报告题目：Characterization of Crystallinity of Gold using X-Rays or Neutrons
报 告 人：Prof. Heinrich Nakotte (New Mexico State University)
报告时间：2017年6月8日（周四）下午15:00
报告地点：励吾科技楼303（材料平台会议室）

报告摘要：Gold is probably the most important precious metal: it is used in various electronics and optics applications, it provides a safe monetary equivalent for the investment market, and it is greatly appreciated in jewelry. Bulk gold crystallizes in a face-centered-cubic structure, but it is currently impossible to grow large single crystals of gold cannot in the laboratory because of difficult and slow growth conditions. In general, large natural gold nuggets are also polycrystalline, with very few exceptions of crystals found in certain parts of the world with favorable geological conditions. These crystals can have dimensions of several centimeters, and they are more rare than similar-sized diamonds. Subsequently, they are highly desirable specimens for collectors, but the market has been plagued by some ‘fake’ gold crystals. Full determination of the crystallinity of such large gold crystals requires a highly penetrating probe, such as neutrons. I will discuss our neutron diffraction studies on some of the large gold crystals that came up for auction, including the largest known crystal in the world. On the other end, micron-sized gold crystals grown under non-equilibrium conditions, tend to crystallize in highly geometrical shapes, such as five-fold decahedral ones, rather than forming the extended bulk face-centered-cubic (fcc) structure found in bulk gold. The five-fold symmetry of decahedral particles is inconsistent with a continuous crystal lattice and it is generally explained by multiple twinning of a tetrahedral subunit about a symmetry axis, with or without structural modification to the fcc motif. The small size of decahedral gold particles severely limits the application of traditional techniques, such as Bragg diffraction. I will discuss the sensitivity and the application of Pair Distribution Function (PDF) analysis for modeling of total (neutron and X-ray) scattering data of decahedral gold nanoparticles.

报告人简介：Heinrich (Heinz) Nakotte，1994年获得阿姆斯特丹大学物理学博士，现为美国新墨西哥州立大学物理系教授，博士生导师。纳科德教授的研究主要集中在各种材料的磁性以及相关的特性，如电子系统、永磁体、超导体和纳米磁体；主要研究方向是单晶的磁各向异性，特别是研究如何在极端的外部条件（如非常低的温度，高的压力和/或非常高的磁场条件）将磁性材料进行调制改性。Nakotte教授还利用中子衍射和强磁场技术对磁性材料的磁性及相关机理进行了研究。Nakotte教授在其三十多年的磁学和磁性材料研究中取得了多项重要研究成果，发表SCI论文200 多篇，先后主持美国多个国家级纳米磁性材料的合作研究项目，成为美国该领域的领军人物并获得了Gardiner教授等多项殊荣。Nakotte教授目前担任美国国家科学基金会、美国能源部、新墨西哥太空财团及石油研究财团的项目评委，同时还担任国际工程科学杂志的编辑。