Studies of Energy-Relevant Materials by Nuclear Magnetic Resonance

Cui, Jinfang
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In this thesis, we have used nuclear magnetic resonance (NMR) as a local probe to microscopically study three different families of energy-relevant complex materials, namely the 122 Fe-based superconductors Ca(Fe1−xCox)2As2, GeTe-based thermoelectric tellurides GeTe and detonation nanodiamond.

In Chapter 3 and Chapter 4, we investigated the Co substitution effects on static and dynamic magnetic properties of the single-crystalline Ca(Fe1−xCox)2As2 (x = 0, 0.023, 0.028, 0.033, 0.059) via 75As NMR and resistivity measurements. Robustness of the Fe magnetic moments was evidenced by only slight decreases of Hint, although TN is strongly suppressed with Co substitution in antiferromagnetic (AFM) state. In the paramagnetic (PM) state, the temperature dependence of Knight shift K for all crystals shows similar T-dependence of magnetic susceptibility χ. The spin fluctuations with the q = 0 components are suppressed with ∆/kB. On the other hand, the growth of the stripe-type AFM fluctuations with q = (π, 0) or (0, π) upon cooling in the PM state for all samples is evidenced by the T-dependence of (1/T1Tχ). A pseudogap-like phenomenon, i.e., suppression of the AFM spin fluctuations, was discovered with decreasing temperature below a x-independent characteristic temperature T* (∼ 100 K) in samples with x ≥ 0.028. In addition, clear evidence for the coexistence and competition of the stripe-type antiferromagnetic and ferromagnetic (FM) spin correlations was given by modified Korringa ratio analysis in Chapter 4.

In Chapter 5, we have carried out 125Te NMR measurements to study the electronic properties of Ge50Te50, Ag2Ge48Te50 and Sb2Ge48Te50. NMR shift K and 1/T1T of Ge50Te50 are nearly temperature independent at T < 50 K and both increase slightly with increasing temperature at high temperatures. A two-band model, where one band overlaps the Fermi level and the other band is separated from the Fermi level by an energy gap, has been used to explain these behaviors. The first-principle calculation revealed that the metallic band originates from the Ge vacancy while the semiconductor-like band may be related to the fine structure of the density of states near the Fermi level. At low temperature, we found conduction carriers are free carriers with no significant electron correlations, while Korringa ratio increases slightly at high temperature, suggesting the slight enhancement of the electron correlation.

In Chapter 6 and Chapter 7, we have used 13C NMR spectral editing technique to accurately analyze the surface composition of pristine purified, heat-treated (at 800 ◦C), and air-oxidized detonation nanodiamond. We have resolved ten peaks of C=O, COO, C=C, O–C–O, C–OH, C–N, CH, subsurface C, core C, and C–C–N. The aromatic fraction is only ∼1.1%, which corresponds to less than 1/20 of an aromatic surface layer. We have also shown that other surface functional groups (CH, COH, etc.) accounts for most of the surface sites, making up ∼11.5% of all C in pristine nanodiamond. The signal of carbon bonded to nitroge was observed selectively based on increased chemical-shift anisotropy due to breaking of the local symmetry. Furthermore, we used modified 13C{1H} REDOR experiments to observe the signals from carbons at different depths from the surface and estimate their quantities.

Iron-based Superconductor, Nanodiamond, NMR Shift, Nuclear Magnetic Resonance, Relaxation, Thermoelectric