A visualization method for probing grain boundaries of single layer graphene via molecular beam epitaxy

Abstract

Graphene, a member of layered two-dimensional (2D) materials, possesses high carrier mobility, mechanical flexibility, and optical transparency, as well as enjoying a wide range of promising applications in electronics. Adopting the chemical vaporization deposition method, the majority of investigators have ubiquitously grown single layer graphene (SLG), which inevitably involves polycrystalline properties. Here we demonstrate a simple method for the direct visualization of arbitrarily large-size SLG domains by synthesizing one-hundred-nm-scale MoS single crystals via a high-vacuum molecular beam epitaxy process. The present study based on epitaxial growth provides a guide for probing the grain boundaries of various 2D materials and implements higher potentials for the next-generation electronic devices.

Spin Coating Promotes the Epitaxial Growth of AgCN Microwires on 2D Materials

“…Therefore, we can verify that the present method is an advanced tool for the crystallographic identification of 2D materials, compared to previous methods including TEM, − STM (scanning tunneling microscopy), LEED (low-energy electron diffraction), − POM (polarized optical microscopy) of liquid crystals, − spectroscopic approaches based on nonlinear optical properties, − and direct imaging of epitaxial materials ,,,,− ,− (see Table S2 for details). For example, TEM − and LEED − are suitable only for conductive or ultrathin substrates, POM of liquid crystals cannot measure absolute crystallographic orientations, − and many epitaxial materials cannot be easily removed. ,, One can be concerned that crystallographic identification might become unnecessary because of the recent progress on wafer-scale synthesis of single-crystalline 2D materials. However, wafer-scale single crystals can be synthesized with limited types of 2D materials such as graphene, hBN, and MoS 2 . , Wrinkles of 2D materials are also a main source for disturbing crystallographic control, but wrinkle-minimized synthesis and transfer methods are achieved only for graphene.…”

“…For example, TEM 48−50 and LEED 51−53 are suitable only for conductive or ultrathin substrates, POM of liquid crystals cannot measure absolute crystallographic orientations, 54−57 and many epitaxial materials cannot be easily removed. 15,16,68 One can be concerned that crystallographic identification might become unnecessary because of the recent progress on wafer-scale synthesis of single-crystalline 2D materials. However, wafer-scale single crystals can be synthesized with limited types of 2D materials such as graphene, hBN, and MoS 2 .…”

Effect of graphene grain boundaries on MoS2/graphene heterostructures*

“…The growth of MoS 2 on graphene depends on van der Waals interaction, which can be used to mark the graphene GBs. [23] Therefore, it is crucial to investigate the influence of graphene GBs on the photoelectric response of MoS 2 /graphene heterostructures.…”

Critical Annealing Temperature for Stacking Orientation of Bilayer Graphene

“…For testing the quality of MoS 2 /BLG heterostructures, characteristic Raman vibration bands ( Figure a) have been measured comprehensively. A 20 cm −1 separation of two Raman bands, namely, and A 1g of MoS 2 , which are located at 382 and 402 cm −1 , suggests that the MoS 2 crystal indeed represents the monolayer (Figure b). Additionally, two facts of I G / I 2D = 1.1 and FWHM 2D = 50 cm −1 for graphene (Figure c) indicate the existence of strongly coupled BLG .…”

“…Based on the epitaxial relationship between MoS 2 and graphene, we can determine stacking orientations of BLG by collecting and analyzing crystal orientations of MoS 2 crystals that are grown on SLG and BLG regions. Triangular MoS 2 crystals are aligned in two preferential orientations in our meticulously selected annealed BLG substrate ( Figure a), namely, 0° (red triangles) and 60° (yellow triangles).…”