Ultrathin Films Research Articles

Ferroelectric behavior arising from polar topologies in epitaxially strained SrTiO3-δ ultrathin films

In this work, we show that epitaxially strained SrTiO3-δ thin films, grown by ion-beam sputtering onto (001)Nb:SrTiO3 substrates, exhibit a ferroelectric behavior. At the atomic-scale, through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, it was possible to identify the presence of polar nanoregions with non-trivial polar topological structures in the SrTiO3-δ film, which are induced through oxygen vacancies. To further confirm the presence of strained regions with a polar structure, Raman spectroscopy and high-resolution X-ray diffraction were employed and it was possible to confirm the presence of a tetragonal structure in the SrTiO3-δ film, with a tetragonality ratio (c/a) of 1.005. Scanning probe microscopy and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior with maximum polarization of ∼1.5 μC/cm2, remnant polarization of ∼0.4 μC/cm2 and coercive field of ∼0.3 MV/cm. This work opens a window for exploring novel polar topological effects in sub-10 nm thin film materials for non-volatile memory application.

Fusion Bonding Technique for Solvent-Free Fabrication of All-Solid-State Battery with Ultrathin Sulfide Electrolyte.

For preparing next-generation sulfide all-solid-state batteries (ASSBs), the solvent-free manufacturing process has huge potential for the advantages of economic, thick electrode, and avoidance of organic solvents. However, the dominating solvent-free process is based on the fibrillation of polytetrafluoroethylene, suffering from poor mechanical property and electrochemical instability. Herein, a continuously solvent-free paradigm of fusion bonding technique is developed. A percolation network of thermoplastic polyamide (TPA) binder with low viscosity in viscous state is constructed with Li6PS5Cl (LPSC) by thermocompression (≤5MPa), facilitating the formation of ultrathin LPSC film (≤25µm). This composite sulfide film (CSF) exhibits excellent mechanical properties, ionic conductivity (2.1 mS cm-1), and unique stress-dissipation to promote interface stabilization. Thick LiNi0.83Co0.11Mn0.06O2 cathode can be prepared by this solvent-free method and tightly adhered to CSF by interfacial fusion of TPA for integrated battery. This integrated ASSB shows high-energy-density feasibility (>2.5 mAh cm-2 after 1400 cycles of 9200 h and run for more than 10 000 h), and energy density of 390 Wh kg-1 and 1020 Wh L-1. More specially, high-voltage bipolar cell (≥8.5V) and bulk-type pouch cell (326 Wh kg-1) are facilely assembled with good cycling performance. This work inspires commercialization of ASSBs by a solvent-free method and provides beneficial guiding for stable batteries.

Continuous protein-density gradients: A new approach to correlate physical cues with cell response.

To assess cellular behavior within heterogeneous tissues, such as bone, skin, and nerves, scaffolds with biophysical gradients are required to adequately replicate the in vivo interaction between cells and their native microenvironment. In this study, we introduce a strategy for depositing ultrathin films comprised of laminin-111 with precisely controlled biophysical gradients onto planar substrates using the Langmuir-Blodgett (LB) technique. The gradient is created by controlled desynchronization of the barrier compression and substrate withdrawal speed during the LB deposition process. Characterization of the films was performed using techniques such as atomic force microscopy and confocal fluorescence microscopy, enabling the comprehensive analysis of biophysical parameters along the gradient direction. Furthermore, human adipose-derived stem cells were seeded onto the gradient films to investigate the influence of protein density on cell attachment, showing that the distribution of the cells can be modulated by the arrangement of the laminin at the air-water interface. The presented approach not only allowed us to gain insights into the intricate interplay between biophysical cues and cell behavior within complex tissue environments, but it is also suited as a screening approach to determine optimal protein concentrations to achieve a target cellular output.

Temperature Induced Reversible Switching of the Magnetic Anisotropy in a Neodymium Complex Adsorbed on Graphite.

Controlling the magnetic anisotropy of molecular layers assembled on a surface is one of the challenges that needs to be addressed to create the next-generation spintronic devices. Recently, metal complexes that show a reversible solid-state switch of their magnetic anisotropy in response to physical stimuli, such as temperature and magnetic field, have been discovered. The complex Nd(trensal) (H3trensal=2,2',2''-tris(salicylideneimino)triethylamine) is predicted to exhibit such property. An ultra-thin film of Nd(trensal) is deposited on highly ordered pyrolytic graphite as a proof-of-concept system to show that this property can be retained at the nanoscale on a layered material. By combining single crystal magnetometric measurements and synchrotron X-ray-based absorption techniques, supported by multiplet ligand field simulations based on the trigonal crystal field surrounding the lanthanide centre, it is demonstrated that changing the temperature reverses the magnetic anisotropy of an ordered film of Nd(trensal), thus opening significant perspectives for the realization of a novel family of temperature-controlled molecular spintronic devices.

Guiding light with surface exciton–polaritons in atomically thin superlattices

Abstract Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton–polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor–insulator–semiconductor superlattices comprising monolayer WS2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stacking additional WS2 monolayers into superlattices further enhances the waveguiding properties. Our results underscore the potential of monolayer-based superlattices as a platform for visible-range nanophotonics with promising optical, electrical, and magnetic tunability.

Left and Right‐Handed Light Reflection and Emission in Ultrathin Cellulose Nanocrystals Films with Printed Helicity

AbstractNatural polymers, particularly plant‐derived nanocelluloses, self‐organize into hierarchical structures, enabling mechanical robustness, bright iridescence, emission, and polarized light reflection. These biophotonic properties are facilitated by the assembly of individual components during evaporation, such as cellulose nanocrystals (CNCs), which exhibit a left‐handed helical pitch in a chiral nematic state. This work demonstrates how optically active films with pre‐programmed opposite handedness (left or right) can be constructed via shear‐induced twisted printing with clockwise and counter‐clockwise shearing vectors. The resulting large‐area thin films are transparent yet exhibit pre‐determined mirror‐symmetrical optical activity, enabling the distinction of absorbed and emitted circularly polarized light. This processing method allows for sequential printing of thin and ultrathin films with twisted layered organization and on‐demand helicity. The complex light polarization behavior is due to step‐like changes in linear birefringence within each deposited layer and circular birefringence, different from that of conventional CNC films as revealed with Muller matrix analysis. Furthermore, intercalating an achiral organic dye into printed structures induces circularly polarized luminescence while preserving high transmittance and controlled handedness. These results suggest that twisted sequential printing can facilitate the construction of chiroptical metamaterials with tunable circular polarization, absorption, and emission for optical filters, encryption, photonic coatings, and chiral sensors.

Ultrathin Al2O3 film modification on waterborne epoxy coatings by atomic layer deposition for augmenting the corrosion resistance

The polar channels formed by the curing of waterborne anticorrosive coatings compromise their water resistance, leading to coating degradation and metal corrosion. To enhance the anticorrosive performance of waterborne coatings, this study proposed a novel method of depositing ultrathin Al2O3 films on the surface of waterborne epoxy coatings by atomic layer deposition, a technique that can modify the surface properties of polymer materials by depositing functional films. The Al2O3-modified coatings exhibited improved sealing and barrier properties by closing the polar channels and surface defects and cracks. The surface structure and morphology of the modified coatings were characterized by x-ray photoelectron spectroscopy and scanning electron microscopy. The hydrophilicity and corrosion resistance of the modified coatings were evaluated by water contact angle measurement, Tafel polarization curve, and electrochemical impedance spectroscopy. The results indicated that the water contact angle of the Al2O3-modified coating increased by 48° compared to the unmodified coating, and the protection efficiency of the modified coating reached 99.81%. The Al2O3-modified coating demonstrated high anticorrosive efficiency and potential applications for metal anticorrosion in harsh marine environments.

Assembly Structure Formation in Bulk and Ultrathin Films of Poly(substituted methylene) Having an Azobenzene Side Chain.

The density of the side chain introduced to a polymer main chain greatly influences the properties and functions of the polymer. This work first reports on the packing structure and properties at an interface of a poly(substituted methylene) where an azobenzene side chain is introduced at every carbon atom in the main chain (C1PAz). The structure and properties are compared with those of a conventional vinyl polymer [poly(methacrylate)] possessing an identical side-chain structure (C2PAz). The packing structure in the bulk state analyzed by X-ray measurements revealed that C1PAz adopts a highly ordered rectangular unit cell structure, whereas C2PAz shows a less ordered lamellar one. Langmuir film balance experiments indicated that both polymers with the trans-azobenzene give essentially the identical 2D side-chain occupying area on water, which agrees well with the smectic B (hexatic packing) model based on the X-ray data. Upon transfer onto a solid substrate, only C1PAz shows a conformational transformation to a spread bilayer-type layer, most probably due to conformational frustration stemming from the crowding of the side chains. This study proposes new insights into the effects of side-chain density on the self-assembly and photoreaction of azobenzene-containing polymers, which are expected to expand the possibilities of polymer design for various applications.

Interface Effects in the Stability of 2D Silica, Silicide, and Silicene on Pt(111) and Rh(111).

Ultrathin two-dimensional silica films have been suggested as highly defined conductive models for fundamental studies on silica-supported catalyst particles. Key requirements in this context are closed silica films that isolate the gas phase from the underlying metal substrate and stability under reaction conditions. Here, we present silica bilayer films grown on Pt(111) and Rh(111) and characterize them by scanning tunneling microscopy and X-ray photoelectron spectroscopy. We provide the first report of silica bilayer films on Rh(111) and have further successfully prepared fully closed films on Pt(111). Interestingly, surface and interface silicide phases play a decisive role in both cases: On platinum, closed films can be stabilized only when silicon is deposited in excess, which results in an interfacial silicide or silicate layer. We show that these silica films can also be grown directly from a surface silicide phase. In the case of rhodium, the silica phase is less stable and can be reduced to a silicide in reductive environments. Though similar in appearance to the "silicene" phases that have been controversially discussed on Ag(111), we conclude that an interpretation of the phase as a surface silicide is more consistent with our data. Finally, we show that the silica film on platinum is stable in 0.8 mbar CO but unstable at elevated temperatures. We thus conclude that these systems are only suitable as model catalyst supports to a limited extent.

Effect of surface states and in-plane deformation on transport properties of charge carriers in bismuth films

Since the energy spectrum of bismuth charge carriers is highly sensitive to mechanical deformations, studying their influence, leading to an increase or decrease in the overlap of actual extrema, on the transport properties of charge carriers in ultrathin bismuth films makes it possible to investigate the combination of manifestations of the quantum size effect and metallic surface states. In this work, the temperature dependencies of the electrical and galvanomagnetic properties of thin bismuth films (10–1250 nm) are studied under conditions of in-plane tensile and compressive deformation. Deformations are created as a result of the difference in the coefficients of thermal expansion of the film and substrate materials. Silicon with an oxidized surface, mica, and polished cleavage (111) CaF2 are used in order to create either tensile and compressive deformations. The presence of film deformations is confirmed by XRD. The magnitude of deformation is calculated based on the CTEs of the materials. The absence of mechanical stress relaxation or the formation of additional defects during temperature measurements is confirmed by the absence of hysteresis of properties in the range of 77–300 K. Based on the experimental results, the mobilities and concentration of charge carriers are calculated within the framework of the two-band model. As the film thickness decreases to less than 100 nm, an increase in the charge carrier concentration is observed, which is associated with metallic surface states. It is shown that the magnitude of the effect of deformation on the concentration of charge carriers in films of all thicknesses remains unchanged.

Symmetry Engineering of Epitaxial Hf0.5Zr0.5O2 Ultrathin Films.

Robust ferroelectricity in HfO2-based ultrathin films has the potential to revolutionize nonvolatile memory applications in nanoscale electronic devices because of their compatibility with the existing Si technology. However, to fully exploit the potential of ferroelectric HfO2-based thin films, it is crucial to develop strategies for the controlled stabilization of various HfO2-based polymorphs in nanoscale heterostructures. This study demonstrates how substrate-orientation-induced anisotropic strain can engineer the crystal symmetry, structural domain morphology, and growth orientation of ultrathin Hf0.5Zr0.5O2 (HZO) films. Epitaxial ultrathin HZO films were grown on the heterostructures of (001)- and (110)-oriented La2/3Sr1/3MnO3/SrTiO3 (LSMO/STO) substrate. Various structural analyses revealed that the (110)-oriented substrate promotes a higher degree of structural order (crystallinity) with improved stability of the (111)-oriented orthorhombic phase (Pca21) of HZO. Conversely, the (001)-oriented substrate not only induces a distorted orthorhombic structure but also facilitates the partial stabilization of nonpolar phases. Electrical measurements revealed robust ferroelectric properties in epitaxial thin films without any wake-up effect, where the well-ordered crystal symmetry stabilized by STO(110) facilitated better ferroelectric characteristics. This study suggests that tuning the epitaxial growth of ferroelectric HZO through substrate orientation can improve the stability of the metastable ferroelectric orthorhombic phase and thereby offer a better understanding of device applications.

Ionic control of magnetism in all-solid-state CoOx/yttria-stabilized zirconia heterostructures

Magneto-ionic gating, a procedure that enables the modulation of materials' magnetic properties by voltage-driven ion motion, offers alternative perspectives for emerging low-power magnetic storage and spintronic applications. Most previous studies in all-solid-state magneto-ionic systems have focused on the control of interfacial magnetism of ultrathin (i.e., 1–3 nm) magnetic films, taking advantage of an adjacent ionic conducting oxide, usually GdOx or HfOx, that transports functional ionic species (e.g., H+ or O2−). Here, we report on room-temperature OFF–ON ferromagnetism by solid-state magneto-ionics in relatively thick (25 nm) patterned CoOx films grown on an yttria-stabilized zirconia (YSZ) layer, which acts as a dielectric to hold electric field and as an O2− ion reservoir. Upon negatively biasing, O2− ions from the CoOx tend to migrate toward the YSZ gate electrode, leading to the gradual generation of magnetization (i.e., OFF-to-ON switching of a ferromagnetic state). X-ray absorption and magnetic circular dichroism studies reveal subtle changes in the electronic/chemical characteristics, responsible for the induced magnetoelectric effects in such all-oxide heterostructures. Recovery of the initial (virtually non-magnetic) state is achieved by application of a positive voltage. The study may guide future development of all-solid-state low-power CMOS-compatible magneto-ionic devices.

Preparation of polycyclosiloxane/ZnO nanoparticle hybrid ultrathin films by Layer-by-Layer assembly methods

Layer-by-layer (LbL) assembly methods through alternative dipping processes were used to prepare hybrid nanoassemblies of 5 nm ZnO nanoparticles (ZnONPs) with catechol-functionalized polycyclosiloxane (CFPS60, 51.9 mol% catechol content). ZnONPs were assembled on solid substrates regularly and firmly through interactions with 4 nm-thick CFPS60, proved using UV-vis absorption measurements, atomic force microscopy and peel-off test. ZnONPs exhibited yellow photoluminescence (PL) in CFPS60/ZnONPs films, and the PL intensity increased notably as the number of bilayers increased. The work demonstrates that CFPS60 has a good hybrid nanoassembly formation capability, which is promising for next-generation optoelectronic device applications.

Implantable Wet‐Adhesive Flexible Electronics with Ultrathin Gelatin Film

AbstractImplantable flexible electronic has attracted significant research interest in various fields. However, it still faces the challenge of simultaneously achieving tight adhesion to tissues in a mildly wet environment and possessing excellent biocompatibility to reduce immune rejection reactions after implantation. Here, a degradable wet‐adhesive flexible electronic device based on liquid metal and ultrathin gelatin film is developed. The ultrathin gelatin film forms numerous hydrogen bonds with tissue in a slightly humid environment, rapidly constructing a wet‐adhesive interface without damaging tissue structure. Inkjet printing is utilized to pattern the mixture of liquid metal and PVP on the surface of the ultrathin gelatin to create flexible patch. With the excellent conductivity of liquid metal, low toxicity, and similarity to natural tissue components of gelatin, flexible patch exhibits outstanding biocompatibility and fatigue resistance. It can be implanted in the body for up to 6 weeks, retaining monitoring capabilities and resisting 1 000 000 cycles of bending fatigue. This study provides a novel strategy for the future development of implantable flexible electronics.

Centrifugal-Gravity-Enforced Deposition of MXene Electrodes for High-Performance and Ultrastable Microsupercapacitors.

Two-dimensional (2D) transition metal carbides, known as MXenes, have captured much attention for their excellent electrical conductivity and electrochemical capability. However, the susceptibility of MXenes to oxidation, particularly Ti3C2Tx transforming into titanium dioxide upon exposure to ambient air, hinders their utilization for extended operational life cycles. This work introduces a simple and straightforward method for producing ultrathin MXene electrode films tailored for energy storage applications, employing centrifugal-gravity force. Our approach significantly suppresses the oxidation phenomenon that arises in MXene materials and also effectively prevents the recrystallization of potentially residual LiF during the film formation. Additionally, the utilization of this MXene electrode in an all-solid-state microsupercapacitor (MSC) with an interdigitated pattern demonstrates an exceptionally improved and stable electrochemical performance. This includes a high volumetric capacitance of approximately 467 F cm-3, an energy density of around 65 mWh cm-3, and impressive long-term cycle stability, retaining about 94% capacity after 10 000 cycles. Moreover, a downsized MSC device exhibits remarkable mechanical durability, retaining over 98% capacity even when folded and sustaining stability over extended periods. Therefore, we believe that this study provides valuable insights for advancing highly integrated energy storage devices, ensuring exceptional electrochemical efficiency and prolonged functionality in diverse environments, whether ambient or humid.

Polar Bloch points in strained ferroelectric films

Topological domain structures have drawn great attention as they have potential applications in future electronic devices. As an important concept linking the quantum and classical magnetism, a magnetic Bloch point, predicted in 1960s but not observed directly so far, is a singular point around which magnetization vectors orient to nearly all directions. Here we show polar Bloch points in tensile-strained ultrathin ferroelectric PbTiO3 films, which are alternatively visualized by phase-field simulations and aberration-corrected scanning transmission electron microscopic imaging. The phase-field simulations indicate local steady-state negative capacitance around the Bloch points. The observation of polar Bloch points and their emergent properties consequently implies novel applications in future integrated circuits and low power electronic devices.

Ultrathin Freestanding Nanocellulose Film Prepared from TEMPO-Mediated Oxidation and Homogenized Hydrogel.

This paper presents a versatile method to fabricate ultrathin nanofibrillated cellulose (NFC) films as thin as 800 nm by blade coating, which is compatible with a roll-to-roll process on a large scale. Our approach allows obtaining a dried nanocellulose film within a span of 1 h subsequent to 2,2,6,6-tetramethylpiperidine-1-oxyl radical-assisted oxidation and homogenization procedures. One of the thinnest freestanding NFC films with a thickness of 800 nm is achieved using a blade coating of nanocellulose after 72 h of oxidation followed by homogenization with a channel size of 65 μm. Incorporating water-soluble CdTe core-type quantum dots into the nanocellulose film led to a uniform emission under 385 nm UV irradiation, indicating excellent material compatibility. We anticipate nanocellulose developed in our study to be beneficial in biomimicry flying objects, environmentally friendly encapsulation, color filters, and energy storage device membranes, to name a few.

Construction of multi(iron)porphyrin array multilayers on quartz surface as peroxidase mimetics for H2O2 and glucose colorimetric, fluorometric and color-read detection

Multi(metallo)porphyrin arrays are important artificial biomimetics for mimicking the biological catalytic reactions in nature. Here, we report construction of multi(iron)porphyrin array ultrathin films on the quartz surface by coordination-driven layer-by-layer (LBL) assembling method with the Pd(II) ion as a connector and iron(III) tetrapyridylporphyrin (FeTPyP) as a linker. It is revealed that the (Pd-FeTPyP)n LBL multilayers can catalyze oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2 with peroxidase-like activity, resulting in the formation of the blue-colored oxidized TMB (oxTMB). Coupled with glucose oxidase, three sensitive probes based on the colorimetric, fluorometric and color-read methods have been developed by using modified quartz plates as the heterogenous catalyst, with a glucose detection limit of about 0.89, 1.5 and 0.65 μM, respectively. Mechanism studies confirm an enhanced affinity between (Pd-FeTPyP)n and H2O2/TMB molecules and an improved catalytic performance, which are attributed to the reason that the FeTPyP macrocycles are well-dispersed and arranged on the two-dimensional quartz surface. Furthermore, owing to chemically anchored on the transparent quartz surface, the (Pd-FeTPyP)n films exhibit good stability with naked-eye observation of color change under moderate conditions, thus providing a potential candidate for developing colorimetric, fluorometric and color-read glucose biosensors.

Performance improvement of an ultra-thin film CIGS tandem solar cell using new plasmonic cluster nanostructures

This work demonstrates an innovative plasmonic nanostructure configuration to enhance the performance of an ultra-thin film CIGS-based tandem solar cell. Silver nanoparticle clusters with optimized cubic and spherical shapes are incorporated into the bottom CIGS layer to create a light-trapping effect. The optimal nanoparticle dimensions and positioning are determined through analysis of optical and electrical parameters. The plasmonic nanostructures achieve a short-circuit current density of 20.24 mA/cm2 and a power conversion efficiency of 22.9 % - a 12.6 % enhancement over the non-plasmonic case. This improvement is attributed to the near-field enhancement from localized surface plasmon resonance, which significantly boosts the absorption in the critical long-wavelength region. The strategic use of plasmonic nanostructures proves effective in overcoming the light absorption limitations of ultra-thin CIGS tandem solar cells.

Ultrathin Boundary-Less SnO2 Films with Surface-Activated Two-Dimensional Nanograins Enable Fast and Sensitive Hydrogen Gas Sensing.

Fast and reliable semiconductor hydrogen sensors are crucially important for the large-scale utilization of hydrogen energy. One major challenge that hinders their practical application is the elevated temperature required, arising from undesirable surface passivation and grain-boundary-dominated electron transportation in the conventional nanocrystalline sensing layers. To address this long-standing issue, in the present work, we report a class of highly reactive and boundary-less ultrathin SnO2 films, which are fabricated by the topochemical transformation of 2D SnO transferred from liquid Sn-Bi droplets. The ultrathin SnO2 films are purposely made to consist of well-crystallized quasi-2D nanograins with in-plane grain sizes going beyond 30 nm, whereby the hydroxyl adsorption and grain boundary side-effects are effectively suppressed, giving rise to an activated (101)-dominating dangling-bond surface and a surface-controlled electrical transportation with an exceptional electron mobility of 209 cm2 V-1 s-1. Our work provides a new cost-effective strategy to disruptively improve the gas reception and transduction of SnO2. The proposed chemiresistive sensors exhibit fast, sensitive, and selective hydrogen sensing performance at a much-reduced working temperature of 60 °C. The remarkable sensing performance as well as the simple and scalable fabrication process of the ultrathin SnO2 films render the thus-developed sensors attractive for long awaited practical applications in hydrogen-related industries.