7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

On-Insulator Growth of Nanocarbons Ranging from 1D to 3D Morphology
through Catalyst- and Seed-Free Plasma Chemical Vapor Deposition

R. Hatakeyama1, H. Ueno2,4, E. Kwon3 and F. Misaizu4
1
New Industry Creation Hatchery Center, Tohoku University, 2 Frontier Research Institute forInterdisciplinary Science, Tohoku University, 3 Research and Analytical Center for Giant
Molecules, Tohoku University, 4 Department of Chemistry, Tohoku University
e-mail (speaker): rikizo.hatakeyama.b1@tohoku.ac.jp
Particular interest has been given to 1D single-walledcarbon nanotubes (SWNTs) and 2D single-layer graphene(sGPN) due to their unique physical and chemical
properties among
nanoscale carbon members(nanocarbons) with different dimensions [1]. Althoughmetal catalysts have usually been used for their synthesis,
their direct growth over insulators to bypass the transfer
process would be highly desirable for extending theirapplication fields [2]. However, this line of strategy has
been handicapped by thermal chemical vapor deposition(TCVD) routes with extremely high reaction temperatures
especially in the case of sGPN (e.g., 1000—1600 ℃). Here,
plasma-enhanced chemical vapor deposition (PECVD) isfor the first time applied to on-insulator catalyst- and seedfree direct growth of nanocarbons ranging from 1D to 3D,
lowering the growth temperature down to 700 ℃.
Our experiments are performed using a PECVD system
consisting of an RF(power: PRF)-discharge quartz tube
and an electric furnace surrounding its centerneighborhood. A quartz glass of typical insulator isadopted as a nanocarbon-growth substrate. Generated
plasmas of electron density ne and temperature Te areprovided to the substrate of temperature Tsub placed within
the furnace site. Here an attention is given to the influx
intensity and incident energy of ions on the substratesurface, which are measured as ion current Ii anddifference between floating (𝜑f ) and space (𝜑𝑠 )potentials, 𝛥𝜑 = 𝜑𝑓 − 𝜑𝑠 , yielding ion accelerationenergy Ui(= −𝑒Δφ). Then, the plasmas are classified inthree categories in accordance with Ii and Ui; (1) “verymild ” (PRF < 20 W: ne <108 cm-3, Te < 0.3 eV, −Δφ < 1.0V) , (2) “mild” (20 ≾ PRF < 40 W: 108 ≾ ne < 109 cm-3,0.3 ≾ Te < 1.0 eV, 1.0 ≾ −Δφ < 5.0 V), and (3) “harsh”(40 W ≾PRF : 109 ≾ ne < 1010 cm-3, 1.0 ≾ Te < 4.0 eV, 5.0≾ −Δφ < 15 V) plasmas (Fig. 1, Ii is normalized by 0.4mA/cm2). In an etching-effect experiment using graphene
oxide (GO) ready beforehand, it is found that the perfectetching of GO occurs between Tsub =350 ℃ and 700 ℃.
Then, the critical temperature 700 ℃ is designated as Tsuband the pressure ratio of H2 is fixed to 12 % for 43 Pa of
C2H5OH (carbon source) pressure. Raman scattering
spectroscopy is mainly adopted on nanocarboncharacterization technique. In the case of TCVD (PRF=0W) only a very small peak of G(graphite)-band (〜1600cm-1) is observed in Raman spectra, indicating the growthof only some sort of carbonaceous matter (CM).
(1) Very mild plasma (0 < Ii < 0.2, 0 < Ui < 1.5 eV) :Since apparent peaks of G-band and D(defect)-band (〜1330 cm-1) appear, a material intermediate between

amorphous carbon and GO is grown in this region.
(2) Mild plasma (0.2 ≾ Ii < 1.2, 1.5 ≾ Ui < 5.0 eV) :Here the carbon-source supply and energy of ions
impinging upon the quartz substrate are gradually
increased to some extent. In addition to G- and D-band,clear Raman-spectrum peaks are often observed in thelow-frequency range of 100-300 cm-1, which is called as
the radial breathing mode (RBM), verifying the growth ofSWNTs. Concerning graphene remarkable peaks around2660 cm-1 (2D band) in the high-frequency region areobserved, the intensity of which is larger (sGPN) orsmaller (a few-layers graphene: fGPN) than that of G band.
Besides, the hybrid of 1D SWNT and 2D GPN, 3D
nanocarbon, is confirmed to exist (e.g., inset of Fig. 1).
The order of observation frequency can be summarized
as follows: SWNTs 〜 fGPN > hybrid > sGPN.
(3) Harsh plasma (1.2 ≾ Ii < 5.0, 5.0 ≾ Ui <15 eV) :When PRF exceeds 35 W, Ii and Ui jump about fivefold
compared to those below it. Concomitantly, SWNTs,
hybrid and fGPN disappear, and only sGPN grows in a
uniformly-full coverage of fashion on the substratesurface instead. As for SWNTs, this phenomenon isunderstandable according to our previous study, whereSWNTs were confirmed to suffer strong etching from ionsin the Ui range of 10 eV [1]. The Raman study reveals thatthe grain size of sGPN (La) varies from the order of 100
nm to 10 nm when PRF increases from 20 W over 40 W.

Fig. 1. Dependences of potential difference (Δφ) andnormalized ion current (Ii) on RF power (PRF). Insets show
Raman-spectrum example of SWNT/sGPN hybrid anddependence of grain size La of sGPN on RF power.
References[1] R. Hatakeyama, Rev. Mod. Plasma Phys. 1, 7 (2017).
[2] B. Liu et al., ChemNanoMat 6, 483 (2020).