Performed Scanning Tunneling Microscopy Research Articles

Flash sample heating for scanning tunneling microscopy: Desorption of 1-octanethiolate self-assembled monolayers in air

The authors have developed a method of performing scanning tunneling microscopy experiments with a sample that can be controllably heated and quickly cooled. Temperatures in excess of 100 °C are achievable, and the same scanning area can be imaged multiple times before and after repeated heating cycles. This opens up for study any physical process or chemical reaction where the reactants, products, and/or intermediates can be kinetically trapped on a conductive surface at room temperature. As a demonstration of this approach, the authors have investigated desorption from 1-octanethiolate self-assembled monolayers on Au(111).

Evidence for Dirac Fermions in a Honeycomb Lattice Based on Silicon

Silicene, a sheet of silicon atoms in a honeycomb lattice, was proposed to be a new Dirac-type electron system similar to graphene. We performed scanning tunneling microscopy and spectroscopy studies on the atomic and electronic properties of silicene on Ag(111). An unexpected √3 × √3 reconstruction was found, which is explained by an extra-buckling model. Pronounced quasiparticle interferences (QPI) patterns, originating from both the intervalley and intravalley scatter, were observed. From the QPI patterns we derived a linear energy-momentum dispersion and a large Fermi velocity, which prove the existence of Dirac fermions in silicene.

One-dimensional Electronic Order in Fe1.07Te Probed by Scanning Tunneling Spectroscopy

We have performed scanning tunneling microscopy on Fe1.07Te that is the parent compound of the iron-chalcogenide superconductors (SCs). The results indicate the existence of the unidirectional electronic order with a period of a 0 (a 0 interchalcogen distance) along one Te–Te direction. From the one-dimensionality of the electronic order, it is believed that the structural or antiferromagnetic transition is closely related to the formation of this electronic order. The results in this study will be crucial key in understanding the parent state of iron-chalcogenide SCs.

Scanning tunneling microscopy reveals LiMnAs is a room temperature anti-ferromagnetic semiconductor

We performed scanning tunneling microscopy and spectroscopy on a LiMnAs(001) thin film epitaxially grown on an InAs(001) substrate by molecular beam epitaxy. While the in situ cleavage exposed only the InAs(110) non-polar planes, the cleavage continued into the LiMnAs thin layer across several facets. We combined both topography and current mappings to confirm that the facets correspond to LiMnAs. By spectroscopy we show that LiMnAs has a band gap. The band gap evidenced in this study, combined with the known Néel temperature well above room temperature, confirms that LiMnAs is a promising candidate for exploring the concepts of high temperature semiconductor spintronics based on antiferromagnets.

Surface Scattering via Bulk Continuum States in the 3D Topological InsulatorBi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.

Inducing Electronic Changes in Graphene through Silicon (100) Substrate Modification

We have performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements as well as ab initio calculations for graphene monolayers on clean and hydrogen(H)-passivated silicon (100) (Si(100)/H) surfaces. In order to experimentally study the same graphene piece on both substrates, we develop a method to depassivate hydrogen from under graphene monolayers on the Si(100)/H surface. Our work represents the first demonstration of successful and reproducible depassivation of hydrogen from beneath monolayer graphene flakes on Si(100)/H by electron-stimulated desorption. Ab initio simulations combined with STS taken before and after hydrogen desorption demonstrate that graphene interacts differently with the clean and H-passivated Si(100) surfaces. The Si(100)/H surface does not perturb the electronic properties of graphene, whereas the interaction between the clean Si(100) surface and graphene changes the electronic states of graphene significantly. This effect results from the covalent bonding between C and surface Si atoms, modifying the π-orbital network of the graphene layer. The local density of states shows that the bonded C and Si surface states are highly disturbed near the Fermi energy.

Surface of underdoped YBa2Cu3O7- δ as revealed by STM/STS

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.

Self-assembly and aggregation of melamine and melamine–uric/cyanuric acid investigated by STM and AFM on solid surfaces

The self-assembly and aggregation of melamine (M), and a mixture of melamine with uric/cyanuric acid (UA/CA) are investigated by performing scanning tunneling microscopy (STM) and atomic force microscopy (AFM) on graphite and mica surfaces at room temperature. Two-dimensional (2D) porous molecular networks and disordered adlayers are found, respectively, in the M and UA-M assembly on graphite. Uniformly dispersed nanoparticles of M and UA-M are found at various concentrations by AFM. Owing to complementary NH...O and NH...N hydrogen bonds, the CA-M mixture forms 2D densely packed adlayers and nanoscale crystals on graphite and aggregates into three-dimensional (3D) clusters on the surfaces. The size of the cluster is dependent on the molecular concentration. The results provide important information for the self-assembly of triazine compounds and are significant in the study of kidney disease related to melamine.

Observation of the local density-of-states modulation in Bi 2Sr 1.6Gd 0.4CuO 6+δ by scanning tunneling microscopy

We performed scanning tunneling microscopy measurements in the pseudogap state and the superconducting state in optimally doped Bi 2Sr 1.6Gd 0.4CuO 6+ δ . The electronic states showed two-dimensional local density-of-states (LDOS) modulation above and below T c. In both of the states, the modulation period did not show the energy dispersion in our experimental accuracy, and was about five lattice constants. This indicates that the modulations observed above and below T c are the same entity.

STM observation of local density-of-states modulation in lanthanide substituted Bi2Sr1.6Ln0.4CuO6+δ

We performed scanning tunneling microscopy (STM) experiments on optimally doped Bi2Sr1.6Ln0.4CuO6+δ (Ln=La, Nd, and Gd) at 4.2K to explore the lanthanide ion dependence of the local density-of-states (LDOS) modulation. STM image at high bias voltage showed the structural superlattice modulation along the b-axis. At low bias voltage, we observed the LDOS modulation along the Cu–O–Cu direction in all materials. The period of the modulation was independent of the lanthanide ion substituted, and was about 5a0 (a0 is the lattice constant).

Anomalous energy gap evolution in Bi2Sr1.6La0.4CuO6+δ

We performed scanning tunneling microscopy and spectroscopy experiments on the optimally doped Bi2Sr1.6La0.4CuO6+δ to explore the detailed local change in the superconducting energy gap. The discontinuous change in the superconducting gap Δs was observed at the boundary between a patch and the surrounding region where Δs evolved from 10meV to 60meV. We found that the gap value of such patches coincided with the lowest value of the gap distribution.

STM studies on structural modulation and two-phase microstructures in Pb-doped Bi2201 single crystals

We have performed scanning tunneling microscopy studies on the structural modulation of Bi2.12−xPbxSr1.88CuOy (Pbx-Bi2201, x=0.18, 0.27, 0.32, 0.37) single crystals. The observed STM images show the clear atomic arrangement with the four-fold symmetry on the Bi(Pb)–O layer. The wavelength λ of incommensurate structural modulations along the b-axis increases with Pb content (i.e., λ∼43Å for x=0.18) and the modulations completely disappear for x∼0.37. In the intermediate content (x∼0.27, 0.32), we find two-phase microstructures which consist of the modulated region with λ∼46Å and the modulation-free region.

Theoretical and experimental characterization of damaged graphite surfaces

We modeled damaged-irradiated carbon plasma facing component PFCs by graphite surfaces with vacancies. We combined theoretical and experimental investigations with a view to building tools to make a better assignment of the observed defects from the scanning tunneling microscope (STM) images. The theoretical part of this work is based on periodic density functional theory calculations. The electronic structure is calculated and the density of state along with the electronic density near the Fermi level are plotted. STM images are further simulated and their features are interpreted. On the experimental side, we performed scanning tunneling microscopy (STM) imaging at low bias voltage in order to probe the electronic modifications of the graphite surface induced either by H+ (or D+) ion bombardment and/or atomic H (respectively D) adsorption. A connection between STM images and the reactivity of defective graphite surfaces towards H is also proposed.

Two-dimensional glasses and their concentration dependent re-ordering: Dy on Mo(1 1 2)

We performed scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) experiments for Dy adsorbed on Mo(1 1 2) in the monolayer regime in order to clarify the concentration dependent reordering of the surface glass that exists for coverages above 0.58 of a monolayer (ML) after annealing to temperatures higher than 400 K. The partial reaction model developed earlier is corroborated. The Dy defect structure formed initially in Dy–Mo surface alloy acts as nucleation sites for Dy so that clusters with a wide distribution of lateral distances are formed, as found in particular at a coverage of 0.28 ML. The change in bonding character at coverages above 0.58 ML leads to reordering of the defects and the concentration dependent modulation of the adsorbed Dy layers. Examples at coverages of 0.7, 0.9 and 1.15 ML are shown and compared.

Scanning tunneling microscopy detection of individual dopant atoms on wet-prepared Si(111):H surfaces

We have performed scanning tunneling microscopy (STM) observation of individual acceptor and donor atoms on hydrogen-terminated Si(111)-1×1 surfaces prepared by wet etching in a NH4F aqueous solution. Separate measurements of p- and n-type substrates showed that acceptors appear as protrusions in filled-state images and as depressions in empty-state images, while for donors the topography is reversed in both filled- and empty-state images. The same relation between the bias polarity and the dopant appearance is preserved for codoped substrates. These results demonstrate that the STM on the Si(111):H surface can detect acceptors and donors distinguishably, enabling us to measure dopant profiles across codoped areas such as p-n junctions.

Microstructural origin of the optical black state in Mg 2NiH x thin films

Hydrogen absorption by a Pd capped thin Mg 2Ni film results in the nucleation of the Mg 2NiH 4 phase at the film/substrate interface and thus induces a self-organized two-layer system. This leads to the optical black state in Mg 2Ni thin films upon hydrogenation. This unusual hydrogenation behaviour is completely unexpected since the hydrogen enters through the top film surface. To explain the nucleation of Mg 2NiH 4 close to the substrate/film interface we performed scanning tunneling microscopy (STM) on as-prepared Mg 2Ni films with various thicknesses (20–150 nm). For films thinner than 50 nm, the film consists of small grains and clusters of small grains whereas on further growth the grain size increases and a columnar microstructure develops. We propose, therefore, that close to the substrate, the relatively porous structure of the film with small Mg 2Ni grains locally reduces the nucleation barrier for Mg 2NiH 4 formation.

Scanning tunneling microscopy and spectroscopy in lanthanide substituted Bi 2Sr 2CuO y

We have performed scanning tunneling microscopy and spectroscopy experiments on Bi 2Sr 1.57La 0.43CuO y and Bi 2Sr 1.52Nd 0.48CuO y . The atomic images showing the incommensurate modulated structure and the tunneling spectra have been successfully obtained at 4.2 K. The observed superconducting gap values in both samples have evolved spatially on the length scale of about 20 nm. This length scale is much longer than that reported in Bi 2Sr 2CaCu 2O y . The distribution of the gap value seems to originate from the inhomogeneous distribution of the lanthanide ions.

STM investigation of porphyrin isomers on metal substrates

Abstract We performed scanning tunneling microscopy (STM) experiments on mono- or sub-monolayer deposited porphyrin molecules on metal substrates. We observed substitution-dependent structure changes of the molecules. Obtaining STM images of the structure and arrangement of functional molecules would contribute valuable information towards creation of nano-molecular-devices. scanning probe microscopy (SPM) shows the greatest potential for reaching this goal, at present. STM is able to obtain images with nano-meter resolution of molecules on metal surfaces. These high-resolution images reveal the molecular structure and arrangement of the molecules on the surface and contain information about the molecule–substrate interaction and the molecule–molecule interaction. STM will be useful for the assembly of nano-devices made from molecules on a substrate.

A molecular approach to self-assembly of trimethylsilylacetylene derivatives on gold.

We recently discovered that a linear multifunctional trimethylsilylacetylene (TMSA) compound forms long-range and highly stable self-assembled monolayers (SAMs) on reconstructed Au(111). To better understand the interactions governing self-assembly in this new system, we synthesized a series of homologue organosilanes and performed scanning tunneling microscopy (STM) measurements at the Au(111)/n-tetradecane interface. The four TMSA-terminated linear silanes that we synthesized self-assemble in similar ways on gold, with the molecules standing upright on the surface. In contrast, compounds with a slightly modified terminal group but the same polyunsaturated linear chain above the TMSA head do not self-assemble. In particular, substituting a methyl group of TMSA with a more bulky one prevents self-assembly. Removing the C triple bond C triple bond of TMSA or substituting the Si atom by a C atom also hinders self-assembly. Finally, substituting one methyl group of TMSA by a hydrogen atom induces self-assembly but in a different geometry, with the molecules lying flat on the gold surface in a quasi-epitaxy mode. Our molecular approach demonstrates the key role played by the TMSA head in self-assembly, its origin being twofold: 1) the TMSA layers are commensurate to the Au(111) adlattice along the <112> direction, and 2) the C triple bond C triple bond of TMSA activates the Si atom and induces the creation of a surface Si-Au chemical bond. The highly stable TMSA-based SAMs appear then as promising materials for applications in surface modification.

Defects in vortex lattice observed by scanning tunneling spectroscopy

We have performed scanning tunneling microscopy and spectroscopy on YNi 2B 2C under the magnetic fields. The vortex lattice has been imaged up to 3 T by mapping the local density of states at the Fermi energy. In addition to the regular vortex lattice, we have observed defects in the vortex lattice above 1.5 T; they are single line vacancies and dislocations. The appearance of the defects seems to be a precursor of the disordered configuration of vortices above 3 T.