Plasma etching was performed at 100 W for 90 min by using 71.4% O2 in the feed gas. Figure 2c shows SEM image of the top surface of VACNT/parylene composite
after plasma etching. Large numbers of bright spots were found, which were believed to be the extending CNT tips agglomerated together, sine parylene was etched faster than CNTs by oxidative plasma [9–11]. HRTEM observation (Figure 3d) confirms the protruding of CNTs from the above of the composite surface after plasma treatment. Alvocidib Furthermore, the marked area highlighted the opened CNT tips, which provides a direct proof for the opening of the exposed CNTs by oxidative plasma. Subsequently, HF acid was used to remove the VACNT/parylene composite from the Si substrate to produce a freestanding membrane. Another Idasanutlin 5-min plasma etching was performed
on the backside to expose the CNTs from the bottom surface. After these procedures, freestanding composite membranes with vertically aligned CNTs embedded in the parylene matrix were successfully fabricated. Raman spectroscopy was employed to characterize the structure of CNTs during membrane fabrication. Figure 4 shows Raman spectra of the as-synthesized CNT forest, the VACNT/parylene composite membrane, and the composite membrane after plasma etching treatment. The G-band at 1,590 cm-1 is associated with the E2g in-plane stretching vibration mode on the basal plane of graphite, which indicates the existence of crystalline graphitic carbon in the CNT samples. The peak at 1,304 cm-1 (D-band) is assigned to the imperfections in CNTs and amorphous carbon. The intensity ratio between G-band click here (I G) and D-band (I D) is sensitive to chemical modification and is a measure of the defects in CNTs. The I G/I D ratio is determined to be 2.56 for the as-synthesized CNTs, suggesting good crystallinity of the CNT array grown by water-assisted CVD. As shown in Figure 4, the G-band and D-band peak positions do not change, and the two bands (1,003 and 687 cm-1) ascribed to parylene appear fantofarone in the Raman spectrum of CNT array after parylene deposition. Although no distinctive change in terms of the Raman shift
of G-band or D-band is found, the I G/I D ratio decreases from 2.56 for the as-synthesized CNT to 1.02 for the composite membrane treated by plasma etching. The Raman analyses suggest that the deposition of parylene into the CNT array does not cause any damage to CNTs, while the plasma etching induced structural defects on CNT tips above the membrane surface. Figure 4 Raman spectra of the CNTs and the composite membranes. Raman spectra of the as-synthesized CNTs and VACNT/parylene (CP) composite membranes and composite membranes after plasma etching (PE). Figure 5 shows Ar permeances versus pressure gradient across the composite membrane at various temperatures. Obviously, at the temperature between 30°C to 70°C, the Ar permeance through the CNT membrane is independent of the applied pressure gradient.