Impact of activator incorporation on red emitting rods of ZnGa2O4:Cr3+ phosphor

Materials Science & Engineering C 94 (2019) 1037–1043

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Impact of activator incorporation on red emitting rods of ZnGa2O4:Cr3+ phosphor T.A. Safeeraa, Jacob Johnyb, Sadasivan Shajib, Yung Tang Nienc, E.I. Anilaa,

T

⁎

a

Optoelectronic and Nanomaterial's Research Lab, Department of Physics, U C College, Aluva, India Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico c Department of Materials Science and Engineering, National Formosa University, 64 Wunhwa Rd, Huwei Township, Yunlin County 63201, Taiwan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Cr3+ activator Microrods Quenching Band tailing

Chromium doped zinc gallium oxide (ZnGa2O4:Cr3+) microrods were synthesized by simple solid state reaction method. The transformation on crystal structure and optical properties with molar concentration of Cr3+ were analyzed. The cubic spinel nature of ZnGa2O4:Cr3+phosphor and their crystalline nature were confirmed from xray diffractogram. The average grain size of the samples range between 24 and 29 nm, with lattice parameter values greater than that of bulk. Lattice strain produced in the lattice on doping was estimated from the Williamson–Hall plot. It increases on Cr3+ doping up to 3 mol% and then decreases. Rod like nature of zinc gallate was observed from the surface morphological analysis using SEM. X-ray photoelectron spectroscopy was used for the chemical state identification of the constituent elements in the compound. The photoluminescense spectra consists of various emission lines originated from the chromium ion in the spinel lattice. The purity of red emissions were observed from chromaticity diagram with a concentration quenching initiated from the dipole–dipole interaction, with increase in dopant concentration. Band gap of the samples were estimated using Kubelka-Munk equation which exhibited red shift compared to bulk due to band tailing effect.

1. Introduction Zinc gallium oxide [ZnGa2O4] is a well-known oxide semiconductor having wide band gap, leading to the elevated optoelectronic applications. The role of this spinel oxide in the field of display devices is widespread from earlier times. It is also a good transparent conducting oxide having enhanced photovoltaic applications [1–3]. The improvements over sulfides, makes it suitable for the enormous range of applications. This self-activated blue phosphor can be tuned to a green and red phosphor by doping with suitable activators like Mn2+/Tb3+ and Cr3+/Eu3+ respectively [4–6]. ZnGa2O4:Cr3+ is a good red emitting phosphor, where the Ga3+ ions are replaced by Cr3+ ions as depicted in Fig. 1. This red oxide phosphor is now renowned by its feature of persistent luminescence [7,8]. It is an optical phenomenon whereby, long running emission, mainly in visible range is observed from a material even after the termination of irradiation. The defects present in the host materials are responsible for this phenomenon of afterglow, by trapping the charge carriers. There are several reports on persistent phosphors which are exploited in optoelectronic application [9–12]. However, ZnGa2O4:Cr3+ is a much preferred material to study this phenomenon, due to the well resolved energy levels of Cr3+ and the

⁎

simpler crystal structure [13]. Also there are reports on its better role as biomarker in in vivo imaging techniques, the detection of cancerous cells and drug delivery [14–16]. Various synthesis techniques like solid state reaction [17], sol gel [18], hydrothermal method [19], pulsed laser deposition [20] etc. were used for the synthesis of zinc gallate. Among the various synthesis techniques solid state reaction (SSR) retain its position, by disregarding all the drawbacks, in material synthesis. We are looking for such a common and simple method for the preparation of our phosphor. This is the most widely used method for the synthesis of polycrystalline bulk phosphors by providing large range of selection of starting materials like, oxides, carbonates, etc. SSR allows the solid reactants to react chemically without the presence of any solvent at high temperatures yielding a product which is stable and in more amount than a normal reaction can. The major advantage of SSR method is, that final product in solid form is structurally pure with the desired properties. Since SSR is a solvent free method, there is no waste to remove at the end of the reaction making this an environment friendly, cost less technique. Hence the final products do not require any purification to remove traces of solvent and impurities making the method more economic. Thus SSR has considerable importance in the rapidly emerging field of

Corresponding author. E-mail address: [email protected] (E.I. Anila).

https://doi.org/10.1016/j.msec.2018.10.059 Received 22 October 2017; Received in revised form 15 September 2018; Accepted 15 October 2018 Available online 17 October 2018 0928-4931/ © 2018 Elsevier B.V. All rights reserved.

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Table 1 Variation in structural parameters with dopant concentration. Cr3+ content (mol %)

Grain size (nm)

Strain (×10−3)

Lattice parameter (A°)

1 2 3 4

24 28 25 29

0.12 1.07 6.40 2.16

8.362 8.356 8.34 8.353

spectrometer. Using Varian, Cary 5000 UV–Vis-NIR Spectrophotometer, diffuse reflectance spectra were recorded. Effect of Cr3+ doping on photoluminescence and decay measurements was done using Horiba Flouromax-4C Spectrofluorometer. The dependence of temperature on luminescence emission is verified by using Hitachi F7000 spectrometer. 3. Results and discussions

Fig. 1. (a) Crystal structure and (b) local surrounding of Cr3+ ion in ZnGa2O4:Cr3+.

3.1. Analysis of phase and structural parameters of ZnGa2O4:Cr3+ phosphor

Green Chemistry which has resulted in major changes in the way of material synthesis. Authors have previously reported a comparative study of zinc gallate, with ZnO and Ga2O3 using the same (SSR) synthesis technique [17]. Researchers have used this technique for the synthesis of ZnGa2O4:Cr3+ phosphor also [7,8]. In the present work, we have employed the above method for studying the effect of Cr3+ doping on the structural and optical properties of zinc gallium oxide which are investigated using x-ray diffraction (XRD), scanning electron microscopy (SEM), UV–Vis-NIR absorption spectroscopy, diffuse reflectance spectroscopy (DRS), ray photoelectron spectroscopy (XPS) and photoluminescence (PL).

Fig. 2.a depicts the XRD pattern for the Cr3+ doped zinc gallate. The major reflections are from (220), (311), (511) and (440) planes indicating the spinel cubic phase of synthesized zinc gallate with Fd3m space group, matching with JCPDS 86-0415. It can be seen that there is no prominent change in crystallinity of zinc gallate on doping. This indicates the effective mixing of dopant with the host. The average grain size of the samples were calculated from Scherrer formula,

D=

0.9λ β cos θ

(1)

where D, λ, β and θ denotes the grain size, wavelength of x-rays used, full width at half maximum and glancing angle respectively. From Williamson–Hall (WH) plot, the strain (ξ) produced in the lattice during crystal formation can be evaluated, which is based on the relation 2 [21,22]. The strain values are directly obtained from the slope of the 2sinθ vs βcosθ plot [WH plot] and is depicted in Fig. 2.b.

2. Experimental Micro rods of ZnGa2O4:Cr3+ phosphor were prepared by solid state reaction method by varying the Cr3+ concentration. In our previous paper, the procedure for making pure zinc gallate has been explained in detail [12]. For Cr3+ doping, chromium nitrate [Cr(NO3)3, Sigma] was also added with the initial components before mixing. The final powder, after grinding using agate mortar and pestle was analyzed using various characterization techniques. The structural analysis was done by XRD using Bruker AXS D8 advance x-ray diffractometer. TESCAN VEGA 3 SBH scanning electron microscope was used for the morphological study of phosphor. Elemental composition and chemical states of the synthesized sample was estimated using Thermo Scientific K-ALPHAX-ray photoelectron

β cos θ =

dhkl =

0.9λ + 2ξ sin θ D

(2)

a h2 + k 2 + l 2

(3)

is the relation used for calculating the lattice parameter, a with (hkl) as miller indices. The estimated lattice parameter, strain and grain size values are in Table 1. The average grain size values are found in the range 24–29 nm. The diffraction peaks of the samples show a shift

Fig. 2. (a) x-ray diffraction pattern and (b) WH plot for ZnGa2O4 for different molar concentrations of Cr3+. 1038

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Fig. 3. a, b, c and d represent surface morphology of ZnGa2O4:Cr3+ phosphor with Cr3+ concentrations 1, 2, 3 and 4 mol% respectively.

Fig. 4. (a) XPS survey spectra and fine spectra of Zn2p(b), Ga2p(c), Ga3d(d), O1s(e) & Cr2p(f) of ZnGa2O4:Cr3+with Cr3+ concentration[1–4 denotes 1–4 mol% respectively].

than Ga3+(r = 76 pm). The SEM micrographs of zinc gallate with varying Cr3+ are given in Fig. 3. The porous nature of zinc gallate, exhibited in the pure state [17] is transformed to refined rectangular rod structures on doping. XPS analyses were used for the identification of chemical states and elemental composition of synthesized ZnGa2O4:Cr3+phosphor. Fig. 4.a represents the whole survey spectra which indicate the presence of Zn 2p, Ga 2p, Ga 3d, O 1 s and Cr 2p elemental states, with prominent binding energy peaks. Another noticeable binding energy peak in the spectrum is from the C 1 s (~284.6 eV), which is used for the charge correction [23]. Fig. 4(b)–(d) give the individual high resolution spectra for the observed chemical states of the elements. The peak energy values for every elemental species of ZnGa2O4:Cr3+with increase in Cr3+ concentrations are included in Table 2. The binding energy difference (ΔE) between Zn 2p3/2-Zn 2p1/2 and Ga 2p3/2-Ga 2p1/2 due to spin orbit coupling are in accordance with the reference values of 22.97 and 26.84 eV respectively, which corresponds to the bonding of Zn2+ and Ga3+ with O2−, rather than formation of metallic clusters [23–25]. Likewise the energy separation between Zn 2p3/2 and Ga 2p3/2 values (~96 eV) implies the formation of zinc gallate spinel [26]. There is another peak around 20 eV which is attributed to the 3d state of Ga3+

Table 2 Effect of Cr3+ incorporation on binding energy of ZnGa2O4:Cr3+ phosphor. Binding energy (eV)

Chemical state Zn2p3/2 Zn2p1/2 Ga 2p3/2 Ga 2p1/2 Ga3d O1s Cr2p3/2 Cr 2p1/2

1%

2%

3%

4%

1021.55 1044.18 1116.74 1143.61 19.87 530.48 576.20 585.87

1021.34 1043.98 1116.74 1143.59 19.81 530.33 575.89 585.44

1021.68 1044.40 1117.95 1144.82 20.09 530.69 576.24 586.02

1021.74 1044.48 1118.02 1144.88 20.09 530.7 576.35 586.29

22.64 26.85

22.72 26.87

22.73 26.86

Energy separation, ΔE (eV) Zn 2p (3/2–1/2) 22.63 Ga 2p (3/2–1/2) 26.88

towards left compared to bulk (JCPDS) and hence the lattice parameter values of all samples are greater than that of bulk zinc gallate [8.335A°]. This is due to the higher ionic radius of Cr3+(r = 114 pm)

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650 to 750 nm, on UV excitation of 410 nm. The formation of new octahedral coordination system by replacing the Ga3+ ions by Cr3+ give rise to this peculiar emission behavior of Cr3+ ion, exhibiting a spectra with several narrow peaks. These emission lines were originated from the transition between the levels 2E(2G) ➔ 4A2(4F) of Cr3+ [7,8,14,31,32]. The spectra can be resolved into zero phonon lines (ZPL) and Stokes (S)/anti-Stokes (AS) phonon side bands (PSB). ZPL is represented by the line at 686 nm (R line), originated from the undistorted octahedral field of Cr3+ at Ga3+ center [32,33]. When the Cr3+ ion is neighboring with an antisite defect, the octahedral field gets distorted and an intense peak at 693 nm (N2 line) is attained. These defects are introduced in the Cr3+ field, by the inversion between Zn2+ and Ga3+ within the spinel structure of zinc gallate i.e. Zn2+and Ga3+ occupies the octahedral and tetrahedral sites, contrary to the original spinel nature. They are represented as [ZnGa]− and [GaZn]+. The PSB accompanies these ZPL, with anti-Stokes lines at 668 and 677 nm and Stokes at 705, 710 and 742 nm, derived from the transitions involving different vibrational energy levels [7,8,14,32,33]. The energy band diagram representing the various energy levels of ZnGa2O4:Cr3+ phosphor is depicted in Fig. 7.a. Only the energy levels of Cr3+, which are taking part in the energy transfer process is portrayed in the diagram. In the case of Cr3+ in ideal octahedral site of zinc gallate, 4A2 is the ground state and the other levels are of excited state. On excitation with an energy 3.03 eV, electrons from the ground level move to the 4T1 band from where, non-radiative transition to 2E band occurs followed by the visible emission, R line [Fig. 7.a]. As mentioned earlier, the R line together with phonon side bands were originated from the Cr3+ions in the zinc gallate spinel lattice. In perturbed condition (⁎Cr3+), i.e. in the presence of antisite defects, N2 line with a lower energy is evolved from the transition between the same bands. Fig. 7.b represents the Commission Internationale de l'Eclairage (CIE) chromaticity diagram for the phosphor. The corresponding coordinates values are in Table 2. The purity of red emission for the sample with 3% molar concentration is clearly observable from the diagram. Its CIE values are comparable to the x = 0.65and y = 0.35, CRT coordinates of red color. It also visually illustrates the quenching process accurately. The correlation of molar concentration of Cr3+ with intensity of emissions for different peaks is portrayed in Fig. 8(a). From the plot it is evident that, for lower amount of Cr3+ the intensity is short due to the insufficient number of luminescent centers. But as the concentration rises, emission intensity increases upto 3% and then decreases. This diminishing nature is known as concentration quenching and it is the outcome of a decrease in Cr-Cr distance with doping. As the distance reduces, non-radiative energy migration within the dopant ion gets activated [32]. The critical distance (RC) for energy transfer between the activator ions can be estimated using the relation based on the Dexter theory as follows:

Fig. 5. PL Excitation spectra of ZnGa2O4:Cr3+ with molar concentration of Cr3+.

Fig. 6. Effect of molar concentration of Cr3+ on photoluminescence of ZnGa2O4:Cr3+ phosphor [#-anti-Stokes band, *-Stokes band].

[27]. The presence of O2– ion in zinc gallate was verified from the energy peak values around 530 eV [28]. The fine spectra of Cr 2p shown in Fig. 4(f), with peaks representing 2p3/2 and 2p1/2 levels, signifies the octahedral site occupation of chromium ions with an oxidation state of 3+ [29,30]. The binding energies of each element exhibit almost a blue shift with Cr3+ concentration, which may be due to the lattice distortion introduced with dopant concentration. 3+

3.2. Evaluation of optical features of ZnGa2O4:Cr

1

3V ⎤3 Rc ≈ 2 ⎡ ⎢ 4 X ⎣ cπ Z ⎥ ⎦

(4)

where V, Xc and Z respectively represents the volume of unit cell, critical concentration of activator ion and number of host cations. In the present case V = 579.89 Å3, Z = 5 and Xc = 0.03. Thus the value of RC between the Cr3+ ions for the effective energy transfer is approximately 20 Å. The electric multipolar interaction which motivates the transfer of energy between the sensitizers, or sensitizer-activator ions can be calculated using the relation formulated by Dexter and Van Uitert. It is an expression connecting emission intensity (I) and concentration of activator ion beyond optimal value(x), which is as follows:

phosphor

Photoluminescence excitation spectra of ZnGa2O4:Cr3+ rods for an emission wavelength of 693 nm are portrayed in Fig. 5. There exist two broad excitations centered on 410 and 550 nm, which are attributed to the electronic transitions of Cr3+ such as 4A2 (4F) ➔ 4T1 (4F); 410 nm and 4A2 (4F) ➔ 4T2 (4F); 550 nm. The spin-orbit coupling at Ga3+ center together with trigonal distortion is the reason for this particular double hump excitation spectra [14]. Fig. 6 shows the photoluminescence spectra of the ZnGa2O4:Cr3+phosphor, exhibiting a broad emission extending from

Q −1 I = k ⎡1 + β [x ] 3 ⎤ ⎣ ⎦ x

1040

(5)

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Fig. 7. (a) Energy band diagram and (b) CIE chromaticity diagram of ZnGa2O4:Cr3+ microrods [1–4 represents the 1–4 mol% of Cr3+].

Fig. 8. (a) Variation in PL intensity with dopant concentration and (b) log (x) Vs log (I/x) graph [for N2 line] of ZnGa2O4:Cr3+.

Fig. 9. (a) Temperature dependent PL spectra and (b) life time decay curve for ZnGa2O4: Cr3+ for the optimum concentration (3 mol%).

only and hence an approximate result can be obtained from the slope = −Q/3 = −2.076. Therefore Q = 6, indicates a tentative conclusion that dipole-dipole interaction is responsible for the non-radiative energy transfer among Cr3+ ions in ZnGa2O4:Cr3+phosphor, which induces the concentration quenching in the phosphor [Fig. 8.b]. Fig. 9.a represents the temperature-dependent PL intensity of

where Q represents the multipolar interaction and it is 6, 8 and 10 respectively for dipole-dipole, dipole-quadrupole and quadrupolequadrupole interaction. K and β are constants of the host lattice, for a given interaction [34–36]. From the slope of log(x) vs log (I/x) graph, which is a linear graph having negative slope, the type of interaction can be estimated. In the present case the plot is consists of two points 1041

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Fig. 10. (a) Diffuse reflectance spectra and (b) Kubelka–Munk plot for Cr3+ doped ZnGa2O4.

CIE coordinates

bulk zinc gallate [4.4 eV], which is ascribed to the band tailing effect produced as a consequence of sizeable number of defects in the samples, caused by activator incorporation.

x

y

4. Conclusions

0.59 0.54 0.61 0.57

0.41 0.46 0.39 0.43

Table 3 Variation in optical parameters with molar concentration of Cr3+. Molar concentrations of Cr3+ (mol%)

1 2 3 4

Band gap (eV)

4.11 3.96 4.01 3.92

Chromium doped ZnGa2O4 rods were synthesized by simple solid state reaction method and the effect of doping were analyzed in detail. From the detailed assessment, the optimum Cr3+ concentration is found to be 3 mol% The bright red luminescence emission of ZnGa2O4:Cr3+ phosphor observed from the photoluminescence spectra with noticeable quenching effect was confirmed from chromaticity diagram. When the temperature is raised to 150 °C, the phosphor present an intensity degradation of 40%. The efficiency in red emission of the synthesized ZnGa2O4:Cr3+microrods leads them to have appreciable applications in the optoelectronic and biomedical fields.

ZnGa2O4:Cr3+ (3 mol%) phosphor powder measured from 45 °C to 150 °C. The inset presents the PL spectra at various temperatures. Note that the PL intensity was integrated from 650 nm to 750 nm. The ZnGa2O4:Cr3+ phosphor powder present an intensity degradation of approximately 40% as heated to 150 °C, which is comparable to commercial YAG:Ce phosphors (~70%). The red emission decay curve for ZnGa2O4:Cr3+ for an excitation at 390 nm, with 3 mol% of Cr3+ is portrayed in Fig. 9.b. The curve shows fast decay in the beginning and then slowed. In our case the decay curves can be well fitted by a triple exponential equation having 3 decay constants, which decide the rate of exponential decay. The average life time for the optimum sample is about 123 ns with decay constants τ1 = 27 ns, τ2 = 3 ns and τ3 = 60 ns. The diffuse reflectance spectra with varying Cr3+ concentration is shown in Fig. 10.a. There are two absorption bands in the spectra around 413 nm and 557 nm, which are originated from the 4A2 (4F) ➔ 4T1 (4F) and 4A2 (4F) ➔ 4T2 (4F) electronic transitions of Cr3+ respectively, as observed from the PLE spectra. In the case of 2% Cr3+ doping, there is an absorption peak at 360 nm which can be attributed to the O2– ➔ Cr6+ d-d impurity transition [32,33]. In the visible region of electromagnetic spectrum, as doping concentration increases, the intensity of these d-d transitions enhances consistently. But no such an ordered absorption is observable in the UV region. Using diffuse reflectance values (R), we can estimate the band gap values using Kubelka-Munk equation [37,38],

k (1 − R)2 = s 2R

Acknowledgements EIA thanks Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for financial support under EMR scheme. T A Safeera thanks University Grants Commission (UGC) for Maulana Azad fellowship. References [1] Xiao Meng Chen, Guang Tao Fei, Jian Yan, Yan Quing Zhu, Li De Zhang, Synthesis of ZnGa2O4 Hierarchical Nanostructure by Au Catalysts Induced Thermal Evaporation, 5 Springer, 2010, pp. 1387–1392. [2] L.E. Shea, Low-voltage cathodoluminescent phosphors, Electrochem. Soc. Interface 14 (1998) 24–27. [3] J.S. Kim, H.L. Park, C.M. Chon, H.S. Moon, T.W. Kim, The origin of emission color of reduced and oxidized ZnGa2O4 phosphors, Solid State Commun. 129 (2004) 163–167. [4] Zhihua Xu, Yongxiang Li, Zhifu Liu, Dong Wang, UV and x-ray excited luminescence of Tb3+doped ZnGa2O4 phosphors, J. Alloys Compd. 391 (2005) 202–205. [5] J.S. Kim, T.W. Kim, H.L. Park, Y.G. Kim, S.K. Chang, S.D. Han, Energy transfer among three luminescent centers in full-color emitting ZnGa2O4:Mn2+, Cr3+ phosphors, Solid State Commun. 131 (2004) 493–497. [6] D. Philip, Rack, Eu3+ and Cr3+ doping for red cathodoluminescence in ZnGa2O4, J. Mater. Res. 16 (2001) 1429–1433. [7] Aurelie Bessiere, Sylvaine Jacquart, Kaustubh Priolkar, Aurelie Lecointre, Bruno Viana, Didier Gourier, ZnGa2O4:Cr3+: a new red long-lasting phosphor with high brightness, Opt. Express 19 (2011) 10131–10137. [8] Aurelie Bessiere, Suchinder K. Sharma, Neelima Basavaraju, Kaustubh R. Priolkar, Laurent Binet, Bruno Viana, Adrie J.J. Bos, Thomas Maldiney, Cyrille Richard, Daniel Scherman, Didier Gourier, Storage of visible light for long-lasting phosphorescence in chromium-doped zinc gallate, Chem. Mater. 26 (2014) 1365–−1373. [9] Yahong Jin, Yihua Hu, Haoyi Wu, He Duan, Li Chen, Yinrong Fu, Guifang Ju,

(6)

where k and s respectively represents the absorption and scattering coefficients. By extrapolating the linear portion of Kubelka–Munk plot i.e. hʋ vs [(k/s)hʋ]2 graph to the x axis [Fig. 10.b], we can find out the corresponding band gap values and here the values are tabulated in Table 3. All the samples show a decrease in band gap compared with the 1042

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