Carbon nanotubes growth in AlPO4-5 zeolites: Evidence for density dependent field emission characteristics

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO4-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO4-5 crystallites were impregnated, for a period of ∼ 10–60 min, in the Fe-catalyst solution and subjected to Electron Spectroscopy for Chemical Analysis (E.S.C.A.). The analysis revealed that the concentration of Fe-catalyst, CFe, was increased from ∼ 1.7% to ∼ 8.6%, respectively, with increase in impregnation time, IT. The HRTEM results showed that Fe nano-clusters, with diameter ∼ 7–10 nm, were formed in the surface region of the crystallites. These crystallites were sprayed on the conducting substrates, under identical spraying conditions. SEM study revealed that the coverage of the crystallites on the substrates was ∼ 103–104 crystallites/cm2. These substrates were subjected to direct current plasma enhanced chemical vapor deposition (dc-PECVD) process, to grow CNTs. The SEM micrographs were recorded for the CNT-grown substrates and the average areal density of CNTs, (σT)av, on the crystallites (t/cm2) was estimated. The analysis indicated that (σT)av increased from ∼ 6.24 ± 0.19 × 1010 to 2.04 ± 0.61 × 1011 t/cm2 with gradual increase in CFe. The field emission study of the samples revealed that the optimum values of the turn-on electric field, ∼ 3.69 V/μm and the field emission current density, ρd, ∼ 1.78 × 103 μA/cm2 were achieved for (σT)av, ∼ 6.24 ± 0.19 × 1010 t/cm2, at a concentration of Fe, CFe, ∼ 3.0%, encapsulated in the AlPO4-5 crystallites.