Synthesis of bilayer graphene via chemical vapor deposition and its optoelectronic devices

Abstract

Due to its unique properties, graphene is a promising two-dimensional material in optoelectronic and energy applications. While the mobility of single layer graphene is extremely high, it has a zero bandgap. This feature restricts various applications of graphene in the field of semiconductor devices. Bilayer graphene, despite the nature of zero bandgap in its pristine form, can be tuned to open bandgap via a dual-gated vertical electrical field in a controlled manner. However, the size and layer number of mechanically exfoliated and liquid phase exfoliated graphene are poorly controlled. Controllable synthesis of large-sized bilayer graphene is an important research direction. This review summarizes a series of work including the controlled synthesis of bilayer graphene by chemical vapor deposition method and bilayer graphene devices. Specifically, growth mechanism of bilayer graphene is dependent on the type of supporting substrate and experimental condition. In the case of Ni substrate, bilayer graphene is grown along the segregation route. On the other hand, graphene growth on Cu is a surface-mediated process due to the extremely low solubility of C in Cu bulk. Depending on the concentration ratio between CH4 and H2, the growth mode of bilayer graphene can be tuned to be similar to Volmer-Weber or Stranski-Krastanov mode, in which the second layer is either grown under or above the first graphene layer. The dynamic growth of bilayer graphene can be further understood by a chemical gate effect and the process in a confined space. Moreover, here in this paper we present several approaches to realize the better control of bilayer graphene growth by modulating the experimental conditions. In terms of device applications for bilayer graphene, in this review we mention two typical applications including field-effect-transistors and hot-electron bolometers. Compared with conventional silicon-based hot-electron bolometer, the bilayer graphene based hot-electron bolometer has a small heat capacity and weak electron-phonon coupling, leading to high sensitivity, fast response, and small thermal noise-equivalent power. Such a bilayer graphene bolometer shows an exceptionally low noise-equivalent power and intrinsic speed three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures. Finally, the outlook and challenge for future research are also given. While significant progress has been made in the past several years, the controlled growth of bilayer or multi-layer graphene is still a key challenge, and the growth mechanism of bilayer graphene is not yet understood clearly. There is still much room for controlling graphene layer numbers, twisted angles, size, quality, and yield by optimizing the conditions. On the other hand, for the device applications of bilayer graphene, it is highly desired to develop high-performance bilayer graphene-based electronic devices.