One‐Step Self‐Assembly, Alignment, and Patterning of Organic Semiconductor Nanowires by Controlled Evaporation of Confined Microfluids

Patterning of Organic Semiconductors Leads to Functional Integration: From Unit Device to Integrated Electronics

The use of organic semiconductors in electronic devices, including transistors, sensors, and memories, unlocks innovative possibilities such as streamlined fabrication processes, enhanced mechanical flexibility, and potential new applications. Nevertheless, the increasing technical demand for patterning organic semiconductors requires greater integration and functional implementation. This paper overviews recent efforts to pattern organic semiconductors compatible with electronic devices. The review categorizes the contributions of organic semiconductor patterning approaches, such as surface-grafting polymers, capillary force lithography, wettability, evaporation, and diffusion in organic semiconductor-based transistors and sensors, offering a timely perspective on unconventional approaches to enable the patterning of organic semiconductors with a strong focus on the advantages of organic semiconductor utilization. In addition, this review explores the opportunities and challenges of organic semiconductor-based integration, emphasizing the issues related to patterning and interconnection.

The control of dry-out patterns using bubble-containing droplets

HYPOTHESIS

For an evaporating nanofluid droplet that contains a bubble inside, we suspect the bubble boundary remains pinned during evaporation whereas the droplet perimeter recedes. Thus, the dry-out patterns are mainly determined by the presence of the bubble and their morphology can be tuned by the size and location of the added bubble.

EXPERIMENTS

Bubbles with varying base diameters and lifetimes are added into evaporating droplets that contain nanoparticles with different types, sizes, concentrations, shapes, and wettability. The geometric dimensions of the dry-out patterns are measured.

FINDINGS

For a droplet containing a long-lifetime bubble, a complete ring-like deposit forms, and its diameter and thickness increases and decreases with the bubble base diameter, respectively. The ring completeness, i.e., the ratio of actual ring length to its imaginary perimeter, decreases with the decrease in bubble lifetime. The pinning of droplet receding contact line by particles near the bubble perimeter has been found to be the key factor leading to ring-like deposits. This study introduces a strategy of producing ring-like deposit and allows a control of the ring morphology in a simple, cheap, and impurity-free fashion, which is applicable to various applications associated with evaporative self-assembly.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

Photocontrolled Hierarchical Self-Assembly of Anisotropic Micropatterns of Nanofibers onto Isotropic Surfaces

Hierarchical self-assembly is achieved using a visible light triggered photoreaction. A pro-gelator, α-diketone-2,3-didecyloxyanthracene, is photoconverted into a low molecular weight gelator, 2,3-didecyloxyanthracene (DDOA), that self-assembles into nanofibers. Spatial confinement and patterns of these nanofibers onto a surface are achieved by localizing initial nucleation with a focused laser and photogenerate subsequent fiber growth with the laser or gentler wide-field irradiation. Remarkably, collective growth of nanofibers results in anisotropic micropatterns with orientation factors (OF) reaching 79%, resulting in collective emission of linearly polarized light. The OF, distance of collective growth and fiber density, are controlled by the photoirradiation conditions and the balance of interactions between DDOA aggregates and the glass surface. An unprecedented juxtaposition of orthogonally oriented nanofiber patterns on an isotropic surface is achieved with individual control of the fibers' main direction. In perspective, this photochemical method can be extended to a large variety of self-assembling molecules.

In-plane aligned assemblies of 1D-nanoobjects: recent approaches and applications

One-dimensional (1D) nanoobjects have strongly anisotropic physical properties which are averaged out and cannot be exploited in disordered systems. We reviewed the in plane alignment approaches and potential applications with perspectives shared.

Precise Positioning of Organic Semiconductor Single Crystals with Two-Component Aligned Structure through 3D Wettability-Induced Sequential Assembly

Highly ordered organic semiconductor single-crystal (OSSC) arrays are ideal building blocks for functional organic devices. However, most of the current methods are only applicable to fabricate OSSC arrays of a single component, which significantly hinders the application of OSSC arrays in integrated organic circuits. Here, we present a universal approach, termed three-dimensional (3D) wettability-induced sequential assembly that can programmatically and progressively manipulate the crystallization locations of different organic semiconductors at the same spatial position using a 3D microchannel template, for the fabrication of the two-component OSSC arrays. As an example, we successfully prepared two-component, bilayer structured OSSC arrays consisting of n-type N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide and p-type 6,13-bis(triisopropylsilylethynyl)pentacene microbelts. The bicomponent OSSCs show ambipolar carrier transport properties with hole and electron mobilities of 0.342 and 0.526 cm² V^-1 s^-1, respectively. Construction of complementary inverters is further demonstrated based on the two-component OSSCs. The capability of integration of multicomponent OSSC arrays opens up unique opportunities for future high-performance organic complementary circuits.

A Facile One-Step Approach for Constructing Multidimensional Ordered Nanowire Micropatterns via Fibrous Elastocapillary Coalescence

Nanowire (NW) based micropatterns have attracted research interests for their applications in electric microdevices. Particularly, aligning NWs represents an important process due to the as-generated integrated physicochemical advantages. Here, a facile and general strategy is developed to align NWs using fibrous elastocapillary coalescence of carbon nanotube arrays (ACNTs), which enables constructing multidimensional ordered NW micropatterns in one step without any external energy input. It is proposed that the liquid film of NW solution is capable of shrinking unidirectionally on the top of ACNTs, driven by the dewetting-induced elastocapillary coalescence of the ACNTs. Consequently, the randomly distributed NWs individually rotate and move into dense alignment. Meanwhile, the aggregating and bundling of ACNTs is helpful to produce carbon nanotube (CNT) yarns connecting neighboring bundles. Thus, a micropatterned NW network composed of a top-layer of horizontally aligned NWs and an under-layer of vertical ACNT bundles connected by CNT yarns is prepared, showing excellent performance in sensing external pressure with a sensitivity of 0.32 kPa^-1 . Moreover, the aligned NWs can be transferred onto various substrates for constructing electronic circuits. The strategy is applicable for aligning various NWs of Ag, ZnO, Al₂ O₃ , and even living microbes. The result may offer new inspiration for fabricating NW-based functional micropatterns.

Large-Scale Alignment of Polymer Semiconductor Nanowires for Efficient Charge Transport via Controlled Evaporation of Confined Fluids

Long-range alignment of conjugated polymers is as critical as polymer chain packing for achieving efficient charge transport in polymer thin films used in electronic and optoelectronic devices. Here, the present study reports a facile, scalable strategy that enables the deposition of macroscopically aligned polymer semiconductor nanowire (NW)-array films with highly enhanced charge carrier mobility, using a modified controlled evaporative self-assembly (MCESA) technique. Organic field-effect transistors (OFETs) based on highly oriented poly(3-hexylthiophene) (P3HT)-NW films exhibit more than 10-fold enhancement of carrier mobility, with the highest mobility of 0.13 cm² V^-1 s^-1, compared to the OFETs based on pristine P3HT films. Significantly, large-area aligned P3HT NW-films, which are deposited over 12 arrays of transistors on a 4 in. wafer by an MCESA coating, result in lower device performance variation (i.e., standard deviation ≈ ±0.0172 (16%) cm² V^-1 s^-1) as well as an excellent average device performance (i.e., average charge mobility ≈ 0.11 cm² V^-1 s^-1), compared to those obtained using the conventional CESA coating, overcoming a critical challenge in the field of OFETs.

Multiscale assembly of solution-processed organic electronics: the critical roles of confinement, fluid flow, and interfaces

Organic semiconducting small molecules and polymers provide a rich phase space for investigating the fundamentals of molecular and hierarchical assembly. Stemming from weak intermolecular interactions, their assembly sensitively depends on processing conditions, which in turn drastically modulate their electronic properties. Much work has gone into molecular design strategies that maximize intermolecular interactions and encourage close packing. Less understood, however, is the non-equilibrium assembly that occurs during the fabrication process (especially solution coating and printing) which is critical to determining thin film morphology across length scales. This encompasses polymorphism and molecular packing at molecular scale, assembly of π-bonding aggregates at the tens of nanometers scale, and the formation of domains at the micron-millimeter device scale. Here, we discuss three phenomena ubiquitous in solution processing of organic electronic thin films: the confinement effect, fluid flows, and interfacial assembly and the role they play in directing assembly. This review focuses on the mechanistic understanding of how assembly outcomes couple closely to the solution processing environment, supported by salient examples from the recent literature.

Guided Growth of Ag Nanowires by Galvanic Replacement on a Flexible Substrate

An aligned Ag nanowire array was directly synthesized by galvanic replacement on curved poly(ethylene terephthalate) (PET) by using a Cu₂O microcrystal as a reductant. A more orderly aligned nanowire array was obtained when the curvature radius was reduced. A second growth with different orientation produced Ag nanowire networks. The guided growth was also achieved when using Zn as a reductant or polystyrene as a substrate. This plain method with facile control over the orientation and density of the Ag nanowire array enriches the grow-in-place methodology and can potentially be applied to various fields.

"Capillary-Bridge Lithography" for Patterning Organic Crystals toward Mode-Tunable Microlaser Arrays

A versatile "capillary-bridge lithography" technique is developed for patterning 1D organic single crystals and microring structures through controlling the generation and dewetting of capillary bridges on an interface with asymmetric wettability. High-performance Fabry-Pérot and whispering-gallery mode lasing emission with tunable modes are achieved on these 1D and microring structures.

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.

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.

Wafer-Scale Precise Patterning of Organic Single-Crystal Nanowire Arrays via a Photolithography-Assisted Spin-Coating Method

A photolithography-assisted spin-coating approach is developed to produce single-crystal organic nanowire (NW) arrays at designated locations with high precision and high efficiency. This strategy enables the large-scale fabrication of organic NW arrays with nearly the same accuracy, reliability, and flexibility as photolithography. The high mobilities of the organic NWs enable the control of the switch of multicolored light-emitting devices with good stability.

In situ observation of meniscus shape deformation with colloidal stripe pattern formation in convective self-assembly

Vertical convective self-assembly is capable of fabricating stripe-patterned structures of colloidal particles with well-ordered periodicity. To unveil the mechanism of the stripe pattern formation, in the present study, we focus on the meniscus shape and conduct in situ observations of shape deformation associated with particulate line evolution. The results reveal that the meniscus is elongated downward in a concave fashion toward the substrate in accordance with solvent evaporation, while the concave deformation is accelerated by solvent flow, resulting in the rupture of the liquid film at the thinnest point of the meniscus. The meniscus rupture triggers the meniscus to slide off from the particulate line, followed by the propagation of the sliding motion of the three-phase contact line, resulting in the formation of stripe spacing.

Self-assembly of large-scale P3HT patterns by confined evaporation in the capillary tube

Large-scale P3HT stripe patterns based on a controlled evaporation self-assembly method in a capillary tube have been fabricated.

Fabrication and physical properties of self-assembled ultralong polymer/small molecule hybrid microstructures

The present work showed a novel approach to fabricate polymer/small molecule hybrid microstructures, and the transport characteristics and morphologies of the as-fabricated wires with different ratios of the two different components.

Large-scale highly ordered hierarchical structures of conjugated polymer via self-assembly from mixed solvents

Highly-ordered large scale patterns of poly(3-hexylthiophene) (P3HT) can be produced via confined evaporative self-assembly (CESA) from mixed solvents in one-step.

Directed assembly of nanodiamond nitrogen-vacancy centers on a chemically modified patterned surface

Nitrogen-vacancy (NV) centers in nanodiamond (ND) particles are an attractive material for photonic, quantum information, and biological sensing technologies due to their optical properties-bright single photon emission and long spin coherence time. To harness these features in practical devices, it is essential to realize efficient methods to assemble and pattern NDs at the micro-/nanoscale. In this work, we report the large scale patterned assembly of NDs on a Au surface by creating hydrophobic and hydrophilic regions using self-assembled monolayer (SAM). Hydrophobic regions are created using a methyl (-CH3) terminated SAM of octadecanethiol molecules. Evaporating a water droplet suspension of NDs on the SAM patterned surface assembles the NDs in the bare Au, hydrophilic regions. Using this procedure, we successfully produced a ND structures in the shape of dots, lines, and rectangles. Subsequent photoluminescence imaging of the patterned NDs confirmed the presence of optically active NV centers. Experimental evidence in conjunction with computational analysis indicates that the surface wettability of the SAM modified Au surface plays a dominant role in the assembly of NDs as compared to van der Waals and other substrate-ND interactions.

Nanoscale size-selective deposition of nanowires by micrometer scale hydrophilic patterns

Controlling the post-growth assembly of nanowires is an important challenge in the development of functional bottom-up devices. Although various methods have been developed for the controlled assembly of nanowires, it is still a challenging issue to align selectively heterogeneous nanowires at desired spatial positions on the substrate. Here we report a size selective deposition and sequential alignment of nanowires by utilizing micrometer scale hydrophilic/hydrophobic patterned substrate. Nanowires dispersed within oil were preferentially deposited only at a water/oil interface onto the hydrophilic patterns. The diameter size of deposited nanowires was strongly limited by the width of hydrophilic patterns, exhibiting the nanoscale size selectivity of nanowires deposited onto micrometer scale hydrophilic patterns. Such size selectivity was due to the nanoscale height variation of a water layer formed onto the micrometer scale hydrophilic patterns. We successfully demonstrated the sequential alignment of different sized nanowires on the same substrate by applying this size selective phenomenon.

"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.

In-situ device integration of large-area patterned organic nanowire arrays for high-performance optical sensors

Single-crystalline organic nanowires (NWs) are important building blocks for future low-cost and efficient nano-optoelectronic devices due to their extraordinary properties. However, it remains a critical challenge to achieve large-scale organic NW array assembly and device integration. Herein, we demonstrate a feasible one-step method for large-area patterned growth of cross-aligned single-crystalline organic NW arrays and their in-situ device integration for optical image sensors. The integrated image sensor circuitry contained a 10 × 10 pixel array in an area of 1.3 × 1.3 mm(2), showing high spatial resolution, excellent stability and reproducibility. More importantly, 100% of the pixels successfully operated at a high response speed and relatively small pixel-to-pixel variation. The high yield and high spatial resolution of the operational pixels, along with the high integration level of the device, clearly demonstrate the great potential of the one-step organic NW array growth and device construction approach for large-scale optoelectronic device integration.

Large-area aligned growth of single-crystalline organic nanowire arrays for high-performance photodetectors

Due to their extraordinary properties, single-crystalline organic nanowires (NWs) are important building blocks for future low-cost and efficient nano-optoelectronic devices. However, it remains a critical challenge to assemble organic NWs rationally in an orientation-, dimensionality- and location-controlled manner. Herein, we demonstrate a feasible method for aligned growth of single-crystalline copper phthalocyanine (CuPc) NW arrays with high density, large-area uniformity and perfect crossed alignment by using Au film as a template. The growth process was investigated in detail. The Au film was found to have a critical function in the aligned growth of NWs, but may only serve as the active site for NW nucleation because of the large surface energy, as well as direct the subsequent aligned growth. The as-prepared NWs were then transferred to construct single NW-based photoconductive devices, which demonstrated excellent photoresponse properties with robust stability and reproducibility; the device showed a high switching ratio of ∼180, a fast response speed of ∼100 ms and could stand continuous operation up to 2 h. Importantly, this strategy can be extended to other organic molecules for their synthesis of NW arrays, revealing great potential for use in the construction of large-scale high-performance functional nano-optoelectronic devices.

Self-assembly and hierarchical patterning of aligned organic nanowire arrays by solvent evaporation on substrates with patterned wettability

The controlled growth and alignment of one-dimensional organic nanostructures at well-defined locations considerably hinders the integration of nanostructures for electronic and optoelectronic applications. Here, we demonstrate a simple process to achieve the growth, alignment, and hierarchical patterning of organic nanowires on substrates with controlled patterns of surface wettability. The first-level pattern is confined by the substrate patterns of wettability. Organic nanostructures are preferentially grown on solvent wettable regions. The second-level pattern is the patterning of aligned organic nanowires deposited by controlling the shape and movement of the solution contact lines during evaporation on the wettable regions. This process is controlled by the cover-hat-controlled method or vertical evaportation method. Therefore, various new patterns of organic nanostructures can be obtained by combing these two levels of patterns. This simple method proves to be a general approach that can be applied to other organic nanostructure systems. Using the as-prepared patterned nanowire arrays, an optoelectronic device (photodetector) is easily fabricated. Hence, the proposed simple, large-scale, low-cost method of preparing patterns of highly ordered organic nanostructures has high potential applications in various electronic and optoelectronic devices.

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.

Ordering Ag nanowire arrays by a glass capillary: a portable, reusable and durable SERS substrate

Assembly of nanowires into ordered macroscopic structures with new functionalities has been a recent focus. In this Letter, we report a new route for ordering hydrophilic Ag nanowires with high aspect ratio by flowing through a glass capillary. The present glass capillary with well-defined silver nanowire films inside can serve as a portable and reusable substrate for surface-enhanced Raman spectroscopy (SERS), which may provide a versatile and promising platform for detecting mixture pollutions. By controlling the flow parameters of nanowire suspensions, initially random Ag nanowires can be aligned to form nanowire arrays with tunable density, forming cambered nanowire films adhered onto the inner wall of the capillary. Compared with the planar ordered Ag nanowire films by the Langmuir-Blodgett (LB) technique, the cambered nanowire films show better SERS performance.

Kinetically driven self-assembly of a binary solute mixture with controlled phase separation via electro-hydrodynamic flow of corona discharge

This feature article describes a new and facile process to fabricate a variety of thin films of non-volatile binary solute mixtures suitable for high performance organic electronic devices via electro-hydrodynamic flow of conventional corona discharge. Both Corona Discharge Coating (CDC) and a modified version of CDC, Scanning Corona Discharge Coating (SCDC), are based on utilizing directional electric flow, known as corona wind, of the charged uni-polar particles generated by corona discharge between a metallic needle and a bottom plate under a high electric field (5-10 kV cm(-1)). The electric flow rapidly spreads out the binary mixture solution on the bottom plate and subsequently forms a smooth and flat thin film in a large area within a few seconds. In the case of SCDC, the static movement of the bottom electrode on which a binary mixture solution is placed provides further control of thin film formation, giving rise to a film highly uniform over a large area. Interesting phase separation behaviors were observed including nanometer scale phase separation of a polymer-polymer binary mixture and vertical phase separation of a polymer-organic semiconductor mixture. Core-shell type phase separation of either polymer-polymer or polymer-colloidal nanoparticle binary mixtures was also developed with a periodically patterned microstructure when the relative location of the corona wind was controlled to a binary solution droplet on a substrate. We also demonstrate potential applications of thin functional films with controlled microstructures by corona coating to various organic electronic devices such as electroluminescent diodes, field effect transistors and non-volatile polymer memories.

Small molecular nanowire arrays assisted by superhydrophobic pillar-structured surfaces with high adhesion

Elaborately programmed fluorescent calcein nanowire arrays can be prepared with the aid of superhydrophobic pillar-structured surfaces with high adhesion. Each nanowire can be precisely positioned by well designed tip-structured micropillars, indicating an advance in the methodologies of controlling small molecular 1D organic nanostructures. The as-prepared fluorescent nanowire arrays can serve as a ferrous salt sensing device.

Optical materials based on molecular nanoparticles

A major part of contemporary nanomaterials research is focused on metal and semiconductor nanoparticles, constituted of extended lattices of atoms or ions. Molecular nanoparticles assembled from small molecules through non-covalent interactions are relatively less explored but equally fascinating materials. Their unique and versatile characteristics have attracted considerable attention in recent years, establishing their identity and status as a novel class of nanomaterials. Optical characteristics of molecular nanoparticles capture the essence of their nanoscale features and form the basis of a variety of applications. This review describes the advances made in the field of fabrication of molecular nanoparticles, the wide spectrum of their optical and nonlinear optical characteristics and explorations of the potential applications that exploit their unique optical attributes.