NaCl Films Research Articles

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.

Size-dependent dissociation of small cobalt clusters on ultrathin NaCl films

Mass-selected anionic cobalt clusters of two different sizes (Co2 and Co13) were deposited onto ultrathin NaCl films grown on an Au(111) substrate. Using scanning tunneling microscopy experiments and density functional theory simulations, we show that the deposited Co2 cluster dissociates and that the resulting Co atoms dope the NaCl surface by substituting Na ions. In contrast, the larger Co13 cluster does not dissociate and remains stable on top of the NaCl film. The size-dependent fragmentation of clusters is an important aspect in the understanding of the chemical interaction between size-selected small aggregates of atoms and supporting surfaces.

Homoepitaxial Growth of Metal Halide Crystals Investigated by Reflection High-Energy Electron Diffraction

We report the homoepitaxial growth of a metal halide on single crystals investigated with in situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM). Epitaxial growth of NaCl on NaCl (001) is explored as a function of temperature and growth rate which provides the first detailed report of RHEED oscillations for metal halide growth. Layer-by-layer growth is observed at room temperature accompanied by clear RHEED oscillations while the growth mode transitions to an island (3D) mode at low temperature. At higher temperatures (>100 °C), RHEED oscillations and AFM data indicate a transition to a step-flow growth mode. To show the importance of such metal halide growth, green organic light-emitting diodes (OLEDs) are demonstrated using a doped NaCl film with a phosphorescent emitter as the emissive layer. This study demonstrates the ability to perform in situ and non-destructive RHEED monitoring even on insulating substrates and could enable doped single crystals and crystalline substrates for a range of optoelectronic applications.

Size-Dependent Penetration of Gold Nanoclusters through a Defect-Free, Nonporous NaCl Membrane.

Membranes and their size-selective filtering properties are universal in nature and their behavior is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation. Exploring and understanding penetration and transmission mechanisms of nanoparticles in thin-film systems may provide new opportunities for size selective deposition or embedding of the nanoparticles. Here, we demonstrate an unexpected finding that the sieving of metal nanoparticles through atomically thin nonporous alkali halide films on a metal support is size dependent and that this sieving effect can be tuned via the film thickness. Specifically, relying on scanning tunneling microscopy and spectroscopy techniques, combined with density functional theory calculations, we find that defect-free NaCl films on a Au(111) support act as size-dependent membranes for deposited Au nanoclusters. The observed sieving ability is found to originate from a driving force toward the metal support and from the dynamics of both the nanoparticles and the alkali halide films.

Effects of molecule-insulator interaction on geometric property of a single phthalocyanine molecule adsorbed on an ultrathin NaCl film

The adsorption structure and orientation of a metal-free phthalocyanine $({\mathrm{H}}_{2}\mathrm{Pc})$ and a magnesium phthalocyanine (MgPc) on a bilayer of NaCl films were investigated both theoretically and experimentally by means of first-principles calculations based on density functional theory and by scanning tunneling microscopy. ${\mathrm{H}}_{2}\mathrm{Pc}$ is adsorbed with its center over the sodium cation, and H-N bonds in the molecule are aligned with the [100] or [010] surface direction of a bilayer (001)-terminated NaCl film. The most stable structures of MgPc on the NaCl film show two kinds of orientations corresponding to the molecule rotated by $\ifmmode\pm\else\textpm\fi{}{7}^{\ensuremath{\circ}}$ relative to the [110] surface direction, with the Mg cation positioned over the chlorine anion in both cases. The energetic barrier for switching between these orientations is as low as 9.0 meV, and during an STM measurement, an orientational change of MgPc can be observed. The interaction between the adsorbed molecule and the NaCl film were analyzed in terms of dispersion interaction, Mg-Cl chemical bonding, and electrostatic interaction. It is found that the small electrostatic interaction between the molecule and the film gives a dominant contribution to determining the molecular orientation. Our detailed and comprehensive studies of the molecule-insulator interaction will provide knowledge to understand and control the properties of molecules on an insulating material.

Strength and Mechanical Response of NaCl Using In-Situ Transmission Electron Microscopy Compression and Nanoindentation.

Strength and mechanical properties of single crystal sodium chloride (NaCl) are characterized. Critical deformation variations of NaCl pillared structures and films are estimated using in-situ transmission electron microscope (TEM) compression tests and nanoindentation experiments. Young's modulus and contact stiffness of NaCl pillars with diameters of 300 to 500 nm were 10.4-23.9 GPa, and 159-230 N/m, respectively. The nanohardness and Vickers hardness of the NaCl (001) film were 282-596 and 196-260 MPa, respectively. The results could provide useful information for understanding the mechanical properties, contact and local deformation of NaCl pillars and films.

Reversible Bergman cyclization by atomic manipulation.

The Bergman cyclization is one of the most fascinating rearrangements in chemistry, with important implications in organic synthesis and pharmacology. Here we demonstrate a reversible Bergman cyclization for the first time. We induced the on-surface transformation of an individual aromatic diradical into a highly strained ten-membered diyne using atomic manipulation and verified the products by non-contact atomic force microscopy with atomic resolution. The diyne and diradical were stabilized by using an ultrathin NaCl film as the substrate, and the diyne could be transformed back into the diradical. Importantly, the diradical and the diyne exhibit different reactivity, electronic, magnetic and optical properties associated with the changes in the bond topology, and spin multiplicity. With this reversible, triggered Bergman cyclization we demonstrated switching on demand between the two reactive intermediates by means of selective C-C bond formation or cleavage, which opens up the field of radical chemistry for on-surface reactions by atomic manipulation.

Charging single Co atoms on ultrathin NaCl films

Single Co adatoms adsorbed on a double-layer NaCl film supported by Cu(111) were negatively charged after applying a positive voltage pulse to the sample in a scanning tunnelling microscope. Density functional calculations showed that the magnetic moment of Co changed from 3μB to 2.2μB after charge state manipulation.

Toggling the Local Electric Field with an Embedded Adatom Switch.

By means of scanning probe microscopy we demonstrate that Au(+) on NaCl films adsorbs in an embedded, slightly off-centered Cl-Cl bridge position and can be switched between two equivalent mirror-symmetric configurations using the attractive force exerted by a scanning probe tip. Density functional theory calculations demonstrate that the displacement of the Au atom from the centered position of the bridge configuration is accompanied by a large lifting of the closest Cl atom leading to significant changes in the local electrostatic field. Our findings suggest that Au(+) can be used to toggle the local electrostatic field.

Chemically modified STM tips for atomic-resolution imaging of ultrathin NaCl films

Cl-functionalized scanning tunneling microscopy (STM) tips are fabricated by modifying a tungsten STM tip in situ on islands of ultrathin NaCl(100) films on Au(111) surfaces. The functionalized tips are used to achieve clear atomicresolution imaging of NaCl(100) islands. In comparison with bare metal tips, the chemically modified tips yield drastically enhanced spatial resolution as well as contrast reversal in STM topographs, implying that Na atoms, rather than Cl atoms, are imaged as protrusions. STM simulations based on a Green’s function formalism reveal that the experimentally observed contrast reversal in the STM topographs is due to the highly localized character of the Cl-pz states at the tip apex. An additional remarkable characteristic of the modified tips is that in dI/dV maps, a Na atom appears as a ring with a diameter that depends crucially on the tip-sample distance.

Adsorption of Benzene on Cu(100) and on Cu(100) Covered with an Ultrathin NaCl Film: Molecule–Substrate Interaction and Decoupling

We present a theoretical study of the adsorption of benzene C6H6 on the Cu(100) metal surface. The insulating effect of ionic films on this system has also been investigated by adsorbing C6H6 on the same surface covered with 1, 2, and 3 monolayers of NaCl. For this purpose, we employed density functional theory (DFT) including the van der Waals dispersion forces via a DFT-D2 scheme. For all the studied systems we analyzed the adsorption energies and geometries as well as the density of states in order to get a complete description of the type of binding, the charge transfer between the molecule and the surface, and the electronic level alignment after adsorption. We show that the molecule–substrate interaction is weak and mainly governed by dispersion forces, with an almost insignificant charge transfer between the substrate and the adsorbate. We found a progressive decoupling of the molecule from the metal surface when the size of the ultrathin insulating NaCl film increases.

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.

Spontaneous doping of two-dimensional NaCl films with Cr atoms: aggregation and electronic structure.

Scanning tunneling microscopy (STM) experiments combined with density functional theory (DFT) calculations reveal that deposited Cr atoms replace either Na or Cl ions, forming substituting dopants in ultrathin NaCl/Au(111) films. The Cr dopants exchange electrons with the support thus changing the electronic properties of the film and in particular the work function. The Cr atoms spontaneously aggregate near the edges of the bilayer (2L) NaCl islands, forming a new phase in the insulator with a remarkably dense population of Cr dopants. The spectra of differential conductance yield evidence that, compared to the undoped or Cr-poor 2L NaCl films on Au(111), the Cr-rich region shows different interface states, shifted image-potential states, and a reduced work function. This demonstrates the potential of doping ultrathin films to modify their adsorption properties in a desired manner.

Graphitic Phase of NaCl. Bulk Properties and Nanoscale Stability.

We applied the ab initio approach to evaluate the stability and physical properties of the nanometer-thickness NaCl layered films and found that the rock salt films with a (111) surface become unstable with thickness below 1 nm and spontaneously split to graphitic-like films for reducing the electrostatic energy penalty. The observed sodium chloride graphitic phase displays an uncommon atomic arrangement and exists only as nanometer-thin quasi-two-dimensional films. The graphitic bulk counterpart is unstable and transforms to another hexagonal wurtzite NaCl phase that locates in the negative-pressure region of the phase diagram. It was found that the layers in the graphitic NaCl film are weakly bounded with each other with a binding energy order of 0.1 eV per stoichiometry unit. The electronic band gap of the graphitic NaCl displays an unusual nonmonotonic quantum confinement response.

Linkage isomerism of CO adsorbed on alkali halides.

Variable-temperature FTIR spectra of CO adsorbed on NaCl and KBr films are shown to reveal linkage isomerism, that is, formation of C- and O-bonded adsorption complexes with the same cationic sites. Not all of the sites can be involved in that, only the three- or four-coordinated cations located on kinks, edges, or steps of microcrystals. For NaCl, these adsorption complexes account for the high-frequency (HF) C-O stretching bands at 2175 and 2160 cm(-1). Coresponding LF bands of O-bonded CO were found at 2115 and 2124 cm(-1). In the spectrum of KBr, the HF band is poorly resolved, and only one LF band can be clearly seen, near 2124 cm(-1). The value of the isomerization enthalpy for complexes that account for the bands at 2160 and 2124 cm(-1) of CO adsorbed on NaCl, estimated from the experiment, is 4.0 ± 0.2 kJ/mol. Quantum mechanical calculations by DFT methods applied to NaCl model clusters interacting with the CO molecule confirm the existence of two potential wells with adsorption and isomerization energies close to the experimentally measured values, and enable us to estimate the height of the potential barrier between the two adsorption states.

Femtosecond Trapping of Free Electrons in Ultrathin Films of NaCl on Ag(100).

We report the excited-state electron dynamics for ultrathin films of NaCl on Ag(100). The first three image potential states (IPSs) were initially observed following excitation. The electrons in the spatially delocalized n = 1 IPS decayed on the ultrafast time scale into multiple spatially localized states lower in energy. The localized electronic states are proposed to correspond to electrons trapped at defects in the NaCl islands. Coverage and temperature dependence of the localized states support the assignment to surface trap states existing at the NaCl/vacuum interface. These results highlight the importance of electron trapping in ultrathin insulating layers.

Distributed-Type Equivalent Circuit to Determine Charge Transfer Resistance of Metals Under a Thin Solution Film

Background Atmospheric corrosion proceeds under an extremely thin solution film. In actual environment, the thickness or concentration of thin solution film increases and decreases continuously with the change in temperature and humidity. These changes act as a very important role in the atmospheric corrosion process. However, it is quite difficult to monitor corrosion rate of metals under a thin solution film with conventional electrochemical methods which are suitable for studying corrosion behavior in bulk solution. The largest problem is an uneven current distribution over working electrode which leads to serious errors in the measurements of the corrosion rate because an extremely high solution resistance. By using Electrochemical Impedance Spectroscopy (EIS), current distribution over electrode surface can be evaluated from frequency dependency of the phase shift1. The current distribution becomes uneven if the phase shift goes no further than -45° on Bode plot when constant phase element α=1. In such a condition, a correct charge transfer resistance R ct cannot be obtained. In the present study, some approaches were proposed to measure the R ct under a thin solution film with uneven current distribution. Experimental A two-electrode cell composed of a pair of identical carbon steel plates (1 cm × 1 cm) in parallel with a gap of 0.2 mm was used. NaCl or NaNO3 solution film with different thickness was covered over cell surface. The thickness of thin film was controlled using a micropipette to render a pre-determined volume, which was assumed to spread over cell surface uniformly. When thickness was smaller than 25 μm, ethanol was mixed into initial solution to guarantee a uniform spread of thin film. The EIS measurements were performed at OCP with a 10 mV amplitude. The frequency range was from 10 kHz to 10 mHz, with 5 points per decade. Transmission line (TML) model of a metal covered with a thin electrolyte layer With EIS measurement, the metal-thin film interface can be presented by a one-dimensionally distributed constant equivalent circuit, i.e. Transmission line (TML) circuit, as shown in Fig. 1. The total impedance Ztotal of single electrode is given byZtotal =Zw coth γ Xw + Rsg /2 (1)γ = (Rs */ Z*)1/2 (2)Zw =L-1(Rs *Z*)1/2 (3)where Xw and L are width and length of electrode respectively, Rs* and Z* are a solution resistance over electrode and a metal-thin electrolyte interfacial impedance per unit length, respectively, and Rsg is a solution resistance over gap between two electrodes. When the value of (γ Xw) is very large, which means current distribution tends to be uneven, Eq. (1) can be converted toZtotal =Zw + Rsg /2 = (RsRct)1/2 + Rsg /2 (4)by eliminating the term of coth γXw. Results and discussion Current distribution over working electrode tends to be uneven when thickness or concentration of electrolyte film decreases. Fig. 2 shows the EIS data of carbon steel covered with a 500μm NaCl solution film with concentration from 0.002M to 2M. Current distribution was evaluated from frequency dependency of phase shift. Uneven current distribution established over electrode surface in whole frequency range when concentration is smaller than 0.02M. Hence, TML model was applied to determine the accurate corrosion rate. The fitting results show quite good consistency with the original data. In the present study, the TML model was applied to eliminate the effect of uneven current distribution during monitoring of atmospheric corrosion rate. Furthermore, the rate-determining step of a metal covered with an extremely thin solution film is discussed.

Local thickness determination of thin insulator films via localized states

We propose measuring the lifetime of localized states below the conduction band to determine the local thickness of thin insulating films using scanning tunneling microscopy. The lifetime, which is a characteristic fingerprint of the film thickness, is inversely proportional to the saturation value of the tunnel current through the localized state at close tip-sample separation and is readily measured using scanning tunneling spectroscopy. We demonstrate the method for 5–11 monolayer thick NaCl films grown on Cu(111).

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...

An unconventional bilayer ice structure on a NaCl(001) film.

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.