Effect of gas adsorption on the electronic structures of carbon nanotubes

Abstract

Tuning Chirality of Single-wall Carbon Nanotubes by selective Etching with Carbon Dioxide We propose a new approach of controlling chirality of single-wall carbon nanotubes by a selective adsorption and subsequent etching procedure with various gas adsorbates. Our density functional calculations show that heterogeneous molecules such as CO, NO, $NO_2$, and $CO_2$ prefer to chemisorb on zigzag tube edges with large adsorption energies compared to armchair tube edges. In particular, $CO_2$ molecule physisorbs on armchair tube edge, whereas it chemisorbs strongly on a zigzag tube edge with large adsorption energy of -4.82 eV. We propose that annealing with ambient $CO_2$ gas can lead to a selective etching via initial CO desorption, followed by subsequent CO desorption from the tube edge. Growth Energetics of Single-Wall Carbon Nanotubes with Carbon Monoxide The microscopic growth energetics of single-wall carbon nanotubes (SWNTs) with gas-phase CO molecules is investigated. Our density functional calculations show that CO molecules can form carbon networks directly at an open edge of SWNT by adsorption and subsequent desorption procedure. CO molecules adsorb to the carbon atom in an open edge of SWNT, not through the oxygen atom but through the carbon atom, because the frontier orbital of CO molecule has more of carbon character. Such form of adsorption results in the formation of a carbon-carbon bond. Formation of a hexagonal carbon ring is thermodynamically more favorable than that of a pentagonal carbon ring, and becomes a driving force for the growth of SWNTs. A possible growth mechanism through formation of hexagonal ring structure is suggested from the results of this study. Our results of energy calculations suggest that growth in the direction of a zigzag wall is more favorable. The roles of a catalyst, which nucleates the growth of SWNT and holds to stabilize an open edge with dangling bonds, are further discussed.