NaCl Films Research Articles

Controlling the Resonance Raman Effect in Tip-Enhanced Raman Spectroscopy Using a Thin Insulating Film.

Both surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) are widely used for the investigation of nanoscale materials. One of the most critical aspects of both SERS and TERS is the control of both the plasmon and molecular resonance precisely. Here, we demonstrate single-molecule TERS under molecular resonance conditions using a scanning tunneling microscope. This was achieved by placing the molecule on a sodium chloride (NaCl) film in order to directly compare the absorption with Raman excitation spectra. Varying the NaCl film thickness changes the degree of screening effect from the metal surface, which leads to a variation of the molecular resonance phenomena. Although it is generally accepted that the target molecule should be directly attached to the metal surface in SERS, our observation using TERS suggests that this is not always optimal, especially under molecular resonance Raman conditions. Our work demonstrates the possibility of controlling molecular resonance by carefully modifying the local environment. This will be useful for future investigation of isolated single molecules or even two-dimensional molecular assemblies.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation.

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

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.

An Endergonic Synthesis of Single Sondheimer–Wong Diyne by Local Probe Chemistry

AbstractRecent advances in scanning probe microscopy on surface enable not only direct observation of molecular structures but also local probe reactions, in which unstable short‐lived products have been synthesized and analyzed. Now, an endergonic reaction to synthesize a single Sondheimer–Wong diyne from 6,13‐dibromopentaleno[1,2‐b:4,5‐b′]dinaphthalene by local probe chemistry on a ultra‐thin film of NaCl formed on a Cu(111) surface at 4.3 K is presented. The structures of the precursor, two intermediates, and the final product were directly identified by the differential conductance imaging with a CO functionalized tip. DFT calculations revealed that the multiple‐step reaction, being endergonic overall, is facilitated by temporal charging and discharging of the molecule placed in the nanometric junction between the Cu tip and the Cu substrate underneath the ultra‐thin NaCl film. This local probe reaction expands possibilities to synthesize nanocarbon materials in a bottom‐up manner.

An Endergonic Synthesis of Single Sondheimer-Wong Diyne by Local Probe Chemistry.

Recent advances in scanning probe microscopy on surface enable not only direct observation of molecular structures but also local probe reactions, in which unstable short-lived products have been synthesized and analyzed. Now, an endergonic reaction to synthesize a single Sondheimer-Wong diyne from 6,13-dibromopentaleno[1,2-b:4,5-b']dinaphthalene by local probe chemistry on a ultra-thin film of NaCl formed on a Cu(111) surface at 4.3 K is presented. The structures of the precursor, two intermediates, and the final product were directly identified by the differential conductance imaging with a CO functionalized tip. DFT calculations revealed that the multiple-step reaction, being endergonic overall, is facilitated by temporal charging and discharging of the molecule placed in the nanometric junction between the Cu tip and the Cu substrate underneath the ultra-thin NaCl film. This local probe reaction expands possibilities to synthesize nanocarbon materials in a bottom-up manner.

Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 clusters.

The free-standing Au20 cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au20 has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters. However, direct real-space ground-state characterization at the atomic scale is still lacking, and obtaining fundamental information about the corresponding structural, electronic, and dynamical properties, is challenging. Here, using cluster-beam deposition and low-temperature scanning tunneling microscopy, atom-resolved topographic images and electronic spectra of supported Au20 clusters are obtained. We demonstrate that individual size-selected Au20 on ultrathin NaCl films maintains its pyramidal structure and large HOMO-LUMO gap. At higher cluster coverages, we find sintering of the clusters via Smoluchowski ripening to Au20n agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size.

Charge-Induced Structural Changes in a Single Molecule Investigated by Atomic Force Microscopy

Intramolecular structural relaxations occurring upon electron transfer are crucial in determining the rate of redox reactions. Here, we demonstrate that subangstrom structural changes occurring upon single-electron charging can be quantified by means of atomically resolved atomic force microscopy (AFM) for the case of single copper(II)phthalocyanine (CuPc) molecules deposited on an ultrathin NaCl film. Imaging the molecule in distinct charge states (neutral and anionic) reveals characteristic differences in the AFM contrast. In comparison to density functional theory simulations these changes in contrast can be directly related to relaxations of the molecule's geometric structure upon charging. The dominant contribution arises from a nonhomogeneous vertical relaxation of the molecule, caused by a change in the electrostatic interaction with the surface.

Molecular structure elucidation with charge-state control.

The charge state of a molecule governs its physicochemical properties, such as conformation, reactivity, and aromaticity, with implications for on-surface synthesis, catalysis, photoconversion, and applications in molecular electronics. On insulating, multilayer sodium chloride (NaCl) films, we controlled the charge state of organic molecules and resolved their structures in neutral, cationic, anionic, and dianionic states by atomic force microscopy, obtaining atomic resolution and bond-order discrimination using carbon monoxide (CO)-functionalized tips. We detected changes in conformation, adsorption geometry, and bond-order relations for azobenzene, tetracyanoquinodimethane, and pentacene in multiple charge states. Moreover, for porphine, we investigate the charge state-dependent change of aromaticity and conjugation pathway in the macrocycle. This work opens the way to studying chemical-structural changes of individual molecules for a wide range of charge states.

Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules.

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

Selective triplet exciton formation in a single molecule

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs)1-7. According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T1) excitons in a 1:3 ratio2-4. After the first report of a highly efficient OLED2 based on phosphorescence, which is produced by the decay of T1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage2-6. Because T1 excitons have lower energy than S1 excitons (owing to the exchange interaction), use of the energy difference could-in principle-enable exclusive production of T1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yetbeen established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope8-20 and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule21-25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T1 excitons without creating their S1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.

Highly efficient solution-processed CZTSSe solar cells based on a convenient sodium-incorporated post-treatment method

Abstract In CZTSSe solar cells, a simple sodium-incorporation post-treatment method toward solution-processed Cu2ZnSnS4 precursor films is presented in this work. An ultrathin NaCl film is deposited on Cu2ZnSnS4 precursor films by spin-coating NaCl solution. In subsequent selenization process, the introduction of NaCl is found to be benefacial for the formation of Cu2-xSe, which can further facilitate the element transportation, leading to dense and smooth CZTSSe films with large grains and less impurity Cu2Sn(S,Se)3 phase. SIMS depth profiles confirm the gradient distribution of the sodium element in Na-doped absorbers. Photoluminescence spectra show that the introduction of appropriate sodium into the absorber can inhibit the band tail states. As high as 11.18% of power conversion efficiency (PCE) is achieved for the device treated with 5 mg mL−1 NaCl solution, and an average efficiency of Na-doped devices is 10.71%, 13% higher than that of the control groups (9.45%). Besides, the depletion width and the charge recombination lifetime can also have regular variation with sodium treatment. This work offers an easy modification method for high-quality Na-doped CZTSSe films and high-performance devices, in the meantime, it can also help to further understand the effects of sodium in CZTSSe solar cells.

Ultra-thin NaCl films as protective layers for graphene.

The ageing of graphene is an important issue that limits its technological applications. Capping layers are a good option for circumventing this problem. In this work, we propose the use of ultra-thin NaCl films as easily-removable protective layers. We have carried out a detailed characterization of the NaCl/graphene interface on metal substrates, namely Cu(111) and Ir(111), by means of complementary microscopy, electron diffraction and spectroscopic techniques. Interestingly, we show that NaCl neither interacts in a chemical way with graphene nor intercalates through it. We demonstrate that the NaCl film is stable under ambient conditions, protecting the graphene surface from oxidation. In addition, after removing the protective layer, graphene remains intact.

Energy-level alignment of a single molecule on ultrathin insulating film

Elucidation of the energy-level alignment mechanism at a molecule/insulator/metal interface is a key to understanding the molecular and interfacial phenomena. Herein, we provide a detailed investigation into the electronic structures of a free-base phthalocyanine on NaCl films of various thicknesses using scanning tunneling microscopy/spectroscopy (STM/STS). The energy of the ionization and the affinity levels of the molecule were deduced from the STS spectra, and we determined their dependence on the NaCl-film thickness, which can be explained based on three effects: a voltage drop within the NaCl films, the degree of electric-field screening around the molecule, and a variation in the work function of the substrates. We further found that the energy levels relative to the vacuum level are independent of the work function of the substrate, and that the size of the energy gap increases with the thickness. Our results suggest that it is possible to predict the energy levels at the interfaces based on the energy levels of the molecules in a gas phase, the work function of the substrate, and the thickness of the insulating films.

Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy.

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-Ångström spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.

Reorganization energy upon charging a single molecule on an insulator measured by atomic force microscopy.

Intermolecular single-electron transfer on electrically insulating films is a key process in molecular electronics1-4 and an important example of a redox reaction5,6. Electron-transfer rates in molecular systems depend on a few fundamental parameters, such as interadsorbate distance, temperature and, in particular, the Marcus reorganization energy 7 . This crucial parameter is the energy gain that results from the distortion of the equilibrium nuclear geometry in the molecule and its environment on charging8,9. The substrate, especially ionic films 10 , can have an important influence on the reorganization energy11,12. Reorganization energies are measured in electrochemistry 13 as well as with optical14,15 and photoemission spectroscopies16,17, but not at the single-molecule limit and nor on insulating surfaces. Atomic force microscopy (AFM), with single-charge sensitivity18-22, atomic-scale spatial resolution 20 and operable on insulating films, overcomes these challenges. Here, we investigate redox reactions of single naphthalocyanine (NPc) molecules on multilayered NaCl films. Employing the atomic force microscope as an ultralow current meter allows us to measure the differential conductance related to transitions between two charge states in both directions. Thereby, the reorganization energy of NPc on NaCl is determined as (0.8 ± 0.2) eV, and density functional theory (DFT) calculations provide the atomistic picture of the nuclear relaxations on charging. Our approach presents a route to perform tunnelling spectroscopy of single adsorbates on insulating substrates and provides insight into single-electron intermolecular transport.

Studying an antiaromatic polycyclic hydrocarbon adsorbed on different surfaces

Antiaromatic and open-shell molecules are attractive because of their distinct electronic and magnetic behaviour. However, their increased reactivity creates a challenge for probing their properties. Here, we describe the on-surface and in-solution generation and characterisation of a highly reactive antiaromatic molecule: indeno[1,2-b]fluorene (IF). In solution, we generated IF by KI-induced dehalogenation of a dibromo-substituted precursor molecule and found that IF survives for minutes at ambient conditions. Using atom manipulation at low temperatures we generated IF on Cu(111) and on bilayer NaCl. On these surfaces, we characterised IF by bond-order analysis using non-contact atomic force microscopy with CO-functionalised tips and by orbital imaging using scanning tunnelling microscopy. We found that the closed-shell configuration and antiaromatic character predicted for gas-phase IF are preserved on the NaCl film. On Cu(111), we observed significant bond-order reorganisation within the s-indacene moiety because of chemisorption, highlighting the importance of molecule surface interactions on the π-electron distribution.

Damping by sequentially tunneling electrons

Sequential tunneling of electrons through a single chlorine vacancy on a multilayer NaCl film on Cu(100) in the junction of a non-contact atomic force microscope with a conductive tip can lead to a negative damping of the cantilever. The characteristic features in the damping signal as a function of NaCl-layer thickness, tip position and tip-sample bias can be explained in a simple rate-equation model for the sequential tunneling process through the double-barrier tunnel junction. The first barrier results from the vacuum gap between tip and vacancy and its tunneling rate is tuned by the tip position and the applied bias. The second barrier results from the NaCl film and its tunneling rate is governed by the film’s thickness. The current-induced damping is strongest if the two tunneling rates through each of the barriers individually are both comparable to the cantilever’s oscillation frequency. The damping signal can be employed for detecting subfemtoampere tunneling currents and furthermore, maps of the damping signal can be used to inspect the mesoscopic tip shape. The observed and described current-induced damping should be a common phenomenon in non-contact atomic force microscopy for double-barrier tunnel junction geometries.

Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation

We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52--60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.

Organic light-emitting diodes based on sodium chloride buffer layer

Since the organic light-emitting diodes (OLEDs) have broad application prospects in the field of display and lighting due to low drive voltage, high brightness and current efficiency, fast response time, easy to realize large area flexible and other advantages, etc. However, the high price of this diode seriously affected the pace of its market. In this paper, the sodium chloride (NaCl) film was used as a cathode buffer layer of OLED device. The device structure is ITO/NPB (60 nm)/Alq3 (50 nm)/NaCl ( x nm)/Al (100 nm) devices, including x =0, 1.5, 2, 2.5, 3, 3.5 nm. The influence of the thickness of NaCl film on the performance of OLED was studied by analyzing the voltage and current characteristics of the device. When a layer of NaCl film was inserted between the cathode and the Alq3, the J - V characteristic curve of the device was obviously shifted to the left, and the turn-on voltage of the device decreased obviously. Moreover, with the increase of the thickness of the NaCl film, the turn-on voltage of the device decreased with the increase of the thickness (0–2 nm) film. When the thickness of the NaCl film is 2 nm, the voltage of the device was the lowest. If the thickness of NaCl film was more than 2.5 nm, the turn-on voltage of the device increased slowly with the increase of the thickness of the NaCl film. However, even if the thickness of the NaCl film increased to 3.5 nm, the turn-on voltage of the device was much lower than that of the NaCl thin film device. When the thickness of the NaCl film is less than 2.5 nm, the current efficiency of the device is much larger than that of the device without NaCl film. At the same time, the physical mechanism was analyzed by combining the tunneling equation. This study gives a new way to reduce the cost of OLED production.

Atomic Structure of Submonolayer NaCl Grown on Ag(110) Surface

We report results on the growth of an NaCl film on Ag(110) under ultrahigh vacuum conditions. At room temperature, low-energy electron diffraction and scanning tunneling microscopy show that the NaCl film forms a (4×1) superstructure. At RT, the film consists of small-sized islands that coalesce into larger islands at 410 K. These large islands preserve the (4×1) superstructure and cover the entire surface. The apparent heights obtained from the STM images show that the initial thickness of the NaCl islands is one atomic layer, and they present a very small height corrugation. The density functional theory calculations, with and without the inclusion of van der Waals effects, confirm the coexistence of two domains in agreement with the observed structure.