Ultrathin Cobalt Oxide Layers as Electrocatalysts for High‐Performance Flexible Zn–Air Batteries

Synergistic improvements in the electrical conductivity and catalytic activity for the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) are of paramount importance for rechargeable metal–air batteries. In this study, one‐nanometer‐scale ultrathin cobalt oxide (CoO_x) layers are fabricated on a conducting substrate (i.e., a metallic Co/N‐doped graphene substrate) to achieve superior bifunctional activity in both the ORR and OER and ultrahigh output power for flexible Zn–air batteries. Specifically, at the atomic scale, the ultrathin CoO_x layers effectively accelerate electron conduction and provide abundant active sites. X‐ray absorption spectroscopy reveals that the metallic Co/N‐doped graphene substrate contributes to electron transfer toward the ultrathin CoO_x layer, which is beneficial for the electrocatalytic process. The as‐obtained electrocatalyst exhibits ultrahigh electrochemical activity with a positive half‐wave potential of 0.896 V for ORR and a low overpotential of 370 mV at 10 mA cm^?2 for OER. The flexible Zn–air battery built with this catalyst exhibits an ultrahigh specific power of 300 W g_cat ^?1, which is essential for portable devices. This work provides a new design pathway for electrocatalysts for high‐performance rechargeable metal–air battery systems.     

Silicon Photovoltaics Using Conducting Photonic Crystal Back‐Reflectors

Currently, research is being directed towards thinning conventional 200–300µm thick silicon photovoltaic cells by an order of magnitude or more. The benefits of reducing the cell thickness include decreased material costs, enhanced cell flexibility, and reduced effects of light‐induced degradation. However, one of the major challenges associated with reducing the active region to this extent is the corresponding reduction of light absorption. To mitigate this effect it has been proposed that the cell should incorporate enhanced light‐trapping strategies. One potential approach to enhance light trapping in thin photovoltaic cells is to structure the back‐reflector in the form of a photonic crystal (PC). It has recently been shown that two fundamental attributes of PC back‐reflectors optically coupled to thin semiconductor films contribute to enhanced absorption in the semiconductor: (i) the PC back‐reflector behaves as a perfect mirror, exhibiting complete reflection over stop‐gap frequencies; and (ii) the PC–semiconductor film interface couples incident light into resonant states that propagate along the plane of the film, thereby further enhancing the absorption. Although the ability of PC back‐reflectors to enhance absorption is encouraging, significant challenges arise when attempting to incorporate this light trapping technique in photovoltaic devices. Herein, we describe the underlying physical mechanisms that give rise to absorption enhancements in thin Si wafers featuring PC back‐reflectors, and describe hurdles that will have to be surmounted in order to reduce‐to‐practice a PC back‐reflector into an actual PV device.     

Camphor‐Enabled Transfer and Mechanical Testing of Centimeter‐Scale Ultrathin Films

Camphor is used to transfer centimeter‐scale ultrathin films onto custom‐designed substrates for mechanical (tensile) testing. Compared to traditional transfer methods using dissolving/peeling to remove the support‐layers, camphor is sublimed away in air at low temperature, thereby avoiding additional stress on the as‐transferred films. Large‐area ultrathin films can be transferred onto hollow substrates without damage by this method. Tensile measurements are made on centimeter‐scale 300 nm‐thick graphene oxide film specimens, much thinner than the ≈2 μm minimum thickness of macroscale graphene‐oxide films previously reported. Tensile tests were also done on two different types of large‐area samples of adlayer free CVD‐grown single‐layer graphene supported by a ≈100 nm thick polycarbonate film; graphene stiffens this sample significantly, thus the intrinsic mechanical response of the graphene can be extracted. This is the first tensile measurement of centimeter‐scale monolayer graphene films. The Young's modulus of polycrystalline graphene ranges from 637 to 793 GPa, while for near single‐crystal graphene, it ranges from 728 to 908 GPa (folds parallel to the tensile loading direction) and from 683 to 775 GPa (folds orthogonal to the tensile loading direction), demonstrating the mechanical performance of large‐area graphene in a size scale relevant to many applications.     

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O₂. Three components of the complex film formed on Al(111) can be identified: an ultrathin under‐stoichiometric Al_xO_y interface layer, an intermediate Al₂O₃ phase with γ‐alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O₂ molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al₂O₃ phase. The electron transfer takes place both from the metal‐oxide interface and the oxide surface to the adsorbed O₂ molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O₂ ⁻ species to capture protons and transform into hydrogen peroxide in a non‐activated process. H₂O₂ is further reduced to two water molecules, in a series of two‐electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al₂O₃ inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.     

Wetting‐Induced Climbing for Transferring Interfacially Assembled Large‐Area Ultrathin Pristine Graphene Film

Owing to inherent 2D structure, marvelous mechanical, electrical, and thermal properties, graphene has great potential as a macroscopic thin film for surface coating, composite, flexible electrode, and sensor. Nevertheless, the production of large‐area graphene‐based thin film from pristine graphene dispersion is severely impeded by its poor solution processability. In this study, a robust wetting‐induced climbing strategy is reported for transferring the interfacially assembled large‐area ultrathin pristine graphene film. This strategy can quickly convert solvent‐exfoliated pristine graphene dispersion into ultrathin graphene film on various substrates with different materials (glass, metal, plastics, and cloth), shapes (film, fiber, and bulk), and hydrophobic/hydrophilic patterns. It is also applicable to nanoparticles, nanofibers, and other exfoliated 2D nanomaterials for fabricating large‐area ultrathin films. Alternate climbing of different ultrathin nanomaterial films allows a layer‐by‐layer transfer, forming a well‐ordered layered composite film with the integration of multiple pristine nanomaterials at nanometer scale. This powerful strategy would greatly promote the development of solvent‐exfoliated pristine nanomaterials from dispersions to macroscopic thin film materials.     

Construction of Large‐Area Ultrathin Conductive Metal–Organic Framework Films through Vapor‐Induced Conversion

On account of unique characteristics, the integration of metal–organic frameworks as active materials in electronic devices attracts more and more attention. The film thickness, uniformity, area, and roughness are all fatal factors limiting the development of electrical and optoelectronic applications. However, research focused on ultrathin free‐standing films is in its infancy. Herein, a new method, vapor‐induced method, is designed to construct centimeter‐sized Ni₃(HITP)₂ films with well‐controlled thickness (7, 40, and 92 nm) and conductivity (0.85, 2.23, and 22.83 S m^?1). Further, traditional transfer methods are tactfully applied to metal–organic graphene analogue (MOGA) films. In order to maintain the integrity of films, substrates are raised up from bottom of water to hold up films. The stripping method greatly improves the surface roughness R_q (root mean square roughness) without loss of conductivity and endows the film with excellent elasticity and flexibility. After 1000 buckling cycles, the conductance shows no obvious decrease. Therefore, the work may open up a new avenue for flexible electronic and magnetic devices based on MOGA.     

Tuning Oxygen Vacancies in Ultrathin TiO2 Nanosheets to Boost Photocatalytic Nitrogen Fixation up to 700 nm

Dinitrogen reduction to ammonia using transition metal catalysts is central to both the chemical industry and the Earth's nitrogen cycle. In the Haber–Bosch process, a metallic iron catalyst and high temperatures (400 °C) and pressures (200 atm) are necessary to activate and cleave N?N bonds, motivating the search for alternative catalysts that can transform N₂ to NH₃ under far milder reaction conditions. Here, the successful hydrothermal synthesis of ultrathin TiO₂ nanosheets with an abundance of oxygen vacancies and intrinsic compressive strain, achieved through a facile copper‐doping strategy, is reported. These defect‐rich ultrathin anatase nanosheets exhibit remarkable and stable performance for photocatalytic reduction of N₂ to NH₃ in water, exhibiting photoactivity up to 700 nm. The oxygen vacancies and strain effect allow strong chemisorption and activation of molecular N₂ and water, resulting in unusually high rates of NH₃ evolution under visible‐light irradiation. Therefore, this study offers a promising and sustainable route for the fixation of atmospheric N₂ using solar energy.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Phase‐Engineered PtSe2‐Layered Films by a Plasma‐Assisted Selenization Process toward All PtSe2‐Based Field Effect Transistor to Highly Sensitive,Flexible, and Wide‐Spectrum Photoresponse Photodetectors

The formation of PtSe₂‐layered films is reported in a large area by the direct plasma‐assisted selenization of Pt films at a low temperature, where temperatures, as low as 100 °C at the applied plasma power of 400 W can be achieved. As the thickness of the Pt film exceeds 5 nm, the PtSe₂‐layered film (five monolayers) exhibits a metallic behavior. A clear p‐type semiconducting behavior of the PtSe₂‐layered film (≈trilayers) is observed with the average field effective mobility of 0.7 cm² V^?1 s^?1 from back‐gated transistor measurements as the thickness of the Pt film reaches below 2.5 nm. A full PtSe₂ field effect transistor is demonstrated where the thinner PtSe₂, exhibiting a semiconducting behavior, is used as the channel material, and the thicker PtSe₂, exhibiting a metallic behavior, is used as an electrode, yielding an ohmic contact. Furthermore, photodetectors using a few PtSe₂‐layered films as an adsorption layer synthesized at the low temperature on a flexible substrate exhibit a wide range of absorption and photoresponse with the highest photocurrent of 9 µA under the laser wavelength of 408 nm. In addition, the device can maintain a high photoresponse under a large bending stress and 1000 bending cycles.     

High‐Performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications

The field of nanophotonics has ushered in a new paradigm of light manipulation by enabling deep subdiffraction confinement assisted by metallic nanostructures. However, a key limitation which has stunted a full development of high‐performance nanophotonic devices is the typical large losses associated with the constituent metals. Although silver has long been known as the highest quality plasmonic material for visible and near infrared applications, its usage has been limited due to practical issues of continuous thin film formation, stability, adhesion, and surface roughness. Recently, a solution is proposed to the above issues by doping a proper amount of aluminum during silver deposition. In this work, the potential of doped silver for nanophotonic applications is presented by demonstrating several high‐performance key nanophotonic devices. First, long‐range surface plasmon polariton waveguides show propagation distances of a few centimeters. Second, hyperbolic metamaterials consisting of ultrathin Al‐doped Ag films are attained having a homogeneous and low‐loss response, and supporting a broad range of high‐k modes. Finally, transparent conductors based on Al‐doped Ag possess both a high and flat transmittance over the visible and near‐IR range.     

Highly Concentrated,Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices

Solution‐based techniques are considered as a promising strategy for scalable fabrication of flexible electronics owing to their low‐cost and high processing speed. The key to the success of these techniques is dominated by the ink formulation of active nanomaterials. This work successfully prepares a highly concentrated two dimensional (2D) crystal ink comprised of ultrathin nickel hydroxide (Ni(OH)₂) nanosheets with an average lateral size of 34 nm. The maximum concentration of Ni(OH)₂ nanosheets in water without adding any additives reaches as high as 50 mg mL^?1, which can be printed on arbitrary substrates to form Ni(OH)₂ thin films. As a proof‐of‐concept application, Ni(OH)₂ nanosheet ink is coated on commercialized carbon fiber yarns to fabricate wearable energy storage devices. The thus‐fabricated hybrid supercapacitors exhibit excellent flexibility with a capacitance retention of 96% after 5000 bending–unbending cycles, and good weavability with a high volumetric capacitance of 36.3 F cm^?3 at a current density of 0.4 A cm^?3, and an energy density of 11.3 mWh cm^?3 at a power density of 0.3 W cm^?3. As a demonstration of practical application, a red light emitting diode can be lighted up by three hybrid devices connected in series.     

GZO厚度对非晶硅电池中复合背电极的影响

在非晶硅太阳能电池中加入复合背电极是提高非晶硅太阳能电池光电转换效率和稳定性的有效手段.本文利用磁控溅射技术在非晶硅薄膜太阳能电池上制备了ZnO :Ga(GZO)/Al复合背电极,研究了GZO厚度对GZO薄膜光电性质及非晶硅电池中GZO/Al复合背电极性能的影响.研究表明：随着GZO层厚度的增加,GZO薄膜的光电性质均表现出较高水平,适合制备GZO/Al复合背电极;相较于单层Al背电极的非晶硅太阳能电池,具有GZO/Al复合背电极的太阳能电池性能大幅提高.当GZO层厚度为100 nm时,太阳能电池的短路电流(I_SC)、开路电压(V_OC)和填充因子(FF)分别达到8.66 mA,1.62 V和54.7%.     

Tunable Magnetism in Nanoporous CuNi Alloys by Reversible Voltage‐Driven Element‐Selective Redox Processes

Voltage‐driven manipulation of magnetism in electrodeposited 200 nm thick nanoporous single‐phase solid solution Cu₂₀Ni₈₀ (at%) alloy films (with sub 10 nm pore size) is accomplished by controlled reduction‐oxidation (i.e., redox) processes in a protic solvent, namely 1 m NaOH aqueous solution. Owing to the selectivity of the electrochemical processes, the oxidation of the CuNi film mainly occurs on the Cu counterpart of the solid solution, resulting in a Ni‐enriched alloy. As a consequence, the magnetic moment at saturation significantly increases (up to 33% enhancement with respect to the as‐prepared sample), while only slight changes in coercivity are observed. Conversely, the reduction process brings Cu back to its metallic state and, remarkably, it becomes alloyed to Ni again. The reported phenomenon is fully reversible, thus allowing for the precise adjustment of the magnetic properties of this system through the sign and amplitude of the applied voltage.     

Thermal and Electrical Conduction in Ultrathin Metallic Films: 7 nm down to Sub‐Nanometer Thickness

For ultrathin metallic films (e.g., less than 5 nm), no knowledge is yet available on how electron scattering at surface and grain boundaries reduces the electrical and thermal transport. The thermal and electrical conduction of metallic films is characterized down to 0.6 nm average thickness. The electrical and thermal conductivities of 0.6 nm Ir film are reduced by 82% and 50% from the respective bulk values. The Lorenz number is measured as 7.08 × 10^?8 W Ω K^?2, almost a twofold increase of the bulk value. The Mayadas‐Shatzkes model is used to interpret the experimental results and reveals very strong electron reflection (>90%) at grain boundaries.     

Inlaying Ultrathin Bimetallic MOF Nanosheets into 3D Ordered Macroporous Hydroxide for Superior Electrocatalytic Oxygen Evolution

Controllable synthesis of ultrathin metal–organic framework (MOF) nanosheets and rational design of their nano/microstructures in favor of electrochemical catalysis is critical for their renewable energy applications. Herein, an in situ growth method is proposed to prepare the ultrathin NiFe MOF nanosheets with a thickness of 1.5 nm, which are vertically inlaid into a 3D ordered macroporous structure of NiFe hydroxide. The well‐designed composite delivers an efficient electrocatalytic performance with a low overpotential of 270 mV at a current density of 10 mA cm^?2 and stable electrolysis as long as 10 h toward the electrochemical oxygen evolution reaction, much superior to the state‐of‐the‐art RuO₂ electrocatalyst. A comprehensive analysis demonstrates that the excellent performance originates from the desirable combination of the highly exposed active centers in the ultrathin bimetallic MOF nanosheets, effective electron conduction between MOF nanosheets and ordered macroporous hydroxide, and efficient mass transfer across the hierarchically porous hybridization. This study sheds light on the exploration of powerful protocols to gain diverse high‐performance MOF nanosheets and may open a perspective to achieve their efficient electrocatalytic performance.     

Hierarchical NiO@N‐Doped Carbon Microspheres with Ultrathin Nanosheet Subunits as Excellent Photocatalysts for Hydrogen Evolution

Achieving highly efficient hierarchical photocatalysts for hydrogen evolution is always challenging. Herein, hierarchical mesoporous NiO@N‐doped carbon microspheres (HNINC) are successfully fabricated with ultrathin nanosheet subunits as high‐performance photocatalysts for hydrogen evolution. The unique architecture of N‐doped carbon layers and hierarchical mesoporous structures from HNINC could effectively facilitate the separation and transfer of photo‐induced electron–hole pairs and afford rich active sites for photocatalytic reactions, leading to a significantly higher H₂ production rate than NiO deposited with platinum. Density functional theory calculations reveal that the migration path of the photo‐generated electron transfer is from Ni 3d and O 2p hybrid states of NiO to the C 2p state of graphite, while the photo‐generated holes locate at Ni 4s and Ni 4p hybrid states of NiO, which is beneficial to improve the separation of photo‐generated electron–hole pairs. Gibbs free energy of the intermediate state for hydrogen evolution reaction is calculated to provide a fundamental understanding of the high H₂ production rate of HNINC. This research sheds light on developing novel photocatalysts for efficient hydrogen evolution.     

Foreign Particle Promoted Crystalline Nucleation for Growing High‐Quality Ultrathin Rubrene Films

Introducing foreign particles or agents as nucleator is an efficient way to promote crystallization in the crystal growth field, with the advantage to speed up the crystallizing rate and control the growth process. However, in the field of organic crystalline film growth, where the crystallization and morphology modulation are of significant importance in optoelectronics, this method has rarely been utilized. Particularly, some potential high‐performance materials such as rubrene face the problem of crystallization during film formation. Here a strategy is reported to promote the crystallization of rubrene films in the initial stage assisted by foreign particles. Highly ordered thin film from the sub‐monolayer stage can be achieved. Efficient charge transport and high mobility up to 2.95 cm² V^?1 s^?1 are achieved on thus ultrathin crystalline films. Such a method enables the well controlling of the film growth from the very early stage and produces uniform crystalline films with good reproducibility, thus highly promising to yield desired optoelectrical properties and applications.     

Structures and photocatalytic behavior of tantalum-oxynitride thin films

Tantalum oxynitride films were created by direct nitridation/oxidation during rapid thermal annealing at temperatures 450-700 °C. Instead of during deposition, this post process may be proved to be an alternative way to make transition metallic oxynitride films. With sufficient supply of oxygen flow (≥ 30 sccm), TaO_xN_y was formed as examined from X-ray diffraction (XRD) analysis. This oxynitride film has a broad optical absorption over the range of visible light and sufficient photocatalytic function. For optical absorption, the films' transmittance and reflectance were measured by a UV-VIS-NIR spectrophotometer with wavelengths ranging from 300 to 900 nm. The broad visible light absorption is associated with the formation of band gap in TaO_xN_y film, which was examined by the theoretical calculations combining the Beer-Lambert law and Tauc formula. Lastly, the photocatalysis of TaO_xN_y was gauged by the photodegradation test which measured the reduction of light absorbance affected by the decomposition of methylene blue (C₁₆H₁₈N₃SCl.3H₂O) on TaO_xN_y under visible light irradiation.     

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary‐Condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO₃ ferroelectric films is reported. Using aberration‐corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm‐thick ultrathin BiFeO₃ film is abnormally increased up to ≈90–100 µC cm^?2 in the out‐of‐plane direction and a peculiar rumpled nanodomain structure with very large variation in c /a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase‐field calculations, it is shown that it is the unique single atomic Bi₂O_3?x layer at the surface that leads to the enhanced polarization and appearance of the MPB‐like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.     

Online in-situ control of lead zirconate titanate film growth in gas discharge chamber

Online in situ monitoring of the gas-discharge deposition of lead zirconate titanate Pb(Zr _x Ti_{1 − x} )O₃ (PZT) films onto a stainless steel substrate revealed oscillatory decay in the intensity of light with a wavelength of 0.640 μm specular reflected from the growing film. It is established that the parameters of the reflectance curves depend on the electric power deposited in discharge and correlate with the type of growing films (possessing pyrochlore or perovskite structures). Using the proposed method, it is possible to control technological parameters of the deposition process, which determine the physical characteristics of the obtained PZT films.