Molecular Beam Epitaxy Growth of Hexagonal Boron Nitride/Graphene Heterostructures, Hexagonal Boron Nitride Layers and Cubic Boron Nitride Nanodots
- Author(s): Khanaki, Alireza;
- Advisor(s): Liu, Jianlin;
- et al.
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.