Highly facetted metallic zinc nanocrystals fabricated by thermal evaporation K.Y. Ng a , Amol Muley a , Y.F. Chan b , A.C.M. Ng c , A.B. Djurišić c , A.H.W. Ngan a, ⁎ a Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China b Electron Microscope Unit, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China c Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China Received 16 November 2005; accepted 11 January 2006 Available online 13 February 2006 Abstract We report the fabrication of highly facetted, hexagonal shaped metallic Zn nanocrystals by a simple catalyst-free thermal evaporation technique on Si (001) substrate using Zn pellets as source material. The Zn nanocrystals were characterized by scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The size of the crystals is of the order of 100nm and they are highly facetted along {0001} and {101¯0} planes. SAD analyses revealed the possibility of the presence of a thin ZnO layer on the surface of the as-deposited Zn nanocrystals. Formation of an epitaxial core-shell structure due to surface oxidation was further confirmed by exposing the Zn nanocrystals to air, which led to the formation of crystalline ZnO having same crystallographic orientation as that of the underlying metallic Zn. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Growth from vapor; Nanomaterials; Metallic Zn 1. Introduction Nanocrystals are of immense interest and increasing importance for future technological applications due to their peculiar and fascinating properties compared to their bulk counterparts [1]. In nanometer-scale structures, finite size effects give rise to novel electronic, magnetic, optical, and mechanical properties, and the desire to identify, understand, and exploit these size-dependent properties motivates the intensive activities on nanocrystals [2]. Semi-conducting nanocrystals such as ZnO can exhibit optical and electronic properties significantly different from bulk counterparts due to quantum confinement effects on their band gap and excitation binding energy [2–8], as well as different surface-to-volume ratios. Metallic nanocrystals, on the other hand, are known to exhibit mechanical strengths one to two orders of magnitude higher than bulk counterparts [9,10]. This immense size effect on strength is thought to be due to the shortage of dislocations [9] and is a phenomenon which must be thoroughly understood before nano- and micro-machineries can be successfully designed. Various techniques, including thermal decomposition, vapor- phase transport and metallorganic chemical vapor deposition (MOCVD) [11–13], are routinely used to synthesize nanos- tructures. Among these, evaporation is of special interest for the fabrication of nanostructures in bulk quantities. In this article, we report the successful synthesis of novel metallic zinc nano- crystals by a simple thermal evaporation technique on Si (100) wafers. In the literature, many different morphological forms of 1D nanostructures and nano-particles of ZnO, including tetrapod nanorods, nanowires, nanobelts, and hierarchical nano- structures, have been reported [14,15], but there is so far no report on the successful synthesis of highly facetted nanorods of metallic Zn. 2. Experimental procedure The zinc nanocrystals were fabricated by an AST PEVA 350T thermal evaporator using 99.99% pure zinc pellets as the source material. Before deposition, the evaporator chamber was thoroughly cleaned and baked, and the inner wall of the cham- ber was lined with aluminum foil. The source holder was an alumina crucible that was heated by a tungsten filament. The substrate was n-type (100) single-crystal silicon wafer, which was pre-cleaned by touluene, acetone, pure ethanol followed by Materials Letters 60 (2006) 2423 – 2427 www.elsevier.com/locate/matlet ⁎ Corresponding author. E-mail address: hwngan@hku.hk (A.H.W. Ngan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.01.069