报 告 人：Dexian Ye，副教授，弗吉尼亚联邦大学
报告题目：Oblique Angle Deposition Technique and Its Application in Optical Nanomaterials
Dexian Ye博士，美国弗吉尼亚联邦大学物理系副教授。1997 年于北京大学地球物理系获学士学位。2001师从著名半导体物理学家和材料学家Toh-Ming Lu教授，2006年于美国伦斯勒理工学院物理系获物理学博士学位，2006年到2009年，继续在伦斯勒理工学院从事博士后研究。2009年至今，在弗吉尼亚联邦大学物理系任职。叶教授主要研究领域为基于物理气相沉积的光子晶体、微结构光子材料设计、制备及应用，多次主持和参与美国国家科学基金重大科研设备项目等课题，先后在Phy. Rev. Lett., Nano Lett., Phy. Rev. B等学术期刊发表论文60余篇。
Oblique angle deposition (OAD) is a facile and versatile nanofabrication technique, which is based on a common vacuum physical vapor deposition system. It manipulates the arrangement of substrates to allow the light-of-sight vapor beam approaching the surface of the substrate at a large incident angle. The atoms are first self-assembled into islands on the surface controlled by thermal dynamics. Since the atoms incident at an angle, they only land on the protrusions and are prohibited to reach certain regions on the surface behind the islands due to the shadowing effect. As such, nanowires with similar structures are formed. They slant toward the PVD source with a same angle, and have uniform sizes and lengths. If the substrate rotates along its normal axis in OAD, the technique is specifically called dynamic OAD or glancing angle deposition (GLAD). The nanostructures can be designed and controlled to form complex shapes, such as nanosprings and nanospheres, by using GLAD technique. General speaking, OAD and GLAD can be used to fabricate nanostructures of a large variety of materials including semiconductors, metals, metal oxides, metal nitrides, polymers, etc. as long as they can be vaporized in PVD systems.
In recent years, combining experiments and computer simulations, we investigated the fabrication of nanostructures made of silicon, metal, titanium nitrides, and transition metal oxides using OAD and GLAD, and studied their growth mechanisms and physical properties. We found that long range capturing of incident atoms through ballistic sticking, non-unity sticking and surface re-emission mechanisms play equally important roles as the shadowing effect and surface diffusion in the growth of nanostructures. In simulations and experiments, we introduced templates to the flat substrates as the seeds to control the arrangement of nanostructures on the surface. The resulted regularly spaced nanostructures have unique optical properties such as the photonic band gaps and plasmonic resonance.
As mentioned above, OAD and GLAD techniques are focused on the manipulation of substrates in common practice. Nevertheless, the control of PVD source is not investigated. Recently, we introduced the partial ionization method to OAD and GLAD. The evaporated atoms are partially ionized through the electron bombardment and accelerated to gain certain amount of kinetic energy. When the ions deposited on the surface with other neutral atoms, the surface diffusion and other thermal dynamics can be enhanced by transferring the kinetic energy of ions to local surface. Under certain conditions, it is possible to promote the crystal growth without substantial substrate heating. We chose tungsten oxide (WO3) as the material to demonstrate this new approach. Single crystal WO3 nanowires can be grown at room temperature with 3 keV kinetic energy of the ions. WO3 is an important optical material with a band gap in the visible region, which can be used as photocatalysts for water splitting.