Analysis of the interaction mechanisms in dynamic mode SFM by means of experimental data and computer simulation

Abstract

The performance of a scanning force microscope (SFM) operated in the dynamic mode at high oscillation amplitudes is determined by the response of the system to a given set of interaction forces between the probing tip and the sample surface. To clarify the details of the cantilever/tip dynamics two different aspects were investigated in experiment and computer simulation. First, the interaction forces dominating the oscillatory motion of the probe were varied by applying an additional electrostatic force field. It is shown that such variations in the attractive part of the interaction potential can cause a switching between two different oscillation states and thereby significantly contribute to the contrast obtained from phase imaging. Secondly, the interaction forces were kept constant but the system response itself was varied by modifying the effective quality factor of the oscillating cantilever with an active feedback circuit. This provides a means to influence the transition from the attractive to the partly repulsive interaction regime, i.e. the onset of the intermittent contact or tapping mode.Operating an SFM in the dynamic mode at high amplitudes (> 10 nm) offers the possibility of minimizing the contact time of the probing tip with the sample surface and thereby reduce lateral or friction forces involved in the scanning process. It also allows the collection of additional data related to different sample properties by recording the phase shift between the force driving the cantilever and its oscillation. In the last few years these features of operating the SFM in the dynamic mode [1, 2] were shown to be very useful to characterize several different kinds of sample surfaces, e.g. thin organic films, polymers, biological samples or even liquid droplets [3]. Although this has led to a steady increase in the number of possible applications, there are still several details of the interaction process between the tip and the sample that need further clarification. The overall goal must be to relate the experimentally accessible data, such as the amplitude and * Corresponding author phase signal, more or less directly to specific sample properties, such as topography, elasticity and viscoelasticity.Because highly nonlinear interaction forces are involved when the oscillating tip is in close proximity to a solid surface, the analysis of the dynamic system becomes quite complex. Therefore supplementary computer simulations based on proper mathematical models are useful to investigate the details of the interaction process. Basically, the equation of motion describing the dynamic properties of the probing tip has to be solved in such a way that the influence of different parameters characterizing the probe as well as the sample surface can be examined. There have been several reports recently on different approaches to this problem, providing analytical [4] as well as numerical [5][6][7][8][9][10][11] solutions. Most of them are based on the point-mass model, but there are also approaches which describe the comp...

Models for quantitative charge imaging by atomic force microscopy

“…9͑a͔͒, suggesting more of a dependence on initial conditions 58 than on the strength or other property of the attractive force. 60 The features we find are similar to those seen by others experimentally and computationally, 57,61 and are qualitatively similar to our experimental traces. Unlike Anczykowski, Krüger, and Fuchs, 61 we calculate a step-like discontinuity for a driving frequency less than the resonant frequency.…”

Controlling the quality factor of a tuning-fork resonance between 9 and 300 K for scanning-probe microscopy

“…The additional feedback circuit (gray dashed box) consisting of an amplifier and a phase shifter (e.g. a fixed phase of 90 • ) is responsible for the quality-factor control and works as described for example in Ref [26]. The signal of this feedback circuit is amplified (gain) and used to excite the tuning fork in addition to the signal from the function generator (Σ).…”

“…Controlling the quality factor of the resonance provides a solution to this problem. It has already been shown [25,26,27,28,29,30,31] that quality factor control is possible. Yet, up to now, it has only been applied to measurements in liquids, in ultrahigh vacuum, or at liquid He temperatures, but it has not been used to measure temperature-dependent system dynamics.…”

Active damping of the scanner for high-speed atomic force microscopy

“…Without counter-balancing (curves (b)), it appears with a much larger amplitude. Active Q control of cantilever vibration has recently been used for various purposes, such as sensitizing the tip-sample interaction by increasing Q [11][12] , or enhancing the cantilever's response speed by decreasing Q. [13][14] This technique can be applied to any resonant system.…”