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小分子多环芳烃在石墨上的自组装:扫描隧道显微镜与理论方法相结合的研究

Self-assembly of small polycyclic aromatic hydrocarbons on graphite: a combined scanning tunneling microscopy and theoretical approach.

作者信息

Florio Gina M, Werblowsky Tova L, Müller Thomas, Berne Bruce J, Flynn George W

机构信息

Department of Chemistry, The Environmental Molecular Sciences Institute, Columbia University, New York, New York 10027, USA.

出版信息

J Phys Chem B. 2005 Mar 17;109(10):4520-32. doi: 10.1021/jp046458v.

Abstract

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to high- and low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.

摘要

在低温、超高真空条件下,于80K温度下利用扫描隧道显微镜(STM)对石墨上的并四苯和茚的自组装单分子层进行了单独观察和表征。这些分子是不含已知可促进二维自组装的烷基链或官能团的小型多环芳烃(PAH)。对吸附在石墨上的小分子团进行的能量最小化和分子动力学模拟,为单层结构和驱动自组装的力提供了深入了解。计算得出单个并四苯分子在模型石墨基底上的吸附能为32千卡/摩尔,而茚的吸附能为17千卡/摩尔。并四苯观察到两种不同的单层结构,分别对应于高密度和低密度组装。用不同偏压极性拍摄的并四苯的高分辨率STM图像揭示了独特的节点结构,这是介导隧穿过程的分子电子态的特征。利用密度泛函理论计算来确定观察到的电子态和可能的隧穿机制。这些结果在PAH和烟尘颗粒形成的背景下进行了讨论,因为并四苯和茚都是已知的小分子烃燃烧的反应产物。它们在纳米技术和分子电子学领域也具有重要的基础研究意义。

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