Comprehensive Application of XFEL Microcrystallography for Challenging Targets in Various Organic Compounds.

There is a growing demand for structure determination from small crystals, and the three-dimensional electron diffraction (3D ED) technique can be employed for this purpose. However, 3D ED has certain limitations related to the crystal thickness and data quality. We here present the application of serial X-ray crystallography (SX) with X-ray free electron lasers (XFELs) to small (a few µm or less) and thin (a few hundred nm or less) crystals of novel compounds dispersed on a substrate. For XFEL exposures, two-dimensional (2D) scanning of the substrate coupled with rotation enables highly efficient data collection. The recorded patterns can be successfully indexed using lattice parameters obtained through 3D ED. This approach is especially effective for challenging targets, including pharmaceuticals and organic materials that form preferentially oriented flat crystals in low-symmetry space groups. Some of these crystals have been difficult to solve or have yielded incomplete solutions using 3D ED. Our extensive analyses confirmed the superior quality of the SX data regardless of crystal orientations. Additionally, 2D scanning with XFEL pulses gives an overall distribution of the samples on the substrate, which can be useful for evaluating the properties of crystal grains and the quality of layered crystals. Therefore, this study demonstrates that XFEL crystallography has become a powerful tool for conducting structure studies of small crystals of organic compounds.

Plastic/Ferroelectric Crystals with Easily Switchable Polarization: Low-Voltage Operation, Unprecedentedly High Pyroelectric Performance, and Large Piezoelectric Effect in Polycrystalline Forms.

Molecular ferroelectric crystals have attracted growing interest as potential alternatives to conventional lead-based ceramic ferroelectrics. We have recently discovered that a class of compounds known as plastic crystals can show multiaxial ferroelectricity, which allows ferroelectric performance even in polycrystalline forms. Here, we report new plastic/ferroelectric ionic molecular crystals that exhibit remarkably small coercive electric fields at room temperature. The easily switchable ferroelectric polarization enables low-voltage switching operations and high-frequency performance. Such ferroelectric crystals can be readily processed into bulk polycrystalline forms with desired shapes that are characterized by unprecedentedly high pyroelectric figures of merit and large piezoelectricity. These multifunctional molecular crystals represent highly attractive prospects for device elements with a diverse range of applications, which will significantly boost the development of molecular ferroelectric crystals.

Inkjet printing of single-crystal films.

The use of single crystals has been fundamental to the development of semiconductor microelectronics and solid-state science. Whether based on inorganic or organic materials, the devices that show the highest performance rely on single-crystal interfaces, with their nearly perfect translational symmetry and exceptionally high chemical purity. Attention has recently been focused on developing simple ways of producing electronic devices by means of printing technologies. 'Printed electronics' is being explored for the manufacture of large-area and flexible electronic devices by the patterned application of functional inks containing soluble or dispersed semiconducting materials. However, because of the strong self-organizing tendency of the deposited materials, the production of semiconducting thin films of high crystallinity (indispensable for realizing high carrier mobility) may be incompatible with conventional printing processes. Here we develop a method that combines the technique of antisolvent crystallization with inkjet printing to produce organic semiconducting thin films of high crystallinity. Specifically, we show that mixing fine droplets of an antisolvent and a solution of an active semiconducting component within a confined area on an amorphous substrate can trigger the controlled formation of exceptionally uniform single-crystal or polycrystalline thin films that grow at the liquid-air interfaces. Using this approach, we have printed single crystals of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C(8)-BTBT) (ref. 15), yielding thin-film transistors with average carrier mobilities as high as 16.4 cm(2) V(-1) s(-1). This printing technique constitutes a major step towards the use of high-performance single-crystal semiconductor devices for large-area and flexible electronics applications.

Flip-flop dynamics in smectic liquid-crystal organic semiconductors revealed by molecular dynamics simulations.

Asymmetric liquid-crystal (LC) organic semiconductors, such as 2-decyl-7-(p-tolyl)-[1]benzothieno[3,2-b][1]benzothiophene (pTol-BTBT-C10), exhibit high mobilities exceeding 10 cm2 V-1 s-1. The LC phases play important roles in thermal stability and self-assembly ordering during film deposition and annealing. In this study, we show molecular dynamics simulations of pTol-BTBT-C10 and reveal a unique mechanism of the molecular flip-flop motion at the smectic E/smectic B phase transition.

Control of Polar/Antipolar Layered Organic Semiconductors by the Odd-Even Effect of Alkyl Chain.

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π-conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side-by-side unidirectionally to form self-organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n, when the molecules are designed to involve effectively weakened head-to-head affinity. They exhibit remarkable odd-even effect in the interlayer stacking: alternating head-to-head and tail-to-tail (antipolar) arrangement in odd-n crystals, and uniform head-to-tail (polar) arrangement in even-n crystals. The films show excellent field-effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second-order nonlinear optical response along normal to the film plane. These findings are key for creating polarity-controlled optoelectronic materials and devices.

Spatiotemporal observation of quantum crystallization of electrons.

Liquids crystallize as they cool; however, when crystallization is avoided in some way, they supercool, maintaining their liquidity, and freezing into glass at low temperatures, as ubiquitously observed. These metastable states crystallize over time through the classical dynamics of nucleation and growth. However, it was recently found that Coulomb interacting electrons on charge-frustrated triangular lattices exhibit supercooled liquid and glass with quantum nature and they crystallize, raising fundamental issues: what features are universal to crystallization at large and specific to that of quantum systems? Here, we report our experimental challenges that address this issue through the spatiotemporal observation of electronic crystallization in an organic material. With Raman microspectroscopy, we have successfully performed real-space and real-time imaging of electronic crystallization. The results directly capture strongly temperature-dependent crystallization profiles indicating that nucleation and growth proceed at distinctive temperature-dependent rates, which is common to conventional crystallization. However, the growth rate is many orders of magnitude larger than that in the conventional case. The temperature characteristics of nucleation and growth are universal, whereas unusually fast growth kinetics features quantum crystallization where a quantum-to-classical catastrophe occurs in interacting electrons.

Giant bulk piezophotovoltaic effect in 3R-MoS2.

Given its innate coupling with wavefunction geometry in solids and its potential to boost the solar energy conversion efficiency, the bulk photovoltaic effect (BPVE) has been of considerable interest in the past decade1-14. Initially discovered and developed in ferroelectric oxide materials2, the BPVE has now been explored in a wide range of emerging materials, such as Weyl semimetals9,10, van der Waals nanomaterials11,12,14, oxide superlattices15, halide perovskites16, organics17, bulk Rashba semiconductors18 and others. However, a feasible experimental approach to optimize the photovoltaic performance is lacking. Here we show that strain-induced polarization can significantly enhance the BPVE in non-centrosymmetric rhombohedral-type MoS2 multilayer flakes (that is, 3R-MoS2). This polarization-enhanced BPVE, termed the piezophotovoltaic effect, exhibits distinctive crystallographic orientation dependence, in that the enhancement mainly manifests in the armchair direction of the 3R-MoS2 lattice while remaining largely intact in the zigzag direction. Moreover, the photocurrent increases by over two orders of magnitude when an in-plane tensile strain of ~0.2% is applied, rivalling that of state-of-the-art materials. This work unravels the potential of strain engineering in boosting the photovoltaic performance, which could potentially promote the exploration of novel photoelectric processes in strained two-dimensional layered materials and their van der Waals heterostructures.

Hybrid energy-minimization simulation of equilibrium droplet shapes on hydrophilic/hydrophobic patterned surfaces.

We have developed an efficient algorithm for simulating the equilibrium shape of a microdroplet placed on a flat substrate that has a fine, discontinuous, and arbitrarily shaped hydrophilic/hydrophobic patterned surface. The method uses a hybrid energy-minimization technique that combines the direct search method to determine the droplet shape around solid/liquid contact lines with the gradient descent method for the other parts of the droplet surface. The method provides high-convergence at a low computational cost with sufficient mesh resolution, providing a useful tool for the optimal design of printed electronic devices.

Insulating Polymer Blend Organic Thin-Film Transistors Based on Bilayer-Type Alkylated Benzothieno[3,2-b]naphtho[2,3-b]thiophene.

Herein, we developed a practical method to produce high-performance organic thin-film transistors (OTFTs) based on highly layered crystalline organic semiconductors (OSCs) that form bilayer-type layered herringbone (b-LHB) packing and exhibit high intrinsic mobility. We applied the insulating polymer blend technique using a typical b-LHB OSC of 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) and fabricated polycrystalline thin-film transistors (TFTs) via short-duration spin coating and subsequent annealing. The use of blends and the choice of polymer additive strongly affected the performance of the polycrystalline TFTs, and poly(methyl methacrylate) (PMMA) blend TFTs exhibited a high mobility exceeding 4 cm2/(V s) and small device-to-device variations. Using extended techniques in atomic force microscopy (AFM), we investigated the thin-film morphologies by bimodal AFM and the carrier transport properties by Kelvin probe force microscopy (KPFM). We demonstrated that the PMMA blend system enables the formation of a well-ordered polycrystalline thin film induced by vertical phase separation between the OSC and PMMA over a large area, resulting in uniform TFT performance. These findings pave the way for obtaining high-performance TFTs using simple processes, representing a substantial advancement toward the realization of printed electronics.

Corrigendum: Protein and Organic-Molecular Crystallography with 300kV Electrons on a Direct Electron Detector.

[This corrects the article DOI: 10.3389/fmolb.2020.612226.].

Distribution of localized states from fine analysis of electron spin resonance spectra in organic transistors.

We developed a novel method for obtaining the distribution of trapped carriers over their degree of localization in organic transistors, based on the fine analysis of electron spin resonance spectra at low enough temperatures where all carriers are localized. To apply the method to pentacene thin-film transistors, we proved through continuous wave saturation experiments that all carriers are localized at below 50 K. We analyzed the spectra at 20 K and found that the major groups of traps comprise localized states having wave functions spanning around 1.5 and 5 molecules and a continuous distribution of states with spatial extent in the range between 6 and 20 molecules.

Competition between charge-transfer exciton dissociation and direct photocarrier generation in molecular donor-acceptor compounds.

The interfacial charge-separation and photovoltaic characteristics of a molecular donor-acceptor charge-transfer compound were examined. Measurements of laser beam-induced currents on the single crystals allowed selective detection of hole and electron photocurrents through the metal-semiconductor interfaces. This method also reveals the exceptionally long diffusion length of 20 µm in the crystal. The transition from charge-transfer exciton dissociation to direct photocarrier generation is discussed on the basis of the photon-energy-dependent diffusion length and photon-to-current conversion spectrum.

Protein and Organic-Molecular Crystallography With 300kV Electrons on a Direct Electron Detector.

Electron 3D crystallography can reveal the atomic structure from undersized crystals of various samples owing to the strong scattering power of electrons. Here, a direct electron detector DE64 was tested for small and thin crystals of protein and an organic molecule using a JEOL CRYO ARM 300 electron microscope. The microscope is equipped with a cold-field emission gun operated at an accelerating voltage of 300 kV, quad condenser lenses for parallel illumination, an in-column energy filter, and a stable rotational goniometer stage. Rotational diffraction data were collected in an unsupervised manner from crystals of a heme-binding enzyme catalase and a representative organic semiconductor material Ph-BTBT-C10. The structures were determined by molecular replacement for catalase and by the direct method for Ph-BTBT-C10. The analyses demonstrate that the system works well for electron 3D crystallography of these molecules with less damaging, a smaller point spread, and less noise than using the conventional scintillator-coupled camera.

Meniscus-controlled printing of single-crystal interfaces showing extremely sharp switching transistor operation.

Meniscus, a curvature of droplet surface around solids, takes critical roles in solution-based thin-film processing. Extension of meniscus shape, and eventual uniform film growth, is strictly limited on highly lyophobic surfaces, although such surface should considerably improve switching characteristics. Here, we demonstrate a technique to control the solution meniscus, allowing to manufacture single-crystalline organic semiconductor (OSC) films on the highest lyophobic amorphous perfluoropolymer, Cytop. We used U-shaped metal film pattern produced on the Cytop surface, to initiate OSC film growth and to keep the meniscus extended on the Cytop surface. The growing edge of the OSC film helped maintain the meniscus extension, leading to a successive film growth. This technique facilitates extremely sharp switching transistors with a subthreshold swing of 63 mV dec-1 owing to the effective elimination of charge traps at the semiconductor/dielectric interface. The technique should expand the capability of print production of functional films and devices.

Regioisomeric control of layered crystallinity in solution-processable organic semiconductors.

The construction and control of 2D layered molecular packing motifs with functionally substituted π-electron cores are crucial for developing organic electronic materials and devices. We investigated a regioisomeric structure-property relationship in high-performance and solution-processable layered organic semiconductors based on mono-octyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene (mono-C8-BTNT). We demonstrated that an isomorphous bilayer-type layered herringbone packing motif is obtainable in a series of four positional isomers of mono-C8-BTNTs whose π-electron core is substituted by an octyl chain at one of the four most peripheral positions with roughly keeping the rod-like molecular shape. These regioisomeric compounds exhibited systematic variations in the solvent solubility and liquid-crystalline phase transitions at elevated temperatures. The analysis of intermolecular interaction energies in the crystals based on dispersion-corrected DFT calculations revealed that the crystals of 2- and 8-mono-C8-BTNTs are more stable than those of 3- and 9-mono-C8-BTNTs owing to the higher ordering of alkyl chain layers in the crystals. Such differences of the stability in their crystal formation are closely correlated with TFT performances, where the single-crystal devices of the 2- and 8-mono-C8-BTNTs substituted at the most peripheral positions exhibit high-performance TFT characteristics with a mobility of approximately 10 cm2 V-1 s-1.

Organic field-effect transistors using single crystals.

Organic field-effect transistors using small-molecule organic single crystals are developed to investigate fundamental aspects of organic thin-film transistors that have been widely studied for possible future markets for 'plastic electronics'. In reviewing the physics and chemistry of single-crystal organic field-effect transistors (SC-OFETs), the nature of intrinsic charge dynamics is elucidated for the carriers induced at the single crystal surfaces of molecular semiconductors. Materials for SC-OFETs are first reviewed with descriptions of the fabrication methods and the field-effect characteristics. In particular, a benchmark carrier mobility of 20-40 cm2 Vs-1, achieved with thin platelets of rubrene single crystals, demonstrates the significance of the SC-OFETs and clarifies material limitations for organic devices. In the latter part of this review, we discuss the physics of microscopic charge transport by using SC-OFETs at metal/semiconductor contacts and along semiconductor/insulator interfaces. Most importantly, Hall effect and electron spin resonance (ESR) measurements reveal that interface charge transport in molecular semiconductors is properly described in terms of band transport and localization by charge traps.

Unique coexistence of dispersion stability and nanoparticle chemisorption in alkylamine/alkylacid encapsulated silver nanocolloids.

Surface encapsulation of metal nanoparticles (NPs) is fundamental to achieve sufficient dispersion stability of metal nanocolloids, or metal nanoink. However, the feature is incompatible with surface reactive nature of the metal NPs, although these features are both essential to realizing the functional applications into printed electronics technologies. Here we show that two different kinds of encapsulation for silver NPs (AgNPs) by alkylamine and alkylacid together are the key to achieve unique compatibility between the high dispersion stability as dense nanoclolloids and the AgNP chemisorption printing on activated patterned polymer surfaces. Advanced confocal dynamic light scattering study reveals that an additive trace amount of oleic acid is the critical parameter for controlling the dispersion and coagulative (or surface-reactive) characteristics of the silver nanocolloids. The composition of the disperse media is also important for obtaining highly concentrated but low-viscosity silver nanocolloids that show very stable dispersion. The results demonstrate that the high-resolution AgNP chemisorption printing is possible only by using unique silver nanocolloids composed of an exceptional balance of ligand formulation and dispersant composition.

Semiconductive Single Molecular Bilayers Realized Using Geometrical Frustration.

A unique solution-based technology to manufacture self-assembled ultrathin organic-semiconductor layers with ultrauniform single-molecular-bilayer thickness over an area as large as wafer scale is developed. A novel concept is adopted in this technique, based upon the idea of geometrical frustration, which can effectively suppress the interlayer stacking (or multilayer crystallization) while maintaining the assembly of the intralayer, which originates from the strong intermolecular interactions between π-conjugated molecules. For this purpose, a mixed solution of extended π-conjugated frameworks substituted asymmetrically by alkyl chains of variable lengths (i.e., (πCore)-Cn 's) is utilized for the solution process. A simple blade-coating with a solution containing two (πCore)-Cn 's with different alkyl chain lengths is effective to provide single molecular bilayers (SMBs) composed of a pair of polar monomolecular layers, which is analogical to the cell membranes of living organisms. It is demonstrated that the chain-length disorder does not perturb the in-plane crystalline order, but acts effectively as a geometrical frustration to inhibit multilayer crystallization. The uniformity, stability, and size scale are unprecedented, as produced by other conventional self-assembly processes. The obtained SMBs also exhibit efficient 2D carrier transport as organic thin-film transistors. This finding should open a new route to SMB-based ultrathin superflexible electronics.

Surface pressure induced charge transfer between fullerene and tetrathiafulvalene derivative in Langmuir-Blodgett films.

Structures and electronic states of a 1:1 mixture of bis-tetrathiafulvalene annulated macrocycle (1) and C60 in Langmuir films at the air-water interface and Langmuir-Blodgett (LB) films on solid substrates were examined. Compression of the Langmuir films induced for the first time a phase transition from a weakly interacting state without charge transfer (CT) to a neutral CT state. The scanning force microscope images of LB films transferred onto mica by a single withdrawal showed quite different spatial patterns depending on the CT states. When deposited at around 1 mNm(-1), a domain structure with 3 nm height was obtained, which corresponded to the state without CT interaction. Contrastingly, once the CT interaction was induced by applying surface pressure, a network structure was observed with a height of 6 nm. The CT band, whose transition moment was almost parallel to the substrate surface, was observed at 11.5 x 10(3) cm(-1) in the polarized UV-VIS-NIR spectra of the films deposited at 9 mNm(-1). The phase transition was irreversible, although the surface pressure-area isotherm showed a reversible behavior below 9 mNm(-1). The morphology and electronic state of the film was controllable merely by changing the surface pressure at the air-water interface.

Nanoparticle chemisorption printing technique for conductive silver patterning with submicron resolution.

Silver nanocolloid, a dense suspension of ligand-encapsulated silver nanoparticles, is an important material for printing-based device production technologies. However, printed conductive patterns of sufficiently high quality and resolution for industrial products have not yet been achieved, as the use of conventional printing techniques is severely limiting. Here we report a printing technique to manufacture ultrafine conductive patterns utilizing the exclusive chemisorption phenomenon of weakly encapsulated silver nanoparticles on a photoactivated surface. The process includes masked irradiation of vacuum ultraviolet light on an amorphous perfluorinated polymer layer to photoactivate the surface with pendant carboxylate groups, and subsequent coating of alkylamine-encapsulated silver nanocolloids, which causes amine-carboxylate conversion to trigger the spontaneous formation of a self-fused solid silver layer. The technique can produce silver patterns of submicron fineness adhered strongly to substrates, thus enabling manufacture of flexible transparent conductive sheets. This printing technique could replace conventional vacuum- and photolithography-based device processing.