Temperature dependence of the surface electric field gradient for indium metal

The electric field gradient at indium surfaces was studied with¹¹¹In PAC-probes as a function of temperature. The surface field gradient was determined to be temperature independent up to 100 K and the fraction of¹¹¹In surface sites shows a pronounced annealing behaviour between 150 K and 250 K.     

Bromine atom diffusion on stepped and kinked copper surfaces

The rates of Br atom diffusion on several single crystalline Cu surfaces have been studied because of the potential impact of Br diffusion on the selectivity of alkyl bromide surface chemistry on Cu. Density functional theory (DFT) has been used to study the diffusion of isolated bromine atoms on a flat Cu surface, Cu(1 1 1), two Cu surfaces with straight steps, Cu(2 2 1) and Cu(5 3 3), and two kinked Cu surfaces, Cu(6 4 3) and Cu(5 3 1). Bromine diffusion is rapid on the flat Cu(1 1 1) surface with a barrier of ΔE_diff = 0.06 eV and a hopping frequency of ν = 4.8 × 10¹⁰ s⁻¹ at 150 K. On the stepped and kinked surfaces the effective diffusion barriers lie in the range ΔE_diff = 0.18-0.31 eV. Thus the rates of diffusion are many orders of magnitude slower on stepped and kinked Cu surfaces than on the Cu(1 1 1) surface. Nonetheless, at temperatures relevant for alkyl bromide debromination on Cu surfaces, bromine atoms remain sufficiently mobile that they can explore all available binding sites on the timescale of the debromination reaction.     

Adsorption sites of111In and111mCd on Pd(111) surfaces characterized by the electric field gradient

PAC measurements of the electric field gradient at the nucleus of isolated probe atoms on Pd(111) surfaces lead to the identification of five adsorption sites successively occupied by the parents¹¹¹In and^111mCd during annealing between 80K and 600K. The data are consistent with an estimated activation energy for Pd surface self diffusion of E_a=0.76(8)eV.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Oxidation of indium implanted aluminium studied by the Rutherford backscattering and perturbed angular correlation methods

Aluminium foils oversaturated with 5.10¹⁵/cm² implanted¹¹⁵In⁺ ions were oxidized in 200 mbar oxygen or vacuum annealed at 370–870 K. The samples were analyzed by the Rutherford backscattering (RBS) and perturbed angular correlation (PAC) techniques, using some 10¹¹/cm² implanted radioactive¹¹¹In tracers. Furthermore, the oxygen surface profiles were also scanned with high resolution by using the nuclear resonance technique (NRA). The formation of passivating Al₂O₃ surface layers, preventing deeper oxygen diffusion and the indium diffusion into these oxidized surface layers and into the bulk, were studied. Several quadrupole interaction frequencies previously attributed to strained cubic indium precipitates and indium (-vacancy) clusters were observed. When the samples were oxidized above 750 K, the formation of In-O complexes and of substitutional¹¹¹In in Al₂O₃ was observed.     

Study of electric quadrupole interactions of111In and111Cd following ion implantation of111In into graphite

The nuclear electric quadrupole interaction of¹¹¹In ion-implanted in hightly oriented pyrolytic graphite has been observed by means of low-temperature nuclear orientation and by means of perturbed angular correlations. From the first kind of experiment, it is concluded that a relatively large number of indium nuclei experience a well-defined macroscopic orientation, which is partly lost after the radioactive decay to cadmium. Indeed, the second kind of experiment revealed a broad distribution of electrid field gradients interacting with the 245 keV Cd excited state, as well as a small faction experiencing a unique electric field gradient. The experimental results are compared with theoretical calculations of the electric field gradient at various lattice positions, in which carbon and indium electronic wavefunctions are allowed to hybridize. Lattice positions of the covalent indium atom between the graphite layers can explain the measured electric field gradient ofV ₂₂=+1.47(11)·10²² V/m², directed parallel to the graphitec-axis.     

Oxygen adsorption on Cu(110): Determination of atom positions with low energy ion scattering

The position of adsorbed oxygen on Cu($\text{1}$10) surfaces was determined with Low Energy Ion Scattering (LEIS). The experiments were performed by bombarding the copper surface at small angles of incidence with low energy Ne⁺ ions (3–5 keV). Measurements of the Ne⁺ ions scattered by adsorbed oxygen showed regular peaks in the azimuthal distribution of the scattered ions due to a shadowing effect. From the symmetry of the azimuthal distributions it follows that the centre of an adsorbed oxygen atom on the Cu(1̄10) surface lies about 0.6 Å below the midpoint between two neighbouring Cu atoms in a 〈001〉 row. A comparison of the azimuthal distributions of Ne⁺ ions scattered from clean Cu surfaces and oxygen-covered Cu surfaces showed that hardly any surface reconstruction had occurred in the oxygen-covered surfaces. The applied method seems to be an appropriate one for locating adsorbed atoms because it uses only simple qualitative considerations about azimuthal distributions of scattered ions.     

Mechanism of Cu transport along clean Si surfaces

Cu diffusion along clean Si(111), (110) and (100) surfaces are investigated by Auger electron spectroscopy and low energy electron diffraction. The effective diffusion coefficients of copper are measured in the temperature range from 500 to 650°C. It is shown that the Cu transport along silicon surface occurs by the diffusion of Cu atoms through Si bulk and the segregation of Cu atoms to the surface during the diffusion process. It is found that the segregation coefficients of Cu to silicon surface during the diffusion process depend on surface orientation.     

Cavity formation at indium impurities in bcc metals

The defect formation in the bcc metals W and Mo above annealing stage III and the influence of rare gases on this process were investigated by means of the perturbed angular correlation technique using¹¹¹In as radioactive probe. In both metals a relatively high electric field gradient (EFG) could be observed at the indium site, characterized by the quadrupole interaction frequencies υ_Q=263 MHz, ν=0 and υ_Q=220 MHz, ν≈0.15 for W and Mo, respectively. The observations are assigned to the growth of threedimensional vacancy clusters at the probe atoms with the indium atoms situated in the inner surface of this cavities, thus experiencing the corresponding surface EFG.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Growth and melting behaviour of thin in films on Ge(100)

Growth and melting behaviour of thin indium films on Ge(100) have been investigated by Auger-electron spectroscopy (AES), atomic force microscopy (AFM) and perturbed angular correlation (PAC) spectroscopy, respectively. At room temperature inidium is found to grow in three-dimensional islands even at submonolayer coverages. A very rough film surface is observed for thicknesses up to 230 ML. The melting behaviour of such films has been studied by PAC. A reduction of the melting temperature T _m as well as a strong supercooling of the films is observed. The electric field gradient for ¹¹¹In(¹¹¹Cd) in the indium islands is determined as a function of temperature and is used to monitor the local crystalline order of the films up to temperatures just below the melting point.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

Vacancy diffusion in the Cu(001) surface I: an STM study

We have used the indium/copper surface alloy to study the dynamics of surface vacancies on the Cu(0 0 1) surface. Individual indium atoms that are embedded within the first layer of the crystal, are used as probes to detect the rapid diffusion of surface vacancies. STM measurements show that these indium atoms make multi-lattice-spacing jumps separated by long time intervals. Temperature dependent waiting time distributions show that the creation and diffusion of thermal vacancies form an Arrhenius type process with individual long jumps being caused by one vacancy only. The length of the long jumps is shown to depend on the specific location of the indium atom and is directly related to the lifetime of vacancies at these sites on the surface. This observation is used to expose the role of step edges as emitting and absorbing boundaries for vacancies.     

Chemisorption and dissociation of carbon monoxide on rhodium surfaces

The adsorption, desorption and decomposition of CO on Rh surfaces have been investigated using field emission microscopy and thermal desorption spectroscopy. Thermal dissociation of CO cannot be detected on clean Rh surfaces at pressures up to 10^?1 Torr and temperatures below 1000 K. This holds also for atomically rough surfaces like (210). CO dissociation can be promoted under the influence of an electron beam directed to the surface, a high electric field in the presence of CO in the gas phase and by means of discharge techniques. The growth of crystallites formed by CO dissociation and the diffusion of carbon into the bulk has been followed as a function of temperature and surface structure. The tip regions around (110) are very active in these processes. Carbon crystallites on these surfaces disappear around 1000 K by diffusion into the lattice whereas crystallites present around (311) surfaces persist up to 1150 K. The results are discussed in relation to the activity of Rh in CO/H₂ reactions.     

3D KMC simulations of crater growth during the reduction of oxide nanoislands on metal surfaces

The homoepitaxial growth of Cu nanocraters induced by thermal reduction of Cu₂O nanoislands on Cu(100) surfaces is simulated using a three-dimensional (3D) kinetic Monte Carlo (KMC) model by incorporating surface diffusion, attachment and detachment Cu adatoms dislodged from reducing Cu₂O islands. The craters are observed to grow continuously in rim height and rim slopes while remaining relatively constant in rim width in the course of the oxide decomposition. Such a growth behavior is attributed to the climbing uphill of Cu adatoms released from the perimeter of the reducing Cu₂O island at the crater bottom. The observed decay of the rim height and slopes after completion of the reduction of oxide islands suggests that these surface craters are thermodynamically unstable at high temperatures.     

Muon/muonium surface interactions

The interactions of muonium (μ ⁺ e ⁻, Mu) with the surfaces of fine silica powders have been extensively studied using zero, longitudinal and transverse field μSR techniques. These studies indicate diffusion and trapping behavior of the Mu atoms on the silica surface, which is strongly influenced by the surface hydroxyl (OH) concentration. Specifically, the presence of the surface OH groups is observed to inhibit the surface mobility of the Mu atoms at low temperatures. Information provided by zero and longitudinal field data suggest a random anisotropic distortion of the Mu hyperfine interaction (RAHD) as the principal relaxation mechanism. A recently developed RAHD spin relaxation theory is used to interpret these data. Additional investigations, using platinum loaded silica, have yielded the first observed surface reaction of Mu. Studies of the interactions of positive muons with surfaces have been also extended to single crystals, where low energy (<10 eV)μ ⁺ andMu ⁻ ions are observed to be reemitted from some materials (e.g., the <100> surface of lithium fluoride). Future applications of these emission phenomena toward the development of a slow^847-3 (or Mu⁻) beam are considered.     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

Cu dimer diffusion on strained Cu(0 0 1)

Cu dimer diffusion energy barrier on strained Cu(0 0 1) surfaces has been studied with nudged elastic band method (NEB) and embedded atom method (EAM). Dimer exchange and hopping mechanisms are chosen as the initial diffusion paths in the NEB method. It is shown here that the dimer exchange is dominant on tensile surfaces and the dimer hopping is dominant on compressive surfaces. For most strain conditions Cu dimer diffusion energy barrier is lower than Cu monomer diffusion barrier. The concerted movement of the remaining adatom toward the hopping adatom lowers the dimer hopping barrier. The adsorption induced relaxation makes the dimer exchange barriers lower than the monomer exchange barriers on tensile surfaces. Transition state theory is used to calculate the diffusion frequencies as a function of temperature. No surface crowdion is observed on the shear strained surfaces for the dimer diffusion.     

Preserving long-term emission stability of carbon nanotube field emitter using aluminum layer

Carbon nanotube (CNT) field emitter was fabricated, and then its emission stability was evaluated with three different anode structures; indium tin oxide (ITO)/glass, ZnS:Cu,Al(green phosphor)/ITO/glass, and Al/ZnS:Cu,Al/ITO/glass. It was found that the electron emission from CNTs to the phosphor layer degrades much faster than the emission to ITO layer does. The current decay time from 100 μA/cm² to 50 μA/cm² for ITO/glass and ZnS:Cu,Al/ITO/glass were 250 h and 20 h, respectively. Such rapid decay in emission current with the phosphor-coated anode was found to be attributed to the formation of Zn particles on CNTs during the field emission. However, the deposition of aluminum layer on the phosphor, in other words, using the anode structure of Al/ZnS:Cu,Al/ITO/glass recovered the stability that is comparable to that with an ITO/glass. The aluminum layer was found to efficiently prevent phosphor elements from being degassed, preserving the long-term emission stability of carbon nanotubes.