Dynamic nuclear polarisation of diamond

Abstract

This study is presented in nine chapters as follows: Chapter one reviews the reported literature on the NMR of natural diamond. The NMR signal of diamond consists on a single line at 39 ppm from TMS and two hyperfine lines due to 13C interactions. The reported relaxation times, measured in natural diamond, synthetic diamonds and 13C enriched diamonds, are discussed. The second chapter introduces the apparatus used, which included a Bruker Avance NMR spectrometer, a Bruker ESP380E pulsed EPR spectrometer and a high powersband DNP system. The availability of this excellently equiped laboratory presented a unique opportunity to perform this investigation. Chapter three outlines the experimental techniques used as well as the manner in which the acquired data was processed. The fourth chapter presents an overview of the most common defects found in diamond. Proposed models of these defects are presented and the resulting EPR spectra displayed. The methods developed to determine the paramagnetic impurity concentration from the EPR line width and the spin-spin relaxation times are presented in the fifth chapter. The line width gives the total paramagnetic impurity concentration to about 10 ppm. The spin-spin relaxation time allows the determination of Pl and P2 paramagnetic impurity concentrations individually, to much lower levels from measurements on the central and hyperfine lines. This information was used in the explanation of the relaxation behaviour for the various diamonds investigated. The temperature dependence of the paramagnetic electron relaxation times is reported in the sixth chapter. The results obtained are consistent with the findings in prior work that Pl impurities are typical Jahn Teller centres. Two diamonds, however, display trends that depart from this theory. These diamonds contain N3 defect centres, which appear to be responsible for this behaviour. It was found in these experiments that, bar thermal expansion effects, the spin-spin relaxation time is essentially independent of temperature. The seventh chapter deals with the solid state and thermal mixing effects. The relevant theory, results obtained and a discussion of these results, are presented. The effect of impurity concentration, defect types, microwave power, the exposure time and the offset from resonance on the polarisation rates and the 13C polarisation are investigated in depth. Finally the effect of applying the DNP treatment on the central and hyperfine lines is discussed. The pulsed DNP process is presented in the eighth chapter. The relevant theory, the effects of matching of the Hartmann-Hahn condition, impurity concentrations and types, on the polarisation rate and signal enhancement of JJC nuclei is given. A comparison to the continuous wave techniques is then made. The ninth chapter summarises the achievements and recommendations for further work.