chapter 5 effects of ni and cr doping on the

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.



, 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


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



blue emission bands centred at 462 and

and the low intensity green emission band at 520 nm is attributed


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.

Chapter-5

123

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.

chapter 5 effects of ni and cr doping on the

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