Unambiguous identification of the role of a single Cu atom in the ZnO structured green band

High quality and purity single crystal ZnO samples doped with single isotopes of (63)Cu and (65)Cu, with equal concentrations of both these isotopes, and with natural Cu using a wet chemical atomic substitution reaction and anneal were studied using low temperature optical spectroscopy. Our data on the zero phonon line of the structured green band in ZnO confirm unambiguously the involvement of a single Cu atom in this defect emission. These data allow us to confirm the main features of the assignment proposed by Dingle in 1969 and to comment further on the defect structure.     

Photoluminescence analysis of semiconductors using radioactive isotopes

The combination of photoluminescence spectroscopy with the radioactive isotopes ⁷Be, ⁷¹As, ¹¹¹Ag, ¹¹¹In, ¹⁹¹Pt, ¹⁹³Au and ¹⁹⁷Hg is shown to provide definitive proof of the chemical identity of impurities producing photoluminescence spectra in all classes of semiconductors. The isotope ⁷¹As is used to show that radioactive isotopes can provide a powerful means of producing and studying a fundamental crystal defect such as an anti-site. Factors governing the luminescence intensities which can lead to apparently anomalous results are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photoelectron spectroscopy of phosphites and phosphates

The ultraviolet photoelectron spectra (UPS) of trimethyl and triethyl phosphite, trimethyl and triethyl phosphate and four substituted phosphates are presented. Assignments are based on analogies to the UPS of phosphorus trichloride and phosphoryl trichloride and are substantiated by CNDO/2 computations. The mechanism of PO (axial) bond formation is discussed.     

Differential pulse polarographic determination of cephalosporins and their degradation products

The differential pulse polarography (d.p.p.) of several cephalosporins — cephalothin, cephalosporin C, cephaloridine, cephalonium, cefuroxime, cephoxazole, cephalexin, cephradine, desacetylcephalosporin C, 7-aminocephalosporanic acid and 7-aminodesacetoxy-cephalosporanic acid — and some degradation products has been studied. Cephalexin, cephradine and 7-aminodesacetoxycephalosporanic acid, which contain an unsubstituted 3-methyl group but no reducible group, do not give d.p.p. peaks at the dropping mercury electrode, whereas certain of their degradation products do. The other cephalosporins, in which the 3-methyl group is substituted, give a d.p.p. peak at about—1 V (pH 2–4). Although these peaks are near the cathodic limit under the conditions used, these cephalosporins can be determined down to about 0.1 μg ml^-1. Cephaloridine exhibits two d.p.p. peaks in this region, and can be determined. Cefuroxime, in addition to the peak at —IV, has a larger peak at —0.45 V (pH 2). Cephalonium shows a large well-defined peak at —0.72 V (pH 3) and two overlapping peaks. D.p.p. data for several degradation products including the diketopiperazines formed from cephalexin and cephradine are reported.     

Electronic spectroscopy of highly-polar aromatics. V. The polar excimer of N,N-dialkyl-p-cyanoaniline

The fluorescence spectra of N, N-dialkyl-p-cyanoanilines in fluid media have been re-examined. The long-wavelength fluorescence, previously attributed to a solvated ¹L_a → ¹A₁ fluorescence, is interpreted as an excimer fluorescence, an assignment which is supported by concentration dependence studies and quenching experiments. The temperature dependence of the excimer/ monomer fluorescence intensity ratio indicates that excimer formation is a diffusion controlled process. The binding energy of the excimer is found to be independent of the nature of the solvent and to be smaller than those values estimated for excimers of polycyclic aromatic hydrocarbons. Solvent viscosity, solvent polarity and specific solvent-solute interactions affect the excimer/monomer fluorescence intensity and these effects are interpreted. The geometry of the excimer is also discussed.     

A copper-catalyzed Petasis reaction for the synthesis of tertiary amines and amino esters

We have developed a copper-catalyzed process for the coupling of aldehydes, amines, and boronic acids. This allows greater reactivity with simple aryl boronic acids and allows coupling reactions to proceed that previously failed. Initial mechanistic studies support a process involving transmetalation from boron to copper.     

Defect-mediated ferromagnetism in ZnO:Mn nanorods

In this work, the structural, chemical and magnetic properties of ZnO:Mn nanorods were investigated. Firstly, well-aligned ZnO nanorods with their long axis parallel to the crystalline c-axis were successfully grown by the vapor phase transport technique on Si substrates coated with a ZnO buffer layer. Mn metal was then diffused into these nanorods at different temperatures in vacuum. From SEM results, ZnO:Mn nanorods were observed to have diameters of ～100 nm and lengths of 4 μm. XPS analysis showed that the Mn dopant substituted into the ZnO matrix with a valence state of +2. Magnetic measurements performed at room temperature revealed that undoped ZnO nanorods exhibit ferromagnetic behavior which may be related to oxygen vacancy defect-mediated d ⁰ ferromagnetism. ZnO:Mn samples were seen to show an excess room temperature ferromagnetism that is attributed to the presence of oxygen vacancy defects forming bound magnetic polarons involving Mn.     

Photoelectron spectroscopy of some biological molecules

The ultraviolet photoelectron spectra (UPS) of the following biologically-active compounds are reported and/or assigned: 2,4-dinitrophenol, nicotinic acid, nicotinamide, barbituric acid, xanthine, hypoxanthine, uric acid, uracil, thymine, cytosine, adenine, guanine, β-carotene, menadione, purine and pyrimidine. The importance of UPS data is exemplified in two ways: First, by investigating the validity of the Pullman k-index approach ¹⁴ to ionization energies; and, second, by generating an experimental scale of electron-donating ability.     

Electronic states and “Geometric Isomerism” of p-disubstituted cyanoanilines

Computations of CNDO/s CI type show that the two lowest energy ¹Γ

states of p-disubstituted cyanobenzenes are 1¹B₁(ππ^*) and 2¹A₁(ππ^*), in order of increasing energy. If the (CH₃)₂N-group of N,N-dimethyl-p-cyanoaniline (NNDMPCA or dimethylaminobenzonitrile) is twisted out of the plane of the benzenoid ring, the two transitions 1¹B₁ ← 1¹A₁ and 2¹A₁ ← 1¹A₁ undergo an increase of energy and a decrease of intensity. The postulated state in which an electron is promoted from the twisted (90°) (CH₃)₂N-group to a vacant in-plane π_χ^* orbital of the cyano group is predicted to lie ≈35409 cm^?1 above the 1¹B₁(ππ^*) state. This prediction contradicts an essential postulate of the excited state isomerization model which has been used to interpret the anomalous fluorescence behavior of NNDMPCA.