Phase transitions of H2 adsorbed on the surface of single carbon nanotubes

TL;DR

This study uses Diffusion Monte Carlo to map the phase diagram of H2 molecules adsorbed on carbon nanotubes, revealing incommensurate solid phases and phase transitions depending on nanotube radius.

Contribution

First comprehensive phase diagrams of H2 on carbon nanotubes at zero temperature using Diffusion Monte Carlo, identifying unique incommensurate solid phases and phase transitions.

Findings

Incommensurate solid phases dominate most nanotubes.

Wider nanotubes exhibit a $\,\sqrt{3} \times \sqrt{3}$ phase.

First order phase transitions occur in (5,5) and (6,6) tubes.

Abstract

By means of Diffusion Monte Carlo calculations, we obtained the complete phase diagrams of H$_2$ adsorbed on the outer surface of isolated armchair carbon nanotubes of radii ranging from 3.42 to 10.85 \AA. We only considered density ranges corresponding to the filling of the first adsorption layer in these curved structures. In all cases, the zero-temperature ground state was found to be an incommensurate solid, except in the widest tube, in which the structure with lowest energy is an analogous of the $\sqrt{3} \times \sqrt{3}$ phase found in planar substrates. Those incommensurate solids result form the arrangement of the hydrogen molecules in circumferences whose plane is perpendicular to the main axis of the carbon nanotube. For each tube, there is only one of such phases stable in the density range considered, except in the case of the (5,5) and (6,6) tubes, in which two of these incommensurate solids are separated by novel first order phase transitions.