Showing papers by "Yue Wu published in 2001"

Confinement Effect on Dipole-Dipole Interactions in Nanofluids

Abstract: Intermolecular dipole-dipole interactions were once thought to average to zero in gases and liquids as a result of rapid molecular motion that leads to sharp nuclear magnetic resonance lines. Recent papers have shown that small residual couplings survive the motional averaging if the magnetization is nonuniform or nonspherical. Here, we show that a much larger, qualitatively different intermolecular dipolar interaction remains in nanogases and nanoliquids as an effect of confinement. The dipolar coupling that characterizes such interactions is identical for all spin pairs and depends on the shape, orientation (with respect to the external magnetic field), and volume of the gas/liquid container. This nanoscale effect is useful in the determination of nanostructures and could have unique applications in the exploration of quantum space.

Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film

Abstract: A nuclear magnetic resonance technique for precisely measuring the bulk magnetic susceptibility of micron-thick hydrogenated amorphous silicon (a-Si:H) film is introduced. The large disorder-induced diamagnetic enhancement exhibited by a-Si:H is shown to provide a sensitive bulk measurement for detecting variations in structural order in a-Si:H films. Furthermore, this approach is shown to be effective in revealing the details of microstructure in a-Si:H, including the presence of microstructural anisotropy.

Proton NMR and Magnetic Susceptibility in a-Si:H

Abstract: Proton nuclear magnetic resonance (NMR) is an important tool for characterizing the structure of a-Si:H over a wide range of length scales including short- and medium-range order as well as nanostructures. Some of the structural information obtained by NMR is based directly on proton NMR characteristics such as the lineshape and dipolar interactions that probe distances between hydrogen atoms and hydrogen environment. Based on such information it is shown that hydrogenated multi-vacancy model for Si-H clusters is fully consistent with both experimental and theoretical observations. It is demonstrated thatchanges of short- and medium-range order can be detected by measuring the magnetic susceptibility χ. Here i is not directly related to NMR characteristics and NMR is used merely as a sensitive and accurate magnetometer. The result indicates that a-Si:H prepared by hot-wire CVD (HWCVD) and plasma-enhanced CVD (PECVD) with high H-dilution have higher structural order compared to conventional a-Si:H. By carrying out NMR measurements on single a-Si:H thin film new NMR features were observed such as the orientation dependence of the proton NMR spectrum with respect to the magnetic field. Based on such orientation dependence, strong evidence of aligned nano-channels was obtained in some device quality a-Si:H films.