The particular advantages that atomic force microscopy (AFM) has over other types of microscopy are well-known, but it has the one major disadvantage of low imaging rates in conventional instruments in which each image requires typically a minute or more to collect. This has two major detrimental effects: (i) only processes that occur on a timescale greater than minutes can be followed, and (ii) because there is no low magnification mode, it is not possible to image large areas of the specimen on a practical timescale.
The atomic force microscope is a mechanical microscope and, as such, imaging speeds are limited by the inertia and resonances of the scanning systems and of the AFM cantilever. At high imaging speeds, the bandwidth of the feedback system (to maintain, for example, constant force or RMS tapping amplitude) also becomes a limiting factor. A logical solution which has been developed over the last decade by several groups worldwide is to decrease the mass and increase the stiffness of the scanning system and the cantilever in order to decrease their response times by reducing inertial effects and increasing resonant frequencies, that is, to move the microscope’s response into a different time regime. However, in this presentation, alternative methods of high-speed AFM imaging will be described and results shown for a variety of specimens.
The alternative solution to high-speed AFM imaging presented here is different to the above method. Instead of avoiding resonance in the scanning system, resonance is used to scan the probe in the fast-scan direction. The original implementation was for scanning near-field optical microscopy1, which achieved 120 frames per second. This represents an increase in imaging speed of over 100,000 times. In the case of the high-speed AFM2, the specimen has either been mounted on the resonant scanner or on a flexure stage capable of scanning at up to 40kHz and an AFM cantilever probe is brought in to continuous contact with the specimen and the topographic structure derived for the deflection of the cantilever. A piezo stack provides the slow scan. Imaging at video rate is routinely achieved even on soft biological or organic molecules materials. Damage to specimens resulting from this high-speed contact-mode imaging is surprisingly very considerably less than would be caused at normal speeds. The nature of tribological forces at the nanoscale and high shear rates is of great interest in general and in this new technique in particular.
Developments of the instrument in the last month have now allow imaging at over 1000 frames per second on soft specimens. With each frame recorded in less than 1 ms, it should be possible to follow a wide range of processes in both biology and materials science.
- ADL Humphris, JK Hobbs, & MJ Miles, Applied Physics Letters 83 (2003) 6-8.
- Humphris, MJ Miles, & JK Hobbs, Applied Physics Letters 86 (2005) art no. 034106.
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