Epitaxial Ferroelectric Heterostructures with Nanocolumn‐Enhanced Dynamic Properties

The possibility to tailor ferroelectricity by controlling epitaxial strain in thin films and heterostructures of complex metal oxides is well established. Here it is demonstrated that apart from this mechanism, 3D film growth during heteroepitaxy can be used to favor specific domain configurations that lead to step‐like polarization switching and a giant nonlinear dielectric response in sub‐switching ac electric fields. A combination of cube‐on‐cube epitaxial growth and the formation of columnar structures during pulsed laser deposition of Pb_0.5Sr_0.5TiO₃ films on La_0.5Sr_0.5CoO₃ bottom electrode layers and MgO (001) substrates stabilizes ferroelectric nanodomains with enhanced dynamic properties. In the Pb_0.5Sr_0.5TiO₃ films, a‐ and c‐oriented epitaxial columns grow from the bottom to the top of the film leading to random polydomain architectures with strong associations between the ferroelectric domains and the nanocolumns. Polarization switching in the two domain populations is initiated at distinctive fields due to domain wall pinning on column boundaries. Moreover, piezoelectric coupling between ferroelectric domains leads to strong interdomain elastic interactions, which result in an enhanced Rayleigh‐type dielectric nonlinearity. The growth of epitaxial films with 3D columnar structures opens up new routes towards the engineering of enhanced ferroelectric and electromechanical functions in a broad class of complex oxide materials.     

Functional Free‐Standing Graphene Honeycomb Films

Fabricating free‐standing, three‐dimensional (3D) ordered porous graphene structure can service a wide range of functional materials such as environmentally friendly materials for antibacterial medical applications and efficient solar harvesting devices. A scalable solution processable strategy is developed to create such free‐standing hierarchical porous structures composed of functionalized graphene sheets via an “on water spreading” method. The free‐standing film shows a large area uniform honeycomb structure and can be transferred onto any substrate of interest. The graphene‐based free‐standing honeycomb films exhibit superior broad spectrum antibacterial activity as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli as model pathogens. Functional nanoparticles such as titanium dioxide (TiO₂) nanoparticles can be easily introduced into conductive graphene‐based scaffolds by premixing. The formed composite honeycomb film electrode shows a fast, stable, and completely reversible photocurrent response accompanying each switch‐on and switch‐off event. The graphene‐based honeycomb scaffold enhances the light‐harvesting efficiency and improves the photoelectric conversion behavior; the photocurrent of the composite film is about two times as high as that of the pure TiO₂ film electrode. Such composite porous films combining remarkably good electrochemical performance of graphene, a large electrode/electrolyte contact area, and excellent stability during the photo‐conversion process hold promise for further applications in water treatment and solar energy conversion.     

Free‐Standing Functionalized Graphene Oxide Solid Electrolytes in Electrochemical Gas Sensors

A free‐standing sulfonic acid functionalized graphene oxide (fSGO)‐based electrolyte film is prepared and used in an electrochemical gas sensor, an alcohol fuel cell sensor (AFCS), for the detection of alcohol. The fSGO electrolyte film‐based AFCS detects ethanol vapor with excellent response, linearity, and sensitivity, since it possesses a high proton conductivity (58 mS cm^?1 at 55 °C). An ethanol detection limit level as low as 25 ppm is achieved and high selectivity for ethanol over acetone is demonstrated. These results do not only show the promising potential of fSGO films in an electrochemical gas sensors, specifically a portable breathalyzer, but also open an alternative pathway to investigate the application of graphene derivatives in the field of gas sensors.     

Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K, of the films from ≈3 to ≈61 W m^?1 K^?1 at room temperature. The cross‐plane thermal conductivity, K_⊥, reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m^?1 K^?1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K_⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □^?1. The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K_⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m^?1 K^?1 with the increase in the sp² domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.     

A Controllable Self‐Assembly Method for Large‐Scale Synthesis of Graphene Sponges and Free‐Standing Graphene Films

A simple method to prepare large‐scale graphene sponges and free‐standing graphene films using a speed vacuum concentrator is presented. During the centrifugal evaporation process, the graphene oxide (GO) sheets in the aqueous suspension are assembled to generate network‐linked GO sponges or a series of multilayer GO films, depending on the temperature of a centrifugal vacuum chamber. While sponge‐like bulk GO materials (GO sponges) are produced at 40 °C, uniform free‐standing GO films of size up to 9 cm² are generated at 80 °C. The thickness of GO films can be controlled from 200 nm to 1 µm based on the concentration of the GO colloidal suspension and evaporation temperature. The synthesized GO films exhibit excellent transparency, typical fluorescent emission signal, and high flexibility with a smooth surface and condensed density. Reduced GO sponges and films with less than 5 wt% oxygen are produced through a thermal annealing process at 800 °C with H₂/Ar flow. The structural flexibility of the reduced GO sponges, which have a highly porous, interconnected, 3D network, as well as excellent electrochemical properties of the reduced GO film with respect to electrode kinetics for the [Fe(CN)₆]^3?/4? redox system, are demonstrated.     

Oxygen‐Free Highly Conductive Graphene Papers

Graphene papers have a potential to overcome the gap from nanoscale graphene to real macroscale applications of graphene. A unique process for preparation of highly conductive graphene thin paper by means of Ar⁺ ion irradiation of graphene oxide (GO) papers, with carbon/oxygen ratio reduced to 100:1, is presented. The composition of graphene paper in terms of carbon/oxygen ratio and in terms of types of individual oxygen‐containing groups is monitored throughout the process. Angle‐resolved high resolution X‐ray photoelectron spectroscopy helps to investigate the depth profile of carbon and oxygen within reduced GO paper. C/O ratios over 100 on the surface and 40 in bulk material are observed. In order to bring insight to the processes of oxygen removal from GO paper by low energy Ar⁺ ion bombardment, the gases released during the irradiation are analyzed by mass spectroscopy. It is proven that Ar⁺ ion beam can be applied as a technique for fabrication of highly reduced graphene papers with high conductivities. Such highly conductive graphene papers have great potential to be used in application for construction of microelectronic and sensor devices.     

MEL‐type Pure‐Silica Zeolite Nanocrystals Prepared by an Evaporation‐Assisted Two‐Stage Synthesis Method as Ultra‐Low‐k Materials

A MEL‐type pure‐silica zeolite (PSZ), prepared by spin‐on of nanoparticle suspensions, has been shown to be a promising ultra‐low‐dielectric‐constant (k) material because of its high mechanical strength, hydrophobicity, and chemical stability. In our previous works, a two‐stage synthesis method was used to synthesize a MEL‐zeolite nanoparticle suspension, in which both nanocrystal yield and particle size of the zeolite suspension increased with increasing synthesis time. For instance, at a crystal yield of 63%, the particle size is 80 nm, which has proved to be too large because it introduces a number of problems for the spin‐on films, including large surface roughness, surface striations, and large mesopores. In the current study, the two‐stage synthesis method is modified into an evaporation‐assisted two‐stage method by adding a solvent‐evaporation process between the two thermal‐treatment steps. The modified method can yield much smaller particle sizes (e.g., 14 vs. 80 nm) while maintaining the same nanocrystal yields as the two‐stage synthesis. Furthermore, the nanoparticle suspensions from the evaporation‐assisted two‐stage synthesis show a bimodal particle size distribution. The primary nanoparticles are around 14 nm in size and are stable in the final suspension with 60% solvent evaporation. The factors that affect nanocrystal synthesis are discussed, including the concentration, pH value, and viscosity. Spin‐on films prepared by using suspensions synthesized this way have no striations and improved elastic modulus (9.67 ± 1.48 GPa vs. 7.82 ± 1.30 GPa), as well as a similar k value (1.91 ± 0.09 vs. 1.89 ± 0.08) to the previous two‐stage synthesized films.     

Single‐Phase Filamentary Cellular Breakdown Via Laser‐Induced Solute Segregation

Nanosecond melting and quenching of materials offers a pathway to novel structures with unusual properties. Impurity‐rich silicon processed using nanosecond‐pulsed‐laser‐melting is known to produce nanoscale features in a process referred to as “cellular breakdown” due to destabilization of the planar liquid/solid interface. Here, atom probe tomography combined with electron microscopy is applied to show that the morphology of cellular breakdown in these materials is significantly more complex than previously documented. Breakdown into a complex, branching filamentary structure topped by a few nm of a cell‐like layer is observed. Single‐phase diamond cubic silicon highly supersaturated with at least 10% atomic Co and no detectable silicides is reported within these filaments. In addition, the unprecedented spatio‐chemical accuracy of the atom probe allows to investigate nanosecond formation dynamics of this complex material. Previously reported properties of these materials can now be reconsidered in light of their true composition, and this class of inhomogeneous metastable alloys in silicon can be explored with confidence.     

Self‐Assembly of Flexible Free‐Standing 3D Porous MoS2‐Reduced Graphene Oxide Structure for High‐Performance Lithium‐Ion Batteries

Flexible freestanding electrodes are highly desired to realize wearable/flexible batteries as required for the design and production of flexible electronic devices. Here, the excellent electrochemical performance and inherent flexibility of atomically thin 2D MoS₂ along with the self‐assembly properties of liquid crystalline graphene oxide (LCGO) dispersion are exploited to fabricate a porous anode for high‐performance lithium ion batteries. Flexible, free‐standing MoS₂–reduced graphene oxide (MG) film with a 3D porous structure is fabricated via a facile spontaneous self‐assembly process and subsequent freeze‐drying. This is the first report of a one‐pot self‐assembly, gelation, and subsequent reduction of MoS₂/LCGO composite to form a flexible, high performance electrode for charge storage. The gelation process occurs directly in the mixed dispersion of MoS₂ and LCGO nanosheets at a low temperature (70 °C) and normal atmosphere (1 atm). The MG film with 75 wt% of MoS₂ exhibits a high reversible capacity of 800 mAh g^?1 at a current density of 100 mA g^?1. It also demonstrates excellent rate capability, and excellent cycling stability with no capacity drop over 500 charge/discharge cycles at a current density of 400 mA g^?1.     

The Effect of Membrane Thickness on a Microvascular Gas Exchange Unit

For reducing anthropogenic CO₂ emissions, carbon capture and sequestration technologies benefit from the creation of new and efficient gas exchange systems. Vascularized systems provide a means of exchanging CO₂ by providing high specific surface areas and patterned, intimate contact between capture fluids and gases. The well‐defined geometrical arrangement of fluid and gas channels, separated by semipermeable membranes, also provides a new platform for augmenting the function of liquid chemical solutions to carbon capture. In particular, the separation distance of the channels, or polymer membrane thickness, is closely related to the absorption rate as gases must permeate through the membrane before reacting with a fluid. Here, a study of the relationship between the membrane thickness in 3D microvascular contactors and absorption rates via a selective etching process is reported. By decreasing the membrane thickness, the mass transport rate of CO₂ in the vascular contactor is increased by up to 160%.     

Self‐Healing Reduced Graphene Oxide Films by Supersonic Kinetic Spraying

The industrial scale application of graphene and other functional materials in the field of electronics has been limited by inherent defects, and the lack of simple deposition methods. A simple spray deposition method is developed that uses a supersonic air jet for a commercially available reduced graphene oxide (r‐GO) suspension. The r‐GO flakes are used as received, which are pre‐annealed and pre‐hydrazine‐treated, and do not undergo any post‐treatment. A part of the considerable kinetic energy of the r‐GO flakes entrained by the supersonic jet is used in stretching the flakes upon impact with the substrate. The resulting “frozen elastic strains” heal the defects (topological defects, namely Stone‐Wales defect and C₂ vacancies) in the r‐GO flakes, which is reflected in the reduced ratio of the intensities of the D and G bands in the deposited film. The defects can also be regenerated by annealing.     

Revisiting the Structure of Graphene Oxide for Preparing New‐Style Graphene‐Based Ultraviolet Absorbers

Despite sustained effort over the years, the exploration of an effective strategy toward understanding the structure and properties of graphene oxide (GO) is still highly desirable. Herein, a facile route to revisit the structure of GO is demonstrated by elucidating its chemical‐conversion process solely in the presence of ammonia. Such a strategy can contribute to settling some arguments in recent models of GO, and also offers a prerequisite to identify critical components that can act as ultraviolet absorbers (UVAs) in resulting dispersions of nitrogen‐doped graphene sheets (NGSs). Inspired by this, for the first time, the performance of NGSs, serving as new‐style UVAs, is investigated through directly assessing the effect of NGSs on the photofastness of azo dyes (Food Black). These studies reveal that, distinct from the common understanding, the as‐prepared NGSs can dramatically enhance the photostability of Food Black under UV irradiation and exhibit greatly applied potential as a multifunctional UVA for new‐generation inkjet inks that can simultaneously integrate the advantages of dye‐based and pigment‐based inks.     

All‐Solution‐Processed Cu2ZnSnS4 Solar Cells with Self‐Depleted Na2S Back Contact Modification Layer

The thin‐film photovoltaic material Cu₂ZnSnS₄ (CZTS) has drawn worldwide attention in recent years due to its earth‐abundant, nontoxic element constitution, and remarkable photovoltaic performance. Although state‐of‐the‐art power conversion efficiency is achieved by hydrazine‐based methods, effort to fabricate such devices in a high throughput, environmental‐friendly way is still highlydesired. Here a hydrazine‐free all‐solution‐processed CZTS solar cell with Na₂S self‐depleted back contact modification layer for the first time is demonstrated, using a ball‐milled CZTS as light absorber, low‐temperature solution‐processed ZnO electron‐transport layer as well as silver‐nanowire transparent electrode. The inserting of Na₂S self‐depleted layer is proven to effectively stabilize the CZTS/Mo interface by eliminating a detrimental phase segregation reaction between CZTS and Mo‐coated soda lime glass, thus leading to a better crystallinity of CZTS light absorbing layer, enhanced carrier transportation at CZTS/Mo interface as well as a smaller series resistance. Furthermore, the self‐depletion feature of the Na₂S modification layer also averts hole‐transportation barrier within the devices. The results show the vital importance of interfacial engineering for these CZST devices and the Na₂S interface layer can be extended to other optoelectronic devices using Mo contact.     

Interaction of Zoospores of the Green Alga Ulva with Bioinspired Micro‐ and Nanostructured Surfaces Prepared by Polyelectrolyte Layer‐by‐Layer Self‐Assembly

The interaction of spores of Ulva with bioinspired structured surfaces in the nanometer–micrometer size range is investigated using a series of coatings with systematically varying morphology and chemistry, which allows separation of the contributions of morphology and surface chemistry to settlement (attachment) and adhesion strength. Structured surfaces are prepared by layer‐by‐layer spray‐coating deposition of polyelectrolytes. By changing the pH during application of oppositely charged poly(acrylic acid) and polyethylenimine polyelectrolytes, the surface structures are systematically varied, which allows the influence of morphology on the biological response to be determined. In order to discriminate morphological from chemical effects, surfaces are chemically modified with poly(ethylene glycol) and tridecafluoroctyltriethoxysilane. This chemical modification changes the water contact angles while the influence of the morphology is retained. The lowest level of settlement is observed for structures of the order 2 µm. All surfaces are characterized with respect to their wettability, chemical composition, and morphological properties by contact angle measurement, X‐ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy.     

Manganese Oxide Octahedral Molecular Sieves (OMS‐2) Multiple Framework Substitutions: A New Route to OMS‐2 Particle Size and Morphology Control

Self‐assembled multidoped cryptomelane hollow microspheres with ultrafine particles in the size range of 4–6 nm, and with a very high surface area of 380 m² g^?1 have been synthesized. The particle size, morphology, and the surface area of these materials are readily controlled via multiple framework substitutions. The X‐ray diffraction and transmission electron microscopy (TEM) results indicate that the as‐synthesized multidoped OMS‐2 materials are pristine and crystalline, with no segregated metal oxide impurities. These results are corroborated by infrared (IR) and Raman spectroscopy data, which show no segregated amorphous and/or crystalline metal impurities. The field‐emission scanning electron microscopy (FESEM) studies confirm the homogeneous morphology consisting of microspheres that are hollow and constructed by the self‐assembly of pseudo‐flakes, whereas energy‐dispersive X‐ray (EDX) analyses imply that all four metal cations are incorporated into the OMS‐2 structure. On the other hand, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) demonstrate that the as‐synthesized multidoped OMS‐2 hollow microspheres are more thermally unstable than their single‐doped and undoped counterparts. However, the in‐situ XRD studies show that the cryptomelane phase of the multidoped OMS‐2 hollow microspheres is stable up to about 450°C in air. The catalytic activity of these microspheres towards the oxidation of diphenylmethanol is excellent compared to that of undoped OMS‐2 materials.     

Enhancement of Thermoelectric Figure‐of‐Merit by a Bulk Nanostructuring Approach

Recently a significant figure‐of‐merit (ZT) improvement in the most‐studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high‐energy ball milling and a direct‐current‐induced hot‐press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.     

膜厚对氧化钒薄膜结构和光学性质的影响

利用真空热蒸发在石英基片上制备了不同厚度的氧化钒薄膜, 研究厚度对薄膜的结构、形貌和光学特性的影响。薄膜的结构由X射线衍射(XRD)仪和拉曼(Raman)光谱仪测得, 表面形貌用原子力显微镜(AFM)观测。利用分光光度计测量薄膜的光学透射率, 并且采用Forouhi-Bloomer模型与修正的德鲁德(Drude)自由电子模型相结合的方法拟合透射率来确定薄膜的折射率、消光系数和带隙。结果表明, 热蒸发的氧化钒薄膜呈非晶态, 薄膜的主要成分为五氧化二钒, 且含有少量的二氧化钒。薄膜表面的颗粒粘结在一起, 随着薄膜厚度的增加, 薄膜表面粗糙度以及颗粒尺寸变小, 膜层表面平整度越来越好, 颗粒之间的空隙变小, 导致折射率随膜厚的增加而增大, 消光系数减小。另外, 随着薄膜厚度从200 nm增加到450 nm, 光学带隙从2.67 eV减小到2.45 eV。     

A Free‐Standing and Ultralong‐Life Lithium‐Selenium Battery Cathode Enabled by 3D Mesoporous Carbon/Graphene Hierarchical Architecture

High capacity cathode materials for long‐life rechargeable lithium batteries are urgently needed. Selenium cathode has recently attracted great research attention due to its comparable volumetric capacity to but much better electrical conductivity than widely studied sulfur cathode. However, selenium cathode faces similar issues as sulfur (i.e., shuttling of polyselenides, volumetric expansion) and high performance lithium‐selenium batteries (Li–Se) have not yet been demonstrated at selenium loading >60% in the electrode. In this work, a 3D mesoporous carbon nanoparticles and graphene hierarchical architecture to storage selenium as binder‐free cathode material (Se/MCN‐RGO) for high energy and long life Li–Se batteries is presented. Such architecture not only provides the electrode with excellent electrical and ionic conductivity, but also efficiently suppresses polyselenides shuttling and accommodates volume change during charge/discharge. At selenium content of 62% in the entire cathode, the free‐standing Se/MCN‐RGO exhibits high discharge capacity of 655 mAh g^?1 at 0.1 C (97% of theoretical capacity) and long cycling stability with a very small capacity decay of 0.008% per cycle over 1300 cycles at 1 C. The present report demonstrates significant progress in the development of high capacity cathode materials for long‐life Li batteries and flexible energy storage device.