Electronic structures and optical properties for Ag-N-codoped ZnO nanotubes
© Feng et al.; licensee Springer. 2013
Received: 30 May 2013
Accepted: 1 August 2013
Published: 27 August 2013
The structural and electronic/optical properties of pure and Ag-N-codoped (8,0) ZnO nanotubes have been studied using first-principles calculations in the framework of the local spin density approximation. The configurations for Zn atoms replaced by Ag atoms are p-type semiconductor materials, and the bandgap increases when N atoms are doped into ZnO nanotube configurations. The optical studies based on dielectric function and reflectivity indicate that new transition peaks in the visible light range are observed, which can be ascribed to the Ag and N doping. Furthermore, there is a red shift observed with the increase of N concentration.
Since the discovery of single-walled carbon nanotubes (SWCNTs) in the early 1990s , the research on tubular nanostructures has attracted increasing interest because their unique structures can provide some unique properties, such as high Young's modulus, high thermal conductivity, and high aspect ratio structure. Besides SWCNTs, many other tubular nanostructures such as boron nitride nanotubes, gallium nitride (GaN) nanotubes, and zinc oxide (ZnO) nanotubes have been intensively investigated in recent years. Density functional theory (DFT) calculations have shown that the single-walled GaN, AlN, and InN nanotubes are all metastable, and they are semiconductors with either a direct bandgap (zigzag tubes) or an indirect bandgap (armchair tubes) [2–5].
Recently, Shen et al. found that ZnO single-walled nanotube (SWNT) is more/less stable than its nanowire or nanobelt if the diameter is smaller/bigger than that of (24,0) ZnO SWNT . Hence, the small-diameter (8,0) ZnO SWNT is expected to be more stable. Additionally, Zhou et al. also studied the size- and surface-dependent stability of (8,0) ZnO nanotube, and found that the (8,0) ZnO nanotube had a good surface texture .
To get p-type doped ZnO, group V, group IA, and group IB elements have been used as dopants [8–13]. Different doping elements are favorable in O-poor/rich conditions to realize p-type doped ZnO, and the doping will easily produce oxygen vacancy defects. For example, N doping is only favorable in O-poor conditions but will easily produce oxygen vacancy defects. For element Ag, it has smaller diameter and larger ionization energy than group IA elements, and its doping process is favorable in O-rich conditions, which can suppress the defects in ZnO; thus, element Ag is a better candidate for p-type ZnO doping.
Codoping ZnO with transition metal/nonmetal ions is an effective way to modify its electronic/optical properties [14, 15]. In this paper, the structure and formation energies of Ag-N-codoped ZnO nanotubes were firstly calculated using DFT and followed by the calculations on the electronic and optical properties with the optimized structures.
The first-principles full-potential linearized augmented plane wave method based on the generalized gradient approximation  is used for the exchange-correlation potential within the framework of DFT to perform the computations, as implemented in the WIEN2K simulation package. Special k points were generated with the 1 × 1 × 4 grid based on Monkhorst-Pack scheme. Good convergence was obtained with these parameters. The total energy was converged to be 1.0 × 10−4 eV/atom in the optimized structure.
Results and discussion
Geometry structures and formation energies
Bandgap ( E gap ), Zn-N bond lengths ( R Zn-N ), and formation energies ( E f ) of Ag-N-codoped ZnO nanotubes
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) = ϵ1(ω)+ iϵ2(ω) , which is mainly connected with the electronic structures. The real part ϵ1(ω) is derived from the imaginary part ϵ2(ω) by the Kramers-Kronig transformation. All the other optical constants, such as the absorption coefficient, reflectivity, and energy loss spectrum, are derived from ϵ1(ω) and ϵ2(ω).
In summary, we have studied the structural, electronic, and optical properties of pure and Ag-N-codoped (8,0) ZnO nanotubes using DFT. The configurations with Zn atoms replaced by Ag atoms are p-type semiconductor materials. For the N-doped ZnO nanotube configurations, the bandgap increases with the N concentration. When N atom replaces the second (Ag1N5) and third neighbor (Ag1N6) sites for Ag atom, the bandgap has a slight difference with the N that replaced the nearest neighbor site (Ag1N2). The calculated dielectric function and reflectivity show obvious peaks in the visible light region which are due to the electronic transition from doped Ag 4d states to the Zn 4s conduction band for the configuration with Ag atoms replacing Zn atoms (Ag1) and Ag 4d state to N 2p state transitions for the Ag-N-codoped configurations, respectively. The peaks at about 0.5- to 2.0-eV energy region for the dielectric function have a red shift with the increase of N concentration. For the reflectivity, the transmissivity of the ZnO nanotube gets better in the visible light range compared with bulk ZnO.
This work was supported by the National Natural Science Foundation of China (grant nos. 61172028, 61076088, and 11274143), Natural Science Foundation of Shandong Province (grant no. ZR2010EL017), Doctor Foundation of University of Jinan (grant no. xbs1043), and Technological Development Program in Shandong Education Department (grant no. J10LA16).
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