Vertically Aligned Boron-Doped Diamond Hollow Nanoneedle Arrays for Enhanced Field Emission

Lower turn-on field (ETO) and threshold field (ETHR), as well as a higher current density, are desired characteristics of materials and structures for field emission (FE) applications. In this work, the properties of a boron-doped diamond (BDD) hollow nanoneedle array (HNNA) structure were remarkably enhanced through surface state modulation. By designedly controlling the surface roughness and boron concentration of the BDD film, a uniform and dense HNNA with a high aspect ratio (∼21.3) was constructed by applying O2 + Cl2 inductively coupled plasma reactive ion etching (ICP-RIE) for an optimal duration (90 min). Although the maximum HNNA density (∼5.5 × 106 mm-2) is produced at a shorter duration, the overall FE performance is even worse than that of the BDD film without an HNNA, arising from the field shielding effect and incomplete elimination of defective needles. The finally formed Actiniaria tentacle-like HNNA structure is associated with the masking role of oxidized Si and amorphous carbon on the top edge of the needle embryos, resulting from the electric field edge effect and Si supply accompanying the substrate. Meanwhile, the sp2 carbon generated by ICP-RIE on the surface of the as-etched HNNA promotes the FE, showing a minimum ETO of 0.11 V/μm. After moderate hydrogen plasma treatment, owing to the negative electron affinity (NEA) of C-H on p-type BDD with downward band bending of the conduction band minimum and local electric field enhancement induced by the hollow nanostructure, the hydrogen-terminated BDD-HNNA shows excellent FE stability and a linear Fowler-Nordheim relation with a lower ETO of 0.38 V/μm and ETHR of 2.21 V/μm and a desirable current density of 6532 μA/cm2 at 3.75 V/μm. The comprehensive FE properties of the surface-modulated high-quality BDD-HNNA exceed those of the vast majority of other conventional or popular nanostructural counterparts.