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Time of Flight Size Control of Carbon Nanoparticles Using Ar+CH_{4} Multi-Hollow Discharge Plasma Chemical Vapor Deposition Method

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## Abstract

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_{4}multi-hollow discharge plasmas. Using the plasmas, we succeeded in continuous generation of hydrogenated amorphous carbon nanoparticles with controlled size (25–220 nm) by the gas flow. Among the nanoparticle growth processes in plasmas, we confirmed the deposition of carbon-related radicals was the dominant process for the method. The size of nanoparticles was proportional to the gas residence time in holes of the discharge electrode. The radical deposition developed the nucleated nanoparticles during their transport in discharges, and the time of flight in discharges controlled the size of nanoparticles.

## 1. Introduction

## 2. Materials and Methods

_{4}gases were introduced from the chamber’s left side, passed through the holes, and later evacuated by the pump system. The FR ratio of Ar and CH

_{4}was 6:1. The total FR was controlled in a range of 10–120 sccm. During this process, gas pressure was kept at 266 Pa. The substrate holder was set at 100 mm apart from the electrode in the downstream region, and it was grounded. The powered electrode was connected to a 60 MHz radio frequency (rf) power supply through an impedance matching box. The discharge power and discharge period were 40 W and 90 min, respectively, and corresponding discharge and self-bias voltages were 230 and 80 V, respectively.

## 3. Results and Discussion

_{p}is the CNP size (diameter). The deposited nanoparticles were stacked for FR above 50 sccm. Thus, we estimated the probability of CNPs deposited on the mesh grid. Two group sizes were produced for FR = 10 sccm; (1) smaller group size has a size range between 20 and 90 nm, and (2) larger group size has a range between 170 and 250 nm. For FR = 20 sccm, two peaks at 60 and 150 nm were detected, but these peaks overlap and form one size group with a wide range between 30–200 nm. At the same time, one group size was obtained for FR above 50 sccm. Therefore, as the FR increases from 50 to 120 sccm, the peak size gradually shifts toward a smaller size from 45 to 20 nm, respectively. The size dispersion became narrower for higher FR from 50 and 120 sccm.

_{p}on FR, as shown in Figure 4. At FR below 20 sccm, the larger-sized nanoparticles seem to be separated from the smaller size group and grow in a monodisperse way. Similar growth behavior was observed for Si nanoparticles in silane plasmas in the earlier study [26]. Considering the larger size of CNPs at FR = 10 sccm, the d

_{p}decreases monotonically with increasing FR.

^{−1}), and G (1580 cm

^{−1}) bands. The area intensity ratio of D/G band was around 1.8; this indicates the structure of the CNPs were polymer like a-C:H [25,28,29,30]. Similar spectra were also observed at other FRs.

_{res}of holes corresponds to discharge duration in the conventional CCP. In this study, gas residence time was calculated from FR. For the CNP, growth involves two growth processes like the coagulation of CNPs during transport toward substrates and radical deposition on CNPs.

_{p1}and number density n

_{p1}of CNPs after the collision are expressed by ${d}_{\mathrm{p}1}={2}^{\frac{1}{3}}{d}_{\mathrm{p}0}$ and ${n}_{\mathrm{p}1}={n}_{\mathrm{p}0}-1$, respectively, where d

_{p0}and n

_{p0}are the size and density of CNPs before the collision. To figure out the effects of the coagulation of CNPs, we examined the CNPs deposition at three positions in the transport region. Figure 7 shows the dependence of the size and surface density of deposited CNPs on the position L far from the electrode. For L = 100, 120, and 140 mm, the size is irrelevant to the position. The area density monotonically decreases with increasing L.

_{r}of CNP expressed as equation 1

_{p}is the size (diameter) of CNPs and DR

_{r}is the deposition rate of radicals on CNPs. If we assume the sticking probability of radicals on CNPs is unity and carbon atoms are responsible for the mass of CNPs, the G

_{r}is given by

_{C}the mass of a carbon atom (2.00 × 10

^{−26}kg), n

_{r}the number density of the radicals in plasmas, and v

_{thr}the thermal velocity of the radicals. The size and density of CNPs affect the radical density. The loss of radicals to the chamber wall is dominant if their size and density are low, while the loss to CNPs is prevalent if their size and density are high. The loss mode is determined by the coupling parameter Γ of CNPs in plasmas [32], given by the following equation.

_{w}is the characteristic length of the reactor. For the Γ >> 1, the coupling among CNPs through radicals is strong, results in the deceased radical density with the time after the nucleation of CNPs. If the coupling is weak, the wall loss of radicals is dominant, resulting in no radical density change with the time. Further, to detect the Γ value, the n

_{p}was deduced from the result in Figure 7.

^{2}during the deposition time of 60 min. The flux of the CNPs can be calculated if the sticking probability of CNPs is unity. Considering the solid angle, the flux at the end of the holes deduced to be 1.30 × 10

^{11}cm

^{−2}s

^{−1}. Raman results show that the structure of CNPs was polymer-like carbon, and the mass density of the CNPs was assumed 1.6 g/cm

^{3}. If the temperature of CNPs equal to that of the electrode (433 K), the n

_{p}was 1.20 × 10

^{9}cm

^{−3}, and d

_{p}was 25 nm, as shown in Figure 6. D

_{w}(D

_{w}= 2.5 mm) assumed as the radius of hole, then Γ was 6.78 at the end of the discharge region. In the discharge region, the size of CNPs was smaller than 25 nm, and Γ value should be less than one. It suggests that the loss of radicals through the wall was predominant, which results in a constant rate of radical loss. To discuss the generation of the radicals, we have measured emission spectra in plasma. We measured two Ar I emission intensities at 425.9 nm I

_{425.9}and 750.4 nm I

_{750.4}with upper-level excitation energy of 14.7 eV (3p

_{1}) and 13.5 eV (2p

_{1}), respectively. These emission processes have little effect on quenching and radiation trapping. The upper excitation level has small cross sections for electron-impact excitation from metastable states [33,34]. The FR dependence of an emission intensity ratio I

_{425.9}/I

_{750.4}, shown in Figure 8. The ratio indicates the information of the high energy tail of the electron energy distribution, which relates to the radical generation.

_{r}is proportional to the density of CH

_{4}because the electron density and the loss rate of the radicals can be assumed to be constant based on the above discussion. Integrating Equation (2), the following formula gives the CNP size.

_{CH4}the density of CH

_{4}, and t the interaction time of CNPs and radicals. The k value is related to the depletion rate of the CH

_{4}molecules. In the current study, CNPs were nucleated in the discharge generated in the holes of the electrode. They grew in the discharge, transport with the gas flow, and growth was stopped outside the holes. We assumed that the growth of CNPs starts when the CH

_{4}molecules enter into the holes where plasmas were generated. Thus, d

_{p}was assumed to be equal to zero at t = 0, and the growth of CNPs stops at t = τ

_{res}. Figure 8 shows the dependence of d

_{p}on τ

_{res}, based on FR dependence, together with the results reported earlier [23]. Considering the larger size of CNPs for FR = 10 sccm, the size of CNPs linearly increases with increasing the τ

_{res}. In this study, n

_{CH4}and v

_{thr}was 6.36 × 10

^{21}m

^{−3}and 8.24 × 10

^{2}m/s, respectively. The calculated value using Equation (4) as a parameter of k and the experimental results were well fitted for k = 0.035 (Figure 9). For the conventional CCP, the depletion rate of CH

_{4}was about 3% for 1.33 Pa pure CH

_{4}gas and 0.15 W/cm

^{2}in discharge power density [35]. The depletion rate monotonically increases with CH

_{4}pressure.

^{2}, much higher than the conventional plasma CVD (above-mentioned), and the partial pressure of CH

_{4}was 38 Pa. The radical loss to CNPs was small but cannot be ignored as it affects the Γ value. Thus, the fitted value of k is reasonable. Based on our results, the CNP in MHDPCVD was grown by the deposition of carbon-related radicals.

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statements

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**TEM images of carbon nanoparticles produced for (

**a**) FR = 10 sccm, (

**b**) FR = 20 sccm, (

**c**) FR = 50 sccm, and (

**d**) FR = 120 sccm. Insets in (

**a**) and (

**b**) show their high magnification TEM images.

**Figure 3.**The size distribution of CNPs as a parameter of FR where n is the number of measured CNPs.

**Figure 7.**Dependence of the size and surface density of deposited CNPs on the position L far from the electrode for FR = 100 sccm. Error bar shows the standard deviation.

**Figure 9.**Dependence of d

_{p}on τ

_{res}. Open circles indicate the results reported earlier [23]. Lines were obtained by Equation (4). Error bar shows the standard deviation.

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**MDPI and ACS Style**

Hwang, S.H.; Koga, K.; Hao, Y.; Attri, P.; Okumura, T.; Kamataki, K.; Itagaki, N.; Shiratani, M.; Oh, J.-S.; Takabayashi, S.;
et al. Time of Flight Size Control of Carbon Nanoparticles Using Ar+CH_{4} Multi-Hollow Discharge Plasma Chemical Vapor Deposition Method. *Processes* **2021**, *9*, 2.
https://doi.org/10.3390/pr9010002

**AMA Style**

Hwang SH, Koga K, Hao Y, Attri P, Okumura T, Kamataki K, Itagaki N, Shiratani M, Oh J-S, Takabayashi S,
et al. Time of Flight Size Control of Carbon Nanoparticles Using Ar+CH_{4} Multi-Hollow Discharge Plasma Chemical Vapor Deposition Method. *Processes*. 2021; 9(1):2.
https://doi.org/10.3390/pr9010002

**Chicago/Turabian Style**

Hwang, Sung Hwa, Kazunori Koga, Yuan Hao, Pankaj Attri, Takamasa Okumura, Kunihiro Kamataki, Naho Itagaki, Masaharu Shiratani, Jun-Seok Oh, Susumu Takabayashi,
and et al. 2021. "Time of Flight Size Control of Carbon Nanoparticles Using Ar+CH_{4} Multi-Hollow Discharge Plasma Chemical Vapor Deposition Method" *Processes* 9, no. 1: 2.
https://doi.org/10.3390/pr9010002