Molecular Beam Epitaxy Growth of Hexagonal Boron Nitride/Graphene Heterostructures, Hexagonal Boron Nitride Layers and Cubic Boron Nitride Nanodots

Low-dimensional materials continue to attract and involve the minds and labor of scientists and engineers around the world as they are offering a set of fascinating properties which can be controlled by composition, size, and morphology. In this thesis, our focus is to explore the growth and characterization of some of such low-dimensional materials with molecular beam epitaxy (MBE) method. A technologically relevant example of low-dimensional systems is two-dimensional (2D) materials including graphene (G) and h-BN. To date, it is still challenging to reliably grow high-quality uniform 2D h-BN and h-BN/G heterostructures in a wafer scale mainly due to their complicated growth process.In this first project, i.e., Chapter 2, we perform a systematic study of h-BN/G heterostructure growth on cobalt (Co) foil substrate in an MBE system and investigate the growth mechanisms of individual G and h-BN layers in the structure. We demonstrate with the increase of C incorporation in Co, three distinct h-BN/G growth regions can be observed: (1) the C saturation is not attained at the growth temperature (900 °C) and G is grown only by precipitation during cooling process to form a “G network” underneath the h-BN film; (2) the Co substrate is just saturated by C atoms at the growth temperature and a part of G growth occurs isothermally to form G islands and another part by precipitation, resulting in a non-uniform h-BN/G film; and region (3): a continuous layered G structure is formed at the growth temperature and precipitated C atoms add additional G layers to the system, leading to a uniform h-BN/G film. We show that in all three growth regions, a 3-hrs h-BN growth at 900 ºC leads to h-BN film with a thickness of 1~2 nm, regardless of the underneath G layers’ thickness or morphology.Following the growth of h-BN/G heterostructures, in the next project, i.e, Chapter 3, we demonstrate that the dissolution of C atoms into heated Co substrate can also facilitate the growth of 2D h-BN and alter its morphology from 2D layer-plus-3D islands to homogeneous 2D few-layers. A high breakdown electric field of 12.5 MV/cm was achieved for a continuous 3-layer h-BN. Density functional theory calculations reveal that the interstitial C atoms can increase the adsorption of B and N atoms on the Co (111) surface, and in turn, promote the growth of 2D h-BN. In the last project, i.e., Chapter 4, we discuss the growth and characterization of another low-dimensional form of BN family materials, namely, cubic boron nitride nanodots (c-BN NDs) which offers a variety of novel opportunities in battery, biology, deep ultraviolet light emitting diodes, sensors, filters, and other optoelectronic applications. To date, the attempts towards producing c-BN NDs were mainly performed under extreme high-temperature/high-pressure conditions and resulted in c-BN NDs with micrometer sizes, a mixture of different BN phases, and containing process-related impurities/contaminants. To enhance device performance for those applications by taking advantage of size effect, pure, sub-100 nm c-BN NDs are necessary. In this chapter, we demonstrate the self-assembled growth of sub-100 nm c-BN NDs on Co and Ni foil substrates by plasma-assisted MBE for the first time. We found that the nucleation, formation, and morphological properties of c-BN NDs can be closely correlated with the nature of substrate including catalysis effect, lattice-mismatch-induced strain, and roughness, and growth conditions, in particular, growth time and growth temperature. The mean lateral size of c-BN NDs on cobalt scales from 175 nm to 77 nm with the growth time. The growth mechanism of c-BN NDs on metal substrates is concluded to be Volmer-Weber (VW) mode. A simplified two-dimensional numerical modeling shows that the elastic strain energy plays a key role in determining the total formation energy of c-BN NDs on metals

Electrophoretic Deposition of CuIn_1–<i>x</i>Ga_<i>x</i>Se₂ Thin Films Using Solvothermal Synthesized Nanoparticles for Solar Cell Application

CuIn_1–<i>x</i>Ga_<i>x</i>Se₂ (CIGS) thin films are successfully prepared by convenient electrophoretic deposition, using colloidal nanoparticles. This is the first report which focuses on the electrophoretic deposition (EPD) of presynthesized CIGS nanoparticles directly from their colloid for solar cell application. In this research, CIGS nanoparticles are first synthesized via solvothermal process and then dispersed in a media containing a mixture of ethanol as the solvent and triethylenetetramine as the additive to be used for the film deposition via the electrophoretic method. By simple adjustment of the electrophoretic parameters, including applied voltage, pH, deposition time, and composition of nanoparticles, CIGS thin films with controlled thickness and optoelectronic properties can be fabricated. The highest photovoltaic efficiency of 5.57% is obtained in the CuIn_0.75Ga_0.25Se₂ sample. It is believed that this fabrication approach may open up a new gate to reduce the production cost of a highly demanding CIGS absorber layer used in thin film solar cells

Role of Carbon Interstitials in Transition Metal Substrates on Controllable Synthesis of High-Quality Large-Area Two-Dimensional Hexagonal Boron Nitride Layers

Reliable and controllable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) layers is highly desirable for their applications as 2D dielectric and wide bandgap semiconductors. In this work, we demonstrate that the dissolution of carbon into cobalt (Co) and nickel (Ni) substrates can facilitate the growth of h-BN and attain large-area 2D homogeneity. The morphology of the h-BN film can be controlled from 2D layer-plus-3D islands to homogeneous 2D few-layers by tuning the carbon interstitial concentration in the Co substrate through a carburization process prior to the h-BN growth step. Comprehensive characterizations were performed to evaluate structural, electrical, optical, and dielectric properties of these samples. Single-crystal h-BN flakes with an edge length of ∼600 μm were demonstrated on carburized Ni. An average breakdown electric field of 9 MV/cm was achieved for an as-grown continuous 3-layer h-BN on carburized Co. Density functional theory calculations reveal that the interstitial carbon atoms can increase the adsorption energy of B and N atoms on the Co(111) surface and decrease the diffusion activation energy and, in turn, promote the nucleation and growth of 2D h-BN