Autoperforation of 2D materials for generating two-terminal memristive Janus particles

High energy density picoliter-scale zinc-air microbatteries for colloidal robotics

The recent interest in microscopic autonomous systems, including microrobots, colloidal state machines, and smart dust, has created a need for microscale energy storage and harvesting. However, macroscopic materials for energy storage have noted incompatibilities with microfabrication techniques, creating substantial challenges to realizing microscale energy systems. Here, we photolithographically patterned a microscale zinc/platinum/SU-8 system to generate the highest energy density microbattery at the picoliter (10-12 liter) scale. The device scavenges ambient or solution-dissolved oxygen for a zinc oxidation reaction, achieving an energy density ranging from 760 to 1070 watt-hours per liter at scales below 100 micrometers lateral and 2 micrometers thickness in size. The parallel nature of photolithography processes allows 10,000 devices per wafer to be released into solution as colloids with energy stored on board. Within a volume of only 2 picoliters each, these primary microbatteries can deliver open circuit voltages of 1.05 ± 0.12 volts, with total energies ranging from 5.5 ± 0.3 to 7.7 ± 1.0 microjoules and a maximum power near 2.7 nanowatts. We demonstrated that such systems can reliably power a micrometer-sized memristor circuit, providing access to nonvolatile memory. We also cycled power to drive the reversible bending of microscale bimorph actuators at 0.05 hertz for mechanical functions of colloidal robots. Additional capabilities, such as powering two distinct nanosensor types and a clock circuit, were also demonstrated. The high energy density, low volume, and simple configuration promise the mass fabrication and adoption of such picoliter zinc-air batteries for micrometer-scale, colloidal robotics with autonomous functions.

Dynamics of a self-interacting sheet in shear flow

Solution processing of 2D materials such as graphene is important for applications thereof, yet a complete fundamental understanding of how 2D materials behave dynamically in solution is lacking. Here, we extend previous work by Silmore et al., Soft Matter, 2021, 17(18), 4707-4718 by adding short-ranged Lennard-Jones interactions to 2D sheets in shear flow. We find that the addition of these interactions allows for a rich landscape of conformations which depend on the balance between shear strength, bending rigidity, and interaction strength as well as the initial configuration of the sheet. We explore this conformational space and classify sheets as flat, tumbling, 1D folded, or 2D folded based on their conformational properties. We use kinetic and energetic arguments to explain why sheets adopt certain conformations within the folded regime. Finally, we calculate the stresslet and find that, even in the absence of thermal fluctuations and multiple sheet interactions, shear-thinning followed by shear-thickening behavior can appear.

Meta-Structured Covalent Organic Framework Nanocoatings with Active and Angle-Independent Structural Coloration

High-performance multifunctional nanocoatings not only protect and enhance substrate materials but also offer additional functionalities. This demands a sophisticated coordination of the coating's inherent properties and microstructural features. Here, a multifunctional active nanocoating via meta-structural engineering of covalent organic framework (COF) deposition materials is presented. This COF nanocoating, characterized by well-defined micropores (1-2 nm), meta-structured textures (30-300 nm), tailored thickness (100-300 nm), and good uniformness, showcases a unique combination of angle-independent structural coloration and ultrafast responsiveness to gaseous stimuli. Remarkably, it demonstrates good compatibility with a wide range of inert substrate materials, from rigid ones like glass and metal to flexible elastomers and nanomaterial films of various shapes and sizes. This versatility enables the facile development of devices that can optically report information about their environments. Examples include chemically active coatings with ultrafast (≈10 ms) color-changing behaviors and programmable actuation behaviors upon exposure to gaseous stimuli, and mechanically active coatings that can detect substrate strain up to 50% yet maintain structural robustness and consistent coloration hue. It is believed that meta-structural engineering of COF nanocoatings on inert substrates can enable them to respond to environmental stimuli, potentially indicating a new trend in developing multifunctional materials and smart devices.

Colloidal robotics

Robots have components that work together to accomplish a task. Colloids are particles, usually less than 100 µm, that are small enough that they do not settle out of solution. Colloidal robots are particles capable of functions such as sensing, computation, communication, locomotion and energy management that are all controlled by the particle itself. Their design and synthesis is an emerging area of interdisciplinary research drawing from materials science, colloid science, self-assembly, robophysics and control theory. Many colloidal robot systems approach synthetic versions of biological cells in autonomy and may find ultimate utility in bringing these specialized functions to previously inaccessible locations. This Perspective examines the emerging literature and highlights certain design principles and strategies towards the realization of colloidal robots.

Synapse-Mimetic Hardware-Implemented Resistive Random-Access Memory for Artificial Neural Network

Memristors mimic synaptic functions in advanced electronics and image sensors, thereby enabling brain-inspired neuromorphic computing to overcome the limitations of the von Neumann architecture. As computing operations based on von Neumann hardware rely on continuous memory transport between processing units and memory, fundamental limitations arise in terms of power consumption and integration density. In biological synapses, chemical stimulation induces information transfer from the pre- to the post-neuron. The memristor operates as resistive random-access memory (RRAM) and is incorporated into the hardware for neuromorphic computing. Hardware composed of synaptic memristor arrays is expected to lead to further breakthroughs owing to their biomimetic in-memory processing capabilities, low power consumption, and amenability to integration; these aspects satisfy the upcoming demands of artificial intelligence for higher computational loads. Among the tremendous efforts toward achieving human-brain-like electronics, layered 2D materials have demonstrated significant potential owing to their outstanding electronic and physical properties, facile integration with other materials, and low-power computing. This review discusses the memristive characteristics of various 2D materials (heterostructures, defect-engineered materials, and alloy materials) used in neuromorphic computing for image segregation or pattern recognition. Neuromorphic computing, the most powerful artificial networks for complicated image processing and recognition, represent a breakthrough in artificial intelligence owing to their enhanced performance and lower power consumption compared with von Neumann architectures. A hardware-implemented CNN with weight control based on synaptic memristor arrays is expected to be a promising candidate for future electronics in society, offering a solution based on non-von Neumann hardware. This emerging paradigm changes the computing algorithm using entirely hardware-connected edge computing and deep neural networks.

Wearable chemical sensors based on 2D materials for healthcare applications

Chemical sensors worn on the body could make possible the continuous, noninvasive, and accurate monitoring of vital human signals, which is necessary for remote health monitoring and telemedicine. Attractive for creating high-performance, wearable chemical sensors are atomically thin materials with intriguing physical features, abundant chemistry, and high surface-to-volume ratios. These advantages allow for appropriate material-analyte interactions, resulting in a high level of sensitivity even at trace analyte concentrations. Previous review articles covered the material and device elements of 2D material-based wearable devices extensively. In contrast, little research has addressed the existing state, future outlook, and promise of 2D materials for wearable chemical sensors. We provide an overview of recent advances in 2D-material-based wearable chemical sensors to overcome this deficiency. The structure design, manufacturing techniques, and mechanisms of 2D material-based wearable chemical sensors will be evaluated, as well as their applicability in human health monitoring. Importantly, we present a thorough review of the current state of the art and the technological gaps that would enable the future design and nanomanufacturing of 2D materials and wearable chemical sensors. Finally, we explore the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

Mechanical Janus Structures by Soft-Hard Material Integration

Engineering Janus structures that possess anisotropic features in functions have attracted growing attention for a wide range of applications in sensors, catalysis, and biomedicine, and are yet usually designed at the nanoscale with distinct physical or chemical functionalities in their opposite sides. Inspired by the seamless integration of soft and hard materials in biological structures, here a mechanical Janus structure composed of soft and hard materials with a dramatic difference in mechanical properties at an additively manufacturable macroscale is presented. In the combination of extensive experimental, theoretical, and computational studies, the design principle of soft-hard materials integrated mechanical Janus structures is established and their unique rotation mechanism is addressed. The systematic studies of assembling the Janus structure units into superstructures with well-ordered organizations by programming the local rotations are further shown, providing a direct route of designing superstructures by leveraging mechanical Janus structures with unique soft-hard material integration. Applications are conducted to demonstrate the features and functionalities of assembled superstructures with local ordered organizations in regulating and filtering acoustic wave propagations, thereby providing exemplification applications of mechanical Janus design in functional structures and devices.

Spatially Resolved Janus Patterning of Graphene by Direct Laser Writing

Covalently patterned Janus-functionalized graphene featuring a spatially defined asymmetric bifacial addend binding motif remains a challenging synthetic target. Here, a facile and universal laser writing approach for a one-step covalent Janus patterning of graphene is reported, leading to the formation of up to now elusive graphene architectures, solely consisting of antaratopically functionalized superlattices. The structurally defined covalent functionalization procedure is based on laser-triggered concurrent photolysis of two different photosensitizers situated on both sides of the graphene plane, generating radicals and subsequent addend binding in the laser-irradiated areas only. Careful structure analysis was performed by Raman spectroscopy and Kelvin probe force microscopy. In terms of the advantages of our newly established concept, including a simple/easy-to-operate patterning procedure, arbitrary pattern availability, and a high degree of addend binding, an easy access to tailor-designed Janus-functionalized graphene devices with spatially resolved functional entities can be envisaged.

Two-dimensional cobalt telluride as a piezo-tribogenerator

Two-dimensional (2D) materials have been shown to be efficient in energy harvesting. Here, we report the use of waste heat to generate electricity via the combined piezoelectric and triboelectric properties of 2D cobalt telluride (CoTe₂). The piezo-triboelectric nanogenerator (PTNG) produced an open-circuit voltage of ∼5 V under 1 N force and the effect of temperature in the range of 305-363 K shows a four-fold energy conversion efficiency improvement. The 2D piezo-tribogenerator shows excellent characteristics with a maximum voltage of ∼10 V, fast response time, and high responsivity. Density functional theory was used to gain further insights and validation of the experimental results. Our results could lead to energy harvesting approaches using 2D materials from various thermal sources and dissipating waste heat from electronic devices.

Soil-based nano-graphene oxide and foliar selenium and nano-Fe influence physiological responses of 'Sultana' grape under salinity

Salinity is a worldwide stressor that influences the growth and productivity of plants. Some novel compounds like; graphene oxide and nutrients such as Se and Fe especially as nano form may improve plant responses to the environmental stress factors. The soil-based graphene oxide (0, 50, and 100 g kg⁻¹) and the foliar applications of Se and nano-Fe (control and 3 mg L⁻¹) were assayed on grapevine cv. Sultana under salinity (0, 50, and 100 mM NaCl). The top flavonoids, chlorophyll b, and plant dry weight belonged to graphene oxide and nano-Fe applications. CAT activity was improved in response to Se, nano-Fe, and graphene oxide (50 g kg⁻¹). The least Fe, K, Se, N, Mg, Mn, and Zn content was recorded for 100 mM NaCl. In contrast, the higher data for K, Se, Ca, Mg, Zn and Mn were acquired with graphene oxide × foliar treatments. In general, graphene oxide treatment (50 g kg⁻¹) × nano-Fe and Se foliar use ameliorated the adverse salinity effects with the improved biochemical and physiological responses of Sultana grape.

Thermally fluctuating, semiflexible sheets in simple shear flow

We perform Brownian dynamics simulations of semiflexible colloidal sheets with hydrodynamic interactions and thermal fluctuations in shear flow. As a function of the ratio of bending rigidity to shear energy (a dimensionless quantity we denote S) and the ratio of bending rigidity to thermal energy, we observe a dynamical transition from stochastic flipping to crumpling and continuous tumbling. This dynamical transition is broadened by thermal fluctuations, and the value of S at which it occurs is consistent with the onset of chaotic dynamics found for athermal sheets. The effects of different dynamical conformations on rheological properties such as viscosity and normal stress differences are also quantified. Namely, the viscosity in a dilute dispersion of sheets is found to decrease with increasing shear rate (shear-thinning) up until the dynamical crumpling transition, at which point it increases again (shear-thickening), and non-zero first normal stress differences are found that exhibit a local maximum with respect to temperature at large S (small shear rate). These results shed light on the dynamical behavior of fluctuating 2D materials dispersed in fluids and should greatly inform the design of associated solution processing methods.

Janus 2D materials via asymmetric molecular functionalization

Janus two-dimensional materials (2DMs) are a novel class of 2DMs in which the two faces of the material are either asymmetrically functionalized or are exposed to a different local environment. The diversity of the properties imparted to the two opposing sides enables the design of new multifunctional materials for applications in a broad variety of fields including opto-electronics, energy storage, and catalysis. In this perspective, we summarize the most enlightening experimental methods for the asymmetric chemical functionalization of 2DMs with tailored made (macro)molecules by means of a supratopic binding (one side) or antaratopic binding (two sides) process. We describe the emergence of unique electrical and optical characteristics resulting from the asymmetric dressing of the two surfaces. Representative examples of Janus 2DMs towards bandgap engineering, enhanced photoresponse and photoluminescence are provided. In addition, examples of Janus 2DMs for real applications such as energy storage (batteries and supercapacitors) and generation (photovoltaics), opto-electronics (field-effect transistors and photodetectors), catalysis, drug delivery, self-healing materials, chemical sensors and selective capture and separation of small molecules are also described. Finally, we discuss the future directions, challenges, and opportunities to expand the frontiers of Janus 2DMs towards technologies with potential impact in environmental science and biomedical applications.

Self-Assembly of Poly(Janus particle)s into Unimolecular and Oligomeric Spherical Micelles

Using shape-persistent Janus particles to construct poly(Janus particle)s and studying their self-assembly behaviors are of great interest, but remain largely unexplored. In this work, we reported a type of amphiphiles constructed by the ring-opening metathesis polymerization of nonspherical molecular Janus particles (APOSS-BPOSS), called poly(Janus particle)s (poly(APOSS-BPOSS)_n, n = 12, 17, 22, and 35, and M_n = 35-100 kg/mol). Unlike traditional bottlebrush polymers consisting of flexible side chains, these poly(Janus particles) consist of rigid hydrophilic and hydrophobic polyhedral oligomeric silsesquioxane (POSS) cages as side chains. Interestingly, instead of maintaining an expected extended chain conformation, they could also collapse and then self-assemble to form unconventional unimolecular or oligomeric spherical micelles in solutions with a feature size smaller than 7 nm. More importantly, unlike traditional amphiphilic polymer brushes that could form unimolecular micelles at a relatively high degree of polymerization by self-assembly, these poly(Janus particles)s could accomplish self-assembly at a quite low degree of polymerization because of their unique chemical structure and molecular topology. The formation of unimolecular and oligomeric micelles was also further confirmed by dissipative particle dynamics simulations. This study of introducing the POSS-based poly(Janus particle)s as a class of shape amphiphiles will provide a model system for generating unimolecular and oligomeric micellar nanostructures through solution self-assembly.

Artificial Visual Perception Nervous System Based on Low-Dimensional Material Photoelectric Memristors

The visual perception system is the most important system for human learning since it receives over 80% of the learning information from the outside world. With the exponential growth of artificial intelligence technology, there is a pressing need for high-energy and area-efficiency visual perception systems capable of processing efficiently the received natural information. Currently, memristors with their elaborate dynamics, excellent scalability, and information (e.g., visual, pressure, sound, etc.) perception ability exhibit tremendous potential for the application of visual perception. Here, we propose a fully memristor-based artificial visual perception nervous system (AVPNS) which consists of a quantum-dot-based photoelectric memristor and a nanosheet-based threshold-switching (TS) memristor. We use a photoelectric and a TS memristor to implement the synapse and leaky integrate-and-fire (LIF) neuron functions, respectively. With the proposed AVPNS we successfully demonstrate the biological image perception, integration and fire, as well as the biosensitization process. Furthermore, the self-regulation process of a speed meeting control system in driverless automobiles can be accurately and conceptually emulated by this system. Our work shows that the functions of the biological visual nervous system may be systematically emulated by a memristor-based hardware system, thus expanding the spectrum of memristor applications in artificial intelligence.

Preparation and applications of freestanding Janus nanosheets

In the family of Janus nanomaterials, Janus nanosheets possess not only the advantages of Janus nanomaterials, but also the advantages of two-dimensional nanosheets, endowing them with many extraordinary properties. Therefore, Janus nanosheets have great potential in the fields of interfacial engineering, catalysis, and molecular recognition. This review summarizes and discusses the recent advances in both the preparation and applications of freestanding Janus nanosheets. After a short introduction to different types of Janus nanosheets, a variety of methods for preparing freestanding Janus nanosheets are introduced, including the surface reaction, interface reaction, emulsion reaction, self-assembly, and stripping of non-Janus nanosheets, as well as selective grafting of existing Janus nanosheets. Then, the wide applications of Janus nanosheets in the fields of emulsification, catalysis, polymer reinforcement, nanomotors, and molecular recognition are summarized in detail. Finally, a discussion on the remaining challenges and future perspectives in this field is included. This review will not only deepen the understanding of Janus nanosheets, but also benefit the designs and fabrications of extraordinary and multi-functional Janus nanosheets.

Hierarchical Assembly of Two-Dimensional Polymers into Colloidosomes and Microcapsules

Hierarchical assembly of two-dimensional (2D) polymers to 3D microstructures provides new means of creating functional materials with exotic properties for extensive applications. Herein, we report an approach of assembling 2D covalent organic framework (COF) colloidosomes or microcapsules from small molecules. We polymerized monomers to produce narrowly distributed COF particles with average particle sizes greater than 490 nm, which were further used as stabilizers to prepare various water-in-oil Pickering emulsions with droplet sizes of 10-120 μm on average. The emulsion droplets were subsequently applied as templates for interfacial polymerization of the same monomers. The COF microcapsules with varied diameters and shell thicknesses of 0.2-3.1 μm were thus obtained, which possessed good stability, high crystallinity, and surface areas no less than 540 m²/g. The approach also permits facile loading of water-soluble substances such as salts, dyes, or proteins. The loaded molecules demonstrated different permeability against the shell, in which 98% of the encapsulated salts could be released in 1 h while only 18% of dye molecules and almost none of the fluorescent proteins diffused out from the microcapsules. Such an assembling approach may greatly extend the applications of 2D polymers and their microcapsules.

Buckling, crumpling, and tumbling of semiflexible sheets in simple shear flow

As 2D materials such as graphene, transition metal dichalcogenides, and 2D polymers become more prevalent, solution processing and colloidal-state properties are being exploited to create advanced and functional materials. However, our understanding of the fundamental behavior of 2D sheets and membranes in fluid flow is still lacking. In this work, we perform numerical simulations of athermal semiflexible sheets with hydrodynamic interactions in shear flow. For sheets initially oriented near the flow-vorticity plane, we find buckling instabilities of different mode numbers that vary with bending stiffness and can be understood with a quasi-static model of elasticity. For different initial orientations, chaotic tumbling trajectories are observed. Notably, we find that sheets fold or crumple before tumbling but do not stretch again upon applying greater shear.

Autoperforation of two-dimensional materials to generate colloidal state machines capable of locomotion

A central ambition of the robotics field has been to increasingly miniaturize such systems, with perhaps the ultimate achievement being the synthetic microbe or cell sized machine. To this end, we have introduced and demonstrated prototypes of what we call colloidal state machines (CSMs) as particulate devices capable of integrating sensing, memory, and energy harvesting as well as other functions onto a single particle. One technique that we have introduced for creating CSMs based on 2D materials such as graphene or monolayer MoS₂ is "autoperforation", where the nanometer-scale film is fractured around a designed strain field to produce structured particles upon liftoff. While CSMs have been demonstrated with functions such as memory, sensing, and energy harvesting, the property of locomotion has not yet been demonstrated. In this work, we introduce an inversion moulding technique compatible with autoperforation that allows for the patterning of an external catalytic surface that enables locomotion in an accompanying fuel bath. Optimal processing conditions for electroplating a catalytic Pt layer to one side of an autoperforated CSM are elucidated. The self-driven propulsion of the resulting Janus CSM in H₂O₂ is studied, including the average velocity, as a function of fluid surface tension and H₂O₂ concentration in the bath. Since machines have to encode for a specific task, this work summarizes efforts to create a microfluidic testbed that allows for CSM designs to be evaluated for the ultimate purpose of navigation through complex fluidic networks, such as the human circulatory system. We introduce two CSM designs that mimic aspects of human immunity to solve search and recruitment tasks in such environments. These results advance CSM design concepts closer to promising applications in medicine and other areas.

Toward Covalent Organic Framework Metastructures

The bottom-up assembly of periodically ordered structures provides a scalable way for producing metastructured materials with exotic optical and mechanical properties. However, direct self-assembly of small molecules into such metastructures beyond the nanoscale remains an unresolved issue. Here we demonstrate that metastructured assemblies of two-dimensional (2D) polymers, specifically 2D covalent organic frameworks (COFs), can be directly synthesized in solution. We applied 2D COF monomer polycondensation to prepare flower-shaped particles consisting of highly crystalline "petals" with sizes larger than 20 μm. The petal comprises periodically arranged COF nanoflake units with tunable lengths of 490-850 nm, thicknesses about 20 nm, interflake spacing around 14 nm, and Hermans orientation factors up to 0.998. Such a metastructure is mechanically robust and remains almost intact even after full pyrolysis at 900 °C. It also demonstrates unique birefringence and polarization-dependent resonances under visible-near-infrared light not observed in its constituents, 2D COF polycrystals, and with well-defined nanopores of 1.8 nm and the high surface area of 1576 m²/g. Such metastructured particles with nanopores are well-suited as novel particulate optical devices for collecting and storing information about their surroundings that can be easily read out by polarization imaging with high sensitivity, as demonstrated by their explosive detection and anticounterfeiting applications. Self-assembly of 2D polymers into metastructures may become an important method for developing functional materials with unprecedented properties and extensive applications.

Leaching of graphene oxide nanosheets in simulated soil and their influences on microbial communities

With the wide use of graphene-like nanosheets, especially in agriculture, their release into the environment, it is crucial to grasp the fate of nanosheets in soil and minimum ecological risks. The present work discovered that leaching and migration of nanosheets (rGO) in soil is affected by soil porosity and adsorption processes. And the contents of rGO-Pd in soil layers and leachate increased and then decreased with the decreased of soil porosity. Moreover, physicochemical properties of rGO-Pd nanosheets changed by leaching processes, especially the changes of morphology, thickness and oxygen functional groups. Leaching of rGO-Pd also interfered the soil microbial homeostasis accompanied by the increase of microbial species richness and community diversity. In addition, rGO-Pd altered the usage of carbon sources by edaphon. The utilization of carbon sources by soil microbes, such as polymers, sugars, phenolic acids, carboxylic acids, amino acids, and amines, was also reduced by nanosheets. These findings provide new insights into environmental behaviors of nanomaterials and nanogeochemistry.

Two-dimensional conjugated microporous polymer films: fabrication strategies and potential applications

This review describes the latest advances in the preparation and application of two-dimensional conjugated microporous polymers, as well as the future research directions of this field.

A 2D-SnSe film with ferroelectricity and its bio-realistic synapse application

Catering to the general trend of artificial intelligence development, simulating humans' learning and thinking behavior has become the research focus. Second-order memristors, which are more analogous to biological synapses, are the most promising devices currently used in neuromorphic/brain-like computing. However, few second-order memristors based on two-dimensional (2D) materials have been reported, and the inherent bionic physics needs to be explored. In this work, a second-order memristor based on 2D SnSe films was fabricated by the pulsed laser deposition technique. The continuously adjustable conductance of Au/SnSe/NSTO structures was achieved by gradually switching the polarization of a ferroelectric SnSe layer. The experimental results show that the bio-synaptic functions, including spike-timing-dependent plasticity, short-term plasticity and long-term plasticity, can be simulated using this two-terminal devices. Moreover, stimulus pulses with nanosecond pulse duration were applied to the device to emulate rapid learning and long-term memory in the human brain. The observed memristive behavior is mainly attributed to the modulation of the width of the depletion layer and barrier height is affected, at the SnSe/NSTO interface, by the reversal of ferroelectric polarization of SnSe materials. The device energy consumption is as low as 66 fJ, being expected to be applied to miniaturized, high-density, low-power neuromorphic computing.

Black Phosphorus Based Multicolor Light-Modulated Transparent Memristor with Enhanced Resistive Switching Performance

Light-modulated transparent memristors combining photoresponse and data storage are promising as multifunctional devices. Herein, a multicolor light-modulated transparent memristor based on black phosphorous (BP) is designed, fabricated, and investigated. BP is a class of emerging two-dimensional (2D) materials with a natural direct band gap and a broad light absorption. Herein, BP nanosheets (BP@PS NSs) coated with polystyrene (PS) are prepared and serve as the resistive switching (RS) layer in the ITO/BP@PS/ITO memristor, which shows >75% transmittance between 350 and 1100 nm. With the aid of PS, the BP@PS-based memristor has excellent RS characteristics such as no initial preforming, low operating voltage, and long retention time. According to the energy band model, the RS mechanism of the high and low resistance states contributes to the transformation from ohmic contact to Schottky contact. During light illumination ranging from ultraviolet (380 nm) to near infrared (785 nm), the Schottky barrier height is elevated further so that the resetting voltages and power consumption decrease. Moreover, the ON/OFF ratios are improved and the maximum enhancement is demonstrated to be more than 10 times. BP is a promising RS material in light-modulated memristors, and the novel device configuration provides insights into the development of multifunctional microelectronic devices based on 2D materials.

Functional Janus-SiO2 Nanoparticles Prepared by a Novel "Cut the Gordian Knot" Method and Their Potential Application for Enhanced Oil Recovery

Currently available methods (e.g., interfacial protection and phase separation) for preparing Janus nanoparticles are often complex and expensive. Furthermore, the preparation of Janus nanoparticles with a particle size below 10 nm is challenging. In this work, we combine an in situ surface-modification route with a chemical etching route to establish a novel "cut the Gordian knot" method for the preparation of functional Janus-SiO₂ nanoparticles. Hydrophobic SiO₂ nanoparticles with a three-dimensional network structure prepared via an in situ surface-modification route were dispersed in NaOH solution containing surfactant or ethanol to enable corrosion close to the modifier-nanoparticle interface with a relatively low content of surface modifiers. Thus, amphipathic Janus-SiO₂ nanoparticles with a hydrophilic surface containing Si-OH species and a hydrophobic surface containing -CH₃ fragments were generated. The as-prepared Janus-SiO₂ nanoparticles with a size of 4-9 nm and a specific surface area of up to 612.9 m²/g can be easily dispersed in water, and they also can transfer from the water phase to the oil phase by tuning the surface polarity. Moreover, they can be tuned to achieve bidirectional regulation of surface wettability plus a reduction of the oil/water interface tension. Hence, a significant reduction (by 33∼50%) of water injection pressure and an enhanced oil recovery (EOR) (by 21.1% ∼ 26.6%) can be achieved. Apart from that, Janus-SiO₂ nanoparticles are able to increase the viscosity of partially hydrolyzed polyacrylamide by 282.9% and significantly decrease its viscosity loss ratio in brine, causing an EOR of about 36.6%. With simple, low-cost, and scalable procedures, the following approach could be well applicable to fabricating Janus-SiO₂ nanoparticles with a high potential for augmented water injection as well as EOR of low-permeability reservoirs.

Hierarchical Hollow-Pore Nanostructure Bilayer Heterojunction Comprising Conjugated Polymers for High-Performance Flash Memory

We report the design and preparation of hierarchical hollow-pore nanostructure bilayer conjugated polymer films for high-performance resistive memory devices. By taking the merits of chemical and structural stabilities of a two-dimensional conjugated microporous polymer (2D CMP), a poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) film with a hollow surface was spin-coated onto 2D CMP nanofilm directly, constructing a bilayer heterojunction. A two-terminal diode with a configuration of indium tin oxide/2D CMP/hollow MEH-PPV/Al was fabricated by employing the prepared bilayer heterojunction. The device poses flash feature with a high on/off ratio (>10⁵) and a long retention time (>3.0 × 10⁴ s), which is higher than that of most of the reported conjugated polymers memories. Our work offers a general guideline to construct high on/off ratio polymer memories via hierarchical nanostructure engineering in memristive layer.

Solution-processed black phosphorus nanoflakes for integrating nonvolatile resistive random access memory and the mechanism unveiled

In this study, we demonstrated the integration of black phosphorus (BP) nanoflakes in a resistive random access memory (RRAM) with a facile and complementary metal-oxide-semiconductor-compatible process. The solution-processed BP nanoflakes embedded in polystyrene (PS) as an active layer were sandwiched between aluminum electrodes (Al/BP:PS/Al). The device shows a figure of merit with typical bipolar behavior and forming-free characteristics as well as excellent memory performances such as nonvolatile, low operation voltage (1.75 V) and high ON/OFF ratio (>10²) as well as the long retention time (>1500 s). The improved device performances were attributed to the formation of effective trap sites from the hybrid structure of the active layer (BP:PS), especially the BP nanoflakes and the partly oxidized species (P _x O _y ). Moreover, the extrinsic aluminum oxide layer was observed after the device operation. The mechanism of switching behavior was further unveiled through the carrier transport models, which confirms the conductive mechanisms of space-charge-limited current and Ohmic conductance at high resistance state (HRS) and low resistance state, respectively. Additionally, in the high electric field at HRS, the transfer curve was well fitted with the Poole-Frenkel emission model, which could be attributed to the formation of the aluminum oxide layer. Accordingly, both the trapping/de-trapping of carriers and the formation/rupture of conductive filaments were introduced as transport mechanisms in our devices. Although the partial P _x O _y species on BP were inevitable during the liquid phase exfoliation process, which was regarded as the disadvantages for various applications, it turns to a key point for improving performances in memory devices. The proposed approach to integrating BP nanoflakes in the active layer of the RRAM device could pave the way for next-generation memory devices.