The invention of scanning tunneling microscopy [1] and atomic force microscopy [2] allowed surface imaging with atomic resolution. Recently [3], it was realized that a lateral resolution limit of the scanning probe microscopy can be reduced even further, down to sub-atomic scale. Actually, the effects of the tip electronic structure were observed in AFM experiments on Si(111) and graphite [3,4]. The latest results [5] demonstrated both experimentally and theoretically that similar fine effects may play a significant role in dynamic STM experiments on Si(111) with a silicon probe tip where the observed sub-atomic features could be explained by scattering of electrons into double dangling bonds of the silicon tip atoms.
In this report we present new high resolution experimental data obtained with UHV room temperature STM GPI-300 operated in constant current mode. The Si(111)-(7×7) images measured at different scanning parameters (bias voltages and tunneling currents) demonstrate that not only adatoms and rest atoms can be clearly revealed by STM. In a series of images taken at smaller tip-surface distances fingerprints of the tip apex atom electronic structure could be clearly seen. The appearance of sub-atomic features in the experiment was rather dependent on the sample bias voltage polarity as well as applied voltages and tunneling currents. The observed shapes of the sub-atomic features are rather similar to one observed in AFM experiments [3,4]. The tip-sample distance dependence of the measured images and reproducibility of the image modifications with changing gap resistance allow us to suggest imaging of atomic orbitals in the STM experiments on the Si(111)-(7×7) surface.
This work was supported by the RAS programs ("Effect of Atomic, Crystal, and Electronic Structure on the Properties of Condensed Matters" and "New Materials").
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