Subsurface Alloying Effect In Modulating The Energetic And Kinetic Properties Of Carbon Nucleation In The Initial Stages Of Graphene Epitaxial Growth On Metal Surfaces

Graphene, a single atomic layer of hexagonally packed carbon atoms, viewed also as basic building blocks for graphitic materials of all other dimensionalities, has attracted considerable attention due to its remarkable electrical, mechanical, and chemical properties and various potential applications, such as in electronics, optoelectronics, and chemical and biological sensing.For all of these potential applications, the primary requirement is cost-efficient, high-quality, reliable, and high-throughput synthesis of single-crystal monolayer graphene. Correspondingly, various methods for synthesizing large-area graphene monolayers have been established, such as graphitization of silicon carbide surfaces, and catalytic chemical vapor deposition(CVD) of carbon sources on transition metals(TM). Among these different avenues, CVD approach stands in stark contrast to others and has been recognized as a highly appealing one to synthesize large-scale excellent monolayer graphene that can be readily transferred to other substrates via chemical etching. However, up to date, the mass production of graphene are chiefly polycrystalline consisting of undesirable grain boundaries(GBs) that may degrade their functionalities in practical applications.The delicate competition between C-metal and C-C interactions is crucial for graphene nucleation in the initial growth stages on metal substrates in CVD fabrication. It is found that the stronger the binding of the C-metal, the more difficult for Cdimerization and graphene nucleation. On the contrary, a relatively weak C-metal interaction potential compared to C-C interaction enables fast diffusion of carbon atoms and results in multi-site nucleation of carbon islands over the whole surfaces, and leads to grain boundaries when different graphene grains coalesce together with different orientations, as reported in the case of graphene growth on Cu. Some efforts have been devoted to suppress the prevalence of grain boundaries by selecting highly appropriate substrates. For examples, superstructured surface alloy Mn-Cu(111) was invoked to suppress the grain orientation disordering; and more recently, and Wafer-scale single-crystal graphene with predefined orientation was fabricated by the aid of anisotropic twofold symmetry of the hydrogen-terminated Ge(110) surface.In this thesis, we propose another new approach of using subsurface alloy(SSA) effect to modulate the C-metal interactions, which essentially determines both the energetics and kinetics of carbon nucleation in the initial stages of graphene epitaxial growth on metal surfaces. The central idea proposed here is to establish an approach to tune the delicate competition between the C-C and C-metal bonding and simultaneously to identify the design principle in suppressing the orientation disordering of the graphene domains over the entire flat terraces of the SSA. An SSA substrate proposed here consists of mainly one type of TM atoms on the topmost surface, which may be relatively more readily fabricated than a rigorous surface superstructure and possesses significantly higher catalytic activities than the hydrogenterminated Ge(110) surface. Taking Rh(111) as a prototypical example, we find that on Rh(111), two C atoms prefer separation, counter-intuitively, when the subsurface atoms are replaced by solute metal elements going to the left side of Rh, such as earlier transition metal(TM) Ru and Tc with higher d-band centers, the two C atoms tend to form dimer. Furthermore, when the solute metal components are adopted by the late TMs, such as relatively more inert Pd and Ag elements possessing lower d-band centers, the repulsion between the two C atoms are enhanced, hindering the formation of graphene-nucleation. Furthermore, such an SSA effect also significantly modulates the diffusion barrier of the C atoms and the stabilities of the C clusters on the metal surface. More interestingly, the rotational barrier of graphene domain on the TM surface can also be significantly modulated, providing us another approach to suppress the grain disordering for graphene growth on TM surface in CVD experiment.