In 2010, R. S. Ajimsha et al. [20] presented the transmission and photoluminescence spectra of
Dy doped ZnO thin films. Dy doped ZnO thin films with different Dy concentration of 0.25, 0.5, 1, 2,
and 5 wt% were deposited on the (0001) sapphire substrates by buffer assisted pulsed laser
deposition. Figure 2.20 shows the room temperature optical transmission spectra of pure and Dy
doped ZnO films. It can be seen that all the films are highly transparent with average transmission of
approximately 85% in the visible range and sharp fundamental absorption edges corresponding to
respective band gaps.
Figure 2.21 represents the variation of band gap of Dy doped ZnO films with Dy concentration.
It can be observed that band gap of Dy:ZnO films increased to a maximum value of approximately
3.42 eV at a Dy concentration of 0.45 at% and then decreased up to 3.3 eV at 4.12 at% of Dy doping.
Variation of band gap with carrier concentration was modeled considering the combined effect of
Burstein-Moss shift and band gap narrowing.
Figure 2.22 shows the room temperature photoluminescence spectra of Dy doped ZnO films. It
can be observed that the decreased intensity of near band edge emission with Dy concentration can
be attributed to deterioration of the crystalline quality of the film and enhancement of non-radiative
compensating native defects density with increase of Dy content. The luminescence emission peak in
the visible spectral region at approximately 575 nm is related to 4F9/2-6H13/2 transition of Dy3+ ion.
In 2013, O. Yayapao et al. [57] presented the Perkin Elmer RX Fourier transform infrared
(FTIR) spectra and photocatalytic properties of Dy-doped ZnO nanostructures. ZnO nanostructures
with Dy concentration of 1, 2, and 3% were synthesized by a sonochemical method. FTIR spectra of
the 0, 1, 2, and 3% Dy-doped ZnO samples are shown in Fig. 2.23. The strong absorption bands at
425 ~ 565 cm-1 were specified as the Zn-O stretching vibration of wurtzite hexagonal structured ZnO
crystal. The O-H stretching broad absorption bands, which is assigned as the vibration of hydroxyl
group of rare earth hydroxide, were at 3013 ~ 3633 cm-1 and were increased with the increasing in
Dy contents.
They measured the absorption intensity of methylene blue (MB) at 664 nm for 0 ~ 300 min
using the pure and Dy-doped ZnO samples as photocatalysts under UV light. Figure 2.24 shows MB
degradation efficiency of the as-synthesized 0, 1, 2, and 3% Dy-doped ZnO samples. The Dy-doped
ZnO samples exhibited higher photocatalytic activities than that of the pure ZnO sample. The 3%
Dy-doped ZnO showed the highest photocatalytic activity. The rapid decrease of the MB
concentration was mainly ascribed to the Dy dopant in ZnO. The increase of Dy doping content
obviously enhanced the photocatalytic activity.
In 2012, X. Ma et al. [21] examined the absorption and photoluminescence spectra of Gd doped
ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The
ZnO nanocrystals were with stoichiometries of 5, 10, and 15 mol% Gd. Figure 2.25 shows the
absorption spectra of 5, 10, and 15 mol% Gd doped ZnO nanocrystals (sample A, B, and C). They
found the absorption intensity is enhanced, the number of absorption bands has also increased, and
the bands are shifted slightly to a longer wavelength with increasing Gd doping concentration. These
features are due to Gd doped into the ZnO nanocrystals in two ways, substitutional and interstitial.
For low doping concentrations, the doping is usually substitution doping. For high doping
concentrations, most of Gd impurities are present as interstitial atoms in the ZnO crystals.
Figure 2.26 shows the photoluminescence (PL) spectra of the pure and 5 mol% Gd doped ZnO
nanocrystals measured at room temperature. The PL spectrum of ZnO nanocrystal exhibits a strong
UV emission peaking at 375 nm, which is attributed to the excited electrons undergo transitions from
the conduction band to valence band. In addition, a small blue peak is located at 432 nm, which is
attributed to the surface defect from oxygen vacancies or zinc interstitials. For comparison, the PL
spectrum of the Gd doped ZnO nanocrystal (sample A) is given as a black line. In addition to the UV
emission from ZnO, there are two blue PL emissions. One is located at 397 nm and the other is at
432 nm. After doping with Gd, the intensity of the UV and the blue emissions is reversed.
Figure 2.27 shows the photoluminescence spectra of 5, 10, and 15 mol% Gd doped ZnO
nanocrystals (sample A, B, and C) measured at room temperature. They found the intensities of the
UV and blue emissions of ZnO change significantly with the ratio of Zn/Gd in the source material.
As increasing the Gd doping concentration, both the UV and blue emissions from ZnO nanocrystals
are initially enhanced then decrease, and finally the blue emission is completely quenched.
At low Gd concentrations, strong UV and blue emissions were observed, which are likely due to
Gd impurities introducing a mid-gap state into ZnO. For high doping concentrations, both the UV
and blue emissions decrease, while a large broad defect emission appears due to the large numbers of
defects, impurities, and the excess Gd2O3 produced.
In 2014, S. Kumar et al. [60] presented the Raman scattering and photoluminescence spectra of
Gd3+ incorporated ZnO nanoparticles. ZnO nanoparticles doped with Gd (0.01, 0.03, and 0.05 mol%)
were synthesized by wet chemical route method. Figure 2.28 shows the measured Raman scattering
spectra for Gd doped ZnO samples with different concentrations of Gd at room temperature. They found that the intensity of both E2low and E2high modes decreased gradually with increasing Gd
concentration, which is attributed to the lattice distortion in ZnO matrix. Figure 2.29 displays the
Lorentzian curve fitting results of the 1LO (581 cm-1) mode of pure and Gd doped ZnO nanoparticles.
The intensity of 1LO mode increases with an increase in Gd concentration. The 1LO mode can be
deconvoluted into two peaks positioned at 563 and 581 cm-1 and assigned to A1 (LO) and E1 (LO).
As Gd is increased, the separation between A1 (LO) and E1 (LO) mode decreases, indicating the
reduced crystallinity or increased lattice distortion of the samples.
Figure 2.30 shows the measured PL spectra for pure and Gd doped ZnO nanoparticles with
different concentrations of Gd at room temperature. The PL spectrum of ZnO nanoparticles exhibited
a strong visible emission centred at 562 nm and a sharp near band edge emission at approximately
384 nm. In the inset of Fig. 2.30, there is a very slight shift in band edge peak, which indicates the
existence of doping. They found a slight blue shift appears in the position of near band edge emission
with increasing doping concentration and this shift is attributed to the enhancement in the band gap
energy and increase in the particle size of the ZnO nanoparticles. In addition, the broad emission
band was observed in the visible region because of the superposition of green, yellow-orange, and
red emissions.