Spontaneous Assembly of Extremely Long,Horizontally‐Aligned,Conductive Gold Micro‐Wires in a Langmuir Monolayer Template

“Bottom‐up” technologies are based upon the premise that organized systems – from the nano‐scale up to the macro‐scale – can be assembled spontaneously from basic building blocks in solution. We demonstrate a simple strategy for the generation of extremely long (up to several centi­meters), horizontally‐aligned gold micro‐wires, produced through a surfactant monolayer template deposited from gold thiocyanate [Au(SCN)₄⁻] aqueous solution. Specifically, we show that the surfactant, octyl‐maleimide (OM), spontaneously forms oriented micro‐wires at the air/water interface, which constitute a template for deposition of metallic gold through binding and crystallization of the soluble gold complex. The Au micro‐wires can be subsequently transferred onto solid substrates, and following plasma treatment and gold enhancement exhibit excellent conductivity even at electrode spacings of several centimeters. Importantly, the micro‐wire alignment determines the direction of electrical current, demonstrating that long‐range ordering of the micro‐wires can be accomplished, significantly affecting the physical properties of the system. The new approach is simple, robust, and can be readily exploited for bottom‐up fabrication of micro‐wire assemblies and transparent conductive electrodes.     

Gold Binary‐Structured Arrays Based on Monolayer Colloidal Crystals and Their Optical Properties

A simple and flexible route is presented to fabricate a gold binary‐structured ordered array by one step based on non‐shadow deposition on a plasma etching‐induced dualistic monolayer colloidal crystal. Such a Au binary‐structure array is built of hexagonally arranged nanoshells and nanorings which stand between two adjacent nanoshells. Six gold nanorings surround each nanoshell. The obtained arrays exhibit both the controllable surface‐plasmon‐resonance (SPR) properties of Au nanoshells and the strong electromagnetic‐field‐enhancement effects of Au nanorings, with the high structural stability of ordered arrays, and show promising potential as the substrate of surface‐enhanced Raman scattering (SERS)‐based devices. The method could also be suitable for fabrication of other material binary‐structured arrays. This study is important in designing and fabricating basal materials for the next generation of multifunctional nanostructured devices.     

Wafer‐Scale Fabrication of High‐Performance n‐Type Polymer Monolayer Transistors Using a Multi‐Level Self‐Assembly Strategy

Wafer‐scale fabrication of high‐performance uniform organic electronic materials is of great challenge and has rarely been realized before. Previous large‐scale fabrication methods always lead to different layer thickness and thereby poor film and device uniformity. Herein, the first demonstration of 4 in. wafer‐scale, uniform, and high‐performance n‐type polymer monolayer films is reported, enabled by controlling the multi‐level self‐assembly process of conjugated polymers in solution. Since the self‐assembly process happened in solution, the uniform 2D polymer monolayers can be facilely deposited on various substrates, and theoretically without size limitations. Polymer monolayer transistors exhibit high electron mobilities of up to 1.88 cm² V^?1 s^?1, which is among the highest in n‐type monolayer organic transistors. This method allows to easily fabricate n‐type conjugated polymers with wafer‐scale, high uniformity, low contact resistance, and excellent transistor performance (better than the traditional spin‐coating method). This work provides an effective strategy to prepare large‐scale and uniform 2D polymer monolayers, which could enable the application of conjugated polymers for wafer‐scale sophisticated electronics.     

Single‐Step Assembly of Large‐Area,Transparent Conductive Patterns Induced Through Edge Adsorption of Template‐Confined Au‐Thiocyanate

Fabrication of patterned metallic films through chemical means is a primary objective in the emerging field of “bottom‐up” lithography. We present a simple technology for generating large area, highly uniform patterns of conductive gold microwires. The approach is based upon dissolution of a gold complex, Au(SCN)₄⁻, in an organic‐solvent/water mixture and confining the solution underneath polymeric molds. We show that Au(SCN)₄⁻ undergoes spontaneous crystallization/reduction, producing metallic Au microwires tracing the contours of the templates. Importantly, no shape‐directing molecules or reducing agents are required for Au microwire formation, as the thiocyanate ligands both donate the reducing electrons as well as direct the crystallization process of the Au patterns. We demonstrate application of the new technology for creating highly transparent conductive films.     

Ultrafast Growth of High‐Quality Monolayer WSe2 on Au

The ultrafast growth of high‐quality uniform monolayer WSe₂ is reported with a growth rate of ≈26 µm s^?1 by chemical vapor deposition on reusable Au substrate, which is ≈2–3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter‐size single‐crystal WSe₂ domains in ≈30 s and large‐area continuous films in ≈60 s. Importantly, the ultrafast grown WSe₂ shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm² V^?1 s^?1 and ON/OFF ratio up to 9 × 10⁶ at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe₂ is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe₂ on Au substrate.     

A Centimeter‐Scale Inorganic Nanoparticle Superlattice Monolayer with Non‐Close‐Packing and its High Performance in Memory Devices

Due to the near‐field coupling effect, non‐close‐packed nanoparticle (NP) assemblies with tunable interparticle distance (d) attract great attention and show huge potential applications in various functional devices, e.g., organic nano‐floating‐gate memory (NFGM) devices. Unfortunately, the fabrication of device‐scale non‐close‐packed 2D NPs material still remains a challenge, limiting its practical applications. Here, a facile yet robust “rapid liquid–liquid interface assembly” strategy is reported to generate a non‐close‐packed AuNP superlattice monolayer (SM) on a centimeter scale for high‐performance pentacene‐based NFGM. The d and hence the surface plasmon resonance spectra of SM can be tailored by adjusting the molecular weight of tethered polymers. Precise control over the d value allows the successful fabrication of photosensitive NFGM devices with highly tunable performances from short‐term memory to nonvolatile data storage. The best performing nonvolatile memory device shows remarkable 8‐level (3‐bit) storage and a memory ratio over 10⁵ even after 10 years compared with traditional devices with a AuNP amorphous monolayer. This work provides a new opportunity to obtain large area 2D NPs materials with non‐close‐packed structure, which is significantly meaningful to microelectronic, photovoltaics devices, and biochemical sensors.     

ZIF‐8/ZIF‐67‐Derived Co‐Nx‐Embedded 1D Porous Carbon Nanofibers with Graphitic Carbon‐Encased Co Nanoparticles as an Efficient Bifunctional Electrocatalyst

Herein, an approach is reported for fabrication of Co‐N_x‐embedded 1D porous carbon nanofibers (CNFs) with graphitic carbon‐encased Co nanoparticles originated from metal–organic frameworks (MOFs), which is further explored as a bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Electrochemical results reveal that the electrocatalyst prepared by pyrolysis at 1000 °C (CoNC‐CNF‐1000) exhibits excellent catalytic activity toward ORR that favors the four‐electron ORR process and outstanding long‐term stability with 86% current retention after 40 000 s. Meanwhile, it also shows superior electrocatalytic activity toward OER, reaching a lower potential of 1.68 V at 10 mA cm^?2 and a potential gap of 0.88 V between the OER potential (at 10 mA cm^?2) and the ORR half‐wave potential. The ORR and OER performance of CoNC‐CNF‐1000 have outperformed commercial Pt/C and most nonprecious‐metal catalysts reported to date. The remarkable ORR and OER catalytic performance can be mainly attributable to the unique 1D structure, such as higher graphitization degree beneficial for electronic mobility, hierarchical porosity facilitating the mass transport, and highly dispersed CoN_xC active sites functionalized carbon framework. This strategy will shed light on the development of other MOF‐based carbon nanofibers for energy storage and electrochemical devices.     

Surface Engineering of Spherical Metal Nanoparticles with Polymers toward Selective Asymmetric Synthesis of Nanobowls and Janus‐Type Dimers

New synthetic methods capable of controlling structural and compositional complexities of asymmetric nanoparticles (NPs) are very challenging but highly desired. A simple and general synthetic approach to designing sophisticated asymmetric NPs by anisotropically patterning the surface of isotropic metallic NPs with amphiphilic block copolymers (BCPs) is reported. The selective galvanic replacement and seed‐mediated growth of a second metal can be achieved on the exposed surface of metal NPs, resulting in the formation of nanobowls and Janus‐type metal–metal dimers, respectively. Using Ag and Au NPs tethered with amphiphilic block copolymers of poly(ethylene oxide)‐block‐polystyrene (PEO‐b‐PS), anisotropic surface patterning of metallic NPs (e.g., Ag and Au) is shown to be driven by thermodynamical phase segregation of BCP ligands on isotropic metal NPs. Two proof‐of‐concept experiments are given on, i) synthesis of Au nanobowls by a selective galvanic replacement reaction on Janus‐type patched Ag/polymer NPs; and ii) preparation of Au–Pd heterodimers and Au–Au homodimers by a seed‐mediated growth on Janus‐type patched Au/polymer NPs. The method shows remarkable versatility; and it can be easily handled in aqueous solution. This synthetic strategy stands out as the new methodology to design and synthesis asymmetric metal NPs with sophisticated topologies.     

Regenerating optical properties of individual gold nanoparticles in alcoholic solvents without any surfactant

We compared the optical properties of gold nanoparticles (GNPs) in various solvents with those of strawberry-like composite particles (Au/SiO2) consisting of a silica core and single attached GNPs. The results show that Au/SiO2 without any surfactant could regenerate well optical properties of individual GNPs in alcoholic solvents. By the electrophoretic light-scattering measurements, the high dispersibility of Au/SiO2 composite particles dispersed in alcoholic solvents has been demonstrated. In addition, using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, we proposed a possible mechanism to qualitatively account for the dispersibility of Au/SiO2 in organic solvents such as alcoholic solvents and cyclohexane, which may provide an opportunity to manipulate optical signals of single nanoparticle in organic solvents.     

Tetrakis(4‐sulfonatophenyl)porphyrin‐Directed Assembly of Gold Nanocrystals: Tailoring the Plasmon Coupling Through Controllable Gap Distances

It is known that universality and controllability over nanocrystal orientation must be accomplished to facilitate the potential applications of metal nanocrystals in the areas of photonics, electronics, and optics. The facile fabrication of linear chains of Au nanorods and bifurcated junctions of nanorods/nanospheres is achieved via the crosslinking of H‐type tetrakis(4‐sulfonatophenyl)porphyrin aggregates in solution. The tuning of the plasmon coupling between the Au nanocrystals is demonstrated by varying the porphyrin concentration and thus the interparticle gap distances. Finite‐difference time‐domain calculations show that the red shift of the plasmon band exhibits a nearly exponential decay with increasing interparticle gap distances, thus giving rise to a “plasmon ruler equation.” The gap distances determined according to this equation agree well with the experimental observations and further confirm the porphyrin‐directed assembly process. The interaction mechanism between the Au nanorods and porphyrins is further investigated by a biological procedure using the dark‐field light scattering technique.     

Realization of In‐Plane p–n Junctions with Continuous Lattice of a Homogeneous Material

Two‐dimensional (2D) in‐plane p–n junctions with a continuous interface have great potential in next‐generation devices. To date, the general fabrication strategies rely on lateral epitaxial growth of p‐ and n‐type 2D semiconductors. An in‐plane p–n junction is fabricated with homogeneous monolayer Te at the step edge on graphene/6H‐SiC(0001). Scanning tunneling spectroscopy reveals that Te on the terrace of trilayer graphene is p‐type, and it is n‐type on monolayer graphene. Atomic‐resolution images demonstrate the continuous lattice of the junction, and mappings of the electronic states visualize the type‐II band bending across the space‐charge region of 6.2 nm with a build‐in field of 4 × 10⁵ V cm^?1. The reported strategy can be extended to other 2D semiconductors on patternable substrates for designed fabrication of in‐plane junctions.     

Organic Ferroelectric‐Based 1T1T Random Access Memory Cell Employing a Common Dielectric Layer Overcoming the Half‐Selection Problem

Organic electronics based on poly(vinylidenefluoride/trifluoroethylene) (P(VDF‐TrFE)) dielectric is facing great challenges in flexible circuits. As one indispensable part of integrated circuits, there is an urgent demand for low‐cost and easy‐fabrication nonvolatile memory devices. A breakthrough is made on a novel ferroelectric random access memory cell (1T1T FeRAM cell) consisting of one selection transistor and one ferroelectric memory transistor in order to overcome the half‐selection problem. Unlike complicated manufacturing using multiple dielectrics, this system simplifies 1T1T FeRAM cell fabrication using one common dielectric. To achieve this goal, a strategy for semiconductor/insulator (S/I) interface modulation is put forward and applied to nonhysteretic selection transistors with high performances for driving or addressing purposes. As a result, high hole mobility of 3.81 cm² V^?1 s^?1 (average) for 2,6‐diphenylanthracene (DPA) and electron mobility of 0.124 cm² V^?1 s^?1 (average) for N ,N ′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDI‐FCN₂) are obtained in selection transistors. In this work, we demonstrate this technology's potential for organic ferroelectric‐based pixelated memory module fabrication.     

3D Gold‐Modified Cerium and Cobalt Oxide Catalyst on a Graphene Aerogel for Highly Efficient Catalytic Formaldehyde Oxidation

A hard template method is used to prepare porous gold‐doped cerium and cobalt oxide (Au‐Ce_xCo_y) materials. A series of 3D Au‐Ce _xCo_y/graphene aerogel (GA) composites is then fabricated by a facile heating method. The obtained catalysts possess a well‐defined structure of ordered arrays of nanotubes and good performance in formaldehyde (HCHO) oxidation. The composition and surface elemental valence states of the catalysts are modulated by the Ce/Co molar ratio. The Au‐Ce_xCo_y catalyst and graphene oxide sheets are well compounded within 60 s through a diamine cross‐linker to form 3D Au‐Ce_xCo_y/GA composites. In addition, the resulting catalyst of 3 wt% Au‐Ce₃Co/GA achieves ≈55% conversion at room temperature and 100% conversion when the reaction temperature is raised at 60 °C. The synergistic effect between CeO₂ and Co₃O₄ promotes the migration of oxygen species and the activation of Au, which facilitates HCHO oxidation. The method used to prepare the 3D catalyst could be used to produce other catalytic materials with good replication of the template. In addition, these findings provide a simple method for rapid fabrication of catalyst/GA composites. The superior activity and stability of the 3D Au‐Ce₃Co/GA catalyst make it potentially applicable in HCHO removal.     

Graphene-Quantum-Dots-Induced Centimeter-Sized Growth of Monolayer Organic Crystals for High-Performance Transistors

Monolayer organic crystals have attracted considerable attention due to their extraordinary optoelectronic properties. Solution self-assembly on the surface of water is an effective approach to fabricate monolayer organic crystals. However, due to the difficulties in controlling the spreading of organic solution on the water surface and the weak intermolecular interaction between the organic molecules, large-area growth of monolayer organic crystals remains a great challenge. Here, a graphene quantum dots (GQDs)-induced self-assembly method for centimeter-sized growth of monolayer organic crystals on a GQDs solution surface is reported. The spreading area of the organic solution can be readily controlled by tuning the pH value of the GQDs solution. Meanwhile, the π–π stacking interaction between the GQDs and the organic molecules can effectively reduce the nucleation energy of the organic molecules and afford a cohesive force to bond the crystals, enabling large-area growth of monolayer organic crystals. Using 2,7-didecyl benzothienobenzothiopene (C₁₀-BTBT) as an examples, centimeter-sized monolayer C₁₀-BTBT crystal with uniform molecular packing and crystal orientation is attained. Organic field-effect transistors based on the monolayer C₁₀-BTBT crystals exhibit a high mobility up to 2.6 cm² V⁻¹ s⁻¹, representing the highest mobility value for solution-assembled monolayer organic crystals. This work provides a feasible route for large-scale fabrication of monolayer organic crystals toward high-performance organic devices.     

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet

Monolayer VSe₂, featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition‐metal dichalcogenides (2D‐TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X‐ray and angle‐resolved photoemission, and X‐ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T‐phase and vanadium 3d¹ electronic configuration in monolayer VSe₂ grown on graphite by molecular‐beam epitaxy. Element‐specific X‐ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe₂ as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long‐range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic‐scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D‐TMDs in the search for exotic low‐dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.     

Atom‐by‐Atom Fabrication of Monolayer Molybdenum Membranes

Fabrication of materials in the monolayer regime to acquire fascinating physical properties has attracted enormous interest during the past decade, and remarkable success has been achieved for layered materials adopting weak interlayer van der Waals forces. However, the fabrication of monolayer metal membranes possessing strong intralayer bonding remains elusive. Here, suspended monolayer Mo membranes are fabricated from monolayer MoSe₂ films via selective electron beam (e‐beam) ionization of Se atoms by scanning transmission electron microscopy (STEM). The nucleation and subsequent growth of the Mo membranes are triggered by the formation and aggregation of Se vacancies as seen by atomic resolution sequential STEM imaging. Various novel structural defects and intriguing self‐healing characteristics are unveiled during the growth. In addition, the monolayer Mo membrane is highly robust under the e‐beam irradiation. It is likely that other metal membranes can be fabricated in a similar manner, and these pure metal‐based 2D materials add to the diversity of 2D materials and introduce profound novel physical properties.     

Colloidally Stable Monolayer Nanosheets with Colorimetric Responses

Despite the discovery of chromogenic‐layered materials for decades of years, fabrication of colloidally stable monolayer organic 2D nanosheets in aqueous media with colorimetric responses is still challenging. Herein reported is the first solution synthesis of chromic monolayer nanosheets via the topochemical polymerization of self‐assembled amphiphilic diacetylenes in aqueous media. The polydiacetylene (PDA) nanosheets are ≈3–4 nm thick in solution and only ≈1.9 nm thick in the dried state, while the lateral size can reach several micrometers. Moreover, the aqueous stability endows PDA nanosheets with excellent processability, which can further assemble into films via vacuum filtration or act as an ink for high‐resolution inkjet printing. The filtrated films and printed patterns exhibit fully reversible blue‐to‐red thermochromism, and the film also displays an interesting reversible colorimetric transition in response to near‐infrared light, which is not reported for other PDA‐only systems. The present colloidal PDA nanosheets should represent a new kind of chromic organic 2D nanomaterials that may be applied as novel building blocks for developing intelligent hybrid materials and may also find diverse sensing, display and/or anticounterfeiting applications.     

Crystallized Monolayer Semiconductor for Ohmic Contact Resistance,High Intrinsic Gain,and High Current Density

The contact resistance limits the downscaling and operating range of organic field-effect transistors (OFETs). Access resistance through multilayers of molecules and the nonideal metal/semiconductor interface are two major bottlenecks preventing the lowering of the contact resistance. In this work, monolayer (1L) organic crystals and nondestructive electrodes are utilized to overcome the abovementioned challenges. High intrinsic mobility of 12.5 cm² V⁻¹ s⁻¹ and Ohmic contact resistance of 40 Ω cm are achieved. Unlike the thermionic emission in common Schottky contacts, the carriers are predominantly injected by field emission. The 1L-OFETs can operate linearly from V_DS = −1 V to V_DS as small as −0.1 mV. Thanks to the good pinch-off behavior brought by the monolayer semiconductor, the 1L-OFETs show high intrinsic gain at the saturation regime. At a high bias load, a maximum current density of 4.2 µA µm⁻¹ is achieved by the only molecular layer as the active channel, with a current saturation effect being observed. In addition to the low contact resistance and high-resolution lithography, it is suggested that the thermal management of high-mobility OFETs will be the next major challenge in achieving high-speed densely integrated flexible electronics.     

Plasmon Resonance Energy Transfer: Coupling between Chromophore Molecules and Metallic Nanoparticles

Plasmon resonance energy transfer (PRET) from a single metallic nanoparticle to the molecules adsorbed on its surface has attracted more and more attentions in recent years. Here, a molecular beacon (MB)‐regulated PRET coupling system composed of gold nanoparticles (GNPs) and chromophore molecules has been designed to study the influence of PRET effect on the scattering spectra of GNPs. In this system, the chromophore molecules are tagged to the 5′‐end of MB, which can form a hairpin structure and modified on the surface of GNPs by its thiol‐labeled 3′‐end. Therefore, the distance between GNPs and chromophore molecules can be adjusted through the open and close of the MB loop. From the peak shift, the PRET interactions of different GNPs‐chromophore molecules coupling pairs have been calculated by discrete dipole approximation and the fitting results match well with the experimental data. Therefore, the proposed system has been successfully applied for the analysis of PRET situation between various metallic nanoparticles and chromophore molecules, and provides a useful tool for the potential application in screening the PRET‐based nanoplasmonic sensors.     

Active Sites Engineering toward Superior Carbon‐Based Oxygen Reduction Catalysts via Confinement Pyrolysis

Developing efficient and low‐cost defective carbon‐based catalysts for the oxygen reduction reaction (ORR) is essential to metal–air batteries and fuel cells. Active sites engineering toward these catalysts is highly desirable but challenging to realize boosted catalytic performance. Herein, a sandwich‐like confinement route to achieve the controllable regulation of active sites for carbon‐based catalysts is reported. In particular, three distinct catalysts including metal‐free N‐doped carbon (NC), single Co atoms dispersed NC (Co–N–C), and Co nanoparticles‐contained Co–N–C (Co/Co–N–C) are controllably realized and clearly identified by synchrotron radiation‐based X‐ray spectroscopy. Electrochemical measurements suggest that the Co/Co–N–C catalyst delivers optimized ORR performance due to the rich Co–N_x active sites and their synergistic effect with metallic Co nanoparticles. This work provides deep insight for rationally designing efficient ORR catalyst based on active sites engineering.