Multifrequency Atomic Force Microscopy (AFM) involves several force microscopy methods that aim to improve spatial resolution, data acquisition times and quantitative mapping of surface properties with respect to the level that has been achieved by conventional AFM. About twenty years ago, amplitude modulation AFM (tapping-mode) was introduced to avoid the sample modifications introduced by lateral forces that existed during contact AFM operation.
Conventional Dynamic AFM
In conventional dynamic AFM the microcantilever is excited at a single frequency and the oscillation signal is also recorded at that frequency. One of the main features that distinguishes multifrequency AFM from conventional dynamic AFM methods is that several frequencies of the microcantilever are involved in the excitation and/or the detection processes. The key issue, in terms of high resolution imaging and material contrast, is that under some circumstances the use of several frequencies is accompanied by a reduction of the applied forces together with the capacity to deal simultaneously with different interactions.
The advantages of multifrequency AFM approaches are more easily understood when the microcantilever-tip system is under the influence of interactions of different nature, for example, mechanical and magnetic forces. The detection of several frequencies readily provides a procedure to discriminate between their effects in a single step. For example, in bimodal AFM [1-3] the cantilever is simultaneously excited by two driving forces. The excitation frequencies are tuned to match the first two flexural eigenmodes of the cantilever. The output signal of the amplitude of the first mode is used to image the topography of the surface. An output signal of the second mode either the amplitude or the phase shift is used to map changes in mechanical, magnetic or electrical properties of the surface.
Various Applications of Multifrequency AFM
Multifrequency AFM images provide atomic or molecular resolution images of several systems and at the same time the mapping of another surface property. In the few years that have passed since the first Multifrequency AFM conference (Madrid, September 2008) torsional harmonic  and bimodal AFM  have been applied to measure the flexibility of several proteins in liquid with molecular resolution.
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In both cases, the flexibility maps are recorded simultaneously with the topography. This capacity brings more information about the relationship between the protein structure and its function. Multifrequency AFM also offers the framework to extend nanoscale spatial resolution for imaging structures that are beneath the surface . The sensitivity of multifrequency AFM together with its capacity to minimize the topography cross-talk in the electrostatic or magnetic interactions maps have led to use the instrument in more technological oriented problems. For example, the band excitation method has been applied to map the ion diffusion in a lithium-ion battery cathode with nanoscale spatial resolution .
Many multifrequency AFM applications involve liquid or air environment, however, it has also been shown that bimodal AFM could also be applied in ultra-high vacuum to image layered materials with atomic resolution . These images were hard to achieve with conventional non-contact AFM because the forces involved were rather high and they produced a displacement of the surface atoms.
Multifrequency AFM will be an inseparable feature of force microscopy as it represents the more consistent framework to improve the capabilities of nanoscale microscopes in terms of spatial resolution and quantitative measurements without slowing down the operation of the microscope. However, the gains in spatial resolution and quantitative mapping require a theoretical understanding that is still under development.
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Prof. Dr. Ricardo Garcia
Dr. Elena T. Herruzo
Dr. Christian Dietz
IMM - Madrid Microelectronics Institute, CSIC
Tres Cantos, Spain
Nicolas F. Martinez, Ph. D.
Mecwins Research Dept.,
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