NaCl Thin Films Research Articles

Molecular beam epitaxy of GaAs templates on water soluble NaCl thin films

Expensive III-V substrates are cost limiting for the adoption of many technologies. Thus, being able to reuse the original substrate is highly desirable. Existing substrate reuse techniques have significant drawbacks, but this work discusses a new method using molecular beam epitaxy deposition of water soluble NaCl thin films on commonly employed (001) GaAs substrates. Single-crystal GaAs templates are grown on continuous NaCl layers utilizing careful exposure of the NaCl to an in-situ electron beam and a low temperature nucleation layer. The template layers can be quickly removed from the substrate via dissolution of the NaCl. After liftoff, the original wafer shows an increase in rms surface roughness of only 0.2 nm.

Electron beam analysis induces Cl vacancy defects in a NaCl thin film

This work reports on the electron-induced modification of NaCl thin film grown on Ag(110). We show using low energy electron diffraction that electron beam bombardment leads to desorption and formation of Cl vacancy defects on NaCl surface. The topographic structure of these defects is studied using scanning tunneling microscopy (STM) showing the Cl defects as depressions on the NaCl surface. Most of the observed defects are mono-atomic vacancies and are located on flat NaCl terraces. Auger electron spectroscopy confirms the effect of electron exposure on NaCl thin films showing Cl atoms desorption from the surface. Using density functional theory taken into account the van der Waals dispersion interactions, we confirm the observed experimental STM measurements with STM simulation. Furthermore, comparing the adsorption of defect free NaCl and defective NaCl monolayer on Ag(110) surfaces, we found an increase of the adhesion energy and the charge transfer between the NaCl film and the substrate due to the Cl vacancy. In details, the adhesion energy increases between the NaCl film and the metallic Ag substrate from 30.4 meV Å−2 for the NaCl film without Cl vacancy and from 39.5 meV Å−2 for NaCl film with a single Cl vacancy. The charge transfer from the NaCl film to the Ag substrate is enhanced when the vacancy is created, from 0.63e− to 1.25e−.

Exotic Two-Dimensional Structure: The First Case of Hexagonal NaCl.

NaCl is one of the simplest compounds and was thought to be well-understood, and yet, unexpected complexities related to it were uncovered at high pressure and in low-dimensional states. Here, exotic hexagonal NaCl thin films on the (110) diamond surface were crystallized in the experiment following a theoretical prediction based on ab initio evolutionary algorithm USPEX. State-of-the-art calculations and experiments showed the existence of a hexagonal NaCl thin film, which is due to the strong chemical interaction of the NaCl film with the diamond substrate.

Verification Study of Nanostructure Evolution with Heating Treatment between Thin and Thick Fullerene-Like Hydrogen Carbon Films

Fullerene-like hydrogen carbon films with a thin film grown on a NaCl substrate are usually employed to show the nanostructure of films (usually of hundred nanometers thick grown on Si substrates) under high resolution transmission electron microscopy (HRTEM) tests because it is easier floated off, where dependability and reasonability has never been seriously contested. Thus, in this paper, thin and thick hydrogen carbon films have been deposited on NaCl (thin films) and Si (thick films) substrates and annealed under room temperature to 500 °C, of which nanostructures have been investigated by HRTEM, Raman spectroscopy, and X-ray photoelectron spectroscopy, to verify the dependability and reasonability of the NaCl method. The results showed heating induced graphitization but with hydrogen content nearly unchanged. HRTEM results revealed that under annealing of 200, 250, and 300 °C, the curved graphene structures gradually increase in films. However, beyond 400 °C, onions structures are present. However, both Raman and XPS spectra show us that after annealed treatment, for original films, both thin and thick films have the near sp2 bonding content and size, but with the annealing temperature increase, sp2 bonding content increases more quickly for thick FL-C:H films due to the higher internal stress compared to thin films. In one word, the NaCl method used for nanostructure detection for films might be a good choice for an easier and quicker analysis, but it is still insufficient, because the heating effect induced by plasma cannot be ignored.

Changes of Oxygen Diffusion Behavior in NaCl Thin Films Formed on Zn Sheets with Temperature

Changes of Oxygen Diffusion Behavior in NaCl Thin Films Formed on Zn Sheets with Temperature

Water-Soluble Epitaxial NaCl Thin Film for Fabrication of Flexible Devices

We studied growth mechanisms of water-soluble NaCl thin films on single crystal substrates. Epitaxial growth of NaCl(100) on Si(100) and domain-matched growth of NaCl(111) on c-sapphire were obtained at thicknesses below 100 nm even at room temperature from low lattice mismatches in both cases. NaCl thin film, which demonstrates high solubility selectivity for water, was successfully applied as a water-soluble sacrificial layer for fabrication of several functional materials, such as WO3 nano-helix and Sn doped In2O3 nano-branches.

Donor-Acceptor Properties of a Single-Molecule Altered by On-Surface Complex Formation.

Electron donor-acceptor molecules are of outstanding interest in molecular electronics and organic solar cells for their intramolecular charge transfer controlled via electrical or optical excitation. The preservation of their electronic character in the ground state upon adsorption on a surface is cardinal for their implementation in such single-molecule devices. Here, we investigate by atomic force microscopy and scanning tunneling microscopy a prototypical system consisting of a π-conjugated tetrathiafulvalene-fused dipyridophenazine molecule adsorbed on thin NaCl films on Cu(111). Depending on the adsorption site, the molecule is found either in a nearly undisturbed free state or in a bound state. In the latter case, the molecule adopts a specific adsorption site, leading to the formation of a chelate complex with a single Na+ alkali cation pulled out from the insulating film. Although expected to be electronically decoupled, the charge distribution of the complex is drastically modified, leading to the loss of the intrinsic donor-acceptor character. The chelate complex formation is reversible with respect to lateral manipulations, enabling tunable donor-acceptor molecular switches activated by on-surface coordination.

Charge-State-Dependent Diffusion of Individual Gold Adatoms on Ionic Thin NaCl Films.

It is known that individual metal atoms on insulating ionic films can occur in several different (meta)stable charge states, which can be reversibly switched in a controlled fashion. Here we show that the diffusion of gold adatoms on NaCl thin films depends critically on their charge state. Surprisingly, the anionic species has a lower diffusion barrier than the neutral one. Furthermore, for the former we observe that the diffusion atop a bilayer of NaCl is strongly influenced by the interface between NaCl and the underlying copper substrate. This effect disappears for a trilayer of NaCl. These observations open the prospect of controlling the diffusion properties of individual metal atoms on thin insulating films.

Self-Doping of Ultrathin Insulating Films by Transition Metal Atoms

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e., at Na and at Cl locations. Using density functional based simulations of the STM images, we show that the Co atoms substitute with either a Na or Cl atom of the NaCl surface, resulting in cationic and anionic Co dopants with a high thermal stability. The dependence of the magnetic coupling between neighboring Co atoms on their separation is investigated via spatially resolved measurement of the local density of states.

Thin NaCl films on silver (001): island growth and work function

The surface work function (WF) and substrate temperature dependence of the NaCl thin-film growth on Ag(001) have been studied by noncontact atomic force microscopy and Kelvin probe force microscopy. In the sub-monolayer range, the NaCl film is composed of large crystalline islands, which decrease in density and increase in size with increasing temperature during deposition. Each island is composed of a large base island 2 monolayers (ML) thick (for T > 343 K), which collects impinging NaCl molecules that form ad-islands on top. Kelvin probe force microscopy (KPFM) measurements show a reduction of the silver WF by with no dependence on the film thickness (1–6 ML), in agreement with recent theoretical calculations. The previously observed nanometer-sized moiré pattern on islands that are 45° rotated with respect to the silver lattice could be observed in the scanning tunneling microscopy mode. However, no contrast could be obtained in KPFM images.

Design and fabrication of a multi-purpose soft x-ray array diagnostic system for KSTAR

A multi-purpose soft x-ray array diagnostic system was developed for measuring two-dimensional x-ray emissivity profile, electron temperature, Ar impurity transport, and total radiation power. A remotely controlled filter wheel was designed with three different choices of filters. The electron temperature profile can be determined from the ratio of two channels having different thickness of Be layer, and the Ar impurity concentration transport can be determined from a pair of Ar Ross filters (CaF(2) and NaCl thin films). Without any filters, this diagnostic system can also be used as a bolometer.

Force Mapping of the NaCl(100)/Cu(111) Surface by Atomic Force Microscopy at 78 K

A new atomic force microscopy (AFM) force mapping technique has been used to investigate insulating thin (100) films of NaCl on conducting Cu(111) substrate at 78 K. This technique was able to map the interaction forces between the AFM tip and the surface ions of the sample. The site-specific force curves of the (100) surface of the NaCl thin films are presented. We observed only an attractive short-range interaction force at the Na+ and Cl- sites. We propose simple models to explain the behavior of the force curves at the different ion sites.

Force Mapping of the NaCl(100)/Cu(111) Surface by Atomic Force Microscopy at 78 K

A new atomic force microscopy (AFM) force mapping technique has been used to investigate insulating thin (100) films of NaCl on conducting Cu(111) substrate at 78 K. This technique was able to map the interaction forces between the AFM tip and the surface ions of the sample. The site-specific force curves of the (100) surface of the NaCl thin films are presented. We observed only an attractive short-range interaction force at the Na+ and Cl- sites. We propose simple models to explain the behavior of the force curves at the different ion sites.

Local modification of NaCl thin films on Cu(111) under different bias voltages

The local modification of NaCl thin films on Cu(111) under different bias voltages is investigated using a scanning tunneling microscope (STM) at room temperature. We find that the type of modification of NaCl thin films is dependent on sample bias voltage ( V s). Defects in a triple-layer-thick NaCl film are destabilized and repelled away from the region beneath an STM tip by applying V s in the range of 0.3 V ≤ | V s| ≤ 0.5 V. When V s is larger than + 1.2 V or smaller than − 4.0 V, the removal of NaCl films takes place and a bare Cu surface appears. In this case, the removed NaCl is transferred to the STM tip and can be supplied back to the surface from the STM tip. The redeposition of NaCl enables not only the reformation of single-crystalline NaCl films on a bare Cu surface but also the formation of additional NaCl films on a clean NaCl film surface.

Hydrogen‐Bonded Molecular Networks of Melamine and Cyanuric Acid on Thin Films of NaCl on Au(111)

Self-assembly of organized molecular structures on insulators is technologically very relevant, but in general rather challenging to achieve due to the comparatively weak molecule-substrate interactions. Here the self-assembly of a bimolecular hydrogen-bonded network formed by melamine (M) and cyanuric acid (CA) on ultrathin NaCl films grown on a Au(111) surface is reported. Using scanning tunneling microscopy under ultrahigh-vacuum conditions it is demonstrated that it is possible to exploit strong intermolecular forces in the M-CA system, resulting from complementary triple hydrogen bonds, to grow 2D bimolecular networks on an ultrathin NaCl film that are stable at a relatively high temperature of approximately 160 K and at a coverage below saturation of the first molecular monolayer. These hydrogen-bonded structures on NaCl are identical to the self-assembled structures observed for the M-CA system on Au(111), which indicates that the molecular self-assembly is not significantly affected by the isolating NaCl substrate.

Misfit driven azimuthal orientation of NaCl domains on Ag(1 0 0)

We investigated the growth of thin NaCl films on Ag(1 0 0) by spot-profile-analysis low energy electron diffraction (SPA-LEED), varying extensively the growth temperature (200–500 K) and the film thickness (0.5–14 ML). The incommensurate growth of NaCl on Ag(1 0 0) yields (1 0 0)-terminated epitaxial NaCl domains, which are preferentially oriented with their [0 1 0] axis parallel to that of the substrate. At 300 K, the NaCl domains exhibit an azimuthal mosaicity by ∼14° around this orientation and the NaCl unit cell is laterally contracted in the first layers by 0.9% with respect to the bulk. At higher growth temperatures, the azimuthal mosaic distribution sharpens and additional distinct orientations appear, presumably due to a higher-order commensurability. The evolution of the azimuthal mosaic distribution with increasing temperature can be ascribed to both the NaCl thermal expansion and higher diffusion rates of NaCl on Ag(1 0 0). The best epitaxy, i.e. that with the highest selectivity of a specific azimuthal domain orientation, is achieved by growing NaCl films at low deposition rate (⩽0.1 ML min −1) on the Ag(1 0 0) substrate at constant high temperature (450–500 K). The observations made here can probably be applied more generally to other heterogeneous interfaces and, in particular, be used to improve the quality of thin insulating films.

Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 × 1) NaCl atomic structure for sample bias voltage V s < −2.5 V and the bright protrusions should be assigned to the Cl − ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)- p(2 × 2) surface as the result of weak interface interaction between the dangling bonds coming from valence π states of the Ge substrate and the 3 p states of the interfacial Cl − ion. These results are consistent with the experiments of STM, LEED and EELS.

Screening in two dimensions:GWcalculations for surfaces and thin films using the repeated-slab approach

In the context of photoelectron spectroscopy, the $GW$ approach has developed into the method of choice for computing excitation spectra of weakly correlated bulk systems and their surfaces. To employ the established computational schemes that have been developed for three-dimensional crystals, two-dimensional systems are typically treated in the repeated-slab approach. In this work we critically examine this approach and identify three important aspects for which the treatment of long-range screening in two dimensions differs from the bulk: (1) anisotropy of the macroscopic screening, (2) $\mathbf{k}$-point sampling parallel to the surface, (3) periodic repetition and slab-slab interaction. For prototypical semiconductor (silicon) and ionic (NaCl) thin films we quantify the individual contributions of points (1) to (3) and develop robust and efficient correction schemes derived from the classic theory of dielectric screening.

Effect of NaCl buffer layer on the performance of organic light-emitting devices (OLEDs)

The characteristics of organic light-emitting devices based on tris-(8-hydroxyquinoline) aluminum (Alq 3 ) with NaCl thin films inserted at both anode/organic and organic/cathode interfaces are investigated. We optimize the thicknesses of NaCl at both interfaces, and we get the optimal device with the structure of ITO/NaCl(2.0 nm)/N,N'-bis(l-naphthyl)-N,N'-diphenyl-1,1' biphenyl-4,4'-dimaine (NPB)/Alq 3 /NaCl(2.0 nm)/Al. The device shows much better electroluminescent (EL) performance than that of the as-stated standard device with only LiF buffer layer. The maximal current efficiency and power efficiency is improved by 31% and 67%, the turn-on voltage is also reduced from 4.2 V for the standard device to 3.1 V for our target device. This is due to enhanced electron injection and blocked hole injection, which results in more balanced hole and electron currents.

STM Studies of NaCl Thin Films on Cu(111) Surface at Low Temperature

NaCl thin films on Cu(111) surfaces have been studied by scanning tunneling microscopy at 78 K. Standing wave patterns appeared not only on the Cu(111) but also on the NaCl films. The wave vector of the standing waves on the NaCl films was shorter than that on the Cu(111), indicating that the energy dispersion of the interface states on the NaCl films shifted upward from that of the Cu(111).