Controlled Synthesis and Scanning Tunneling Microscopy Study of Graphene and Graphene-Based Heterostructures

Graphene, a two-dimensional material consisted of sp² hybridization carbon atoms, has fascinated much attention owing to its extraordinary electronic, optical, magnetic, thermal, and mechanical properties, such as high carrier mobility (~10⁵ cm² V⁻¹ s⁻¹), high Young’s modulus (~1.0 TPa), high thermal conductivity (~5000 W m⁻¹ K⁻¹) and optical transmittance (~97.7%).

Compared with monolayer graphene, bilayer graphene displays even more complex electronic band structures and intriguing properties. Recent studies reveal that the low-energy band structure of bilayer graphene is extremely sensitive to the stacking order. Two low-energy VHSs, which originate from the two saddle points in the band structure, were observed in the twisted graphene bilayer as two pronounced peaks in the DOS. The VHSs will induce novel physical properties, such as, superconductivity and magnetism. Therefore, the preparation of large area non-AB-stacked bilayer graphene is an efficient way to modify the energy band structure near Fermi level. Combined with the preparation methods of graphene introduced in Chap. 1, especially the growth method of bilayer graphene, I choose segregation growth as a method for preparing non-AB stacking bilayer graphene.

In the 1980s, the invention of scanning tunneling microscope (STM) opened the door to observe the world from the atomic scale. For STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm (10 pm) depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated.

The heterostructure of graphene and h-BN is predicted to show many excellent physical properties, such as, bandgap opening, ultra-high carrier mobility, antiferromagnetic and half-semimetallic characteristics. In the first section of this chapter, I will give a brief review of the novel properties and the reported synthesis methods of h-BN-G heterostructures. The process of preparation of h-BN-G in-plane heterostructures is maturing, but some important basic scientific problems are still not solved. For example, the atomic structures and electronic properties on the interface between graphene and h-BN. The second section of this chapter introduces the UHV two-step growth method and the weak influence substrate Ir(111) single crystal with little electron doping effect on graphene.

Notably, the physical characteristics of h-BN-G heterostructures are related to the linking type in the interface. Thus, it becomes an important subject that how to form particular-type boundaries during growth process and how to identify the atomic linking type. This chapter mainly introduces the study of interface type and interfacial electron state of h-BN-G heterostructures.

We used the APCVD growth system to carry out the segregation growth of graphene by using Rh as substrates. By controlling the growth conditions and the cooling rate after growth, we can controllably obtain monolayer graphene, bilayer graphene and multilayer graphene on Rh foils. Wrinkles can be observed on graphene synthesized on Rh foils, which is due to the mismatch of coefficient of thermal expansion between graphene and Rh substrates. The height of wrinkles on monolayer graphene and multilayer graphene is ~3 and ~10 nm, respectively, and the wrinkles have specific orientations, which are closely related to the lattice orientation of the Rh facets. We can also controllable synthesize bilayer graphene on Rh(111) substrate. However, there is no wrinkles observed, which is indicated that graphene wrinkles are originated on Rh grain boundaries and they are the main channel of stress release during the cooling process. Based on the growth characteristics of graphene, We propose the combined CVD and segregated growth mechanism of graphene on Rh substrates. The main path of carbon segregation is Rh grain boundaries, meanwhile the Rh atom steps and terraces are also the way for carbon segregation.