Single-Walled Carbon Nanotubes are fluorinated within a Radio-Frequency Plasma Reactor using SF6 as a source gas. To develop an understanding of how varying experimental parameters affect the plasma, and hence the resultant modification of the nanotubes, a Langmuir Probe has been employed to study the plasma system. Information about plasma densities, electron temperatures, and the plasma potential is presented and discussed. An interesting oscillatory behavior within the plasma has also been observed and studied

Barlow, Anders Jack

Quinton, Jamie Scott

Flinders Academic Commons

Radio Frequency SF6 Plasma Modified Single-Walled Carbon Nanotubes: Synchrotron Spectroscopy and Plasma Characterisation Studies A.J. Barlow and J.S. Quinton  Smart Surface Structures Group School of Chemical and Physical Sciences Flinders University, GPO Box 2100 Adelaide SA 5001 Email: Jamie.Quinton@flinders.edu.au    Abstract—Single-Walled Carbon Nanotubes are fluorinated within a Radio-Frequency Plasma Reactor using SF6 as a source gas. To develop an understanding of how varying experimental parameters affect the plasma, and hence the resultant modification of the nanotubes, a Langmuir Probe has been employed to study the plasma system. Information about plasma densities, electron temperatures, and the plasma potential is presented and discussed. An interesting oscillatory behavior within the plasma has also been observed and studied. Keywords - carbon; nanotube; fluorination; plasma I.  INTRODUCTION Carbon nanotubes (CNTs) have received much attention recently and spanning the last decade, due to their attractive intrinsic properties. These tubules of graphitic carbon show great promise in molecular electronics applications as wires, diodes and field effect transistors [1]. Fabrication of such devices and nanostructures however requires that any chemistry being performed on the nanotubes to enhance physical properties or to improve dispersion, be controllable and predictable [2]. Fluorination of carbon nanotubes has been investigated by many research groups as a possible means for improving solubility and for a route towards further modification [3-5]. These studies have often used high temperature gaseous processes to obtain the desired result. Only recently has plasma modification been investigated as a means towards fluorination, with results showing promise for an efficient functionalisation technique [5-9]. An investigation into the mechanism behind plasma modification of carbon nanotubes has only recently been researched in depth, with Osuna et al. presenting Density Functional Theory calculations that predict fluorine addition mechanisms for both graphene and CNTs [10]. However, experimental evidence to support such models is still lacking in literature. Plasma as a state of matter has been studied in depth for several decades, particularly in fusion research, but discharges involving attaching gases (gases that readily accept/attach electrons) such as SF6 are receiving more attention in recent years due to the rising interest for plasma processing in industry, particularly for silicon etching in the microprocessor industry [11]. Whilst in many cases these plasmas may seem quiescent, spatial instabilities downstream from the plasma source have been documented [12-14]. So-called relaxation oscillations have been observed in both pure SF6 discharges, and mixtures of Ar/SF6, with ~kHz oscillation frequencies [15]. Understanding these downstream instabilities is of importance for overall understanding of the surface chemistry of materials exposed to electronegative discharges. In this paper we will discuss certain aspects of our investigation into the attachment of fluorine onto the sidewalls of Single-Walled Carbon Nanotubes via exposure to radio-frequency plasma. We briefly discuss the modification of CNT surfaces based on characterisation studies using X-ray Photoelectron Spectroscopy (lab and synchrotron based XPS). Also, the behaviour of SF6 plasma with varying experimental parameters is of particular interest to our research group. We discuss recent preliminary data on Langmuir Probe studies of the plasma generated in our research reactor, including the curious oscillatory behavior in the spatial density distribution of the plasma. II. EXPERIMENTAL A. Carbon Nanotube Preparation Single-walled carbon nanotubes were purchased from Carbon Solutions and Unidym. The nanotubes were dispersed by placing 0.010g of the nanotube soot into 10mL of dimethylformamide (DMF) and treating with ultra-sonication for 2 hours at room temperature. High purity metallic substrates were used for XPS studies. For each sample, the substrate was covered with the nanotube solution (0.5mL), and then baked at 150°C under vacuum (10-2 Torr) overnight to remove the DMF. The result was a dense paper of nanotubes across the substrate. 978-1-4244-5262-0/10/$26.00 © 2010 IEEE ICONN 2010120B. Plasma Reactor Samples were fluorinated in a cylindrical vacuum reaction chamber (4x10-2 m3 made from stainless steel). Prior to treatment, the chamber was reduced to 5x10-6 Torr. Fluorination was performed via the decomposition of SF6 using a radio frequency plasma source (internal single-loop antenna) operating at 13.56MHz and an applied power of 10-100W. C. Surface Characterisation Synchrotron photoemission studies were performed on the soft X-ray (SXR) beamline at the Australian Synchrotron, with the end station utilising a SPECS Phoibos hemispherical analyser. Survey spectra were obtained using 1486.6eV X-rays to mimic that of a lab-based X-ray source for comparison. High resolution S2p scans were taken using both 1486.6eV and 249eV excitation. The latter X-ray energy was determined experimentally to maximise the ionisation cross section of the Sulphur 2p peak without causing undesirable overlap of Auger peaks (which are known to shift with X-ray energy on a binding energy scale). D. Plasma Characterisation The Langmuir Probe used in this research is an in-house constructed cylindrical probe consisting of a 0.3mm tungsten wire tip of 20mm length protruding from an isolated stainless steel compensating electrode. The voltage ramp is generated by an Agilent 33220 Waveform Generator, and controlled by a Virtual Instrument within the LabVIEW programming environment using a PCI-6024E card which also acts as the data acquisition device. III. RESULTS AND DISCUSSION A. X-ray Photoemission Studies The effects of varying experimental parameters such as applied plasma power and exposure time on the resultant functionalisation have been presented previously [9,16]. Results from these studies suggest that the process occurs without contamination by sulphur due to the absence of a peak at 164eV (the primary S2p photoemission line, shown in the XPS spectrum of Fig. 1). After plasma exposure, a notable presence of fluorine can be seen via the peak at 688eV. The presence of oxygen in the spectrum is attributed to the fact that the nanotubes used in this initial experiment are of RFP type (and hence have been exposed to an acid treatment resulting in carboxyl groups on the tube sidewalls and ends). High resolution scans of the S2p region using 1486.6eV excitation revealed no discernable features (see Fig. 2 inset), however the advantage of synchrotron based photoemission is the continuously tunable excitation energy. Tuning the excitation closer to the S2p energy reveals the presence of a photo-peak that we have attributed to a low concentration of surface sulphur (Fig. 2). At this stage the peak assignments are speculative, but it is likely that there is a convolution of peaks from sulphur attached to carbon and to fluorine. Note that in the spectrum the six peak assignments are paired (2 solid, 2 dashed, and 2 dotted) due the orbital splitting of the S2p line. Experiments to determine the state of the sulphur on the surface remain ongoing. B. Plasma Characterisation An important part of the research currently being performed on our research reactor is that of characterizing plasma used for surface modification. Fig. 3 shows results for important plasma parameters; the floating potential, plasma potential, ion and electron densities, and the electron temperature (shown in eV due to convention). It is important to note that a Langmuir Probe like the one used in this research cannot give information about the ion temperature (only electron temperature), however the plasma is assumed to be “cold”, meaning <1% is ionized, and so the global average ion temperature is be expected to be similar to room temperature. Fig. 3A shows the power delivered to the antenna within the chamber versus the applied peak-to-peak voltage (Vp-p). Across the power range investigated, the power delivered to the plasma (the absorbed power, defined as [forward power] – [reflected power]) is linear up until 30W, beyond which the impedance match between the drive electronics and the plasma diverges, and the reflected power increases. Figure 1. XPS spectra for clean and SF6 plasma treated nanotubes.  Figure 2. High resolution photoemission scans of the S2p region of fluoro-NTs. The inset spectrum was taken using 1486.6eV synchrotron light, replicating that of a lab-based X-ray source, whilst the larger spectrum was taken on the same sample across the same region, using 249eV synchrotron radiation, greatly enhancing the ionization cross-section for sulphur. 01002003004005006007008009001000Binding Energy (eV)C1sO1sF1sOKLLFKLLFeFeF2sNSF6 TreatedPristineS2p121The floating and plasma potentials (B and C respectively) relate to the size of the barrier (known as the sheath, a charged layer surrounding a surface nearby to the plasma) that a species must overcome before being able to leave the plasma, and impact a surface. After an initial sharp increase, the plasma potential increases linearly at a slow rate beyond 20W absorbed power, with a total increase of only 8V over a 50W power range. This indicates that a species will require a similar amount of energy to transit to the surface being modified at around 20W of absorbed power, as those at 70W of power. Over the power range presented the electron and ion densities monotonically increase but do not quite plateau to a maximum value. They also follow a similar trend to one another, which is to be somewhat expected as it is generally accepted that the net charge on a plasma is zero. As the absorbed power is increased, the density of charged species increases, and accordingly, when modifying a surface, the level of surface functionality after exposure would be increased [9]. The electron temperature shows a similar trend to that of the plasma potential, which due to the co-dependency in the way in which these values are calculated, is expected. Etching rates on carbon surfaces have been shown to increase with absorbed power. Since the electron temperature does not increase greatly beyond 20W absorbed power, it is concluded that the increased etching rates are due to greater density, rather than energy. C. Low-frequency Oscillations The I-V (current-voltage) characteristic curves used to generate the values shown in Fig. 3 were averages of 100 measurements. This was necessary as single I-V curves from SF6 plasma displayed prominent distortions due to oscillatory behaviour in the plasma density, radially downstream from the antenna. This effect has been documented in the literature, with oscillation frequencies in the order of ~kHz. Fig. 4 shows a typical probe current oscillation for an SF6 plasma in our reactor (10W, 2.5mTorr, probe held at constant bias voltage).  Figure 3. A: Power delivered to the plasma from the drive electronics, all plots use the absorbed power as the abscissa which is the difference between the applied and reflected power. B: The floating voltage of the plasma, the voltage at which the ion and electron currents are equal. C: The plasma voltage, which needs to be overcome for a charged species to cross the plasma sheath. D: The electron density. E: The ion density. F: The electron temperature (in eV). The abscissa for B-F is the absorbed power as this is what generates the plasma. 01020304050607080900 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Power (W)Input Wave Vp-p (V)Applied PowerReflected PowerAbsorbed PowerA02468101214160 10 20 30 40 50 60 70 80Absorbed Power (W)Vf (V)B01020304050600 10 20 30 40 50 60 70 80Absorbed Power (W)Vp (V)C0123456780 10 20 30 40 50 60 70 80Absorbed Power (W)kTe (eV)F0E+001E+092E+093E+094E+095E+096E+090 20 40 60 80Absorbed Power (W)Ne(cm-3)D0E+005E+081E+092E+092E+093E+093E+094E+094E+090 10 20 30 40 50 60 70 80Absorbed Power (W)Ni (cm-3)E   Figure 4. Probe current with constant probe bias voltage.  Figure 5. FFTs for various gases under the same conditions as the SF6. Only the SF6 displays an oscillation in the probe current. 0 100 200 300 400 500 600 700 800 900 1000Oscillation Frequency (Hz)ArH2N2O2SF6r222F6122The time scale of one oscillation equates in this case to a frequency of 47 Hz. From this point, Fast Fourier Transforms (FFTs) of the probe current allow the frequencies of any oscillatory behavior to be studied in real time. Fig. 5 displays two important points; that of the gases studied, only SF6 shows oscillatory behaviour and that there is a fundamental frequency, and also overtones of this frequency making up the oscillation. Upon first impression it was thought that this was 50Hz noise convoluting the signal (due to mains power), however it was found that the oscillation frequency exhibited a strong dependency on gas pressure, as shown in Fig. 6. As the gas pressure is increased, the fundamental frequency decreases to the point that the oscillatory behaviour can be seen in the plasma with the naked eye as a “fluttering” of the discharge, indicating there is an optical component to the periodic instabilities. This fluttering was very stable and well-defined. Varying the absorbed power reduced the oscillation frequency to 30Hz within 15-50W power, but beyond 50W returned to near the original value (Fig. 7). Within this power region, the FFTs were not well defined, and the plasma was visibly unstable, lacking the defined periodicity that could be seen for varying pressure. Research continues in our laboratory on plasma functionalisation of single-walled carbon nanotubes, but also on the characterization of various plasma, specifically that of the SF6 plasma oscillations. Of particular interest is mapping out a regime within which these oscillations are most stable, and potentially a regime within which the plasma is quiescent. IV. CONCLUSIONS Single-walled carbon nanotubes are functionalized in SF6 plasma with only minimal contamination by sulphur containing species, with its presence only elucidated when using synchrotron X-rays tuned to near the energy of the primary sulphur peak. Oscillations in the SF6 plasma have been observed at much lower frequencies than previously documented in literature, oscillations which do not appear for other gases under study. The oscillation frequency is dependent on both the gas pressure and the absorbed power with a largely linear decrease in frequency with increasing pressure. Increasing the power resulted in a frequency minimum within which the plasma was extremely unstable. ACKNOWLEDGMENTS The authors would like to thank the assistance of Australian Synchrotron staff during the photoemission study, particularly Drs B. Cowie and L. Thomsen of the SXR beamline. REFERENCES [1] N.O.V. Plank, and R. Cheung, "Functionalisation of Carbon Nanotubes for Molecular Electronics". Microelectron. Eng., vol. 73-74, pp. 578-582, 2004. [2] S. Banerjee, M.G.C. Kahn, and S.S. Wong, "Rational Chemical Strategies for Carbon Nanotube Functionalization". Chem. Eur. J., vol. 9, pp. 1898-1908, 2003. [3] E.T. Mickelson, et al., "Fluorination of single-walled carbon nanotubes". Chem. Phys. Lett., vol. 296, pp. 188-194, 1998. [4] H. Touhara, and F. Okino, "Property control of carbon materials by fluorination". Carbon, vol. 38, pp. 241-267, 2000. [5] Y.-Q. Wang, and P.M.A. Sherwood, "Studies of Carbon Nanotubes and Fluorinated Nanotubes by X-ray and Ultraviolet Photoelectron Spectroscopy". Chem. Mater., vol. 16, pp. 5427-5436, 2004. [6] J.S. Quinton, A. Deslandes, A. Barlow, and J.G. Shapter, "Plasma modified carbon surfaces for supporting sensor architectures". in: H.V. Nicolas, (Ed.), Smart Materials, Nano- and Micro-Smart Systems IV, SPIE, Adelaide, South Australia, 2006, pp. 9. [7] J.S. Quinton, et al., "RF plasma functionalized carbon surfaces for supporting sensor architectures". Current Applied Physics, vol. 8, pp. 376-379, 2008. [8] K.A. Park, Y.S. Choi, and Y.H. Lee, "Atomic and Electronic Structures of Fluorinated Single-Walled Carbon Nanotubes". Phys. Rev. B, vol. 68, pp., 2003. [9] A.J. Barlow, and J.S. Quinton, "Synchrotron Soft X-ray Studies of Single-Walled Carbon Nanotubes Functionalized by Radio-Frequency SF6 Plasma", ADVANCED MATERIALS AND NANOTECHNOLOGY: Proceedings of the International Conference (AMN-4), AIP, Dunedin (New Zealand), 2009, pp. 21-24. [10] S. Osuna, et al., "Reaction Mechanisms for Graphene and Carbon Nanotube Fluorination". The Journal of Physical Chemistry C, vol., pp.,  [11] P. Chabert, A.J. Lichtenberg, and M.A. Lieberman, "Theory of a double-layer in an expanding electronegative plasma". Physics of Plasmas, vol. 14, pp. 093502-11, 2007. [12] M.A. Lieberman, A.J. Lichtenberg, and A.M. Marakhtanov, "Instabilities in low-pressure inductive discharges with attaching gases". Applied Physics Letters, vol. 75, pp. 3617-3619, 1999. [13] M. Tuszewski, "An electronegative inductive discharge instability". Journal of Applied Physics, vol. 79, pp. 8967-8975, 1996. [14] M. Tuszewski, and S.P. Gary, "Downstream instabilities of electronegative plasma discharges". Physics of Plasmas, vol. 10, pp. 539-545, 2003. [15] P. Chabert, A.J. Lichtenberg, M.A. Lieberman, and A.M. Marakhtanov, "Instabilities in low-pressure electronegative inductive discharges". Plasma Sources Science and Technology, vol. 10, pp. 478-489, 2001. [16] A. Barlow, and J. Quinton, "Fluorinated Carbon Surfaces Using Radio Frequency Plasma Modification", AIP 18th National Congress 2006 Refereed Papers, Adelaide, South Australia, 2008.     Figure 6. Oscillation frequency with varying gas pressure.  Figure 7. Oscillation frequency with varying absorbed power. 01020304050600 1 2 3 4 5 6 7 8 9 10Primary Oscillation Frequency (Hz)Pressure (mTorr)01020304050600 10 20 30 40 50 60 70Primary Oscillation Frequency (Hz)Absorbed Power (W)123

Radio frequency SF6 plasma modified single-walled carbon nanotubes: Synchrotron spectroscopy and plasma characterisation studies

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Radio frequency SF6 plasma modified single-walled carbon nanotubes: Synchrotron spectroscopy and plasma characterisation studies

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