Nanocomposites and nanomaterials

Structural and electronic properties of ZnO-GaN heterostructured nanosheets: an ab initio study
Kovalenko M.V.1, Bovgyra O.V.1, Bovhyra R.V.2
1 Faculty of Physics, Ivan Franko National University of Lviv, 8a, Kyrylo and Mefodiy str., 79005 Lviv, Ukraine. 
E-mail: kovalenkomarry@gmail.com.com
2 Pidstryhach Institute for Applied Problems of Mechanics and Mathematics NAS Ukraine, 3-b, Naukova str., 79060 Lviv, Ukraine.

Compounds of GaN and ZnO have recently attracted attention as promising photocatalyst candidates for solar-driven water splitting, with possible applications in alternative energy production. Band offset, a fundamental parameter of ZnO/GaN heterostructures, is crucial for their promising applications in solar harvesting.
In this work through density functional theory, we computationally designed two-dimensional ZnO–GaN heterostructured nanosheets, and investigated their structural, electronic and optical properties. Density functional theory (DFT) calculations of the electronic structure of proposed GaN:ZnO superlattices were shown to underestimate the band gap, but this is typical for DFT calculations in wide-gap semiconductors. To correct describe the electronic structures, we adopted the DFT+ Ud+Up method. The calculated bandgap for ZnO with GGA+U (Ud,Zn = 3 eV ang Up,O=7 eV) is 3,38 eV. Using GGA+U for the calculated bandgap of GaN we have Eg = 3.47 eV (Ud,Ga = 0,3 eV ang Up,N=5 eV). These received values for band gaps are in good agreement with experimentally values for ZnO (3,37 eV) and GaN (3,44).
All nanosheets are constructed from a 3D wurtzite structure () and terminated with non-polar (100) surface. All the nanosheets have direct band gaps at the Г point. As a result of the quantum size effect, the (ZnO)4(GaN)4 nanosheets have the smallest band gap and the following sequence is (ZnO)3(GaN)3 < (ZnO)2(GaN)2. All bare nanosheets have band gaps smaller than 2.8 eV, indicating optical absorption in the visible light spectrum.
We find that the valence band offset is 0.82 eV for the (ZnO)4(GaN)4 heterostructured nanosheet, whereas ΔEVBO = 0,74 eV and 0,56 eV for the (ZnO)3(GaN)3 and (ZnO)2(GaN)2, respectively.