Structural and field emission properties of effective nanocomposite cathodes CNT@TiO$_2$

A comprehensive study of the composition and field emission properties of field emission cathodes based on CNT@TiO$_2$ core-shell nanocomposites is presented. Coatings with arrays of vertical carbon nanotubes (CNTs) were produced by the plasma-chemical method on silicon substrates with a Ni catalyst, and thin layers of TiO$_2$ were produced by subsequent atomic layer deposition. It was found that the work function of the coating material with the initial CNT array was 4.98 eV; for the case of CNT@TiO$_2$, it took values of 4.29 and 3.82 eV for oxide thicknesses of 3 and 6 nm, respectively. The developed technique for comparing emission characteristics showed that a decrease in the work function of structures with CNT@TiO$_2$ was accompanied by a decrease in local electric fields at the tips. A cathode with CNT@TiO$_2$ arrays (6 nm) required the lowest electric field in the group of samples to ensure an emission current density of 1 mA/cm$^2$ about 5 $\cdot$ 10$^9$ V/m. This is 1.6 times less than for a similar sample with an array of “pure” CNTs. The average values of the effective field enhancement coefficient tended to decrease when going from CNTs to CNTs@TiO$_2$, probably due to an increase in the radius of curvature of tubular nanoparticles upon deposition of an additional layer. Modification with an oxide coating led to an increase in the effective emission area of the cathode.