Field emitters based on porous aluminum oxide templates

The field-emission properties of cold cathodes produced using nano-porous anodic aluminum oxide (AAO) templates are reported. Several types of field emitters were fabricated: aligned copper nanowires grown halfway up the parallel nano-pores of the AAO; aligned multiwalled carbon nanotubes grown to the top of the pores; surfaces overgrown with random tangles of carbon nanotubes; and empty AAO templates. Significant field-emission currents (field enhancement values ∼ 1800) were obtained at threshold voltages as low as 80 V (corresponding to fields of 3–4 V/μm) on samples of nanotube tangles. Perfectly aligned carbon nanotubes were less efficient field emitters and had lower field enhancement values. These observations are explained in terms of the mean separation of active tips in the two sets of samples. Empty templates and metal nanowire arrays show lower field enhancements and higher threshold electric fields (40–70 V/μm). In these samples significant field-emission currents are produced at relatively low applied voltages of 110–300 V due to the small inter-electrode separations achieved on depositing a metal grid directly on the surface of the porous template.