Synthetic two dimensional hexagonal boron nitride and gallium sulfide crystals using chemical vapor deposition growth

Thesis

Two dimensional (2D) materials have promising potential in the next generation electronics and optoelectronics devices. This requires scalable production methods which can provide high quality material. This DPhil project is about using chemical vapor deposition (CVD) method to grow multilayer hexagonal boron nitride (h-BN) and monolayer gallium sulfide (GaS). A range of characterisation methods is used to examine the morphology of these grown materials, as well as their structural, vibrational, and optical properties. In addition, the growth mechanism of h-BN and GaS are discussed based on the observed results.

Multilayer h-BN films are grown on copper substrates using an atmospheric pressure CVD (APCVD) method and the commonly used precursor, ammonia borane (AB). The h-BN films produced from AB precursor are compared to the films produced from two other precursors, such as dimeric diborazane (DAB) and trimeric triborazane (TAB). SEM results indicate that the DAB leads to fully continuous h-BN film in a shorter period of time. Quantitative surface analysis reveals that DAB and TAB precursors increase the coverage of contaminant nanoparticles on the surface of the grown h-BN films. Finally, the growth mechanism of h-BN on copper substrate is discussed.

Gallium sulfide mixture phase (GaS/Ga₂S₃) thin film is produced by a one-step thermolysis of single source precursor Ga₂S₃. It is deposited on both SiO2/Si and glassy carbon substrates via an APCVD method. The as-grown films have flakes and its thickness is between 4.5 nm and 30 nm depending on the growth time. The GaS/Ga₂S₃ thin films on glassy carbon substrates are explored as hydrogen evolution reaction (HER) catalysts. One of the samples exhibits a 126 mV/dec Tafel slope with 5.50 A/cm². Its cathodic current behaves almost the same after a 1000-cycle durability test.

At last, the monolayer GaS film with triangular domains is achieved. The diagonal of the monolayer GaS continuous film is about 0.7 cm. The modified APCVD method results the sharp precursor concentration gradient and the change in the kinetic growth dynamics at substrate edge. This project provides important insights of grown 2D GaS material, in term of enlarging the domain size and understanding its growth mechanism.

Authors: Wang, X

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford