Moiré superlattices (MSLs) are special laterally modulated 2D heterostructures that have emerged as an exciting playground for discovering new fascinating phenomena and modulating 2D materials properties. This is rooted in new global crystalline symmetry of the MSLs, in accompany with spatial potential and strain field modulation. Several emergent materials properties have been discovered, e.g. the recent cases of superconductivity in magic-angle MSL graphene [1], dynamic band-structure tuning in compressed graphene MSL [2], and multiple Dirac cones in quasi-crystalline graphene MSL [3].

    Such MSLs are mostly composed of two layers that have lattice mismatch or twisting angle between them, usually achieved by van der Waals mechanically stacking which demands sophisticated registering and prevention of contamination. Especially for the homogeneous MSL, to our best knowledge thus far it is only achieved in stacked graphenes. It is therefore strongly desirable to find ways of fabricating new MSLs consisting of alternative 2D material sublayers and exploring their emergent new properties.

    We unexpectly find a new way of realization of homogeneous MSL based on a wide-gap 2D semiconductor BiOCl completely from a bottom-up synthetic approach. Unlike the usual approaches fabricating MSLs, our MSLs are prepared through one-pot solvothermal chemical growth of spiral-type nanosheets under a screw-dislocation driven mechanism (see Figure 1). The MSLs are composed of multiple BiOCl layers spirally stacked along the out-of-layer direction. The structural advantage of such MSLs is that they can naturally incorporate various twisting angles controlling MSL properties. To our best knowledge, this is the first example of an MSL that is grown by a facile, scalable chemical method with good reproducibility.

Figure 1: The Moiré superlattice formed by the spiral-type BiOCl nanosheets driven by screw-dislocation.

    We find that such spiral MSLs of BiOCl exhibit fascinating physical properties including unusually large ~0.6 eV band-gap reduction (see Figure 2, compared to the small modulation of <0.05 eV observed in graphene MSL), and two-fold increase in carrier lifetime. Our first-principles electronic structure calculations reproduce the large band-gap reduction (Figure 2) and identify the charge separation behavior across the MSL that is responsible for the increased carrier lifetime. Further analyses indicate that such unusual properties can be ascribed to the locally enhanced inter-layer coupling effect associated with the Moiré potential modulation.

Figure 2: The Moiré superlattice exhibits large ~0.6 eV band-gap reduction (left panel, according to the optical absorption measurement). First-principles modelling reproduces the large band-gap reduction (middle and right panels).

    These emergent properties render BiOCl an optically active material in visible-light region with enhanced optoelectronic conversion effeciency. More importantly, we find emergent chemical properties, specifically in photocatalysis. To our best knowledge, this is also the first example of an MSL giving emergent chemical functionality rather than only novel physical behaviors.

    We hope that our work not reports the discovery of a completely new class of 2D MSL structure with emergent physical and chemical properties but also offers a facile protocol for large-scale production, which hold promise for applications of 2D MSLs in electronics, optics, and practical environmental purifications.

    This project is completed by the three-year and half joint experiment-theory efforts. The experimental part is performed by the Low-Dimensional Material Group led by Prof. Weitao Zheng and the Surface/Interface Modulation and Properties of Low-Dimensional Material Group led by Prof. Xiaoqiang Cui. The theoretical part is done by the Theory&Design of Optoelectronic Semiconductors Group led by Prof. Lijun Zhang. Dr. Lulu Liu (in experiment) and Dr. Yuanhui Sun (in theory) contributed equally to this work.

    Also this work cannot be done without the valuable collaborative efforts from Prof. Qiaoliang Bao (Monash University), Prof. Jiong Lu (National University of Singapore), Prof. Peng Zhang (Dalhousie University), Prof. Lirong Zheng (Institute of High Energy Physics, CAS), Prof. Liping Yu (University of Maine), Prof. David J. Singh (University of Missouri), and Prof. Qihua Xiong (Nanyang Technological University, Singapore). Read the complete story at Nature Communications[4].

Reference:

1. Cao, Y. et al. Unconventional superconductivity in magic-angle graphene superlattices. Nature 556, 43–50 (2018).

2. Yankowitz, M. et al. Dynamic band-structure tuning of graphene moiré superlattices with pressure. Nature 557, 404–408 (2018).

3. Ahn, S. J. et al. Dirac electrons in a dodecagonal graphene quasicrystal. Science 361, 782–786 (2018).

4. Liu, L. et al. Bottom-up growth of homogeneous Moiré superlattices in bismuth oxychloride spiral nanosheets. Nature Communications 10, 4472 (2019).