Photometric properties of Sm3+ doped LaOF nanostructures prepared via green combustion route suitable for display and fingerprint applications
C. Suresh1, 2*, R.B. Basavaraj1, D. Kavyashree3
1Prof. C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur- 572 103, India
2Department of Physics, Govt. First Grade College, Tumkur -572 103, India
3Department of Physics, HMSIT, Tumkur -572 103, India
Samarium (Sm3+) doped LaOF nanostructures were prepared by green combustion synthesis using Mimosa pudica(M.P.)extract as fuel. The final product was well characterized by Powder X-ray diffraction (PXRD), Scanning electron microscopy (SEM), and Photoluminescence (PL) spectroscopy. The PXRD profiles confirm the tetragonal phase of the product. The morphology was studied in detail by varying the concentration of the fuel. The 4f-4f emission bands of Sm3+ ions were observed at 561, 607 and 653 nm ascribed to 5G5/2→6HJ (J = 5/2, 7/2 and 9/2) transitions respectively under 406 nm (6H5/2→4F5/2 + 4M7/2) excitation. The results indicated that the obtained phosphors emit strong orange red colour useful for the fabrication of white light emitting diode. The green combustion route used was scalable to industrial applications for mass production as well as for lightning device applications in the field of advanced photonics and fingerprint applications in forensic science.
Keywords: Green combustion synthesis, Photoluminescence, Displays devices.
* Corresponding author: E-mail: sureshcphy13@gmail.com(C. Suresh).
In recent years, rare earth activated phosphor materials have been attracting much attention based on their applications in various fields, such as plasma display panels (PDP), mercury free lamps, fluorescent lamps, cathode ray tubes, field emission displays (FEDs) and white light-emitting diodes (WLEDs) due to their potential luminescent properties such as high brightness, high efficiency, and long working time.[1–3] Among them, WLEDs and FEDs have gained abundant interest. Recently, Oxide phosphors for FEDs have gained much interest due to their better thermal and chemical stability and environmental friendliness compared with sulfides.The former has been recognized as one of the most promising technologies in the solidstate lighting industry due to their low electricity consumption by taking advantage of the conversion from direct electricity to light [4]. The latter has also been considered as one of the most promising next generation flat panel displays due to their potential to provide displays with thin panels, self-emission, wide viewing, quick response time, high brightness, high contrast ratio, light weight, and low power consumption[5]. Actually, phosphors with an ideally spherical shape, a narrow size distribution, and non-agglomeration are even more important, because ideally spherical phosphors will lead to low scattering of light, high packing densities, high brightness and high resolution. Therefore it is even more important to develop a method to control the size and distribution of LaOF based phosphors. Rare earth doped LaOF materials have been synthesized by modified pechini sol–gel technique, hydrothermal method, ultrasound sonochemical synthesis and solid-state reactions. From the various fabrications techniques mentioned above, combustion synthesis method proved to be advantageous because of the fast production of the powder, using relatively low temperature reactors, low cost, and the easy way to achieve high purity and single phase nano materials [6].In the present work, we report samarium doped LaOF nanostructures prepared by combustion synthesis methodusing Mimosa pudica(M.P.) extractas fuel. To find the potential applicability of the synthesized LaOF nanophosphors the photometric properties (PL, CIE, CCT and color purity) were evaluated.The Samarium ion (Sm3+) is well-known as an orange red emitting activator due to its5G5/2→6HJ (J = 5/2, 7/2 and 9/2) transitions respectively under 406 nm (6H5/2→4F5/2 + 4M7/2) excitation.Usually ranging from 500 nm for J = 0 to 750 nm for J = 7/2, with 5G5/2 → 6H7/2 around 610–625 nm as the most prominent group). These emissions have found an important application in the lighting and display fields.Fingerprints (FPs) are universal investigative protocol for the recognizing of individuals and also conveying added records of the individual in forensic field.
2. Experimental
2.1. Synthesis
Sm3+ doped LaOF nanophosphors were prepared by combustion synthesis using reagent-grade samarium nitrate Sm(NO3)3·6H2O(99.9%, Aldrich) and lanthanum nitrate La(NO3)3·6H2O (99.9%,Aldrich).as oxidizers and Mimosa pudica(M.P.) extract as fuel., The corresponding metal nitrates, in a proper molar ratio, were dissolved in a minimum quantity of de-ionized water forming a homogeneous solution. Subsequently, the appropriate amount of M.P. (5-30 ml) was added to this solution and the mixture was continuously stirred at room temperature for ~ 5 min. Then, an aqueous solution of ammonium fluoride NH4F (Sigma Aldrich) was added drop wise under constant stirring. Thereafter, the mixed solution was kept under constant stirring, at a room temperature, for ~10 min, to form a milky solution and placed in a furnace preheated at 500 ± 100C for 15 min, until excess free water evaporated and spontaneous ignition occurred resulting in a fine powder product. Finally, the as-prepared powders were sintered at 700 0C for 2 h in an air atmosphere for characterizations.
Phase purity and crystallinity of nanostructures were measured using a powder X-ray diffractometer (PXRD, Shimadzu 7000). Scanning electron microscopy (SEM) measurements were performed on a Hitachi table top microscope (TM 3000).The Diffuse reflectance spectroscopy of the samples was recorded on spectrometer Perkin Elmer (Lambda-35). The Jobin Yvon Spectroflourimeter Fluorolog-3 operational with 450 W Xenon lamp as an excitation source was used for photoluminescence (PL) measurement.
The PXRD patterns of pure and Sm3+ (1-11 mol %) doped LAOF were shown in Fig.1 (a). The results confirmed the tetragonal phase of LaOF: Sm3+with JCPDS card no. 89-5168 having space group P4/n mm (No.129).The average crystallite size was estimated by using Debye – Scherrer’s equation; and found to be in the range of 20 – 40 nm. The calculated values of crystalline size were summarized in Table 1.
-------------------- (1)
Where: β; full width at half maximum (FWHM in radian) caused by the crystallites, λ; wavelength of the X-ray (1.542 Å), θ; Bragg angle. The calculation of crystalline size and strain present within the prepared sample by W – H approach was given by the equation[7];
------------------- (2)
Where, b; FWHM of peaks in radians, e ; the strain in the sample, D; the crystalline size and 
; Bragg’s angle. The above equation gives a straight line between 4sinθ (X – axis) and fact that dopants in the host matrices induces the distortion. bcosθ (Y – axis) for LaOF: Sm3+ (Fig. 1(b)).
Fig. 1(a) PXRD patterns and (b) W-H plot of pure & Sm3+ (1-11 mol %) doped LaOF NSs.
The estimated crystallite size, stain and energy gap values are tabulated in Table 1.
Table.1 Estimated crystallite size, strain of Pure & Sm3+ (1- 11 mol %) doped LaOF: Sm3+ nano structures.
|
Sm3+ (mol %) |
Crystallite size (nm) |
Strain ε x (10-3) |
|
|
Scherrer’s
|
W-H
|
||
|
undoped |
38 |
40 |
1.25 |
|
1 |
32 |
38 |
1.30 |
|
3 |
30 |
36 |
1.32 |
|
5 |
26 |
30 |
1.36 |
|
7 |
23 |
27 |
1.39 |
|
9 |
20 |
22 |
1.42 |
|
11 |
19 |
21 |
1.45 |
3.2 Morphological analysis
Fig.2 (a-d) shows the SEM micrographs of LaOF: Sm3+ (3 mol %) NPs prepared with different concentrations of Mimosa pudica(M.P.) leaves extract (5-30 ml). It can be observed from the figure that at initial concentration (5 ml) of M.P. extract the particles were agglomerated with random shape and size ((Fig. 2(a)). As the M.P. concentration was increased to 10 ml these particles start to segregate with each other to form a flake like morphology (Fig.2b). Further increase in the M.P. concentration to 20 ml highly porous molecules with large voids was formed (Fig.2c). Finally an elongated dumbbell shaped structures were formed at 30 ml M.P. concentration (Fig.2d). The obtained result indicates that the morphology was highly influenced by the M.P. concentration.
Fig.2 SEM micrographs of Sm3+ (3 mol%) doped LaOF NSs at different concentrations of M.P. (a) 5 ml, (b) 10 ml, (c) 20 ml and (d) 30 ml.
3.3.Photoluminescence (PL) studies
Fig.3(a) shows the PL excitation spectrum of Sm3+ (1-11 mol %) doped LaOF NSs monitored at 607 nm emission wavelength. The excitation spectrum shows the sharp peak at 406 nm corresponding to 6H5/2→4F5/2+4M7/2 transition. Fig. 3(b) shows the emission spectra recorded in the range of 500 – 750 nm at 406 nm excitation wavelength. The spectra exhibits strong emission peaks at 566, 653 and 708 nm and broad emission peak centered at about 607 nm. These emission bands characteristic of Sm3+ which corresponds to 4G5/2 → 6H5/2, 4G5/2 → 6H9/2, 4G5/2 → 6H11/2and 4G5/2 → 4H7/2 transitions respectively[8]. The Commission International de I’Eclairage (CIE) 1931 x-y chromaticity diagram of LaOF:Sm3+ NSs (1-11 mol %) were presented in Fig.3(c) excited under 406 nm [9]. As shown in the inset of Fig.3(c) the CIE chromaticity coordinates were located in the orangered region. To identify technical applicability of this orange red emission, correlated color temperature (CCT) was determined from CIE coordinates. Fig. 3(d) shows the CCT diagram of LaOF:Sm3+ NSs (1-11 mol%) excited under 406 nm. The CCT is a specification of the color appearance of the light emitted by a light source, relating its color to the color of light from a reference source when heated to particular temperature[10]. The correlated color temperature (CCT) was one of the essential parameter to know the color appearance of the light emitted by a light source with respect to a reference light source when heated up to a specific temperature, in Kelvin (K). CCT is estimated by transforming the (x, y) coordinates of the light source to (U0, V0) by using the equations 3 and 4, and by determining the temperature of the closest point of the Planckian locus to the light source on the (U0, V0) uniform chromaticity diagram (Fig. 3(d));
---------------- (3)
----------------- (4)
Also, the quality of white light in terms of color correlated temperature (CCT) was given by
McCamy empirical formula 
(theoretical) where
; the inverse slope line and chromaticity epicenter was at xc=
0.3320 andyc= 0.1858. Generally, a CCT value greater than 5000 K indicates the cold white light used for commercial lighting purpose [11]. In the present study, the CCT of LaOF: Sm3+ NSs (1-11 mol %) were found to be ~1817 K which was well within the range of vertical daylight. Thus it can be useful for artificial production of white light in illumination devices.In the present study, the estimated CIE co-ordinates (x, y), (U0, V0) and CCT values of LaOF: Sm3+ (1-11 mol %) NP was tabulated in Table 2. Thus present phosphor can be useful for artificial production of white light in illumination devices.
The Quantum efficiency (QE) of the optimized LaOF: Sm3+ (3 mol %) NP was estimated using the relation reported elsewhere [12]. The estimated value of QE was found to be ~ 86.12 %. Color purity (CP) of the prepared sample is also checked by using following relation [13];
---------- (5)
where (
,
) ; the coordinates of a sample point, (
,
) ; the coordinates of the dominant wavelength and (
, 
) ; the coordinates of the illuminant point. The CP of the prepared samples were estimated and tabulated in Table 2.The CP of optimized sample was found to be ~ 83.5%, indicates that the prepared phosphor might be excellent materials for WLED applications.
Fig.3(a) PL excitation (b) Emission spectra (c) CIE & (d) CCT diagram of LaOF: Sm3+ (1-11 mol %) NS.
Table.2 Photometric characteristics of Sm3+ (1- 11 mol %) doped LaOF nanostructures.
|
Sm3+ (mol %)
|
CIE |
CCT
|
CCT (K) |
CRI (%) |
||
|
X |
Y |
U׳ |
V׳ |
|
||
|
1 |
0.5235 |
0.3587 |
0.3231 |
0.5133 |
1762.12 |
85.03 |
|
3 |
0.5315 |
0.3525 |
0.3256 |
0.5112 |
1778.13 |
85.13 |
|
5 |
0.5225 |
0.3505 |
0.3210 |
0.5111 |
1792.03 |
83.24 |
|
7 |
0.5127 |
0.3492 |
0.3278 |
0.5057 |
1810.12 |
81.53 |
|
9 |
0.5016 |
0.3382 |
0.3162 |
0.5042 |
1846.42 |
80.42 |
|
11 |
0.4956 |
0.3345 |
0.3124 |
0.5026 |
1911.05 |
79.62 |
Fig.4. shows the finger print visualization on glass slide by using optimized LaOF:Sm3+ 3 mol% from powder dusting method. From the figure clear ridge details (Type I-III) can be seen without any ambiguity. The enlarged ridge details are presented in Fig.4. Further, unquestionably visualized level I (core), level II (bifurcation, bridge, ridge ending, crossover, island, enclosure, scar, eye and delta) and level III (sweat pores) by means of optimized LaOF: Sm3+ (3 mol %) labeling agent under normal light (Fig.4). The images clearly indicate that, the powder particles uniformly distributed on the ridges due to uniform sized particles with superior adhesive ability.
Fig.3. FPs visualized by using LaOF: Sm3+ (3 mol %) NP on aluminium foil under normal light of ridge patterns. (Highlighted parts clearly displayed the various ridge details).
Herein we have successfully synthesized LaOF: Sm3+ (1– 11mol %) nanophosphors via green combustion route using Mimosa pudica(M.P.) extract as fuel. Elongated dumbbell shaped structures were observed from SEM studies. The excellent orange red emission properties and the estimated CIE chromaticity co-ordinates (x, y) were very close to NTSC standard value and CCT was found to be 1817 K. Hence the optimized LaOF: Sm3+ (3 mol %)nanophosphors were potentially used as orange red emitting component in LEDs. The powder particles uniformly distributed on the ridges due to identical sized particles with superior adhesive ability.
Acknowledgment
Authors thank Prof. C.N.R. Rao Centre for Advanced Material Research for providing characterization facilities.
References