Introduction. It is well-known that the electrical resistivity of insulating polymers can be decreased by dispersing a conductive filler, e.g., carbon black (CB) throughout the polymer matrix.1-7 The critical amount of filler necessary to build up a continuous conductive

The electronic structure of tris͑8-hydroxyquinoline͒ aluminum (Alq 3 ) has been studied in the pristine molecular solid state as well as upon interaction ͑doping͒ with potassium and lithium. We discuss the results of a joint theoretical and experimental investigation, based on a combination of x-ray and ultraviolet photoelectron spectroscopies with quantum-chemical calculations at the density functional theory level. Upon doping, each electron transferred from an alkali metal atom is stored on one of the three ligands of the Alq 3 molecule, resulting in a new spectral feature ͑peak͒ in the valence band that evolves uniformly when going from a doping level of one to three metal atoms per Alq 3 molecule.

Self-assembly of block copolymers is attractive for nanodevice fabrication because the association of different polymers by covalent bonding and the interplay between the sequences allows one to generate a variety of structures with well-defined shapes. We present a straightforward approach to generate self-organized organic semiconducting nanostructures. This approach is based on the spontaneous molecular organization of block copolymers containing one fully conjugated segment associated with a nonconjugated sequence. Such systems show high local contrast in their properties and are of interest for applications in nanoscale patterning for quantum confinement of light emission or fabrication of nanowire networks.

We investigate theoretically the geometric structure and vibrational properties of complexes of polyenes with aluminum atoms that constitute model systems for the species formed at the interface between aluminum and polyacetylene. The calculations are performed with two quantum-mechanical techniques: ab initio Hartree-Fock and density functional theory in the local spin density approximation. These methods are first applied to a polyene molecule, all-trans octatetraene, and the calculated vibrational spectra are compared to existing experimental and theoretical data. The molecule is then made to interact with two aluminum atoms in various configurations. Since the metal atoms form covalent bonds with carbon atoms in the central part of octatetraene, strong geometric modifications occur along the conjugated system, which in turn deeply affect the vibrational spectra. These results allow us to derive the expected infrared signature of the chemical species present at the interface.

Following the discovery of light emitting diodes"] based on conjugated polymers, ['] research on the physics and chemistry of the conjugated polymer materials in their semiconducting state has evolved considerably.[31 In particular, the materials science underlying light-emission-device development has progressed at a rapid pace.L4] Even though complex multilayer devices exist, in the simplest configuration, polymer-based light emission devices (LEDs) consist of an ITO-covered glass substrate, a spincoated active conjugated polymer layer, and an electron-injecting, low-work-function metal e l e~t r o d e .~~] Among the most widely used of the conjugated polymers for polymerbased LEDs are poly(p-phenylenevinylene), or PPV, and its substituted derivatives. Most of the substituted PPVs are soluble in common organic solvents. However, it has been established that the performance characteristics of devices based on the PPVs are unfortunately sensitive to exposure to air.[5361 The dependence of the electronic properties of the PPVs upon air exposure may thus prove to be a central issue in the development of polymer-based LEDs.This work reports some of the results of a study of the effects of air exposure on the electronic structure of unsub-stituted PPV, as studied by both X-ray and ultaviolet photoelectron spectroscopy (XPS and UPS, respectively), optical absorption spectroscopy, and model quantum chemical calculations. Reversible effects associated with the absorption of water vapor, which lead to detrimental changes in the frontier electronic structure, have been observed. The nature of these effects is identified through the use of quantum chemical calculations and verified experimentally. Sample treatment and photoelectron spectroscopy were carried out in a unique UHV system of our own design and construction. The system consists of separate sample introduction, preparation, and analysis chambers, pumped by a special combination of turbo-, ion-, and cryogenic pumps, enabling studies under UHV (53 x lo-'' mbar) conditions even for polymer samples inserted from the atmo~phere.'~] Optical absorption spectroscopy was carried out either in a commercial PE Lambda-9 spectrometer, or in a special UHV system, also of our own design and construction.[*] Figure 1 shows the O(1s) XPS spectra of a PPV film in the converted (clean) state, following overnight exposure to air. Note that the residual oxygen signal in the freshly converted PPV corresponds to 51.5 at.-% oxygen. Upon exposure, the O(1s) signal increases, and the increased signal can be deconvoluted from the original (residual) O(1s) peak as shown. The O(1s) signal decreases at elevated temperatures. Upon heat treating overnight in UHV at 180 "C, the O(1s) signal associated with the air exposure is completely removed. The process is thus fully reversible. The low[*] Prof.