Wide band-gap materials show promise for applications in coating of field emission tips. Recently nanocrystalline hexagonal GaN crystallites as small as 12 nm average diameter have been formed using reactive laser ablation of gallium metal in nitrogenating ambient [T. J. Goodwin et al., Appl. Phys. Lett. 70, 3122 (1997)]. In this article we investigated the performance of ungated emitters with and without surface coating. Silicon tip arrays are coated by dielectrophoresis of gallium nitride nanoparticles or nanocrystalline diamond clusters from an ethanol suspension. The emitters were evaluated and compared before and after the surface treatment using SEM image and I-V measurements in the diode configuration. The results suggest that the emitters benefit, from coating the surface with nanocrystalline diamond clusters in terms of reduction in the turn on voltage by 100 V and increase uniformity in emission during low voltage operation. The long-term emission stability for both types of the coated cathodes was studied over a period of 90 h. The emitters coated with GaN nanoparticle clusters show a significant improvement in the current fluctuation from 150% (for untreated cathodes) to 50%. (C) 2003 American Vacuum Society.