Ultrathin Layer Research Articles

Construction of ultrathin and microporous polyesteramide nanofilms for efficient organic solvent nanofiltration (OSN) using diacyl chloride

The integration of high permeability and exceptional selectivity, thereby overcoming the conventional “trade-off” effect, remains a persistent challenge for the practical implementation of organic solvent nanofiltration (OSN) membranes. Herein, at the molecular level of design, we utilized the 5,5′-diamino-2,2′-biphenol (amino-BIPOL) and diacyl chloride as IP monomers to prepare OSN membranes with high perm-selectivity. The amino-BIPOL, a contorted biphenol molecule with four reactive sites, serves as a molecular-level unit to hinder efficient packing of polymer chains and induce the formation of interconnected micropores in the resulting polyesteramide layers. By employing diacyl chloride terephthaloyl dichloride (TPC) and isophthaloyl dichloride (IPC) as the oil-phase cross-linker, two kinds of TFC membranes were prepared. The resulting membranes both present smooth surface morphology, with ultrathin and high microporous selective layer allowing for efficient mass transfer. Specifically, the methanol permeance of amino-BIPOL/TPC and amino-BIPOL/IPC membrane reaches up to 15.1 and 15.6 LMH/bar, while maintaining a molecular weight cut-off (MWCO) value of 300 and 373 Da, respectively. Overall, this work illustrates the different oil-phase cross-linkers will result in diverse nanofilm microstructure and thus with distinct perm-selectivity properties, offering new insights for the design and modification of advanced OSN membranes.

Single-atom iridium doped ultrathin MoS2 layers as a highly efficient catalyst for the hydrodeoxygenation of 5-hydroxymethylfurfural

The catalytic transformation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) is of great importance within the context of biomass valorization, but remains a major challenge due to the complicated reaction networks. Herein, single-atom Ir doped ultrathin MoS2 layers were synthesized and used as catalyst for the hydrodeoxygenation of HMF, yielding 96.5% of DMF. The single atomic structure of Ir was determined by X-ray diffraction (XRD), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption fine structure analysis (XAFS), and X-ray photoelectron spectroscopy (XPS). Furthermore, the Ir doped MoS2 catalyst showed excellent stability and did not deactivate after five reaction cycles. This work expands the applications of single-atom catalysts and introduces a new and highly efficient catalyst for the sustainable production of DMF.

Ultra-broadband and wide-angle plasmonic light absorber based on all-dielectric gallium arsenide (GaAs) metasurface in visible and near-infrared region

High-performance and cost-effective broadband light absorbers are crucial for various optical applications, including solar energy harvesting, thermophotovoltaic systems, and integrated photoelectric devices. In this study, an ultra-broadband and wide-angle plasmonic light absorber (PLA) based on an all-dielectric gallium arsenide (GaAs) metasurface in the visible and near-infrared regions is proposed and numerically investigated. The unit-cell of the proposed PLA is consisted of all-dielectric GaAs cylinder resonator (CR) adhered on an ultra-thin GaAs spacer layer backed with a tungsten (W) substrate. Numerical simulation results show that the absorption peak of the proposed PLA is up to 99.85 %, 99.9 % and 99.98 % at 0.452 μm, 0.712 μm and 1.596 μm, respectively. In addition, the ultra-broadband strong absorption of over 90 % also can be achieved from 0.39 μm to 2.09 μm with a relative bandwidth of 137.1 %. The ultrabroadband strong absorption of the proposed PLA is further demonstrated by the equivalent circuit model (ECM), which displays excellent agreement with the results of the full-wave numerical simulation performed using the finite element method (FEM). The simulated electromagnetic (EM) field distributions reveal that the ultra-broadband strong absorption of the designed PLA is mainly from the excited propagating surface plasmon (PSP), located surface plasmon (LSP), cavity resonance modes combined with coupling effect between adjacent CRs. The proposed PLA is also polarization-insensitive and wide incident angles for both transverse electric (TE) and transverse magnetic (TM) modes. The absorption properties of the designed PLA can be adjusted by varying geometric parameters of the unit-cell. The PLA achieves a remarkable short-circuit current density of over 64.75569 mA/cm2 and exhibits nearly perfect solar energy trapping due to the effective coupling of multiple resonances, resulting in enhanced photovoltaic performance. Thus, the proposed PLA can be potential application in the photodetectors, thermal imaging, and solar energy harvesting devices.

Improvement of high energy X-ray optical performance of W/Si supermirror by optimizing interface compounds using ultra-thin buffer layer

The high energy spectral properties of X-ray supermirrors are governed mainly by the interface diffusion and compound formation at the interfaces. Using ion beam sputtering technique, a set of W/Si periodic multilayers with C buffer layer (BL) and without C BL have been prepared and the effect of BL has been studied by grazing incidence specular X-ray reflectivity, diffused X-ray scattering and X-ray photoelectron spectroscopy. Using C BL at the W-on-Si interfaces, reduction of interface diffusion and minimization of WSix formation at the interfaces has been observed though signature of carbide formation has been noticed. Using the results an interface diffusion model has been proposed explaining the effect of C BL. Block method of X-ray supermirror design has been effectively used for reducing the number of layers of W/Si supermirror targeting the improvement of the performance. Finally, 36-layer W/Si supermirror with C BL and without C BL have been fabricated and the performance evaluated by reflectivity measurement using 25 keV synchrotron X-rays and cross sectional TEM measurements. This study has both technological implication in X-ray optical devices and also offers atomistic insight into the mechanism of improvement of the interface quality by application of ultra-thin buffer layer.

Carbon Molecular Sieve Membranes with Ultra-Thin Selective Skin Layer by Pyrolysis of Porous Hollow Fibers.

Translating carbon molecular sieve (CMS) membranes into highly scalable hollow fiber geometry with ultra-thin selective layer (<1µm) for gas separation remains as great challenge. The porous support layer of precursor hollow fiber membranes is prone to collapse during pyrolysis, which induces thick skin layer (15-50µm) of CMS hollow fiber membranes. Here, a novel strategy is present to obtain an ultra-thin selective skin layer by carbonization of hollow fiber membranes with porous skin. P84-based defect-free CMS hollow fiber membranes with ultra-thin selective skin layer (0.9µm) for gas separation are prepared without any coating or complex chemical pretreatment. Compared with the carbon membranes derived from defect-free fibers, the H2 permeance (93.9 GPU) of CMS membranes derived from the porous fibers increases ≈1353% with comparable selectivity of H2/CH4 (143) and higher H2/N2 (120). Furthermore, the porous fibers are pre-aged near the Tg in N2 conditions before carbonization, and the H2 permeance of the derived CMS hollow fiber membranes reached 147 GPU (increased 2180%). It is a new facile way to prepare CMS hollow fiber membranes with ultra-thin selective layer by porous fibers, demonstrating its versatile potential in gas separation or organic liquids separation.

Al2O3 Thin Layer Formed inside Porous Membrane Using Spray Synthesis Method and Its Application

Aluminum oxide (Al2O3) films have been investigated for use in various applications, and numerous deposition techniques have been reported. The spray synthesis method has the advantage of forming a thin layer of crystal at low temperatures using the appropriate precursors. A precursor prepared by diluting Methylaluminoxane with N-methyl pyrrolidone was sprayed onto a porous membrane while varying conditions such as the substrate temperature, feeding speed, and spray amount. The solution penetrated the film during spray application, and the ultra-thin layers deposited on the side wall of the internal pores were observed using a cross-sectional transmission electron microscope (XTEM). The lattice image obtained using the TEM and the composition analysis conducted using a scanning TEM and an energy-dispersive X-ray spectroscope suggest that this thin layer is a layer of Al2O3. The formation of Al2O3 occurred at lower temperatures than in previous reports. This is a major advantage for applications with low-melting-point materials. The most suitable spraying conditions were determined based on the state of deposition on the surface and inside the membrane. These conditions were applied to a three-layer separator for lithium-ion batteries and their effect on thermal stability was investigated. Through heating experiments and XRD analysis, it was confirmed that the shrinkage and melting of the separator are suppressed by spraying. This process can be expected to have wide applications in low-melting-point materials such as polyolefin.

Improvement of Efficiency in Kesterite Solar Cells by Intentionally Inserting a Thin MoS2 Layer into the Back Interface.

A Mo(S,Se)2 interfacial layer is formed inevitably and uncontrollably between the Mo electrode and Cu2ZnSn(S,Se)4 (CZTSSe) absorber during the selenization process, which significantly influences the performance of CZTSSe solar cells. In this work, an ultrathin MoS2 layer is intentionally inserted into Mo/CZTSSe to reduce the recombination and thus optimize the interface quality. It is revealed that the absorber exhibits a continuous and compact morphology with bigger grains and remarkably without pinholes across the surface or cross-sectional regions after MoS2 modification. Benefitting from this, the shunt resistance (RSh) of the device increased evidently from ∼395 to ∼634 Ω·cm2, and simultaneously, the reverse saturation current density (J0) realized an effective depression. As a result, the power conversion efficiency (PCE) of the MoS2-modified device reaches 9.64% via the optimization of the thickness of the MoS2 layer, indicating performance improvements with respect to the MoS2-free case. Furthermore, the main contribution to the performance improvement is derived and analyzed in detail from the increased RSh, decreased J0, and diode ideality factor. Our results suggest that the Mo/CZTSSe interface quality and performance of CZTSSe solar cells can be modulated and improved by appropriately designing and optimizing the thickness of the inserted MoS2 layer.

Construction of gradient SA-TiO2 hydrogel coated PVDF-g-IL fibre membranes with high hydrophilicity and self-cleaning for the efficient separation of oil-water emulsion and dye wastewater

The industrial wastewater filtered by water treatment membranes is very complex in practical applications, therefore, it is a challenge to prepare a membrane with large pore size and high flux that can efficiently separate various wastewater containing pollutants of different scales. To solve this problem, an SA-TiO2 hydrogel-coated PVDF-g-IL composite fibre membrane with super hydrophilicity, anti-pollution, and pH-responsive self-cleaning properties has been designed and prepared by electrostatic spinning and layer-by-layer coating techniques in the present work. The super hydrophilic gel layer on the surface gives the composite membrane efficient separation capability while maintaining rapid passage of the aqueous phase; the underlying PVDF-g-IL fibre membrane is positively charged and removes not only anionic/cationic pollutants by electrostatic repulsion or attraction but also creates an ionic bond with oppositely assigned SA so that the gel layer tightly bond to the base membrane. The PVDF-g-IL@SA-TiO2 membrane achieves stable and efficient separation of oil-water emulsions and dye wastewaters with a high filtration flux while retaining a retention rate higher than ∼95%. After multiple self-cleaning cycles, the ultra-thin gel layer still offers satisfactory durability with ≈100% flux recovery. This study provides a promising new strategy for designing efficient multiple contaminant separation and simpler self-cleaning membranes.

Ultra-Low-Loaded Platinum Bonded Hexagonal Boron Nitride as Stable Electrocatalyst for Hydrogen Generation.

Chemical stability of hexagonal boron nitride (hBN) ultrathin layers in harsh electrolytes and the availability of nitrogen site in hBN to stabilize metals like Pt are used here to develop a high intrinsic activity hydrogen evolution reaction (HER) catalyst having low loaded Pt (5 weight% or <1 atomic%). A catalyst having a nonzero oxidation state for Pt (with a Pt-N bonding) is shown to be HER active even with low catalyst loadings (0.114 mgcm-2). Electronic modification of the shear exfoliated hBN sheets is achieved by Au nanoparticle-based surface decoration (hBN_Au), and further anchoring with Pt develops a catalyst (hBN_Au_Pt) with high turnover frequency for HER (∼15). The hBN_Au_Pt is shown to be a highly durable catalyst even after the accelerated durability test for 10000 cycles and temperature annealing at 100 °C. Density functional theory based calculations gave insights in to the electronic modifications of hBN with Au and the catalytic activity of the hBN_Au_Pt system, in line with the experimental studies, indicating the demonstration of a new class of catalyst system devoid of issues such as carbon corrosion and Pt leaching.

All-Inorganic Hydrothermally Processed Semitransparent Sb2S3 Solar Cells with CuSCN as the Hole Transport Layer.

An inorganic wide-bandgap hole transport layer (HTL), copper(I) thiocyanate (CuSCN), is employed in inorganic planar hydrothermally deposited Sb2S3 solar cells. With excellent hole transport properties and uniform compact morphology, the solution-processed CuSCN layer suppresses the leakage current and improves charge selectivity in an n-i-p-type solar cell structure. The device without the HTL (FTO/CdS/Sb2S3/Au) delivers a modest power conversion efficiency (PCE) of 1.54%, which increases to 2.46% with the introduction of CuSCN (FTO/CdS/Sb2S3/CuSCN/Au). This PCE is a significant improvement compared with the previous reports of planar Sb2S3 solar cells employing CuSCN. CuSCN is therefore a promising alternative to expensive and inherently unstable organic HTLs. In addition, CuSCN makes an excellent optically transparent (with average transmittance >90% in the visible region) and shunt-blocking HTL layer in pinhole-prone ultrathin (<100 nm) semitransparent absorber layers grown by green and facile hydrothermal deposition. A semitransparent device is fabricated using an ultrathin Au layer (∼10 nm) with a PCE of 2.13% and an average visible transmittance of 13.7%.

New avenues for residual stress analysis in ultrathin atomic layer deposited free-standing membranes through release of micro-cantilevers

The fabrication of thinnest, yet undeformed membrane structures with nanometer resolution is a prerequisite for a variety of Microelectromechanical systems (MEMS). However, functionally relevant thin films are susceptible to growth-generated stress. To tune the performance and reach large aspect ratios, knowledge of the intrinsic material properties is indispensable. Here, we present a new method for stress evaluation through releasing defined micro-cantilever segments by focused ion beam (FIB) milling from a predefined free-standing membrane structure. Thereby, the cantilever segment is allowed to equilibrate to a stress-released state through measurable strain in the form of a resulting radius of curvature. This radius can be back-calculated to the residual stress state. The method was tested on a 20 nm and 50 nm thick tunnel-like ALD ▪ membrane structure, revealing a significant amount of residual stress with 866 MPa and 6104 MPa, respectively. Complementary finite element analysis to estimate the stress distribution in the structure showed a 97% and 90% agreement in out-of-plane deflection for the 20 nm and 50 nm membranes, respectively. This work reveals the possibilities of releasing entire membrane segments from thin film membranes with a significant amount of residual stress and to use the resulting bending behavior for evaluating stress and strain by measuring their deformation.

Influence of surface acoustic wave (SAW) on nanoscale in-plane magnetic tunnel junctions

The use of voltage induced strain to switch magnetic tunnel junctions (MTJs) is a promising solution for reducing the switching energy in MRAM technologies. The MTJ is integrated with a piezoelectric layer to generate the strain. A very thin layer is needed to switch with small voltages and small energy dissipation. It is challenging to synthesize ultrathin piezoelectric layers that retain a high degree of piezoelectricity. An alternate approach is to use time-varying strain generated by a surface acoustic wave (SAW). This approach does not require a thin piezoelectric layer since the SAW is confined to the surface of the layer. In this study, we fabricated in-plane MTJs on piezoelectric LiNbO3 substrates and used IDTs to generate the SAW signal within the substrate. Our results showed that the SAW signal had a significant influence on the resistance and the tunneling magnetoresistance (TMR) ratio of the MTJs. The influence was much less significant in nanometer size MTJs than in micrometer sized ones. Most surprisingly, the SAW signal caused the tunneling magnetoresistance ratio (TMR) to drop below zero for the micrometer size MTJ, meaning that the antiparallel resistance RAP is temporarily less than the parallel resistance RP under SAW excitation. Our results provide insight into the dynamic behavior of MTJs under periodic strain and the dependence of this behavior on the device dimensions as they are scaled down to nanometer sizes.

In Situ Construction of a Nanostructured Ultrathin Catalyst Layer on Both Sides of a Membrane toward Fuel Cell Application

In Situ Construction of a Nanostructured Ultrathin Catalyst Layer on Both Sides of a Membrane toward Fuel Cell Application

Density of Unoccupied Electronic States of the Ultrathin Layers of Dibromo-Bianthracene on the Surface of Layer-by-Layer Grown ZnO

Density of Unoccupied Electronic States of the Ultrathin Layers of Dibromo-Bianthracene on the Surface of Layer-by-Layer Grown ZnO

Formation of ultra-thin nickel silicide on SiO2 using Si/Ni/Si structures for oxidation control

We have demonstrated the formation of ultrathin Ni silicide on SiO2 by annealing Si/Ni/Si structures and have systematically evaluated the impacts of the Si layer thickness on oxidation, surface roughening and the silicidation reaction. X-ray photoelectron spectroscopy analyses revealed that suppression of Ni oxidation due to the top Si layer makes it possible to form an ultrathin Ni silicide layer with a thickness of around 2 nm. Then, it turned out that the composition ratio of Ni and Si depends on not only the annealing temperature but also the initial thickness ratio of the top and bottom Si layers. Furthermore, this work clarified that the ultra-thin top Si layer has a large impact on the surface morphology during Ni silicide formation with diffusion and preferential oxidation.

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.

Ultrathin TiO2 Coatings via Atomic Layer Deposition Strongly Improve Cellular Interactions on Planar and Nanotubular Biomedical Ti Substrates.

This work aims to investigate the chemical and/or structural modification of Ti and Ti-6Al-4V (TiAlV) alloy surfaces to possess even more favorable properties toward cell growth. These modifications were achieved by (i) growing TiO2 nanotube layers on these substrates by anodization, (ii) surface coating by ultrathin TiO2 atomic layer deposition (ALD), or (iii) by the combination of both. In particular, an ultrathin TiO2 coating, achieved by 1 cycle of TiO2 ALD, was intended to shade the impurities of F- and V-based species in tested materials while preserving the original structure and morphology. The cell growth on TiO2-coated and uncoated TiO2 nanotube layers, Ti foils, and TiAlV alloy foils were compared after incubation for up to 72 h. For evaluation of the biocompatibility of tested materials, cell lines of different tissue origin, including predominantly MG-63 osteoblastic cells, were used. For all tested nanomaterials, adding an ultrathin TiO2 coating improved the growth of MG-63 cells and other cell lines compared with the non-TiO2-coated counterparts. Here, the presented approach of ultrathin TiO2 coating could be used potentially for improving implants, especially in terms of shading problematic F- and V-based species in TiO2 nanotube layers.

Adaptive Ion Channels Formed in Ultrathin and Semicrystalline Polymer Interphases for Stable Aqueous Batteries.

Aqueous Zn batteries have recently emerged as promising candidates for large-scale energy storage, driven by the need for a safe and cost-effective technology with sufficient energy density and readily accessible electrode materials. However, the energy density and cycle life of Zn batteries have been limited by inherent chemical, morphological, and mechanical instabilities at the electrode-electrolyte interface where uncontrolled reactions occur. To suppress the uncontrolled reactions, we designed a crystalline polymer interphase for both electrodes, which simultaneously promotes electrode reversibility via fast and selective Zn transport through the adaptive formation of ion channels. The interphase comprises an ultrathin layer of crystalline poly(1H,1H,2H,2H-perfluorodecyl acrylate), synthesized and applied as a conformal coating in a single step using initiated chemical vapor deposition (iCVD). Crystallinity is optimized to improve interphase stability and Zn-ion transport. The optimized interphase enables a cycle life of 9500 for Zn symmetric cells and over 11,000 for Zn-MnO2 full-cell batteries. We further demonstrate the generalizability of this interphase design using Cu and Li as examples, improving their stability and achieving reversible cycling in both. The iCVD method and molecular design unlock the potential of highly reversible and cost-effective aqueous batteries using earth-abundant Zn anode materials, pointing to grid-scale energy storage.

Ultrathin Zincophilic Interphase Regulated Electric Double Layer Enabling Highly Stable Aqueous Zinc-Ion Batteries

HighlightsElectric double-layer regulation enabled by an ultrathin multifunctional solid electrolyte interphase layer with zincophilicity and rapid transport kinetics.Lowered potential drop over the Helmholtz layer and suppressed diffuse layer.Inhibited side reactions and uniform zinc deposition.

Fully-vertical GaN-on-SiC Schottky barrier diode with ultrathin AlGaN buffer layer

In this letter, vertically conductive GaN epilayer on SiC substrate was achieved without the typically used conductive buffer layer. Here, in order to reduce the impacts of band offset of different layers on vertical conductivity and improve the vertical carrier transportation, an ultrathin AlGaN buffer layer was employed to replace the thick conductive buffer layer. Fully-vertical Schottky barrier diode (SBD) based on this vertically conductive epi-stack demonstrated a much lower specific on-resistance of 0.84 mΩ·cm2 with superior thermal stability. Moreover, the SBD also exhibited an on/off ratio of ∼5 × 109 and a nearly unity ideality factor of 1.08. This approach lays the foundation for the heterogeneous integration of GaN/SiC based devices.