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School of Physics and Telecommunication Engineering/NEWS 2022-06-16 15:09:51 From：School of Physics and Telecommunication Engineering Hits： Favorite
Available online 16 June 2022.
Consisting of one to a few atomic layers, two-dimensional (2D) materials have atomically perfect surfaces that are free of dangling bonds and exhibit various exotic physical and chemical properties [1]. Even more exciting, one can construct new materials by stacking different 2D layers as they are generally bonded by van der Waals interactions [2]. Therefore, in the past decade, 2D materials have attracted much interest in the field of materials science, physics, chemistry and electrical/optical engineering. To implement the most anticipated applications of the 2D materials-based devices in the future, the growth of large-scale 2D single crystals is a prerequisite. Only single crystals can ensure the ultimate intrinsic performance of the materials and the uniformity of the devices.
Generally, there are two ways to realize the growth of 2D single crystals: (1) Controlling the nucleation. One should ensure the material grows gradually from only one nucleus, which could eventually grow into a large single crystal [3], [4]. (2) Controlling the orientation. There are massive nuclei formations during the growth and one should ensure that the lattice orientations of all the 2D islands are the same. When these islands meet each other, they can seamlessly merge together and form large single-crystal films [5], [6], [7], [8], [9], [10], [11]. After more than ten years efforts, the epitaxy of 2D single crystals by controlling the orientation has been widely chosen due to its high efficiency, general applicability, stable controllability and better compatibility with industrial production.
The realization of epitaxial growth of 2D single crystals relies on suitable single-crystal substrates. Driven by the urgency of preparing large 2D single crystals, the production of large-size single-crystal substrates has been developed rapidly in recent years. Taking copper (Cu) as an example, the size of single-crystal Cu foils increases quickly from millimetre- to meter-scale, and the surface index also develops from only low index to a lot of high indices through recently developed temperature-gradient-driven [5], contact-free annealing [12] or seeded growth techniques [13].
Having a single-crystal substrate, it is then necessary to select applicable epitaxy substrates and proper mechanisms to achieve the unidirectional alignment of 2D islands according to the lattice symmetry of the 2D materials and substrates. For centrosymmetric graphene, the identical orientations can be achieved by van der Waals coupling between graphene islands and the triple symmetric Cu(111). Regulated by the periodic potential fields on Cu(111) surface, all graphene islands exhibit the same orientation and then seamlessly merge into a single-crystal film (Fig.1a) [5]. Later, this epitaxy has also been extended to the CuxNiy (111) substrates, where the presence of minor nickel can enhance the catalytic activity of Cu, and realize the rapid growth of graphene single crystals [6].
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