Ultrahigh-vacuum Tunneling Microscopy Research Articles

Assessing the insulating properties of an ultrathin SrTiO3 shell grown around GaAs nanowires with molecular beam epitaxy

We have studied electronic transport in undoped GaAs/SrTiO3 core–shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/μm. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO3 shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3 shells as gate insulators.

Growth Dynamics and Electron Reflectivity in Ultrathin Films of Chiral Heptahelicene on Metal (100) Surfaces Studied by Spin‐Polarized Low Energy Electron Microscopy

Interaction of electrons with chiral matter gives rise to interesting phenomena such as the chirality‐induced spin selectivity. The interdependence of reflectivity of spin‐polarized low energy electrons and the absolute handedness of chiral molecules is investigated. First, the growth of homochiral films of helical aromatic hydrocarbons, so‐called helicenes, on a Cu(100) surface is studied by means of low energy electron microscopy and scanning tunneling microscopy in ultrahigh vacuum. As soon as the coverage exceeds one monolayer, double‐layer nucleation and growth is favored such that depletion in the first layer occurs. Spatially resolved work function measurements show that second‐layer patches have a lower work function than first‐layer areas. Reflectivity spectra of spin‐polarized electrons do not show any asymmetry between homochiral films of the enantiomers. Laterally resolved work function measurements do not confirm work function differences such as those reported earlier for photoelectron studies of chiral peptide films on ferromagnetic substrates.

How Surface Species Drive Product Distribution during Ammonia Oxidation: An STM and Operando APXPS Study

The oxidation of ammonia is a key reaction for the production of artificial fertilizers and for environmental protection. Depending on the area of application, the catalytic reaction needs to be tuned toward the production of either N2 or NO, and this selectivity is controlled by temperature, pressure, reactant ratio, and the type of catalyst. PtRh alloys are highly useful catalytic materials for the oxidation of ammonia, and they can be employed at different reaction conditions. In contrast to pure Pt and Rh catalysts, for which a large number of studies of ammonia oxidation reaction mechanism are available, for PtRh alloys, direct spectroscopic evidence for structure–performance relationship is still lacking. To understand the behavior of PtRh alloys, namely, what is their active phase under reaction conditions and how the alloy composition leads to a particular product distribution, we study the oxidation of ammonia over PtRh/Pt(111) surfaces by simultaneous operando ambient pressure X-ray photoelectron spectroscopy and mass spectrometry at 1 mbar total reaction pressure. These data are complemented by a catalyst surface characterization by scanning tunneling microscopy in ultrahigh vacuum. We establish that the predominant surface structure during NH3 oxidation strongly depends on the degree of Pt enrichment and the O2/NH3 mixing ratio. At the nanoscale, the selectivity toward N2 or NO production is driven by the surface populations of N and O species. These, in turn, are controlled by the nature of the alloying of Pt with Rh.

Self-Assembled 2D-Coordination Kagome, Quadratic, and Close-Packed Hexagonal Lattices Formed from a Cyano-Functionalized Benzoporphyrin on Cu(111)

We investigated the adsorption of 2H-5,10,15,20-tetracyanophenyl-tetrabenzoporphyrin (2H-TCNPTBP) molecules on Cu(111) by scanning tunneling microscopy in ultrahigh vacuum at room temperature. Thre...

Methylisatin Structural Isomers Have Different Kinetic Pathways to Self-Assembly

Monolayers of methylisatin structural isomers are prepared on the Au(111) surface and imaged using scanning tunneling microscopy in ultrahigh vacuum. Monolayer preparation via solution deposition r...

Interface-controlled two-dimensional cuprous oxide structures

The synthesis, structures and properties of two-dimensional oxides have great fundamental significance and application potentials for the fields of catalysis, electronic devices and superconductors. In this article, the growth and synthesis of two-dimensional cuprous oxide were investigated on the surfaces of Ag(111) and Cu(111) single crystals. The structures of these two-dimensional cuprous oxides were characterized at the atomic scale using scanning tunneling microscopy in ultrahigh vacuum (UHV). On Ag(111), two-dimensional cuprous oxide nanostructures exhibit an ordered honeycomb structure with the lattice distance between six-membered rings at ~6.0 A and the stoichiometry of Cu3O2. The hexagonal lattice of Cu3O2 also contains a small amount of “5-7” defect structure. In comparison, two-dimensional cuprous oxide nanostructures on Cu(111) also exhibit the stoichiometry of Cu3O2, but their surfaces are dominated by the large number of “5-7” defect structure. As a result, their surface lattice does not display the long-range order. Two-dimensional Cu3O2 nanostructures on Ag(111), as well as their zig-zag edges, could remain stable in 10−6 mbar O2 at 400 K or in UHV at 700 K. Under similar oxidation conditions, the Cu3O2 structure without long-range order would grow on Cu(111), due to the continuous oxidation of the exposed Cu surface. The oxidation of the Cu3O2 structure on Cu(111) eventually leads to the formation of the ordered “44”- or “29”-cuprous oxide structure. The comparison of the structure and stability of two-dimensional cuprous oxide on Ag(111) and Cu(111) thus demonstrated that the properties of two-dimensional cuprous oxide could be determined by the interfacial interaction. Our study provides not only a method for the preparation of model catalysts for the interfaces between metal and cuprous oxide, but also a pathway for further exploration of the physical and chemical properties of two-dimensional cuprous oxides.

Stability and Catalytic Performance of Reconstructed Fe3O4(001) and Fe3O4(110) Surfaces during Oxygen Evolution Reaction

Earth-abundant oxides are promising candidates as effective and low-cost catalysts for the oxygen evolution reaction (OER) in alkaline media, which remains one of the bottlenecks in electrolysis and artificial photosynthesis. A fundamental understanding of the atomic-scale reaction mechanism during OER could drive further progress, but a stable model system has yet to be provided. Here we show that Fe3O4 single crystal surfaces, prepared in ultrahigh vacuum (UHV) are stable in alkaline electrolytein the range pH 7-14 and under OER conditions in 1 M NaOH. Fe3O4(001) and Fe3O4(110) surfaces were studied with X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy in UHV, and atomic force microscopy in air. Fe3O4(110) is found to be more reactive for oxidative water splitting than (001)-oriented magnetite samples. Magnetite is electrically conductive, and the structure and properties of its major facets are well understood in UHV. With these newly obtained results, we propose magnetite (Fe3O4) as a promising model system for further mechanistic studies of electrochemical reactions in alkaline media and under highly oxidizing conditions.

Scanning Tunneling Microscopy Investigation of Two-Dimensional Polymorphism of Structural Isomers

Monolayers of two pairs of structural isomers were deposited on Au(111) and observed via scanning tunneling microscopy in ultrahigh vacuum. We observe exclusively cyclic pentamers of isatin (1H-indole-2,3-dione), whereas its structural isomer, phthalimide (isoindole-1,3-dione), self-assembles primarily into close-packed arrays, with alternate structures that include kinetically locked disordered clusters and tetramer networks. Removal of the phthalimide NH group and its replacement with a CH2 group produces 1,3-indandione, which, despite the loss of the hydrogen-bond donor site, self-assembles into similar structures: close-packed areas and tetramer networks. The equivalent analog for isatin, 1,2-indandione, does not form pentamers and instead forms only close-packed areas and disordered regions. By iteratively altering the chemical structure, we demonstrate the influence that the chemical structure has on the resulting two-dimensional self-assembly.

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111).

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111) were investigated via low-temperature scanning tunneling microscopy in ultra-high vacuum. The dominant molecular hierarchical superstructure could be selectively controlled to dimer, tetramer, or pentamer patterns, which were stabilized by hydrogen bonds and van der Waals interactions.

Molecular beam epitaxial growth and scanning tunneling microscopy studies of the gallium rich trench line structure on N-polar w-GaN( 000 1 ¯ )

In addition to the usual set of the well-known reconstructions that have been observed on the N-polar GaN surface, namely 1 × 1, 3 × 3, 6 × 6, and c(6 × 12), an additional structure is occasionally seen at high Ga coverage, which can extend over a large area of the surface. This structure, which is referred to as trench line structure, is partially ordered and consists of parallel-running dark (trench) lines separating wide and narrow strips of atomically ordered regions. There are also randomly placed defects interrupting the ideal ordering. Reflection high energy electron diffraction and scanning tunneling microscopy in ultrahigh vacuum are applied to investigate this trench line structure on samples prepared using molecular beam epitaxy. It is found that the trench line structure results from annealing the Ga-rich c(6 × 12) at high temperature followed by quenching to room temperature. By careful comparison of the scanning tunneling microscopy images with those from neighboring c(6 × 12) regions, it is found that the trench line structure can be decomposed into subunits of the c(6 × 12). Using these subunits, some simple models for the trench line structure are created. It is proposed that the trench line structure is composed of two primary c(6 × 12) subunits consisting of first and second layer Ga adatoms and that the trench lines are regions devoid of Ga adatoms, going down to the Ga adlayer.

Identifying Molecular Species on Surfaces by Scanning Tunneling Microscopy: Methyl Pyruvate on Pd(111)

The structures of low coverages of methyl pyruvate on a Pd(111) surface at 120 K were studied using scanning tunneling microscopy in ultrahigh vacuum. The experimentally observed images were assigned to adsorbate structures using a combination of density functional theory calculations and by simulating the images using the Bardeen method. Two forms of methyl pyruvate were identified. The first, previously found using reflection–absorption infrared spectroscopy, was a flat-lying, keto form of cis-methyl pyruvate. It was characterized by elongated, two-lobed images with the long axes of the images oriented at ∼0 and ∼30° to the close-packed directions. The structure was simulated using clean, CO- and methyl-functionalized gold tips, and the simulated images agreed well with those found experimentally. The simulated structures were not strongly dependent on the tip structure or tip bias. This approach was used to identify the nature of the second species as the enol form of cis-methyl pyruvate with the carbo...

Growth of self-assembled Mn, Sb and MnSb nanostructures on highly oriented pyrolytic graphite

The surface morphology of Mn, Sb and MnSb nanostructures grown on highly-oriented pyrolytic graphite (HOPG) has been studied in detail with scanning tunneling microscopy in ultrahigh vacuum. At the initial stage of growth, three dimensional (3D) clusters and islands of Mn, Sb and MnSb are formed at step edges and other defect sites. In middle stage, crystalline 2D islands and 1D nanorods of Sb can be observed together with crystalline 3D islands, whereas single- and double-layer cluster chains of Mn are obtained at room temperature (RT). MnSb crystallites form after deposition of both Mn and Sb on HOPG surface and annealing at T~375K. The ex-situ X-ray photoemission spectroscopy measurement revealed the formation of MnSb compound on graphite. Ferromagnetism was observed at RT for the 50-nm MnSb thin film using vibrating sample magnetometer.

Effect of Alkyl Chain-Length on Dissociative Attachment: 1-Bromoalkanes on Si(100)-c(4×2)

Chemical reactivity as a function of chain-length was investigated for three 1-bromoalkanes on silicon. The physisorption and subsequent thermal dissociative attachment of bromoethane (EtBr), 1-bromopropane (PrBr), and 1-bromobutane (BuBr) on Si(100)-c(4×2) were examined by scanning tunneling microscopy in ultrahigh vacuum from 50 to 180 K, and interpreted by ab initio theory. These 1-bromoalkanes were found to physisorb and react exclusively over the inter-row sites of Si(100)-c(4×2), with activation barriers, Ea, increasing with alkyl chain-length: Ea = 343 ± 5 meV for EtBr, Ea = 410 ± 6 meV for PrBr, and Ea = 536 ± 2 meV for BuBr. Extensive ab initio calculations gave increasing barriers along the series: Ec = 317 meV for EtBr, Ec = 406 meV for PrBr, and Ec = 430 meV for BuBr. On the basis of our calculated geometries, we interpret this dependence of thermal barrier on chain-length as due to the additional energy required with increasing chain-length in order to lift the alkyl chain away from the surface, in going from the initial physisorbed state to the reactive transition state. For BuBr, the measured Ea significantly exceeded the calculated value. This increase in effective barrier-height could be due to a “dynamical delay” in optimizing the configuration of the alkyl chain.

Chiral Domains Achieved by Surface Adsorption of Achiral Nickel Tetraphenyl- or Octaethylporphyrin on Smooth and Locally Kinked Au(111)

The self-assembly of either nickel tetraphenylporphyrin (NiTPP) or nickel octaethylporphyrin (NiOEP) on a reconstructed Au(111) surface is studied using scanning tunneling microscopy in ultrahigh vacuum. Even though the porphyrins are achiral in the gas phase, they are shown to form racemic chiral domains on the surface. For NiTPP we observe the expected roughly “square” unit cell as well as a new “parallelogram” unit cell. For NiOEP we find a hexagonally packed structure in which alternating rows are rotated 15° with respect to each other. We discuss the forces responsible for the formation of chiral domains and find intermolecular interactions to be the dominant factor in the assembly when the porphyrins are on a flat surface. Moreover, we observe that it is possible to break the formation of racemic domains of NiTPP on Au(111) by using a kinked vicinal surface as a symmetry-breaking template to locally lift the structural equivalence of the racemic surface structures. This study shows that a delicate interplay of intermolecular forces and adsorbate−surface interactions leads to the formation of complex structures and chiral phenomena.

Relationship between the c(4×2) and the (√3×√3)R30° phases in alkanethiol self-assembled monolayers on Au(111)

Transformation between the two well-known phases of alkanethiol monolayers on Au(111), c(4×2) and (√3×√3)R30°, has been studied using scanning tunneling microscopy in ultra-high vacuum. Among the many versions of the c(4×2) phases observed, one particular structure where a lateral shift of adsorbate by as much as 0.17 nm within the unit cell is found. This lateral shift along the [112[combining macron]] direction corresponds to the movement of one adsorbed unit, towards its nearest neighbour from one hollow site to another (fcc to hcp, or hcp to fcc).

Experimental and theoretical analysis of H-bonded supramolecular assemblies of PTCDA molecules

Using a systematic method based on considering all possible hydrogen bond connections between molecules and subsequent density-functional theory (DFT) calculations, we investigated planar superstructures that the perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) molecules can form in one and two dimensions. Structures studied are mostly based on two molecule unit cells and all assemble in flat periodic arrays. We show that 42 different monolayer structures are possible, which can be split into eight families of distinct structures. A single representative of every family was selected and relaxed using DFT. We find square, herringbone and brick wall phases (among others) which were already observed on various substrates. Using scanning tunneling microscopy in ultrahigh vacuum, we also observed herringbone and square phases after sublimation of PTCDA molecules on the Au(111) surface at room temperature, the square phase being observed for the first time on this substrate. The square phase appears as a thin stripe separating two herringbone domains and provides a perfect structural matching for them. A similar structural formation serving as a domain wall between two other phases has been recently reported on the same surface formed by melamine molecules [F. Silly et al., J. Phys. Chem. C 112, 11476 (2008)]. Our theoretical analysis helps to account for these and other observed complex structures.

Commensurate ordering of iron phthalocyanine on Ag(1 1 1) surface

Epitaxial growth of iron phthalocyanine on a silver (1 1 1) surface has been studied with scanning tunneling microscopy in ultrahigh vacuum. While several structures were observed, this report concentrates on what appeared to be the most stable phase. This phase has commensurate two-dimensional ordering with oblique unit vectors ( b 1, b 2) where b 1 = 5 a 1 + 2 a 2 and b 2 = 5 a 2, where a 1 and a 2 are unit vectors of Ag(1 1 1) surface. The rotation angle of iron phthalocyanine relative to the silver [1 0 1 ¯ ] direction was determined to be 16.63° from the high-resolution image analysis. The oblique structure is more commensurate than that of iron phthalocyanine on graphite and of copper phthalocyanine on the Ag(1 1 1) surface. The observed FePc on Ag(1 1 1) structure is similar to that of FePc on Au(1 1 1), but has significantly higher surface density. Bias dependence of the images was observed and is consistent with the expected small HOMO–LUMO gap.

Regioregular poly(3-hexyl-thiophene) helical self-organization on carbon nanotubes

Mixtures of regioregular poly(3-hexyl-thiophene) (rrP3HT) and multiwall carbon nanotubes have been investigated by scanning tunneling microscopy in ultrahigh vacuum. Carbon nanotubes covered by rrP3HT have been imaged and analyzed, providing a clear evidence that this polymer self-assembles on the nanotube surface following geometrical constraints and adapting its equilibrium chain-to-chain distance. Largely spaced covered nanotubes have been analyzed to investigate the role played by nanotube chirality in the polymer wrapping, evidencing strong rrP3HT interactions along well defined directions.

Scanning tunneling microscopy investigation of growth of self-assembled indium and aluminum nanostructures on inert substrates

The growth and surface morphology of indium and aluminum nanostructures on highly oriented pyrolytic graphite (HOPG) and molybdenum disulphide (MoS 2) surfaces were investigated using scanning tunneling microscopy in ultra-high vacuum. Mostly triangular and hexagonal In islands with (111) orientation were obtained along the steps of HOPG at room temperature (RT). Triangular, round-shape and large irregular In islands were found on MoS 2 surfaces at different growth conditions. Al island chains as well as isolated triangular islands were found on HOPG whereas nanoparticles and ramified Al islands were obtained on MoS 2 at RT. The shapes of these self-assembled metal nanostructures were controlled by varying growth conditions. The different shapes of nanostructures on these inert substrates can be explained in terms of differences in energetic and kinetic properties of atoms and clusters of each elements as well as the nature of substrates.

Scanning tunneling microscopy study of nitrogen incorporated HfO2

The impact of nitrogen incorporation on the physical and electrical characteristics of the HfO2 is examined. X-ray photoelectron spectroscopy shows that nitrogen can be incorporated into the HfO2 via a two-step thermal anneal—first in ultrahigh vacuum (UHV) and subsequently in N2. Following the N2 anneal, scanning tunneling microscopy in UHV reveals a marked reduction in the low-voltage leakage current under gate injection biasing. From band theory and existing first-principles simulation results, one may consistently attribute this improvement to the passivation of oxygen vacancies in the HfO2 by nitrogen. Improvement in the breakdown strength of the HfO2 subjected to ramp-voltage stress (substrate injection) is also observed after the N2 anneal. The local current-voltage curves acquired concurrently during the ramp-voltage stress exhibit “space-charge limited conduction,” which implies that the observed improvement in breakdown strength may be related to a limitation of the current flow through the gate...