The electronic and magnetic properties of IIIA group doped ZnO nanosheets (ZnONSs) are investigated by the first principles. The results show that the band gap of ZnO nanosheets increases gradually along with Al, Ga, and In ions occupying Zn sites and O sites. The configuration of Al atoms replacing Zn atoms is more stable than other doped. The system shows half-metallic characteristics for In-doped ZnO nanosheets.
As one of the wide-band-gap oxides, ZnO has received considerable interest for its wide variety of practical applications, such as liquid crystal display transistors, gas sensors, and ferroelectric transparent thin-film transistors [
In general, the electron conductivity of pure ZnO is not low enough to be used as a transparent conductive oxide (TCO). It is necessary that the behavior of the ZnO must be changed effectively by substitutional doping into ZnO. Zuo et al. [
In the present work, we explored the electronic properties of ZnO doped with IIIA ions (Al, Ga, and In) based on first-principles calculations to the ZnO nanosheets (ZnONSs). These studies provide us with a deep understanding of the novel properties of intrinsic defect ZnO nanosheets, which is essential to employ ZnO nanosheet as building blocks for the future nanodevices.
We use first-principles full-potential linearized augmented plane wave (FLAPW) method based on the generalized gradient approximation (GGA) [
The ZnO is wurtzite structure, in which all atoms are in sp3 hybridization with each Zn (O) atom surrounded by four neighboring O (Zn) atoms at the corners of a tetrahedron. The Zn–O bond length is calculated to be 1.973 Å, in good agreement with the values of 1.86 Å by Tu [
First we consider a 3D wurtzite bulk ZnO structure in which all atoms are fourfold coordinated through hexagonal directed sp3 orbital. After optimization, the hexagonal lattice constants under equilibrium are
Band structure of a 3D bulk ZnO crystal with an optimized lattice constant. The valence band maximum is set to zero.
Next we studied 2D nanosheets derived from wurtzite ZnO. The nanosheets are based on the model of (
(a) (Color online) Structure of ZnO nanosheet, the red ball stands for O atom and the gray one is Zn atom. (a) Top view of the configuration without being relaxed. (b) Side view of configuration after being relaxed. (c) DOS of the pristine ZnONSs.
We also find that the calculated band structure of the ZnONSs exhibits a direct band gap at the
For the structures that are doped, we considered two configurations: one IIIA ion replacing one Zn atom (M–Zn) and O atom (M–O). Figure
Band gap evolution of graphitic nanosheets as a function of Al, Ga, In.
The formation energies of M-doped ZnONSs were calculated to evaluate their stability. To make comparison, we also calculated the formation energies of bulk ZnO. The formation energy [
Table
Formation energies (eV) of M-doped (M = Al, Ga, In) ZnONSs.
Configurations | Formation energies (eV) | |
---|---|---|
M–Zn | M–O | |
Al-ZnONSs | 3.97 | 5.76 |
Ga-ZnONSs | 6.60 | 6.81 |
In-ZnONSs | 7.78 | 8.83 |
Bulk ZnO | 1.26 |
In Figure
Band structure, total DOS of (a) Al-doped ZnONSs, (b) Ga-doped ZnONSs, and (c) In-doped ZnONSs, respectively. (d) The band gap of undoped ZnO nanosheet. The arrows in the energy band mean spin direction.
In order to further explain the origin of the half-metallic ferromagnet, we also give the partial DOS for the In-doped ZnONSs (Figure
PDOS (a) and isosurface of spin densities (b) for In-doped ZnONSs. The red ball stands for O atom and the gray one is Zn atom; the dark gray one is In atom.
To analyze spin polarization induced by In doped, we calculated spin density distribution around the In atom of ZnONSs; electrons of In and O atoms are both spin-polarized, and In-5s and O-2p electrons couple at the Fermi level. As presented in Figure
In summary, we have studied the electronic structures and magnetic properties of IIIA group-doped ZnO nanosheets. Our results clearly demonstrate that the system’s band gap increases along with the increase of the IIIA atom number due to the Goldschmidt-Pauling rule of bond contraction induced by undercoordination. The formation energies of M–Zn are all smaller than M–O and increases go along with the increase of the IIIA proton number for M–Zn. After Al-doped in the ZnO nanosheet, we find that the system shows half-metallic character and leads ZnO nanosheet to an n-type semiconductor. Ga-doped ZnO nanosheets shows metallic character with strong spin polarization. The half-metallicity is also found when In-doped in the ZnO nanosheets. The hole states resulting from In doped are spin-polarized and therefore lead to a high-spin state with a magnetic moment of about
This work was supported by the National Natural Science Foundation of China (Grant nos. 61172028 and 61076088), the Natural Science Foundation of Shandong Province (Grant no. ZR2010EL017), the Doctor Foundation of University of Jinan (Grant no. xbs1043), Foundation for Young Scientist in Shandong Province (Grant no. BS2009CL012), and Technological Development Program in Shandong Education Department (Grant no. J10LA16).