Formation of carbon nanospheres via ultrashort pulse laser irradiation of methane

Irradiation of methane with intense, spatially- and temporally-shaped femtosecond laser pulses forms a spatially confined microplasma that produces carbon nanospheres. The morphology and composition of the nanospheres are characterized by transmission electron microscopy (TEM), ultraviolet (UV) Raman spectroscopy, and infrared spectroscopy (IR). Increasing the pressure of methane from 6.7 to 133.3 kPa results in a decrease of the median diameter of the spheres from ∼500 nm to 85 nm. At pressures of 101.3 kPa and higher, particles with non-spherical morphologies are observed in TEM analysis. The morphology of the nanospheres is determined to be amorphous, containing both sp2 and sp3 hybridized carbon atoms, based on the presence of the carbon D and T peaks in the UV Raman spectrum. The red shift of the G peak and a high fluorescence background in the Raman spectrum indicates that the hydrogen content of the spheres is at least 30%. The presence of hydrogen is supported by the IR spectrum, which shows the presence of C-H bonds in the nanosphere product.