The role of Ru promotion of Fe catalysts in the growth of small-diameter single-wall carbon nanotubes (SWCNTs) has been investigated using an Autonomous Research System (ARES)a high throughput, laser-induced chemical vapor deposition (CVD) system capable of in situ Raman spectroscopy. Growth experiments in the ARES were conducted using a standard feedstock (ethylene) at 800 °C for Fe-Ru and Fe catalysts. The results show that Fe-Ru (with a composition ∼10%) supports the growth of small-diameter SWCNTs (less than 1.1 nm) compared to pure Fe. In addition, density functional theory (DFT) calculations on Fe nanoparticle clusters of 55 and 59 atoms with and without adding Ru were performed to probe the effect of Ru on the catalyst stability. The DFT results show that, irrespective of the exact distribution of Ru atoms within these clusters, the addition of Ru to Fe clusters increases the cohesive energy of the catalyst particle with respect to the pure Fe cluster. The combined Raman and DFT data indicate that high-melting-point transition metals can stabilize the catalyst nanoparticles and suppress sintering, thus increasing small-diameter selectivity of SWCNTs. This finding has been verified in a conventional hot-wall CVD system that utilizes industrial gaseous waste as a feedstock using multiexcitation Raman spectroscopy.

In this study, the growth of high-quality graphite single crystals from a molten metal flux at atmospheric pressure was optimized. The crystals were precipitated from a saturated iron-carbon solution by slowly cooling (4 °C/h) from a maximum temperature to reduce the carbon solubility. The graphite flakes were >25 square millimeters in area and >10 of microns thick, with individual crystal grains as large as 1.2 mm2. The crystals were (0002) oriented, as determined by x-ray diffraction. The high structural quality of the graphite crystals was verified by Raman spectroscopy. For graphite with the natural distribution of carbon isotopes, the G-peak at 1580 cm-1 was narrow (~12 cm-1) and the defect peak (D-peak) was absent. To demonstrate the process versatility, graphite crystals enriched in the 13C isotope were grown. The Raman peak shifted to 1520 cm-1 for graphite crystals en-riched to 99% 13C. The etch densities from defect sensitive etching ranged from 0 to 1.6x108 etch pits per cm2. The process was refined by examining the crystal size and quality as functions of the carbon concentration in the starting sources, the carrier gas composition, and maximum temperature. The simplicity of this process suggests it can be scaled to produce very large graphite crystals that would be suitable for a wide range of technologies.

In this study, the growth of high-quality graphite single crystals from a molten metal flux at atmospheric pressure was optimized. The crystals were precipitated from a saturated iron–carbon solution by slowly cooling (4 °C/h) from a maximum temperature to reduce the carbon solubility. The graphite flakes were >25 square millimeters in area and >10 microns thick, with individual crystal grains as large as 1.2 mm². The crystals were (0002) oriented, as determined by X-ray diffraction. The high structural quality of the graphite crystals was verified by Raman spectroscopy. For graphite with the natural distribution of carbon isotopes, the G-peak at 1580 cm⁻¹ was narrow (~12 cm⁻¹) and the defect peak (D-peak) was absent. To demonstrate the process versatility, graphite crystals enriched in the ¹³C isotope were grown at 5 degrees of enrichment. The Raman G-peak linearly shifted from 1580 cm⁻¹ to 1520 cm⁻¹ for graphite crystals enriched from 1 to 99% ¹³C. The etch pit densities from defect-sensitive etching ranged from 0 to 1.6 × 10⁸ per cm². The process was refined by examining the grain size and quality as functions of the carbon concentration in the starting sources, the carrier gas composition, and maximum temperature. The simplicity of this process suggests it can be scaled to produce very large graphite crystals that would be suitable for a wide range of technologies.