Charge Transport Measurements Of Vertically Aligned Carbon Nanofibers

Vertically aligned carbon nanofibers (VACNFs) have found a variety of electronic applications. To further realize these applications, a good understanding of the charge transport properties is essential. In this work, charge transport properties have been systematically measured for three types of VACNF forests with Ni as catalyst, namely VACNFs grown by direct current PECVD, and inductively coupled PECVD at both normal pressure (3.6 Torr) and low pressure (50 mTorr). The structure and composition of these nanofibers have also been investigated in detail prior to the charge transport measurements. It has been found that the dc VACNF body consists of three parts: a 10-15 nm thick graphitic outer layer, cross-struts, and a layer with darker contrast in between. Carbon, nitrogen, silicon, nickel and oxygen are all present in the dc VACNF body. Ni is distributed along the entire dc VACNF body, as first reported in this work. Auger electron spectroscopy results indicate that Ni is primarily located in fiber walls, not in the center catalytic part. Four-probe I-V measurements on individual nanofibers have been enabled by the fabrication of multiple metal ohmic contacts on individual fibers that exhibited resistance of only a few k[Omega]. An O2 plasma reactive ion etch method has been used to achieve ohmic contacts between the nanofibers and Ti/Au, Ag/Au, Cd/Au, and Cr/Au electrodes. Dc VACNFs exhibit linear I-V behavior at room temperature, with a resistivity of approximately 4.2x10−3 [Omega]·cm. Gate effect is not observed when the heavily doped Si substrate is used as a back gate. Our measurements are consistent with a dominant transport mechanism of electrons traveling through intergraphitic planes in the dc VACNFs. The resistivity of these fibers is almost independent of temperature, and the contact resistance decreases as temperature increases. Further studies reveal that the 10-15 nm thick graphitic outer layer dominates the charge transport properties of dc VACNFs. This is demonstrated by comparison of charge transport properties of as-grown VACNFs and VACNFs with the outer layer partially removed by oxygen plasma reactive ion etch. The linear I-V behavior of the fibers does not vary as this outer layer becomes thinner, but displays a drastic shift to a rectifying behavior when this layer is completely stripped away from some regions of the nanofiber. This shift may be related with the compositional differences in the outer layer and the inner core of the nanofibers. Our results imply that by varying the extent of graphitization and structure of the outer layer, it may be possible to achieve controllable charge transport properties for dc VACNFs. VACNFs grown by inductively coupled PECVD at normal and low pressure both have a defective outer layer and a more crystalline inner core. The composition of these fibers is predominately carbon, and Ni is not observed along the fiber body. Nitrogen is present possibly as a result of sample storage in air. Two-probe charge transport measurements indicate linear I-V behavior, and the resistivity of both types of inductively coupled PECVD grown VACNFs is on the order of 10−3 to 10−4 [Omega]·cm.