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We present the synthesis and characterization of gallium nitride (GaN) pine tree-like nanostructures (PTLNs) grown by low-pressure chemical vapor deposition. A high yield of PTLNs is densely arranged with each PTLN having a typical length of 15.46 ± 3.38 μm. From Raman spectroscopy, we observe anE2Hpeak at 570 ± 6 cm-1which is the primary characteristic of wurtzite. X-ray and ultraviolet photoemission spectroscopy reveal that the electronic structures of GaN PTLNs indicate an n-type character, while the work function and valence band maximum are determined to be 3.30 ± 0.05 and 3.85 ± 0.08 eV, respectively. We confirm the electronic nature of our structure from the current-voltage characteristics exhibiting rectifying behavior. Density functional theory calculations of GaN PTLNs modeled by germanium-doped GaN nanowires are consistent with our experimental findings. To summarize, the energy-band diagram is presented for the future of GaN PTLNs in the optoelectronic and sensing applications. Ministry of Education (MOE) This research is supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 2, grant no. MOE2016-T2-1-052 and Tier 1, grant no. MOE2019-T1-002063. F.A. and H.H. would like to acknowledge Toray Science and Technology Research Grant 2021.

We report the use of Germanium (Ge) as catalyst for Gallium Nitride (GaN) nanowires growth. High-yield growth has been achieved with Ge nanoparticles obtained by dewetting a thin layer of Ge on a Si (100) substrate. The nanowires are long and grow straight with very little curvature. The GaN nanowires are single-crystalline and show a Wurtzite structure growing along the [0001] axis. The growth follows a metal-free Vapor–Liquid–Solid (VLS) mechanism, further allowing a CMOS technology compatibility. The synthesis of nanowires has been done using an industrial Low Pressure Chemical Vapor Deposition (LPCVD) system.

Three different configurations of GaN are analysed to show robust and tunable light emitters from localised point defects induced in GaN crystal by a femtosecond laser (fs-laser). Localised irradiations of GaN are achieved using a fs-laser. The laser-induced damage threshold is found at a fluence of 130 ± 10 mJ/cm2. Raman spectroscopy allows for the characterization of irradiated GaN crystal while quasi-resonant photoluminescence mapping reveals a defect-related visible emission corresponding to the fs-laser irradiated area. From three different configurations of GaN, emission peaks vary from 620 to 680 nm-wavelengths for thin film, MBE intrinsically doped and Mg-doped NWs of GaN, respectively. The red emission in GaN is localized thanks to the new laser-induced fabrication and the engineering of the defect emission paves the way to further lighting applications.

Semiconductor structures offer unique properties at the nanoscale. Mastering the growth of semiconductor crystals and fabrication processes is crucial towards developing better opto-electronic devices. Morphological variations at nanoscale have a direct effect on available electronics states, impacting fundamental electronic properties and photonics capabilities of the material. This thesis focuses on one-dimensional Gallium Nitride (GaN) nanowires, Graphene Quantum Dots (GQDs) and two-dimensional hexagonal-Boron Nitride (hBN). First, synthesis of semiconductor nanostructures with a focus on nanoparticle mediated growth of one-dimensional Gallium Nitride crystal is developed. Catalyst-assisted growth of GaN nanowires using a Low Pressure Chemical Vapour Deposition (LPCVD) system is shown. The choice of growth catalyst influences directly the GaN crystal and the morphology of nanowires along with growth parameters such as temperature, vacuum pressure, precursor gases or time. An uncommon nanostructure of GaN in the shape of pine trees is introduced. Next, promising emission from GaN nanowires and hBN monolayers in the visible range is reported. Defects in LPCVD-grown GaN nanowires show yellow and green luminescences. Besides, femto-second pulsed laser is used, on defect-free nanowires grown by molecular beam epitaxy, to intentionally induce optically-active defects. Above a laser threshold power the irradiated GaN nanowires melt, higher power sublimates the crystal. Irradiated GaN is characterised as Wurtzite crystal and show emission in the visible range, additionally to the band-to-band UV luminescence. Similar experiment on hBN monolayers show defect-related visible emission near the irradiated areas. Contrary to GaN nanowires, where a wide emission is observed, hBN defects show strong and sharp emission. Point defects created in hBN - monolayers and flakes - are shown to be single photons emitters. Finally, fabrication of efficient micro- and nano-scale emitters is of crucial importance for optical nano-devices. In conventional semiconductors, such as Si or GaAs, fabrication processes can involve expensive and time consuming electron beam lithography and etching. Using femto-second pulsed laser, efficient and cost-effective GaN and hBN-based localised micro and nanometer-size light emitters can be fabricated using optically active defect states. Coupling these nano-emitters with surface plasmons of another nanostructure can improve the emission rate as shown by coupling GQDs to single Bi2Te3 nanoplates. From synthesis by chemical vapour deposition methods to structural and optical characterisation, this thesis will show how emission wavelength can be modulated and localised micro-/nano-emitters be fabricated before coupling them with surface plasmons to improve the emission rate for more efficient and bright emitters. Doctor of Philosophy

The light–matter interaction between nitrogen-doped graphene quantum dots (N-GQDs) and bismuth telluride (Bi2Te3) nanoplates is investigated. A maximum of (2.9 ± 0.3)-fold emission rate enhancement is observed at room temperature due to the coupling of N-GQD emission with the breathing mode of surface plasmon of single Bi2Te3 nanoplates. The enhancement varies with different emission wavelengths and nanoplate diameters in accordance with results obtained through the dipole radiation power in the electromagnetic simulations. From experiment, the quantum yield of N-GQDs is obtained to be almost unity, while Bi2Te3 nanoplates may replace the conventional antenna. Such combination of novel active and plasmonic materials is promising for efficient lighting applications with multiple functionalities, especially tunable plasmonic metamaterial based on topological insulators.

The low pressure chemical vapor deposition (LPCVD) method is used to synthesize GaN nanowires. It is an alternative technique to the more conventional molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE). Nanowires grown by LPCVD are shown to have a single-crystal Wurtzite structure and present a strong luminescence at a near-band-gap energy. A sub-band-gap defect-related luminescence is also observed in the visible range. Identified as yellow (YL) and green (GL) luminescences, these emissions are very similar to the one reported from bulk and thin 2D GaN film samples and may be attributed to comparable defects. In this work, using photoluminescence (PL) spectroscopy with an above-band-gap excitation and a time-resolved PL with a below-band-gap excitation, we investigate whether these luminescences originate from bulk and/or surface defects. We demonstrate that the YL defect-related band can be significantly suppressed by 88% after passivating the surface of nanowires with aluminum oxide. This suppression is in favor of the localized surface defects responsible for the yellow luminescence, while the green luminescence band originates from deeper bulk defects.

Primary neuroendocrine carcinomas of the genitourinary tract are rare and aggressive tumors carrying a bad prognosis. With squamous cell and transitional cell carcinoma being the most commonly reported urethral malignancies, primary small cell carcinoma (SCC) of the urethra is extremely rare. To date, only 5 cases have been reported in the literature. We present the first case of primary SCC occurring in the bulbar urethra in an 89-year-old male. We discuss the clinical, histological and immunohistochemical features of SCC of the urethra. Furthermore, we summarize the available literature and discuss the possible treatment options for this rare yet aggressive neoplasm.