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The geometric and electronic properties of the double-walled nanotubes (DWNTs), constructed by the two coaxial single-walled nanotubes (SWNTs) rolling the hexagonal titanium dioxide (TiO2) nanosheet along with the armchair (ac-) and the zigzag (zz-) directions, have been investigated systematically using the methods based on the density functional theory. For the optimized structures, the bandgap values of the TiO2 DWNTs are significantly reduced from that of the constituent SWNTs, falling in the visible light range. Further detailed analysis reveals that the reduction is caused by the band misalignment due to the different curvatures of the inner and outer TiO2 SWNTs. [ABSTRACT FROM AUTHOR]

Using the method based on the density functional theory, the geometric and electronic properties of the TiO2 single-wall nanotubes, constructed by rolling the most stable nanosheet along the (n, 0) and (n, n) directions, have been investigated systematically. The nanotubes with size from n = 6 up to n = 20 have been modeled and studied. The strain energies of the nanotubes decrease monotonically as the radii of the nanotubes increase, regardless of the rolling direction. The band gaps of the nanotubes are increasing with the increase of the n value, approaching the value of the nanosheet. However, there is one nanotube significantly different from the others, i.e., the (6, 0) nanotube. The substantial structural change of (6, 0) nanotube causes a reduction of the band gap. Then, the isovalent sulfur (S) substitution and adsorption with the (6, 0) nanotube have been studied. Energetically, S adsorption at the inner surface is preferred. Electronically, the band gaps are further reduced by 35% for S substitution of oxygen and 22% for S adsorption, respectively, making the nanotube visible light-sensitive. [ABSTRACT FROM AUTHOR]

Abstract: Based on density functional theory, the electronic structures, formation energy and transition energy level of a p-type Mg-doped GaN nanosheet are investigated. Numerical results show that the transition energy level decreases monotonously with increasing Mg doping concentration in Mg-doped GaN nanosheet systems, which is lower than that of the Mg-doped bulk GaN case. Moreover, the formation energy calculations indicate that Mg-doped GaN nanosheet structures can be realized under N-rich experimental growth conditions. [Copyright &y& Elsevier]

Based on density functional theory, the electronic structure, formation energy and transition level of group V and VII atoms-doped SnS 2 are investigated by means of first-principles methods. Numerical results show that the formation energy and transition level are dependent highly on the atom number in the periodic table. Group V atom substituting S atom has high formation energy and can create deep acceptor impurity level inside the band gap of SnS 2 . However, our calculations also show that group VII atom substituting S atom may serve as a promising n -type doping in the SnS 2 due to its negative formation energy and shallow transition level under the Sn-rich growth conditions. [ABSTRACT FROM AUTHOR]