Nanocomposites and nanomaterials

Zinc oxide nanotubes: An ab initio investigation of their structural and electronic properties 
Tkach P.V., Kovalenko M.V., Bovgyra O.V.
Faculty of Physics, Ivan Franko National University of Lviv, 8a, Kyrylo and Mefodiy str., 79005 Lviv, Ukraine. 
E-mail: mariya.kovalenko@lnu.edu.ua

One-dimensional nanomaterials including nanotubes, nanorods, and nanowires, have been attracting enormous interest due to their unique electronic, optical, and mechanical properties and big potential for nanotechnology applications compared to corresponding bulk materials. In particular, zinc oxide (ZnO) is a wide bandgap II−VI group semiconductor and is well known for its excellent performance in various application areas such as piezoelectrics, pyroelectrics, optoelectronics, and spintronics. At present, scientific research on ZnO nanostructure has made considerable progress. Research groups in the world have successfully synthesized ZnO nanomaterials with different structures, such as low-dimension ZnO nanowire, nanorod, nanotube, nanobelt, and other nanomaterials with unique structures, for example, ZnO nanodisk, and nanoflowers. The ZnO material research shows a potential application in nanometer optoelectronic systems, gas sensors, optoelectronic memory, catalysis, optical-electrical converter, laser, and transparent conductive film.
In this work, based on the density functional theory was performed the first-principles modeling of different chirality ZnO nanotube, particular, with armchair and zigzag structures. Structure optimization and band energy calculations were carried out using the generalized gradient approximation (GGA) in the PBEsol parameterization. We used the GGA+U approximation to more accurately describe the electronic structure of the ZnO nanotube. In our study, all nanotube structures were obtained by rolling up a graphitic ZnO single layer with subsequent structural optimization.
The calculations results show that after structure relaxation ZnO nanotubes have tubular structures in which zinc and oxygen atoms were slightly shifted inward and outward to the axis, forming homocentric cylinders with different diameters similar to carbon nanotubes. The electronic structure calculations show that all ZnO nanotubes are wide bandgap semiconductors with a bandgap bigger than wurtzite ZnO. It is established that the bandgaps of all investigated the ZnO nanotubes increase with their diameters. We hope that such a diameter dependence of the bandgap can perhaps be used to control the bandgap of the ZnO nanotubes by growing the ZnO nanotubes with a predefined diameter.