Hexagonal Boron Nitride Nanostructures for Optofluidic Biosensing

Abstract

Hexagonal boron nitride (hBN) has emerged as a unique platform for room temperature quantum photonics, yet translating its optically active defects into a practical single molecule sensing technology requires two stringent conditions: (i) deterministic and spatially controlled generation of emitters and (ii) engineering nanoscale confinement geometries that reliably bring labelled biomolecules in proximity to the hBN emitters while suppressing background fluorescence. This dissertation develops and connects these two capabilities through complementary routes based on optical and strain nano-engineering in layered hBN.

We initially establish a microsphere-assisted femtosecond-laser approach to enhance light–matter interaction during defect formation and readout. By exploiting the combination of photonic nanojets with whispering gallery mode-assisted signal collection, the method enables deterministic emitter generation with improved spatial confinement and higher collection efficiency compared to microsphere-free processing. Specifically, the approach reduces the emission area by a factor of five and increases fluorescence collection efficiency by approximately tenfold.

A second result is the generation and characterization of hBN wrinkle networks in multilayer hexagonal boron nitride,which form from thermal expansion coefficient mismatch with the substrate during annealing. We demonstrate that wrinkles function as planar nanoscale confinements, and can therefore be used as a feature rather than a limitation. Liquid infiltration and retention are validated by time dependent optical imaging, Raman mapping of the water OH stretch band, and capacitance gradient mapping, consistent with liquid retention exceeding 10 h. This self-assembly process provides a lithography-free route to obtain 1D nanochannels and multi-junctions directly on-chip.

For the purposes of biomolecule confinement and imaging, however, such confinements alone do not guarantee clear optical readouts because wide-field imaging remains limited by fluorescence background from surface adsorbed molecules. This shortcoming motivated a background suppression strategy that we implement via vertical hBN/graphene stacks. By using few-layer hBN as a precise physical spacer between biomolecules and graphene, non-radiative energy transfer can be tuned in a predictable manner. This yields a parameter i.e. spacer thickness, that can be exploited to control the degree of quenching and fluorescence recovery. In this way, graphene suppresses unwanted background fluorescence from molecules adsorbed on hBN wrinkles, while preserving the emission from molecules confined deeper inside the wrinkle volumes. As a result, the imaging contrast is starkly improved.

Overall, this dissertation demonstrates how hBN emitter engineering, strain defined confinement, and interface controlled background suppression can be combined into a framework for high-throughput, fluorescence based biosensing using hBN, forming the first steps towards optical protein fingerprinting at 2D material interfaces.