Cu-phthalocyanine overlayers on ZnO (1\bar 100) surfaces

Photoconductivity measurements and ellipsometric spectroscopy have been used to study Cu-phthalocyanine (CuPc) overlayers on nonpolar ZnO (1 \(\bar 1\) 00) surfaces which have been prepared in ultrahigh vacuum (UHV). In the spectral range between 1.5 and 2 eV, i.e. in theπ→π ^* absorption band of CuPc a sensitized photoconductivity in the ZnO substrate is found. The analysis of the ellipsometric data suggests a chemisorption bond of the first monolayer of sublimated CuPc molecules accompanied by charge transfer between the ZnO substrate and the adsorbate. Bonding is most probably done through theπ-orbitals of the CuPc molecules.     

Effect of Cu adsorption on the tungsten (100) surface resonance

It has recently been observed that adsorbed Au and Hg do not quench the W (100) surface resonance up to monolayer coverage. We report here that Cu does quench the W (100) surface resonance at θ≧0.5 monolayers. This suggests that the effective potential of Cu differs more strongly from that of W than do the effective potentials of Au or Hg.     

Single domain nonpolar (13$ \bar 4 $0) ZnO on (114) LaAlO3

An unconventional nonpolar plane (13$ \bar 4 $ 0) ZnO epitaxial film was grown on a 2‐inch (114) LaAlO₃ (LAO) substrate by pulsed laser deposition. Reflection high energy electron diffraction (RHEED) patterns of the grown ZnO surface demonstrate single crystalline characteristics with the orientation inclined with the a‐axis. Atomic force microscopy (AFM) shows that the grown ZnO film exhibits a stripe‐like surface morphology with the longitudinal direction parallel to the c‐axis. Cross‐sectional transmission electron microscopy (TEM) with selected area electron diffraction (SAED) was used to characterize the microstructure and to determine the growth plane of ZnO grown film as (13$ \bar 4 $ 0). In addition, XRD pole‐figure measurements confirm the single domain growth of (13$ \bar 4 $ 0) ZnO on (114) LAO. Room temperature photoluminescence spectra of the ZnO film measured across the substrate show the same near band edge emission peak at 3.29 eV, indicating that the nonpolar (13$ \bar 4 $ 0) ZnO film has excellent uniform optical properties. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Calorimetric absorption spectroscopy of J-aggregate dye monolayers below 0.1 Kelvin

We describe an extremely sensitive method to investigate nonradiative processes following optical excitation. In a low-temperature calorimeter, the sample is irradiated with monochromatic light. The absorbed energy which is converted into phonons leads to a measurable increase of the sample temperature. The lowest detectable dissipated energy is 2·10^–12J at a working temperature of 0.03 K. We demonstrate our method with a calorimetric absorption spectrum of J-aggregate monolayers of 1-methyl-1-octadecyl-2,2-cyanine. From this spectrum the emission quantum efficiency is obtained as a function of excitation wavelength. A relative minimum is found at the long wavelength edge of the absorption band at 582.5 nm.SFB 126     

The formation of subsurface oxygen on Pt(100)

PhotoEmission Electron Microscopy (PEEM) enables imaging a surface via its work function. If a CO covered Pt(100) surface is exposed to oxygen patches are formed which appear dark in the PEEM image due to their high work function. As the surface is heated to temperatures above 650 K we observe the conversion of these dark islands into very bright ones with work functions much lower than even that of the clean surface. These findings are attributed to a change in the dipole moment of the adsorbed oxygen induced by their migration beneath the surface. A total work-function decrease of up to 1.2 eV has been evaluated independently using a Scanning Photoemission Microscope (SPM). The properties of this new kind of oxygen were also further investigated with thermal desorption spectroscopy and with Auger-electron spectroscopy.     

Entropy-controlled site occupation of CO adsorbed on Ni(100)

The adsorption of CO on a Ni(100) surface has been studied by FT-IRAS in the temperature range from 85 K to 300 K. At 300 K and for =0.5, the CO molecules are predominantly adsorbed in on-top sites with only a minor fraction located at two-fold bridge sites. Measurements on a Ni(100) surface pre-covered with sulphur, oxygen and carbon indicate that the occupation of bridge sites may be caused by small amounts of surface impurities. The relative broadness of the infrared bands is explained by CO molecules occupying intermediate positions at domain walls. Upon lowering the temperature, the bridge sites are increasingly occupied at the expense of terminal sites. This process is completely reversible and is explained by a contribution of the hindered translations of the adsorbed CO molecules to the entropy. At 85 K and for low initial coverages, we observe an unusual high CO stretching frequency at 2205 cm^–1 which cannot be explained at present.     

Weak carrier localization on the (10\bar 10) tellurium surface

A theory of weak localization responsible for the negative magnetoresistance effect is developed for quantum wells at the tellurium surface, which retains, in contrast to the surface considered earlier, a trigonal spectrum distortion with opposite signs near the M and P extrema, with the latter playing the part of states with opposite spins. It is shown that as a result of the motion in momentum and spin spaces being correlated the spin relaxation due to this distortion is smaller than that for the (0001) surface. Fiz. Tverd. Tela (St. Petersburg) 39, 1659–1664 (September 1997)     

Calculation ofn^3 S_1 (Q\bar Q) \to \gamma + (pseudo)scalar

The decays of³ S ₁ quarkonia into a photon and a scalar or pseudoscalar Higgs particle are examined, taking into account the bound-state dynamics in the framework of a nonrelativistic potential model. We find that for realistic quark potentials the naive calculation [1] overestimates the scalar rate. Numerical results are obtained for the \(\bar bb\) and \(\bar tt\) quark systems.     

Characteristics of surface acoustic waves in (11$$\bar 2$$ 0)ZnO film/ R-sapphire substrate structures

(11\(\bar 2\)0)ZnO film/R-sapphire substrate structure is promising for high frequency acoustic wave devices. The propagation characteristics of SAWs, including the Rayleigh waves along [0001] direction and Love waves along [1ī00] direction, are investigated by using 3 dimensional finite element method (3D-FEM). The phase velocity (v _p), electromechanical coupling coefficient (k ²), temperature coefficient of frequency (TCF) and reflection coefficient (r) of Rayleigh wave and Love wave devices are theoretically analyzed. Furthermore, the influences of ZnO films with different crystal orientation on SAW properties are also investigated. The results show that the 1st Rayleigh wave has an exceedingly large k ² of 4.95% in (90°, 90°, 0°) (11\(\bar 2\)0)ZnO film/R-sapphire substrate associated with a phase velocity of 5300 m/s; and the 0th Love wave in (0°, 90°, 0°) (11\(\bar 2\)0)ZnO film/R-sapphire substrate has a maximum k ² of 3.86% associated with a phase velocity of 3400 m/s. And (11\(\bar 2\)0)ZnO film/R-sapphire substrate structures can be used to design temperature-compensated and wide-band SAW devices. All of the results indicate that the performances of SAW devices can be optimized by suitably selecting ZnO films with different thickness and crystal orientations deposited on R-sapphire substrates.     

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H₂ and D₂ on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases at _H =0.5 and at _H =1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (₁,₂ and) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of the state is invariant with the hydrogen isotope, the D ₁ state appears at a 8 Klower and theD ₂ state at a 5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D₂ adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases.Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.     

Infrared-visible sum-frequency generation spectra of CO on Ni(100)

Infrared-visible sum-frequency generation (SFG) spectra of the stretching region of carbon monoxide adsorbed on Ni(100) are measured as a function of overlayer coverage and substrate temperature. The frequency and linewidth of the CO vibrational resonance in SFG have a similar dependence on coverage and temperature to that observed in linear absorption and emission spectroscopies. The lineshape of the SFG resonance, however, is influenced by the nonlinear response of the underlying metal surface. A simple model indicates that the nonresonant response of the substrate is out of phase with respect to that of the resonant CO response.     

Properties ofe^ + e^ - \to h\bar h in the\Upsilon (b\bar b)-continuum

The unflavored hadrons frome ⁺ e ^? annihilation in the \(\Upsilon (b\bar b)\) -continuum are analyzed in terms of the covariant Boltzmann distribution with an equilibrium temperature 151 MeV according to the Stefan's law. It is found that the effective fugacities of quark contents are approximately equal to the reciprocals of those from othere ⁺ e ^? annihilations outside the γ-continuum, whereas the fireballs are practically stationary, reflecting the decay at rest of \(B\bar B\) mesons produced by thee ⁺ e ^? annihilation.     

Manifestation of the bound state \bar NN (1870) in the annihilation process \bar pd \to p_s 5\pi

It is shown that the existence of an N?N bound state with a mass of approximately 1870 MeV and width 10 MeV follows from analysis of the experimental data on the momentum distribution of recoil protons in the annihilation process p?d→p _s5π.     

Semipolar (11\bar 2\bar 2) ZnO thin films grown on LaAlO3‐buffered LSAT (112) single crystals by pulsed laser deposition

Semipolar (11\bar 2 \bar 2) ZnO was successfully grown on (112) LaAlO₃/(LaAlO₃)_0.29(Sr₂AlTaO₆)_0.35 substrate by pulsed laser deposition. The epitaxial relationship is [11\bar 23]_{\rm ZnO} // [11\bar 1]_{\rm LAO/LSAT} with the polar axis of [000\bar 1]_{\rm ZnO} pointing to the surface. For ZnO films with thickness of 1.6 μm, the threading dislocation density is ～1 × 10⁹ cm^–2, and the density of basal stacking faults is below 1 × 10⁴ cm^–1. The (11\bar 2 \bar 2) ZnO exhibits strong D⁰X emissions with a FWHM of 9 meV and very few green–yellow emissions in the low‐temperature (10 K) and room‐temperature photoluminescence spectra, respectively.

     

The dielectric constant of PbTe at 4.2 K and\tilde v=84.15 cm-1, 96.97cm−1, 103.60cm−1

The dielectric constant of a PbTe epitaxial layer has been measured by surface wave spectroscopy using an optically pumped far-infrared laser and the technique of attenuated total reflection.     

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v₁, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v₁ wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.     

Growth kinetics of graphene and graphite films on the \operatorname{Re} (10\bar 10) surface

Growth kinetics of graphene and graphite layers on the surface of carbon-saturated rhenium has been investigated. The experimental data agree well with the results of the mathematical analysis of diffusion processes in the metal bulk with the participation of carbon atoms and the subsequent precipitation of carbon on the surface. It has been established that graphene layers of rigorously specified thickness in the range of 1–50 layers can be grown on metal in the temperature range of 1300–1500 K.     

Photoemission spectroscopy at MOVPE-prepared InGaAs(100) surface reconstructions

Angular resolved ultraviolet photoemission spectroscopy at BESSY was employed to study the electronic structure of the three different, (4 × 3)-, (2 × 4)-, and (4 × 2)-surface reconstructions of In_0.53 Ga_0.47As, which was grown lattice-matched to InP(100). The surfaces have been prepared using metal organic vapor phase epitaxy (MOVPE). For spectroscopy, a dedicated transfer system was employed and samples were transferred contamination-free from the MOVPE reactor to UHV-based analysis tools. For the different surface reconstructions, the Γ ? Δ ? X direction was scanned while varying the photon energy between 10 eV and 28 eV. We observed two surface states in the photoelectron spectra on all of these surface reconstructions in addition to the bulk derived valence band emissions. Different binding energies of the surface states originating from different surface band bending were detected and described.     

Plastic Relaxation of Stressed Semipolar AlN( $$10\bar {1}1$$ ) Layer Synthesized on a Nanopatterned Si(100) Substrate

Technical Physics - Plastic relaxation of a stressed semipolar AlN( $$10\bar {1}1$$ ) layer synthesized on a nanopatterned Si(100) substrate has been investigated using scanning electron...