Growth and Valence Excitations of ZnO:M(Al, In, Sn) Hierarchical Nanostructures

Comb- and fishbone-like doped ZnO nanostructures were synthesized by introducing different dopants in alloying vapor deposition process. Whereas, Al and Sn doping could induce comb-like structures, In doping introduced fishbone-like structures due to different chemical activity of dopants involved. While belts exhibit the growth direction of [01 (1) over bar0] for fishbone-like structures or [2 (1) over bar(1) over bar0] for comb-like structures, all branches grow only along the [0001] direction. However, the morphology of the belts and the nucleation of the branches are remarkably different among these three structures. Elemental mapping with electron energy loss spectra (EELS) indicated that all the dopants are incorporated rather uniformly into ZnO, consistent with the expansion of the (0001) lattice spacing. All the interband and intraband transitions have been probed by valence EELS. The results show that the projected band gap transitions vary with dopants, resulting in 2.03 eV for ZnO:Al, 2.3 eV for ZnO:In, and 2.7 eV for ZnO:Sn, realized by the shift of the green emission maximum arisen from impurity deep levels.