Multiscale modeling approach to catalytic growth of carbon nanotubes

0731246Yakobson，Boris表示：“碳纳米管的生产和质量控制取决于充当纳米反应器的微小金属催化剂颗粒”。他们分解原料气体以溶解释放的碳原子，然后碳原子迅速扩散，以促进碳茎-纳米管的生长。这个过程的健壮性、产量和新出现的小管的特殊物理类型--它的直径和手性对称性--都由这几个立方纳米内的原子动力学和化学相互作用的戏剧性决定。尽管在纳米管的合成和理论建模方面具有重要的技术意义和长达十年的进展，但对指导纳米管成核和生长的过程缺乏基本的了解。基于以往的经验，我们建议进行系统的研究，以促进纳米管生长科学的发展。在几个重要的方面中，我们主要关注(I)催化剂颗粒的结构和状态及其与纳米管边缘的界面，(Ii)催化剂中毒的机制以及如何延缓或防止它，以及(Iii)管生长过程中的缺陷退火；后者将进一步与寻找定义出露管的手性的可能机制以及如何控制它有关。这项拟议研究的学术价值在于促进了对碳纳米管生长的密切原子机制的理解。特别是，它应该揭示催化剂颗粒内部和表面的原子间物理化学相互作用。我们计划将三个研究尺度(从头算、原子经典MD和连续统现象学模型)连接起来，从多尺度建模的角度来看，这具有额外的优点。当然，主要的智力价值将在于揭示催化剂中毒的机理和如何减少它，催化剂颗粒-纳米管界面的结构，以及碳原子积累和生长管体结构缺陷的退火的确切机制，以保持其完美性。基于积累的知识，我们希望提出可能的途径来实现可控的手性(和直径)特定的纳米管生长--这是材料化学家，特别是碳纳米管研究人员和技术使用者的圣杯。除了碳纳米管之外，这项工作还应该为其他近一维结构的催化生长提供更全面的见解。为碳管确立的一般特征可能适用于氮化硼管，以及更广泛类别的纳米线(硅、硼和其他重要的无机成分)。在这个为期三年的项目中，教育活动的要素无缝地融合在一起。一名本科生和一名研究生将与PI和Co-PI(以及目前在PI小组中的其他十几名学生和博士后)密切互动。该项目的跨学科性质，涉及计算量子化学、分子动力学、纳米力学和位错理论的元素，鼓励培养真正的跨学科科学家。这项研究的内容最终将被纳入已经开设的(由PI开设的)课程MSCI614纳米材料原理“。研究生将通过在课程中提供与本项目相关的主题的几堂课(在项目后期)获得一些教学经验。赖斯一直在非常积极地努力，通过有针对性的招聘努力，增加代表不足的群体的参与。在这种情况下，在该项目中，我们预计将邀请来自墨西哥的莱斯本科生和学生(他们之前在PI的小组中度过了夏天)。我们将与专门对新型碳纳米管材料感兴趣的国防部空军研究实验室(材料局)以及对碳纳米结构合成路线和改进感兴趣的美国能源部ORNL同事密切合作。

0731246Yakobson, Boris Production and quality control of carbon nanotubes hinges on the tiny metal catalyst particles which serve as nano-reactors". They decompose the feedstock gas to dissolve the released carbon atoms, which then rapidly diffuse to feed the growth of a carbon stem, a nanotube. The robustness of this process, its yield and the particular physical type of the emerging tubule-its diameter and chiral symmetry-are all determined by the drama of atomic dynamics and chemical interactions within this few cubic nanometers. In spite of great technological importance and decade-long progress in nanotube synthesis and theoretical modeling, the fundamental understanding of the processes guiding nanotube nucleation and growth is lacking. Based on previous experience, we propose a systematic study to advance the science of nanotube growth. Among several important aspects, we focus mainly on the (i) structure and state of the catalyst particle and its interface with the nanotube edge, (ii) the mechanisms of catalyst poisoning and how it can be delayed or prevented, and (iii) the defect annealing in the course of tube growth; the latter will further be connected with finding possible mechanisms defining the chirality of the emerging tube and how it can possibly be controlled. The intellectual merit of the proposed research lies in advancing the understanding of the intimate atomic mechanisms of carbon nanotube growth. In particular, it should reveal the interatomic physico-chemical interactions inside and at the surface of the catalyst particle. We plan to connect three scales of study (ab initio, atomistic-classical MD, and continuum phenomenological model), which has additional merit from the multiscale modeling viewpoint. The main intellectual value of course will lie in revealing the catalyst poisoning mechanism and how to reduce it, the structure of catalyst particle-nanotube interface, and the exact mechanisms of atomic accretion of carbon and the annealing of the structural defects in the body of the growing tube, in order to maintain its perfection. Based on accumulated knowledge we hope to suggest possible paths to achieving controlled chirality (and diameter) specific growth of nanotubes-a holy grail of materials chemists and especially carbon nanotube researchers and technology users. Beyond carbon nanotubes, the effort should provide more general insights into catalytic growth of other nearly-one-dimensional structures. General features established for carbon tubules may apply to boron-nitride tubes, and yet broader class of nanowires (silicon, boron, and other important inorganic compositions). Elements of educational activities are seamlessly integrated in this three-year project. An undergraduate and a graduate student will interact closely with the PI and Co-PI (as well as with nearly a dozen of other students and postdocs presently in the PI's group). The cross-disciplinary nature of the project, involving elements of computational quantum chemistry, molecular dynamics, nanomechanics, and dislocation theory, encourages training of truly interdisciplinary scientists. Elements of the research will be eventually incorporated into the already-offered (by PI) course MSCI614 Principles of Nanoscale Materials". The graduate student will gain some teaching experience through offering (later in the project) a few lectures within the course, on the topics related to this project. Rice has been working very aggressively at increasing the participation of underrepresented groups through targeted recruiting efforts. In this case, within the project we anticipate to engage Rice undergraduates and students from Mexico (who have previously spent the summer in the PI's group). We will work closely with DOD Air Force Research Laboratory (Materials Directorate) specifically interested in novel CNT-based materials, and with DOE ORNL colleagues interested in carbon nanostructure synthetic routs and improvements.