(a), (b), and (c) show TEM images of Gr-I spherical nanostructures of average size 5 nm, 10 nm, and 15 nm, respectively. Lower panel shows the Gr-II samples of rodlike (d), hexagonal platelets (e), and complex morphology with rods and square pillars (f).
X-ray diffraction pattern for all the samples investigated show wurtzite symmetry (peaks are indexed). Gradual increase in the average size of the nanostructures can be seen from decreasing peak width.
Comparison of experimental value of and calculated value of from two-state trapping model, denoted by ™.
Variation in measured lifetime values and as a function of nanostructure size.
Variation in intensities and corresponding to positron lifetimes and respectively, as a function of nanostructure size.
Positron trapping rates and calculated by three state trapping model are plotted as a function of nanostructure size.
The emission bands of the ZnO nanostructures of different size and shape. The excitation was at 325 nm. The sharp dependence of the emission band at 490–565 nm on the size can be seen. A representative two Gaussian fitting has been shown for the 15 nm spherical nanoparticle.
The NBE emission energy has been plotted as a function of diameter of the nanostructures. An approximate 1/diameter dependence can be observed.
Intensity ratio of NBE to visible emission for Gr-I and Gr-II samples. The data are fitted by a surface recombination model with surface layer thicknesses of 1.5 nm and 3.6 nm for Gr-I and Gr-II samples, respectively.
There exists a surface layer of thickness t where defects such as singly and doubly charged oxygen vacancies are mostly located. The horizontal arrows indicate the energy barrier seen by the positrons due to the presence of positively charged oxygen vacancies at the surface. Spherical particles (Gr-I) of size having higher positive surface charge offer higher barrier for positrons confined in the bulk core region than rods and hexagonal platelets (Gr-II) of size .
Size and morphology of ZnO nanostructures.
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