Dr Qianqian Shi

Plasmene is recently defined as 2D arrays of plasmonic nanoparticles, which could be fabricated by the bottom-up self-assembly approach and demonstrated a wide range of applications in sensing, energy harvesting, nanophotonics and encryption. Here, this work further demonstrates a 3D helical plasmonic nanostructures that can be fabricated from 2D plasmene nanosheet. Inspired by chocolate curls-making process, a micro-spatula-based strategy is developed to selectively scrape substrate-supported plasmene to free space, which spontaneously folds the plasmene nanosheet into various complex helical nanostructures with controlled dimensions. 3D nanospirals can also be obtained by focus ion beam (FIB)-based lithography on free-standing plasmene. Helical plasmene structures are robust, exhibiting elastic mechanical properties and chiral optical response. This methodology represents a versatile fabrication route combining both bottom-up and top-down approaches to create soft plasmonic helical structures for potential applications in next-generation flexible nanophotonic devices.

Tissues, which consist of groups of closely packed cell arrays, are essentially sheet-like biosynthesis plants. In tissues, individual cells are discrete microreactors working under highly viscous and confined environments. Herein, soft polystyrene-encased nanoframe (PEN) reactor arrays, as analogous nanoscale ¿sheet-like chemosynthesis plants¿, for the controlled synthesis of novel nanocrystals, are reported. Although the soft polystyrene (PS) is only 3¿nm thick, it is elastic, robust, and permeable to aqueous solutes, while significantly slowing down their diffusion. PEN-associated palladium (Pd) crystallization follows a diffusion-controlled zero-order kinetics rather than a reaction-controlled first-order kinetics in bulk solution. Each individual PEN reactor has a volume in the zeptoliter range, which offers a unique confined environment, enabling a directional inward crystallization, in contrast to the conventional outward nucleation/growth that occurs in an unconfined bulk solution. This strategy makes it possible to generate a set of mono-, bi-, and trimetallic, and even semiconductor nanocrystals with tunable interior structures, which are difficult to achieve with normal systems based on bulk solutions.

Solar-driven hydrogen generation is emerging as an economical and sustainable means of producing renewable energy. However, current photocatalysts for hydrogen generation are mostly powder-based or rigid-substrate-supported, which suffer from limitations, such as difficulties in catalyst regeneration or poor omnidirectional light-harvesting. Here, we report a two-dimensional (2D) flexible photocatalyst based on elastomer-supported black gold nanotube (GNT) arrays with conformal CdS coating and Pt decoration. The highly porous GNT arrays display a strong light-trapping effect, leading to near-complete absorption over almost the entire range of the solar spectrum. In addition, they offer high surface-to-volume ratios promoting efficient photocatalytic reactions. These structural features result in high H2 generation efficiencies. Importantly, our elastomer-supported photocatalyst displays comparable photocatalytic activity even when being mechanically deformed, including bending, stretching, and twisting. We further designed a three-dimensional (3D) tree-like flexible photocatalytic system to mimic Nature's photosynthesis, which demonstrated omnidirectional H2 generation. We believe our strategy represents a promising route in designing next-generation solar-to-fuel systems that rival natural plants.

Chiral metallic nanoparticles can exhibit novel plasmonic circular dichroism (PCD) in the ultraviolet and visible range of the electromagnetic spectrum. Here, we investigate how thermoresponsive dielectric nanoenvironments will influence such PCD responses through poly(N-isopropylacrylamide) (PNIPAM) modified chiral gold nanorods (AuNRs). We observed the temperature-dependent chiral plasmonic responses distinctly from unmodified counterparts. As for the modified systems, the PCD peaks for both l-AuNRs and d-AuNRs at 50 °C red shifted simultaneously with enhanced intensities compared to the results at 20 °C. In contrast, the unmodified l-AuNRs and d-AuNRs exhibited no peak shift with reduced intensities. Subsequent simulation and experimental studies demonstrated that the enhanced PCD was attributed to PNIPAM chain collapse causing the increase of the refractive index by expelling minute water out of the corona surrounding chiral plasmonic AuNRs. Notably, such thermoresponsive chiral plasmonic responses are reversible, general, and extendable to other types of chiral plasmonic nanoparticles.

Time-lapse mechanical properties of stem cell derived cardiac organoids are important biological cues for understanding contraction dynamics of human heart tissues, cardiovascular functions and diseases. However, it remains difficult to directly, instantaneously and accurately characterize such mechanical properties in real-time and in situ because cardiac organoids are topologically complex, three-dimensional soft tissues suspended in biological media, which creates a mismatch in mechanics and topology with state-of-the-art force sensors that are typically rigid, planar and bulky. Here, we present a soft resistive force-sensing diaphragm based on ultrasensitive resistive nanocracked platinum film, which can be integrated into an all-soft culture well via an oxygen plasma-enabled bonding process. We show that a reliable organoid-diaphragm contact can be established by an ¿Atomic Force Microscope-like¿ engaging process. This allows for instantaneous detection of the organoids¿ minute contractile forces and beating patterns during electrical stimulation, resuscitation, drug dosing, tissue culture, and disease modelling.

In parallel to the burgeoning field of soft electronics, soft plasmonics focuses on the design and fabrication of plasmonic structures supported on elastomers and to understand how their properties respond to mechanical deformations. Here, we report on a partial ligand-stripping strategy to fabricate elastomer-supported gold nanobipyramid (NBP) plasmene nanosheets. Unlike spherelike building blocks, NBP-building blocks display complex orientation-dependent plasmonic responses to external strains. By collecting polarized plasmonic resonance spectra in conjunction with electrostatic eigenmode modeling, we reveal simultaneous changes in interparticle spacing and spatial orientations of NBP building blocks under mechanical strains. Such changes are directly related to initial NBP packing orders. Further analysis of strain sensitivities for various NBP plasmenes indicated that plasmonic spectra of ~45° oriented samples are mostly susceptible to strain at acute polarized angles. The results presented may enable novel applications in future soft optoelectronic devices in sensing, encryption, and data storage.

Two-dimensional (2D) plasmonic nanoassemblies are programmable ultrathin materials that one can, in principle, adjust the size and shape of their constituent building blocks to fine-tune collective optical, electrical, and mechanical properties. Here, we report a new 2D nanoassembly from structurally complex plasmonic building blocks, namely, matryoshka-like gold (Au) nanoframes. Using a seed-mediated alternating deposition of Au and silver (Ag) elements in conjunction with a selective etching process, we obtain monodisperse matryoshka-like Au nanoframes with a nesting number (Nn) of up to 5. A cubic nanoframe displays dominant intraplasmonic coupling attributed to bonding and antibonding dipolar modes, which shift to blue and red, respectively, with an increased Nn. Combined with polystyrene (PS) ligand exchange and drying-mediated self-assembly, the approach mentioned above can be used to produce 2D plasmonic matryoshka nanoassemblies. Both experimental and simulation results demonstrate the presence of intra- and inter-ridge plasmonic coupling in the nanoassemblies. The 5-nested nanomatryoshka assemblies exhibit a Raman enhancement 14-fold greater than those with 1-nested cubic nanomatryoshka, demonstrating the dominant intraridge hot spot effects. Taking advantage of interparticle distance-dependent optical transparency of 2D matryoshka nanoframe assemblies, we further demonstrate temperature-enabled encryption/decryption using thermoresponsive polymers.

Epitaxy has been demonstrated to be a powerful technology to precisely engineer strains at the atomic level to modulate semiconductor device properties. However, it is not suitable for strain engineering in nanoscale lattice structures, such as two-dimensional ligand-based plasmonic nanoparticle superlattices (termed as soft plasmene nanosheets). Although unidirectional tensile strain has been applied to soft plasmene nanosheets, strain engineering was not omnidirectional across the entire lattice structures. Here, we report on omnidirectional strain engineering of soft plasmene nanosheets by utilizing thermoresponsive hydrogels in conjunction with temperature programming. Pristine plasmene nanosheets were naturally under compressive strain states upon formation, which could be transferred onto poly(N-isopropylacrylamide) hydrogel. In the model gold@silver nanocube plasmene system, the constituent building blocks could be switched between non-coupling and strongly-coupling states simply by modulating the temperature, corresponding to the highest tensile and compressive strain conditions, respectively. This led to a significantly broad plasmonic spectral shift of over 200 nm. Consequently, it caused drastically different surface enhanced Raman scattering (SERS) enhancements of model Raman dyes, which are reversibly switchable simply by heating and cooling. Our temperature-enabled omnidirectional strain engineering is extendable to other plasmonic nanosheets, indicating its generality and versatility.

A leaf is a free-standing photocatalytic system that can effectively harvest solar energy and convert CO2 and H2O into carbohydrates in a continuous manner without the need for regeneration or tedious product extraction steps. Despite encouraging advances achieved in designing artificial photocatalysts, most of them function in bulk solution or on rigid surfaces. Here, we report on a 2D flexible photocatalytic system based on close packed Janus plasmene nanosheets. One side of the Janus nanosheets is hydrophilic with catalytically active palladium, while the opposite side is hydrophobic with plasmonic nanocrystals. Such a unique design ensures a stable nanostructure on a flexible polymer substrate, preventing dissolution/degradation of plasmonic photocatalysts during chemical conversion in aqueous solutions. Using catalytic reduction of 4-nitrophenol as a model reaction, we demonstrated efficient plasmon-enhanced photochemical conversion on our flexible Janus plasmene. The photocatalytic efficiency could be tuned by adjusting the palladium thickness or types of constituent building blocks or their orientations, indicating the potential for tailor-made catalyst design for desired reactions. Furthermore, the flexible Janus plasmene nanosheets were designed into a small 3D printed artificial tree, which could continuously convert 30 mL of chemicals in 45 minutes.

Natural leaves are virtually two-dimensional (2D) flexible photocatalytic system. In particular, seagrass can efficiently harvest low-intensity sunlight to drive photochemical reactions continuously in an aqueous solution. To mimic this process, we present a novel 2D hydrogel-integrated photocatalytic sheet based on free-standing nanoassemblies of multifunctional nanohexagons (mNHs). The mNHs building blocks is made of plasmonic gold nanohexagons (NHs) decorated with Pd nanoparticles in the corners and CdS nanoparticles throughout their exposed surfaces. The mNHs can self-assemble into free-standing 2D nanoassemblies and be integrated with thin hydrogel films, which can catalyze chemical reactions under visible light illumination. Hydrogels are translucent, porous, and soft, allowing for continuous photochemical conversion in an aqueous environment. Using methylene blue (MB) as a model system, we demonstrate a soft seagrass-like photodegradation design, which offers high efficiency, continuous operation without the need of catalyst regeneration, and omnidirectional light-harvesting capability under low-intensity sunlight irradiation, defying their rigid substrate-supported random aggregates and solution-based discrete counterparts. This journal is

Core¿shell nanocrystals with plasmonic metals as cores and catalytic metals as shells, have recently received considerable attention due to their enhanced catalytic properties under visible light irradiation. However, it remains illusive how sizes/shapes of cores and shells influence catalytic efficiency. Here, using Au@Pd nanocrystals as a model system, key parameters including Au core size/shape and Pd shell thickness are scrutinized to systematically examine how they influence reaction rate. The comparison is based on normalizing Pd amount, which is important from economics standpoint. The results show that the reaction rate decreases with the increase in shell thickness. With the fixed shell thickness, the maximum reaction rate occurs in the smallest core. Among four different core shapes (nanocube, nanorod, nanohexagon, and nanostar), Au@Pd nanorods display the highest reaction rate and the highest plasmonic enhancement factor. These findings may provide a rational route to screen desired plasmonic nanocatalyst with a balanced consideration of efficiency and economics.

Nanotechnology's rapid development of nanostructured materials with disruptive material properties has inspired research for their use as electrocatalysts to potentially substitute enzymes. Herein, a novel electrocatalytic nanomaterial was constructed by growing gold nanograss (AuNG) on 2D nanoassemblies of gold nanocubes (AuNC). The resulting structure (NG@NC) was used for the detection of H2O2 via its electrochemical reduction. The NG@NC electrode displayed a large active surface area, resulting in improved electron transfer efficiency. On the nanoscale, AuNG maintained its structure, providing high stability and reproducibility of the sensing platform. Our nanostructured electrode showed excellent catalytic activity towards H2O2 at an applied potential of -0.5 V vs Ag/AgCl. This facilitated H2O2 detection with excellent selectivity in an environment like human urine, and a linear response from 50 µM to 30 mM, with a sensitivity of 100.66 ± 4.0 µA mM-1 cm-2. The NG@NC-based sensor hence shows great potential in nonenzymatic electrochemical sensing.

Free-standing 2D nanoassemblies are ultrathin nanomembranes or nanosheets constructed from constituent nanoscale building blocks including metal nanoparticles, quantum dots, and magnetic nanoparticles, typically by a bottom-up self-assembly approach. Such free-standing nanoassemblies are a new class of advanced functional materials that can integrate the unique optoelectronic properties of nanomaterials with thin film mechanics into confined 2D space. This offers attributes such as minimizing substrate effects, facile transfer, and soft devices in comparison to the corresponding substrate-supported system. This review covers the recent progress in fabrication, characterization, and application of the free-standing 2D nanoassemblies. To begin with, the attributes of free-standing 2D nanoassemblies are discussed, followed by the description of fabrication methodologies. Then their novel optical, electronic, mechanical, magnetic, and stimuli responsible properties are covered, and their potential applications in filtration membrane, nanomechanical devices, and chemical sensing are further discussed. Finally, perspectives on the challenges and future opportunities of the free-standing 2D nanoassemblies are shared.

Understanding the structure and chemical composition at the liquid-nanoparticle (NP) interface is crucial for a wide range of physical, chemical, and biological processes. In this study, direct imaging of the liquid-NP interface by atom probe tomography (APT) is reported for the first time, which reveals the distributions and the interactions of key atoms and molecules in this critical domain. The APT specimen is prepared by controlled graphene encapsulation of the solution containing nanoparticles on a metal tip, with an end radius in the range of 50 nm to allow field evaporation. Using gold nanoparticles (AuNPs) in suspension as an example, analysis of the mass spectrum and three-dimensional (3D) chemical maps from APT provides a detailed image of the water-gold interface at near-atomic resolution. A locally dense region of Au+ ions has been reconstructed, representing a portion of an individual AuNP. A large number of water-related ions have also been identified, confirming the AuNP in the hydrated state. At the water-gold interface, a trisodium citrate layer has been observed based on Na+ and C-containing ions.

Sweat pH is a key health indicator related to metabolism and homeostasis level through hydrogen ion concentration in biological bio-fluid. Therefore, increasing research efforts have been directed to develop wearable pH sensors towards continuous non-invasive monitoring of sweat pH values in the out-of-hospital environments. Herein, we report a stretchable gold fiber-based electrochemical pH sensor based on our recently developed elastomer-bonded gold nanowire coating technology. The densely packed gold film offers superior strain-insensitive conductivity, high stretchability and large electrochemical active surface area (EASA). By electrodepositing polyamine (PANI) and Ag/AgCl onto the gold fibers, we could selectively detect the pH based on open circuit potentials in an ion-selective electrode design. The obtained fiber-based pH sensors feature a great sensitivity (60.6 mV per pH), high selectivity against cationic interference and high stretchability (up to 100% strain). One of the attributes for the fiber-based sensors is that they can be weaved into textiles, holding great potential for integration into everyday clothing for ¿unfeelable¿ personal health monitoring.

Skin-like optoelectronic sensors can have a wide range of technical applications ranging from wearable/implantable biodiagnostics, human-machine interfaces, and soft robotics to artificial intelligence. The previous focus has been on electrical signal transduction, whether resistive, capacitive, or piezoelectric. Here, we report on "optical skin"strain sensors based on elastomer-supported, highly ordered, and closely packed plasmonic nanocrystal arrays (plasmene). Using gold nanocubes (AuNCs) as a model system, we find that the types of polymeric ligands, interparticle spacing, and AuNC sizes play vital roles in strain-induced plasmonic responses. In particular, brush-forming polystyrene (PS) is a "good"ligand for forming elastic plasmenes which display strain-induced blue shift of high-energy plasmonic peaks with high reversibility upon strain release. Further experimental and simulation studies reveal the transition from isotropic uniform plasmon coupling at a non-strained state to anisotropic plasmon coupling at strained states, due to the AuNC alignment perpendicular to the straining direction. The two-term plasmonic ruler model may predict the primary high-energy peak location. Using the relative shift of the averaged high-energy peak to the coupling peak before straining, a plasmene nanosheet may be used as a strain sensor with the sensitivity depending on its internal structures, such as the constituent AuNC size or inter-particle spacing.

Invited for this month's cover are the collaborating groups of Prof. Xiaojun Han from Harbin Institute of Technology, China and Prof. Wenlong Cheng from Monash University, Australia. The cover picture shows how a stretchable sensor made from a 3D structure comprising mesostructured silica fibers decorated with gold nanowires exhibits reliable resistance signals for real-time response to radial artery blood pulses. This work reinforces the idea of smart material hybridization and may offer flexible materials for real-time, in situ health monitoring applications. Read the full text of the article at 10.1002/cplu.201900043.

Development of high-performance fiber-shaped wearable sensors is of great significance for next-generation smart textiles for real-time and out-of-clinic health monitoring. The previous focus has been mainly on monitoring physical parameters such as pressure and strains associated with human activities. Development of an enzyme-based non-invasive wearable electrochemical sensor to monitor biochemical vital signs of health such as the glucose level in sweat has attracted increasing attention recently, due to the unmet clinical needs for the diabetic patients. To achieve this, the key challenge lies in the design of a highly stretchable fiber with high conductivity, facile enzyme immobilization, and strain-insensitive properties. Herein, we demonstrate an elastic gold fiber-based three-electrode electrochemical platform that can meet the aforementioned criteria toward wearable textile glucose biosensing. The gold fiber could be functionalized with Prussian blue and glucose oxidase to obtain the working electrode and modified by Ag/AgCl to serve as the reference electrode; and the nonmodified gold fiber could serve as the counter electrode. The as-fabricated textile glucose biosensors achieved a linear range of 0-500 µM and a sensitivity of 11.7 µA mM-1 cm-2. Importantly, such sensing performance could be maintained even under a large strain of 200%, indicating the potential applications in real-world wearable biochemical diagnostics from human sweat.

2D freestanding nanocrystal superlattices represent a new class of advanced metamaterials in that they can integrate mechanical flexibility with novel optical, electrical, plasmonic, and magnetic properties into one multifunctional system. The freestanding 2D superlattices reported to date are typically constructed from symmetrical constituent building blocks, which have identical structural and functional properties on both sides. Here, a general ligand symmetry-breaking strategy is reported to grow 2D Janus gold nanocrystal superlattice sheets with nanocube morphology on one side yet with nanostar on the opposite side. Such asymmetric metallic structures lead to distinct wetting and optical properties as well as surface-enhanced Raman scattering (SERS) effects. In particular, the SERS enhancement of the nanocube side is about 20-fold of that of the nanostar side, likely due to the combined ¿hot spot + lightening-rod¿ effects. This is nearly 700-fold of SERS enhancement as compared with the symmetric nanocube superlattices without Janus structures.

Freestanding plasmonic nanoparticle (NP) superlattice sheets are novel 2D nanomaterials with tailorable properties that enable their use for broad applications in sensing, anticounterfeit measures, ionic gating, nanophotonics and flat lenses. We recently developed a robust, yet general, two-step drying-mediated approach to produce freestanding monolayer, plasmonic NP superlattice sheets, which are typically held together by holey grids with minimal solid support. Within these superlattices, NP building blocks are closely packed and have strong plasmonic coupling interactions; hence, we termed such freestanding materials ¿plasmene nanosheets¿. Using the desired NP building blocks as starting material, we describe the detailed fabrication protocol, including NP surface functionalization by thiolated polystyrene and the self-assembly of NPs at the air¿water interface. We also discuss various characterization approaches for checking the quality and optical properties of the as-obtained plasmene nanosheets: optical microscopy, spectrophotometry, transmission/scanning electron microscopy (TEM/SEM) and atomic force microscopy (AFM). With regard to different constituent building blocks, the key experimental parameters, including NP concentration and volume, are summarized to guide the successful fabrication of specific types of plasmene nanosheets. This protocol, from initial NP synthesis to the final fabrication and characterization, takes ~33.5 h.

Droplets suspended by acoustic levitation provide genuine substrate-free environments for understanding unconventional fluid dynamics, evaporation kinetics, and chemical reactions by circumventing solid surface and boundary effects. Using a fully levitated air-water interface by acoustic levitation in conjunction with drying-mediated nanoparticle self-assembly, here, we demonstrate a general approach to fabricating free-standing nanoassemblies, which can totally avoid solid surface effects during the entire process. This strategy has no limitation for the sizes or shapes of constituent metallic nanoparticle building blocks and can also be applied to fabricate free-standing bilayered and trilayered nanoassemblies or even three-dimensional hollow nanoassemblies. We believe that our strategy may be further extended to quantum dots, magnetic particles, colloids, etc. Hence, it may lead to a myriad of homogeneous or heterogeneous free-standing nanoassemblies with programmable functionalities.

Thiol-polystyrene (SH-PS)-capped plasmonic nanoparticles can be fabricated into free-standing, one-nanoparticle-thick superlattice sheets (termed plasmene) based on physical entanglement between ligands, which, however, suffer from irreversible dissociation in organic solvents. To address this issue, we introduce coumarin-based photo-cross-linkable moieties to the SH-PS ligands to stabilize gold nanoparticles. Once cross-linked, the obtained plasmene nanosheets consisting of chemically locked nanoparticles can well maintain structural integrity in organic solvents. Particularly, arising from ligand-swelling-induced enlargement of the interparticle spacing, these plasmene nanosheets show significant optical responses to various solvents in a specific as well as reversible manner, which may offer an excellent material for solvent sensing and dynamic plasmonic display.

The ability to control the site-selective deposition of a noble metal with nanoscale accuracy is vital for the synthesis of well-defined heterogeneous core-shell bimetallic nanoparticles for various applications ranging from nanophotonics to catalysis. Here, precise site-specific Ag coating onto concave gold nanoarrows (GNAs) is reported by tuning the concentration of the surfactant-cetyltrimethylammonium chloride (CTAC). Three distinct nanocoating structures, namely, anisotropic coating, middle coating, and conformal coating are obtained, which are achieved under low, medium and high CTAC concentrations, respectively. The site-specific nanoscale coating on GNAs is proved by scanning transmission electron microscopy imaging in conjunction with the elemental mapping. The CTAC concentration-dependent, facet-specific passivation may be the cause for the three distinct nanoparticles obtained. The morphology differences resulted in discrete plasmonic features, and a linear relationship between the resonance peak and the CTAC concentration is found for the conformal-coated GNAs. We further fabricate free-standing monolayer nanosheets out of the three kinds of nanoparticles, which display strong shape-dependent SERS enhancements.

Light-matter interactions are extremely important, as they sustain life on Earth and can be tailored for diverse applications in areas such as solar energy conversion, chemical sensing, and information storage. One key process of these interactions is the absorption of photons. We demonstrate a novel material capable of absorbing up to 98% of incident visible light. The material comprises a thin sheet of a tightly packed two-dimensional lattice of metal nanoparticles, called plasmene, supported by a thin (subwavelength) dielectric film deposited on top of a mirror. We demonstrate how the resulting metasurface absorbers are useful in surface-enhanced spectroscopy and in the generation of plasmonic hot carriers. These structures hold great promise for applications in structural color, sensing, and photocatalysis.

Ultrathin Fresnel lens may revolutionize current optical imaging system, leading to thinner and lighter optoelectronic devices with a myriad of technical applications. To date, evaporated bulk metal films and top-down grown graphene represent viable material choices toward the design of ultrathin Fresnel lenses. Despite recent advances, it is still lack of a scalable fabrication strategy to achieve ultrathin lens with high focusing efficiency. Here, we report a new self-assembled metamaterials-based strategy to design ultrathin Fresnel lens using our recently reported plasmene nanosheets. With comparable thickness, our plasmene-based Fresnel lens offers a much better focusing efficiency than that based on continuous metallic films. This may be attributed to the dual Huygens¿ effects from both slits and plasmene-constituent nanoparticle building blocks. Importantly, internal structural features of plasmene can be precisely tuned simply by adjusting sizes and shapes of its constituent building blocks, allowing for maximizing the focusing efficiency at a desired operating wavelength ¿ a capability impossible to achieve with continuous metal films or graphene. Our plasmene-based strategy opens a new route to design tailor-made flat lens with finely tunable internal and overall structural properties, which offers new dimensionalities in controlling light-matter interactions for a myriad of technological applications.

Thin, skin-conformal, transparent and stretchable energy devices are ideal for powering future wearable and implantable electronics. However, it is difficult to achieve such "unfeelable" and "invisible" devices with traditional materials and design methodologies because of the challenge of simultaneously achieving high optical transparency, high electrical conductivity and high mechanical stretchability. Here, we report a two-step nanowire growth approach for fabricating gold nanorime mesh conductors, enabling skin-thin, transparent and stretchable supercapacitors. Solution-state oleylamine-capped 2 nm-thin gold nanowires self-assemble into highly transparent nanomeshes, which then serve as templates for growing highly conductive vertically aligned nanowires. This two-step solution-plus-surface nanowire growth strategy leads to elastic gold nanorime mesh conductors with an optical transparency up to 90.3% at 550 nm, a low sheet resistance as low as 1.7 ± 0.8 O sq-1, and a stretchability of over 100% strain. Such elastic conductors are successfully used to construct symmetrical supercapacitors that can simultaneously achieve high areal capacitance and high stretchability, demonstrating the potential to power future bio-integratable electronics.

Human skin can sense an external object in a location-specific manner, simultaneously recognizing whether it is sharp or blunt. Such tactile capability can be achieved in both natural and stretched states. It is impractical to mimic this tactile function of human skin by designing pixelated sensor arrays across our whole curvilinear human body. Here, we report a new tactile electronic skin sensor based on staircase-like vertically aligned gold nanowires (V-AuNWs). With a back-to-back linear or spiral assembly of two staircase structures into a single sensor, we are able to recognize pressure in a highly location-specific manner for both non-stretched and stretched states (up to 50% strain); with a concentric design on the fingertip, we can identify the sharpness of an external object. We believe that our strategy opens up a new route to highly specific second-skin-like tactile sensors for electronic skin (E-skin) applications.

The electronic, optical, thermal, and magnetic properties of an extrinsic bulk semiconductor can be finely tuned by adjusting its dopant concentration. Here, it is demonstrated that such a doping concept can be extended to plasmonic nanomaterials. Using two-dimensional (2D) assemblies of Au@Ag and Au nanocubes (NCs) as a model system, detailed experimental and theoretical studies are carried out, which reveal collective semiconductor n/p-doping-like plasmonic properties. A threshold doping concentration of Au@Ag NCs is observed, below which p-doping dominates and above which n-doping prevails. Furthermore, Au@Ag NC dopants can be converted into corresponding Au seed ¿voids¿ dopants by selectively removing Ag without changing the overall structural integrity. The results show that the plasmonic doping concept may serve as a general design principle guiding synthesis and assembly of plasmonic metamaterials for programmable optoelectronic devices.

Self-assembly of nanoparticles represents a simple yet efficient route to synthesize designer materials with unusual properties. However, the previous assembled structures whether by surfactants, polymer, or DNA ligands are "static" or "frozen" building block structures. Here, we report the growth of transformable self-assembled nanosheets which could enable reversible switching between two types of nanosheets and even evolving into diverse third generation nanosheet structures without losing pristine periodicity. Such in situ transformation of nanoparticle building blocks can even be achieved in a free-standing two-dimensional system and three-dimensional origami. The success in such in situ nanocrystal transformation is attributed to robust "plant-cell-wall-like" ion-permeable reactor arrays from densely packed polymer ligands, which spatially define and confine nanoscale nucleation/growth/etching events. Our strategy enables efficient fabrication of nanocrystal nanosheets with programmable building blocks for innovative applications in adaptive tactile metamaterials, optoelectronic devices, and sensors.

Nanoparticles were called "artificial atoms" about two decades ago due to their ability to organize into regular lattices or supracrystals. Their self-assembly into free-standing, two-dimensional (2D) nanoparticle arrays enables the generation of 2D metamaterials for novel applications in sensing, nanophotonics and energy fields. However, their controlled fabrication is nontrivial due to the complex nanoscale forces among nanoparticle building blocks. Here, we report a new type of 2D plasmonic superlattice from high-index gold trisoctahedron (TOH) nanoparticles. TOH is an anisotropic polyhedron with 24 facets and 14 vertices. By using polymer ligands in conjunction with drying-mediated self-assembly, we obtained highly ordered 2D superlattices as quantified by synchrotron based grazing-incidence small-angle X-ray scattering (GISAXS). The plasmonic properties were optimized by adjusting the ligand length and particle size. The excellent surface-enhanced Raman scattering (SERS) performance enables us to demonstrate TOH superlattices as uniform SERS immunosubstrates with a detection limit down to 1 pg ml-1 and a dynamic range from 1 pg ml-1 to 100 ng ml-1.

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine-tune the interparticle potential as well as program lattice structures and interparticle distances ¿ the two key parameters governing superlattice properties. This article aims to review the up-to-date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft-ligand-based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Thermosensitive polymer capped plasmonic nanoparticles are novel thermal nanofluids with potential sensing applications. Previous research efforts have been focused only on monitoring plasmonic resonance peak shifts caused by aggregation as temperature varied. However, to date, no linear relationship between the resonance peak shift and temperature has been established. Here, we systematically investigate how plasmonic resonance peak intensity responds to solution temperature using poly(N-isopropylacrylamide)-capped gold nanorods (AuNRs) and nanobipyramids (AuNBPs) under aggregation-free conditions. Our results clearly reveal the linear correlation between longitudinal resonance peak intensity and solution temperature for both types of particles. AuNBPs have sharper ends than AuNRs, resulting in greater thermo-sensitivity due to the presence of stronger 'hot spots'. Further analytical and numerical studies demonstrate chemical interface damping effects by surface-capping ligand configurational changes and these theoretical results agree well with our experimental observations. In addition, this damping-based sensing is reversible with excellent durability, indicating the possibility of potential real-world temperature sensing applications.

High transparency, conductivity and stretchability are very difficult to achieve simultaneously in a single type of capacitance device. Usually, high optical transparency requires very thin conductive film, which will inevitably cause cracking or breakage when a strain is applied. In this case, most of the currently developed electrodes are either transparent or stretchable but not both. Herein, we report a simple yet efficient method to fabricate self-assembled monolayer gold nanowires (AuNWs) conductive thin film. Such film is both transparent and stretchable due to the unique flexible hairy structure of ultrathin AuNWs (2 nm in diameter, aspect ratio >10,000). In addition, the resultant AuNWs film could be used as nanostructured electrode for transparent and stretchable supercapacitors. The entire supercapacitor showed a high transparency of 79 % at the wavelength of 550 nm, and could be stretched up to 30 % strain without degradation over 80 stretching cycles.

Anisotropic plasmonic nanoparticles have been successfully used as constituent elements for growing ordered nanoparticle arrays. However, orientational control over their spatial ordering remains challenging. Here, we report on a self-assembled two-dimensional (2D) nanoparticle liquid crystalline superstructure (NLCS) from bipyramid gold nanoparticles (BNPs), which showed four distinct orientational packing orders, corresponding to horizontal alignment (H-NLCS), circular arrangement (C-NLCS), slanted alignment (S-NLCS), and vertical alignment (V-NLCS) of constituent particle building elements. These packing orders are characteristic of the unique shape of BNPs because all four packing modes were observed for particles with various sizes. Nevertheless, only H-NLCS and V-NLCS packing orders were observed for the free-standing ordered array nanosheets formed from a drying-mediated self-assembly at the air/water interface of a sessile droplet. This is due to strong surface tension and the absence of particle-substrate interaction. In addition, we found the collective plasmonic coupling properties mainly depend on the packing type, and characteristic coupling peak locations depend on particle sizes. Interestingly, surface-enhanced Raman scattering (SERS) enhancements were heavily dependent on the orientational packing ordering. In particular, V-NLCS showed the highest Raman enhancement factor, which was about 77-fold greater than the H-NLCS and about 19-fold greater than C-NLCS. The results presented here reveal the nature and significance of orientational ordering in controlling plasmonic coupling and SERS enhancements of ordered plasmonic nanoparticle arrays.

Transparent electrodes simultaneously require high electrical conductivity and high optical transparency, which have been achieved with mesh metal structures. However, most currently fabricated micro- and nanoelectronic devices are produced via top-down lithography methods. Here, a bottom-up self-assembly approach to fabricate mesh electrode using ultrathin gold nanowires (AuNWs) at the air/water interface is reported. Slow partial ligand removal during the aging process is the key for the formation of such self-assembled mesh structures. The resulting mesh film has a typical mesh pore size of 8¿52 µm, with a sheet resistance of ¿40 times smaller than our previously reported nonmeshed AuNWs electrodes under similar optical transmittance. Our self-assembled mesh electrodes are mechanically flexible, easily transferrable to a variety of substrates, and also patternable by marker pen lithography or cutting machine lithography, and washable, and can be directly used for touch screen and flexible interconnects for light-emitting devices. The entire fabrication process is under ambient conditions without the need for any special equipment. These attributes indicate the potential applications of self-assembled gold mesh electrodes in flexible solar cells, touch screen displays, and wearable electronics.