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首页> 外文期刊>The Journal of Chemical Physics >A free energy study of carbon clusters on Ir(111): Precursors to graphene growth
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A free energy study of carbon clusters on Ir(111): Precursors to graphene growth

机译：IR（111）上碳簇的自由能研究：石墨烯生长的前体

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It is widely accepted that the nucleation of graphene on transition metals is related to the formation of carbon clusters of various sizes and shapes on the surface. Assuming a low concentration of carbon atoms on a crystal surface, we derive a thermodynamic expression for the grand potential of the cluster of N carbon atoms, relative to a single carbon atom on the surface (the cluster work of formation). This is derived taking into account both the energetic and entropic contributions, including structural and rotational components, and is explicitly dependent on the temperature. Then, using ab initio density functional theory, we calculate the work of formation of carbon clusters C-N on the Ir(111) surface as a function of temperature considering clusters with up to N = 16 C atoms. We consider five types of clusters (chains, rings, arches, top-hollow, and domes), and find, in agreement with previous zero temperature studies, that at elevated temperatures the structure most favoured depends on N, with chains and arches being the most likely at N < 10 and the hexagonal domes becoming the most favourable at all temperatures for N > 10. Our calculations reveal the work of formation to have a much more complex character as a function of the cluster size than one would expect from classical nucleation theory: for typical conditions, the work of formation displays not one but two nucleation barriers, at around N = 4-5 and N = 9-11. This suggests, in agreement with existing LEEM data, that five atom carbon clusters, along with C monomers, must play a pivotal role in the nucleation and growth of graphene sheets, whereby the formation of large clusters is achieved from the coalescence of smaller clusters (Smoluchowski ripening). Although the main emphasis of our study is on thermodynamic aspects of nucleation, the pivotal role of kinetics of transitions between different cluster types during the nucleation process is also discussed for a few cases as illustrative examples. Published by AIP Publishing.
机译：众所周知，石墨烯对过渡金属的成核与地面各种尺寸和形状的形成碳簇。假设晶体表面上的低浓度的碳原子，我们相对于表面上的单个碳原子（形成的簇工作）来导出N个碳原子簇的巨大电位的热力学表达。这是考虑到具有结构和旋转部件的能量和熵贡献，并明确依赖于温度。然后，使用AB初始密度函数理论，我们计算IR（111）表面上的碳簇C-N的工作作为考虑簇的函数，考虑到最多n = 16c原子。我们考虑五种类型的集群（链，戒指，拱形，顶部空心和圆顶），并与先前的零温度研究一起找到，在升高的温度下，最有利的结构取决于N，带有链和拱门最有可能在N <10和六角形圆顶处变得最有利的是N> 10。我们的计算揭示了形成的形成更复杂的性格，作为群集大小的函数，而不是一个古典成核的函数理论：对于典型条件，形成的工作不是一个而不是两个成核屏障，在n = 4-5和n = 9-11周围。这表明，与现有的leem数据一致，五个原子碳簇以及C单体必须在石墨烯片的成核和生长中发挥枢轴作用，从而从较小簇的聚结会形成大簇（ Smoluchowski成熟）。尽管我们研究的主要重点是核心的热力学方面，但在少量病例中还讨论了在核切种过程中不同聚类类型之间的转变之间的动力学的关键作用。通过AIP发布发布。

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