Contact Injection into Polymer Light-Emitting Diodes

The variation of current I with voltage V for poly(phenylene vinylene) and other polymer light-emitting diodes has been attributed to carriers tunneling into broad conduction and valence bands. In actuality the electrons and holes tunnel into polaron levels and transport is by hopping among these levels. We show that for small injection the I-V characteristic is determined mainly by the image force, for large injection by space charge effects, but in both cases the strong variation of mobility with field due to disorder plays an important role.Comment: 9 pages, two Postscript figures are aviable upon reques

Kohn anomaly and interplay of electron-electron and electron-phonon interactions in epitaxial graphene

The interplay of electron-phonon (el-ph) and electron-electron (el-el) interactions in epitaxial graphene is studied by directly probing its electronic structure. We found a strong coupling of electrons to the soft part of the A1g phonon evident by a kink at 150+/-15 meV, while the coupling of electrons to another expected phonon E2g at 195 meV can only be barely detected. The possible role of the el-el interaction to account for the enhanced coupling of electrons to the A1g phonon, and the contribution of el-ph interaction to the linear imaginary part of the self energy at high binding energy are also discussed. Our results reveal the dominant role of the A1g phonon in the el-ph interaction in graphene, and highlight the important interplay of el-el and el-ph interactions in the self energy of graphene.Comment: accepted to Phys. Rev.

An ultrasensitive photoelectrochemical nucleic acid biosensor

A simple and ultrasensitive procedure for non-labeling detection of nucleic acids is described in this study. It is based on the photoelectrochemical detection of target nucleic acids by forming a nucleic acid/photoreporter adduct layer on an ITO electrode. The target nucleic acids were hybridized with immobilized oligonucleotide capture probes on the ITO electrode. A subsequent binding of a photoreporter—a photoactive threading bis-intercalator consisting of two N,N′-bis(3-propyl-imidazole)-1,4,5,8-naphthalene diimides (PIND) linked by a [Formula: see text] (bpy = 2,2′-bipyridine) complex (PIND–Ru–PIND)—allowed for photoelectrochemical detection of the target nucleic acids. The extremely low dissociation rate of the adduct and the highly reversible photoelectrochemical response under visible light illumination (490 nm) make it possible to conduct nucleic acid detection, with a sensitivity enhancement of four orders of magnitude over voltammetry. These results demonstrate for the first time the potential of photoelectrochemical biosensors for PCR-free ultrasensitive detection of nucleic acids

2005-06 Managing and Editorial Boards

Managing and Editorial Boards for the Academic Year 2005-0

Modelling the effects of mesostructure on electronic applications of polymer thin layers

There is considerable experimental evidence that the microstructure has important consequences for polymer-based electronic and optoelectronic applications, but few theoretical and computational models account for it. We produced several realisations of polymer networks exhibiting specific arrangements of C4n+2 H2n+4 molecules at the mesoscopic scale and we carried out computer experiments in which bipolar charge carriers were injected in the polymer system from the appropriate electrodes. Our results show that polymer microstructures resulting from different arrangements of polymer molecules have significant effects on the competition between charge trapping, current transport and recombination within the polymer layer. It was found that current efficiency increases non-linearly with the external applied electric field, the effect being more pronounced for molecular orientations parallel to the electrode surface. In contrast, recombination efficiency shows an opposite behaviour since no significant charge accumulation within the polymer layer is predicted. However, the space-charge effects due to electrons and holes are responsible for most of the recombination events not occurring in neither long or short chains.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001 e SFRH/BD/11231/200

Pengaruh Jenis dan Konsentrasi Pelarut terhadap Kinerja Devais Organic Light Emitting Diode

Penelitian ini bertujuan untuk meningkatkan kualitas kinerja LED dari bahan organik dengan melakukan optimasi parameter fabrikasinya. Fabrikasi film tipis MEH-PPV (poly[2–methoxy–5-(2 ethylhexyloxy)–1,4-phenylenevinylene]) dilakukan dengan teknik spin coating dan menggunakan dua jenis pelarut yaitu toluen dan THF (tetrahidro furran). Untuk mengetahui kualitas film dan frekuensi emisinya, dilakukan pengukuran spektroskopi UV-Vis. Pengujian kualitas kinerja LED di-lakukan dengan pengukuran kurva karakteristik I-V. Berdasarkan hasil pengukuran spektroskopi UV-Vis, panjang gelombang emisi rata-rata berada pada kisaran 600 nm yang bertepatan dengan emisi warna jingga. Selain itu film tipis MEH-PPV yang difabrikasi menggunakan pelarut THF mengalami pergeseran merah (bathochromic shift). Sedangkan film tipis yang difabrikasi dengan pelarut toluen mengalami pergeseran biru (hypsochromic shift). Hasil pengujian kinerja devasi LED dengan konfigurasi ITO/MEH-PPV/Al, memperlihatkan kinerja yang cukup baik. Kualitas kinerja terbaik LED diperoleh pada devais yang menggunakan film MEH-PPV yang difabrikasi dengan pelarut THF pada konsentrasi (w/w) 0,5%. Tegangan operasional rata-ratanya adalah 2,6 Volt dengan warna emisi jingga

High-throughput three-dimensional lithographic microfabrication

A 3D lithographic microfabrication process has been developed that is high throughput, scalable, and capable of producing arbitrary patterns. It offers the possibility for industrial scale manufacturing of 3D microdevices such as photonic crystals, tissue engineering scaffolds, and microfluidics chips. This method is based on depth-resolved wide-field illumination by temporally focusing femtosecond light pulses. We characterized the axial resolution of this technique, and the result is consistent with the theoretical prediction. As proof-of-concept experiments, we demonstrated photobleaching of 3D resolved patterns in a fluorescent medium and fabricating 3D microstructures with SU-8 photoresist.Deshpande Center for Technological Innovation (Massachusetts Institute of Technology. School of Engineering)Singapore-MIT Alliance for Research and Technology (SMART

CHROMIC TRANSITIONS IN PHENYL-SUBSTITUTED POLYTHIOPHENES

The solvatochromic and thermochromic behaviours of phenyl-substituted polythiophenes were studied. The pristine polymers, upon dissolution in chloroform, exhibited blue-shifted absorption. The solid films of the polymers showed significant blue-shifted as well as red-shifted absorptions when heated. While the addition of methanol to the chloroform solutions of the polymers caused dramatic chromic changes and development of red-shifted spectra for many of the polymers investigated, the symmetrically phenyl-substituted and sterically hindered polymer (polymer 1) does not show significant changes. These chromic behaviours have been examined in terms of substituent effects and attempt has been made to explain these effects by calculating the energy barrier for rotation to a planar structure using the HF SCF method and 3-21G* basis set. KEY WORDS: Chromic transitions, Phenyl-substituted polythiophenes Bull. Chem. Soc. Ethiop. 2005, 19(2), 267-276

UV-induced electrical and optical changes in PVC blends

2-Chloro-polyaniline (2-Cl-PANI) in its non-conducting (emeraldine base, EB) form, prepared by a chemical route, was dissolved together with poly-(vinylchloride) (PVC) in THF for casting into thin (10-50 μm) films. Upon exposure to UV radiation, the electrical conductivity of these films increased by more than 4 orders of magnitude (from 10-6 to 10-2 S/cm). This is attributed to the dehydrochlorination of PVC by exposure to energetic photons and subsequent doping of 2-Cl-PANI (i.e. conversion to emeraldine salt, ES) by in situ created HCl. The doped films could be returned to their undoped form by exposure to NH3 vapours. The UV-induced doping/NH3 undoping cycles could be repeated several times. Various spectroscopic techniques were employed to follow the changes in the films upon exposure to UV radiation. The same photo-dehydrochlorination process has also been utilized for optical and/or lithographic purposes by preparing PVC blends containing methyl violet, and acid-base indicator dye. The photo-dehydrochlorination can be effectively sensitized by incorporating hydroquinone into the PVC blends containing methyl violet

Optical studies of triplet excitations in poly(p-phenylene vinylene)

Journal ArticleWe have studied energy states in the triplet manifold in films of poly(p-phenylene vinylene) (PPV) using a multitude of optical spectroscopies including photoinduced absorption, photoluminescence, their optically detected magnetic resonances, as well as their photogeneration action spectra. We found that singlet fission into two triplets with opposite spins is an efficient route for triplet generation in PPV, similar in strength to the intersystem crossing process. From the threshold energy for singlet fission we deduced that the lowest-lying, odd-parity triplet excitonic state, 13Bµ , is located at 1.55 eV from the ground state, which is about 0.9 eV lower than the lowest-lying, odd-parity singlet state 11Bµ