Atomic-Scale Imaging of Clay Mineral Nanosheets and Their Supramolecular Complexes through Electron Microscopy: A Supramolecular Chemist's Perspective

Recent advancements in electron microscopy techniques have revolutionized the ability to directly visualize and understand the intricate world of supramolecular chemistry. This paper provides a concise overview of a study delving into the atomic-scale imaging of monolayer clay mineral nanosheets and their associated supramolecular complexes. The imaging is conducted by harnessing the power of aberration-corrected scanning transmission electron microscopy (STEM). Clay mineral nanosheets, with their anionic charge and ultrathin thickness (of 1 nm), serve as a stable Coulombic host material for cationic guest molecules through electrostatic interactions, facilitating exceptional stability and control during observation. By incorporation of heavy-metal atom markers coordinated within the target molecules, high-angle annular dark field STEM enables a clear visualization of these supramolecular complexes. This approach helps to overcome the limitations of graphene-based systems and expands the possibilities of atomic-scale imaging of nonperiodic molecular assemblies formed by weak supramolecular interactions. The fusion of electron microscopy techniques with the principles of supramolecular and material chemistry offers exciting opportunities for studying the structure, behavior, and properties of complex supramolecular systems. It sheds light on the intricate molecular architectures and design principles governing these systems. This study showcases the immense potential of electron microscopy in supramolecular chemistry and invites researchers from various disciplines to explore the transformative possibilities of atomic-scale imaging in the field.

Enhanced NO2 Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.

Artificial Optoelectronic Synapse Based on Violet Phosphorus Microfiber Arrays

Memristor-based artificial synapses are regarded as the most promising candidate to develop brain-like neuromorphic network computers and overcome the bottleneck of Von-Neumann architecture. Violet phosphorus (VP) as a new allotrope of available phosphorus with outstanding electro-optical properties and stability has attracted more and more attention in the past several years. In this study, large-scale, high-yield VP microfiber vertical arrays have been successfully developed on a Sn-coated graphite paper and are used as the memristor functional layers to build reliable, low-power artificial synaptic devices. The VP devices can well mimic the major synaptic functions such as short-term memory (STM), long-term memory (LTM), paired-pulse facilitation (PPF), spike timing-dependent plasticity (STDP), and spike rate-dependent plasticity (SRDP) under both electrical and light stimulation conditions, even the dendritic synapse functions and simple logical operations. By virtue of the excellent performance, the VP artificial synapse devices can be conductive to building high-performance optic-neural synaptic devices simulating the human-like optic nerve system. On this basis, Pavlov's associative memory can be successfully implemented optically. This study provides a promising approach for the design and manufacture of VP-based artificial synaptic devices and outlines a direction with multifunctional neural devices.

A review on advanced band-structure engineering with dynamic control for nonvolatile memory based 2D transistors

With advancements in information technology, an enormous amount of data is being generated that must be quickly accessible. However, conventional Si memory cells are approaching their physical limits and will be unable to meet the requirements of intense applications in the future. Notably, 2D atomically thin materials have demonstrated multiple novel physical and chemical properties that can be used to investigate next-generation electronic devices and breakthrough physical limits to continue Moore's law. Band structure is an important semiconductor parameter that determines their electrical and optical properties. In particular, 2D materials have highly tunable bandgaps and Fermi levels that can be achieved through band structure engineering methods such as heterostructure, substrate engineering, chemical doping, intercalation, and electrostatic doping. In particular, dynamic control of band structure engineering can be used in recent advancements in 2D devices to realize nonvolatile storage performance. This study examines recent advancements in 2D memory devices that utilize band structure engineering. The operational mechanisms and memory characteristics are described for each band structure engineering method. Band structure engineering provides a platform for developing new structures and realizing superior performance with respect to nonvolatile memory.

Switching the memory behaviour from binary to ternary by triggering S62- relaxation in polysulfide-bearing zinc-organic complex molecular memories

The use of crystalline metal-organic complexes with definite structures as multilevel memories can enable explicit structure-property correlations, which is significant for designing the next generation of memories. Here, four Zn-polysulfide complexes with different degrees of conjugation have been fabricated as memory devices. ZnS₆(L)₂-based memories (L = pyridine and 3-methylpyridine) can exhibit only bipolar binary memory performances, but ZnS₆(L)-based memories (L = 2,2'-bipyridine and 1,10-phenanthroline) illustrate non-volatile ternary memory performances with high ON2/ON1/OFF ratios (10^4.22/10^2.27/1 and 10^4.85/10^2.58/1) and ternary yields (74% and 78%). Their ON1 states stem from the packing adjustments of organic ligands upon the injection of carriers, and the ON2 states are a result of the ring-to-chain relaxation of S₆^2- anions. The lower conjugated degrees in ZnS₆(L)₂ result in less compact packing; consequently, the adjacent S₆^2- rings are too long to trigger the S₆^2- relaxation. The deep structure-property correlation in this work provides a new strategy for implementing multilevel memory by triggering polysulfide relaxation based on the conjugated degree regulation of organic ligands.

On the switching mechanism and optimisation of ion irradiation enabled 2D MoS2 memristors

Memristors are prominent passive circuit elements with promising futures for energy-efficient in-memory processing and revolutionary neuromorphic computation. State-of-the-art memristors based on two-dimensional (2D) materials exhibit enhanced tunability, scalability and electrical reliability. However, the fundamental of the switching is yet to be clarified before they can meet industrial standards in terms of endurance, variability, resistance ratio, and scalability. This new physical simulator based on the kinetic Monte Carlo (kMC) algorithm reproduces the defect migration process in 2D materials and sheds light on the operation of 2D memristors. The present work employs the simulator to study a two-dimensional 2H-MoS₂ planar resistive switching (RS) device with an asymmetric defect concentration introduced by ion irradiation. The simulations unveil the non-filamentary RS process and propose routes to optimize the device's performance. For instance, the resistance ratio can be increased by 53% by controlling the concentration and distribution of defects, while the variability can be reduced by 55% by increasing 5-fold the device size from 10 to 50 nm. Our simulator also explains the trade-offs between the resistance ratio and variability, resistance ratio and scalability, and variability and scalability. Overall, the simulator may enable an understanding and optimization of devices to expedite cutting-edge applications.

Redox Switching Behavior in Resistive Memory Device Designed Using a Solution-Processable Phenalenyl-Based Co(II) Complex: Experimental and DFT Studies

We herein report a novel square-planar complex [Co^IIL], which was synthesized using the electronically interesting phenalenyl-derived ligand LH₂ = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one). The molecular structure of the complex is confirmed with the help of the single-crystal X-ray diffraction technique. [Co^IIL] is a mononuclear complex where the Co(II) ion is present in the square-planar geometry coordinated by the chelating bis-phenalenone ligand. The solid-state packing of [Co^IIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [Co^IIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [Co^IIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/Co^IIL/Al, and characterized using the write-read-erase-read cycle. The device has interestingly shown a stable and reproducible switching between two different resistance states for more than 2000 s. Observed bistable resistive states of the device have been explained by corroborating the electrochemical characterizations and density functional theory studies, where the role of the Co^II metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed.

Enhancing the Performance of Electret-Free Phototransistor Memory by Using All-Conjugated Block Copolymer

Conjugated polymers are of great interest owing to their potential in stretchable electronics to function under complex deformation conditions. To improve the performance of conjugated polymers, various structural designs have been proposed and these conjugated polymers are specially applied in exotic optoelectronics. In this work, a series of all-conjugated block copolymers (PII2T-b-PNDI2T) comprising poly(isoindigo-bithiophene) (PII2T) and poly(naphthalenediimide-bithiophene) (PNDI2T) are developed with varied compositions and applied to electret-free phototransistor memory. Accordingly, these memory devices present p-type transport capability and electrical-ON/photo-OFF memory behavior. The efficacy of the all-conjugated block copolymer design in improving the memory-photoresponse properties in phototransistor memory is revealed. By optimizing the composition of the block copolymer, the corresponding device achieves a wide memory window of 36 V and a high memory ratio of 7 × 10⁴ . Collectively, the results of this study indicate a new concept for designing electret-free phototransistor memory by using all-conjugated block copolymer heterojunctions to mitigate the phase separation of conjugated polymer blends. Meanwhile, the intrinsic optoelectronic properties of the constituent conjugated polymers can be well-maintained by using an all-conjugated block copolymer design.

A Highly Crystalline Single Layer 2D Polymer for Low Variability and Excellent Scalability Molecular Memristors

Large-scale growth of highly crystalline single layer 2D polymers (SL-2DPs) and their subsequent integration into memristors is key to advancing the development of high-density data storage devices. However, leakage problems resulting from the porous structure of 2DPs continue to make such advances extremely challenging. Herein, we overcome this issue by incorporating long alkoxy chains into key molecular building blocks to obtain a highly crystalline 2DP, as visualized by scanning tunneling microscopy, and prevent metal permeation in the subsequent device fabrication process. SL-2DP memristors constructed via direct evaporation of the top electrodes exhibit low variability (σ_Vset  = 0.14) due to the single-monomer-thick feature together with the high regular structure and coordination ability which minimizes the stochastic spatial distribution of conductive filaments (CFs) in both vertical and lateral dimensions. The variability is further decreased to 0.04 by confining the formation and fracture of CFs to the interface through the utilization of bilayer junctions. Using peak force tunneling atomic force microscopy, the nanometer scalability (< 50 nm² ) and low power consumption of these molecular memristor devices are demonstrated.

Integrated Memory Devices Based on 2D Materials

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Optically Encodable and Erasable Multilevel Nonvolatile Flexible Memory Device Based on Metal-Organic Frameworks

Multilevel and flexible nonvolatile memory (NVM) is a promising candidate for data storage in next-generation devices but its high bias and low mobility of conducting channels are often its drawbacks. In this study, we demonstrate a low bias of smaller than 0.1 V and a high-mobility graphene layer as a conducting channel for flexible optoelectronic NVM based on a composite thin film of indium-based MOF-derived InCl₃ and 4,4-oxydiphthalic anhydride (odpta), Na[In₃(odpt)₂(OH)₂(H₂O)₂](H₂O)₄, and reduced graphene oxide (rGO). The optoelectronic NVM device can be encoded and erased optically by ultraviolet (UV) light and visible light, respectively. Our device also achieves memory states over 192 (6-bit storage) distinct levels, which can emerge as mass data storage. It also shows an excellent endurance of write-erase cycles under irradiation with a laser of varying wavelengths, the mechanical stability of more than 1000 bending cycles, and stable retention for longer than 10 000 s. These results open an alternative route for developing low bias and innovative optoelectronic technologies.

Wafer-scale solution-processed 2D material analog resistive memory array for memory-based computing

Realization of high-density and reliable resistive random access memories based on two-dimensional semiconductors is crucial toward their development in next-generation information storage and neuromorphic computing. Here, wafer-scale integration of solution-processed two-dimensional MoS2 memristor arrays are reported. The MoS2 memristors achieve excellent endurance, long memory retention, low device variations, and high analog on/off ratio with linear conductance update characteristics. The two-dimensional nanosheets appear to enable a unique way to modulate switching characteristics through the inter-flake sulfur vacancies diffusion, which can be controlled by the flake size distribution. Furthermore, the MNIST handwritten digits recognition shows that the MoS2 memristors can operate with a high accuracy of >98.02%, which demonstrates its feasibility for future analog memory applications. Finally, a monolithic three-dimensional memory cube has been demonstrated by stacking the two-dimensional MoS2 layers, paving the way for the implementation of two memristor into high-density neuromorphic computing system.

Recent advances in biological applications of nanomaterials through defect engineering

In recent years, defect engineering sprung up in the artificial nanomaterials (NMs) has attracted significant attention, since the physical and chemical properties of NMs could be largely optimized based on the rational control of different defect types and densities. Defective NMs equipped with the improved electric and catalytic ability, would be widely utilized as the photoelectric device and catalysts to alleviate the growing demands of industrial production and environmental treatments. In particular, considering that the features of targeting, adsorptive, loading and optical could be adjusted by the introduction of defects, numerous defective NMs are encouraged to be applied in the biological fields including bacterial inactivation, cancer therapy and so on. And this review is devoted to summarize the recent biological applications of NMs with abundant defects. Moreover, the opportunity of these defective NMs released into the surrounding environment continue to increase, the direct and indirect contact with biological molecules and organisms would be inevitable. Due to its high reactivity and adsorption triggered by defects, NMs tend to exhibit overestimate biological behaviors and effects on organisms. Thus, the sections regarding toxicological effects of NMs with abundant defects are also carried out to supplement the safety assessments of NMs and guide further applications in the industrial production and living.

Covalent RGD-graphene-phthalocyanine nanocomposite for fluorescence imaging-guided dual active/passive tumor-targeted combinatorial phototherapy

Poor tumor selectivity, low stability and quenched fluorescence are the main challenges to be overcome for nanomedicine, and are mainly caused by the dissociation of the nanostructure and aggregation of chromophores in the biological environment. Herein, covalently connected nanoparticles RGD-graphene-phthalocyanine (RGD-GO-SiPc) were constructed based on RGD peptide, silicon phthalocyanine (SiPc) and graphene oxide (GO) via a conjugation reaction for fluorescence imaging-guided cancer-targeted combinatorial phototherapy. The prepared RGD-GO-SiPc exhibited supreme biological stability, high-contrast fluorescence imaging, significantly enhanced NIR absorption, high photothermal conversion efficiency (25.6%), greatly improved cancer-targeting capability, and synergistic photodynamic (PDT) and photothermal therapy (PTT) efficacy along with low toxicity. Both in vitro and in vivo biological studies showed that RGD-GO-SiPc is a kind of promising multifunctional nanomedicine for fluorescence imaging-guided combined photothermal and photodynamic therapy with dual active/passive tumor-targeting properties.

Stimuli-Responsive Memristive Materials for Artificial Synapses and Neuromorphic Computing

Neuromorphic computing holds promise for building next-generation intelligent systems in a more energy-efficient way than the conventional von Neumann computing architecture. Memristive hardware, which mimics biological neurons and synapses, offers high-speed operation and low power consumption, enabling energy- and area-efficient, brain-inspired computing. Here, recent advances in memristive materials and strategies that emulate synaptic functions for neuromorphic computing are highlighted. The working principles and characteristics of biological neurons and synapses, which can be mimicked by memristive devices, are presented. Besides device structures and operation with different external stimuli such as electric, magnetic, and optical fields, how memristive materials with a rich variety of underlying physical mechanisms can allow fast, reliable, and low-power neuromorphic applications is also discussed. Finally, device requirements are examined and a perspective on challenges in developing memristive materials for device engineering and computing science is given.

Polymer-Assisted Space-Confined Strategy for the Foot-Scale Synthesis of Flexible Metal-Organic Framework-Based Composite Films

At the gas-liquid interface, the confined synthesis of metal-organic framework (MOF) films has been extensively developed by spreading an ultrathin oil layer on the aqueous surface as a reactor. However, this interface is susceptible to various disturbances and incapable of synthesizing large-area crystalline MOF films. Herein, we developed a polymer-assisted space-confined strategy to synthesize large-area films by blending poly(methyl methacrylate) (PMMA) into the oil layer, which improved the stability of the gas-liquid interface and the self-shrinkage of the oil layer on the water surface. Meanwhile, the as-synthesized MOFs as a quasi-solid substrate immobilized the edge of the oil layer, which maintained a large spreading area. Thanks to this synergistic effect, we synthesized the freestanding MOF-based film with a foot-level (0.66 ft) lateral dimension, which is the largest size reported so far. Besides, due to the phase separation of the two components, the MOF-PMMA composite film combined the conductivity of MOFs (1.13 S/m) with the flexibility of PMMA and exhibited excellent mechanical properties. More importantly, this strategy could be extended to the preparation of other MOFs, coordination polymers (CPs), and even inorganic material composite films, bringing light to the design and large-scale synthesis of various composite films for practical applications.

Local Interactions of Atmospheric Oxygen with MoS2 Crystals

Thin and single MoS2 flakes are envisioned to contribute to the flexible nanoelectronics, particularly in sensing, optoelectronics and energy harvesting. Thus, it is important to study their stability and local surface reactivity. Their most straightforward surface reactions in this context pertain to thermally induced interactions with atmospheric oxygen. This review focuses on local and thermally induced interactions of MoS2 crystals and single MoS2 flakes. First, experimentally observed data for oxygen-mediated thermally induced morphological and chemical changes of the MoS2 crystals and single MoS2 flakes are presented. Second, state-of-the-art mechanistic insight from computer simulations and arising open questions are discussed. Finally, the properties and fate of the Mo oxides arising from thermal oxidation are reviewed, and future directions into the research of the local MoS2/MoOx interface are provided.

Resistive switching on individual V2O5 nanoparticles encapsulated in fluorinated graphene films

Memristors currently attract much attention as basic building blocks for future neuromorphic electronics. Due to their unusual electronic, optical, magnetic, electrochemical, and structural properties, transition metal oxides offer much potential in the development of memristors. Recent trends in the design and fabrication of electronic devices have led to miniaturization of their working elements, with nanometer-sized structures enjoying increasing demand. In the present study, we investigated resistive switching on individual vanadium oxide (V₂O₅) crystal-hydrate nanoparticles, 2 to 10 nm in size, encapsulated in fluorinated graphene (FG). Measurements using a conductive atomic force microscope (c-AFM) probe showed that the core-shell V₂O₅/FG nanoparticles make it possible to achieve bipolar resistive switching, reproducible during 10⁴ switching cycles, with the ON/OFF current ratio reaching 10³-10⁵. The switching voltage of the structures depends on the thickness of the FG shells of the composite particles and equals ∼2-4 V. It is shown that the encapsulation of V₂O₅ particles in fluorinated graphene ensures a high stability of the resistive switching effect and, simultaneously, prevents the escape of water from the crystalline vanadium oxide hydrates. A qualitative model is proposed to describe the bipolar resistive switching effect in examined structures. Results reported in the present article will prove useful in creating bipolar nanoswitches.

A Bio-Inspired Molecular Design Strategy toward 2D Organic Semiconductor Crystals with Superior Integrated Optoelectronic Properties

Inspired by the 2D bilayer lipid membranes in nature, a unique supramolecular "push-pull" synergetic strategy toward self-assembled 2D organic crystals (2DOCs) is proposed in this work, which can effectively suppress the interlayer 3D stacking while maintaining the assembly of the intralayer for 2D growth. For this purpose, a model molecule PF-Py consisting of a planar supramolecular "attractor" and a nonplanar steric "repellor" is designed for the solution self-assembly process. Well-defined 2DOCs including crystal nanosheets and millimeter-sized crystal films with layered amphiphile-like packing are obtained, which is analogical to the cell membranes of living organisms. Thanks to the special packing mode, the 2DOCs have fascinating integrated photoelectric property, with high mobility of 7.8 × 10^-2 cm² V^-1 s^-1 , high crystalline state photoluminescence quantum yield of 55%, and superior deep-blue laser characteristics with a low threshold of 5.51 µJ cm^-2 . This supramolecular synergetic strategy advances the design of 2D organic semiconductor crystals for high performance optoelectronics.

Thermal-Responsive Polyoxometalate-Metalloviologen Hybrid: Reversible Intermolecular Three-Component Reaction and Temperature-Regulated Resistive Switching Behaviors

The development of new-type memristors with special performance is of great interest. Herein, an inorganic-organic hybrid crystalline polyoxometalate (POM) with usual dynamic structures is reported and used as active material for fabricating memristor with unique temperature-regulated resistive switching behaviors. The hybrid POM not only exhibits tunable thermochromic properties, but also thermal-induced reversible aggregation and disaggregation reactions, leading to reversible structural transformations in SCSC fashion. Further, the memory device using the hybrid POM as active layer exhibits uncommon performance, which can keep resistive switching silent in the low temperature range of 30-150 °C, but show nonvolatile memory behavior in the high temperature range of 150-270 °C. Particularly, the silent and working states at three special temperatures (30, 150 and 270 °C) can be monitored by chromism. The correlation between structure and resistive switching property of the material has been discussed. The work demonstrates that crystalline inorganic-organic hybrid POMs are promising materials for making memristors with superior performance.

Hot Injection-Based Synthesized Colloidal CdSe Quantum Dots Embedded in Poly(4-vinylpyridine) (PVP) Matrix Form a Nanoscale Heterostructure for a High On-Off Ratio Memory-Switching Device

Chalcogenide-based quantum dots are useful for the application of memory-switching devices because of the control in the trap states in the materials. The control in the trap states can be achieved using a hot-injection colloidal synthesis method that produces temperature-dependent size-variable quantum dots. In addition to this, formation of a nanoscale heterostructure with an insulating material adds to the charge-trapped switching mechanism. Here, we have shown that the colloidal monodispersed CdSe quantum dots and poly(4-vinylpyridine) (PVP) formed a nanoscale heterostructure between themselves when taken in a suitable ratio to fabricate a device. This heterostructure helps realize memory-switching in the device with a maximum on-off current ratio of 10⁵. The switching in the device is mainly due to the trap states in the CdSe quantum dots. The conduction in the off state is due to thermal charge injection and space charge injection conduction and in the on state, due to the Ohmic conduction mechanism.

Physicochemical defect guided dewetting of ultrathin films to fabricate nanoscale patterns

Pathways to fabricate self-organized nanostructures have been identified exploiting the instabilities of ultrathin (<100 nm) polystyrene (PS) film on the polydimethylsiloxane (PDMS) substrates loaded with discrete and closely packed gold nanoparticles (AuNPs). The AuNPs were deposited on the PDMS substrates by chemical treatment, and the size and periodicity of the AuNPs were varied before coating the PS films. The study unveils that the physicochemical heterogeneity created by the AuNPs on the PDMS surface could guide the hole-formation, influence the average spacing between the holes formed at the initial dewetting stage, and affects the spacing and periodicity of the droplets formed at the end of the dewetting phase. The size and spacing of the holes and the droplets could be tuned by varying the nanoparticle loading on the PDMS substrate. Interestingly, as compared to the dewetting of PS films on the homogeneous PDMS surfaces, the AuNP guided dewetted patterns show ten-fold miniaturization, leading to the formation of the micro-holes and nanodroplets. The spacing between the droplets could also see a ten-fold reduction resulting in high-density random patterns on the PDMS substrate. Further, the use of a physicochemical substrate with varying density of physicochemical heterogeneities could impose a long-range order to the dewetted patterns to develop a gradient surface. The reported results can be of significance in the fabrication of high-density nanostructures exploiting the self-organized instabilities of thin polymers films.

Recent advances in MoS2-based photothermal therapy for cancer and infectious disease treatment

Photothermal therapy (PTT) is a treatment combining laser irradiation and a photothermal transduction agent (PTA) to generate hyperthermia, which is used to efficiently and effectively treat cancer and prevent bacteria-induced infectious diseases. MoS2, an increasingly used two-dimensional transition metal dichalcogenide, which shows high absorbance in the near infrared (NIR) laser region, has been extensively utilized as a novel PTA in biomedical applications. The use of MoS2 as an advanced photoabsorbing agent has introduced a more efficient cancer therapy and improved antibacterial efficacy. In this review, we firstly summarize the recent advances in the MoS2-based platform for PTT in cancer and bacteria-induced infectious diseases treatments. We then discuss that the combination of MoS2-based PTT and other biomedical methods along with multimodality imaging, such as chemotherapy, photodynamic therapy (PDT) and immunotherapy, might be a promising strategy for cancer treatment. Furthermore, a new concept is proposed wherein MoS2-based PTT and combined therapies based on this could be more effective for the treatment of various bacteria-induced infectious diseases. Finally, research progress, challenges, and perspectives for the future development of this MoS2-based platform in cancer and bacteria-induced infectious disease treatments are discussed and concluded. Collectively, we think that MoS2-based PTT with high therapeutic efficacy and minimal side-effects could be potentially applied in clinical settings to improve cancer and infectious disease treatments.

Transition-metal adatoms on 2D-GaAs: a route to chiral magnetic 2D materials by design

Using relativistic density-functional calculations, we examine the magneto-crystalline anisotropy and exchange properties of transition-metal atoms adsorbed on 2D-GaAs. We show that single Mn and Mo atom (Co and Os) strongly bind on 2D-GaAs, and induce local out-of-plane (in-plane) magnetic anisotropy. When a pair of TM atoms is adsorbed on 2D-GaAs in a close range from each other, magnetisation properties change (become tunable) with respect to concentrations and ordering of the adatoms. In all cases, we reveal presence of strong Dzyaloshinskii-Moriya interaction. These results indicate novel pathways towards two-dimensional chiral magnetic materials by design, tailored for desired applications in magneto-electronics.

A Library of Atomically Thin 2D Materials Featuring the Conductive-Point Resistive Switching Phenomenon

Non-volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few-layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX₂ , M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS₂ /MoS₂ ), and an atomically thin insulator (h-BN). These results indicate the universality of the phenomenon in 2D non-conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming-free characteristic. A dissociation-diffusion-adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive-point mechanism supported by first-principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

Recent Progress in Solution-Based Metal Oxide Resistive Switching Devices

Metal oxide resistive switching memories have been a crucial component for the requirements of the Internet of Things, which demands ultra-low power and high-density devices with new computing principles, exploiting low cost green products and technologies. Most of the reported resistive switching devices use conventional methods (physical and chemical vapor deposition), which are quite expensive due to their up-scale production. Solution-processing methods have been improved, being now a reliable technology that offers many advantages for resistive random-access memory (RRAM) such as high versatility, large area uniformity, transparency, low-cost and a simple fabrication of two-terminal structures. Solution-based metal oxide RRAM devices are emergent and promising non-volatile memories for future electronics. In this review, a brief history of non-volatile memories is highlighted as well as the present status of solution-based metal oxide resistive random-access memory (S-RRAM). Then, a focus on describing the solution synthesis parameters of S-RRAMs which induce a massive influence in the overall performance of these devices is discussed. Next, a precise analysis is performed on the metal oxide thin film and electrode interface and the recent advances on S-RRAM that will allow their large-area manufacturing. Finally, the figures of merit and the main challenges in S-RRAMs are discussed and future trends are proposed.

Wafer-scale graphene-ferroelectric HfO2/Ge-HfO2/HfO2 transistors acting as three-terminal memristors

In this paper we report a set of experiments at the wafer level regarding field-effect transistors with a graphene monolayer channel transferred on the ferroelectric HfO₂/Ge-HfO₂/HfO₂ three-layer structure. This kind of transistor has a switching ratio of 10³ between on and off states due to the bandgap in graphene induced by the ferroelectric structure. Both top and back gates effectively control the carriers' charge flow in graphene. The transistor acts as a three-terminal memristor, termed a memtransistor, with applications in neuromorphic computation.

Backbone Engineering of Diketopyrrolopyrrole-Based Conjugated Polymers through Random Terpolymerization for Improved Mobility-Stretchability Property

Conjugated polymers synthesized through random terpolymerization have recently attracted great research interest due to the synergetic effect on the polymer's crystallinity and semiconducting properties. Several studies have demonstrated the efficacy of random terpolymerization in fine-tuning the aggregation behavior and optoelectronic property of conjugated polymers to yield enhanced device performance. However, as an influential approach of backbone engineering, its efficacy in modulating the mobility-stretchability property of high-performance conjugated polymers has not been fuller explored to date. Herein, a series of random terpolymers based on the diketopyrrolopyrrole-bithiophene (DPP-2T) backbone incorporating different amounts of isoindigo (IID) unit are synthesized, and their structure-mobility-stretchability correlation is thoroughly investigated. Our results reveal that random terpolymers containing a low IID content (DPP95 and DPP90) show enhanced interchain packing and solid-state aggregation to result in improved charge-transporting performance (can reach 4 order higher) compared to the parent polymer DPP100. In addition, owing to the enriched amorphous feature, DPP95 and DPP90 deliver an improved orthogonal mobility (μ_h) of >0.01 cm² V^-1 s^-1 under a 100% strain, higher than the value (∼0.002 cm² V^-1 s^-1) of DPP100. Moreover, DPP95 even yields 20% enhanced orthogonal μ_h retention after 800 stretching-releasing cycles with 60% strain. As concluded from a series of analyses, the improved mobility-stretchability property exerted by random terpolymerization arises from the enriched amorphous feature and enhanced aggregation behavior imposed by the geometry mismatch between different acceptors (DPP and IID). This study demonstrates that backbone engineering through rational random terpolymerization not only enhances the mobility-stretchability of a conjugated polymer but also realizes a better mechanical endurance, providing a new perspective for the design of high-performance stretchable conjugated polymers.

Ammonium phosphomolybdate: a material for dielectric crossover and resistive switching performance

The yellow ammonium phosphomolybdate [(NH₄)₃PMo₁₂O₄₀] (YAPM) is a robust and elegant compound that has found innumerable field applications. Herein, we have shown that this inorganic polymer serves as a novel dielectric material and a compound for memory device fabrication. It displays changeable dielectric performance and ac conductivity under UV (∼365 nm) irradiation. Drastic lowering of the dielectric constant (ε') was observed with the increase in dielectric loss factor, which was ascertained due to electron accumulation under UV exposure producing green APM (GAPM). The contributions of the Maxwell-Wagner polarization and the dipolar relaxation are correlated with the charge transfer and dielectric contribution of the material. Interestingly, the pressure-induced reduction of Mo(vi) to Mo(v) is reported for the first time and is similar to UV-exposed mixed-valence GAPM, which was corroborated by EPR spectra. In the ac signal, the crossover from quantum mechanical tunneling to hopping conduction is an adequate explanation for YAPM under UV irradiation. The fabricated device Au‖YAPM‖Au on a flexible paper substrate shows a resistive memory behavior that is modeled as a Schottky-type emission (SE) and Poole-Frenkel (PF) carrier transport for the OFF and ON states, respectively. The device exhibited a constant ON-OFF current ratio of 2 × 10² for YAPM. The OFF state endurance of the device (with 3 V pulses having 1 s time-period) under UV showed a steady increment current strength with time. After 100 s of UV exposure, the Au‖YAPM‖Au device became Au‖GAPM‖Au, and the conductivity completely shifted to a stable ON-state (at 300 s).

Logic-in-memory based on an atomically thin semiconductor

The growing importance of applications based on machine learning is driving the need to develop dedicated, energy-efficient electronic hardware. Compared with von-Neumann architectures, brain-inspired in-memory computing uses the same basic device structure for logic operations and data storage^1–3, thus promising to reduce the energy cost of data-centric computing significantly⁴. While there is ample research focused on exploring new device architectures, the engineering of material platforms suitable for such device designs remains a challenge. Two-dimensional materials^5,6 such as semiconducting MoS2 could stand out as a promising candidate to face this obstacle thanks to their exceptional electrical and mechanical properties^7–9. Here, we explore large-area grown MoS2 as an active channel material for developing logic-in-memory devices and circuits based on floating-gate field-effect transistors (FGFET). The conductance of our FGFETs can be precisely and continuously tuned, allowing us to use them as building blocks for reconfigurable logic circuits where logic operations can be directly performed using the memory elements. After demonstrating a programmable NOR gate, we show that this design can be simply extended to implement more complex programmable logic and functionally complete sets of functions. Our findings highlight the potential of atomically thin semiconductors for the development of next-generation low-power electronics.

Probing the Dielectric Effects on the Colloidal 2D Perovskite Oxides by Eu3+ Luminescence

Solution-processing technique of two-dimensional (2D) materials requires the knowledge of the dielectric effects on mutual interaction of each component and the layer structure variation. This research uses the magnetic dipole (MD) transition of intentionally doped Eu³⁺, which is dependent on the dielectric environments, as an optical probe to study the dielectric effects on the colloidal charge-bearing [Ca_1.8Eu_0.1Na_0.1Nb₃O₁₀]^- perovskite nanosheets (NSs) in various solvents. Results reveal that the solvent molecules with longer alkyl chain could more easily impact the ligands on the surface of the NSs, leading to a weaker interaction between the ligands and the NSs as well as less distortion of Eu³⁺ site (Ca²⁺ site) at the inner layer of the NSs. The large-sized ligands would impede the stacking of the NSs, while H⁺ would make the H⁺-modified NSs restack more easily. With the assistance of density functional theory (DFT) simulation, it is found that the ligands or the dielectric solvents could distort or relax the surface covalent polyhedra [NbO₆]^7- to a larger extent than the inner polyhedra. Small-sized ligands and a large thickness with more atomic layers of the NSs can resist structural variation caused by solvents. The acquired knowledge in this research benefits the understanding of the solution-processing technique for industrial application of 2D materials.

Programmable electronic synapse and nonvolatile resistive switches using MoS2 quantum dots

Brain-inspired computation that mimics the coordinated functioning of neural networks through multitudes of synaptic connections is deemed to be the future of computation to overcome the classical von Neumann bottleneck. The future artificial intelligence circuits require scalable electronic synapse (e-synapses) with very high bit densities and operational speeds. In this respect, nanostructures of two-dimensional materials serve the purpose and offer the scalability of the devices in lateral and vertical dimensions. In this work, we report the nonvolatile bipolar resistive switching and neuromorphic behavior of molybdenum disulfide (MoS2) quantum dots (QD) synthesized using liquid-phase exfoliation method. The ReRAM devices exhibit good resistive switching with an On-Off ratio of 104, with excellent endurance and data retention at a smaller read voltage as compared to the existing MoS2 based memory devices. Besides, we have demonstrated the e-synapse based on MoS2 QD. Similar to our biological synapse, Paired Pulse Facilitation / Depression of short-term memory has been observed in these MoS2 QD based e-synapse devices. This work suggests that MoS2 QD has potential applications in ultra-high-density storage as well as artificial intelligence circuitry in a cost-effective way.

Memtransistors Based on Nanopatterned Graphene Ferroelectric Field-Effect Transistors

The ultimate memristor, which acts as resistive memory and an artificial neural synapse, is made from a single atomic layer. In this manuscript, we present experimental evidence of the memristive properties of a nanopatterned ferroelectric graphene field-effect transistor (FET). The graphene FET has, as a channel, a graphene monolayer transferred onto an HfO₂-based ferroelectric material, the channel being nanopatterned with an array of holes with a diameter of 20 nm.

Neuromorphic nanoelectronic materials

Memristive and nanoionic devices have recently emerged as leading candidates for neuromorphic computing architectures. While top-down fabrication based on conventional bulk materials has enabled many early neuromorphic devices and circuits, bottom-up approaches based on low-dimensional nanomaterials have shown novel device functionality that often better mimics a biological neuron. In addition, the chemical, structural and compositional tunability of low-dimensional nanomaterials coupled with the permutational flexibility enabled by van der Waals heterostructures offers significant opportunities for artificial neural networks. In this Review, we present a critical survey of emerging neuromorphic devices and architectures enabled by quantum dots, metal nanoparticles, polymers, nanotubes, nanowires, two-dimensional layered materials and van der Waals heterojunctions with a particular emphasis on bio-inspired device responses that are uniquely enabled by low-dimensional topology, quantum confinement and interfaces. We also provide a forward-looking perspective on the opportunities and challenges of neuromorphic nanoelectronic materials in comparison with more mature technologies based on traditional bulk electronic materials.

Two-Dimensional Unipolar Memristors with Logic and Memory Functions

Two-dimensional materials have been widely used in electronics due to their electrical properties that are not accessible in traditional materials. Here, we present the first demonstration of logic functions of unipolar memristors made of functionalized HfSe_2-xO_x flakes and memtransistors made of MoS₂/graphene/HfSe_2-xO_x van der Waals heterostructures. The two-terminal memristors exhibit stable unipolar switching behavior with high switching ratio (>10⁶), high operating temperature (106 °C), long-term endurance (>10⁴ s), and multibit data storage and can operate as memory latches and logic gates. Benefiting from these superior memristive properties, the three-terminal heterostructure memtransistors show wide tunability in electrical switching behaviors, which can simultaneously implement logic operation and data storage. Finally, we investigate their application prospect in logical units with memory capability, such as D-type flip-flop. These results demonstrate the potential of two-dimensional materials for resistive switching applications and open up an avenue for future in-memory computing.

Memristive and Memory Impedance Behavior in a Photo-Annealed ZnO–rGO Thin-Film Device

An oxygen-rich ZnO-reduced graphene oxide (rGO) thin film was synthesized using a photo-annealing technique from zinc precursor (ZnO)–graphene oxide (GO) sol–gel solution. X-ray diffraction (XRD) results show a clear characteristic peak corresponding to rGO. The scanning electron microscope (SEM) image of the prepared thin film shows an evenly distributed wrinkled surface structure. Transition Metal Oxide (TMO)-based memristive devices are nominees for beyond CMOS Non-Volatile Memory (NVRAM) devices. The two-terminal Metal–TMO (Insulator)–Metal (MIM) memristive device is fabricated using a synthesized ZnO–rGO as an active layer on fluorine-doped tin oxide (FTO)-coated glass substrate. Aluminum (Al) is deposited as a top metal contact on the ZnO–rGO active layer to complete the device. Photo annealing was used to reduce the GO to rGO to make the proposed method suitable for fabricating ZnO–rGO thin-film devices on flexible substrates. The electrical characterization of the Al–ZnO–rGO–FTO device confirms the coexistence of memristive and memimpedance characteristics. The coexistence of memory resistance and memory impedance in the same device could be valuable for developing novel programmable analog filters and self-resonating circuits and systems.

Advances of 2D bismuth in energy sciences

Since graphene has been successfully exfoliated, two-dimensional (2D) materials constitute a vibrant research field and open vast perspectives in high-performance applications. Among them, bismuthene and 2D bismuth (Bi) are unique with superior properties to fabricate state-of-the-art energy saving, storage and conversion devices. The largest experimentally determined bulk gap, even larger than those of stanene and antimonene, allows 2D Bi to be the most promising candidate to construct room-temperature topological insulators. Moreover, 2D Bi exhibits cyclability for high-performance sodium-ion batteries, and the enlarged surface together with the good electrochemical activity renders it an efficient electrocatalyst for energy conversion. Also, the air-stability of 2D Bi is better than that of silicene, germanene, phosphorene and arsenene, which could enable more practical applications. This review aims to thoroughly explore the fundamentals of 2D Bi and its improved fabrication methods, in order to further bridge gaps between theoretical predictions and experimental achievements in its energy-related applications. We begin with an introduction of the status of 2D Bi in the 2D-material family, which is followed by descriptions of its intrinsic properties along with various fabrication methods. The vast implications of 2D Bi for high-performance devices can be envisioned to add a new pillar in energy sciences. In addition, in the context of recent pioneering studies on moiré superlattices of other 2D materials, we hope that the improved manipulation techniques of bismuthene, along with its unique properties, might even enable 2D Bi to play an important role in future energy-related twistronics.

Effect of tryptophan residues on gold mineralization by a gold reducing peptide

AuBP1, obtained by phage display selection, was previously shown to produce gold nanoparticles without reducing agents. The tryptophan (Trp) residue located at the N-terminus of this peptide contributes to the reduction of Au³⁺ to Au⁰ and is involved in the nucleation and crystal growth of gold nanoparticles. However, clear guidelines for relationships between the number of Trp residues in the peptide and its gold reducing ability have not been established. We focused on gold mineralization and attempted to elucidate aspects of the underlying mechanism. We performed a detailed evaluation of the effects of modifying the N-terminus of the core sequence on gold mineralization without reducing agents. Besides, advantages of utilizing peptides in manufacturing gold nanoparticles are shown. UV-Vis measurements, TEM observations, and kinetic analyses were used to show that increasing the number of Trp residues in the peptide increases the reducing ability, causing predominance of the nucleation reaction and the production of small gold nanoparticles. In addition, these peptides also had the ability as a dispersant to protect the surface of gold nanoparticles. Furthermore, the catalytic activity of mineralized gold nanoparticles with peptides was higher than that of a commercial gold nanoparticle. This study should help to elucidate the relationship between peptide sequence and mineralization ability for use in materials chemistry.

Increasing the number of tryptophan (Trp) in peptides led to higher gold reducing ability and the peptides could disperse the generated gold-nanoparticles.

Broadband Nonlinear Optical Response of InSe Nanosheets for the Pulse Generation From 1 to 2 μm

Few-layered InSe nanosheets were fabricated by the simple liquid-phase exfoliation method. The morphology and crystal structure features of InSe nanosheet sample were characterized comprehensively. The photoluminescence (PL) spectrum indicated that the liquid-phase exfoliated InSe nanosheets contained variously layered nanoflakes, where eight layers nanosheets dominate. In addition, the first-principle simulation was carried out to describe the electron density of states (DOS) and the electronic band structures. Moreover, the few-layered InSe nanosheets performed excellent nonlinear absorption properties in a broad spectral band. As an application, the stable passively Q-switched (PQS) lasers with few-layered InSe nanosheets saturable absorbers (SAs) were realized with the operating wavelengths at 1.06, 1.34, and 1.91 μm. The shortest pulse durations were 599, 520, and 210 ns, respectively. Our results confirmed that the few-layered InSe nanosheets could be an excellent candidate for pulsed lasers in wide spectral bands.