For example, the electrical conductivity rose from 21 to 54 S/cm with a density increase from 0.25 to 0.65 g/cm3. Significantly, we observed that the taller the forest used in the buckypaper fabrication,
the higher the electrical conductivity. Comparing buckypapers with almost the same density, the buckypaper obtained from forests with heights of 1,500 μm exhibited approximately twice the electrical conductivity of buckypaper made from 350-μm forests, (i.e., 45 vs. 19 S/cm at 0.50 g/cm3, and 27 vs. 16 S/cm around 0.35 g/cm3). Figure 2 Electrical conductivity of buckypapers MK-4827 (a) and sheet resistance of SWCNT forest (b). (a) The electrical conductivity of buckypapers as a function of the mass density of buckypapers. Red, black, and blue dots indicate the buckypaper fabricated from SWCNT forest with the heights of 1,500, 700, and 350 μm, respectively. (b) Sheet resistance
of SWCNT forest with different heights measured by a micro 4-probe. Red, black, and blue dots indicate the SWCNT forest with the heights of 1,500, 700, and 350 μm, respectively. Inset shows the photograph of the gold electrode Selleckchem MK-1775 on Si substrate used as a micro 4-probe. In order to verify that this apparent height-dependent variation in buckypaper conductivity was not due to differences in CNT quality, which has been shown to be essential for the various properties of buckypaper in previous works , Raman spectroscopy and electrical resistivity measurements of the as-grown SWCNT forests were carried out. The intensity ratios of the G-band (1,600/cm) and the D-band (1,350/cm) in the Raman spectra (see additional file 1: Figure S2), an indicator of CNT quality, were very similar (approximately 7). Peak positions and intensities in the radial breathing modes (RBM; 100 to 300/cm) were also nearly identical for all SWCNT forest heights. As the RBM peak position w (cm-1) is reported to be inversely proportional to the SWCNT diameter (nm), i.e., w = 248/d, these findings indicate that the effect of forest
height on SWCNT diameter distribution was small. Furthermore, electrical conductivity of raw material forest was evaluated by applying a micro 4-probe onto the sides of SWCNT forests. Since the distances between the probes (50 μm) in a micro 4-probe was sufficiently short compared Bacterial neuraminidase with the forest height, CNT length had almost no influence on the resistance values observed with this measurement. The measured resistance was nearly identical (206 to 220 Ω/sq) regardless of forest height (Figure 2b), indicating that quality of the SWCNTs did not degrade when growing forests of height to 1,500 μm, in accordance with the results of Raman spectroscopy. As shown in the previous paragraph, taking into consideration the fact that forest height did not influence CNT quality, we conclude that the increase in buckypaper conductivity accompanying forest height was a result of the increased length of individual SWCNTs.