Experimental investigations of laser-induced forward transfer process of organic thin films

This paper deals with transfer induced by laser of thin layers of a conducting polymer, the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), for applications in plastic electronics. This relatively simple technique of direct writing offers the ability to make surface micro-patterning by localized deposits of material. The study of the various mechanisms (ablation, transfer and deposit) has been carried out according to different conditions of irradiation: wavelength (from ultraviolet to infrared radiation), pulse duration (nanosecond and sub-nanosecond) and fluence. The morphology of the transferred patterns has been analyzed by optical microscopy and scanning electronic microscopy. Our objective is to understand the different mechanisms involved in the process in order to optimize it in terms of geometrical resolution while preserving the properties of the transferred material.     

有机分子束外延技术与研究进展

本文介绍了超高真空分子束外延生长有机薄膜的技术及其研究进展 ,讨论了外延材料的纯化过程和杂质对外延薄膜结构的影响 ;从理论和实验观点评论了薄膜的生长性质和膜的有序结构。超高真空有机分子束外延技术是一种多用途的高技术 ,可以生长有机、无机、有机 /无机混和的薄膜结构。这种薄膜结构是未来光学和电子器件有希望应用的新一类工程材料。     

有机分子束外延技术与研究进展

本介绍了超高真空分子束外延生长有机薄膜的技术及基研究进展,讨论了外延材料的纯化过程和杂质对外延薄膜的影响;从理论和实验观点评论了薄膜的生长性质和膜的有序结构。超高真空有机分子束外延技术是一种多用途的高技术,可以生长有机、无机、有机／无机混和的薄膜结构。这种薄膜结构是示来光学和电子器件有希望应用的新一类工程材料。     

Surprising electronic-magnetic properties of closed packed organized organic layers

Cooperative effects generate new electronic and magnetic properties in closed packed organized organic layers. In layers made from chiral molecules, unexpectedly large electronic dichroism is observed, which manifests itself as spin specific electron transmission. For many thiolated molecules self-assembled on gold, a surprisingly large ferromagnetism is observed. All the observations can be rationalized by assuming orbital ferromagnetism of the organic thin layer. This is a new type of magnetism that is caused by the formation of closed packed layers of organic molecules on metal. In particular, charge transfer occurs between the substrate and the adsorbed layer. This charge is responsible for the appearance of magnetism.     

Efficient broadband simulations for thin optical structures

The thickness of the layers comprising optical structures is usually very thin. When modelling such thin features using a traditional numerical method, for instance the transmission-line modelling (TLM) method, a very small space step is often used to properly discretize the material geometry. This consequently results in large memory storage and longer run time. In this paper a new technique embedding thin structures between TLM nodes is investigated. The key features of this technique are the acquisition of the formulations in the frequency domain and the utilisation of digital filter theory and an inverse Z transform to change the formulations to the time domain. This technique has been successfully applied to calculate the reflection and transmission coefficients of optical structures incorporating thin layers, including antireflection coatings and fibre Bragg grating structures.     

Hall mobility and characteristics of gas-phase polymerized poly(3-iodothiophene) thin films

The semi-conductive poly(3-iodothiophene)(P3IT) films were fabricated by gas-phase polymerization through a chemical vapor deposition process. The P3IT nanoscale films have a high crystalline morphologies, and possessed a high Hall mobility up to 10 cm²/Vs. The degree of crystalline and the mobility values measured through Scanning Electron Microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy with structural analysis. These conductive thin films, possessing polycrystalline structures, have a very high mobility and are capable of being applied to organic electronic layers for electrical devices such as the thin film transistors and organic photovoltaic cells.     

Progress in Modification of Indium-Tin Oxide/Organic Interfaces for Organic Light-Emitting Diodes

The oxide/organic interfaces play crucial roles in the hole injection from the anode electrodes to the emitting organics in organic light-emitting diodes (OLEDs), and hence have strong impacts on the efficiencies and other properties of the devices. Indium-tin oxide (ITO) is currently the most popular anode material used in OLEDs due to several merits, such as good etch ability, good adherence, high transparency, low resistivity, and high work function. Interfacial engineering between the ITO electrode and the overlying organic layers is an important process to obtain the high performance of the diode devices. In this article, recent progress in modification of the ITO/organic interfaces is reviewed, as these interfaces are important to the development of the technologies aiming at improving the electroluminescence, and efficiencies as well as reducing the operation voltages of OLEDs. ITO/Organic interfacial properties can be controlled or modified by simply changing the surface properties of ITO using chemical or physical treatments, and by adding a buffer layer (e.g., metal, oxide, or organic thin films) between the ITO and hole transport or emitting organic layers. The literature data showed that the electroluminescence, efficiencies, and lifetimes of the OLEDs could be greatly increased and the operation voltage considerably decreased when the ITO/organic interfaces have been properly improved.     

Surface scattering resonance absorption of free electron in nano structured materials

A physical model has been established for calculating the electronic transportation in nano structured material when it is irradiated by a laser pulse with a power density in a magnitude of GW/cm². The Monte Carlo technique has been selected to deal with the electron scattering process. In the process of electrons absorbing laser energy, a surface scattering resonance absorption mechanism and a small-size effect can be found. The regularities of electron energy changes with laser wavelength, structure dimension, and orientation of nano-line have been analyzed.     

Determination of elastic modulus of a beam by using electronic speckle pattern interferometry

This paper proposes a sonic resonance test for an elastic modulus measurement which is based on the electronic speckle pattern interferometry. Previous measurement technique of elastic constant has the limitation of application for thin film or polymer material because contact to specimen affects the result. It has been developed as a non-contact optical measurement technique which can visualize resonance vibration mode shapes with whole field. The maximum vibration amplitude at each vibration mode shape is a clue to find the resonance frequencies. The dynamic elastic constant of test material can be easily determined from vibration of a beam equation using the measured resonance frequencies. The proposed technique is able to give high accurate elastic modulus of materials through a simple experimental set-up and analysis. The experimental result also compared to the theoretical result.     

Quantum size effects in the optical properties of organic superlattices containing 3, 4, 9, 10 perylene tetracarboxylic dianhydride (PTCDA)

A Frenkel exciton model is applied to ultrathin films of PTCDA confined between layers of a material with electronic transitions in a different energetic region. This model accounts for the elongation of an effective internal vibration in the relaxed excited geometry of a molecule and for the transfer of an electronic excitation between different molecular sites. The comparison of the model calculations with experimental findings reveals that due to the modified conditions at the organic interfaces, boundary layers of the PTCDA films show blue-shifted transition energies with respect to inner molecular layers surrounded by the same material. The resulting changes of the lowest vibronic subband contribute further to the distinct absorption line shapes.     

The role of free carbon in the transport and magnetic properties of boron carbide

Boron carbide is a ceramic which has a wide field of application because of its mechanical and nuclear properties. This material is difficult to characterise due to the presence of different levels of disorder and inhomogeneities which are found in the usual available samples. The transport and magnetic properties of several samples of boron carbide have been measured from liquid helium to room temperature as a function of temperature and composition. We have attempted to attribute the different features of these properties to the different levels of disorder. The role of free carbon, in form of thin layers of graphite within the disordered semi-conducting matrix, was investigated in particular details, because it was either ignored or neglected by others. Free carbon is found to dominate the D.C. transport when its concentration is larger than 5%; while the principal features of the electron spin resonance (E.S.R.) line show a dominance of free carbon when the concentration is larger than 3.5%. Below these concentrations conductivities as well as spin relaxation rates do not depend very much on free carbon; neither these have been found to be correlated in a simple way to the stoichiometry: the disorder in the boron sublattice is probably responsible for the electronic properties in this range of concentrations.     

Cross-section preparation for transmission electron microscopy of phases and interfaces in C/BN heterostructures

A technique is described for the preparation of transmission electron microscopy cross-sectional samples of pyrolytical carbon layers deposited on polycrystalline boron nitride substrates. To solve the problem of different abrasion rates of C and BN a filler material, Si wafers, has been bonded to both sides of the pre-thinned BN substrate. Correspondence between color and thickness of Si wafers facilitates controlled sample thickness reduction during dimpling. The samples prepared by this technique even without ion milling are thin enough for HRTEM studies.     

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.     

Two-layer light emitting diodes prepared by the sol–gel route

We have elaborated organic–inorganic hybrid light-emitting diodes (HLED). These devices emitting in the green are formed of two hybrid thin layers, exhibiting different functionalities, which are sandwiched between indium–tin oxide (ITO) and metallic electrodes. These layers have been prepared from silane precursors modified with hole transporting units and light-emitting naphthalimide moieties by the sol–gel technique. The hole transporting sol–gel layers exhibit about the same charge mobility as organic polymers having equivalent active units. The maximum external quantum efficiency of the best diode using LiF/Al cathode is about 1% and the luminance reaches 4000 cd · m ⁻² .     

有机电致发光白光器件的研究进展

在十多年的时间里,有机电致发光二极管(Organic Lightemitting Diodes,OLEDs)的研究和应用取得了长足的进展。有机电致发光器件具有许多优点,例如:自发光、视角宽、响应快、发光效率高、温度适应性好、生产工艺简单、驱动电压低、能耗低、成本低等,因此有机电致发光器件极有可能成为下一代的平板显示终端。有机电致发光白光器件因为可以用于全彩色显示和照明,已成为OLED研究中的热点。介绍了有机电致发光白光器件的研究进展,按发光的性质将白光器件分为荧光器件和磷光器件两类,按发光层数将白光器件分为单层和多层器件,对相关材料、器件结构、发光机理等方面进行了讨论。     

Characterization of amorphous organic thin films, determination of precise model for spectroscopic ellipsometry measurements

The optical properties of tris(8-hydroxyquinoline) aluminum (Alq₃), N,N′-diphenyl-N,N′-bis(1-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD) and other amorphous organic materials for OLEDs application, e.g. 4,4-bis(2,2-diphenyl vinyl)-1,1-biphenyl (DPVBI) and Spiro-DPVBI have been studied by multi-angle spectroscopic ellipsometry (SE). The thin films of these materials have been deposited by organic vapor phase deposition (OVPD). The structural characterization has been performed using atomic force microscopy (AFM) and X-ray reflectometry (XRR). Comparison of the measurements using these different independent techniques enables the precise determination of the optical model for dielectric function of these thin films. The detail analyses on Alq₃ and α-NPD show that the Kim model with Gaussian broadening provides a significantly better fit to the ellipsometry data than the frequently used harmonic oscillator model. This conclusion is further proved by performing similar measurements on other amorphous organic samples for OLEDs application, e.g. DPVBI and Spiro-DPVBI. This result can be explained by the characteristic features of electronic states in organic molecules.     

A single‐crystal Raman and infrared study of the nonlinear optical crystal MBANP

Single‐crystal Raman and polycrystalline thin‐film infrared measurements have been obtained for the polar organic nonlinear optical material 2‐(α‐methylbenzylamino)‐5‐nitropyridine (MBANP). For comparison, thin‐film polycrystalline infrared measurements were also made on 2‐(α‐methylbenzylamino)‐3,5‐dinitropyridine (MBADNP). The long wavelength electronic absorption was measured in several solvents and as a thin solid film. The Raman spectra are dominated by three intense bands attributed to vibrations of the ring, the NO₂ substituent, and the N H bond. The most intense scattering and absorption arose from the α_bb component of the polarisability tensor. This implies that the most significant contribution to the transition polarisability arises from the electronic transition near 383 nm, polarised along the b‐axis of the crystal. The strongest bands in the infrared spectra are also associated with the same three bands, consistent with the predictions of the effective conjugation coordinate (ECC) theory, implying efficient electron–phonon coupling (or electronic delocalisation) in the conjugated system. DFT calculations of vibrational wavenumbers and eigenvectors were used to assign relevant vibrational features and to derive useful information about the molecular structure. This single‐crystal material is also a strong candidate for an efficient laser Raman converter with a large wavenumber shift of 3404 cm⁻¹ and a high damage threshold. Copyright © 2010 John Wiley & Sons, Ltd.     

On the problem of coupling of ultrasonic receivers to layered structures for process control

Different layers in the construction of ultrasonic receiving transducers have been identified as a significant factor in the frequency response of these transducers. The discussion has been extended to the reception of signals through the layers of a process vessel-wall to determine whether the resulting system can be made independent of frequency over a significant spectral range and whether a system can be identified that is essentially independent, in its spectral response, to the thickness of one of the layers (the couplant to the vessel). While the present results are limited to a frequency range of 0–2 MHz, they can readily be extended to other frequency ranges. These computations have as variables the backing material, the transducer material, a wear plate, one or two quarter wavelength layers, the couplant and the structure of the vessel wall. The effect of thin bonding layers for the quarter wavelength plates is also briefly considered. As may have been expected each extra layer introduced into the system produces its own series of resonances. The case of a number of quarter wavelength layers can give a flat frequency response over almost a megahertz of bandwidth but great care needs to be taken with the bonding layers. A system in which the thickness of the couplant had little effect on the overall spectral response was not found.     

Organic materials in future integrated optoelectronic circuits

During the last two decades, lithium niobate has been extensively studied for applications in integrated optical circuits. However, it is difficult to integrate lithium niobate optical devices with semiconductor electronic devices because the materials are incompatible. In recent years, semiconductor materials have been emerging as the main contenders in applications; these materials have the advantage of allowing both optical and electronic devices to be integrated. Further, the semiconductor technology has advanced rapidly, allowing us to engineer device parameters very precisely. In semiconductor optoelectronic devices, that is, bulk and quantum well structures, electroabsorption has mainly been used for amplitude modulation of light. The electrorefraction effect is the most useful for devices employing phase-modulation techniques, but this effect cannot be effectively utilized in semiconductors since the strongest electrorefraction effect is near the absorption edge of the material. Recently, organic materials have been shown to have electro-optic coefficients equal to or larger than that of lithium niobate. There are major advantages of organic materials: (1) the organics can be deposited on semiconductor substrates, and therefore both electronic and optical circuits can be integrated; (2) in organic materials the electrorefraction can be effectively utilized to obtain both amplitude and phase modulation; (3) the organic material composition can be adjusted to satisfy some device requirements. In this paper, a comparison of these material systems are made in terms of device applications.     

Thickness effects on the optical properties of layer-by-layer poly(p-phenylene vinylene) thin films and their use in energy-modulated structures

Poly(p-phenylene vinylene) (PPV) thin films were produced by layer-by-layer (LbL) method, using soluble PPV-precursor and dodecylbenzenesulfonate salt (DBS). The amount of deposited layers strongly influences the optical properties of the thermally converted PPV film. The absorbance and luminescence spectra of ultra-thin films (consisting of only two or three PPV layers) are shifted to smaller wavelengths with respect to spectra of thicker films. This is related to the smaller average conjugation length of polymer chains, resulting in a higher HOMO-LUMO gap energy of the material. However, if a thick film is produced by repeating the deposition process and thermal conversion of ultra-thin layers, the optical spectra are still displaced to higher energies in comparison with those of thicker films produced by the conventional continuous deposition of layers. This result enabled the production of multilayered polymeric films with modulated energy profile, taking the number of deposited layers as the only variable in the manufacturing process of the structure. The aim is to guide the excitation to specific regions of the material through the Förster-type energy transfer processes. Such systems can be used at interfaces electrode/polymer and/or electrode/polymeric active layers in order to improve the performance of organic optoelectronic devices.