Room‐Temperature Growth of Silicon Oxide Nanofilms: New Opportunities for Plastic Electronics

A new generation of plastic transistors consisting primarily of light and flexible organic materials requires new fabrication methods which combine low‐temperature, solution‐phase processing with precise control in the nanometer range over the component dimensions. Ultrathin silicon oxide films, which serve as gate dielectric layers in these transistors, were recently grown at room temperature from polymer precursor films by a novel layer‐by‐layer deposition/oxidation process.

     

桥连氧化钨纳米线的可控合成及气敏性质

The rapid development of industrialization has resulted in severe environmental problems. A comprehensive assessment of air quality is urgently required all around the world. Among various technologies used in gas molecule detection, including Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, mass spectroscopy (MS), electrochemical sensors, and metal oxide semiconductor (MOS) gas sensors, MOS gas sensors possess the advantages of small dimension, low power consumption, high sensitivity, low production cost, and excellent silicon chip compatibility. MOS sensors hold great promise for future Internet of Things (IoT) sensors, which will have a profound impact on indoor and outdoor air quality monitoring. The development of nanotechnology has significantly enhanced the development of MOS gas sensors. Among various nanostructures like nanoparticles, nanosheets and nanowires, the emergence of quasi-one-dimensional (q1D) nanowires/nanorods/nanofibers, with unique q1D geometry (facilitating fast carrier transport) and large surface-to-volume ratio, potentially act as ideal sensing channels for MOS sensors with extremely small dimension, and good stability and sensitivity. These structures have thus been the focus of extensive research. Among the various MOS nanomaterials available, tungsten oxide (WO_3-x, 0 ≤ x < 1) nanowires feature the characteristic properties (multiple oxidation states, rich substoichiometric oxides with distinct properties, photo/electrochromism, (photo)catalytic properties, etc.), and unique q1D geometry (single-crystalline pathway for fast carrier transport, large surface-to-volume ratio, etc.). WO_3-x nanowires have broad applications in smart windows, energy conversation & storage, and gas sensing devices, and have thus become a focus of attention. In this paper, the fundamental properties of tungsten oxide, synthesis methods and growth mechanism of tungsten oxide nanowires are reviewed. Among various (vapor-liquid-solid (VLS), vapor-solid (VS) and thermal oxidation) growth methods, the thermal oxidation method enables an in situ integration of WO_3-x nanowires on predefined electrodes (so-called bridged nanowire devices) via the oxidation of lithographically patterned W film at relatively low growth temperature (~500 ℃) because of interfacial strain, defects and oxygen on the surface of the W film. The novel bridged nanowire-based sensor devices outperform traditional lateral nanowire devices in terms of larger exposure area, low power consumption via self-heating, and greater convenience in device processing. Recent progress in bridged WO_3-x nanowire devices and sensitive NO_x molecule detection under low power consumption have also been reviewed. Power consumption of as low as a few milliwatts was achieved, and the detection limit of NO₂ was reduced to 0.3 ppb (1 ppb = 1 × 10^-9, volume fraction). In situ formed bridged WO_3-x nanowire devices potentially satisfy the strict requirements of IoT sensors (small dimension, low power consumption, high integration, low cost, high sensitivity, and selectivity), and hold great promises for future IoT sensors.     

Real‐time Observation of Deep Lithiation of Tungsten Oxide Nanowires by In Situ Electron Microscopy

An in‐depth mechanistic understanding of the electrochemical lithiation process of tungsten oxide (WO₃) is both of fundamental interest and relevant for potential applications. One of the most important features of WO₃ lithiation is the formation of the chemically flexible, nonstoichiometric Li_xWO₃, known as tungsten bronze. Herein, we achieved the real‐time observation of the deep electrochemical lithiation process of single‐crystal WO₃ nanowires by constructing in situ transmission electron microscopy (TEM) electrochemical cells. As revealed by nanoscale imaging, diffraction, and spectroscopy, it is shown that the rapid and deep lithiation of WO₃ nanowires leads to the formation of highly disordered and near‐amorphous Li_xWO₃ phases, but with no detectable traces of elemental W and segregated Li₂O phase formation. These results highlight the remarkable chemical and structural flexibility of the Li_xWO₃ phases in accommodating the rapid and deep lithiation reaction.     

Surfactant‐Free Synthesis of Plasmonic Tungsten Oxide Nanowires with Visible‐Light‐Enhanced Hydrogen Generation from Ammonia Borane

WO_3?x nanowires were successfully synthesized through a simple surfactant‐free solvothermal method. These nanowires exhibit strong plasmonic absorption in the visible and near‐infrared region owing to the abundant oxygen vacancies. The plasmon excitation of these WO_3?x nanowires provide five times enhancement on the hydrogen generation from ammonia borane.     

Correlation of Molecular Assembly and Interactions in Crystals and Langmuir–Blodgett Films of N‐(2,4‐dinitrophenyl)‐n‐octadecylamine

Correlation of molecular organization in crystals and in ultrathin films is of fundamental interest in the design of molecular materials based on thin films. We have chosen as a test case, N‐(2,4‐dinitrophenyl)‐n‐octadecylamine (DNPOA), a potential candidate for the fabrication of Langmuir–Blodgett (LB) films for quadratic nonlinear optical applications. Like several other 4‐nitroaniline derivatives, DNPOA does not form stable monolayers at the air–water interface. This has precluded investigations of their organization in LB films. We have stabilized composite Langmuir films of DNPOA with the phospholipid molecule DSPC and fabricated their LB films. Successful growth of single crystals of DNPOA allowed structure determination and detailed analysis of molecular associations in the solid state. Electronic absorption spectra of DNPOA in solution, in the solid state and in the LB film are investigated. Modeling of the various spectral signatures by semiempirical computations on molecular clusters extracted from the crystal lattice provides insight into the correlation between the molecular organization in crystals and in LB films.     

Three‐Component Langmuir–Blodgett Films Consisting of Surfactant,Clay Mineral,and Lysozyme: Construction and Characterization

The Langmuir–Blodgett (L–B) technique has been employed for the construction of hybrid films consisting of three components: surfactant, clay, and lysozyme (Lys). The surfactants are octadecylammonium chloride (ODAH) and octadecyl ester of rhodamine B (RhB18). The clays include saponite and laponite. Surface pressure versus area isotherms indicate that lysozyme is adsorbed by the surfactant–clay L–B film at the air–water interface without phase transition. The UV‐visible spectra of the hybrid film ODAH–saponite–Lys show that the amount of immobilized lysozyme in the hybrid film is (1.3±0.2) ng mm^?2. The average surface area (Ω) per molecule of lysozyme is approximately 18.2 nm² in the saponite layer. For the multilayer film (ODAH–saponite–Lys)_n, the average amount of lysozyme per layer is (1.0±0.1) ng mm^?2. The amount of lysozyme found in the hybrid films of ODAH–laponite–Lys is at the detection limit of about 0.4 ng mm^?2. Attenuated total reflectance (ATR) FTIR spectra give evidence for clay layers, ODAH, lysozyme, and water in the hybrid film. The octadecylammonium cations are partially oxidized to the corresponding carbamate. A weak 1620 cm^?1 band of lysozyme in the hybrid films is reminiscent of the presence of lysozyme aggregates. AFM reveals evidence of randomly oriented saponite layers of various sizes and shapes. Individual lysozyme molecules are not resolved, but aggregates of about 20 nm in diameter are clearly seen. Some aggregates are in contact with the clay mineral layers, others are not. These aggregates are aligned in films deposited at a surface pressure of 20 mN m^?1.     

Synthesis of an Open‐Cage Structure POSS Containing Various Functional Groups and Their Effect on the Formation and Properties of Langmuir Monolayers

Recently, silsesquioxanes have been recognized as a new group of film‐forming materials. This study has been aimed at determining the effect of the kind of functional groups present in two different open‐cage structure POSS molecules on the possibility of the formation of Langmuir monolayers and their properties. To achieve this goal, two new POSS derivatives (of open‐cage structures) containing polyether and fluoroalkyl functional groups have been synthesized on the basis of a hydrosilylation process. An optimization of the process was performed, which makes it possible to obtain the above‐mentioned derivatives with high yields. In the next step, the Langmuir technique was applied to measurements of the surface pressure (π) ? the mean molecular area (A) isotherms during the compression of monolayers formed by molecules of the two POSS derivatives considered. Subsequently, the monolayers were transferred onto quartz plates according to the Langmuir–Blodgett technique. Both derivatives are able to form insoluble Langmuir films at the air–water interface, which can be transferred onto a solid substrate and effectively change its wetting properties.     

Interfacial reactions of nickel ion containing multilayers and nickel ultra‐thin films with an interpenetrating polymer network substrate

An interpenetrating polymer network (IPN) substrate was prepared by dip‐pulling a silicon wafer or glass plate into polymer precursors, followed by solidification at room temperature. The nickel ion containing Langmuir–Blodgett (LB) multilayers was subsequently fabricated onto the IPN substrate by a LB technique, and then the nickel ultra‐thin films were deposited by chemical reduction. The interfacial reactions, the metal transformations and the non‐metal bond types during the film formation were detected by the X‐ray photoelectron spectroscopy, atomic force microscope and attenuated total reflection infrared spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

Classical Benzotriazole‐Mediated α‐Aminoalkylations of Alkynes: Synthesis and Characterization of Alk‐2‐yn‐1‐amines as Amphiphilic Materials

Reactions of readily available and stable benzotriazolemethanamines 1a – l , obtained from aldehydes and secondary amines (Scheme 2), gave the expected alk‐2‐yn‐1‐amines 3a – t (Scheme 3). The amphiphilic character of the synthesized products was responsible for physicochemical measurements. Specific aggregation properties of the obtained compounds make them useful as electroactive materials in the Langmuir–Blodgett technique.     

Cobalt Phosphide Nanowires: Efficient Nanostructures for Fluorescence Sensing of Biomolecules and Photocatalytic Evolution of Dihydrogen from Water under Visible Light

The detection of specific DNA sequences plays an important role in the identification of disease‐causing pathogens and genetic diseases, and photochemical water splitting offers a promising avenue to sustainable, environmentally friendly hydrogen production. Cobalt–phosphorus nanowires (CoP NWs) show a high fluorescence quenching ability and different affinity toward single‐ versus double‐stranded DNA. Based on this result, the utilization of CoP NWs as fluorescent DNA nanosensors with a detection limit of 100 pM and a selectivity down to single‐base mismatch was demonstrated. The use of a thrombin‐specific DNA aptamer also enabled the selective detection of thrombin. The photoinduced electron transfer from the excited dye that labels the oligonucleotide probe to the CoP semiconductor led to efficient fluorescence quenching, and largely enhanced the photocatalytic evolution of hydrogen from water under visible light.     

pH‐Modulated Molecular Assemblies and Surface Properties of Metal–Organic Supercontainers at the Air–Water Interface

The orientation of metal–organic supercontainer (MOSC) molecules in Langmuir films was systematically studied at the air–water interface. The acidity of the aqueous subphases plays a significant role in tuning the orientation of MOSC molecules in the Langmuir films. Furthermore, Langmuir–Blodgett films of MOSCs were prepared and the uniform multilayer structures demonstrated various surface properties, depending on their conditions of fabrication. Our use of Langmuir films provides a novel approach to access tunable assemblies of MOSC molecules in two‐dimensional thin films.     

Oriented structure of octadecyl acridine orange intercalated in the monolayer and Langmuir–Blodgett film of octadecyl adenine‐thymine base pairs

The oriented structure of acridine orange (AO) in both monolayer and Langmuir–Blodgett (LB) film has been studied by optical waveguide (OWG) spectroscopy using polarized incident light. Mixed monolayer and LB films, consisting of octadecyl acridine orange (C₁₈‐AO) incorporated in stacked base pairs of octadecyl adenine (C₁₈‐Ade) and octadecyl thymine (C₁₈‐Thy), were prepared on a quartz waveguide. Absorption of transverse electric field (TE) polarized light was about twice that of transverse magnetic field (TM) polarized light. Both OWG spectra have λ_max at 500 nm, which is characteristic of monomeric AO molecules. This result strongly suggests that C₁₈‐AO molecules were dispersed uniformly in the mixed monolayer and were excited more effectively by the TE polarized light. Since the absorption moment of AO molecules is related to their long axis, it is proposed that C₁₈‐AO molecules are incorporated in C₁₈‐Ade/C₁₈‐Thy pairs with the long axis parallel to the layer surface. The absorbance at 500 nm was proportional to the number of layers on the waveguide. The dichroic ratio of the absorbance at 500 nm for TE polarized light to that for TM polarized light was constant regardless of the number of layers. The C₁₈‐AO molecules were uniformly incorporated in each layer with the long axis relatively parallel to the layer surface. Copyright © 2004 John Wiley & Sons, Ltd.     

Highly ordered amphiphilic cyclopalladated arylimine self‐assembly films for catalyzing Heck and Suzuki coupling reactions

A series of new cyclopalladated arylimine compounds ( 3a , 3b , 3c , 4a , 4b , 4c ) were synthesized and characterized. Their catalytic properties for Heck and Suzuki coupling reactions in a homogeneous system were preliminarily investigated using water as solvent, in which no ligands, air isolation or assistant solvents were needed in cross‐coupling reactions. The optimization of the homogeneous system provided a basis for research on the heterogeneous catalytic reaction catalyzed by ordered self‐assembly films. Organized monolayers of 3a , 3b , 3c were prepared and utilized as C? C coupling catalysts. Monolayers of 3a , 3b , 3c were deposited using Langmuir–Blodgett techniques and analyzed using π–A isotherms, UV–visible and X‐ray photoelectron spectroscopies and atomic force microscopy, which showed near orientation on the surface and stability under the optimized experimental conditions suitable for exploring Heck and Suzuki coupling reactions. The activity of immobilized 3c monolayer is enhanced relative to homogeneous reaction, in which the ordered monolayers are efficient with a catalyst loading as low as 10^?5 mol%, turnover number as high as 79 200 and turnover frequency as high as 2640 h^?1. The catalytic efficiency is 100 times higher than that in the homogeneous case using the same amount and ratio of reagent. The increased activity of immobilized 3c monolayer is due to a combination of its structure and changes in conformation when deposited onto the substrate. The topographic changes of catalyst films, stability of films and catalytic activity were investigated with atomic force microscopy, cyclic voltammetry, X‐ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry, from which a heterogeneous catalytic mechanism for Suzuki coupling reaction is proposed. The study demonstrates that careful monolayer studies can provide useful models for the design and study of supported molecular catalyst systems. Copyright © 2016 John Wiley & Sons, Ltd.     

Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room‐Temperature Phosphorescence

Although persistent room‐temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited‐state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.     

Design and Synthesis of Aviram–Ratner‐Type Dyads and Rectification Studies in Langmuir–Blodgett (LB) Films

The design and synthesis of Aviram–Ratner‐type molecular rectifiers, featuring an anilino‐substituted extended tetracyanoquinodimethane (exTCNQ) acceptor, covalently linked by the σ‐spacer bicyclo[2.2.2]octane (BCO) to a tetrathiafulvalene (TTF) donor moiety, are described. The rigid BCO spacer keeps the TTF donor and exTCNQ acceptor moieties apart, as demonstrated by X‐ray analysis. The photophysical properties of the TTF‐BCO‐exTCNQ dyads were investigated by UV/Vis and EPR spectroscopy, electrochemical studies, and theoretical calculations. Langmuir–Blodgett films were prepared and used in the fabrication and electrical studies of junction devices. One dyad showed the asymmetric current–voltage (I–V) curve characteristic for rectification, unlike control compounds containing the TTF unit but not the exTCNQ moiety or comprising the exTCNQ acceptor moiety but lacking the donor TTF part, which both gave symmetric I–V curves. The direction of the observed rectification indicated that the preferred electron current flows from the exTCNQ acceptor to the TTF donor.     

Mn Oxide‐Silver Composite Nanowires for Improved Thermal Stability,SERS and Electrical Conductivity

Redox transformation reaction between aqueous AgNO₃ and Mn(CH₃COO)₂ at low temperature (～80 °C) has been adopted for industrial‐scale production of uniform Ag–MnOOH composite nanowires for the first time. Varying amounts of incorporated Ag in the composite retain the 1D morphology of the composite. Nanowires upon annealing evolve Ag–MnO₂ nanocomposites, once again with the retention of the parental morphology. Just 4 % of silver incorporation in the composite demonstrates metal‐like conducting performance from the corresponding semiconducting material. Transition of MnO₂ to Mn₂O₃ to Mn₃O₄ takes place upon heat treatment in relation to successive increase in Ag concentrations in the nanowires. The composites offer resistance to the observed oxide transformation. This is evidenced from the progressive increase in transition temperature. In situ Raman, ex situ thermal and XRD analysis corroborate the fact. The composite with 12 % Ag offers resistance to the transformation of MnO₂, which is also verified from laser heating. Importantly, Ag nanoparticle incorporation is proved to offer a thermally stable and better surface enhanced Raman scattering (SERS) platform than the individual components. Both the Ag–MnOOH and Ag–MnO₂ nanocomposites with 8 atomic % Ag show the best SERS enhancement (enhancement factor ～10¹⁰). The observed enhancement relates to charge transfer as well as electromagnetic effects.