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MATERIALS RESEARCH

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Synthesis of cobalt oxide nanocrystal self-assembled materials J. S. Yin and Z. L. Wanga) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Received 12 August 1997; accepted 3 May 1998)

Self-assembling of size, shape, and phase-selected nanocrystals into superlattices in a new approach for synthesizing a new generation of advanced materials with functionality. In this paper, high purity and monodispersive tetrahedral of CoO, with edge lengths of 4.4 6 0.2 nm, have been synthesized and separated from Co nanocrystals using colloidal chemistry and magnetic separation. The tetrahedral CoO nanocrystals behave like a molecular matter, and their assembling forms superlattices with translational symmetry. The phase transformation of the CoO nanocrystals is examined by ex situ annealing in oxygen, and the results showed the formation of Co3 O4 with spinel structure.

I. INTRODUCTION

Nanocrystal materials have many potential applications owing to their unique particle sizes and surface effects.1–10 Very recently, there has been a great deal of interest in making monodispersive nanocrystals and assembling these nanocrystals into well-organized twoand three-dimensional “quantum” crystals. The quantum crystals can be used in data storage, solar absorber, optical gratings, and other microelectronics devices. In addition to the fascinating properties offered by the nanocrystals themselves, one unique advantage of the quantum crystals is the flexibility of controlling the interparticle distance, allowing tunable interparticle interaction and coupling properties. The size of the nanocrystals is controlled to be smaller than 10 nm, below which the quantum effect is significant. Up to now self-assembly passivated nanocrystal superlattices (NCS’s) or nanocrystals arrays (NCA) have been successfully fabricated using metal,11–16 semiconductor,17–19 oxide,20 and sulfite21 clusters. Well-defined ordered solids prepared from tailored nanocrystalline building blocks provide new opportunities for optimizing and enhancing the properties and performance of the materials. This is a new initiative of research on cluster engineered materials. There are two key components for constructing the NCS’s. First, the nanocrystal cores are required to be monodispersive, even with controlled shape. Second, the surface passivation molecules need to be chemically active to cap tightly on the surfaces of the nanocrystals. This surface-coating layer separates, protects, and links the nanocrystal cores, forming ordered superlattices with translation and even orientational order. The length and density of the surface-passivated molecules as well as the a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 2, Feb 1999

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size of the nanocrystals cores determine the crystallography of the three-dimensional packing. The length of the molecules can be easi