NaCl Thin Films Research Articles

Formation of Two-Dimensional Polarons that are Absent in Three-Dimensional Crystals

We report the direct time domain observation of a many-body process in a two-dimensional system: small polaron formation from the localization of a conduction band electron in NaCl thin films of unit cell thickness. Contrary to theoretical prediction for bulk NaCl crystal where an electron polaron does not exist, time-resolved two-photon photoemission reveals small polaron formation from delocalized conduction band electrons in crystalline NaCl thin films. The increased deformability and the reduced electronic bandwidth of a crystalline lattice in the thin film format are both responsible for the formation of small polarons that are absent in bulk solids.

Interaction of NaCl with solid water

The interaction of NaCl with solid water, deposited on tungsten at 80 K, was investigated with metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) (He I). We have studied the ionization of Cl(3p) and the 1b(1), 3a(1), and 1b(2) bands of molecular water. The results are supplemented by first-principles density functional theory (DFT) calculations of the electronic structure of solvated Cl(-) ions. We have prepared NaCl/water interfaces at 80 K, NaCl layers on thin films of solid water, and H(2)O ad-layers on thin NaCl films; they were annealed between 80 and 300 K. At 80 K, closed layers of NaCl on H(2)O, and vice versa, are obtained; no interpenetration of the two components H(2)O and NaCl was observed. However, ionic dissociation of NaCl takes place when H(2)O and NaCl are in direct contact. Above 115 K solvation of the ionic species Cl(-) becomes significant. Our results are compatible with a transition of Cl(-) species from an interface site (Cl in direct contact with the NaCl lattice) to an energetically favored configuration, where Cl species are solvated. The DFT calculations show that Cl(-) species, surrounded by their solvation shell, are nevertheless by some extent accessed by MIES because the Cl(3p)-charge cloud extends through the solvation shell. Water desorption is noticeable around 145 K, but is not complete before 170 K, about 15 K higher than for pure solid water. Above 150 K the NaCl-induced modification of the water network gives rise to gas phase like structures in the water spectra. In particular, the 3a(1) emission turns into a well-defined peak. This suggests that under these conditions water molecules interact mainly with Cl(-) rather than among themselves. Above 170 K only Cl is detected on the surface and desorbs around 450 K.

Heterogeneous Reaction of HOI with Sodium Halide Salts

The interaction of gaseous HOI with crystalline grains of NaCl and sea-salt and thin films of NaBr crystals has been studied in a wall coated tubular flow reactor coupled to a quadrupole mass spectrometer over a concentration range (0.2−8) × 1012 molecules cm-3 at 278 and 298 K. On a fresh surface, the uptake coefficients determined were independent of temperature with γ = 0.034 ± 0.009, γ = 0.016 ± 0.004, and γ = 0.061 ± 0.021 for NaBr, NaCl, and sea-salt, respectively. No increase in reactivity was observed on addition of water vapor between 0 and 23% relative humidity at 278 K. It was also shown that the reactivity of the salt surface decreased with time of exposure to HOI and that steady-state uptake was slower on “aged” salt surfaces. Products of the reactions released into the gas phase were IBr, ICl, and IBr + ICl for the reaction of HOI on NaBr, NaCl, and sea-salt surfaces, respectively. The atmospheric implications of the results obtained are briefly discussed.

Modifications in desorption kinetics of physisorbed species induced by colour centres on NaCl(100)

Low-energy electrons (65 eV) were used to produce anion vacancies (colour centres) at temperatures below 100 K in epitaxial thin films of NaCl(100) which were grown about ten monolayers thick on a Ge(100) substrate. A low-energy electron diffraction profile analysis of the NaCl diffraction spots during electron bombardment revealed that the defects created are randomly distributed across the NaCl surface at these temperatures. Whereas at low electron exposures predominantly colour centres are produced, at high exposures molecular vacancies are also formed. We present data of the desorption kinetics of physisorbates on both the undamaged surface and the damaged surface in the regime of dominant colour centre formation carried out by isosteric heat analysis and the leading-edge method. Zeroth-order desorption kinetics is found in the full range up to one monolayer, except for the first few percent, with constant parameters of desorption. The presence of colour centres causes a change in desorption kinetics for the physisorbed noble gases Ar, Kr and Xe and of CO 2 from zeroth- to first-order desorption in the low coverage regime up to 0.2 ML, providing evidence that the equilibrium between the two-dimensional (2D) gas and the 2D condensate is effectively destroyed. On the defect sites the atoms and also CO 2 are bound more strongly by 20–25% compared with the perfect NaCl(100) surface. From the experimental data and from simulations, a saturation concentration of colour centres on this surface of 10% is deduced.

Interface structure of alkali-halide heteroepitaxial films studied by x-ray-absorption fine structure

We have investigated the interface structures of KCl and NaCl thin films grown on NaBr(100) by means of Cl K-edge x-ray-absorption fine structure. The x-ray-absorption near-edge structure of 1-ML-thick KCl on NaBr indicates that the mixed crystal does not form at the interface. Through the analysis of extended x-ray-absorption fine structure, it was revealed that with a decrease in film thickness, the K-Cl bond distance is slightly shortened in KCl/NaBr(100), while the Na-Cl bond is elongated in NaCl/NaBr(100). The 1-ML-thick film does not, however, form coherent bonds with the substrate. This implies that the in-plane bond distance of the film does not coincide with that of the substrate but is close to the bond distance of the bulk material. For further understanding of the bond character between the film and substrate, Monte Carlo calculations have been carried out for the KCl/KBr system which gave a similar conclusion in our previous study. The calculated results were found to agree with the experimental results reasonably. These findings are contradictory with the previous diffraction results that have suggested the formation of coherent bonding at the interface. Such a discrepancy implies that the short-range order (bond distance) does not always reflect the long-range order (lattice constant).

Tensor low-energy electron diffraction analysis of the surface structure of NaCl(100) thin films grown on Pd(100) and Pt(111)

Thin films of NaCl were molecularly deposited and subsequently ordered on Pd(100) and Pt(111). These films exhibited the same adsorbate–substrate interaction on Pd(100) and Pt(111) as evidenced by the lone multilayer desorption peak observed in the temperature programmed desorption spectra of NaCl on these two substrates. The ordered structures of the films were produced by exposing a heated substrate to a flux of NaCl emanating from a Knudsen cell and investigated through the use of low-energy electron diffraction (LEED). On Pd(100), the adsorbed multilayer yielded a (1×1) NaCl(100) LEED pattern. The (1×1) pattern was seen as the result of the film's thickness being greater than the sampling depth of the elastically scattered electrons used in the LEED experiment, thus removing the contribution of the substrate's lattice to the observed pattern. For the (1×1) pattern of NaCl(100) on Pd(100), it was found that the NaCl film grew along the [010]-type surface directions of Pd(100) to take advantage of a near 1:1 lattice match. On Pt(111), the ordered NaCl multilayer gave a (1×1) NaCl(100) LEED pattern also, but, unlike Pd(100), one of the overlayer's unit cell vectors lies parallel to that of the substrate's with the other rotated by 90° from the previous one to preserve the geometry of the NaCl(100) lattice. Multiple intensity versus electron energy ( IV) data sets were gathered for the NaCl(100) multilayer film on both Pd(100) and Pt(111). An analysis of the I– V curves of the multilayer NaCl(100) pattern on both substrates showed that the multilayer has the same structure on both substrates to a depth sampled by the electrons in the experimental energy range. Data from both systems were used in a fully dynamical LEED calculation. In the final structure, which refined down to a Pendry R-factor ( R P) of 0.16, the largest deviation from the ideally terminated NaCl(100) structure was the movement of the surface Na + towards the bulk, thereby causing a 0.12±0.03 Å corrugation of the surface layer. This shift of the surface cations was not followed by a corresponding displacement of the underlying anions, so the first interlayer spacing, d(Cl 2–Na 1), was reduced to 2.72±0.03 Å from its bulk value of 2.82 Å. Deviations from the bulk were smaller in subsequent layers, e.g. the second interlayer spacing, d(Na 3–Cl 2), was 2.77±0.03 Å, and there was essentially no buckling of the second layer.

Holographic inversion of Kikuchi electron diffraction patterns for thin epitaxial NaCl films grown on GaAs(001)

Normal-incidence, multiple-energy Kikuchi electron diffraction patterns (holograms) have been measured for clean GaAs(001) surface and for thin NaCl films epitaxially grown on a GaAs(001) substrate. Three-dimensional atomic images of both structures have been obtained directly by the integral-energy phase-summing method. For the clean semiconductor surface, the superimposed images of Ga and As bulk atoms were well resolved within an accuracy of 0.5 Å. A distinctly different local atomic configuration was found after the deposition of the epitaxial NaCl film. Possible models of the NaCl/GaAs(001) epitaxy are discussed. A detailed analysis of the reconstructed diffraction patterns revealed that an initial formation of the Cl–Ga bond occurred in the system, and after deposition of 5 ML NaCl, the substrate was uniformly covered by a layer at least 3 ML thick.

Adsorption geometry of OH adsorbed at F centers on a NaCl(100) surface.

The orientation of molecular ${\mathrm{H}}_{2}$O in a bilayer on defect-free NaCl(100) and of chemisorbed OH adsorbed at F centers on the (100) surface of NaCl was investigated using angle-resolved O K edge near-edge x-ray-absorption fine-structure spectroscopy (NEXAFS). High-quality single crystalline thin films of NaCl(100) of about four monolayers thickness were grown on Ge(100). These films were bombarded with 250 eV electrons so that F and F\ensuremath{'} centers of a maximum density of about 10% of a monolayer were created. Whereas on the defect-free surface only molecular water is adsorbed, dissociation occurs at the F centers and ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ is bound. NEXAFS of molecular water shows almost no angular dependence even in the monolayer. This finding is consistent with formation of a hydrogen bonded bilayer of water suggested earlier. The OH species formed at the F centers is found to be inclined with respect to the surface normal by 39\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}. An adsorption model is proposed. \textcopyright{} 1996 The American Physical Society.

Multiple losses in off-specular electron energy loss spectra of thin NaCl films individually resolved in energy and momentum

A LEED system which provides high resolution in energy and momentum simultaneously has been used to record electron energy loss spectra of thin NaCl films epitaxially grown on Ge(100). While measurements in specular geometry show energetically equidistant dipole losses due to single and multiple excitations of the surface phonon polariton, off-specular spectra show multiple loss features which evidently are the result of a combination of one impact and several dipole scattering processes with individually resolved energy and momentum.

Ordering and phase diagrams of xenon adsorbed on thin epitaxial NaCl(100) films and on Ge(100).

An adsorption study of Xe on both the bare Ge(100) surface, as well as on thin NaCl films grown pseudomorphically on Ge(100), was carried out using high-resolution low-energy electron diffraction. Ordered phases and phase diagrams were investigated in quasiequilibrium with room-temperature gas. Starting from low pressures, a transition from a disordered lattice gas to a distorted hexagonal phase was found for Xe on NaCl(100) in addition to the hexagonal bulk phase, which grows as a multilayer phase. The distorted hexagonal phase was found to be commensurate with the substrate in only one direction. At lower temperatures and/or higher pressures, the onset of three-dimensional growth is observed in the form of a hexagonal nanocrystalline phase (average grain size 80 \AA{}) with Xe bulk lattice constant which is still oriented along the [110] direction with one of its axes. Both phase transitions are of first order, with pronounced hysteresis effects for the second. On Ge(100), no additional superstructure spots are induced by Xe before the bulk phase grows. The (2\ifmmode\times\else\texttimes\fi{}1) reconstruction is maintained, but characteristic jumps in intensity of integral order as well as of half-order beams are observed as a function of temperature and pressure so that Xe seems to lock on defined sites in (2\ifmmode\times\else\texttimes\fi{}1) registry. Phase diagrams for Xe adsorbed on both surfaces were derived and heats of transition at the phase boundaries were determined.

Morphology of thin NaCl films grown epitaxially on Ge(100)

A LEED profile analysis of NaCl films grown epitaxially on Ge(100) was carried out in order to determine the morphology of these films. Films of 3 and 8 double layers thickness were analysed. Due to steps of monatomic height of the Ge substrate, which do not fit to the NaCl lattice, the NaCl lattice is elastically strained over many lattice constants forming an elastic “carpet” across these steps. Far away from these steps the films are in registry with the Ge substrate. This model describes quantitatively the measured beam profiles, as tested both by analytic calculations and by computer simulations, and correctly predicts an increase of the strained areas as a function of film thickness.

Morphology of thin NaCl films grown epitaxially on Ge(100)

Morphology of thin NaCl films grown epitaxially on Ge(100)

Electron-beam patterning of materials with nanometer resolution

It was first reported in 1978 that intense electron irradiation of thin films of NaCl leads to the formation of holes with a diameter of the order of the probe. Since that time the same phenomena has been observed in a variety of materials, typical examples being AlF3, MgO and Al2O3. Quantitative and qualitative descriptions of the damage have been given by a number of workers in an attempt to understand the underlying mechanisms. While groups of chemically or structurally similar materials often show the same general sequence of events in the hole formation process, it is a common observation that polymorphs or different preparation techniques result in very different behavior, Furthermore, examples of hole formation have been given which appear to show characteristics specific to that material system.In an effort to gain insight into the hole formation process we decided to concentrate on a set of observations which are reasonably common.

Surface states on epitaxial thin films of NaCl and KCl

Thin epitaxial layers of NaCl and KC1 have been grown on clean Ge(100) in UHV and investigated in situ. It is shown that good quality films are produced with thicknesses up to 5 nm with no observable charging up. The electronic structure is measured both by UPS and EELS. On clean surfaces the well known bulk transitions are reproduced. Surface states within the forbidden gaps are indentified after water adsorption with a filled band at 3.4 and 2.2 eV above the valence band edge for NaCl and KC1 respectively. With EELS also an empty surface state band is found.

Surface states on epitaxial thin films of NaCl and KCl

Abstract Thin epitaxial layers of NaCl and KC1 have been grown on clean Ge(100) in UHV and investigated in situ. It is shown that good quality films are produced with thicknesses up to 5 nm with no observable charging up. The electronic structure is measured both by UPS and EELS. On clean surfaces the well known bulk transitions are reproduced. Surface states within the forbidden gaps are indentified after water adsorption with a filled band at 3.4 and 2.2 eV above the valence band edge for NaCl and KC1 respectively. With EELS also an empty surface state band is found.

Electron induced dissociation of the NaCl(111) surface

Thin NaCl films, epitaxially grown under ultra-high vacuum conditions, were electron bombarded and analyzed by reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES) and quadrupole mass spectrometry (QMS). A comprehensive analysis of the results indicates that the impinging electrons cause surface dissociation, and consequently desorption of neutral chlorine and sodium at a rate governed by an Elovich-type mechanism. Simultaneously, the increasing coverage of carbon-oxygen compounds seem to indicate that metallic sodium clusters are formed and oxidized by gas phase residuals, to form a disordered inert layer at the surface. A mechanism of this process is suggested and the implications to subsequent epitaxial growth are discussed.

Investigation of the phonon broadening of core levels in NaCl

We have investigated the temperature dependence of the Na +-2p-photoemission linewidth from thin films of NaCl. We found a large contribution to the linewidth due to coupling to lattice vibrations. Widths of the Na +-2s, Cl −-3s, Cl −-2p lines are given at T=300 K.

The dielectric response of alkali halide thin films

The infrared reflectivity of NaCl and KBr metal-backed thin films has been investigated. The experiments confirm the occurrence of anomalous structure in the frequency dependent self energy of the TO mode near the LO frequency. The results are compared with theoretical predictions based on the shell model, and fairly good agreement is obtained.

Low frequency dielectric losses in thin films of NaCl and NaBr

Abstract Measurements of permittivity and loss tangent in the frequency range 0.01 to 1000 c/s have been made on thin polycrystalline films of sodium chloride and sodium bromide. Two regions of dielectric dispersion have been found to exist, having loss tangent maxima typically near 1 c/s and 0.003 c/s at room temperature. The activation energies associated with these peaks are respectively 0.9±0.2, 0.95±0.1 for NaCl, and 0.7±0.1, 1.0±0.1 for NaBr. Permittivity values exceed 104 at high temperatures and low frequencies. It is proposed that the results may be interpreted in terms of two interfacial polarization mechanisms, which involve the blocking of cation vacancies, firstly at intercrystalline boundaries, and secondly at the electrodes, the latter process predominating at sufficiently low frequencies.

Reflection Spectrum and Structure in the Exciton Absorption Peak of NaCl and KCl

Measurements in the vacuum ultraviolet have been made on the transmission of thin films of NaCl and KCl and on the reflectivity of single crystals of these same salts. Comparison is made between the two types of measurements and with the absorptions measured in thin films by Schneider and O'Bryan. The agreement with the latter in the case of NaCl is good only over the region of the first absorption while in the case of KCl it is good over the whole region over which the measurements overlap. A new absorption is found in NaCl near 1100 A and in KCl just below 1000 A. In both salts the first fundamental absorption near 1600 A, presumed to be that of the exciton, as inferred from the reflectivity is at least a doublet at liquid nitrogen temperatures; but further multiplicity of the type predicted by Overhauser is not established, although possibly indicated. Absolute reflectivities of the order of 60% are measured at low temperatures.