Two-dimensional atomic crystals

A chemical compound’s properties can depend strongly on its arrangement. Even something as simple as crystalline carbon takes on different characteristics if it is zero-dimensional (as for a buckyball), 1D (a nanotube), or 3D (graphite or diamond). Largely absent, however, have been experimental samples of 2D crystals. Unsuccessful attempts have been made to chemically remove such arrangements from layered materials, which have strong in-plane bonds but weak coupling between adjacent planes. A group at the University of Manchester (UK) has now found a simple, low-tech route to success: The researchers scrape the fresh surface of a layered crystal against another solid surface. Surprisingly, among the many flakes left on the substrate are always some macroscopic flakes that are just one atomic layer thick. As an added surprise, the monolayer flakes are stable and retain a high crystalline quality under ambient conditions. The 2D crystals were identified among the detritus using optical microscopy followed by atomic force microscopy. Thus far, the technique has succeeded on five different materials: graphite, BSCCO, BN, NbSe₂, and MoS₂. The researchers, led by Andre Geim, also made electrical conductivity measurements on their samples. For example, the carrier concentration in a layer of NbSe₂ in a 3D crystal is 100 times higher than in an isolated plane of the same material. (K. S. Novoselov et al., Proc. Natl. Acad. Sci. USA 102 , 10451, 2005 http://dx.doi.org/10.1073/pnas.0502848102 .)