Organic-Inorganic Rubrene/WS2 Heterostructure for Broadband Detection and Polarization Imaging.

Organic single crystals exhibit improved carrier mobility, longer exciton diffusion length, anisotropic charge transport, and unique linear dichroism, while its high exciton binding energy seriously limits the free-carrier generation and photoelectric conversion efficiency. Layered van der Waals heterostructures, which integrate organic crystals with high mobility two-dimensional (2D) inorganic semiconductors, are promising for promoting exciton dissociation and boosting sensitivity by utilizing the interfacial potential and photogating effect. In this work, organic single-crystal rubrene is integrated with a few-layer WS2 to design the high-performance photodetector. The device exhibits an excellent responsivity of 1000 A W-1, and a fast speed of 180 µs, which is far superior to the individual WS2 device. Equally importantly, this device provides excellent polarization detection performance by virtue of the anisotropic properties of rubrene, and the dichroic ratios are 1.56, 1.5, and 1.7 for 375, 405, and 658 nm irradiation, respectively. Finally, several high-resolution single-pixel broadband polarization imaging was demonstrated. Our work shows that organic-inorganic heterostructure is an essential candidate for improving optoelectronics performance and has potential for polarization imaging.

Asymmetric Schottky Barrier in Rubrene Transistor via Monolayer Graphene Insertion toward Self-Powered Imaging.

Organic semiconductor materials featuring lightweight, and flexibility may play a significant role in various future applications, such as foldable displays, wearable devices, and artificial skin. For developing high-performance organic devices, organic crystals are highly desired, while a remaining fundamental issue is their contact problem. Here, we have grown a high-quality rubrene single crystal by utilizing a simple in-air sublimation technique. The contact characteristics (barrier height and contact resistance) are detail-studied by resist-free transfer electrodes (Au metal or graphene/Au). The Schottky barrier of the rubrene/graphene interface is lower and can be also modulated by gate bias, which is confirmed by spatial photocurrent mapping. Finally, we demonstrated the zero-bias photocurrent imaging application by constructing the asymmetrical device employing different electrode contacts. Our work would be of significance for studying the contact issue of organic crystals and wireless imaging.

Bulk Heterojunction Upconversion Thin Films Fabricated via One-Step Solution Deposition.

Upconversion of near-infrared light into the visible has achieved limited success in applications due to the difficulty of creating solid-state films with high external quantum efficiency (EQE). Recent developments have expanded the range of relevant materials for solid-state triplet-triplet annihilation upconversion through the use of a charge-transfer state sensitization process. Here, we report the single-step solution-processed deposition of a bulk heterojunction upconversion film using organic semiconductors. The use of a bulk heterojunction thin film enables a high contact area between sensitizer and annihilator materials in this interface-triplet-generation mechanism and allows for a facile single-step deposition process. Demonstrations of multiple deposition and patterning methods on glass and flexible substrates show the promise of this materials system for solid-state upconversion applications.

Two new cases of polymorphism in diagonally substituted rubrene derivatives.

The crystal structures of two rubrene derivatives, 5,11-diphenyl-6,12-bis-[4-(tri-fluoro-meth-yl)phen-yl]tetra-cene, C44H26F6, and 5,11-bis-(4-tert-butyl-phen-yl)-6,12-di-phenyl-tetra-cene, C50H44, are presented. Each are substituted on diagonal (5/11) phenyl rings. Each derivative has one polymorph reported previously. A discussion of the differences between each derivative and its previously reported polymorph is provided. The triclinic packing of the CF3-substituted structure is similar to the packing of the parent rubrene's triclinic polymorph. In the tert-butyl-substituted structure, a planar tetra-cene core formed, which has been hypothesized but never published. Crystallization conditions are provided as they differ from previous reports.

Wireframe Orbit-Accelerated Bipedal DNA Walker for Electrochemiluminescence Detection of Methyltransferase Activity.

In spite of the DNA walkers executing the signal accumulation task in the process of moving along the predetermined paths, the enhancement of walking dynamics and walking path controllability are still challenging due to the unprogrammed arrangements of DNA orbits. Taking these dilemmas into account, a bipedal DNA walker was designed skillfully by the virtue of wireframe orbits assembled by DNA cubes in order, which improved the efficiency and the continuity of walking. It could be attributed to the fact that both the contact chance and the dynamic interaction between walking strands and designated orbits were beneficial to minimize the possibility of derailment and improve the accumulation of signal. In addition, the hollow titanium dioxide nanospheres coated with rubrene (Rub@TiO2 NSs) were prepared by the etching of inner silicon dioxide nanoparticles (SiO2 NPs) to regulate the distribution pattern of rubrene (Rub) molecules and expose more electrochemically active sites for high-efficient electrochemiluminescence (ECL). Benefiting by the pore confinement-enhanced ECL, the electron and mass transfer was significantly accelerated because of the hollow structure of Rub@TiO2 NSs. Subsequently, endogenous dissolved oxygen as the coreactant and palladium nanoparticles (Pd NPs) as the coreaction accelerator were employed to constitute a ternary ECL system with explosive signal response. Combining with this ECL platform, the bipedal walker activated by the target can autonomously and directionally move on the DNA wireframe orbits to release the quenching probes continuously. In this way, the biosensor displayed a low detection limit (2.30 × 10-8 U·mL-1) and a wide linear range (1.0 × 10-7 to 1.0 × 10-1 U·mL-1) for the sensitive detection of Dam methyltransferase (Dam MTase) activity. Therefore, a novel strategy for the accurate quantification of epigenetic targets was developed by virtue of improving the walking dynamics of DNA walker and amplifying the ECL of Rub molecules.

Rubrene-Directed Structural Transformation of Fullerene (C60) Microsheets to Nanorod Arrays with Enhanced Photoelectrochemical Properties.

One-dimensional (1D) nanostructures possess huge potential in electronics and optoelectronics, but the axial alignment of such 1D structures is still a challenging task. Herein, we report a simple method that enables two-dimensional (2D) C60 microsheets to evolve into highly ordered nanorod arrays using rubrene as a structure-directing agent. The structural transformation is accomplished by adding droplets of rubrene-m-xylene solution onto C60 microsheets and allowing the m-xylene solvent to evaporate naturally. In sharp contrast, when rubrene is absent from m-xylene, randomly oriented C60 nanorods are produced. Spectroscopic and microscopic characterizations collectively indicate a rather plausible transformation mechanism that the close lattice match allows the epitaxial growth of rubrene on C60 microsheets, followed by the reassembly of dissolved C60 along the aligned rubrene due to the intermolecular charge-transfer (CT) interactions, leading to the formation of ordered nanorod arrays. Due to the aligned structures and the CT interactions between rubrene and C60, the photocurrent density of the nanorod arrays is improved by 31.2% in the UV region relative to the randomly oriented counterpart. This work presents a facile and effective strategy for the construction of ordered fullerene nanorod arrays, providing new ideas for the alignment of fullerene and other relevant organic microstructures.

Highly Sensitive and Ultrafast Organic Phototransistor Based on Rubrene Single Crystals.

Rubrene single crystals have received a lot of attention for their great potential in electronic and wearable nanoelectronics due to their high carrier mobility and excellent flexibility. While they exhibited remarkable electrical performances, their intrinsic potential as photon detectors has not been fully exploited. Here, we fabricate a sensitive and ultrafast organic phototransistor based on rubrene single crystals. The device covers the ultraviolet to visible range (275-532 nm), and the responsivity and detectivity can reach up to ∼4000 A W-1 and 1011 jones at 532 nm, respectively. Furthermore, the response times are highly gate-tunable down to sub-90 µs, and the cutoff frequency is ∼4 kHz, which is one of the fastest organic material-based phototransistors reported so far. Equally important is that the fabricated device exhibits stable light detection ability even after 8 months, indicating great long-term stability and excellent environmental robustness. The results suggest that the high-quality rubrene single crystal may be a promising material for future flexible optoelectronics with its intrinsic mechanical flexibility.

Pressure-Induced Enhancement of Thermoelectric Performance in Rubrene.

In this work, the thermoelectric performance of a typical small-molecule organic semiconductor rubrene under different hydrostatic pressures was studied by first-principles calculation and molecular dynamics simulation. The ZT value of rubrene can reach 1.6 at 400 K due to an unprecedented increase in hole mobility under hydrostatic pressure. The underlying mechanism is ascribed to the suppression of low-frequency phonons (which weakens electron-phonon scattering) and the increase in the intermolecular electronic coupling. The effect of uniaxial stress has also been investigated to confirm this conclusion. Our results provide meaningful insights to understand the relationship between thermoelectric properties and hydrostatic pressure in organic semiconductors.

Rubrene Thin Films with Viably Enhanced Charge Transport Fabricated by Cryo-Matrix-Assisted Laser Evaporation.

Among organic semiconductors, rubrene (RB; C42H28) is of rapidly growing interest for the development of organic and hybrid electronics due to exceptionally long spin diffusion length and carrier mobility up to 20 cm2V-1s-1 in single crystals. However, the fabrication of RB thin films resembling properties of the bulk remains challenging, mainly because of the RB molecule's twisted conformation. This hinders the formation of orthorhombic crystals with strong π-π interactions that support the band transport. In this work, RB films with a high crystalline content were fabricated by matrix-assisted laser evaporation and the associated structure, composition, and transport properties are investigated. Enhanced charge transport is ascribed to the crystalline content of the film. Spherulitic structures are observed on top of an amorphous RB layer formed in the initial deposition stage. In spherulites, orthorhombic crystals dominate, as confirmed by X-ray diffraction and the absorption and Raman spectra. Surprisingly, nanowires several microns in length are also detected. The desorption/ionization mass and X-ray photoelectron spectra consistently show minimal material decomposition and absence of RB peroxides. The observed carrier mobility up to 0.13 cm2V-1s-1, is close to the technologically accepted level, making these rubrene films attractive for spintronic and optoelectronic applications.

Doped Highly Crystalline Organic Films: Toward High-Performance Organic Electronics.

Today's organic electronic devices, such as the highly successful OLED displays, are based on disordered films, with carrier mobilities orders of magnitude below those of inorganic semiconductors like silicon or GaAs. For organic devices such as diodes and transistors, higher charge carrier mobilities are paramount to achieve high performance. Organic single crystals have been shown to offer these required high mobilities. However, manufacturing and processing of these crystals are complex, rendering their use outside of laboratory-scale applications negligible. Furthermore, doping cannot be easily integrated into these systems, which is particularly problematic for devices mandating high mobility materials. Here, it is demonstrated for the model system rubrene that highly ordered, doped thin films can be prepared, allowing high-performance organic devices on almost any substrate. Specifically, triclinic rubrene crystals are created by abrupt heating of amorphous layers and can be electrically doped during the epitaxial growth process to achieve hole or electron conduction. Analysis of the space charge limited current in these films reveals record vertical mobilities of 10.3(49) cm2 V-1 s-1. To demonstrate the performance of this materials system, monolithic pin-diodes aimed for rectification are built. The f 3 d b of these diodes is over 1 GHz and thus higher than any other organic semiconductor-based device shown so far. It is believed that this work will pave the way for future high-performance organic devices based on highly crystalline thin films.

The role of orientation in the MEL response of OLEDs.

Magneto electroluminescence (MEL) is emerging as a powerful tool to study spin dynamics in organic light emitting diodes (OLEDs). The shape of the MEL response is typically used to draw qualitative inference on the dominant process (singlet fission or triplet fusion) in the device. In this study, we develop a quantitative model for MEL and apply it to devices based on Rubrene, and three solution processable anthradithiophene emitters. The four emitters allow us to systematically vary the film structure between highly textured, poly-crystalline to amorphous. We find significant diversity in the MEL, with the textured films giving highly structured responses. We find that the additional structure does not coincide with energy anti-crossings, but intersections in the singlet character between adjacent states. In all cases the MEL can be adequately described by an extended Merrifield model. Via the inclusion of charge injection, we are able to draw additional information on underlying physics in OLED devices.

Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces.

Rubrene (C42H28) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C42H28 consistently reflect the progressive reduction of the molecule-substrate hybridization. Separate C42H28 clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C42H28 on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C42H28 on graphene. They are due to different molecular vibrational quanta with distinct Huang-Rhys factors.

Suppression of Persistent Photoconductivity of Rubrene Crystals using Gate-Tunable Rubrene/Bi2 Se3 Diodes with Photoinduced Negative Differential Resistance.

Organic single-crystalline semiconductors show great potential in high-performance photodetectors. However, they suffer from persistent photoconductivity (PPC) due to the charge trapping, which has severely hindered high-speed imaging applications. Here, a universal strategy of solving the PPC by integrating with topological insulator Bi2 Se3 is provided. The rubrene/Bi2 Se3 heterojunctions are selected as an example for general demonstration due to the reproducibly high mobility and broad optoelectronic applications of rubrene crystals. By virtue of high carrier concentration on Bi2 Se3 surface and the strong built-in electrical field, the photoresponse of the heterotransistor is significantly reduced for more than two orders (from over 10 s to 54 ms), meanwhile the photoresponsivity can reach 124 A W-1 . To the best of knowledge, this operating speed is among the fastest responses in organic-inorganic heterojunctions. The heterotransistor also shows unique negative differential resistance under positive gate bias, which can be explained by photoinduced de-trapping of electron trap states in the bulk rubrene crystals. Besides, the rubrene/Bi2 Se3 heterojunction behaves as a gate-tunable backward-like diode due to the inhomogenous carrier distribution in the thick rubrene crystal and inversion of relative Fermi level positions. The findings demonstrate versatile functionalities of the rubrene/Bi2 Se3 heterojunctions for various emerging optoelectronic applications.

Donor/Acceptor Photovoltaic Cells Fabricated on p-Doped Organic Single-Crystal Substrates.

In this study, the operation of donor/acceptor photovoltaic cells fabricated on homoepitaxially grown p-doped rubrene single-crystal substrates is demonstrated. The photocurrent density is dominated by the sheet conductivity (σ□) of the p-type single-crystal layer doped to 100 ppm with an iron chloride (Fe2Cl6) acceptor. A 65 mm thick p-type rubrene single-crystal substrate is expected to be required for a photocurrent density of 20 mA·cm-2. An entire bulk doping technique for rubrene single crystals is indispensable for the fabrication of practical organic single-crystal solar cells.

Crystal Structures of Dimethoxyanthracens: A Clue to a Rational Design of Packing Structures of π-Conjugated Molecules.

We carried out a systematic investigation of packing structures of a series of dimethoxyanthracenes, i. e., 1,4- (1), 1,8- (2), 1,5- (3), 2,6- (4), and 2,7-derivatives (5). The packing structures of the dimethoxyanthracenes are classified into two types, a rubrene-like pitched π-stack (1-3) and a typical herringbone packing (4 and 5), which evidently show that the position of methoxy groups is crucial to determine the packing structure of dimethoxyanthracenes. Effects of the substitution position on intermolecular interactions are analyzed by the noncovalent intermolecular interaction (NCI) method, Hirshfeld surface analysis, and symmetry-adapted perturbation theory (SAPT) method, thus clarifying active roles of the methoxy groups in the formation of rubrene-like pitched π-stack. The present results shed light on a molecular design strategy to realize the rubrene-like pitched π-stack in the solid state, which had been regarded as a packing structure limited for rubrene and its closely related derivatives.

A DFT study on the photoelectric properties of rubrene and its derivatives.

The way to obtain new materials was usually to introduce some groups to molecules. Correlations among the molecular structure and photoelectric properties of rubrene and its eight derivatives have been studied in this paper, and the influences of the introduction of different electron-donating and withdrawing substituents on the molecular orbital, reorganization energies, absorption spectra, and fluorescence spectra of rubrene and its derivatives were discussed. In the present work, density functional theory calculations were performed at the B3LYP/6-311G(d,p) level to optimize the structure, and TD-DFT was used to calculate the absorption and emission spectra. Quantum chemistry calculation results indicated that the maximum absorption wavelengths are redshifted due to the introduction of cyclopentadienyl or furan groups, the maximum absorption wavelengths are blueshifted while methoxy groups substituted on tetracene backbone. We also discussed the influence of substituents on the molecule structure, which suggests that introducing furan and cyclopentadienyl substituents on the tetracene backbone can increase the rigidity of rubrene and improve the fluorescence intensity. The results of reorganization show that the introduction of cyclopentadienyl or furan groups into rubrene is advantageous to the holes transportation, and the introduction of F groups contributes to the electrons transportation. This study provides an insight into the properties of rubrene and its derivatives and supplies an effective method to design new organic semiconductor materials.

Monolayer rubrene functionalized graphene-based eletrochemiluminescence biosensor for serum cystatin C detection with immunorecognition-induced 3D DNA machine.

Only surficial molecules of eletrochemiluminescent (ECL) nanomaterials are the most reactive species in the typical ECL reaction. Herein, monolayer rubrene was assembled on the surface of graphene sheet to obtain monolayer rubrene functionalized graphene composite (G/mRub) with strong ECL emission by maximizing the surficial rubrene molecules. Based on G/mRub as the strong ECL emitter, an ultrasensitive "on-off" biosensor was developed to detect cystatin C (Cys C) in human serum with the help of a novel immunorecognition-induced enzyme-free 3D DNA machine. Benefiting from the strong ECL emission of G/mRub and the efficient signal amplification of 3D DNA machine, the established biosensor achieved high sensitivity for Cys C detection with linear range from 1.0 fg mL-1 to 10 ng mL-1 and limit of detection down to 0.38 fg mL-1. In addition, this enzyme-free biosensing method was adopted to successfully detect the concentration of Cys C in human serum. Therefore, the G/mRub based ECL biosensor might provide a potential tool for protein detection in clinical diagnosis and a new avenue to prepare high-performance luminescent nanomaterials.

Two-Dimensional Electronic Transport in Rubrene: The Impact of Inter-Chain Coupling.

Organic semi-conductors have unique electronic properties and are important systems both at the fundamental level and also for their applications in electronic devices. In this article we focus on the particular case of rubrene which has one of the best electronic transport properties for application purposes. We show that this system can be well simulated by simple tight-binding systems representing one-dimensional (1D) chains that are weakly coupled to their neighboring chains in the same plane. This makes in principle this rubrene system somehow intermediate between 1D and isotropic 2D models. We analyse in detail the dc-transport and terahertz conductivity in the 1D and in the anisotropic 2D models. The transient localisation scenario allows us to reproduce satisfactorily some basics results such as mobility anisotropy and orders of magnitude as well as ac-conductivity in the terahertz range. This model shows in particular that even a weak inter-chain coupling is able to improve notably the propagation along the chains. This suggest also that a strong inter-chain coupling is important to get organic semi-conductors with the best possible transport properties for applicative purposes.

Application of Rubrene Air-Gap Transistors as Sensitive MEMS Physical Sensors.

Micro-electromechanical systems (MEMS) made of organic materials have attracted efforts for the development a new generation of physical, chemical, and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single-crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations in drain current: an unparalleled gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward in the development of high-performance flexible sensors.

Application of Antibody-Powered Triplex-DNA Nanomachine to Electrochemiluminescence Biosensor for the Detection of Anti-Digoxigenin with Improved Sensitivity Versus Cycling Strand Displacement Reaction.

The accurate and rapid quantitative detection of antibodies had a significant influence in controlling and preventing disease or toxin outbreaks. In this work, we first introduce the antibody-powered triplex-DNA nanomachine to release cargo DNA as a substitute target for sensitive electrochemiluminescence (ECL) detection of anti-digoxigenin based on a novel ternary ECL system. It is worth noting that the cargo DNA as a substitute target of antibody can further participate in an enzyme-assisted cycling strand displacement reaction to achieve ECL signal amplification and improve the sensitivity of antibody detection. Additionally, porous palladium nanospheres with a considerable catalytic activity were first applied as a coreaction accelerator to efficiently enhance the intensity of the ECL system of rubrene microblocks as luminophore and dissolved O2 as an endogenous coreactant. With the resultant ternary ECL system as a biosensing platform, a significantly enhanced initial signal was achieved in advance. Then, the ferrocene-labeled quenching probes were employed to reduce initial signal and obtain the low-background signal. Eventually, the cargo DNA made the quenching probes release and recover the signal in the presence of anti-digoxigenin. Thereupon, the wide linear range (0.01-200 nM) and low limit of detection (6.7 pM) were obtained, and this method not only reduces conjugation steps but also provides a sensitive and novel ECL analysis platform for the trace detection of other antibodies and antigen.