Original paper Synthesis of nanocrystalline GaN from Ga 2 O 3 nanoparticles derived from salt-assisted spray pyrolysis Takashi Ogi a , Yutaka Kaihatsu a , Ferry Iskandar a , Eishi Tanabe b , Kikuo Okuyama a, * a Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, Higashi Hiroshima, Hiroshima 739-8527, Japan b Hiroshima Prefectural Institute of Industrial Science and Technology, 3-10-31 Kagamiyama, Higashi Hiroshima, Hiroshima 739-0046, Japan article info Article history: Received 26 November 2007 Received in revised form 1 January 2008 Accepted 7 January 2008 Keywords: Nanomaterials Spray pyrolysis Semiconductors Luminescence abstract Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga 2 O 3 ) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga 2 O 3 nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga 2 O 3 particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga 2 O 3 nanoparticles with relatively low crystallinity were used as the start- ing material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm. Ó 2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved. 1. Introduction GaN and its related compounds have recently received consid- erable attention for their potential use in applications, such as blue-light-emitting and laser diodes, high-speed field-effect tran- sistors, UV photodetectors, and high-temperature electronic de- vices [1,2]. In particular, GaN colloidal nanocrystals, also termed ‘‘quantum dots” (QDs), are expected to have good thermal, chemi- cal, and radiation stability, in addition to excellent optical proper- ties [3]. For these reasons, a rational method for the dispersion of nanoparticles or colloids of free-standing nanocrystals of GaN is highly desirable. GaN nanoparticle preparation has been investigated in many studies, which examined various methods, such as metal organic chemical vapor deposition (MOCVD) [4], in situ synthesis of amor- phous GaN nanoparticles in a polymer [5], and formation of colloi- dal GaN QD using non-aqueous solvothermal reactions [6]. However, these methods require expensive precursors and produce GaN nanoparticles at relatively low rates. Therefore, a simple, scalable, low-cost method for the preparation of highly crystalline GaN nanoparticle powder is desirable for industrial- scale synthesis. Our research group previously reported a simple scheme for the production of highly crystalline GaN nanoparticles based on the transformation of Ga 2 O 3 nanoparticles into their nitride-form under a flow of ammonia [7,8]. Although this method produced GaN nanoparticles at a high rate, the as-prepared GaN nanoparti- cles were agglomerated and had a broad particle size distribution. The Ga 2 O 3 nanoparticles that were used as the starting material were prepared using a liquid synthesis method. It is difficult to produce non-agglomerated nanoparticles with narrow size distri- bution using this liquid synthesis method without a strong dis- persing agent. Thus, the production of GaN nanoparticles with a narrow size distribution is highly dependent on the particle distri- bution of the starting material—Ga 2 O 3 nanoparticles. A salt-as- sisted spray pyrolysis (SASP) method has the potential to successfully produce Ga 2 O 3 nanoparticles with homogeneous composition and a narrow size distribution. SASP is a modified spray pyrolysis (SP) process, in which salts are added to the pre- cursor solution before atomization. The addition of flux salts to the precursor, which inhibit agglomeration of the as-prepared par- ticles, resulted in nanometer-sized oxide particles. This modified SP method is quite simple because nanometer-sized particles form at atmospheric pressure. Therefore, the present study investigated a novel method of synthesizing highly crystalline GaN nanoparti- cles with a narrow size distribution using Ga 2 O 3 nanoparticles de- rived from salt-assisted spray pyrolysis as the starting material. The effects of the crystallinity of the Ga 2 O 3 particles and of the nitridation time on transformation to GaN were also investigated. 0921-8831/$ - see front matter Ó 2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2008.10.005 * Corresponding author. Tel.: +81 82 424 7716; fax: +81 82 424 7850. E-mail address: okuyama@hiroshima-u.ac.jp (K. Okuyama). Advanced Powder Technology 20 (2009) 29–34 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt