Conventional imaging and recognition systems require an extensive amount of data storage, pre-processing, and chip-to-chip communications as well as aberration-proof light focusing with multiple lenses for recognizing an object from massive optical inputs. This is because separate chips (i.e., flat image sensor array, memory device, and CPU) in conjunction with complicated optics should capture, store, and process massive image information independently. In contrast, human vision employs a highly efficient imaging and recognition process. Here, inspired by the human visual recognition system, we present a novel imaging device for efficient image acquisition and data pre-processing by conferring the neuromorphic data processing function on a curved image sensor array. The curved neuromorphic image sensor array is based on a heterostructure of MoS2 and poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane). The curved neuromorphic image sensor array features photon-triggered synaptic plasticity owing to its quasi-linear time-dependent photocurrent generation and prolonged photocurrent decay, originated from charge trapping in the MoS2-organic vertical stack. The curved neuromorphic image sensor array integrated with a plano-convex lens derives a pre-processed image from a set of noisy optical inputs without redundant data storage, processing, and communications as well as without complex optics. The proposed imaging device can substantially improve efficiency of the image acquisition and recognition process, a step forward to the next generation machine vision.

Interlayer excitons in heterobilayers of transition-metal dichalcogenides (TMDCs) have generated enormous interest due to their permanent vertical dipole moments and long lifetimes. However, the effects of mechanical strain on the optoelectronic properties of interlayer excitons in heterobilayers remain relatively uncharacterized. Here, we experimentally demonstrate strain tuning of Γ–K interlayer excitons in molybdenum disulfide and tungsten diselenide (MoS2/WSe2) wrinkled heterobilayers and obtain a deformation potential constant of ∼107 meV/% uniaxial strain, which is approximately twice that of the intralayer excitons in the constituent monolayers. We further observe a nonmonotonic dependence of the interlayer exciton photoluminescence intensity with strain, which we interpret as being due to the sensitivity of the Γ point to band hybridization arising from the competition between in-plane strain and out-of-plane interlayer coupling. Strain engineering with interlayer excitons in TMDC heterobilayers offers higher strain tunability and new degrees of freedom compared to their monolayer counterparts.

Flexible electrodes that allow electrical conductance to be maintained during mechanical deformation are required for the development of wearable electronics. However, flexible electrodes based on metal thin-films on elastomeric substrates can suffer from complete and unexpected electrical disconnection after the onset of mechanical fracture across the metal. Here we show that the strain-resilient electrical performance of thin-film metal electrodes under multimodal deformation can be enhanced by using a two-dimensional (2D) interlayer. Insertion of atomically-thin interlayers — graphene, molybdenum disulfide, or hexagonal boron nitride — induce continuous in-plane crack deflection in thin-film metal electrodes. This leads to unique electrical characteristics (termed electrical ductility) in which electrical resistance gradually increases with strain, creating extended regions of stable resistance. Our 2D-interlayer electrodes can maintain a low electrical resistance beyond a strain in which conventional metal electrodes would completely disconnect. We use the approach to create a flexible electroluminescent light emitting device with an augmented strain-resilient electrical functionality and an early-damage diagnosis capability.

The emergence of two-dimensional (2D) materials as functional surfaces for sensing, electronics, mechanics, and other myriad applications underscores the importance of understanding 2D material-liquid interactions. The thinness and environmental sensitivity of 2D materials induce novel surface forces that drive liquid interactions. This complexity makes fundamental 2D material-liquid interactions variable. In this review, we discuss the (1) wettability, (2) electrical double layer (EDL) structure, and (3) frictional interactions originating from 2D material-liquid interactions. While many 2D materials are inherently hydrophilic, their wettability is perturbed by their substrate and contaminants, which can shift the contact angle. This modulation of the wetting behavior enables templating, filtration, and actuation. Similarly, the inherent EDL at 2D material-liquid interfaces is easily perturbed. This EDL modulation partially explains the wettability modulation and enables distinctive electrofluidic systems, including supercapacitors, energy harvesters, microfluidic sensors, and nanojunction gating devices. Furthermore, nanoconfinement of liquid molecules at 2D material surfaces arising from a perturbed liquid structure results in distinctive hydrofrictional behavior, influencing the use of 2D materials in microchannels. We expect 2D material-liquid interactions to inform future fields of study, including modulation of the chemical reactivity of 2D materials via tuning 2D material-liquid interactions. Overall, 2D material-liquid interactions are a rich area for research that enables the unique tuning of surface properties, electrical and mechanical interactions, and chemistry. Origin of 2D material interactions with liquids The ideal interaction between a surface and a liquid is dictated by surface forces. Surface forces can be subdivided into three components: van der Waals forces, which exist at any interface between solids and liquids; electrostatic interactions, which exist between charged or polar surfaces and fluids; and structural forces, principally hydrogen bonding 1. These interactions influence the wetting behavior and determine whether the interaction with water is hydrophilic or hydrophobic, control the electronic structure at the liquid-solid interface, influence the frictional interaction, and modulate the chemical activity. However, this ideal picture of the surface interactions is complicated when considering surface heterogeneities, including roughness 2 and contamination 3 , which interfere with the formation of an equilibrium configuration between the liquid and the surface, leading to eccentric behaviors, including superwetting, superslipping, and superhydrophobicity. These surface heterogeneities also allow the surface liquid interactions to be tuned, enabling an array of applications in sensing, chemical transport, and actuation. Two-dimensional (2D) materials, which are extremely thin, have unique electrical properties, and have a tendency to deform and accumulate contamination during proce...

Multifunctional piezoelectric materials with controllable mechanical and thermal properties could enable the next generation of soft interconnect materials, energy harvesters, and health monitors. [1] Boron nitride nanotubes (BNNTs) possess unique properties that have piqued the interest of the broader research community as the basis for these multifunctional materials. [2] BNNTs exhibit high mechanical strength, possessing a Young's modulus (Y) as high as 1.3 TPa [3] which outperforms most structural materials. BNNTs also show enhanced thermal properties including extreme thermal stability in air (>800 °C) [4] and thermal conductivity competitive with carbon nanotubes (CNT) [5] which, when combined with their electrically insulating nature, may enable the development of novel thermally conductive, electrically insulating materials. [6] Of particular interest are BNNTs' piezoelectric properties, [7] arising from polarization due to the electronegativity difference between boron and nitrogen atoms, which is predicted to result in the generation of a coupled electric dipole in response to deformation. [8] With the advent of techniques to produce large volumes of BNNTs such as the high temperature pressure (HTP) method, [9,10] interest has shifted to production of ensembles of BNNTs which translate the nanoscale properties of BNNTs to macroscale systems. The most widely employed technique to produce functional structures of BNNTs is suspension in a matrix material to form a functional composite. For instance, the dispersion of BNNTs in soft polymers has been shown to augment mechanical properties, nearly doubling the compressive modulus of polyurethane. [11] Similarly, BNNTs can be added to thermally insulating materials to enhance thermal conductivity, with addition of BNNTs to polymer-derived ceramic (PDC) enhancing thermal conductivity by 2100%. [12] Most saliently, BNNT/polymer composites represent the first realization of BNNT piezoelectricity at the macroscale. Polyimide composites containing strainaligned BNNTs demonstrate piezoelectric coefficients up to 4.81 pm V −1 , [13] and nanoporous BNNT thin films are predicted to possess piezoelectric responses competitive with commercial piezoelectric polymers. [14] Of particular interest are stretchable BNNT composites which are optimal for use as flexible thermal interconnects [15] and platforms for strain aligning nanotubes [16] Boron nitride nanotubes (BNNT) uniformly dispersed in stretchable materials, such as poly(dimethylsiloxane) (PDMS), could create the next generation of composites with augmented mechanical, thermal, and piezoelectric characteristics. This work reports tunable piezoelectricity of multifunctional BNNT/PDMS stretchable composites prepared via co-solvent blending with tetrahydrofuran (THF) to disperse BNNTs in PDMS while avoiding sonication or functionalization. The resultant stretchable BNNT/PDMS composites demonstrate augmented Young's modulus (200% increase at 9 wt% BNNT) and thermal conductivity (120% increase at 9 wt% BNNT) without losin...

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