Field emission studies of boron and nitrogen doped carbon nanotubes grown on pointed and flat substrates

Abstract

The idea of doping carbon nanotubes (CNTs) with boron (B) and/or nitrogen (N) looks attractive in view of the changes brought in their electronic structures by the dopants. The field electron emission studies of boron (B) and nitrogen (N) doped CNTs grown in situ on pointed tungsten (W) tip and flat silicon (Si) substrate surfaces have been reported in this paper. The CNTs were grown by pyrolysis of ferrocene with suitable dopants. The morphology of the B-doped and N-doped CNTs on flat Si substrate was observed under SEM (Fig 1a & b). The B-doped CNTs on flat Si and pointed W tip have rope like structure and are also found to be considerably long (/spl sim/50 /spl mu/m). This may be attributed to the dopant atoms of boron, which act as catalysts for further growth of the CNTs along the tube axis. N-doped CNTs were, however, short (/spl sim/5 /spl mu/m), uniform and very densely packed. The Fowler-Nordheim (F-N) plots obtained from the I-V curves of doped field emitters show non-linear behavior (Fig 2 a and b). This non-linearity may be attributed to strong field penetration into the emitter apex region resulting into the local large variations of the electric field in the electron tunneling region. The local field enhancement factor (/spl beta/) and the current density (J) have been calculated from the slopes of the F-N plots. Field emission micrographs from the B- and N- doped CNTs on W tips reveal geometrical structures, typical of CNT bundles. The FEM images corresponding to B and N-doped CNTs on flat Si substrates show streaky structures. Typical field emission currents upto 200 /spl mu/A drawn from both B and N-doped CNTs are remarkably stable over periods greater than 3 hours (Fig 3 a and b). But in case of B-doped CNTs on flat Si, the set current of 400 /spl mu/A decreases slowly for about an hour and then gets stabilised at 300 /spl mu/A. The turn-on voltage to draw 0.1 nA current was found to be 3.32 kV for B-doped CNTs/W tip and 3.0 kV for B-doped CNTs /Si flat substrate. The threshold voltage to draw 1 /spl mu/A current was found to be 5.52 kV for B-doped CNTs/W tip and 4.6 kV for CNTs/Si flat substrate. For N-doped CNTs, the turn-on voltage to draw 0.1 nA current was found to be 3.12 kV for W tip and 3.88 kV for Si flat substrate. The threshold voltage to draw 1 /spl mu/A current was found to be 4.8 kV for N-doped CNTs/W tip and 7.0 kV for N-doped CNTs/Si flat substrate. So a lower voltage is needed to draw a fixed emission current from B-doped CNTs/Si than B-doped CNTs/W tip substrate, whereas a lower voltage is needed to draw a fixed emission current from N-doped CNTs/W tip than N-doped CNTs/Si substrates. Thus, it can be inferred from this study that the B-doped CNTs are better suited for large-area emitter applications such as flat panel display devices whereas the N-doped CNTs on pointed W tips would be promising candidates for high current density single electron beam applications.