Vertical and transverse confinement of electromagnetic radiation in planar organic light emitting diodes (OLED) has been widely addressed in order to maximize their external quantum efficiency. Here we propose a planar micro-cavity OLED with the aim to tailor the angular emission pattern and obtain light emission suppression in the vertical direction. Vertical confinement was obtained by depositing a conventional molecular OLED on a glass substrate coated with a thin gold semitransparent layer and a silica spacer. The conventional OLED was obtained by depositing under vacuum an indium tin oxide hole-injection layer, an organic hetero-junction constituted by N,N-diphenyl-N,N-bis(3-methyl phenyl)-1,1-biphenyl-4,4-diamine (TPD) and tris(8-hydroxy-quinoline)-aluminium (Alq3) layers and a LiF/Al electron-injection layer. Performance of the micro-cavity OLEDs was characterized by measuring the current/voltage characteristics and the angular dependence of the brightness and emission spectrum. The results were compared to those obtained for reference conventional OLEDs, without vertical confinement. Suppression of the green emission in the vertical direction was observed, confirming what was expected by calculations of dipole emission in a micro-cavity structure.

By studying the photoluminescence of Alq(3) films as a function of time exposure to open and dry atmospheres, it has been found that the emission intensity can be described in a meaningful physical way by four, and only four, exponential decays. If the morphological structure of the film is also taken into account, then it is deduced that the bulk of the film is composed of three different states of aggregation that have been called species 1, 2, and 3. This unexpected discovery opens new research scenarios in the basic and applicative aspects of the Alq(3) molecule. (c) 2005 The Electrochemical Society.

We review the status of FEL source activity of the ongoing SPARC FEL experiment, developed within the framework of a collaboration among ENEA, CNR, INFN, INFM, Sincrotrone Trieste and University of Rome Tor Vergata. The project is aimed at realizing a SASE-FEL source, operating in the visible (around 500 nm), with an extended range of tunability down to the VUV (100 nm) by the use of the mechanism of non-linear harmonic generation. The development of the relevant activities foresees the realization of an advanced 150 MeV photo-injector source, aimed at producing a high brightness electron beams, needed to drive a SASE-FEL experiment, and a 14 m long undulator. We present the status of the design and construction of SPARC FEL device. In particular we discuss the choice of the project parameters, their optimization and the sensitivity of the SPARC performance to any parameter variation. We will show, using start-to-end simulations, what is the impact of the e-beam and of the undulator parameters on the characteristics of the output laser field and in particular on the amount of the non-linearly generated power at higher harmonics.

Recent annealing experiments have shown that Alq(3) films are highly susceptible to various atmospheres, going to their destruction in particular environments well below 200degreesC. Moreover, before this abrupt transition a sizable increase in photoluminescence has been observed especially by annealing in humid air. This property, which hints at a new phase of the material with some analogies to the recently discovered delta-phase except for a much lower temperature, displays a remarkable stability in ambient air, a result with important implications both for the development of environmentally stable organic light-emitting diodes devices and for basic understanding of the morphological structure of Alq3 systems. (C) 2004 The Electrochemical Society.

Local fluorescent probes based on low‐energy electron beam irradiated lithium fluoride (LiF) thin films were preparated and tested for their applications in Scanning Near‐Field Optical Microscopy.

We report the results of the morphological, electrical and optical characterisation of double-layer Alq3-based organic emitting diodes with a lithium fluoride (LiF)/Al cathode. A detailed electron microscopy investigation of their cross-section shows the presence of LiF isolated grains underneath the Al film. Due to the introduction of the LiF layer, luminance was larger than 30,000cd/m2 at a bias voltage VB=25V with a maximum external luminous efficiency as large as 46lm/W at VB=20V. Performing on/off VB cycles at a very low frequency, each time a recovery of the initial electric and luminous performance was observed. The non-exponential decay of both electric current and luminance during each on-cycle has been interpreted as the signature of dispersive transport controlled by multiple trapping on localised states in an amorphous system.

In this work we report about the fabrication and characterization of heterostructure organic electroluminescent devices based on the system N,N'-diphenyl-N,N'-bis(3-methyl phenyl)1,1'-biphenyl-4,4'-diamine (TPD) and tris (8-hydroxy-quinoline) aluminum (Alq(3)). We present the electric and photometric characterization of Alq(3)/TPD diodes, using ITO (Indium-Tin-Oxide) and Al or Al/LiF as transparent anode and metallic cathode, respectively, and discuss the results.

Tris(8-hydroxyquinoline)aluminum (Alq(3))-based organic light emitting diodes are highly susceptible to environmental conditions which severely limit their lifetime. In particular, moisture and oxygen play an important role in the degradation processes. In order to study those effects more deeply, annealing processes of Alq(3) films have been performed in various atmospheres, and they have confirmed the physical and chemical destructive actions of temperature, oxygen, and water, the last one more effective than the previous two agents. However, also a sizeable increase of the photoluminescence has been discovered by annealing in humidified atmospheres, especially air. Due to the complexity of the matter, it is not yet clear whether this important effect can be ascribed to the temperature alone or to the combined action of temperature and water, but almost certainly it is related to the phase transitions of the molecular material itself. (C) 2003 The Electrochemical Society.

Polycrystalline thick lithium fluoride (LiF) films have been grown by thermal evaporation on heated glass substrates. Excellent agreement between the values obtained from effective index measurements carried out by the m-line technique and those provided by computer simulations has shown that leaky modes are supported by these dielectric films, as they have a gradually increasing refractive index distribution along the thickness moving from the substrate interface towards the air interface. In some cases the films present also optical anisotropy which results in TM modes having higher effective indices than TE ones.

8-hydroxyquinoline metal chelate complexes were use d in the past by chemists for gravimetric determination f various metal cations in solutions [1], but their u se subsided in time with the advent of new spectroscopic techni ques. More recently, the discovery of efficient electrolu minescent devices based on tris(8-hydroxyquinoline)aluminum, Alq3, attracted renewed interest in this class of materia ls to be utilized as optically active media in organic light -emitting diodes (OLED) [2]. Nowadays, this area of research is currently in a period of rapid growth with many ind ustrial and academic groups from all over the world already running active research programs. However, there ar e still consistent obstacles on the road to full commercial production [3]. Indeed, polymers and organic compou nds used in the OLED layers are sensitive to oxygen and moisture [4], and degradation processes have also b een attributed to various mechanisms, including crystal liz tion of the emitting layers, electrochemical reactions at the electrode/organic interface, migration of ionic spe cies, and electrochemical reactions [5].

X-ray microradiographs of small biological objects, such as animals and plant materials at micrometric resolution, are currently performed by various methods, all of which are limited by the resolution or the dynamic range of the image detectors. Here a novel X-ray image detector is discussed, in which the previous limitations have been overcome. A film of lithium fluoride salt is used as a detector, in which the stored biological image is read by observing the optically stimulated visible luminescence of the active color centers, efficiently produced by the X-rays.

It is argued that some organic materials play an important role in preventing the formation of nonemissive species and enhancing the intrinsic stability of Alq3 thin films. In the present work, films (Alq3 pure, Alq3+ORGMAT (top), and Alq3 doped with same) were grown on glass and silica substrates by thermal evaporation. Excitation, fluorescence, and transmission spectra have been measured, and the intensity of the green photoluminescence as a function of time has been monitored. Preliminary results show an improved stability of the emission in the doped Alq3 films compared with the undoped ones.

Amorphous tris(8-hydroxyquinoline) aluminium (Alq(3)) thin films were deposited on fused silica substrates by thermal evaporation. Optical absorption. emission and excitation spectra were measured at room temperature and. for the first time, at 80K, on an Alq(3) film in a spectral range extending from the near ultraviolet to the visible. Spectroscopic features were carefully investigated as a function of temperature.

Over the last 10 years, several organic compounds have been used to realize efficient electroluminescent devices, organic light-emitting diodes (OLED), which have now become competitive with the well-known and older inorganic semiconductor diodes, LED. Among these new luminescent compounds, the metallorganic molecule Alq(3) has an efficient emission band in the green region of the electromagnetic spectrum, and is easily handled. However, OLEDs realized with Alq(3) and other organic compounds display a lifetime defined as the time required by the emission to reduce by half its initial value, equal to about 5000 h, which is considered to be too short for practical applications. Several studies have been performed on organic compounds to understand their degradation, which has been ascribed to both intrinsic and extrinsic effects; among the latter, exposition to atmospheric agents is considered to be very important. Simple experiments have been made to verify the oxidation processes and the possibility to halt or retard them by using appropriate chemical compounds. Butylated hydroxytoluene, a molecule belonging to the big family of phenols, proved to be effective in preserving the photoluminescence of Alq(3) films. Our results support the idea that phenols, which are known to be strong antioxidant products, can increase the lifetime of OLEDs realized with small organic molecules and polymers. (C) 2002 The Electrochemical Society.

Low energy electron beam irradiation of LiF single crystals and polycrystalline films induces efficient formation of stable laser active defects emitting in the visible spectral range at room temperature, together with a consistent increase of the real part of the refractive index in the same wavelength interval. The use of electron lithography techniques look promising for the realization of active channel waveguides.