Understanding plasma nucleation for a priori control of synthesis of carbon allotropes at the nanoscale

1335990碳纳米材料是一类特殊的材料，具有多种结构和无与伦比的电子、光子和化学性质，引起了广泛技术应用的兴趣。本项目将研究一种新型微等离子体工艺中碳纳米材料的均匀成核。微等离子体是一种非平衡的常压等离子体，在受限的电极几何形状中形成，允许粒子成核并迅速淬火，以限制它们的最终尺寸和团聚。初步结果表明，在远离金刚石热力学稳定性的环境条件下，通过微等离子体中乙醇蒸气的解离，可以制备出尺寸小于5纳米的金刚石相碳团簇。该项目将把这些实验扩展到其他碳同素异形体的形成，包括富勒烯和石墨烯。碳前体在不同C:H:O比率的微等离子体中的解离将通过光学发射光谱来表征，从而将碳二聚体和氢原子等自由基与粒子成核联系起来。粒子成核将通过气溶胶迁移率测量实时监测。将收集生长的碳纳米材料，并通过高分辨率透射电子显微镜、微拉曼光谱、x射线衍射和x射线光电子能谱进一步表征。这项工作的一个特别重要的方面将是通过不同频率的脉冲微等离子体来探索淬火速率对粒子形成的影响。该项目将为碳纳米材料的形成提供新的见解。通过将等离子体化学与材料结构联系起来，建立了一种类似于薄膜化学气相沉积（CVD）的均匀成核经验模型。通过改变微等离子体中的停留时间和淬火速率，可以建立纳米金刚石的稳定性等尺寸-结构关系。最终，这项研究将使碳纳米材料的结构在成核处得到控制，从而有可能调整它们的性质以用于各种应用。这些研究活动将为研究生、本科生和高中生提供教育机会，并成为当地一所高中纳米技术选修课的一部分。

1335990SankaranCarbon nanomaterials are a special class of materials with a diverse range of structures and unparalleled electronic, photonic, and chemical properties that have elicited interest for a wide-range of technological applications. This project will study the homogeneous nucleation of carbon nanomaterials in a novel microplasma process. Microplasmas are non-equilibrium, atmospheric-pressure plasmas formed in confined electrode geometries that allow particles to be nucleated and rapidly quenched to limit their final size and agglomeration. Preliminary results have shown that nanodiamond, carbon clusters less than 5 nm in size exhibiting diamond phase, can be produced at near ambient conditions, far from the thermodynamic stability of diamond, by dissociation of ethanol vapor in a microplasma. This project will extend these experiments to the formation of other carbon allotropes including fullerene and graphene. Dissociation of carbon precursors in the microplasma with varying C:H:O ratios will be characterized by optical emission spectroscopy to relate radical species such as carbon dimers and atomic hydrogen to particle nucleation. Particle nucleation will be monitored in real time by aerosol mobility measurements. The as-grown carbon nanomaterials will be collected and further characterized by high-resolution transmission electron microscopy, micro Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. A particularly important aspect of the work will be to explore the influence of quenching rate on particle formation by pulsing the microplasma at different frequencies.This project will provide new insight into the formation of carbon nanomaterials. By linking plasma chemistry to material structure, an empirical model for homogeneous nucleation analogous to chemical vapor deposition (CVD) of thin films will be developed. Size-structure relationships such as the stability of nanodiamond will be established by varying the residence time and quench rate in the microplasma. Ultimately, the research will enable the structure of carbon nanomaterials to be controlled at nucleation so that it is possible to tune their properties for various applications. The research activities will provide educational opportunities for graduate, undergraduate, and high school students, and be a part of an elective course on nanotechnology at a local high school.