Bias-dependent Scanning Tunneling Microscopy Research Articles

Defect pairing in Fe-doped SnS van der Waals crystals: a photoemission and scanning tunneling microscopy study.

We investigate the effect of low concentrations of iron on the physical properties of SnS van der Waals crystals grown from the melt. By means of scanning tunneling microscopy (STM) and photoemission spectroscopy we study Fe-induced defects and observe an electron doping effect in the band structure of the native p-type SnS semiconductor. Atomically resolved and bias dependent STM data of characteristic defects are compared to ab initio density functional theory simulations of vacancy (VS and VSn), Fe substitutional (FeSn), and Fe interstitial (Feint) defects. While native SnS is dominated by acceptor-like VSn vacancies, our results show that Fe preferentially occupies donor-like interstitial Feint sites in close proximity to VSn defects along the high-symmetry c-axis of SnS. The formation of such well-defined coupled (VSn, Feint) defect pairs leads to local compensation of the acceptor-like character of VSn, which is in line with a reduction of p-type carrier concentrations observed in our Hall transport measurements.

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.

Identifying Native Point Defects in the Topological Insulator Bi2Te3.

We successfully identified native point defects that occur in Bi2Te3 crystals by combining high-resolution bias-dependent scanning tunneling microscopy and density functional theory based calculations. As-grown Bi2Te3 crystals contain vacancies, antisites, and interstitial defects that may result in bulk conductivity and therefore may change the insulating bulk character. Here, we demonstrate the interplay between the growth conditions and the density of different types of native near-surface defects. In particular, scanning tunneling spectroscopy reveals the dependence on not only the local atomic environment but also on the growth kinetics and the resulting sample doping from n-type toward intrinsic crystals with the Fermi level positioned inside the energy gap. Our results establish a bias-dependent STM signature of the Bi2Te3 native defects and shed light on the link between the native defects and the electronic properties of Bi2Te3, which is relevant for the synthesis of topological insulator materials and the related functional properties.

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.

Universal building block for (1 1 0)-family silicon and germanium surfaces

The universal building block which is an essential part of all atomic structures on (1 1 0) silicon and germanium surfaces and their vicinals is proposed from first-principles calculations and a comparison of results with available experimental data. The atomic models for the (1 1 0)-(16 × 2), (1 1 0)-c(8 × 10), (1 1 0)-(5 × 8) and (17 15 1)-(2 × 1) surface reconstructions are developed on the basis of the building block structure. The models exhibit very low surface energies and excellent agreements with bias-dependent scanning tunneling microscopy (STM) images. It is shown experimentally using STM that the Si(47 35 7) surface shares the same building block. Our study closes the long-debated pentagon structures on (1 1 0) silicon and germanium surfaces.

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.

Bias-Dependent Scanning Tunneling Microscopy Signature of Bridging-Oxygen Vacancies on Rutile TiO2(110).

The rutile TiO2(110) surface has long-served as a well-characterized, prototypical transition-metal oxide surface used in heterogeneous catalysis and photocatalytic water splitting. Naturally occurring defects on this surface, called bridging-oxygen (BO) vacancies, are important as they determine the overall reactivity of the surface. Herein, we report a bias-dependent, scanning tunneling microscopy (STM) signature of the BO vacancies on TiO2(110): for sample bias voltages past a threshold of +3 V, the bright vacancies are flanked on either side (along the oxygen row) by two dark spots approximately shaped like half-moons. The BO vacancies have a bright aspect below the threshold bias also but are not surrounded by half-moon dark depressions. Using generalized gradient approximation calculations with Hubbard correction (GGA + U) for projected density of states (DOS) and simulated STM images, we find that the bias-dependent STM signature originates from (i) local DOS maxima of all BOs (lighter background that occurs above the threshold bias) and (ii) the increased separation between the first and second BO atoms neighboring the vacancy which leads to an apparent dip between these neighboring oxygens. These results offer a new striking example of the STM signature that appears without switching the polarity of the bias. Similar approaches can be employed for seeking distinguishing features on the surfaces of other large band gap semiconductors and insulators.

Investigation of the structural anisotropy in a self-assembling glycinate layer on Cu(100) by scanning tunneling microscopy and density functional theory calculations

Self-assembling organic molecule-metal interfaces exhibiting free-electron like (FEL) states offers an attractive bottom-up approach to fabricating materials for molecular electronics. Accomplishing this, however, requires detailed understanding of the fundamental driving mechanisms behind the self-assembly process. For instance, it is still unresolved as to why the adsorption of glycine ([NH2(CH2)COOH]) on isotropic Cu(100) single crystal surface leads, via deprotonation and self-assembly, to a glycinate ([NH2(CH2)COO–]) layer that exhibits anisotropic FEL behavior. Here, we report on bias-dependent scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations for glycine adsorption on Cu(100) single crystal surface. We find that after physical vapor deposition (PVD) of glycine on Cu(100), glycinate self-assembles into an overlayer exhibiting c(2×4) and p(2×4) symmetries with non-identical adsorption sites. Our findings underscore the intricacy of electrical conductivity in nanomolecular organic overlayers and the critical role the structural anisotropy at molecule-metal interface plays in the fabrication of materials for molecular electronics.

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.

Electronic State of Oxidized Nanographene Edge with Atomically Sharp Zigzag Boundaries

Combined scanning tunneling microscopy (STM) and density functional theory (DFT) characterizations of the electronic state were performed on the zigzag edge of oxidized nanographene samples. The oxidized zigzag edge with atomically sharp boundaries was prepared by electrochemical oxidation of the graphite surface in aqueous sulfuric acid solution. Bias-dependent STM measurements demonstrated the presence of the edge state at the zigzag edges with local density of states (LDOS) split into two peaks around the Fermi level. Our DFT-based analysis showed that the two-peak structure of the edge state was due to the termination of the zigzag edge by carbonyl functional groups. The LDOS arising from the edge states was slowly dampened in the bulk at the carbonyl-terminated zigzag edges (~1.5 nm). This result is in clear contrast to the strongly localized edge states at hydrogenated zigzag edges in previous reports. The oxygen atoms in the carbonyl functional groups act as additional π sites at the edges; thus, the topology of the π electron network changes from "zigzag" to "Klein" type, leading to drastic modification of the edge states at the oxidized edges.

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.

Selective molecular adsorption in sub-nanometer cages of a Cu2O surface oxide

In this study the identity of diverse adsorption sites on a 5-7 Cu2O/Cu(111) surface oxide structure has been identified. The 5-7 membered rings formed by a topological defect on stoichiometric Cu2O present different electronic structures from the originating hexagonal rings, as shown by combined bias dependent scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The adsorption of CO as a probe molecule on the 5-7 structure, studied using infrared reflection-absorption spectroscopy (IRRAS), shows the existence of special adsorption sites. By combining experimental and theoretical results, it is determined that CO molecules can be selectively confined inside the 7-membered oxide rings with internal dimensions of ∼0.85 nm, leading to a marked different adsorbate-substrate interaction than in either clean Cu(111) or Cu2O. The implication of these newly discovered sites on the chemistry of copper for catalytic reactions is discussed.

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.

Doping of the step-edge Si chain: Ag on a Si(557)-Au surface

Structural and electronic properties of monatomic Ag chains on the Au-induced, highly ordered Si(557) surface are investigated by scanning tunneling microscopy (STM)/spectroscopy and first-principles density functional theory (DFT) calculations. The STM topography data show that a small amount of Ag (0.25 ML) very weakly modifies the one-dimensional structure induced by Au atoms. However, the bias-dependent STM topography and spectroscopy point to the importance of the electronic effects in this system, which are further corroborated by the DFT calculations. The obtained results suggest that Ag atoms act as electron donors leaving the geometry of the surface almost unchanged.

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.

In and Si adatoms onSi(111)5×2-Au: Scanning tunneling microscopy and first-principles density functional calculations

Structural properties of monatomic indium chains on $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}$ surface are investigated by scanning tunneling microscopy (STM) and first-principles density functional calculations (DFT). The STM topography data show that submonolayer coverage of indium leads to a well-ordered chain structure with the same periodicity as the Si adatoms form on $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}$ surface. Bias-dependent STM topography and spectroscopy reveal two different mechanisms of In-atoms adsorption on the surface: bonding to Si adatoms and substitution for Si atoms in the adatom positions. Those mechanisms are further corroborated by DFT calculations. The obtained structural model of In-modified $\text{Si}(111)5\ifmmode\times\else\texttimes\fi{}2\text{-Au}$ surface remains in good agreement with the experimental data.

Structure of the fivefold surface of the Ag-In-Yb icosahedral quasicrystal

The surface structural study of an icosahedral $(i)$ quasicrystal of the Cd-Yb family is presented. Comparison of bias-dependent scanning tunneling microscopy data from the fivefold surface of $i\text{-Ag-In-Yb}$ with the refined bulk model of isostructural $i\text{-Cd-Yb}$ indicates that surfaces are formed at bulk planes intersecting the center of the rhombic triacontahedral clusters, the building blocks of the Cd-Yb family quasicrystals. These observations open up the possibility of the use of this material as a template for epitaxial structures.

One-Dimensional Growth of PTCDA Molecular Rows on Si(111)-(2√3 × 2√3)R30°-Sn Surfaces

We report an investigation on the adsorption of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on Si(111)-(2√3 × 2√3)R30°-Sn surfaces using scanning tunneling microscopy (STM) under ultrahigh vacuum. A novel PTCDA structure, consisting of quasi-one-dimensional molecular chains, is obtained at a submonolayer range. A fine balance between intermolecular and molecule−substrate interactions favors and stabilizes the formation of this molecular structure, which extends homogeneously, creating a highly ordered overlayer with regularly spaced rows in a commensurate (4√3 × 2√3)R30° PTCDA reconstruction. The visualization of PTCDA molecular orbitals in bias-dependent high resolution STM images allows for identifying the bonding configuration and the molecular adsorption site.