TWO-DIMENSIONAL MATERIAL-SUBSTRATE INTERACTIONS FOR EPITAXIAL GROWTH

Abstract

Department of Materials Science and EngineeringDue to their unique dimensionality, two-dimensional (2D) materials exhibit many excellent electronic, magnetic, mechanical and thermal properties that are absent from their 3D counterparts, which makes them hold great potential in various applications, such as electronics, optoelectronics and photovoltaics, etc. To maximize the advantages of 2D materials and realize their practical applications in 2D devices, synthesizing single crystals of the 2D materials with a wafer scale is highly required. Over the past decade, various strategies for synthesizing 2D materials have been developed, and there are mainly two routes towards the fabrication of 2D single crystals especially with a wafer scale via chemical vapor deposition (CVD): (i) nucleating only one 2D nucleus on the whole substrate and growing it to wafer scale and (ii) seamless stitching of a large number of unidirectionally aligned 2D material islands on a substrate. Although route (i) can produce wafer scale single crystalline (WSSC) 2D materials with a very high quality, the synthesis process usually requires tens of hours and delicate experimental setups. Route (ii) is much more cost-effective because the synthesis process is mediated by the simultaneous growth of all the 2D islands, while it requires substrates that can template the synthesis of unidirectionally aligned 2D islands. For route (ii), an in-depth understanding on the alignment of 2D materials on substrates is of critical importance for choosing proper substrates that can template the synthesis of unidirectionally aligned 2D islands. Experimentally, various 2D material islands with both multi- and mono- alignment orientations have been observed on different substrates, including unidirectionally aligned graphene islands on Cu(111) and Cu(110) substrates, graphene islands with two orientations on Cu(100) substrates, multi-orientations of hexagonal boron nitride (hBN) islands on all low-index Cu substrates, unidirectionally aligned hBN islands on vicinal Cu(110) surfaces and on stepped Cu(111) surfaces, unidirectionally aligned MoS2 islands on vicinal Au(111) surfaces, etc. Up to now, WSSC graphene, hBN and MoS2 have been successfully synthesized by route (ii) on Cu(111) substrates, vicinal Cu(110) or stepped Cu(111) surfaces, and vicinal Au(111) surfaces, respectively. In principle, the interactions between 2D materials and their substrates are responsible for the epitaxial growth of the 2D materials and the behaviors of the 2D materials on the substrates after growth. However, up to now, an in-depth understanding on the alignment mechanism of 2D materials on substrates at atomic scale is still lacking. In this dissertation, we carry out theoretical investigations on the alignment of 2D materials on both high-symmetric and low symmetric transition metal (TM) surfaces. Firstly, we find that a high-symmetric direction of a 2D material usually prefers to align along a high symmetric direction of the substrate, which determines the most stable configuration of a 2D material island on the substrate. Moreover, we reveal that the interplay between the symmetry of the 2D material and that of the substrate determines the alignments of 2D islands on the substrate. To grow unidirectionally aligned 2D islands, the most stable orientation of the 2D material on the substrate cannot be changed by any symmetrical operation of the substrate, i.e., the symmetry group of the substrate should be a subgroup of that of the 2D material. Therefore, low symmetric TM surfaces are more promising for templating unidirectional 2D islands. This part is presented in Chapter 4. We then further investigated the alignment of 2D materials on an arbitrary high-index low symmetric TM substrate in Chapters 5-9. The high-index low symmetric FCC TM substrates are classified into three categories according to their surface configuration, which are FCC{111}-based, FCC{100}-based and FCC{110}-based low symmetric surfaces, and the alignment of various 2D materials, including graphene, hBN and transition metal dichalcogenides (TMDCs) monolayers, on these three types of low symmetric FCC substrates are systematically explored by density functional theory (DFT) calculations. It is revealed that the interaction differences between various edges of a 2D material and a unidirectional TM step edge determines the sole orientation of the 2D island along this step edge. However, TM step edges usually present a meandering direction in real cases because of surface roughness, the alignments of 2D islands along such meandering step edges are then investigated by structural analysis. We find that the similar kink heights of substrate step edges and the edges of the 2D island are critical for the unidirectional alignment of 2D islands along meandering step edges. Besides, the orientations of hBN and TMDCs on substrates are also affected by the ambient conditions in experiments due to their binary compositions. As a promising substrate for the epitaxial growth of graphene, Cu{111} foils attract lots of attentions. By collaborating with experimental groups, a single crystalline Cu{111} foil with a size up to 32 cm2 is realized using a contact-free annealing method. We theoretically revealed the transition mechanism of single crystalline Cu{111} from polycrystalline Cu foils by classical molecular dynamic (MD) simulations combined with DFT calculations in Chapter 10. It is found that the high annealing temperature and Cu{112} grains in the raw Cu foils are essential for obtaining the single crystalline Cu{111} foil. After growth, 2D materials usually show novel moir?? structures and properties modulated by the underlying substrate, and in Chapter 11 of this dissertation, the behaviors of graphene on various TM substrates that have a large lattice mismatch with graphene are explored by the DFT calculations. Depending on the rotation angles between graphene and the substrate, two kinds of graphene moir?? structures are revealed, i.e., highly corrugated graphene moir?? superstructures under small rotation angles and ultra-flat graphene layers under large rotation angles, and we further find that such different behaviors of graphene are determined by the competition between the graphene-substrate interaction and curvature energy of the graphene/TM superstructures. Compared with the ultra-flat ones, the corrugated graphene/TM superstructures show anisotropic properties and are found to be capable of templating size-tunable metal clusters. We finally formulate the evolution of the graphene-substrate interaction and curvature energy, and the morphology of graphene on various TM substrate can be estimated effectively.clos