Metallic thin fi lms on stepped surfaces: lateral scattering of quantum well states

Quantum well states of Ag films grown on stepped Au(111) surfaces are shown to undergo lateral scattering, in analogy with surface states of vicinal Ag(111). Applying angle resolved photoemission spectroscopy we observe quantum well bands with zone-folding and gap openings driven by surface/interface step lattice scattering. Experiments performed on a curved Au(111) substrate allow us to determine a subtle terrace-size effect, i.e., a fine step-density-dependent upward shift of quantum well bands. This energy shift is explained as mainly due to the periodically stepped crystal potential offset at the interface side of the film. Finally, the surface state of the stepped Ag film is analyzed with both photoemission and scanning tunneling microscopy. We observe that the stepped film interface also affects the surface state energy, which exhibits a larger terrace-size effect compared to surface states of bulk vicinal Ag(111) crystals

Corrugated surfaces formed on GaAs (331)a substrates: The template for laterally ordered InGaAs nanowires

Morphology evolution of high-index (331)A surfaces during molecular beam epitaxy (MBE) growth have been investigated in order to uncover their unique physic properties and fabricate spatially ordered low dimensional nanostructures. Atomic Force Microscope (AFM) measurements have shown that the step height and terrace width of GaAs layers increase monotonically with increasing substrate temperature in conventional MBE. However, this situation is reversed in atomic hydrogen-assisted MBE, indicating that step bunching is partly suppressed. We attribute this to the reduced surface migration length of Ga adatoms with atomic hydrogen. By using the step arrays formed on GaAs (331)A surfaces as the templates, we fabricated laterally ordered InGaAs self-aligned nanowires.

Untersuchung magnetischer Nanostrukturen mittels Rastertunnelspektroskopie und Kerr-Magnetometrie am Beispiel von Fe, Co, Co-Fe und Fe-Mn Nanostrukturen

Nach einer kurzen Einführung in die Entwicklung der magnetischen Anwendungen, werden in Kapitel 2 und 3 die physikalischen Grundlagen der Messmethoden, insbesondere die Rastertunnelspektroskopie und Kerr-Magnetometrie, sowie der experimentelle Aufbau erläutert. Kapitel 4 beschäftigt sich mit den magnetischen Eigenschaften von quasi ein-dimensionalen ferromagnetischen Nanostreifen und Monolagen, die durch Selbstorganisation auf einem Wolfram(110)-Einkristall mit vizinaler und glatter Oberfläche präpariert werden. Hierbei wird die Temperaturabhängigkeit der magnetischen Größen, wie Remanenz, Sättigungsmagnetisierung und Suszeptibilität, sowie die Auswirkung einer Abdeckung des Systems auf die Domänenwandenergie und Anisotropie untersucht. Zusätzlich wird die Kopplung von parallelen Nanostreifen in Abhängigkeit des Streifenabstandes betrachtet. In Kapitel 5 werden das Wachstum und die Morphologie von Co-Monolagen auf W(110) untersucht. Der Übergang von pseudomorphem zu dicht gepacktem Wachstum in der Monolage wird mit Hilfe der Rastertunnelspektroskopie sichtbar gemacht, ebenso wie unterschiedliche Stapelfolgen in Tripellagen Co-Systemen. Atomar aufgelöste Rastertunnelmikroskopie erlaubt die genauen Atompositionen der Oberfläche zu bestimmen und mit theoretischen Wachstumsmodellen zu vergleichen. Auf die Untersuchung zwei-dimensionaler binärer Co-Fe und Fe-Mn Legierungen auf W(110) wird in Kapitel 6 eingegangen. Mit einer Präparationstemperatur von T=520 K ist es möglich, atomar geordnete Co-Fe Legierungsmonolagen wachsen zu lassen. Ein direkter Zusammenhang zwischen der Magnetisierung und der lokalen Zustandsdichte in Abhängigkeit der Legierungszusammensetzung wird gezeigt.

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520 degrees C, and then exhibits a sudden decrease at 535 degrees C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates. (c) 2006 American Institute of Physics.

Initial growth stages of epitaxial BaTiO3 films on vicinal SrTiO3 : Nb (001) substrates

The growth mechanism of epitaxial BaTiO3 films on vicinal Nb-doped SrTiO3 (srTiO(3):Nb) (001) substrate surfaces was studied in terms of surface morphology, crystalline orientation, microstructure, and film/substrate interface. Well-oriented BaTiO3 thin films were grown on SrTiO3 substrates with well-defined terraces by pulsed laser deposition. The regularly terraced TiO2-terminated surfaces of vicinal SrTiO3:Nb (001) substrates were prepared by a definite chemical and thermal treatment. Under our conditions, BaTiO3 seems to grow with a layer-then-island (Stranski-Krastanov) growth mechanism. In order to investigate the orientation and crystallinity of the BaTiO3 films, x-ray diffraction and high-resolution transmission election microscopy were performed. Ferroelectricity of the BaTiO3 films was proved by electrical measurements performed on Pt/BaTiO3/SrTiO3:Nb heterostructures.

Voltammetric characterization of stepped platinum single crystal surfaces vicinal to the (110) pole

Platinum stepped surfaces vicinal to the (1 1 0) crystallographic pole have been investigated voltammetrically in 0.1 M HClO(4) and 0.1 M H(2)SO(4) solutions. Changes in the voltammetric profile with the step density suggest the existence of two types of surface sites, that has been ascribed to linear and bidimensional domains. This result indicates the existence of important restructuring processes that separate the real surface distribution from the nominal one. The electronic properties of the surfaces have been characterized with the CO charge displacement method and the potential of zero total charge has been calculated as a function of the step density. (c) 2009 Elsevier B.V. All rights reserved.

Oxidation of ethanol and its derivatives on well defined pt single crystal electrodes vicinal to pt(111): a comparative study

The oxidation of ethanol (EtOH) at Pt(111) electrodes is dominated by the 4e path leading to acetic acid. The inclusion of surface defects such as those present on stepped surfaces leads to an increase of the reactivity towards the most desirable 12e path leading to CO2 as final product. This path is also favored when the methyl group is more oxidized, as in the case of ethylene glycol (EG) that spontaneously decomposes to CO on Pt(111) electrodes, thus showing a more effective breaking of the C-C bond. Some trends in reactivity can be envisaged when other derivative molecules are compared at well-ordered electrodes. This strategy was used in the past, but the improvement in the electrode pretreatment and the overall information available on the subject suggest that relevant information is still missing.

In situ control of Si(100) and Ge(100) surface preparation for the heteroepitaxy of III-V solar cell architectures

Si(100) and Ge(100) substrates essential for subsequent III-V integration were studied in the hydrogen ambient of a metalorganic vapor phase epitaxy reactor. Reflectance anisotropy spectroscopy (RAS) enabled us to distinguish characteristic configurations of vicinal Si(100) in situ: covered with oxide, cleaned by thermal removing in H2, and terminated with monohydrides when cooling in H2 ambient. RAS measurements during cooling in H2 ambient after the oxide removal process revealed a transition from the clean to the monohydride terminated Si(100) surface dependent on process temperature. For vicinal Ge(100) we observed a characteristic RA spectrum after annealing and cooling in H2 ambient. According to results from X-ray photo electron spectroscopy and Fourier-transform infrared spectroscopy the spectrum corresponds to the monohydride terminated Ge(100) surface.

In situ control of the GE(100)surface domain structure for III-V multijunction solar cells

Vicinal Ge(100) is the common substrate for state of the art multi-junction solar cells grown by metal-organic vapor phase epitaxy (MOVPE). While triple junction solar cells based on Ge(100) present efficiencies mayor que 40%, little is known about the microscopic III-V/Ge(100) nucleation and its interface formation. A suitable Ge(100) surface preparation prior to heteroepitaxy is crucial to achieve low defect densities in the III-V epilayers. Formation of single domain surfaces with double layer steps is required to avoid anti-phase domains in the III-V films. The step formation processes in MOVPE environment strongly depends on the major process parameters such as substrate temperature, H2 partial pressure, group V precursors [1], and reactor conditions. Detailed investigation of these processes on the Ge(100) surface by ultrahigh vacuum (UHV) based standard surface science tools are complicated due to the presence of H2 process gas. However, in situ surface characterization by reflection anisotropy spectroscopy (RAS) allowed us to study the MOVPE preparation of Ge(100) surfaces directly in dependence on the relevant process parameters [2, 3, 4]. A contamination free MOVPE to UHV transfer system [5] enabled correlation of the RA spectra to results from UHV-based surface science tools. In this paper, we established the characteristic RA spectra of vicinal Ge(100) surfaces terminated with monohydrides, arsenic and phosphorous. RAS enabled in situ control of oxide removal, H2 interaction and domain formation during MOVPE preparation.

Intramolecular surfaces for vicinal proton–proton coupling constants 3JHH

Equations for the intramolecular surfaces of the 3JHH coupling constants in ethane, ethylene, and acetylene are formulated, and the corresponding coefficients are estimated from calculations at the DFT/B3LYP level. The chosen variables are changes in bond lengths, in the torsion angle φ between the coupled protons Ha and Hb, in bond angles, and in dihedral angles. The 3JHH surface of ethane is formulated as an extended Karplus equation with the coefficients of a truncated Fourier series on the torsion angle φ expanded as second-order Taylor series in the chosen variables taking into account the invariance of 3JHH under reflections and rotations of nuclear coordinates. Partial vibrational contributions from linear and square terms corresponding to changes in the geometry of the Ha − Ca − Cb − Hb fragment are important while those from cross terms are small with a few exceptions. The 3JHH surface of ethane is useful to predict contributions to 3JHH from changes in local geometry of derivatives but vibrational contributions are predicted less satisfactorily. The predicted values at the B3LYP/BS2 level of the 3JHH couplings (vibrational contributions at 300 K) from equilibrium geometries are 9.79 (−0.17) for acetylene, and 17.08 (1.93) and 10.73(0.93) for the trans and cis couplings of ethylene.

Oxidation of Ethanol and Its Derivatives on Well Defined Pt Single Crystal Electrodes Vicinal to Pt(111): A Comparative Study

The oxidation of ethanol (EtOH) at Pt(111) electrodes is dominated by the 4e path leading to acetic acid. The inclusion of surface defects such as those present on stepped surfaces leads to an increase of the reactivity towards the most desirable 12e path leading to CO2 as final product. This path is also favored when the methyl group is more oxidized, as in the case of ethylene glycol (EG) that spontaneously decomposes to CO on Pt(111) electrodes, thus showing a more effective breaking of the C-C bond. Some trends in reactivity can be envisaged when other derivative molecules are compared at well-ordered electrodes. This strategy was used in the past, but the improvement in the electrode pretreatment and the overall information available on the subject suggest that relevant information is still missing.

On the behavior of the Pt(1 0 0) and vicinal surfaces in alkaline media

We address in this paper a voltammetric study of the charge transfer processes characteristic of Pt(1 0 0) and vicinal surfaces in alkaline media. The electrochemical behavior of a series of stepped surfaces of the type Pt(S)[n(1 0 0) × (1 1 1)] has been characterized using cyclic voltammetry at different pHs, charge displacement measurements and FTIR experiments for adsorbed CO. The results from these techniques allow assigning the different peaks appearing in the voltammogram to hydrogen and/or OH adsorption on the different sites of these surfaces, namely, terrace and step sites. Additionally, the potential of zero total charge (pztc) of the electrodes was determined. The resulting pztc values shift to more negative values when the step density increases on the surface up to n = 5. FTIR spectroscopy experiments have been used to monitor the adsorption of CO on the different surfaces as well as the consequent CO oxidation, accompanying a positive potential sweep. The oxidation of adsorbed CO on (1 0 0) terraces is catalyzed by the presence of the (1 1 1) steps. The FTIR spectra revealed that CO is mostly bonded in bridge configuration at low potentials interconverting to on-top when the electrode potential is increased.