Defect luminescence and lattice strain in Mn2+ doped ZnGa2O4

Physica B 491 (2016) 79–83

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Defect luminescence and lattice strain in Mn2 þ doped ZnGa2O4 K. Somasundaram a, K.P. Abhilash a, V. Sudarsan b,n, P. Christopher Selvin a,n, R.M. Kadam c a b c

Department of Physics, Nallamuthu Gounder Mahalingam College, Pollachi, 642001 Coimbatore, India Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

art ic l e i nf o

a b s t r a c t

Article history: Received 23 January 2016 Received in revised form 12 March 2016 Accepted 14 March 2016 Available online 15 March 2016

Undoped and Mn2 þ doped ZnGa2O4 phosphors were prepared by solution combustion method and characterized by XRD, SEM, luminescence and electron paramagnetic resonance (EPR) techniques. Based on XRD results, it is inferred that, strain in ZnGa2O4 host lattice increases with incorporation of Mn2 þ ions in the lattice. Mn2 þ doping at concentration levels investigated, lead to significant reduction in the defect emission and this has been attributed to the formation of higher oxidation states of Mn ions in the lattice. Electron Paramagnetic Resonance studies confirmed that majority of Mn ions exist as Mn2 þ species and they occupy tetrahedral Zn2 þ site in ZnGa2O4 lattice with an average hyperfine coupling constant, Aiso∼82 G. & 2016 Elsevier B.V. All rights reserved.

Keywords: ZnGa2O4 Photoluminescence EPR SEM Strain

1. Introduction In this modern era of energy depletion, one of the most important use of nano-phosphors is their application potential in field emission displays (FEDs), electroluminescence displays and vacuum florescent display systems (VFDs) [1,2]. During recent years multicolor phosphors for White-Light Emitting Diode (WLED) applications are being extensively investigated [3–5], as the phosphors possess high color rendering properties. Oxide phosphors are potential candidates for the above mentioned applications, due to their high thermal stability and improved luminescence characteristics. Photoluminescence (PL) from most of the oxide phosphors generally occurs upon UV excitation. Among different oxide phosphors, those with blue light emission characteristics are of particular importance as they can mix with yellow or red & green light emitting phosphors to generate white light [5,6]. ZnGa2O4 is an oxide phosphor which is known to emit blue light upon both optical and electrical excitations [7–9]. Of late ZnGa2O4 phosphor is gaining significant attention because of its higher chemical stability, low energy consumption and low voltage cathodoluminescent characteristics [10]. It is well known that ZnGa2O4exists in normal spinel structure with a wide band gap of about 4.4 eV. In normal spinel structure, oxygen ions form face-centered cubic closed packing and all Zn2 þ ions occupy n

Corresponding authors. E-mail addresses: [email protected] (V. Sudarsan), [email protected] (P. Christopher Selvin). http://dx.doi.org/10.1016/j.physb.2016.03.022 0921-4526/& 2016 Elsevier B.V. All rights reserved.

tetrahedrally coordinated A-sites, whereas Ga3 þ ions occupy the octahedral positions (B sites). Different dopants like Cr3 þ , Mn2 þ , Eu3 þ etc., in ZnGa2O4 lattice, leads to emission of different colors upon UV excitation [7,8,11–16]. Singh et al. [11], have prepared Mn2 þ doped ZnGa2O4 phosphor by urea assisted combustion route followed by annealing at 900 °C in air. Based on EPR and photo-luminescence (PL) studies, it has been inferred that Mn2 þ ions occupy Zn2 þ site in the lattice and the green emission around 528 nm is arising due to the 4T1-6A1 transition of Mn2 þ ions [11]. Kim et al. [17], have investigated in detail the optical properties and local environment around Mn2 þ ions in ZnGa2O4:Mn2 þ phosphors prepared by sol–gel method followed by annealing at different temperatures. From this study it is understood that, Mn2 þ ions occupy both tetrahedral (Td) and octahedral (Oh) sites in ZnGa2O4 lattice and relative concentration of tetrahedrally occupying Mn2 þ sites increases at the expense of Mn2 þ at octahedral sites with increase in annealing temperatures. Two distinct emission peaks have been observed at liquid nitrogen temperatures from Mn2 þ ions occupying tetrahedral and octahedral sites in ZnGa2O4 lattice [12]. Kim et al., [7] also reported that luminescence from ZnGa2O4:Mn2 þ can be improved by co-doping with Ge4 þ and Li þ ions in the host. Enhancement in luminescence properties has been attributed to the combined effect of Ge4 þ acting as a donor species in the lattice of ZnGa2O4 and Li þ doping in increasing the conductivity of host. Two emission bands at 468 and 502 nm have also been observed from Mn2 þ doped ZnGa2O4 phosphors annealed at 800 °C in reducing atmosphere, by Kumar et al. [18]. Authors have attributed this to the trap state transition

K. Somasundaram et al. / Physica B 491 (2016) 79–83

was 40 ms with a field-sweeping rate of 100 G/164 s. Standard samples of DPPH were used to calibrate the g-factor.

3. Results and discussions 3.1. X-ray diffraction (XRD) Rietveld refined XRD patterns for undoped and Mn2 þ doped ZnGa2O4 samples are shown in Fig. 1. All the peaks are characteristic of the spinel structure of ZnGa2O4. No other peaks characteristic of impurity phase could be detected in the XRD patterns. Lattice parameters obtained from the refinement of XRD pattern are shown in Table 1. Lattice parameter systematically increases with increase in Mn2 þ concentration in the lattice. This can be explained based on replacement of smaller ionic radii Zn2 þ (0.60 A) with larger ionic radii Mn2 þ (0.66 A) in the lattice. Based on line width of XRD patterns average crystallite size has been found to be ∼19 nm. With increase in Mn2 þ doping average crystallite size is found to decrease as can be seen from Table 1.

(6 4 2)

Difference Calculated Observed Background (6 2 0) (5 3 3) (6 2 2) (4 4 4)

(4 4 0)

(5 1 1)

(4 2 2)

1500 0

20

40

60

(b)

Intensity

Undoped and Mn2 þ doped ZnGa2O4 phosphor materials were prepared by solution combustion method. The undoped ZnGa2O4 sample has been synthesized by using Zinc nitrate hexahydrate (AR-99%, Aldrich, 1.61 g), Gallium nitrate hydrate (AR-99.9%, Aldrich, 3 g) and Urea (2.19 g) as starting materials. Metal nitrates (oxidizers) and urea (fuel) were used for the process of combustion. Starting materials were crushed and ground in a china dish with minimum quantity of the de-ionized water to form a transparent solution. It was then inserted in a preheated furnace maintained at 500 °C. The prepared solution undergoes rapid dehydration and decomposition with an evolution of large amount of gaseous by-products. The solution undergoes ignition, burns completely, yielding a voluminous solid mass. The entire combustion process was over in less than five minutes. The dish was immediately removed from the furnace. The resultant voluminous product was crushed into fine powder using pestle and mortar. Similar procedure was also used for preparing Mn2 þ doped samples except that manganese chloride tetra hydrate was initially added (at the expense of Zn2 þ ) along with Zinc and gallium salts. Mn2 þ ion in two different concentrations in ZnGa2O4 namely Zn0.97Mn0.03Ga2O4, Zn0.95Mn0.05Ga2O4 has been prepared by this method. As prepared undoped and Mn2 þ doped ZnGa2O4 phosphor samples have been characterized by powder X-Ray Diffraction technique (XRD-SHIMAZDU-6000) using Cu-Kα radiation source. Morphological characterizations of the samples were carried out using a JEOL-JSM 6390 Scanning Electron Microscope (SEM). All steady-state luminescence and lifetime measurements were carried out by using an Edinburgh Instruments’ FLSP 920 system with a 450-W Xe lamp, nanosecond hydrogen flash lamp as the excitation sources. EPR spectra was recorded at room temperature using Bruker X-band spectrometer (EMX series; EMM – 1843) using a standard rectangular cavity ER4119HS operating at 9.60 GHz with a 100 kHz modulation frequency. The spectrometer is equipped with an electromagnet capable of providing a magnetic field ranging from 50 G to 12 kG. The EPR parameters were chosen to provide the maximum signal-to-noise ratio for nondistorted signals. The microwave power and modulation amplitude were 2 mW and 1 G respectively. The response time constant

(3 1 1)

(4 0 0)

2. Experimental

(a)

(2 2 2)

3000

(2 2 0)

of host and 4T1-6A1 transitions of Mn2 þ respectively [18]. Lee et al. [19] have demonstrated that, ZnGa2O4 nanoparticles prepared by solution combustion method have improved luminescence properties compared to bulk micron sized ZnGa2O4 phosphors prepared by solid-state reaction. From the above studies it is clear that luminescence properties of ZnG2O4 and Mn doped ZnGa2O4 phosphors strongly depends on method of preparation, heat treatment temperatures, presence of co-dopants and associated defects (traps) etc. In the present study Manganese doped ZnGa2O4 phosphors in nano-size dimensions were prepared at a relatively low temperature of 500 °C by solution combustion method. The annealing time duration was maintained as low as few minutes at 500 °C with view to enhance the defect concentration in the host as well as to understand its modification with Mn2 þ doping in the lattice. It will be of interest to investigate luminescence properties of such samples prepared at a relatively low temperature and subjected to annealing for such short time durations. Nature of environment around Mn2 þ in these phosphors has been investigated using electron paramagnetic resonance (EPR) technique. To the best of our knowledge, this is the first time that, investigations are being carried out on such low temperature synthesized ZnGa2O4:Mn2 þ samples, annealed for short durations with relatively high concentrations of Mn2 þ ions.

(1 1 1)

80

80 Difference Calculated Observed Background

4000 2000 0 20 8000

40

60

(c)

80 Difference Calculated Observed Background

4000 0 20

40

60

80

2 /° Fig. 1. Rietveld refined XRD patterns of ZnGa2O4 samples containing (a) 0%, (b) 3% and (c) 5% Mn2 þ ions. The patterns represented in blue and red correspond to experimentally observed and calculated peak profiles, respectively. Green lines represent the background and the black line is the difference between experimentally observed and calculated peak profiles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

K. Somasundaram et al. / Physica B 491 (2016) 79–83

Table 1 Unit-cell parameters obtained from Rietveld refinement of ZnGa2O4 samples containing different amounts of manganese ions. Concentration of Mn2 þ ions (at%)

a (Å)

Average Crystallite Size (nm)

0 3 5

8.329(4) 8.340(2) 8.354(4)

19 25 14

0.007 2+

5%Mn doped ZnGa2O4

Cos

0.006

81

the effective particle size. Depending upon compressive and expansive strains, η, can have negative and positive values respectively. For undoped and Mn2 þ samples, slope is found to be negative confirming that compressive strains are existing in the lattice. ZnGa2O4 sample doped with 5% Mn2 þ shows higher value of compressive strain when compared to undoped sample as can be seen from increased magnitude of slope (  0.007) of former sample compared to latter ( 0.002). Hence based on the above results, it is inferred that Mn2 þ doping in the host is associated with increase in the extent of strain in the lattice. The effective particle size has been calculated from the intercept of the straightline plots and found to be ∼25 nm for undoped and ∼15 nm for 5% Mn2 þ doped samples. 3.2. Scanning electron microscopy (SEM)

0.005

undoped ZnGa2O4

0.004

0.16

0.20

0.24

0.28

0.32

0.36

Sin Fig. 2. Williamson–Hall strain plots for undoped and 5% Mn2 þ doped ZnGa2O4 samples.

One of the possible reason for this could be the increased strain in the lattice brought about by Mn2 þ incorporation and associated difference in the nucleation and growth rate. To check this aspect, nature of strain in the lattice has been evaluated using Williamson–Hall plot for both doped and undoped ZnGa2O4 samples and the results are described below. Fig. 2, shows Williamson Hall plots obtained by plotting βcos θ/ λ versus ηSin θ/λ of Eq. (1), [20].

βCos θ 1 ηSin θ = + λ ϵ λ

(1)

where, θ is the angle of diffraction, β is the full width at half maximum (FWHM) of the diffraction peaks, η is the coefficient of strain associated with the lattice, ε is the effective particle size and λ is the wavelength of X-ray. A plot of β Cos θ versus Sin θ is exλ

λ

pected to give a straight line with slope η , indicating the nature of the strain associated with the lattice and intercept 1 a measure of ε

(a)

The SEM micrographs recorded from different regions of undoped ZnGa2O4 and Mn2 þ doped ZnGa2O4 samples are shown in Fig. 3. (a–c). SEM micrographs of undoped ZnGa2O4 nanophosphors consist of irregular shaped particles with significant extent of aggregation (Fig. 3 (a and b) and their insets). Average particle/ aggregate size for ZnGa2O4 sample is found to be around 100 nm. SEM images confirm that Mn2 þ doped phosphors also exhibit the same morphology as that of undoped sample as can be seen from Fig. 3(b). It is also observed form SEM images that, there exists number of pores of varying dimensions (Fig. 3(c)), as can be seen from the 5% Mn2 þ doped ZnGa2O4 sample. These pores arose due to the evolution of large amount of gaseous by-products during the combustion process involved in sample preparation [11]. In order to evaluate the luminescence properties of these samples, photo luminescence studies were carried out and the results are described below. 3.3. Photoluminescence (PL) studies Emission spectra from undoped ZnGa2O4 sample along with that of Mn doped samples obtained after excitation at 270 nm are shown in Fig. 4. The two broad overlapping emission peaks with maximum around 450 nm and 500 nm can be clearly seen for undoped ZnGa2O4 sample. The emission peak at 450 nm is characteristic of ZnGa2O4 host lattice and is attributed to the self-activated centers originating from the octahedral Ga–O structural units [20,21]. The broad emission band around 500 nm occurs mainly due to recombination of charge carriers from oxygen vacancies present in the lattice [21]. Broad peak indicates that there exists variety of defects with range of energy values. With incorporation of Mn2 þ in ZnGa2O4 lattice, emission gets blue shifted and peak gets narrowed as can be seen from the Figure. The results indicate that certain defect centers in the host gets removed

(b) Fig. 3. SEM micrographs of (a) undoped, (b) 3% and (c) 5% Mn2 þ doped ZnGa2O4 samples.

(c)

82

K. Somasundaram et al. / Physica B 491 (2016) 79–83

Table 2 Lifetime values of undoped and Mn2 þ doped samples. Sample

τ1 (ns)

Undoped ZnGa2O4 (492 nm Em-270 Ex) 3% Mn2 þ doped ZnGa2O4 (420 nm Em-270 Ex) 5% Mn2 þ doped ZnGa2O4(420 nm Em-270 Ex)

1.0 0.86 0.85

Undoped 2+ 3% Mn doped 2+ 5% Mn doped

1.0

Intensity (a.u)

ZnGa2O4 ZnGa2O4:Mn(5%)

10

1

0.8 Intensity

ZnGa2O4:Mn(3%)

100

25

30

35

40

45

Time (ns)

0.6

Fig. 5. Decay corresponding to respective emission from undoped ZnGa2O4, 3% and 5% Mn2 þ doped ZnGa2O4 samples. Emission wavelength was 492 nm for undoped sample and 420 nm for Mn doped samples.

0.4 0.2 0.0 400

450 500 550 600 Wavelength (nm)

650

Fig. 4. Emission spectra of (a) undoped, (b) 3% and (c) 5% Mn2 þ doped ZnGa2O4 samples obtained after excitation at 270 nm.

(nullified) once Mn ions are incorporated in the lattice. Possible explanation for this could be the partial oxidation of Mn2 þ to higher oxidation states like Mn3 þ , Mn4 þ , Mn5 þ etc. Incorporation of such higher valent Mn ions in the lattice (at Zn2 þ or Ga3 þ site) leads to creation of cation vacancies which get nullified with the inherent anion vacancies (generated during the combustion process involved in the sample preparation) in the lattice. This accounts for the significant narrowing of the line shapes observed for Mn doped samples. In both the samples no emission characteristic of Mn2 þ at 506 nm could be observed. It is worthwhile to mention here that Zhitari et al. [22] and López et al. [23], observed emission characteristic of Mn2 þ ions (∼506 nm) from ZnGa2O4:Mn samples with Mn concentration comparable or even higher than that of the present study. However, in those studies, the samples were annealed at relatively higher temperatures (more than 900 °C) for a considerable period of time (say for example 10 h., 24 h., etc). Such high temperature annealing for longer time durations removes most of the defects and quenching centers in the lattice and facilitate emission from Mn2 þ ions. Liu et al. [24] also observed that the green emission from Mn2 þ ions in ZnGa2O4 host increases with increase in annealing temperatures. However, with annealing at higher temperatures, the nanoparticles particles undergo significant aggregation, leading to the formation of bulk samples. In the present study annealing was carried at a relatively low temperature of 500 °C for a duration of five minutes, which may not be sufficient for removing all the defects or quenching centers from the lattice of ZnGa2O4, leading to quenching of excited state of Mn2 þ and this explains the lack of green emission from the samples. Unlike the emission spectra, the lifetime values are found to remain same for both undoped and Mn2 þ doped samples. Decay curves corresponding to the emission from all the three samples are shown in Fig. 5 (Emission

wavelength was 492 nm for undoped sample and 420 nm for Mn doped samples (Table 2)). Lifetime value is found to be ∼1.0 ns for all the samples, suggesting that defect present in the samples are of similar type in both doped and undoped samples. To understand the nature of environment around Mn2 þ ions in the host, EPR studies were carried out on the samples and the results are described below. 3.4. Electron paramagnetic resonance (EPR) studies Local structural information and symmetry of paramagnetic ions incorporated in the host lattice [25–27] can be probed by EPR technique. In the present study, paramagnetic species is Mn2 þ (total electronic spin, S ¼5/2) and its EPR spectrum is very sensitive to local environment [25] around Mn2 þ in the lattice. In addition to this, it has a long spin-lattice relaxation time thereby making its EPR spectrum readily observable over a wide temperature range. In the present investigations, EPR studies were conducted on Mn2 þ doped ZnGa2O4 to understand the occupancy of dopant ions in the host lattice and also the local environment around it. The EPR spectra of 3% and 5% Mn2 þ incorporated ZnGaO4

*

*

*

*

(A)

*

(B)

3000

3250

3500 H (GAUSS)

3750

4000

Fig. 6. EPR spectra recorded at X band frequency at room temperature: (A) 3% Mn2 þ doped ZnGa2O4 sample and (B) 5% Mn2 þ doped ZnGa2O4 sample; * indicate forbidden transition arising due to ΔMS¼ 7 1 and ΔMI ¼ 71.

K. Somasundaram et al. / Physica B 491 (2016) 79–83

samples are shown in Fig. 6. The observed spectrum is due to MS ¼  1/2 to MS ¼ þ1/2 transition of Mn2 þ in tetrahedral environment. The spectrum is explained by Spin Hamiltonian used to represent the spin states of S ¼5/2, I (nuclear spin) ¼5/2 system of 55 Mn2 þ (natural abundance for 55Mn is 100%)

H = βe S.ge .B+D [Sz2−1/3S (S + 1)] +E [Sx2 −S y2⎤⎦ + S.A.I

83

increase in extent of strain as confirmed by XRD studies. Defect emission from ZnGa2O4 lattice decreases significantly by doping high concentrations of Mn2 þ ions. From EPR studies it is concluded Mn2 þ occupy Zn2 þ site in the lattice with higher extent of Mn–Mn interactions.

(2)

Where ge and βe are the Lande's “ge” factor and Bohr magneton respectively. The first term in the Hamiltonian is electron Zeeman term, the second and third terms are zero field spin–spin interaction (for cubic symmetry D ¼0 and E¼ 0, for axial distortion D≠0 and E ¼0, for orthorhombic distortion D≠0 and E≠0), and the fourth term is electron spin-nuclear spin interaction respectively. The six principal lines observed at g∼2.00 are due to MS ¼  1/2 to MS ¼ þ1/2 transitions having MI values in the range of 5/2 to  5/2 with increase in magnetic field. The weaker lines, in between, arise from double spin transitions which are forbidden transitions where both MS and MI change simultaneously (ΔMS ¼ 71and ΔMI ¼ 7 1). Presence of electronic zero field interaction and/or the nuclear quadrupole interaction causes a mixing of spin states, leading to these forbidden transitions becoming weakly allowed. The spinel-type oxides (AB2O4) have a structure described as a densely packed oxygen array with A and B cations in tetrahedral and octahedral sites, respectively. In zinc gallium oxide (ZnGa2O4) lattice, divalent Mn ions occupy tetrahedral sites (A sites) while Ga ions being trivalent, can occupy B sites (octahedral sites). There are numerous reports existing in literature [25–35] on EPR studies of Mn2 þ in tetrahedral and octahedral environment. For example, Mn2 þ present in tetrahedral sites in cubic structure of ZnS showed EPR spectra consisting of gxx ¼2.0064, gyy ¼ 2.0064, gzz ¼2.0066 with D ¼37.4  10  4 cm  1, E¼ 12.5  10  4 cm  1; Axx ¼ 63.9  10  4 cm  1 [28–30]. Small anisotropy existing in the six hyperfine lines indicated that there is a distortion of Td site symmetry around Mn2 þ in cubic ZnS structure. In an another study [31], EPR spectrum of Mn2 þ in face centered cubic structure of ZnAl2O4 exhibited g ¼2.0010, D¼ 170  10  4 cm  1 and E¼ 55  10  4 cm  1 with Aiso ¼84  10  4 cm  1 indicting that there is a moderate distortion for tetrahedral site around Mn2 þ ions in the host. EPR of Mn2 þ doped Ga2O3 showed signals with spin Hamiltonian parameter gx ¼ 2.0140, gy ¼2.0120, gz ¼2.0010, |A⊥| ¼ 82.7  10  4 cm  1, |A║| ¼80.7  10  4 cm  1, D¼ 510  10  4 cm  1, E ¼116  10  4 cm  1 [31–33]. Existence of very large zero field (D and E term) suggested the presence of substantial amounts of distortion for Mn2 þ at Ga3 þ sites. This is due to the mismatch between the sizes of Mn2 þ and Ga3 þ and presence of charge compensating vacancies combined with the low symmetry of monoclinic structure. ZnGa2O4 samples in the present study showed a sextet hyperfine structure with average Aiso∼82 G which is in agreement with all the reported literature for Mn2 þ occupying tetrahedral Zn2 þ sites [34,35]. However, in the present case, absence of fine structure splitting (D and E terms) in the EPR spectra (at X band frequency) is because of the relatively higher concentration of Mn2 þ in the sample and associated Mn–Mn interactions Thus from EPR studies it is confirmed that Mn2 þ ions occupy 2þ Zn site in ZnGa2O4 lattice with significant extent of Mn–Mn interactions.

4. Conclusion Doping of manganese ions in ZnGa2O4 host lattice leads to

Acknowledgment The authors indebted to acknowledge the Board of Research in Nuclear Sciences-DAE (BRNS) F.No: 2012/34/70/BRNS for providing the financial support for this work.

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