Optical properties and charge-transfer excitations in edge-functionalized all-graphene nanojunctions

We investigate the optical properties of edge-functionalized graphene nanosystems, focusing on the formation of junctions and charge transfer excitons. We consider a class of graphene structures which combine the main electronic features of graphene with the wide tunability of large polycyclic aromatic hydrocarbons. By investigating prototypical ribbon-like systems, we show that, upon convenient choice of functional groups, low energy excitations with remarkable charge transfer character and large oscillator strength are obtained. These properties can be further modulated through an appropriate width variation, thus spanning a wide range in the low-energy region of the UV-Vis spectra. Our results are relevant in view of designing all-graphene optoelectronic nanodevices, which take advantage of the versatility of molecular functionalization, together with the stability and the electronic properties of graphene nanostructures.Comment: J. Phys. Chem. Lett. (2011), in pres

Density functional theory calculations of the carbon ELNES of small diameter armchair and zigzag nanotubes: core-hole, curvature and momentum transfer orientation effects

We perform density functional theory calculations on a series of armchair and zigzag nanotubes of diameters less than 1nm using the all-electron Full-Potential(-Linearised)-Augmented-Plane-Wave (FPLAPW) method. Emphasis is laid on the effects of curvature, the electron beam orientation and the inclusion of the core-hole on the carbon electron energy loss K-edge. The electron energy loss near-edge spectra of all the studied tubes show strong curvature effects compared to that of flat graphene. The curvature induced $\pi-\sigma$ hybridisation is shown to have a more drastic effect on the electronic properties of zigzag tubes than on those of armchair tubes. We show that the core-hole effect must be accounted for in order to correctly reproduce electron energy loss measurements. We also find that, the energy loss near edge spectra of these carbon systems are dominantly dipole selected and that they can be expressed simply as a proportionality with the local momentum projected density of states, thus portraying the weak energy dependence of the transition matrix elements. Compared to graphite, the ELNES of carbon nanotubes show a reduced anisotropy.Comment: 25 pages, 15 figures, revtex4 submitted for publication to Phys. Rev.

Change of Structural Behaviors of Organo-Silane Exposed Graphene Nanoflakes

[[abstract]]The electronic structures of graphene nanoflakes (GNFs) exposed to an organo-silane precursor [tetramethylsilane, TMS, Si(CH3)4] were studied using electron field emission (EFE), Raman spectroscopy, X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and first-principles calculation. The results of XANES, XPS, and Raman spectroscopy indicate that the silyl radical strong covalent bonds were formed in GNFs, which induced local structural relaxations and enhanced sp3 hybridization. Comparison of calculated electronic structure, XANES, and XES spectra of Sitreated GNFs suggests that the Si atom substitutes one 3-fold coordinated C atom in a given graphene layer and relaxes outward to form sp3 bonding with another C atom in the adjacent graphene layer. The EFE measurements show an increase in the turn-on electric field with the increase of the Si content, which suggests an enhancement of the nonmetallic sp3 bonding[[journaltype]]國外[[incitationindex]]SCI[[booktype]]紙本[[countrycodes]]US

Cidadania mediada : processos de democratização da política municipal no Brasil

This article discusses the notion that the persistence of &ldquo;traditional&rdquo; political practices weakens Brazil&rsquo;s democracy.Drawing on the cases of three Brazilian municipalities administered by the Workers&rsquo; Party (PT), the author examines the space between &ldquo;traditional&rdquo; and &ldquo;modern&rdquo; and argues that successful democratization does not eradicate practices such as clientelism and patronage, but it tends to incorporate and build on these traditional political elements. Moreover, the article maintains that the democratization of municipal politics is inextricably bound up with the eradication of poverty and the construction of a responsive, state-based social safety net.<br /

A metamorfose do orçamento participativo: mudança de governo e seus efeitos em Porto Alegre

As diferentes experiências participativas surgidas no Brasil na década de 1990 colaboraram para que o país tornasse-se, como diz Tatagiba, em "um celeiro de experiências inovadoras de gestão". Um dos destaques dentre as várias experiências de governança é o Orçamento Participativo (OP): programa que ao longo dos anos tem sido palco de intensos debates e estudos e colecionado admiradores e críticos. Entretanto, apesar de sua fama e dos diversos estudos, poucos buscaram avaliar o desempenho e a sensibilidade do programa ao governo eleito no município. Este artigo investiga o programa de orçamento participativo do município de Porto Alegre entre 1989 e 2008, analisando especificamente a sensibilidade do programa às mudanças de governo entre as gestões do Partido dos Trabalhadores (PT) de 2001-2004 e do Partido do Movimento Democrático Brasileiro (PMDB) de 2005-2008. Ou seja, investigamos se a mudança de partido no controle da prefeitura, mantendo o OP em funcionamento, afetou, no entanto, seu desempenho. Nossa análise indica que o funcionamento do programa é bastante afetado pelo partido político que o implementa e que, portanto, o programa peca por baixa institucionalização, mesmo em seu caso de maior sucesso: o município de Porto Alegre