Layered van der Waals materials have played a pivotal role in expanding the scope of condensed matter physics by examining the effects of reduced dimensionality in various systems. These include semiconductors, ferromagnets, and charge density wave materials, among many others. Hexagonal boron nitride (hBN) is often used as a passivation/encapsulation layer for air-sensitive materials in optical and electronic studies owing to its effectiveness as a substrate for graphene in transport measurements. In this thesis, samples probed by Raman spectroscopy and as well as those measured through electronic transport were first encapsulated during fabrication. The specific experimental details are found in each corresponding chapter. This thesis aims to characterize several 2-D materials and explore physical phenomena arising from combinations thereof through optical and electronic means. Before delving into the specific research projects, it provides a motivation for each, descriptions of the material(s) involved, and sample fabrication techniques used to assemble the various heterostructures. Topics to be covered include the effects of encapsulation on the transition metal dichalcogenide (TMD) 1T’-MoTe2 subject to elevated temperatures, how the nearly commensurate to commensurate phase transition of another TMD, the charge density wave material 1T-TaS2, in its few-layer form can be tuned electronically, preliminary results of electronic transport in graphene-ferromagnet heterostructures, and an outline of other optical studies on mono- to few-layered forms of related materials and possible future directions that may be pursued.

Boron nitride (BN) materials have unique properties, which has led to interest in them in the last few years. The deformation of boron nitride materials including hexagonal boron nitride, boron nitride nanosheets (BNNSs) and boron nitride nanotubes have been studied by Raman spectroscopy. Both mechanical and liquid exfoliations were employed to obtain boron nitride nanostructures. Boron nitride glass composites were synthesised and prepared in thin films to be deformed by bending test in-situ Raman spectroscopy. Hexagonal boron nitride in the form of an individual flake and as flakes dispersed in glass matrices has been deformed and Raman measurement shows its response to strain. The shift rates were, -4.2 cm-1/%, -6.5 cm-1/% for exfoliated h-BN flake with thick and thin regions and -7.0 cm-1/%, -2.8 cm-1/% for the h-BN flakes in the h-BN/ glass (I) and glass (II) composites. Boron nitride nanosheets (BNNSs) shows a G band Raman peak at 1367.5 cm-1, and the deformation process of BNNSs/ glass composites gives a shift rate of -7.65 cm-1/% for G band. Boron nitride nanotubes (BNNTs) have a Raman peak with position at 1368 cm-1, and their deformation individually and in composites gives Raman band shift rates of -25.7 cm-1/% and -23.6 cm-1/%. Glass matrices shows compressive stresses on boron nitride fillers and this was found as an upshift in the frequencies of G band peak of boron nitride materials. Grüneisen parameters of boron nitride (BN) were used to calculate the residual strains in glass matrices of BNNSs nanocomposites as well as to estimate the band shift rates which found to be in agreement with the experimental shift rate of bulk BN and BNNTs.

Yu Wei-jian. / Thesis submitted in: November 2000. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 70-71). / Text in English; abstracts in English and Chinese. / Yu Wei-jian. / TITLE PAGE --- p.i / THESIS COMMITTEE --- p.ii / ABSTRACT (English) --- p.iii / ABSTRACT (Chinese) --- p.iv / ACKNOWLEDGEMENTS --- p.v / TABLE OF CONTENTS --- p.vi / LIST OF FIGURES --- p.viii / LIST OF TABLES --- p.ix / Chapter CHAPTER 1 --- INTRUDUCTION --- p.1 / Chapter Section 1.1 --- Background of the BN System --- p.1 / Chapter 1.1.1 --- Desirable Properties of c-BN --- p.1 / Chapter 1.1.2 --- Phases in the BN System --- p.2 / Chapter 1.1.3 --- Phase Diagram of BN --- p.4 / Chapter 1.1.4 --- Synthesis Techniques for c-BN Films --- p.5 / Chapter 1.1.5 --- Characterization of BN Films --- p.6 / Chapter Section 1.2 --- Background of Theory --- p.7 / Chapter Section 1.3 --- Objectives --- p.9 / Chapter 1.3.1 --- Determination of Stable State of BN --- p.9 / Chapter 1.3.2 --- Phonon-dispersion Relations of BN --- p.9 / Chapter 1.3.3 --- "Phase (p, T) Diagram" --- p.10 / Chapter 1.3.4 --- Transformation Paths in Direct Compressions --- p.10 / Chapter Section 1.4 --- Roadmap --- p.11 / Chapter CHAPTER 2 --- METHODS --- p.12 / Chapter Section 2.1 --- Density Functional Theory (DFT) for E0 Calculation --- p.12 / Chapter Section 2.2 --- Direct Force-constant Method --- p.16 / Chapter Section 2.2 --- Quasi-harmonic Approximation --- p.26 / Chapter CHAPTER 3 --- RESULTS --- p.27 / Chapter Section 3.1 --- Stable State of BN --- p.27 / Chapter Section 3.2 --- Phonon-dispersion Relations --- p.29 / Chapter Section 3.3 --- "Phase (p, T) Diagram of BN" --- p.36 / Chapter Section 3.4 --- Transformation Paths via Direct Compression --- p.44 / Chapter 3.4.1 --- Direct Compression of h-BN and r-BN --- p.46 / Chapter 3.4.2 --- Direct Compression of t-BN --- p.50 / Chapter Section 3.5 --- Energy Barriers in the Transformation of h-BN to c-BN --- p.52 / Chapter CHAPTER 4 --- DISCUSSION --- p.58 / Chapter Section 4.1 --- Transition States in the Transformation of h-BN to c-BN --- p.58 / Chapter Section 4.2 --- Phonon-dispersion Relations --- p.60 / Chapter Section 4.3 --- Phase Diagrams --- p.62 / Chapter Section 4.4 --- Future Studies --- p.63 / Chapter 4.4.1 --- Cubic BN Films Formation --- p.63 / Chapter 4.4.1.1 --- Nanoarches Nucleation --- p.64 / Chapter 4.4.1.2 --- Growth: Interfaces Between h-BN {0001} and c-BN Planes --- p.64 / Chapter 4.4.2 --- Transformation Paths of w-BN to c-BN and h-BN to r-BN --- p.65 / Chapter CHAPTER 5 --- CONCLUSION --- p.66 / APPENDIX: Mechanistic Models in c-BN Films Formation --- p.67 / REFERENCES --- p.70

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Thesis (Ph. D.)--University of Texas at Austin, 2009. / Title from PDF title page (University of Texas Digital Repository, viewed on Sept. 14, 2009). Vita. Includes bibliographical references.

Thesis (M.Sc.)--City University of Hong Kong, 2005. / At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.

The growth of single and multilayer BN films on several substrates was investigated. A typical atomic layer deposition (ALD) process was demonstrated on Si(111) substrate with a growth rate of 1.1 Å/cycle which showed good agreement with the literature value and a near stoichiometric B/N ratio. Boron nitride films were also deposited by ALD on Cu poly crystal and Cu(111) single crystal substrates for the first time, and a growth rate of ~1ML/ALD cycle was obtained with a B/N ratio of ~2. The realization of a h-BN/Cu heterojunction was the first step towards a graphene/h-BN/Cu structure which has potential application in gateable interconnects.

The search for alternative methods of synthesizing cubic boron nitride (cBN), one of the hardest known materials, at low thermo-baric conditions has stimulated considerable research interest due to its great potential for numerous practical industrial applications. The practical applications are motivated by the material’s amazing combination of extraordinarily superior properties. The cBN phase is presently being synthesized from graphite-like boron nitride modifications at high thermo-baric conditions in the presence of catalytic solvents or by ion–beam assisted (chemical and physical) deposition methods. However, the potential and performance of cBN have not been fully realized largely due to central problems arising from the aforementioned synthesis methods. The work reported in this dissertation is inspired by the extensive theoretical investigation of the influence of defects in a ecting the transformation of the hexagonal boron nitride (hBN) phase to the cBN phase that was carried out by Mosuang and Lowther (Phys Rev B 66, 014112 (2002)). From their investigation, using an ab-initio local density approach, for the B, C, N, and O simple defects in hBN, they concluded that the defects introduced into hBN could facilitate a low activation–energy hexagonal-to-cubic boron nitride phase transformation, under less extreme conditions. We use ion implantation as a technique of choice for introducing ‘controlled’ defects into the hot–pressed polycrystalline 99.9% hBN powder samples. The reasons are that the technique is non–equilibrium (not influenced by dilusion laws) and controllable, that is the species of ions, their energy and number introduced per unit area can be changed and monitored easily. We investigate the structural modifications of hBN by ion implantation. Emphasis is given to the possibilities of influencing a low activation–energy hBN-to-cBN phase transformation. The characterization of the structural modifications induced to the hBN samples by implanting with He+ ions of energies ranging between 200 keV and 1.2 MeV, at fluences of up to 1.0 1017 ionscm2, was accomplished by correlating results from X-Ray Di raction (XRD), micro-Raman (-Raman) spectroscopy measurements, and two-dimensional X-Y Raman (2D-Raman) mapping measurements. The surface to pography of the samples was investigated using Scanning Electron Microscopy (SEM). E orts to use Surface Brillouin Scattering (SBS) were hampered by the transparency of the samples to the laser light as well as the large degree of surface roughness. All the implantations were carried out at room temperature under high vacuum. 2D-Raman mapping and -Raman spectroscopy measurements done before and after He+ ion irradiation show that an induced hBN-to-cBN phase transformation is possible: nanocrystals of cBN have been observed to have nucleated as a consequence of ion implantation,the extent of which is dictated by the fluences of implantation. The deviationof the measured spectra from the Raman spectra of single crystal cBN is expected, has been observed before and been attributed to phonon confinement e ects. Also observed are phase transformations from the pre-existing hBN modification to: (a) the amorphous boron nitride (aBN), (b) the rhombohedral boron nitride (rBN) modifications, (c) crystalline and amorphous boron clusters, which are a result of the agglomeration of elementary boron during and immediately after ion implantation. These transformations were observed at high energies. Unfortunately, the XRD measurements carried out could not complement the Raman spectroscopy outcomes probably because the respective amounts of the transformed materials were well below the detection limit of the instrument used in the former case.