Direct observation and mechanism of increased emission sites in Fe-coated microcrystalline diamond films

The electron field emission (EFE) properties of microcrystalline diamond (MCD) films are significantly enhanced due to the Fe coating and post-annealing processes. The 900 °C post-annealed Fe coated diamond films exhibit the best EFE properties, with a turn on field (E₀) of 3.42 V/μm and attain EFE current density (J_e) of 170 μA/cm² at 7.5 V/μm. Scanning tunnelling spectroscopy (STS) in current imaging tunnelling spectroscopy mode clearly shows the increased number density of emission sites in Fe-coated and post-annealed MCD films than the as-prepared ones. Emission is seen from the boundaries of the Fe (or Fe₃C) nanoparticles formed during the annealing process. In STS measurement, the normalized conductance

dI/dV   I/V

versus V curves indicate nearly metallic band gap, at the boundaries of Fe (or Fe₃C) nanoparticles. Microstructural analysis indicates that the mechanism for improved EFE properties is due to the formation of nanographite that surrounds the Fe (or Fe₃C) nanoparticles.