In situ integration of squaraine-nanowire-array-based Schottky-type photodetectors with enhanced switching performance

Volatile Memristive Devices with Analog Resistance Switching Based on Self-Assembled Squaraine Microtubes as Synaptic Emulators

In this work, the discovery of volatile memristive devices that exhibit analog resistive switching (RS) and synaptic emulation based on squaraine materials is presented. Specifically, organic microtubes (MTs) based on 2,4-bis[(4-(N,N-diisobutyl)-2-6-hydroxyphenyl]squaraine (SQ) are prepared by evaporation-induced self-assembly (EISA). The MTs are ca. 2 μm in diameter (aspect ratio: 10-130). While powder X-ray diffraction data for MTs identify monoclinic and orthorhombic polymorphs, optical data report the monoclinic phase with energetic disorder. By favorable energetic alignment of the Au work function with the SQ HOMO energy, unipolar (hole-only) symmetric metal-insulator-metal devices are formed by EISA of MT meshes on interdigitated electrodes. The DC I-V characteristics acquired exhibit pinched hysteretic I-V loops, indicative of memristive behavior. Analysis indicates Ohmic transport at low bias with carrier extraction by thermionic emission. At high bias, space-charge-limited conduction in the presence of traps distributed in energy, enhanced by a Poole-Frenkel effect and with carrier extraction by Fowler-Nordheim tunneling, is observed. These data indicate purely electronic conduction. I-V hysteresis attenuates at smaller voltage windows, suggesting that carrier trapping/detrapping underpins the hysteresis. By applying triangular voltage waveforms, device conductance gradually increases sweep-on-sweep, with wait-time-erase or voltage-erase options. Using square waveforms, repeated erase-write-read of multiple distinct conductance states is achieved. Such analog RS behavior is consistent with trap filling/emptying effects. By waveform design, volatile conductance states may also be written so that successive conductance states exhibit identical current levels, indicating forgetting of previously written states and mimicking the forgetting curve. Finally, advanced synaptic functions, i.e., excitatory postsynaptic current, paired-pulse facilitation, pulse-dependent plasticity, and a transition from short- to long-term memory driven by post-tetanic potentiation, are demonstrated.

Nanowire Assemblies for Flexible Electronic Devices: Recent Advances and Perspectives

The fabrication of nanowire (NW)-based flexible electronics including wearable energy storage devices, flexible displays, electrical sensors, and health monitors has received great attention both in fundamental research and market requirements in our daily lives. Other than a disordered state after synthesis, NWs with designed and hierarchical structures would not only optimize the intrinsic performance, but also create new physical and chemical properties, and integration of individual NWs into well-defined structures over large areas is one of the most promising strategies to optimize the performance of NW-based flexible electronics. Here, the recent developments and achievements made in the field of flexible electronics composed of integrated NW structures are presented. The different assembly strategies for the construction of 1D, 2D, and 3D NW assemblies, especially the NW coassembly process for 2D NW assemblies, are comprehensively discussed. The improvements of different NW assemblies on flexible electronics structure and performance are described in detail to elucidate the advantages of well-defined NW assemblies. Finally, a short summary and outlook for future challenges and perspectives in this field are presented.

Precise Patterning of Organic Single Crystals via Capillary-Assisted Alternating-Electric Field

Owing to the extraordinary properties, organic micro/nanocrystals are important building blocks for future low-cost and high-performance organic electronic devices. However, integrated device application of the organic micro/nanocrystals is hampered by the difficulty in high-throughput, high-precision patterning of the micro/nanocrystals. In this study, the authors demonstrate, for the first time, a facile capillary-assisted alternating-electric field method for the large-scale assembling and patterning of both 0D and 1D organic crystals. These crystals can be precisely patterned at the photolithography defined holes/channels at the substrate with the yield up to 95% in 1 mm² . The mechanism of assembly kinetics is systematically studied by the electric field distribution simulation and experimental investigations. By using the strategy, various organic micro/nanocrystal patterns are obtained by simply altering the geometries of the photoresist patterns on substrates. Moreover, ultraviolet photodetectors based on the patterned Alq3 micro/nanocrystals exhibit visible-blind photoresponse with high sensitivity as well as excellent stability and reproducibility. This work paves the way toward high-integration, high-performance organic electronic, and optoelectronic devices from the organic micro/nanocrystals.

Controlled Growth of Large-Area Aligned Single-Crystalline Organic Nanoribbon Arrays for Transistors and Light-Emitting Diodes Driving

ABSTRACT

Organic field-effect transistors (OFETs) based on organic micro-/nanocrystals have been widely reported with charge carrier mobility exceeding 1.0 cm² V^-1 s^-1, demonstrating great potential for high-performance, low-cost organic electronic applications. However, fabrication of large-area organic micro-/nanocrystal arrays with consistent crystal growth direction has posed a significant technical challenge. Here, we describe a solution-processed dip-coating technique to grow large-area, aligned 9,10-bis(phenylethynyl) anthracene (BPEA) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) single-crystalline nanoribbon arrays. The method is scalable to a 5 × 10 cm² wafer substrate, with around 60% of the wafer surface covered by aligned crystals. The quality of crystals can be easily controlled by tuning the dip-coating speed. Furthermore, OFETs based on well-aligned BPEA and TIPS-PEN single-crystalline nanoribbons were constructed. By optimizing channel lengths and using appropriate metallic electrodes, the BPEA and TIPS-PEN-based OFETs showed hole mobility exceeding 2.0 cm² V^-1 s^-1 (average mobility 1.2 cm² V^-1 s^-1) and 3.0 cm² V^-1 s^-1 (average mobility 2.0 cm² V^-1 s^-1), respectively. They both have a high on/off ratio (I _on/I _off) > 10⁹. The performance can well satisfy the requirements for light-emitting diodes driving.

Electro-optical Properties of Neutral and Radical Ion Thienosquaraines

Thienosquaraines are an interesting class of electroactive dyes that are useful for applications in organic electronics. Herein, the redox chemistry and electrochromic response of a few newly synthesized thienosquaraines are presented. These properties are compared to those of the commercial 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine. The stability of the radical ions formed in electrochemical processes strongly affects these properties, as shown by cyclic voltammetry, in situ spectroelectrochemistry, and quantum chemical calculations. Furthermore, all of the dyes show aggregation tendency resulting in panchromatic absorption covering the whole UV/Vis spectral range.

Alignment and Patterning of Ordered Small-Molecule Organic Semiconductor Micro-/Nanocrystals for Device Applications

Large-area alignment and patterning of small-molecule organic semiconductor micro-/nanocrystals (SMOSNs) at desired locations is a prerequisite for their practical device applications. Recent strategies for alignment and patterning of ordered SMOSNs and their corresponding device applications are highlighted.

Porphyrin-Based Nanostructures for Photocatalytic Applications

Well-defined organic nanostructures with controllable size and morphology are increasingly exploited in optoelectronic devices. As promising building blocks, porphyrins have demonstrated great potentials in visible-light photocatalytic applications, because of their electrical, optical and catalytic properties. From this perspective, we have summarized the recent significant advances on the design and photocatalytic applications of porphyrin-based nanostructures. The rational strategies, such as texture or crystal modification and interfacial heterostructuring, are described. The applications of the porphyrin-based nanostructures in photocatalytic pollutant degradation and hydrogen evolution are presented. Finally, the ongoing challenges and opportunities for the future development of porphyrin nanostructures in high-quality nanodevices are also proposed.

Aligned Single-Crystalline Perovskite Microwire Arrays for High-Performance Flexible Image Sensors with Long-Term Stability

A simple, low-cost blade-coating method is developed for the large-area fabrication of single-crystalline aligned CH3NH3PbI3 microwire (MW) arrays. The solution-coating method is applicable to flexible substrates, enabling the fabrication of MW-array-based photodetectors with excellent long-term stability, flexibility, and bending durability. Integrated devices from such photodetectors demonstrate high performance for high-resolution, flexible image sensors.

Precisely Patterned Growth of Ultra-Long Single-Crystalline Organic Microwire Arrays for Near-Infrared Photodetectors

Owing to extraordinary properties, small-molecule organic micro/nanocrystals are identified to be prospective system to construct new-generation organic electronic and optoelectronic devices. Alignment and patterning of organic micro/nanocrystals at desired locations are prerequisite for their device applications in practice. Though various methods have been developed to control their directional growth and alignment, high-throughput precise positioning and patterning of the organic micro/nanocrystals at desired locations remains a challenge. Here, we report a photoresist-assisted evaporation method for large-area growth of precisely positioned ultralong methyl-squarylium (MeSq) microwire (MW) arrays. Positions as well as alignment densities of the MWs can be precisely controlled with the aid of the photoresist-template that fabricated by photolithography process. This strategy enables large-scale fabrication of organic MW arrays with nearly the same accuracy, uniformity, and reliability as photolithography. Near-infrared (NIR) photodetectors based on the MeSq MW arrays show excellent photoresponse behavior and are capable of detecting 808 nm light with high stability and reproducibility. The high on/off ratio of 1600 is significantly better than other organic nanostructure-based optical switchers. More importantly, this strategy can be readily extended to other organic molecules, revealing the great potential of photoresist-assisted evaporation method for future high-performance organic optoelectronic devices.

Highly sensitive broadband flexible photodetectors based on a blend film with zinc octaethylporphyrin long nanowires embedded in an insulating polymer

Blend films with long nanowires of zinc octaethylporphyrin (ZnOEP) embedded in an insulating polymer of poly(methyl methacrylate) (PMMA) have been successfully fabricated by a one-step spin-coating process. Concerning photoactive blends based on small-molecule semiconductors, this is quite a novel strategy and allows us to greatly reduce the issues related to low device performance, such as phase-separation, poor connectivity of the semiconducting layer, and higher densities of interfacial defects. Intensive studies on the correlation between the film morphology and device performance have revealed that excellent photodetector performance is derived from efficient charge transport and good connectivity observed in highly crystalline, interconnected ZnOEP nanowires embedded in an insulating PMMA matrix. To the best of our knowledge, this is the first demonstration of a blend-film-based organic photodetector, which exhibits high sensitivity, high stability, high I(on)/I(off) ratio, excellent mechanical flexibility, and a broadband responsivity region extending up to 1050 nm. The unique characteristics of facile fabrication, high sensitivity, excellent mechanical stability, and broadband responsivity can make the blend film of ZnOEP and PMMA promising in large-area flexible photodetectors.

Patterned growth of single-crystal 3, 4, 9, 10-perylenetetracarboxylic dianhydride nanowire arrays for field-emission and optoelectronic devices

3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) organic nanostructures possess extraordinary electronic and optoelectronic properties. However, it remains a challenge to achieve patterned growth of PTCDA nanowire (NW) arrays for integrated device applications. Here, we demonstrated the high-density, large-area, uniform, and cross-aligned growth of single-crystalline PTCDA NW arrays by using Au nanoparticles (NPs) as the growth templates. The high surface energy of Au NPs led to the cross-aligned growth of organic NWs, enabling the growth of PTCDA NW arrays with any desirable patterns by pre-patterning the Au films on a Si substrate. The PTCDA NW arrays as field emitters show good performance with a large emission current density and high emission stability. Furthermore, photodetectors based on PTCDA NW arrays were constructed via a simple in-situ growth approach, which exhibited high sensitivity to a wideband light ranging from 400-800 nm and surpassed the individual NW-based photodetectors in terms of higher photocurrent and faster response speed. Successful applications of PTCDA NW arrays in field emission and photodetectors show a great potential application of organic NW arrays in future efficient electronic and optoelectronic devices.

Organic nanowire/crystalline silicon p-n heterojunctions for high-sensitivity, broadband photodetectors

Organic/inorganic hybrid devices are promising candidates for high-performance, low-cost optoelectronic devices, by virtue of their unique properties. Polycrystalline/amorphous organic films are widely used in hybrid devices, because defects in the films hamper the improvement of device performance. Here, we report the construction of 2,4-bis[4-(N,N-dimethylamino)phenyl]squaraine (SQ) nanowire (NW)/crystalline Si (c-Si) p-n heterojunctions. Thanks to the high crystal quality of the SQ NWs, the heterojunctions exhibit excellent diode characteristics in darkness. It is significant that the heterojunctions have been found to be capable of detecting broadband light with wavelengths spanning from ultraviolet (UV) light, to visible (Vis) light, to near-infrared (NIR) light, because of the complementary spectrum absorption of SQ NWs with Si. The junction is demonstrated to play a core role in enhancing the device performance, in terms of ultrahigh sensitivity, excellent stability, and fast response. The photovoltaic characteristics of the heterojunctions are further investigated, revealing a power conversion efficiency (PCE) of up to 1.17%. This result also proves the potential of the device as self-powered photodetectors operating at zero external bias voltage. This work presents an important advance in constructing single-crystal organic nanostructure/inorganic heterojunctions and will enable future exploration of their applications in broadband photodetectors and solar cells.

High performance few-layer GaS photodetector and its unique photo-response in different gas environments

Layered GaS nanosheets have been attracting increasing research interests due to their highly anisotropic structural, electrical, optical, and mechanical properties, which are useful for many applications. However, single-layer or few-layer GaS-based photodetectors have been rarely reported. Here a few-layer GaS two-terminal photodetector with a fast and stable response has been fabricated. It shows different photo-responses in various gas environments. A higher photo-response (64.43 A W(-1)) and external quantum efficiency (EQE) (12,621%) is obtained in ammonia (NH3) than in air or oxygen (O2). A theoretical investigation shows that the charge transfer between the adsorbed gas molecules and the photodetector leads to the different photo-responses.