Surface Superstructures Research Articles

Interband interaction between bulk and surface resonance bands of a Pb-adsorbed Ge(001) surface

We investigated the valence band structure of a Pb-adsorbed Ge(001) surface by angle-resolved photoelectron spectroscopy. Three Ge bands, G1, G2, and G3, were observed in a Ge(001) 2 × 1 clean surface. In addition to these three bands, a fourth band (R band) is found on the surface with 2 ML of Pb. The R band continuously appeared even when the surface superstructure was changed. The position of the R band does not depend on Pb coverage. These results indicate that the R band derives from Ge subsurface states, known as surface resonance states. Furthermore, the effective mass of G3 is significantly reduced when the R band exists. We found that this reduction of G3 effective mass was explained by the interaction of the G3 and R bands. Consequently, the surface resonance band is considered to penetrate into the Ge subsurface region affecting the Ge bulk states. We determine the hybridization energy to be 0.068 eV by fitting the observed bands.

Surface structure of thin pseudomorphous GeSi layers

Reflection high-energy electron diffraction (RHEED) was used to study the evolution of thin GexSi1−x film surface superstructures s in the course of molecular beam epitaxy. The (2×N) superstructure of the epitaxial film surface at periodicity N from 14 to 8, the latter being characteristic of pure germanium at the Si(100) surface. The epitaxial film thickness that is required for the formation of the (2×8) superstructure depends on the deposition temperature and germanium content in the solid solution. The germanium segregation on the growing film surface is shown to be responsible for the observed superstructural changes.

Formation of nanosize silicides films on the Si(111) and Si(100) surfaces by low-energy ion implantation

The formation of nanosize silicides films by implantation of B, P, Ba, and alkali metal atoms in Si(111) and Si(100) followed by thermal annealing is studied by electron spectroscopy and slow-electron diffraction methods. It is shown that implantation of ions with a large dose D > 1016 cm−2 and short-term heating lead to the formation of thin silicides films with new surface superstructures: \(Si(111) - (\sqrt 3 \times \sqrt 3 )R30^ \circ - B\), Si(100)-2 × 2Ba, Si(111)-1 × 1P, etc.

Novel Ultrathin Nanoflake Assembled Porous MnO2/Carbon Strip Microspheres for Superior Pseudocapacitors

A novel hierarchical MnO2/carbon strip (MnO2/C) microsphere is synthesized via galvanostatic charge–discharge of a MnO@C matrix precursor where the carbon is from a low-cost citric acid. This hierarchical structure is composed of manganese oxides nanoflakes and inlaid carbon strips. The ultrathin nanoflakes assemble to form porous microspheres with a rippled surface superstructure. Due to its improved conductivity and remarkable increased phase contact area, this novel structure exhibits an excellent electrochemical performance with a specific capacitance of 485.6 F g–1 at a current density of 0.5 A g–1 and an area capacitance as high as 4.23 F cm–2 at a mass loading of 8.7 mg cm–2. It also shows an excellent cycling stability with 88.9% capacity retention after 1000 cycles. It is speculated that the present low-cost novel hierarchical porous microspheres can serve as a promising electrode material for pseudocapacitors.

Ordered surface-alloys formation in the Zr/W(1 0 0) adsorption system

The Zr/W(100) adsorption system was examined in situ, applying STM, LEED and AES techniques. Zr mono-layers of average thickness 1.7–2.0, vapor-deposited under UHV on W(100) surface, were gradually annealed from 1200 to 2100K. Up to 1700K Zr layers were isomorphic with the substrate. Two surface superstructures of W(100) –(5×5)R26.6 and W(100)−c(2×4) were identified one at the temperature range 1700–2000K, the other above 2000K. The analysis of the experimental data combined with the ab initio structural calculations made it possible to ascribe the superstructures respectively to Zr4W and Zr3W ordered surface alloys.

Anisotropic electronic conduction in metal nanofilms grown on a one-dimensional surface superstructure

We performed in situ conductivity measurements by multiple microscopic probes of Ag nanofilms grown on a periodic array of indium atomic wires on Si(111). Within the investigated thickness range the transport properties differ markedly from the bulk case. The ratio between the in-plane conductivity along and perpendicular to the In wires is 1.4 for the 3 monolayer (0.7 nm) thick Ag film and approaches unity with increasing film thickness. The observed anisotropy at the initial stage of the film growth is far larger than expected from the quasi-one-dimensional quantum well electronic structure of the films and ascribed to the carrier scattering at the film/vacuum and film/substrate interfaces. Our findings show that an ordered monolayer inserted at the interface enables drastic changes of the transport behavior of the whole metal nanofilm.

Reflection high energy electron diffraction studies on SixSnyGe1 − x − y on Si(100) molecular beam epitaxial growth

In situ reflection high energy electron diffraction was used to study the surface micromorphology of SixSnyGe1−x−y/Si(100) heterostructures obtained by molecular beam epitaxy. The obtained reflection high energy electron diffraction data allowed us to conclude that the epitaxial SixSnyGe1−x−y films are grown by the Stranski–Krastanov mechanism. It was established that SixSnyGe1−x−y and Ge1−zSnz wetting layer thicknesses depend on the substrate temperature. The observed surface superstructures changed during the wetting layer growth.

Influence of Si(111) surface superstructure on quantum size effect in Au films

Abstract The influence of Si/Au interface on the appearance of the quantum size effect (QSE) in ultrathin Au films on Si(1 1 1) surface is investigated. Angle-resolved photoemission measurements indicate QSE states in Au layers grown on the Si(1 1 1) surface with the (7 × 7), (6 × 6)Au and ( 3 × 3 ) Ag reconstructions and show no QSE for the ( 3 × 3 ) Pb reconstruction. Reflection high energyelectron diffraction patterns indicate that the existence of the QSE states depends on the Au film morphology.

From novel PtSn/Pt(110) surface alloys to SnOx/Pt(110) nano-oxides

UHV evaporation of Sn on (1 × 2) Pt(110), followed by UHV annealing, provides three different PtSn/Pt(110) surface alloys, each characterized by a specific low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) pattern. When annealed in a controlled oxygen background, the PtSn/Pt(110) surface alloys can be transformed into SnOx/Pt(110) nano-oxides. This paper reports the experimental phase diagram of the metallic and oxidized surface superstructures.

Model of the nucleation and growth of amorphous and crystalline films of diamond-like materials: The (100) plane

The nucleation and growth processes of amorphous and crystalline films of diamond-like materials deposited on the (100) planes of monocrystals with a diamond structure of diamond are modeled. The mechanisms of the formation of monolayers are analyzed from the results of calculating the potential energies of carbon clusters by means of modern semiempirical quantum chemical methods of computer chemistry. The nucleation and growth of an epitaxial film is considered as a replication process, and the deposition of amorphous films is described from the viewpoint of modifying the surface superstructure. Equations establishing the relationship between the structure of amorphous and crystalline films and conditions of the synthesis are obtained as a result of the performed studies.

Surface Electrical Conductivity Measurement System with Micro-Four-Point Probes at Sub-Kelvin Temperature under High Magnetic Field in Ultrahigh Vacuum

We have constructed an ultrahigh vacuum system in which surface conductivity is measured in situ by microfour-point probe method down to 0.8 K, combined with conventional surface preparation/analysis capability. In order to reduce the data scattering due to changes of the probe spacing, we have employed a dual configuration method in which the combinations with current/voltage probes are switched. The surface-state superconductivity of Si(111)- √ 7 × √ 3-In surface superstructure was confirmed with the critical temperature of 2.8 K. The critical magnetic field was 0.3-0.4 T in the surface-normal direction. [DOI: 10.1380/ejssnt.2012.400]

Electronic structure of graphene on single-crystal copper substrates

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low-energy electron microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of $\ensuremath{\sim}$$\ensuremath{-}$0.3 eV and the opening of a $\ensuremath{\sim}$250 meV gap. Exposure of the samples to air resulted in intercalation of oxygen under the graphene on Cu(100), which formed a ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}2\sqrt{2}$)R45${}^{\mathrm{o}}$ superstructure. The effect of this intercalation on the graphene $\ensuremath{\pi}$ bands is to increase the offset of the Dirac crossing ($\ensuremath{\sim}$$\ensuremath{-}$0.6 eV) and enlarge the gap ($\ensuremath{\sim}$350 meV). No such effect is observed for the graphene on the Cu(111) sample, with the surface state at $\ensuremath{\Gamma}$ not showing the gap associated with a surface superstructure. The graphene film is found to protect the surface state from air exposure, with no change in the effective mass observed, as for one monolayer of Ag on Cu(111).

Scanning tunneling microscopy observation of surface superstructures during the growth of In on In/Si(111) surface

Surface superstructures are studied with scanning tunneling microscopy during the growth of In on In/Si(111). On the inhomogeneous substrate of Si(111) 4 × 1/ 3 × 3 -In coexisting surface, the deposition of 1.5 monolayer (ML) In at about 0 °C leads to 7 × 3 reconstruction on 3 × 3 surface and the formation of one-dimensional nanowires on 4 × 1 surface (1.0 ML = 7.8 × 10 14 atoms/cm 2). Subsequent deposition of In at − 100 °C gives rise to the appearance of a hexagonal superstructure with 37 × 4 3 periodicity on 7 × 3 reconstructed surface, while self-alignment of In dots along one-dimensional nanowires is observed on the initial 4 × 1 surface. On Si(111) 3 × 3 -In surface, the growth of 2.5 ML In at about − 100 °C yields 6 × 6 superstructure. The strain in the epitaxial In thin films provides a driving force for the formation of self-organized surface structures.

Computational design of virus-like protein assemblies on carbon nanotube surfaces.

There is a general need for the engineering of protein-like molecules that organize into geometrically specific superstructures on molecular surfaces, directing further functionalization to create richly textured, multilayered assemblies. Here we describe a computational approach whereby the surface properties and symmetry of a targeted surface define the sequence and superstructure of surface-organizing peptides. Computational design proceeds in a series of steps that encode both surface recognition and favorable intersubunit packing interactions. This procedure is exemplified in the design of peptides that assemble into a tubular structure surrounding single-walled carbon nanotubes (SWNTs). The geometrically defined, virus-like coating created by these peptides converts the smooth surfaces of SWNTs into highly textured assemblies with long-scale order, capable of directing the assembly of gold nanoparticles into helical arrays along the SWNT axis.

Chirality Transfer from a Single Chiral Molecule to 2D Superstructures in Alaninol on the Cu(100) Surface

The formation of 2D chiral monolayers obtained by self-assembly of chiral molecules on surfaces has been widely reported in the literature. Control of chirality transfer from a single molecule to surface superstructures is a challenging and important aspect for tailoring the properties of 2D nanostructures. However, despite the wealth of investigations performed in recent years, how chiral transfer takes place on a large scale still remains an open question. In this paper we report a coupling of scanning tunneling microscopy and low energy electron diffraction measurements with an original theoretical approach, combining molecular dynamics and essential dynamics with density functional theory, to investigate self-assembled chiral structures formed when alaninol adsorbs on Cu(100). The peculiarity of this system is related to the formation of tetrameric molecular structures which constitute the building blocks of the self-assembled chiral monolayer. Such characteristics make alaninol/Cu(100) a good candidate to reveal chiral expression changes. We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage or when geometrical confinement prevents long-range order. Conversely, a rotation of 14° with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. An insight to the process leading to a 2D globally chiral surface has been obtained by monitoring molecular assemblies as they grow from the early stages of adsorption, suggesting that the distinctive orientation of the self-assembled monolayer originates from a balance of cooperating forces which start acting only when tetramers pack together to form small clusters.

Ultraviolet photoelectron spectroscopic study of boron adsorption and surface segregation on Si(111)

The adsorption and surface segregation behavior of elemental boron (B) deposited on Si(111) have been studied by ultraviolet photoelectron spectroscopy and accompanying reflection high-energy electron diffraction as a function of the B coverage (${c}_{B}$) and annealing temperature ($T$). Our results clearly demonstrate an effective incorporation of B into subsurface sites at $T$ g 800 K and formation of a well-ordered $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ifmmode^\circ\else\textdegree\fi{}}$ surface superstructure. Thereby, a critical ${c}_{B}$ of about 0.6 monolayers (ML) was determined for different conditions to prevent surface defects resulting from Si dangling bonds, which appear as surface states at 0.4 eV below the Fermi level. Annealing of the defect-free $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ifmmode^\circ\else\textdegree\fi{}}$ superstructure covered by several MLs Si at $T$ \ensuremath{\geqslant} 1040 K results in a renewal of the perfect B-induced Si surface structure, even after several deposition and annealing cycles. This indicates a dominance of B surface segregation over bulk diffusion. Significant B bulk diffusion commences only above 1100 K. Differences were found for spectra obtained for the B-induced surface structures formed after deposition and surface segregation, respectively. An additional surface state appeared at 2.1 eV below the Fermi level after deposition of 0.6 ML B. The state could be attributed to boron-boron interaction due to the presence of small clusters at the surface. This surface state did not disappear after high-$T$ annealing and was not observed for lower ${c}_{B}$ and after B surface segregation, respectively.

SrTiO3(100) − √5 × √5 − R26.6 surface observed by high-resolution scanning tunneling microscopy

SrTiO3(100)–(√5×√5)–R26.6 surfaces were studied by means of high-resolution scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. By varying the bias voltage in the occupied state, it was possible to observe the arrangement of titanium and oxygen atoms in the unit cells of a (√5×√5) surface superstructure, which revealed that the TiO2 layer is the terminating plane of the (√5×√5) surface. In the STM images, peculiar protrusions were seen at the oxygen fourfold hollow site responsible for √5×√5 periodicity. The protrusions are asymmetrical in contrast, which would be an important consideration in proposing accurate structural models for (√5×√5) surface superstructures.

Towards controlled molecular beam epitaxial growth of artificially stacked Si: Study of boron adsorption and surface segregation on Si(1 1 1)

Creating rotation twins periodically in a defined distance within Si layers could lead to the formation of miscellaneous Si crystal structures. This could be realized by several growth and annealing cycles on heavily B-covered Si(111) exhibiting (√3×√3)R30° surface superstructure. However, surface defects due to imperfections of the B-induced surface structure give rise to an inhomogeneous Si nucleation, which limits the structure size. Therefore, surface structure formation induced by both adsorption and surface segregation of B on Si(111) and its influence on the Si molecular beam epitaxial growth mode has been investigated using ultraviolet photoelectron spectroscopy and accompanying reflection high-energy electron diffraction. Based on these studies, conditions have been established to prevent surface defects. Furthermore, annealing samples with 0.6 monolayers (ML) B buried below several ML Si at 1080K results in a renewal of the B-induced Si surface structure without any defects. This indicates a dominance of B surface segregation over bulk diffusion, which becomes significant only above 1100K.

Magnetic anisotropy of Fe and Co adatoms and monolayers: Need for a proper treatment of the substrate

The magnetocrystalline contribution to the magnetic anisotropy energy (MAE) of Fe and Co adatoms, monolayers, and surface superstructures on Pt(111) is investigated. It is shown that the thickness of the slab representing the substrate and the interaction between the atoms in neighboring surface supercells affect the calculated MAE much more profoundly than they affect magnetic moments. Reliable theoretical values of MAE cannot be obtained if the substrate is represented by a slab of less than about ten atomic layers. If a surface superstructure is meant to represent an adatom by means of supercell approach, then decoupling has to be ensured by using very large supercells.

Vibrational states of the Pt(111)– $$ \left( {\sqrt {3} \times \sqrt {3} } \right) $$ R30°–K surface structure

Vibrational spectrum of the ordered Pt(111)– $$ \left( {\sqrt {3} \times \sqrt {3} } \right) $$ R30°–K surface superstructure formed on the platinum surface with adsorption of 1/3 ML potassium is calculated with the use of the interatomic interaction potentials obtained in the strong bond approximation. Relaxation of the surface, dispersion of the surface phonons, local density of vibrational states, and polarization of phonon modes of adatoms and atoms of the substrate are discussed in the work. The theoretical results obtained agree well with the available experimental data.