Positron Spectrosopy of 2-D and Nanostructured Materials

该项目利用一束低能正电子（电子的反物质）来提供关于新型纳米材料（包括石墨烯和纳米多孔金）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，两个粒子的质量以伽马射线的形式完全转化为能量。从原子中突然移除电子会导致高度激发态，并发射出称为俄歇电子的电子。湮灭引起的伽马射线和俄歇电子的能量提供了与正电子湮灭的原子的化学信息。此前由美国国家科学基金会资助的研究表明，低能量正电子可以用来选择性地探测表面原子顶层的性质，因为在湮灭时，低能量位置很有可能被困在材料表面。该项目的研究旨在探索利用正电子光谱学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、非破坏性的方法，以便了解吸附原子可以用来改变这些材料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在一种被称为纳米多孔金属的新型材料中，利用正电子产生的伽马射线在纳米尺寸孔的内表面湮灭的可行性，以提供理解这些材料为什么能大大加快理想化学反应的原因所需的信息。伽马射线的穿透性允许对内部表面进行测量。该活动的结果将最终促进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应用。该项目为高技术工业表面科学领域的研究生和本科生提供培训，包括低维物理、超高真空技术、带电粒子束、辐射检测、纳米结构合成、材料表征等。技术摘要：低能正电子在湮灭时极有可能被困在像势态，因此对材料表面极为敏感。本项目的研究旨在了解低能正电子（~ 1eV）与二维材料（包括石墨烯/氧化石墨烯片）的相互作用。利用类似俄歇的俄歇介导粘着过程以及正电子湮灭诱导俄歇电子发射和正电子形成来研究正电子在二维材料上的捕获。测量正电子粘附在石墨烯和氧化石墨烯表面的概率旨在提供这些材料表面特性对比的信息，并证明使用低能正电子作为二维结构的多参数光谱技术的可行性。石墨烯电子的低背景发射有望揭示正电子湮灭产生的俄歇发射中低能电子尾巴的物理起源。该项目的第二个主要目标是探索使用多普勒增宽伽马能谱探测纳米多孔金属内部表面的能力。正电子被困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，表明正电子可以用来表征孔隙的内表面。这些研究旨在利用从正电子光谱中获得的独特信息来了解气体与纳米多孔金属应变内表面的相互作用。这些研究将有助于更好地控制这些新材料的催化活性，并有助于利用纳米多孔金属开发高密度储氢材料。该项目代表了首次探索使用正电子束来研究功能化石墨烯、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.