In 2006, G. S. Wu et al. [56] presented the structural analysis of Dy-doped ZnO nanowires
prepared by sol-gel template approach. Figure 2.2 shows X-ray powder diffraction (XRD) pattern of
the as-obtained Dy-doped ZnO nanowires. All the diffraction peaks of nanowires can be indexed to
the hexagonal structured ZnO. No characteristic peaks were observed for the other impurities,
implying the synthesis of high-purity ZnO samples.
In 2010, R. S. Ajimsha et al. [20] examined the structural and electrical properties 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
2.3 displays the typical ω−2θ high-resolution x-ray diffractometer (HRXRD) patterns of intrinsic
and Dy doped ZnO films. The Dy doped ZnO thin films exhibit only (0002) peak of ZnO, which
indicates that all the as grown films show a preferred orientation with c-axis perpendicular to the
surface. No characteristic peaks corresponding to Dy2O3 phase was observed, which implies that Dy
substitute the Zn sites in the ZnO matrics.
Figures 2.4 and 2.5 show the electrical resistivity (ρ ) and carrier concentration of Dy:ZnO thin
films with different Dy concentrations. All the samples were found to be n-type semiconductor. Dy
concentration of 0.45 at% was found to be the optimum doping concentration for lowest resistivity.
The dependence of the free carrier mobility (µ ) of the film on Dy concentration is shown in Fig. 2.5.
It can be seen that the carrier mobility of Dy:ZnO thin films with Dy concentration of 0.2 at% which
was approximately 27.2 cm2/Vs decreased up to approximately 22.3 cm2/Vs with an increase in Dy
concentration up to 4.12 at%. The decrease in carrier mobility in Dy:ZnO thin films with increasing
Dy concentration can be explained from the deterioration of crystalline quality of the film and thus
enhanced scattering of free carriers with defects and grain boundaries.
In 2013, O. Yayapao et al. [57] presented the structural properties of Dy-doped ZnO
nanostructures. ZnO nanostructures with Dy concentration of 1, 2, and 3% were synthesized by a
sonochemical method. Figure 2.6 shows XRD patterns of the Dy-doped ZnO samples. For doping 1,
2, and 3% Dy in ZnO samples, their XRD patterns are the same as that of pure wurtzite hexagonal
ZnO structure. No other peaks corresponding to Dy2O3, Zn(OH)2, and other impurities were detected.
It should be noted that 2θ angles of the (100), (002), and (101) planes at 32.11, 34.75, and
36.57 for pure ZnO were shifted to the lower diffraction values with the increasing in the doping
concentrations until reaching at 32.99 , 34.65 , and 36.49 by doping with 3% Dy. This phenomenon can be illustrated by the expansion of ZnO lattice caused by the larger radius of Dy3+
(0.91 Å) than that of Zn2+ (0.74 Å).
In 2012, X. Ma [58] reported the magnetic properties of Gd doped ZnO nanowires grown on Si
substrates by means of a chemical vapor deposition process. Their sample was grown with a Gd 5%
mole in a mixed Zn/Mn source under a constant O2/Ar gas mixture flowing at 580℃ followed by
annealing at 800℃. Figure 2.7 shows the X-ray powder diffraction (XRD) pattern of the as grown
ZnO:Gd nanowire sample. From the diffraction peaks, they can confirm that ZnO nanowires are a
wurtzite structure. The diffraction spectrum of ZnO:Gd nanowire is almost the same as that of pure
ZnO. That is to say, for a low doping concentration of Gd, the structure of ZnO remains unchanged,
and Gd atoms simply replace Zn atoms. By the results of the Rutherford backscattering measurement,
the concentration of Gd in the ZnO nanowires is about 1 × 1015 cm-3. The stoichiometry of
Zn0.95Gd0.05O was obtained from the experimental data, as shown in the Rutherford backscattering
spectrum (Fig. 2.8).
Figure 2.9 shows the magnetization properties obtained from the as grown and annealed
ZnO:Gd nanowires samples at 77 K. In the presence of the magnetic field H, a large opening up of
the hysteresis loop is visible for as grown nanowires and is even more pronounced for the annealed
sample. The two samples have qualitatively similar hysteresis loops, although the annealed sample
shows a remarkable enhancement of the magnetic properties. This appearance implies that the
annealing process removes some of the structural defects in ZnO nanowires and increases the
magnetic properties of Gd.
Figure 2.10 displays the magnetization properties obtained from the as grown and annealed
ZnO:Gd nanowires samples at room temperature. The wide opening hysteresis loops indicate the
presence of a ferromagnetic phase in the nanowires at room temperature, and the steep rise in
magnetization reveals the samples to be intrinsic diluted magnetic semiconductors. The results for
the annealed sample show a clear separation ∆M . They found that the ferromagnetism and the
colossal moment of Gd observed in these samples are closely related to the interactions between the
Gd ions. Notably, their ZnO:Gd nanowires show the high-temperature ferromagnetism.
Figure 2.11 (a) shows the magnetic susceptibility of the as grown and annealed ZnO:Gd
nanowires at room temperature and as a function of the applied magnetic field. The results of
magnetic susceptibility further confirm that ZnO:Gd nanowires are highly ferromagnetic. The effective magnetic moment per Gd atom peff can be derived from the value of the saturation
magnetization Ms ( peff =Ms /NGd; NGd = ×1 10 cm15 −3). Figure 2.11 (b) illustrates peff as a
function of the applied magnetic field H. The maximum peff of the annealed and as growen sample is 3279 μB and 1284 μB, respectively. The large effective moment is closely connected with
the exchange interaction modes of Gd ions. Such a colossal moment can be explained in terms of a
very effective Ruderman-Kittel-Kasuya-Yosida exchange interaction.
Because the moment of the rare-earth metals originates from the 4f electrons, which are
confined to the inner shell with a radius of 0.3 × 10-10 m and screened by the outer shell 5s2p6d106s2
electrons, two neighboring rare-earth metals ions are unable to exchange couple directly or interact
via a superexchange interaction because the space between two neighboring Gd ions is large.
In the same year, V. Ney et al. [59] presented the structural and magnetic analysis of Gd-doped
ZnO epitaxial films prepared by reactive magnetron sputtering. The Gd:ZnO films with Gd
concentrations of nominally 1.3%, 4%, 7%, and 16% were grown on c-plane sapphire substrates.
Figure 2.12 shows the wide range of XRD 2θ scans recorded for ZnO(002) of the 1.3%, 4%, 7%,
and 16% Gd:ZnO films. They found that the intensity of the ZnO(002) peak is correlated to the
thickness of the samples, which were measured by x-ray reflectometry (XRR). The inset of Fig. 2.12
illustrates the full width at half maximum (FWHM), depending extremely on the amount of Gd in the
films.
Figure 2.13 shows the isotropic x-ray absorption near edge spectra (XANES) and the x-ray
linear dichroism (XLD) signals at the Zn K-edge for the sputtered 1.3%, 4%, 7%, and 16% Gd:ZnO
films. XANES shows a small reduction of the spectral fine structure with increasing Gd content. The
changes in the isotropic XANES indicate a change in the oxidation state of the corresponding
element. The XLD spectra reveal the typical signature of a wurtzite crystal structure. The XLD also
reflects the local structural environment of the Zn atom and therefore the quality of the ZnO films.
The size of the XLD is strongly reduced for the higher Gd concentrations. With an increase in the Gd
content, the ZnO films show the degradation of the local structural quality of the ZnO host lattice in
accordance with the integral structural characterization using x-ray diffraction (XRD).
Figure 2.14 (a) and (b) display the isotropic XANES and the XLD signals at the Gd L3-edge for
the sputtered 1.3%, 4%, 7%, and 16% Gd:ZnO films. For all sputtered samples, the XANES are
nearly identical. But a strong dependence of the Gd concentration is seen in the XLD, which is
extremely reduced with increasing Gd content. As for the Zn, the presence of a characteristic Gd
XLD confirms that at least a fraction of the Gd atoms are present in a wurtzite crystal environment.
The fraction of substitutional Gd is further reduced for higher Gd concentrations. This phenomenon
can be either due to rotated grains, or Gd atoms, which are not located on substitutional Zn lattice
sites. In conclusion, with an increasing concentration of Gd, the crystal structure of the ZnO is more
and more disturbed. However, the Gd:ZnO films remain essentially in the wurtzite structure.
Figure 2.14 (c) shows the element-specific magnetic properties of the samples analyzed by
measuring the x-ray magnetic circular dichroism (XMCD) at the Gd L3-edge. With increasing Gd
concentration, the XMCD decreases, but the shape of the spectra remain the same. Figure 2.15
displays the XMCD (H) signal taken at a maximum of the XMCD signal. The magnetic field was
swept between ± 60 kOe. For all samples, the XMCD (H) curves exhibit a clear S shape, which
shows no hysteresis. This indicated paramagnetic behavior. The curvature of the XMCD (H)
decreases with increasing Gd concentration, indicating a reduced effective Gd magnetic moment.
Figure 2.16 shows the XMCD (H), SQUID M H , and Brillouin function for
( )
J = =S 7 / 2 experimental data of the 1.3% Gd:ZnO sample. Both integral and elementspecific magnetizationmeasurements demonstrate that the 1.3% Gd:ZnO sample behaves purely paramagnetically. Figure
2.17 displays the integral magnetic characterization by the SQUID for the entire Gd concentration
series. No separation between field-cooled (FC) and zero-field-cooled (ZFC) magnetization occurs at
any temperature for any sample. Therefore, all samples have to be considered to behave solely
paramagnetically.
The geometry for Gd located on substitutional and interstitial sites is shown in Fig. 2.18. They
calculated the structure of Gd sample by using the density-functional theory. They found that for
small doping concentrations of nominally 1.3% Gd, a large fraction of the Gd atoms are substituional
on Zn lattice sites within the wurtzite crystal structure. With higher Gd concentrations, an increasing
amount of Gd is not substitutional anymore. The interstitial center becomes the most stable position,
which is accompanied by lattice distortions.
In 2014, S. Kumar et al. [60] presented the magnetic properties 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 2.19 shows the magnetiztion versus magnetic field (M-H) curves
for ZnO:Gd (0.01, 0.03, and 0.05 mol%) nanoparticles. Gd doped ZnO nanoparticles exhibit room
temperature ferromagnetic properties which support bound magnetic polarons (BMP) model. With
increasing Gd concentration, the doping in ZnO nanoparticles increases the magnetic properties of
ZnO. The coercivity is also increased with Gd concentration (inset of Fig. 2.19).