Nanoscale Self-Assembly: Energetic Irradiation Triggering And In Situ Monitoring

Abstract

Origami inspired assembly of three-dimensional (3D) micro and nano-structures arise to be a broad topic in the past two decades due to their ability of property engineering, 3D space utilization, and controlled motion, which have been widely used in the applications of metamaterial and plasmonic devices, electronics, and biomedical devices. However, the present techniques for the assembly of 3D nanostructures, such as by using DNA technology, reactive ion etching, atomic layer deposition, and metal-assisted etching, do not allow real-time visualization, which bring great challenges in controlling the shape with nanoscale precision, resulting in an extremely low yield and significant geometric and topological constrains. To address the issues of the traditional nanoscale self-assembly, my Ph.D. work involves developing novel in situ monitored self-assembly techniques triggered by energetic irradiation, such as ion and electron beam. The energetic irradiations offer two functions in the self-assembly: 1) on one hand, an excited ion or electron beam is able to deliver energy to the specific irradiated material, triggering localized material phase change, such as Sn grain coalescence, crystallization of amorphous material, or polymer reflow and shrinkage. Associated with the material phase changes, stress is induced in the thin film, folding the suspended 2D thin film up to 3D nanostructures; 2) on the other hand, the imaging capability of ion or electron beam enables real-time monitoring of the self-assembly process, making it possible to precise tune the energy delivery to reach the desired assembly status. Because of the localized energy delivery and real-time imaging, ultra-high self-assembly process with sub-10 nm scale precision is achieved. With further understanding of the material-irradiation interaction and careful design of the 2D patterns and material layout, more advanced functions have been achieved, leading to programmable, sequential, multidirectional, and reversible self-assembly in nanoscale. Further, the energetic irradiation triggered self-assembly processes have been used to build functional materials with advanced properties. I develop a strategy to build 3D graphene based nanostructures (i.e. nanocube and nanotube) via self-assembly process, which is one of the pioneer works in this field. By transforming graphene into 3D format, its amazing properties could be modified by the extra dimensionality, achieving enhanced or novel behaviors that does not exist in 2D. For instance, the plasmonic near-field enhancement of planar graphene undergoes severe exponential decay in the vertical direction away from the surface of the graphene, resulting in a relatively small spatial overlap between the specimens and the volume of high field confinement. I find that 3D graphene nanostructures exhibit novel plasmon hybridizations, which result in a near-field enhancement across the entire surface of these 3D structures as well as within their spatial volume. As the sensitivity is directly related to the field intensity in the vicinity of the analyte, the strong volumetric electric field confinement in these 3D nanostructures are proposed to be candidates for high sensitivity detection of proteins and other biological specimens. In addition, self-assembled nanocylinders with automatically formed plasmonic nanogaps have been developed into a nanofluidic based plasmonic sensor. The sequential and reversible self-assembly processes enable the realization of nano-machine and nano-robotics. Overall, this in situ monitored self-assembly technique provides a solid foundation to build 3D nanostructures with various advanced functions, which push the limit for further exploration of the next generation devices.