Huge Volume Expansion and Structural Transformation of Carbon Nanotube   Aligned Arrays during Electrical Breakdown in Vacuum
  We observed a huge volume expansion of aligned single walled carbon nanotube (SWNT) arrays accompanied by structural transformation during electrical breakdown in vacuum. The SWNT arrays were assembled between prefabricated Pd source and drain electrodes of 2 \mu m separation on Si/SiO_2 substrate via dielectrophoresis. At high electrical field, the SWNT arrays erupt into large mushroom-like structure. Systematic studies with controlled electrical bias show that above a certain field SWNTs swell and transform to nanoparticles and flower-like structures with small volume increase. Further increase in electrical bias and repeated sweeping results into amorphous carbon as determined from scanning and transmission electron microscopy (TEM). Cross sectional studies using focused ion beam and TEM show the height of 2-3 nm SWNT array increased to about 1 \mu m with a volume gain of ~ 400 times. The electron energy loss spectroscopy reveals that graphitic sp^2 networks of SWNTs are transformed predominantly to sp^3. The current-voltage measurements also show an increase in the resistance of the transformed structure. 
Shashank Shekhar, Helge Heinrich, and Saiful I. Khondaker
cond-mat.mes-hall cond-mat.mtrl-sci