Nanoscale light-matter interactions in van der Waals materials

In this thesis, I study nanoscale light-matter interactions in two-dimensional (2d) and van der Waals (vdW) materials, with a particular focus on graphene and graphene-based heterostructures.
In part I, I begin by reviewing some key background information, with an overview of the physics of graphene and bilayer graphene (BLG) in chapter 1, and an overview of the wider family of 2d materials and vdW heterostructures in chapter 2.
Then in part II, I follow this with a review of the characterisation techniques used in the rest of this thesis, with a discussion of laser-coupled scanning probe microscopy (SPM) methods in chapter 3, and of optical spectroscopy in chapter 4
Finally in part III, I present several original research chapters based on work I conducted during my PhD:
Chapter 5 demonstrates spatial mapping of nanoscale strains and doping in graphene heterostructures with Raman vector decomposition analysis, in which I reveal that variations in Raman spectra correlate with nanoscale features such as cracks, bubbles and wrinkles.
In chapter 6, I build upon this by investigating bubbles in boron nitride-encapsulated graphene with scanning near-field optical microscopy (SNOM), and reveal strongly absorbing subwavelength domains for infrared light, which relate to the complex strain configurations of the bubbles.
Chapter 7 focuses on a different vdW material indium selenide, and I use Raman and photoluminescence (PL) spectroscopy to probe strains induced by depositing the material on a patterned substrate.
Lastly, in chapter 8 I showcase different ways that data cluster analysis, a machine learning tool, can accelerate the identification and analysis of twisted bilayer graphene from Raman spectra.

The research presented here demonstrates that the combination of advanced microscopy and spectroscopy with modern computational techniques, such as vector decomposition and machine learning, can reveal a wealth of important material properties. With these techniques, I show that nanoscale features of vdW materials and heterostructures, such as fractures folds and bubbles, play a significant role in determining their optoelectronic properties, particularly via their local effects on strain and doping. This is fundamentally important for both the design and characterisation of 2d material-based optoelectronic devices.