High Resolution Lithography Techniques for Applications in Plasmonic Nanomaterials

Abstract

Metallic nanoparticles are of great scientific interest owing to their ability to support collective oscillations of electrons across their surface, known as localised surface plasmon resonances (LSPRs), when illuminated with light. These LSPRs amplify optical fields at the surface of the nanoparticle. The enhanced fields yield increased light-matter interactions, which have exciting applications in light-emitting devices, sensing, and the modification of the reactivity of chemical bonds via the formation of strongly-coupled systems. The enhanced fields can also result in the excitation of hot charge carriers which have applications in charge-transfer catalysis. The decay of hot carriers results in a localised heating of the nanoparticle that also has applications in catalysis. The LSPR wavelength is dependent on the metal used, the background medium, and the size and shape of the metal nanoparticle. As such, precise control over the dimensions of the metallic nanoparticle is essential for exploiting LSPRs for the aforementioned applications. Top-down fabrication methods such as electron-beam lithography (EBL) and helium ion beam lithography (HIBL) are ideal methods for the fabrication of plasmonic nanoparticles as they allow for the fabrication of arbitrary 2-D geometries with precise control of their on-chip location.

In this thesis, the limits of HIBL are investigated via the simulation and measurement of the point-spread functions of the He+ ion beam in both poly (methyl methacrylate) (PMMA) and a fullerene-derivative resist suspended on ultrathin membranes. PSFs were measured by performing point exposures with varying He+ ion beam dose. Transmission electron microscopy (TEM) was then employed to measure the average radius of the features produced as a function of beam dose. The experimental PSFs of both PMMA and the fullerene-derivative were then compared to a similar study carried out in PMMA exposed using a state-of-the-art aberration-corrected STEM. The results indicate that HIBL has the potential to outperform aberration-corrected EBL for the patterning of dense arrays of nanostructures regardless of the pitch of the array.

In this work we used simulations alongside high-resolution EBL to design and fabricate various plasmonic Au nanoparticles. The enhanced optical properties the plasmonic nanoparticles were exploited to probe the coupling between a vibrational mode in a carbon nanodot and a plasmonic antenna. Finite-difference time domain (FDTD) simulations were used to determine the dimension of Au antennas capable of supporting both a mid-IR LSPR mode to couple with a graphitic G band peak, as well as a near-IR mode to couple with a Raman laser to allow the coupled system to be probed via surface-enhanced Raman spectroscopy (SERS). EBL was used to fabricate the segmented antennas, and the NIR resonance was confirmed using optical spectroscopy. To date, the presence of the IR LSPR mode has not been confirmed.

Finally, the application of the localised heating of plasmonic Au nanoparticles was investigated via the LSPR-induced growth of semiconductor materials. FDTD simulations were again used to design plasmonic Au nanoantennas having LSPRs resonant with an excitation laser. Au nanoparticles were fabricated using EBL and their resonance was confirmed using optical spectroscopy. Scanning electron microscopy (SEM), EDX, and TEM were used to confirm the growth of both crystalline Ge nanowires, and amorphous Ge. Preliminary results indicate the ZnO nanowires can also be grown via using the same experimental setup. It was determined that crystalline Ge is grown via a solution-liquid-solid growth mechanism. Amorphous Ge was determined to grow via the coffee-ring deposition of elemental Ge onto the surface of the Au nanoparticles due to the formation of air bubbles caused by the plasmonic heating of the precursor material. ZnO nanowires were also grown via a hydrothermal synthesis method. The growth of nanoscale semiconductor materials via a range of different mechanisms highlights the versatility of the LSPR-induced growth process.