Identifying Atoms by Feel

Hank Hogan

Atomic force microscopes (AFMs) and other scanning probe devices can map surfaces on an atomic scale but have not been able to distinguish one type of atom from another using purely mechanical means. Now researchers have used an AFM to identify atoms by feel, translating the short-range stickiness between atoms into a chemical fingerprint.

Using an AFM in noncontact mode, the investigators moved a minuscule yet sharp vibrating tip mounted on a cantilever across a silicon surface covered with an alloy of silicon, tin and lead. Via a reflected laser beam, they read out changes in the cantilever’s oscillations resulting from interactions between the tip and the surface. This form of AFM, called dynamic force microscopy (DFM), images atoms by detecting the short-range forces present.

The investigation was conducted by Óscar Custance of Osaka University in Japan, and by his colleagues there and at Universidad Autónoma de Madrid in Spain, by the PRESTO (Precursory Research for Embryonic Science and Technology) group of the Japan Science and Technology Agency in Saitama and by a group at the Academy of Sciences of the Czech Republic in Prague.

Extracting atomic identity from dynamic force microscopy readings is not a trivial task because the structure and chemical termination of the probing tip also affect the readings, and these parameters vary from one tip to the next.

Normalized data

The researchers solved this problem by measuring the force readings with very high accuracy, normalizing the data by dividing the readings over each atom by the strongest attractive force — that of silicon. The result was a reproducible atomic fingerprint, which matched theory. They found, for example, that the measured force ratio of tin to silicon was 0.77, close to the calculated ratio of 0.71. Using normalization, they successfully identified individual silicon, tin and lead atoms on the probed surface.

The ability to distinguish atoms could someday be combined with a previously demonstrated ability to place them where desired, benefiting surface chemistry and materials science research as well as nanotechnology and nanoscience.

“The capabilities of DFM to identify and manipulate individual atoms at semiconductor surfaces will allow, for instance, the selective doping of semiconductors, arranging dopants of different nature in particular arrays to enhance the performance of nanoscale transistors and, therefore, electronic devices,” Custance said.

Nature, March 1, 2007, pp. 64-67.