Efficient Directed Energy Transfer through Size‐Gradient Nanocrystal Layers into Silicon Substrates

Spectroscopic evidence of directed excitonic energy transfer (ET) is presented through size‐gradient CdSe/ZnS nanocrystal quantum dot (NQD) layers into an underlying Si substrate. NQD monolayers are chemically grafted on hydrogen‐terminated Si surfaces via a self‐assembled monolayer of amine modified carboxy‐alkyl chains. Subsequent NQD monolayers are linked with short alkyldiamines. The linking approach enables accurate positioning and enhanced passivation of the layers. Two different sizes of NQDs (energy donors emitting at 545 nm, and energy acceptors emitting at 585 nm) are used in comparing different monolayer and bilayer samples grafted on SiO₂ and oxide‐free Si surfaces via time‐resolved photoluminescence measurements. The overall efficiency of ET from the top‐layer donor NQDs into Si is estimated to approach ≈90% through a combination of different energy relaxation pathways. These include sequential ET through the intermediate acceptor layer realized mainly via the non‐radiative mechanism and direct ET into the Si substrate realized by means of the radiative coupling. The experimental observations are quantitatively rationalized by the theoretical modeling without introducing any extraneous energy scavenging processes. This indicates that the linker‐assisted fabrication enables the construction of defect‐free, bandgap‐gradient multilayer NQD/Si hybrid structures suitable for thin‐film photovoltaic applications.     

High‐Nanofiller‐Content Graphene Oxide–Polymer Nanocomposites via Vacuum‐Assisted Self‐Assembly

Highly ordered, homogeneous polymer nanocomposites of layered graphene oxide are prepared using a vacuum‐assisted self‐assembly (VASA) technique. In VASA, all components (nanofiller and polymer) are pre‐mixed prior to assembly under a flow, making it compatible with either hydrophilic poly(vinyl alcohol) (PVA) or hydrophobic poly(methyl methacrylate) (PMMA) for the preparation of composites with over 50 wt% filler. This process is complimentary to layer‐by‐layer assembly, where the assembling components are required to interact strongly (e.g., via Coulombic attraction). The nanosheets within the VASA‐assembled composites exhibit a high degree of order with tunable intersheet spacing, depending on the polymer content. Graphene oxide–PVA nanocomposites, prepared from water, exhibit greatly improved modulus values in comparison to films of either pure PVA or pure graphene oxide. Modulus values for graphene oxide–PMMA nanocomposites, prepared from dimethylformamide, are intermediate to those of the pure components. The differences in structure, modulus, and strength can be attributed to the gallery composition, specifically the hydrogen bonding ability of the intercalating species     

Quantum‐Dot‐Functionalized Poly(styrene‐co‐acrylic acid) Microbeads: Step‐Wise Self‐Assembly,Characterization, and Applications for Sub‐femtomolar Electrochemical Detection of DNA Hybridization

A novel nanoparticle label capable of amplifying the electrochemical signal of DNA hybridization is fabricated by functionalizing poly(styrene‐co‐acrylic acid) microbeads with CdTe quantum dots. CdTe‐tagged polybeads are prepared by a layer‐by‐layer self‐assembly of the CdTe quantum dots (diameter = 3.07 nm) and polyelectrolyte on the polybeads (diameter = 323 nm). The self‐assembly procedure is characterized using scanning and transmission electron microscopy, and X‐ray photoelectron, infrared and photoluminescence spectroscopy. The mean quantum‐dot coverage is (9.54 ± 1.2) × 10³ per polybead. The enormous coverage and the unique properties of the quantum dots make the polybeads an effective candidate as a functionalized amplification platform for labelling of DNA or protein. Herein, as an example, the CdTe‐tagged polybeads are attached to DNA probes specific to breast cancer by streptavidin–biotin binding to construct a DNA biosensor. The detection of the DNA hybridization process is achieved by the square‐wave voltammetry of Cd²⁺ after the dissolution of the CdTe tags with HNO₃. The efficient carrier‐bead amplification platform, coupled with the highly sensitive stripping voltammetric measurement, gives rise to a detection limit of 0.52 fmol L^?1 and a dynamic range spanning 5 orders of magnitude. This proposed nanoparticle label is promising, exhibits an efficient amplification performance, and opens new opportunities for ultrasensitive detection of other biorecognition events.     

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.     

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.     

Ultra‐Thin Self‐Assembled Protein‐Polymer Membranes: A New Pore Forming Strategy

Self‐assembled membranes offer a promising alternative for conventional membrane fabrication, especially in the field of ultrafiltration. Here, a new pore‐making strategy is introduced involving stimuli responsive protein‐polymer conjugates self‐assembled across a large surface area using drying‐mediated interfacial self‐assembly. The membrane is flexible and assembled on porous supports. The protein used is the cage protein ferritin and resides within the polymer matrix. Upon denaturation of ferritin, a pore is formed which intrinsically is determined by the size of the protein and how it resides in the matrix. Due to the self‐assembly at interfaces, the membrane constitutes of only one layer resulting in a membrane thickness of 7 nm on average in the dry state. The membrane is stable up to at least 50 mbar transmembrane pressure, operating at a flux of about 21 000–25 000 L m^?2 h^?1 bar^?1 and displayed a preferred size selectivity of particles below 20 nm. This approach diversifies membrane technology generating a platform for “smart” self‐assembled membranes.     

Nanoscale Refractive Index Tuning of Siloxane‐Based Self‐Assembled Electro‐Optic Superlattices

The refractive indices of self‐assembled organic electro‐optic superlattices can be tuned by intercalating high‐Z optically transparent group 13 metal oxide sheets into the structures during the self‐assembly process. Microstructurally regular acentricity and sizable electro‐optic responses are retained in this straightforward synthetic procedure. This “one‐pot” all wet‐chemistry approach involves: i) layer‐by‐layer covalent self‐assembly of intrinsically acentric multilayers of high‐hyperpolarizability chromophores on inorganic oxide substrates, ii) protecting group cleavage to generate a large density of reactive surface hydroxyl sites, iii) self‐limiting capping of each chromophore layer with octachlorotrisiloxane, iv) deposition of metal oxide sheets derived from THF solutions of Ga(OⁱC₃H₇)₃ or In(OⁱC₃H₇)₃, and v) covalent capping of the resulting superlattices.     

Integrative Self‐Assembly of Graphene Quantum Dots and Biopolymers into a Versatile Biosensing Toolkit

Hybrid self‐assembly has become a reliable approach to synthesize soft materials with multiple levels of structural complexity and synergistic functionality. In this work, photoluminescent graphene quantum dots (GQDs, 2–5 nm) are used for the first time as molecule‐like building blocks to construct self‐assembled hybrid materials for label‐free biosensors. Ionic self‐assembly of disc‐shaped GQDs and charged biopolymers is found to generate a series of hierarchical structures that exhibit aggregation‐induced fluorescence quenching of the GQDs and change the protein/polypeptide secondary structure. The integration of GQDs and biopolymers via self‐assembly offers a flexible toolkit for the design of label‐free biosensors in which the GQDs serve as a fluorescent probe and the biopolymers provide biological function. The versatility of this approach is demonstrated in the detection of glycosaminoglycans (GAGs), pH, and proteases using three strategies: 1) competitive binding of GAGs to biopolymers, 2) pH‐responsive structural changes of polypeptides, and 3) enzymatic hydrolysis of the protein backbone, respectively. It is anticipated that the integrative self‐assembly of biomolecules and GQDs will open up new avenues for the design of multifunctional biomaterials with combined optoelectronic properties and biological applications.     

Self‐Assembled Graphene–Enzyme Hierarchical Nanostructures for Electrochemical Biosensing

The self‐assembly of sodium dodecyl benzene sulphonate (SDBS) functionalized graphene sheets (GSs) and horseradish peroxidase (HRP) by electrostatic attraction into novel hierarchical nanostructures in aqueous solution is reported. Data from scanning electron microscopy, high‐resolution transmission electron microscopy, and X‐ray diffraction demonstrate that the HRP–GSs bionanocomposites feature ordered hierarchical nanostructures with well‐dispersed HRP intercalated between the GSs. UV‐vis and infrared spectra indicate the native structure of HRP is maintained after the assembly, implying good biocompatibility of SDBS‐functionalized GSs. Furthermore, the HRP–GSs composites are utilized for the fabrication of enzyme electrodes (HRP–GSs electrodes). Electrochemical measurements reveal that the resulting HRP–GSs electrodes display high electrocatalytic activity to H₂O₂ with high sensitivity, wide linear range, low detection limit, and fast amperometric response. These desirable electrochemical performances are attributed to excellent biocompatibility and superb electron transport efficiency of GSs as well as high HRP loading and synergistic catalytic effect of the HRP–GSs bionanocomposites toward H₂O₂. As graphene can be readily non‐covalently functionalized by “designer” aromatic molecules with different electrostatic properties, the proposed self‐assembly strategy affords a facile and effective platform for the assembly of various biomolecules into hierarchically ordered bionanocomposites in biosensing and biocatalytic applications.     

Sub‐Micrometer Zeolite Films on Gold‐Coated Silicon Wafers with Single‐Crystal‐Like Dielectric Constant and Elastic Modulus

A low‐temperature synthesis coupled with mild activation produces zeolite films exhibiting low dielectric constant (low‐k) matching the theoretically predicted and experimentally measured values for single crystals. This synthesis and activation method allows for the fabrication of a device consisting of a b‐oriented film of the pure‐silica zeolite MFI (silicalite‐1) supported on a gold‐coated silicon wafer. The zeolite seeds are assembled by a manual assembly process and subjected to optimized secondary growth conditions that do not cause corrosion of the gold underlayer, while strongly promoting in‐plane growth. The traditional calcination process is replaced with a nonthermal photochemical activation to ensure preservation of an intact gold layer. The dielectric constant (k), obtained through measurement of electrical capacitance in a metal–insulator–metal configuration, highlights the ultralow k ≈ 1.7 of the synthetized films, which is among the lowest values reported for an MFI film. There is large improvement in elastic modulus of the film (E ≈ 54 GPa) over previous reports, potentially allowing for integration into silicon wafer processing technology.     

Eliminating the Trade‐Off between the Throughput and Pattern Quality of Sub‐15 nm Directed Self‐Assembly via Warm Solvent Annealing

The directed self‐assembly (DSA) of block copolymers (BCPs) has been suggested as a promising nanofabrication solution. However, further improvements of both the pattern quality and manufacturability remain as critical challenges. Although the use of BCPs with a high Flory‐Huggins interaction parameter (χ) has been suggested as a potential solution, this practical self‐assembly route has yet to be developed due to their extremely slow self‐assembly kinetics. In this study, it is reported that warm solvent annealing (WSA) in a controlled environment can markedly improve both the self‐assembly kinetics and pattern quality. A means of avoiding the undesirable trade‐off between the quality and formation throughput of the self‐assembled patterns, which is a dilemma which arises when using the conventional solvent vapor treatment, is suggested. As a demonstration, the formation of well‐defined 13‐nm‐wide self‐assembled patterns (3σ line edge roughness of ≈2.50 nm) in treatment times of 0.5 min (for 360‐nm‐wide templates) is shown. Self‐consistent field theory (SCFT) simulation results are provided to elucidate the mechanism of the pattern quality improvement realized by WSA.     

Enhanced Performance of Self‐Assembled Monolayer Field‐Effect Transistors with Top‐Contact Geometry through Molecular Tailoring,Heated Assembly,and Thermal Annealing

Low‐voltage self‐assembled monolayer field‐effect transistors (SAMFETs) that operate under an applied bias of less than ?3 V and a high hole mobility of 10^?2 cm² V^?1 s^?1 are reported. A self‐assembled monolayer (SAM) with a quaterthiophene semiconducting core and a phosphonic acid binding group is used to fabricate SAMFETs on both high‐voltage (AlO_x/300 nm SiO₂) and low‐voltage (HfO₂) dielectric platforms. High performance is achieved through enhanced SAM packing density via a heated assembly process and through improved electrical contact between SAM semiconductor and metal electrodes. Enhanced electrical contact is obtained by utilizing a functional methylthio head group combined with thermal annealing post gold source/drain electrode deposition to facilitate the interaction between SAM and electrode.     

Multi‐Atomic Layers of Metallic Aluminum for Ultralong Life Lithium Storage with High Volumetric Capacity

Metallic aluminum (Al) have been explored as potential anode materials for lithium storage because of its high theoretical capacity (993 mAh g^–1) and low voltage plateaus. Al possesses high electric conductivity, low cost and environmental friendliness. Unfortunately, Al suffers from huge volume change (>100%) during the lithiation/delithiation process, which inevitably results in the pulverization of electrode and rapid capacity decay during cycling processes. To circumvent above issues, a simple but efficient strategy is demonstrated to fabricate free‐standing multi‐atomic layers of metallic Al by harnessing the good ductility of Al under pressure. The resultant multi‐atomic Al layers are ultrathin, ≈3 nm, and have a large aspect ratio. Such unique features enable multi‐atomic Al nanosheets to construct uniform and compact films with graphene. Thus, the hybrid films with different ratios are achieved, in which the notorious volume change of metallic Al can be efficiently circumvented via the good flexibility of graphene, and the density of whole electrode can be significantly enhanced. As a consequence, the optimized multi‐atomic Al layers‐graphene (AlL‐G) film exhibits a very high volumetric capacity of 1089 mAh cm^–3, high‐rate capability and ultralong cycle life up to 20 000 cycles for lithium storage.     

Hierarchical Ordered Dual‐Mesoporous Polypyrrole/Graphene Nanosheets as Bi‐Functional Active Materials for High‐Performance Planar Integrated System of Micro‐Supercapacitor and Gas Sensor

Planar integrated systems of micro‐supercapacitors (MSCs) and sensors are of profound importance for 3C electronics, but usually appear poor in compatibility due to the complex connections of device units with multiple mono‐functional materials. Herein, 2D hierarchical ordered dual‐mesoporous polypyrrole/graphene (DM‐PG) nanosheets are developed as bi‐functional active materials for a novel prototype planar integrated system of MSC and NH₃ sensor. Owing to effective coupling of conductive graphene and high‐sensitive pseudocapacitive polypyrrole, well‐defined dual‐mesopores of ≈7 and ≈18 nm, hierarchical mesoporous network, and large surface area of 112 m² g^?1, the resultant DM‐PG nanosheets exhibit extraordinary sensing response to NH₃ as low as 200 ppb, exceptional selectivity toward NH₃ that is much higher than other volatile organic compounds, and outstanding capacitance of 376 F g^?1 at 1 mV s^?1 for supercapacitors, simultaneously surpassing single‐mesoporous and non‐mesoporous counterparts. Importantly, the bi‐functional DM‐PG‐based MSC‐sensor integrated system represents rapid and stable response exposed to 10–40 ppm of NH₃ after only charging for 100 s, remarkable sensitivity of NH₃ detection that is close to DM‐PG‐based MSC‐free sensor, impressive flexibility with ≈82% of initial response value even at 180°, and enhanced overall compatibility, thereby holding great promise for ultrathin, miniaturized, body‐attachable, and portable detection of NH₃.     

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.     

Evolution‐Based Design of an Injectable Hydrogel

A new class of simple, linear, amphiphilic peptides are developed that have the ability to undergo triggered self‐assembly into self‐supporting hydrogels. Under non‐gelling aqueous conditions, these peptides exist in a random coil conformation and peptide solutions have the viscosity of water. On the addition of a buffered saline solution, the peptides assemble into a β‐sheet rich network of fibrils, ultimately leading to hydrogelation. A family of nine peptides is prepared to study the influence of peptide length and amino acid composition on the rate of self‐assembly and hydrogel material properties. The amino acid composition is modulated by varying residue hydrophobicity and hydrophilicity on the two opposing faces of the amphiphile. The conformation of peptides in their soluble and gel state is studied by circular dichroism (CD), while the resultant material properties of their gels is investigated using oscillatory sheer rheology. One weight percent gels formed under physiological conditions have storage modulus (G′) values that vary from ≈20 to ≈800 Pa, with sequence length and hydrophobic character playing a dominant roll in defining hydrogel rigidity. Based on the structural and functional data provided by the nine‐peptide family members, an optimal sequence, namely LK13, is evolved. LK13 (LKLKLKLKLKLKL‐NH₂) undergoes triggered self‐assembly, affording the most rigid gel of those studied (G′=797 ± 105). It displays shear thin‐recovery behavior, allowing its delivery by syringe and is cytocompatibile as assessed with murine C3H10t1/2 mesenchymal stem cells.     

Tunable Near‐Infrared Organic Nanowire Nanolasers

Organic semiconductor nanowires have inherent advantages, such as amenability to low‐cost, low‐temperature processing, and inherent four‐level energy systems, which will significantly contribute to the organic solid‐state lasers (OSSLs) and miniaturized laser devices. However, the realization of near‐infrared (NIR) organic nanowire lasers is always a big challenge due to the difficultly in fabrication of organic nanowires with diameters of ≈100 nm and material issues such as low photoluminescence quantum efficiency in the red‐NIR region. What is more, the achievement of wavelength‐tunable OSSLs has also encountered enormous challenge. This study first demonstrates the 720 nm NIR lasing with a low lasing threshold of ≈1.4 µJ cm^?2 from the organic single‐crystalline nanowires, which are self‐assembled from small organic molecules of (E )‐3‐(4‐(dimethylamino)‐2‐methoxyphenyl)‐1‐(1‐hydroxynaphthalen‐2‐yl)prop‐2‐en‐1‐one through a facile solution‐phase growth method. Notably, these individual nanowires' Fabry–Pérot cavity can alternatively provide the red‐NIR lasing action at 660 or 720 nm from the 0–1 or 0–2 radiative transition channels, and the single (660 or 720 nm)/dual‐wavelength (660 and 720 nm) laser action can be achieved by modulating the length of these organic nanowires due to the intrinsic self‐absorption. These easily‐fabricated organic nanowires are natural laser sources, which offer considerable promise for coherent light devices integrated on the optics microchip.     

BN–Graphene Composites Generated by Covalent Cross‐Linking with Organic Linkers

Composites of boron nitride (BN) and carboxylated graphene are prepared for the first time using covalent cross‐linking employing the carbodiimide reaction. The BN_1–xG_x (x ≈ 0.25, 0.5, and 0.75) obtained are characterized using a variety of spectroscopic techniques and thermogravimetric analysis. The composites show composition‐dependent electrical resistivity, the resistivity decreasing with increase in graphene content. The composites exhibit microporosity and the x ≈ 0.75 composite especially exhibits satisfactory performance with high stability as an electrode in supercapacitors. The x ≈ 0.75 composite is also found to be a good electrocatalyst for the oxygen reduction reaction in fuel cells.     

Bioinspired,Stimuli‐Responsive,Multifunctional Superhydrophobic Surface with Directional Wetting,Adhesion, and Transport of Water

A novel smart stimuli responsive surface can be fabricated by the subsequent self‐assembly of the graphene monolayer and the TiO₂ nanofilm on various substrates, that is, fabrics, Si wafers, and polymer thin films. Multiscale application property can be achieved from the interfacial interaction between the hierarchical graphene/TiO₂ surface structure and the underlying substrate. The smart surface possesses superhydrophobic property as a result of its hierarchical micro‐ to nanoscale structural roughness. Upon manipulating the UV induced hydrophilic conversion of TiO₂ on graphene/TiO₂ surface, smart surface features, such as tunable adhesiveness, wettability, and directional water transport, can be easily obtained. The existence of graphene indeed enhances the electron–hole pair separation efficiency of the photo‐active TiO₂, as the time required for the TiO₂ superhydrophilic conversion is largely reduced. Multifunctional characteristics, such as gas sensing, droplet manipulation, and self‐cleaning, are achieved on the smart surface as a result of its robust superhydrophobicity, tunable wettability, and high photo‐catalytic activity. It is also revealed that the superhydrophilic conversion of TiO₂ is possibly caused by the atomic rearrangement of TiO₂ under UV radiation, as a structural transformation from {101} to {001} is observed after the UV treatment.     

A Non‐Covalent Approach for Depositing Spatially Selective Materials on Surfaces

We describe a new method for depositing patterned materials, based on non‐covalent trapping of ligands in solvent‐templated nanocavities created in aromatic, self‐assembled monolayer or polymer films. A model has been developed and tested to describe nanocavity formation and the ligand adsorption process, which occurs via ligand exclusion from ambient, aqueous solution into the hydrophobic nanocavities. Ligand adsorption rates and ligand adsorbate reactivity with solution species are governed by ligand size/geometry design factors identified using the model. Spatial control of adsorption is achieved via film photochemical changes that inhibit subsequent ligand adsorption/accessibility (UV or X‐ray) or displacement of entrapped ligands (50 keV electron‐beam) during film patterning. The reactivity of the adsorbed ligand is illustrated by the selective binding of Pd^II species that catalyze electroless metal deposition. Fabrication of high‐resolution (≈ 50 nm), positive‐tone patterns in nickel with acceptable feature‐edge acuity and critical dimension control (≈ 5 %) is demonstrated.