Taniguchi et al. 1984, p. 867: "... black phosphorus ... [is] characterized by the wide valence bands with rather delocalized nature."; Morita 1986, p. 230; Carmalt & Norman 1998, p. 7: "Phosphorus ... should therefore be expected to have some metalloid properties."; Du et al. 2010. Interlayer interactions in black phosphorus, which are attributed to van der Waals-Keesom forces, are thought to contribute to the smaller band gap of the bulk material (calculated 0.19 eV; observed 0.3 eV) as opposed to the larger band gap of a single layer (calculated ~0.75 eV). Carmalt CJ & Norman NC 1998, 'Arsenic, antimony and bismuth: Some general properties and aspects of periodicity', in Norman NC (ed.), Chemistry of Arsenic, Antimony and Bismuth, Blackie Academic & Professional, London, pp. 1–38, ISBN0-7514-0389-X Du Y, Ouyang C, Shi S & Lei M 2010, "Ab initio studies on atomic and electronic structures of black phosphorus", Journal of Applied Physics, vol. 107, no. 9, pp. 093718–1–4, doi:10.1063/1.3386509