NMR Studies of Hydrogen-Hydrate Clathrates and Large Band-Gap Amorphous Semiconductors and Development of Ultra-High Sensitivity NMR

提出了对水合氢包合物结构和非晶Ga(Al)N膜的核磁共振研究，以了解其结构细节。包合物在储氢方面具有重要意义，非晶态氮化物是有用的电子材料。氢和氘核磁共振将用于研究一种新发现的笼形化合物，这种化合物在2000巴的温度下形成，氢（或氘）被保存在水冰框架的两个不同大小的笼子里。0.45:1的氢水比导致氢的有效载荷为5重量%，因此与储氢技术有关。笼形物为研究少量相互作用的H2分子提供了一个干净的环境：两个在小笼中，四个在大笼中。特别是，低温下的定向冻结和对位转化预计将与散装固体H2有很大不同，并将通过核磁共振进行检查。在较高的温度下，小笼和大笼之间的h2交换速率将通过核磁共振谱线变窄来确定。初步实验将研究氢和氘在普通散装水冰（Ih相）中的溶解。最终的目标是了解在高压下形成的1:1的氢：水的cII包合物。宽带隙GaN和AlGaN合金非晶态相的电子结构预测表明，与其他III-V型非晶态半导体相比，GaN和AlGaN合金的非晶态相具有无缺陷能带隙和更多的离域带尾。这些特性使得非晶态Ga(Al)N作为电子材料具有更大的潜力。本研究将利用核磁共振技术研究MBE生长的GaN和AlGaN薄膜中的局部非晶结构。这些实验与其他光电测量相结合，将用于确定局部原子和电子结构，以便与理论预测进行比较。通过培养研究生，这项研究将产生更广泛的影响。他们将学习磁共振，实验室高压技术，半导体制造和表征。这种广泛的知识和经验基础将加强科学工作队伍。氢被认为是汽车和卡车的理想燃料，燃烧时几乎没有污染，而且燃料电池的效率更高。但是机载氢存储仍然是一个未解决的问题。目前正在考虑的每一种技术都有一个或多个明显的缺点，如括号中所示：高压气瓶（安全性）、液氢（不切实际的-253℃温度）和金属氢化物（重量和费用）。这项提议的工作是研究一种新发现的冰和氢化合物，其中冰分子在2或4个氢分子周围形成笼子。虽然这种化合物本身不适合用于实际的储氢，但它可能指向更实用的系统。本研究将利用核磁共振来研究笼中氢分子的旋转运动及其在笼间的扩散。数据的高密度光存储需要短波长的光（蓝光或紫外线），因为最小的焦斑直径约为一个波长（衍射极限）。目前，大多数半导体光源和蓝光探测器都使用氮化材料的薄膜晶体（例如氮化镓）。然而，这种晶体薄膜的制造是昂贵的，并且仅限于几英寸的设备（不适合大型显示器）。最近的计算表明，非晶（非晶）氮化物可以提供合适的性能作为蓝色发射器和探测器；非晶材料的生长方法不受尺寸限制，而且相对便宜。我们将制备非晶氮化物材料，并利用核磁共振方法以及电学和光学技术表征其结构。这项研究将跨越传统的学科界限，为研究生提供培训。现实世界的问题（如储氢和氮化物半导体）需要多种技能和知识的结合。在建议的研究中，学生将学习高压技术、核磁共振（所有磁共振成像的基础）、半导体物理和半导体生长。

NMR studies on hydrogen-hydrate clathrate structures and amorphous Ga(Al)N films are proposed to understand their structural details. The clathrate compound is important from hydrogen storage aspects and amorphous nitrides are useful electronic materials. Hydrogen and deuterium NMR will be used to study a newly discovered clathrate compound, which forms at only 2000 bars and has hydrogen (or deuterium) held in two different size cages of an water-ice framework. The 0.45:1 hydrogen:water ratio results in a 5 weight-% payload of hydrogen and so is relevant to hydrogen-storage technologies. The clathrate offers a clean environment in which to study small numbers of interacting H2 molecules: two in the small cages and four in the large cages. In particular, orientational freezing at low temperatures and ortho-para conversion are expected to be profoundly different than for bulk solid H2 and will be examined by NMR. At higher temperatures, the rate of H2-exchange between the small and large cages will be determined from NMR line-narrowing. Preliminary experiments will investigate hydrogen and deuterium dissolved in ordinary bulk water-ice (phase Ih). An eventual goal is to understand the 1:1 hydrogen:water cII clathrate which forms at higher pressures. The predicted electronic structure of the amorphous phase of wide-band-gap GaN and AlGaN alloys indicates a defect-free energy band gap and more delocalized band tails than that found in other III-V amorphous semiconductors. These features make amorphous Ga(Al)N potentially more useful as electronic materials. The proposed research will use nuclear magnetic resonance to study the local amorphous structure in GaN and AlGaN thin films grown by MBE. These experiments combined with other optoelectronic measurements will be used to determine the local atomic and electronic structure for comparison with theoretical predictions. The research will produce broader impacts, through the training of graduate students. They will learn magnetic resonance, laboratory high-pressure techniques, and semiconductor fabrication and characterization. This broad base of knowledge and experience will strengthen the scientific workforce.Hydrogen has been proposed as an ideal fuel for automobiles and trucks, burning with essentially zero pollution and yielding higher efficiency in fuel cells. But the on-board storage of hydrogen remains an unsolved problem. Each of the technologies being considered now, has one or more substantial drawbacks indicated in parentheses: high-pressure gas cylinders (safety), liquid hydrogen (impractical temperature of -253 C), and metal-hydrides (weight and expense). The proposed work investigates a newly discovered compound of ice and hydrogen, in which ice molecules form cages around 2 or 4 hydrogen molecules. While this compound itself is not appropriate for practical hydrogen storage, it may point towards more practical systems. The present research will use nuclear magnetic resonance to study the rotational motions of the caged hydrogen molecules and how they diffuse from cage to cage. High-density optical storage of data requires short-wavelength light (blue or ultraviolet), because the smallest focal spot is about a wavelength in diameter (diffraction limit). Currently, most semiconducting sources and detectors of blue light use thin-film crystals of nitride materials (e.g., gallium nitride). However, the manufacture of such crystalline films is expensive and limited to devices of a few inches (not suitable for large displays). Recent calculations have indicated that amorphous (non-crystalline) nitrides may provide suitable performance as blue emitters and detectors; the growth methods for amorphous materials are not size limited and are comparatively inexpensive. We will prepare amorphous nitride materials and characterize their structures with nuclear magnetic resonance methods as well as electrical and optical techniques. The research will provide training to graduate students, crossing the traditional discipline boundaries. Real-world problems (such as hydrogen storage and nitride semiconductors) require a combination of diverse skills and knowledge. In the proposed research, students will learn high-pressure techniques, nuclear magnetic resonance (the basis for all magnetic resonance imaging, MRI), semiconductor physics, and semiconductor growth.