Coherent electron transport through an azobenzene molecule: a light-driven molecular switch

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.     

Conformational changes of single molecules induced by scanning tunneling microscopy manipulation: a route to molecular switching

A detailed experimental and theoretical investigation of the processes involved in the manipulation of individual specially designed porphyrin-based molecules by scanning tunneling microscopy at low temperature is presented. On a stepped Cu(211) surface, the interaction between tip and molecule was used to locally modify in a reversible way the internal configuration of a single molecule, thus drastically changing the tunneling current passing through it. Model calculations confirm that this manipulation realizes the principle of a conformational molecular switch.     

Selective analysis of molecular states by functionalized scanning tunneling microscopy tips

Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the pi states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.     

光子扫描隧道显微镜的光电成像系统研究

本文论述了光子扫描隧道显微镜（ＰＳＴＭ）的显微成像机理、成像规律，针对具体的物理模型进行数值模拟计算，并得到了与实际探测相一致的场分布规律。采用自行研制的光子扫描隧道显微镜（ＰＳＴＭ）的显微实验系统对多种样品进行了表面显微成像研究，获得了关于样品表面三维立体图像信息，通过多种图像处理手段对原始图像进行后期处理，得到了更具视觉效果、更为逼真的样品表面图像。     

Theory of light rectification in scanning tunneling microscopy in the presence of an adsorbed molecule

We consider the problem of the rectified current induced by laser radiation in the STM junction when the tip is placed above a small molecule like CO or NO. This is calculated assuming a simple tight-binding model for the tunneling junction including the adsorbate and using nonequilibrium Green's functions techniques. The coupling between tunneling electrons and the molecule vibrational modes is taken into account by a local electron-phonon interaction term. In a second step we estimate the excitation rate of the molecule vibrations for a given laser power. This value is then used to obtain the relative change in the rectified current when the laser is in resonance with a molecule vibration. For a moderate laser power of 2 kW/cm² a relative change of 1 to 3% is predicted.     

First-principles theory of scanning tunneling microscopy

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.     

Energy-filtered scanning tunneling microscopy using a semiconductor tip

The use of cleaved, [111]-oriented monocrystalline InAs probe tips enables state-specific imaging in constant-current filled-state scanning tunneling microscopy. On Si(111)-(7 x 7), the adatom or rest-atom dangling-bond states can thus be mapped selectively at different tip-sample bias. This state-selective imaging is made possible by energy gaps in the projected bulk band structure of the semiconductor probe. The lack of extended bulk states in these gaps gives rise to efficient energy filtering of the tunneling current, to which only sample states not aligned with a gap contribute significantly.     

Atomic-scale structure of dislocations revealed by scanning tunneling microscopy and molecular dynamics

The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width at the surface, in agreement with the STM observations. Implications for surface-induced cross slip are discussed.     

Imaging the phenol molecule adsorbed on TiO2(110) by scanning tunneling microscopy

The image of a phenol molecule adsorbed on a TiO₂(110) single crystal was obtained, for the first time, by using the STM. TiO₂(110) has a tunneling active free region from + 1.35 eV below the Fermi level to −0.25 eV above the Fermi level. This region was found after measuring the surface density of states by using tunneling spectroscopy. The phenol molecule was fixed on TiO₂ utilizing the amphoteric nature of the TiO₂ surface. The image of the phenol molecule was measured at a dp bias of + 443 mV, which almost matches the energy of the OH and C-H stretching bond of the phenol molecule, in the free of energy levels region of the TiO₂(110) substrate. The electron density of phenol ring is apparent in the STM picture.     

Application of scanning tunneling microscopy to a superconducting surface

It is shown that the scanning tunneling microscope as developed by Binnig and Rohrer can be expected to resolve the spatial variations of the superconducting order parameter in inhomogeneous superconductors.     

The image potential in scanning transmission electron microscopy and scanning tunneling microscopy

Abstract

We sketch developments in the theory of the self-energy of charged particles moving near condensed matter surfaces. Some applications to experimental results from spectroscopy with electrons localized in microprobe beams and to electrons tunneling across a gap between two metals are considered.     

Theoretical aspects of scanning tunneling microscopy andspectroscopy

In spite of the huge potential of scanning tunneling microscopy (STM), a firm interpretation of experimental data is often difficult. Theoretical simulation of STM images andSTS spectra plays a very important role to derive detailed information from experiments. In the present article, a method of the first-principles simulation based on the local density functional approach is introduced, andapplied to some interesting surface systems. It is clarified how the atomistic structure of the tip influences the STM image. An example is presented in which a naive interpretation of the STM image fails. The exotic phenomenon of transparency of the adsorbed molecule is discussed.     

自旋极化扫描隧道显微镜的研究进展

自旋极化扫描隧道显微镜(spin-polarized scanning tunneling microscope,SP-STM)将扫描隧道显微镜(scanning tunneling microscope,STM)的实空间分辨率和对自旋敏感的磁成像技术结合起来,已经成为人们研究纳米磁性物理的最有效工具之一.文章介绍了SP-STM的工作原理及其在低维磁性物理领域的应用和最新进展,如对磁性薄膜、磁性纳米岛、磁性原子及不共线结构的研究等,并对我国在这一领域的研究现状和发展前景进行简要评述.     

自旋极化扫描隧道显微镜的研究进展

自旋极化扫描隧道显微镜（spin-polarized scanning tunneling microscope,SP-STM）将扫描隧道显微镜（scanning tunneling microscope,STM）的实空间分辨率和对自旋敏感的磁成像技术结合起来,已经成为人们研究纳米磁性物理的最有效工具之一.文章介绍了SP-STM的工作原理及其在低维磁性物理领域的应用和最新进展,如对磁性薄膜、磁性纳米岛、磁性原子及不共线结构的研究等,并对我国在这一领域的研究现状和发展前景进行简要评述.     

Contrast mechanisms in photothermal scanning tunneling microscopy

By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.