  Almost regular hexagonal arrays of a few micrometers tall and wide
micropyramids consisting of soot nano-particles are formed on the surface of
graphitized hollow filaments, which are resistively heated to
~1800$^\circ$C-2400$^\circ$C in an Ar atmosphere containing trace amounts of
oxygen (~300 p.p.m.). At the higher temperatures (T>2300$^\circ$C,
approximately) the soot particles are represented mainly by multi-shell carbon
nano-onions. The height and the width of the pyramids is strongly dependent on
the temperature of the resistive heating, diminishing from 5-10mkm at
T=1800$^\circ$C to 1mkm at 2300-2400$^\circ$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 are
pointing always 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 easy destroyed by touching them, but can be
hardened by heating them in the oxygen-free atmosphere. The pyramids are
observed only on the exterior surface of the microtubes, but 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 micropyramids 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 micro-nanoprobing station, mounted in a scanning electron
microscope.
