Abstract
Deposition of vertically oriented carbon nanofibers (CNFs) has been studied in an atmospheric pressure radio frequency discharge without dielectric barrier covering the metallic electrodes. When the frequency is sufficiently high so that ions reside in the gap for more than one rf cycle ("trapped ions"), the operating voltage decreases remarkably and the transition from a uniform glow discharge to an arc discharge is suppressed even without dielectric barriers. More importantly, the trapped ions are able to build up a cathodic ion sheath. A large potential drop is created in the sheath between the bulk plasma and the electrode, which is essential for aligning growing CNFs. At the same time, the damage to CNFs due to ion bombardment can be minimized at atmospheric pressure. The primary interest of the present work is in identifying the cathodic ion sheath and investigating how it influences the alignment of growing CNFs in atmospheric pressure plasma-enhanced chemical-vapor deposition. Spectral emission profiles of He (706 nm), Hα (656 nm), and CH (432 nm) clearly showed that a dark space is formed between the cathode layer and the heated bottom electrode. However, increasing the rf power induced the transition to a nonuniform γ -mode discharge which creates intense plasma spots in the dark space. Aligned CNFs can be grown at moderate input power during the initial stage of the deposition process. Catalyst particles were heavily contaminated by precipitated carbon in less than 5 min. Alignment deteriorates as CNFs grow and deposition was virtually terminated by the deactivation of catalyst particles.
| Original language | English (US) |
|---|---|
| Article number | 024310 |
| Journal | Journal of Applied Physics |
| Volume | 99 |
| Issue number | 2 |
| DOIs | |
| State | Published - Jan 15 2006 |
Bibliographical note
Funding Information:This project was partly supported by The Research Grants in Engineering School of Tokyo Institute of Technology (Tokyo Tech). Two of the authors (U.K. and T.N.) acknowledge partial support through the U.S. Department of Energy through Grant No. DE-FG02-00ER54583. The authors would like to thank Jun Koki and Akira Genseki (Center for Advanced Materials Analysis of Tokyo Tech) for supporting TEM analysis. Another author (T.N.) kindly acknowledges Professor Rikizo Hatakeyama at Tohoku University for very helpful discussion on the characterization of APRFD. Table I. Experimental condition. 2 in. Si wafer (100) coated with Cr ∕ Ni ( 20 nm ∕ 20 nm ) Discharge area: 12.6 cm 2 ( ϕ 40 mm ) Deposition time: 5 min , 10 min Parameters He ( cm 3 ∕ min ) H 2 ( cm 3 ∕ min ) C H 4 ( cm 2 ∕ min ) Time(min) Power(W) Gap(mm) Press.(Torr) Temp.(°C) Annealing 1000 10 ⋯ 20 ⋯ Pretreatment 1000 4 ⋯ 10 40 4 760 700 Deposition 1000 4,6,8 2 5 45–60 Table II. Comparison of cathode characteristics in low-pressure and high-pressure regimes. Low-pressure dc discharge a High-pressure rf discharge b Substrate positioned on Powered electrode Ground electrode Total pressure 8 Torr 600 Torr Applied voltage (rms)-gap 750 V – 15.0 mm 160 V ( 21.2 mA ∕ cm − 2 ) - 2.4 mm Maximum electric field 30 kV ∕ cm 4 kV ∕ cm Ion density: bulk → cathode C 2 H 2 + : 5 × 10 12 → 10 9 cm − 3 N 2 + : 5 × 10 10 → 10 11 cm − 3 Radical density: bulk → cathode H : 10 17 → 10 17 cm − 3 N : 2 × 10 14 → 4 × 10 13 cm − 3 a See Ref. 11 . b See Ref. 7 . FIG. 1. Atmospheric pressure radio frequency discharge reactor. Upper rf electrode: diffusion electrode ( ϕ 40 mm ) , bottom GND electrode: resistive heater ( ϕ 60 mm ) , background pressure: 10 − 2 Torr , capacity: 10 l . rf: rf power source, MB: matching circuit, OSC: digital oscilloscope. FIG. 2. Transition of the α -mode discharge to a γ -mode discharge. Discharge gap: 4 mm , discharge area: 12.56 cm 2 . Emission of helium 706 nm line: bandpass filter ( 710 nm ± 6 nm ) . FIG. 3. Axial emission profile of He ( 706 nm ) at different operation regimes. Bottom electrode: 0 mm , upper electrode: 4 mm , conditions: see Fig. 2 . FIG. 4. Voltage and current waveforms of APRFD. He ∕ H 2 ∕ C H 4 = 1000 ∕ 4 ∕ 2 cm 3 ∕ min , temperature of the bottom electrode: 700 ° C , P rf : 45 W , P eff : 32 W , ϕ : − 56 ° . FIG. 5. Applied voltage (rms) vs conduction current at different H 2 contents. He: 1000 cm 3 ∕ min , C H 4 : 2 cm 3 ∕ min . 700 ° C . FIG. 6. Axial emission profiles of He ( 706 nm ) , H α ( 656 nm ) , and CH ( 432 nm ) . Bottom electrode: 0 mm , Upper electrode: 4 mm , discharge area: 12.56 cm 2 , He ∕ H 2 ∕ C H 4 = 1000 ∕ 6 ∕ 2 cm 3 ∕ min , rf 45 W , V rf : 209 V , I rf : 268 mA . Optical characteristics of bandpass filters: He ( 706 nm : 710 nm ± 6 nm ), H α ( 656 nm : 660 nm ± 7 nm ), CH ( 432 nm : 430 nm ± 6 nm ). FIG. 7. SEM images of carbon nanofibers obtained at different H 2 ∕ C H 4 ratios. rf power: 60 W , deposition time: 5 min , helium flow rate: 1000 cm 3 ∕ min . FIG. 8. SEM image of vertically oriented carbon nanofibers and high-resolution TEM images of catalyst nanoparticles.