Sodium Chloride Surface Research Articles

Electron energy loss spectroscopy of nanometer-sized Cr crystallites in MgO single crystals

Conposite films including nanometer-sized metal crystallites and atomic clusters in ceramics or insulators are currently of great interest from the points of view of basic science and technology. We have developed new single crystalline composite films of magnesium oxide(MgO) and nanometer-sized metal crystallites. The crystalline structures were studied in detail by HREM and nanometer-area diffraction. In the present study, electron energy loss spectroscopy(EELS) of Chromium(Cr)-MgO composite films was performed to study the oxidation state of the Cr crystallites in the MgO matrix.Preparation methods of the composite films were described in detail in a previous paper. Cr and MgO were simultaneously deposited on a (001) cleaved surface of sodium chloride in UHV conditions of 10−6 Pa, the mean thickness of Cr and MgO being 3 nm and 30 nm, respectively. The composite films were separated from the substrate in water and mounted on a perforated carbon film. The EELS was carried out at 100 kV by using an electron microscope with a field emission gun (Philips FEG 400ST) equipped with a parallel detection spectrometer (Gatan 666).

ChemInform Abstract: Predesorption of CO from the Sodium Chloride (100) Surface: Study by the Complex Coordinate Method.

ChemInform Abstract: Predesorption of CO from the Sodium Chloride (100) Surface: Study by the Complex Coordinate Method.

Predesorption of CO from the sodium chloride (100) surface: Study by the complex coordinate method

The rotational predesorption resonances of CO adsorbed on NaCl(100) were calculated by the quantum mechanical complex coordinate method, the CO bond length being varied adiabatically. It is shown that the internal vibrational excitation of adsorbed CO by an infrared laser does not have a strong effect on the resonance positions and lifetimes, i.e., predesorption states, and the desorption of CO from the NaCl(100) surface is probably a rotational predissociation phenomenon.

Growth of Al-Mn quasi-crystals by vacuum deposition

Thin films of Al-Mn quasi-crystals are prepared by a successive deposition technique in vacuum onto air-cleaved sodium chloride surfaces. At a substrate temperature of 280 to 330°C, the growth of the quasi-crystals is found to produce polycrystalline films with a grain size of 20 to 50 nm, the composition ratio of aluminium and manganese being about 5:1. No structure change can be found after a heat treatment of the film at 280°C for 8 h, though a trend of grain growth is detected. For a specimen with a composition ratio above 8:1 grown at 280°C, a diffraction pattern due to aluminium with (111) orientation is recognized in addition to that of quasi-crystals, the lattice relation being that the [110] direction of aluminium is parallel to the two-fold axis of the quasi-crystal. A heat treatment at 330°C for a few hours brings about a transformation from quasi-crystalline films to an alloy phase, Al 6Mn. High resolution electron microscopy is applied to the quasi-crystal films and the edge length of the fundamental hexahedron of the Penrose tiling has been determined to be 0.47 nm from the structure images.

Thin film nucleation and growth kinetics

The nucleation and growth of thin films generally proceeds via one of three growth modes: particle growth, layer growth or layer-plus-island growth. For situations where the substrate is atomically flat and defect-free, a well defined theory to describe the kinetics of the process has now been developed. However the extent to which the relevant theoretical expressions have been tested in unperturbed situations is still limited. A discussion is presented of the basis and relevant predictions of the theory, together with some examples of the type of experiments being used to test these predictions. In the case of particle growth, emphasis is given to describing recent theoretical and experimental studies relating to the deposition of gold on UHV cleaved sodium chloride surfaces. This work gives an example of how perturbing effects can be recognised and included in the theoretical treatment whilst at the same time it finally provides an analysis of what is probably the most studied of all nucleation systems.

Growth of AlMn quasicrystals by vacuum deposition

Thin films of AlMn quasicrystals are prepared by a successive deposition technique in a vacuum onto air-cleaved sodium chloride surfaces at a substrate temperature of 280–330°C. The growth of the quasicrystals is found as random orientated films with a grain size of 20–50 nm for Al-21 at.% Mn. For a specimen of Al-5at.% Mn grown at 280°C, quasicrystals and aluminum with (111) orientation are recognized, the lattice relation being that the [110] direction of the aluminum is parallel to the twofold axis of the quasicrystal. Heat treatment at 330°C for a few hours brings about a transformation from quasicrystalline films to an equilibrium phase Al 6Mn. High resolution electron microscopy is applied to the quasicrystal films, by which the edge length of the fundamental hexahedron of the Penrose tiling is determined to be 0.47 nm from the structural images.

The role and structure of microclusters in metal deposition onto weak-bonding substrates

The structure of the smallest clusters of atoms forming during thin film deposition is considered. It is proposed that, for films forming via the Volmer- Weber or three-dimensional particle growth mode, these microclusters will have well-defined, tightly bound, low energy, non-crystalline structures. These predicted structures are based on published least energy configuration calculations. Evidence to support the proposed structures is presented from observations of gold deposits made on sodium chloride surfaces cleaved under ultrahigh vacuum conditions at temperatures ranging down to −150°C. The direct observation of such structures may soon be possible through scanning tunnelling microscopy.

Direct determination of the surface potential on sodium chloride single crystals. II. Separate anion and cation defect parameters

For pt.I see ibid., vol.18, p.4845 (1985). By measurement of the variation with temperature of the potential on sodium chloride surfaces, freshly cleaved in ultrahigh vacuum, it has been found possible to obtain a value for the enthalpy h- and entropy s- of formation of an individual cation vacancy. The method depends on the location of the isoelectric temperature Ti at which the surface potential is zero and of the knee temperature Tk that characterises the change from extrinsic to intrinsic behaviour. Location of Ti requires an experimental method for determining the interface potential between a metal electrode and the crystal surface opposite to that on which surface potential is being measured. The values obtained are h-=1.15+or-0.02 eV and s-=4.1 k+or-0.2 k eV K-1 and hence, by using known values of the Schottky formation enthalpy and entropy, the corresponding quantities for the anion vacancy may also be determined.

Atomic force microscopy of liquid-covered surfaces: Atomic resolution images

Images of graphite surfaces that are covered with oil reveal the hexagonal rings of carbon atoms. Images of a sodium chloride surface, protected from moisture by oil, exhibit a monoatomic step. Together, these images demonstrate the potential of atomic force microscopy (AFM) for studying both conducting and nonconducting surfaces, even surfaces covered with liquids. Our AFM uses a cross of double wires with an attached diamond stylus as a force sensor. The force constant is ≊40 N/m. The resonant frequency is ≊3 kHz. The lateral and vertical resolutions are 0.15 nm and 5 pm.

The role of preferred surface sites in the nucleation and growth of gold on sodium chloride II: Reanalysis of two comprehensive studies

The role of preferred sites induced by low energy electrons in the nucleation of gold on sodium chloride surfaces was demonstrated in the first of two papers sharing the above title. In this second paper, data from two previous kinetic studies of gold deposition onto sodium chloride are reanalysed, taking into account the above-mentioned mechanism of generating preferred sites. This has succeeded in removing several significant inconsistencies which were apparent in the previous analyses of these studies. This treatment of a situation where preferred nucleation sites are being created continuously during the deposition serves to illustrate the power and adaptability of the rate equation approach for analysing nucleation and growth kinetic data once the active processes occurring have been determined.

Fundamentals and present state of surface decoration

Recent studies of nucleation at surface steps have elucidated the basic mechanisms of step decoration. A systematic study of the condensation of various metals on sodium chloride surfaces demonstrates the influence of the deposition parameters on resolution and contrast of this electron microscope method. In the standard process of decoration which is based on three-dimensional nucleation, the decoration of atomic steps pre-supposes extremely high supersaturations. At low supersaturations involving large critical nuclei only high steps can be decorated. By means of a combined coarse and fine decoration the nuclei can be localized with respect to the step level. A survey of crystal materials accessible to step decoration as well as future areas of application is given.

Dynamics of the gas-surface interaction and the mechanism of condensation of RbI molecules incident on NaCl (100) surfaces

Reflection coefficients and complete angular distributions have been measured at angles of incidence between normal and near grazing for thermal beams of RbI molecules which were scattered from clean and beam-covered NaCl (100) surfaces. For fixed angles of incidence and scattering the intensity of scattered molecules was measured as a function of coverage of RbI on the sodium chloride surfaces. Dynamics of the gas-surface interactions are discussed and a mechanism for condensation of RbI is proposed.

Infrared spectroscopic investigation of the adsorption of fluoromethane on sodiumchloride surfaces under ultra high vacuum

The adsorption of CH 3F on NaCl between 77 K and 250 K was investigated using infrared spectroscopy under ultra high vacuum conditions. All internal normal modes of the adsorbate were observed and assigned. Three adsorption phases were detected. The largest frequency shift with respect to the gas takes place for the most intense mode, the C-F stretch ν 3, − 102 cm −1 ( 10 %) for α-CH 3F-NaCl(film) on NaCl(100), while the other modes were much less influenced. No splitting of the vibrations degenerate in the gas occurs. Adsorption isotherms and the isosteric heats of adsorption were determined.

E.s.r. studies of the formation of hydrocarbon radicals on the clean surfaces of evaporated ionic solids

Cyclohexyl and allyl radicals are detected by e.s.r. spectroscopy when cyclohexane and 2,5-dimethylhex-3-ene are deposited on to the uncontaminated, continuously renewed surface of sodium chloride evaporated on to the cold surface of a rotating cryostat at 77 K.

The nucleation and growth of thin films on sodium chloride cleavage steps

In the present investigation we used scanning and transmission electron microscopy to study the cleaved sodium chloride surface. Several metals with different melting points were used to decorate the cleavage steps. It was observed that step decoration can be achieved with epitaxial, polycrystalline and amorphous thin films. The difference in the nucleation density at steps and edges and the nucleation density at smooth surfaces is attributed to the smaller activation energy required to nucleate an island in the former case. The pitting on NaCl could be caused by a contaminated vacuum system or by the exposure of the cleaved face to air.

The application of the gas–solid virial expansion to argon absorbed on the (100) face of sodium chloride

The gas–solid virial expansion is developed for argon adsorbed on the (100) sodium chloride surface. Limiting parameters for the attractive components of the gas-ion interaction are suggested and the corresponding repulsive parameters for the Li+, Na+, K+, Rb+, Cs+, F–, Cl–, Br–and I–ions are determined from an analysis of alkali halide bulk crystal data. The second two dimensional virial coefficient for argon absorbed on (100) sodium chloride is determined over a range of temperatures by using the generalized ‘exponential-six’ equation for all interatomic interactions. The results indicate large changes from the ideal perfectly mobile two-dimensional second virial coefficient caused by the inclusion of perturbation terms that take account of the periodic structured surface. These changes are shown to be sensitive to the exact structure of the crystal at the interface as shown by adsorption of argon on the (100) sodium chloride surface which has been allowed to relax to its equilibrium conformation. The third body (surface) interactions at the interface are considered together with the applicability of the two dimensional approximation for the adsorbate self interactions. The zero coverage isosteric enthalpy and Henry’s law constants at 80 K are computed for each of the gas-ion potentials that are chosen. The results are found to agree with those obtained by using the hindered translation model and from a comparison with experimental data, suggest a larger attractive gas–crystal interaction than previously thought.

The influence of ordered surface point defect arrays on the epitaxial deposition of gold upon (100) surfaces of sodium chloride

The effect of an ordered array of sodium ion vacancies (incorporated into a (100) surface of sodium chloride) upon the epitaxial temperature associated with gold being deposited onto that (100) surface has been investigated. A dramatic reduction in the epitaxial temperature has been observed, and single-crystal correctly oriented gold films have been achieved at - 30 °C upon those areas of the sodium chloride surface containing the point defects.

Gold Decoration of Etched Polycrystalline KCl

The gold decoration method described by Bassett was shown to be capable of displaying unit lattice steps on cleavage surfaces of sodium chloride. By this technique slip steps could be readily distinguished from cleavage steps. Bassett's technique was refined by others to include cleaving the crystal in vacuum and heating to obtain thermal etch patterns from which information on the sites and nature of dislocations could be deduced. Another use for the technique was to decorate PbCl2 inclusions in KCl where the resulting patterns suggested the presence of double electric layers surrounding the precipitate.

Molecular orientation by surface forces: I. On the replication of surface forces by plastic materials

We have attempted to replicate the surface electrical profile of freshly cleaved sodium chloride crystals using materials with known electret characteristics such as; polyethylene; polyvinyl acetate; polymethyl methacrylate; polyvinyl chloride; polystyrene and carnauba wax. Only P.V.C. replicas exhibit surface orientational properties. A theory is presented to account for the orientation of polar organic molecules on polyvinyl chloride replicas of sodium chloride surfaces and evidence is reported for the existence of long range surface forces arising from gross crystallographic defects.

Epitaxial Growth of CdSe Films Evaporated on Sodium Chloride

The orientation and structure of CdSe films evaporated on air-cleaved surfaces of sodium chloride were studied at various stages in film growth by means of a transmission electron microscope. The substrate temperature at which oriented overgrowth occurred was found to be about 130°C lower on step-rich regions than on step-free regions of the cleavage surface. This result is explained as the effect of coalescence in the initial stage of growth. In process of the growth in size of nuclei, thin layers of wurtzite type crystal grow on {111} planes of zincblende type crystals. The layers were found to be formed preferentially on the inclined {111} planes which intersect the rocksalt surface along <110>NaCl. This fact suggests that zincblende type crystals are apt to grow due to stacking of close-packed layers on these {111} planes. The structure of CdSe films on (111) surfaces of sodium chloride was also studied in connection with results in the cleavage surface.