Browsing by Subject "Scanning Tunneling Microscopy"

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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 theory of the scanning tunneling microscope
(1988) Tekman, Ahmet Erkan
The Scanning Tunneling Microscope is proven to be one of the most powerful tools for surface structure determination. Present theories are able to explain the operation of the microscope when the tip is far from the surface. For the small tip height case the atomic-scale interaction of the tip and the surface has to be included in the theory. The electronic structure of the combined system of the tip and the surface is calculated with an Empirical Tight Binding approach for graphite. It is found that in the vicinity of the tip some Tip Induced Localized States are formed. These states play an important role in the tunneling phenomenon. The contribution of these states to the tunneling current is calculated.
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.
Open Access
Scanning probe microscopy for optoelectronic characterization at the nanoscale
(2010) Ürel, Mustafa
In this work, we propose methods for electrical characterization of nanostructured surfaces using electrostatic force and tunneling current measurements in scanning probe microscopy. Resolution smaller than 10 nm in electrostatic force microscopy (EFM) is attained and reasons for this attainment is explained in terms of the tip-sample capacitance and mechanical vibrations of tip design. Dynamic measurements are done in EFM using a lumped model for tip-sample electrostatic interaction instead of a simple tip-sample capacitance model. Surface photovoltage measurements are done and assured in EFM using frequency response techniques. Also, combining tunneling current measurements by EFM measurements, optoelectonic properties of graphene/graphene oxide samples are characterized.