Raman and infrared-active modes in MgO nanotubes

Finite magnesium oxide nanotubes are investigated. Stacks of squares and hexagons of MgO clusters are constructed and studied by the density-functional theory (DFT). Optimized structures are slightly distorted stacks of polygons. Frequencies of radial breathing modes and radial twisting modes show strong size effect. Raman and infrared activity of the modes present a dependence on parity of layer. The activity can be deduced in the low-dimensional system, based on the symmetry and selection rules. A suggestion for experimental characterization between squares and hexagons cross-sections of nanotubes by Raman and infrared is proposed.