Synergy of Resonant Oscillatory Modes in Atomic Force Microscopy of Polymers
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Synergy of Resonant Oscillatory Modes in Atomic Force Microscopy of Polymers Sergei Magonov, Sergey Belikov, John Alexander and Marko Surtchev NT-MDT Development Inc., 7910 S. Kyrene Rd, Tempe, AZ 85284, U.S.A.
ABSTRACT The set of oscillatory resonance AFM modes is expanded with frequency modulation mode and frequency imaging in amplitude modulation mode. The backgrounds of these modes are discussed and their capabilities are compared on the practical examples. The data show how these techniques complement the amplitude modulation with phase imaging. The frequency imaging enhances the compositional mapping of heterogeneous samples. Frequency modulation mode provides a superior capability in imaging at low tip-sample forces.
INTRODUCTION The tip-sample interactions in Atomic Force Microscopy (AFM) change the probe characteristics (amplitude, phase or frequency) when it oscillates near its resonance. The drop of the probe amplitude is used to track the surface profile in amplitude modulation mode, also known as taping mode [1]. The technique has been further expanded by recording the phase changes, which provide the image contrast distinguishing the dissimilar components of heterogeneous materials [2]. We denote this primary AFM tool for studies in air and under liquid as amplitude modulation with phase imaging (AM-PI). In UHV studies, the high-quality factor of the oscillating probe makes the use of amplitude changes for surface profiling impractical, and in such environment the frequency changes are applied for imaging of surfaces in frequency modulation (FM) mode [3]. The FM applications in air and liquid, which are performed by few researchers, are mostly focused at atomic-scale imaging and precise detection of tip-sample forces [4]. In the present study we describe the common background of the above modes and a less known amplitude modulation with frequency imaging (AM-FI) and illustrate their synergy in AFM applications on a number of samples. EXPERIMENTAL DETAILS The measurements in the resonant oscillatory modes were performed with NEXT scanning probe microscope (NT-MDT), which was placed in a quiet temperature stable cabinet. The sample temperature was kept approximately 3 degrees above room temperature with deviations less than 0.01˚C. High temperature stability ensures a low thermal drift 0.2 nm/min of the sample. The applied AM-PI, AM-FI and FM modes are implemented in the applied microscope with a controller, which incorporates lock-in amplifiers (LIA) and phase-locked loop (PLL). In our studies we have applied the Si probes with spring constant in the 1 – 40 N/m range. The scanning rates were varied from 0.5 Hz for imaging of large areas to 1Hz at smaller areas.
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RESULTS The tip-sample interactions in AFM resonant modes can be described by solving the EulerBernoul
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