Surfactant and temperature influence on Co nanocrystal structure and shape
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Surfactant and temperature influence on Co nanocrystal structure and shape V.F. Puntes1,*, D. Zanchet2, C. K. Erdonmez3, A.P. Alivisatos1 1 Chemistry Department, University of California, Berkeley, 94720, USA 2 Laboratório Nacional de Luz Síncrotron, Campinas-SP, 13084-971, BRAZIL 3 Materials Science Department, University of California, Berkeley, 94720, USA * Present address: Physics Department, University of Barcelona, 08028 SPAIN
ABSTRACT Inorganic nanoparticles of a variety of materials have been obtained by chemical methods. Under the kinetic control, small variations in the synthesis recipe may yield crystals of different sizes, shapes and structures. Co nanocrystals are of particular interest: besides achieving size control, at least three nearly isoenergetic crystal structures (face-centered cubic, hexagonal closepacked and epsilon) can be synthesized, and shape control (spheres, prisms, cubes, rods and disks) is also possible. Although shape and structure should be intrinsically related, the relationship between these two characteristics has not been deeply explored up to this moment. Here, we present our latest results on Co nanoparticles, analyzing the influence of some parameters on particle shape and structure. In particular, the influence of ligand type and temperature has been tackled.
INTRODUCTION Colloidal synthesis is a powerful route to obtain large collections of monodisperse inorganic nanocrystals with well-defined characteristics. The advances in this field have been exciting, allowing nowadays the development of new materials with properties tuned at the molecular level [1,2]. One main target has been the generation of homogeneous nanoparticles with controlled sizes and more recently shapes, to use them as building blocks for more complex materials or as specific hybrid molecules to interact in bio-medical processes. In this approach, it has been observed that during nucleation and first stages of particle growth, small variations in the synthesis recipe may yield crystals of different sizes, shapes and crystal structures [1,3-5]. In other words, a large assortment of nanocrystals may be obtained by slightly perturbing the system at the earlier stages of growth. In addition, particle size, shape and crystal symmetry are closely related and the predominant mechanism is a very important issue and worth to be understood. The case of Co is of particular interest: besides the size control, at least three nearly isoenergetic crystal structures (face-centered cubic - fcc, hexagonal close-packed - hcp and epsilon - ε-Co) have been synthesized [5-7] and shape control (spheres, prisms, cubes, rods and disks) has also been achieved [6,8]. Similar to the pioneering example of shape control in CdSe [3], Co anisotropic growth (disks) is favored by a certain type of ligand (in this case long chain amines) [5]. The proposed mechanism is based on the different affinity of the ligand for different crystallographic facets, leading to changes in their relative growth rate. In the Co case, however, the
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