Sep 20, 2007
Measuring true nano-distance
Feynman’s suggestion of single-atom manipulation could become a reality after the introduction of scanned-probe microscopes. The arrangement of xenon atoms on a nickel surface by a scanning tunneling microscope to compose a notorious logo is recognized as the beginning of the nano-age.
Recently, the atomic force microscope (AFM) has been used to form single atomic bonds in a controlled fashion, to recognize and manipulate single atoms at room temperature, and to visualize atomic orbitals achieving sub-atomic resolution. In essence, a sharp tip is combined with an extremely sensitive force detector – commonly an elastic cantilever – and used to sense and control its distance from the surface. Cantilevers with high stiffness – to overcome thermal fluctuations – are employed in the dynamic mode, in which a tiny oscillation of the probe is perturbed by the approaching surface. Compact, self-sensing piezoelectric resonators are better suited than cantilevers in ultra-high vacuum or cryogenic environments. Among those are quartz tuning forks – like the one in quartz watches.
When attached to one tuning fork prong, AFM tips may present much lower stiffness than the tuning fork itself, so that actual tip vibration may differ substantially from what the tuning fork measures. This point is often disregarded and may affect interpretation of distance-dependence approach curves and accurate distance control. In this work, tuning forks with attached tips were modelled as coupled oscillators, quantitatively reproducing previously published approach curves. It is observed that true inter-atomic distance changes only slightly near to physical contact, even in spite of a relatively large (apparent) approach of the AFM probe. Improved knowledge of interaction distance-dependence may impact many AFM-based nanotechnological applications.
About the author
Massimiliano Labardi is a researcher at CNR, polyLab in Pisa, Italy. His research involves surface physics investigated by nanometer-scale resolution microscopy and spectroscopy, with special focus on local surface dissipation phenomena.