Zhang et al. 10.1073/pnas.0507064102. |
Fig. 7. The 4-inch CVD system used for the vertical SWNT synthesis. A schematic drawing of the system used for our nanotube synthesis.
Fig. 8. Comparison of growth results with various O2 and H2 conditions. (a-e) Growth results (SEM or AFM images) under various conditions. (f) Raman spectra obtained with the various samples. Spectra for #3 and #5 are not shown because of the low SWNT yield. All of the growth experiments were carried out on identically prepared substrates with the same submonolayer Fe nanoparticle catalyst. All growth runs were carried out at the same temperature (720°C), total pressure (0.34 torr), and plasma power (70 W) for the same amount of time (10 min). Specific growth conditions are as follows. Growth run #1: Regular (normal) growth condition result with CH4/H2/O2. Partial pressure of H2:O2 = 12%:1% (very high yield; SWNTs grown into vertical films). Growth run #2: H2-free growth condition (no H2 flow used in growth). CH4/O2 only. Partial pressure of O2 = 0.8% (very high yield; SWNTs grown into vertical films). Removal of H2 from the regular growth condition can still afford high-yield growth, after adjusting down O2 partial pressure. H2 is not essential in our regular condition but can be added for balancing purposes. Growth run #3: O2-free growth condition (no O2 flow used in growth.) CH4/H 2 only. Partial pressure of H2 = 7.4% (very low yield of SWNT with H2 but no O2 in growth). Growth run #4: A CH4/H2/O2 growth condition with high H2 concentration. H2:O2 = 22%:1% (>12%:1% in regular optimum growth condition in #1). (Adding excessive H2 gives lower yield than #1and fails to produce vertically packed SWNTs. Rather, a mat of SWNTs lying on the substrates was formed.) Growth run #5: CH4-only growth (yield is higher than CH4/H2 growth in #3, but packed V-SWNTs cannot be grown). General trends: (i) Vertical SWNTs were never grown without O2 for the CH4 PECVD method (e.g., run #3, #4, and #5). (ii) Higher H2 conditions give lower yield of SWNTs. For CH4/H2/O2 conditions, increasing H2 gives lower yield (e.g., run #4 vs. #1). CH4/H2 (#3) yield was lower than CH4 alone (#5) growth.