Determination of surface atomic structures of Bi₂Se₃(111)-(2X2) film and ZnO nano-rods by low energy electron diffraction

Abstract

The emergence of topological insulators and nano-materials brings the importance of surface science to a new level. The properties of topological insulator depend strongly on surface state. The novel properties of nano-materials are strictly related to their surfaces structure due to the large surface-area-to-volume ratio. In this thesis, I investigated the surface morphology of the Bi2Se3(111)-(2x2) reconstruction and the ZnO nano-rods by means of low energy electron diffraction(LEED).

Bi2Se3(111)-(2x2) was prepared by molecular beam epitaxy, and the reconstruction is believed to be caused by Se adsorption. An atomic model with Se on the top site is proposed by the virtue of multiple incidence angles LEED IV Patterson inversion.

Well aligned ZnO nano-rods array were prepared by template-assisted hydrothermal growth and vapour phase transport growth. Nono-rods prepared by vapour phase transport method were able to give the LEED IV spectra. The side facet orientation is found to be {101 ̅0} in the LEED perspective. The peak shifts of nano-rod IV spectra suggest that the nano-rods undergo a different extent of relaxation compared with that of single crystal. Here, I emphasise it is the first ever successful extraction of LEED IV from free standing nano-material surfaces.

Although the above proved LEED is a powerful technique in surface morphology analysis, conventional LEED optics suffers electric charge up when probing high-resistivity material. In order to bring LEED to its widest potential, microchannel plate low energy electron diffraction (MCP-LEED) system was implemented. MCP-LEED avoids the charge up by bringing the electron beam current further down to the order of nA, when compared with mA of the conventional LEED system. An automate LEED data acquisition scheme was installed on the MCP-LEED system and the validity of the MCP-LEED system was verified by comparing its LEED IV spectra with that of the conventional LEED. The capability of MCP-LEED is justified by probing the high-resistivity ZnO single crystal manufactured by Tokyo Denpa. Pros and cons, as well as suggested future improvements of MCP-LEED are discussed.