Quantum confinement and interface structure of Si nanocrystals of sizes 3–5 nm embedded in a-SiO2

Spectroscopic ellipsometry and Monte Carlo simulations are employed to answer the fundamental question whether the energy gaps of Si nanocrystals with sizes in the range of 3–5 nm, which are embedded in amorphous silica, follow or deviate from the quantum confinement model, and to examine their interfacial structure. It is shown that the optical properties of these nanocrystals are well described by the Forouhi–Bloomer interband model. Analysis of the optical measurements over a photon-energy range of 1.5–5 eV shows that the gap of embedded nanocrystals with a mean size of 3.9 nm follows closely quantum confinement theory. A large band gap expansion (0.65 eV) compared to bulk Si is observed. The Monte Carlo simulations reveal a non-abrupt interface and a large fraction of interface oxygen bonds. This, in conjunction with the experimental observations, indicates that oxygen states and the chemical disorder at the interface have a negligible influence on the optical properties of the material in this size regime.     

Harmonic Analysis on Chord Arc Domains

In the present paper we study the solvability of the Dirichlet problem for second order divergence form elliptic operators with bounded measurable coefficients which are small perturbations of given operators in rough domains beyond the Lipschitz category. In our approach, the development of the theory of tent spaces on these domains is essential.     

Divergence form operators in Reifenberg flat domains

We study the boundary regularity of solutions of elliptic operators in divergence form with C ^0,α coefficients or operators which are small perturbations of the Laplacian in non-smooth domains. We show that, as in the case of the Laplacian, there exists a close relationship between the regularity of the corresponding elliptic measure and the geometry of the domain.     

HPLC determination of residual monomers released from heat-cured acrylic resins

HPLC was used to examine the leachability of three non-phthalic and four phthalic post-polymerized residual monomers from three commercially available heat-cured acrylic resins. Specimens of equal dimensions were constructed from each brand of material by following the standardized procedure and were stored under three different conditions, namely, distilled water, artificial saliva, and a binary mixture of ethanol/water. The resulting liquids provided samples for analysis by HPLC. Three different experiments were performed for each brand of acrylic and each storage condition in order to examine the effects of parameters, particularly time and temperature. The results obtained from this study suggest that a wide spectrum of residues diffuse out of the three examined acrylic resin materials. The non-phthalic compounds were leached at high concentrations, whereas all the phthalates examined exhibited different degrees of elusion commensurate with the storage condition, brand of material, and type of experiment. It seems that a significant quantity of non-phthalic and phthalic residues diffuse out of the acrylic resin materials examined. The main component extracted was methyl methacrylate, the level of which seems to be time-dependent and decreases for a period of up to 5 days when resins are stored in distilled water at room temperature.     

Improved chromium-catalyzed allylic oxidation of Δ-steroids with t-butyl hydroperoxide

Various chromium^VI-catalyzed conditions have been tested to improve the oxidation of Δ⁵-steroids with t-BuOOH to their corresponding 5-en-7-ones. The use of PDC or the association of CrO₃ with an amine as the catalyst and CH₂Cl₂ or PhCF₃ as the solvent led usually to the best yields. A minor reaction pathway was the epoxidation of the double bond.     

Use of ultrafast dispersed pump-dump-probe and pump-repump-probe spectroscopies to explore the light-induced dynamics of peridinin in solution

Optical pump-induced dynamics of the highly asymmetric carotenoid peridinin in methanol was studied by dispersed pump-probe, pump-dump-probe, and pump-repump-probe transient absorption spectroscopy in the visible region. Dispersed pump-probe measurements show that the decay of the initially excited S2 state populates two excited states, the S1 and the intramolecular charge-transfer (ICT) state, at a ratio determined by the excitation wavelength. The ensuing spectral evolution occurs on the time scale of a few picoseconds and suggests the equilibration of these states. Dumping the stimulated emission of the ICT state with an additional 800-nm pulse after 400- and 530-nm excitation preferentially removes the ICT state contribution from the broad excited-state absorption, allowing for its spectral characterization. At the same time, an unrelaxed ground-state species, which has a subpicosecond lifetime, is populated. The application of the 800-nm pulse at early times, when the S2 state is still populated, led to direct generation of the peridinin cation, observed for the first time in a transient absorption experiment. The excited and ground electronic states manifold of peridinin has been reconstructed using target analysis; this approach combined with the measured multipulse spectroscopic data allows us to estimate the spectra and time scales of the corresponding transient states.     

Plasmon-enhanced light emission based on lattice resonances of silver nanocylinder arrays

Diffractive arrays of silver nanocylinders are used to increase the radiative efficiency of InGaN/GaN quantum wells emitting at near-green wavelengths. Large enhancements in luminescence intensity (up to a factor of nearly 5) are measured when the array period exceeds the emission wavelength in the semiconductor material. The experimental results and related numerical simulations indicate that the underlying mechanism is a strong resonant coupling between the light-emitting excitons in the quantum wells and the plasmonic lattice resonances of the arrays. These excitations are particularly well suited to light-emission-efficiency enhancement, compared to localized surface plasmon resonances at similar wavelengths, due to their larger scattering efficiency and larger spatial extension across the sample area.     

Excited-state dynamics of carotenoids in light-harvesting complexes. 1. Exploring the relationship between the S1 and S* states

Dispersed transient absorption spectra collected at variable excitation intensities in combination with time-resolved signals were used to explore the underlying connectivity of the electronic excited-state manifold of the carotenoid rhodopin glucoside in the light-harvesting 2 complex isolated from Rhodopseudomonas acidophila. We find that the S state, which was recently identified as an excited state in carotenoids bound in bacterial light-harvesting complexes, exhibits a different response to the increase of excitation intensity than the S(1) state, which suggests that the models used so far to describe the excited states of carotenoids are incomplete. We propose two new models that can describe both the time-resolved and the intensity-dependent data; the first postulates that S(1) and S* are not populated in parallel after the decay of the initially excited S(2) state but instead result from the excitation of distinct ground-state subpopulations. The second model introduces a resonantly enhanced light-induced transition during excitation, which promotes population to higher-lying excited states that favors the formation of S* over S(1). Multiwavelength target analysis of the time-resolved and excitation-intensity dependence measurements were used to characterize the involved states and their responses. We show that both proposed models adequately fit the measured data, although it is not possible to determine which model is most apt. The physical origins and implications of both models are explored.