# Kwon Group, Physics, Kyung Hee University

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lab:research:cnt

## Carbon Nanotubes

Using ab initio density functional theory and semi-empirical theories, we have studied various properties and applications of carbon nanotubes (CNTs), which are seamless cylinders wrapped from graphene.

### Equilibrium and electronic structures of singlewall CNTs

There are variety of different computational results on the same CNTs with the same computational method. To resolve this issue, we swapped completely the configurational spaces of each CNT considering its structural symmetry determined by either as well as achiral (armchair and zigzag) and chiral CNTs. As a result, We were able to identify the most stable structure and the coupling effects between structural and electronic properties.

### Orientational melting in CNT ropes

Using Monte Carlo simulations, we have investigated the possibility of an orientational melting transition within a “rope” of CNTs. hen twisting nanotubes bundle up during the synthesis, orientational dislocations or twistons arise from the competition between the anisotropic interbute interactions, which tend to align neighboring tubes, and the torsion rigidity that tends to keep individual tubes straight. We mapped the energetics of a rope containing twistons onto a alttice gas model and found that the onset of a free “diffusion” of twistons, conrresponding to orientational melting, occurs at TOM ≥ 160 K.

### Effect of intertube interaction in CNTs

We explored the effect of intertube interaction in bundles of single-wall CNTs and multiwall CNTs. Because of small intertube interaction and lattice frustration, we predicted very soft libration modes to occur and to disappear above the orientational melting temperature which marks the onset of free tube rotations about their axis. We also predicted such weak intertube coupling causes an additional electronic band dispersion of < 0.2 eV and opens up a pseudogap of the same magnitude at the Fermi level in the rope of (10,10) SWCNTs. In a (5,5)@(10,10) double wall CNT, the weak interwall interaction and the symmetry reduction causes four pseudogaps to open and close periodically near the Fermi level during the soft librational motaion. This electron-libration coupling, absent in solids composed of fullerenes and SWCNTs, may yield superconductivity in MWCNTs.

### Microscopic growth mechansim of multi-wall CNTs

We investigated the morphology and structural stability at the growing edge of multi-wall CNTs using ab initio and parametrized calculations. We found that the open-ended structures are stabilized against dome closure by strong covalent bonds connecting the exposed edgs of adjacent walls. Growth at the open edge involves rearrangement of these bonds, which are mediated by carbon atoms bridging the gap and change the tip morphology significantly. Presence of a strong lip-lip'' interaction can explain formation of carbon nanotubes under annealing conditions.