基于第一性原理对氟化石墨烯纳米带性能的研究

（限3000字符）Graphene has gained great attention since it in nature owns superb properties in almost every respect as applications are concerned. More research efforts have recently been exerted on theoretical studies or techniques to open its band gap thus to boost its practical applications. It was already shown that band gap can be clearly opened when graphene is functionalized by external atoms such as hydrogen, oxygen or fluorine. Among the three options more evident stability can be observed when graphene is functionalized by fluorine than by the other two atoms, especially in environment with or above room temperatures (R.R. Nair et al., Small 6, 2877-2884, 2010; J.T. Robinson et al., Nano Letter 10, 3001-3005, 2010). In practical applications either graphene or its derivatives will exist as ribbons or in other finite-sized forms. Studies on the edge morphologies and related properties of graphene nano ribbons or strips have already been carried out (S. Jun, Phys. Rev. B 78, 073405, 2008; V.B. Shenoy et al., Phys. Rev. Lett. 101, 245501, 2008; C.K. Gan and D.J. Srolovitz, Phys. Rev. B 81, 125445, 2010). Their studies all showed that compressive edge stress exists in graphene ribbons with either armchair or zigzag edges. This compressive edge stress can lead to the warping effect near edge zones. Edge re-constructings (with Stone-Wales defects or being modified locally by hydrogen atoms) will clearly weaken the warping effect by reducing the compressive edge stress. This proposed research project will be focused on edge related properties of fluorinated graphene nano ribbons. Atomic scale calculations based on first-principles will be performed. Investigation foci will mainly cover three respects: edge energy and edge stress; effective in-plane elastic modulus and residual stress, and the band gap energy.

（限400字）因石墨烯本质上在各个方面具有优越的性能，其最近得到更多的科研关注。这些研究的重点在于从理论和技术上扩展其带隙能量，从而促进其实际应用价值。研究发现，尽管石墨烯的带隙能量在其被功能化后（用氢、氧或者氟原子）都能得到扩展，但氟化后的石墨烯在室温下具有更显著的稳定性。实际应用中石墨烯是以带状等有限大小尺寸存在的，尤其不可避免地有边界的存在。对石墨烯纳米带性能的研究表明，无论扶椅型还是锯齿型的边界，石墨烯纳米带的边界应力均为压应力。这一性质与石墨烯带边界区域的翘曲效应有关。石墨烯带的边界有缺陷或者部分被氢化后，边界应力显著减小，从而附近区域的翘曲效应也减弱不少。这个科研课题将专注于氟化石墨烯纳米带与其边界相关性能的研究。研究的主要方法是基于第一性原理的原子尺度计算。研究内容主要着眼于揭示完全氟化石墨烯带三方面的性能：边界能和边界应力，面内弹性模量和残余应力，以及带隙能量。

全面理解氟化石墨烯纳米带的电学和力学性能是其从科学研究到实践应用的重要环节。在本项目中，我们应用第一性原理对氟化石墨烯纳米带的边界性能以及边界对纳米带整体性能的影响进行了数值计算研究。对于选取的具有对称/反对称、扶椅型/锯齿型边界的四种纳米带，研究发现，它们的边界能在宽度达到0.5nm时快速到达一个平台高度，然后随着宽度的增加而缓慢增长。无论氟化石墨烯纳米带具有对称还是反对称的扶椅型边界，其边界应力总是负的；然而锯齿型边界情形的氟化石墨烯纳米带正好相反，是正的。氟化石墨烯纳米带的边界应力对带的宽度很敏感，且比同样宽度的石墨烯纳米带的边界应力要小。这说明石墨烯氟化后厚度的增加降低了边界应力。相应地，边界应力导致扶椅型和锯齿型边界的氟化石墨烯纳米带分别具有拉伸和压缩残余应变。就面内弹性模量而言，随着宽度的增加，所有类型纳米带的值接近无限大氟化石墨烯的面内弹性模量大小。最后，氟化石墨烯纳米带不同边界时的电学性能差别很大：扶椅型边界的纳米带象无限大的一样，仍为半导体，但能带宽度降低了约50%；然而锯齿型边界的纳米带则变为导电体，具有金属性质。这一性质差别对氟化石墨烯纳米带的应用产生直接的影响：在电子领域应用时必须注意其边界形状。