Flexible 2D Crystals of Polycyclic Aromatics Stabilized by Static Distortion Waves.
Flexible 2D Crystals of Polycyclic Aromatics Stabilized by Static Distortion Waves.
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DOI：
10.1021/acsnano.6b00935
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发表时间：
2016-07-26
期刊：
影响因子：
17.1
通讯作者：
Fritz T
中科院分区：
文献类型：
--
作者：
Meissner M;Sojka F;Matthes L;Bechstedt F;Feng X;Müllen K;Mannsfeld SC;Forker R;Fritz T
许多有机薄膜在无机衬底上的外延可以在刚性晶格的框架内分类，这有助于理解驱动薄膜外延的能量增益的起源。然而，存在具有不同相互取向的吸附物-基底组合，这种分类失败，并且无法在刚性晶格概念内解释外延。有人提出，原子位置远离理想晶格点的微小位移，即所谓的静态畸变波（SDW），是在这种情况下观察到的取向外延的原因。利用低能电子衍射和扫描隧道显微镜，我们提供了直接的实验证据SDW在有机吸附膜，即六-正-六苯并冠在石墨上。它们表现为远离理想吸附质晶格的波状亚-nigström分子位移，该理想吸附质晶格与石墨不相称。通过基于密度泛函理论的模型，我们表明，由于在吸附层的灵活性，分子-衬底的能量是通过应变的分子间键和所得到的总能量是最小的观察到的域的取向，构成取向外延。虽然在界面处的结构弛豫是一个常见的假设，结合精确测定的无公度外延关系，SDW在真实的空间中的直接观察，以及它们作为外延能量增益的唯一来源的识别构成了这种效果的全面证明。
The epitaxy of many organic films on inorganic substrates can be classified within the framework of rigid lattices which helps to understand the origin of energy gain driving the epitaxy of the films. Yet, there are adsorbate–substrate combinations with distinct mutual orientations for which this classification fails and epitaxy cannot be explained within a rigid lattice concept. It has been proposed that tiny shifts in atomic positions away from ideal lattice points, so-called static distortion waves (SDWs), are responsible for the observed orientational epitaxy in such cases. Using low-energy electron diffraction and scanning tunneling microscopy, we provide direct experimental evidence for SDWs in organic adsorbate films, namely hexa-peri-hexabenzocoronene on graphite. They manifest as wave-like sub-Ångström molecular displacements away from an ideal adsorbate lattice which is incommensurate with graphite. By means of a density-functional-theory based model, we show that, due to the flexibility in the adsorbate layer, molecule–substrate energy is gained by straining the intermolecular bonds and that the resulting total energy is minimal for the observed domain orientation, constituting the orientational epitaxy. While structural relaxation at an interface is a common assumption, the combination of the precise determination of the incommensurate epitaxial relation, the direct observation of SDWs in real space, and their identification as the sole source of epitaxial energy gain constitutes a comprehensive proof of this effect.