(247d) Epitaxial Matching As Route to Grow Chirally Selective Single-Walled Carbon Nanotubes

One of the major goals in nanoscience involves the synthesis of single-walled carbon nanotubes (SWCNTs) with a predefined chirality because of its potential impact on a wide range of emerging technologies including nanoelectronics, renewable energy, nanocomposites, sensor and medical.  SWCNTs are typically grown using the chemical vapor deposition (CVD) method by passing acetylene gas over small metallic nanoparticles. Recently, we showed that changing the composition of Ni_xFe_1-x bimetallic nanoparticles via a novel direct-current atmospheric-pressure microplasma synthesis route resulted in chiral-selective growth of specific nanotube chiralities.  In this work, an epitaxial nucleation model for single-walled carbon nanotube (SWCNT) growth on bimetallic catalysts surfaces is reported in support of experimental observations of chiral enrichment. Tuning composition will perturb the crystal structure, and affect the lattice matching of the nanoparticle with certain chiralities of the SWCNT, promoting chiral-selective growth. We model the bimetallic catalyst surfaces as a 2D (111) surface consisting of Ni or a combination of Ni and Fe atoms, with varying average bond length between nearest neighbor atoms which corresponds to the crystal structure of the alloys. The energies associated with nanotube cap formation on these various surfaces are calculated using density functional theory (DFT). We find that certain cap chiralities, such as (8, 4), are more stably bound to a surface that resembles a Ni0.27Fe0.73 bimetallic catalyst, whereas other chiralities, such as (9, 4), are more stable on a pure Ni surface. These results help explain the predominance of certain chiralities on specific bimetallic catalysts and provide a potential route to controlling the chirality of as-grown SWCNTs.