Surface Superstructures Research Articles

Dynamic Equilibrium at the HCOOH-Saturated TiO2(110)-Water Interface.

Carboxylic acids bind to titanium dioxide (TiO2) dissociatively, forming surface superstructures that give rise to a (2 × 1) pattern detected by low-energy electron diffraction. Exposing this system to water, however, leads to a loss of the highly ordered surface structure. The formate-covered surface was investigated by a combination of diffraction and spectroscopy techniques, together with static and dynamic ab initio simulations, with the conclusion that a dynamic equilibrium exists between adsorbed formic acid and water molecules. This equilibrium process is an important factor for obtaining a better understanding of controlling the self-cleaning properties of TiO2, because the formic acid monolayer is responsible for the amphiphilic character of the surface.

Observing an ordered surface phase by B deposition on Cu(110)

Growths of novel two-dimensional materials have recently attracted interests to develop chemistry and physics at surfaces. By the boron deposition on Cu(110), we discovered a 1D anisotropic ordered phase, 3×1, as a pattern of electron diffraction. The electronic analysis by photoelectron spectroscopy unveiled that the surface superstructure is a layer of the Cu boride with a component of the negatively charged boron atoms. This surface boron atoms likely form the chain structure that is isoelectronic to carbon polymers, such as bumulene that was recently reported on Cu(111). Considering the two-fold rotation symmetry of Cu(110) substrate, the novel surface phase likely becomes an ideal system to examine 1D boron materials.

Skill-Agnostic analysis of reflection high-energy electron diffraction patterns for Si(111) surface superstructures using machine learning

Skill-Agnostic analysis of reflection high-energy electron diffraction patterns for Si(111) surface superstructures using machine learning

Investigation on the Electronic Structure of Nanosized Barium Monosilicide Films Produced by Low-energy Implantation of Ba<sup>+</sup> Ions in Si

The processes of formation of nanoscale silicide films during the implantation of Ba⁺ ions into Si (111) and Si (100) and subsequent thermal annealing were studied by electron spectroscopy. It is shown that implantation of ions with a high dose D > 10¹⁶ cm^-2 and short-term heating leads to the formation of thin films of barium monosilicide with new surface superstructures. The optimal modes of formation and band-energy parameters of BaSi films obtained by low-energy high-dose implantation of barium ions in Si are determined. It is shown that BaSi films are a narrow-gap semiconductor with a band gap of 0.7 eV and have good emission and thermoelectric properties corresponding to the solar spectrum and have high photoelectric and thermoelectric characteristics. It should be noted that, in addition to the formation of a chemical compound, the narrowing of the Si band gap upon implantation of large doses of Ba⁺ ions also contributes to defects formed as a result of strong disordering of the crystal lattice.

Optimum technological modes of ion implantation and subsequent annealing for formation of thin nanosized silicide films

Using the methods of electron spectroscopy and slow electron diffraction, we studied the processes of the formation of nanosized metal silicide films in the near-surface region of Si (111) and Si (100) during low-energy implantation of Ba ions and alkaline elements. The optimal technological modes of ion implantation and subsequent annealing for the formation of thin nanoscale films of silicides were determined. The type of surface superstructures of thin silicide films has been established.

Superconducting dome revealed by surface structure dependence in single unit cell FeSe on SrTiO3(001)

We report the superconducting properties of single unit cell FeSe prepared on ${\mathrm{SrTiO}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{STO})\phantom{\rule{0.28em}{0ex}}(001)\text{\ensuremath{-}}c(6\ifmmode\times\else\texttimes\fi{}2)$ using scanning tunneling microscopy/spectroscopy (STM/STS). A surface superstructure of $c(6\ifmmode\times\else\texttimes\fi{}2)$ was obtained by annealing the substrate under an oxygen atmosphere. A $3\ifmmode\times\else\texttimes\fi{}1$ periodicity, which corresponds to the topmost Ti atoms of the substrate, was observed in high-resolution STM images, and the superconducting gap size was $11.5\ifmmode\pm\else\textpm\fi{}2.5\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. A nonsuperconducting region with additional $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ periodicity was also found, and this originates from the difference in the arrangement of the Sr and Ti atoms at the surface. We conclude that there is a universal superconducting dome structure in the (electron-doping level)---(superconducting gap size) phase diagram by comparing with previous results that controlled the surface of the substrate/film and the gap size can be maximized/minimized by selecting the appropriate STO surface superstructure.

Molecular beam epitaxy growth and surface structure of Sr1−xNdxCuO2 cuprate films

We report on the epitaxial growth and surface structure of infinite-layer cuprate Sr1-xNdxCuO2 films on SrTiO3(001) substrates by combining ozone-assisted molecular beam epitaxy and in situ scanning tunneling microscopy. Careful substrate temperature and flux control has been used to achieve single-phase, stoichiometric, and c-axis oriented films. The surface of the films is usually characterized by a mixed CuO2 surface and gridlike superstructure. The superstructure exhibits a periodicity of 3.47 nm that corresponds to a coincidence lattice between the overlayer peroxide SrO2 and underlying CuO2 plane, and gives rise to a conductance spectrum that is distinct from the Mott-Hubbard band structure of CuO2. At a higher Nd composition x > 0.1, a (2 x 2) surface characteristic of the hole-doped CuO2 emerges, which we ascribe to the intake of apical oxygens in the intervening Sr planes.

Controllable Manipulation of the Surface Superstructures and Stoichiometry of Single-Crystal CoO(001)

The surface stoichiometry of transition metal oxides plays a vital role in their physical and chemical properties. However, the controllable manipulation of the surface stoichiometry on the host st...

Metallic band structure of CaF2 thin films grown on silicon(1 1 1): Possible formation of CaSi2

We grew epitaxial 10 monolayer (ML) CaF2 films on a Si(111) substrate and measured its electronic structure with angle-resolved photoemission spectroscopy. Reflection high-energy electron diffraction measurements showed the 1×1 streaks of the grown film with no sign of the formation of the Ca-Si surface superstructure at the growth temperature of 400–500 °C. The band dispersion of the films was metallic in contrast to the highly insulating nature of bulk CaF2. The metallic band dispersion did not change with additional Si deposition (1–10 ML) on the CaF2 film. We suggest that CaSi2 may be formed near the surface layers and this shows that the CaF2/Si is not an ideal insulator/semiconductor heterostructure for application in atomically thin electronic devices.

Robust structured Cu-based film catalysts with greatly enhanced catalytic hydrogenation property

Presently, developing efficient and stable structured heterogenous catalysts with fabulous advantages of excellent ability for heat/mass transfer, facile recovery and separation, and superior stability is of great importance in terms of economic production of chemicals. Here, structured Cu-based film catalysts with novel surface superstructure composed of large quantities of interlaced and nearly vertically grown nanosheets for aqueous-phase hydrogenation of furfural toward the production of furfuryl alcohol (FOL) and cyclopentanone (CPO) were successfully fabricated from structured Cu-Zn-Al layered double hydroxide (CuZnAl-LDH) film precursors. As-fabricated structured Cu-based film catalyst possessing a Cu/(Zn + Cu) molar ratio of 4:5 exhibited a superior catalytic performance in the above reaction to other Zn-free and Zn-rich Cu-based film catalysts, as well as pristine CuZnAl-LDH-derived powdered one, due to the synergistic effect between highly dispersive Cu species and favorable surface acidic sites originating from LDH-derived Zn-Al mixed metal oxide (ZnAl-MMO), as well as the presence of abundant Cu-MMO interface within surface nanosheets, being responsible for the enhanced adsorption and activation of reactant and reaction intermediates. Furthermore, due to strong interactions between Cu species and MMO matrix within nanosheets and between surface nanosheets and anodic aluminum oxide/aluminum substrate, the present structured catalyst showed high structural stability and reusability.

Real-Space Imaging of Orbital Selectivity on SrTiO3(001) Surface.

Real-space access of the orbital degree of freedom in complex oxides is still challenging due to intricate electronic hybridization. Here, we report a direct observation of reproducible orbital-selective tunneling on a novel SrTiO3(001) surface by scanning tunneling microscopy. The electronic structures reversibly switch between two different sets of symmetries depending on the sample bias, which is accompanied by a remarkable change in energy-dependent spectroscopy data. Tunneling spectrum combined with density functional theory calculations elucidates that symmetry-breaking at the surface determines the crystal-splitting field of eg/t2g orbitals with a strong in-plane anisotropy so that electrons alternatingly fill eg and t2g orbitals during the imaging process with different biases. This surface superstructure provides a new strategy toward understanding orbital textures and orbital selectivity in complex oxides.

Diamagnetic response of a superconducting surface superstructure: Si(111)- √7×√3 -In

How does one detect the Meissner effect in ultrathin superconducting films that are usually extremely sensitive to air? Here, the authors succeed in observing the diamagnetic response of a superconducting surface superstructure by using an $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ two-coil mutual inductance technique embedded in a specially designed scanning tunneling microscope. The diamagnetic measurement allows for the calculation of the penetration depth, which provides invaluable information about the nature of superconductivity and the symmetry of the underlying order parameter.

How adsorbates alter the metallic behavior of quasi-1D electron systems of the Si(5 5 3)-Au surface

The plasmonic signals of quasi-1D electron systems are a clear and direct measure of their metallic behavior. Due to the finite size of such systems in reality, plasmonic signals from a gold-induced superstructure on Si(5 5 3) can be studied with infrared spectroscopy. The infrared spectroscopic features have turned out to be extremely sensitive to adsorbates. Even without geometrical changes of the surface superstructure, the effects of doping, of the adsorbate induced electronic surface scattering, and of the electronic polarizability changes on top of the substrate surface give rise to measurable changes of the plasmonic signal. Especially strong changes of the plasmonic signal have been observed for gold, oxygen, and hydrogen exposure. The plasmonic resonance gradually disappears under these exposures, indicating the transion to an insulating behavior, which is in accordance with published results obtained from other experimental methods. For C70 and, as shown here for the first time, TAPP-Br, the plasmonic signal almost retains its original intensity even up to coverages of many monolayers. For C70, the changes of the spectral shape, e.g. of electronic damping and of the resonance position, were also found to be marginal. On the other hand, TAPP-Br adsorption shifts the plasmonic resonance to higher frequencies and strongly increases the electronic damping. Given the dispersion relation for plasmonic resonances of 1D electron systems, the findings for TAPP-Br indicate a push-back effect and therefore stronger confinement of the free charge carriers in the quasi-one-dimensonal channel due to the coverage by the flat TAPP-Br molecules. On the gold-doped Si(5 5 3)-Au surface TAPP-Br acts as counter dopant and increases the plasmonic signal.

Superconductivity of single unit cell FeSe/SrTiO3 (001): Substrate-surface superstructure dependence

We report the relationship between the surface superstructure of ${\mathrm{SrTiO}}_{3}$(001) (STO) substrates and the superconducting properties of single unit cell (1-UC) FeSe films using scanning tunneling microscopy/spectroscopy (STM/STS). Under reflection high-energy-electron diffraction observation, we controlled the periodicity of the STO surface to $2\ifmmode\times\else\texttimes\fi{}1$ and $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and grew high-quality FeSe films. We found that the substrate periodicity can be imaged through the 1-UC FeSe from high-resolution STM observations and obtained clear evidence that the superconducting gap size depends on the surface periodicity (10--15 meV on $2\ifmmode\times\else\texttimes\fi{}1$, 12--17 meV on $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$, and 9--13 meV for $2\ifmmode\times\else\texttimes\fi{}1$ domains on the $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ surface). Our results strongly suggest the important role of the film/substrate interface on high-temperature superconductivity.

Bidimensional phases in Co–Pt surface alloys: A theoretical study of ordering and surface segregation

Surface segregation and bulk ordering are characterized by Monte Carlo simulations using a Tight-Binding interatomic potential within the Second Moment Approximation of the density of states (TB-SMA potential) which reproduces the bulk ordered phases and the surface segregation reversal between the dense (111) and (100) surfaces (Pt segregation) and the more opened (110) one (Co segregation). Among the different surface superstructures, most are simply the surface termination of bulk ordered phases and have been observed experimentally, but we predict also purely bidimensional phases such as the (3×3)R30(111) or the c(2 × 2)(110) ones with no equivalence in the bulk alloy. Such (3×3)R30(111) phase should be stable on nanoalloys where the (111) facets are prevailing. Finally, we show that the c(2 × 2) superstructure of the CoPt(100) surface, which represents the mixed variant of the L10 phase, remains stable above the bulk order/disorder critical temperature.

Phonon-induced linewidth of quantum-well states in monolayer Pb on Si(111)

We report a study of the electron–phonon contribution to the linewidth of quantum-well states in a surface superstructure formed by a 4/3-monolayer of Pb on Si(111), a dense phase with a 3×3 unit cell. Ab initio calculations based on the density-functional theory were carried out using a linear response approach in the mixed-basis pseudopotential representation. The phonon-induced contribution to the lifetime broadening is analyzed for both excited electrons and holes. The phonon contribution is found to be generally very sensitive to the energy position of the excited electron (hole) except for unoccupied Pb electronic states inside the Si band gap where the phonon-induced linewidth varies smoothly around ∼ 13 meV irrespective of electron energy. This differs for occupied Pb electronic states, which exhibit larger linewidths, or shorter lifetimes, because numerous phonon-mediated transitions to Si electronic bands substantially increase the electron–phonon coupling.

Publisher's Note: Observation of surface superstructure induced by systematic vacancies in the topological Dirac semimetal Cd3As2 [Phys. Rev. B 95 , 081410(R) (2017)

Publisher's Note: Observation of surface superstructure induced by systematic vacancies in the topological Dirac semimetal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cd</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> [Phys. Rev. B <b>95</b> , 081410(R) (2017)

Publisher's Note: Observation of surface superstructure induced by systematic vacancies in the topological Dirac semimetal Cd3As2 [Phys. Rev. B 95 , 081410(R) (2017)

Received 2 March 2017DOI:https://doi.org/10.1103/PhysRevB.95.119902©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCrystal structureCrystal symmetrySurface & interfacial phenomenaTopological materialsPhysical SystemsDirac semimetalSingle crystal materialsTechniquesDensity functional theoryScanning tunneling microscopyCondensed Matter, Materials & Applied Physics

Sn influence on MBE growth of GeSiSn/Si MQW

Temperature and composition dependencies of the critical thickness of transition from two-dimensional to three-dimensional growth for GeSiSn films on Si(100) with a lattice mismatch of 1 – 5% were experimentally determined. To understand the Sn influence on growth of SiGeSn/Si multi-quantum wells, the phase diagram of surface superstructures during the growth of pure Sn on Si(100) was created. A possibility of synthesizing multilayer structures by molecular beam epitaxy was shown, and the crystal lattice constants were determined using high-resolution transmission electron microscopy. We obtained GeSiSn/Si MQW structures which demonstrated photoluminescence for the Sn content in GeSiSn layers of up to 6%.

A prospective submonolayer template structure for integration of functional oxides with silicon

Epitaxial integration of oxides with the ubiquitous silicon technological platform promises devices with dreamed-of functionality but is difficult to achieve due to chemical processes at the Si/oxide interface. The standard approach suggests a protective template – a surface superstructure of SrSi2 stoichiometry; still, only a handful of oxides couple to Si with atomically abrupt interface. We advocate an alternative approach – the use of metal-rich submonolayer templates as a general recipe, in particular, robust 1×5 reconstructions of Sr and Eu on Si. Their fail-safe synthesis is reported, diffraction techniques and electron microscopy reveal the structure and thermal stability of the reconstructions. The surface protection is discussed in terms of oxidation kinetics. The advantages of the new templates are demonstrated recruiting the example of the paradigmatic spintronic system EuO/Si.