Valence band structure of rubrene single crystals in contact with an organic gate dielectric

Organic field effect transistors (OFETs) using crystalline organic semiconductors are of great interest because of their well-defined structural and electronic properties to study the intrinsic charge carrier transport mechanisms in π-conjugated molecular solids, as well as to unravel their potential to be applied as a novel type of electronic device. In the present study, the valence band structure of the channel region of an OFET is proposed based on photoemission results of a well-defined interface between a dielectric molecular monolayer and single crystals of 5,6,11,12-tetraphenyltetracene (rubrene) which is known to exhibit the highest field effect mobility of all organic semiconductors at room temperature. Commensurate growth of clusters of tetratetracontane (TTC; n-C₄₄H₉₀) on the rubrene single crystal surface and their morphological transformation into a uniform overlayer were observed by atomic force microscopy. Photoelectron spectroscopy measurements at various electron take-off angles were then conducted to derive the valance band width of the rubrene single crystal covered by the TTC overlayers. The valence band width at this hetero-interface was found to be equivalent to that of the pristine rubrene, which suggests an unchanged ‘band effective mass ?²(d²E/dk_||²)’ of accumulated holes even at the vicinity of hydrocarbon-based gate dielectrics.     

Liquid phase epitaxy for the production of YBa2Cu3O7-δ coated conductor

The liquid phase epitaxy (LPE), which is performed under ambient pressure with simple growth apparatus, allows us to fabricate high-Jc YBa₂Cu₃O_7-δ (YBCO) crystalline thick films at high growth rate. We propose that the LPE growth of YBCO films on single crystalline oxide fibers is another way to produce YBCO-coated conductors for power application. In the conventional case of using BaO–CuO melt as a solvent, by which the YBCO crystals grow at about 1000°C, YBCO coating on yttria-stabilized zirconia (YSZ) or SrTiO₃ (STO) single crystalline fibers has not been successfully achieved because of their reaction with the solvent. Recently we have discovered that BaF₂ added as a solution decreases the crystallization temperature of YBCO down to 900°C and suppress the reactivity with no degradation of superconductivity of the YBCO crystal. The YBCO-coated fibers are successfully prepared using this solution. The behavior of critical current under magnetic fields shows that these YBCO-coated fibers have the potential to be used under a high magnetic field at the temperature of liquid nitrogen.     

Structural Determination and Imaging of Charge Ordering and Oxygen Vacancies of the Multifunctional Oxides REBaMn2O6‐χ (RE = Gd,Tb)

Charge ordering and oxygen vacancy ordering are revealed in REBaMn₂O_6‐δ (RE = Gd, Tb) oxides with perovskite‐related structures. Electron diffraction and transmission electron microscopy results indicate a modulation of the crystal structure. The average oxidation state of Mn and the oxygen stoichiometry are determined by means of electron energy‐loss spectroscopy, giving a REBaMn₂O_5.75 general formula. A 1:3 Mn⁴⁺:Mn³⁺ charge ordering model is confirmed by neutron powder diffraction, and oxygen vacancies‐Mn³⁺ association is suggested by pair distribution function analysis. Direct imaging of the oxygen sublattice is obtained by phase image reconstruction. Location of the oxygen vacancies in the anion sublattice is achieved by analysis of the intensity of the averaged phase image. Both ionic conduction and multiferroic behavior are predicted from the crystal structures of these oxides.     

Boosting Photovoltaic Performance for Lead Halide Perovskites Solar Cells with BF4− Anion Substitutions

Composition engineering is a particularly simple and effective approach especially using mixed cations and halide anions to optimize the morphology, crystallinity, and light absorption of perovskite films. However, there are very few reports on the use of anion substitutions to develop uniform and highly crystalline perovskite films with large grain size and reduced defects. Here, the first report of employing tetrafluoroborate (BF₄^?) anion substitutions to improve the properties of (FA = formamidinium, MA = methylammonium (FAPbI₃)_0.83(MAPbBr₃)_0.17) perovskite films is demonstrated. The BF₄^? can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite films and derived solar cells. These advantages benefit the performance of perovskite solar cells (PVSCs), resulting in an improved power conversion efficiency (PCE) of 20.16% from 17.55% due to enhanced open‐circuit voltage (V_OC) and fill factor. This is the highest PCE for BF₄^? anion substituted lead halide PVSCs reported to date. This work provides insight for further exploration of anion substitutions in perovskites to enhance the performance of PVSCs and other optoelectronic devices.     

Boron Nitride Fibers Prepared from Symmetric and Asymmetric Alkylaminoborazines

The thermolysis of 2,4,6‐[(CH₃)₂N]₃B₃N₃H₃ ( 1 ), 2,4‐[(CH₃)₂N]₂‐6‐(CH₃HN)B₃N₃H₃ ( 2 ), and 2‐[(CH₃)₂N]‐4,6‐(CH₃HN)₂B₃‐N₃H₃ ( 3 ) led to polyborazines 4 , 5 , and 6 respectively. The polymers display direct B–N bonds between borazinic B₃N₃ rings and, in addition, a proportion of –N(CH₃)– bridges for 5 and 6 , as clearly underlined by ¹³C NMR spectroscopy. Melt‐spinning of these three polymeric precursors exemplified that their ease of processing increases in the order 4 < 5 < 6 . Nevertheless, polyborazine filaments could be prepared from each of them and a subsequent thermal treatment up to 1800 °C resulted in the formation of crystalline hexagonal boron nitride fibers, which were characterized by X‐ray diffraction analysis, Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Scanning electron microscopy (SEM) images showed that the ceramic fibers are circular and dense without major defects. The mechanical properties for 4 ‐derived fibers could not be measured because of their brittleness, whereas measurements on 5 ‐ and 6 ‐derived fibers gave tensile strength σ^R = 0.51 GPa, Young’s modulus E = 67 GPa, and σ^R = 0.69 GPa, E = 170 GPa, respectively. The improvement in mechanical properties for ceramic fibers prepared respectively from 4 , 5 , and 6 could be explained to a large extent by the improvement of the processing properties of the preceramic polymers. This evolution could be related to the increased ratio of bridging –N(CH₃)– groups between the B₃N₃ rings within the polymers 4 , 5 , and 6 and therefore to the functionalities of the starting monomers 1 , 2 , and 3 .     

Microemulsion-assisted hydrothermal preparation and infrared radiation property of TiO2 nanomaterials with tunable morphologies and crystal form

TiO₂ nanomaterials with adjustable crystalline phases and morphologies were prepared via a mild condition of microemulsion-assisted hydrothermal method. The effects of various reaction conditions, such as the species of inhibitor acid, the concentrations of acid and the reaction temperatures on the crystal forms and morphologies of the TiO₂ nanoparticles were investigated. The particles were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and Fourier transformed infrared spectrometer. Experimental results showed that the inhibitor acid of hydrochloric acid in the high concentration was conducive to the formation of rutile TiO₂. Nitric acid led to the formation of anatase TiO₂. The crystallinity of the TiO₂ was improved and the particle size was increased with the increase of the reaction temperature and the acid concentration. The morphologies of the TiO₂ nanoparticles also depended on the species and the concentration of the acids. TiO₂ nanoparticles with various structures of irregular granular, spherical, cosh and donuts-like were presented. Moreover, the infrared emissivity values of the TiO₂ nanoparticles were measured at wavelength of 8–14 μm. The irregular TiO₂ granular with single crystal phase exhibited the optimal infrared radiation property owing to the enhanced scattering ability and attenuation effect.     

Effect of Rapid Thermal Annealing on Sputtered CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> Thin Films

Calcium copper titanium oxide (CaCu₃Ti₄O₁₂, abbreviated to CCTO) films were deposited on Pt/Ti/SiO₂/Si substrates at room temperature (RT) by radiofrequency magnetron sputtering. As-deposited CCTO films were treated by rapid thermal annealing (RTA) at various temperatures and in various atmospheres. X-ray diffraction patterns and scanning electron microscope (SEM) images demonstrated that the crystalline structures and surface morphologies of CCTO thin films were sensitive to the annealing temperature and ambient atmosphere. Polycrystalline CCTO films could be obtained when the annealing temperature was 700°C in air, and the grain size increased signifi- cantly with annealing in O₂. The 0.8-μm CCTO thin film that was deposited at RT for 2 h and then annealed at 700°C in O₂ exhibited a high dielectric constant (ε′) of 410, a dielectric loss (tan δ) of 0.17 (at 10 kHz), and a leakage current density (J) of 1.28 × 10⁻⁵ A/cm² (at 25 kV/cm).     

Amorphous structures of buried oxide in SiC-on-insulator

A buried amorphous layer in high-dose oxygen ion-implanted silicon carbide (SiC) has been characterized by transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS), and energy dispersive x-ray spectroscopy. Single crystalline (0001)-oriented 6H-SiC wafers were irradiated with 180 keV oxygen ions at 650°C to a fluence of 1.4 ×10¹⁸/cm². A fully amorphous SiO₂ layer was formed insido the irradiated crystal, while the surrounded 6H-SiC exhibited minimal damage. This SiO₂ layer included self-bonded carbon atoms and showed a layered structure due to compositional variations of silicon, carbon, and oxygen.     

Emerging Porous Solid Electrolytes for Hydroxide Ion Transport

Anion exchange membrane fuel cells (AEMFCs) offer several advantages over proton exchange membrane fuel cells, such as the use of a non-precious metal catalyst, but these cells suffer from various issues related to OH^–-conducting electrolytes, including low conductivity and the formation of K₂CO₃ salt. These issues need to be resolved for the widespread use of AEMFCs. Recently, many studies have focused on developing excellent ion-conductive electrolytes using porous materials based on metal–organic and covalent organic frameworks. However, most of this research is biased toward proton-conducting electrolytes; to the best of the authors’ knowledge, reviews addressing OH^–-conducting electrolytes using porous materials have not been reported thus far. This review discusses OH^–-conducting porous crystalline materials and their membranes in terms of different synthetic strategies, conduction mechanisms, and experimental modalities for the design and development of future anion conductive electrolytes in fuel cells.     

Formation of Chiral Mesopores in Conducting Polymers by Chiral‐Lipid‐Ribbon Templating and “Seeding” Route

Conducting polymer nanofibers with controllable chiral mesopores in the size, the shape, and handedness have been synthesized by chiral lipid ribbon templating and “seeding” route. Chiral mesoporous conducting poly(pyrrole) (CMPP) synthesized with very small amount of chiral amphiphilic molecules (usually < 3%) has helically twisted channels with well‐defined controllable pore size of 5–20 nm in central axis of the twisted fibers. The structure and chirality of helical mesopores have been characterized by high‐resolution transmission electron microscope (HRTEM), scanning electron microscope (SEM) and electron tomography. The average pore diameters of chiral mesopores were approximately estimated from the N₂ adsorption–desorption data and calculated by the conversion calculation from helical ribbons to a rectangular straight tape. The pore size of CMPP has been controlled by choosing different alkyl chain lengths of chiral lipid molecules or precisely adjusting the H₂O/EtOH volume ratio.     

Optimization of N2/Ar gas flow ratio for the realization of nitrogen-doped p-type ZnCdO thin films synthesized by RF magnetron sputtering

Nitrogen doped ZnCdO films [ZCO:N] have been grown on quartz substrates by radio frequency (RF) reactive magnetron sputtering technique, and the effect of the ratio of nitrogen to argon gas flow [N₂:Ar] on their electrical, microstructure and optical properties were investigated by Hall effect, energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscope (TEM), optical absorbance and photoluminescence (PL) measurements. The results indicate that all the ZCO:N films are of hexagonal wurtzite structure with highly (002) preferential orientation. As the N₂:Ar increases from 0:1 to 4:1, the absorption edge for the samples exhibits blue shift. Hall effect measurement results indicate that the N₂:Ar exerts an immense influence on the p-type conduction conversion for ZCO:N film. It is found that ZCO:N film deposited at the N₂:Ar of 1:2 shows the optimal p-type behavior, which has a carrier concentration of 1.10×10¹⁷ cm⁻³, a mobility of 3.28 cm²V⁻¹s⁻¹ and a resistivity of 17.3 Ω cm. Compared with the other samples, ZCO:N film fabricated at the relatively lower N₂:Ar possesses the superior crystal quality, luminescent and electrical properties. Additionally, a possible mechanism of p-type conduction for ZCO:N film was discussed in this work.     

Controlling the crystal formation in solution-process for organic field-effect transistors with high-performance

We control the growth of high-quality organic semiconducting crystals in the aim of transistor application. By finely tuning the processing parameters, both isolated crystals showing characteristic facet angles and irregular-shaped, thin crystalline domains are obtained in large sizes (>400 μm). Structural investigations indicate that the various shapes of crystals are in the same crystal structure, and reveal that the irregular-shaped crystalline domains are composed by terrace of molecularly flat regions, which can be up to hundreds of microns in size. When applied in field-effect transistors, the thin crystalline domains exhibit the best performance showing μ_FET up to 4.4 cm²/V s. This is an order of magnitude higher than that of the transistors made from as-spun films and thick crystals. The approach well demonstrates the importance of fine control of crystal formation and can be generally used for getting organic crystal transistors.     

Organized Nanostructured Complexes of Polyoxometalates and Surfactants that Exhibit Photoluminescence and Electrochromism

A variety of functional nanostructured organic/inorganic hybrid materials from the europium‐exchanged derivative of a Preyssler‐type polyoxometalate (POM), [EuP₅W₃₀O₁₁₀]^12?, and functional organic surfactants were prepared by the ionic self‐assembly (ISA) route. The effect of organic surfactants on the structure, photoluminescent, electrochemical and electrochromic properties of the POM anions was investigated in detail. All obtained hybrid materials are amphotropic, i.e., exhibit both thermotropic and lyotropic liquid‐crystalline phase behaviour. Investigations of their photophysical properties have shown that the interactions of the various surfactants with the polyanions influence the coordination environments and site symmetry of Eu³⁺ in different ways. The functional groups in the organic surfactants significantly influence the electrochromic properties and photoluminescence of POMs. Different from normal and pyridine‐containing complexes, no photoluminescence and no electrochromism were observed from the ferrocene‐containing complexes. This may be explained in view of charge transfer between the POM anion and the ferrocenyl group.     

Study on the diffusion of Cd and Zn IN InP

 The study on the diffusion of Cd and Zn in InP at a temperature range of 450—700℃ and their compared results are described in this paper.The effects of various impurity sources,such as Cd and Zn as well as their compounds on the results of diffusion are investigated intensively.Using X_j~2/t ratio defined by the square of the diffusion depth X_j~2 and time t as a measure of the diffusion velocity,we show the plots of X_j~2/t vs temperature T.It is found that the diffusion velocity of Cd, especially CdP_2 diffusion sources is slower and thus easier to control the diffusion depth and concentration.However they are more ideal diffusion impurity sources.We explain the complication phenomena of Cd-and Zn-diffusion in InP with neutral complex presented by Tien(1979).     

Fibres optiques pour l’infrarouge moyen

This contribution is a review of a new field between material science and optics. Two kinds of materials are used for the fabrication of IR waveguides: glasses and crystalline compounds. Metallic halides mainly fluorides and chalcogenides are the most promising candidats because of their predicted ultralow losses, 10^?2, 10^?3 dB/km, and their broad transmission range. Special attention is paid to fibers operating in the CO₂ laser emission region near 10.6 μm. Fluoride glasses, because of their potential ultratransparency, appear to be the candidates for optical fibers of the second generation operating in the 2–4 μm band. IR optical fibers will find many applications in the field of power and informations transmission as well as in the broad field of censors technology.     

Synthesis of the PZT films deposited on pt-coated (100) Si substrates for nonvolatile memory applications

Ferroelectric lead-zirconate-titanate (PZT) thin films were deposited by the pulsed laser deposition technique on Pt-coated (100) Si substrates. This study was focused on the investigation of the PZT film growth on (100) Si substrate at varying deposition parameters and electrical characterization of the films including hysteresis loop and fatigue properties by RT66A Standardized Ferroelectric Test System. PZT deposited at higher temperature (575°C in 450 mTorr O₂ partial pressure) showed the best crystalline structure. The remnant polarization and the retained polarization of the ferroelectric capacitors were 13 μC/cm² and 20 μC/cm², respectively. The crystallographic properties of the films were determined using the x-ray diffractometer method. The cross-sectional transmission electron microscope results showed very smooth interfaces among different layers of films.     

Quantum chemical calculations,synthesis and properties of novel organic molecules with photoinduced intramolecular electron transfer and large change in electric dipole moment in the excited state

The topic of this paper is bipolar organic compounds containing both charged electron donor and electron acceptor groups interconnected by various kinds of bridges (–D–X–A⁺). Such betaines are subject to photoinduced intramolecular electron transfer (PIET), large change in dipole moment in the excited state and considerable hyperpolarisability, which causes large non‐linear optical effects in solutions and Longmuir–Blodgett (LB) films. Quantum chemical calculations (CNDO/S, ZINDO/S) of some types of betaines containing a 1,3‐indandione anion or diazole anion electron donor part and an N‐pyridinium cation electron acceptor part show that the HOMO and LUMO are strongly localized and an effective PIET takes place. The calculated change in dipole moment in the excited state, δμ=μ_g−μ_ex , is substantial (8–20 D) and in many cases the direction of μ is reversed. The character of the bridge X in betaines shows the largest effect on μ_g and δμ, especially in the case of X=p‐phenylene. The pyrazole anion is a better electron donor than the 1,3‐indandione anion. The synthesis of betaines containing a 1,3‐indandione anion part of an N‐pyridinium cation part interconnected directly or through a p‐phenylene bridge has been achieved and their electron absorption spectra have been investigated. Preliminary experiments confirm NLO effects in solutions and LB films. The synthesis of novel substituted betaines is unlimited and the synthesis of polymer‐bonded or surface‐bonded betaines is possible. Copyright © 1999 John Wiley & Sons, Ltd.     

Synergistic Crystallization Modulation and Defects passivation via Additive Engineering Stabilize Perovskite Films for Efficient Solar Cells

Organic-inorganic lead halide perovskite are promising photovoltaic materials, but their intrinsic defects and crystalline quality severely deteriorate the solar cells efficiency and stability. Herein, potassium 1,1,2,2,3,3-hexafluoroprop-ane-1,3-disulfonimide (KHFDF) is introduced into PbI₂ precursor solution to passivate various defects and improve the crystalline quality of perovskite films. It is found that KHFDF can inhibit PbI₂ crystallization, thus tuning the crystal orientation and growth of perovskite films. Furthermore, KHFDF with dual-functional sulfonyl group cannot only passivate grain boundaries (GBs), but also passivate the defects at GBs via strong interaction with undercoordinated Pb²⁺ and/or hydrogen bonding with FA⁺, while the K⁺ counter cations allow ionic interaction with undercoordinated I⁻. As a result, the KHFDF-modified films exhibit high quality with a larger grain size and a reduced trap-state density, thereby suppressing the trap-state nonradiative recombination. And the devices show a champion efficiency up to 24.15%, benefiting from a sharp enhancement of open-circuit voltage (V_oc) of 1.183 V and fill factor of 81.78%. In addition, due to the enhanced humidity tolerance and chemical structure stability, the devices exhibit excellent long-term humidity and thermal stability without encapsulation.     

Developments of new concept analytical instruments for failure analyses of sub-100 nm devices

We have developed analytical instruments based on new concepts for failure analyses of devices of 100 nm dimensions or less. They are a sputtered neutral mass spectrometer with focused ion beam for highly sensitive element analysis in microarea (10¹⁸ atoms/cm³ in 30 nm area), transmission electron microscope (TEM) with electron energy loss spectrometer for chemical bond analysis in less than 2 nm area, Nanoprober for electrical characteristics inspection in actual circuits, computed tomography-TEM for three-dimensional observation of crystalline defect with 1 nm spatial resolution, atmospheric pressure ionization mass spectrometer for trace impurities ppq (parts per quadrillion) analysis in gases, and glow discharge optical emission spectrometer for rapid and precise composition analysis. Using these instruments, it was found that the formation of SiO₂ or TiO_x film by water from titanic acid (TiO_x·H₂O) is the cause of the high resistivity in a contact (CVD-W/CVD-TiN/Ti/Si) and vaporization of silicon dioxide by phosphorus trifluoride (PF₃) is the cause of voids in interlayer dielectric film borophosphosilicate glass.     

Electric field-enhanced metal-induced lateral crystallization and P-channel poly-Si TFTs fabricated by it

The poly-Si thin film was obtained by electric field-enhanced metal-induced lateral crystallization technique at low temperature. Raman spectra, X-ray diffraction (XRD) and scan electron microscope (SEM) were used to analyze the crystallization state, crystal structure and surface morphology of the poly-Si thin film. Results show that the poly-Si has good crystallinity, and the electric field has the effect of enhancing the crystallization when DC electric voltage is added to the film during annealing. Secondary ion mass spectroscopy (SIMS) shows that the metal Ni improves the crystallization by diffusing into the a-Si thin film, so the crystallization of the lateral diffused region of Ni is the best. The p-channel poly-Si thin film transistors (TFTs) were fabricated by this large-size grain technique. The I_DS−V_DS and the transfer characteristics of the TFTs were measured, from which, the hole mobility of TFTs was 65 cm²/V s, the on and off current ratio was 5×10⁶. It is a promising method to fabricate high-performance poly-Si TFTs at low temperature for applications in AMLCD and AMOLED.