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

All-in-One 2D Molecular Crystal Optoelectronic Synapse for Polarization-Sensitive Neuromorphic Visual System

Neuromorphic visual systems (NVSs) hold the potential to not only preserve but also enhance human visual capabilities. One such augmentation lies in harnessing polarization information from light reflected or scattered off surfaces like bees, which can disclose unique characteristics imperceptible to the human eyes. While creating polarization-sensitive optoelectronic synapses presents an intriguing avenue for equipping NVS with this capability, integrating functions like polarization sensitivity, photodetection, and synaptic operations into a singular device has proven challenging. This integration typically necessitates distinct functional components for each performance metric, leading to intricate fabrication processes and constraining overall performance. Herein, a pioneering linear polarized light sensitive synaptic organic phototransistor (OPT) based on 2D molecular crystals (2DMCs) with highly integrated, all-in-one functionality, is demonstrated. By leveraging the superior crystallinity and molecular thinness of 2DMC, the synaptic OPT exhibits comprehensive superior performance, including a linear dichroic ratio up to 3.85, a high responsivity of 1.47 × 104 A W-1, and the adept emulation of biological synapse functions. A sophisticated application in noncontact fingerprint detection achieves a 99.8% recognition accuracy, further highlights its potential. The all-in-one 2DMC optoelectronic synapse for polarization-sensitive NVS marks a new era for intelligent perception systems.

Sensing Interfaces Engineering for Organic Thin Film Transistors-Based Biosensors: Opportunities and Challenges

Organic thin film transistors (OTFTs) enable rapid and label-free high-sensitivity detection of target analytes due to their low cost, large-area processing, biocompatibility, and inherent signal amplification. At the same time, the freedom of synthesis, tailorability, and functionalization of organic semiconductor materials and their ability to be combined with flexible substrates make them one of the ideal platforms for biosensing. However, OTFTs-based biosensors still face significant challenges, such as unexpected surface adsorption, disordered conformation, inhomogeneous graft density, and flexibility of probe molecules that biological sensing probes would face during immobilization. In this review, efficient immobilization strategies based on OTFTs biological sensing probes developed in the last 5 years are highlighted. First, the biosensors are classified according to their sensing interface. Second, a comprehensive discussion of the types of biological sensing probes is presented. Third, three commonly used methods for immobilizing biological sensing probes and their challenges are briefly described. Finally, the applications of OTFTs-based biosensors for liquid phase detection are summarized. This review provides a comprehensive and timely review of optimization in sensing interface engineering so that efficient immobilization of biological sensing probes with sensing interfaces will contribute to the development of high-performance OTFTs-based biosensors.

Negative Photoconductivity Transistors for Visuomorphic Computing

Visuomorphic computing aims to simulate and potentially surpass the human retina by mimicking biological visual perception with an artificial retina. Despite significant progress, challenges persist in perceiving complex interactive environments. Negative photoconductivity transistors (NPTs) mimic synaptic behavior by achieving adjustable positive photoconductivity (PPC) and negative photoconductivity (NPC), simulating "excitation" and "inhibition" akin to sensory cell signals. In complex interactive environments, NPTs are desired for visuomorphic computing that can achieve a better sense of information, lower power consumption, and reduce hardware complexity. In this review, it is started by introducing the development process of NPTs, while placing a strong emphasis on the device structures, working mechanisms, and key performance parameters. The common material systems employed in NPTs based on their functions are then summarized. Moreover, it is proceeded to summarize the noteworthy applications of NPTs in optoelectronic devices, including advanced multibit nonvolatile memory, optoelectronic logic gates, optical encryption, and visual perception. Finally, the challenges and prospects that lie ahead in the ongoing development of NPTs are addressed, offering valuable insights into their applications in optoelectronics and a comprehensive understanding of their significance.

Centimeter-Scale Assembly of Fractal Organic Crystals with Crisscross Structures for Flexible Photosynapses

Fractal assembly technology enables scalable construction of organic crystal patterns for emerging nanoelectronics and optoelectronics. Here, a polymer-templating assembly strategy is presented for centimeter-scale patterned growth of fractal organic crystals (FOCs). These structures are formed by drop-coating perylene solution directly onto a gelatin-modified surface, resulting in the formation of crisscross fractal patterns. By adjusting the tilt angle of the template, the morphology of FOCs can be effectively controlled, with the diameter distribution of each level branch ranging from hundreds to ten micrometers. The planar FOC device exhibits flexible photoreception and photosynaptic capabilities, with a high specific detectivity of 1.35 × 109 Jones and paired-pulse facilitation (PPF) index of 104%, withstanding a 0.5 cm bending radius during bending test. These findings present a reliable route for large-scale assembly of flexible organic crystalline materials toward neuromorphic electronics.

Temperature-Modulated Evolution of Surface Structures Induces Significant Enhancement of Two-Photon Fluorescent Emission from a Dye Molecule

Fluorescence is an essential property of molecules and materials that plays a pivotal role across various areas such as lighting, sensing, imaging, and other applications. For instance, temperature-sensitive fluorescence emission is widely utilized for chemo-/biosensing but usually decreases the intensity upon the increase in temperature. In this study, we observed a temperature-induced enhancement of up to ∼150 times in two-photon fluorescence (TPF) emission from a dye molecule, 4-(4-diethylaminostyry)-1-methylpyridinium iodide (D289), as it interacted with binary complex vesicles composed of two commonly applied surfactants: sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB). By employing second harmonic generation (SHG) and TPF techniques, we clearly revealed the temperature-dependent kinetic behavior of D289 on the surface of the vesicles and utilized it to interpret the origin of the significant TPF enhancement. Additionally, we also demonstrated a similar heating-induced enhancement of the TPF emission from D289 on the membrane of phospholipid vesicles, indicating the potential application of TPF in temperature sensing in the biology systems. The embedding of D289 in the tightly packed alkane chains was identified as the key factor in enhancing the TPF emission from D289. This finding may provide valuable information for synthesizing fluorescence materials with a high optical yield.

Stepwise Aggregation Control of PEDOT:PSS Enabled High-Conductivity, High-Resolution Printing of Polymer Electrodes for Transparent Organic Phototransistors

Electrohydrodynamic (EHD) jet printing is a widely employed technology to create high-resolution patterns and thus has enormous potential for circuit production. However, achieving both high conductivity and high resolution in printed polymer electrodes is a challenging task. Here, by modulating the aggregation state of the conducting polymer in the solution and solid phases, a stable and continuous jetting of PEDOT:PSS is realized, and high-conductivity electrode arrays are prepared. The line width reaches less than 5 μm with a record-high conductivity of 1250 S/cm. Organic field-effect transistors (OFETs) are further developed by combining printed source/drain electrodes with ultrathin organic semiconductor crystals. These OFETs show great light sensitivity, with a specific detectivity (D*) value of 2.86 × 1014 Jones. In addition, a proof-of-concept fully transparent phototransistor is demonstrated, which opens up new pathways to multidimensional optical imaging.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

Conductive Nanosheets Fabricated from Au Nanoparticles on Aqueous Metal Solutions under UV Irradiation

Highly transparent, conductive nanosheets are extremely attractive for advanced opto-electronic applications. Previously, we have demonstrated that transparent, conductive Au nanosheets can be prepared by UV irradiation of Au nanoparticle (AuNP) monolayers spread on water, which serves as the subphase. However, thick Au nanosheets cannot be fabricated because the method is not applicable to large Au NPs. Further, in order to fabricate nanosheets with different thicknesses and compositions, it is necessary to prepare the appropriate NPs. A strategy is needed to produce nanosheets with different thicknesses and compositions from a single type of metal NP monolayer. In this study, we show that this UV irradiation technique can easily be extended as a nanosheet modification method by using subphases containing metal ions. UV irradiation of 4.7 nm AuNP monolayers on 480 µM HAuCl4 solution increased the thickness of Au nanosheets from 3.5 nm to 36.5 nm, which improved conductivity, but reduced transparency. On the other hand, the use of aqueous AgNO3 and CH3COOAg solutions yielded Au-Ag hybrid nanosheets; however, their morphologies depended on the electrolytes used. In Au-Ag nanosheets prepared on aqueous 500 µM AgNO3, Au and Ag metals are homogeneously distributed throughout the nanosheet. On the other hand, in Au-Ag nanosheets prepared on aqueous 500 µM CH3COOAg, AuNPs still remained and these AuNPs were covered with a Ag nanosheet. Further, these Au-Ag hybrid nanosheets had high conductivity without reduced transparency. Therefore, this UV irradiation method, modified by adding metal ions, is quite effective at improving and diversifying properties of Au nanosheets.

Ultrahigh-gain organic transistors based on van der Waals metal-barrier interlayer-semiconductor junction

Intrinsic gain is a vital figure of merit in transistors, closely related to signal amplification, operation voltage, power consumption, and circuit simplification. However, organic thin-film transistors (OTFTs) targeted at high gain have suffered from challenges such as narrow subthreshold operating voltage, low-quality interface, and uncontrollable barrier. Here, we report a van der Waals metal-barrier interlayer-semiconductor junction-based OTFT, which shows ultrahigh performance including ultrahigh gain of ~104, low saturation voltage, negligible hysteresis, and good stability. The high-quality van der Waals-contacted junctions are mainly attributed to patterning EGaIn liquid metal electrodes by low-energy microfluidic processes. The wide-bandgap semiconductor Ga2O3 as barrier interlayer is achieved by in situ surface oxidation of EGaIn electrodes, allowing for an adjustable barrier height and expected thermionic emission properties. The organic inverters with a high gain of 5130 and a simplified current stabilizer are further demonstrated, paving a way for high-gain and low-power organic electronics.

Wafer-Scale Epitaxial Growth of Two-dimensional Organic Semiconductor Single Crystals toward High-Performance Transistors

The success of state-of-the-art electronics and optoelectronics relies heavily on the capability to fabricate semiconductor single-crystal wafers. However, the conventional epitaxial growth strategy for inorganic wafers is invalid for growing organic semiconductor single crystals due to the lack of lattice-matched epitaxial substrates and intricate nucleation behaviors, severely impeding the advancement of organic single-crystal electronics. Here, an anchored crystal-seed epitaxial growth method for wafer-scale growth of 2D organic semiconductor single crystals is developed for the first time. The crystal seed is firmly anchored on the viscous liquid surface, ensuring the steady epitaxial growth of organic single crystals from the crystal seed. The atomically flat liquid surface effectively eliminates the disturbance from substrate defects and greatly enhances the 2D growth of organic crystals. Using this approach, a wafer-scale few-layer bis(triethylsilythynyl)-anthradithphene (Dif-TES-ADT) single crystal is formed, yielding a breakthrough for organic field-effect transistors with a high reliable mobility up to 8.6 cm2 V-1 s-1 and an ultralow mobility variable coefficient of 8.9%. This work opens a new avenue to fabricate organic single-crystal wafers for high-performance organic electronics.

Two-Dimensional Self-Assembly Driven by Intermolecular Hydrogen Bonding in Benzodi-7-azaindole Molecules on Au(111)

The control of molecular structures at the nanoscale plays a critical role in the development of materials and applications. The adsorption of a polyheteroaromatic molecule with hydrogen bond donor and acceptor sites integrated in the conjugated structure itself, namely, benzodi-7-azaindole (BDAI), has been studied on Au(111). Intermolecular hydrogen bonding determines the formation of highly organized linear structures where surface chirality, resulting from the 2D confinement of the centrosymmetric molecules, is observed. Moreover, the structural features of the BDAI molecule lead to the formation of two differentiated arrangements with extended brick-wall and herringbone packing. A comprehensive experimental study that combines scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory theoretical calculations has been performed to fully characterize the 2D hydrogen-bonded domains and the on-surface thermal stability of the physisorbed material.

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.

Reducing Contact Resistance in Organic Field-Effect Transistors: A Comprehensive Comparison between 2D and Microrod Single Crystals

A comprehensive comparison of organic single crystals based on a single material but with different dimensions provides a unique approach to probe their carrier injection mechanism. In this report, both two-dimensional (2D) and microrod single crystals with the same crystalline structure of an identical thiopyran derivative, 7,14-dioctylnaphtho[2,1-f:6,5-f']bis(cyclopentane[b]thiopyran) (C₈-SS), are grown on a glycerol surface with the space-confined method. Organic field-effect transistors (OFETs) based on the 2D C₈-SS single crystal exhibit superior performance compared with those based on the microrod single crystal, particularly in their contact resistance (R_C). It is demonstrated that the resistance of the crystal bulk in the contact region plays a key role in R_C of the OFETs. Thus, among the 30 devices tested, the microrod OFETs typically appear contact-limited, whereas the 2D OFETs possess significantly reduced R_C arising from the tiny thickness of the 2D single crystal. The 2D OFETs show high operational stability and high channel mobility up to 5.7 cm²/V·s. The elucidation of the contact behavior highlights the merits and great potential of 2D molecular single crystals in organic electronics.

Homogeneously Oriented Organic Single-Crystalline Patterns Grown by Microspacing In-Air Sublimation

Fabrication of single-crystalline organic semiconductor patterns is of key importance to enable practical applications. However due to the poor controllability on nucleation locations and the intrinsic anisotropic nature of single-crystals, growth of single-crystal patterns with homogeneous orientation is a big challenge especially by the vapor method. Herein a vapor growth protocol to achieve patterned organic semiconductor single-crystals with high crystallinity and uniform crystallographic orientation is presented. The protocol relies on the recently invented microspacing in-air sublimation assisted with surface wettability treatment to precisely pin the organic molecules at desired locations, and inter-connecting pattern motifs to induce homogeneous crystallographic orientation. Single-crystalline patterns with different shapes and sizes, and uniform orientation are demonstrated exemplarily by using 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT). Field-effect transistor arrays fabricate on the patterned C8-BTBT single-crystal patterns show uniform electrical performance: a 100% yield with an average mobility of 6.28 cm²  V^-1  s^-1 and in a 5 × 8 array. The developed protocols overcome the uncontrollability of the isolated crystal patterns in vapor growth on non-epitaxial substrates, making it possible to align the anisotropic electronic nature of single-crystal patterns in large-scale devices integration.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

New Approaches to Produce Large-Area Single Crystal Thin Films

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

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.

Intrinsic Linear Dichroism of Organic Single Crystals toward High-Performance Polarization-Sensitive Photodetectors

The ability to detect light in photodetectors is central to practical optoelectronic applications, which has been demonstrated in inorganic semiconductor devices. However, so far, the study of polarization-sensitive organic photodetectors, which have unique applications in flexible and wearable electronics, has not received much attention. Herein, the construction of polarization-sensitive photodetectors based on the single crystals of a superior optoelectronic organic semiconductor, 2,6-diphenyl anthracene (DPA), is demonstrated. The systematic characterization of two-dimensionally grown DPA crystals with various techniques definitely show their strong anisotropy in molecular vibration, optical reflectance and optical absorption. In terms of polarization sensitivity, DPA-crystal based photodetectors exhibit a linear dichroic ratio up to ≈1.9. Theoretical calculations confirm that intrinsic linear dichroism, originated from the anisotropic in-plane crystal structure, is responsible for the polarization sensitivity of DPA crystals. This work opens up a new door for exploiting organic semiconductors for developing highly compact polarization photodetectors and providing new functionalities in novel flexible optical and optoelectronic applications.

Organic Semiconductor Crystal Engineering for High-Resolution Layer-Controlled 2D Crystal Arrays

2D organic semiconductor crystals (2DOSCs) have extraordinary charge transport capability, adjustable photoelectric properties, and superior flexibility, and have stimulated continuous research interest for next-generation electronic and optoelectronic applications. The prerequisite for achieving large-area and high-throughput optoelectronic device integration is to fabricate high-resolution 2DOSC arrays. Patterned substrate- and template-assisted self-assembly is an effective strategy to fabricate OSC arrays. However, the film thickness is difficult to control due to the complicated crystallization process during solvent evaporation. Therefore, the manufacturing of 2DOSC arrays with high-resolution and controllable molecular-layer numbers through solution-based patterning methods remains a challenge. Herein, a two-step strategy to produce high-resolution layer-controlled 2DOSC arrays is reported. First, large-scale 2DOSCs with well-defined layer numbers are obtained by a solution-processed organic semiconductor crystal engineering method. Next, the high-resolution layer-controlled 2DOSC arrays are fabricated by a polydimethylsiloxane mold-assisted selective contact evaporation printing technique. The organic field-effect transistors (OFETs) based on 2DOSC arrays have high electrical performance and excellent uniformity. The 2,6-bis(4-hexylphenyl)anthracene 2DOSC arrays-based OFETs have a small variation of 12.5% in mobility. This strategy can be applied to various organic semiconductors and pattern arrays. These demonstrations will offer more opportunities for 2DOSCs for integrated optoelectronic devices.

Controlled Growth and Self-Assembly of Multiscale Organic Semiconductor

Currently, organic semiconductors (OSs) are widely used as active components in practical devices related to energy storage and conversion, optoelectronics, catalysis, and biological sensors, etc. To satisfy the actual requirements of different types of devices, chemical structure design and self-assembly process control have been synergistically performed. The morphology and other basic properties of multiscale OS components are governed on a broad scale from nanometers to macroscopic micrometers. Herein, the up-to-date design strategies for fabricating multiscale OSs are comprehensively reviewed. Related representative works are introduced, applications in practical devices are discussed, and future research directions are presented. Design strategies combining the advances in organic synthetic chemistry and supramolecular assembly technology perform an integral role in the development of a new generation of multiscale OSs.

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.

Temperature-Triggered Supramolecular Assembly of Organic Semiconductors

Supramolecular assembly is a promising bottom-up approach for producing materials that behave as charge transporting components in electronic devices. Although extensive advances have been made during the past two decades, formidable challenges exist in controlling the local randomness present in supramolecular assemblies. Here, a temperature-triggered supramolecular assembly strategy using heat to heal defects and disorders is reported. The central concept of the molecular design-named the "Tetris strategy" in this research-is to: i) increase the rotational freedom of the molecules through thermal perturbation, ii) induce conformation-fitting of adjacent molecules through two different kinds of intermolecular [π···π] interactions, and finally iii) lock the nearby molecules in inactive co-conformations. Experimentally, upon heating to 57 °C, amorphous solid-state films undergo spontaneous assembly, leading to the growth of uniform and highly ordered microwire arrays. Temperature-triggered supramolecular assembly provides an approach closer to the precision control of assembled structures and presents with a broad canvas to work on in approaching a new generation of supramolecular electronics. Tetris is a registered trademark of Tetris Holding, LLC, used with permission.

Wall-Confined Spreading Dynamics on the Surface of Surfactant Solution

A liquid spreading over another is a universal physical process in the nature, which was investigated by the scaling law to reveal the underlying mechanical mechanism over the decades. However, scaling laws are restricted to piecewise physical stages, respectively. It is a challenge to present a full physical picture for a dynamic spreading process covering a wide-spectrum speed. We propose a general wall-confined spreading dynamics (WCSD) model originating from molecular kinetic theory (MKT). It creatively illustrates the order and domination between driving energy and energy dissipation (or transfer) using a phase diagram according to theory and experiments. This work reveals the deep mechanical mechanism of WCSD which provides an indirect guidance on the solution processing methods of two-dimensional molecular crystals (2DMCs) growth.

Infinite possibilities of ultrathin III-V semiconductors: Starting from synthesis

Ultrathin III-V semiconductors have been receiving tremendous research interest over the past few years. Owing to their exotic structures, excellent physical and chemical properties, ultrathin III-V semiconductors are widely applied in the field of electronics, optoelectronics, and solar energy. However, the strong chemical bonds in layers are the bottleneck of the two-dimensionalization preparation process, which hinders the further development of ultrathin III-V semiconductors. Some effective methods to synthesize ultrathin III-V semiconductors have been reported recently. In this perspective, we briefly introduce the structures and properties of ultrathin III-V semiconductors firstly. Then, we comprehensively summarize the synthetic strategies of ultrathin III-V semiconductors, mainly focusing on space confinement, atomic substitution, adhesion energy regulation, and epitaxial growth. Finally, we summarize the current challenges and propose the development directions of ultrathin III-V semiconductors in the future.

Effect of Strong Intermolecular Interaction in 2D Inorganic Molecular Crystals

Strong intermolecular interactions in 2D organic molecular crystals arising from π-π stacking have been widely explored to achieve high thermal stability, high carrier mobility, and novel physical properties, which have already produced phenomenal progress. However, strong intermolecular interactions in 2D inorganic molecular crystals (2DIMCs) have rarely been investigated, severely limiting both the fundamental research in molecular physics and the potential applications of 2DIMCs for optoelectronics. Here, the effect of strong intermolecular interactions induced by unique short intermolecular Se-Se and P-Se contacts in 2D α-P₄Se₃ nanoflakes is reported. On the basis of theoretical calculations of the charge density distribution and an analysis of the thermal expansion and plastic-crystal transition, the physical picture of strong intermolecular interactions can be elucidated as a higher charge density between adjacent P₄Se₃ molecules, arising from an orderly and close packing of P₄Se₃ molecules. More importantly, encouraged by the strong intermolecular coupling, the in-plane mobility of α-P₄Se₃ nanoflakes is first calculated with a quantum nuclear tunneling model, and a competitive hole mobility of 0.4 cm² V^-1 s^-1 is obtained. Our work sheds new light on the intermolecular interactions in 2D inorganic molecular crystals and is highly significant for promoting the development of molecular physics and optoelectronics.

Origin of Intense Luminescence from Supramolecular 2D Molecular Crystals

Luminescence enhancement in 2D molecular crystals (2D crystals) is promising for a variety of optical applications, yet the availability is limited because of unclear mechanism and inefficient design strategy of luminescence control. Herein, the room temperature phosphorescence from micron long molecular thin free-standing 2D crystals of a mono-cyclometalated Ir(III) complex designed at the water surface is reported. A large luminescence enhancement is observed from the 2D crystals at 300 K, which is comparable with the rigidified solution at 77 K suggesting room temperature phosphorescence origin of the luminescence. In situ synchrotron grazing incidence X-ray diffraction measurements determine the constituent centered rectangular unit cells with precise molecular conformation that promotes the formation of 2D crystals. The molecular crystal design leads to a reduced singlet-triplet energy gap (ΔE_ST ) and mixing of singlet-triplet states by spin-orbit coupling (SOC) for efficient intersystem crossing, which explains the phosphorescence origin at room temperature and luminescence enhancement. The supramolecular assembly process provides an elegant design strategy to realize room temperature phosphorescence from 2D crystals by rigid intermolecular interactions.

Ultrasmooth Organic Films Via Efficient Aggregation Suppression by a Low-Vacuum Physical Vapor Deposition

Organic thin films with smooth surfaces are mandated for high-performance organic electronic devices. Abrupt nucleation and aggregation during film formation are two main factors that forbid smooth surfaces. Here, we report a simple fast cooling (FC) adapted physical vapor deposition (FCPVD) method to produce ultrasmooth organic thin films through effectively suppressing the aggregation of adsorbed molecules. We have found that thermal energy control is essential for the spread of molecules on a substrate by diffusion and it prohibits the unwanted nucleation of adsorbed molecules. FCPVD is employed for cooling the horizontal tube-type organic vapor deposition setup to effectively remove thermal energy applied to adsorbed molecules on a substrate. The organic thin films prepared using the FCPVD method have remarkably ultrasmooth surfaces with less than 0.4 nm root mean square (RMS) roughness on various substrates, even in a low vacuum, which is highly comparable to the ones prepared using conventional high-vacuum deposition methods. Our results provide a deeper understanding of the role of thermal energy employed to substrates during organic film growth using the PVD process and pave the way for cost-effective and high-performance organic devices.

Oil-Water-Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets

Ionic covalent organic framework nanosheets (iCOFNs) with long-range ordered and mono-dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil-water biphase method, herein, we explore an oil-water-oil triphase method based on phase engineering strategy for the bottom-up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few-layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra-high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect-free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs.

Multi-level aggregation of conjugated small molecules and polymers: from morphology control to physical insights

Aggregation of molecules is a multi-molecular phenomenon occurring when two or more molecules behave differently from discrete molecules due to their intermolecular interactions. Moving beyond single molecules, aggregation usually demonstrates evolutive or wholly emerging new functionalities relative to the molecular components. Conjugated small molecules and polymers interact with each other, resulting in complex solution-state aggregates and solid-state microstructures. Optoelectronic properties of conjugated small molecules and polymers are sensitively determined by their aggregation states across a broad range of spatial scales. This review focused on the aggregation ranging from molecular structure, intermolecular interactions, solution-state assemblies, and solid-state microstructures of conjugated small molecules and polymers. We addressed the importance of such aggregation in filling the gaps from the molecular level to device functions and highlighted the multi-scale structures and properties at different scales. From the view of multi-level aggregation behaviors, we divided the whole process from the molecule to devices into several parts: molecular design, solvation, solution-state aggregation, crystal engineering, and solid-state microstructures. We summarized the progress and challenges of relationships between optoelectronic properties and multi-level aggregation. We believe aggregation science will become an interdisciplinary research field and serves as a general platform to develop future materials with the desired functions.

Tailored Solution-Based N-heterotriangulene Thin Films: Unravelling the Self-Assembly

The ability of a series of bridged triarylamines, so‐called N‐heterotriangulenes, to form multilayer‐type 2D‐extended films via a solution‐based processing method was examined using complementary microscopic techniques. We found that the long‐range order, crystallinity, and layer thickness decisively depend on the nature of the substituents attached to the polycyclic backbone. Owing to their flat core unit, compounds exhibiting a carbonyl unit at the bridge position provide a superior building block as compared to thioketone‐bridged derivatives. In addition, nature and length of the peripheral substituents affect the orientation of the aromatic core unit within highly crystalline films. Hence, our results stress the significance of a suitable molecular framework and provide deeper understanding of structure formation in 2D‐confined surroundings for such compounds.

Molecular cocrystal odyssey to unconventional electronics and photonics

Cocrystal has been discovered and studied for more than 170 years since 1844, while the applications to optoelectronics only begin in the last decade. Several general questions that chemists and materials scientists currently seek to answer are: can we design and control the molecular self-assembly and cocrystal growth, what's the packing-property correlations, as well as how can we improve device parameters for real applications in industry. In this contribution, we review our and other groups' recent advances in the cocrystal research field sequentially including: (1) nucleation and growth mechanisms for selective preparation of cocrystals with different donor/acceptor ratio and morphology; (2) charge transport and electronic devices, particularly field-effect transistor (FET) and photo-response device. We discuss the in-situ single crystal device fabrication method, ambipolar charge transport, and molecular packing-charge separation correlation; (3) photonic and optical property, focusing on optical waveguide, photonic logic computation, and nonlinear optics (NLO). We present unusual optical properties revealed by advanced instruments and general structure-function relations for future study. Importantly, the extensive investigations described herein yield in-depth and detailed understandings of molecular cocrystals, and show that such bi-component material systems together with the developed instrument measurement methodologies have the potential to initiate unconventional electronic and photonic science and technology.

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.

B/N-Enriched Semi-Conductive Polymer Film for Micro-Supercapacitors with AC Line-Filtering Performance

Microsupercapacitors (MSCs) have drawn great attention for use as miniaturized electrochemical energy storage devices in portable, wearable, as well as implantable electronics. Many materials have been developed as electrodes for MSCs. However, the thin-film fabrication for most of these materials involves multistep operations, including filtration, spray coating, and sputtering. Most importantly, these methods present challenges for the preparation of thin films at the atomic or molecular scale. Therefore, the understanding of performance of ultrathin-film-based MSCs remains challenge. Herein, a B/N-enriched polymer film is successfully prepared using the photoassisted interfacial approach. The as-synthesized polymer film exhibits typical semiconductive characteristics and can be easily scaled up to a large area of up to tens of square centimeters. This ultrathin polymer film can be directly transferred to silicon wafers to fabricate MSC through laser scribing. The prepared MSC exhibits specific volumetric capacitance as high as 20.9 F cm^-3, corresponding to volumetric energy density of 2.9 mWh cm^-3 (at 0.1 V s^-1). Moreover, the volumetric power density can reach 1461 W cm^-3, surpassing most existing semiconductive polymer film-based MSC devices. In addition, the prepared MSC exhibits typical AC line-filtering ability (-67° at 120 Hz). This study offers a facile interfacial approach to preparing semiconductive polymer films with aromatic moieties for microsized energy storage devices.

Water-Surface Drag Coating: A New Route Toward High-Quality Conjugated Small-Molecule Thin Films with Enhanced Charge Transport Properties

Electronic properties of organic semiconductor (OSC) thin films are largely determined by their morphologies and crystallinities. However, solution-processed conjugated small-molecule OSC thin films usually exhibit abundant grain boundaries and impure grain orientations because of complex fluid dynamics during solution coating. Here, a novel methodology, water-surface drag coating, is demonstrated to fabricate high-quality OSC thin films with greatly enhanced charge transport properties. This method utilizes the water surface to alter the evaporation dynamics of solution to enlarge the grain size, and a unique drag-coating process to achieve the unidirectional growth of organic crystals. Using 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (Dif-TES-ADT) as an example, thin films with millimeter-sized single-crystal domains and pure crystallographic orientations are achieved, revealing a significant enhancement (4.7 times) of carrier mobility. More importantly, the resulting film can be directly transferred onto any desired flexible substrates, and flexible transistors based on the Dif-TES-ADT thin films show a mobility as high as 16.1 cm² V^-1 s^-1 , which represents the highest mobility value for the flexible transistors reported thus far. The method is general for the growth of various high-quality OSC thin films, thus opening up opportunities for high-performance organic flexible electronics.

Nontrivial band geometry in an optically active system

Optical activity, also called circular birefringence, is known for two hundred years, but its applications for topological photonics remain unexplored. Unlike the Faraday effect, the optical activity provokes rotation of the linear polarization of light without magnetic effects, thus preserving the time-reversal symmetry. In this work, we report a direct measurement of the Berry curvature and quantum metric of the photonic modes of a planar cavity, containing a birefringent organic microcrystal (perylene) and exhibiting emergent optical activity. This experiment, performed at room temperature and at visible wavelength, establishes the potential of organic materials for implementing non-magnetic and low-cost topological photonic devices.

Lithographical Fabrication of Organic Single-Crystal Arrays by Area-Selective Growth and Solvent Vapor Annealing

Miniaturized organic single-crystal arrays that are addressed by reading-out circuits are crucial for high performance and high-level integration organic electronics. Here, we report a lithography compatible strategy to fabricate organic single-crystal arrays via area-selective growth and solvent vapor annealing (SVA). The organic semiconducting molecules can first selectively grow on photographically patterned drain-source electrodes, forming ordered amorphous aggregates that can further be converted to discrete single-crystal arrays by SVA. This strategy can be applied to self-align the microsized organic single crystals on predesigned locations. With this method, suppression of cross-talk among devices, organic field-effect transistors, and basic logic gate arrays with reading-out electrodes are further demonstrated.

Graphene-Quantum-Dots-Induced Centimeter-Sized Growth of Monolayer Organic Crystals for High-Performance Transistors

Monolayer organic crystals have attracted considerable attention due to their extraordinary optoelectronic properties. Solution self-assembly on the surface of water is an effective approach to fabricate monolayer organic crystals. However, due to the difficulties in controlling the spreading of organic solution on the water surface and the weak intermolecular interaction between the organic molecules, large-area growth of monolayer organic crystals remains a great challenge. Here, a graphene quantum dots (GQDs)-induced self-assembly method for centimeter-sized growth of monolayer organic crystals on a GQDs solution surface is reported. The spreading area of the organic solution can be readily controlled by tuning the pH value of the GQDs solution. Meanwhile, the π-π stacking interaction between the GQDs and the organic molecules can effectively reduce the nucleation energy of the organic molecules and afford a cohesive force to bond the crystals, enabling large-area growth of monolayer organic crystals. Using 2,7-didecyl benzothienobenzothiopene (C₁₀-BTBT) as an examples, centimeter-sized monolayer C₁₀-BTBT crystal with uniform molecular packing and crystal orientation is attained. Organic field-effect transistors based on the monolayer C₁₀-BTBT crystals exhibit a high mobility up to 2.6 cm² V^-1 s^-1, representing the highest mobility value for solution-assembled monolayer organic crystals. This work provides a feasible route for large-scale fabrication of monolayer organic crystals toward high-performance organic devices.

Solution-Processed, Large-Area, Two-Dimensional Crystals of Organic Semiconductors for Field-Effect Transistors and Phototransistors

Organic electronics with π-conjugated organic semiconductors are promising candidates for the next electronics revolution. For the conductive channel, the large-area two-dimensional (2D) crystals of organic semiconductors (2DCOS) serve as useful scaffolds for modern organic electronics, benefiting not only from long-range order and low defect density nature but also from unique charge transport characteristic and photoelectrical properties. Meanwhile, the solution process with advantages of cost-effectiveness and room temperature compatibility is the foundation of high-throughput print electrical devices. Herein, we will give an insightful overview to witness the huge advances in 2DCOS over the past decade. First, the typical influencing factors and state-of-the-art assembly strategies of the solution-process for large-area 2DCOS over sub-millimeter even to wafer size are discussed accompanying rational evaluation. Then, the charge transport characteristics and contact resistance of 2DCOS-based transistors are explored. Following this, beyond single transistors, the p-n junction devices and planar integrated circuits based on 2DCOS are also emphasized. Furthermore, the burgeoning phototransistors (OPTs) based on crystals in the 2D limits are elaborated. Next, we emphasized the unique and enhanced photoelectrical properties based on a hybrid system with other 2D van der Waals solids. Finally, frontier insights and opportunities are proposed, promoting further research in this field.

Enhanced emission in organic nanocrystals via asymmetrical design of spirocyclic aromatic hydrocarbons

Two spirocyclic aromatic hydrocarbon derivatives were prepared to clarify the molecular geometry effects on the regulation of the crystalline morphologies and photophysical behaviors of organic nanocrystals. Due to the different structural symmetry of a spiro-center, distinguishing nanocrystal morphologies with unique crystallization-enhanced/quenched emission was achieved.

Preparing two-dimensional crystalline conjugated polymer films by synergetic polymerization and self-assembly at air/water interface

Conjugated polymer (CP) films with high molecular order are attractive in polymeric optoelectronics, but challenging.

High-performance, semiconducting membrane composed of ultrathin, single-crystal organic semiconductors

Organic thin film transistors (OTFTs) are promising building blocks in next-generation electronic devices due to the compatibility with the solution process of organic semiconductors (OSCs). Generally, OTFT processes inevitably face the serious issue that the available substrates are limited to those that are solution-process compatible. In a striking contrast to conventional OTFT fabrication processes, we successfully demonstrate simple water exfoliation together with a transfer method for solution-processed organic single-crystal semiconductors. An ideal single-crystalline form can be maintained during the process, which results in excellent electronic performance. The methodology presented in this study allows the ideal production of OTFTs on a wide range of destination substrates with sufficient scalability and expands the possibility for OSCs to be employed in printed electronic devices.

Thin film transistors (TFTs) are indispensable building blocks in any electronic device and play vital roles in switching, processing, and transmitting electronic information. TFT fabrication processes inherently require the sequential deposition of metal, semiconductor, and dielectric layers and so on, which makes it difficult to achieve reliable production of highly integrated devices. The integration issues are more apparent in organic TFTs (OTFTs), particularly for solution-processed organic semiconductors due to limits on which underlayers are compatible with the printing technologies. We demonstrate a ground-breaking methodology to integrate an active, semiconducting layer of OTFTs. In this method, a solution-processed, semiconducting membrane composed of few-molecular-layer–thick single-crystal organic semiconductors is exfoliated by water as a self-standing ultrathin membrane on the water surface and then transferred directly to any given underlayer. The ultrathin, semiconducting membrane preserves its original single crystallinity, resulting in excellent electronic properties with a high mobility up to 12 cm2⋅V−1⋅s−1. The ability to achieve transfer of wafer-scale single crystals with almost no deterioration of electrical properties means the present method is scalable. The demonstrations in this study show that the present transfer method can revolutionize printed electronics and constitute a key step forward in TFT fabrication processes.