Investigation of Growth Behavior of ZnS Nanocrystal by HR-TEM and STEM-Tomography
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Investigation of Growth Behavior of ZnS Nanocrystal by HR-TEM and STEMTomography Masato Uehara Measurement Solution Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 807-1, Shuku-machi, Tosu, Saga 841-0052, Japan ABSTRACT We obtained interestingly-shaped ZnS nanocrystals in oleylamine solution and characterized their structures using HR-TEM and HAADF tomography. These interesting nanocrystals had a conical head rather than the frequently-reported pyramidal one. The cone consisted of not only low-energy planes but high Miller-index planes. In addition, we threedimensionally and crystallographically investigated the particle growth of these nanocrystals by the same method. INTRODUCTION Nanocrystals (NCs) have received much attention because of their attractive properties. There are many reports on the synthesis and structure control of NCs, and these materials with high colloidal stability and narrow size distribution have been developed. The attractive properties of NCs are strongly influenced by their size and shape in addition to their crystalline structure; consequently, it is very important to control their morphology [1-3]. In particular, we must characterize and control the surface structure of NC materials because those structures govern particle growth behaviour. HR-TEM is a powerful tool for the study of the atomic structures of individual particles. However, HR-TEM imaging alone is not sufficient for a discussion of NC surface structure. NCs have various surface configurations; flat, curved and convex segments, so their surfaces must be discussed three-dimensionally [4-6]. Using HR-TEM alone, it is not easy to understand three-dimensional surface configurations[5]. Contrast based on thickness is low, and information on configuration along the observational direction is lacking. Meanwhile, the contrast in STEM-HAADF (High Angle Annular Dark Field) images depends heavily on atomic mass and specimen thickness[7]. Then, using HAADF-tomography, we can obtain threedimensional information about a configuration on a nanoscale. By combining these techniques, HR-TEM and HAADF-tomography, we can crystallographically and three-dimensionally discuss surface atomic structures. Indeed, computational investigation and other analyses are needed for clarification of an actual surface structure, but the above analyses would be useful for understanding the crystallographic structure of a surface, leading to the ability to control attractive properties based on NC surface characteristics. ZnS is an attractive material as an optoelectronic device and catalyst[8, 9]. In a previous study[10], we achieved the controlled-synthesis of ZnS nanodots and nanorods from the same raw material solution by varying the heating rate. We concluded that the particle shape and crystalline phase were influenced by the adsorption properties of oleylamine ligands incorporating changes in the particle surface structure. Especially, we obtained nanorods at a slow heating rate. The nanorods had interesting crysta
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