Monolayer Thin Films Research Articles

Dynamics of interacting interphases in polymer bilayer thin films

We investigate how the local glass-transition temperature (T{ing}) depends on film thickness in monolayer and bilayer thin films with a polystyrene (PS) upper-layer and a poly(methyl methacrylate) (PMMA) lower-layer using coarse-grained simulations. Interactions between overlapping interphases demonstrate a superposition principle for describing their glass-transition behaviors. For supported bilayer films, the free surface effect on a PS film upper-layer is effectively eliminated due to an enhanced local T{ing} near the PS-PMMA interface, which cancels out depressed T{ing} near the free surface. However, at very low PMMA lower-layer thicknesses, the PMMA-substrate effect can penetrate through the polymer-polymer interface, leading to enhanced T{ing} in the PS upper-layer.

Solution-Processable transparent conducting electrodes via the self-assembly of silver nanowires for organic photovoltaic devices

Solution-processed transparent conducting electrodes (TCEs) were fabricated via the self-assembly deposition of silver nanowires (Ag NWs). Glass substrates modified with (3-aminopropyl)triethoxysilane (APTES) and (3-mercaptopropyl)trimethoxysilane (MPTES) were coated with Ag NWs for various deposition times, leading to three different Ag NWs samples (APTES-Ag NWs (PVP), MPTES-Ag NWs (PVP), and APTES-Ag NWs (COOH)). Controlling the deposition time produced Ag NWs monolayer thin films with different optical transmittance and sheet resistance. Post-annealing treatment improved their electrical conductivity. The Ag NWs films were successfully characterized using UV–Vis spectroscopy, field emission scanning electron microscopy, optical microscopy and four-point probe. Three Ag NWs films exhibited low sheet resistance of 4–19Ω/sq and high optical transmittance of 65–81% (at 550nm), which are comparable to those of commercial ITO electrode. We fabricated an organic photovoltaic device by using Ag NWs as the anode instead of ITO electrode, and optimized device with Ag NWs exhibited power conversion efficiency of 1.72%.

Exploiting Click Chemistry for the Covalent Immobilization of Tetra (4-Propargyloxyphenoxy) Metallophthalocyanines onto Phenylazide-Grafted Gold Surfaces

In this study, tetra-(4-propargyloxy)phenoxy phthalocyanines (MTPrOPhOPc) were covalently immobilized as thin films onto gold surfaces via click reaction. The gold electrode surfaces were pre-functionalized with phenylazide (Au-PAz) thin film using in-situ diazonium generation followed by electrografting. Copper (I) catalyzed alkynyl-azide cycloaddition (CuCAAC) reaction was used to covalently immobilize the MTPrOPhOPcs onto the gold electrode surfaces to form Au-PAz-MTPrOPhOPc. The MTPrOPhOPcs were further studied for their electrocatalytic and electroanalytical properties towards the detection of hydrogen peroxide. Au-PAz-MTPrOPhOPc exhibited good reproducibility and stability in various electrolyte conditions. Electrochemical and spectroscopic surface characterization of the functionalized gold electrode surfaces confirmed the presence of the phenylazide and MTPrOPhOPc thin monolayer films. The excellent electroanalysis of hydrogen peroxide with the limit of detection (LoD) and limit of quantification (LoQ) in the μM range was obtained. The electrocatalytic reduction peaks for H2O2 were observed at −0.37V for Au-PAz-Mn(OAc)TPrOPhOPc and −0.31V for Au-PAz-CoTPrOPhOPc when Ag|AgCl pseudo-reference electrode was used. The Au-PAz-Mn(OAc)TPrOPhOPc and Au-PAz-CoTPrOPhOPc gold electrode surfaces showed good sensitivity and reproducibility towards the electrocatalytic reduction of hydrogen peroxide in pH 7.4 phosphate buffer solution.

A facile approach for the fabrication of Au/ZnO-hollow-sphere-monolayer thin films and their photocatalytic properties

In this study, we present a facile strategy for the fabrication of Au/ZnO-hollow-sphere-monolayer thin films. ZnO hollow spheres were synthesized by a template technique based on polystyrene (PS). ZnO and Au nanoparticles (NPs) were simultaneously deposited by DC magnetron sputtering. The as-prepared ZnO spheres exhibit a hollow structure with an obvious contrast between the thin ZnO shell and the interior. The Au/ZnO-hollow-sphere-monolayer thin films showed significantly enhanced photocatalytic performance in the degradation of MO compared with that of the ZnO thin film and ZnO-hollow-sphere-monolayer thin films. The observed degradation constant k of MO for the Au/ZnO–400nm sample was 0.0415min−1, which is approximately 17 times that of the ZnO thin film (0.0024min−1) and 3 times that of the ZnO–400nm sample (0.0128min−1). The enhancement in photocatalytic activity was attributed to the synergistic effect of the transfer of effective charges from the ZnO conduction band to the plasmonic Au NPs and the light trapping within the ZnO hollow spheres.

Quality enhancement of AZO thin films at various thicknesses by introducing ITO buffer layer

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d(002), strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (Eg) of AZO film are increased from 4.62 × 1019 to 8.21 × 1019 cm−3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6 × 10−4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19 × 104 (Ω cm)−1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.

Structures of spin-coated and annealed monolayer and multilayer poly(3-dodecylthiophene) thin films

The effects of film thickness and post annealing on the edge-on ordering and grain-size of spin-coated P3DDT films, which are of immense importance for their better device performances, were investigated using complementary XR and AFM techniques.

Interacting topological phases in thin films of topological mirror Kondo insulators

We study the interaction effects on thin films of topological mirror Kondo insulators (TMKI), where the strong interaction is expected to play an important role. Our study has led to the following results: (1) We identify a rich phase diagram of non-interacting TMKI with different mirror Chern numbers in the monolayer and bilayer thin films; (2) We obtain the phase diagram with interaction and identify the regimes of interaction parameters to mimic bosonic symmetry protected topological phases with either gapless bosonic modes or spontaneous mirror symmetry breaking at the boundary; (3) For the spontaneous mirror symmetry breaking boundary, we also study various domain-wall defects between different mirror symmetry breaking order parameters at the boundary. Our results reveal that the thin film TMKI serves as an intriguing platform for the experimental studies of interacting topological phases.

Influence of Carbon in Metalorganic Chemical Vapor Deposition of Few-Layer WSe2 Thin Films

Metalorganic chemical vapor deposition (MOCVD) is a promising technique to form large-area, uniform films of monolayer or few-layer transition metal dichalcogenide (TMD) thin films; however, unintentional carbon incorporation is a concern. In this work, we report the presence of a defective graphene layer that forms simultaneously during MOCVD growth of tungsten diselenide (WSe2) on sapphire at high growth temperature and high Se:W ratio when using tungsten hexacarbonyl (W(CO)6) and dimethyl selenide ((CH3)2Se, DMSe) as precursors. The graphene layer alters the surface energy of the substrate reducing the lateral growth and coalescence of WSe2 domains. The use of hydrogen selenide (H2Se) instead of DMSe eliminates the defective graphene layer enabling coalesced monolayer and few-layer WSe2 films.

Large-scale synthesis of WSe2 atomic layers on SiO2/Si

We report a systematic study of large-scale growth of high-quality WSe2 atomic layers directly on SiO2/Si substrates using a convenient method. Various parameters, especially growth temperatures, flow rate of carrier gas and tube pressure, are investigated in affecting the properties of as-grown WSe2 flakes in terms of their sizes, shapes and thickness. The pre-annealing step is demonstrated to be a key role in achieving the large-scale growth. Under an optimized condition, the lateral size of triangular single-crystal monolayer WSe2 is up to 30 [Formula: see text]m and the area of the monolayer thin film can be up to 0.25 mm2. And some other interesting features, such as nanoflowers, are observed, which are a promising for catalyzing research. Raman spectrum and microphotoluminescence indicate distinct layer dependent efficiency. Auger electron spectroscopy (AES) studies demonstrate the atomic concentration of the as-grown WSe2. Electrical transport further shows that the [Formula: see text]-type WSe2 field-effect transistors exhibit excellent electrical properties with carrier mobility of [Formula: see text][Formula: see text]64 cm[Formula: see text]V[Formula: see text]s[Formula: see text] and current on/off ratio over 105. These results are comparable to the exfoliated materials.

ChemInform Abstract: Ruthenium‐Amine Conjugated Organometallic Materials for Multistate Near‐IR Electrochromism and Information Storage

AbstractReview: [61 refs.

Ab initiostudy of cross-interface electron-phonon couplings in FeSe thin films onSrTiO3andBaTiO3

We study the electron-phonon coupling strength near the interface of monolayer and bilayer FeSe thin films on SrTiO$_3$, BaTiO$_3$, and oxygen-vacant SrTiO$_3$ substrates, using ab initio methods. The calculated total electron-phonon coupling strength $\lambda=0.2\text{--}0.3$ cannot account for the high $T_c\sim 70$ K observed in these systems through the conventional phonon-mediated pairing mechanism. In all of these systems, however, we find that the coupling constant of a polar oxygen branch peaks at $\mathbf{q}=0$ with negligible coupling elsewhere, while the energy of this mode coincides with the offset energy of the replica bands measured recently by angle-resolved photoemission spectroscopy experiments. But the integrated coupling strength for this mode from our current calculations is still too small to produce the observed high $T_c$, even through the more efficient pairing mechanism provided by the forward scattering. We arrive at the same qualitative conclusion when considering a checkerboard antiferromagnetic configuration in the Fe layer. In light of the experimental observations of the replica band feature and the relatively high $T_c$ of FeSe monolayers on polar substrates, our results point towards a cooperative role for the electron-phonon interaction, where the cross-interface interaction acts in conjunction with a purely electronic interaction. We also discuss a few scenarios where the coupling strength obtained here may be enhanced.

Ruthenium-Amine Conjugated Organometallic Materials for Multistate Near-IR Electrochromism and Information Storage.

Monometallic and dimetallic complexes with the ruthenium-amine conjugated structural unit have been prepared. These complexes display consecutive redox waves with low potentials and rich and intense absorptions in the near-infrared region. The electrochemical and spectroscopic properties can be modulated using substituents or auxiliary ligands with different electronic natures. Through simple functionalization, electropolymerized or monolayer thin films of these complexes have been prepared. These films display multistate near-infrared electrochromism with good contrast ratios and long optical retention times. In addition, flip-flop and flip-flap-flop memories have been demonstrated on the basis of these thin films.

A SERS and electrical sensor from gas-phase generated Ag nanoparticles self-assembled on planar substrates.

Optical excitation of coupled plasmonic nanoparticles supports intense localized electromagnetic "hot-spots" which enable a variety of surface enhanced spectroscopies with the best known example being surface enhanced Raman scattering (SERS), currently of great interest for sensing applications. In this study, we present a novel SERS and electrical dual transduction chemical sensor based on gas-phase generated, negatively charged, silver nanoparticles self-assembled on glass slide forming a close-packed plasmonic monolayer thin-film that supports both SERS and electrical sensing. We demonstrate broad tunability of the localized surface plasmon resonance (LSPR) of the close-packed plasmonic nanoparticle monolayer thin-film sensors through control of the nanoparticle (NP) deposition time which directly influences the plasmonic coupling between neghibouring NPs. This broad tunability supports strong SERS activity from visible to near infrared (NIR) excitation wavelengths. We performed SERS and electrical measurements of a non-resonant molecule 4-mercaptobenzonitrile (4-MBN) as a sample Raman reporter molecule to determine the SERS enhancement factor of our SERS substrate. We measured an average SERS enhancement factor of 10(7) from our close-packed plasmonic nanoparticle monolayer thin-film sensor. Films which were grown below or above one nanoparticle monolayer both exhibited significantly lower SERS performance in one or more of SERS enhancement factor (EF), uniformity or repeatability. Our close-packed plasmonic nanoparticle monolayer thin-film sensors are highly uniform from point-to-point across the entire substrate and showed good reproducibility from batch-to-batch. These qualities are highly desirable for quantifiable detection of chemical and biological molecules. As an example application, this type of substrates provides an affordable and reliable sensing and identification capability for combatting new and emerging chemical and biological threats in support of security applications.

Anisotropic optical and conductive properties of oriented 1D-nanoparticle thin films made by spray-assisted self-assembly.

We report on the fabrication of oriented anisotropic metal nanoparticle thin films made by Grazing Incidence Spraying (GIS) and on the anisotropic plasmonic properties of the resulting thin films. Gold nanorods of two different aspect ratios and silver nanowires were self-assembled as a uniaxially aligned monolayer with the GIS approach. In particular, we examine the influence of the nanowire/nanorod length and diameter on the degree of ordering determined by electron microscopy pictures. Furthermore, we show that the anisotropy of the optical properties (probed by polarized UV-visible-near infrared spectroscopy) strongly depend on the quality of alignment. The prepared monolayer thin films have an orientation order parameter of up to 0.83 for silver nanowires, which is reflected in an optical anisotropy of 0.57 in the UV-visible and 0.76 in the near infrared through the selective excitation of transverse and longitudinal surface plasmon resonance modes. The electronic transport in oriented silver nanowire monolayers is also shown to be highly directional, with the sheet resistance varying over almost an order of magnitude depending on the transport direction. Such anisotropic conductive plasmonic thin films may find applications in various fields like biochemical sensing, energy transport and harvesting or optoelectronic devices.

Preparation and high-frequency soft magnetic property of FeCo-based thin films

A series of FeCo-based thin films were prepared by magnetron sputtering without applying an induced magnetic field. The microstructure, electrical properties, magnetic properties and thermal stability of FeCo, FeCoSiN monolayer thin film and [FeCoSiN/SiN x ] n multilayer thin film were investigated systematically. When FeCo thin film was doped with Si and N, the resistivity and soft magnetic properties of the obtained FeCoSiN thin film can be improved effectively. The coercivity (H c), resistivity (ρ) and ferromagnetic resonance frequency (f r) can be further optimized for the [FeCoSiN/SiN x ] n multilayer thin film. When the thickness of FeCoSiN layer and SiN x layer is maintained at 7 and 2 nm, the H c, ρ and f r for [FeCoSiN/SiN x ] n multilayer thin film are 225 A·m−1 392 µΩ·cm−1 and 4.29 GHz, respectively. In addition, the low coercivity of easy axis (H ce ≈ 506 A·m−1) of [FeCoSiN/SiN x ] n multilayer thin film can be maintained after annealing at 300 °C in air for 2 h.

Optical properties of multilayer (III–V semiconductors) thin films within the framework of DFT + characteristic matrix

We present a new model of calculations based on DFT and Abeles matrix theory to understand the optical properties of semiconductor thin films which form the active layers in many optoelectronic applications. We show how optical quantities such as refractive index, extinction coefficient and band gap can change the optical representations of thin films. In this study two main types of calculations are carried out. The first is the calculation of the dielectric functions within the density functional theory framework (DFT). The second is the calculation of the optical transmittance, absorbance and reflection of monolayer and multilayer thin films within the Abeles matrix theory. The output of this combination (DFT+characteristic matrix) is very helpful and the effect of many parameters like thickness, incidence angle and light polarization on optical properties of multilayer systems can be easily investigated.

Glutamine-reinforced silica gel microassembly as protective coating for aluminium surface

The present article comprises the formation of novel glutamine reinforced silica gel microassembly over aluminium and the use thereof as promising protective coating. The aluminium sheets were immersed in a glutamine solution and then in silica gel solutions with four different concentrations. Epichlorohydrin as coupling agent and Ca2+ ions as crosslinking agents were introduced in dropwise addition. Formation of monolayer and bilayer thin films, verified by inhibition efficiency and surface morphology, demonstrated successful protective coating of aluminum surface against severe corrosive environment for prolonged time. In monolayer, maximum inhibition efficiency obtained was 98.3% at 10 °C using silica gel concentration = 1200 ppm. Higher activation energy, enthalpy and entropy of activation obtained at higher silica gel concentration. The adsorption process demonstrated appropriate Langmuir isotherm fit. The formation of layer-by-layer comprises firm and stable thin films over aluminium surface with inhibition efficiency ∼99%. Such coatings performed by building-up construction have some guideline specifics that include simplicity, low cost manufacturing, and superior corrosion protection.

Determination of structural, mechanical and corrosion properties of titanium alloy surface covered by hybrid system based on graphene monolayer and silicon nitride thin films

We report the comparative studies on structural, mechanical and corrosion properties of SiN, SiN/graphene and graphene/SiN hybrid systems deposited on titanium alloy surface. The deposited silicon nitride thin film and silicon nitride/graphene coatings systems were crack free and exhibited good adherence to the substrate and the surface morphology was homogeneous. The graphene/silicon nitride coating system demonstrated a tendency to peeling off and had poor adhesion to the graphene monolayer deposited on the titanium alloy surface. Graphene transferred on titanium alloy surface and silicon nitride thin film surface was a single layer without defects. The hardness for silicon nitride thin film and SiN/graphene coatings was the same (22.4 GPa). Graphene coating has no effect on surface hardness but it decreases the electrochemical activity of the system. The best corrosion resistance has the Ti6Al4V sample coated with SiN/graphene coating system. This sample retains the most stable mechanical and structural parameters during corrosion process. The SiN/graphene coating system greatly improves the mechanical and corrosion properties of examined titanium alloy surface.

Investigation of band-offsets at monolayer-multilayer MoS₂ junctions by scanning photocurrent microscopy.

The thickness-dependent band structure of MoS2 implies that discontinuities in energy bands exist at the interface of monolayer (1L) and multilayer (ML) thin films. The characteristics of such heterojunctions are analyzed here using current versus voltage measurements, scanning photocurrent microscopy, and finite element simulations of charge carrier transport. Rectifying I-V curves are consistently observed between contacts on opposite sides of 1L/ML junctions, and a strong bias-dependent photocurrent is observed at the junction. Finite element device simulations with varying carrier concentrations and electron affinities show that a type II band alignment at single layer/multilayer junctions reproduces both the rectifying electrical characteristics and the photocurrent response under bias. However, the zero-bias junction photocurrent and its energy dependence are not explained by conventional photovoltaic and photothermoelectric mechanisms, indicating the contributions of hot carriers.

Ionization of covalent immobilized poly(4-vinylphenol) monolayers measured by ellipsometry, QCM and SPR

Covalently immobilized poly(4-vinylphenol) (PVP) monolayer films were fabricated by spin coating PVP on perfluorophenyl azide (PFPA)-functionalized surfaces followed by UV irradiation. The pH-responsive behavior of these PVP ultrathin films was evaluated by ellipsometry, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR). By monitoring the responses of these films to pH in situ, the ionization constant of the monolayer thin films was obtained. The apparent pKa value of these covalently immobilized PVP monolayers, 13.4 by SPR, was 3 units higher than that of the free polymer in aqueous solution.