For N-doped ZnO nanotube (configurations Ag1N2, Ag1N3,4, and Ag1N2,3,4), the bandgaps increase with the N concentrations (1.10, 1.20, and 1.25 eV, respectively) increasing. Some levels pass through the Fermi level, indicating that N impurity acts as an acceptor doping in ZnO nanotube. In Ag1N2,3,4 system, it follows Figure 3e that the host valence band (VB) is surpassed and two gap states are introduced above the VB. The lowest defect level is occupied and locates at

about 0.19 eV above the host Selleckchem PD0325901 VBM. Another gap state is occupied and locates at 0.22 eV above the Fermi level. However, the lowest acceptor level in Ag1N3,4 is occupied and is located at 0.04 eV around the Fermi level. All these results illustrate that Ag1N3,4 demonstrates the better p-type behavior than the Ag1N2,3,4 system. For the

Ag1N5 and Ag1N6 system, the bandgaps are 1.15 and 1.17 eV, which are different to PD 332991 the Ag1N2 system (1.17 eV), indicating that the bandgap has nothing with the distance of Ag atom and N atom. Before investigating the Ag doping effect on the ZnO nanotubes’ optical properties, we calculated the density of states (DOS) of Ag-N-codoped (8,0) ZnO nanotubes as shown in Figure 4, which indicates that Ag-doped ZnO nanotube shows typical characters of p-type semiconductor. Figure 4a,b shows that the states located at the Fermi level are dominated by Ag 4d states and N 2p states, demonstrating the occurrence of the N 2p to Ag 4d hybridization. As discussed above, more impurity

states will be introduced in the band structure with the increase of N dopant concentration. From Figure 4 (c′), we find that the hybridization between Ag atom dopant and its neighboring host atoms results in the splitting of the energy levels near the Fermi level, which shifts Paclitaxel supplier to the majority spin states downward and minority spin states upward to lower the total energy of the system. Figure 2 The calculated band structures of 3D bulk ZnO crystal. Figure 3 Band structures of pure and Ag-N-codoped (8,0) ZnO nanotubes. (a) Pure (8,0) ZnO nanotube, (b) Ag1 configuration, (c) Ag1N2 configuration, (d) Ag1N3,4 configuration, (e) Ag1N2,3,4 configuration, (f) Ag1N5 configuration, and (g) Ag1N6 configuration. Figure 4 Total DOS (a) and PDOS (b) of Ag 1 , Ag 1 N 2 , Ag 1 N 3,4 , and Ag 1 N 2,3,4 configurations. Optical properties As discussed, the optical properties of pure and Ag-N-codoped (8,0) ZnO nanotubes are based on the dielectric function, absorption coefficient, and reflectivity. In the linear response range, the solid macroscopic optical response function can usually be described by the frequency-dependent dielectric function ϵ(ω) = ϵ 1(ω)+ iϵ 2(ω) [19], which is mainly connected with the electronic structures. The real part ϵ 1(ω) is derived from the imaginary part ϵ 2(ω) by the Kramers-Kronig transformation. All the other optical constants, such as the absorption coefficient, reflectivity, and energy loss spectrum, are derived from ϵ 1(ω) and ϵ 2(ω).