Many-body multiple quantum dynamics in dipolar coupled spin one-half systems, with an application to hydrogen networks in hydrogenated amorphous silicon.

摘要

The dynamics of multiple quantum (MQ) excitation in dipolar-coupled spin one-half nuclei is investigated. General properties of the even-order selective sequence are discussed, and general solutions for a three-body system are developed. Numerical simulations for larger systems of spins are also presented.; The microstructures of 2-3 hydrogen at.% hot-wire (HW) CVD a-Si:H (hydrogenated amorphous silicon) and 8-10 hydrogen at.% conventional plasma-enhanced CVD (glow discharge, or GD) a-Si:H are characterized by {dollar}rmsp1H{dollar} nuclear magnetic resonance (NMR). Significant differences are shown to exist between the hydrogen distribution in these two materials. Among other things, the broad resonance line in the HW a-Si:H is 50 kHz wide, which is much broader than that observed (22-36 kHz) in previously measured GD a-Si:H films. In HW a-Si:H, large H clusters account for 90% of the 2-3 H at.%, with the remaining H atoms more dispersed, but aggregated in a small volume fraction of the material. Computer simulations of the observed free induction decays (FID's) for the {dollar}rmsp1H{dollar} nuclei indicate that the distribution of the nearest-neighbor distances between H atoms in the H clusters is quite narrow in HW a-Si:H, whereas the distribution is larger in GD a-Si:H. This is clear evidence of improved structural order in HW a-Si:H. The relaxed hydrogenated divacancy and multi-vacancy models reproduce the main features of the observed FID in HW a-Si:H very well. Furthermore, computer simulations of the MQ NMR spectra indicate that a relaxed hydrogenated divacancy configuration leads lo good agreement with experimental observations in device quality GD a-Si:H. Results of simulations based on other H cluster configurations are also discussed, providing restrictions on the possible models for H clusters in a-Si:H. These results suggest that an ideal a-Si:H network with low defect density and high structural stability must be characterized by low hydrogen content, high structural order, and nonuniform H distribution.; Also, the dynamics of spin locking for half-integer quadrupolar nuclei under sample rotation is investigated. It is shown theoretically and experimentally that the spin locking efficiency decreases significantly when the MAS rotation frequency is equal to the amplitude of the spin locking RF field. This can be explained as a rotation-induced resonance effect. Furthermore, it is demonstrated that spin locking under first-order quadrupolar coupling with any given time dependence has a vector model interpretation which leads to an exact analytical expression for the signal intensity at any given time. If the second order quadrupolar interaction is stronger than the RF interaction, it is shown that the spin locking efficiency is reduced and a significant lineshape distortion occurs.