Liao Yunlong, Cao Wei, Connell John W, Chen Zhongfang, Lin Yi
National Institute of Aerospace, 100 Exploration Way, Hampton, VA, 23666, USA.
Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico, 00931, USA.
Sci Rep. 2016 May 18;6:26084. doi: 10.1038/srep26084.
Two-dimensional (2D) van der Waals (vdW) superstructures, or vdW solids, are formed by the precise restacking of 2D nanosheet lattices, which can lead to unique physical and electronic properties that are not available in the parent nanosheets. Moiré patterns formed by the crystalline mismatch between adjacent nanosheets are the most direct features for vdW superstructures under microscopic imaging. In this article, transmission electron microscopy (TEM) observation of hexagonal Moiré patterns with unusually large micrometer-sized lateral areas (up to ~1 μm(2)) and periodicities (up to ~50 nm) from restacking of liquid exfoliated hexagonal boron nitride nanosheets (BNNSs) is reported. This observation was attributed to the long range crystallinity and the contaminant-free surfaces of these chemically inert nanosheets. Parallel-line-like Moiré fringes with similarly large periodicities were also observed. The simulations and experiments unambiguously revealed that the hexagonal patterns and the parallel fringes originated from the same rotationally mismatched vdW stacking of BNNSs and can be inter-converted by simply tilting the TEM specimen following designated directions. This finding may pave the way for further structural decoding of other 2D vdW superstructure systems with more complex Moiré images.
二维（2D）范德华（vdW）超结构，即vdW固体，是由二维纳米片晶格精确重新堆叠形成的，这会导致母体纳米片中不存在的独特物理和电子特性。相邻纳米片之间的晶体失配形成的莫尔条纹是vdW超结构在微观成像下最直接的特征。本文报道了通过透射电子显微镜（TEM）观察到的，由液体剥离的六方氮化硼纳米片（BNNSs）重新堆叠形成的具有异常大的微米级横向面积（高达1μm²）和周期（高达50nm）的六方莫尔条纹。这一观察结果归因于这些化学惰性纳米片的长程结晶性和无污染物表面。还观察到了具有类似大周期的平行线状莫尔条纹。模拟和实验明确表明，六方图案和平行条纹源自BNNSs相同的旋转失配vdW堆叠，并且可以通过沿指定方向简单倾斜TEM样品进行相互转换。这一发现可能为进一步解析其他具有更复杂莫尔图像的二维vdW超结构系统的结构铺平道路。
