Room‐Temperature Metallic Fusion‐Induced Layer‐by‐Layer Assembly for Highly Flexible Electrode Applications

To fabricate flexible electrodes, conventional silver (Ag) nanomaterials have been deposited onto flexible substrates, but the formed electrodes display limited electrical conductivity due to residual bulky organic ligands, and thus postsintering processes are required to improve the electrical conductivity. Herein, an entirely different approach is introduced to produce highly flexible electrodes with bulk metal–like electrical conductivity: the room‐temperature metallic fusion of multilayered silver nanoparticles (NPs). Synthesized tetraoctylammonium thiosulfate (TOAS)‐stabilized Ag NPs are deposited onto flexible substrates by layer‐by‐layer assembly involving a perfect ligand‐exchange reaction between bulky TOAS ligands and small tris(2‐aminoethyl)amine linkers. The introduced small linkers substantially reduce the separation distance between neighboring Ag NPs. This shortened interparticle distance, combined with the low cohesive energy of Ag NPs, strongly induces metallic fusion between the close‐packed Ag NPs at room temperature without additional treatments, resulting in a high electrical conductivity of ≈1.60 × 10⁵ S cm^?1 (bulk Ag: ≈6.30 × 10⁵ S cm^?1). Furthermore, depositing the TOAS–Ag NPs onto cellulose papers through this approach can convert the insulating substrates into highly flexible and conductive papers that can be used as 3D current collectors for energy‐storage devices.     

A Transparent,Flexible, Low‐Temperature,and Solution‐Processible Graphene Composite Electrode

The synthesis and preparation of a new type of graphene composite material suitable for spin‐coating into conductive, transparent, and flexible thin film electrodes in ambient conditions is reported here for the first time. Solution‐processible graphene with diameter up to 50 μm is synthesized by surfactant‐assisted exfoliation of graphite oxide and in situ chemical reduction in a large quantity. Spin‐coating the mixing solution of surfactant‐functionalized graphene and PEDOT:PSS yields the graphene composite electrode (GCE) without the need for high temperature annealing, chemical vapor deposition, or any additional transfer‐printing process. The conductivity and transparency of GCE are at the same level as those of an indium tin oxide (ITO) electrode. Importantly, it exhibits high stability (both mechanical and electrical) in bending tests of at least 1000 cycles. The performance of organic light‐emitting diodes based on a GCE anode is comparable, if not superior, to that of OLEDs made with an ITO anode.     

Silver‐Nanowire‐Based Interferometric Optical Tweezers for Enhanced Optical Trapping and Binding of Nanoparticles

Light‐induced self‐assembly offers a new route to build mesoscale optical matter arrays from nanoparticles (NPs), yet the low stability of optical matter systems limits the assembly of large‐scale NP arrays. Here it is shown that the interferometric optical fields created by illuminating a single Ag nanowire deposited on a coverslip can enhance the electrodynamic interactions among NPs. The Ag nanowire serves as a plasmonic antenna to shape the incident laser beam and guide the optical assembly of colloidal metal (Ag and Au) and dielectric (polystyrene) NPs in solution. By controlling the laser polarization direction, both the mesoscale interactions among multiple NPs and the near‐field coupling between the NPs and nanowire can be tuned, leading to large‐scale and stable optical matter arrays consisting of up to 60 NPs. These results demonstrate that single Ag nanowires can serve as multifunctional antennas to guide the optical trapping and binding of multiple NPs and provide a new strategy to control electrodynamic interactions using hybrid nanostructures.     

Layer‐Controlled and Wafer‐Scale Synthesis of Uniform and High‐Quality Graphene Films on a Polycrystalline Nickel Catalyst

Chemical vapor deposition (CVD) provides a synthesis route for large‐area and high‐quality graphene films. However, layer‐controlled synthesis remains a great challenge on polycrystalline metallic films. Here, a facile and viable synthesis of layer‐controlled and high‐quality graphene films on wafer‐scale Ni surface by the sequentially separated steps of gas carburization, hydrogen exposure, and segregation is developed. The layer numbers of graphene films with large domain sizes are controlled precisely at ambient pressure by modulating the simplified CVD process conditions and hydrogen exposure. The hydrogen exposure assisted with a Ni catalyst plays a critical role in promoting the preferential segregation through removing the carbon layers on the Ni surface and reducing carbon content in the Ni. Excellent electrical and transparent conductive performance, with a room‐temperature mobility of ≈3000 cm² V^?1 s^?1 and a sheet resistance as low as ≈100 Ω per square at ≈90% transmittance, of the twisted few‐layer grapheme films grown on the Ni catalyst is demonstrated.     

The Swiss‐Army‐Knife Self‐Assembled Monolayer: Improving Electron Injection,Stability, and Wettability of Metal Electrodes with a One‐Minute Process

A novel Self‐assembled Monolayer (SAM) forming molecule bisjulolidyldisulfide (9,9'‐disulfanediylbis(2,3,6,7‐tetrahydro‐1H,5H‐pyrido[3,2,1‐ij]quinoline)) is demonstrated which lowers the work function of metal surfaces by ≈1.2 eV and can be deposited in a 1 min process. Bisjulolidyldisulfide exists in a stable disulfide configuration prior to surface exposure and can therefore be stored, handled, and processed in ambient conditions. SAM from bisjulolidyldisulfide are deposited on metal surfaces (Au and Ag), including inkjet printed Ag on polyethylene terephthalate substrates, investigated by photoelectron and infrared spectroscopy, and used as electrodes in n‐type organic field effect transistor (OFET). Treatment of electrodes in OFET devices with with bisjulolidyldisulfide‐SAMs reduces the contact resistance by two orders of magnitude and improves shelf life with respect to pristine metal electrodes. The presented treatment also increases the surfaces wettability and thereby facilitates solution processing of a subsequent layer. These beneficial properties for device performance, processing, and stability, combined with ease of preparation and handling, render this SAM‐forming molecule an excellent candidate for the high‐throughput production of flexible electronic devices.     

An Interface‐Induced Co‐Assembly Approach Towards Ordered Mesoporous Carbon/Graphene Aerogel for High‐Performance Supercapacitors

Hierarchically porous composites with mesoporous carbon wrapping around the macroporous graphene aerogel can combine the advantages of both components and are expected to show excellent performance in electrochemical energy devices. However, the fabrication of such composites is challenging due to the lack of an effective strategy to control the porosity of the mesostructured carbon layers. Here an interface‐induced co‐assembly approach towards hierarchically mesoporous carbon/graphene aerogel composites, possessing interconnected macroporous graphene networks covered by highly ordered mesoporous carbon with a diameter of ≈9.6 nm, is reported. And the orientation of the mesopores (vertical or horizontal to the surface of the composites) can be tuned by the ratio of the components. As the electrodes in supercapacitors, the resulting composites demonstrate outstanding electrochemical performances. More importantly, the synthesis strategy provides an ideal platform for hierarchically porous graphene composites with potential for energy storage and conversion applications.     

Large‐Area Schottky Barrier Transistors Based on Vertically Stacked Graphene–Metal Oxide Heterostructures

The fabrication of all‐transparent flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene–metal oxide–metal heterostructures and ion gel gate dielectrics is demonstrated. The vertical SB transistor structure is formed by (i) vertically sandwiching a solution‐processed indium‐gallium‐zinc‐oxide (IGZO) semiconductor layer between graphene (source) and metallic (drain) electrodes and (ii) employing a separate coplanar gate electrode bridged with a vertical channel through an ion gel. The channel current is modulated by tuning the Schottky barrier height across the graphene–IGZO junction under an applied external gate bias. The ion gel gate dielectric with high specific capacitance enables modulation of the Schottky barrier height at the graphene–IGZO junction over 0.87 eV using a voltage below 2 V. The resulting vertical devices show high current densities (18.9 A cm^?2) and on–off current ratios (>10⁴) at low voltages. The simple structure of the unit transistor enables the successful fabrication of low‐power logic gates based on device assemblies, such as the NOT, NAND, and NOR gates, prepared on a flexible substrate. The facile, large‐area, and room‐temperature deposition of both semiconducting metal oxide and gate insulators integrates with transparent and flexible graphene opens up new opportunities for realizing graphene‐based future electronics.     

Simultaneous Electrochemical Dual‐Electrode Exfoliation of Graphite toward Scalable Production of High‐Quality Graphene

Developing scalable methods to produce large quantities of high‐quality and solution‐processable graphene is essential to bridge the gap between laboratory study and commercial applications. Here an efficient electrochemical dual‐electrode exfoliation approach is developed, which combines simultaneous anodic and cathodic exfoliation of graphite. Newly designed sandwich‐structured graphite electrodes which are wrapped in a confined space with porous metal mesh serve as both electrodes, enabling a sufficient ionic intercalation. Mechanism studies reveal that the combination of electrochemical intercalation with subsequent thermal decomposition results in drastic expansion of graphite toward high‐efficiency production of graphene with high quality. By precisely controlling the intercalation chemistry, the two‐step approach leads to graphene with outstanding yields (85% and 48% for cathode and anode, respectively) comprising few‐layer graphene (1–3 layers, >70%), ultralow defects (I_D/I_G < 0.08), and high production rate (exceeding 25 g h^?1). Moreover, its excellent electrical conductivity (>3 × 10⁴ S m^?1) and great solution dispersibility in N‐methyl pyrrolidone (10 mg mL^?1) enable the fabrication of highly conductive (11 Ω sq^?1) and flexible graphene films by inkjet printing. This simple and efficient exfoliation approach will facilitate the development of large‐scale production of high‐quality graphene and holds great promise for its wide application.     

Highly Robust Indium‐Free Transparent Conductive Electrodes Based on Composites of Silver Nanowires and Conductive Metal Oxides

A hybrid approach for the realization of In‐free transparent conductive layers based on a composite of a mesh of silver nanowires (NWs) and a conductive metal‐oxide is demonstrated. As metal‐oxide room‐temperature‐processed sol–gel SnO_x or Al:ZnO prepared by low‐temperature (100 °C) atomic layer deposition is used, respectively. In this concept, the metal‐oxide is intended to fuse the wires together and also to “glue” them to the substrate. As a result, a low sheet resistance down to 5.2 Ω sq^‐1 is achieved with a concomitant average transmission of 87%. The adhesion of the NWs to the substrate is significantly improved and the resulting composites withstand adhesion tests without loss in conductivity. Owing to the low processing temperatures, this concept allows highly robust, highly conductive, and transparent coatings even on top of temperature sensitive objects, for example, polymer foils, organic devices. These Indium‐ and PEDOT:PSS‐free hybrid layers are successfully implemented as transparent top‐electrodes in efficient all‐solution‐processed semitransparent organic solar cells. It is obvious that this approach is not limited to organic solar cells but will generally be applicable in devices which require transparent electrodes.     

Seriography‐Guided Reduction of Graphene Oxide Biopapers for Wearable Sensory Electronics

Novel nacre‐mimic bio‐nanocomposites, such as graphene‐based laminates, are pushing the boundaries of strength and toughness as flexible engineering materials. Translating these material advances to functional flexible electronics requires methods for generating print‐scalable microcircuits (conductive elements surrounded by dielectric) into these strong, tough, lightweight bio‐nanocomposites. Here, a new paradigm for printing flexible electronics by employing facile, eco‐friendly seriography to confine the reduction of graphene oxide biopapers reinforced by silk interlayers is presented. Well‐defined, micropatterned regions on the biopaper are chemically reduced, generating a 10⁶ increase in conductivity (up to 10⁴ S m^?1). Flexible, robust graphene‐silk circuits are showcased in diverse applications such as resistive moisture sensors and capacitive proximity sensors. Unlike conductive (i.e., graphene‐ or Ag nanoparticle‐loaded) inks printed onto substrates, seriography‐guided reduction does not create mechanically weak interfaces between dissimilar materials and does not require the judicious formation of ink. The unimpaired functionality of printed‐in graphene‐silk microcircuits after thousands of punitive folding cycles and chemical attack by harsh solvents is demonstrated. This novel approach provides a low‐cost, portable solution for printing micrometer‐scale conductive features uniformly across large areas (>hundreds of cm²) in layered composites for applications including wearable health monitors, electronic skin, rollable antennas, and conformable displays.     

Non‐Invasive High‐Throughput Metrology of Functionalized Graphene Sheets

The utilization of fluorescence quenching microscopy (FQM) for quick visualization of chemical functionalization in relatively large regions of graphene, grown via chemical vapor deposition (CVD), is discussed. Through reactive ion plasma etching, patterns of p‐type CVD‐grown graphene functionalized with fluorine are generated. 4‐(dicyanomethylene)‐2‐methyl‐6‐(4‐dimethylaminostyryl)‐4H‐pyran (DCM) is used as the fluorescent agent. The emission of DCM is quenched to a different extent by fluorinated and pristine graphene, which provides the fluorescence‐imaging contrast essential for this metrology. To probe the functionalized surface patterns with DCM, the dye is dispersed in polymethylmethacrylate (PMMA) then the graphene surface is coated, forming a 30‐nm‐thick DCM‐PMMA layer. Fluorescence images of dye‐coated graphene distinctly reveal the difference between the chemically treated and as‐grown regions. The pristine graphene quenches the DCM emission more efficiently than the fluorinated graphene. Therefore, the regions with pristine graphene appear darker on the fluorescence images than the regions with fluorinated graphene, enabling large‐scale mapping of the functionalized regions in CVD grown graphene sheets Due to its simplicity and consistent results, FQM is now poised for widespread adoption by graphene manufacturers as a basis for facile and high throughput metrology of large‐scale graphene sheets.     

Alternating‐Current MXene Polymer Light‐Emitting Diodes

MXenes (Ti₃C₂) are 2D transition‐metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes suitable for polymer light‐emitting diodes (PLEDs) have rarely been demonstrated. With the discovery of the excellent electrical stability of MXene under an alternating current (AC), herein, PLEDs that employ MXene electrodes and exhibit high performance under AC operation (AC MXene PLEDs) are presented. The PLED exhibits a turn‐on voltage, current efficiency, and brightness of 2.1 V, 7 cd A^?1, and 12 547 cd m^?2, respectively, when operated under AC with a frequency of 1 kHz. The results indicate that the undesirable electric breakdown associated with heat arising from the poor interface of the MXene with a hole transport layer in the direct‐current mode is efficiently suppressed by the transient injection of carriers accompanied by the alternating change of the electric polarity under the AC, giving rise to reliable light emission with a high efficiency. The solution‐processable MXene electrode can be readily fabricated on a flexible polymer substrate, allowing for the development of a mechanically flexible AC MXene PLED with a higher performance than flexible PLEDs employing solution‐processed nanomaterial‐based electrodes such as carbon nanotubes, reduced graphene oxide, and Ag nanowires.     

Highly Conductive Ordered Mesoporous Carbon Based Electrodes Decorated by 3D Graphene and 1D Silver Nanowire for Flexible Supercapacitor

Ordered mesoporous carbon (OMC) is considered one of the most promising materials for electric double layer capacitors (EDLC) given its low‐cost, high specific surface area, and easily accessed ordered pore channels. However, pristine OMC electrode suffers from poor electrical conductivity and mechanical flexibility, whose specific capacitance and cycling stability is unsatisfactory in flexible devices. In this work, OMC is coated on the surface of highly conductive three‐dimensional graphene foam, serving as both charge collector and flexible substrate. Upon further decoration with silver nanowires (Ag NWs), the novel architecture of Ag NWs/3D‐graphene foam/OMC (Ag‐GF‐OMC) exhibits exceptional electrical conductivity (up to 762 S cm^?1) and mechanical robustness. The Ag‐GF‐OMC electrodes in flexible supercapacitors reach a specific capacitance as high as 213 F g^?1, a value five‐fold higher than that of the pristine OMC electrode. Moreover, these flexible electrodes also exhibit excellent long‐term stability with >90% capacitance retention over 10 000 cycles, as well as high energy and power density (4.5 Wh kg^?1 and 5040 W kg^?1, respectively). This study provides a new procedure to enhance the device performance of OMC based supercapacitors, which is a promising candidate for the application of flexible energy storage devices.     

Hybrid Silver Nanowire and Graphene‐Based Solution‐Processed Transparent Electrode for Organic Optoelectronics

The research on transparent conductive electrodes is in rapid ascent in order to respond to the requests of novel optoelectronic devices. The synergic coupling of silver nanowires (AgNWs) and high‐quality solution‐processable exfoliated graphene (EG) enables an efficient transparent conductor with low‐surface roughness of 4.6 nm, low sheet resistance of 13.7 Ω sq^?1 at high transmittance, and superior mechanical and chemical stabilities. The developed AgNWs–EG films are versatile for a wide variety of optoelectronics. As an example, when used as a bottom electrode in organic solar cell and polymer light‐emitting diode, the devices exhibit a power conversion efficiency of 6.6% and an external quantum efficiency of 4.4%, respectively, comparable to their commercial indium tin oxide counterparts.     

Structure‐Properties Relationship in Iron Oxide‐Reduced Graphene Oxide Nanostructures for Li‐Ion Batteries

Non‐aqueous sol‐gel routes involving the reaction of metal oxide precursors in organic solvents (e.g., benzyl alcohol) at moderate temperature and pressure, offer advantages such as high purity, high reproducibility and the ability to control the crystal growth without the need of using additional ligands. In this paper, a study carried out on a series of iron oxide/reduced graphene oxide composites is presented to elucidate a structure‐properties relationship leading to an improved electrochemical performance of such composites. Moreover, it is demonstrated that the easy production of the composites in a variety of temperature and composition ranges, allows a fine control over the final particles size, density and distribution. The materials obtained are remarkable in terms of the particle's size homogeneity and dispersion onto the reduced graphene oxide surface. Moreover, the synthesis method used to obtain the graphene oxide clearly affects the performances of the final composites through the control of the restacking of the reduced graphene oxide sheets. It is shown that a homogeneous and less defective reduced graphene oxide enables good electrochemical performances even at high current densities (over 500 mAh/g delivered at current densities as high as 1600 mA/g). The electrochemical properties of improved samples reach the best compromise between specific capacity, rate capability and cycle stability reported so far.     

Low‐Resistant Electrical and Robust Mechanical Contacts of Self‐Attachable Flexible Transparent Electrodes with Patternable Circuits

Flexible, transparent, conductive electrodes are key elements of emerging flexible electronic and energy devices. Such electrodes should form an intimate physical contact with various active components of flexible devices to ensure stable, low‐resistant electrical contacts. However, contact formation techniques are based largely on conventional soldering, conductive pastes, mechanical clamping, and thin film deposition. These generally result in damaged, contaminated, bulky, and uncontrollable contact interfaces. A self‐attachable, flexible, transparent, and conductive electrode that is based on a distinctive design of regular grid patterns into which bioinspired adhesive architectures and percolating Ag nanowires are integrated is proposed. Based on this integrated design, the proposed electrode forms reliable, low‐resistant electrical contacts; strong mechanical adhesive contacts; and ultra‐clean, damage‐free contact interfaces with active device components by attaching onto the components without using additional conductive pastes, mechanical pressing, or vacuum deposition processes. The contact interfaces of the electrode and device components remain stable even when the electrode is extremely bent. Moreover, specific electronic circuits can be generated on the electrode surface by a selective deposition of Ag nanowires. This enables simple interconnections of diverse electronic components on its surface.     

Insulator‐Conductor Type Transitions in Graphene‐Modified Silver Nanowire Networks: A Route to Inexpensive Transparent Conductors

Silver nanowire coatings are an attractive alternative to indium tin oxide for producing transparent conductors. To fabricate coatings with low sheet resistance required for touchscreen displays, a multi‐layer network of silver nanowires must be produced that may not be cost effective. This problem is counteracted here by modifying the electrical properties of an ultra‐low‐density nanowire network through local deposition of conducting graphene platelets. Unlike other solution‐processed materials, such as graphene oxide, our pristine graphene is free of oxygen functional groups, resulting in it being electrically conducting without the need for further chemical treatment. Graphene adsorption at inter‐wire junctions as well as graphene connecting adjacent wires contributes to a marked enhancement in electrical properties. Using our approach, the amount of nanowires needed to produce viable transparent electrodes could be more than 50 times less than the equivalent pristine high density nanowire networks, thus having major commercial implications. Using a laser ablation process, it is shown that the resulting films can be patterned into individual electrode structures, which is a pre‐requisite to touchscreen sensor fabrication.     

Graphene‐Based Mesoporous SnO2 with Enhanced Electrochemical Performance for Lithium‐Ion Batteries

Graphene‐based metal oxides generally show outstanding electrochemical performance due to the superior properties of graphene. However, the aggregation of active metal oxide nanoparticles on the graphene surface may result in a capacity fading and poor cycle performance. Here, a mesostructured graphene‐based SnO₂ composite is prepared through in situ growth of SnO₂ particles on the graphene surface using cetyltrimethylammonium bromide as the structure‐directing agent. This novel mesoporous composite inherits the advantages of graphene nanosheets and mesoporous materials and exhibits higher reversible capacity, better cycle performance, and better rate capability compared to pure mesoporous SnO₂ and graphene‐based nonporous SnO₂. It is concluded that the synergetic effect between graphene and mesostructure benefits the improvement of the electrochemical properties of the hybrid composites. This facile method may offer an attractive alternative approach for preparation of the graphene‐based mesoporous composites as high‐ performance electrodes for lithium‐ion batteries.     

Arrays of Cone‐Shaped ZnO Nanorods Decorated with Ag Nanoparticles as 3D Surface‐Enhanced Raman Scattering Substrates for Rapid Detection of Trace Polychlorinated Biphenyls

A new, highly sensitive and uniform three‐dimensional (3D) hybrid surface‐enhanced Raman scattering (SERS) substrate has been achieved via simultaneously assembling small Ag nanoparticles (Ag‐NPs) and large Ag spheres onto the side surface and the top ends of large‐scale vertically aligned cone‐shaped ZnO nanorods (ZnO‐NRs), respectively. This 3D hybrid substrate manifests high SERS sensitivity to rhodamine and a detection limit as low as 10^?11 M to polychlorinated biphenyl (PCB) 77—a kind of persistent organic pollutants as global environmental hazard. Three kinds of inter‐Ag‐NP gaps in 3D geometry create a huge number of SERS “hot spots” that mainly contribute to the high SERS sensitivity. Moreover, the supporting chemical enhancement effect of ZnO‐NRs and the better enrichment effect ascribed to the large surface area of the substrate also help to achieve a lower detection limit. The arrays of cone‐shaped ZnO‐NRs decorated with Ag‐NPs on their side surface and large Ag spheres on the top ends have potentials in SERS‐based rapid detection of trace PCBs.     

Large‐Scale Fabrication of Three‐Dimensional Surface Patterns Using Template‐Defined Electrochemical Deposition

A new strategy to achieve large‐scale, three‐dimensional (3D) micro‐ and nanostructured surface patterns through selective electrochemical growth on monolayer colloidal crystal (MCC) templates is reported. This method can effectively create large‐area (>1 cm²), 3D surface patterns with well‐defined structures in a cost‐effective and time‐saving manner (<30 min). A variety of 3D surface patterns, including semishells, Janus particles, microcups, and mushroom‐like clusters, is generated. Most importantly, our method can be used to prepare surface patterns with prescribed compositions, such as metals, metal oxides, organic materials, or composites (e.g., metal/metal oxide, metal/polymer). The 3D surface patterns produced by our method can be valuable in a wide range of applications, such as biosensing, data storage, and plasmonics. In a proof‐of‐concept study, we investigated, both experimentally and theoretically, the surface‐enhanced Raman scattering (SERS) performance of the fabricated silver 3D semishell arrays.