Abstract
This thesis aims to advance the application of hBN as a single-photon source by investigations of native
hexagonal boron nitride (hBN) single-photon emitters (SPE), unreported luminescent hBN-substrate
layers, and the positioning of emitters using an atomic force microscopy (AFM) nano-indentation
method.
Mechanically exfoliated mono-, bi-, and multi-layer hBN flakes are thermally annealed to optically
activate native defects in the hBN crystal lattice, which are known to produce SPE. Photoluminescence
(PL) mapping is used to survey flakes and locate emitters. Emitter zero phonon lines (ZPL) with
wavelengths between 560 – 620 nm are observed, accompanied by a characteristic phonon sideband
detuned by ~160 meV. Emission in mono- and bi-layer hBN was found to be highly unstable, and it
was not possible to revisit emitters to confirm SPE. SPE was confirmed from multilayer emitters using
Hanbury-Brown and Twiss (HBT) interferometry. Only one emitter initially passed the criteria for SPE
of g(2)(0) < 0.5, but many displayed anticorrelation dips with 0.5 < g(2)(0) < 1, indicating the additional
presence of background photons or multiple emitters. All emitters displayed blinking or bleaching,
and this was leveraged to separate data into groups by emission intensity (‘bright’, ‘dim’ or ‘off’ states)
for analysis. This approach allowed careful accounting for background photons in some HBT data,
resulting in a background corrected g(2)(0) value of -0.009 ± 0.032 for one emitter. This represents one
of only two measurements of g(2)(0) = 0 within experimental error for hBN SPE at room temperature
currently. Further analysis using the intensity grouping method allowed identification of two distinct
energy level configurations for a rapid blinking defect, likely caused by charge state switching similar
to colour centres in diamond, supported by an associated single exponential state duration switching
time.
Annealing of hBN can produce broadband luminescence in addition to localised emission from SPEs.
Here, it was discovered a significant source of this luminescence is from previously unreported, ultrathin residue layers located between the hBN flake and substrate. Residues were isolated and identified
by transferring parent hBN flakes to a different substrate using a homebuilt all-dry transfer system
and performing PL and AFM imaging of the flake’s original location. Residues bore a striking
resemblance to monolayer hBN, with a height of just 0.3 nm and displaying the same luminescence
pattern as the parent flake. Furthermore, SPE like spectra are identified originating from residues at
2
multiple locations. However, it is established, by correlation of the rate of occurrence of bubbles and
decline of residue height, the most likely origin of the residue layers is a thin uniform layer of
contamination trapped between hBN and substrate during fabrication. It is also found the key step to
activating the strong residue luminescence is high temperature annealing.
To create arrays of deterministically placed emitters, nano-indentation of polymer supported hBN was
performed using an AFM. A detailed account of the nano-indentation method is given for hBN of
different thicknesses, including the limitations found. For 2 nm and monolayer hBN, no indent related
emission was identified. Optical characterisation of emitters demonstrated poor photostability and
ZPLs spread over a wide wavelength range for 15 nm hBN. Thermal light contributions from the
underlying polymer due to high incident laser power impeded quantification of HBT data. 57% of
indent sites displayed signs of anticorrelation, with no optimisation of indent parameters,
demonstrating excellent potential for this method. The lowest anti-correlation dip measured was
g
(2)(0) = 0.87 ± 0.04, comparable to some other fabrication techniques. The results of this thesis
advance the knowledge of native hBN SPE, will assist in background correction and elimination in HBT
experiments, and presents a practical method of deterministically positioning emitters.