Catalyst-free growth of needle-shaped carbon filaments at low temperature in a near-critical binary fluid

Ultraviolet laser irradiation into a mixture of supercritical carbon dioxide and benzene at 266 nm wavelength in an electric field of 8 kV/cm resulted in production of needle-shaped carbon filaments at low temperature, observed T min being 33.5 °C. Structures could only be produced under near- and supercritical conditions, and did not appear in sub-critical conditions, i.e., in the gas–liquid coexistence state. The filaments are straight, with maximum observed length approximately 20 μm, and are 0.5–1 μm in diameter at the base, and approximately 300 nm at the tip. They do not bundle together, but grow individually, aligned radially from the sharp edges of the silicon substrates attached to the electrodes. No catalyst particles were found inside, at the base or the tip of the filaments. We suggest that the primary facilitator in the filament growth is the local electric field strength, which corresponds to the substrate morphology, possibly further assisted by the unusual properties of the critical fluid phase, such as increased miscibility and high isothermal compressibility. The structures were observed by scanning electron microscopy, and further characterized by energy dispersive X-ray spectroscopy and visible and ultraviolet Raman spectroscopy.