NaCl Layer Research Articles

Integer versus Fractional Charge Transfer at Metal(/Insulator)/Organic Interfaces: Cu(/NaCl)/TCNE.

Semilocal and hybrid density functional theory was used to study the charge transfer and the energy-level alignment at a representative interface between an extended metal substrate and an organic adsorbate layer. Upon suppressing electronic coupling between the adsorbate and the substrate by inserting thin, insulating layers of NaCl, the hybrid functional localizes charge. The laterally inhomogeneous charge distribution resulting from this spontaneous breaking of translational symmetry is reflected in observables such as the molecular geometry, the valence and core density of states, and the evolution of the work function with molecular coverage, which we discuss for different growth modes. We found that the amount of charge transfer is determined, to a significant extent, by the ratio of the lateral spacing of the molecules and their distance to the metal. Therefore, charge transfer does not only depend on the electronic structure of the individual components but, just as importantly, on the interface geometry.

The effects of sodium chloride (NaCl) and residues of cellulosic fibres derived from sodium carboxymethylcellulose (NaCMC) synthesis on thermal and mechanical properties of CMC films

The main aim of this work is to provide an accurate determination of the morphological, thermal and mechanical properties of NaCMC films. Indeed, their main applications require good thermal stability and mechanical strength and the literature lacks a good description of these properties. We analyse two types of NaCMC, an analytical (Fluka) and a crude (Niklacell) grade. For both grades, optical micrographs of NaCMC solutions show partially soluble cellulosic fragments. As evidenced by SEM, films prepared with Niklacell display the formation of opaque (CMC-fNo) and semi-transparent (CMC-fNt) regions due to the combined effect of the formation of a NaCl layer and the presence of fragments of cellulosic fibres. After the elimination of NaCl by precipitation of the acid form of the crude grade of CMC, the films (CMC-fNp) are slightly opaque compared to films prepared with analytical NaCMC grade (CMC-fF). This is due to the residues of cellulosic fibres not eliminated during the precipitation and filtration procedures. The TGA curves of the NaCMC films display a three-step degradation. The results of DSC analyses show that NaCl has an impact on the water content of films due to its hygroscopic behaviour. As evidenced by DSC and DMA, NaCl does not influence the Tm (which is determined at ca. 240°C) of NaCMC films. However, NaCl has a considerable impact on the thermal and mechanical properties close to the Tg. CMC-fF (analytical) and CMC-fNp (purified grade) exhibit similar Tg values at ca. 40°C and CMC-fNt and CMC-fNo (crude grade samples) show the highest temperature values for Tg and a remarkable broadening of this transition. NaCl also influences the E' modulus of NaCMC films in the temperature range studied as observed from DMA. The CMC-fF has the highest E' values.

Manipulation of the charge state of single Au atoms on insulating multilayer films.

We show charge-state manipulation of single Au adatoms on 2-11 monolayer (ML) thick NaCl films on Cu surfaces by attaching or detaching single electrons via the tip of an atomic force microscope (AFM). Tristate charge control (neutral, negatively charged, and positively charged) is achieved. On Cu(100) and Cu(111) supports, charge tristability is achieved independently of the NaCl layer thickness. In contrast, on Cu(311), only Au anions are stable on the thinnest NaCl films, but neutral and positive charge states become sufficiently long lived on films thicker than 4 ML to allow AFM-based charge-state-manipulation experiments.

Engineering the Band Structure of Nanoparticles by an Incommensurate Cover Layer

The electronic properties of nanometer size Au(111) vacancy islands are modified by covering the islands with an atomically thin NaCl(001) layer. The large-scale commensurability between the atomic lattices of the cover layer and the islands gives rise to various Moire-effect-induced standing wave patterns of which the periodicity and orientation depend on the relative angle between the lattices. The observed standing wave patterns are accompanied by the formation of multiple band gaps resulting from the Moire-effect-related periodic potential. Moreover, the energy level quantization and the coverlayer-induced energy shift give rise to partial depopulation of the NaCl/Au interface state. Targeted use of a thin covering layer and the resulting Moire-effect-related potential offers yet unexploited possibilities to conveniently modify the electronic structure of nanoparticles.

Effect of NaCl doped into Bphen layer on the performance of tandem organic light-emitting diodes

To improve the performance of tandem organic light-emitting diodes (OLEDs), we study the novel NaCl as n-type dopant in Bphen:NaCl layer. By analyzing their relevant energy levels and carrier transporting characteristics, we discuss the mechanisms of the effective charge generation layer (CGL) of Bphen:NaCl (6 wt%)/MoO3. In addition, we use the Bphen:NaCl (20 wt%) layer as the electron injection layer (EIL) combining the CGL to further improve the performance of tandem device. For this tandem device, the maximal current efficiency of 9.32 cd/A and the maximal power efficiency of 1.93 lm/W are obtained, which are enhanced approximately by 2.1 and 1.1 times compared with those of the single-emissive-unit device respectively. We attribute this improvement to the increase of electron injection ability by introducing of Bphen:NaCl layer. Moreover, the CGL is almost completely transparent in the visible light region, which is also important to achieve an efficient tandem OLEDs.

Adsorption Properties of Two-Dimensional NaCl: A Density Functional Theory Study of the Interaction of Co, Ag, and Au Atoms with NaCl/Au(111) Ultrathin Films

Recent experiments have shown that Co atoms deposited on NaCl/Au(111) ultrathin films give rise to spontaneous substitutional doping of the NaCl layer (Li et al. Phys. Rev. Lett. 2014, 112, 026102 ). In this work, the interaction of Co atoms deposited on NaCl/Au(111) films is investigated by means of density functional theory calculations. Co atoms have been adsorbed on various sites of the surface of unsupported and supported two- and three-layer NaCl films; incorporation of the Co atoms in interstitial positions or replacement of Na and Cl lattice ions has also been investigated. The Co adsorption properties are compared with those of Ag and Au atoms. We found that all metal atoms interact strongly with NaCl/Au(111) films, giving rise to large structural distortions of the insulating layer. Co, Ag, or Au incorporation in interstitial positions between the first and second NaCl layers is thermodynamically preferred over adsorption on surface sites. The results show the reactivity of NaCl with Co is highe...

Impact of Fe/NaCl (0 0 1) interface structure on electronic, magnetic and spin-polarized transport properties of Fe/NaCl/Fe (0 0 1) heterojunctions: An ab initio study

Structural, electronic and spin-dependent transport properties of Fe/NaCl/Fe (001) tunnel junctions with c(2×2) structure are studied by means of first-principles calculations. Several Fe/NaCl (001) interfaces are considered and their stability is analyzed. The interfacial charge transfer has an important role in stabilizing Fe/NaCl (001) interfaces. Interfacial iron magnetic moments are enhanced over the bulk value. Fe induced gap states are observed in barrier layers on both Na and Cl ions. Small exchange couplings with exponential decays as function of the barriers thickness are evidenced. Resonant tunnelling plays a major role on the spin-polarized transport properties. High magnetoresistance ratios comparable to Fe/MgO/Fe junctions are predicted. Total energy calculations show that interfacial interdiffusion is not favourable but the disorder at interfaces may strongly affect the electronic transport properties. Absence of interfacial interdiffusion together with the small lattice mismatch between Fe and NaCl layers are important advantages of Fe/NaCl/Fe (001) junctions for the purpose of spintronic applications.

The Anisotropy of Replicated Aluminum Foams

The replication casting process gives the open-cell aluminum foams that can be used in many industrial applications as well as in filtering technology. The essential requirement for filters is the uniformity of filtering degree which is defined by the minimal pore size. However the structure of replication castings is often inhomogeneous and the minimal pore radius is decreasing in the direction of melt infiltration. The objective of this investigation is to study the dynamics of melt impregnation of the porous medium by vacuum suction to identify the possibility of reducing the anisotropy. Theoretical data illustrate the processes at the boundary between melt and gas medium. The experiments were carried out using the replication aluminum samples produced according to commercial technology. It was found that the permeability coefficient varies throughout the height of castings. A method for estimation of pressure on the line of melt movement was proposed. The resistance of NaCl layer and circular vents of the mold causes the inhomogeneity of castings. Finally the ways of minimizing the anisotropy were offered.

The Effect of Sodium Chloride on the Process of Drying of Porous Media

The high temperature drying process of corundum cylinders partially saturated with aqueous solutions of sodium chloride was investigated with neutron imaging technique accompanied with mass and temperature measurements. The statistical analysis of neutron images was applied in order to quantify the time evolution of the process. The substantial slowing down of drying with increasing salt concentration was found. This e ect was attributed to increase of viscosity of the solution with increasing salt content as well as to the formation of an outer layer of NaCl on the sample surface.

Nanostripe Pattern of NaCl Layers on Cu(110)

A sodium chloride monolayer on a Cu(110) surface gives rise to a highly corrugated periodic nanostripe pattern of the (100) lattice as observed by scanning tunneling microscopy and low-energy electron diffraction. As revealed by density-functional calculations, this pattern is a consequence of the frustration of the overlayer-substrate chemical bonding produced by epitaxial mismatch. The coexistence of regions of strong Cu-Cl covalent and weak nonbonding interactions leads to a chemically induced topographic modulation here realized in a two-dimensional dielectric. The carpetlike growth of the NaCl layer across Cu step edges induces a distinct contrast inversion in the stripe pattern as a result of the change in epitaxial relationship due to the stacking sequence of the (110) Cu layers. It is demonstrated that the competition between local substrate-overlayer and intraoverlayer interactions can support a well-defined heteroepitaxial relationship of a ionic dielectric film and a metal surface, with important consequences for the nanoscale morphology and related properties.

Effect of Salt Coatings on Low Cycle Fatigue Behavior of Nickel -base Superalloy GTM-SU-718

Nickel-base superalloys are used as components of gas turbines both of jet engines as well as marine engines. Sin e these components are subjected to high temperature and oxidizing environment, their performance is drastically affected by the environmental conditions. Marine environment further aggravates the situation due to presence of salt (NaCl) particles in air. This salt along with sulphur and vanadium present in the fuel oil, leads to formation of compounds like sodium sulfate (Na2SO4) and vanadium pentaoxide (V2O5) during combustion and causes hot corrosion and stress corrosion cracking of engine components. Strain controlled low cycle fatigue tests were conducted on the nickel base superalloy GTM-SU-718 in air, at room temperature on unexposed, exposed at 550 °C for 25 h, exposed at 650 °C for 25 h as well as on the specimens coated with layers of NaCl, 25 wt.%NaCl + 75 wt.%Na2SO4 and 90 wt.%Na2SO4 + 5 wt.%NaCl + 5 wt.%V2O5 salt/salt mixtures separately and exposed at elevated temperatures for 25 h. While the NaCl coated sample was exposed at 550 °C, those coated with other two salt mixtures were exposed at 650 °C. It was observed that fatigue life of the NaCl coated sample, exposed at 550 °C for 25 h was reduced, however, there was little effect on fatigue life of the other specimens referred to above, including even those coated with salt mixtures and exposed at 650 °C.

Water nanodroplet impacts on surfaces: Effect of the substrate nature

The impact of 1–5km/s (H2O)n nanodroplets on flat surfaces, with normal or oblique incidence (60°), is modeled using molecular dynamics simulations. The focus is placed on the effect of the substrate, one of the targets being a rigid atomic layer with a repulsive interaction to the droplet, while the other one is a polar surface modeled by three layers of NaCl. In our simulations at 60° incidence, the velocity limit for droplet fragmentation is between 1.5 and 3km/s for both substrates. However, the dynamics and the energetics of the interaction are very dependent on the substrate nature. While ⩽2km/s droplets glide on the repulsive substrate, keeping most of their translational energy, they stick and stop on the polar substrate, transforming their energy into heat. The influence of the substrate is also pronounced for velocities above the fragmentation threshold, with much higher internal energies and more extensive fragmentation observed for the polar substrate. The results are mainly discussed on the basis of the particle distributions and energy partitions obtained upon interaction. In conclusion, our simulations demonstrate that the nature of the substrate cannot be overlooked in such impact processes.

Dependence of the NaCl/Au(111) interface state on the thickness of the NaCl layer

We investigated the growth and the electronic properties of crystalline NaCl layers on Au(111) surfaces by means of cryogenic scanning tunneling microscopy and spectroscopy under ultra-high vacuum conditions. Deposition of NaCl on Au(111) at room temperature yields bilayer NaCl islands, which can be transformed into trilayer NaCl islands by post-annealing. Upon NaCl adsorption, the Au(111) Shockley surface state becomes an interface state (IS) at the NaCl/Au(111) interface. Using Fourier-transform images of maps of the local density of states, the energy versus wave vector dispersions of the IS and the Au(111) bulk states are determined. The dispersion of both states is found to depend strongly on the thickness of the adsorbed NaCl layer.

Robust Surface Nano-Architecture by Alkali–Carboxylate Ionic Bonding

Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na-carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na-TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.

Coulomb attraction during the carpet growth mode of NaCl

The submonolayer growth of NaCl bilayer high-rectangular shaped islands on Ag(111) is investigated at around room temperature by using low temperature scanning tunneling microscopy. The growth at the step edges is preferred. Two kinds of islands are observed. They either grow with their non-polar edge at the step edge of Ag(111) or the islands overgrow in a carpet-like mode with the polar direction parallel to the edge. In the latter case, the Ag step is rearranged and considerable, while the NaCl layer is bent. This study clarifies the nature of the interaction of an alkali halide nanostructure with a metal step edge.

Atomic and Electronic Processes during the Formation of an Ionic NaCl Monolayer on a Covalent Si(100) Surface

An atomic layer of stoichiometric NaCl was formed on a covalent Si(100) surface after two successive half-reactions at room temperature. The first half-reaction due to Cl2 exposure generates a square array of Cl adatoms with a distance close to that in a NaCl(100) surface plane. By utilizing scanning tunneling microscopy (STM), core-level photoemission spectroscopy, and ab initio density functional theory (DFT) calculations, it was found that progressive deposition of Na in the second-half reaction results in surface-supported Na3Cl clusters, one-dimensional cluster chains, and (2 × 2) patches, and eventually turns the Cl-adlayer into a single-terrace, wavy NaCl layer at one monolayer Na coverage. The grown NaCl monolayer rolls over atomic steps like a carpet and covers the entire surface.The atomic and electronic structure of the topmost Si layer underneath the NaCl layer resembles that of the initial silicon surface layer with buckled dimers. Results of the comprehensive investigation together suggest t...

Imaging the charge distribution within a single molecule

Scanning tunnelling microscopy and atomic force microscopy can be used to study the electronic and structural properties of surfaces, as well as molecules and nanostructures adsorbed on surfaces, with atomic precision, but they cannot directly probe the distribution of charge in these systems. However, another form of scanning probe microscopy, Kelvin probe force microscopy, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity that is closely related to the charge distribution on the surface. Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules.

Atomic Force Microscopy Reveals Bistable Configurations of Dibenzo[a,h]thianthrene and their Interconversion Pathway

We investigated dibenzo[a,h]thianthrene molecules adsorbed on ultrathin layers of NaCl using a combined low-temperature scanning tunneling and atomic force microscope. Two stable configurations exist corresponding to different isomers of free nonplanar molecules. By means of excitations from inelastic electron tunneling we can switch between both configurations. Atomic force microscopy with submolecular resolution allows unambiguous determination of the molecular geometry, and the pathway of the interconversion of the isomers. Our investigations also shed new light on contrast mechanisms in scanning tunneling microscopy.

Photoisomerization for a molecular switch in contact with a surface

The photoisomerization properties of an azobenzene derivative on a thin insulating NaCl layer on Ag(111) are investigated with a low-temperature scanning tunneling microscope and density functional calculations. Illumination with UV light at 365 nm induces the reversible direct isomerization of the adsorbed species, while visible light does not lead to any changes. This unexpected behavior cannot be explained by the change of the electronic structure upon adsorption on the inert surface. It is rationalized in terms of electrostatic interactions caused by the atomistic details of the surface.

Thermal diffusivity measurement in a diamond anvil cell using a light pulse thermoreflectance technique

Apparatus to measure the thermal diffusivity of a plate between diamond anvils at high pressure was developed. The Mg(OH)2 plates coated with Pt films on both sides were used for testing the apparatus. One side of the Pt film played the role of an absorber of a laser pulse, and the other side was used for the detection of the temperature change. The sample was loaded into a hole drilled in a rhenium foil gasket together with the insulation layers of NaCl as a pressure medium. Subsequently, the sample was compressed with 150 µm culet beveled diamond anvils. The pressure in this study reached 57 GPa. Then, a front face of the sample was heated by laser pulses with a duration of 2 ns and a wavelength of 1064 nm. To probe the temperature change at the back face of the sample, a continuous wave laser beam with a wavelength of 782 nm was irradiated. Using the reflected intensity of the continuous wave laser beam, a transient temperature curve was measured when the heat transferred across Pt/Mg(OH)2/Pt layers. The thermal diffusivity of Mg(OH)2 was calculated by analyzing the transient temperature curves. The thermal diffusivity of Mg(OH)2 is 5.9 × 10−6 m2 s−1 at 57 GPa, which is larger than that at ambient pressure by a factor of ≈6.