Topmost Surface Layer Research Articles

Cross-sectional transmission electron microscopy investigation of the dead layer of ZnS:Ag,Al phosphors in field emission displays

The dead surface layer of blue-emitting ZnS:Ag,Al phosphor with Al metallized thin film in high-voltage field emission displays (FEDs) has been investigated by means of cross-sectional transmission electron microscopy. From these observations, it was found that electron irradiation at 6 keV excitation causes the decomposition of ZnS and the subsequent evolution of sulfur in the topmost surface layer ∼30 nm, and also causes the formation of lattice defects within the electron penetration depth of ∼300 nm in a life-end stage. When this evidence was taken into account, it was estimated that the decomposition rate of ZnS and the formation rate of lattice defects depend mainly on the degree of crystallinity and the atomic-scale surface roughness of ZnS phosphor particles. Necessary characteristics of ZnS:Ag,Al phosphors for longer luminescence lifetime in FEDs were suggested in this work.

Unambiguous interpretation of atomically resolved force microscopy images of an insulator.

The (111) surface of CaF2 was imaged with dynamic mode scanning force microscopy and modeled using atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy.

Physicochemical and electrochemical hydriding-dehydriding characteristics of amorphous MgNi x ( x =1.0, 1.5, 2.0) alloys prepared by mechanical alloying

Physicochemical and electrochemical hydriding-dehydriding characteristics of amorphous MgNi x (x=1.0, 1.5, 2.0) alloys prepared by mechanical alloying have been investigated. It was found that the increase of Ni content in the alloys resulted in greatly enhanced kinetics for both absorption and desorption of hydrogen, while the saturation capacity showed the reverse tendency. Charge–discharge tests showed that the increase in Ni content lead to a significant enhancement in cycle performance of MgNi x alloy electrodes. X-ray photoelectron spectroscopic and X-ray excited Auger electron spectroscopic investigations indicated the existence of a significantly thicker Ni-enriched layer for the MgNi1.5 and MgNi2.0 alloys than for the MgNi alloy. These results reveal that excess of Ni in MgNi x alloys may improve the cycle performance of alloy electrodes by suppressing the segregation of Mg during electrochemical cycling, and the Ni in the topmost surface layer is mainly responsible for the improvement in the kinetics of hydrogen absorption.

Computational simulations of pulsed laser induced melting and solidification of monocrystalline GaSb

Numerical analysis of pulsed laser induced phase change processes in the near-surface region of monocrystalline bulk GaSb is done using a thermal nonequilibrium model. The calculations of basic parameters of the processes such as surface melt duration, maximum reflectivity of the probe laser beam, etc., are verified using results of experimental work on GaSb samples irradiated by the ruby (694 nm, 35 ns FWHM) and ArF (193 nm, 13 ns FWHM) lasers. The comparison of experimental data with numerical predictions shows that while for the ruby laser a reasonable agreement in surface melt duration is achieved, the results for the ArF laser differ quite a lot. The amorphization of the topmost surface layer is identified as a main reason for these differences. The subsequent experimental analysis of the surface structure by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) confirms the theoretical predictions and clearly indicates the appearance of an amorphous GaSb structure after ArF laser irradiation.

Observation of an exchange-split alloy surface state

The spin-resolved electronic structure at the (110) surface of the magnetic alloy ${\mathrm{FeNi}}_{3}$ has been studied with spin-polarized inverse photoemission spectroscopy in conjunction with first-principles electronic structure calculations. An unoccupied $\overline{Y}$ Schockley surface state in a bulk band gap is observed that is exchange split by 320 meV, indicating that the topmost surface layer is ferromagnetically ordered.

Structure analysis of the TiC(001) surface with transition metal impurities (Zr, Nb, Mo)

The surface structures of TiC(001) crystals with dilute transition metal impurities heated up to around 2000 K (0.6 wt.% of Zr, Nb and Mo) have been investigated by an Li + impact collision ion scattering spectroscopy combined with a three dimensional three-atom computer simulation. The TiC(001) surface shows the 1×1 structure before and after segregation of the impurities and the impurity atom of these metals substitutes for the Ti atom at the topmost surface layer. The group VIa impurity, such as Mo, is segregated at the TiC(001) surface with carbon vacancies in their nearest neighbors, but the group IVa and Va metal (Zr and Nb) impurities are segregated without the carbon vacancies. The segregated impurity atoms are located at levels shifted somewhat from the TiC(001) surface.

Segregation of K at the TiO 2(100) surface

We have studied the segregation of potassium to the surface of TiO 2(100) using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), direct recoil spectroscopy (DRS), and mass-spectroscopy of recoiled ions (MSRI). It is found that the concentration of potassium at the surface is strongly influenced by the sample temperature. MSRI results indicate the presence of potassium starting at annealing temperatures of 675 K and the subsequent disappearance above 875 K. Conversely, AES only indicates the presence of potassium after annealing the sample to 975 K. Together these results indicate that below 875 K, potassium exists in the very topmost surface layer, while above 875 K, potassium is located below this layer.

Exchange splitting of FeNi 3(1 1 0) bulk and surface bands

We have investigated the electronic structure and magnetic properties of the magnetic alloy FeNi 3 with the methods of spin-polarised inverse photoemission spectroscopy and the magneto-optic Kerr effect. We have observed bulk as well as surface states in the unoccupied electronic structure, exhibiting dispersion resembling that measured on Ni(1 1 0). However, the observed exchange splitting on FeNi 3(1 1 0) is significantly larger and the surface states are extremely sensitive to contamination. The presence of an exchange-split surface state indicates that the topmost surface layer is magnetic.

Surface Structure and Particulate Generation in Pb0.97Nd0.02(Zr0.55Ti0.45)O3 Ablation Targets During Pulsed Laser Deposition

A pulsed XeCl excimer laser was used to deposit amorphous Nd‐modified lead zirconate titanate thin films with thicknesses between 25 and 500 nm. Atomic force microscopy was used to study the thin‐film surface morphology and to determine the particulate number density (PND) on film surfaces. The dependence of the PND and the growth rate of the film on the laser‐beam fluence and the density of Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets was also studied. It was found that the PND of particulates with diameters larger than 150 nm increased with increasing laser‐beam fluence and decreased with increasing target density. The surface morphology and quality of the target after ablation were studied by scanning electron microscopy. The generation of laser‐cone structures and particulates at the target surface was also studied. X‐ray diffraction measurements and element analyses by the energy dispersive spectroscopy of X‐rays revealed the formation of extra compounds and phases at the target surface during ablation processes. Raman spectroscopy was also used to study the structure of the topmost surface layer of the target after ablation.

Capture and loss of valence electrons during low energy H + and H − scattering from LaB 6(100), Cs/Si(100), graphite and LiCl

The mechanism of electron attachment to low-energy (100 eV) protons has been investigated during scattering from graphite, Si(100), LaB 6(100), and LiCl. The primary H + ion survives neutralization when scattered from highly ionized target species, such as Cs +, La 3+, Li +, and Cl −, existing on the topmost surface layer. In terms of H − ion formation at the metal and semiconductor surfaces, multiple scattering from the solid is dominant relative to single surface scattering because the former has a much larger cross-section than the latter, indicating that H − is caused by the non-local resonant tunneling process. At the LiCl surface, on the other hand, a close atomic encounter with individual target ions is found to be important for H − ion formation. This is because the non-local resonance process is prohibited due to the existence of the large band gap and the charge state of scattered hydrogen is determined by the transient chemisorption state very close to the surface.

Interaction of methanol and water on MgO(100) studied by ultraviolet photoelectron and metastable impact electron spectroscopies

The coadsorption of methanol (CH3OH) and water (D2O) on the MgO(100)/Mo(100) surface at 100 K has been studied by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy [UPS (HeI)], and by thermal programmed desorption (TPD). Methanol wets the MgO surface and adsorbs nondissociatively within the first monolayer with the hydroxyl group oriented toward the substrate. In coadsorption experiments, methanol wets a water precovered MgO surface; however, adding water to a methanol precovered MgO surface does not yield a water-only surface. Essentially, a constant fraction of the preadsorbed methanol remains within the topmost surface layer during the buildup of the water multilayer. Temperature-dependent measurements show that the adsorption and desorption dynamics of both water and methanol are governed by intermolecular interactions rather than adsorbate–substrate interactions.

Orientation of molecules at a liquid surface revealed by ion scattering and recoiling spectroscopy

Analysis of the kinetic energy of scattered inert gas ions and ejected recoil atoms reveals the identity of atoms in the topmost atomic surface layers of liquids. Spectra from bis(2-ethylhexyl)phthalate (DOP) and bis(2-ethylhexyl)chlorendate (BEHC), two molecules with similar `head–tails' structures, show that the molecules have somewhat different surface orientations. Atom ratios derived from peak intensity ratios show that neither molecule has an exclusively `heads-up' orientation at the surface. A `head-down' orientation is favored for DOP. For BEHC, part of the ring (head) that includes 2–3 Cl atoms is at the surface.

Surface analysis of a heat-treated, Al-containing, iron-based superalloy

The surface composition of MA 956 superalloy both as-received and after four exposure times at 1100 °C has been investigated by energy dispersive x-ray spectrometry (EDX) and x-ray photoelectron spectroscopy (XPS). The passive layer of the as-received sample is mainly formed by Cr- and Fe-oxides. Heat treatment leads to the formation of an alumina layer on which small nodules grow. XPS spectra evidence the presence of titanium and yttrium oxides at the surface of the heat-treated samples, suggesting Y and Ti outward diffusion through the alumina layer. Iron and chromium oxides at the topmost surface layers are observed only for short heat-treatment times.

Structure of the (100) surface of the icosahedral borideYB66

The composition and structure of the (100) surface of ${\mathrm{YB}}_{66}$ has been investigated with the techniques of angle-resolved x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The LEED and STM results reveal that the surface possesses a structure with the same periodicity as a bulk (100) plane. Angle-resolved XPS data shows that the topmost surface layer has a higher B to Y ratio than the bulk, while the ratio in the near surface region is approximately the same as the bulk crystal. STM images under a variety of tunneling conditions are compared with structural images drawn for various assumed bulklike surface terminations. A specific surface termination is proposed that is consistent with all of the STM, XPS, and LEED data.

The behaviour of phase transfer catalysts in polar solvents

Abstract Tetra(alkyl)onium salts are hydrophobic in polar solvents due to their CH2-/CH3-groups. The segregation leads to formation of an equilibrium structure at the liquid surface which is monitored with the extremely surface sensitive method MIES (Metastable Induced Electron Spectroscopy). It is found that the surface activity as well as the formation of a surface layer strongly depends on the molecular structure of the surfactant: in the case of the symmetric tetra(butyl)ammonium iodide (TBAI, (C4H9)4N+I−)) anions and cations are both traced in the topmost surface layer. The exchange of anions I−→Br− leads to a change in the depth profile: TBABr− has a lower surface activity with the formation of a diffuse anion layer below the surface. In constrast a change of the cation central atom N→P enhances the surface activity. Deviating from the symmetrical shape the salt tetradecyl tri(methyl)ammonium bromide ((CH2)13CH3 Met3N+Br−) with three of four alkane chaines substituted by methyl groups was investigated. As a first result no anion signal is found in the surface, and second one finds an orientation of salt molecules which can be compared with organic monolayers on solid substrates or layers of alkane on liquids: they form a closed layer consisting of flat lying molecules exposing mainly CH2-groups. For the above mentioned surfactants the surface activity is caused by CH2-/CH3-groups. Therefore tetra(phenyl)phosphonium bromide ((C6H5)4PBr) was the first aromatic system studied to see how the transition from alkanes to aromates influences the solvation behaviour. One finding is that the ring structure leads to a strongly decreased surface activity.

Self-surfactant effect on Fe/Au(100):: place exchange plus Au self-diffusion

In this paper, we report on the submonolayer growth of Fe on Au(100), a system where a monolayer of the substrate floats at the external surface during growth facilitating the layer-by-layer growth of the Fe film. STM and STS demonstrate that this self-surfactant action results from a combination of several processes: fast place exchange between Fe and the Au atoms of the topmost surface layer, anisotropic diffusion of the Au atoms ejected due to the local lifting of the surface reconstruction, and preferential nucleation on the isolated Fe atoms embedded within the surface. The Au atoms diffuse over the surface in a way almost identical to the homoepitaxial growth of Au on Au(100). As a result, stable, reconstructed islands composed of Au atoms are formed and end up burying the deposited Fe.

MgO(100) surface relaxation by symmetrized automated tensor low energy electron diffraction analysis

We present a low energy electron diffraction study of the surface relaxation of MgO(100) at T=80 K down to the second atomic layer using in situ cleaved MgO crystals. We find that the main perturbation from the bulk structure is a topmost surface layer rumpling of 3.3±1.5% and a very small second layer rumpling of 0.2±2%. In both cases the oxygen atom is displaced outward. The first interlayer spacing is slightly reduced by 0.2±0.7% whereas there is no change in the second interlayer spacing.

Reflectance anisotropy of silicon surfaces: Discrete dipole calculation

The reflectivity and reflectance anisotropy (RA) spectra of five silicon surfaces [(001), (113), (112), (111), and (110)] are calculated using the discrete dipole model. The structures used have bulk-terminated surfaces so that RA is found at the optical gap and above. A comparison is made between experimental RA spectra of H-covered (001), (113), and (110) surfaces and the discrete dipole spectra, and it is found that there is agreement for the (001) and (110) surfaces but the best fit to the experimental (113) surface spectrum is found for a (112) surface discrete dipole calculation. RA spectra are obtained using the McIntyre-Aspnes three-layer model with surface layer and bulk dielectric functions taken from discrete dipole calculations. Surface excess dielectric functions are shown for these surfaces. It is found that, although RA spectra of the different surfaces are quite distinct, the gross features of the surface excess dielectric functions that are used to calculate them are quite similar. Thus small shifts in maxima and minima and differences in strength of these functions are responsible for the observed optical anisotropy, rather than distinct spectral features that differ from surface to surface. The surface dielectric function converges to the bulk value within 10 to 20 \AA{} of the vacuum-solid interface, depending on frequency, so that RA at frequencies corresponding to the optical gap energy and above arises from this 20 \AA{} region, rather than the topmost surface layer or layers.

Effects of defect structures at surfaces and thin films on grazing scattering of fast ions

Studies on grazing scattering of 25 keV ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ ions from clean Fe(001) and submonolayer films of Cr and Mn on Fe(001) are reported. We find that angular distributions of reflected ions directly depend on the defect structure of the topmost surface layer. Surface defects of different dimensions (thermal displacements, surface steps, islands) can be separated due to characteristic effects on the scattering process. Computer simulations based on the binary collision approximation permit a quantitative analysis of data.

LEED structural determination of the c(2 × 2) [formula omitted] surface alloy

Deposition of half atomic layer of Si on the Cu(110) surface leads to the formation of a stable c(2 × 2) superstructure. Our structural study by quantitative analysis of low-energy electron diffraction intensities finds a two-dimensional alloyed top layer in which Si atoms occupy substitutional Cu sites and are displaced inwards by 0.24 Å with respect to the surface Cu atoms. An oscillatory behavior of the interlayer spacing is observed in the last three layers: the topmost surface layer exhibits a contraction of 15%, the second an expansion of 9% and the third a contraction of 3%. The third layer presents a corrugation of 0.1 Å.