Coaxial heterostructure formation of highly crystalline graphene flakes on boron nitride nanotubes by high-temperature chemical vapor deposition

概要

Title

Coaxial heterostructure formation of highly
crystalline graphene flakes on boron nitride
nanotubes by high-temperature chemical vapor
deposition

Author(s)

Kato, Masakiyo; Inoue, Taiki; Chiew, Yi Ling et
al.

Citation

Applied Physics Express. 2023, 16(3), p. 035001

Version Type AO
URL

https://hdl.handle.net/11094/90187

rights
Note

Osaka University Knowledge Archive : OUKA
https://ir.library.osaka-u.ac.jp/
Osaka University

This is the version of the article before peer review or editing, as submitted by an author to Applied
Physics Express on December 27th, 2022.  IOP Publishing Ltd is not responsible for any errors or
omissions in this version of the manuscript or any version derived from it.  The Version of Record is
available online at https://doi.org/10.35848/1882-0786/acbd0e. 

Coaxial heterostructure formation of highly crystalline
graphene flakes on boron nitride nanotubes by hightemperature chemical vapor deposition
Masakiyo Kato1, Taiki Inoue1*, Yi Ling Chiew2, Yungkai Chou1, Masashi Nakatake3,
Shoichi Takakura3,4, Yoshio Watanabe3, Kazu Suenaga2,5, Yoshihiro Kobayashi1
1

Department of Applied Physics, Osaka University, Osaka 565-0871, Japan

2

The Institute of Scientific and Industrial Research (ISIR-SANKEN), Osaka University,

Osaka 567-0047, Japan
3

Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi 489-

0985, Japan
4

Synchrotron Radiation Research Center, Nagoya University, Aichi 464-8603, Japan

5

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), Tsukuba 305-8565, Japan
E-mail: inoue.taiki@ap.eng.osaka-u.ac.jp
We develop a high-temperature chemical vapor deposition of highly crystalline graphene on
the surface of boron nitride nanotubes (BNNTs). The growth of few-layer graphene flakes
on BNNT templates was confirmed by scanning transmission electron microscopy. Based
on an investigation of the effect of growth temperature and growth time on defect density,
graphene with a relatively high crystallinity was obtained at 1350 C. The absence of
undesirable alterations in the BNNT lattice during graphene growth was verified by multiple
analyses. The high-temperature growth of heterolayers would assist in the advancement of
nanodevices that coaxially combine graphene and boron nitride.

1

Carbon nanotubes (CNTs),1) boron nitride (BN) nanotubes (BNNTs),2) and transition metal
dichalcogenide nanotubes (TMDNTs)3) are representative nanotube materials that possess
unique properties originating from their quasi-one-dimensional structures. The closed lattice
structure without dangling bonds on the nanotube surfaces provides a potential for nested
structures of different nanotubes via van der Waals (vdW) force.4),5) Following the rapid
development of vdW heterostructures of two-dimensional layered materials,6) coaxial
heterostructures of nanotube materials, i.e., heteronanotubes, have attracted an increasing
attention.7) A controlled synthesis of BNNTs and TMDNTs coaxially on single-walled CNTs
(SWCNTs) as a template was realized by chemical vapor deposition (CVD).7) The ternary
heteronanotubes function well as a diode based on semiconductor–insulator–semiconductor
junctions.8) By controllably combining nanotubes with different elements and properties,
various electronic and optical functionalities are expected to be integrated into the small
body of each heteronanotube. As post-growth manipulation is not readily applicable to nest
nanotubes, in contrast to the transfer techniques used for layered vdW heterostructures,6)
development of a direct heterolayer growth is strongly demanded to obtain
heteronanotubes.9)
As CNTs are semiconducting or metallic depending on their atomic structures,10) they
have the potential to be versatile components in heteronanotube devices not only as inner
walls7),8) but also as outer walls. While non-carbon nanotubes have been grown on the
outsides of SWCNTs7),9) and multi-walled CNTs (MWCNTs),5),11)–13) CNTs have scarcely
been synthesized on the outside of non-carbon nanotubes.14) A few studies have reported
growth of outer-wall CNTs, i.e., over-layer graphene, on MWCNT15) and BNNT14) templates,
but the crystallinity of the obtained graphene is relatively low. Carbon deposition followed
by a graphitization treatment can provide a highly crystalline graphene on MWCNTs but is
limited to thick graphene layers.16) Nucleation and growth of single- to few-layer graphene
have to be investigated for the application of high-quality thin CNTs as outer walls. Because
of the difficulty in employing catalytic metals in coaxial growth, a high growth temperature
is preferred to overcome energetic barriers, e.g., for diffusion of precursor carbon adatoms
on a template surface and incorporation of the adatoms to graphene at the growth edges.17),18)
Simultaneously, since graphene and BN can form hybridized phases (hexagonal BNC),19)
alteration and atomic substituion20) of the template structures should be avoided for
heterolayer growth at high temperatures.
In this study, we carried out a high-temperature CVD for a synthesis of graphene using
BNNT films as a template. The growth of graphene on the surface of BNNTs was evidenced
2

by scanning transmission electron microscopy (STEM). To obtain a highly crystalline
graphene, we analyzed the effects of the growth temperature and growth time. We also
characterized the electrical conduction of the graphene-coated BNNT films. The possibility
of alteration and atomic substitution of BNNT structures during graphene growth was
excluded by vibrational spectroscopy and chemical state analyses.
As a template for graphene growth, suspended BNNT films were prepared. A BNNT
powder (BNNT Materials SP10R, diameter of 4

2 nm, wall number of one to five)21) (~0.2

mg) was lightly dispersed in iso-propanol by stirring over-night,22) followed by bath
sonication for 10 min. BNNT films with a diameter of ~16 mm were formed on filter
membranes by vacuum filtration. A graphite sheet (Toyo Tanso PERMA-FOIL, 0.66 mm
thick) was processed into a ring structure with an inner diameter of ~8 mm by belt punches
and used as a supporting substrate that can withstand a high-temperature process.23) Note
that common substrates such as silicon and quartz are not stable above ~1200 C. After the
membrane filter was dissolved in acetone, the BNNT films floating on the acetone surface
were picked up by the ring substrate to obtain a suspended BNNT film on graphite (top
optical image in Fig. 1(a)).
Graphene growth by CVD was carried out with an infrared radiation furnace (Thermo
Riko SR1800G) where the sample stage was locally heated by a focused infrared light.
Ethanol was selected as a carbon source because its decomposition provides oxygen atomcontaining species, which can assist the efficient growth of nanocarbon materials.24),25) The
growth condition was determined according to previous studies on non-catalytic growth of
graphene on graphene templates.26)–28) After the sample was set in the chamber, and the
chamber was pumped down, the temperature was increased to the growth temperature with
an Ar flow (100 sccm at ~1150 Pa). The growth temperature was 1150 to 1550 C. After the
sample reached the growth temperature, an ethanol vapor (0.5 sccm) was introduced in
addition to Ar (100 sccm), where the total pressure was ~1150 Pa, corresponding to an
ethanol partial pressure of ~5.7 Pa. The growth time was 5 to 60 min.
The samples were characterized by STEM-electron energy loss spectroscopy (-EELS)
(JEOL ARM200F, acceleration voltage of 60 kV), Raman spectroscopy (HORIBA HR800,
excitation wavelength of 532 nm), four-probe sheet resistance measurement (Nittoseiko
MCP-T370), Fourier transform infrared (FTIR) spectroscopy (Bruker Hyperion 2000), and
X-ray absorption near edge structure (XANES) spectroscopy (BL7U of Aichi Synchrotron
Radiation Center, total fluorescence yield method). For the observation by STEM, the grown
samples were lightly dispersed in ethanol and dropped on holey carbon-coated copper grids.
3

Other characterizations were carried out with as-grown film samples.
After the ethanol CVD process, the color of the BNNT films turned from white to black
(bottom optical image in Fig. 1(a)), which indicates the growth of carbonaceous materials.
A representative STEM image of the sample grown at 1350 C for 60 min is shown in Fig.
1(b). In addition to amorphous structures, partially crystalized layered structures with an
interlayer distance of ~0.35 nm were observed on the surface of nanotubes. The
corresponding EELS mapping indicates the strong B and N signals along the continuous
tubular structure and C signal from the layered flakes on it (Fig. 1(c)). These observations
confirm that few-layer graphene flakes were grown on the surface of the BNNT template.
We investigated the growth temperature dependence of the structure of the graphene
grown on the BNNT templates by Raman spectroscopy. Figure 2(a) shows Raman spectra
of the samples grown at different temperatures for 5 min. Except for the sample grown at
1550 C, which exhibits only broad fluorescent peaks, the other samples exhibit Raman
signals that are characteristic of graphitic materials. Note that Raman signals from BNNTs
were not apparent due to the weak resonance with the excitation laser. The spectra indicate
growth of graphene on BNNTs at 1150 1450 C and absence of graphene growth at 1550
C under this carbon feeding condition. The lack of graphene growth at 1550 C can be
explained as the density of precursor carbon adatoms on the BNNT surface does not reach
the nucleation threshold18),29) due to an increase in the desorption rate of adatoms at a high
temperature. Another possibility is that the chemical potential of carbon atoms in the gas
phase becomes lower than that in the solid phase at high temperature, preventing graphene
growth. ...

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