Optical spectroscopy and langmuir probe diagnostics of microwave plasma in synthesis of graphene-based nanomaterials
Along with the revolutionary discovery and development of carbon nanostructures, such as carbon nanotubes and graphitic sheets, has arrived the potential for their application in the fields of medicine, bioscience and engineering due to their exceptional structural, thermal and electrical properties. As roll-to-roll plasma deposition systems begin to provide means for large scale production of these nanodevices, a detailed understanding of the environment responsible for their synthesis is imperative in order to more accurately design and control the growth of carbon nanodevices. To date, the understanding of the chemistry and kinetics that govern the synthesis of carbon nanodevices is only partially understood. In response to this need, the plasma environment of a microwave plasma chemical vapor deposition reactor has been studied. Coherent anti-Stokes Raman scattering spectroscopy was used to probe the H2 molecules in the plasma under various parametric conditions. The rotational temperature of H2 was found to increase with reactor pressure, plasma generator power, and distance from the deposition surface. At 10 Torr, the temperature range is approximately 700 to 1200 K while at 30 Torr it is 1200 to 2000 K. Also, the introduction of CH4 and N2 to the plasma increases the rotational temperature consistently. However, the number density of H2 in the plasma does not significantly deviate from theoretical values corresponding to conditions without the plasma indicating that the microwave plasma is weakly ionized and that the rotational temperatures obtained approximate the translation temperature of H2. The spectral region of the vibrational hot band was also inspected but no transitions were found indicating that there is little vibrational nonequilibrium. Additionally, a Langmuir probe was used to obtain the electron energy density function of the plasma. Due to the mismatch between the probe and microwave plasma system, however, the electron energy density function was distorted resulting in an overestimation of the electron temperature and an underestimation of its number density in what is known as the Druyvesteynization effect. In spite of this, an upper limit for the electron temperature was established at 4 eV and a lower limit for its number density at 1E10 m -3. Furthermore, the collisional frequency was estimated to be on the same order as the microwave electric field frequency indicating that the plasma is dominated by collisions. In conclusion, the plasma in the microwave plasma chemical vapor deposition reactor was found to be weakly ionized and not collisionless. These findings help to identify the plasma and understand its kinetics. Gas temperature profiles were established at 10 and 30 Torr based on the rotational temperature of H2 at parametric conditions conducive to the growth of graphene-based nanomaterials. An upper limit to for the electron temperature was also obtained.
Lucht, Purdue University.
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