Vertically aligned multiwalled carbon nanotubes were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates at temperatures as low as 400 degreesC. Negative self-biases were induced to the substrates by radio frequency plasma to give ion bombardment to the growing surface. An increase of self-bias voltages from -50 to -200 V resulted in an evolution of the microstructures from amorphous carbon to nanorods, subsequently to nanotubes. Nanotubes grown above - 150 V were more straight in morphology and better in crystallinity than nanorods grown at - 100 V. In the field emission (FE) measurements, the electric fields to obtain 1 muA/cm(2) were 4.6 and 11.1 V/mum for the nanorods and nanotubes grown at - 100 and - 200 V, respectively. The emission areas, calculated from the Fowler-Nordheim plots, were much larger in the nanorods than the nanotubes. It is considered that a larger amount of crystalline defects in nanorods plays a major role in improving their FE characteristics. (C) 2003 American Vacuum Society.