We explore the atomic and electronic structures of single-crystalline aluminum nitride nanowires (AlNNWs) and thick-walled aluminum nitride nanotubes (AlNNTs) with the diameters ranging from 0.7 to 2.2 nm by using first-principles calculations and molecular dynamics simulations based on density functional theory (DFT). We find that the preferable lateral facets of AlNNWs and thick-walled AlNNTs are {1010} surfaces, giving rise to hexagonal cross sections. Quite different from the cylindrical network of hexagons revealed in single-walled AlNNTs, the wall of thick-walled AlNNTs displays a wurtzite structure. The strain energies per atom in AlNNWs are proportional to the inverse of the wire diameter, whereas those in thick-walled AlNNTs are independent of tube diameter but proportional to the inverse of the wall thickness. Thick-walled AlNNTs are energetically comparable to AlNNWs of similar diameter, and both of them are energetically more favorable than single-walled AlNNTs. Both AlNNWs and AlNNTs are wide band gap semiconductors accompanied with surface states located in the band gap of bulk wurtzite AlN.

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The defect effect on hydrogen adsorption on single-walled carbon nanotubes ͑SWNTs͒ has been studied by using extensive molecular dynamics simulations and density functional theory ͑DFT͒ calculations. It indicates that the defects created on the exterior wall of the SWNTs by bombarding the tube wall with carbon atoms and C 2 dimers at a collision energy of 20 eV can enhance the hydrogen adsorption potential of the SWNTs substantially. The average adsorption energy for a H 2 molecule adsorbed on the exterior wall of a defected ͑10,10͒ SWNT is ϳ150 meV, while that for a H 2 molecule adsorbed on the exterior wall of a perfect ͑10,10͒ SWNT is ϳ104 meV. The H 2 sticking coefficient is very sensitive to temperature, and has a maximum value around 70 to 90 K. The electron density contours, the local density of states, and the electron transfers obtained from the DFT calculations clearly indicate that the H 2 molecules are all physisorbed on the SWNTs. At temperatures above 200 K, most of the H 2 molecules adsorbed on the perfect SWNT are soon desorbed, but the H 2 molecules can still remain on the defected SWNTs at 300 K. The detailed processes of H 2 molecules adsorbing on and desorbing from the ͑10,10͒ SWNTs are demonstrated.