Role of a covalent bonding interaction in noncontact-mode atomic-force microscopy on Si(111)7×7

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Abstract
We experimentally investigated a force interaction between a Si tip and the Si(111)7×7 reconstructed surface using a noncontact-mode atomic-force microscope (AFM) in ultrahigh vacuum. Two types of force gradient curves with and without discontinuity were found. Furthermore, the correlation between the force gradient curves and the noncontact AFM image contrasts has been clarified. The image contrast was very weak in the case of the force gradient curve without discontinuity, while it was clearly enhanced and the contrast between inequivalent adatoms was obtained in the case of the force gradient curve with discontinuity. The discontinuity of the force gradient curve can be explained by a model which considers a crossover between physical bonding and chemical bonding interactions between tip and sample. This model suggests that the weak contrast image reflects the variation in van der Waals and/or electrostatic force interactions, while the strong contrast image reflects the variation in a chemical reactivity on Si adatoms.