Masaru Tsukada

First Principles Study of the Effect of Tip Shape on Scanning Tunneling Microscopy Images

Effects of the shape of probe tip on observed scanning tunneling microscopy images have been investigated by first-principles calculations by taking the Si(111)√3×√3-Ag surface as an example of sample. By using three tungsten clusters with different shapes and by tilting their symmetry axes, various shapes of the tip were represented. Even for those tips which produce images distinctly different from experimental ones when their symmetry axes are normal to the surface, the images change so as to agree with experiments as the axes are tilted gradually. The results indicate that only one atom at the tip apex usually makes an essential contribution to the tunneling current and can explain why an experimental tip prepared quite arbitrarily usually produces good images.

Theory of scanning tunneling microscopy of oxygen adsorption on Ag(110) surface

Abstract Oxygen adsorption on Ag(110) surface has been investigated by means of first-principles total-energy and force calculations for repeated-slab geometries within the local density approximation (LDA). The added row model composed of oxygen atom and silver atom has been believed to be one of the most realistic models. We have already obtained the optimized geometry of the added row model. Using the optimized positions, the simulation of STM images and STS spectra has been performed based on the Bardeens perturbation theory. Compared with the experimental data, the theoretical results seem to show a fairly good correspondence.

Step-induced state of a Si(001) reconstructed surface

Abstract We calculate the electronic structure of a Si(001) stepped surface, applying the DV-Xα-LCAO method in the density functional approach. For the surface, a slab model which consists of 4 Si-layers, including two different types of single-layer steps is applied. The dimers of the outermost Si atoms are assumed to take an asymmetric geometry. A localized state is found at the dimer on the lower terrace just below the S A step. We also simulate scanning tunneling microscopy (STM) images and scanning tunneling spectroscopy (STS) spectra in the first-principles approach. In the STS spectra, the localized state at the S A step gives a sharp peak, such as observed in the experiment by Iwawaki et al. [J. Vac. Sci. Technol. B 9 (1991) 711]. However, the up- and down-atoms of the dimer at the step edge of the upper terrace do not show any significant difference in the spectra. In the STM images, the dimer seems buckled no more than the geometry. Furthermore, to clarify the origin of the localized state, we analyze a phenomenological tight-binding model for the π-band of dimer rows. One of the π-states localizes to the dimer site just below the S A step, due to a potential induced at the step edge.

Mechanism of transparent STM images of chemisorbed molecules and outermost layers

Abstract There have been many examples of STM image reported, where depending on the bias value the adsorbate molecules become transparent or a moire pattern is formed between the adlayer and the substrate surface. Analytical theory for the mechanism of the transparency is presented based on the transfer matrix formalism of the surface wavefunction. Examples of the graphite/benzene adsorption systems and TiC(111)/graphite monolayer are investigated by the first-principles LDA calculations.

Theory of scanning tunneling microscopy of oxygen‐adsorbed Ag(110) and Cu(110) surfaces

Though recent observation of scanning tunneling microscopy (STM) images of Ag(110) (n×1)–O(n=1,...,7) surface revealed the unique added row structure, detailed atomic arrangement for this model has been an open question. In previous work, the optimized geometries for the (2×1) and (3×1) oxygen‐adsorbed surfaces by the first‐principles local density functional approximation (LDA) have been determined. The STM images for various bias voltages and scanning tunneling spectroscopy (STS) spectra are calculated for this optimized structure based on the theoretical simulation method by LDA developed in this group. The results reproduced well the clear ridge structure along the added row. Though the oxygen atom is closer to the tip, Ag atoms are more highlighted for the surface positive bias. On the other hand, the local density of states map shows the maximum amplitude at the O site. This strange feature can be explained by the effect of the tip electronic states. The similar calculation has also been performed f...

Novel features of surface electronic structure revealed by the theoretical simulation of scanning tunneling microscopy/spectroscopy

Theoretical simulation based on the first-principles local density functional approximation (LDA) calculation reveals the mechanism of STM as well as significant effects of microscopic states of the tip on experimental data. Moreover, various exotic properties of clean or chemisorbed surfaces can be explained with the theory of STM/STS. How the tip influences images of STM is discussed by case studies with a numerical simulation for Si(100) dimer rows and step structures and Si(111)3 × 3Ag and Ag(110)-O chemisorption surfaces.

Microscopic theory of scanning tunneling microscope for finite electric field and current

A new method of obtaining a self‐consistent surface electronic structure for a finite electric field and current within the local density functional formalism is presented. In the method, two different chemical potentials are assigned to the left and right electrodes, and the scattering waves are calculated by the step‐by‐step recursion matrix method. This method is applied to the Na bimetallic planar junction. Effective potential, electric current, and charge density distribution are presented for given values of the separation and the bias voltage.

Theory of electron tunneling in scanning tunneling microscopy and field ion microscopy

Abstract Electron tunneling in microscopic systems such as STM and FIM is discussed, based on first-principles electronic state theory. The mechanisms achieving the atomic resolution of the images are discussed both for STM and FIM and compared to each other. The influence of the wave-like and the particle-like behavior in the tunneling processes are studied mainly for the case of STM.

Theory of tip-surface gap dependence in laser-assisted scanning tunneling microscopy

Nonlinear characteristics of laser-assisted tunneling in the context of scanning tunneling microscopy (STM) are investigated by combining Bardeens formalism and the WKB approximation. Atomic-scale images of the second-order nonlinear response calculated for graphite are dependent on the tip-sample distance. At small distance, this response is proportional to the square of the derivative of the surface local density of states (LDOS), giving trigonal images peaking atop A sites. However, at large distance, the images are proportional to the square of the LDOS, giving trigonal images of B sites. The present formalism can also predict atomic-scale STM images induced by laser irradiating other surfaces such as silicon.

Atomic and electronic structure of the Si(001) surface induced by hydrogen-adsorption

Abstract The initial stage of hydrogen (H) adsorption on the Si(001) surface is theoretically investigated to clarify the atomic and electronic structure induced by the adsorption. For this purpose, the electronic states are calculated in the density functional approach with the DV(discretized variation)-Xα-LCAO method. We also simulate the scanning tunneling microscopy (STM) image and the scanning tunneling spectroscopy (STS) spectrum in the first-principles approach. Our results of the STM image do not support the asymmetric dimer structure of the substrate with a H atom bonded to the upper Si atom. They conclude that the bright ball-like spot in the observed images comes from the free dangling bond induced on the remaining Si atom of the reacted dimer. However, the single particle picture cannot reproduce the observed features of the STS spectrum at the spot. We discuss that all the features can be well explained by the Coulomb blockade effect due to the electron correlation in the dangling bond state.