NaCl Layer Research Articles

Gating Single-Molecule Fluorescence with Electrons.

Tip-enhanced photoluminescence (TEPL) measurements are performed with subnanometer spatial resolution on individual molecules decoupled from a metallic substrate by a thin NaCl layer. TEPL spectra reveal progressive fluorescence quenching with decreasing tip-molecule distance when electrons tunneling from the tip of a scanning tunneling microscope are injected at resonance with the molecular states. Rate equations based on a many-body model reveal that the luminescence quenching is due to a progressive population inversion between the ground neutral (S_{0}) and the ground charge (D_{0}^{-}) states of the molecule occurring when the current is raised. We demonstrate that the bias voltage and the lateral tip position can be used to gate the molecular emission. Our approach can be applied to any molecular system, providing unprecedented control over the fluorescence of a single molecule.

Theoretical study of tip-enhanced photoluminescence of single molecule in nanocavity formed by apex protrusion and substrate

Tip-enhanced photoluminescence techniques, characterized by ultra-high detection sensitivity and spatial resolution, have exhibited unprecedented capabilities in exploring optoelectronic phenomena and unraveling intrinsic physical mechanism at the single-molecule level. To suppress the quenching of molecular luminescence and achieve high-resolution optical imaging, a rational design of the tip-substrate configuration is required. In this paper, we systematically investigate the tip-enhanced photoluminescence characteristics of single molecules located within the plasmonic nanocavity formed by the tip protrusion and the substrate from a theoretical perspective. The impacts of electron tunneling and electron-hole pair creation on localized electric field and quantum yield are discussed using the quantum correction model and nonlocal dielectric theory, respectively. The crucial roles of atomic-scale protrusion at the tip apex and the decoupling layer on the substrate in enhancing vertical and horizontal electric fields as well as quantum yields have been emphasized. The results indicate that in smaller sub-nanometer cavities, electron tunneling and nonlocal dielectric effects greatly weaken the electric field intensity and fluorescence quantum yield. Meanwhile, due to the sharp lightning rod effect, the presence of atomic-level protrusion at the tip apex can provide a strong and highly localized plasmonic field. We find that the added NaCl layer not only blocks charge transfer between molecules and the substrate but also effectively prevents fluorescence quenching. The maximum fluorescence and Raman enhancement factors near the tip apex approach up to 5 and 12 orders of magnitude, respectively, achieving sub-nanometer spatial resolution. Finally, compared to vertical electric field and dipole, sharp protrusion can excite a stronger horizontal electric field and couple with horizontal dipoles to yield a higher quantum yield, resulting in a three orders of magnitude higher in the fluorescence enhancement of horizontal dipole compared to the tip without protrusion. Our theoretical research not only confirms the experimental results of spectral enhancement with the proximity of the tip to the molecule in tip-enhanced photoluminescence, but also provides effective guidance for a deeper understanding of the mechanism of tip-enhanced fluorescence and Raman spectroscopy, as well as optimizing the experimental system.

Single-Molecule Time-Resolved Spectroscopy in a Tunable STM Nanocavity.

Spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However, coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between a tip and sample in a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters, depending on their coupling strength to the nanocavity. With such a setup, we determine the excitation lifetimes from the direct time-resolved measurements of phthalocyanine fluorescence decays, decoupled from the metal substrates by ultrathin NaCl layers. We find that when the tip is approached to single molecules, their lifetimes are reduced to the picosecond range due to the effect of coupling with the tip-sample nanocavity. On the other hand, ensembles of the adsorbed molecules measured without the nanocavity manifest nanosecond-range lifetimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission line widths.

Exotic hexagonal NaCl atom-thin layer on methylammonium lead iodide perovskite: new hints for perovskite solar cells from first-principles calculations.

Alkali halides are simple inorganic compounds extensively used as surface modifiers in optoelectronic devices. In perovskite solar cells (PSCs), they act as interlayers between the light absorber material and the charge selective layers improving their contact quality. They introduce surface dipoles that enable the fine tuning of the relative band alignment and passivate surface defects, a well-known drawback of hybrid organic-inorganic perovskites, that is responsible for most of the issues hampering the long-term performances. Reducing the thickness of such salt-based insulating layer might be beneficial in terms of charge transfer between the perovskite and the electron/hole transport layers. In this context, here we apply density functional theory (DFT) to characterize the structure and the electronic features of atom-thin layers of NaCl adsorbed on the methylammonium lead iodide (MAPI) perovskite. We analyze two different models of MAPI surface terminations and find unexpected structural reconstructions arising at the interface. Unexpectedly, we find an exotic honeycomb-like structuring of the salt, also recently observed in experiments on a diamond substrate. We also investigate how the salt affects the perovskite electronic properties that are key to control the charge dynamics at the interface. Moreover, we also assess the salt ability to improve the defect tolerance of the perovskite surface. With these results, we derive new hints regarding the potential benefits of using an atom-thin layer of alkali halides in PSCs.

Boosting-photothermal properties of Cu/ Black TiO2 nanoparticles on biomimetics texture structure as high‐performance and self‐regenerating solar-evaporator

To address the global water crisis, solar vapor generation emerged as a promising technology for clean-water uptake from seawater. Black TiO2 with wide-spectrum absorbancy, non-toxicity, cost-effectively, and chemical stability has attracted great interest in efficient solar-driven evaporators. Herein, the black TiO2 nanoparticles were synthesized through one step and facile reduction process of white oxide powder squeezed between two layers of NaCl. The prepared powder was deposited on the bare and copper-treated cotton fabrics as light absorbers in the proposed solar steam generation (SSG) systems. Various structural and morphology characterization methods have been utilized to verify of synthesis procedure. Heat localization was performed by the floated yarn-stitched polyurethane (PU) foam as a holder for the fabrics. Continuous water transmission from the saline water container to the evaporation surface was done via capillary forces in the yarns. The hybrid absorber introduced a superior solar-thermal conversion efficiency of about 98.56 % and 1.94 kg.m−2 h−1 evaporation rate which was 3.13 times larger than the control one. Antifouling and recyclability properties of the samples were studied in outdoor conditions. The suggested hybrid-based (Cu/black TiO2) SSG with significant properties of simplicity, non-toxicity, and high evaporating activity opens new shortcuts toward green water desalination and also salt extraction demands.

Character of Electronic States in the Transport Gap of Molecules on Surfaces.

We report on scanning tunneling microscopy (STM) topographs of individual metal phthalocyanines (MPc) on a thin salt (NaCl) film adsorbed on a gold substrate, at tunneling energies within the molecule's electronic transport gap. Theoretical models of increasing complexity are discussed. The calculations for MPcs adsorbed on a thin NaCl layer on Au(111) demonstrate that the STM pattern rotates with the molecule's orientations─in excellent agreement with the experimental data. Thus, even the STM topography obtained for energies in the transport gap represent the structure of a one atom thick molecule. It is shown that the electronic states inside the transport gap can be rather accurately approximated by linear combinations of bound molecular orbitals (MOs). The gap states include not only the frontier orbitals but also surprisingly large contributions from energetically much lower MOs. These results will be essential for understanding processes, such as exciton creation, which can be induced by electrons tunneling through the transport gap of a molecule.

First steps of silicene growth on an insulating thin-film: effect of the substrate temperature

Silicene is a two-dimensional (2D) material with very promising electronic properties for applications in silicon modern technology. However, the first experimental synthesis of silicene on metallic surfaces shows strong interactions between the silicene and its substrate, which can alter its electronic properties. Here, we report on the first steps of silicene growth on an insulating surface (NaCl) using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and angle-resolved photoemission spectroscopy (ARPES). We demonstrate the importance of temperature annealing in the growth of silicene on NaCl. Indeed, after deposition of silicon on the NaCl/Ag(110) surface, we observe the following stages: (i) at room temperature, the silicon atoms accumulate on top of the NaCl layer without any given order. (ii) At 60 °C, silicon dimers start to grow on the NaCl. (iii) At 140 °C, these dimers form a 2D silicon chains on the surface. (iv) After a post-annealing at 200 °C, evident 2D silicon nanoribbons with a honeycomb-like structure were observed. Our results of the first silicene growth stages on an insulating surface are a necessary step for exploring its growth mechanism further.

Galvanic Coupling of Copper and Carbon Steel in the Presence of Bentonite Clay and Chloride

The long-term containment plan for Canada’s used nuclear fuel employs copper-coated carbon steel (CS) containers encased in compacted bentonite clay buffer boxes. In the hypothetical case of a through-coating defect, galvanic interactions between the Cu coating and the CS vessel are dependent on the conditions at the container surface. A zero-resistance ammeter was used to measure the galvanic current between Cu and CS electrodes at various area ratios in the presence of aerated 1 M NaCl and obstructive layers of bentonite slurry. The presence of bentonite resulted in an order of magnitude decrease in the corrosion rate of the steel. As shown by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and Raman spectroscopy, the extent of steel corrosion decreased significantly in the presence of bentonite slurry, favouring mild surface etching over oxide growth. Electrochemical impedance spectroscopy (EIS) measurements showed that the total impedance on carbon steel generally increased with bentonite layer thickness, but the total impedance on Cu plateaued when the layer exceeded 6 mm, consistent with trends in galvanic current density on carbon steel. The benign conditions produced by the bentonite slurry are attributed to the slightly alkaline pH and the hindrance of O2 diffusion.

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.

Nonbenzenoid High-Spin Polycyclic Hydrocarbons Generated by Atom Manipulation.

We report the on-surface synthesis of a nonbenzenoid triradical through dehydrogenation of truxene (C27H18) on coinage metal and insulator surfaces. Voltage pulses applied via the tip of a combined scanning tunneling microscope/atomic force microscope were used to cleave individual C-H bonds in truxene. The resultant final product truxene-5,10,15-triyl (1) was characterized at the single-molecule scale using a combination of atomic force microscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Our analyses show that 1 retains its open-shell quartet ground state, predicted by density functional theory, on a two monolayer-thick NaCl layer on a Cu(111) surface. We image the frontier orbital densities of 1 and confirm that they correspond to spin-split singly occupied molecular orbitals. Through our synthetic strategy, we also isolate two reactive intermediates toward the synthesis of 1, derivatives of fluorenyl radical and indeno[1,2-a]fluorene, with predicted open-shell doublet and triplet ground states, respectively. Our results should have bearings on the synthesis of nonbenzenoid high-spin polycyclic frameworks with magnetism beyond Lieb's theorem.

Biotite supports long-range diffusive transport in dissolution–precipitation creep in halite through small porosity fluctuations

Abstract. Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution–precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources and geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl–biotite samples to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4-D) microtomographic data to capture the dynamic evolution of the porosity in layered NaCl–NaCl/biotite samples over 1619 and 1932 h of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the sample by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the sample suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl–biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are complemented by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl–biotite mixture over distances of several hundred micrometres and several grain diameters. Such a mechanism was first postulated by Merino et al. (1983).

Ladder Phenylenes Synthesized on Au(111) Surface via Selective [2+2] Cycloaddition.

Ladder phenylenes (LPs) composed of alternating fused benzene and cyclobutadiene rings have been synthesized in solution with a maximum length no longer than five units. Longer polymeric LPs have not been obtained so far because of their poor stability and insolubility. Here, we report the synthesis of linear LP chains on the Au(111) surface via dehalogenative [2+2] cycloaddition, in which the steric hindrance of the methyl groups in the 1,2,4,5-tetrabromo-3,6-dimethylbenzene precursor improves the chemoselectivity as well as the orientation orderliness. By combining scanning tunneling microscopy and noncontact atomic force microscopy, we determined the atomic structure and the electronic properties of the LP chains on the metallic substrate and NaCl/Au(111). The tunneling spectroscopy measurements revealed the charged state of chains on the NaCl layer, and this finding is supported by density functional theory calculations, which predict an indirect bandgap and antiferromagnetism in the polymeric LP chains.

Bottom‐up Fabrication and Atomic‐Scale Characterization of Triply Linked, Laterally π‐Extended Porphyrin Nanotapes**

AbstractPorphyrin nanotapes (Por NTs) are promising structures for their use as molecular wires thanks to a high degree of π‐conjugation, low HOMO—LUMO gaps, and exceptional conductance. Such structures have been prepared in solution, but their on‐surface synthesis remains unreported. Here, meso–meso triply fused Por NTs have been prepared through a two‐step synthesis on Au(111). The diradical character of the on‐surface formed building block PorA2, a phenalenyl π‐extended ZnIIPor, facilitates intermolecular homocoupling and allows for the formation of laterally π‐extended tapes. The structural and electronic properties of individual Por NTs are addressed, both on Au(111) and on a thin insulating NaCl layer, by high‐resolution scanning probe microscopy/spectroscopy complemented by DFT calculations. These Por NTs carry one unpaired electron at each end, which leads to magnetic end states. Our study provides a versatile route towards Por NTs and the atomic‐scale characterization of such tapes.

Energetic effects analysis of shear waves and their second harmonics in a waveguide with non-ideal component contact

In this article the energetic and kinematic effects that occur in the elastic shear wave and its second harmonics propagation are investigated. The waveguide consists of anisotropic elastic crystal layer of cubic system m3m class enclosed between crystal halfspaces of same anisotropy class. A slipping contact is assumed in the contact zone of waveguide parts. The research is based on a model of general geometrical end physical nonlinearity in dynamic deformation processes. It allows to use elastic potential with the quadratic and cubic deformation components and the deformations with nonlinear terms. The approach of nonlinear elastic wave characteristics expansion into rows of a small parameter is used. Due to this approach at the first stage it's necessary to solve the problem of finding the components of the localized shear wave displacement vector (the problem of the first approximation). In the second stage, using the obtained results of the first approximation problem, the representation of the components of the displacement vector for the second harmonics of the localized elastic wave is solved in analytical form (the problem of the second approximation). By using the obtained kinematic results, the energy effects can be evaluated in the form of a vector of the average for the period of power flow. Specific results of the study of the amplitude-frequency and energy characteristics of the localized shear type elastic waves in the considered waveguide structure were obtained using computer algebra methods.The calculations of cinematic and energetic characteristics (that in contrast to linear SH harmonic are P-SV type waves) have been carried out for NaCl layer and germanium halfspaces waveguide.

A study on hot corrosion performance of high velocity arc-sprayed FeCrNiAlMnB/Cr3C2 coating exposed to Na2SO4 + K2SO4 and Na2SO4 + NaCl

Hot corrosion has been considered as a serious problem in boiler tubes because of poor quality fuels which result in the presence of salt contaminants such as Na2SO4, K2SO4 and NaCl. To solve this problem, FeCrNiAlMnB/Cr3C2 coating was fabricated on AISI 1020 steel substrate by high velocity arc spray technique, and the hot corrosion behavior of the coating was investigated in the presence of Na2SO4 + 25 wt% K2SO4 and Na2SO4 + 25 wt% NaCl at 650 °C and 750 °C. Phase composition, microstructure and corrosion products of the coating were characterized by XRD and SEM/EDS techniques. The results showed that the coating exhibited much less mass gain and superior hot corrosion resistance as compared with bare AISI 1020 steel, and kinetics studies indicated parabolic characteristic for the coating under different corrosion conditions. Different morphologies attributed to corrosion products which consisted of (Fe, Cr)2O3, FeCr2O4 (Na2SO4 + 25 wt% K2SO4) and (Fe, Cr)2O3, FeO, Na2CrO4, NaFeO2 (Na2SO4 + 25 wt% NaCl) were observed after exposition to above two molten salts. The destructive effect of Na2SO4 + 25 wt% NaCl on the coating was much greater than that of Na2SO4 + 25 wt% K2SO4 due to the reaction between NaCl and protective oxide layer, which formed Cl2 and further destroyed the uncorroded coating as well as led to the formation of cracks in the protective oxide layer.

Substrate protection and deprotection with salt films to prevent surface contamination and enable selective atomic layer deposition

Surface contamination can decrease surface reactivity, and removal of surface contaminants adds additional steps to a process. Here we demonstrate surface protection from contamination and one-sided atomic layer deposition (ALD) by means of protective, sacrificial, thermally evaporated NaCl layers. This approach allows clean silicon surfaces to be stored under ambient conditions for extended periods of time in a pristine state, and ALD to be performed selectively on one side of a planar material. For the ALD depositions, planar substrates are (i) selectively coated on one side with ca. 100 nm of NaCl by thermal evaporation, (ii) non-selectively coated using traditional ALD, and (c) rinsed with water to remove the NaCl layer. Water treatment removes the salt film and any ALD deposition on top of it. Salt film deposition, surface protection, and selective ALD are confirmed by X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). Selective, thermal ALD of alumina and ZnO are demonstrated, and protective salt films of different thicknesses (10, 50, and 100 nm) are investigated.

Novel preparation system of an organic–inorganic hybrid chemical conversion layer with high chlorine tolerance: A characterization and mechanism study

The quality of the pretreatment layer on steel was obviously affected by the chloridion concentration in the layer bath. In this study, the effects of chloridion on the micro- anti-corrosion performance, structure and chemical composition of polyacrylic acid (PAA) based layer were investigated. PAA-Zn layer and PAA-Zn-NaCl layers were fabricated on steel surface through immersion the steel in the layer bath without and with different concentrations of chloridion for 20 min. The electrochemical impedance spectroscopy and polarization curve results indicated that the anti-corrosion performance of the PAA–Zn layer could be improved due to the existence of chloridion in the layer bath and reached the best anti-corrosion performance when the concentration of chloridion was 2 wt%. Scanning electron microscopy results reflected that the thickness of the PAA-Zn-2%NaCl layer was higher than that of PAA-Zn layer. X-ray photoelectron spectroscopy results demonstrated that the concentrations of iron and zinc for the PAA-Zn-2%NaCl layer were higher than that of the PAA-Zn layer. The all experimental results indicated that chloridion could accelerate the steel corrosion during the initial immersion and thus promote PAA ionization and produce numerous iron ions. The formed PAA anions could chelate iron and zinc ions to form layers on steel, leading to the high chlorine tolerance characteristic of the PAA layer bath.

Probing the Magnetism of Topological End States in 5-Armchair Graphene Nanoribbons.

We extensively characterize the electronic structure of ultranarrow graphene nanoribbons (GNRs) with armchair edges and zigzag termini that have five carbon atoms across their width (5-AGNRs), as synthesized on Au(111). Scanning tunneling spectroscopy measurements on the ribbons, recorded on both the metallic substrate and a decoupling NaCl layer, show well-defined dispersive bands and in-gap states. In combination with theoretical calculations, we show how these in-gap states are topological in nature and localized at the zigzag termini of the nanoribbons. In addition to rationalizing the driving force behind the topological class selection of 5-AGNRs, we also uncover the length-dependent behavior of these end states which transition from singly occupied spin-split states to a closed-shell form as the ribbons become shorter. Finally, we demonstrate the magnetic character of the end states via transport experiments in a model two-terminal device structure in which the ribbons are suspended between the scanning probe and the substrate that both act as leads.

Building an artificial solid electrolyte interphase with high-uniformity and fast ion diffusion for ultralong-life sodium metal anodes

An artificial SEI consisting of an Na–Sn alloy and NaCl layer possesses high-uniformity and fast ion diffusion, stabilizing Na metal anodes.

Mechanism study on highly efficient polymer light-emitting diodes utilizing double-layered alkali halide electron injection layer

Enhancing the injection of electron is an effective strategy to improve the performance of polymer light-emitting diodes (PLEDs). In this work, we reported a 286% improvement in current efficiency (CE) of PLEDs by using double-layered alkali halide electron injection layer (EIL) NaCl/LiF instead of LiF. A significant enhancement of electron injection was observed after inserting the NaCl layer. To understand the mechanism of such improvement, the devices with KBr/LiF and CsF/LiF as EILs were also investigated. Experimental results show that metal cation migrated under the effect of built-in electric field (Vbi), which plays the main role on the improvement of electron injection in PLEDs.