Multi-Frequency Atomic Force Microscopy Combining Amplitude- and Frequency-Modulation Techniques
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Multi-Frequency Atomic Force Microscopy Combining Amplitude- and FrequencyModulation Techniques Santiago D. Solares and Gaurav Chawla Department of Mechanical Engineering, University of Maryland, 2181 Glenn L. Martin Hall, College Park, MD 20742, U.S.A. ABSTRACT Multi-frequency atomic force microscopy (AFM) offers additional response signals in comparison to traditional dynamic AFM. Furthermore, depending on the mode of operation used, the higher eigenmode responses are generally not directly influenced by the topographical acquisition control loops, such that they can explore a fuller range of tip-sample interactions. In this work we describe the implementation of multi-frequency imaging schemes that enable the acquisition of topographical, phase and frequency shift contrast in tapping-mode operation. This type of characterization can be especially useful for soft, highly dissipative samples, such as polymers, for which the various response channels can exhibit significantly different response, thus providing complementary information. We discuss typical results obtained as well as important challenges that need to be addressed in order to develop a fully quantitative technique. INTRODUCTION Recently, developments in multi-frequency AFM have made possible the simultaneous mapping of a larger number of sources of contrast during each sample scan. The first implementation of this technique reported by Garcia and coworkers [1, 2] involved a bimodal scheme applied in non-contact imaging, which has since been extended to repulsive conditions and to liquid environments. The method consists of controlling the fundamental cantilever eigenmode using amplitude modulation (AM-AFM [6]) to acquire the sample topography, while simultaneously exciting the second eigenmode with a smaller amplitude in open loop mode to obtain a phase contrast (note that it is also possible to use a different higher eigenmode). This approach has the advantage over traditional single-frequency tapping-mode AFM that the higherorder phase contrast can offer superior sensitivity with respect to the fundamental phase [1, 3]. Similarly, Meyer and coworkers [7] and Sugawara and coworkers [8] reported on the development of a bimodal imaging scheme for ultra-high vacuum in which both the fundamental and higher-order eigenmodes were operated using phase-locked-loop (PLL) implementations of frequency-modulation AFM (FM-AFM). These authors also operated the higher eigenmode with a smaller amplitude than that of the fundamental mode and observed higher sensitivity with respect to single-mode operation. Other multi-frequency AFM characterization approaches have been proposed and/or implemented, using direct or indirect excitation of the higher eigenmodes of the cantilever, including dual-frequency resonance tracking [9], intermodulation atomic force microscopy [10], band excitation AFM [11], higher-harmonics AFM [12-14], dual-frequency
phase modulation AFM [15], and dual-frequency modulation schemes with self-excitation of the higher eigenmode for 3D mapping of
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