www.acsnano.org
Crystalline Bilayer Graphene with Preferential Stacking from Ni−Cu Gradient Alloy Zhaoli Gao,† Qicheng Zhang,†,‡ Carl H. Naylor,† Youngkuk Kim,#,§ Irfan Haider Abidi,‡ Jinglei Ping,† Pedro Ducos,†,∥ Jonathan Zauberman,† Meng-Qiang Zhao,† Andrew M. Rappe,# Zhengtang Luo,‡ Li Ren,*,⊥ and Alan T. Charlie Johnson*,† †
Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong # The Makineni Theoretical Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-632, United States § Department of Physics, Sungkyunkwan University, Suwon 16419, Korea ∥ Departamento de Física, Universidad San Francisco de Quito, Quito 170901, Ecuador ⊥ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510006, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni− Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni−Cu uniform alloys and “copper pocket” structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either “surface limited” (for Cu) or carbon precipitation (for uniform Ni−Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon backdiffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni−Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature. KEYWORDS: bilayer graphene, single-crystal, Ni−Cu gradient alloy, Bernal and 30° stacking order, high-yield synthesis
B
controlled stacking structure is thus of great significance for both basic scientific research and potential applications. Chemical vapor deposition (CVD) is a promising approach to grow large-area BLG on metal substrates (e.g., Cu, Ni/Cu alloys), and progress has been made toward growth of BLG in the Bernal-stacked form.16−22 However, synthesis of BLG with large crystalline domains remains a significant experimental challenge. BLG synthesis on flat Cu and uniform Ni−Cu substrates has been reported where growth was either surfacemediated17,22,23 or by a carbon precipitation mechanism (for Ni−Cu).16,18,21 Typically, the BLG domain size was small and
ilayer graphene (BLG) consists of two graphene monolayers and is typically found in the AB form (commonly referred to as Bernal-stacked), the AA form (hexagonal on-top stacked), or a twisted configuration, where the layers are rotated with respect to each other at a given angle (tBLG).1 BLG offers a rich array of physical properties such as a tunable band gap,2,3 topological valley transport,4,5 van Hove singularities,6−9 coherent commensurate electronic states,10,11 superlattice-induced insulating states,12 and helical edge states.13 Production of BLG crystals based on mechanical exfoliation2,3 has low yield. Preparation of BLG by stacking one graphene monolayer on top of another requires a multiple-step transfer process to yield tBLG with a controlled twist angle.12,14,15 Scalable synthesis of highly crystalline BLG with © XXXX American Chemical Society
Received: October 2, 2017 Accepted: March 6, 2018 Published: March 6, 2018 A
DOI: 10.1021/acsnano.7b06992 ACS Nano XXXX, XXX, XXX−XXX
Article
Cite This: ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano
The Ni−Cu foils were annealed at 1050 °C for 5 min to allow for Ni diffusion to create a gradient alloy substrate with a Nirich side and a Ni-poor side, and then 1.8 sccm methane (CH4) was introduced as the carbon source. To visualize the results, as-grown samples were transferred onto 250 nm SiO2/Si substrates. For an optimized Ni−Cu gradient alloy (160 nm Ni on Cu; see below for further discussion), the Ni-rich side was covered by BLG (∼20%) and monolayer graphene (MLG; ∼80%) with an MLG nucleation density of 23 ± 7 mm−2 (Figure 2a, left panel). The Ni-poor
variable, leading to undesirable stacking structure transitions within a BLG region.17,18,24−26 A recent report detailed synthesis of single-crystal BLG with large domains through a carbon back-diffusion mechanism,27 but the growth was based on a manually constructed Cu pocket that is less suitable for industrial scale-up compared to conventional flat growth substrates. It is also highly desirable to obtain tBLG with reproducible twist angle,27−31 which is crucial for investigations of superlattice-induced transport11,12 and helical edge states.13 Progress in the field has been slowed due to the lack of control over the grain size and interlayer stacking in CVD-grown BLG; for example, we are not aware of any reports to date of scalable fabrication of transistor arrays based on CVD-grown crystalline Bernal-stacked BLG where the band gap could be measurably tuned with an applied electric field at room temperature. Here we report the use of an optimized Ni−Cu gradient alloy for high-yield synthesis of large crystalline BLG domains (200 ± 50 μm for the top graphene layer and 20 ± 4 μm for the bottom layer) with layer stacking that is almost exclusively Bernal or tBLG with a twist angle of 30°. By proper design of the Ni−Cu gradient alloy, we enabled BLG growth through a carbon back-diffusion mechanism previously obtained only using a copper pocket structure.27 A scalable photolithography process was used to fabricate arrays of dual-gate field-effect transistors based on Bernal-stacked BLG. The BLG band gap was tunable at room temperature, with an on/off ratio of 5.0 ± 1.1. We also identified strain solitons32 in crystalline Bernalstacked BLG, which is useful to the study of topologically protected states and valley physics in graphene.4,33
RESULTS Figure 1 is a schematic of the proposed growth mechanism for BLG on the Ni−Cu gradient alloy catalytic substrate. Since Ni Figure 2. Results of BLG growth on both sides of a Ni−Cu gradient alloy substrate, after transfer onto Si/SiO2. Optical micrographs of growth on the (a) top (Ni-rich) and (b) bottom (Ni-poor) surfaces of an optimized Ni−Cu gradient alloy substrate. The Ni-rich side is almost fully covered by graphene (domain size of 200 ± 50 μm), which forms the top layer of BLG. The bottom BLG layer forms by back-diffusion of carbon atoms from the Ni-poor side. The right panel of (a) shows single-crystal BLG regions, where the size of the bottom layer is 20 ± 4 μm. The Ni-poor side of the substrate has a low density of monolayer graphene domains (surface coverage of ∼18%).
side showed a sparse growth of discrete MLG grains with a nucleation density of 1.3 ± 0.4 mm−2 (Figure 2b, left panel). This asymmetric growth allowed for carbon back-diffusion, leading to the formation of BLG with well-controlled layer number and compact hexagonal shapes (Figure 2a, right panel). The grain size for the top (bottom) layer of BLG was 200 ± 50 μm (20 ± 4 μm). The slower growth rate of the bottom layer compared to the top layer can be attributed to the additional decomposition and diffusion energy required for the carbon source to diffuse from the Ni-poor side to the Ni-rich.27 In contrast, for growth on Cu foil with no Ni (Figure S1), graphene coverage was low (
