10:30-12:00
Force Microscopy with Subatomic Resolution
Prof. Franz J. Giessibl (University of Regensburg, Institute of Experimental and Applied Physics)
-
A theoretical analysis of the factors that determine the spatial resolution of the force microscope led to the conclusion that optimal results are expected to occur when the oscillation amplitude of the cantilever in dynamic atomic force microscopy (AFM) has a magnitude similar to the range of the forces at play. When the cantilever is to oscillate with small amplitudes in a stable fashion under the influence of the field of the chemical bonding forces present between tip and sample, cantilevers with a stiffness of roughly 1kN/m in stead of the commonly used 40 N/m are required.1 Soon after these findings, experimental force microscopy data was published that shows "subatomic" resolution, that is the imaging of features within single atoms.2 While these findings were debated, a direct comparison of scanning tunneling microscopy and AFM data showed that when probing a tungsten tip with a graphite surface (a "light-atom probe"), subatomic orbital structures with a spatial resolution of less than one Angstrom can be obtained in the force map, while a map of the tunneling current only shows the known atomic resolution.3 Optimized subatomic contrast is obtained when recording the higher harmonics of the cantilever motion.3 The idea of the light atom probe was carried further in a collaboration with the IBM Low Temperature STM group in Almaden, San Jose where an adsorbed CO molecule was used to probe the tip atom. It requires quite a large force to move a CO molecule laterally4 and this molecule is an excellent probe for the orbital structure of the front atom of the metal tip. Ideally, one wishes to create a probe with a perfectly perpendicularly oriented CO molecule at the very end of the tip. First steps towards that goal will be discussed. Large amplitude dynamic AFM has recently provided stunning breakthroughs including atomic manipulation at room temperature5 and chemical identification of individual atoms.6 These recent achievements show that the noise level that can be accomplished by large amplitude AFM is at least similar to the noise in small amplitude AFM. However, subatomic resolution has apparently not yet been demonstrated by large-amplitude AFM. Potential answers of that open question are discussed.
1 F.J. Giessibl, H. Bielefeldt, S. Hembacher and J. Mannhart, Appl. Surf. Sci. 140, 352 (1999); F. J. Giessibl, Rev. Mod. Phys. 75, 949 (2003).
2 F.J. Giessibl, H. Bielefeldt, S. Hembacher, J. Mannhart, Science 289, 422 (2000).
3 S. Hembacher, F. J. Giessibl, J. Mannhart, Science 305, 380 (2004).
4 M. Ternes, C. P. Lutz, C.F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich, Science 319, 1066 (2008).
5 Yoshiaki Sugimoto, Masayuki Abe, Shinji Hirayama, Noriaki Oyabu, Oscar Custance and Seizo Morita, Nature Materials 4, 156 (2005).
6 Yoshiaki Sugimoto, Pablo Pou, Masayuki Abe, Pavel Jelinek, Ruben Perez, Seizo Morita and Oscar Custance, Nature 446 , 64 (2007).
