Near uv light excitable sral2o4 eu3þ nanophosphors for display device applications

The color purity CP of the synthesized NPs has been calculated and the value is found to be 88%, which indicates that the prepared phosphor powders are highly useful for the red componen

Original Article

device applications

K.R Ashwinia,b, H.B Premkumarc,*, G.P Darshand, R.B Basavaraja, H Nagabhushanae,

a Department of Physics, BMS Institute of Technology and Management, Bangalore, 560064, India

b Visvesvaraya Technological University, RRC, Bangalore, 560091, India

c Department of Physics, FMPS, MS Ramaiah University of Applied Sciences, Bangalore, 560 054, India

d Department of Physics, Acharya Institute of Graduate Studies, Bangalore, 560 107, India

e Prof C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumkur, 572 103, India

a r t i c l e i n f o

Article history:

Received 12 January 2020

Received in revised form

9 February 2020

Accepted 16 February 2020

Available online xxx

Keywords:

Nanopowders

Rare earth

Combustion method

Photoluminescence

Color purity

a b s t r a c t

Near UV-light excitable inorganic phosphors are widely recommended for white-light emitting diode (wLED) applications Here in this paper we prepared Eu3þdoped SrAl2O4phosphors with various con-centrations of Eu3þ(1e5 mol%) by a green combustion technique The phase structure and purity of all prepared nanopowders (NPs) were studied by X-ray diffraction analysis The prepared NPs showed a fibre-like morphology which is confirmed by scanning electron microscopy results The particle size as obtained by the transmission electron microscopy results is found to be around 40 nm Further, the elemental composition showed the presence of doped Eu3þ ions into the SrAl2O4host The photo-luminescence (PL) spectra of Eu3þions doped SrAl2O4compounds are studied in detail From the PL studies it is observed that the intense peak at 618 nm is due to the5D0/7F2transition of Eu3þions under

393 nm (near UV-light) excitation wavelength The emission color and the chromaticity values of the NPs are studied, which suggest a strong red emission The average estimated co-related temperature value is observed to be ~2688 K This temperature value indicates that these phosphor powders can be used for warm LED applications The color purity (CP) of the synthesized NPs has been calculated and the value is found to be 88%, which indicates that the prepared phosphor powders are highly useful for the red component in white light emitting diodes under UV light excitation

© 2020 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

In recent times, white light-emitting diodes (WLEDs) have

shown keen attention among the researchers as a capable light

source to meet the needs of world's energy requirements WLEDs

have shown high luminous efficiency, more compactness,

long-lifetime, simple design aspects and fast switching [1,2] Currently,

commercial WLEDs have low CRI (Color rendering indexes:< 80)

and high CCT (Correlated color temperature:>4500 K) To overcome

these inadequacies, many materials combinations were reported in

the recent literature as potential candidates [3e7] In these reports,

the utilized synthesis routes requires expensive fuels and leave a

harmful impact on the surrounding environment However, the tough preparation conditions, expensive reagents and harmful chemical compositions hamper their practical applications [8] As an alternative, near-UV LED chips were coupled with multi-phosphor materials which emit a specific ratio of red, green and blue light [9

Lanthanide ions are usually considered as biocompatible ma-terials, especially Eu3þ, Gd3þ, Sm3þ, Yb3þand Y3þions which were found to have a relatively low toxicity [10e13] Many works focus

on rare earth (RE) ion doped luminescence materials because of the peculiar optical properties arising from the intra 4f transitions In fact, compared to metals or semiconductor materials, the emission bandwidth of RE ions is much sharper and, moreover, lifetimes are several orders of magnitudes longer and they have already shown

to be useful for this purpose [14] As a result,fluorescent powders based on rare-earth doped materials can be used for display device applications For these reasons, active research was being carried out to fabricate more effective, stable and consistent RE doped NPs

* Corresponding author.

E-mail address: premhb@gmail.com (H.B Premkumar).

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.2020.02.003

2468-2179/© 2020 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/ ).

[15,16] And no universally recognized conclusions have been

drawn on what material is the best to be used for this application,

yet

Eu3þ(4F6) ions have been casted-off as activator in the utmost

profit-making red luminescent materials [17] Desirable host

ma-terials with Eu3þions aroused an enormously strong interest as

potential candidate for WLEDs [18e20] Generally, aluminates

doped with Eu3þions show exceptional luminescence properties

This makes them suited for LED's, optoelectronic and storage

de-vices, luminous paints, display devices etc [21].Table 1shows the

previous reports of various aluminates doped with RE ions

pre-pared by different methods for WLED applications [22e27] Among

the various aluminates, SrAl2O4NPs were used in LED's, X-ray

im-aging, scintillations, sensors, optical communication,fluorescence

imaging etc., because of their crystal stability, morphological

fea-tures and easily tuneable optical properties [28e30] Generally, the

RE ions doped aluminates were fabricated via classy and elevated

temperature methods [31] Hence, there is an enormous

require-ment for less cost, a non-hazardous and short time production is in

demand Various methodologies have been reported for the

prep-aration of SrAl2O4 NPs viz., hydrothermal, solvothermal, solegel,

co-precipitation and CVD techniques Many of these procedures

require costly equipment's and time consuming experimental

procedures

Hence, plant latex used in green combustion synthesis (GCS) is an

alternative approach which is able to obtain products by a simple

procedure and provides highly pure samples with good yield GCS is

one of the popular and quick methods based on the propellant

chemistry techniques which uses organic and plant extracts as a fuel The advantages of this technique are, the heat energy released at considerably low temperatures by the redox exothermic reaction., This route can offer a distinct size and shape NPs, and additionally tuned physical and chemical properties compared to other existing synthesis techniques [32e34] Plant latex used in the preparation technique is used as both the reducing and the stabilizing chemical agent in the preparation of NPs [35]

2 Materials and methods Semiliquid Milky white latex was extracted from the stems of freshly collected Euphorbia tirucalli (E tirucalli) plants collected from dry lands of the Madhugiri region 13.6643 N, 77.2089E Latex of E tirucalli is used as vesicant and rubefacient for many diseases Extracted latex was sterilized and kept at20C before it

was used as fuel to avoid fungus formation At the time of use of the latex, a 10 ml quantity of this stock solution was further diluted by adding double distilled water of suitable quantity and prepared E tirucalli latex solutions withfinal six desired concentrations varying from 5 ml to 30 ml SrAl2O4:Eu3þ(1e5 mol %) NPs were prepared by

Al(NO3)39H2O (99.99%), Eu(NO3)36H2O (99.99%) as starting ma-terials Stoichiometric quantities of the precursors were dissolved

in 25 ml of deionized water along with desired quantities of plant latex extract solutions withfinal six desired concentrations varying from 5 ml to 30 ml The solution is mixed together thoroughly using

a magnetic stirrer and ultrasonication for about 30 min and kept

Table 1

Various aluminates doped with RE ions prepared by different methods for wLED applications.

Fig 1 (a) PXRD patterns and (b) WeH plots of SrAl 2 O 4 :Eu3þ(1e5 mol%) NPs.

into a muffle furnace that was maintained at 500C At this

tem-perature, the solution boils, and evaporates with large amounts of

gases escaping out by combustion technique The whole process

was completed in about 5e10 min and gives a white ash which was

further calcined at 1200C for 2 h

2.1 Characterization

The structural properties were studied using the X'Pert- Pro

Diffractometer with CuKa radiation and wavelength of 1.5406 Å

The reflectance spectra were recorded using the PerkinElmer

Lambda-35 UV spectrometer The surface morphology and

elemental analysis of the product was studied using the TESCAN

VEGA3 LMU scanning electron microscope The particle size was

estimated by using the JEOL/JEM 2100 Transmission electron

mi-croscope The Jobin YVON Fluorolog-3 Spectrofluorimeter was

utilized to study the photoluminescence

3 Results and discussion

3.1 Structural investigation

The PXRD patterns displayed in Fig 1 correspond to the

SrAl2O4:Eu3þ(1 - 5 mol %) prepared with 30 ml plant latex samples

All the compounds peaks are well matched with the monoclinic

SrAl2O4standard card (ICDD PDF 01-074-0794), showing that the

Eu3þdoping did not modify the lattice parameter significantly.Fig

1(a) depicts the diffraction profiles of Eu3 þ (1e5 mol%) doped

SrAl2O4NPs [36] It can be further noticed that there are no

addi-tional impurity peaks in the samples confirms the successful

incorporation of Eu3þions into the SrAl2O4host lattice This was

confirmed by estimating the acceptable percentage difference (Dr)

between Eu3þions and SrAl2O4lattice sites using following relation

[37];

Dr¼Rs Rd

where, Rsis the ionic radius of either Sr2þor Al3þ, and where Rdis

the ionic radius of the Eu3þion Generally, the value of Drfor the

host site and the dopant ion differ not more than 30% In the

pre-sent work, the estimated value of Drby considering the ionic radii of

the Eu3þion and the Sr2þsite was found to be very close to 30%

This suggest that the dopant Eu3þions were clearly occupying the

Sr2þsite in the SrAl2O4host Further, the Scherer's and WeH plots

techniques were employed in order to calculate the average

crys-tallite size (D) as well as the strain of the obtained NPs using the

relations [38];

D¼ 0:9l

bcosq¼l

where,lis the X-ray wavelength,bis the PXRD peaks full width at half maximum,ε is the strain in the prepared powders.Fig 1(b) displays the WeH plots of the NPs The calculated crystalline pa-rameters were listed inTable 2

3.2 Energy band gap estimate

In order to evaluate the energy band gap (Eg) values of the synthesized powders their diffuse reflectance (DR) spectra were studied and depicted inFig 2(a) From thisfigure it is seen that there is a strong absorption band appearing in the lower wave-length range which is ascribed to the host lattice absorption However, in the larger wavelength range some sharp absorption bands are observed which are due to the incorporation of Eu3þions These sharp absorption bands are ascribed to excitations from the ground level 7F0 to the numerous excited levels of Eu3þ ions (7F0/5D4, 7F0/5G2, 7F0/5L6, and 7F0/5D2) [39] Further, KubelkaeMunk (K-M) relations [40] (Equations(4) and (5)) were employed to estimate the Egplots (Fig 2(b))

Table 2

The estimated average crystallite size, micro strain and energy gap values of

SrAl 2 O 4 :Eu3þ(1e5 mol%) NPs.

Eu3þConc.

(mol %)

Crystallite size (nm) Micro strain x 103 Energy

gap (eV) Refractive index (n) Scherer's

method

WeH plots

Fig 2 (a) diffuse reflectance spectra and (b) energy band gap plots of SrAl 2 O 4 :Eu3þ (1e5 mol%) NPs.

FðR∞Þ ¼ð1  R∞Þ2

where R∞is the coefficient of reflection,lis the wavelength of

absorption The evaluated Egvalues are listed inTable 2 Also the

refractive index of the prepared samples are estimated and

tabu-lated inTable 2

3.3 Morphological analysis

Fig 3shows the SEM micrographs of SrAl2O4:Eu3þ(3 mol %) NPs

prepared with different concentrations (5, 10, 15, 20, 25 and 30 ml)

of E Tirucalli milk extract For the lower concentrations of the

extract (5 and 10 ml), the SEM micrographs shows irregular units

with rigidfibres (Fig 3(a, b)) At a further increase in concentration

to ~15 ml, a mesh-like network is observed as shown inFig 3(c)

However, when the concentration is further increased to 20 ml,

short lengthfibres are observed (Fig 3(d)) Finally, when the

con-centration is 25e30 ml, uniform broom-like structures are

observed (Fig 3(e, f)) It is stated that the dispersion persuaded

splitting growth mechanism is accountable for such broom-like

structure Under non-equilibrium conditions, these irregular

branching structures were moulded through a self-organization procedure The separation between the nucleation center and the driving force are responsible for the crystallization arrangement of hierarchical structures It is assumed that at lower concentrations

of the milk extract, a concentric dispersionfield was set round the crystal soon after the formation of nuclei The reactants present closer to the surface will undergo this diffusion process very quickly On the other hand when the milk extract is higher (25e30 ml), the ends of the crystals join together quicker than the dominant parts which lead to the formation of a broom-like morphology

Fig 4(aee) shows the TEM images with different magnifications

of the optimized NPs (3 mol%) with 30 ml of the plant extract From thefigure, it is evident that the TEM image of the NPs is broom-like

in nature with the approximate length of 445.21 nm (Fig 4b) Further, the tip of the broom-fibres is projected outward at the edges The obtained results are in agreement with that observed for SEM images The HRTEM image is depicted inFig 4(f) From, the figure the interatomic distance for different domains is found to be 0.36 and 0.23 nm The selected area electron diffraction (SAED) pattern is shown inFig 4(g) The image indicates that the obtained powders are crystalline in nature Further, the elemental analysis is shown inFig 4(h) From the EDAX spectra, it is evident that the compound consists of the elements Sr, Al, O and Eu This result shows that the doped Eu ions are effectively incorporated into the host matrix

Fig 3 SEM micrographs of SrAl 2 O 4 :Eu3þ(3 mol%) NPs with different plant extract concentrations, (1) 5 ml, (b) 10 ml, (c) 15 ml, (d) 20 ml, (e) 25 ml and (f) 30 ml.

3.4 Spectroscopic studies

The effect of Eu3þions on the photoluminescence properties of

SrAl2O4was studied and is depicted inFig 5(a, b) The excitation

spectrum of the Eu3þ(3 mol %) doped SrAl2O4NPs with 30 ml latex

is depicted inFig 5(a) The prominent peak observed at 393 nm

corresponds to the7F0/5L6transition of the Eu3þions [41] It is

observed that the intensity of the 393 nm peak is stronger than the

467 nm peak This excellent absorption band at 393 nm peak

in-dicates that SrAl2O4:Eu3þNPs have a considerable potential for red

light emitting diode applications

The emission spectrum of various concentrations (1e5 mol %) of

Eu3þdoped SrAl2O4NPs under 393 nm excitation wavelength is

depicted inFig 5(b) The emission peaks observed between 500

and 700 nm are ascribed to 4f transitions of the Eu3þ ions

(5D0/7FJ: J¼ 0, 1, 2, 3, 4) [42] The strong emission peak detected at

618 nm is attributed to the5D0/7F2transition of the Eu3þions

This transition is attributed to hypersensitive transition [43]

Among the two prominent emission peaks observed at 610

(5D0/7F1) and 618 (5D0/7F2) the latter is more intense, which

indicates that Eu3þions occupy a non-inversion symmetry site in

the SrAl2O4host lattice FromFig 5(c) it can be clearly noticed that

the PL emission intensity increases with increase in Eu3þion con-centration till 3 mol% and thereafter shrinks owing to the famous concentration quenching phenomenon [44] Therefore, the optimal concentration of Eu3þions in the SrAl2O4phosphor system is 3 mol

% This quenching mechanism can be explained based on the non-radiative energy transfer method which includes: exchange inter-action, radiation re-absorption and multipolar interaction [45] To find out the particular type of the above interactions it was essential to estimate the critical distance (Rc) between the adjacent

Eu3þions This value of Rc was evaluated by utilizing the Blasse relation [46]:

Rc¼ 2

3V=4pNXc

1=3

(6)

Here, Xc is equal to 3, the optimal dopant concentration, V is 425.32 Å3(volume of the unit cell), N is the coordination number for the unit cell By utilizing these values in the above equation, the

Rcvalue was found to be ~18.56 Å Further, the multipolar inter-action among the dopant ions was estimated by utilizing the Dexter's relation [47]:

Fig 4 (aee) TEM images with different magnifications, (f) HRTEM image, (g) SAED patterns and (h) EDAX spectra of SrAl 2 O 4 :Eu3þ(1e5 mol%) NPs prepared using 30 ml plant latex.

c¼ K

2

41 þbðcÞQ

3

5

1

(7)

wherecis the dopant ion concentration and Q is the constant of the

multipolar interaction K andbare constants for the present host

Fig 5(d) shows the plot of log (I/x) versus log x and the slope is

found to be0.5998 Hence, the value of Q is found to be ~1.8,

which specifies that the electric multipoleemultipole interaction

where the charge transfer mechanism is the major one for energy

transfer among Eu3þ which is responsible for the concentration

quenching phenomenon

3.5 Color parameters

In order to evaluate the productivity of the prepared phosphor powders in thefield of display devices, it is necessary to validate its color parameters according to the Commission Internationale de I'Eclairage (CIE) and the correlated colour temperature (CCT) [48,49].Fig 5(e) depicts the CIE diagram of SrAl2O4:Eu3þ(1e5 mol

%) phosphors under 393 nm near UV-excitation It is evidently observed that the entire color co-ordinates lies well inside the red area indicating the use of the present phosphor as red component

in white light emitting diodes The CCT values were estimated by utilizing the relations given elsewhere [50] and were found to be

~2668 K, which indicates that the prepared powders can be effec-tively used in the fabrication of warm LEDs (Fig 5f) Further, to

Fig 5 (a) Excitation spectrum for SrAl 2 O 4 :Eu3þ(3 mol%) NPs., (b) emission spectra, (c) Variation of PL intensity versus Eu3þions concentrations, (d) log (I/x) versus log (x) plot, (e) CIE and (f) CCT diagrams of SrAl 2 O 4 :Eu3þ(1e5 mol%) NPs prepared with 30 ml plant latex.

signify the phosphor usage in the LED industry it was necessary to

evaluate its color purity (CP) value The CP values of the prepared

NPs were evaluated by utilizing the following relation [33]:

color purity¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðxs xiÞ2þ ðys yiÞ2

q

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ðxd xiÞ2þ ðyd yiÞ2

where, (xs, ys), (xi, yi) and (xd, yd) are colour co-ordinates of the light

source, CIE illuminate and dominant wavelength of the light source,

respectively The estimated CP values are recorded inTable 3 The

CP value for the optimized composition (3 mol %) is found to be

~88%, which indicates that the prepared NPs are exceedingly

suit-able for display device applications

4 Conclusions

In this report, we prepared Eu3þ doped SrAl2O4 fluorescent

powder by the green combustion synthesis technique with the aim

to obtain an efficient component in display device applications The

key points of the present study includes, the synthesis route

fol-lowed is eco-friendly, which requires less time, low cost, and high

yield The structural and spectroscopic studies of the obtained

samples showed a good crystalline structure allowing high dopant

concentrations The SEM results indicate afiber-like morphology

The EDAX results showed the presence of all the stated elements in

the compound The particles size was in the nano-metric scale The

PL results shows an intense red emission of the powders at 618 nm

wavelength under near UV-excitation (393 nm) The PL emission

intensity increases up to 3 mol% Eu3þconcentration and thereafter,

declines due to concentration quenching The chromaticity

dia-grams indicated that the prepared powders emit an intense red

color and their average CCT value was estimated to be ~2688 K

Also, the CP of the optimized powder was estimated to be ~88%

These obtained results clearly indicate that the prepared powder

SrAl2O4:Eu3þ(3 mol%) using 30 ml plant latex can be utilized as the

red component in display device applications

Declaration of Competing Interest

The article is original, has been written by the stated authors

who are all aware of its content and approve its submission, no

conflict of interest exists, or if such conflict exists, the exact nature

of the conflict must be declared and if accepted, the article will not

be published elsewhere in the same form, in any language, without

the written consent of the publisher

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Table 3

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