Frequency-Dependent Multistep Ferroelectric Polarization Switching Mechanism in P(VDF-TrFE)-Based Capacitors Induced by Polystyrene Electret-like Modulation.

In this work, we investigate multistep ferroelectric polarization switching dynamics of a series of poly(vinylidene fluoride-trifluoroethylene)/polystyrene, P(VDF-TrFE)/PS, as active layers in ferroelectric capacitors with variable P(VDF-TrFE)/PS thickness ratios and a wide range of driving voltage frequencies (1-1000 Hz). The PS electret-like modulation effects on the depolarized field fluctuation are proven to be responsible for this multistep ferroelectric polarization switching process. To be specific, the switching current density peak splits into two peaks in both positive and negative voltage ranges according to the stimulus-response (S-R) data from the metal-ferroelectric-electret-metal capacitor driven by a periodic triangular voltage wave. The double-peak current trough appears when the transitorily suppressed ferroelectric polarization switching occurs while the discharge and recharge of the PS electret by external voltage brings a specific dynamic change in the electric field across ferroelectric (EFE). We also propose a theoretical model to simulate the ferroelectric polarization switching process at a current trough zone. This phenomenon provides new concepts on the electret-modulated multistep ferroelectric switching dynamics, and such switching mechanisms are critical for realizing reliable nonvolatile memory applications in flexible electronics.

High-Contrast Bidirectional Optoelectronic Synapses based on 2D Molecular Crystal Heterojunctions for Motion Detection.

Light-stimulated optoelectronic synaptic devices are fundamental compositions of the neuromorphic vision system. However, there are still huge challenges to achieving both bidirectional synaptic behaviors under light stimuli and high performance. Herein, a bilayer 2D molecular crystal (2DMC) p-n heterojunction is developed to achieve high-performance bidirectional synaptic behaviors. The 2DMC heterojunction-based field effect transistor (FET) devices exhibit typical ambipolar properties and remarkable responsivity (R) of 3.58×104 A W-1 under weak light as low as 0.008 mW cm-2 . Excitatory and inhibitory synaptic behaviors are successfully realized by the same light stimuli under different gate voltages. Moreover, a superior contrast ratio (CR) of 1.53×103 is demonstrated by the ultrathin and high-quality 2DMC heterojunction, which transcends previous optoelectronic synapses and enables application for the motion detection of the pendulum. Furthermore, a motion detection network based on the device is developed to detect and recognize classic motion vehicles in road traffic with an accuracy exceeding 90%. This work provides an effective strategy for developing high-contrast bidirectional optoelectronic synapses and shows great potential in the intelligent bionic device and future artificial vision.

Dragon-Boat-Type Heptathienoacenes: Synthesis, Structures, and Their Applications in OFETs.

A novel synthetic strategy for efficient construction of dragon-boat-type heptathienoacenes (DBHTs) via the α-ß position carbon-carbon cross-coupling between two dithienothiophenes is employed. Their crystal structures are confirmed by X-ray single-crystal analysis. The first thin film FET devices of heptathienoacenes are fabricated using OTS-treated SiO2/Si substrates, in which DBHT-5-based devices exhibit an unprecedented highest hole mobility value of 1.15 cm2 V-1 s-1 and on/off ratios over 106 with a threshold voltage of 0 V.

Challenging Bendable Organic Single Crystal and Transistor Arrays with High Mobility and Durability toward Flexible Electronics.

Bendable organic single crystals are promising candidates for flexible electronics owing to their superior charge-transport properties. However, large-area high-quality organic single crystals are rarely available on the polymer substrates generally used in flexible electronics. Here, a surface-assisted assembly strategy based on a polymer modification, poly(amic acid) (PAA), is developed to grow large-area organic singe crystals on polymer substrates using a simple drop-casting method. The unique surface properties of PAA that enable molecular solution superwetting and promote molecular ordered assembly produce an extraordinary self-driven "meniscus-guided coating" behavior, enabling the fabrication of millimeter-sized, highly aligned organic single crystals for a variety of organic semiconductors. Organic field-effect transistors based on a mode molecule of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene demonstrate the highest (average) mobility of 18.6 cm2 V-1 s-1 (15.9 cm2 V-1 s-1 ), attractively low operating voltage of -3 V, and high flexible durability. The results shed light on the large-area fabrication of organic single crystals on polymer dielectrics toward high-performance and integrated plastic electronics.

Negative Phototransistors with Ultrahigh Sensitivity and Weak-Light Detection Based on 1D/2D Molecular Crystal p-n Heterojunctions and their Application in Light Encoders.

Anomalous negative phototransistors in which the channel current decreases under light illumination hold potential to generate novel and multifunctional optoelectronic applications. Although a variety of design strategies have been developed to construct such devices, NPTs still suffer from far lower device performance compared to well-developed positive phototransistors (PPTs). In this work, a novel 1D/2D molecular crystal p-n heterojunction, in which p-type 1D molecular crystal (1DMC) arrays are embedded into n-type 2D molecular crystals (2DMCs), is developed to produce ultrasensitive NPTs. The p-type 1DMC arrays act as light-absorbing layers to induce p-doping of n-type 2DMCs through charge transfer under illumination, resulting in ineffective gate control and significant negative photoresponses. As a result, the NPTs show remarkable performances in photoresponsivity (P) (1.9 × 108 ) and detectivity (D*) (1.7 × 1017 Jones), greatly outperforming previously reported NPTs, which are one of the highest values among all organic phototransistors. Moreover, the device exhibits intriguing characteristics undiscovered in PPTs, including precise control of the threshold voltage by controlling light signals and ultrasensitive detection of weak light. As a proof-of-concept, the NTPs are demonstrated as light encoders that can encrypt electrical signals by light. These findings represent a milestone for negative phototransistors, and pave the way for the development of future novel optoelectronic applications.

Ultra-thin two-dimensional molecular crystals grown on a liquid surface for high-performance phototransistors.

A novel strategy for the growth of molecularly thin two-dimensional molecular crystals (2DMCs) of organic semiconductors with poor solubility was developed. Large-area bilayer 2DMCs were grown on a liquid surface at elevated temperatures, with record mobility and superior photoresponse.

A "Phase Separation" Molecular Design Strategy Towards Large-Area 2D Molecular Crystals.

2D molecular crystals (2DMCs) have attracted considerable attention because of their unique optoelectronic properties and potential applications. Taking advantage of the solution processability of organic semiconductors, solution self-assembly is considered an effective way to grow large-area 2DMCs. However, this route is largely blocked because a precise molecular design towards 2DMCs is missing and little is known about the relationship between 2D solution self-assembly and molecular structure. A "phase separation" molecular design strategy towards 2DMCs is proposed and layer-by-layer growth of millimeter-sized monolayer or few-layer 2DMCs is realized. High-performance organic phototransistors are constructed based on the 2DMCs with unprecedented photosensitivity (2.58 × 107 ), high responsivity (1.91 × 104 A W-1 ), and high detectivity (4.93 × 1015 Jones). This "phase separation" molecular design strategy provides a guide for the design and synthesis of novel organic semiconductors that self-assemble into large-area 2DMCs for advanced organic (opto)electronics.

Cocrystal Engineering: A Collaborative Strategy toward Functional Materials.

Cocrystal engineering with a noncovalent assembly feature by simple constituent units has inspired great interest and has emerged as an efficient and versatile route to construct functional materials, especially for the fabrication of novel and multifunctional materials, due to the collaborative strategy in the distinct constituent units. Meanwhile, the precise crystal architectures of organic cocrystals, with long-range order as well as free defects, offer the opportunity to unveil the structure-property and charge-transfer-property relationships, which are beneficial to provide some general rules in rational design and choice of functional materials. In this regard, an overview of organic cocrystals in terms of assembly, containing the intermolecular interactions and growth methods, two functionality-related factors including packing structure and charge-transfer nature, and those advanced and novel functionalities, is presented. An outlook of future research directions and challenges for organic cocrystal is also provided.

A Robust Nonvolatile Resistive Memory Device Based on a Freestanding Ultrathin 2D Imine Polymer Film.

Here, the synthesis of a wafer-scale ultrathin 2D imine polymer (2DP) film with controllable thickness from simple benzene-1,3,5-tricarbaldehyde (BTA) and p-phenylenediamine (PDA) building blocks is reported using a Schiff base polycondensation reaction at the air-water interface. The synthesized freestanding 2DP films are porous, insulating, and more importantly, covalently linked, which is ideally suited for nonvolatile memristors that use a conductive filament mechanism. These devices exhibit excellent switching performance with high reliability and reproducibility, with on/off ratios in the range of 102 to 105 depending on the thickness of the film. In addition, the endurance and data retention capability of 2DP-based nonvolatile resistive memristors are up to 200 cycles and 8 × 104 s under constant voltage stress at 0.1 V. The intrinsic flexibility of the covalent organic polymer enables the fabrication of a flexible memory device on a polyimide film, which exhibits as reliable memory performance as that on the rigid substrate. Moreover, the 2DP-based memory device shows outstanding thermal stability and organic solvent resistance, which are desirable properties for applications in wearable devices.

Thermally Activated Delayed Fluorescence in an Organic Cocrystal: Narrowing the Singlet-Triplet Energy Gap via Charge Transfer.

Harvesting non-emissive spin-triplet charge-transfer (CT) excitons of organic semiconductors is fundamentally important for increasing the operation efficiency of future devices. Here we observe thermally activated delayed fluorescence (TADF) in a 1:2 CT cocrystal of trans-1,2-diphenylethylene (TSB) and 1,2,4,5-tetracyanobenzene (TCNB). This cocrystal system is characterized by absorption spectroscopy, variable-temperature steady-state and time-resolved photoluminescence spectroscopy, single-crystal X-ray diffraction, and first-principles calculations. These data reveal that intermolecular CT in cocrystal narrows the singlet-triplet energy gap and therefore facilitates reverse intersystem crossing (RISC) for TADF. These findings open up a new way for the future design and development of novel TADF materials.

Scalable Fabrication of Highly Crystalline Organic Semiconductor Thin Film by Channel-Restricted Screen Printing toward the Low-Cost Fabrication of High-Performance Transistor Arrays.

Control over the morphology and crystallinity of small-molecule organic semiconductor (OSC) films is of key importance to enable high-performance organic optoelectronic devices. However, such control remains particularly challenging for solution-processed OSC devices because of the complex crystallization kinetics of small-molecule OSC materials in the dynamic flow of inks. Here, a simple yet effective channel-restricted screen-printing method is reported, which uses small-molecule OSCs/insulating polymer to yield large-grained small-molecule OSC thin-film arrays with good crystallization and preferred orientation. The use of cross-linked organic polymer banks produces a confinement effect to trigger the outward convective flow at two sides of the channel by the fast solvent evaporation, which imparts the transport of small-molecule OSC solutes and promotes the growth of small-molecule OSC crystals parallel to the channel. The small-molecule OSC thin-film array produced by screen printing exhibits excellent performance characteristics with an average mobility of 7.94 cm2 V-1 s-1 and a maximum mobility of 12.10 cm2 V-1 s-1 , which are on par with its single crystal. Finally, screen printing can be carried out using a flexible substrate, with good performance. These demonstrations bring this robust screen-printing method closer to industrial application and expand its applicability to various flexible electronics.

Organic crystalline materials in flexible electronics.

Flexible electronics have attracted considerable attention recently given their potential to revolutionize human lives. High-performance organic crystalline materials (OCMs) are considered strong candidates for next-generation flexible electronics such as displays, image sensors, and artificial skin. They not only have great advantages in terms of flexibility, molecular diversity, low-cost, solution processability, and inherent compatibility with flexible substrates, but also show less grain boundaries with minimal defects, ensuring excellent and uniform electronic characteristics. Meanwhile, OCMs also serve as a powerful tool to probe the intrinsic electronic and mechanical properties of organics and reveal the flexible device physics for further guidance for flexible materials and device design. While the past decades have witnessed huge advances in OCM-based flexible electronics, this review is intended to provide a timely overview of this fascinating field. First, the crystal packing, charge transport, and assembly protocols of OCMs are introduced. State-of-the-art construction strategies for aligned/patterned OCM on/into flexible substrates are then discussed in detail. Following this, advanced OCM-based flexible devices and their potential applications are highlighted. Finally, future directions and opportunities for this field are proposed, in the hope of providing guidance for future research.

Free-Standing 2D Hexagonal Aluminum Nitride Dielectric Crystals for High-Performance Organic Field-Effect Transistors.

The existence of defects and traps in a transistor plays an adverse role on efficient charge transport. In response to this challenge, extensive research has been conducted on semiconductor crystalline materials in the past decades. However, the development of dielectric crystals for transistors is still in its infancy due to the lack of appropriate dielectric crystalline materials and, most importantly, the crystal morphology required by the gate dielectric layer, which is also crucial for the construction of high-performance transistor as it can greatly improve the interfacial quality of carrier transport path. Here, a new type of dielectric crystal of hexagonal aluminum nitride (AlN) with the desired 2D morphology of combing thin thickness with large lateral dimension is synthesized. Such a suitable morphology in combination with the outstanding dielectric properties of AlN makes it promising as a gate dielectric for transistors. Furthermore, ultrathin 2,6-diphenylanthracene molecular crystals with only a few molecular layers can be prepared on AlN crystal via van der Waals epitaxy. As a result, this all-crystalline system incorporating dielectric and semiconductor crystals greatly enhances the overall performance of a transistor, indicating the importance of minimizing defects and preparing high-quality semiconductor/dielectric interface in a transistor configuration.

Low-Voltage Organic Single-Crystal Field-Effect Transistor with Steep Subthreshold Slope.

Anodization is a promising technique to form high- k dielectrics for low-power organic field-effect transistor (OFET) applications. However, the surface quality of the dielectric, which is mainly inherited from the metal electrode, can be improved further than other fabrication techniques, such as sol-gel. In this study, we applied the template stripping method to fabricate a low-power single-crystalline OFET based on the anodized AlO x dielectric. We found that the template stripping method largely improves the surface roughness of the deposited Al and allows for the formation of a high-quality AlO x high- k dielectric by anodization. The ultraflat AlO x/SAM dielectric combined with a single-crystal 2,6-diphenylanthracene (DPA) semiconductor produced a nearly defect-free interface with a steep subthreshold swing (SS) of 66 mV/decade. The current device is a promising candidate for future ultralow-power applications. Other than metal deposition, template stripping could provide a general approach to improve thin-film quality for many other types of materials and processes.

Space-Confined Strategy toward Large-Area Two-Dimensional Single Crystals of Molecular Materials.

Two-dimensional molecular crystals (2DMCs) are a promising candidate for flexible and large-area electronics. Their large-area production requires both low nuclei density and 2D crystal growth mode. As an emerging type of material, their large-area production remains a case-by-case practice. Here we present a general, efficient strategy for large-area 2DMCs. The method grows crystals on water surface to minimize the density of nuclei. By controlling the interfacial tension of the water/solution system with a phase transfer surfactant, the spreading area of the solvent increases tens of times, leading to the space-confined 2D growth of molecular crystals. As-grown sub-centimeter-sized 2DMCs floating on the water surface can be easily transferred to arbitrary substrates for device applications.

2D Organic Materials for Optoelectronic Applications.

The remarkable merits of 2D materials with atomically thin structures and optoelectronic attributes have inspired great interest in integrating 2D materials into electronics and optoelectronics. Moreover, as an emerging field in the 2D-materials family, assembly of organic nanostructures into 2D forms offers the advantages of molecular diversity, intrinsic flexibility, ease of processing, light weight, and so on, providing an exciting prospect for optoelectronic applications. Herein, the applications of organic 2D materials for optoelectronic devices are a main focus. Material examples include 2D, organic, crystalline, small molecules, polymers, self-assembly monolayers, and covalent organic frameworks. The protocols for 2D-organic-crystal-fabrication and -patterning techniques are briefly discussed, then applications in optoelectronic devices are introduced in detail. Overall, an introduction to what is known and suggestions for the potential of many exciting developments are presented.

Molecular cocrystals: design, charge-transfer and optoelectronic functionality.

Organic cocrystals, formed by a combination of electron-rich donors and electron-poor acceptors, play an important role in tailoring the optoelectronic properties of molecular materials. Charge transfer interactions in cocrystals not only endow them with an ordered three-dimensional (3D) supramolecular network in different constituent units, but also render them ideal scaffolds to control the intermolecular interactions in multicomponent solids. In this perspective, we firstly introduce preparation methods, molecular packing modes and charge transfer in organic cocrystals. Then, we focus on the novel and promising optoelectronic properties of organic cocrystals based on charge transfer interactions. Finally, we briefly discuss the outlook for the future development of these multicomponent crystalline materials.

Intermolecular Charge-Transfer Interactions Facilitate Two-Photon Absorption in Styrylpyridine-Tetracyanobenzene Cocrystals.

Cocrystals of 4-styrylpyridine and 1,2,4,5-tetracyanobenzene were successfully prepared by supramolecular self-assembly. Donor-acceptor interactions between the molecular components are the main driving force for self-assembly and contribute to intermolecular charge transfer. The cocrystals possess two-photon absorption properties that are not observed in the individual components; suggesting that two-photon absorption originates from intermolecular charge-transfer interactions in the donor-acceptor system. The origin of two-photon absorption in multichromophore systems remains under-researched; thus, the system offers a rare demonstration of two-photon absorption by cocrystallization. Cocrystal engineering may facilitate further design and development of novel materials for nonlinear optical and optoelectronic applications.

Ternary NiCo2 Px Nanowires as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction.

A bimetallic-structured ternary phosphide (NiCo2 Px ) as a novel pH-universal electrocatalyst for hydrogen evolution reaction is presented. It exhibits both high activity and long-term stability in all the tested alkaline, neutral, and acidic media. The excellent catalytic performance endows it with a bright future in the large-scale electrochemical water splitting industry.

Mass Production of Nanogap Electrodes toward Robust Resistive Random Access Memory.

Nanogap electrodes arrays are fabricated by combining atomic layer deposition, adhesive tape, and chemical etching. A unipolar nonvolatile resistive-switching behavior is identified in the nanogap electrodes, showing stable, robust performance and the multibit storage ability, demonstrating great potential in ultrahigh-density storage. The formation and dissolution of Si conductive filaments and migration of Au atoms is the mechanism behind the resistive switching.