Anthony H. Francis

A photoluminescence study of C60 and C70

Abstract The luminescence spectra of C 60 and C 70 have been recorded in glassy toluene solution at 77 K and as neat solids at 5 K. The C 70 spectrum consists of two band systems each with resolved vibronic structure. The red system of C 70 begins at 15012 cm −1 , exhibits a lifetime of 1 → S 0 fluorescence. The infrared emission system begins at 12614 cm −1 , decays with a 30 ms lifetime and is assigned as T 1 → S 0 phosphorescence. A weak luminescence maximum at 13579 cm −1 is assigned tentatively as the S 1 → S 0 fluorescence of C 60 .

Is indium tin oxide a suitable electrode in organic solar cells? Photovoltaic properties of interfaces in organic p∕n junction photodiodes

The charge generation properties at all interfaces of a p∕n junction, bilayer photodiode have been investigated by means of the photoaction spectrum (PAS) as a function of applied bias. The organic photodiode was fabricated with a low-glass transition temperature (Tg) polysiloxane with pendant hydrazone groups as the p-type material and a perylene diimide derivative as the n-type material. The PAS under short circuit and reverse bias showed an antibatic response at the high-energy region (3.0–3.5eV), and a symbatic response at the low-energy region (2.0–3.0eV). However, under forward bias, the PAS showed the opposite behavior. These results are interpreted in terms of the band structure of tin-doped indium oxide (ITO) that prevents effective photoinjection of electrons at the polymer/ITO interface and the relative energy levels of the constituent materials.

The ground state fundamentals of p-benzoquinone and p-benzoquinone-d4

Abstract The solid state infrared and Raman spectra of p -benzoquinone and several isotopic derivatives are reported and compared with the results of previous solution and vapor phase studies and with the results of previously published normal coordinate analyses. The vibrational assignments are discussed in terms of the vibronic activity observed in the phosphorescence spectra of the individual isotopically related quinones in several host lattices at 2°K. The infrared and Raman inactive fundamentals of p -benzoquinone- h 4 obtained from vibronic analyses of the vapor phase resonance fluorescence are reported. The six Raman active lattice modes of p -benzoquinone- h 4 and - d 4 are assigned with the aid of isotope shifts and thermal-anisotropic-motion amplitudes from previously published X-ray diffraction data.

Evolution of structural and optical properties of ion-beam synthesized GaAsN nanostructures

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.

A particle-on-a-sphere model for C60

A perimeter model for the x electrons of C, is developed and implemented within the independent particle approximation. Physically, the model gives the energy levels of a particle on a sphere perturbed by the icosahedral potential set up by the 60 carbon atomic cores. Computational techniques are adapted from well known crystal field formalisms. The molecular orbitals are represented by linear combinations of spherical harmonics. One or two adjustable parameters are used to give reasonable agreement with the observed UV-visible spectrum and with semi-empirical and ab initio calculations at a similar level of approximation. A closed-shell ground state, appreciable HOMO-LUMO gap, and electric dipole forbidden HOMO-LUMO transition are predicted. 1. Intmduction Since its discovery in 1985 [ 11, CsO has been the subject of many quantum-mechanical calculations [ 2-91. Techniques ranging from HiIckel theory to rigorous Hartree-Fock SCF computations have predicted the two distinct bond lengths, closed-shell ground state and appreciable HOMO-LUMO gap observed experimentally. In addition, papers which report results at the orbital level are in agreement as to the ordering of the molecular orbitals in terms of energy. These calculations have provided much useful information about Cso but fail to give a simple conceptual basis for understanding the electronic structure of the molecule. Many of the authors have addressed this issue by noting the similarity between the energy levels of the C& R system and those of a particle on a sphere [2-4,7-g 1. We present here an elaboration of this idea, in which the K electrons are treated as independent particles sliding on the surface of a sphere onto which is impressed an icosahedral potential generated by point charges at the carbon atomic positions. The present treatment is restricted to this conceptually simple case and ignores electron correlation. The predictions are therefore limited to results at the orbital level of approximation.

Unusual photoluminescence of porous CdS (CdSe) crystals

Abstract Low temperature photoluminescence (PL) is used to study photoetched (PE) CdS (CdSe). This surface treatment produces a porous fractal-type morphology, with superior photovoltaic properties. While most of the usual features in the PL spectrum are suppressed after PE, a new broad spectral band is observed which is deeper than the original bound exciton (I2) line. In contrast with previously known centers, this band shows remarkably large shift as a function of light intensity, which can be described by scaling laws. A theoretical model is proposed, which considers the excitonic emission in porous media. According to this model, the coulombic energy of the exciton increases due to the reduced polarizability of the composite media. Good agreement is obtained between theory and experimental data, and self-consistency is established for the parameters of the theory.

The electronic absorption spectrum of crystallinechromyl chloride at 1.7°K

The spectrum of chromyl chloride, CrO 2 Cl 2 , in the region 6000 A-3800 A hasbeen obtained for the crystalline substance at temperatures down to 1.7°K. Three absorption systems have been observed, together with the start of a fourth system at λ

Spectroscopic and optically detected magnetic resonance studies of PO2−in potassium chloride. I. Electronic and vibrational states

Abstract The phosphorescence and phosphorescence excitation spectra of phosphorus activated single crystals of potassium chloride have been examined at 4.2 K. The “color center” responsible for the blue emission in these samples is identified as PO 2 − by isotopic substitution and optically detected magnetic resonance measurements. The observed vibrational frequencies of the PO 2 − ion have been used to compute the normal coordinates of the ground state totally symmetric modes as well as the valuence angle in the valence force approximation. The vibrational analysis of the phosphorescence spectrum has been used together with the computed ground state normal coordinate description to compute the geometrical changes associated with excitation to the lowest triplet state of PO 2 − . The results for PO 2 − are compared with the results of similar calculations and measurements for other 18-electron AB 2 type molecules of the isoelectronic sequence PO 2 − , SO 2 , NO 2 − and O 3 .

Selective separation of fullerenes on hydroxyphenyl-triphenylporphyrin-silica stationary phases

Abstract A novel immobilization method for the preparation of tetraphenylporphyrin–silica stationary phases is investigated. Stationary phases consisting of immobilized hydroxyphenyl-triphenylporphyrin (HPTPP) on silica are shown to exhibit unmatched selectivity (α=kC70′/kC60′) and improved efficiency for the separation of fullerenes, with α=7 using 100% toluene as the mobile phase. The HPTPP species are immobilized via reaction with glycidoxypropyltrimethoxysilane. The high shape selectivity of columns packed with HPTPP–silicas enables the single step separation of higher molecular mass fullerenes and higher fullerene isomers using strong fullerene solvents as mobile phases.

An optically detected magnetic resonance study of Mn4+ in Cs2GeF6 and K2GeF6

Abstract The photoexcitation and photoluminescence spectra (2E ↔ 4A2) of MnF62− diluted in Cs2GeF6 and K2GeF6 host lattices have been recorded at liquid helium temperature. The manganese coordination geometry is Oh in Cs2GeF6 and D3d in K2GeF6. Optically detected magnetic resonance (ODMR) has been employed to determine the g value of the 2E state in both coordination geometries. The trigonal distortion has a pronounced effect on the g value and produces a measurable splitting in the optical spectrum, but exerts relatively little effect on the radiative lifetime. The g value in the Oh site (g(2E) = 2.0) is as predicted theoretically; however, the value obtained for the D3d site is unexpectedly large (g(2 E ) = 2.95). The data obtained are compared with theoretical predictions and the results of similar studies of d3 ions in a variety of lattices. The differences are shown to arise, in part, from the relative magnitude of the spin-orbit coupling and the trigonal field splitting.