Island Height Research Articles

Observation of the quantum-confinement effect in individual β-FeSi2 nanoislands epitaxially grown on Si (111) surfaces using scanning tunneling spectroscopy

The quantum-confinement effect in two-dimensional β-FeSi2 nanoislands epitaxially grown on Si (111) by codeposition of Fe and Si was observed using scanning tunneling spectroscopy at room temperature. The energy band gaps of the H-terminated β-FeSi2 nanoislands increased by approximately 0.4eV when island height decreased from 5to2nm. This size dependence was explained by the quantum-confinement effect in β-FeSi2 nanoislands.

The role of precursor gases on the surface restructuring of catalyst films during carbon nanotube growth

Catalyst films undergo considerable surface morphology restructuring prior to carbon nanotube nucleation, deeply influencing the nanostructures obtained. Here we study the influence of different gaseous atmospheres on the structure of thin Fe films. The morphology is influenced by process temperature and substrate interactions and varying the gas type and pressure can control the average catalyst island height.

Electronic properties of Au nano-particles supported on stoichiometric and reduced TiO 2(1 1 0) substrates

Growth modes and electronic properties were analyzed for Au nano-particles grown on stoichiometric and reduced TiO 2(1 1 0) substrates by medium energy ion scattering (MEIS) and photoelectron spectroscopy(PES) using synchrotron-radiation-light. Initially, two-dimensional islands (2D) with a height of one and two atomic layers grow and higher coverage increases the islands height to form three-dimensional (3D) islands for the stoichiometric TiO 2(1 1 0) substrate. In contrast, 3D islands start to grow from initial stage with a small Au coverage (⩾0.1 ML, 1 ML = 1.39 × 10 15 atoms/cm 2: Au(1 1 1)) probably due to O-vacancies acting as a nucleation site. Above 0.7 ML, all the islands become 3D ones taking a shape of a partial sphere and the Au clusters change to metal for both substrates. We observed the Au 4f and Ti 3p core level shifts together with the valence band spectra. The Ti 3p peak for the O-deficient surface shifts to higher binding energy by 0.25 ± 0.05 eV compared to that for the stoichiometric surface, indicating downward band bending by an electron charge transfer from an O-vacancy induced surface state band to n-type TiO 2 substrate. Higher binding energy shifts of Au 4f peaks observed for both substrates reveal an electron charge transfer from Au to TiO 2 substrates. The work functions of Au nano-particles supported on the stoichiometric and reduced TiO 2 substrates were also determined as a function of Au coverage and explained clearly by the above surface and interface dipoles.

Ostwald ripening of manganese silicide islands on Si(001)

The deposition of Mn onto Si(001) in the submonolayer regime has been studied with scanning tunneling microscopy to gain insight into the bonding and energetics of Mn with Si. The as-deposited Mn films at room temperature are unstructured. Upon annealing to 300–700 °C three-dimensional islands of Mn or MnxSiy form while between the islands the Si(001)-(2×1) reconstruction becomes visible. With increasing annealing time the density of islands per surface area decreases while the average height of the remaining islands increases. The large islands grow in size at the expense of the small ones, which can be understood in the context of Ostwald [Z. Phys. Chem. 34, 495 (1900)] ripening theory. The average island height shows a time dependence of H∼t1∕4, indicating that surface diffusion is the growth limiting process.

Edge-dimer row – The reason of three-bilayer steps and islands stability on Si(1 1 1)-7 × 7

A hypothesis of perpendicular dimer row formation along three-bilayer (3 BL) step was suggested. The hypothesis, explains the stability of 3 BL steps on the vicinal Si(1 1 1) surface deflected in 〈 1 ¯ 1 ¯ 2 〉 direction as well as the limitation of Ge and Si island height by 3 BL at the initial nucleation stages on Si(1 1 1) surface. The detailed examinations of STM images of 3 BL steps were carried out. New peculiarities of atomic structure of 3 BL single step on Si(1 1 1) and 3 BL steps on Si(5 5 7) surfaces were revealed. The results of STM images examination verify the hypothesis of perpendicular dimer row formation along the boundary of the 3 BL step.

Influence of Quantum Size Effects on Island Coarsening

Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si(111)-(7 x 7). It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times tau are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., tau is approximately 1/F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.

In situ monitoring of internal strain and height of InAs nanoislands grown on GaAs(001)

A monitoring technique for molecular beam epitaxial growth of InAs∕GaAs(001) nanoislands is presented. With the help of synchrotron radiation, x-ray diffraction intensity mapping in reciprocal space has been measured during growth. The internal strain distribution and height of the Stranski-Krastanov islands were monitored at a temporal resolution of 9.6s. The relaxation process of internal strain inside the Stranski-Krastanov islands displayed significant dependence on the growth temperature.

Self-organization and geometry control of Pb islands grown on anisotropic substrates

The growth of uniform nanostructures requires discovering robust and reproducible ways to control the grown size and geometry. It is found that the growth of Pb on the anisotropic substrate Si(111)-In(4x1) leads to the control of both the island height and island width by exploiting the combined effects of two different stabilization mechanisms. Quantum size effects control the height and result in uniform four-layer islands while the strain anisotropy, due to the underlying reconstruction, controls the width selection.

Strain and electronic structure of Ge nanoislands on Si(111)-7×7surface

To study the relation between the strain and the electronic structure in two-dimensional (2D) Ge nanoislands on the Si(111)-$7\ifmmode\times\else\texttimes\fi{}7$ surface, we calculated the relation based on the density functional theory and measured with the angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and the scanning tunneling microscopy (STM). In the calculation, the dangling-bond state at the strained adatom on the 2D Ge island (${S}_{Ge}^{R}$ state) shifts to lower energy at the rate of $\ensuremath{-}8.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{nm}$ with the height of the adatoms. The ARUPS spectrum shows a characteristic peak corresponding to the ${S}_{Ge}^{R}$ state, whose energy depends on the size of the Ge nanoisland. The state appears at $\ensuremath{-}0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level for the small Ge islands (3.8--5 nm) and at $\ensuremath{-}0.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the large Ge islands. The height of the Ge islands estimated from the energy difference is consistent with a result of the STM measurement. The strain of Ge islands can be estimated from the electronic structure.

Quantum Size Effects in Metal Thin Films Grown on Quasicrystalline Substrates

We have investigated by scanning tunneling microscopy the growth of Bi and Ag thin films on the fivefold surface of Al63Cu24Fe13 and Al72Pd19.5Mn8.5 quasicrystal, respectively. For both systems, we observe the formation of islands with magic height, corresponding to the stacking of a specific number of atomic layers. We interpret this unusual growth morphology in terms of quantum size effects, arising from the confinement of the electron within the film. The magic island heights are thus a direct manifestation of the electronic structure of the quasicrystalline substrates.

Quantum size effects in the surface energy ofPb∕Si(111)film nanostructures studied by surface x-ray diffraction and model calculations

We have used surface x-ray diffraction from a synchrotron source, along with models based upon a free-electron gas confined to a quantum well, to study quantum size effects in the surface energy of ultrathin Pb films grown on pretreated Si(111) substrates. Films grown at $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are smooth, but as they are annealed to near room temperature, their morphology is observed evolving through various metastable states and eventually to a roughened state in local equilibrium. Strong variations in the stability of different island heights are observed, consistent with quasibilayer oscillations in the surface energy found from the theoretical free-electron calculations. By analyzing the quasiequilibrium distribution of thicknesses, empirical information on the film surface energy is obtained for a wide range of thicknesses. The morphological annealing behavior of the films is also found to be explained by the deduced surface energy.

Central Island Treatment Using Technolas 217 Based on Orbscan II Assessment

To present a case of central island after myopic laser in situ keratomileusis and the treatment technique using Technolas 217 laser based on topography findings. Serial corneal topographies with Orbscan were taken and an ablation profile was generated with subsequent treatment using the phototherapeutic keratectomy mode and surface ablation technique. Marked reduction was noted in the height of central island and the best spectacle-corrected visual acuity improved from 20/40 to 20/25. Ablation profile based on corneal topography can provide a possible treatment option for patients with central island.

Poly(ethylene glycol) Self-Assembled Monolayer Island Growth

Here, we report a study of the morphology and growth dynamics of a self-assembled monolayer (SAM) of the amide containing poly(ethylene glycol) (PEG) thiol (CH3O(CH2CH2O)17NHCO(CH2)2SH) on atomically flat Au(111) surfaces. SAM growth from a 20 muM ethanolic solution reveals island growth through three distinct steps: island nucleation, island growth, and coalescence. The coalescence-step, filling voids in the SAM, is by far slowest. The fine structure study reveals dendritic island formation, an observation which can be explained by attractive intermolecular interactions and surface diffusion-limited aggregation. We have also observed a change in the island height, which peaks during the island growth phase. This height change can be associated with a molecular conformational transition.

Human foetal osteoblastic cell response to polymer-demixed nanotopographic interfaces

Nanoscale cell-substratum interactions are of significant interest in various biomedical applications. We investigated human foetal osteoblastic cell response to randomly distributed nanoisland topography with varying heights (11, 38 and 85 nm) produced by a polystyrene (PS)/polybromostyrene polymer-demixing technique. Cells displayed island-conforming lamellipodia spreading, and filopodia projections appeared to play a role in sensing the nanotopography. Cells cultured on 11 nm high islands displayed significantly enhanced cell spreading and larger cell dimensions than cells on larger nanoislands or flat PS control, on which cells often displayed a stellate shape. Development of signal transmitting structures such as focal adhesive vinculin protein and cytoskeletal actin stress fibres was more pronounced, as was their colocalization, in cells cultured on smaller nanoisland surfaces. Cell adhesion and proliferation were greater with decreasing island height. Alkaline phosphatase (AP) activity, an early stage marker of bone cell differentiation, also exhibited nanotopography dependence, i.e. higher AP activity on 11 nm islands compared with that on larger islands or flat PS. Therefore, randomly distributed island topography with varying nanoscale heights not only affect adhesion-related cell behaviour but also bone cell phenotype. Our results suggest that modulation of nanoscale topography may be exploited to control cell function at cell-biomaterial interfaces.

Heteroepitaxial Nucleation and Oriented Growth of Manganese Oxide Islands on Carbonate Minerals under Aqueous Conditions

Manganese redox cycling and the accompanying dissolution and precipitation reactions are important processes in natural waters. In the present study, Mn2+ (aq) is reacted with O2(aq) at circumneutral pH to form manganese oxide islands on the (1014) surface of MnCO3. The islands grow heteroepitaxially. The effects of the substrate surface morphology, the substrate atomic structure, and the aqueous concentration of Mn2+ are investigated. On terraces, rhombohedral oxide islands form with 90 degrees rotation relative to the crystallographic axis of the underlying carbonate substrate. Although the island heights self-limit between 2 and 3 nm depending on reaction conditions, the islands grow laterally to several square microns before separate islands collide and coalesce. The islands do not grow over substrate steps or down dissolution-pit edges. Comparison studies done with MgCO3 and CaCO3 show that the former also promotes heteroepitaxial growth whereas the latter does not. This difference is explained by the relative bond length mismatch between the structures of the carbonate substrates and the atomic structures of manganese oxides. A free energy model is also presented to explain why the heights of the manganese oxide islands self-limit. Our results provide an improved basis both for the development of predictive models of contaminant fate and transport and for the modeling of hydraulic flow through carbonate aquifers.

Effect of Island Size on the Packing Density in the Early Stages of Alkylsilane-Based Monolayer Self Assembly

The early stage of the self assembly for octadecyltrichlorosilane (OTS)-based monolayers is investigated using atomic force microscopy (AFM). Height measurements using AFM prove that the island height of the monolayers gradually increases with increasing the island size, and is close to the limiting value (h = <TEX>$\sim$</TEX>25 <TEX>$\AA$</TEX>) after d = <TEX>$\sim$</TEX>600 nm in size. Since the theoretical length of a covalently bound OTS molecule is 26.2 Å, the limiting value of the island height means that the islands with d <TEX>${\geq}$</TEX> 600 nm consist of close-packed, fully extended chains. The heights for the islands with d < 600 nm are lower than the limiting value and decrease with decreasing the island sizes. This observation indicates that the OTS molecules in the small islands are less densely packed, and that the packing densities of the islands increase as the islands grow in size.

スマトラ沖地震津波によるタイ西部海岸での被害実態調査の報告

The great earthquake of M9 was generated in the North Sumatra offing at 7: 58 on December 26th, 2004, and the serious tsunami calamity was produced in every coast of Indian Ocean. We report on the damage survey that we carried out from January 9th to 12th in Thai western coast where typical tsunami damage was produced.(1) The tsunami height of Khoa Lak coast was about 9m on the sea level, and almost all walls and pillars of buildings within 30m of coastal line were toppled. The flood region had reached to lkm or more inland.(2) The tsunami height of Patong beach was about 6m on the sea level, and almost all walls and pillars of buildings within 30m of coastal line were not toppled but the flood depth was about 2m.(3) The tsunami height of Phi Phi island is 6-7m on the sea level at the north side and 3.5-5m on the sea level at the south side.

The evolution of the structure of quantum size effect Pb nanocrystals on Si(1 1 1) 7×7

We have carried out diffuse X-ray scattering measurements of the growth of Pb nanocrystalline islands on Si(1 1 1). Analysis of our data shows that islands growing on an initially rough wetting layer transforms the portion of the wetting layer below them into ordered fcc sites. Therefore, the islands grow directly on top of the Si surface with a disordered wetting layer occupying the region between the islands and, consequently, the island height responsible for the quantum well depth is one layer thicker than reported by LEED and STM. These islands have an extremely good vertical order until the islands coalesce into a closed film. At that point the disorder of the film increases consistent with misfit strain relaxation.

Structural and optical properties of InAs/GaAs quantum dots emitting at 1.5 μm

We have demonstrated 1.5 μm light emission from InAs quantum dots (QDs) capped with a thin GaAs layer. The extension of the emission wavelength can be assigned to the large QD height. We also investigate the effect of growth interruption on the PL properties and the shape of InAs QDs fabricated by migration-enhanced growth (MEG). Contrary to expectation, we observed a remarkable blueshift of the emission energy with the growth interruption in MEG mode. Detailed investigations reveal that the blueshift is related to the reduced island height with the growth interruption, which is confirmed by reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) measurement results. Accordingly, the structure changes of the islands are interpreted in terms of thermodynamic and kinetic theories.

Wetting-layer transformation for Pb nanocrystals grown on Si(111)

We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si(111). It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. These results have important implications for predicting stable Pb island heights.