Gunar Vingre da Silva Mota

Photoluminescence, photoabsorption and photoemission studies of hydrazone thin film used as hole transporting material in OLEDs

A fotoluminescencia de filmes finos de 1-(3-metilfenil)-1,2,3,4-tetrahidroquinolina-6carboxialdeido-1,1’-difenilhidrazona foi monitorada em funcao da irradiacao com luz UV. A intensidade da emissao decresce exponencialmente com o tempo de exposicao, sugerindo degradacao das amostras. Com o objetivo de investigar os mecanismos de degradacao e determinar a estrutura eletronica desse material orgânico usado com sucesso como camada transportadora de buracos na fabricacao de diodos orgânicos emissores de luz (OLEDs), foram empregadas as tecnicas de fotoabsorcao e de fotoemissao nas bordas 1s do carbono e do nitrogenio bem como na banda de valencia. A influencia da luz solar foi simulada usando radiacao sincrotron nao-monocromatica. Apos exposicao, todos os espectros apresentam um decrescimo nos sinais de fotoabsorcao e de fotoemissao, que e menos acentuado na borda do carbono, apresentando, entretanto, um decrescimo drastico na borda do nitrogenio e na regiao de valencia. O estudo sugere que a perda de nitrogenio e a principal causa para a quebra do sistema π, levando, dessa forma, a falha do dispositivo fabricado com esse composto. Photoluminescence (PL) emission of 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6carboxyaldehyde-1,1 ’ -diphenylhydrazone (MTCD) thin films was monitored as a function of UV irradiation, and it was found to decrease exponentially with the exposure time. In order to gain insight into the degradation mechanisms and evaluate the electronic structure of this organic material used with good results as hole transporting layer (HTL) in the fabrication of organic light emitting diodes (OLEDs), synchrotron radiation-based photoabsorption and photoemission techniques at the carbon and nitrogen 1s edges as well as at the valence band were employed. The influence of sunlight was simulated using non-monochromatized synchrotron radiation. After exposure all the spectra show a decrease of the photoabsorption and photoemission signals, however, while it is less accentuated at the carbon edge, at the nitrogen edge and at the valence region it decreases drastically. The loss of nitrogen is suggested to be the main step in the disruption of the π system, leading to the failure of the devices fabricated with this compound as hole transporting layer.

Time-Dependent Density Functional Theory Analysis of Triphenylamine-Functionalized Graphene Doped with Transition Metals for Photocatalytic Hydrogen Production

The electronic structures and optical properties of triphenylamine-functionalized graphene (G-TPA) doped with transition metals, using water as a solvent, were theoretically investigated to verify the efficiency of photocatalytic hydrogen production with the use of transition metals. This study was performed by Density Functional Theory and Time-dependent Density Functional Theory through Gaussian 09W software, adopting the B3LYP functional for all structures. The 6-31g(d) basis set was used for H, C and N atoms, and the LANL2DZ basis set for transition metals using the Effective Core Potentials method. Two approaches were adopted: (1) using single metallic dopants (Ni, Pd, Fe, Os and Pt) and (2) using combinations of Ni with the other dopants (NiPd, NiPt, NiFe and NiOs). The DOS spectra reveal an increase of accessible states in the valence shell, in addition to a gap decrease for all dopants. This doping also increases the absorption in the visible region of solar radiation where sunlight is most intense (400 nm to 700 nm), with additional absorption peaks. The results lead us to propose the G-TPA structures doped with Ni, Pd, Pt, NiPt or NiPd to be novel catalysts for the conversion of solar energy for photocatalytic hydrogen production, since they improve the absorption of solar energy in the range of interest for solar radiation; and act as reaction centers, reducing the required overpotential for hydrogen production from water.

Photon stimulated ion desorption of condensed CO2 at ~ 85 K studied by synchrotron radiation

Photon stimulated ion desorption (PSID) from condensed carbon dioxide has been studied for photon excitation energies ranging from 93 to 193 eV. PSID studies have been performed at the Brazilian synchrotron light source (LNLS), Campinas, during a multi-bunch operation mode of the storage ring. The results showed that after photon excitation several ions desorbed from the CO2 films: C+ , O+ , CO+ and O2+. PSID experiments showed that ion desorption was enhanced only at the Si resonance excitations. When the thickness of the CO2 was ~ 500 L or higher, almost no desorption yield was observed. The study of the dependence of the relative partial ion yield on the photon excitation showed that the X-ray induced Electron Stimulated Desorption (XESD) mechanism has to be invoked to explain the origin of the desorbed ions in the energy region studied.