Scanning Tunneling Microscopy Modeling Research Articles

Modeling the Kondo effect of a magnetic atom adsorbed on graphene

The Kondo effect due to a magnetic atom adsorbed on graphene is investigated using two theoretical approaches, including the non-equilibrium Green’s function method within the slave-boson mean-field approximation and a newly developed tight-binding (TB) model with an effectively infinite Hubbard U term. Both methods reveal the presence of a Kondo peak in the local density of state (LDOS) of graphene near the Fermi level. A sharp Kondo peak is only observed in the odd-neighboring C sites of the C atom directly below the magnetic atom placed in top position. The peak intensity is found to decay quickly with respect to the distance between the C site and the magnetic atom. A theoretical model of scanning tunneling microscopy (STM) of graphene is presented as a means to identify the manifestation of the Kondo effect via direct topographic STM measurements. STM simulations show that in the proximity of the magnetic atom, only the C atoms of one sublattice are visible at low bias potential and thus a triangular lattice can be seen, in agreement with recent experiments. Further away from the magnetic atom, the Kondo effect dies down and eventually both sublattices are visible, leading to the recovery of the hexagonal lattice. This work not only provides a new TB method to study the Kondo effect in graphene, but also presents both scanning tunneling spectroscopy and microscopy fingerprints of the Kondo effect to facilitate its experimental verification.

Cold Emission Model of Scanning Tunneling Microscopy

Cold Emission Model of Scanning Tunneling Microscopy

Modeling realistic tip structures: Scanning tunneling microscopy of NO adsorption on Rh(111)

We have performed a joint experimental and theoretical scanning tunneling microscopy (STM) study of NO adsorbed on Rh(111). The experimental STM images showed a strong sensitivity to the tip conditions that could be altered by dipping the tip into the sample or by application of voltage pulses. Only via STM simulations performed over an exhaustive range of tip-apex terminations, including several contaminants in different adsorption geometries and with different spatial orientations, we have been able to reproduce the rich variety of measured images. From the analysis, we are able to infer a realistic structural model for the ultimate tip-apex structure involving apex geometries considerably more complex than those typically employed in STM modeling.

Dynamical effects and conductance asymmetry in metal-insulator-metal systems with different electrodes

We investigated the electron tunneling process in a planar system consisting of two semi-infinite metal electrodes of different dielectric properties separated by a narrow vacuum gap. This system can be viewed as a model of a metal-insulator-metal structure, or even as a very rough model of scanning tunneling microscopy. We used a local limit of a self-consistent dynamic calculation to evaluate effective tunneling barriers, and discussed the influence of different screening properties of the metallic electrodes on these barriers as well as on the conductance. We found that the barriers are significantly additionally lowered due to the difference in the charge fluctuation frequencies of the electrodes, and that the conductance minimum in such systems is shifted to a nonzero value. We compared our results with some measurements of I-V characteristics and work functions and found that our calculations contribute to the understanding of the observed conductance asymmetry.

Ab initio molecular orbital model of scanning tunneling microscopy. Benzene and benzene adsorbed on a Ag surface

The molecular orbital (MO) model of scanning tunneling microscopy (STM) developed previously is applied to the Pd 2 tip C 6H 6 sample system to examine the effect of tilting the tip, and to benzene adsorbed on a silver surface to investigate the effect of the surface-adsorbate interaction on the STM image. The effect of tilting the tip is shown to depend on the nature of the tip MOs responsible for the STM current. The possibility of elucidating the site and geometry of the adsorbate by a combined use of experimental and theoretical STM images is discussed.

Ab initio molecular orbital model of scanning tunneling microscopy

An ab initio model of scanning tunneling microscopy (STM) is presented in the framework of the perturbation theory of Bardeen. The tip and sample are represented by the cluster model, and the first-order wave functions are calculated by ab initio Hartree–Fock (HF), configuration interaction (CI), and symmetry adapted cluster (SAC)/SAC-CI methods. This model was applied to simple tip–sample systems such as Pd2–Ag2, Pd2–C6H6, Pd2–(C6H6)2, and Li2–Li2. The calculated STM images were related to the HOMO, LUMO, etc. orbital symmetries of the tip–sample systems. The contribution of the Rydberg orbital was examined at different tip–sample distances. Electron correlations were found to modify the behavior of the tunneling transition probability of the Li2–Li2 system vs that calculated by the HF method.

A theoretical model of scanning tunneling microscopy: Application to the graphite (0001) and Au(111) surfaces

An expression for the scanning tunneling microscopy (STM) current between the tip and sample is presented using first-order perturbation theory for a two-Hamiltonian formalism (‘‘reactants’’ and ‘‘products’’). The calculated STM current depends on the square of the sample-tip matrix elements, averaged over a selection of random points in wave vector space. In the limit of low voltage and temperature, this averaging is over the Fermi surface of the sample. The model is applied to the graphite (0001) and Au(111) surfaces using a simple model (chain) of a tungsten tip and the tight-binding approximation. Comparisons with experiments and with the result for graphite obtained by Tersoff and Lang using a molybdenum tip are given. The theory is applied elsewhere to STM of adsorbates.