Electron Diffraction and HRTEM Structure Analysis of Nanowires

In this work an electron diffraction analysis and an electron microscope structure study of nanowires are presented. Nanowires, as one-dimensionally nanostructured materials, have become the focus of intensive research due to their great potential for use as building blocks in the fabrication of electronic, optoelectronic, and sensor devices with nanoscale dimensions. Therefore, the importance of studying and understanding the electron diffraction phenomena and their implications over their electron microscopy images is great. It has been indicated that the silver nanowires are highly faceted at nanometric scales. A comprehensive electron diffraction study on the structure of nanowires, firstly in a general approach and then in the specific case of silver nanowires, will be presented in this chapter. The important role of twinning in determining the habit of the final morphology will also be discussed. One interesting feature of silver, gold, and copper pentagonal cross-section nanowires, is their remarkable structure (Gao et al., 2003; Giersig et al., 2004; Hofmeister et al., 2002; Sun et al., 2002; 2003; Zhao et al., 2005). It has been proposed that this structure evolved from a multi-twin decahedral nanoparticle growing in the [110] direction by stabilizing more effectively the newly formed 100 facets than the 111 facets and their electron diffraction patterns present "forbidden" spots. The pentagonal arrangement in the multiple twinned particles (MTP) is quite known. MTP nanoparticles of transition metals with face-centered cubic (FCC) lattice (Heinemann et al., 1979; Howie M Sun et al., 2002) have been reported. On the basis of these studies, the basic structure of a decahedral particle was described as the junction of five tetrahedral single crystals with twin-related adjoining faces along a common [110] edge in such a way that the [110] direction parallel to the five-fold axis is perpendicular to the direction of the edges, and that the normal vectors to all of them are parallel to . The theoretical angle between two [111] planes is 70.5◦ , so by joining 5 tetrahedrons, which are bounded by 111 facets, a gap of 7.5◦ is generated. Thus, to fill this gap some internal strain is necessary, giving place to dislocations and other structure defects (Heinemann et al., 1979; Howie & Marks, 1984; Sun et al., 2002). These defects have been observed in the TEM cross-sectional images of the mentioned penta-twinned nanowires (Chen et al., 2004). When observing nanowires with the high-resolution transmission electron microscope (HRTEM) very often we observe a contrast that can be interpreted at first sight as coming Electron Diffraction and HRTEM Structure Analysis of Nanowires