Capped ZnO (3, 0) nanotubes as building blocks of bare and H passivated wurtzite ZnO nanocrystals

•ZnO wurtzite nanocrystals relation to nanotubes.•Size-dependent energy gap, bond length, cohesive energy.•Vibrational spectra, reduced masses, force constants.•sp2 and sp3 bonding of bare and H passivated ZnO structures.

In the present work we propose building blocks of hexagonal type crystals and nanocrystals of zinc oxide including wurtzite structure that are called wurtzoids. These molecules are ZnO (3, 0) nanotubes capped at their two terminals with Zn or O atoms. Hexagonal part of these molecules is included in the central part of these molecules. This part can be repeated to increase hexagonal structure ratio. Hydrogen passivated and bare ZnO wurtzoids are investigated. Results show bare wurtzoids have shorter and stronger surface sp2 bonds than H passivated sp3 bonded wurtzoids. The calculated energy gap of these molecules shows the expected trend of gaps. Calculated binding energy per atom shows that wurtzoids are tight and stable structures which are not the case of ZnO diamondoids. Vibrational frequencies manifest the expected trends of hexagonal type structures. Reduced mass and force constant of these vibrations are investigated to illustrate the sp2 and sp3 bonding effects of bare and H passivated ZnO nanocrystals respectively.