanical properties of molecular ordering in water. EXPERIMENTAL The two AFM used in this study were entirely home-built and incorporated a variety of new features not generally present in commercial AFM. One of the instruments is located in an ultra-high vacuum system (base pressure 4x10-11 mbar), while the other is designed to operate in air or liquids. Both AFM use levers of sufficient stiffness to avoid instabilities (> 100 N/m in the ultra-high vacuum experiments) and sub-Ångstrom amplitudes. This requires
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a very high sensitivity sensor to accurately measure lever deflection and amplitude. In our case, we used a fiber interferometer which can be tuned to give a sensitivity up to 2 x 10-4 Å Hz-1/2. The levers were typically vibrated far below their lowest resonance at amplitudes of less than 1 Å (peak-to-peak). As the lever moved towards the surface
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