Synthesis Of Bilayer Graphene Research Articles

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

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.

Synchronous growth of AB-stacked bilayer graphene on Cu by simply controlling hydrogen pressure in CVD process

AB-stacked bilayer graphene has attracted considerable attention due to its feasibility of band gap tuning. Although synthesis of bilayer graphene on Cu has been reported using chemical vapor deposition (CVD) through a layer-by-layer growth mechanism, the process is long and complicated due to lack of catalytic assistance of Cu to the second graphene layer growth. Here we show that theoretical modeling demonstrates an alternative synchronous growth of bilayer graphene on Cu is possible by passivating the top graphene nuclei edges with hydrogen to allow carbon diffusion underneath the top graphene nuclei for bottom graphene layer formation. Moreover, such a growth mechanism has been achieved experimentally in a facile CVD method by simply controlling the H2 pressure. Bilayer graphene with high coverage of over ∼95% and a high AB stacking ratio of up to ∼90% has been obtained within a short growth time of 30min. Also, graphene with single, double and multiple layers can be obtained by simply controlling the hydrogen pressure. This result represents the demonstration of the fast synchronous AB-stacked bilayer graphene growth, which is important to scalable manufacture of graphene with controllable layer number and stacking required for practical applications.

Easy Approach and Scalable Synthesis of Bi Layer Graphene

Easy Approach and Scalable Synthesis of Bi Layer Graphene

Role of Hydrogen in Graphene Chemical Vapor Deposition Growth on a Copper Surface

Synthesizing bilayer graphene (BLG), which has a band gap, is an important step in graphene application in microelectronics. Experimentally, it was broadly observed that hydrogen plays a crucial role in graphene chemical vapor deposition (CVD) growth on a copper surface. Here, by using ab initio calculations, we have revealed a crucial role of hydrogen in graphene CVD growth, terminating the graphene edges. Our study demonstrates the following. (i) At a low hydrogen pressure, the graphene edges are not passivated by H and thus tend to tightly attach to the catalyst surface. As a consequence, the diffusion of active C species into the area beneath the graphene top layer (GTL) is prohibited, and therefore, single-layer graphene growth is favored. (ii) At a high hydrogen pressure, the graphene edges tend to be terminated by H, and therefore, its detachment from the catalyst surface favors the diffusion of active C species into the area beneath the GTL to form the adlayer graphene below the GTL; as a result, the growth of BLG or few-layer graphene (FLG) is preferred. This insightful understanding reveals a crucial role of H in graphene CVD growth and paves a way for the controllable synthesis of BLG or FLG. Besides, this study also provides a reasonable explanation for the hydrogen pressure-dependent graphene CVD growth behaviors on a Cu surface.

Graphene monolayer rotation on Ni(111) facilitates bilayer graphene growth

Synthesis of bilayer graphene by chemical vapor deposition is of importance for graphene-based field effect devices. Here, we demonstrate that bilayer graphene preferentially grows by carbon-segregation under graphene sheets that are rotated relative to a Ni(111) substrate. Rotated graphene monolayer films can be synthesized at growth temperatures above 650 °C on a Ni(111) thin-film. The segregated second graphene layer is in registry with the Ni(111) substrate and this suppresses further C-segregation, effectively self-limiting graphene formation to two layers.

Large‐Scale Synthesis of Bi‐layer Graphene in Strongly Coupled Stacking Order

AbstractLarge‐scale synthesis of single‐layer graphene (SLG) by chemical vapor deposition (CVD) has received a lot of attention recently. However, CVD synthesis of AB stacked bi‐layer graphene (BLG) is still challenging. Here, we report synthesis of BLG homogeneously at large scale by thermal CVD. The 2D Raman band of CVD BLG splits into four components, suggesting splitting of electronic bands due to strong interlayer coupling. The splitting of electronic bands in CVD BLG is further evidenced by the study of near infrared absorption and carrier dynamics are probed by transient absorption spectroscopy. UV photoelectron spectroscopy invesigation also indiates CVD BLG possesses different electronic structures to those of CVD SLG. The growth mechanism of BLG is found to be related to catalytic activity of the copper (Cu) surface, which is determined by the purity of Cu foils employed in the CVD process. Our work shows that strongly coupled or even AB stacked BLG can be grown on Cu foils at large scale, which is of particular importance for device applications based on their split electronic bands.