Harnessing the damping properties of materials for high-speed atomic force microscopy

Abstract

The success of high-speed atomic force microscopy in imaging molecular motors 1 , enzymes 2 and microbes 3 in liquid environments suggests that the technique could be of significant value in a variety of areas of nanotechnology. However, the majority of atomic force microscopy experiments are performed in air, and the tapping-mode detection speed of current highspeed cantilevers is an order of magnitude lower in air than in liquids. Traditional approaches to increasing the imaging rate of atomic force microscopy have involved reducing the size of the cantilever 4,5 , but further reductions in size will require a fundamental change in the detection method of the microscope [6][7][8] . Here, we show that high-speed imaging in air can instead be achieved by changing the cantilever material. We use cantilevers fabricated from polymers, which can mimic the high damping environment of liquids. With this approach, SU-8 polymer cantilevers are developed that have an imaging-in-air detection bandwidth that is 19 times faster than those of conventional cantilevers of similar size, resonance frequency and spring constant.A primary research goal in atomic force microscopy (AFM) is to increase the imaging speed, improve its ease of use and expand its potential range of applications 9 . In the most widely used AFM mode (a.c. mode or tapping mode) the detection speed (mechanical bandwidth, BW) of the AFM cantilever fundamentally limits the imaging speed. The bandwidth is a measure of the maximum rate of topography change the cantilever can accurately detect. It is related to the cantilever resonance as BW ∝ f 0 /Q, where the cantilever resonance frequency f 0 is primarily determined by the cantilever mass and elastic modulus, and the quality factor Q is determined by the cantilever damping 10 . When the oscillating cantilever experiences a change in boundary condition (that is, topography), it requires several cycles to reach a new steady-state amplitude (Fig. 1a). A cantilever with higher resonance frequency runs through the required number of cycles more quickly, thereby enabling faster imaging (Fig. 1b). The number of required oscillatory cycles is determined by the damping of the cantilever, characterized by Q. A cantilever with low resonance frequency and low Q can therefore be equally as fast as a cantilever with high resonance frequency and high Q (compare Fig. 1b and c). From a detection bandwidth perspective, the ideal combination is a high resonance frequency and low quality factor (Fig. 1d).The development of current high-speed AFM (HS-AFM) technology was enabled by the miniaturization of silicon and silicon nitride (SiN) cantilevers to x-y dimensions below 10 µm (the approach taken in Fig. 1b), resulting in cantilevers with megahertz resonance frequencies 4,5 . Modelling and an improved understanding of cantilever behaviour in fluids has greatly benefited this geometric optimization. For nearly all cantilevers, viscous damping in the surrounding medium determines Q (ref. 11). In liquid, viscous damping yields Q ≈ 2...

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