Single-Crystal Organic Nanowires of Copper–Tetracyanoquinodimethane: Synthesis, Patterning, Characterization, and Device Applications

Copper Tetracyanoquinodimethane: From Micro/Nanostructures to Applications

Copper tetracyanoquinodimethane (CuTCNQ) has been investigated around 40 years as a representative bistable material. Meanwhile, micro/nanostructures of CuTCNQ is considered as the prototype of molecular electronics, which have attracted the world's attention and shown great potential applications in nanoelectronics. In this review, methods for synthesis of CuTCNQ micro/nanostructures are first summarized briefly. Then, the strategies for controlling morphologies and sizes of CuTCNQ micro/nanostructures are highlighted. Afterwards, the devices based on these micro/nanostructures are reviewed. Finally, an outlook of future research directions and challenges in this area is presented.

The Emergence of Organic Single-Crystal Electronics

Organic semiconducting single crystals are perfect for both fundamental and application-oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low-temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm²  V^-1  s^-1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single-crystal electronic devices, including field-effect transistors, phototransistors, p-n heterojunctions, and circuits, are summarized. Organic two-dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state-of-the-art overview of organic single-crystal electronics, with their challenges and prospects, is also provided.

Intermediate phase-assisted solution preparation of two dimensional CsPbCl3 perovskite for efficient ultraviolet photodetection

Fully-inorganic halide perovskites (HPs) have realized respectable progress in multiple optoelectronic applications. However, Cl-based fully-inorganic HPs that are ideal for ultraviolet (UV) photodetection applications in high demand still remain rarely explored mainly due to the poor solution processability compared with other counterparts. Here we propose a facile solution method to fabricate CsPbCl₃ with not only high crystallinity but also a two dimensional (2D) morphology for efficient UV photodetection. 2D Ruddlesden-Popper perovskites (RPPs) are firstly prepared as the intermediate phase, which habitually grow into microplates owing to an intrinsic 2D structure. Then Cs⁺ was introduced in the form of highly soluble cesium acetate to exchange with the organic cations in the RPPs to produce 2D CsPbCl₃ with preserved morphology and micron scale size. By this chemical route, the poor solubility issue can be addressed. All the procedures are conducted at room temperature in open air. The perfect band gap, high crystallinity and 2D morphology promise superior UV light sensing capability, one of the best overall performances featuring high responsivity, fast response speed, low driving voltages and good stability is obtained. This work is believed to fill in the "Cl-gap" for this promising class of material.

Correlating growth mechanism and morphology in Cu-TCNQ organometallic complex: A microscopic study

The structure-property correlation in the Cu-TCNQ organometallic complex is very important for explaining its unusual electrical, optical and magnetic properties. Consequently several morphological studies and their correlation with the properties of these materials can be found in the literature, although no systematic study of various morphologies with growth conditions and their correlation has been reported to the best of our knowledge. Therefore in this manuscript the interconversion of various morphologies is reported using electron and probe microscopies. A conventional Cu TEM grid acted as the copper source to form a Cu-TCNQ complex and the complex, which formed at the surface of the TEM grid. The complex thus prepared was characterized by FTIR and Raman spectroscopic techniques. The shifting of ̵-CN from 2221 cm^-1 (TCNQ) to 2201 cm^-1 indicates formation of a complex and the identical nature of IR spectra in two phases indicates that they are polymorphs. The morphologies of Cu-TCNQ were followed through FE-SEM and TEM studies. Various morphologies such as needle, square tube, platelet etc. were observed as a function of time. A distinct transition from needle to platelet morphology was observed as the complex grew. The conductance of various morphologies in phase-I as well as phase-II were also measured and compared by Spreading Resistance Imaging (SRI) at different bias voltage i.e. 1 V, 3 V and 5 V.

Organic semiconductor crystals

A comprehensive overview of organic semiconductor crystals is provided, including the physicochemical features, the control of crystallization and the device physics.

Organometallic MTCNQ films: a comparative study of CuTCNQ versus AgTCNQ

We performed a systematic study of electron-acceptor molecules in two closely related organometallic solids, namely, CuTCNQ and AgTCNQ, proposing a model for the conductive switching behavior of these materials.

Localized Synthesis of Conductive Copper-Tetracyanoquinodimethane Nanostructures in Ultrasmall Microchambers for Nanoelectronics

In this work, the microfluidic-assisted synthesis of copper-tetracyanoquinodimethane (Cu-TCNQ) nanostructures in an ambient environment is reported for the first time. A two-layer microfluidic device comprising parallel actuated microchambers was used for the synthesis and enabled excellent fluid handling for the continuous and multiple chemical reactions in confined ultrasmall chambers. Different precautions were applied to ensure the reduction state of copper (Cu) for the synthesis of Cu-TCNQ charge-transfer compounds. The localized synthesis of Cu and in situ transformation to Cu-TCNQ complexes in solution were achieved by applying different gas pressures in the control layer. Additionally, various diameters of the Cu-TCNQ nano/microstructures were obtained by adjusting the concentration of the precursors and reaction time. After the synthesis, platinum (Pt) microelectrode arrays, which were aligned at the microchambers, could enable the in situ measurements of the electronic properties of the synthesized nanostructures without further manipulation. The as-prepared Cu-TCNQ wire bundles showed good conductivity and a reversible hysteretic switching effect, which proved the possibility in using them to build advanced nanoelectronics.

One-minute self-assembly of millimetre-long DAST crystalline microbelts via substrate-supported rapid evaporation crystallization

We propose a substrate-supported rapid evaporation crystallization method to rapidly self-assemble microbelts of DAST, a benchmark organic NLO crystal. DAST microbelt formation depends on substrate properties and surfactant.

Unveiling the Switching Riddle of Silver Tetracyanoquinodimethane Towards Novel Planar Single-Crystalline Electrochemical Metallization Memories

The switching riddle of AgTCNQ is shown to be caused by the solid electrolyte mechanism. Both factors of bulk phase change and contact issue play key roles in the efficient work of the devices. An effective strategy is developed to locate the formation/disruption of Ag conductive filaments using the planar asymmetric configuration of Au/AgTCNQ/AlOx /Al. These novel electrochemical metallization memories demonstrate many promising properties.

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.

Controlled Growth of Rubrene Nanowires by Eutectic Melt Crystallization

Organic semiconductors including rubrene, Alq₃, copper phthalocyanine and pentacene are crystallized by the eutectic melt crystallization. Those organic semiconductors form good eutectic systems with the various volatile crystallizable additives such as benzoic acid, salicylic acid, naphthalene and 1,3,5-trichlorobenzene. Due to the formation of the eutectic system, organic semiconductors having originally high melting point (T_m > 300 °C) are melted and crystallized at low temperature (T_e = 40.8–133 °C). The volatile crystallizable additives are easily removed by sublimation. For a model system using rubrene, single crystalline rubrene nanowires are prepared by the eutectic melt crystallization and the eutectic-melt-assisted nanoimpinting (EMAN) technique. It is demonstrated that crystal structure and the growth direction of rubrene can be controlled by using different volatile crystallizable additives. The field effect mobility of rubrene nanowires prepared using several different crystallizable additives are measured and compared.

Solution-Processed Large-Area Nanocrystal Arrays of Metal-Organic Frameworks as Wearable, Ultrasensitive, Electronic Skin for Health Monitoring

Pressure sensors based on solution-processed metal-organic frameworks nanowire arrays are fabricated with very low cost, flexibility, high sensitivity, and ease of integration into sensor arrays. Furthermore, the pressure sensors are suitable for monitoring and diagnosing biomedical signals such as radial artery pressure waveforms in real time.

Air-Stable and Solution-Processable Perovskite Photodetectors for Solar-Blind UV and Visible Light

Stable perovskite CH3NH3PbI3-xClx for a photodetector was prepared through spin-coating of a fluorous polymer as a light protection layer. The best responsivity of photodetector was 14.5 A/W to white light and 7.85 A/W for solar-blind UV light (λ = 254 nm). The response time was in the submicrosecond range. The fluorous polymer coating increases the lifetime of the devices to almost 100 days.

Low-temperature fabrication of alkali metal-organic charge transfer complexes on cotton textile for optoelectronics and gas sensing

A generalized low-temperature approach for fabricating high aspect ratio nanorod arrays of alkali metal-TCNQ (7,7,8,8-tetracyanoquinodimethane) charge transfer complexes at 140 °C is demonstrated. This facile approach overcomes the current limitation associated with fabrication of alkali metal-TCNQ complexes that are based on physical vapor deposition processes and typically require an excess of 800 °C. The compatibility of soft substrates with the proposed low-temperature route allows direct fabrication of NaTCNQ and LiTCNQ nanoarrays on individual cotton threads interwoven within the 3D matrix of textiles. The applicability of these textile-supported TCNQ-based organic charge transfer complexes toward optoelectronics and gas sensing applications is established.

Organic semiconductors for device applications: current trends and future prospects

With the rich experience of developing silicon devices over a period of the last six decades, it is easy to assess the suitability of a new material for device applications by examining charge carrier injection, transport, and extraction across a practically realizable architecture; surface passivation; and packaging and reliability issues besides the feasibility of preparing mechanically robust wafer/substrate of single-crystal or polycrystalline/amorphous thin films. For material preparation, parameters such as purification of constituent materials, crystal growth, and thin-film deposition with minimum defects/disorders are equally important. Further, it is relevant to know whether conventional semiconductor processes, already known, would be useable directly or would require completely new technologies. Having found a likely candidate after such a screening, it would be necessary to identify a specific area of application against an existing list of materials available with special reference to cost reduction considerations in large-scale production. Various families of organic semiconductors are reviewed here, especially with the objective of using them in niche areas of large-area electronic displays, flexible organic electronics, and organic photovoltaic solar cells. While doing so, it appears feasible to improve mobility and stability by adjusting π-conjugation and modifying the energy band-gap. Higher conductivity nanocomposites, formed by blending with chemically conjugated C-allotropes and metal nanoparticles, open exciting methods of designing flexible contact/interconnects for organic and flexible electronics as can be seen from the discussion included here.

Facilely and efficiently tuning metal-organic nanostructures of a charge-transfer complex based on a water controlled nanoreaction and the chemistry of 7,7,8,8-tetracyanoquinodimethane (TCNQ)

Metal-organic charge-transfer complexes based on 7,7,8,8-tetracyanoquinodimethane (TCNQ) have received considerable attention because of their unique solid-state physical properties for potential applications in nanoscale opto-electronic devices. To address the challenge in preparing novel metal-TCNQ (MTCNQ) nanostructures, here we introduce a facile and efficient way for synthesizing MTCNQ, taking Ni[TCNQ]2(H2O)2 as an example. By finely tuning the amount of water added into TCNQ solution, well-ordered and large-scale patterns of Ni[TCNQ]2(H2O)2 were successfully obtained in a controllable manner. This facile method will not only be beneficial for the tailored preparation of nanoscale MTCNQ complexes, but also enrich the chemistry of TCNQ.

Decorating Semiconductor Silver-Tetracyanoquinodimethane Nanowires with Silver Nanoparticles from Ionic Liquids

Hybrid semiconducting silver-tetracyanoquinodimethane (AgTCNQ) nanowires decorated with Ag nanoparticles have been synthesized at room temperature in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. Hydroquinone was applied to reduce Ag+ and TCNQ to silver nanoparticles, and TCNQ–, respectively, under ambient conditions. AgTCNQ nanowires were formed via spontaneous electrolysis between Ag metal nanoparticles and TCNQ, and reaction between Ag+ and TCNQ–. Microscopic, spectroscopic, and X-ray characterizations all confirmed the formation of crystalline Ag nanoparticle–AgTCNQ nanowire hybrid structures. The ionic liquid was used as a reaction medium, but also as a stabilizing (or blocking) agent to control the nucleation and growth rate of AgTCNQ wires.

Reversible iodine absorption of nonporous coordination polymer Cu(TCNQ)

Polycrystalline powders of Cu(TCNQ) absorb iodine to form Cu(TCNQ)I₄upon solid grinding with iodine or immersion in a hexane solution of iodine.

"Liquid-liquid-solid"-type superoleophobic surfaces to pattern polymeric semiconductors towards high-quality organic field-effect transistors

Precisely aligned organic-liquid-soluble semiconductor microwire arrays have been fabricated by "liquid-liquid-solid" type superoleophobic surfaces directed fluid drying. Aligned organic 1D micro-architectures can be built as high-quality organic field-effect transistors with high mobilities of >10 cm(2) ·V(-1) ·s(-1) and current on/off ratio of more than 10(6) . All these studies will boost the development of 1D microstructures of organic semiconductor materials for potential application in organic electronics.

Deterministic growth of AgTCNQ and CuTCNQ nanowires on large-area reduced graphene oxide films for flexible optoelectronics

We describe a synchronous reduction and assembly procedure to directly produce large-area reduced graphene oxide (rGO) films sandwiched by a high density of metal nanoparticles (silver and copper). Further, by using the sandwiched metal NPs as sources, networks consisting of AgTCNQ and CuTCNQ nanowires were deterministically grown from the rGO films, forming structurally and functionally integrated rGO/metal-TCNQ hybrid films with outstanding flexibility, bending endurance, and electrical stability. Interestingly, due to the p-type nature of the rGO film and the n-type nature of the metal-TCNQ NWs, the hybrid films are essentially thin-film p-n junctions which are useful in ubiquitous electronics and optoelectronics. Measurements of the optoelectronic properties demonstrate that the rGO/metal-TCNQ hybrid films exhibit substantial photoconductivity and highly reproducible photoswitching behaviours. The present approach may open the door to the versatile and deterministic integration of functional nanostructures into flexible conducting substrates and provide an important step towards producing low-cost and high-performance soft electronic and optoelectronic devices.

Three alkaline-earth metal complexes with 3D networks constructed from a 7,7,8,8-tetracyanoquinodimethane ligand: synthesis, structure and electrochemical properties

Three new 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline-earth metal complexes, namely {[M2(TCNQ)3(H2O)6]·TCNQ}n (M = Ca (1), Sr (2) and Ba (3)) have been synthesized by salt elimination reactions. X-ray crystallographic analysis reveals that complexes 1, 2 and 3 are isomorphic featuring a unique 3D structure. Cyclic and differential pulse voltammograms for complexes 1-3 show a reversible one-electron oxidation and a reversible one-electron reduction within the electrochemical window of CH3CN. Their electrochemical HOMO-LUMO gap and reversibility are examined.

Confined synthesis and integration of functional materials in sub-nanoliter volumes

We present a novel microchip-based approach to combine the synthesis, characterization, and utilization of different functional materials on a single platform. A two-layer microfluidic device comprising 10 parallel actuated reaction chambers with volumes of a few hundred picoliters is used to localize and confine the synthesis, while the surfaces of the reaction chambers comprise an electrode array for direct integration and further characterization of the created crystalline assemblies without the need for further manipulation or positioning devices. First we visualized and evaluated the dynamics of our method by monitoring the formation of a fluorescent metal-organic complex (Zn(bix)). Next, we induced the site-specific growth of two types of organic conductive crystals, AuTTF and AgTCNQ, directly onto the electrode arrays in one- and two-step reactions, respectively. The performance of the created AgTCNQ crystals as memory elements was thoroughly examined. Moreover, we proved for first time that AuTTF composites can be used as label-free sensing elements.

Mass-production of single-crystalline device arrays of an organic charge-transfer complex for its memory nature

Controllable synthesis of single-crystalline lamellae of copper tetracyano-p- quinodimethane (CuTCNQ, phase II) is achieved, and mass-produced devices or device arrays based on symmetrical Cr/Au gap electrodes are fabricated in situ. The devices exhibit semiconductor properties important for the understanding of CuTCNQ.

Nanofriction visualized in space and time by 4D electron microscopy

In this letter, we report a novel method of visualizing nanoscale friction in space and time using ultrafast electron microscopy (UEM). The methodology is demonstrated for a nanoscale movement of a single crystal beam on a thin amorphous membrane of silicon nitride. The movement results from the elongation of the crystal beam, which is initiated by a laser (clocking) pulse, and we examined two types of beams: those that are free of friction and the others which are fixed on the substrate. From observations of image change with time we are able to decipher the nature of microscopic friction at the solid-solid interface: smooth-sliding and periodic slip-stick friction. At the molecular and nanoscale level, and when a force parallel to the surface (expansion of the beam) is applied, the force of gravity as a (perpendicular) load cannot explain the observed friction. An additional effective load being 6 orders of magnitude larger than that due to gravity is attributed to Coulombic/van der Waals adhesion at the interface. For the case under study, metal-organic crystals, the gravitational force is on the order of piconewtons whereas the static friction force is 0.5 μN and dynamic friction is 0.4 μN; typical beam expansions are 50 nm/nJ for the free beam and 10 nm/nJ for the fixed beam. The method reported here should have applications for other materials, and for elucidating the origin of periodic and chaotic friction and their relevance to the efficacy of nano(micro)-scale devices.

Functional organic field-effect transistors

Functional organic field-effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed.

Modified thermodynamics in ionic liquids for controlled electrocrystallization of nanocubes, nanowires, and crystalline thin films of silver-tetracyanoquinodimethane

Electrocrystallization of nanocubes, nanorods, nanowires, and crystalline thin films of silver-tetracyanoquinodimethane (AgTCNQ) onto glassy carbon, indium tin oxide, and platinum electrodes can be achieved from ionic liquids containing dissolved TCNQ and Ag(I) salts. In conventional molecular organic solvents, such as acetonitrile, the reduction of TCNQ and Ag(+) occurs at almost the same potential. In contrast, the different thermodynamics that apply to the room temperature ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF(4)), give rise to a large potential separation in the two processes, which enables electrocrystallization of AgTCNQ to be undertaken via two distinctly different, potential-dependent mechanisms. Cyclic and microelectrode voltammetric, chronoamperometric, together with microscopic and spectroscopic techniques reveal that AgTCNQ nanostuctures of controlled morphology, size, density, and uniformity can be achieved by tuning the electrocrystallization parameters such as potential, stoichiometric ratio of Ag(+) and TCNQ, and their concentrations, time, and ionic liquid viscosity by altering the water content. In the potential range of -0.1 to 0.3 V vs Fc(0/+) (Fc = ferrocene), electrocrystallization occurs when Ag is deposited at electrode defect sites via a progressive nucleation and 3-D growth mechanism followed by reaction with TCNQ to produce structures ranging from nanocubes to nanowires. At higher stoichiometric concentrations of Ag(+) and more negative potentials (<-0.1 V vs Fc(0/+)), extremely thin crystalline films could be obtained via overpotential deposition. Infrared and Raman spectroscopy, elemental analysis, together with X-ray diffraction and scanning electron microscopy all confirm the formation of highly pure AgTCNQ nanomaterials, which exhibit differences in morphology but not phase. The study highlights the capability of the electrocrystallization method to precisely control the morphology of nanomaterials, and also the unprecedented opportunities provided by using ionic liquids as the medium for preparation of technologically important metal-TCNQ charge transfer complexes.

Solution-processed, high-performance n-channel organic microwire transistors

The development of solution-processable, high-performance n-channel organic semiconductors is crucial to realizing low-cost, all-organic complementary circuits. Single-crystalline organic semiconductor nano/microwires (NWs/MWs) have great potential as active materials in solution-formed high-performance transistors. However, the technology to integrate these elements into functional networks with controlled alignment and density lags far behind their inorganic counterparts. Here, we report a solution-processing approach to achieve high-performance air-stable n-channel organic transistors (the field-effect mobility (mu) up to 0.24 cm(2)/Vs for MW networks) comprising high mobility, solution-synthesized single-crystalline organic semiconducting MWs (mu as high as 1.4 cm(2)/Vs for individual MWs) and a filtration-and-transfer (FAT) alignment method. The FAT method enables facile control over both alignment and density of MWs. Our approach presents a route toward solution-processed, high-performance organic transistors and could be used for directed assembly of various functional organic and inorganic NWs/MWs.