Recombination dynamics of free and bound excitons in bulk GaN

We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively.     

Optimization of instrumental neutron activation analysis for studying the chemical composition of silicate rocks

Software for optimization of instrumental neutron activation analysis of geological samples, preferentially silicate rocks, and a technique for evaluation and minimization of interference of gamma-ray lines are given. The maximum relevant signal-to-noise ratio was taken as a criterion for optimization by taking into account information available from a large number of lines in the gamma spectra studied.     

Optimization of instrumental neutron activation analysis for studying the chemical composition of silicate rocks

Software for optimization of instrumental neutron activation analysis of geological samples, preferentially silicate rocks, and a technique for evaluation and minimization of interference of gamma-ray lines are given. The maximum relevant signal-to-noise ratio was taken as a criterion for optimization by taking into account information available from a large number of lines in the gamma spectra studied. Received: 17 July 1997 / Revised: 7 November 1997 / Accepted: 16 November 1997     

AlGaN nanostructures with extremely high quantum yield at 300 K

Theoretical optimization of a quantum well heterostructure based on AlGaN solid solutions is implemented in order to attain the maximum charge carrier activation energy and the maximum exciton binding energy at a radiation wavelength of ~300 nm. An optimized structure sample with the radiative recombination dominating over the temperature range of 5 to 300 K and the room temperature internal quantum yield as high as 80% of the value measured at 5 K has been manufactured via plasma-assisted molecular beam epitaxy.     

Ferrocene-containing tri- and tetranuclear cyclic copper(<Emphasis Type="SmallCaps">i</Emphasis>) and silver(<Emphasis Type="SmallCaps">i</Emphasis>) pyrazolates

New tri- and tetranuclear macrocyclic silver(i) and copper(i) 3-ferrocenyl-5-(trifluoromethyl)pyrazolates were prepared: [{(3-((η⁵-C₅H₄)Fe(η⁵-C₅H₅))-5-(CF₃)-Pz}M]₃ (M = Cu (1), Ag (2)) and [{(3-(( η⁵-C₅H₄)Fe(η⁵-C₅H₅))-5-(CF₃)-Pz}Cu]₄ (3). The structures of compounds were established by X-ray diffraction analysis. In the crystalline state, a planar trinuclear silver-containing macrocycliс pyrazolate and a saddle-shaped tetranuclear copper-containing macrocycle are formed. The introduction of a bulky substituent, ferrocene, into the pyrazole ligand results in complete shielding of the acidic metal sites, which precludes the coordination of base molecules.     

Determination of absolute stereochemistry of natural alicyclic glycosides by 1H NMR spectroscopy without application of chiral reagents--an indication

A study has been made of the 1H NMR spectra of peracetylated beta-glucopyranosides and alpha-arabinopyranosides obtained by reaction of D- and L-glucoses, and L- and D-arabinoses with either (R)- or (S)-2-octanols. The obtained and literature data show that 1H NMR spectra may be used to determine the absolute configuration of the aglycone moieties of some alicyclic glycosides without the need to synthesize derivatives with chiral reagents, as long as the absolute configuration of their monosaccharide moiety is known or vice versa. Spectra of marine steroid glycosides and their acetates containing glycosylated side chains as alicyclic fragments were also examined. It was shown that analysis of 1H NMR spectra for the determination of the absolute configuration is more applicable in the cases when glycosides have the same substitution in the D-ring of the aglycone moiety.     

Influence of electron doping on the hydrogenation of fullerene C60 : a theoretical investigation

The influence of electron attachment on the stability of the mono- and dihydrogenated buckminsterfullerene C(60) was studied using density functional theory and semiempirical molecular orbital techniques. We have also assessed the reliability of computationally accessible methods that are important for investigating the reactivity of graphenic species and surfaces in general. The B3LYP and M06L functionals with the 6-311+G(d,p) basis set and MNDO/c are found to be the best methods for describing the electron affinities of C(60) and C(60)H(2) . It is shown that simple frontier molecular orbital analyses at both the AM1 and B3LYP/6-31G(d) levels are useful for predicting the most favourable position of protonation of C(60)H(-) , that is, formation of the kinetically controlled product 1,9-dihydro[60]fullerene, which is also the thermodynamically controlled product, in agreement with experimental and previous theoretical studies. We have shown that reduction of exo- and endo-C(60)H makes them more stable in contrast to the reduction of the exo,exo-1,9-C(60)H(2) , reduced forms of which decompose more readily, in agreement with experimental electrochemical studies. However, most other dihydro[60]fullerenes are stabilized by reduction and the regioselectivity of addition is predicted to decrease as the less stable isomers are stabilized more by the addition of electrons than the two most stable ones (1,9 and 1,7).     

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

A theoretical study on two series of electron‐rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen‐bonded adducts of [(PP₃)MH₂] and [Cp*MH(dppe)] (M=Fe, Ru, Os; PP₃=κ⁴‐P(CH₂CH₂PPh₂)₃, dppe= κ²‐Ph₂PCH₂CH₂PPh₂) with CF₃CH₂OH (TFE) as proton donor. The results are compared with those of adduct [Cp₂NbH₃] ? TFE featuring a “pure” dihydrogen bond, and classical hydrogen bonds in pyridine ? TFE and Me₃N ? TFE. Deviation of the H ??? H? A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH ??? HA interaction to be described as a bifurcate hydrogen bond. The impact of the M ??? HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton‐transfer reactions via MH ??? HA intermediates. Strengthening of the M ??? H interaction in the case of electron‐rich 5d metal hydrides leads to direct proton transfer to the metal atom.     

Coordination chemistry of diphenylphosphinoferrocenylthioethers on cyclooctadiene and norbornadiene rhodium(i) platforms

Complexes [RhCl(diene)(P,SR)] with chiral ferrocenyl phosphine-thioethers ligands (diene = norbornadiene, NBD, 1(R), or 1,5-cyclooctadiene, COD, 3(R); P,SR = CpFe(1,2-η(5)-C(5)H(3)(PPh(2))(CH(2)SR); R = tBu, Ph, Bz, Et) and the corresponding [Rh(diene)(P,SR)][BF(4)] (diene = NBD, 2(R); COD, 4(R)) have been synthesized from [RhCl(diene)](2) and the appropriate P,SR ligand. The molecular structure of the cationic complexes 2(tBu), 4(Ph) and 4(Bz), determined by single-crystal X-ray diffraction, shows the expected slightly distorted square planar geometry. For the neutral chloride complexes, a combination of experimental IR and computational DFT investigations points to an equally four coordinate square planar geometry with the diene ligand, the chlorine and the phosphorus atoms in the coordination sphere and with a dangling thioether function. However, a second isomeric form featuring a 5-coordinated square planar geometry with the thioether function placed in the axial position is easily accessible in some cases.     

Biexciton in II–VI quantum dots with different localization potentials

We present a comparative study of the influence of the form of a localization potential on the binding energy of the biexciton in spherically symmetric quantum dots based on II–VI compounds. The proposed criterion for the comparison of potentials of different forms—the box potential, the harmonic oscillator, and the Gaussian potential—is based on the identical localization of charge carriers of the same sign in these potentials. Calculations of the biexciton binding energy have been performed using the variational method within the framework of the kp-perturbation theory taking into account additional polarization terms in the wave functions of the electron and hole subsystems, as well as the complex structure of the valence band. The obtained results have demonstrated that the presence of a smoothly varying finite-height potential in Cd(Zn)Se/ZnSe quantum dots can lead to a more efficient localization in the case of the biexciton in comparison with the exciton, which is of interest for the implementation of fast-acting quantum light emitters.