Study on True Atomic Resolution Imaging Using Noncontact Atomic Force Microscopy

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1) With an atomic force microscope (AFM) operating in the noncontact mode in an ultrahigh vacuum (UHV), the InP (110) 1x1 and GaAs (110) surfaces were observed. Rectangle lattice including the atomic-scale point defects have been clearly and reproducibly resolved. These results clearly show that the noncontact UHV AFM has true atomic-scale lateral resolution and is quite effective for atomic surface structure analysis in real space.2) The constant vibration mode and the constant excitation mode in noncontact atomic force microscopy were compared to investigate the force interaction between tip and surface. When the tip touch the surface, the repulsive force interaction is weakened in the constant excitation mode because of the decrease of the vibration amplitude of the cantilever, while it is maintained in the constant amplitude mode. This means that the constant excitation mode is much more gentle than the constant vibration mode.3) With an atomic force microscope (AFM) operating in the noncontact mode in an ultrahigh vacuum (UHV), the reactive Si (111) 7*7 surface was observed. The individual adatoms and the corner holes in the 7*7 reconstruction, described by the dimer-adatom-stacking fault (DAS) model and also missing adatoms, can be observed.4) We found the discontinuity of the force gradient curve on reactive Si (111) 7*7 reconstructed surface. A strong contrast has been obtained with discontinuity, while weak contrast has been obtained without discontinuity. Thus, the image contrast is drastically increased by the presence of the discontinuity of the force gradient curve. The discontinuity of the force gradient curve can be explained by the crossover between the physical and chemical bonding interaction.