Nobuyuki Isshiki

First-principles theory of scanning tunneling microscopy

Abstract Scanning tunneling microscopy/spectroscopy (STM/STS), which has been so epoch-making in surface science experiments introduced many challenging problems also to the theory of condensed matter physics. Recent progress in theories of STM/STS contributed to revealing the relation between the atomic structure of the tip and the STM/STS data, and to clarify various strange phenomena observed. The present article reviews various important issues of the fundamentals of STM/STS from theoretical view points. After surveying the so far presented theoretical approaches, the first-principles simulation method based on the microscopic electronic state of both the sample surface and the tip is introduced. Several examples of the simulation such as graphite and Si surfaces, are described. Some novel phenomena of the microscopic tunnel system of STM such as the negative differential resistance in STS and single electron tunneling through fine supported particles are also discussed, as well as the many-body effect or electron-phonon coupling effect on STM/STS.

Effect of electronic states of the tip on the STM image of graphite

Abstract A first-principles calculation of the STM images of graphite is performed for three different shapes of a tungsten tip. In the calculation the tip is approximated by a small cluster. One of the tips, with more than one atom at the top, produces an abnormal image, while the others, with only one atom at the top, produce normal trigonal images. The abnormal image is caused by current interference through the multi peaks of the squared amplitude of the tunnel-active orbital at the tip.

Effect of tip atomic and electronic structure on scanning tunneling microscopy/spectroscopy

Abstract The current image and the tunnel conductance in the scanning tunneling microscopy/spectroscopy (STM/STS) of graphite are simulated by the first-principles local density functional calculation of the electronic states. As models of the tip, W 10 [111], W 14 [110], W 13 [110] and Pt 10 [111] clusters are used. For the tips with a single apex atom, normal STM images are obtained. However, abnormal images are generated by the removal of the apex atom. STS spectra are changed in a systematic way by the apex angle of the tip. The negative differential resistance observed in the STS of the Si(111) 3 × 3 -B/defect surface is reproduced by the theoretical simulation with the cluster models of the tip.

First‐principle simulation of scanning tunneling microscopy/spectroscopy with cluster models of W, Pt, TiC, and impurity adsorbed tips

Tunneling current and conductance in scanning tunneling microscopy/spectroscopy (STM/STS) are calculated for various kind of tips using first‐principle local density approximation (LDA) method with the tunneling‐Hamiltonian formalism. As models of tip, tungsten cluster, platinum cluster, carbon adsorbed tungsten cluster, and titanium carbide cluster are used. Graphite surface is used for the simulation. STM image and qualitative feature of STS spectrum mainly depend on the arrangement of atoms at the tip apex and not so much affected by the atomic compositions of the tip because the most of tunneling current comes from a single or a few atom at the tip apex.

Theory of scanning tunneling microscopy/spectroscopy for adsorbed surfaces and layer crystal surfaces

A method for theoretical simulation of the sccanning tunneling microscopy (STM) image and scanning tunneling spectroscopy (STS) spectra based on the LDA (local density functional approach) is applied for some chemisorption surfaces. For the Si(111)√3×√3-Ag surface, it is demonstrated that the modified HCT model results in a honeycomb-like distribution of the bright spots in the STM image. Effects of the microscopic geometry of the tip on the STM image are discussed for this surface. A strange transparent feature of the STM image for chemisorbed molecues or outermost layers is investigated both by analytical consideration and by theoretical simulation. The unique structure of the Ag(110)-O added row model is determined by optimization calculation and its experimental STM/STS data are reproduced by first-principles calculations.

Connected vacuum tail method and its application to scanning tunneling microscopy

Abstract A new improved method, the so-called connected vacuum tail (CVT) method, is proposed for the first-principles theoretical simulation of the STM/STS. The CVT method is especially powerful in reproducing the tail part of the surface electronic states extending far into vacuum. In this method, the natural asymptotic decay functions are used as the basis for the wave function in the far-vacuum tail. The expansion coefficients are obtained by solving the Schrodinger equation in the intermediate region by the Laue representation, and connecting these to the solutions of the wave function inside the surface obtained by the standard LDA method. The CVT method is utilized for the theoretical simulation of the STM/STS of Si(100) reconstructed surfaces. In this simulation, the dependence of the STM images on the tip-surface distance shows an interesting feature which corresponds well with experiment.

Scanning tunneling microscopy/scanning tunneling spectroscopy simulation of Si(111)√3×√3‐B surface

Mechanism of the negative differential resistance (NDR) observed in the scanning tunneling spectroscopy of the Si(111)√3×√3‐B surface is discussed based on the simulation with the first‐principles electronic states calculation. The NDR is reproduced by the W10[111] tip model, but not by the W14[110] and Pt10[111] models. The presence of the localized dangling bond state near EF for the surface in conjunction with a single tunnel active orbital at the tip apex causes the NDR.

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.

First principle simulation of STM images of benzene molecules adsorbed on a rhodium surface

Abstract STM images of Rh(111)-(3 × 3)-(C 6 H 6 +2CO) are simulated by the first principle local density approximation (LDA) method and the tunneling Hamiltonian formalism. Our simulation reproduces well the experimentally obtained STM images at both high and low bias voltage. The bias dependence of the STM images can be explained by the interaction between the π orbitals of benzene molecules and the rhodium atoms. Our simulation also shows that the π orbitals of benzene shift to higher binding energy while the σ orbitals remain unchanged, which is in good agreement with experiment.

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.