Nanoscale structural characteristics and electron field emission properties of transition metal–fullerene compound TiC60 films
Introduction
In the past decades, field electron emission (FEE) from diamond and related carbon films has been a popular topic due to their promising potential application for cold-cathode devices [1], [2]. The interest in electron emission from these carbonaceous materials, such as polycrystalline diamond, diamond like films, various forms of carbon, is due to superior properties of the materials (e.g., high thermal conductivity, chemical stability, low or even negative electron affinity) [3], [4]. Among these carbon films, fullerenes could also be considered as good candidates for field electron emission. However, except that numerous reports on FEE of carbon nanotubes [5], few studies on FEE in other fullerenes have been reported [6], [7].
Toshiki Hara et al. [8] found that the field emission of C60 films could be enhanced after irradiated by a 3 keV electron-beam for 20 h. After performing a measurement on the electron emission characteristics of individual C60 supported on a tungsten substrate, Lin et al. [9] suggested that a properly doped C60 might ultimately prove to be an easily fabricated, stable, subnanometer electron emission source. Jayatissa et al. measured the field emission from a carbon film produced by the pulsed ArF laser ablation of C60. They found that electron emission from the film was enhanced by increasing the number of C–H bonds (sp3 clusters) [6]. Chen et al. [10] also indicated the promising electron emission of C60 film with a turn-on field of 3 MV/m and an enhancement factor of 791, which was attributed to a quasi-direct tunneling process.
In addition, although the extensive studies on field electron emission of diamond and related carbon materials have been made, to date there have been very few reports on local FEE on a nanometer scale. In this paper, along with the structural characteristics and electronic transport properties, the conventional FEE characteristics of transition metal–fullerene compound TiC60 thin films are investigated, and the mechanism of field emission enhancement is discussed in terms of structural variation of TiC60 and electrical conducting channels formed by titanium nanocrystallites. Furthermore, local FEE characterization of the thin films is conducted for the first time on a nanometer-scale.
Section snippets
Experimental
TiC60 thin films were deposited on the copper substrate by using a radical beam titanium source and a conventional ionized cluster beam (ICB) deposition technique (for C60 deposition) that was first suggested by Takagi [11]. During deposition the substrate was heated to 100 °C and the pressure was kept at about 10−5 Torr. The deposition time was 3–4 h. Profile metric measurement showed that the thickness of the TiC60 thin film was in the range of 350–600 nm.
Micro-Raman spectrum was measured by
Structural and morphological characterization and electronic property
Raman spectroscopy is sensitive to the change of translational symmetry and therefore is a proper means for characterization of crystalline, nanocrystalline, and amorphous carbons. Fig. 1 shows a micro-Raman spectrum of TiC60 film on copper substrate, which is similar to the C60 vibration spectrum by Bethune et al. [12]. The C60 molecule gives rise to 46 normal modes: ΓC60=2Ag+3F1g+4F2g+6Gg+8Hg+Au+4F1u+5F2U+6GU+7HU, among which only two non-degenerate Ag modes and eight fivefold degenerate Hg
Conclusion
In summary, transition-metal compound TiC60 films demonstrate a deformed C60 structure with sp3 bonding and a corrugated surface with a myriad of nanoprotrusions. z–V measurement suggests that FEE and SCLC charge transport mechanisms are dominated as the tip penetrates the thin film. Conventional FEE characteristics show a high emission current density of 10 mA/cm2 and a low turn-on field less than 8 MV/m. The non-linearity in the F–N plot is observed and discussed. The enhancement of FEE is
Acknowledgements
This work is partially supported by Research Grants Council of the Hong Kong SAR, under Grant Nos. CUHK4390/99E and CUHK 4203/01E.
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