We have synthesized high-purity double-walled carbon nanotubes (DWCNTs) by an arc-discharge method in hydrogen ambient. The DWCNTs were synthesized using a mixture of Fe catalyst and FeS promoter. Without FeS promoter, we only obtained single-walled carbon nanotubes (SWCNTs). The synthesized DWCNTs had outer diameters in the range of 3.0−3.4 nm and an average interlayer distance of 0.38 nm between graphene layers. The FeS promoter played a key role for the DWCNT growth. The DWCNTs indicated high electronic density of states in the binding energy region between 3.88 and 13.23 eV below Fermi energy, indicating that the DWCNTs had alot of delocalized graphite σ and σ/π electrons. For field emission properties, the typical turn-on field of DWCNTs was about 3.0 V/μm at the emission current density of 0.1 μA/cm2, and the emission current density of DWCNTs was about 10 mA/cm2 at the applied field of 6.5 V/μm. It is considered that the higher current densities of DWCNTs were mainly attributed to the emitted delocalized graphite σ and σ/π state electrons of DWCNTs. Moreover, DWCNTs have higher emission stability than SWCNTs due to two neighboring graphene layers.

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Single-walled carbon nanotubes (SWCNTs) were directly synthesized by a hydrogen arc-discharge method by using only Fe catalyst. The synthesized carbon materials indicated high-purity SWCNTs without amorphous carbon materials from SEM observation. The SWCNTs had diameters of 1.5-2.0 nm from TEM and Raman observation. After a simple purification, TGA indicated that SWCNTs had a purity of ca. 90.1 wt %. Field emission from the SWCNT emitters which were fabricated by using a spray method was measured by a diode structure. The vertically aligned SWCNT emitters showed the low turn-on voltage of 0.86 V/microm and a high emission-current density of 3 mA/cm2 at an applied field of about 3 V/microm. From a Fowler-Nordheim plot, the vertically aligned SWCNT revealed a high field enhancement factor of 2.35 x 10(4). The photoemission measurements, excited by a photon energy of 360 eV, showed significantly delocalized graphite-pi states at the purified SWCNTs. Here, we investigated that the field-emission properties of SWCNTs would be attributed to the high electronic density of states near Fermi energy, including the delocalized graphite-pi states.