99
CHAPTER 5
EFFECTS OF Ni AND Cr DOPING ON THE
PHOTOLUMINESCENCE AND MAGNETIC PROPERTIES OF
ZnO
In this chapter, our focus is on the properties of the two other TM (Ni and Cr) doped ZnO
systems. First, we discuss the effects of Ni doping on the optical and magnetic properties of ZnO
which is a potential candidate from the view point of transparency and magnetism for future
spintronic and optical applications. Due to its unique chemical stability on zinc sites it is
recognized it as one of the most efficient doping elements to improve and tune the properties of
ZnO materials. Because of low solubility limit of Ni, the magnetic properties of Ni doped ZnO
are not very well understood [191]. In literature, there are only few studies on optical and
magnetic properties of TM metal doped ZnO [207, 208] Ni doped ZnO system with diverse
magnetic properties explained in various manners. First-principle calculations indicated that the
stability of the FM state in Cr-doped ZnO is more than a spin-glass state and also be more
energetically favorable than Co-doped ZnO. Among the TM dopants, trivalent Cr3+ ions exhibit
3d3 high-spin configuration, which may help to generate large magnetic moments in the host
semiconductors [209]. In particular, Cr is an intrinsically nonmagnetic transition metal, its
clusters or compounds (except nanocrystalline CrO2) do not contribute to FM. Therefore, to
some extent, the study of the Cr-doped ZnO can be more effective to tell whether the FM
originates from the existence of magnetic. It is also an attractive candidate for optoelectronic
devices. From the literature, we observed that some Cr doped ZnO systems show the blue shift in
the band gap [210]. On the other hand, red shift in the band gap [211] is also reported. Thus, the
optical properties seems to be controversial.
Under this scenario, magnetic and optical properties of these systems are a subject of
debate which demand careful investigations. In this chapter, we present the detailed analysis of
the following series of samples:
(i) Zn1-xNixO (x= 0.02, 0.04 and 0.6) samples prepared by sol-gel route.
Chapter-5
100
(ii) Zn1-xCrxO (x= 0.02, 0.04 and 0.6) nanoparticles prepared by thermal
decomposition method.
For the sol-gel route, Ni doped ZnO samples were prepared by using 2-methaxyethanol as
stabilizing agent, high purity zinc acetate [Zn(CH3COO)2·(2H2O)] and nickel acetate
[Ni(CH3COO)2] were used as starting materials. Samples were calcined at 600 °C for 2h.
Further, samples were compacted into pellets and sintered at 950 °C.
For Cr doped ZnO system, nitrates of Zn and Cr were used as starting materials and citric acid
and DEA were taken as precursors. Samples are calcined at 400°C for two hours.
5.1 Ni DOPED ZnO SYSTEM PREPARED BY SOL-GEL ROUTE:
5.1.1 STRUCTURAL PROPERTIES
5.1.1.1 X-RAY DIFFRACTION
Rietveld refined XRD patterns of Ni doped ZnO samples are shown in figure 5.1 in which all
Bragg peaks are indexed in the wurtzite type hexagonal structure with space group P63mc.
Parameters Rp (profile fitting R-value), Rwp (weighted profile R-value) and χ2 (goodness- of-fit
quality factor) obtained after the final cycle of refinement for all samples are presented in Table
5.1. Low values of χ2 and profile parameters (Rp and Rwp) suggest that derived samples are of
good quality and refinements are effective. Observed and calculated values were perfectly
matching as can be seen from figures.
There is no detectable impurity peak upto the 4% of Ni doping but a few traces of NiO (111)
phase around 42°, were detected for x > 0.04 which shows that nickel content x > 0.04 is beyond
the solid solubility limit of Ni in ZnO. Lattice parameters slightly decreased with increase in Ni
concentration, which can be assigned to a smaller ionic radius of Ni2+ (0.55 Å) than that of the
Zn2+ (0.60 Å) which shows the incorporation of Ni ions on Zn sites. A doping induced peak shift
(101) toward higher values (inset of figure 5.1) is also observed.
Chapter-5
101
Figure 5.1: Reitveld refined XRD patterns of Zn1-xNixO (inset shows the doping-induced peak shift).
The average bond length shows a continuous decrease with doping concentration
confirming the substitution of Ni on Zn sites. Average crystallite sizes determined by Scherrer’s
formula vary from 45 to 48 nm which is dependent on the increasing broadening of peaks with
doping concentration.
Table 5.1: Calculated parameters from Rietveld refinement.
% of Ni
Cell parameters a (Å) c(Å) V(Å3)
Bond length
R-factors D (nm)
2 3.253 5.203 47.63 1.978 Rp= 9.84, Rwp= 16.2, Rexp= 12.4, χ
2=1.7. 48
4 3.251 5.202 47.61 1.977 Rp= 10.6, Rwp= 18.0, Rexp= 13.2, χ
2 = 1.8.
47
6 3.250 5.201 47.58 1.976 Rp= 10.8, Rwp = 16.7,Rexp = 16.7, χ
2= 1.9. 45
Chapter-5
102
5.1.1.2 SEM
Figure 5.2 shows the FESEM image of 2% Ni doped ZnO sample which shows
accurately separated grains with hexagonal structure on the surface.
Figure 5.2: FESEM image of 2% Ni doped ZnO.
5.1.1.3 FTIR STUDIES
FTIR spectra were performed in the range of 400 to 4000 cm-1 using the KBr method at
room temperature to study the composition, quality and molecular structure of samples. Figure
5.3a shows the full scan transmittance spectra of Zn1-xNixO (x=0.02, 0.04 and 0.06). The broad
absorption peak around 3500 cm-1 represents the stretching vibration of the O-H group.
Absorption peaks observed at around 2350 are assigned to the CO2 mode which may be due to
atmospheric CO2 [212]. The absorption peak at 1420 cm-1 corresponds to the symmetric
stretching υs (COO-) vibrations of acetate species but the absorption peak (1580 cm-1)
corresponding to asymmetric stretching υas (COO-) [163] may have merged with the principle
absorption peak at 1620 cm-1 corresponding to bending vibrations of the interlayer water
molecule.
Chapter-5
103
Figure 5.3: (a) The full range transmittance spectra of Zn1-xNixO (x=0.02, 0.04 and 0.06). Inset shows the variation
of IR bands corresponding with Ni content. (b) FTIR absorbance spectra corresponding to Zn-O bonds with
Gaussian fitting.
The IR active characteristic broad band (optical phonon modes) of ZnO is observed in the
spectral range 400-600 cm-1. Absorption bands are found to blue shift with increasing Ni doping
which reflects that the Zn-O-Zn network is perturbed by the presence of Ni in its environment.
To locate exact positions of Zn-O bands, IR band in the region 400-600 cm-1 is shown in the
figure 5.3b fitted by the Gaussian and showing three bands X1, X2 and X3. The band X1 at around
445 cm-1 corresponds to the E1 (TO) mode. The bands at 484 cm-1 (X2) and 533 cm-1 (X3) are
surface phonon modes (SPM) and named as SPM [A1 (TO)] and SPM [E1(TO)], respectively,
generally appear when the size of prepared particles is smaller than the incident IR wavelength
[213]. These IR bands corresponding to Zn show a variation in vibrational frequencies with Ni
concentration, as shown in the inset of figure 5.3a, which may be due to the difference in ionic
radii of Zn and Ni as well as due to induced structural changes on doping [213]. The average
bond length of Zn(Ni)-O in Zn1-xNixO system is determined from the band position of E1(TO)
Chapter-5
104
and . Calculated values of effective mass, force constant and bond length are summarized in
table 5.2 calculated by the formula mentioned in chapter 3.
Table 5.2: The IR band and local structure data of Zn Ni-O bonds of Zn1−xNixO.
T
The effective mass of Zn (Ni)-O bond decreased with Ni substitution because of the
lower atomic weight of Ni than Zn. Also, a decrease of the average force constant is observed
with substitution of Ni which results in an increment in the average Zn (Ni)–O bond length. The
variation of the bond length due to Ni substitution is consistent with the trend observed by
Rietveld analysis.
5.2.2 OPTICAL PROPERTIES
5.2.2.1 UV-VIS SPECTROSCOPY
Optical properties of samples were studied by UV-visible spectroscopy in the range 300-
800 nm. The position of the absorption edge is observed to be shifting towards lower wavelength
side with increase in Ni concentration in ZnO (figure 5.4), indicating an increase in the band gap
with Ni doping.
The band gaps of samples were estimated by extrapolation of linear portion of (αhν)2
versus hν curve (Figure 5.4b) by using the Tauc plot relation 2/1)( gEh −∝ να for the direct
band gap semiconductor between the absorption coefficient (α) and the energy band gap (Eg),
where h is the planks constant and ν is the frequency of incident photon. The band gaps are
observed to vary from 3.29 to 3.32 eV with increase in the nickel concentration as shown in the
inset of figure 5.4 b.
Samples (Zn1-xNixO)
Wavenumber (cm-1)
Effective mass (atomic weight)
Force constant (N m-1)
Bond length
(Å) x=0.02 445 12.8426 150.11 2.2457 x=0.04 448 12.8327 152.15 2.2356
x=0.06 451 12.8322 154.07 2.2263
Chapter-5
105
Figure 5.4: (a) Absorption spectra of Zn1-xNixO (Ni= 2%, 4%, 6%). (b) Tauc plot of the samples, Inset shows the
variation of band gap with Ni doping.
This optical energy band gap widening and the absorption edge blueshift can be attributed to an
increase be the carrier concentration and in principle be explained by the Moss-Burstein band
filling effect, which is frequently observed in n-type semiconductors [214].
5.2.2.2 PHOTOLUMINESCENCE SPECTROSCOPY
The intrinsic and extrinsic defects and the change in the optical band edge are also
examined from the PL spectroscopy. Room temperature PL (RT PL) spectra of Ni doped ZnO
samples excited at 325 nm are shown in figure 5.5 which show the near band edge (NBE) UV
emission around 375 nm, slightly shifting to the longer wavelength with increase in Ni
concentration which is in good agreement with the absorbance spectra and further confirms the
substitution of Ni 2+ ions on Zn2+ sites. The UV emission is followed by the high intensity broad
visible band in the range 420-520 nm. The origin of the visible emission in ZnO is still a
controversial issue as it is not easy separate the produced emissions by different type of defects.
For instance, it was suggested by Chiorescu et al.
electronic transition between the interstitial
bottom of the conduction band and the Zn
ZnO being mediated by the oxygen vacancy (
Thus as shown in Figure 5.5b, the broad visible emission is deconvoluted for different
defect states with the help of Gaussian fitting. The defect level emission band at 410 nm
corresponds to Oi, emission band at 434 nm t
482 nm corresponds to the VZn and the low intensity green emission band at 520 nm is attributed
to oxygen and zinc vacancies. Thus, we conclude that Ni doping leads to an increase in electron
concentration and decrease in the intrinsic defect density.
Chapter-5
106
For instance, it was suggested by Chiorescu et al. [192] that violet emission originates from an
electronic transition between the interstitial-zinc (Zni) level and the valence band or between the
bottom of the conduction band and the Zn-vacancy level (VZn). However, green luminescence of
y the oxygen vacancy (Vo) defects.
Thus as shown in Figure 5.5b, the broad visible emission is deconvoluted for different
defect states with the help of Gaussian fitting. The defect level emission band at 410 nm
, emission band at 434 nm to the Zni, blue emission bands centred at 462 and
and the low intensity green emission band at 520 nm is attributed
to oxygen and zinc vacancies. Thus, we conclude that Ni doping leads to an increase in electron
and decrease in the intrinsic defect density.
] that violet emission originates from an
) level and the valence band or between the
). However, green luminescence of
Thus as shown in Figure 5.5b, the broad visible emission is deconvoluted for different
defect states with the help of Gaussian fitting. The defect level emission band at 410 nm
blue emission bands centred at 462 and
and the low intensity green emission band at 520 nm is attributed
to oxygen and zinc vacancies. Thus, we conclude that Ni doping leads to an increase in electron
Figure 5.5: (a) PL spectra of Ni doped ZnO samples inset shows the blue shift in NBE. (b) PL spectra of
Zn
5.2.3 MAGNETIC PROPERTIES
The field dependent magnetization (M
shown in figure 5.6. All samples show the RTFM behaviour. A decrease in the saturation
magnetization with increase in the doping concentration is observed and values of
polarization and coercivity are approximately same for 2 and 4% Ni doped ZnO samples but
slightly decreased for 6%.
Chapter-5
107
(a) PL spectra of Ni doped ZnO samples inset shows the blue shift in NBE. (b) PL spectra of
Zn0.98Ni0.02O along with the Gaussian fit.
MAGNETIC PROPERTIES
magnetization (M-H) curves recorded at room t
6. All samples show the RTFM behaviour. A decrease in the saturation
magnetization with increase in the doping concentration is observed and values of
ercivity are approximately same for 2 and 4% Ni doped ZnO samples but
(a) PL spectra of Ni doped ZnO samples inset shows the blue shift in NBE. (b) PL spectra of
H) curves recorded at room temperature are
6. All samples show the RTFM behaviour. A decrease in the saturation
magnetization with increase in the doping concentration is observed and values of remnant
ercivity are approximately same for 2 and 4% Ni doped ZnO samples but
Chapter-5
108
Chapter-5
109
Figure 5.6: M-H curves of Zn1-xNixO samples (a) x=0.02, (b) x=0.04 and (c) x=0.06. Insets (i) in a, b and c show the
initial portion of the M-H curve fitted with BMP model, and inset (ii) zoomed M-H curves.
The origin of RTFM in oxide based DMSs is still not clear as there is an incomplete
understanding whether it is an extrinsic effect due to direct interaction between the local
moments in magnetic impurity clusters or is indeed an intrinsic property caused by exchange
coupling between the spin of carriers and local moments. There are various theories proposed in
the literature to explain the mechanism of the origin of magnetism in DMSs [214, 215].
As oxygen vacancies are inherently present in our samples due to the stabilization of the
structure, it can be predicted that oxygen vacancy defect constituted bound magnetic polarons
(BMPs) are of the promising candidates for the origin of RTFM in this system. According to the
BMP model, bound electrons in defects, like oxygen vacancies (Vo), can couple with Ni ions and
cause ferromagnetic regions to overlap giving rise to long range ferromagnetic ordering.
According to this theory of defect mediated RTFM, the large density of oxygen vacancy help to
provide more BMPs and enhancing FM. The evolution observed in our case is decrease in
magnetization with decrease in oxygen vacancies, indicating that percolation of BMPs may be
responsible for ferromagnetism.
Chapter-5
110
Table 5.3: List of parameters obtained from experimental M-H curve along with the fitted data in BMP model.
The suitability of BMP model is cheked by fitting the M-H data, it can be observe from the
fitting (inset of figure 5.6) that filled data closely follow the experimental data and fitted
parameters are listed in the table 5.3. The spontaneous moment per BMP is found to be of the
order of 10-15 emu and the number of BMPs which were determined from Mo and meff values are
of be the order of 1015 per cm3 which is very small to the concentration necessary for percolation
in ZnO. The required concentration for percolation of BMPs is in the range of 1020 per cm3
which is five orders larger than observed value. Thus BMP model alone is insufficient to explain
RTFM in the Ni doped ZnO system.
We further try to explain the FM on the basis of RKKY interaction which explains the
magnetic phases on the basis concentration of free carriers apart from the concentration of
magnetic ions. To achieve ferromagnetism in Ni doped ZnO, the electron concentration must be
low, as ZnO is a native n-type material. Furthermore, the addition of electrons to the system will
move the Fermi energy level up, resulting a decrease in hole density and a reduction in
magnetization. In our case, oxygen rich stoichiometery with increased Zn-O bonding favors the
indirect Ni-O-Ni ferromagnetic exchange coupling. Furthermore, the enhanced Zn-O bonding in
2% and 4% in turn reduces the Vo (donors) and leads to strong hybridization (p-d exchange
coupling) of Ni in ZnO host matrix which is responsible for RTFM. It may, however, be noted
that for 2% sample, the presence of DLE (~520 nm) in PL spectra indicates the presence of
oxygen vacancy (acceptors) which according to Dietl’s prediction [201] might also contribute
towards the ferromagnetic ordering for this sample as holes are required to mediate RTFM in Ni
Samples
Zn1-xNixO
Experimental
Data
Fitting parameters extracted from BMP Model
Mr* 10-3
(emu/g)
Hc
(Oe)
S Mo
(emu/g)
meff *10-16
(emu)
χχχχm*10-5
(egs)
N*1015
(cm3)
x=0.02 2.51 53 3/2 3 1.5 0.4 1.1
x=0.04 2.52 54 3/2 2.3 1.5 9.9 0.86
x=0.06 2.06 49 3/2 2.1 1.32 1.5 0.89
Chapter-5
111
doped ZnO. However, the VSM data show that the ferromagnetism in 4% Ni doped ZnO
samples is stronger than that of 2% indicating that the ferromagnetic contribution of Ni-O-Ni
exchange coupling due to Zn-O bonding is much more significant as compared to defected
mediated ferromagnetism. However, there is decrease in the magnetization for 6% Ni doped ZnO
which may be due to occurrence of an anti-ferromagnetic ordering of spins. It is well known that
Ni ions belonging to antiferromagnetic clusters do not contribute to the increase in magnetic
signal rather they reduce the net magnetization [35] or due to the enhanced antiferromagnetic
interaction between neighbouring Ni–Ni ions suppresses the ferromagnetism at higher doping
concentrations of Ni2+.
5.3 Cr DOPED ZnO NANOPARTICLES PREPARED BY THERMAL
DECOMPOSITION METHOD
5.3.1 STRUCTURAL CHARACTERIZATION
5.3.1.1 X-RAY DIFFRACTION
Figure 5.7 shows the reitveld refined XRD patterns of Cr doped ZnO nanoparticles with the 2%,
4%, 6% and 8% concentration of Cr. All diffraction peaks were indexed to wurtzite, the
hexagonal ZnO (space group P63mc). In addition, no hints of the Cr metal or the Cr oxides were
observed. Also, from the Reitveld refinement, we observed the perfect matching between the
experimental and theoretical values which was confirmed by the almost straight difference line.
Also, the values of the profile fitting parameters after the final cycle of refinement are low and
effective (table 5.4). Values of the cell constants (a and c) decreased with increase in the Cr
concentration. Such a lattice contraction can be qualitatively explained in terms of sizes of ions
and their local co-ordinations.
Chapter-5
112
Figure 5.7: Reitveld refined XRD patterns of Zn1-xCrxO nanoparticles.
To sum up, this result may originate from the substitution of Cr ions (0.63 Å) at Zn (0.74 Å) sites
[216]. The particle size calculated by Scherrer’s formula of Zn1-xCrxO (x=0.02, 0.04 and 0.08)
are 18, 19, 21, and 24 nm by, respectively. Many theoretical and experimental studies have
demonstrated size-dependent lattice contraction [216, 217]. So, the size effect should be another
possible reason of the lattice contraction due to the variation of the surface stresses.
Chapter-5
113
Table 5.4: Calculated parameters from Rietveld refinement
5.3.1.2 FTIR
The FTIR spectra in Figure 5.8 show several absorption bands. The band centered at around
3400 cm-1 is assigned to the O-H stretching vibrations for hydroxide group or for interlayer water
molecule as the O-H mode appears in all hexagonal ZnO structures due to the stacking of
positively charged Zn2+ ions and negatively charged O2- ions in planes perpendicular to the c-
axis, the Zn2+ plane tends to absorb the hydroxide [OH]- while the O2- plane tends to absorb the
hydrogen H+ ions due to electrostatic instability of these planes. Small band at ~2900 cm-1 is
featured to the C-H bond of organic compounds and all other bands corresponding to different
chemical species are as mentioned in previous chapter.
% of
Mn
Cell parameters
a (Å) c(Å) V(Å3)
Bond
length
R-factors D
(nm)
2 3.2506 5.2091 47.67 1.973 Rp= 9.4, Rwp= 8.9 Rb= 2.27 , Rf=2.28,
χ2= 2.92
26
4 3.2504 5.2077 47.65 1.975 Rp= 8.78, Rwp= 10.2, Rb= 2.15, Rf=
1.47, χ2=2.14.
28
6 3.2500 5.2070 47.63 1.976 Rp= 11.1, Rwp = 11.7, Rb= 3.77, Rf=
5.08, χ2=2.12.
28
4 3.2498 5.2068 47.62 1.978 Rp= 9.10, Rwp= 8.59, Rb= 2.14,
Rf=1.35, χ2=2.71.
32
Chapter-5
114
Figure 5.8: The full range transmittance spectra of Zn1-xCrxO
The band corresponding to the ZnO appeared in the range of 400-600 cm-1. For the exact
position of modes of ZnO, it was fitted by Gaussian and shown in figure 5.9. Gaussian fitted IR
spectra shows three bands named as X1, X2 and X3, shown in Figure 5.9. The high intensity X1
mode is appeared at ~ 435 cm-1 corresponding to the the E1(TO) mode and the other two bands
X2 and X3 centered at ~480 cm-1 and ~535 cm-1, respectively, corresponds to the surface phonon
modes and named as SPM [A1 (TO)] and SPM [E1 (TO)]. Peaks at around 435 and 535 cm-1
correspond to the stretching vibration modes of Zn-O which is an indication to the complete
transformation from zinc nitrate to zinc oxide [215].
Chapter-5
115
Figure 5.9: FTIR absorbance spectra corresponding to Zn-O bonds with Gaussian fitting
The slight shift in the absorption frequency with Cr concentration confirms the incorporation of
Cr ions into ZnO lattice. The force constant, effective mass and bond length were determined by
the formula as given in the previous chapter and summarized in table 5.5. Bond lengths are the
samples is in the same order as determined by XRD. The effective mass of Zn (Cr)-O bond
decreased with Cr substitution because of the lower atomic weight of Cr than Zn. Also, a
decrease of the average force constant is observed with substitution of Cr which resulted in a
decrease in the average Zn (Cr)–O bond length.
Chapter-5
116
Table 5.5: The IR band and local structure data of Zn Ni-O bonds of Zn1−xCrxO.
Samples
(Zn1-xCrxO)
Wavenumber
(cm-1)
Effective mass
(atomic weight)
Force
constant
(N m-1)
Bond
length
(Å)
x=0.02 435 12.8323 150.20 2.2678
x=0.04 437 12.8271 152.24 2.2546
x=0.06 440 12.8105 154.56 2.2476
x=0.08 443 12.8012 156.08 2.2314
5.3.2. OPTICAL PROPERTIES
5.3.2.1 UV-VISIBLE SPECTROSCOPY
UV–Vis absorption spectroscopy is used to study the optical properties of Cr doped ZnO
nanoparticles. The energy band gap of semiconductor changes as the dopants create crystal
imperfections which depend upon particle size, oxygen deficiency, defects in grain structure etc.
The absoprption spectra of nanoparticles are shown in figure 5.10 and strong optical absorbance
were found between 370-388 nm. The position of the absorption spectra was not affected with
change in Cr doping concentration in ZnO but the maximum absorption peak moved toward the
larger wavelength (red shift).
Chapter-5
117
Chapter-5
118
Figure 5.10: Absorption spectra of Zn1-xCrxO, inset shows Tauc plot of the samples
Band gap were calculated by Tauc plot relation for direct band gap semiconductor. An
extrapolation of the linear region of a plot of hν vs (αhν)2 gives the value of the optical band gap
Eg as shown in inset of figure 5.10.
Band gaps of samples are decreased from 3.23 to 3.04 eV with the increase of Cr doping. This
band gap interpreted in terms of the s, p–d spin-exchange interactions between delocalized s- or
p-type band electrons of Zn and O atoms, respectively and localized d electrons of transition
metal replacing the cation [218, 219]. This shift occurs most probably due to band structure
deformation by Cr ion doping in the lattice structure of ZnO. In s–d and p–d exchange
interactions, the conduction band edge decreases and the valence band edges increases, resulting
reduction of energy bandgap [220]. The tunability of in the band gap in ZnO due to Cr doping
makes it more suitable for various nano photoelectronics applications.
5.3.2.2 PHOTOLUMINESCENCE SPECTROSCOPY
Figure 5.11 shows the PL spectra of Cr-doped ZnO nanoparticles excited by 325nm at room
temperature. Inset (a) of Figure 5.11 shows the zoomed UV-emission peak which shows the
decrease in the peak intensity with increase in Cr concentration. On the other hand inset (b)
shows an increase in the intensity of the peak corresponding
increase in the Cr concentration.
Figure 5.11: (a) PL spectra of Cr doped ZnO samples. (b) the red
In the PL spectra of the Cr doped ZnO nanoparticles
appeared at around 380 nm and a broad deep level emission band in the range of 430
was observed. The UV emission b
exhibits red shift, and the intensity of deep level emission
Chapter-5
119
ntensity of the peak corresponding to the deep level emission
ectra of Cr doped ZnO samples. (b) the red shift in NBE and (c) increase in defect state
of the Cr doped ZnO nanoparticles, UV emission peak is dominant which
around 380 nm and a broad deep level emission band in the range of 430
. The UV emission band which is related to a near band-edge transition of ZnO
shift, and the intensity of deep level emission (DLE) band has increased
to the deep level emission with
(c) increase in defect state.
UV emission peak is dominant which
around 380 nm and a broad deep level emission band in the range of 430–700 nm
edge transition of ZnO
increased with Cr
Chapter-5
120
doping indicating an increase in defect states. As far as the DLE is concerned, the visible
emissions observed are generally originated from various intrinsic defects, such as Zni, VO, VZn,
and Oi. DL emissions in the visible region might also be due to energy levels formed by
impurities or doping. Moreover, XRD spectra of these samples showed no peak other than ZnO.
Therefore, it may be assumed that the visible emissions observed were due to intrinsic defects.
Figure 5.12: PL spectra of Zn0.90Cr0.1O along with the Gaussian fit.
The DLE spectra was deconvoluted with seven peaks, centered at 448, 465, 495, 537, 590, 630
and 666 nm, respectively, using Gaussian fit (shown in figure 5.12). Deconvoluted peaks at 448
and 494 nm are assigned to the energy of transition of electron from interstitial Zn (Zni) to Zn
vacancies (VZn) while the green emission peak (~505 nm) could be due to the transition from Zni
levels to Oi [55-58]. The yellow emission peak at 580 nm can be attributed to the presence of
oxygen interstitial (Oi) in nanocrystalline powders. The peak present at 630 and 666 nm are
associated with the excess oxygen, Oi and Zni. We conclude that the Cr-doping leads to an
decrease in the electron concentration and a concomitant increase in the intrinsic defects (such as
VO and OZn) density.
Chapter-5
121
5.3.3 MAGNETIC MEASUREMENTS
Figure 5.13 represents the M-H curves of Zn1-xCrxO (x=0.02, 0.04, 0.06 and 0.08) nanoparticles
at room temperature. M-H curves of all samples show strong signal of ferromagnetism at RT.
Values of the coercive field and magnetization have increased with Cr doping concentration.
Figure 5.13. M-H curves of Zn1-xCrxO samples, insets show zoomed M-H curves.
Also, the tendency of saturation increased with the increase in Cr doping. The coercive field
increased from 770 Oe (for 2% Cr) to 1449 Oe (for 8% Cr). It is well known that ZnO is non-
magnetic and the observed ferromagnetism in the system is purely dopant induced. In the present
case, ferromagnetic ordering have been increased with Cr concentration and this may be due to
the fact that in case of Cr-ions, the exchange coupling is short ranged and the sign of the
exchange interaction depends on the arrangement of the concerned ions in the crystal. But strong
Chapter-5
122
ferromagnetic ordering can be introduced through defect/oxygen vacancies but the interaction
will still remain short ranged [221].
The origin of the observed RTFM in the Cr doped ZnO nanoparticles could be attributed to arise
from a number of sources viz., intrinsic property of doped ions, formation of some nanoscale
“Cr” related secondary phase, precipitation of Cr etc., As there was no indication of any any
secondry phase observed in XRD, the introduction of the ferromagnetism by the any other phase
can be easily ruled out. We tried to explain the occurrence of ferromagnetism by BMP model.
The magnetic exchange interaction between O vacancy and Cr ions align all Cr spins around the
O vacancy, forming BMPs. With the filling up of oxygen vacancies, neighboring Cr ions which
were coupled via an oxygen vacancy (ferromagnetic exchange) are now being coupled by super
exchange interaction (oxygen bond) are responsible for the ferromagnetic ordering.
Table 5.6: List of parameters obtained from experimental M-H curve along with the fitted data in BMP model.
Samples
Zn1-xCrxO
Experimental Data Fitting parameters extracted from BMP Model
Mr* 10-3
(emu/g)
Hc
(Oe)
S Mo
(emu/g)
meff *10-17
(emu)
χχχχm*10-5
(egs)
N*1019
(cm3)
x=0.02 7.8 64 3/2 1.62 8.82 4.1 1.01
x=0.04 8.2 72 3/2 1.76 7.47 4.2 1.2
x=0.06 8.7 78 3/2 1.93 7.07 4.3 1.4
x=0.08 9.2 85 3/2 2.07 5.75 4.7 1.8
In order to explain the ferromagnetism by BMP model, we fitted (figure 5.14) the measured
initial M-H curves in terms of the bound magnetic polaron (BMP) model (similar to the previous
chapter). The number of BMP and effective magnetic moment per BMP estimated from the
fittings are summarized in table 5.6.
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Figure 5.14: The initial portion of the M-H curve fitted with BMP model
The estimated number of BMPs are 1.1 x 10-19, 1.2 x 10-19, 1.4 x 10-19, 1.8 x 10-19 per cm3 for 2,
4, and 8% Cr doped ZnO samples, respectively which is higher than earlier observed values in
other TM metals doped systems and it is above the threshold of condition of percolation in DMS
and this is quite likely due to the high value of magnetization of the present sample.
5.4 CONCLUSIONS:
Ni doped ZnO samples were prepared by sol-gel route, X-ray diffraction pattern of samples
shows all the samples are in single phase, a secondary phase of NiO is appeare in 6% Ni doped
sample. Phonon modes in Ni doped ZnO nanoparticles were studied through FTIR
measurements. Furthermore, the enhancement in optical band gap with Ni has been observed
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through UV-visible spectroscopic analysis. Photoluminescence spectra of Z1-xNixO show the
UV-emission peak showing the blue shift with increase in doping concentration followed by
broad visible (blue) emission. A clear RTFM is observed in all samples but saturation
magnetization decreased with increasing Ni content. The suitability of bound magnetic polarons
(BMP) model is checked and numbers of BMPs are found to be of the order 1015 per cm3, which
is very small for the percolation in ZnO. In the present case, oxygen rich stoichiometry with
enhanced Zn-O bonding favours the indirect Ni-O-Ni ferromagnetic exchange coupling and
reduction of oxygen vacancies leading to strong hybridization of Ni in ZnO host matrix
responsible for RTFM.
Interestingly, band gap decreased with Cr doping from 3.23 to 3.04 eV, this shift is most
probably due to band structure deformation by Cr ions doping in the lattice of ZnO structure. The
estimated number of BMPs is higher than earlier observed values in other TM doped systems
and it is above the threshold of percolation in DMS and this is quite likely due to the high value
of magnetization of the present system.