Graphene has attracted broad interests in the field of condensed mater physics and materials science owning to its superior electronic, optical and mechanical properties. Hexagonal boron nitride (hBN) was recently found an extraordinary substrate on which graphene’s intrinsic properties could be largely preserved, due to its atomic smooth surface, absence of dangle bonds, energy band gap, and minimum charge doping. Such two-dimensional (2D) hetrostructures of graphene/hBN atomic crystals provides a novel platform for addressing novel physics.

The mismatch (1.8%) between the lattice constants of hBN and graphene lead to a hexagonal moiré pattern, which has been observed by scanning probe microscopy. The periods of the moiré pattern can be modulated by the relative rotation angle (φ) between the graphene and hBN crystalline lattices. The moiré potential acts on charge carriers in graphene, resulting in a modification of its electronic spectrum, such as additional superlattice Dirac point, Hofstadter Butterfly spectrum et al. So far it has been assumed that no structural changes occur in graphene after it is brought in contact with hBN. Besides, scientists have always been puzzled by the widely different measuring properties on the graphene/hBN hetrostructures obtained from various research groups.

Recently, the group led by Prof. Hong-Jun Gao from Nanoscale Physics and Devices Laboratory, Institute of Physics, CAS, in collaboration with the Nobel laureates, Prof. A. Geim and Prof. K. Novoselov’s group from University of Manchester, UK, investigated the strain distribution in graphene on hBN for different misorientation angles (φ) between the two crystalline structures. They made a successful process of transferring precisely an exfoliated monolayer graphene onto hBN substartes, and validated a commensurate-incommensurate transition by using atomic force microscopy (AFM), scanning tunneling microscopy (STM) and Raman spectroscopy. When φ is less than 1° (large moiré periodicity, L > 10 nm), graphene stretches locally to achieve an energetically favourable state for van der Waals interactions with hBN, which results in relatively large areas of commensurate stacking and deformations concentrated in narrow strained regions. For φ > 1° (small moiré periodicity), graphene and hBN lattices remain unsynchronized and there are no distinct regions with accumulated strain (the incommensurate state). The different distributions of Young’s modulus of Graphene/hBN with various misorientation angles were first discovered from the AFM results. In order to observe directly the slight changes of lattice constants of graphene at different moiré regions, Ruisong Ma, Jianchen Lu and Dr. Haiming Guo in Gao’s group achieved the large-area, high-resolution STM images at the atomic level on the Graphene/hBN surfaces, and found 2.0 ± 0.6 % variations of C-C atomic separations in graphene among different moiré regions. This slight change in atomic separations in graphene provides a clear and direct evidence of commensurate–incommensurate transition in graphene on hBN. The differences in their structures and properties were further validated from Raman spectra and electronic transport measurements.

The discovery and investigation of such commensurate–incommensurate transition is of importance for us to understand the novel phenomena and resolve the puzzles in graphene/hBN hetrostructures. For example, a band gap can be easily found around the Dirac point for the φ < 1° (L > 10 nm) sample, while it becomes much difficult for the φ > 1° (L < 10 nm) sample. This work is a great breakthrough in the related fields in condensed-matter physics, and the main results were published on Nature Physics [10, 451–456 (2014)]. The project is supported by the National Science Foundation, the Ministry of Science and Technology of China, and the Chinese Academy of Science.

Paper link：http://www.nature.com/nphys/journal/v10/n6/full/nphys2954.html