Photoluminescence and photometric studies of low temperature prepared red emitting MgAl2O4:Cr3þ nanophosphors for solid state displays

It was found that the average CCT was found to be 2109 K ( <5000 K), indicating that the phosphor can be highly useful for the production of arti ﬁcial white light in white LEDs and i[r]

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Original Article

Photoluminescence and photometric studies of low temperature

prepared red emitting MgAl2O4:Cr3ỵ nanophosphors for solid state

displays

C Pratapkumara, S.C Prashanthaa,*, H Nagabhushanab,**, D.M Jnaneshwarac aResearch Center, Department of Physics, East West Institute of Technology, VTU, Bengaluru, 560091, India

bProf CNR Rao Center for Advanced Materials, Tumkur University, Tumkur, 572103, India cDepartment of Physics, SJB Institute of Technology, VTU, Bengaluru, 560060, India

a r t i c l e i n f o

Article history: Received 19 June 2018 Received in revised form 30 August 2018

Accepted September 2018 Available online 13 September 2018

Keywords: MgAl2O4:Cr3ỵ Nanophosphor Diffused reﬂectance Photoluminescence Racah parameters

a b s t r a c t

MgAl2O4:Cr3ỵ(0.01e0.11 mol) doped nanocrystalline powders were prepared by a combustion method at low temperature The Scherer's method was used to estimate crystallite size and values were found to be in the range 20e30 nm and the same was conﬁrmed by transmission electron microscopy (TEM) images The energy band gap of the prepared samples was estimated from diffusion reﬂection spectra (DRS) and found to be 3.7e4.5 eV The characteristic emission peaks recorded at 17123.3 cm1and 14727.5 cm1were attributed to the transitions4T

2/4A2and2E/4A2respectively upon 357 nm exci-tation The emission intensity increases up to mol % Cr3ỵparticles and then decreases, which may be due to concentration quenching The CIE (Commission International de I'E' clairage) chromaticity co-ordinates were calculated from emission spectra They were close to the NTSC (National television standard committee) standard value of red emission The average correlated color temperature (CCT) was found to be 2109 K Therefore, the MgAl2O4:Cr3ỵphosphor is useful for laser and solid state display applications

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Materials having inverse spinel with AB2O4structure with the

space group (Fd3m) were effectively used as a host for glow phosphors when doped with transition-metal/rare-earth ions[1,2] Owing to their special physical, chemical and thermal stability they are quite useful in many applications namely oxide fuel cell, sen-sors, microwave dielectric material, X-ray imaging, solid state dis-plays, environmental monitoring, etc[3] In this type, magnesium aluminate (MgAl2O4) includes cubic oxygen and cations that ingest

octahedral or tetrahedral destinations, in which magnesium and aluminum cations occupy 1/8th of the tetrahedral lattices and 1½ of the octahedral sites, with cation to anion ratio 3:4, so that A2ỵ ions and 16 B3ỵions capture fully tetrahedral and octahedral sites,

respectively, or A2ỵions B3ỵions must be placed in 16 octahedral and the second half in the tetrahedral sites[4,5] MgAl2O4

(MP-2135C) based phosphors have attracted great attention of lumi-nescent researchers due to their excellent chemical and physical stability[6,7]

In recent years, nanotechnology has been developed as an interdisciplinaryfield of research It found wide range applications in variousfields[8e10] Especially, much attention has been paid for the facile development of efficient luminescent phosphors (excited by blue or near UV light) with high brightness, long lasting, sensitive, energy saving, environmentally friendliness which are of great in-terest for material scientists, for example to replace conventional uorescent lamps[11,12] Recently, Cr3ỵactivated nanophosphors such as MgAl2O4:Cr3ỵ [5,12e16] LiAl5O8:Cr3ỵ [17],

Zn3Al2Ge3O12:Cr3ỵ [18], ZnO:Cr3ỵ [19], ZnAl2O4:Cr3ỵ [13,20],

Ba2Mg(BO3)2:Cr3ỵ [21], Ca3Ga2Ge3O12: Cr3ỵ [22], BaZrSi3O9:Cr3ỵ

[23], BaAl2O4:Cr3ỵ[24], Mg2SiO4:Cr3ỵ [25]etc were used for the

development of tunable lasers, displays and bio imaging because its 3d3 electronic conﬁguration is easily affected by the crystal ﬁeld environment of ligand anions in various materials

* Corresponding author ** Corresponding author

E-mail addresses:scphysics@gmail.com(S.C Prashantha),bhushanvlc@gmail com(H Nagabhushana)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.09.002

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MgAl2O4requires high temperature to form pure phase as we

discussed in our previous paper [26] However it is remained challenging for the synthesis of MgAl2O4at low temperatures in its

pristine or doped form In the present investigation, MgAl2O4:Cr3ỵ

(1e11 mol %) was prepared by solution combustion process at very low temperature (280C), which is simple, fast and safe We re-ported the preparation of MgAl2O4:Dy3ỵ(1e11 mol %) earlier[26]

at the same temperature Also, attention is paid to the inuence of Cr3ỵdoping on photoluminescence properties

2 Experimental

Magnesium nitrate (Mg(NO3)2: 99.99%, Sigma Aldrich),

aluminium nitrate (Al(NO3)3: 99.99%, Sigma Aldrich) and

chro-mium nitrate (Cr(NO3)3: 99.9%, Sigma Aldrich) and lab made oxalyl

dihydrazide (ODH) were used as oxidizers and fuel, respectively, for the preparation of MgAl2O4:Cr3ỵ(1e11 mol %) nanophosphors All

the starting materials for were taken in stoichiometric ratios [F/ O¼ 1] with minimum quantity of double ionized water mixed well in a petri dish using magnetic stirrer and the dish containing a homogeneous mixture was placed in a pre-heated mufﬂe furnace The stoichiometry of the redox mixture used for the combustion synthesis was calculated based on the total oxidizing and reducing valencies of the compounds The solution initially boiled and ig-nites at one spot with liberation of large amount of gaseous prod-ucts (oxides of nitrogen and carbon) The combustion propagated throughout the reaction mixture with the formation of white powder Similar procedure was used for other doping concentra-tions, and the detailed synthesis procedure has been reported elsewhere [27,28] The balanced chemical equation for the MgAl2O4:Cr3ỵ(1 mol %) is given as follows

990 Mg(NO3)2ỵ 20 Al(NO3)3ỵ 1234C2H6N4O2ỵ 10 Cr(NO3)3ẳ 10

Mg0.99Al2O4:Cr0.01ỵ 3503 N2ỵ 3702H2Oỵ 2468 CO2

The structural parameters of Cr3ỵdoped MgAl2O4nanoparticles

were examined by X-ray diffractometer (XRD) (Shimadzu, oper-ating at 50 kV and 20 mA by with CuKaradiation at a wavelength of 1.541Å) The lattice parameters were estimated with the Rietveld analysis using the FULLPROF software package and the packing diagram was drawn by using diamond software The structure, crystallinity and morphology were performed with Hitachi table top (SEM) (Model TM 3000) (accelerating voltage up to 20 kV using Tungstenﬁlament) and JEOL (TEM), JEM-2100 (accelerating voltage up to 200 kV, LaB6ﬁlament) equipped with EDS having 1.5 Å

res-olutions respectively The diffuse reﬂectance spectra were recorded using UV-VIS spectrophotometer (Shimadzu UV-2600) using BaSO4

as a reference The room temperature photoluminescence (PL) spectra were recorded on Horiba (model Fluorolog-3) Spectro-ﬂourimeter at RT with Fluor Essence™ software and supplied with aﬂash 450 W Xenon lamp as excitation source

3 Results and discussion

Powder X-ray diffraction (PXRD) patterns of the MgAl2O4doped

with Cr3ỵ(1e11 mol %) is shown inFig Diffraction peaks were well indexed to pure cubic phase (JCPDS card No 21-1152) with space group Fd3m (O7h, No 227) The acceptable percentage dif-ference in ionic radii[26]was found to be 15.3% (<30%) conﬁrming the effective doping of Cr3ỵions at Mg2ỵsites in the host lattice rather than the interstitial sites The lattice parameters and unit cell volume for cubic MgAl2O4were estimated using the following

re-lations for (311) plane and values were found to be 8.0676 Å and 525.089Å3, respectively The average crystallite size (D) in the

range 10e20 nm as estimated using Scherer's method[26]

2dSinq¼ nl (1)

dhkl¼ a ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2ỵ k2ỵ l2

p (2)

Vẳ a3 (3)

D¼bKl

cosq (4)

The observed enhancement in the intensity of diffraction peaks indicates that incorporation of Cr3ỵions induces an improvement in the crystallinity of the sample It also causes expansion of the unit cell (the d value becomes larger) volume indicating that the crystal structure of the host matrix is not inﬂuenced but certainly modiﬁed the lattice parameters (Fig 2) This results in the contin-uous shift of PXRD peaks towards lower angle side as shown in

Fig [18,29]

FULLPROF program and ThomsoneCoxeHasting Pseudo-voigt function[30]was used tofit the several parameters, the Rietveld refinement with packing diagram was shown in Fig GOF (Goodness of Fitting) was found to be ~0.96, which confirms the goodfitting[31]and all the isostructural refined parameters were consolidated inTable

Fig depicts the FTIR spectra of undoped and Cr3ỵ doped MgAl2O4:Cr3ỵ nanophosphors recorded in the range

400e4000 cm1 The peaks at 440, 530, 670 and 850 cm1were

attributed to MgO6octahedral, AlO6octahedral (n4), metaleoxygen

MgeO and AleO (n3, stretching) respectively; where n3;

asym-metric stretching andn4; asymmetric deformation vibrations[32]

in an octahedral coordination state (AlO6 octahedral units) The

peaks at ~3350 and 1700 cm1were attributed to OeH stretching vibration and HeOeH bending vibration, respectively The bands between 1400 and 1600 cm1 were attributed to C¼O, eC¼Ce stretching modes The peak at ~2300 cm1was due to absorption of atmospheric CO2on the metallic cations[33e35]

Fig shows the diffuse reectance spectra (DRS) of MgAl2O4:Cr3ỵ(1e11 mol %) The splitting of the energy levels of

Cr3ỵwas explained by TanabeeSugano energy level (Fig 6) dia-gram[18] The broad absorption bands at ~375 and 556 nm arise from the spin-allowed 3de3d transitions of Cr3ỵ ions, due to

4A

2g(F)/4T1g(F) and 4A2g(F)/4T2g(F) transitions, respectively

The optical band gap energy (Eg) was calculated by Kubelka and

Munk [K-M] relation[36]

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FRịhnẳ Ahne Eg n

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where n is the nature of the sample transition (n¼ 1/2, 2, 3/2, 3; direct allowed, indirect allowed, direct forbidden, indirect forbidden transitions, respectively), Rẳ RRstandardsample and FRị was the KubelkaMunk function, (where Rstd0.973e0.988 for BaSO4) as

discussed elsewhere (Fig 5)[37],

FRị ẳ1 Rị

2R∞ ¼

K

S (6)

wheren: photon energy; C: proportionality constant; h: Planck's constant; k: the molar absorption coefﬁcient, and s: the scattering coefﬁcient

From the extrapolation of the line [F (R) hy]ẵ ẳ (inset of

Fig 5), the energy bandgap calculated from the KeM function was found to vary from 4.5 to 3.7 eV with the doping of Cr3ỵions The decrease in band gap energy (Eg) values with chromium content

may be due to the increase of carrier concentrations or attributed to the band gap renormalization effect in host lattice by the Cr3ỵions Also, it may be due to the inuence of [Cr-O] clusters on the elec-tronic structure of the host materials or the formation of non-bridging oxygen's (NBO) with doping The similar effect was observed earlier by others[38,39]

The Racah parameter, B, was computed to show the impacts of electroneelectron repulsion inside the metal complexes and used to appraise the bond strength between the ligand to metal[13] B can be estimated by measuringDE, the energy difference between the4T1 and the4T2energy states (DE¼ 8577 cm1), that

corre-sponds to energy difference betweeng1(¼ 376 nm; 26595 cm1)

andg2(¼ 555 nm; 18018 cm1) was calculated

Dq¼ 101 g1 (7)

B Dq¼

"D E Dq

2 10

D E Dq

# 15

D E Dq

1

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Cz8:051

E2Eị 7:9 B ỵ1:8 BDq2

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The Racah parameter B was experimentally determined to be 811.21 cm1in the present case and C is nearly¼ of B The ratio of Dq/B deﬁnes the criterion for the evaluation of the crystal eld that surrounds Cr3ỵin the host lattice and was calculated to be 3.27 According to the Tanabe and Sugano diagram, for Cr3ỵions (3d3), when Dq/B> 2.3, Cr3ỵsituates in a strong crystalﬁeld and

gener-ates sharp emissions[13], since emission mainly depends on the crystalﬁeld strength (Dq)

Assessment of Racah parameter in free ion and in complex gives the strength of Nephelauxetic ratio and can be used to predict the nature of bonding between Cr3ỵions and the ligands The B value for free ion was 1030 cm1, whereas in the present case it is 811.21 cm1 B decreased by 21.26% from the free ion value and this reduction was caused by the bond effect, conrming that Cr3ỵin a strong crystalﬁeld exhibited a narrow band emission at 697 nm, which was dominantly attributed to the2E/4A

2transition

h¼

Bfree B

Bfree

k (10)

where h and k (0.21)[18]are nephelauxetic parameters of the li-gands and the central metal ion, respectively, the values of h was found to be 1.01 (>1), indicating the increased delocalization of the d-electrons and it shows a predominant ionic nature of bonding between Cr3ỵions and the ligands

TEM analysis shows almost spherical shaped polycrystalline particles (Fig 7) with size in the range 15e30 nm and was conﬁrmed by both SAED and histogram The high degree of crys-tallinity was proved by well deﬁned lattice fringe patterns with fringe width ~0.5 nm for (111) plane, which is very close to the calculated value of 0.4953 nm[40]

Excitation spectrum of MgAl2O4:Cr3ỵ(5 mol %) nanophosphor

recorded at RT is shown inFig (inset), the peaks at 357 and 413 nm corresponding to 4A2(4F)/4T1(4P) and 4A2(4F)/4T2(4F)

spin allowed transitions of Cr3ỵions and suggested that Cr3ỵwas in octahedral symmetry [16] PL spectra upon 357 nm excitation (Fig 8) present two emission bands centered at 584 nm

Fig Schematic diagram of cell expansion when Cr3ỵions doped in MgAl2O4

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(17123.3 cm1) due to 3d-3d inner transitions of Cr3ỵ from its excited state of4T2/4A2and located in green region and a sharp,

high intensity R band at 679 nm (14727.5 cm1), ascribed to the

2E/4A

2hypersensitive forced electric dipole transition of isolated

single Cr3ỵions in the inversion octahedron (Mg (I) site) and similar emission characteristics was observed in Mg2SiO4:Cr3ỵ [25], all

possible transitions of Cr3ỵ are shown in energy level diagram (Fig 9) MgAl2O4is a normal spinel belonging to the Oh7 group

with a tetrahedral coordination for the Mg2ỵion at 8a positions and a trigonal distorted octahedron (D3dsymmetry) surrounding the

Al3ỵsite at 16d positions and the octahedral Cr3ỵions are being doped the trigonally distorted CrO6 octahedron which is

sur-rounded by Al3ỵions and by Mg2ỵions[41] The weaker lines called N-lines near the R band may be due to the closely coupled pair of Cr3ỵions The R band belongs to regular 16d site of the spinel structure, however the N lines may be due to the Cr3ỵions per-turbed by inversion between Mg2ỵand Al3ỵwithin therst two coordination spheres[42] The present phosphor can be a candidate for solid state display applications, since it was effectively excited by NUV radiations

It is clear that the emission intensity is inuenced by Cr3ỵ concentration upto mol % It increases and reaches the saturation After that the luminescence intensity reduces due to concentration quenching, which occurs due to various defects When Cr3ỵ is doped in MgAl2O4host by substituting Mg2ỵsites, there are many

oxygenO vacancies (Vo), single ionized Mg interstitial (Mg, i) and

ionized Mg (V0Mg) defects created in MgAl2O4 To keep the

elec-troneutrality for the substitution of Cr3ỵon Mg2ỵ, a cation vacancy

Table

Rietveld reﬁned structural parameters for MgAl2O4:Cr3ỵ(1e11 mol %) nanophosphor Compounds MgAl2O4 MgAl2O4:Cr3ỵ

1 mol %

MgAl2O4:Cr3ỵ mol %

MgAl2O4:Cr3ỵ 11 mol % Crystal system Cubic

Space group F d -3 m Hall symbol -F 4vw 2vw Lattice parameters (Å)

a¼ b ¼ c 8.0676 8.0689 8.0672 8.0623 8.0632 8.0703 8.0717

a¼b¼g 90

Unit cell volume (Å3) 525.095 525.33 525.018 524.059 524.225 525.614 525.886

RP 4.24 5.94 4.81 4.15 4.13 3.92 3.32

RWP 5.14 7.62 7.32 6.59 6.48 5.82 4.74

RExp 5.64 8.27 7.61 7.04 6.71 6.45 6.17

c2 0.832 0.85 0.924 0.876 0.934 0.813 0.590

GoF 0.96 0.95 0.96 0.93 0.96 0.94 0.95

RBragg 1.85 7.66 5.59 5.46 6.52 7.2 13.7

RF 1.86 5.95 5.63 4.69 4.49 5.12 15.4

X-ray density (g/cc3) 3.820 5.197 5.327 5.897 5.531 5.317 5.129

Fig FT-IR spectra of undoped and Cr3ỵdoped MgAl2O4nanophosphors

Fig DRS patterns of as formed MgAl2O4:Cr3ỵ(1e11 mol %) nanophosphors (inset: Optical energy gap analysis)

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(a) (b)

(c)

(d)

Fig a) TEM image, b) SAED, c) HRTEM spectra and d) histogram of MgAl2O4

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defect (V0Mg) with negative charge will compensate the induced

positive defects of Cr,Mg Moreover, the negative V0Mg and single ionized Mg interstitial (Mg,i) would form one Mg vacancy (VMg)

The whole process can be expressed as:

3Mg2ỵ/2Cr3ỵỵ 2Cr, Mgỵ 2V

0 Mg

V0Mgỵ Mg,i/VMg

The electrons in the defect transited to the ground state under the action of thermal disturbances and showed red emission The number of resultant defects is larger than that ofﬁlled defects when the concentration is low However, with the increase of Cr3ỵions, Mg,i and V0Mgdefects will be rapidly decrease due to the decreased

Mg2ỵions, leading to decrease in the numbers of defects Hence, the intensity of red emission will decrease with the increment of doping concentration[43] Also, Van Uitert theory was used toﬁnd out the type of interaction involved in energy transfer [25], the graph of log (I/x) vs log x was shown inFig 10 The Q value ob-tained by the linear ﬁtting is ~3, indicating that the exchange interaction was the major mechanism for the concentration quenching in MgAl2O4:Cr3ỵphosphor

The perception of the emission color from MgAl2O4:Cr3ỵ

nanostructures were analyzed with the help of Commission Inter-nationale de l’Eclairage (CIE) 1931 chromaticity coordinate diagram

[26] The CIE chromaticity coordinates of MgAl2O4:Cr3ỵ

nano-structures were evaluated using the tristimulus colorimetry system (X, Y, Z) from the emission spectra excited at 357 nm and found to

lie in red region of CIE spectra (Fig 11), and correlated color tem-perature (CCT) was estimated by CIE coordinates as discussed elsewhere[25,26] It was found that the average CCT was found to be 2109 K (<5000 K), indicating that the phosphor can be highly useful for the production of artiﬁcial white light in white LEDs and in solid state display applications

4 Conclusion

The pure cubic Cr3ỵ (1e11 mol %) doped polycrystalline MgAl2O4red-emitting nanophosphors with an energy band gap of

4 eV were prepared by the combustion technique at very low temperature (280C) using ODH as a fuel The crystalﬁeld splitting (Dq) and Racah parameters (B and C) have been estimated The sharp red emission at 14347.2 cm1is due to2E/4A

2transition of

Cr3ỵions and by the ratio Dq/B (3.27) It was conﬁrmed that Cr3ỵ ions occupy the strong ligand eld sites The CIE coordinates (x¼ 0.633, y ¼ 0.366) lie in the red region and CCT was found to be 2109 K for the optimized phosphor The above results show the present sample can be used as a promising material for lasers, optical displays and for WLEDs

Acknowledgements

The authors CP, SCP and HN thanks to VGST, Govt of Karnataka, India, (VGST/K-FIST-L1/2017-18/GRD-360) and (VGST/K-FIST-L1/ 2017-18/GRD-489) for sanctioning the research project

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