Model calculations of STM imaging and manipulation of oxygen on Pt(111)

We study tip-adsorbate–substrate interactions in scanning tunneling microscopy (STM) manipulation and imaging, and the influence of impurities on the images. Thence, we perform molecular dynamics simulations and calculate qualitative STM images for oxygen on Pt(1 1 1) surface. The adsorption site of the oxygen molecule is found to be in accordance with ab initio calculations. The calculated STM image has a good resemblance to the experimental ones. The contamination of the tip by oxygen or water alters the STM image strongly. Molecular dynamics simulations on manipulations of oxygen on the surface reveal several mechanisms of how molecular oxygen can be either produced or decomposed with STM tip. Finally, we find out that transfer of oxygen from the surface to an STM tip is not very probable.     

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.     

STM manipulation of a subphthalocyanine double-wheel molecule on Au(111)

A new class of double-wheel molecules is manipulated on a Au(111) surface by the tip of a scanning tunneling microscope (STM) at low temperature. The double-wheel molecule consists of two subphthalocyanine wheels connected by a central rotation carbon axis. Each of the subphthalocyanine wheels has a nitrogen tag to monitor its intramolecular rolling during an STM manipulation sequence. The position of the tag can be followed by STM, allowing us to distinguish between the different lateral movements of the molecule on the surface when manipulated by the STM tip.     

Phosphorus and hydrogen atoms on the (0 0 1) surface of silicon: A comparative scanning tunnelling microscopy study of surface species with a single dangling bond

We present a comparative scanning tunnelling microscopy (STM) study of two features on the Si(0 0 1) surface with a single dangling bond. One feature is the Si-P heterodimer—a single surface phosphorus atom substituted for one Si atom of a Si-Si dimer. The other feature is the Si-Si-H hemihydride—a single hydrogen atom adsorbed to one Si atom of a Si-Si dimer. Previous STM studies of both surface species have reported a nearly identical appearance in STM which has hampered an experimental distinction between them to date. Using voltage-dependent STM we are able to distinguish and identify both heterodimer and hemihydride on the Si(0 0 1) surface. This work is particularly relevant for the fabrication of atomic-scale Si:P devices by STM lithography on the hydrogen terminated Si(0 0 1):H surface, where it is important to monitor the distribution of single P dopants in the surface. Based on the experimental identification, we study the lateral P diffusion out of nanoscale reservoirs prepared by STM lithography.     

Scanning tunnelling microscopy imaging and modification of hydrogen-passivated Ge(100) surfaces

Scanning tunnelling microscopy (STM) study and modification of hydrogen (H)-passivated Ge(100) surfaces have been investigated. Thermal oxidation procedures were used to minimise surface roughness. Ge samples were passivated in HF solution after thermal oxidation. STM and atomic force microscope (AFM) imaging showed that, using HF etching after thermal oxidation, we can obtain a natural H-passivatedtopographically and chemically flat Ge(100) surface. The root-mean-square (rms) roughness ofa H-passivatedGe(100) surface measured both by STM and AFM is less than 2 ?. Electric properties of H-passivatedGe(100) surfaces were studied by scanning tunnelling spectroscopy (STS) in nitrogen ambient. STS showed that the H-passivated Ge surfaces were not pinned. Modification on H-passivated Ge(100) surfaces was carried out using STM by applying an electric voltage between the sample and tip in air. Modified features were characterised by STM and AFM imaging. On the H-passivated Ge(100) surfaces, stable, low-voltage, nanometer-scale modified features can be produced.     

Manipulation of organic “needles” using an STM operated under SEM control

Nano-sized P4P (para-quaterphenylene) organic crystallites grown on a gold substrate were removed and redeposited using the tip of an STM. The controlled lift-off of the needle-shaped crystallites was imaged by a simultaneously operated SEM. The forces exerted by the metal STM tip on the nano-crystal during this nano-manipulation process could be determined by monitoring the deflection of the STM tip. After the lift-off process the former contact area was imaged using the STM. These STM micrographs clearly demonstrate that the region between the needles is covered by a P4P layer with a thickness of at least 3 nm.     

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.     

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff-Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.     

Electrochemical behaviour of gold modified with contaminated TMP amine adlayers studied by STM, CV, EPR

Scanning tunnelling microscopy (STM), cyclic voltammetry (CV) and electron paramagnetic resonance spectroscopy (EPR) were used to investigate the influence of the TMP amine derivative on Au (1 1 1). The STM results show that the gold surface covered by the adlayer of the TMP derivative is easily modified (holes formation) after increasing the bias voltage to 0.5 V. The CV and EPR results show the electrochemical origin of observed STM topography changes. It is suggested that TMP could be oxidized to the nitroxyl TEMPO radical which adsorbs on Au in the form of an oxoammonium cation. Such an oxoammonium cation at the potential of 0.5 V forms a permanent complex of gold and the nitroxyl radical which could be easily desorbed during STM imaging.     

简单紧凑高刚性扫描隧道显微镜的研制

本文展示了一台自主研制的扫描隧道显微镜,它的主要特色是利用了一个内壁抛光的蓝宝石导向管作为扫描头的导轨,从而形成了较短的探针到样品的机械回路. 该导向管里面是一个被弹簧片夹持的方形滑杆,而压电扫描头安装在该滑杆上使得扫描隧道显微镜本身结构比较简单、紧凑、刚性. 同时蓝宝石良好的热导率又有利于提高显微镜的热稳定性. 为了证明该扫描隧道显微镜的性能,本文给出了高质量的石墨原子分辨率图像.     

Inelastic electron tunneling spectroscopy of an alkanethiol self-assembled monolayer using scanning tunneling microscopy

We report inelastic electron tunneling spectroscopy (IETS) of a C8 alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.     

Atomic resolution noncontact atomic force and scanning tunneling microscopy of TiO2(110)-(1 x 1) and - (1 x 2): simultaneous imaging of surface structures and electronic states

We present simultaneous imaging of TiO2(110)-(1 x 1) and - (1 x 2) using noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). The surface topography was imaged under NC-AFM feedback, while the surface electronic states were imaged by STM. The image contrasts of NC-AFM and STM were antiphase in (1 x 1) and in phase in (1 x 2). The uppermost oxygen and Ti atoms underneath were, respectively, imaged by NC-AFM and STM. The NC-AFM image contrast was close to the true surface topography in (1 x 2), but reduced in (1 x 1).     

Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

We have used scanning tunnelling microscopes (STMs) operating at liquid helium and liquid nitrogen temperatures to image the charge-density waves (CDWs) in transition metal chalcogenides. The layer structure dichalcogenides TaSe₂, TaS₂, NbSe₂, VSe₂, TiSe₂ and TiS₂ have been studied including representative polytype phases such as 1T, 2H and 4Hb. Experimental results are presented for the complete range of CDW amplitudes and structures observed in these materials. In most cases both the CDW and the surface atomic structure have been simultaneously imaged. Results on the trichalcogenide NbSe₃ are also included.

The formation of the CDW along with the associated periodic lattice distortion gaps the Fermi surface (FS) and modifies the local density-of-states (LDOS) detected by the tunnelling process. The tunnelling microscopes have been operated mostly in the constant current mode which maps the LDOS at the position of the tunnelling tip. The relative amplitudes and profiles of the CDW superlattice and the atomic lattice have been measured and confirm on an atomic scale the CDW structures predicted by X-ray, electron and neutron diffraction. The absolute STM deflections are larger than expected for the CDW induced modifications of the LDOS above the surface and possible enhancement mechanisms are reviewed.

In the 2H trigonal prismatic coordination phases the CDWs involve a relatively small charge transfer and the atomic structure dominates the STM images. In the 1T octahedral coordination phases the charge transfer is large and the CDW structure dominates the STM image with an anomalously large enhancement of the STM profile. Systematic comparison of the STM profiles with band structure and FS information is included.

In the case of the 4Hb mixed coordination phases at the lowest temperatures two nearly independent CDWs form in alternate sandwiches. STM studies on 4Hb crystals with both octahedral and trigonal prismatic surface sandwiches have been carried out. The STM scans detect the relative strengths of the two CDWs as well as the interactions between the two types of CDW structure.

The STM scans are also able to detect defects and domain structure in the CDW image. Several examples will be given demonstrating the potential of the STM to detect these local variations in LDOS on an atomic scale. In contrast to the layer structure crystals the linear chain compound NbSe₃ shows a complex surface atomic structure as well as the formation of two CDWs. The surface atomic structure is resolved in the STM scans and profiles have detected the presence of the CDW modulation at 77K and 4.2K. These results demonstrate the feasibility of detecting CDW structure in the presence of complex atomic structure and using materials where dynamical CDW effects can also be studied by STM.

The range of STM results presented here show that the STM scans are extremely sensitive to the detail of the CDW structure and its effect on the LDOS. Although much of this structure has been deduced from diffraction studies, the ability to examine the CDW structure on an atomic scale with the STM is new. The sensitivity of the STM method suggests potential applications to a wide range of electronic structures in materials.     

Structure of graphene on the Ni(110) surface

The structure of graphene on Ni(110) was studied using scanning tunneling microscopy (STM) and low-energy electron diffraction spectroscopy. STM images show a Moire structure, depending on the orientation of the domains making up the graphene layer. A simple model has been proposed which permits prediction of the Moire structure and interpretation of STM images based on calculation of the distances between the nearest neighbor carbon and nickel atoms. Our theoretical calculation suggests that the final orientation of graphene domains forming in the course of synthesis is defined by the angle of rotation of small clusters in the initial stages of growth.     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Study of scanning tunneling microscopy images and probable relaxations of the SrTiO3(100) surface by electronic structure calculations

Partial electron density plots were calculated for a model SrTiO₃(100) surface with √5 × √5 ordered oxygen vacancy to examine why the bright spots of the scanning tunneling microscopy (STM) images of SrTiO₃(100) observed in ultrahigh vacuum (UHV) correspond to the oxygen vacancy sites. Possible dependence of the image on the polarity and magnitude of the bias voltage was also discussed on the basis of partial electron density plot calculations. Our study strongly suggests that the UHV STM imaging involves the lowest-lying d-block level of every two Ti³⁺ centers adjacent to an oxygen vacancy, the tip-sample distance involved in the UHV STM experiments is substantially larger than that involved in typical ambient-condition STM imaging, and the Ti⁴⁺ and Ti³⁺ sites of SrTiO₃(100) are reconstructed.     

STM and chemistry: a qualitative molecular orbital understanding of the image of CO on a Pt surface

Theoretical calculations of STM images for isolated carbon monoxide on Pt(111) have been performed. These simulations are directly comparable to experimental data from Eigler's group. They also confirm a strong site dependence of the STM molecular contrast and assign the two experimental STM forms of CO with two distinct binding sites (the top and the bridge sites) which have already been well-characterised by other surface techniques. Moreover, the approach used here allows a detailed understanding of the electronic origin of the STM molecular pattern. Hence, we demonstrate that the CO STM pattern results from the superposition, with interference effects, of two current contributions. The direct through space tunneling process between the tip and the metal surface is decreased by the presence of the adsorbate molecule, but this depression in the current is more or less compensated, depending on the binding site, by the tunnel current mediated by the molecular levels of the adsorbate. The frontier π orbitals of CO have a small contribution to the current and the σ framework has the major influence with the 5σ lone pair (HOMO of CO) and also the low-lying 3σ. These two MO contributions, however, show a destructive interference effect, and this, together with the absence of π current, is responsible for the weak amplitude of the CO STM image.     

Hydrogen-induced instability of the Ge(105) surface

The structure and stability of the hydrogen-terminated (105) surface of Ge deposited on Si(105) substrates are investigated by scanning tunneling microscopy (STM). Investigations combining STM, electron energy loss spectroscopy, and theory reveal that Si incorporation into the surface Ge layer of hydrogen-terminated Ge/Si(105) drastically destabilizes the surface. The STM images obtained on this surface are well explained by the recently established rebonded-step structure model.     

STM images apparently corresponding to a stable structure: considerable fluctuation of a phase boundary of the Si(111)-(square root of (3) x square root of (3))-Ag surface

We study scanning tunneling microscopy (STM) images near a phase boundary of the Si(111)- (square root of (3) x square root of (3))-Ag surface by using Monte Carlo simulations based on results of first-principles calculations. The boundary is found to fluctuate from snapshot to snapshot, and the feature of the simulated STM images differs distinctly from the observed one with a straightly extending honeycomb pattern of bright spots. Remarkably, statistical averages of the simulated images reproduce the observed feature. This study gives a warning of our tendency to relate STM images revealing clear arrangement of bright spots with some stable structure.     

Migration-enhanced epitaxy (MEE) growth of five-layer asymmetric coupled quantum well (FACQW) and its cross-sectional STM observation

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well that is promising for ultra-fast and ultra-low-voltage optical modulators and switches. FACQW samples were grown by the migration-enhanced epitaxy (MEE) and the conventional molecular beam epitaxy methods with steep and flat heterointerfaces in the monolayer accuracy. They were characterized with the cross-sectional scanning tunneling microscopy (STM). In the cross-sectional STM image, double-stripe structures with different contrast were observed. The stripe area corresponds to the FACQW (about 10 nm wide), sandwiched with the AlGaAs barrier layers (15 nm wide). A dark line observed at the middle of the FACQW stripe area corresponds to the 3-monolayer-thick AlAs layer. The cross-sectional STM images of the high-quality heterointerface FACQW structures were successfully observed for the samples grown by the MEE method. More detailed studies of this kind of cross-sectional STM observations will be very effective to obtain the optimized growth conditions for fine and complicated ultra-thin structures.