Abstract
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 μm2) 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.
Highlights
“Nanosheets” is a general term referring to two-dimensional (2D) nanostructures typically with large lateral area but only one or a few atoms thick, such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides1–3
Moiré patterns of restacked boron nitride nanosheets (BNNSs) were commonly observed in transmission electron microscopy (TEM) investigations of specimens prepared from BNNS dispersions from direct liquid phase exfoliation using solvents such as N,N′-dimethylformamide (DMF)49,50, water51, and tetrahydrofuran (THF)
Not within the scope of the current work, a general observation was that the likelihood to observe van der Waals (vdW) superstructures from an aged dispersion was typically higher
Summary
“Nanosheets” is a general term referring to two-dimensional (2D) nanostructures typically with large lateral area but only one or a few atoms thick, such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides1–3. The kinetic nature determines that it is highly unlikely for two exfoliated nanosheets to restack in a perfect order, the strong B-N interactions that resulted in AA′stacking might have made the small lattice mismatch much more prone to occur than the case of restacked graphene nanosheets (much less stacking preference), for which large inter-sheet rotational angles were commonly observed.
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