Noncontact Atomic Force Microscopy - Yale School of Engineering ...

AFM probe tips with a small front atom
Th-0920
T. Hofmann, 1 J. Welker, 1 M. Ternes, 2 C. P. Lutz, 2 A. J. Heinrich, 2 Franz J. Giessibl 1
1 University of Regensburg, Institute of Experimental and Applied Physics, D-93040 Regensburg, Germany
2 IBM Research Division, Almaden Research Center, 650 Harry Rd, San Jose, CA 95120, USA
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. 1
Optimized subatomic contrast is obtained when recording the higher harmonics of the
cantilever motion. 1 The idea of the light atom probe was carried further by using an adsorbed
CO molecule to probe the tip atom. It requires quite a large force to move a CO molecule
laterally 2 and this molecule is an excellent probe for the orbital structure of the front atom of
the metal tip. In contrast to previous examples of “subatomic” resolution, where a flat sample
with a periodic array of sample atoms created multiple tip images, using a single adsorbed CO
molecule as a “tip” allows to isolate the force contribution of a single atom-pair contact.
Ideally, one wishes to create a probe with a perfectly perpendicularly oriented CO molecule at
the very end of the tip. First results will be discussed where a CO molecule has been picked
up by a metal tip and used to image adsorbed atoms.
In order to allow simultaneous STM and AFM, high electrical conductivity is another
desirable property of a light atom probe. Because of its small atomic radius, high mechanical
strength and fair electric conductivity Beryllium is a promising choice as a probe material.
However, due to its high reactivity, preparation is a challenge. Results using Be tips prepared
by high-voltage field emission for atomic imaging of Si(111)-(7×7) will be presented.
1. S. Hembacher, F. J. Giessibl, J. Mannhart, Science 305, 380 (2004).
2. M. Ternes, C. P. Lutz, C.F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich, Science 319, 1066
(2008).
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