Depolarization Field-Induced Photovoltaic Effect in Graphene/α-In2Se3/Graphene Heterostructures.

The ferroelectric photovoltaic effect (FPVE) enables alternate pathways for energy conversion that are not allowed in centrosymmetric materials. Understanding the dominant mechanism of the FPVE at the ultrathin limit is important for defining the ultimate efficiency. In contrast to the wide band gap conventional thin-film ferroelectrics, 2D α-In2Se3 has an ideal band gap of 1.3 eV and enables the fabrication of ultrathin and stable heterostructures, providing the perfect platform to explore FPVE in the nanoscale limit. Here, we study the ferroelectric layer thickness-dependent FPVE in vertical few-layer graphene/α-In2Se3/graphene heterostructures. We find that the short-circuit photocurrent is antiparallel to the ferroelectric polarization and increases exponentially with decreasing thickness. We show that the observed behavior is predicted by the depolarization field model, originating from the unscreened bound charges due to the finite density of states in semimetal few-layer graphene. As a result, the heterostructures show enhancement of the power conversion efficiency, reaching 2.56 × 10-3% under 100 W/cm2 in 18 nm thick α-In2Se3, approximately 275 times more than the 50 nm thick α-In2Se3. These results demonstrate the importance of the depolarization field at the nanoscale and define design principles for the potential of harnessing FPVE at reduced dimension.

Strain modulation in crumpled Si nanomembranes: Light detection beyond the Si absorption limit.

Although Si is extensively used in micro-nano electronics, its inherent optical absorption cutoff at 1100-nm limits its photonic and optoelectronic applications in visible to partly near infrared (NIR) spectral range. Recently, strain engineering has emerged as a promising approach for extending device functionality via tuning the material properties, including change in optical bandgap. In this study, the reduction in bandgap with applied strain was used for extending the absorption limit of crystalline Si up to 1310 nm beyond its intrinsic bandgap, which was achieved by creating the crumpled structures in Si nanomembranes (NMs). The concept was used to develop a prototype NIR image sensor by organizing metal-semiconductor-metal-configured crumpled Si NM photosensing pixels in 6 × 6 array. The geometry-controlled, self-sustained strain induction in Si NMs provided an exclusive photon management with shortening of optical bandgap and enhanced photoresponse beyond the conventional Si absorption limit.

Spatial Tuning of Light-Matter Interaction via Strain-Gradient-Induced Polarization in Freestanding Wrinkled 2D Materials.

To date, controlled deformation of two-dimensional (2D) materials has been extensively demonstrated with substrate-supported structures. However, interfacial effects arising from these supporting materials may suppress or alter the unique behavior of the deformed 2D materials. To address interfacial effects, we report, for the first time, the formation of a micrometer-scale freestanding wrinkled structure of 2D material without any encapsulation layers where we observed the enhanced light-matter interactions with a spatial modulation. Freestanding wrinkled monolayer WSe2 exhibited about a 330% enhancement relative to supported wrinkled WSe2 quantified through photoinduced force microscopy. Spatial modulation and enhancement of light interaction in the freestanding wrinkled structures are attributed to the enhanced strain-gradient effect (i.e., out-of-plane polarization) enabled by removing the constraining support and proximate dielectrics. Our findings offer an additional degree of freedom to modulate the out-of-plane polarization and enhance the out-of-plane light-matter interaction in 2D materials.

Dynamically tuning friction at the graphene interface using the field effect.

Dynamically controlling friction in micro- and nanoscale devices is possible using applied electrical bias between contacting surfaces, but this can also induce unwanted reactions which can affect device performance. External electric fields provide a way around this limitation by removing the need to apply bias directly between the contacting surfaces. 2D materials are promising candidates for this approach as their properties can be easily tuned by electric fields and they can be straightforwardly used as surface coatings. This work investigates the friction between single layer graphene and an atomic force microscope tip under the influence of external electric fields. While the primary effect in most systems is electrostatically controllable adhesion, graphene in contact with semiconducting tips exhibits a regime of unexpectedly enhanced and highly tunable friction. The origins of this phenomenon are discussed in the context of fundamental frictional dissipation mechanisms considering stick slip behavior, electron-phonon coupling and viscous electronic flow.

Bend-Induced Ferroelectric Domain Walls in α-In2Se3.

The low bending stiffness of atomic membranes from van der Waals ferroelectrics such as α-In2Se3 allow access to a regime of strong coupling between electrical polarization and mechanical deformation at extremely high strain gradients and nanoscale curvatures. Here, we investigate the atomic structure and polarization at bends in multilayer α-In2Se3 at high curvatures down to 0.3 nm utilizing atomic-resolution scanning transmission electron microscopy, density functional theory, and piezoelectric force microscopy. We find that bent α-In2Se3 produces two classes of structures: arcs, which form at bending angles below ∼33°, and kinks, which form above ∼33°. While arcs preserve the original polarization of the material, kinks contain ferroelectric domain walls that reverse the out-of-plane polarization. We show that these kinks stabilize ferroelectric domains that can be extremely small, down to 2 atoms or ∼4 Å wide at their narrowest point. Using DFT modeling and the theory of geometrically necessary disclinations, we derive conditions for the formation of kink-induced ferroelectric domain boundaries. Finally, we demonstrate direct control over the ferroelectric polarization using templated substrates to induce patterned micro- and nanoscale ferroelectric domains with alternating polarization. Our results describe the electromechanical coupling of α-In2Se3 at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning.

MoS2 Passivated Multilayer Graphene Membranes for Li-Ion Extraction From Seawater.

Abundant Li resources in the ocean are promising alternatives to refining ore, whose supplies are limited by the total amount and geopolitical imbalance of reserves in Earth's crust. Despite advances in Li+ extraction using porous membranes, they require screening other cations on a large scale due to the lack in precise control of pore size and inborn defects. Herein, MoS2 nanoflakes on a multilayer graphene membrane (MFs-on-MGM) that possess ion channels comprising i) van der Waals interlayer gaps for optimal Li+ extraction and ii) negatively charged vertical inlets for cation attraction, are reported. Ion transport measurements across the membrane reveal ≈6- and 13-fold higher selectivity for Li+ compared to Na+ and Mg2+ , respectively. Furthermore, continuous, stable Li+ extraction from seawater is demonstrated by integrating the membrane into a H2 and Cl2 evolution system, enabling more than 104 -fold decrease in the Na+ concentration and near-complete elimination of other cations.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities.

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Plasmonic sensors based on graphene and graphene hybrid materials.

The past decade has witnessed a rapid growth of graphene plasmonics and their applications in different fields. Compared with conventional plasmonic materials, graphene enables highly confined plasmons with much longer lifetimes. Moreover, graphene plasmons work in an extended wavelength range, i.e., mid-infrared and terahertz regime, overlapping with the fingerprints of most organic and biomolecules, and have broadened their applications towards plasmonic biological and chemical sensors. In this review, we discuss intrinsic plasmonic properties of graphene and strategies both for tuning graphene plasmons as well as achieving higher performance by integrating graphene with plasmonic nanostructures. Next, we survey applications of graphene and graphene-hybrid materials in biosensors, chemical sensors, optical sensors, and sensors in other fields. Lastly, we conclude this review by providing a brief outlook and challenges of the field. Through this review, we aim to provide an overall picture of graphene plasmonic sensing and to suggest future trends of development of graphene plasmonics.

Role of Thin Film Adhesion on Capillary Peeling.

The capillary force can peel off a substrate-attached film if the adhesion energy (Gw) is low. Capillary peeling has been used as a convenient, rapid, and nondestructive method for fabricating free-standing thin films. However, the critical value of Gw, which leads to the transition between peeling and sticking, remains largely unknown. As a result, capillary peeling remains empirical and applicable to a limited set of materials. Here, we investigate the critical value of Gw and experimentally show the critical adhesion (Gw,c) to scale with the water-film interfacial energy (≈0.7γfw), which corresponds well with our theoretical prediction of Gw,c = γfw. Based on the critical adhesion, we propose quantitative thermodynamic guidelines for designing thin film interfaces that enable successful capillary peeling. The outcomes of this work present a powerful technique for thin film transfer and advanced nanofabrication in flexible photovoltaics, battery materials, biosensing, translational medicine, and stretchable bioelectronics.

Ultra-thin self-healing vitrimer coatings for durable hydrophobicity.

Durable hydrophobic materials have attracted considerable interest in the last century. Currently, the most popular strategy to achieve hydrophobic coating durability is through the combination of a perfluoro-compound with a mechanically robust matrix to form a composite for coating protection. The matrix structure is typically large (thicker than 10 µm), difficult to scale to arbitrary materials, and incompatible with applications requiring nanoscale thickness such as heat transfer, water harvesting, and desalination. Here, we demonstrate durable hydrophobicity and superhydrophobicity with nanoscale-thick, perfluorinated compound-free polydimethylsiloxane vitrimers that are self-healing due to the exchange of network strands. The polydimethylsiloxane vitrimer thin film maintains excellent hydrophobicity and optical transparency after scratching, cutting, and indenting. We show that the polydimethylsiloxane vitrimer thin film can be deposited through scalable dip-coating on a variety of substrates. In contrast to previous work achieving thick durable hydrophobic coatings by passively stacking protective structures, this work presents a pathway to achieving ultra-thin (thinner than 100 nm) durable hydrophobic films.

Large scale self-assembly of plasmonic nanoparticles on deformed graphene templates.

Hierarchical heterostructures of two-dimensional (2D) nanomaterials are versatile platforms for nanoscale optoelectronics. Further coupling of these 2D materials with plasmonic nanostructures, especially in non-close-packed morphologies, imparts new metastructural properties such as increased photosensitivity as well as spectral selectivity and range. However, the integration of plasmonic nanoparticles with 2D materials has largely been limited to lithographic patterning and/or undefined deposition of metallic structures. Here we show that colloidally synthesized zero-dimensional (0D) gold nanoparticles of various sizes can be deterministically self-assembled in highly-ordered, anisotropic, non-close-packed, multi-scale morphologies with templates designed from instability-driven, deformed 2D nanomaterials. The anisotropic plasmonic coupling of the particle arrays exhibits emergent polarization-dependent absorbance in the visible to near-IR regions. Additionally, controllable metasurface arrays of nanoparticles by functionalization with varying polymer brushes modulate the plasmonic coupling between polarization dependent and independent assemblies. This self-assembly method shows potential for bottom-up nanomanufacturing of diverse optoelectronic components and can potentially be adapted to a wide array of nanoscale 0D, 1D, and 2D materials.

Effects of Layering and Supporting Substrate on Liquid Slip at the Single-Layer Graphene Interface.

Understanding modulation of liquid molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials, such as in sensors, nanofluidic devices, and biological systems. Here, we use force measurements by atomic force microscopy (AFM) to directly measure hydrodynamic, solvation, and frictional forces along the graphene plane in seven liquids. The results show that the greater slip lengths correlate with the interfacial ordering of the liquid molecules, which suggests that the ordering of the liquid forming multiple layers promotes slip. This phenomenon appears to be more relevant than solely the wetting behavior of graphene or the solid-liquid interaction energy, as traditionally assumed. Furthermore, the slip boundary condition of the liquids along the graphene plane is sensitive to the substrate underneath graphene, indicating that the underlying substrate affects graphene's interaction with the liquid molecules. Because interfacial slip can have prominent consequences on the pressure drop, on electrical and diffusive transport through nanochannels, and on lubrication, this work can inspire innovation in many applications through the modulation of the substrate underneath graphene and of the interfacial ordering of the liquid.

Highly Strain-Tunable Interlayer Excitons in MoS2/WSe2 Heterobilayers.

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.

Polarization Control of Deterministic Single-Photon Emitters in Monolayer WSe2.

Single-photon emitters, the basic building blocks of quantum communication and information, have been developed using atomically thin transition metal dichalcogenides (TMDCs). Although the bandgap of TMDCs was spatially engineered in artificially created defects for single-photon emitters, it remains a challenge to precisely align the emitter's dipole moment to optical cavities for the Purcell enhancement. Here, we demonstrate position- and polarization-controlled single-photon emitters in monolayer WSe2. A tensile strain of ∼0.2% was applied to monolayer WSe2 by placing it onto a dielectric rod structure with a nanosized gap. Excitons were localized in the nanogap sites, resulting in the generation of linearly polarized single-photon emission with a g(2) of ∼0.1 at 4 K. Additionally, we measured the abrupt change in polarization of single photons with respect to the nanogap size. Our robust spatial and polarization control of emission provides an efficient way to demonstrate deterministic and scalable single-photon sources by integrating with nanocavities.

Strain-resilient electrical functionality in thin-film metal electrodes using two-dimensional interlayers.

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.

Curved neuromorphic image sensor array using a MoS2-organic heterostructure inspired by the human visual recognition system.

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.

Tunable Piezoelectricity of Multifunctional Boron Nitride Nanotube/Poly(dimethylsiloxane) Stretchable Composites.

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 losing stretchability. Furthermore, BNNT/PDMS composites demonstrate piezoelectric responses that are linearly proportional to BNNT wt%, achieving a piezoelectric constant (|d33 |) of 18 pmV-1 at 9 wt% BNNT without poling, which is competitive with commercial piezoelectric polymers. Uniquely, BNNT/PDMS accommodates tensile strains up to 60% without plastic deformation by aligning BNNTs, which enhances the composites' piezoelectric response approximately five times. Finally, the combined stretchable and piezoelectric nature of the composite was exploited to produce a vibration sensor sensitive to low-frequency (≈1 kHz) excitation. This is the first demonstration of multifunctional, stretchable BNNT/PDMS composites with enhanced mechanical strength and thermal conductivity and furthermore tunable piezoelectric response by varying BNNT wt% and applied strain, permitting applications in soft actuators and vibration sensors.

Heterogeneous deformation of two-dimensional materials for emerging functionalities.

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

Tunable Wettability of Graphene through Nondestructive Hydrogenation and Wettability-Based Patterning for Bioapplications.

The wettability of graphene has been extensively studied and successfully modified by chemical functionalization. Nevertheless, the unavoidable introduction of undesired defects and the absence of systematic and local control over wettability by previous methods have limited the use of graphene in applications. In addition, microscale patterning, according to wettability, has not been attempted. Here, we demonstrate that the wettability of graphene can be systematically controlled and surface patterned into microscale sections based on wettability without creating significant defects, possible by nondestructive hydrogen plasma. Hydrophobic graphene is progressively converted to hydrophilic hydrogenated graphene (H-Gr) that reaches superhydrophilicity. The great contrast in wettability between graphene and H-Gr makes it possible to selectively position and isolate human breast cancer cells on arrays of micropatterns since strong hydrophilicity facilitates the adsorption of the cells. We believe that our method will provide an essential technique for enabling surface and biological applications requiring microscale patterns with different wettability.