Energy Dissipation in the Mechanical-Diode Jump of a Nanoscale Contact
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Energy Dissipation in the Mechanical-Diode Jump of a Nanoscale Contact Juan J. Martínez, and M. Teresa Cuberes Laboratory of Nanotechnology, University of Castilla-La Mancha, Plaza Manuel de Meca, 1, Almadén, E-13400, Spain ABSTRACT In Ultrasonic Force Microscopy, the mechanical diode response refers to the quasistatic cantilever deflexion in the presence of surface out-of-plane ultrasonic vibration of sufficiently high amplitude. The effect has been described by introducing ultrasonic-amplitude-dependent tip-sample force-distance curves [Phys. Rev. 61 (2000) 13997]. Here, we demonstrate that the ultrasonic hysteresis phenomenon is qualitatively explained taking into account that for certain ultrasonic amplitudes, the modified tip-sample forces lead to two stable quasi-static equilibrium states, separated by an energy barrier. Experimental UFM data obtained on mica at ambient conditions are discussed in terms of ultrasonic-induced quasi-static equilibrium states, taking into account the role of the surface water layer. INTRODUCTION In Ultrasonic Force Microscopy (UFM), an Atomic Force Microscopy (AFM) cantilever with the tip in contact with a sample surface behaves as a mechanical diode which deflects in the presence of normal ultrasonic vibration of sufficiently high amplitude [1-4]. Recently, novel methods to obtain information about the work of adhesion and the adhesion hysteresis at the tipsample contact using UFM have been proposed [4-7]. Essentially, they take advantage from the fact that the ultrasonic excitation amplitude at which the UFM cantilever response jumps out when increasing the excitation is different from this at which it jumps in when decreasing the excitation. The jumps out and in consist in an increase and decrease respectively of the mechanical-diode cantilever deflection that occur at certain ultrasonic amplitudes [5-7]. The mechanical diode effect has been described using ultrasonic-amplitude-dependent tip-sample force-distance curves [3]. The quasistatic equilibrium deflexion of the cantilever is reached when the force exerted by the cantilever, considered as a point mass, equals the modified tip-sample force. For certain ultrasonic amplitudes, more than a single quasistatic cantilever position satisfy the equilibrium force condition. Here, we demonstrate that at each of those amplitudes, different minimum energy quasi-static equilibrium states separated by an energy barrier are accessible to the tip. Modification of the ultrasonic excitation amplitude leads to a modification of the state energies, and of barrier height between the energy minima. Transition of the tip from a minimum energy quasistatic equilibrium state to the next explains the mechanical diode jump-out (and jump-in) of the cantilever induced by increasing (and decreasing) the ultrasonic amplitude. EXPERIMENT The experimental set-up for UFM [2, 3] has been implemented in our lab on a commercial AFM (Nanotec). Ultrasonic vibration was excited using a piezotransducer located below the sample. A function generator
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