Mechanism for controlling the shape of Cu nanocrystals prepared by the polyol process
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This study investigated a mechanism for controlling the shape of Cu nanocrystals fabricated using the polyol process, which considers the thermodynamic transition from a faceted surface to a rough surface and the growth mechanisms of nanocrystals with faceted or rough surfaces. The faceted surfaces were stable at relatively low temperatures because of the low entropy of perfectly faceted surfaces. Nanocrystals fabricated using a coordinative surfactant stabilized the faceted surface at a higher temperature than those fabricated using a noncoordinative surfactant. The growth rate of the surface under a given driving force was dependent on the surface structure, i.e., faceted or rough, and the growth of a faceted surface was a thermally activated process. Surface twins decreased the activation energy for growth of the faceted surface and resulted in rod- or wire-shaped nanocrystals.
I. INTRODUCTION
Nanocrystals with uniform sizes and shapes have a wide range of potential applications, such as in biosensoring systems, plasmonics, photonics, and recording media. Simultaneously, nanocrystals can serve as templates for nanostructures or as building blocks for more complicated nanostructures.1–3 It has been reported that the physical and chemical properties of nanocrystals are dependent on the materials, and at the same time, particle size and shape.1–3 For example, the plasmonic resonance and catalytic properties of metal nanocrystals are dependent on the crystallographic characterization of surfaces, and the magnetic properties of Fe or Co nanocrystals are dependent on their size and aspect ratio.1–3,21 Therefore, monodispersed nanoparticles with a controlled shape and size are required for the application of nanocrystals. In recent years, there has been intense research on the fabrication of nanocrystals using various processes.1–6 Rapid thermal decomposition can produce nanocrystals with a uniform size distribution and an average size below 10 nm by separating the nucleation and growth mechanisms.1,3 However, the control of nanocrystal shape during rapid thermal decomposition is relatively difficult. Slow processes, such as reduction with metal salts or slow thermal decomposition, can be used to control the shape of nanocrystals.2 The polyol process is one
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0285 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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such slow process in which the metal salts are reduced by polyols. The polyol process has been used to produce metal nanocrystals of Fe, Co, Ni, Cu, Ag, Au, Pt, and their alloys. Moreover, nanocrystals of various shapes can be fabricated using the polyol process, including cubes, spheres, rods, and wires.2,4–17 The process parameters such as temperature, reaction time, and type of surfactant can influence the shape of nanocrystals.2,4–17 However, the mechanism for controlling the shape of nanocrystals is not fully understood. The equilibrium concentration of vacancies
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