The theme of this thesis is on artificial structures, ranging from the macroscopic to the nanoscale. The first part of the thesis is on metamaterials, which are artificial structures designed to have wave manipulation functionalities beyond those found in nature. An ultra-broadband microwave absorber was designed, fabricated, and experimentally measured. The second part of the thesis is on carbon nanostructures. In particular, the focus is on the search for superconductivity in Mg-doped carbon nanostructures embedded in ZSM-5 zeolite templates. Some elaborations on both projects are given below.

The advent of 5G technology, which means faster and higher capacity communication, is accompanied by higher microwave power in the 5G active space. The motivation for the microwave metamaterial...[ Read more ]

The theme of this thesis is on artificial structures, ranging from the macroscopic to the nanoscale. The first part of the thesis is on metamaterials, which are artificial structures designed to have wave manipulation functionalities beyond those found in nature. An ultra-broadband microwave absorber was designed, fabricated, and experimentally measured. The second part of the thesis is on carbon nanostructures. In particular, the focus is on the search for superconductivity in Mg-doped carbon nanostructures embedded in ZSM-5 zeolite templates. Some elaborations on both projects are given below.

The advent of 5G technology, which means faster and higher capacity communication, is accompanied by higher microwave power in the 5G active space. The motivation for the microwave metamaterial absorber project is based on human health concerns arising from the long term exposure to order of magnitude higher microwave power, as well as on providing a balance between necessary monitoring and privacy. The microwave metamaterial absorber was based on simple elements—metallic rings—that are resonant in the microwave frequency regime, implemented with hierarchical self-similar structures so as to attain an ultra-broadband absorption spectrum. The absorber’s performance is polarization-independent and insensitive to oblique incidence up to 45°. It exhibits an average of 19.4 dB reflection loss from 3-40 GHz with an overall thickness of 14.2 mm, which is only 5% more than the ultimate minimum thickness limit imposed by the causality principle. The absorber is both electrically and magnetically excited, with two-dimensional capacitively coupled (electrical) dipolar resonances in the lateral plane. By placing a metallic boundary at an optimal distance, chosen via simulations, to the resonant array, the electrical dipolar resonances generate two additional high impedance magnetic resonances. By optimizing the resistance value of the chip resistors soldered on the metallic rings, impedance matching with vacuum is achieved, leading to near-total microwave dissipation. With its simple basic structure of printed metallic rings on PCB board, this microwave absorber implies low production cost and, therefore, can be easily mass-produced for diverse applications.

In the nanoscale area, the second part of this thesis is concerned with the search for superconductivity in chemical vapor deposition (CVD) fabricated carbon nanostructures formed in the pores of the ZSM-5 zeolite template. Inspired by Peierls-type metal-insulator transition in carbon nanostructures grown within the pore network of ZSM-5 zeolite, which implies strong electron-phonon interaction, we prepared Mg-doped carbon nanostructures grown within ZSM-5 zeolite to further explore their superconducting properties. The carbon nanostructures showed a distinct Raman radial breathing mode (RBM) feature, characteristic of the (3,0) carbon nanotubes. The magnetic SQUID measurements showed a significant negative magnetoresistance under 30 Oe, which resembled the Meissner effect up to 800 Oe. A one-dimensional superconducting transition at 18 K, and a three-dimensional superconductive-like signals at 20 K, were observed in the transport measurements on two separate samples. The 3D superconducting signal, which displayed a magnetic dependence, was superposed on a semiconductor-type signal. However, in both cases the SQUID and transport results did not show consistency with each other. Hence these observed phenomena cannot be attributed to superconductivity with certainty. Due to the retirement of Prof. Sheng, this line of research is therefore ended. The underlying mechanism(s) of the phenomena remains unknown.