We perform ab initio calculations of optical properties for a typical semiconductor conjugated polymer, poly-para-phenylenevinylene, in both isolated chain and crystalline packing. In order to obtain results for excitonic energies and real-space wavefunctions we explicitly include electron-hole interaction within the density-matrix formalism. We find that the details of crystalline arrangement crucially affect the optical properties, leading to a richer exciton structure and opening non-radiative decay channels. This has implications for the optical activity and optoelectronic applications of polymer films.PACS numbers: 71.15. Qe, 71.20.Rv, Ordered films of organic conjugated polymers are of strategic relevance for novel optoelectronic devices [1]. In addition, they offer an ideal scenario for the study of electronic and excitonic confinement, being composed of quasi-one-dimensional (1D) systems, arranged however in a three-dimensional (3D) crystalline environment. Long linear chains can indeed be thought of as 1D systems with the highest degree of 1D confinement for a polyatomic system (∼ 2-5Å). Since the main optoelectronic characteristics derive from the mobile π-electrons, delocalized along the chain backbone, and non bonding to neighbouring chains, the vast majority of studies adopt the single-chain model: in fact, complete ab initio theoretical studies of the optical properties have been performed for isolated chains, highlighting the strong confinement expected for such systems [2,3]. Recently, very simplified models of polymers in a "crystalline medium" have been approached through ab initio theory [4]; however the effect of crystal structure or side chains is completely neglected. This is also usual practice in the intepretation of experimental data: e.g. data on poly-paraphenylenevinylene (PPV) and its alkylated and metoxilated derivatives (MH-PPV, MEH-PPV, etc. . . ) are usually bundled together as representative of PPV, based on the supposition that details of the 3D structure are not relevant. The picture emerging from this analysis is far from clear, however: quoted exciton binding energies differ by an order of magnitude [5,6,7,8,9,10], there is an on-going controversy about the existence of chargetransfer excitons or excimers [8,11,12,13,14], and about conditions for efficient light emission. The question then arises if, on the contrary, solid state effects cannot be negleted and crystallization entails also structure-specific 3D interchain coupling effects: recent data on oligothiophene crystals seem to suggest that this might be the case [15,16].Here we address these issues through a full ab initio calculation of optical spectra and real-space exciton wavefunctions for PPV, in both the isolated chain and the crystalline phase. In the single PPV chain, we find that the lowest singlet exciton extends over a few monomers along the chain and is optically active, with a binding energy of about 0.6÷0.7 eV. In the crystal we find evidence of significant interchain coupling, with decrease of the first ...

The excitation energies of impurities in semiconductors, as well as their donor and acceptor ionization energies, represent a combination of one-electron and many-electron multiplet effects, where the latter contribution becomes increasingly significant as localized states are formed. Analysis of the absorption and ionization data for 3d impurities is often obscured by the inability of contemporary multiplet theories (e.g. , the Tanabe-Sugano approach) to separate these two contributions and by the inadequacy of mean-field, one-electron theories that neglect multiplet effects altogether. We present a novel theory of the multiplet structure of localized impurities in semiconductors that circumvents the major shortcomings of the classical Tanabe-Sugano approach and at the same time separates many-electron from mean-field effects. Excitation and ionization energies are given as a sum of mean-field (MF) and multiplet corrections (MC): AE = hEMF +AEMc. We determine EEMc from the analysis of the experimental data. This provides a way to compare experimentally deduced mean-field excitation and ionization energies AEMF --AE -AEMc with the results of electronicstructure calculations. The three central quantities of the theorythe eand t2-orbital deformation parameters and the effective crystal-field splittingcan be obtained from mean-field electronicstructure calculations, or, alternatively, can be deduced from experiment. In this paper, we analyze the absorption spectra of 3d impurities in ZnO, ZnS, ZnSe, and GaP, as well as those of the bulk Mott insulators NiO, CoO, and MnO, in light of the new approach to multiplet effects. These mean-field parameters are shown to display simple chemical regularities with the impurity atomic number and the covalency of the host crystal; they combine, however, to produce interesting nonmonotonic trends in the many-electron correction terms AEMC. These trends explain many of the hitherto puzzling discrepancies between one-electron (AEMF ) theory and experiment (hE). This approach unravels the chemical trends underlying the excitation and donor or acceptor spectra, provides predictions for unobserved excitations and donor or acceptor energies, and distinguishes the regime where one-electron theory is applicable (EEMc small) from the region where it is not (aEMc-aE)

We investigated theoretically the effect of covalent edge functionalization, with organic functional groups, on the electronic properties of graphene nanostructures and nanojunctions. Our analysis shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes, and to control the energy alignment of frontier orbitals in nanometer-wide graphene junctions. The stability of the proposed mechanism is discussed with respect to the functional groups, their number as well as the width of graphene nanostructures. The results of our work indicate that different level alignments can be obtained and engineered in order to realize stable all-graphene nanodevices

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