Graphitization of carbon nanofibers:: visualizing the structural evolution on the nanometer and atomic scales by scanning tunneling microscopy

The structural evolution of carbon nanofibers submitted to high-temperature ( 1800, 2300, and 2800C) heat treatments has been investigated at the nanometric and atomic scales by means of scanning tunneling microscopy (STM). To complement the local STM observations, X-ray diffraction and Raman spectroscopy characterization of the samples were also carried out. On the nanometer scale, the as-grown nanofibers displayed an isotropic platelet morphology that developed into striped arrangements of increasing width at 1800 and 2300degreesC, and into large, atomically flat terraces at 2800degreesC. On the atomic scale, the starting nanofibers were characterized by tiny ( less than or similar to 2 nm) crystallites. The crystallites were observed to coalesce at 1800degreesC into appreciably larger (similar to 3 - 4 nm) although still defective units. Atomic structures evidencing truly graphitic ordering (i.e. the typical STM triangular pattern with a periodicity of 0.25 nm) started to develop at 2300degreesC. At this temperature, a segregation of graphitic domains and highly defective areas was noticed and attributed mainly to the mobility and subsequent aggregation of point defects ( atomic vacancies). Long-range atomic-scale order was generally established in the nanofibers heat treated at 2800degreesC, where only some incompletely graphitized, fragmentary graphenes were left on the surface.