NaCl Layer Research Articles

Synthesis, X-ray and optical characterizations of two new oxysulfides: LaInS 2O and La 5In 3S 9O 3

Two new compounds, LaInS 2O and La 5In 3S 9O 3 were synthesized in the La–In–S–O quaternary system. Both compounds crystallize in the orthorhombic system with lattice constants a=20.5421(6) Å, b=14.8490(4) Å, c=3.9829(1) Å for LaInS 2O, and a=4.1018(1) Å, b=26.833(1) Å, c=16.023(1) Å for La 5In 3S 9O 3. The structure of La 5In 3S 9O 3 was solved from single-crystal X-ray data, in the space group Pbcm, with Z=4; it is built from three-atom-thick (100) NaCl layers interleaved with fluorite-type ribbons, and is closely related to the structures of the known lanthanum and indium compounds La 10In 6S 17O 6 and La 4In 5S 13. Both compounds LaInS 2O and La 5In 3S 9O 3 exhibit a yellow color; measurement of their optical gaps gave 2.73 and 2.60 eV, respectively.

ESD thresholds for NaCl

Energy thresholds of electron stimulated desorption (ESD) for thin NaCl layers relative to the target current onset are measured. The applied procedure allows to correct electron beam energy for the contact potential difference in the system. The measured threshold energies are about 16.8 and 21 eV for Cl +. For Na +, Na and Cl the threshold energies are about 8.4 and 16.8 eV. The value of 8.4 eV coincides with the fundamental absorption edge and confirms the so-called “defect-mediated desorption” model of ESD. Basing on the experimental fact that the fundamental absorption edge appears to be a common threshold for different particles it is suggested that the near surface lattice instabilities induced by electronic transitions may result in a direct ejection of the constituents as well. The 16.8 eV threshold for Cl + confirms the involvement of an Auger transition in positive halogen ion emission. The 21 eV threshold is assigned to the shake-up/shake-off processes.

Synthesis and characterization of nanosized titanium particles by gas-phase combustion

Abstract Nanosized titanium particles are produced by gas-phase combustion under low-temperature. Combustion behavior of a TiCl4–Na mixture in a flow of argon was investigated by coflow flame method depending on the processing parameters such as burner shape, argon pressure and temperature. The nanosized titanium particles of 20–100 nm covered by a thin layer of NaCl were obtained at the flame temperatures of 510–585 °C. The optimal condition of nanosized titanium particles synthesis was found. It was established that the titanium particles are well distributed in the range of 50–80 nm sizes after the NaCl was removed by sublimation in vacuum. The formation mechanism of titanium crystal with the salt by coflow method is described.

The growth of NaCl on flat and stepped silver surfaces

The growth of thin NaCl layers at room temperature on a Ag(100) and a Ag(1, 1, 19)substrate has been investigated by LEED spot profile analysis, supplemented byEELS in order to clarify the electronic structure of the film. Our LEED analysisof NaCl/Ag(100) shows that the average orientation of the NaCl unit mesh isthe same as that of the Ag substrate, but rotational mosaics are formed inthe (100) plane. The angular distribution of rotational angles has a width of ± 9° around the average direction. From this result we conclude that the direction of thesubstrate step edges determines the orientation of the NaCl islands. The steps of the vicinalsilver surface are overgrown by elastic NaCl carpets. With increasing coverage, themodulation of the NaCl films by the step structure gets more and more washed out until,at around 10 ML, the whole film is only tilted by the nominal miscut of the sample. Closedfilms are only found at a thickness larger than 4 ML, as concluded from EELS.

Multi‐peak structure of the kinetic‐energy distribution of Na+ desorbed from NaCl during electron irradiation

AbstractResults of electron‐stimulated desorption of positive ions measured by the time‐of‐flight method from thin NaCl layers deposited onto Si(111) crystal are presented. The kinetic‐energy distribution of Na+ is characterized by at least a double‐peak structure suggesting desorption by the wave packet squeezing process, proposed recently for Li+ desorption from LiF. Moreover, a delayed Na+ signal is observed, indicating desorption of core‐excited Na** atoms being in metastable autoionizing states.

Enhancing the electroluminescent properties of organic light-emitting devices using a thin NaCl layer

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.

A Model for Structures Growth by Sodium Electrodiffusion in Quartz Crystals

Dendritic structures were grown inside the quartz crystal by electrodiffusion of sodium ions from a NaCl layer at the anode. The electric field was applied along the optical axis Z and the structures were grown in the xy plane. An experimental arrangement using a pointed cathode and a graphite plate anode was employed. X ray diffraction analysis shows that the structures' chemical composition is a mixture of sodium silicates. To explain the structures' growth a model is proposed. The model is a bidimensional one. The transport of the particles along the Z axis structural channels by the electric field was taken into account by introducing a probability of generating the particles in the growth plane as a function of the distance to the cathode. The computer generated results and the experimental ones were compared and a good correlation was found.

Gy sampling theory in environmental studies. 1. Assessing soil splitting protocols

AbstractFive soil sample splitting methods (riffle splitting, paper cone riffle splitting, fractional shoveling, coning and quartering, and grab sampling) were evaluated with synthetic samples to verify Pierre Gy sampling theory expectations. Individually prepared samples consisting of layers of sand, NaCl and magnetite were left layered until splitting to simulate stratification from transport or density effects. Riffle splitting performed the best, with approximate 99% confidence levels of less than 2%, followed by paper cone riffle splitting. Coning and quartering and fractional shoveling were associated with significantly higher variability and also took much longer to perform. Common grab sampling was the poorest performer, with approximate 99% confidence levels of 100%–150% and biases of 15%–20%. Method performance rankings were in qualitative agreement with expectations from Gy sampling theory. Precision results depended on the number of increments, the type of increment, and other factors influencing the probability of selecting a particle at random, and were all much higher than Pierre Gy's fundamental error estimate of 1%. A critical factor associated with good performance for these methods is a low conditional probability of sampling adjacent particles. Accuracy levels were dominated by the sampling process rather than by the analytical method. Sampling accuracy was at least two orders of magnitude worse than the accuracy of the analytical method. Published in 2002 by John Wiley & Sons, Ltd.

Nanoscale surface patterning by adsorbate-induced faceting and selective growth: NaCl on Cu( [formula omitted

We present a combined high-resolution electron diffraction (SPALEED) and scanning tunneling microscopy study of the insulator/metal growth system NaCl/Cu(2 1 1) which is inherently unstable against lateral pattern formation on the nanometer scale. Cu(2 1 1) is a vicinal surface with (1 1 1) terraces and intrinsic (1 0 0) steps (step distance 6.25 Å). This starting surface restructures upon NaCl deposition when the growth temperature exceeds ∼270 K. The initially flat surface is transformed into a periodic one-dimensional hill-and-valley structure consisting of (3 1 1) and (1 1 1) facets. NaCl grows selectively on (3 1 1) facets only, thereby creating a regular surface pattern with alternating stripes of bare Cu and chemically inert NaCl-covered areas. The lateral stripe spacing can be varied from ∼30 Å at 300 K to ∼230 Å at 600 K via the growth/annealing temperature. The present restructuring process is governed by the interplay between energetics and kinetics, namely by (1) the tendency to form (1 0 0)-terminated NaCl layers, (2) energetically favored interfacial matching between NaCl(1 0 0) and Cu(3 1 1), and (3) sufficient mobility of the substrate surface to allow for Cu mass transport.

The structure of NaCl(1 0 0) and KCl(1 0 0) single crystal surfaces: a tensor low energy electron diffraction analysis

Relaxation and rumpling of the (1 0 0) surfaces of NaCl and KCl single crystals were investigated by means of low energy electron diffraction (LEED). For primary currents in the range of 5 nA, sharp diffraction patterns of (1×1) symmetry were measured in the temperature range between 20 and 295 K. Under these conditions, neither charging effects nor electron-induced surface damage were observed. The temperature dependence of the peak intensities is described by a surface Debye temperature of 174±3 K for NaCl and 146±10 K for KCl, respectively. From the sequence of diffraction patterns measured in the energy range between about 60 and 300 eV, intensity versus electron energy curves ( I( V) curves) were obtained for several diffraction peaks, and analyzed with the tensor LEED approach. Structural as well as non-structural parameters were varied systematically. According to the LEED analysis, the distance between first and second layer of NaCl is contracted by 1.4(7)% relative to the bulk value, and the chloride ions in the first layer are lifted by 0.07(3) Å while the sodium ions are shifted towards the bulk by the same amount. The second and third layer show almost no rumpling and layer distances close to the bulk value. For KCl, the spacing between the top layers is identical to the bulk value within the error bars. The cations of the first layer are relaxed towards the bulk by only 0.03(5) Å and the anions by the same amount outwards. For both substrates, the refined final structures were characterized by Pendry R-factors of 0.13.

Chemical Composition of Structures Obtained Inside Quartz Crystals by Sodium Electrodiffusion

Dendritic structures were grown inside quartz crystals by electrodiffusion of sodium ions from a NaCl layer at the anode. An experimental arrangement using a point cathode and a graphite plate anode was employed. X-ray diffraction experiments were carried out, the structures chemical composition was found to be a mixture of sodium silicates. The chemical composition evolution in time was observed. There were made VIS and UV optical absorption spectroscopy measurements of these structures, a colloidal band being found. Thermoluminescence glow curves were measured. Based on the experimental results and observations a growth model is proposed.

The role of initial perturbations in the development of folds in a rock-analogue

A series of experiments is described in which layered specimens were shortened parallel to the layering. The specimens comprise two salt (NaCl) layers sandwiched between three layers of salt–mica synthetic schist. All specimens were prepared and deformed under the same conditions, except for the amount of shortening, which was varied. The resulting fold-shapes are variable, even where the amounts of shortening are the same. In one specimen, folds are believed to have developed essentially by buckling with very little concomitant bulk homogeneous shortening perpendicular to the axial-plane. The specimen lacks an axial-plane foliation. Other folds are believed to have experienced varying amounts of bulk homogeneous shortening before and during buckling, and all have axial-plane foliations that have developed by grain-scale transposition of the original bedding-parallel mica foliation. The difference in the behaviour of the various specimens is explained in terms of initial perturbations. These irregularities take the form of initial deflections in the almost planar bedding, variation in the degree of preferred orientation of the mica grains, and local compositional variation within individual salt–mica layers.

Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells

ABSTRACTLow substrate temperatures have to be used for polymer substrates. Therefore, using soda- lime glass (SLG) substrates with and without an alkali barrier (Al2O3), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm2 area) has been achieved.

Structure and electronic properties of ionic nano-layers MBE-grown on III–V semiconductors

Thin epitaxial NaCl and KBr layers of various thickness (from 3 to 100 monolayers) have been deposited on (100) surfaces of GaAs and InSb semiconductors by means of molecular beam epitaxy (MBE). Electronic and structural properties of the freshly prepared films were subsequently investigated in the attached UHV analytical chamber by means of low energy electron diffraction, low electron energy loss spectroscopy, Auger electron spectroscopy and an electron holography.Alkali halide growth mode was found to be a two-dimensional layer-by-layer type (Franck van der Merve growth mode). It was possible to demonstrate that the first monolayer of alkali halide on the AIIIBV semiconductor is arranged by the strong bond formed between the halogen ion and the AIII metallic element. In case of NaCl/GaAs(100) system a local atomic configuration was found for an early stage of epitaxy (three to five monolayers) by means of the electron holography. 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 five monolayers NaCl, the substrate was uniformly covered by a layer at least three monolayers thick.

Self-organized patterning of an insulator-on-metal system by surface faceting and selective growth: NaCl/Cu(211)

We report experimental results on an insulator-on-metal system which is inherently unstable against lateral pattern formation on the nanometer scale. NaCl deposition on Cu(211) at substrate temperatures >300 K leads to faceting into (311) and (111) facets and selective NaCl growth on (311) facets only, thereby creating alternating stripes of bare Cu and NaCl-covered areas. The mesoscopic restructuring process is brought about by (1) the tendency to form (100)-terminated NaCl layers, (2) epitaxial matching between NaCl(100) and Cu(311), and (3) sufficient mobility of the Cu substrate surface.

Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations

Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab initio calculations of the local density of states by means of the full-potential linearized augmented plane wave method, the protrusions could be attributed to the anion Cl –. The calculations shows that a higher density of occupied states at the Cl-sites than for the Na-sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the band gap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers.

Visible photoluminescence from the nanophase film prepared by GeAl co-evaporation

Abstract Nanocluster-assembled thin films have been prepared by a GeAl co-evaporation technique. The nanoclusters were deposited on crystalline silicon (c-Si) and quartz substrates and then coated with a layer of NaCl to prevent the nanoclusters from oxidation in air. The composition of the thin films is about 86 at.% Ge and 14 at.% Al. From transmission electron microscopy and X-ray diffraction patterns, it is clear that the thin films consist of Ge nanocrystals (NCs) and amorphous Ge (a-Ge) nanoclusters with incorporated Al atoms, and the mean diameter of Ge NCs is estimated to be about 5 nm. In the absorption spectrum, a shoulder peak appears at 3.44–4.05 eV, which results from the Ge NCs. Under 3.54 eV excitation, the photoluminescence (PL) peaks appear at 2.30, 2.43, and 2.82–3.02 eV. The strong PL peak at 2.82–3.02 eV is ascribed to a-Ge nanoclusters with incorporated Al atoms; the two weak PL peaks at 2.30 and 2.43 eV are assigned to Ge NCs and can be explained by the quantum confinement effect.

Observation of Redshifts of Photoluminescence from Thin Films Assembled by CuCl Nanoclusters

Two samples of thin films, assembled by CuCl nanoclusters, have been prepared by the gas evaporation technique. The CuCl nanoclusters were deposited on monocrystalline silicon and quartz substrates and then coated with a layer of NaCl to prevent oxidation of the CuCl nanoclusters. From transmission electron microscope and selected area diffraction, it is clear that the two samples consist of CuCl nanocrystals and Cu aggregates and the mean diameters are about 3 nm and 6 nm. From the absorption and photoluminescence (PL) spectra of the two samples carried out at ~300 K, we found that the peaks of exciton absorption do not show up and luminescence peaks shift to lower energies, possibly due to the strong coupling between exciton and phonon. At 77 K, the PL peaks of excitons broadened by exciton–phonon coupling have been observed, in approximate agreement with the values calculated by the quantum confinement model. The broad peaks of emission from trapped states or bands have also been observed at 77 K. In addition, the interaction between exciton and phonon is stronger in the sample with mean diameter about 3 nm according to a larger broadening and redshift of the PL peak from excitons, in qualitative agreement with theoretical prediction.

Secondary electron emission from ionic crystals

The emission of secondary electrons excited by 60–1000 eV primary electrons from NaCl, KCl, LiF and CaF 2 layers on W(110) ranging in thickness from less than one to several tens of monolayers is studied with a radial field analyzer. The energy distributions from NaCl and KCl layers are very narrow and have very high peaks at very low energies compared to the energy distributions from metals as exemplified by the substrate. LiF and CaF 2 have wider distributions than NaCl and KCl. Nevertheless, most electrons have energies below the ionization threshold of the halogen atoms; this has important consequences for electron-stimulated neutral and ion desorption.

Initial growth of insulating overlayers of NaCl on Ge(100) observed by scanning tunneling microscopy with atomic resolution.

The epitaxial growth start of an insulating NaCl film on a Ge(100) surface was directly imaged by scanning tunneling microscopy. Atomic resolution was achieved for islands of the first NaCl double layer with upright-standing NaCl dipoles. The tunneling current is preferentially determined by occupied Ge states extending into the NaCl layer. The results corroborate and extend the earlier proposed ``carpetlike'' growth mode of the NaCl layer over monoatomic Ge steps, even for small NaCl islands submonolayer coverage. \textcopyright{} 1996 The American Physical Society.