Since sticky tape was used just over two decades ago to isolate a single layer of graphene, an entire field of research has emerged to find more 2D materials, which have the thickness of just one or a few atoms. (See the article by Andrey Geim and Allan MacDonald, Physics Today, August 2007, page 35.) Materials with atomic-scale dimensions exhibit distinct and exotic properties because of quantum confinement effects. Graphene, for example, has exceptionally high electrical conductivity because its structure hosts mobile electrons that behave like massless particles. (See Physics Today, December 2010, page 14.)
Graphene provided a natural route into the realm of 2D materials because its one-atom-thick sheets have strong internal bonds but connect to other graphene sheets through weaker van der Waals forces. Those features make graphene relatively easy to peel off from its bulk form, graphite. Development of other 2D van der Waals materials, such as the insulating boron nitride and the semiconducting molybdenum disulfide (MoS2), has followed. (See the article by Pulickel Ajayan, Philip Kim, and Kaustav Banerjee, Physics Today, September 2016, page 38.) Theory is used to predict some of the exotic properties of new 2D material phases. Under certain conditions, for example, 2D bismuth is expected to be a topological insulator—a material that conducts charge along its surfaces or edges while its interior acts as an insulator.
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