Tip Oscillation Amplitude Research Articles

Energydissipation above plane terraces of a model crystal innon-contact atomic force microscopy

We re-examine the calculation of the dissipation energy in thenon-contact atomic force microscope for a model flat surface within thestochastic friction force (the `Brownian motion') mechanism.All important aspects of the problem are taken into account. In particular,we have considered for the first time: (i) the effect of the tip onthe surface atom vibrations including the possibility of inducinglocal vibrational modes and (ii) the second term in the expansionof the fluctuating tip-surface force in atomic displacements. We foundthat generally, if the tip does not come very close to the surface,the first effect noticeably reduces the dissipation energy, whilethe second-order correction is very small, at least for the modelplanar surface considered. However, if during its oscillations thetip comes closer than a certain critical distance to the surface,a local vibrational mode is induced which may considerably increasethe dissipation energy depending on its lifetime, i.e. the degreeof anharmonicity in the system coupling the local mode (LM) with the restof the phonons. For certain systems which provide long-livingLMs, this effect may serve as a microscopic mechanism of energydissipation. We also demonstrate that the dissipation power increaseswith the tip oscillation amplitude.

From Images to Interactions: High-Resolution Phase Imaging in Tapping-Mode Atomic Force Microscopy

In tapping-mode atomic force microscopy, the phase shift between excitation and response of the cantilever is used as a material-dependent signal complementary to topography. The localization of information in the phase signal is demonstrated with 1.4-nm lateral resolution on purple membrane of Halobacterium salinarum in buffer solution. In a first-order approximation, the phase signal is found to correlate with modulations of the tip oscillation amplitude, induced by topography. Extending the analysis to contributions of the tip-sample interaction area as a second-order approximation, a method is proposed to extract information about the interaction from the phase signal for surfaces with a roughness in the order of the tip radius.

Viscoelastic and electrical properties of self-assembled monolayers on gold (111) films

The interaction between Pt-13% Rh tips (of {approximately}1000 {angstrom} radius) and (111) oriented gold films covered with alkanethiolate monolayers has been studied with an atomic force microscope (AFM) of the interferometer type. The thiol molecules, with structural formula H-S-(CH{sub 2}){sub n}-CH{sub 3} had 12 and 22 carbon atom chains (n = 11 and 21). The cantilever of the AFM was forced to oscillate near its resonance frequency while the amplitude and phase of the oscillation at the free end supporting the tip were measured. These two parameters changed rapidly when the tip approached within a few nanometers of the surface. This allows us to investigate the viscoelastic properties of the surface films. We found that the thiolate layers respond to the applied load in a reversible way. Upon contact with the layer, the tip oscillation amplitude decreased to 10-20% of its original value and remained at this level until the load reached (7 to 15) x 10{sup -6} N. At loads below 1 x 10{sup -6} N, the tip-sample interaction is mainly elastic and no visible marks can be seen in the ac-AFM (attractive mode) images. Above the value, the gold substrate was found to yield plastically. This plastic thresholdmore » was identical to that found in bare gold. With an applied bias, tunnel current was found upon initial contact of the tip with the alkyl chain. This current is very sensitive to defects of the layer and to the compressive force applied by the tip. A theoretical calculation was performed to explain the effects of the adsorbed organic molecules on tunnel currents. 31 refs., 14 figs.« less