Self-Assembly of Soot Nanoparticles on the Surface of Resistively Heated Carbon Microtubes in Near-Hexagonal Arrays of Micropyramids

Almost regular hexagonal arrays of microscopic pyramids consisting of soot nanoparticles are formed on the surface of graphitized hollow filaments, which are resistively heated to similar to 1800-2400 degrees C under an Ar atmosphere containing trace amounts of oxygen (similar to 300 ppm). At higher temperatures (T > 2300 degrees C, approximately) the soot particles are represented mainly by multishell carbon nano-onions. The height and width of the pyramids are strongly dependent on the temperature of the resistive heating, diminishing from 5 to 10 mu m at T approximate to 1800 degrees C to similar to 1 mu m at 2300-2400 degrees C. Quasi-hexagonal arrays of the micropyramids are organized in the convex craters on the surface of the microtubes, which grow with the time of the thermal treatment. The pyramids always point normally to the surface of the craters, except at the boundaries between the craters, where the normal direction is not well-defined. The pyramids are soft and can be easily destroyed by touching them but can be hardened by heating them under an oxygen-free atmosphere. The pyramids are observed only on the exterior surface of the microtubes, not on their inner surface. This suggests that the thermophoretic force generated by a strong temperature gradient near the external surface of the tubes may be the cause of the micropyramid formation. Electrostatic charging of the soot nanoparticles due to thermionic emission may also be relevant to this phenomenon. The micropyramids can function as field emission point sources, as demonstrated with the use of a micronanoprobing station, mounted in a scanning electron microscope.

Automated summary

Under Ar atmosphere with trace amounts of oxygen, graphitized hollow filaments are resistively heated to 1800-2400 degrees C, forming regular hexagonal arrays of microscopic pyramids consisting of soot nanoparticles on the surface. The height and width of these pyramids depend on the temperature, and they can function as field emission point sources.