ROLE OF A COVALENT BONDING INTERACTION IN NONCONTACT-MODE ATOMIC-FORCE MICROSCOPY ON SI(111)7X7

We experimentally investigated a force interaction between a Si tip an d the Si(111)7x7 reconstructed surface using a noncontact-mode atomic- force microscope (AFM) in ultrahigh vacuum. Two types of force gradien t curves with and without discontinuity were found. Furthermore, the c orrelation between the force gradient curves and the noncontact AFM im age contrasts has been clarified. The image contrast was very weak in the case of the force gradient curve without discontinuity, while it w as 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 mo del 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 ref lects the variation in a chemical reactivity on Si adatoms.