Carbon nanotubes growth in AlPO_4-5 zeolites: Evidence for density dependent field emission characteristics

摘要

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO_4-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO_4-5 crystallites were impregnated, for a period of approx 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, C_(Fe), was increased from approx 1.7 percent to approx 8.6 percent, respectively, with increase in impregnation time, I_T. The HRTEM results showed that Fe nano-clusters, with diameter approx 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 approx 10~3 - 10~4 crystallites/cm~2. 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, (sigma_T)_(av), on the crystallites (t/cm~2) was estimated. The analysis indicated that (sigma_T)_(av) increased from - 6.24 +- 0.19 X 10~(10) to 2.04 +- 0.61 X 10~(11) t/cm~2 with gradual increase in C_(Fe). The field emission study of the samples revealed that the optimum values of the turn-on electric field, approx 3.69 V/mu m and the field emission current density, rho_d, approx 1.78 X 10~3 mu A/cm~2 were achieved for (sigma_T)_(av), approx 6.24 +- 0.19 X 10~(10) t/cm~2. at a concentration of Fe, C_(Fe), approx 3.0 percent, encapsulated in the AlPO_4-5 crystallites.