Mechanisms of 1D Crystal Growth in Reactive Vapor Transport: Indium Nitride Nanowires Sreeram Vaddiraju, ² Aditya Mohite, ‡ Alan Chin, | M. Meyyappan, | Gamini Sumanasekera, § Bruce W. Alphenaar, ‡ and Mahendra K. Sunkara* ,² Departments of Chemical Engineering, Electrical and Computer Engineering, and Physics, UniVersity of LouisVille, LouisVille, Kentucky 40292, and Center for Nanotechnology, NASA Ames Research Center, Moffett Filed, California 94035 Received March 24, 2005; Revised Manuscript Received June 15, 2005 ABSTRACT Indium nitride (InN) nanowire synthesis using indium (In) vapor transport in a dissociated ammonia environment (reactive vapor transport) is studied in detail to understand the nucleation and growth mechanisms involved with the so-called “self-catalysis” schemes. The results show that the nucleation of InN crystal occurs first on the substrate. Later, In droplets are formed on top of the InN crystals because of selective wetting of In onto InN crystals. Further growth via liquid-phase epitaxy through In droplets leads the growth in one dimension (1D), resulting in the formation of InN nanowires. The details about the nucleation and growth aspects within these self-catalysis schemes are rationalized further by demonstrating the growth of heteroepitaxially oriented nanowire arrays on single-crystal substrates and “tree-like” morphologies on a variety of substrates. However, the direct nitridation of In droplets using dissociated ammonia results in the spontaneous nucleation and basal growth of nanowires directly from the In melt surface, which is quite different from the above-mentioned nucleation mechanism with the reactive vapor transport case. The InN nanowires exhibit a band gap of 0.8 eV, whereas the mixed phase of InN and In 2 O 3 nanowires exhibit a peak at ∼1.9 eV in addition to that at 0.8 eV. The synthesis of one-dimensional (1D), group-III-nitride nanostructures such as indium nitride (InN), gallium nitride (GaN), and aluminum nitride (AlN), has been accomplished mainly by using catalyst-assisted or oxide-assisted growth methods. 1-4 In the case of catalyst-metal-cluster-assisted techniques, gold clusters have served mostly as templates for the 1D growth of III-nitrides. 1-3 In the case of oxygen- assisted techniques, oxide sheaths around crystals assisted the growth of III-nitrides in one dimension. 4 A completely different concept, involving the use of low-melting metal mediums for the multiple nucleation and growth of nano- wires, was also reported in the literature. 5-8 The solubility of nitrogen in low-melting metals including Group III metals (Ga, In, Al) is extremely low. Therefore, the dissolution of nitrogen into low-melting metals is expected to yield crust formation or multiple nanowire growth from molten metal pools. 6 This phenomenon should be true even when low- melting metal droplets become smaller. 9 An alternative is to use the vapor transport of In onto substrates in the presence of decomposed ammonia in a reactive vapor transport approach. 10-13 Recent studies have shown that the combined vapor transport of indium (In) and decomposed ammonia onto substrates leads to the formation of 1D InN structures. 10 This nanowire synthesis method is henceforth referred to as “reactive vapor transport” in this manuscript. However, the nucleation and growth mechanisms leading to the formation of 1D structures in reactive vapor transport methods are not understood. Knowledge on the nucleation and early stages of growth could potentially lead to control over the size and the growth direction of the resulting nanowires. Concisely, the main questions are the following: (1) What is the nucleation mechanism that explains nanowire formation in reactive vapor transport experiments? (2) Why does the growth occur in one dimension? Synthesis of heteroepitaxial nanowire arrays on single- crystal substrates is important for the integration of nanowires into device applications. Similarly, rational approaches for synthesizing highly branched nanowire systems (tree-like structures) could be important for increasing the area for interfacial processes in applications such as catalysis and energy conversion. Thus far, only catalyst-metal-assisted techniques have been shown to produce heteroepitaxially oriented arrays of nanowires. 14 In addition, the synthesis of branched nanowires resembling a “tree formation” has been reported recently with the continuous use of catalytic metal * Corresponding author. Tel: 502-852-1558; fax: 502-852-6355; e- mail: mahendra@louisville.edu. ² Department of Chemical Engineering. ‡ Department of Electrical and Computer Engineering. § Department of Physics. | NASA Ames Research Center. NANO LETTERS 2005 Vol. 5, No. 8 1625-1631 10.1021/nl0505804 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005