The invention of AFM by Binnig was triggered by his observations of force effects that occur when performing STM experiments. The publication of first experiments with an "atomic force microscope" and the possibility of once obtaining atomic resolution by AFM1 lead to a surge in global research activities and today, an estimated 10000 AFMs are in use worldwide. However, it turned out that the road to an atom resolving force microscope was a long and winding one, a road that initially led away from STM. Measuring spring deflections through tunneling, the pivotal trick that promised great force resolution in AFM, proved to be impractical and it turned out that long-range interactions that are unimportant in STM are important in AFM. Almost a decade had passed before atomic resolution on silicon - STM's early feat that captured the imagination of surface scientists - could be repeated by AFM in 1994. In these AFM experiments, cantilevers made from silicon with spring constants of about 20 N/m were used in the frequency-modulation mode (FM-AFM).2 In FM-AFM the cantilever is driven at a constant amplitude and forces acting between tip and sample are reflected in frequency changes. Intuitively, one would think that small oscillation amplitudes would be optimal in FM-AFM, because that would maximize the effects of the tip-sample forces on the cantilevers motion. However, amplitudes of about 10 nm were required when using silicon cantilevers with a stiffness of 20 N/m. Large amplitude operation is strikingly different than STM - while STM only records short-range tunneling currents, large-amplitude AFM is strongly susceptible to long-range forces. To minimize the magnitude of long-range forces, sharp tips that were machined on the ends of silicon cantilevers had to be used. Simultaneous tunneling experiments were hard to do under these conditions because first, the average tunneling current is much smaller than the peak current for a tip that oscillates to and from a sample with nm amplitudes. Second, metals are favorable to silicon as a tip material for STM, especially at low temperatures.
The introduction of force sensors that are about two orders of magnitude stiffer than usual silicon cantilevers has allowed stable small-amplitude operation and brought the AFM closer to the STM again in several aspects.3 The qPlus sensor, an implementation of a self-sensing cantilever based on a quartz tuning fork, that is particularly simple to implement in STMs is now used in several laboratories4,5,6 at room temperature and low temperatures. In particular at low temperatures, high-precision force measurements are now possible. Perspectives about the merits of adding AFM capabilities to previously pure STM experiments will be discussed.
1. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986).
2. T. Albrecht, P. Grutter, H. K. Horne, and D. Rugar, J. Appl. Phys. 69, 668 (1991).
3. Franz J. Giessibl, "AFM's path to atomic resolution", Materials Today 8, 32 (2005).
4. M. Maier, Pico Nr. I 2006 , page 4 (Omicron newsletter, http://www.omicron.de).
5. M. Ternes, C. P. Lutz, A. Heinrich et al., unpublished.
6. M. Heyde et al., Appl. Phys. Lett. 89, 263107 (2006).
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