Bias-dependent Scanning Tunneling Microscopy Images Research Articles

Absence of in-gap modes in charge density wave edge dislocations of the Weyl semimetal (TaSe4)2I

Axion electrodynamics in condensed matter could emerge from the formation of charge density waves (CDWs) in Weyl semimetals. (TaSe$_4$)$_2$I was proposed to be the first material platform for realizing axionic CDW that may host topological defects with one-dimensional in-gap modes. The real-space modulation of the CDW phase and the existence of in-gap modes remain elusive. Here, we present a comprehensive scanning tunneling microscopic and spectroscopic study of a CDW on a (TaSe$_4$)$_2$I (110) surface. The tunneling spectroscopic measurements reveal a CDW gap of $\sim 200$ meV, in good agreement with prior studies, while the spectroscopy of CDW edge dislocation shows no indication of in-gap states. The bias-dependent scanning tunneling microscopy images indicate that the CDW in(TaSe$_4$)$_2$I is dominated by a large periodic lattice distortion instead of charge modulation, suggesting a non-Peierls mechanism of the CDW instability.

Single-layer CrI3 grown by molecular beam epitaxy

Single- and few-layer chromium triiodide (CrI3), which has been intensively investigated as a promising platform for two-dimensional magnetism, is usually prepared by the mechanical exfoliation. Here, we report direct growth of single-layer CrI3 using molecular beam epitaxy in ultrahigh vacuum. Scanning tunneling microscopy (STM), together with density functional theory (DFT) calculation, revealed that the iodine trimers, each of which consists of three I atoms surrounding a three-fold Cr honeycomb center, are the basic units of the topmost I layer. Different superstructures of single-layer CrI3 with periodicity around 2–4 nm were obtained on Au(1 1 1), while only the 1 × 1 structure was observed on the graphite substrate. At an elevated temperature of 423 K, single-layer CrI3 began to decompose and transformed into single-layer chromium diiodide. Our bias-dependent STM images suggest that the unoccupied and occupied states are spatial-separately distributed, consistent with the results of our DFT calculation. We also discussed the role of charge distribution in the super-exchange interactions among Cr atoms in single-layer CrI3.

Scanning tunneling microscopy study of PTCDI on Sn/Si(111)-23×23.

Perylene tetracarboxylic diimide molecules were evaporated onto a Sn/Si(111)-23×23 surface and studied using scanning tunneling microscopy (STM) and low energy electron diffraction. At low coverages, single molecules are locked into specific adsorption geometries, which are investigated in detail using high resolution STM. The electronic structure of these individual molecules was studied using bias dependent STM images. The molecules form 1D rows that become more common with increasing coverages. Possible intermolecular O⋯H interactions within the rows have been identified. At around half of a monolayer (ML), the rows of molecules interact with each other and form a commensurate 43×23 reconstruction. In a complete monolayer, several structures emerge as molecules fill in the space between the 43×23 stripes. Possible intermolecular interactions within the 1 ML structures have been discussed. At coverages above 1 ML, the growth is characterized by island growth, where the molecules are arranged according to the canted structure within the layers.

Adsorption of PTCDA on Ge(001)

Adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on the Ge(001) surface was studied using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and the density-functional theory (DFT) calculations. Only single adsorption configuration of the PTCDA molecule was observed at low coverages on the Ge(001) at room temperature, unlike on the Si(001) where several adsorption configurations were reported. This indicates that the PTCDA molecules on the Ge(001) were more mobile than those on the Si(001). Atomic structure of the adsorption configuration on the Ge(001) was determined by comparison between the STM experiments and the DFT calculations. Bias-dependent STM images, STS, and calculated projected density of state curves show nontrivial hybridization of molecular orbitals with surface states of the Ge substrate. Interactions of the PTCDA molecule with the Ge and the Si surfaces were in detail analyzed by the DFT calculation, considering five main competing contributions to the adsorption energy: formation energy of polar covalent Ge–O (Si–O) bonds, energy of molecular deformation, interaction energy of Ge atoms with the perylene core of PTCDA, energy of dimer buckling deformation, and van der Waals interaction energy. The analysis explains different adsorption behaviors between the Ge and the Si substrates.

Imaging of Formaldehyde Adsorption and Diffusion on TiO2(110)

Surface reactions of formaldehyde with reduced TiO2(110) surfaces have been studied using variable-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). STM images taken from a same area at various temperatures clearly show that formaldehyde preferentially adsorbs on the bridge-bonded oxygen (Ob) vacancy (VO) defect sites. Bias-dependent STM images show that the STM features corresponding to both the Ti-bound CH2O and the VO-bound CH2O are positioned between the Ob row and the Ti row. While the VO-bound formaldehyde rotates at 95 K, the Ti-bound CH2O does not. The VO-bound CH2O starts to diffuse along the Ob row as –CH2– at ~170 K and starts to diffuse along the Ti row as an intact molecule at ~215 K. However, the stabilities and the configurations of the Ti-bound and VO-bound formaldehyde calculated using DFT are not in line with the experimental results. The discrepancy between the experiment and theory indicates the presence of a complex charge distribution related to the surface defects.

Oxygen adsorbates on the Si(111)4×1-In metallic atomic wire: Scanning tunneling microscopy and density-functional theory calculations

The Si(111)$4\ifmmode\times\else\texttimes\fi{}1$-In surface is composed of metallic atomic wires, which undergo a transition into a charge density wave phase at a transition temperature $({T}_{c})$ of 125 K. This ${T}_{c}$ was reported recently to substantially increase upon the oxygen adsorption, for which the underlying mechanism is not understood. We investigate the structures of oxygen adsorbates on the Si(111)$4\ifmmode\times\else\texttimes\fi{}1$-In surface by scanning tunneling microscopy (STM) and density-functional theory calculations. We identify three distinct atomic-scale structures induced by the oxygen adsorption with high-resolution STM topography. The calculations find two energetically favorable adsorption sites on and between In zigzag chains, respectively. In conjunction with an additional adsorption configuration, where O is buried underneath the In chain, three stable structures are thus identified that reproduce very well the characteristic bias-dependent STM images. Experimentally, a switching between two specific adsorption structures is observed and is consistent with the structure models proposed. The structural distortions and the charge transfer of In atomic wires around the adsorbates are also characterized. This work provides a solid basis for the microscopic understanding of the intriguing oxygen impurity effect on the phase transition.

Adsorption of phosphorus molecules evaporated from an InP solid source on the Si(100) surface

The adsorption of phosphorus molecules evaporated from InP onto the Si(100) surface has been studied by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Five distinct structures of phosphorus adsorbates (four structures for phosphorus dimers and one for a phosphorus tetramer) were observed on the Si(100) surface by STM. Comparisons between bias-dependent STM images and simulated images based on the DFT calculation determined the adsorption sites and structures of these adsorbates. We also discuss their electronic structures and stabilities. Additionally, STM-induced structural conversions of the phosphorus adsorbates to the state with the minimum energy are shown.

Octithiophene on Cu(111) and Au(111): Formation and Electronic Structure of Molecular Chains and Films

Adsorption and electronic structure of octithiophene (8T) molecules on Cu(III) and Au(III) surfaces are investigated using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. We find a large difference in adsorption behavior of 8T molecules on the two surfaces. At the initial stage of adsorption, 8T molecules are stabilized in the form of molecular chain on a terrace of Cu(III), whereas neither such chain structure nor isolated 8T molecules have been observed on a terrace of Au(III). By increasing the amount of adsorbed molecules, a disordered monolayer film is formed on Cu(III) while a well-ordered monolayer film is formed on Au(III). From the spectroscopic investigations using bias-dependent STM images and STS spectra and by comparing the data with theoretical calculations, it is found that the electronic property of 8T molecules in the molecular chain on Cu(III) is different from that of a free-standing 8T molecule while that in the monolayer film on Au(III) keeps original character of the free-standing 8T molecule. The present study shows that adsorption of 8T molecules on Cu(III) results in a formation of adsorption-induced states near the Fermi level.

Structural and Electronic Properties of Cobalt Germanide Islands on Ge(100)

The structure and electronic properties of cobalt germanide islands were studied by scanning tunneling microscopy (STM). We found that cobalt-rich surfaces (C-surface) and germanium-rich surfaces (G-surface) coexisted in the islands. Scanning tunneling spectroscopy (STS) confirmed that both surfaces displayed metallic characteristics. Structural models are suggested based on the bias-dependent STM images.

Role of a topological defect in the local structure transformation on clean Ge(0 0 1) surface by STM

The transformation rate of the local superstructure from c(4 × 2) to p(2 × 2) is studied on the clean Ge(0 0 1) surface at 80 K by scanning tunneling microscopy (STM). The transformation is reversible and shows hysteresis for the direction of the sample bias voltage change. The rate was found to depend on the width of the terrace. The results are explained by the mechanism that a topological defect between c(4 × 2) and p(2 × 2) structures are formed and moved by the electronic excitation from the tunneling electron to the Ge lattice. The electronic structure of the defect obtained by first-principles calculation is consistent with the bias-dependent STM images.

Semiconducting surface reconstructions of p-type Si(100) substrates at 5 K.

We report scanning tunneling microscopy (STM) studies of the technologically important Si(100) surface that reveal at 5 K the coexistence of stable surface domains consisting of the p(2 x 1) reconstruction along with the c(4 x 2) and p(2 x 2) reconstructions. Using highly resolved tunneling spectroscopic measurements and tight binding calculations, we prove that the p(2 x 1) reconstruction is asymmetric and determine the mechanism that enables the contrast variation observed in the formation of the bias-dependent STM images for this reconstruction.

Absolute In coverage and bias-dependent STM images of the Si(111)4×1-In surface

We report on the absolute measurement of In coverage on the Si(111)4\ifmmode\times\else\texttimes\fi{}1-In surface using medium-energy ion scattering, and present bias-dependent scanning tunneling microscopy (STM) images. The In coverage was determined to be 1 monolayer (ML). This result is in contrast to previous results of 3/4-ML In determined by semidirect measurements using Auger electron spectroscopy and STM. Our result supports a 4\ifmmode\times\else\texttimes\fi{}1-In structural model consisting of four surface In rows, which was constructed from the recent surface x-ray diffraction study and tested by subsequent theoretical calculations. The bias-dependent STM images are well reproduced by ab initio simulations based on this 1-ML-In model.

Substitution of In for Si adatoms and In-induced charge redistribution of the Si(1 1 1)-(7×7) surface

The adsorption mechanism and the origin of the In-induced surface electronic states of the Si(1 1 1)-(7×7) surface have been studied using bias-dependent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). At very low In coverages, bias-dependent STM images show that some of the adatoms have either slightly higher or lower apparent height in the filled-state topographic images, while the empty-state images are completely identical to those of the clean Si(1 1 1)-(7×7). These peculiar features are interpreted as being caused by three different adsorption mechanisms: the substitution of In for Si adatoms in the 7×7 unit cell; In atom adsorption on Si adatoms; and charge redistribution due to the adsorption of In atoms on Si rest-atom dangling bonds (DBs). These mechanisms involve covalent bonding which removes intrinsic metallic surface states caused by partially occupied Si adatom DBs, opening a band gap at the surface and producing a metal–insulator transition.

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.

Scanning tunneling microscopy andab initiocalculations:c(4×8)reconstructions of Pb on Si and Ge(001)

The $c(4\ifmmode\times\else\texttimes\fi{}8)$ reconstructions induced by Pb atoms on Si(001) and Ge(001) surfaces have been studied by scanning tunneling microscopy (STM) and first-principles total-energy calculations. Two different structures were found experimentally. In both cases, there is a coexistence of Pb and Si(Ge) dimers either in perpendicular and/or in parallel orientations. The $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\ensuremath{\alpha}$ phase is similar to the structure previously reported for Pb on Si and Ge(001) and Sn on Si(001). It is formed by three rows of asymmetric Pb dimers forming chains along the [110] direction, and it corresponds to a coverage of 0.75 monolayer (ML). In the second structure, the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\mathrm{l}$ phase, the middle Pb dimers are replaced by Si(Ge) dimers resulting in a Pb coverage of 0.5 ML. The agreement between bias-dependent STM images and calculated local density of states for the two structures is excellent. Our calculations show that the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\ensuremath{\alpha}$ structure is generally the most stable configuration for Si and Ge surfaces covered by Pb. However, if there is a deficit of Pb atoms, the $c(4\ifmmode\times\else\texttimes\fi{}8)\ensuremath{-}\mathrm{l}$ structure becomes more stable for Pb on Si(001).

ATOMIC STRUCTURE OF THE Ge(112)-(4×1)-In RECONSTRUCTION

We have studied the Ge(112)-(4×1)-In reconstruction with scanning tunneling microscopy (STM). Based on our bias dependent STM images and the characteristic electronic structure of stable submonolayer group Ⅲ-metal/group-Ⅳ-semiconductor interfaces, we propose an atomic model of this reconstruction for further investigation. The adsorption sites of In atoms are the same as In atoms' adsorption sites in the Si(112)-(7×1)-In reconstruction, but are different from the adsorption sites of Al and Ga atoms on the Si(112) surface. We suggest that this difference in adsorption sites is mainly due to the longer covalent bond length of In atoms. We further propose that, due to the smaller misfit, Al and Ga atoms may form longer nano wires along the step edges on the Ge(112) surface.

Atomic Structure of theGaAs(001)−(2×4)Surface Resolved Using Scanning Tunneling Microscopy and First-Principles Theory

The atomic arrangement of the technologically important As-rich $\mathrm{GaAs}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed surface is determined using bias-dependent scanning tunneling microscopy (STM) and first-principles electronic structure calculations. The STM images reveal the relative position and depth of the atomic-scale features within the trenches between the top-layer As dimers, which are in agreement with the $\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4)$ structural model. The bias-dependent simulated STM images reveal that a retraction of the topmost dangling bond orbitals is the novel electronic mechanism that enables the STM tip to image the trench structure.

Bias-Dependent STM Image Contrast Study of Phenyloctadecyl Ethers Physisorbed onto Highly Oriented Pyrolytic Graphite

A homologous series of para-substituted phenyloctadecyl ethers (X-POEs, X = H, Cl, Br, and I) was prepared using Williamson's ether synthesis. The structure and purity of the ethers were confirmed using 1H NMR and GC/MS. Scanning tunneling microscopy (STM) images were acquired from monolayers of the ethers which formed at the surface of highly oriented pyrolytic graphite from a 2.5 wt % solution in phenylhexane. For all four ethers, the monolayers displayed a contrast which varied as a function of tip−sample bias. A comparison of STM images of the adsorbed ether molecules with electron density contours calculated using HyperChem suggested a bias-dependent participation to tunneling of individual bonding molecular orbitals (MOs). For example, at biases of −0.26 to −0.70 V, STM images of I-POE resembled the highest occupied molecular orbital (HOMO) exhibiting two bright spots for the pair of lobes of the phenyl ring and one bright spot for the halogen atom. At higher biases, contrast was observed for the phenyl ring alone (−0.8 to −1.2 V) and for the alkyl tail (−1.0 to −1.8 V) which was similar to that of the HOMO-1 and HOMO-4 contours, respectively. Owing to the measurement of an enhanced tunneling current simultaneous with the acquisition of atomically resolved bias-dependent STM images of the X-POE adsorbates, a resonance tunneling mechanism between the tip and substrate via MOs of molecular adsorbates adjacent to and including the HOMO is proposed.

Bias-dependent STM images of oxygen-induced structures on Ti(0001) facets

Titanium films grown on Si(111) surfaces in ultrahigh vacuum were investigated using scanning tunneling microscopy (STM). STM images of adsorbate structures induced by oxygen adsorption on Ti(0001) facets are observed to change from protrusions to depressions depending on the applied bias voltage. Using electron spectroscopic data we interpret our results in terms of local, oxygen-induced modifications of the density of states.