Nanographene growing on free-standing monolayer graphene
Graphical abstract
Introduction
In recent years, the development of aberration-corrected electron optics for electron microscopy has permitted imaging at atomic-level resolution [[1], [2], [3], [4]]. Since the first atomic-resolved imaging of free-standing graphene by aberration-corrected transmission electron microscope (TEM) [5] and scanning transmission electron microscope (STEM) [6], numerous attempts have been made to analyze two-dimensional graphene using aberration-corrected TEM and STEM [[7], [8], [9], [10], [11]]. However, to characterize graphene, it is crucial to eliminate or reduce any contamination that might occur during synthesis, sample preparation, and observation [12]. Contamination is a generic term for carbon, oxygen, silicon, hydrocarbon chains, silicon oxide, and various other contaminating substances. If carbon is the dominant element of the contamination, the contaminating carbon is called adventitious or amorphous carbon. However, the structure of adventitious or amorphous carbon remains controversial. Indeed, the atomic structure of contamination on single-layer graphene remains ambiguous.
In the first report on atomic-resolution observation of a free-standing monolayer graphene using STEM [6], contamination was also observed directly in the vicinity of the region of pristine graphene; the authors described a “surface contamination that presumably consisted of hydrocarbon chains.” Similar contamination was observed when holes in monolayer graphene were reknitted by 5-, 6-, 7- and 8-membered rings [13]. In another study, crystalline graphene grew from the step edge of bilayer graphene at high temperature (500 °C < T < 700 °C), although only amorphous carbon agglomerations were found at T < 500 °C [14]. Also, it was reported that amorphous carbon deposited on graphene using electron-beam-induced deposition could be graphitized by an electron beam in TEM [15].
In the present study, we observed in situ atomic-resolved high-angle annular dark field (HAADF) images of a region of carbon contamination grown on free-standing single-layer pristine graphene at room temperature and analyzed the growth process of the contamination. As a result, we quantitatively identified an aggregation of small islands consisting of several layers of nanoscale graphene flakes stacked on a pristine single layer. Analysis of the three-dimensional atomic structure also revealed that the atoms in the third layer (layer 3) were contributed to formation of the second layer (layer 2) at the step edge. In addition, we found that the lattice constant of pristine graphene in the region of the first layer (layer 1) was larger than that of the bulk graphite crystals, and that it gradually decreased as layer 2 grew on layer 1.
Section snippets
Graphene synthesis and transfer
Cu substrates (25 μm thick, 99.8% purity; Alfa Aesar, No. 46365) were preserved in an acetic acid solution overnight. Graphene was grown by chemical vapor deposition (CVD) on Cu substrates using the following recipe. After the Cu substrates were loaded into a quartz tube with a heater, the pressure in the tube was decreased to 50 Pa. The Cu substrates were annealed at 300 °C by flowing 50 sccm Ar gas for 15 min to induce the growth of uniform oxide film on their surfaces, and the pressure was
EDX analysis to identify contamination
Graphene was grown by CVD on Cu substrates using an optimized recipe and was transferred to a TEM grid [17]. The free-standing graphene sample was studied using an aberration-corrected TEM/STEM equipped with energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) operating at 80 kV. Details of the sample preparation are given in the Experimental section. Before proceeding to the subsequent experiment, we irradiated the graphene sample on the TEM grid with an
Conclusions and discussion
In this paper, we quantitatively confirmed that the contamination growing on pristine single-layer graphene consists of several layers of nanoscale graphene flakes. Although the layered structure remained stable for several layers, the EELS spectrum showed that the thick region of contamination deviated from the pristine hexagonal carbon lattice. This suggests that the pristine hexagonal network of carbon bonding deteriorates into 5-, 7-, and 8-membered rings as the layers mount, as shown by
Author contributions
All authors contributed to the manuscript and approved the final version of the manuscript.
Notes
The authors declare no competing financial interests.
Acknowledgments
This work was partly supported by a JSPS Grant-in-Aid for Scientific Research on the Innovative Area "3D Active-Site Science" (grant number 26105009) and by a JSPS Grant-in-Aid for Scientific Research (grant number 15K17642). A portion of this work was conducted at Hokkaido University and was supported by the Nanotechnology Platform Program of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. We are grateful to Mr. Ohta for his assistance operating the STEM
References (30)
- et al.
Higher-order aberration corrector for an image-forming system in a transmission electron microscope
Ultramicroscopy
(2010) - et al.
On the roughness of single- and bi-layer graphene membranes
Solid State Commun.
(2007) Thermal expansion coefficients of graphite crystals
Carbon
(1972)- et al.
Electron microscopy image enhanced
Nature
(1998) - et al.
Sub-ångstrom resolution using aberration corrected electron optics
Nature
(2002) - et al.
Advancing the hexapole Cs-corrector for the scanning transmission electron microscope
Microsc. Microanal.
(2006) - et al.
The structure of suspended graphene sheets
Nature
(2007) - et al.
Free-standing graphene at atomic resolution
Nat. Nanotechnol.
(2008) - et al.
Graphene at the edge: stability and dynamics
Science
(2009) - et al.
Atom-by-atom spectroscopy at graphene edge
Nature
(2010)
Grains and grain boundaries in single-layer graphene atomic patchwork quilts
Nature
Dislocation-Driven deformations in graphene
Science
Experimental analysis of charge redistribution due to chemical bonding by high-resolution transmission electron microscopy
Nat. Mater.
Graphene annealing: how clean can it be?
Nano Lett.
Graphene reknits its holes
Nano Lett.
Cited by (5)
Effects of uniaxial strain on elastic and electronic properties of hydrogenated XC (X=Si, Ge, and Sn) monolayers by first-principles calculations
2023, Physics Letters, Section A: General, Atomic and Solid State PhysicsIn Situ Observation of the Motion of Platinum and Gold Single Atoms on Graphene Using Aberration-Corrected Electron Microscopy
2022, Journal of Physical Chemistry CSingle Pt Atoms on N-Doped Graphene: Atomic Structure and Local Electronic States
2021, Journal of Physical Chemistry CBiophysical research in Hokkaido University, Japan
2020, Biophysical Reviews