Browsing by Subject "Microscopic Examination"

Now showing 1 - 3 of 3

Open Access
Adhesive energy, force and barrier height between simple metal surfaces
(1992) Çıracı, Salim; Tekman, E.; Gökçedag, M.; Batra, I. P.; Baratoff, A.
Using the self-consistent field pseudopotential method we calculated the adhesive energy, perpendicular and lateral forces and barrier height between two rigid A1(001) slabs modeling the sample and a blunt tip. We found that the adhesive energy and forces are site specific, and can lead to a significant corrugation in the constant force mode with negative force gradient. Lateral forces, which determine friction on the atomic scale are not simply proportional to the perpendicular force, and are typically one order of magnitude smaller. Our results confirm that perpendicular tip force and barrier height are interrelated for separations where the force gradient is positive. © 1992.
Open Access
Adsorption site of alkali metal overlayers on Si(001) 2 × 1
(1992) Batra, I. P.; Çıracı, Salim
The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.
Open Access
Atomic-scale tip-sample interactions and contact phenomena
(1992) Çıracı, Salim
Tip-sample interactions become crucial owing to increased overlap at small tip-sample separation. The potential barrier collapses before the point of maximum attraction on the apex of the tip, but the effective barrier may remain significant owing to the strong confinement of current-carrying states to the constriction between tip and sample. At such separations the perpendicular tip force is still attractive and determined by ion-ion repulsion and redistribution of electronic charge. Electronic states are modified by the tip-induced perturbation of the potential in the vicinity of the tip. Self-consistent calculations reveal that local properties, such as elastic deformation, effective height and width of the tunneling barrier, electronic states and attractive tip force are site-dependent and reversible on the atomic scale. Numerical results suggest a relation between the perpendicular tip force and barrier height as a function of separation. A mechanical contact is formed with relatively strong bonds at separation near the point of zero force gradient. Whether the effective potential can collapse and hence the first channel can open to allow a transition from tunneling to ballistic conduction, and whether the conductance can show quantized steplike changes with increasing plastic deformation depends on material properties. © 1992.