Substrate/semiconductor interface effects on the emission efficiency of luminescent polymers

The importance of interface effects for organic devices has long been recognized, but getting detailed knowledge of the extent of such effects remains a major challenge because of the difficulty in distinguishing from bulk effects. This paper addresses the interface effects on the emission efficiency of poly(p-phenylene vinylene) (PPV), by producing layer-by-layer (LBL) films of PPV alternated with dodecylbenzenesulfonate. Films with thickness varying from similar to 15 to 225 nm had the structural defects controlled empirically by converting the films at two temperatures, 110 and 230 degrees C, while the optical properties were characterized by using optical absorption, photoluminescence (PL), and photoluminescence excitation spectra. Blueshifts in the absorption and PL spectra for LBL films with less than 25 bilayers (<40-50 nm) pointed to a larger number of PPV segments with low conjugation degree, regardless of the conversion temperature. For these thin films, the mean free-path for diffusion of photoexcited carriers decreased, and energy transfer may have been hampered owing to the low mobility of the excited carriers. The emission efficiency was then found to depend on the concentration of structural defects, i.e., on the conversion temperature. For thick films with more than 25 bilayers, on the other hand, the PL signal did not depend on the PPV conversion temperature. We also checked that the interface effects were not caused by waveguiding properties of the excited light. Overall, the electronic states at the interface were more localized, and this applied to film thickness of up to 40-50 nm. Because this is a typical film thickness in devices, the implication from the findings here is that interface phenomena should be a primary concern for the design of any organic device. (C) 2011 American Institute of Physics. [doi:10.1063/1.3622143]

Directed Abelian algebras and their application to stochastic models

With each directed acyclic graph (this includes some D-dimensional lattices) one can associate some Abelian algebras that we call directed Abelian algebras (DAAs). On each site of the graph one attaches a generator of the algebra. These algebras depend on several parameters and are semisimple. Using any DAA, one can define a family of Hamiltonians which give the continuous time evolution of a stochastic process. The calculation of the spectra and ground-state wave functions (stationary state probability distributions) is an easy algebraic exercise. If one considers D-dimensional lattices and chooses Hamiltonians linear in the generators, in finite-size scaling the Hamiltonian spectrum is gapless with a critical dynamic exponent z=D. One possible application of the DAA is to sandpile models. In the paper we present this application, considering one- and two-dimensional lattices. In the one-dimensional case, when the DAA conserves the number of particles, the avalanches belong to the random walker universality class (critical exponent sigma(tau)=3/2). We study the local density of particles inside large avalanches, showing a depletion of particles at the source of the avalanche and an enrichment at its end. In two dimensions we did extensive Monte-Carlo simulations and found sigma(tau)=1.780 +/- 0.005.

Substrate orientation effect on potential fluctuations in multiquantum wells of GaAs/AlGaAs

The photoluminescence (PL) technique as a function of temperature and excitation intensity was used to study the optical properties of multiquantum wells (MQWs) of GaAs/Al(x)Ga(1-x)As grown by molecular beam epitaxy on GaAs substrates oriented in the [100], [311]A, and [311]B directions. The asymmetry presented by the PL spectra of the MQWs with an apparent exponential tail in the lower-energy side and the unusual behavior of the PL peak energy versus temperature (blueshift) at low temperatures are explained by the exciton localization in the confinement potential fluctuations of the heterostructures. The PL peak energy dependence with temperature was fitted by the expression proposed by Passler [Phys. Status Solidi B 200, 155 (1997)] by subtracting the term sigma(2)(E)/k(B)T, which considers the presence of potential fluctuations. It can be verified from the PL line shape, the full width at half maximum of PL spectra, the sigma(E) values obtained from the adjustment of experimental points, and the blueshift maximum values that the samples grown in the [311]A/B directions have higher potential fluctuation amplitude than the sample grown in the [100] direction. This indicates a higher degree of the superficial corrugations for the MQWs grown in the [311] direction. (C) 2008 American Institute of Physics.

The role of quantum confinement and crystalline structure on excitonic lifetimes in silicon nanoclusters

The emission energy dependence of the photoluminescence (PL) decay rate at room temperature has been studied in Si nanoclusters (Si-ncl) embedded in Si oxide matrices obtained by thermal annealing of substoichiometric Si oxide layers Si(y)O(1-y), y=(0.36,0.39,0.42), at various annealing temperatures (T(a)) and gas atmospheres. Raman scattering measurements give evidence for the formation of amorphous Si-ncl at T(a)=900 degrees C and of crystalline Si-ncl for T(a)=1000 degrees C and 1100 degrees C. For T(a)=1100 degrees C, the energy dispersion of the PL decay rate does not depend on sample fabrication conditions and follows previously reported behavior. For lower T(a), the rate becomes dependent on fabrication conditions and less energy dispersive. The effects are attributed to exciton localization and decoherence leading to the suppression of quantum confinement and the enhancement of nonradiative recombination in disordered and amorphous Si-ncl. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3457900]

Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry

A recently developed thermal lens spectrometry configuration has been used to study CdSe/ZnS core-shell quantum dots (QDs) suspended in toluene and tetrahydrofuran (THF) solvents. The special features of this configuration make it very attractive to measure fluorescence quantum yield (eta) excitation spectrum since it simplifies the measurement procedure and consequently improve the accuracy. Furthermore, the precision reached is much higher than in conventional photoluminescence (PL) technique. Two methods, called reference sample and multiwavelength have been applied to determine eta, varying excitation wavelength in the UV-visible region (between 335-543 nm). The eta and PL spectra are practically independent of the excitation wavelength. For CdSe/ZnS QDs suspended in toluene we have obtained eta=76 +/- 2%. In addition, the aging effect on eta and PL has been studied over a 200 h period for QDs suspended in THF. (C) 2010 American Institute of Physics. [doi:10.1063/1.3343517]

4-{2-[4-(Dimethylamino)phenyl]ethylidene}benzonitrile

In the crystal of the title compound, C(17)H(16)N(2), molecules are linked by C-H center dot center dot center dot N hydrogen bonds, forming rings of graph-set motifs R(2)(1) (6) and R(2)(2) (10). The title molecule is close to planar, with a dihedral angle between the aromatic rings of 0.6 (1)degrees. Torsion angles confirm a conformational trans structure.

N-(4-chlorophenyl)maleimide

In the title compound, C10H6ClNO2, the dihedral angle between the benzene and maleimide rings is 47.54 (9)degrees. Molecules form centrosymmetric dimers through C-H center dot center dot center dot O hydrogen bonds, resulting in rings of graph- set motif R2 2(8) and chains in the [100] direction. Molecules are also linked by C-H center dot center dot center dot Cl hydrogen bonds along [001]. In this same direction, molecules are connected to other neighbouring molecules by C-H center dot center dot center dot O hydrogen bonds, forming edge- fused R-4(4)(24) rings.

On the k-restricted structure ratio in planar and outerplanar graphs

A planar k-restricted structure is a simple graph whose blocks are planar and each has at most k vertices. Planar k-restricted structures are used by approximation algorithms for Maximum Weight Planar Subgraph, which motivates this work. The planar k-restricted ratio is the infimum, over simple planar graphs H, of the ratio of the number of edges in a maximum k-restricted structure subgraph of H to the number edges of H. We prove that, as k tends to infinity, the planar k-restricted ratio tends to 1/2. The same result holds for the weighted version. Our results are based on analyzing the analogous ratios for outerplanar and weighted outerplanar graphs. Here both ratios tend to 1 as k goes to infinity, and we provide good estimates of the rates of convergence, showing that they differ in the weighted from the unweighted case.

An algorithmic Friedman-Pippenger theorem on tree embeddings and applications

An (n, d)-expander is a graph G = (V, E) such that for every X subset of V with vertical bar X vertical bar <= 2n - 2 we have vertical bar Gamma(G)(X) vertical bar >= (d + 1) vertical bar X vertical bar. A tree T is small if it has at most n vertices and has maximum degree at most d. Friedman and Pippenger (1987) proved that any ( n; d)- expander contains every small tree. However, their elegant proof does not seem to yield an efficient algorithm for obtaining the tree. In this paper, we give an alternative result that does admit a polynomial time algorithm for finding the immersion of any small tree in subgraphs G of (N, D, lambda)-graphs Lambda, as long as G contains a positive fraction of the edges of Lambda and lambda/D is small enough. In several applications of the Friedman-Pippenger theorem, including the ones in the original paper of those authors, the (n, d)-expander G is a subgraph of an (N, D, lambda)-graph as above. Therefore, our result suffices to provide efficient algorithms for such previously non-constructive applications. As an example, we discuss a recent result of Alon, Krivelevich, and Sudakov (2007) concerning embedding nearly spanning bounded degree trees, the proof of which makes use of the Friedman-Pippenger theorem. We shall also show a construction inspired on Wigderson-Zuckerman expander graphs for which any sufficiently dense subgraph contains all trees of sizes and maximum degrees achieving essentially optimal parameters. Our algorithmic approach is based on a reduction of the tree embedding problem to a certain on-line matching problem for bipartite graphs, solved by Aggarwal et al. (1996).

TESTING STATISTICAL HYPOTHESIS ON RANDOM TREES AND APPLICATIONS TO THE PROTEIN CLASSIFICATION PROBLEM

Efficient automatic protein classification is of central importance in genomic annotation. As an independent way to check the reliability of the classification, we propose a statistical approach to test if two sets of protein domain sequences coming from two families of the Pfam database are significantly different. We model protein sequences as realizations of Variable Length Markov Chains (VLMC) and we use the context trees as a signature of each protein family. Our approach is based on a Kolmogorov-Smirnov-type goodness-of-fit test proposed by Balding et at. [Limit theorems for sequences of random trees (2008), DOI: 10.1007/s11749-008-0092-z]. The test statistic is a supremum over the space of trees of a function of the two samples; its computation grows, in principle, exponentially fast with the maximal number of nodes of the potential trees. We show how to transform this problem into a max-flow over a related graph which can be solved using a Ford-Fulkerson algorithm in polynomial time on that number. We apply the test to 10 randomly chosen protein domain families from the seed of Pfam-A database (high quality, manually curated families). The test shows that the distributions of context trees coming from different families are significantly different. We emphasize that this is a novel mathematical approach to validate the automatic clustering of sequences in any context. We also study the performance of the test via simulations on Galton-Watson related processes.

Gibbs random graphs on point processes

Consider a discrete locally finite subset Gamma of R(d) and the cornplete graph (Gamma, E), with vertices Gamma and edges E. We consider Gibbs measures on the set of sub-graphs with vertices Gamma and edges E` subset of E. The Gibbs interaction acts between open edges having a vertex in common. We study percolation properties of the Gibbs distribution of the graph ensemble. The main results concern percolation properties of the open edges in two cases: (a) when Gamma is sampled from a homogeneous Poisson process; and (b) for a fixed Gamma with sufficiently sparse points. (c) 2010 American Institute of Physics. [doi:10.1063/1.3514605]

An oracle approach for interaction neighborhood estimation in random fields

We consider the problem of interaction neighborhood estimation from the partial observation of a finite number of realizations of a random field. We introduce a model selection rule to choose estimators of conditional probabilities among natural candidates. Our main result is an oracle inequality satisfied by the resulting estimator. We use then this selection rule in a two-step procedure to evaluate the interacting neighborhoods. The selection rule selects a small prior set of possible interacting points and a cutting step remove from this prior set the irrelevant points. We also prove that the Ising models satisfy the assumptions of the main theorems, without restrictions on the temperature, on the structure of the interacting graph or on the range of the interactions. It provides therefore a large class of applications for our results. We give a computationally efficient procedure in these models. We finally show the practical efficiency of our approach in a simulation study.

The Lobel-Komlos-Sos conjecture for trees of diameter 5 and for certain caterpillars

Loebl, Komlos, and Sos conjectured that if at least half the vertices of a graph G have degree at least some k is an element of N, then every tree with at most k edges is a subgraph of G. We prove the conjecture for all trees of diameter at most 5 and for a class of caterpillars. Our result implies a bound on the Ramsey number r( T, T') of trees T, T' from the above classes.

Morpholin-4-ium morpholine-4-carbodithioate

The title compound, C(4)H(10)NO(+)center dot C(5)H(8)NOS(2)(-), is built up of a morpholinium cation and a dithiocarbamate anion. In the crystal, two structurally independent formula units are linked via N-H center dot center dot center dot S hydrogen bonds, forming an inversion dimer, with graph-set motif R(4)(4)(12).

Skewness, splitting number and vertex deletion of some toroidal meshes

The skewness sk(G) of a graph G = (V, E) is the smallest integer sk(G) >= 0 such that a planar graph can be obtained from G by the removal of sk(C) edges. The splitting number sp(G) of C is the smallest integer sp(G) >= 0 such that a planar graph can be obtained from G by sp(G) vertex splitting operations. The vertex deletion vd(G) of G is the smallest integer vd(G) >= 0 such that a planar graph can be obtained from G by the removal of vd(G) vertices. Regular toroidal meshes are popular topologies for the connection networks of SIMD parallel machines. The best known of these meshes is the rectangular toroidal mesh C(m) x C(n) for which is known the skewness, the splitting number and the vertex deletion. In this work we consider two related families: a triangulation Tc(m) x c(n) of C(m) x C(n) in the torus, and an hexagonal mesh Hc(m) x c(n), the dual of Tc(m) x c(n) in the torus. It is established that sp(Tc(m) x c(n)) = vd(Tc(m) x c(n) = sk(Hc(m) x c(n)) = sp(Hc(m) x c(n)) = vd(Hc(m) x c(n)) = min{m, n} and that sk(Tc(m) x c(n)) = 2 min {m, n}.