Scanning Tunneling Microscopy Topography Research Articles

Scanning Tunneling Microscopy for Molecules: Effects of Electron Propagation into Vacuum.

Using scanning tunneling microscopy (STM), we experimentally and theoretically investigate isolated platinum phthalocyanine (PtPc) molecules adsorbed on an atomically thin NaCl(100) film vapor deposited on Au(111). We obtain good agreement between theory and constant-height STM topography. We theoretically examine why strong distortions of STM images occur as a function of distance between the molecule and the STM tip. The images of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) exhibit for increasing distance, significant radial expansion due to electron propagation in the vacuum. Additionally, the imaged angular dependence is substantially distorted. The LUMO image has substantial intensity along the molecular diagonals where PtPc has no atoms. In the electronic transport gap, the image differs drastically from HOMO and LUMO even at energies very close to these orbitals. As the tunneling becomes increasingly off-resonant, the eight angular lobes of the HOMO or of the degenerate LUMOs diminish and reveal four lobes with maxima along the molecular axes, where both, HOMO and LUMO have little or no weight. These images are strongly influenced by low-lying PtPc orbitals that have simple angular structures.

Character of Electronic States in the Transport Gap of Molecules on Surfaces.

We report on scanning tunneling microscopy (STM) topographs of individual metal phthalocyanines (MPc) on a thin salt (NaCl) film adsorbed on a gold substrate, at tunneling energies within the molecule's electronic transport gap. Theoretical models of increasing complexity are discussed. The calculations for MPcs adsorbed on a thin NaCl layer on Au(111) demonstrate that the STM pattern rotates with the molecule's orientations─in excellent agreement with the experimental data. Thus, even the STM topography obtained for energies in the transport gap represent the structure of a one atom thick molecule. It is shown that the electronic states inside the transport gap can be rather accurately approximated by linear combinations of bound molecular orbitals (MOs). The gap states include not only the frontier orbitals but also surprisingly large contributions from energetically much lower MOs. These results will be essential for understanding processes, such as exciton creation, which can be induced by electrons tunneling through the transport gap of a molecule.

Controlling Localized Plasmons via an Atomistic Approach: Attainment of Site-Selective Activation inside a Single Molecule.

Chemical reactions such as bond dissociation and formation assisted by localized surface plasmons (LSPs) of noble metal nanostructures hold promise in solar-to-chemical energy conversion. However, the precise control of localized plasmons to activate a specific moiety of a molecule, in the presence of multiple chemically equivalent parts within a single molecule, is scarce due to the relatively large lateral distribution of the plasmonic field. Herein, we report the plasmon-assisted dissociation of a specific molecular site (C-Si bond) within a polyfunctional molecule adsorbed on a Cu(100) surface in the scanning tunneling microscope (STM) junction. The molecular site to be activated can be selected by carefully positioning the tip and bringing the tip extremely close to the molecule (atomistic approach), thereby achieving plasmonic nanoconfinement at the tip apex. Furthermore, multiple reactive sites are activated in a sequential manner at the sub-molecular scale, and different sets of products are created and visualized by STM topography and density functional theory (DFT) modeling. The illustration of site-selective activation achieved by localized surface plasmons implies the realization of molecular-scale resolution for bond-selected plasmon-induced chemistry.

Subatomic Distortion of Surface Monolayer Lattice Visualized by Moiré Pattern.

Mapping of the local lattice distortion is required to understand the details of the chemical and physical properties of thin films. However, the resolution by the direct microscopic methods was insufficient to detect the local distortion. Here, we have demonstrated that the local distortion of a monatomic film on a substrate is estimated with high resolution using the moiré pattern of the topographic scanning tunneling microscopy image. The analysis focused on the apparently low centers of the moiré pattern of the hexagonal iron nitride monolayer on the Cu(111) substrate. The local distortion was visualized by estimating the displacement of the experimentally detected center position from the ideal position that is extracted from the adjacent center positions. The map of the local distortion revealed that the subsurface impurities are preferentially located on the largely distorted areas. This approach is widely applicable to other thin films on the substrates.

Characterizing armchaired and zigzagged phases: Antimony on oxide layer of Cu(110)

Antimony nanostructures fabricated on metal oxide layers are essential for practical applications in fields of solar cells, electronic and photonic devices. Here the antimony phases were systemically studied on the copper oxide of Cu(110). By high-resolution scanning tunneling microscopy (STM), we revealed that a wrinkled antimony phase was formed on the CuO surface regardless of the initial oxide structure. It exhibited an armchair-like configuration making up of protruded and dipped Sb atoms as confirmed by STM and density functional theory calculations. Further Sb deposition caused the armchair-like phase distorting and subsequently gave rise to formation of a c(2 × 2)-Sb phase. Finally, another kind of wrinkled phase was successfully fabricated. It consisted of two zigzagged antimony chains with protruded and dipped Sb atoms alternately distributing. By analysis of the atomically-resolved STM topographies, several kinds of dislocations have been observed and the corresponding geometric models are addressed. The armchair and zigzag phases both exhibited high stabilities for oxygen exposure. Conductance measurements imply that each phase interacts with the underlying layer. Our results might provide a way for fabricating stable ultrathin materials with a well-defined edge structure on oxide surfaces.

Nanoscale decoupling of electronic nematicity and structural anisotropy in FeSe thin films

In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. This is consistent with experimental observations, where electronic nematicity and structural anisotropy typically appear hand-in-hand. In this work, we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films. We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to spatially varying strain. We map local anisotropic strain by analyzing scanning tunneling microscopy topographs, and visualize electronic nematic domains from concomitant spectroscopic maps. While the domains form so that the energy of nemato-elastic coupling is minimized, we observe distinct regions where electronic nematic ordering fails to flip direction, even though the underlying structural anisotropy is locally reversed. The findings point towards a nanometer-scale stiffness of the nematic order parameter.

Insight into the Charge Density Wave Gap from Contrast Inversion in Topographic STM Images.

Charge density waves (CDWs) are understood in great detail in one dimension, but they remain largely enigmatic in two-dimensional systems. In particular, numerous aspects of the associated energy gap and the formation mechanism are not fully understood. Two long-standing riddles are the amplitude and position of the CDW gap with respect to the Fermi level (E_{F}) and the frequent absence of CDW contrast inversion (CI) between opposite bias scanning tunneling microscopy (STM) images. Here, we find compelling evidence that these two issues are intimately related. Combining density functional theory and STM to analyze the CDW pattern and modulation amplitude in 1T-TiSe_{2}, we find that CI takes place at an unexpected negative sample bias because the CDW gap opens away from E_{F}, deep inside the valence band. This bias becomes increasingly negative as the CDW gap shifts to higher binding energy with electron doping. This study shows the importance of CI in STM images to identify periodic modulations with a CDW and to gain valuable insight into the CDW gap, whose measurement is notoriously controversial.

Nanoscale Work Function Contrast Induced by Decanethiol Self-Assembled Monolayers on Au(111).

In this paper, we obtain maps of the spatial tunnel barrier variations in self-assembled monolayers of organosulfurs on Au(111). Maps down to the sub-nanometer scale are obtained by combining topographic scanning tunneling microscopy images with dI/dz spectroscopy. The square root of the tunnel barrier height is directly proportional to the local work function and the dI/dz signal. We use ratios of the tunnel barriers to study the work function contrast in various decanethiol phases: the lying-down striped β phase, the dense standing-up φ phase, and the oxidized decanesulfonate λ phase. We compare the induced work function variations too: the work function contrast induced by a lying-down striped phase in comparison to the modulation induced by the standing-up φ phase, as well as the oxidized λ phase. By performing these comparisons, we can account for the similarities and differences in the effects of the mechanisms acting on the surface and extract valuable insights into molecular binding to the substrate. The pillow effect, governing the lowering of the work function due to lying-down molecular tails in the striped low density phases, seems to have quite a similar contribution as the surface dipole effect emerging in the dense standing-up decanethiol phases. The dI/dz spectroscopy map of the nonoxidized β phase compared to the map of the oxidized λ phase indicates that the strong binding of molecules to the substrate is no longer present in the latter.

Fingerprint of checkerboard antiferromagnetic order in FeSe monolayer due to magnetic-electric correlation

Though the significance of magnetism in material properties has been well recognized, there are many difficulties and limitations in the identification techniques of magnetism. Noting the magnetic order of FeSe monolayer is important for understanding its superconductivity and remains mysterious, we take it as an example to demonstrate that electric measurements may be applied to identify the magnetic order, by utilizing the magnetic-electric correlation. The contrasts in scanning tunneling microscopy topography, superconducting gaps near defects and evolution of the electronic band structures under in-plane strain for different magnetic orders, are presented as strong evidence, or fingerprints, for the existence of checkerboard antiferromagnetic order in the FeSe monolayer observed in these experiments, and the Fe–Se–Fe exchange interaction is identified to be responsible for the correlation between Fe–Se (electric) bonding states and magnetic orders.

Melting of charge density wave and Mott gap collapse on 1T−TaS2 induced by interfacial water

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\ifmmode\times\else\texttimes\fi{}6$ superstructures on the surface of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ charge order in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.

√3 × 2 and √3 × √7 Charge Density Wave Driven by Lattice Distortion in Monolayer VSe2

A charge density wave (CDW), an ordered modulation of electron distribution and lattice distortion, is one of the intriguing phenomena observed in transition metal dichalcogenides. Recent STM studies reported a new CDW phase with √3 × 2 and √3 × √7 periodicities in monolayer (ML) VSe2 grown on graphene, which is totally different from the 4 x 4 x 3 CDW periodicity in bulk. Although the emergence of new CDW phase is of great research interest, the origin of the new modulation in ML VSe2 has not been clearly investigated. In this report, we conduct a systematic study to understand the nature of the √3 × 2 and √3 × √7 CDW in ML VSe2 using scanning tunneling microscopy (STM). Bias dependent topography and differential conductance (dI/dV) mapping indicate that the √3 × 2 and √3 × √7 modulations are mostly driven by strong lattice distortions of the Se atoms rather than by charge orderings. In addition, STM topography reveals that the v3 × 2 modulation corresponds to a gap feature, and the v3 × v7 modulation corresponds to an isolated Se atom in the distorted lattice structures. Our work provides prerequisite information to understand the emergence of √3, × 2 and √3, × √7 CDW in ML VSe2.

Modeling the Kondo effect of a magnetic atom adsorbed on graphene

The Kondo effect due to a magnetic atom adsorbed on graphene is investigated using two theoretical approaches, including the non-equilibrium Green’s function method within the slave-boson mean-field approximation and a newly developed tight-binding (TB) model with an effectively infinite Hubbard U term. Both methods reveal the presence of a Kondo peak in the local density of state (LDOS) of graphene near the Fermi level. A sharp Kondo peak is only observed in the odd-neighboring C sites of the C atom directly below the magnetic atom placed in top position. The peak intensity is found to decay quickly with respect to the distance between the C site and the magnetic atom. A theoretical model of scanning tunneling microscopy (STM) of graphene is presented as a means to identify the manifestation of the Kondo effect via direct topographic STM measurements. STM simulations show that in the proximity of the magnetic atom, only the C atoms of one sublattice are visible at low bias potential and thus a triangular lattice can be seen, in agreement with recent experiments. Further away from the magnetic atom, the Kondo effect dies down and eventually both sublattices are visible, leading to the recovery of the hexagonal lattice. This work not only provides a new TB method to study the Kondo effect in graphene, but also presents both scanning tunneling spectroscopy and microscopy fingerprints of the Kondo effect to facilitate its experimental verification.

Investigation of CVD graphene as-grown on Cu foil using simultaneous scanning tunneling/atomic force microscopy.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of graphene reveal either a triangular or honeycomb pattern at the atomic scale depending on the imaging parameters. The triangular patterns at the atomic scale are particularly difficult to interpret, as the maxima in the images could be every other carbon atom in the six-fold hexagonal array or even a hollow site. Carbon sites exhibit an inequivalent electronic structure in HOPG or multilayer graphene due to the presence of a carbon atom or a hollow site underneath. In this work, we report small-amplitude, simultaneous STM/AFM imaging using a metallic (tungsten) tip, of the graphene surface as-grown by chemical vapor deposition (CVD) on Cu foils. Truly simultaneous operation is possible only with the use of small oscillation amplitudes. Under a typical STM imaging regime the force interaction is found to be repulsive. Force–distance spectroscopy revealed a maximum attractive force of about 7 nN between the tip and carbon/hollow sites. We obtained different contrast between force and STM topography images for atomic features. A honeycomb pattern showing all six carbon atoms is revealed in AFM images. In one contrast type, simultaneously acquired STM topography revealed hollow sites to be brighter. In another, a triangular array with maxima located in between the two carbon atoms was acquired in STM topography.

Self-assembled triangular graphene nanostructures: Evidence of dual electronic response

Structural and electronic properties of bilayer graphene films and nanostructures obtained through the graphitization of SiC(0001) were investigated in this work using scanning tunneling microscopy and spectroscopy. We report on the observation of triangular nanostructures which result from extended stacking faults in the SiC substrate and their effects on graphene layers that are formed on top of them. Spectroscopic measurements revealed distinct electronic responses as a function of the local hydrogen intercalation. Spectroscopic signatures ranging from single- to double-layer graphene, as well as intermediate states were observed as a consequence of the (in)complete hydrogen intercalation process. High resolution topographic scanning tunneling microscopy images at resonant bias voltages inside triangular nanostructures reveal that the bottom layer of the bilayer graphene film is still bonded to the substrate. Therefore, the triangular nanostructures present edges and facets with the coexistence of carbon atoms in sp3 and sp2 hybridizations. Using atomistic calculations we have modeled the local density of states of these objects reproducing their electronic response. The generation of regions with distinct electronic responses is potentially interesting for high-density data storage with hidden bit capabilities.

Single-molecule chemical reduction induced by low-temperature scanning tunneling microscopy: A case study of gold-porphyrin on Au(111)

Electron transfer in weakly coupled molecules typically occurs via a transient charging of the molecule by tunneling electrons. We report on the permanent electron injection into a single molecule in a controlled manner by employing low-temperature (LT) scanning tunneling microscopy (STM). Our model system is Au(III) 5,10,15,20-tetraphenylporphyrin (AuTPP) single molecules physisorbed on a single-crystal Au(111) surface. The results of our combined topographic STM images, tunnel conductance dI/dV spectra and two-dimensional spatial dI/dV mapping experiments support a switching of the molecular charge state of AuIIITPP on Au(111) induced by STM manipulation. We successfully realized STM tip-induced chemical reduction reactions in single AuTPP molecules, reducing AuIIITPP to AuIITPP, with the extra electron distributed either preferentially in a 5d state of the center gold ion or the aromatic (pi-conjugated) tetrapyrrole macrocycle of the porphyrin ligand. Details of the electron transfer process were identified and visualized with intra-molecular resolution by single-molecule tunneling spectroscopy and orbital mapping.

Influence of the State of the Tungsten Tip on STM Topographic Images of SnSe Surfaces

Tin selenide (SnSe) has recently attracted significant attention because of its excellent thermoelectric properties with a figure of merit (ZT) of 2.6. Previous scanning tunneling microscopy (STM) studies of SnSe surfaces showed that only Sn atoms are resolved in topographic images due to the dominant contribution of the Sn 5pz states in tunneling. However, when the state of the tungsten (W) tip changes from a typical four-lobe d state such as d xy or $${d_{{x^2} - {y^2}}}$$ to a two-lobe $${d_{{z^2}}}$$ state, the atomic features observed on the SnSe surface in STM topography can be dramatically altered. In this report, we present the results of a systematic study on the influence of the W tip’s states on the STM images of SnSe surfaces. Sn atoms are observed with much stronger corrugation amplitude and smaller apparent radius when the tip is in a $${d_{{z^2}}}$$ state. In addition, the atomic features of the Se atoms become visible because of the sharply focused shape of the W $${d_{{z^2}}}$$ state. We expect our results to provide important information for establishing a better understanding of the microscopic nature of SnSe surfaces.

First-Principle Study of the Electronic Structure and Stability of Reconstructed AgInSe2 (112) Polar Surfaces

Chalcopyrite AgInSe2 (AIS) is a candidate material for alloying with Cu(In,Ga)Se2 (CIGS) to increase the band gap and potentially enhance the efficiency of CIGS thin-film photovoltaic materials. As Cu depletion at the heterojunction of CIGS photovoltaic cells plays an important role in its high efficiency, the stoichiometry, stability, and electronic structure of AIS surfaces are a matter of interest. In this work, hybrid density functional theory was implemented to study the (112) polar surface of AIS. We found that, similar to the corresponding CIS surface, as-cleaved AIS (112) surfaces are reconstructed by Ag vacancies or Ag-on-In antisites depending on the thermodynamic environment. The former is found to be more favorable under most typical growth conditions. Moreover, unlike CIS, the fluctuations in the position of the AIS valence band are small for the Ag vacancy reconstruction, but can increase if antisite reconstructions are present. Simulated scanning tunneling microscopy topographs are compared to those obtained from the experiment. Our findings suggest that alloying Ag into CIGS can potentially reduce electron–hole recombination at defects, leading to improved device performance.

Atomic-scale visualization of surface-assisted orbital order.

Orbital-related physics attracts growing interest in condensed matter research, but direct real-space access of the orbital degree of freedom is challenging. We report a first, real-space, imaging of a surface-assisted orbital ordered structure on a cobalt-terminated surface of the well-studied heavy fermion compound CeCoIn5. Within small tip-sample distances, the cobalt atoms on a cleaved (001) surface take on dumbbell shapes alternatingly aligned in the [100] and [010] directions in scanning tunneling microscopy topographies. First-principles calculations reveal that this structure is a consequence of the staggered d xz -d yz orbital order triggered by enhanced on-site Coulomb interaction at the surface. This so far overlooked surface-assisted orbital ordering may prevail in transition metal oxides, heavy fermion superconductors, and other materials.

Electronic structure and switching behavior of the metastable silicene domain boundary

Silicene, a silicon allotrope with a buckled honeycomb lattice, has been extensively studied in the search for materials with graphene-like properties. Here, we study the domain boundaries of a silicene 4 × 4 superstructure on an Ag(111) surface at the atomic resolution using scanning tunneling microscopy (STM) and spectroscopy (STS) along with density functional theory calculations. The silicene domain boundaries (β-phases) are formed at the interface between misaligned domains (α-phases) and show a bias dependence, forming protrusions or depressions as the sample bias changes. In particular, the STM topographs of the silicene–substrate system at a bias of ∼2.0 V show brightly protruding domain boundaries, which can be explained by an energy state originating from the Si 3s and 3pz orbitals. In addition, the topographs depicting the vicinity of the domain boundaries show that the structure does not follow the buckled geometry of the atomic ball-and-stick model. Inside the domain, STS data showed a step-up at ∼0.4 V, which originated from the Si 3p orbitals. We found this step-up to have shifted, which may be attributed to the strain effect at the interface regions between silver and silicene and between the domain and its boundary upon performing spatially resolved STS measurements. The metastable characteristic of the domain boundary (β-phase) causes changes, such as creation or annihilation, in the buckling structures (switching behavior). The observed low activation energy for the buckling change between distinct states may find applications in the electronic control of properties related to domain boundary structures in silicene.

Real-Space Analysis of Scanning Tunneling Microscopy Topography Datasets Using Sparse Modeling Approach

A sparse modeling approach is proposed for analyzing scanning tunneling microscopy topography data, which contains numerous peaks corresponding to surface atoms. The method, based on the relevance vector machine with $\mathrm{L}_1$ regularization and $k$-means clustering, enables separation of the peaks and atomic center positioning with accuracy beyond the resolution of the measurement grid. The validity and efficiency of the proposed method are demonstrated using synthetic data in comparison to the conventional least-square method. An application of the proposed method to experimental data of a metallic oxide thin film clearly indicates the existence of defects and corresponding local lattice deformations.