Nanomechanics Using an Ultra-Small Amplitude AFM

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.

Direct measurement of molecular stiffness and damping in confined water layers

“…The challenge of such a technique is the measurement of exceedingly small signals, since the usual methods of signal enhancement (large amplitudes, low stiffness levers, resonance operation) are not used. Recently, we succeeded in implementing such a technique in UHV 11,12,13 -and in liquids 14 , using an improved fiber interferometric displacement sensor 15 to overcome the reduced signal-tonoise of the technique. Here we report on our direct measurements of the mechanical properties of confined water layers using this novel AFM technique.…”

Theoretical and experimental explorations in atomic force microscopy

“…A suitable choice of cantilever spring constant k and oscillation amplitude in vacuum have resulted in atomic resolution [34,39] rivaling that of an STM. A sub-resonance small amplitude AM technique has been recently developed [40,41] that can be used in liquids [42]. In chapter 5 a new small ampli tude FM technique based on thermal noise is developed that is particularly suitable for studying highly localized slowly evolving atomic or molecular phenomena at ambient temperatures.…”