Dielectrophoretic Frequency Effect on Purification and Field Emission of Carbon Nanotubes

This paper presents the design, fabrication and test of a field emission chip with carbon nanotubes (CNTs). The objective of this study was to utilize positive dielectrophoresis (DEP) to purify CNTs on parallel microelectrodes and then to perform a field emission test on the CNT film. Study parameters include the electrode width and gap, and the frequency of DEP. To understand the locations where the CNTs might be accumulated, a computer simulation was used to analyze the distribution of gradient of the square of electric field, VE, around the electrodes under the electric field. Three combinations of parallel electrodes, with various width and gap, were designed for the purification and field emission of CNTs. A test chip consisting of three parallel micro electrodes and a micro channel, formed by polydimethyl siloxane (PDMS), was manufactured by photolithography. The medium, also served as cathode surfactant and dispersant, was prepared by sodium dodecyl sulfate (SDS), and the medium with CNTs was fulfilled into the micro channel by syringe pump. By applying different DEP frequencies on the electrodes, the experiment on trapping and purification of CNTs was observed and analyzed. The results showed the CNTs would be accumulated around the edges of electrodes where higher values of VE existed. This trend was in good agreement with the simulation. After the CNTs were purified, Raman spectroscopy showed that the relative intensity ratio of G-band to D-band increased from 0.36 to 0.56 for the samples prepared by DEP at frequencies from 1 to 25 MHz, while the value of non-purified CNTs was 0.29. Therefore, higher frequency of DEP resulted in better purification of CNTs. Followed by the field emission test on the CNT film under voltage ranging from 0 to 1100 V. Results revealed that the CNT film prepared at higher frequency of DEP had a lower threshold of electric field for field emission.