Paper NS+SP-TuM4
Homogeneous Nucleation and Characterization of Nanodiamonds Synthesized in an Atmospheric-Pressure Microplasma

Tuesday, October 20, 2015, 9:00 am, Room 212B

Nanodiamonds possess striking properties such as exceptional mechanical and chemical stability, low cytotoxicity, tunable active surface chemistry, and tunable photoluminescence-inducing color centers, but their synthesis remains a challenge. At normal temperature and pressure, graphite is the thermodynamically stable phase of bulk carbon, while the diamond phase requires high temperatures and pressures. Nanodiamonds have thus been produced via high-pressure, high-temperature (HPHT) conversion of graphite in the presence of carbonaceous precursors and via detonation of carbon-containing explosives. Alternatively, nanocrystalline diamond films have been deposited at low pressure (~1 Torr) by chemical vapor deposition (CVD). Supporting theoretical predictions¹ have shown that at the nanoscale, hydrogenated forms of carbon prefer sp³ bonding, even at normal conditions, up to a certain size.

Here, we present a study of a plasma process for the synthesis of nanodiamonds at low temperature (<1000 ^oC) and atmospheric pressure. C-H-O-containing vapor precursors diluted in Argon are continuously fed into a DC hollow cathode plasma discharge. Nanoparticles nucleate from radical moieties such as C₂ and CH, whose radiative transitions (specifically, C₂ 516 nm vibrational band and CH 431 nm electronic band) are monitored by optical emission spectroscopy (OES). Particles are carried as an aerosol to either a filter for ex situ materials analysis or an in situ scanning mobility particle sizer system (SMPS). SMPS measurements confirm particle formation and allow us to correlate particle yield and size distribution with OES results. Specifically, we have compared results for ethanol and methanol precursors and find that C₂ formation is favored by ethanol, while CH formation is favored by methanol, and that an increase in both radical populations (measured as the aforementioned peak intensities normalized to Ar 750 nm intensity) increase with both particle number density and average diameter.

Analogous to CVD of diamond films², we find from TEM analysis that the selectivity of diamond phase is dependent on the C-H-O atomic ratio of the precursor. Specifically, methanol is found to produce a significant amount of sub-5 nm nanodiamonds, while ethanol mostly yields larger amorphous and graphitic carbon. We suggest that the radical chemistry observed via OES (i.e., C₂ versus CH populations) plays an important role in such observations, and we demonstrate that addition of H₂ gas to ethanol vapor will recover the results given by methanol.

References

¹P. Badziag et al., Nature 343, 244-245 (1990).

²P. Bachmann et al., Diamond and Related Materials 1, Issue 1, 1-12 (1991).