Burkhard E. Wagner

Paramagnetic defects in silicon/silicon dioxide systems

Abstract Paramagnetic defect centers in Si/SiO2 systems have been observed by direct ESR, optically-induced ESR, and NMR relaxation of liquids at the outer oxide surface. In general, all the defects reported elsewhere were confirmed, but with some significant discrepancies in character. The PB center was observable even at room temperature. The PC center was found to exist much deeper in the silicon than previously determined, and it is tentatively identified to be neutral iron. Surface liquid relaxation is very strong on oxidized crushed silicon, is not dependent on liquid composition, and suggests a strong wide-line spin center in the outer oxide surface. The optically activated spin center created by HF/HNO3 etches was found not to involve H2O or OH functionalities, and appears to be a nitrogenous radical. The optical defect center lies within the silicon, and its presence warrants caution in use of HNO3-based etches in wafer processing. Oxides prepared at elevated pressures show fewer PA and PC defects than those produced by conventional processing, which indicates potential merit in pressure oxidation methods.

Relaxation of fluorine nuclei by collisions with free radicals

Intermolecular nuclear relaxation during liquid‐state collisions has been examined for 16 combinations of four free radicals and four fluorocarbons. The spectra of electron‐induced scalar relaxation transitions are resolved into high‐ and low‐frequency components. All samples are found to have a spectral component with a short correlation time of around 1×10−11 sec, corresponding to fast diffusion‐controlled collisions. Those samples which show stronger scalar hyperfine coupling show a second component with time up to 44×10−11 sec, corresponding to sluggish stereospecific collisions or transient complexation. Strong scalar coupling is found to occur with a proportionally strong dipolar coupling, which is augmented well beyond that expected from feasible molecular approach distances. Monopoly of spin‐rich sites by the fluorocarbon might account for part of the dipolar augmentation but does not satisfactorily account for the entire range of observations. An induced anisotropic hyperfine coupling, after the ...

Dynamic polarization of carbon-13 nuclei in free radical solutions

Abstract In contrast to previously studied nuclei, solvent carbon-13 NMR enhancements at 74 G for a series of free radicals show increased contribution of scalar coupling for well shielded, saturated carbons. Enriched 13 CCl 4 with BDPA gives an ultimate enhancement of +850.

Hydrogen bonding in systems containing imidazoline nitroxide radicals

Hydrogen bonding between imidazoline nitroxide free radicals and hydrogen donors of varying strength [acetone, chloroform, trifluoroacetic acid (TFA)] has been examined using magnetic resonance techniques. Low‐field dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. Dipolar coupling was found to increase in the order acetone <CHCl3<TFA, while only TFA systems showed substantial scalar coupling. For acetone, observed dipolar relaxation rates are consistent with translational modulation and little, if any, complexation. With CHCl3, both translational and rotational modulation are required; the data suggest that CHCl3 forms weak hydrogen bonds with the radical chromophore with little spin density transferred to the bridge proton. In contrast, the protons of TFA show strong scalar coupling with the various radicals which results in positive proton NMR enhancements or strong hyperfine coupling (AH?5 G). The 1H scalar...

Characterization of silica surfaces by free radical spin labels

Abstract Silanol groups on the surface of various silicon oxide materials have been labeled for esr study by bonding or adsorption of free radicals. The number of bonded radicals was correlated with the relative concentrations of vicinal and isolated silanols. When the vicinal silanols have been removed by treatment above 600°C, the labeling was found to be quantitative and complete. The improved label attachment in the latter case indicates that steric crowding is not a consideration, but that the resistance of vicinal silanols to labeling reagents is responsible for limited labeling at low temperatures. The ratio of trichlorosilane di-labels to dichlorosilane mono-labels was observed to approach the theoretical limiting value 2.0 above 600°C, and the variation of this ratio may be a basis for quantitative determinations under conditions of incomplete labeling. In contrast to covalently bonded labels, adsorbed labels were found to attach to both vicinal and isolated silanols, and observed silanol concentrations are closely comparable to the bonded label values above 600°C. Labeling susceptibility of the silica surface was found to be drastically modified by pretreatment with inorganic and organometallic reagents. The potential usefulness of spin-labeling in electronics was illustrated by labeling of silanols in quartz surfaces (piezoelectric resonators), on oxidized silicon (MOS integrated circuits) and in evaluation of various surface passivation techniques.

Scalar dynamic nuclear polarization of protons in trifluoroacetic acid solutions

Abstract Strongly scalar 1 H intermolecular dynamic nuclear polarization (DNP) of solvent 1 H nuclei in solutions of organic free radicals has now been observed at room temperature. Transient protonation of the CF 3 COOH/pyridyl imidazoline-1-oxyl radical system accounts for the observed effects.

Complexing and protonation of free-radical imidazolin-1-oxyl and imidazolin-1-oxyl 3-oxide ligands: a magnetic-resonance investigation

Free-radical ligands consisting of 4,4,5,5-tetramethyl-Δ2-imidazolin-1-oxyl 3-oxide, (II), or 4,4,5,5-tetramethyl-Δ2-imidazolin-1-oxyl, (I), fragments with diamagnetic 2-substituents R {R = 2-pyridyi(py), 6-bromo-2-pyridyl (6-Brpy), 3-bromophenyl (3-BrPh), and Ph are described and their interactions with dosed-shell ions of nonzero nuclear spins (H+, Li+, Ag+, Zn2+, Cd2+, Hg2+, and Pb2+) are discussed. Unpaired spin density on the ligands is distributed over both imidazoline nitrogen atoms, and metal or proton hyperfine (h.f.) splittings appear in the e.s.r. spectra on interactions, so that electronic effects can be monitored. Protonations are fast on trie e.s.r. time scale (τca. 3 × 10–8), while complex lifetimes are long (τ 10–7). Interaction with Li+ does occur, but is too transient to be observable by e.s.r. spectroscopy. The derivatives (II) can be protonated but do not complex, while (I) both protonate and complex at the N(3) atom. For a given ligand e.s.r. spectrum, changes in 14N h.f. splittings on complexing are closely correlated with determined complex-formation constants, Kf. For (I; R = py), unpaired electron delocalization increases by as much as 0·5 A on co-ordination, and increases proportionately with Kf, as shown by matrix ENDOR. The diamagnetic 2-substituent R has a pronounced effect on the protonation and complexing interactions. Pseudo-first-order rate constants for protonation at N(3) in (I) increase from 3 × 106 to 1 × 109 l mol–1 s–1 on going from R = py to Ph, while complex Kf values decrease by one or two orders of magnitude. Electronic requirements on the imidazoline ring in chelated complexes of (I; R = py) are much smaller than in those of the unidentate ligands where R = 6-Brpy, 3-BrPh, and Ph, as shown by h.f. splittings of the e.s.r. spectra.

Dynamic nuclear polarization studies of intermolecular coupling of deuteron and unpaired electron spins

Observed 2H NMR enhancements with free radicals in solution are far below the dipolar limit. Leakage factors for the deuteron systems are much poorer than for the protonated analogs. This is the result of two factors: (1) unfavorable bulk nuclear relaxation times due to quadrupolar relaxation mechanisms, and (2) weak intermolecular coupling of nuclear and unpaired electron spins. Comparison of radical‐induced nuclear relaxation rates for deuterobenzene with the free radicals GALV and BDPA and the analogous benzene solutions demonstrates the (γN)2 dependence of the dipolar coupling strength. The consequences of these results on future studies for nuclei with I ≠ (1/2) and small γN are discussed.

Reactions of phenothiazine radical cations on ion-exchange resin surfaces. an esr flow study

Abstract The adsorption and reactivity of radical ions on the macroreticular ion-exchange resin Amberlyst 15 have been studied using an ESR flow technique. Diffusion of N-methylphenothiazine (1) to Fe3+-labeled resin bead surfaces, with subsequent electron transfer to produce adsorbed 1•+, occurs about as quickly as the methanol eluant can penetrate into the interior of the resin matrix. In contrast, reactions of adsorbed 1•+ with reagents in the incoming eluant stream are slowed, and second-order reaction rates decrease on changing the reagent from ascorbic acid to diethylamine to cyclohexene. The degree of stabilization of adsorbed 1•+ appears to reflect the differing radical cation mobility requirements in these three reactions. The adsorption and reactivity of unsubstituted phenothiazine, thianthrene, and phenoxathin radical cations have also been investigated. The ESR technique used in these studies should be applicable to other paramagnetic systems as well, providing information on diffusion, adsorption, electron transfer reactions, and other reactions occurring within porous substrates.

Spin density delocalization from a stable free-radical ligand onto diamagnetic metal ions

The novel air-stable, free-radical di-imine ligand 4,4,5,5-tetramethyl-2-(2-pyridyl)-Δ2–imidazoline-1-oxyl forms stable 1 : 1 complexes with bivalent, Zn, Cd, and Hg; upon complexation, e.s.r. ligand hyperfine couplings increase from 4·25 G to 5·2 G for the co-ordinating imine nitrogen, decrease from 9·25 G to 7·6 G for the nitroxide nitrogen, and metal hyperfine splitting of 9·6 G and 46 G is observed due to I=½ isotopes of Cd and Hg, respectively.