Positron Spectrosopy of 2-D and Nanostructured Materials

非技术摘要该项目采用一束低能正电子（电子的反物质）来提供关于包括石墨烯和纳米多孔金在内的新型纳米尺寸材料的独特信息。 当正电子与电子湮灭时，正电子和电子都消失了，两个粒子的质量完全转换成伽马射线形式的能量。 从原子中突然移除电子可以导致高度激发态和称为俄歇电子的电子发射。 湮灭感应伽马射线和俄歇电子的能量提供了正电子与其电子湮灭的原子的化学信息。 研究人员先前由NSF资助的研究表明，低能正电子可以用于选择性地探测表面原子顶层的性质，因为低能位置在湮灭时被捕获在材料表面的可能性很高。该项目的研究旨在探索使用正电子光谱来探测石墨烯等二维材料。 这些材料非常薄。 探测这些材料的最顶层原子层的能力为研究这些材料提供了一种独特的非破坏性方法，以了解吸附原子可用于改变这些材料的重要化学和物理性质的机制。 该项目的另一个重要目标是确定使用伽马射线的可行性，这些伽马射线来自于被称为纳米多孔金属的新型材料内部纳米尺寸孔的内表面处的正电子湮灭，以提供理解这些材料可以大大加速所需化学反应的原因所需的信息。 伽马射线的穿透性质允许在内表面上进行测量。该活动的成果将最终提高基于低能正电子束的光谱技术在化学和生物科学以及工程研究中的使用。该项目为研究生和本科生提供与高科技工业表面科学密切相关的领域的培训，包括低维物理，超高真空技术，带电粒子束，辐射探测，纳米结构合成，低能正电子对材料表面极其敏感，这是由于它们将被捕获在材料表面处的像势状态中的高概率。湮灭的时刻 该项目的研究旨在了解低能正电子（~ 1 eV）与包括石墨烯/氧化石墨烯片在内的二维材料的相互作用。 正电子在二维材料上的捕获将使用称为俄歇介导的粘附的俄歇过程以及通过正电子湮灭诱导的俄歇电子发射和正电子素形成来研究。正电子粘附到石墨烯和氧化石墨烯表面的概率的测量旨在提供关于这些材料的对比表面性质的信息，并证明使用低能量正电子作为二维结构的多参数光谱技术的可行性。来自石墨烯的电子的低背景发射预期将揭示在由正电子湮没产生的俄歇发射中看到的低能电子尾的物理起源。 该项目的第二个主要目标是探索使用多普勒展宽伽马光谱探测纳米多孔金属内表面的能力。 正电子被捕获在内表面的高概率与伽马射线从内表面逃逸的能力相结合，表明正电子可用于表征孔的内表面。这些研究的目的是使用从正电子光谱学获得的独特信息来理解气体与纳米多孔金属的应变内表面的相互作用。这些研究将允许更好地控制这些新材料的催化活性，以及有助于使用纳米多孔金属的高密度储氢材料的开发。该项目是首次探索使用正电子束研究功能化石墨烯、氧化石墨烯片和纳米多孔金属。

Non-Technical AbstractThis project employs a beam of low energy positrons (anti-matter of electrons) to provide unique information about novel nano-dimensioned materials including Graphene and nano-porous gold. When positrons annihilate with electrons both the positron and the electron disappear and the mass of the two particles is converted completely into energy in the form of gamma rays. The sudden removal of the electron from an atom can result in a highly excited state and the emission of electrons called Auger electrons. The energy of the annihilation induced gamma rays and Auger electrons provide chemical information about the atoms with whose electrons the positron annihilates. Previous NSF funded research by the investigators has shown that low energy positrons can be used to selectively probe the properties of the top layer of atoms at the surface due to the high probability of a low energy position becoming trapped at the surfaces of materials at the time of annihilation. The research in this project is aimed at exploring the use of positron spectroscopy to probe two dimensional materials like graphene. These materials are extremely thin. The ability to probe the topmost atomic layers of these materials provides a unique, non-destructive method for studying these materials in order to understand the mechanisms by which adsorbed atoms can be used to change important chemical and physical properties of these materials. Another important goal of the project is to determine the feasibility of using gamma rays from positrons annihilating at the internal surfaces of nanometer sized pores inside a novel class of materials called nanoporous metals to provide the information needed to understand the reason why these materials can greatly speed up desirable chemical reactions. The penetrating nature of the gamma rays permits measurement to be made on internal surfaces. The outcome of the activity will ultimately enhance the usage of low energy positron beam based spectroscopic techniques for research in chemical and biological sciences as well as in engineering. The project provides training to graduate and undergraduate students in fields of great relevance to high tech industry surface science, including low dimensional physics, ultra high vacuum technology, charged particle beams, radiation detection, nanostructure synthesis, material characterization.Technical AbstractLow energy positrons are extremely sensitive to material surfaces due to a high probability that they will become trapped in an image potential state at the time of annihilation. The research in this project is aimed at understanding the interaction of low energy positrons (~ 1eV) with two dimensional materials including graphene/graphene oxide sheets. The trapping of positrons on two dimensional materials will be investigated using Auger like process called Auger mediated sticking as well as through positron annihilation induced Auger electron emission and Positronium formation. Measurements of positron sticking probability to graphene and graphene oxide surfaces are aimed at providing information on the contrasting surface properties of these materials and demonstrate the feasibility of using low energy positrons as a multi-parameter spectroscopic technique for two dimensional structures. Low background emission of electrons from graphene is expected to throw light on the physical origin of low energy electron tail seen in Auger emission produced by positron annihilation. A second major goal of this project is to explore the ability of using Doppler broadening gamma spectroscopy to probe the internal surfaces of nano-porous metals. The high probability that positrons will become trapped at the internal surfaces combined with the ability of gamma rays to escape from the internal surfaces indicate that positrons can be used to characterize the internal surfaces of the pores. These studies are aimed at using the unique information obtained from positron spectroscopy to understand the interaction of gases with the strained internal surfaces of nanoporous metals. These studies will allow for better control of catalytic activity of these novel materials as well as contribute to the development of high density hydrogen storage materials using nanoporous metals. The project represents the first exploration of the use of positron beams for the investigation of functionalized graphene, graphene oxide sheets and nanoporous metals.