Vertically Aligned\nBoron-Doped Diamond Hollow Nanoneedle\nArrays for Enhanced Field Emission

Abstract

Lower turn-on field (<i>E</i>_TO)\nand threshold\nfield (<i>E</i>_THR), as well as a higher current\ndensity, are desired characteristics of materials and structures for\nfield emission (FE) applications. In this work, the properties of\na boron-doped diamond (BDD) hollow nanoneedle array (HNNA) structure\nwere remarkably enhanced through surface state modulation. By designedly\ncontrolling the surface roughness and boron concentration of the BDD\nfilm, a uniform and dense HNNA with a high aspect ratio (∼21.3)\nwas constructed by applying O₂ + Cl₂ inductively\ncoupled plasma reactive ion etching (ICP-RIE) for an optimal duration\n(90 min). Although the maximum HNNA density (∼5.5 × 10⁶ mm^–2) is produced at a shorter duration,\nthe overall FE performance is even worse than that of the BDD film\nwithout an HNNA, arising from the field shielding effect and incomplete\nelimination of defective needles. The finally formed <i>Actiniaria</i> tentacle-like HNNA structure is associated with the masking role\nof oxidized Si and amorphous carbon on the top edge of the needle\nembryos, resulting from the electric field edge effect and Si supply\naccompanying the substrate. Meanwhile, the sp² carbon generated\nby ICP-RIE on the surface of the as-etched HNNA promotes the FE, showing\na minimum <i>E</i>_TO of 0.11 V/μm. After\nmoderate hydrogen plasma treatment, owing to the negative electron\naffinity (NEA) of C–H on p-type BDD with downward band bending\nof the conduction band minimum and local electric field enhancement\ninduced by the hollow nanostructure, the hydrogen-terminated BDD–HNNA\nshows excellent FE stability and a linear Fowler–Nordheim relation\nwith a lower <i>E</i>_TO of 0.38 V/μm and <i>E</i>_THR of 2.21 V/μm and a desirable current\ndensity of 6532 μA/cm² at 3.75 V/μm. The comprehensive\nFE properties of the surface-modulated high-quality BDD–HNNA\nexceed those of the vast majority of other conventional or popular\nnanostructural counterparts.