A structural investigation of (ND+4)0.22(ND3)0.34 TiS0.22-2 by time-of-flight neutron powder diffraction

The structure of the intercalation complex (ND₄⁺)_0.22(ND₃)_0.34TiS₂^0.22- has been determined by Rietveld refinement of time-of-flight neutron powder diffraction data taken at 298 and 12 K. This compound belongs to space group R3̄m, and the lattice parameters are $\text{a}\text{= 3.4170(3)}\text{A}\text{̊}$ and $\text{c}\text{= 26.723(4)}\text{A}\text{̊}$ at 298 K. No phase change was observed on cooling to 12 K. However, the $\text{c}$ lattice parameter was observed to contract about 1.2%. Both ND₄⁺ and ND₃ occupy trigonal prismatic interlayer sites, with ND₄⁺ exhibiting random spherical disorder and ND₃ adopting a planar geometry. Two of the ND₃ deuterium atoms are hydrogen bonded to host TiS₂ sulfur atoms, whereas the third is found in the interlayer midplane and is not hydrogen bonded. The implications of this structure for the properties of such intercalation compounds is discussed.     

Chemical shifts in auger electron spectra from silicon in silicon nitride

The chemisorption of nitric oxide on (110) nickel has been investigated by Auger electron spectroscopy, LEED and thermal desorption. The NO adsorbed irreversibly at 300 K and a faint (2 × 3) structure was observed. At 500 K this pattern intensified, the nitrogen Auger signal increased and the oxygen signal decreased. This is interpreted as the dissociation of NO which had been bound via nitrogen to the surface. By measuring the rate of the decomposition as a function of temperature the dissociation energy is calculated at 125 kJ mol^?1. At ～860 K nitrogen desorbs. The rate of this desorption has been measured by AES and by quantitative thermal desorption. It is shown that the desorption of N₂ is first order and that the binding energy is 213 kJ mol^?1. The small increase in desorption temperature with increasing coverage is interpreted as due to an attractive interaction between adsorbed molecules of ～14 kJ mol^?1 for a monolayer. The (2 × 3) LEED pattern which persists from 500–800 K is shown to be associated with nitrogen only. The same pattern is obtained on a carbon contaminated crystal from which oxygen has desorbed as CO and CO₂. The (2 × 3) pattern has spots split along the (0.1) direction as $\text{(m,}\text{n}\text{3}\text{)}$ and $\text{(}\text{m}\text{2, n}\text{)}$. This is interpreted as domains of (2 × 3) structures separated by boundaries which give phase differences of $\text{2π}\text{3}$ and π. The split spots coalesce as the nitrogen starts to desorb. A (2 × 1) pattern due to adsorbed oxygen was then observed to 1100 K when the oxygen dissolved in the crystal leaving the nickel (110) pattern.     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

Ferroelectric lithium tantalate—III. Temperature dependence of the structure in the ferroelectric phase and the para-electric structure at 940°K

The neutron scattering from a single crystal of LiTaO₃ (Curie temperature 907°K) has been measured at 760, 820, 885 and 940°K, and has been remeasured at 298°K. Least squares structural refinement of the five data sets shows that as the temperature approaches T_c the oxygen atom approaches the position $\text{x,}\text{1}\text{3}\text{,}\text{1}\text{12}$, with respect to Ta at the origin, in space group R3c. The temperature variation of the oxygen y- and z-coordinates is very similar to that of the spontaneous polarization. The lithium atom position below T_c remains essentially invariant as a function of temperature. At T_c, the oxygen atom occupies the position $\text{x,}\text{1}\text{3}\text{,}\text{1}\text{12}$, the lithium atom becomes disordered and distributed over the positions $\text{00z}\text{and}\text{0, 0,}\text{1}\text{2}\text{-z}$, and the tan alum atom becomes located at an inversion center, in space group $\text{R}\text{3}\text{c}$. The lithium atom sites above T_c lie 0.374 Å on either side of the oxygen atom plane at $\text{z =}\text{1}\text{4}$.     

Evidence from photoelectron spectroscopy for dissociative adsorption of oxygen on nickel oxide

XPS and UPS spectra from oxygen adsorption at high temperature on polycrystalline nickel oxide surfaces pre-heated at 700°C and 1450°C are presented. Adsorption results in complete loss of surface charge on both surfaces. There is an increase in intensity of the 0(1s) 529.7 eV peak, attributed to O^2? ions, and the $\text{Ni}\text{(2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ spectra show a shift of intensity to the 854.6 eV peak due to Ni²⁺ ions. The results are compared with previous data from kinetic, conductivity and electrochemical measurements. Agreement with a model of O^2? and nickel vacancy production is demonstrated. UPS spectra indicate considerable reorganisation of electronic charge in the surface of the 1450°C pre-heated oxide, after oxygen adsorption, giving an almost stoichiometric surface similar to that of a cleaved single crystal.     

Oxidation of Co(poly) and Co(1010) surfaces: Structure and temperature effects

The oxidation of Co(poly) and Co(10$\text{1}$0) surfaces has been investigated using AES within the temperature range 300–700 K. It is found that at both surfaces the initial step — dissociative adsorption of oxygen — exhibits identical kinetics, independent of temperature. Approaching the oxide layer formation regime, temperature and surface structure of the substrate as well determine the oxygen uptake. With the aid of sputter profiling through the oxidized surface layers it is seen that at both, single crystal and polycrystal, oxygen is present even far below the actual surface. The diffusion of oxygen into the bulk is found to be faster at the polycrystalline sample.     

Determination of the atomic arrangement of the unreconstructed Ir (110) surface by low-energy electron diffraction a

An Ir(110)-(1 × 1) surface structure has been prepared by adsorbing $\text{1}\text{4}$ monolayer of oxygen at 850 K on a clean, reconstructed (1 × 2) surface. Results of the low-energy electron diffraction structure analysis reveal that the oxygen is probably distributed randomly over the crystal surface, and the (1 × 1) structure is the same as a clean unreconstructed (1 × 1) structure, with a topmost interlayer Ir spacing of 1.26 ± 0.05 Å. This is equivalent to a contraction of approximately 7.5% of the bulk interlayer spacing of 1.36 Å.     

The growth and alloy formation of copper on the platinum (111) and stepped (553) crystal surfaces; characterization by Leed,AES, and CO thermal desorption

Epitaxial layers of copper were formed on Pt(111) and Pt(553) single crystal surfaces by condensation of copper atoms from the vapor. Surface alloys were formed by diffusing the copper atoms into the platinum substrate at temperatures above 550 K. The activation energy for this process was found to be ~ 120 $\text{kJ}\text{mol}$. These Pt/Cu surfaces were characterized by LEED, AES, and TDS of CO. The copper grows in islands on the Pt(111) surface and one monolayer is completed before another begins. There is an apparent repulsive interaction between the copper atoms and the step sites of the Pt(553) surface which causes a second layer of copper to begin forming before the first layer is complete. Epitaxial copper atoms block CO adsorption sites on the platinum surface without affecting the CO desorption energy. When the copper is alloyed with the platinum however, the energy of desorption of CO from the platinum was reduced by as much as 20 $\text{kJ}\text{mol}$. This reduction in the desorption energy suggests an electronic modification that weakens the Pt-CO bond.     

The change of conductivity of erbium films caused by the sorption of hydrogen and oxygen

Films of erbium were evaporated onto the inside wall of a pyrex glass reaction vessel at 1O^?9 torr. Their mean thickness (200–600 Å) was deduced from their mass and geometrical area. Estimates of their surface areas were made from the physical adsorption of krypton at 78 °K (BET method) giving a mean specific surface area of 71 m^?2 g^?1. The number of surface sites was calculated from a relationship given by Brennan et al.¹⁾. The sorption of hydrogen and oxygen was studied separately, by measuring the changes in the electrical resistance of the films as a function of the amount of pure gas admitted to the reaction vessel in measured doses ～ 10¹⁸ atoms per dose. The experiments were repeated at 295 °K, 200 °K, 130–140 °K for both gases (measurements at 78 °K were not reproducible, probably because of a magnetic phase change). Hydrogen at 295 °K [as reported²⁾] caused an initial increase ΔR in the original resistance R to a maximum $\text{ΔR}\text{R}$ ～ 20 %, which was followed by a decrease to $\text{ΔR}\text{R}\text{= 0}$ and then to $\text{ΔR}\text{R}\text{< 0.}$ A similar pattern of resistance changes was observed at 200°K and 130–140°K with smaller changes of $\text{ΔR}\text{R}$, i.e. 16 % and 8 % respectively, but no significant differance in surface (θ) and bulk atomic ratios at which the maxima occurred, e.g. $\text{ΔR}\text{R}$ was a maximum for 1.6?θ?1.9.     

The effect of oxygen coverage on surface relaxation of Ni(110) measured by medium energy ion scattering

Changes have been observed in the upper layer spacing of a clean and an oxygen covered Ni(110) single crystal by employing medium energy ion scattering, combined with channeling and blocking. We find a contraction of 4% for the clean surface and a minor expansion of 1% for a surface with $\text{1}\text{3}$ monolayer of adsorbed oxygen.     

Directional anisotropy of surface self-diffusion on Pt(110)

The rate of surface self-diffusion on a Pt(110) single crystal in the [1$\text{1}$0] and [001] directions was measured at 1200–1750 K by monitoring the decay of a sinusoidal surface profile. The surface diffusion rate in the [1$\text{1}$o] direction was much faster than in the [001] direction. The activation energy of surface self-diffusion was 1.70 and 3.16 eV for the [1$\text{1}$0] and [001] directions, respectively, in good agreement with theoretical estimates. For large amplitudes of the profile the decay rate for the [001] direction was also dependent on the amplitude. This behavior can be explained by the appearance of (111) facets on the profile, which cause a retardation of the profile decay.     

Charge transfer between ZnO crystals and dye layers

The interaction between crystal and adsorbed dye molecules has been studied under well defined conditions by measurements of field effect and spectrally sensitized photoconductivity. The (101&#x0304;0) surfaces of n-type ZnO crystals (band gap 3.3 eV) are cleaned in ultrahigh vacuum. A pretreatment with atomic hydrogen produces an accumulation layer. Merocyanine (polymethine) dye molecules are deposited by sublimation in the same vacuum (coverage (1–2000) × 10¹⁴ cm^?2. Optical excitation of the dye causes a sensitized photoconductivity in the ZnO crystal close to the surface. The spectra distribution resembles the absorption spectrum of the dye with a maximum at 2.3 eV. An electric field applied perpendicular to the dye covered surface induces charge carriers in the crystal and changes the surface conductivity (field effect). Additional excitation of the dye by light causes a slow relaxation of the field-induced change of surface conductivity. This relaxation is observed for both signs of the field. Furthermore a memory of the dye covered crystals has been found. It can be programmed by field and light, read out via the surface conductivity and quenched by light. A phenomenological model for relaxation and memory is refined by kinetic equations and by considerations about charge transport within the dye layer. The observations can only be explained by a charge transfer between crystal and dye operating in both directions. From these results the following conclusion is drawn for the mechanism of spectrally sensitized photoconductivity of the present system: An electron transfer between dye molecules and crystal represents the decisive step rather than an energy transfer.     

H2O adsorption on Ni(100): Evidence for oriented water dimers

H₂O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below $\text{θ ? 0.5}$ to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H₂O coverage $\text{(θ ≧ 0.5)}$, the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at $\text{θ ? 1}$ a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order.     

Mesure de la conductivite electrique totale du protoxyde de cobalt en fonction de sa composition definie par des electrodes d'alliage cobalt-platine,entre 1000 et 1400 K

The total electrical conductivity of cobaltous oxide has been measured from 1000 to 1400 K as a function of its composition, especially in the range of low oxygen pressures up to the limit of equilibrium with cobalt. These measurements have been carried out with monocrystalline samples of oxide placed between cobalt-platinum alloy electrodes; oxide composition is fixed by the activity of cobalt in the alloy which is also the activity of cobalt in oxide. Results are given by isotherm curves of the logarithm of conductivity versus the logarithm of oxygen pressure. The slope of these isotherms shows a regular variation between $\text{1}\text{4}$ and less than $\text{1}\text{6}$; this fact can only be explained by the successive effects of the formation of vacancies V″_Co, V′_Co and of intrinsic conductivity by electrons and free holes.     

A new model for spectral sensitization of photoconduction in zinc oxide powder

Seven organic dyes were adsorbed on photoconductive zinc oxide powder and their sensitizing efficiencies were examined. The adsorption of dye induced a change in the dark conductivity, the magnitude of which appeared to represent the sensitizing power of the dye. The initial slope of sensitized photocurrent was found to be an exponential function of σ₀, the pre-exponential factor in the conductivity equation. A model is proposed, which is consistent with all the observations. In this model, the sensitized photoinjection of electrons is assumed to occur from surface states to the bulk, depending upon the population of the surface states. A critical surface level for the spectral sensitization is introduced to account for the observed temperature effect. Surface states located below the critical energy are assumed not to contribute for the sensitization.     

An IR study of the adsorption of water on Ru(001)

The strong OH stretch at 3400 cm^?1 (fwhm ～ 230 cm^?1) in the IR reflection absorption spectra of the system $\text{H}{}_2\text{O}\text{Ru(001)}$ at 85 K indicates the presence of hydrogen-bonded clusters. These clusters appear to form even at the lowest coverages. On the clean surface there is a linear relationship between integrated absorption intensity and coverage. The presence of small quantities of preadsorbed oxygen delays, however, the onset of absorption. It is thought that the oxygen atoms “bind” the water molecules, thus preventing cluster formation and in turn eliminating the intensity enhancement due to hydrogen bonding. Flash desorption spectra also indicate a second binding state when oxygen is coadsorbed. The relevance of these results to models of the electric double layer at the metal-electrolyte interface is discussed.     

The role of impurities in the stability of ZnO surfaces

Both “as-grown” and “real” etched prism and (000$\text{1}$) oxygen surfaces have been studied by LEED and Auger electron spectroscopy. Heat treatment up to 800 K was sufficient to remove impurities other than calcium on all surfaces and potassium on the polar “real” surface. These could only be removed by ion bombardment. The Ca was associated with a (3 × 1) superstructure on the prism surface and a $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the polar surface. On the “as-grown” polar surface it was also possible to see (3 × 3) structure associated with reduced amounts of Ca. The especially strong binding of the electropositive elements on the negative oxygen polar surface is due to charge transfer, i.e. impurity stabilisation, this in turn can lead to chemical shifts in some of the Zn Auger transitions and to changes in the oxygen peak shape.     

Oxygen adsorption on Ge(111) surface: I. Atomic clean surface

Oxygen adsorption on a clean Ge(111) surface has been studied in the temperature range 300–560 K by means of Auger electron spectroscopy (AES), thermal desorption (TD), work function (WF) measurements, and electron energy loss spectroscopy (ELS). The adsorption and WF kinetics at 300 K exhibit a shape different from those observed at higher adsorption temperatures. At 300 K oxygen only removes the empty dangling bond surface state, whereas at higher temperature new loss transitions involving chemically shifted Ge 3d core levels appear. The findings imply that at 300 K only a chemisorption oxygen state exists on the Ge(111) surface whereas the formation of an oxide phase requires higher temperatures. The shapes of the TD curves show that the desorption of GeO follows $\text{1}\text{2}$ order desorption kinetics.     

On the correlation of geometrical structure and electronic properties at clean semiconductor surfaces

In their LEED patterns most clean semiconductor surfaces show extra spots in addition to the normal beams as expected for a continuation of the bulk lattice up to the topmost layer. This means, that these surfaces are reconstructed. The (111) surfaces of silicon and germanium crystals exhibit another interesting feature: the cleaved surfaces show a metastable superstructure which irreversibly converts to a new modification upon heat treatments above about $\text{1}\text{3}$ of the melting temperatures. The structural changes are correlated with changes in the electronic properties of the surfaces. This has been shown by investigations of surface conductivity, surface photoconductivity, surface photovoltage, internal reflection, electron energy-loss spectroscopy, photoemission, and work function. Recent theoretical studies have also revealed a distinct correlation between geometrical arrangements of the surface atoms and the corresponding band structures of the surface states. However, the atomic positions of the surface atoms used in these calculations are only a model since a detailed analysis of the LEED-intensity-versus-energy curves have not been carried out up to now.