Wanda Andreoni

N-type organic thin-film transistor with high field-effect mobility based on a N,N′-dialkyl-3,4,9,10-perylene tetracarboxylic diimide derivative

N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8H) thin films have been implemented into organic thin-film field-effect transistors. Mobilities up to 0.6 cm2 V−1 s−1 and current on/off ratios >105 were obtained. Linear regime mobilities were typically half of those measured in the saturation regime. X-ray studies in reflection mode suggest a spacing of ∼20 A for thin evaporated films of PTCDI-C8H, which is consistent with the value of ∼21±2 A obtained from our simulations when an interdigitated packing structure is assumed.

ALQ3 : AB INITIO CALCULATIONS OF ITS STRUCTURAL AND ELECTRONIC PROPERTIES IN NEUTRAL AND CHARGED STATES

Abstract A detailed density functional theory study is presented for the structural and electronic properties of Alq3, one of the electroluminescent material most successfully used in organic light-emitting diodes. Both geometrical isomers are considered, and interesting discrepancies are found in their electronic behavior. Calculations are compared with available experimental data and with the results of semiempirical models as well. The effects of hole and electron injection are clarified, and trap energies for the negative charge carriers are estimated.

Structural and electronic properties of sodium microclusters (n=2–20) at low and high temperatures: New insights from ab initio molecular dynamics studies

We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density‐functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low‐energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low‐energy isomers as well as the similarity of the structural pattern with that of Lennard‐Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘non...

Atom-resolved electronic spectra for Alq3 from theory and experiment

The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

Structural and Electronic Properties of La@C82

The structural and electronic properties of the La@C82 fullerene have been investigated by means of the Car-Parrinello method, which is based on the local density approximation of the density functional theory. The topological arrangement of the C82 cage was assumed to be a C3v symmetry isomer. Three configurations were considered, one with the lanthanum atom at the center of the cluster, one with it along the threefold axis, and one with it at a low-symmetry, highly coordinated site. The structure was fully relaxed and it was found that the last of these configurations is energetically preferred. In this position, the lanthanum atom is nearly in a La3+ state and the unpaired electron is somewhat delocalized on the cage, in agreement with available experimental data. This arrangement suggests that the chemical shifts of the 5s and 5p lanthanum states can be used as a structural probe and as a way of further validating this picture. It is argued that this conclusion is not affected by the assumed fullerene structure.

Impurity states in doped fullerenes: C59B and C59N

Abstract Using the Car—Parrinello method we have investigated theoretically the properties of the recently synthesized C 59 B and of the yet hypothetical C 59 N molecules. We show that doping induces important changes in both ionic and electronic properties of C 60 . The highest occupied molecular orbital is strongly localized and can act either as an electron acceptor state (C 59 B) or as a donor state (C 59 N). A parallel is drawn between this and the case of deep impurity states in semiconductors. Based on these results we speculate on potentially interesting properties of materials derived from these molecules.

Lithium–aluminum contacts for organic light-emitting devices

Organic light-emitting devices have been prepared with multilayer Al–Li–Al cathodes in an ultrahigh vacuum molecular beam deposition system. The optimum device characteristics are obtained when there is a single Al layer separating the Li layer from the organic materials. Ab initio molecular dynamics calculations of the Al–Li interaction clarify the role of Al as a blocking layer to Li diffusion into the organic films as well as the behavior of the device when the thickness of this Al interfacial layer is changed.

Gold and platinum microclusters and their anions: comparison of structural and electronic properties

Abstract We present a study of Au 2 to Au 5 and Pt 2 to Pt 5 clusters within density-functional theory in the neutral and anionic states. Results obtained using two exchange-correlation (xc) functionals, local spin density approximation and spin-polarized Becke–Lee–Yang–Parr (BLYP), are compared. The structural characteristics and relative stabilities of different isomers as well as electron affinities and vertical detachment energies are calculated. The latter compare generally well with experimental data, especially in the BLYP case. However, strong dependence of the vertical electron detachment energies on the isomer and on the xc-functional scheme indicates that special care must be taken for any comparison of theoretical results with experiment. Both for platinum and gold tetramers and pentamers, 3D geometries are unfavored. Triplet states are predicted to be more stable than singlets for platinum, for all sizes considered. The relative stability of different geometrical isomers is strongly affected by the addition of one electron. Marked differences emerge in the electronic structure between the clusters of these two metals.

New advances in chemistry and materials science with CPMD and parallel computing

Abstract A short overview is presented of the density functional theory and molecular dynamics (DFT–MD) method and of a code (CPMD) based on a plane wave scheme. Its power is shown through the survey of specific applications to diverse frontier areas of chemistry and materials science that make use of parallel computing.

Thirteen‐atom clusters: Equilibrium geometries, structural transformations, and trends in Na, Mg, Al, and Si

We report the results of an extensive structural study of Na13, Mg13, Al13, and Si13 carried out with the Car–Parrinello method. Several and mostly unforeseen noncrystalline structures are discovered to characterize the low portion of the potential energy surface. Crystalline structures are shown either to correspond to high‐energy local minima or to be highly unstable. The low‐energy structural pattern appears to change significantly from one element to the other. Specific characteristics as well as trends are discussed.