Chapter 4 Results and discussion
4.2 Room-temperature optical properties of the ZnO:MgO nanowires
4.2.3 Effect of annealing treatment on optical property of ZnMgO nanowires
explanation for the growth of theZnO nanowires. The follow-up vaporation of Mg would condensed, and rapidly oxidized to MgO on the ZnO nanowires. This may lead to the formation of ZnO/MgO core-shell nanowires. Although the formation mechanism is important for the fabrication of high quality ZnO-based heterostructures, the detailed growth mechanisms of the nanostructuresare not fully understood.
4.2.3 Effect of annealing treatment on optical property of ZnMgO nanowires
In order to implement the modulation of the optical bandgap, keeping the morphology of nanowire is essential for this purpose if the ternary ZnMgO nanowires could be formed by driving the inter-diffusion between the ZnO/MgO in the core/shell structure during the annealing process. Figure 4-13(a)-(d) show the as-grown ZnO/MgO core/shell nanowires annealed at different temperatures range from 800oC to 1000oC. Their shapes did not change very much after annealing treatment.
Figure 13(e)-(h) show the EDX spectra of the ZnO/MgO core/shell nanowires annealed at different temperatures. The XRD patterns of the nanowires are shownin Fig. 4-14. All relatively sharp diffraction peaks can beperfectly indexed to a high crystallinity of the hexagonal structure of ZnO and cubic structure of MgO.
Moreover, all of the diffraction peaks become sharper with increasing annealing temperature due to the improved crystal quality. However, the lattice constants of
a
andc were calculated according to the following Eqs. (4-1) and (4-2), respectively.
Here, the resulting values a=3.253 Å and c=5.189 Å of ZnMgO nanowire after annealed at 1000°C are smaller then the as-grown ZnMgO nanowire(a=3.248 Å and
c=5.202 Å) due to Mg atoms entering the ZnO lattice. Consequently, the cell
volume (3 3/2
a
2c
) are 142.579 (Å3) and 142.66 (Å3) for the as-grown and annealing samples, respectively. The cell volume hardly changed because the size of Zn2+radius is nearly equal to Mg2+ radius.
Figure 4-13 SEM image of (a) as grown and annealing treatment with annealing temperature at (b)800 oC, (c)900 oC and (d)1000 oC ZnMgO nanowires. The EDS spectra of (e), (f) ,(g), and (h) correspond to (a) as grown and annealing treatment with annealing temperature at (b)800 oC, (c)900 oC
30 35 40 45 50 55 60 65
Figure 4-14 The XRD pattern of (a) as grown and annealing treatment with annealing temperature at (b)800 oC, (c)900 oC and (d)1000 oC ZnMgO nanowires.
Figure 4-15 shows the room temperature full PL spectra of pure ZnO nanowires and the as grown ZnO/MgO core/shell nanowires annealed at different temperatures range from 800oC to 1000oC. As a reference, the spectrum of the pure ZnO nanowire without Mg dopant is also shown in the bottom of Fig. 4-15(a). In the PL spectra of the pure ZnO nanowires, an near band edge (NBE) emission peak at 3.27 eV with a Full Width at Half Maximum (FWHM) of 110 meV is attributed to the free exciton emission have been reported elsewhere45,50. In the as-grown nanowires, only emission lines from pure ZnO can be seen. However, after an annealing treatment under ambient atmosphere pressure for 120 min at 800oC to1000oC, new peak appeared on the higher energy side, as shown in Fig. 4-15(c)-(e). Figure 4-16 shows the UV -PL spectra of pure ZnO nanowires measured at room temperature and the as-grown ZnO/MgO core/shell nanowires annealed at different temperatures range from 800oC to 1000oC. As the annealing temperature is increased,the position of the NBE emission peak tends to shift toward the higher photon energy from 3.27 eV~3.5
eV, as shown in Fig. 4-16(b)-(e). Furthermore, the FWHM of the emission gradually broadens as shown in Fig. 4-17. Another peakappears at 3.23 eV on the low-energy side of the exciton band as shown in Fig. 4-16(c)-(d). While the annealing temperature increases up to 1100 oC, the position of the emission peak reaches to 3.5 eV and the low-energy peak disappears.
The dramatically blueshifted emission is attributed to the formation of ternary ZnMgO nanowires via Mg incorporation. Figure 4-13(e)-(h) show the EDX spectra of the ZnO/MgO core/shell nanowires annealed at different temperatures. Zn content increases with increasing temperature. This indicates that the annealing treatment leads to inter-diffusion of Mg into ZnO core and Zn to MgO shell. The diffused Mg concentration increases while increasing annealing temperature.
According to the relation between NBE emission and Mg content of the ZnMgO thin films, the Mg content in ZnMgO nanowires can be roughly estimated as shown in Fig.
4-18 by60-62
E(Zn
1-xMgxO)=E(ZnO)+1.64X
(eV).
(4-3) The origin of the emission peak at 3.22eV should be attributed to the NBE emission from the residual inner core of ZnO nanostructures.2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Figure 4-15 Room temperature full PL spectra of (a) pure ZnO nanowires and (b) the as grown ZnO/MgO core/shell nanowires annealed at different temperatures in the range from (c)800oC, (d)900
3.0 3.1 3.2 3.3 3.4 3.5 3.6
Figure 4-16 Room temperature UV region PL spectra of (a) pure ZnO nanowires and (b) the as grown ZnO/MgO core/shell nanowires annealed at different temperatures in the range from (c)800oC, (d)900
oC and (e)1000oC.
Figure 4-17 The photon energy of PL emission and FWHM the dependence of at different annealing temperature.
0 800 900 1000
Figure 4-18 The dependence of the photon energy as a function of Mg content at different annealing temperature.
Besides the NBE emission, the defect emissions were also observed at ~2.4 eV in Fig. 4-15. The origin of defect emission results from the transition of defect levels such as singleionized oxygen vacancy, O interstitial. The relative intensity of the defect emission band to the NBE band also changes with the change of Mg content in the Mg-doped ZnO nanostructures. From the PL spectra, it can be seen that the pure ZnO has a much stronger NBE band than the defect emission band, while both Mg-doped ZnO nanowires have a weaker NBE band than the defect emission band.
With an increase of Mg content in the Mg-doped ZnO nanowires, the ratio of the defect emission band to the NBE band increases by a factor of about 0.037 to 0.867, indicating the defects density in ZnO nanowires with low Mg dopant higher than in the ZnO with high Mg dopant. The relative intensity of the defect emission band to the NBE band follows the order of Mg-doped ZnO nanowires > ZnO nanowires.
The difference in the PL spectra among these products can be explained as follows.
ZnO crystal lattice may introduce little lattice distortion in the ZnO nanostructures.
After all, due to the increase of Mg content in the Mg-doped ZnO nanostructures, the intensity of surface oxygen vacancies may also increase due to the insufficiency of oxygen in the reaction system, which also results in the increase of defect emission.
The NBE emission peak weakens accompanied with the defect emission is enhanced.
The band gap broadens for the reason mentioned before, which leads to the blueshifted of the PL spectra and changes the intensity of NBE emission. This will give rise to some new defects, such as oxygen vacancies, which should result in the changes of the PL intensity of defect emission.