We investigated the structural variation and the concomitant change in the emission characteristics of hot-filament chemical-vapor-deposition-grown nanostructured carbon films induced by the addition of ammonia to source gases. At low ammonia concentrations the film morphology was dominated by carbon nanoparticles encapsulating metal and/or carbide cores. As the ammonia concentration was increased nanotube density increased, and in the medium ammonia-concentration range nanotubes dominated the film morphology. However, at 80% of ammonia concentration the number of nanotubes decreased substantially and nanoparticles became the dominant species again. The diameter, length, and shape also showed systematic ammonia-concentration-dependent variations, while the nanoparticle size did not change significantly. Another important variation was found in the quality of carbon layers constituting the carbon nanoparticles and nanotubes. As more and more ammonia was added to the source gas, large structural deterioration, including the increase of disorder in the atomic bonding and the decrease of graphitic sheet size, was observed. The electron-emission characteristics of these films were as diverse as their morphological and structural variations. At the low ammonia-concentration range, the emission was degraded as the ammonia concentration was increased. However, when the ammonia concentration was increased up to 40%, the emission was recovered almost back to the best level. At higher ammonia concentrations, substantial emission degradation recurred. (C) 2003 American Vacuum Society.