Journal of Solid State Chemistry 178 (2005) 937–949 Amorphous and nanocrystalline luminescent Si and Ge obtained via a solid-state chemical metathesis synthesis route Paul F. McMillan a,b,Ã , Jan Gryko c , Craig Bull d , Richard Arledge c , Anthony J. Kenyon e , Barbara A. Cressey f a Christopher Ingold Laboratory, Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK b Davy Faraday Research Laboratory, Royal Institution of Great Britain, London W1S 4BS, UK c Department of Physical and Earth Sciences, Jacksonville State University, AL 36265, USA d School of Physics and Centre for Science at Extreme Conditions, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK e Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK f Science and Engineering Electron Microscopy Centre, School of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK Received 6 October 2004; received in revised form 27 December 2004; accepted 28 December 2004 Abstract A new solid-state metathesis synthesis route was applied to obtain bulk samples of amorphous or microcrystalline Si and Ge. The method involves reaction of Zintl phases such as NaSi or NaGe, with ammonium or metal (e.g., CuCl, CoBr 2 ) halides. The driving force for the solid-state reaction is provided by the formation of alkali halides and the transition metals or metal silicides, or gaseous ammonia and hydrogen. The semiconductors were purified by washing to remove other solid products. The amorphous semiconductors were obtained in bulk form from reactions carried out at 200–300 1C. Syntheses at higher temperatures gave rise to microcrystalline semiconductors, or to micro-/nanocrystalline particles contained within the amorphous material. Similar crystalline/amorphous composites were obtained after heat treatment of bulk amorphous materials. r 2005 Elsevier Inc. All rights reserved. 1. Introduction Silicon and germanium are important semiconducting materials that are used in bulk or thin film form as both crystalline and amorphous solids. The amorphous forms have been developed particularly for use in photovoltaic devices [1]. Nanocrystalline particles of the group IV elements, either free-standing or incorporated in solid matrices, are now under development because of their useful photo- and electro-luminescent properties [2–7]. Here we report a solid-state chemical synthesis route that can be used to obtain bulk samples of amorphous or microcrystalline silicon and germanium materials, that can also yield luminescent nanoparticles embedded within an amorphous solid matrix. The synthesis method involves reacting polyanionic ‘‘Zintl phases’’ (e.g., NaSi, NaGe) together with metal halides (e.g., CuCl, CoBr 2 ) or ammonium salts (NH 4 Cl, NH 4 Br). Sodium silicide is an ionic Zintl phase that contains isolated Si 4 4 tetrahedral clusters surrounded by Na + ions [8] (Fig. 1). The formation reaction proceeds via a chemical ‘‘metathesis’’ process, in which ionic salts (alkali halides) are formed as products, together with solid metals (Cu, Co) or gaseous NH 3 , as well as the elemental semiconductors (Si, Ge); e.g., CuBrðsÞþ NaSiðsÞ¼ NaBrðsÞþ SiðsÞþ CuðsÞ: (1) Depending upon the transition metal used, metal silicides (e.g., FeSi 2 ) are also occasionally formed in the ARTICLE IN PRESS www.elsevier.com/locate/jssc 0022-4596/$-see front matter r 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2004.12.040 Ã Corresponding author. Christopher Ingold Laboratory, Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK. Fax: +442076797463. E-mail address: p.f.mcmillan@ucl.ac.uk (P.F. McMillan).