Generation of quantum emitters in hBN via strain engineering for biosensing applications

Abstract

This master thesis explores strain-induced quantum emitters in hexagonal boron nitride (hBN) as novel optical nanoprobes for Förster resonance energy transfer (FRET)-based biosensors. These types of emitters could outperform conventionally used fluorophores due to their high brightness, stability in harsh environments, biocompatibility and ease of integration with solid state devices. Ultimately, the aim is to combine optically-active hBN emitters with protein fingerprinting devices, which could impact the field of molecular diagnostics by detecting clinically relevant protein biomarkers.

To date, however, it is unclear which parameters are crucial for the generation of hBN quantum emitters with strain in both CVD grown and exfoliated hBN crystals. To address this gap in the field, this thesis systematically investigates the generation of strain by mechanically exfoliating pristine hBN crystals onto a variety of rigid micro/nanostructures with different aspect ratios, including 5 µm and 10 µm microbeads, femtosecond laser-ablated cavities, and CD/Blu-ray micro-nanostructures. We characterised the samples with fluorescence microscopy and atomic force microscopy in order to correlate the optical properties of the hBN with the topography of the substrate. Among the tested structures, samples displayed clear fluorescent emission at the location where the hBN was deposited on the femtosecond laser-ablated cavities with sharp edges. The presence of strain in these regions was verified with Raman spectroscopy, and the spectral properties of the fluorescent regions were determined with photoluminescence spectroscopy. We additionally studied the temporal behavior of the identified emitters and observed effects such as blinking with intensities reduced up to a 38 % and photobleaching with quantum emitters’ lifetimes between 6.57 s and 44.17 s.

While there were no clear threshold values of curvature, substrate structure height, and thickness of hBN that led to reproducible localized fluorescence, these findings open up further research opportunities for the use of strain engineering to generate quantum emitters in hBN.