Letter pubs.acs.org/NanoLett
Seed-Assisted Growth of Single-Crystalline Patterned Graphene Domains on Hexagonal Boron Nitride by Chemical Vapor Deposition Xiuju Song,† Teng Gao,† Yufeng Nie,† Jianing Zhuang,§ Jingyu Sun,† Donglin Ma,† Jianping Shi,‡ Yuanwei Lin,† Feng Ding,§ Yanfeng Zhang,*,†,‡ and Zhongfan Liu*,† †
Center for Nanochemistry (CNC), College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies and ‡Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China § Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hong Kong 999077, China S Supporting Information *
ABSTRACT: Vertical heterostructures based on two-dimensional layered materials, such as stacked graphene and hexagonal boron nitride (G/h-BN), have stimulated wide interest in fundamental physics, material sciences and nanoelectronics. To date, it still remains challenging to obtain high quality G/h-BN heterostructures concurrently with controlled nucleation density and thickness uniformity. In this work, with the aid of the well-defined poly(methyl methacrylate) seeds, effective control over the nucleation densities and locations of graphene domains on the predeposited h-BN monolayers was realized, leading to the formation of patterned G/h-BN arrays or continuous films. Detailed spectroscopic and morphological characterizations further confirmed that ∼85.7% of such monolayer graphene domains were of single-crystalline nature with their domain sizes predetermined throughout seed interspacing. Density functional theory calculations suggested that a self-terminated growth mechanism can be applied for the related graphene growth on h-BN/Cu. In turn, asconstructed field-effect transistor arrays based on such synthesized single-crystalline G/h-BN patterning were found to be compatible with fabricating devices with nice and steady performance, hence holding great promise for the development of nextgeneration graphene-based electronics. KEYWORDS: Graphene and hexagonal boron nitride heterostructures, chemical vapor deposition, seed-assisted growth, controllable growth, characterizations vapor depositing (CVD) graphene onto h-BN films.6,14 Obviously, the transferring method not only introduces unavoidable residual at the interface of the heterostructures but also causes uncontrollability and complexity in device fabrications. With respect to these issues, CVD gives a solution to direct growth of G/h-BN heterostructures, ensuring a contamination-free interface. Yang et al.15 and Tang et al.16 demonstrated that graphene can be grown on mechanically exfoliated h-BN with the dimension of graphene still limited to the scale of the exfoliated h-BN. Lee et al.17,18 subsequently developed a two-step CVD route to obtain continuous G/h-BN stacks through sequentially growing graphene and h-BN on Cu foils. However, the CVD-derived graphene layer on h-BN presented a high density of grain boundaries, therefore hindering the applications in high-performance graphene electronics.
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he van der Waals heterostructures, constructed by the stacking of two-dimensional (2D) layered materials such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides, have emerged as a nice addition to the general paradigm of vertical heterostructures, usually served as very important platforms for material sciences and physics. These versatile heterostructures are efficient for engineering the electronic structures of 2D materials, developing high-performance transistors and exploring intriguing physical properties.1−4 Among these, vertically stacked graphene/h-BN (G/h-BN) presents a remarkably enhanced carrier mobility5,6 due to atomic-flatness and dangling-bond-free nature of the underlying h-BN that greatly suppresses scattering of carriers.7,8 Moreover, owing to the very close lattice parameters between graphene and h-BN, accurately aligned G/h-BN heterostructures can offer a long-range moiré superlattice, which can tailor the electronic structure of graphene, providing a platform for studying many fascinating physical phenomena, such as Hofstadter’s butterfly9−11 and commensurate-incommensurate transition.12 Thus far, fabricating vertical G/h-BN heterostructures mainly relies on transferring mechanically exfoliated5,13 or chemical © 2016 American Chemical Society
Received: June 6, 2016 Revised: August 24, 2016 Published: August 31, 2016 6109
DOI: 10.1021/acs.nanolett.6b02279 Nano Lett. 2016, 16, 6109−6116
Letter
Nano Letters
Figure 1. CVD growth of patterned G/h-BN stacks on Cu foils. (a) Schematic diagrams of four steps in the growth of patterned G/h-BN domains and monolayers using predeposited PMMA seeds on Cu foils as nucleation sites. (b) SEM image of PMMA-seed arrays on bare Cu fabricated by EBL (with a close-up view shown in the inset). (c) SEM image of patterned graphene domains grown from the PMMA seeds on the predeposited monolayer h-BN. (d) SEM image of fully covered G/h-BN heterostructures. (e) SEM image showing graphene domains arranged as G/BN CNC.
fully covered monolayer h-BN at ∼1000 °C using ammonia borane as precursor (see Supporting Information Figure S2). Notably, at the initial h-BN growth stage a preferable nucleation from the PMMA seeds gave rise to the patterned growth of discrete h-BN hexagons, which then merged into fully covered monolayer (see Supporting Information Figure S2). After full-coverage monolayer h-BN growth, BA (sublimated by a heating belt) was then introduced into the system for 10 min to grow graphene domains on h-BN at 850 °C. As shown in Figure 1c, the graphene domains precisely nucleated around the PMMA seeds, leading to patterned graphene domains possessing nearly the same dimension and shape. Note that if the growth time is extended up to 20 min, graphene domains would gradually merge with each other, resulting in the formation of fully covered graphene film on hBN (Figure 1d). For demonstrating the relatively high controllability of the nucleation site, “G/BN CNC” (Figure 1e) graphene patterns were also synthesized with the current route. Moreover, the coexistence of h-BN and graphene from the as-grown heterostructures is confirmed through core level B 1s, N 1s, and C 1s XPS spectra (see Supporting Information Figure S2).21 The stacking sequence of graphene and h-BN is further studied via electrostatic force microscopy (EFM). In EFM measurements, a direct current (dc) bias was applied between the probe and our sample at a fixed height for the surface potential mapping. The acquired surface potential is directly related to the work function of the probed material, given that the probe work function is constant. As shown in the EFM images, a surface potential of graphene island is ∼30 meV lower than that of the surrounding h-BN region in the G/h-BN stacks, which is distinct from the ∼80 meV difference between the in-plane patched graphene and h-BN structures (see Supporting Information Figure S3). These surface potential differences are consistent with our previous observations19 and match the theoretical calculations of the work functions on G/ h-BN/Cu (4.233 eV), h-BN/Cu (4.28 eV), and G/Cu (4.386 eV).22,23 Regarding the G/h-BN stacks, the charging effect from Cu substrate will be screened by the dielectric h-BN layer positioned between graphene and Cu substrate, resulting in the smaller surface potential difference between graphene and hBN regions. Taken together, the aforementioned EFM characterizations along with surface potential analysis are indicative of graphene located on top of h-BN.
Besides the above-mentioned methods, our group also reported the selective growth of in-plane and vertical G/hBN heterostructures on Cu foils, via temperature-triggered chemical switching route.19 The one-batch synthesis of vertical G/h-BN heterostructures were achieved using benzoic acid (C6H5COOH, BA), a precursor of high carbon/hydrogen ratio, at low growth temperature (
