- Email: [email protected]
Eu3+ phosphors with tunable emissions and energy transfer for WLEDs
Accepted Manuscript 3+ 3+ Facile pechini synthesis of Sr3Y2Ge3O12:Bi /Eu phosphors with tunable emissions and energy transfer for WLEDs Sk. Khaja Hussain, L. Krishna Bharat, Dong Hyun Kim, Jae Su Yu PII:
S0925-8388(17)30399-7
DOI:
10.1016/j.jallcom.2017.01.345
Reference:
JALCOM 40712
To appear in:
Journal of Alloys and Compounds
Received Date: 28 November 2016 Revised Date:
29 January 2017
Accepted Date: 30 January 2017
Please cite this article as: S.K. Hussain, L.K. Bharat, D.H. Kim, J.S. Yu, Facile pechini synthesis of 3+ 3+ Sr3Y2Ge3O12:Bi /Eu phosphors with tunable emissions and energy transfer for WLEDs, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.01.345. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Graphical abstract:
ACCEPTED MANUSCRIPT
Facile pechini synthesis of Sr3Y2Ge3O12:Bi3+/Eu3+ phosphors with tunable emissions and energy transfer for WLEDs
RI PT
Sk. Khaja Hussain, L. Krishna Bharat, Dong Hyun Kim and Jae Su Yu*
*Corresponding author
M AN U
E-mail: [email protected] Phone: +82-31-201-3820; FAX: +82-206-2820.
SC
Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Republic of Korea.
Abstract
AC C
EP
TE D
A series of Sr3Y2Ge3O12 (SYGO), SYGO:Bi3+, SYGO:Eu3+ and SYGO:Bi3+/Eu3+ phosphors were synthesized by a facile pechini type sol-gel method. X-ray diffraction patterns exhibited a characteristic cubic lattice structure with a space group of la3(230) after annealing at 1200 °C for 8 h. The morphological properties of the SYGO host lattice were studied by a field-emission scanning electron microscope, which displayed nearly spherical-shaped particles. Under ultraviolet (UV) or near-UV excitations, the SYGO:Bi3+ phosphors showed the blue emission of Bi3+ (3p1 → 1s0) at 467 nm and SYGO:Eu3+ phosphors exhibited orange red emission (5D0 → 7F1) at 594 nm, respectively. The energy transfer process was investigated in between the Bi3+ and Eu3+ ions by the wide spectral overlap of Bi3+ emission spectrum and Eu3+ excitation spectrum. The efficient energy transfer phenomenon from Bi3+ to Eu3+ was studied by adjusting the Eu3+ ion concentration in SYGO:Bi3+/Eu3+ phosphors and it was strongly confirmed by their luminescence decay curves. The critical distance was calculated for the energy transfer process from Bi3+ to Eu3+ due to the concentration quenching effect and it was further demonstrated to be a resonance type of quadrupole-quadrupole interactions. The obtained photoluminescence results revealed tunable emissions from blue to cool white, depending on the Eu3+ ion concentration. Furthermore, thermal properties and quantum yield of the optimized cool white-light emitting SYGO:0.02Bi3+/0.05Eu3+ phosphor sample were studied and it exhibited good thermal stability with 20.5% quantum yield. Commission International de I’Eclairage chromaticity coordinates were calculated for all the prepared samples. Therefore, all these results indicate that the SYGO:0.02Bi3+/0.05Eu3+ is a promising phosphor for the application of white light-emitting diodes. Keywords: Garnet phosphors; Energy transfer; Tunable emissions; White-light emitting diodes
ACCEPTED MANUSCRIPT
1. Introduction White light-emitting diodes (WLEDs) as a next-generation lighting source have appeared
RI PT
to be the most promising in a typical form of solid-state lighting owing to their superior properties such as low power consumption, high brightness, long working lifetime, durable energy saving capability, and eco-friendly feature with mercury-free properties and less thermal radiations [1-3]. Generally, available WLEDs in the market are based on blue light-emitting
SC
InGaN chips in the wavelength range of 450-470 nm, coated with yellow emitting Y3Al5O12:Ce3+
M AN U
(YAG:Ce3+) phosphors, which is one of the widely used techniques [4-6]. The YAG:Ce3+ based WLEDs have some merits such as cost effectiveness, single-chip implementation and circuit flexibility. However, they suffer from low color-rendering index (CRI) which is < 80 due to the deficiency of the green and red color emitting phosphor components in the lower energy region, high correlated color temperature, and thermal quenching at elevated temperatures[7, 8].
TE D
Consequently, ultraviolet (UV) or near-UV (NUV) LEDs coated with novel single-phase tricolor-emitting phosphors are considered as an alternative for getting the white-light emission
EP
with higher CRI (> 90) values.[7, 9, 10]
Recently, increasing attention has been focused on tunable emissions from a single
AC C
component with energy transfer process mechanism from sensitizer to acceptor in many host lattices by doping with the co-activators such as Ce3+/Mn2+[11], Ce3+/Eu2+, Ce3+/Tb3+[12], Ce3+/Tb3+/Mn2+[13], and Eu2+/Tb3+/Mn2+[14, 15]. The tunable emissions occur due to their broadband emission spectra with higher intensities. Besides, Bi3+/Eu3+ have been introduced and co-doped into distinct host lattices to obtain the tunable emissions including white light from a single composition[16-18]. It is well known that Eu3+ ions doped phosphors exhibit pure red or orange-red emissions owing to their 5D0 → 7F2 electric dipole and 5D0 → 7F1 magnetic dipole
ACCEPTED MANUSCRIPT
transitions of Eu3+ ions, respectively. The incorporation of Bi3+ sensitizer enhances the emission intensities into various host lattices such as tungstates [19], silicates [16], molybdates [20], and vanadates [21, 22], exhibiting tunable blue-green emissions along with the red emissions in
RI PT
between 400 to 700 nm wavelength due to the characteristic 1S0 → 3P1 electronic transition under NUV excitation[16]. The energy transfer process from Bi3+ to Eu3+ ions and the white-light emission in a single phase composition have been widely investigated in many phosphor
SC
materials [16-18, 23-25]. In this manner, we made an attempt to enhance the emission intensities and study the energy transfer process from Bi3+ to Eu3+ ions in a novel germanium garnet-type
M AN U
metal oxide phosphor.
The trivalent rare earth (RE3+) ions doped garnet-type A3B2X3O12 (A = Ca, Sr, B= Y, Ga, In, Sc, Al and X = Si, Ge) phosphors such as Ca3Sc2Si3O12 (CSSO) and YAG have been investigated as good host lattice materials for luminescent properties. Sr3Y2Ge3O12 (SYGO) is
TE D
also a garnet-type phosphor which is analogous to CSSO silicate phosphor including the cubic structure with the space group of la3(230) [26]. It could be expected that Sr3Y2Ge3O12 phosphors exhibit excellent photoluminescence (PL) properties by activating with the RE3+ ions
EP
because Si and Ge belong to the same elemental group in the periodic table. Thus, we selected
AC C
Sr3Y2Ge3O12 as the garnet host lattice material to obtain the white-light emission under UV/NUV excitation. To our knowledge, until now, there have been no reports on the Sr3Y2Ge3O12 host lattice co-doped with Bi3+ and Eu3+ ions. In this work, we reported the synthesis of Bi3+ and/or Eu3+ single- and co-doped Sr3Y2Ge3O12 phosphor materials by a facile pechini type sol-gel method which is one of the finest synthetic methods to prepare the phosphor materials with definite morphology. The effect of annealing temperature was investigated on the pure phase formation of Sr3Y2Ge3O12 host lattice. The PL properties of the Sr3Y2Ge3O12:Bi3+/Eu3+
ACCEPTED MANUSCRIPT
phosphors were investigated in details. Also, the thermal properties and quantum yield were studied for the optimized phosphor sample. Tunable emissions with cool white-light are obtained
RI PT
based on the concentration of the Bi3+ and Eu3+ ions in SYGO:Bi3+/Eu3+ phosphors. 2. Experimental procedures
A series of Sr3Y2Ge3O12 host lattice, Sr3Y2-xGe3O12:x (Bi3+ or Eu3+) (x = 0.005, to 0.07
SC
mol) single-doped and Sr3Y1.98-yGe3O12:0.02Bi3+/yEu3+ (y = 0.005 to 0.07 mol) co-doped phosphor samples were synthesized by a facile pechini type sol-gel method using the
M AN U
stoichiometric amounts of high-purity chemicals (Sigma-Aldrich Co.) such as strontium nitrate (Sr(NO3)2), yttrium nitrate hexahydrate (Y(NO3)3 . 6H2O), germanium oxide (GeO2), europium nitrate pentahydrate (Eu(NO3)3) . 5H2O), bismuth nitrate pentahydrate (Bi(NO3)3 . 5H2O), citric acid (HCO(COOH)(CH2COOH)2), and poly (ethylene glycol) (PEG) ((C2H4O)n . H2O). At first,
TE D
in a 50 mL beaker, 3 mol of germanium oxide was dissolved in 10 mL of concentrated nitric acid. After 10 min, 20 mL of de-ionized (DI) water was added to get complete germanium nitrate solution (solution I). The solution II was prepared by mixing the stoichiometric amounts of
EP
strontium nitrate, yttrium nitrate hexahydrate, europium nitrate pentahydrate/bismuth nitrate pentahydrate in 200 mL of DI water. After 30 min of stirring, the solution I was added slowly to
AC C
the solution II. A 16 mol of complexing agent citric acid (2:1 ratio of complexing agent and metal ions) was added to the above solution mixture and maintained stirring for 15 min. At last, 1 g of PEG was added as a cross-linking agent. The supposed citric acid in the solution not only makes a stable metal complex network but also produces polyesterification with the added PEG via polymeric resin process to minimize the segregation in between the metal ions. Later, the beaker was covered with a polyethylene cap and the stirring was continued for 1 h for complete homogenization process of all the reactants. Furthermore, the stable metal complex mixture
ACCEPTED MANUSCRIPT
solution was heated on hot plate at 85 °C for about 2 h with magnetic stirring. After progressive reaction time, the magnetic bead and the cap were removed, and then the solution slowly evaporated, which results in the formation of brownish wet gel. The wet gel was dried in an oven
RI PT
at 120 °C for 1 day in ambient atmosphere, which produces a white colored porous solid matrix called as a xerogel. The precursor xerogel was calcined at 500 °C for 5 h to form black colored flakes. The polymeric resins at a moderate temperature of 500 °C created pure phase
SC
multicomponent metal oxides. Finally, for characterization, the obtained samples were annealed
M AN U
in a muffle furnace at 1200 °C for 8 h.
The X-ray diffraction (XRD) patterns of the all the prepared Sr3Y2Ge3O12 (hereafter referred as SYGO), SYGO:Bi3+ and SYGO:Bi3+/Eu3+ phosphor samples annealed at 1200 °C were characterized on a Mac Science M18XHF-SRA X-ray powder diffractometer. The morphological properties of the SYGO host lattice were measured by using a high-resolution
TE D
field-emission scanning electron microscope (HR FE-SEM: CARL ZEISS, MERLIN). The PL properties of the synthesized samples were studied by using a fluorescence spectrometer (FluoroMate FS-2, Scinco, South Korea) with xenon lamp as an excitation source and the
EP
luminescence decay lifetimes were recorded on a Photon Technology International (PTI, USA)
AC C
phosphorimeter attachment to the main system with a Xe flash lamp of 25 W power. The quantum yield was characterized by using a Hamamatsu Photonics C9920-02 system with an integrating sphere. Furthermore, temperature-dependent PL spectra (30 to 210 °C) were recorded on the same fluorescence spectrometer (FluoroMate FS-2, Scinco, South Korea) equipped with a thermocouple in a temperature controlled chamber (NOVA ST540). 3. Results and discussion 3.1. XRD and elemental analyses of SYGO and SYGO:Bi3+/Eu3+ phosphors
ACCEPTED MANUSCRIPT
Fig. 1(a) shows the XRD patterns of the synthesized SYGO samples and the SYGO samples annealed at different temperatures in the range of 1000 to1300 °C. The SYGO phosphor at 1000 °C exhibited an intermediate phase with major diffraction peak. As the annealing
RI PT
temperature increased to 1100 °C, the corresponding diffraction pattern peak intensities significantly increased, which indicates the good crystallinity of the SYGO samples with increased crystallite sizes. At 1200 °C, all the diffraction peaks are attributed to the pure cubic
SC
crystal phase of garnet with a space group of la3(230) and are in good agreement with the available standard JCPDS # 85-2410. By increasing the annealing temperature to 1300 °C, the
M AN U
SYGO sample revealed the formation of Y2Ge2O7 (JCPDS # 38-0288) impurity peaks. Thus, we confirmed that 1200 °C is an optimum temperature for pure phase formation of the SYGO host lattice. Commonly, the crystallite sizes of the metal oxides can be calculated by the Scherrer equation [27, 28]:
.
TE D
D =
(1)
Here, D is the average grain size or crystallite size, k (0.9) is a shape factor, β is the full width at
EP
half maximum (FWHM), λ is wavelength (1.5406 Å) and θ is the diffraction angle of the noticed peaks, respectively. The major diffraction peaks were used to calculate the crystallite sizes of the
AC C
SYGO samples prepared at 1000, 1100, 1200 and 1300 °C annealing temperatures, yielding the average crystallite sizes of 57, 69.5, 79.4 and 75.8 nm, respectively. Fig. 1(b) shows the XRD patterns of the SYGO:0.05 mol Eu3+ (i.e., SYGO:0.05Eu3+), SYGO:0.02Bi3+ and SYGO:0.02Bi3+/0.05Eu3+ phosphors, respectively. Any impurity peaks or other significant changes were not identified by the optimized single and co-doping of Eu3+ or Bi3+ ions in the current phase form of SYGO host lattice, which demonstrates that it has good stability for the accommodation of dopant ions. The lattice constant a of the SYGO host lattice (a = 13.04),
ACCEPTED MANUSCRIPT
SYGO:0.02Bi3+ phosphor (a = 12.97), SYGO:0.05Eu3+ phosphor (a = 12.96), and SYGO:0.02Bi3+/0.05Eu3+ phosphor (a = 12.98) were calculated based on the formula 1/d2 = (h2+k2+l2)/a2 for cubic structures. The obtained lattice constant values were in good agreement
RI PT
with the values reported in JCPDS # 85-2410 (a = 13.08). Fig. 2 (a) shows the FE-SEM images of the SYGO host lattice annealed at 1200 °C. The FE-SEM images revealed that the particles were nearly spherical in shape with non-identical sizes. The morphology of the SYGO host
SC
lattice materials was not effected distinctly for single- and co-doping of Eu3+ or Bi3+ ions as shown in Fig. S1. The spherical-shaped particles are more desirable, exhibiting better
M AN U
photoluminescent properties due to their lower scattering of light and higher packing density. Additionally, it is also noticed that the FE-SEM image displays particle sizes of >1 µm which are different from the calculated average crystallite size for the SYGO host lattice (~80 nm), because a particle may be composed of one or more crystallites. Fig. 2(b) shows the energy-dispersive X-
TE D
ray spectroscopy (EDS) spectrum of the SYGO host lattice. The EDS spectrum established that all the elements were homogenously distributed in the host lattice and no elements other than the Sr, Y, Ge and O were presented, which further supports the XRD results. Fig. 2(c-f) displays the
lattice.
EP
color mappings of the all the elements in a single spherical-shaped particle of the SYGO host
AC C
3.2. PL properties of SYGO:Bi3+ or Eu3+ single-doped phosphors In general, Bi3+ ion consists of ns2-type of luminescence center.[29] By Seitz models, the
Bi3+ ion is assigned with 6S2 electronic configuration from the ground state to the first 6S6P excited state. The ground state 6S2 electronic configuration of Bi3+ ion is ascribed to the 1S0 electronic transition and the first excited state is composed of three triplet state 3P0, 3P1, 3P2 and one 1P1 singlet state with increasing order of excitation energy. The transitions of 1S0 → 3P1, 1S0
ACCEPTED MANUSCRIPT
→ 3P2 and 1S0 → 1P1 can be noticeable in optical absorption spectroscopy [18, 30]. Due to the existence of the spin-orbital coupling in between the 3P1 and 1P1 excited states, the absorption bands of 1S0 → 3P1 and 1S0 → 3P2 conflict by partial allowance. The 1S0 → 3P2 absorption band is
RI PT
completely spin-forbidden, while the 1P1 → 1S0 transition may occur in the UV region. At the same time, the 3P1 → 1S0 absorption band is commonly observed in the NUV or visible region [18]. Fig. 3 shows the PL excitation (PLE) and PL emission spectra of the SYGO:0.02Bi3+
SC
phosphor monitored by the corresponding emission and excitation wavelengths, respectively. The PLE spectrum of the SYGO:0.02Bi3+ phosphor exhibited a broadband ranging from 225 to
M AN U
450 nm with the band maxima at 374 nm. The PL emission spectrum revealed a broadband blue emission from 425 to 545 nm with an origination at 467 nm due to the 3P1 → 1S0 electronic transition of the Bi3+ ions, exhibiting the FWHM of 40 nm. Therefore, the presence of the broad blue emission of Bi3+ ions with higher FWHM value can be favorable for the formation of cool
TE D
white- light by combining with a red component.
Fig. 4 shows the PL emission spectra of the Bi3+ single-doped SYGO phosphors under 374 nm excitation wavelength. From the spectra, the PL profile of all the prepared phosphor
EP
samples seems to be identical with each other. A series (0.005 to 0.05 mol) of Bi3+ ions were doped into the SYGO host lattice, and the blue emission intensity increased up to the 0.02 mol
AC C
concentration and then it started to decrease monotonously with further increase of Bi3+ ion concentration, which is attributed to the concentration quenching effect. Thus, the optimum concentration was found to be 0.02 mol. The concentration quenching occurred due to the heavier energy transfer between the Bi3+ ions in the SYGO host lattice. By increasing the Bi3+ ion concentration above its optimum level, the distance between the Bi3+-Bi3+ ions was decreased, which may promote the energy transfer, resulting in the decrease of PL emission intensity. The
ACCEPTED MANUSCRIPT
critical distance (Rc) is used to calculate the distance between the Bi3+ ions using the Blasse expression [31, 32]:
" #
!,
(2)
RI PT
R ≈ 2
where V and N are the volume and number of host cations in the unit cell, respectively, and χc is the total concentration of the Bi3+ ions. For the SYGO host lattice, V = 2241.46 Å3, N = 5 and χc
SC
= 0.02 (optimum concentration for Bi3+ ions). Therefore, the calculated critical distance value
M AN U
from the Eq. (2) is Rc = 34.98 Å. Usually, the exchange interaction is preferable for the energy transfer process when the critical distance value is restricted to about 5 to 8 Å [33]. But, the above calculated Rc value exceeds 8 Å, indicating the absence of the exchange interactions in this mechanism. Consequently, electric multipolar interaction grants to the non-radiative concentration quenching mechanism in between the two nearest Bi3+ atoms, which was found by
TE D
Huang’s theoretical expression [34, 35]: (
log ' * = K − ' * log(C),
(3)
EP
)
where K is the activator constant, I is the luminescence intensity, C is the substituted mole
AC C
fraction of the Bi3+ ions at the optimum levels and θ is the electric multipolar parameter, and it is equal to 6, 8 and 10, which corresponds to the dipole-dipole, dipole-quadrupole and quadrupolequadrupole interactions, respectively. The inset of Fig. 4 represents the linear relationship between the log(I/C) versus log(C) of Bi3+ ions and is fitted by a straight line with a slope of 1.93. Thus, θ value was found to be 10.14 and it is very close to 10. Therefore, the quadrupolequadrupole interaction commands the energy transfer process between the Bi3+ ions.
ACCEPTED MANUSCRIPT
Fig. 5 represents the PLE and PL emission spectra of the SYGO:0.05Eu3+ phosphor under their corresponding emission and excitation wavelengths, respectively. The PLE spectrum showed a broad charge transfer band (CTB) of Eu3+-O2- from 210 to 310 nm with band maxima
7
RI PT
at 238 nm and also exhibited the f-f transitions at 380, 393, 463 and 527 nm which correspond to F0 → 5G2, 7F0 → 5L6, 7F0 → 5D2, and 7F0 → 5D1 characteristic electronic transitions, respectively.
Under 238 nm UV excitation, the SYGO:0.05Eu3+ phosphor sample exhibited sharp emission
SC
lines from 500 to 750 nm, corresponding to the 5D0 → 7FJ (J = 0, 1, 2, 3 and 4) electronic transitions of Eu3+ ions. The peaks at 587 and 594 nm are assigned with the 5D0 → 7F0 and 5D0
M AN U
→ 7F1 magnetic-dipole transitions and an intense sharp emission peak occurred at 594 nm. The remaining emission peaks were observed at 610, 654 and 706 nm with the corresponding 5D0 → 7
F2, 5D0 → 7F3 and 5D0 → 7F4 electron transitions, respectively [36, 37]. It is noticeable that the
both emission spectra are not similar at 238 and 393 nm excitation wavelengths. Upon the 238
TE D
nm excitation, an orange-red emission peak at 594 nm is predominant with the 5D0 → 7F1 magnetic-dipole transition, which describes that Eu3+ ions occupied lattice sites with inversion centrosymmetry. Under characteristic 393 nm excitation wavelength, the intense red emission
EP
exhibited with the corresponding 5D0 → 7F2 electric-dipole transition due to the lack of inversion symmetry. When excited at 238 nm, more Eu3+ ions were present in the inversion
AC C
centrosymmety sites which enhanced the 5D0 → 7F1 magnetic-dipole transition, showing an orange red emission as the principal product. Conversely, under 393 nm excitation, the less number of Eu3+ ions appeared in the inversion site, more will be the 5D0 → 7F2 electric-dipole transition and exhibited red luminescence. Thus, the 5D0 → 7F1 magnetic-dipole transition showed stronger orange emission than the red emissive 5D0 → 7F2 electric-dipole transition [38, 39]. The valence state and ionic radius of the added Eu3+ ions are relevant to occupy the Y3+ site
ACCEPTED MANUSCRIPT
in the SYGO host lattice, leading to an intense orange-red emission at 594 nm in the emission spectra. The inset of Fig. 5 shows the PL emission intensity of SYGO:Eu3+ as a function of Eu3+ ion concentration under 238 nm excitation. As the Eu3+ ion concentration increased up to 0.05
RI PT
mol, the orange emission intensity increased and then decreased with further increasing the Eu3+ ion concentration due to the concentration quenching effect. Thus, the optimum Eu3+ ion concentration was found to be 0.05 mol in the SYGO host lattice. Therefore, the SYGO:0.05Eu3+
SC
phosphor can be utilized as a probable orange emissive phosphor material for WLEDs. The spectral overlap occurred between the sharp excitation lines of the Eu3+ ions and the broadband
M AN U
blue emissive Bi3+ ions doped SYGO host lattice as shown in Fig. S2, indicating that the resonance type of energy transfer mechanism happened from Bi3+ to Eu3+ ions according to the Dexter proposal [40]. Consequently, Bi3+ ions are incorporated into the SYGO:0.05Eu3+ phosphor and are expected to act as a sensitizer to enhance the absorption of Eu3+ ions in the
TE D
NUV region and transfer the energy to Eu3+ ions.
3.3. PL properties of SYGO:Bi3+/ Eu3+ phosphors and energy transfer mechanism from Bi3+
EP
to Eu3+ ions.
Fig. 6(a) and (b) shows the PLE and PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+
AC C
phosphor sample monitored at their corresponding emission and excitation wavelengths, respectively. The co-doped SYGO:0.02Bi3+/0.05Eu3+ phosphor sample revealed a broadband blue emission along with a strong orange emission when monitored at 374 nm excitation wavelength. Under 332 and 393 nm excitations, similar emission results were observed with different intensities as shown in Fig. 6(b). By monitoring at 594 nm emission wavelength, the PLE spectrum showed the broadband from 215 to 475 nm with band maxima at 380 nm due to the spectral overlap of the f-f transitions of Eu3+ ions and the excitation spectrum of the Bi3+ (Fig.
ACCEPTED MANUSCRIPT
6(a)). The excitation spectrum consists of the CTB of Eu3+-O2- ions, the Bi3+ excitation band and the characteristic excitation band of Eu3+ ions centered at 237, 374 and 393 nm, respectively. From the spectrum, Bi3+ (374 nm) excitation band exhibited stronger excitation intensity than the
RI PT
CTB of Eu3+-O2- ions (237 nm), which demonstrates that the effective energy transfer occurred from Bi3+ to Eu3+ ions. When monitored at 467 nm emission wavelength, the PLE spectrum of SYGO:0.02Bi3+/0.05Eu3+ phosphor is similar to that of the Bi3+ single-doped SYGO phosphor.
SC
Consequently, the PLE spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor suits with the NUV
M AN U
(360-410 nm) chip and can be potentially applicable for WLEDs.
Fig. 7 shows the PL emission spectra of SYGO:0.02Bi3+/yEu3+ (y = 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mol) phosphors under 374 nm excitation wavelength. When the Eu3+ ion concentration increased, the orange emission intensity gradually increased at 594 nm and the blue emission intensity at 467 nm monotonically decreased due to the efficient energy
TE D
transfer from Bi3+ to Eu3+ ions. The emission intensity of Eu3+ ions increased up to y = 0.05 mol, and then it decreased above y = 0.05 mol due to the general concentration quenching effect. Thus, y = 0.05 mol represents the optimum concentration to the co-doped SYGO:0.02Bi3+/yEu3+
EP
phosphors. The obtained results give one more endorsement to the existence of energy transfer from Bi3+ to Eu3+ ions. Additionally, the percentage of energy transfer efficiency from the Bi3+
AC C
sensitizer to the Eu3+ acceptor can be estimated by the following expression [41, 42]:
η/ = 1 −
(1
(12
.
(4)
Here, ηT is the energy transfer efficiency, Is and Iso are the corresponding emission intensities with and without dopant ions in the SYGO host lattice, respectively. Therefore, with the increase of Eu3+ ion concentration in SYGO:0.02Bi3+/yEu3+ phosphors, the efficient energy transfer from
ACCEPTED MANUSCRIPT
Bi3+ to Eu3+ ions gradually increased and the maximum energy transfer efficiency was found to be 62.90%, as shown in Fig. 8. The possible schematic energy transfer mechanism from Bi3+ to Eu3+ ions was depicted in the inset of Fig. 8. From the schematic diagram, the 1S0 electrons of
RI PT
the Bi3+ ions absorb the excitation energy from the ground state and enter into the 3P1 excited state. The only some of the excited electrons come back to the ground state via the relaxation process to produce the blue broadband luminescence and the remaining electrons are utilized for
SC
the energy transfer mechanism with the Eu3+ ions. In addition, Eu3+ ions can also be excited from the ground state to the 5G2 excited state and then the energy will descend to the first 5D0 excited
M AN U
state. Therefore, the possible Eu3+ emission lines were obtained with their corresponding electronic transitions.
Typically, the energy transfer from the sensitizer to activator happens through the resonance-type mechanism and it is a favor to the exchange interactions and multipolar
TE D
interactions. The both exchange and multipolar interactions strongly depend on the distance between the both the sensitizer and activator ions at higher concentrations. The critical distance between the Bi3+ and Eu3+ ions was determined using the Eq. (2), where the χc is treated as the
EP
critical concentration of the dopant ions at which the luminescence intensity of Bi3+ is half of that
AC C
sample without the existence of Eu3+ ions and it is 0.07. Thus, the Rc value for the energy transfer was determined to be 23.04 Å. The obtained Rc value exceeds the required exchange interaction value (5 to 8 Å). This result also excludes the exchange interaction mechanism. Hence, in this case, the energy transfer between the Bi3+ and Eu3+ ions is mainly dependent on the multipolar interactions. By the below given equation, the multipolar interactions can be identified on account of Dexter’s energy transfer proposal:[43, 44]
ACCEPTED MANUSCRIPT
312 31
:
∝ C(56789) # ,
(5)
where ηso and ηs are the PL quantum efficiencies of the Bi3+ sensitizer without and with the
RI PT
presence of the Eu3+ activator ions, respectively. C represents the sum of the concentrations of the Bi3+ and Eu3+ ions and α is analogous to θ in the Eq. (3). Nevertheless, ηso/ηs is strenuous to determine and it can be approximately estimated by taking Iso/ Is instead of ηso/ ηs. Iso and Is are
SC
the relative PL emission intensities of the Bi3+ sensitizer without and with the presence of the Eu3+ activator ions, respectively. Thus, the above equation can be modified as follows: :
M AN U
(12 (1
∝ C(56789) # .
(6)
The relationship between the Iso/ Is and C(Bi+Eu) at different α (6, 8 and 10) values was depicted in Fig. S3. It is noticed that the fitting in Fig. S3(c) is more linear when compared with Fig. S3(a)
TE D
and (b) and the corresponding α value for the linearly fitted graph is 10. Therefore, quadrupolequadrupole interactions are mainly responsible for the energy transfer mechanism from Bi3+ to Eu3+ ions. Furthermore, the luminescence decay curves were measured for SYGO:0.02Bi3+/yEu3+
EP
(y = 0, 0.005, 0.01, 0.03, 0.05, and 0.07 mol) phosphors under 374 nm excitation and 467 nm emission wavelengths, respectively, as shown in Fig 9. The decay profiles of all the phosphors
AC C
were well fitted to a second-order exponential function. The calculated average lifetime values of SYGO:0.02Bi3+/yEu3+ (y = 0, 0.005, 0.01, 0.03, 0.05, and 0.07 mol) phosphor samples were 13.40, 12.66, 12.14, 8.94, 4.67 and 3.11 µs. From the decay curves, it is clear that the lifetime values of the phosphors decreased from 13.40 to 3.11 µs with increasing the Eu3+ ion concentration. The monotonous decrease of lifetime values under 467 nm emission wavelength confirms the existence of energy transfer from Bi3+ to Eu3+ ions [16, 17].
ACCEPTED MANUSCRIPT
3.4. Thermal properties of the SYGO:0.02Bi3+/0.05Eu3+ phosphor For better WLEDs practical applications, the thermal PL property is one of the most
RI PT
popular and essential parameters for phosphors. Fig. 10(a) represents the temperature-dependent PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor sample under 374 nm excitation in the temperature range of 30 to 210 °C (step of 20 °C). From the spectra, it is noticeable that the luminescence intensity, position and shape gradually changed with increasing the operating
SC
temperature. It is clear that the reason for the decrease of emission intensity is ascribed to the
M AN U
thermal quenching effect. The relative emission intensities of Bi3+ and Eu3+ ions at 150 °C maintained to about 59 and 38%, respectively. The decline rate of emission intensities of Bi3+ and Eu3+ ions in the SYGO:0.02Bi3+/0.05Eu3+ phosphor is completely different due to the energy transfer process from Bi3+ to Eu3+ ions. According to Arrhenius equation, the activation energy (Ea) for the Bi3+ emission in the SYGO:0.02Bi3+/0.05Eu3+ phosphor is estimated by the
TE D
following expression [45, 46]:
(
8
ln ' <* = ln A − > , (
/
(7)
EP
where I0 and I are the luminescence intensities of the optimized SYGO:0.02Bi3+/0.05Eu3+
AC C
phosphor at room temperature and applied temperatures, respectively. A is an Arrhenius’s constant, k is a Boltzmann’s constant (8.617 × 10-5 eV/K). The corresponding plots of ln (I0/I – 1) versus 1/kT could be fit to a straight line with the slope of -0.3033 as shown in Fig. 10(b). The calculated Ea value can be inferred to be 0.3033 eV. Therefore, the higher Ea value means that the prepared SYGO:0.02Bi3+/0.05Eu3+ phosphor has good thermal properties. Fig. S4 shows the shift of Commission International de I’Eclairage (CIE) chromaticity coordinates from (0.2863, 0.2094) to (0.2175, 0.2003) when the temperature increased from 30 to 210 °C. The calculated
ACCEPTED MANUSCRIPT
CIE values at different temperatures are presented in the inset of Fig. S4. The color tunability of the SYGO:0.02Bi3+/0.05Eu3+ phosphor can also assess the application in LEDs.
RI PT
3.5. Quantum yield and CIE chromaticity coordinates Moreover, quantum yield (ηQE) is another important parameter to estimate the efficiency
sphere method based on the following formula [47]: @ A B
@ 8C D@ 8B
,
(8)
M AN U
η?8 =
SC
of the optimized SYGO:0.02Bi3+/0.05Eu3+ phosphor and it was determined by an integrated
where LS represents the emission spectrum of the optimized sample, ES is the spectrum of light which is used for the exciting the sample and ER is the spectrum of excitation light in the absence of the sample, respectively. Accordingly, from the measurement, the estimated quantum yield of
TE D
the optimized SYGO:0.02Bi3+/0.05Eu3+ phosphor under 374 nm excitation wavelength is about 20.5%.
Fig. 11 shows the calculated CIE chromaticity coordinates for the SYGO:0.02Bi3+/yEu3+
EP
(y = 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mol) phosphors. The CIE chromaticity coordinates varied from (0.1277, 0.1091) to (0.2868, 0.2098) corresponding to y = 0 to 0.05 mol,
AC C
locating in the blue and cool white regions, respectively. Accordingly, tunable emissions are possible from blue to cool white under NUV excitation and these emissions occurred due to the energy transfer process from Bi3+ to Eu3+ ions by adjusting the Eu3+ ion concentration in the SYGO host lattice. The calculated energy transfer efficiencies and CIE chromaticity coordinate values are listed in Table 1. 4. Conclusions
ACCEPTED MANUSCRIPT
Bi3+, Eu3+ single-doped and Bi3+/ Eu3+ ions co-doped SYGO phosphors were successfully synthesized by the facile pechini type sol-gel process. After annealing at 1200 °C, the XRD patterns confirmed the cubic structure with a space group of la3(230). The FE-SEM image
RI PT
showed a spherical-shaped morphology for the SYGO host lattice. The SYGO:Bi3+ phosphor exhibited a broadband blue emission at 467 nm with higher FWHM value of 40 nm due to the allowance of the 3P1 → 1S0 transition under 374 nm excitation wavelength. By supplying the
SC
different concentrations of orange emissive Eu3+ ions into the optimized SYGO:0.02Bi3+ phosphor, the emission color was tuned from blue to cool white due to the higher energy transfer
M AN U
process from Bi3+ to Eu3+ ions. The energy transfer process was explained using the schematic energy level diagram, by calculating the energy transfer efficiencies and the spectral overlap of the SYGO:0.02Bi3+ emission and SYGO:0.05Eu3+ excitation. This is further strongly confirmed by the variation of decay times in the SYGO:Bi3+/Eu3+ phosphors. The CIE chromaticity
TE D
coordinate were calculated to the SYGO:Bi3+/Eu3+ phosphors and are well tuned from (0.1277, 0.1091) for the SYGO:0.02Bi3+ phosphor to (0.2963, 0.2074) for the SYGO:0.02Bi3+/0.07Eu3+ phosphor. A cool white-light emission was produced at (0.2868, 0.2098) CIE values for the
EP
SYGO:0.02Bi3+/0.05Eu3+ phosphor with the quantum yield of 20.5%. The optimized phosphor showed good thermal properties with high activation energy. Therefore, from the obtained results,
AC C
we are able to suggest that the single-phased color-tunable emissions from SYGO:Bi3+/Eu3+ phosphors have potential applications in WLEDs under NUV excitations. Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No. 2015R1A5A1037656).
ACCEPTED MANUSCRIPT
References
AC C
EP
TE D
M AN U
SC
RI PT
[1] H. Liu, Y. Luo, Z. Mao, L. Liao, Z. Xia, A novel single-composition trichromatic whiteemitting Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5:Ce3+/Tb3+/Mn2+ phosphor:synthesis, luminescent properties and applications for white LEDs, J. Mater. Chem. C 2 (2014) 1619-1627. [2] G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Single-composition trichromatic white-emitting Ca4Y6(SiO4)6O:Ce3+/Mn2+/Tb3+ phosphor:luminescence and energy transfer, ACS Appl. Mater. Interfaces 4 (2012) 296-305. [3] M. Dalal, V.B. Taxak, J. Dalal, A. Khatkar, S. Chahar, R. Devi, S.P. Khatkar, Crystal structure and Judd-Ofelt properties of a novel color tunable blue-white-red Ba5Zn4Y8O21:Eu3+ nanophosphor for near-ultraviolet based WLEDs, J. Alloys Compd. 698 (2017) 662-672. [4] L. Wu, Y. Zhang, M. Gui, P. Lu, L. Zhao, S. Tian, Y. Kong, J. Xu, Luminescence and energy transfer of a color tunable phosphor: Dy3+-, Tm3+-, and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs, J. Mater. Chem. 22 (2012) 6463-6470. [5] D. Wang, J. Fan, M. Shang, K. Li, Y. Zhang, H. Lian, J. Lin, Pechini-type sol–gel synthesis and multicolor-tunable emission properties of GdY (MoO4)3:RE3+(RE=Eu, Dy, Sm, Tb) phosphors, Opt. Mater. 51 (2016) 162-170. [6] Z.-w. Zhang, J.-w. Hou, J. Li, X.-y. Wang, X.-y. Zhu, H.-x. Qi, R.-j. Lv, D.-j. Wang, Tunable luminescence and energy transfer properties of LiSrPO4: Ce3+, Tb3+, Mn2+ phosphors, J. Alloys Compd., 682 (2016) 557-564. [7] Y. Zhang, W. Gong, J. Yu, Y. Lin, G. Ning, Tunable white-light emission via energy transfer in single-phase LiGd(WO4)2:Re3+(Re = Tm, Tb, Dy, Eu) phosphors for UV-excited WLEDs, RSC Adv. 5 (2015) 96272-96280. [8] Z.-B. Tang, C.-L. Xu, X.-R. Wei, X.-G. Zhang, Y.-B. Chen, Improved photoluminescence intensity and thermal stability brought by increasing Eu3+ content in KBaY1−xEuxSi2O7 solidsolution phosphors, J. Alloys Compd. 695 (2017) 2745-2750. [9] L.K. Bharat, P. Du, J.S. Yu, Long-wave UV-A radiation excited warm white-light emitting NaGdTiO4: Tm3+/Dy3+/Eu3+ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties, J. Alloys Compd. 666 (2016) 270-278. [10] M. Zhang, Y. Liang, R. Tang, D. Yu, M. Tong, Q. Wang, Y. Zhu, X. Wu, G. Li, Highly efficient Sr3Y2(Si3O9)2:Ce3+,Tb3+/Mn2+/Eu2+ phosphors for white LEDs: structure refinement, color tuning and energy transfer, RSC Adv. 4 (2014) 40626-40637. [11] G. Li, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Tunable luminescence of Ce3+/Mn2+coactivated Ca2Gd8(SiO4)6O2 through energy transfer and modulation of excitation: potential single-phase white/yellow-emitting phosphors, J. Mater. Chem. 21 (2011) 13334-13344. [12] Z. Jiang, Y. Wang, Near-UV Excitable White Sr2Al2GeO7 :Ce3 +, Tb3 + phosphor for light emitting diodes, Electrochem. Solid-State Lett. 13 (2010) J68-J70. [13] W. Lü, N. Guo, Y. Jia, Q. Zhao, W. Lv, M. Jiao, B. Shao, H. You, Tunable Color of Ce3+/Tb3+/Mn2+-Coactivated CaScAlSiO6 via energy transfer:A single-component red/whiteemitting phosphor, Inorg. Chem. 52 (2013) 3007-3012. [14] W. Lü, Z. Hao, X. Zhang, Y. Luo, X. Wang, J. Zhang, Tunable full-color emitting BaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+ phosphors based on energy transfer, Inorg. Chem. 50 (2011) 7846-7851. [15] X. Chen, P. Dai, X. Zhang, C. Li, S. Lu, X. Wang, Y. Jia, Y. Liu, A highly efficient white light (Sr3,Ca,Ba)(PO4)3Cl:Eu2+, Tb3+, Mn2+ phosphor via dual energy transfers for white lightemitting diodes, Inorg. Chem. 53 (2014) 3441-3448.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[16] K. Li, J. Fan, M. Shang, H. Lian, J. Lin, Sr2Y8(SiO4)6O2:Bi3+/Eu3+: a single-component white-emitting phosphor via energy transfer for UV w-LEDs, J. Mater. Chem. C 3 (2015) 99899998. [17] H. Zhou, Y. Jin, M. Jiang, Q. Wang, X. Jiang, A single-phased tunable emission phosphor MgY2Si3O10: Eu3+, Bi3+ with efficient energy transfer for white LEDs, Dalton Trans. 44 (2015) 1102-1109. [18] H. Zhou, Q. Wang, Y. Jin, Temperature dependence of energy transfer in tunable white light-emitting phosphor BaY2Si3O10:Bi3+,Eu3+ for near UV LEDs, J. Mater. Chem. C 3 (2015) 11151-11162. [19] M. Thomas, P.P. Rao, S.P.K. Mahesh, L.S. Kumari, P. Koshy, Luminescence properties of Eu3+, Bi3+ coactivated CaLaNbWO8 red phosphors under near UV and blue excitations, Phys. Status Solidi A 208 (2011) 2170-2175. [20] H. Xie, F. Li, H. Xi, R. Tian, X. Wang, Luminescent properties of sol–gel processed redemitting phosphor Eu3+, Bi3+ co-doped (Ca,Sr)(Mo,W)O4, J. Mater. Sci.: Mater. Electron. 26 (2015) 23-31. [21] L. Chen, K.-J. Chen, C.-C. Lin, C.-I. Chu, S.-F. Hu, M.-H. Lee, R.-S. Liu, Combinatorial approach to the development of a single mass YVO4:Bi3+,Eu3+ phosphor with red and green dual colors for high color rendering white light-emitting diodes, J. Comb. Chem. 12 (2010) 587-594. [22] J. Su, X. Mi, J. Sun, L. Yang, C. Hui, L. Lu, Z. Bai, X. Zhang, Tunable luminescence and energy transfer properties in YVO4: Bi3+, Eu3+ phosphors, J. Mater. Sci. 52 (2017) 782-792. [23] M. Noh, S.-H. Cho, S. Park, Tunable luminescence in Bi3+ and Eu3+ co-doped Sr3AlO4F oxyfluorides phosphors, J. Lumin. 161 (2015) 343-346. [24] W. Ran, L. Wang, L. Tan, D. Qu, J. Shi, Remote Control Effect of Li+, Na+, K+ ions on the super energy transfer process in ZnMoO4:Eu3+, Bi3+ phosphors, Sci. Rep. 6 (2016) 27657. [25] R. Cao, T. Fu, D. Peng, C. Cao, W. Ruan, X. Yu, Synthesis, energy transfer and tunable emission properties of SrSb2O6:Eu3+, Bi3+ phosphor, Spectrochim. Acta, Part A 169 (2016) 192196. [26] C. Liu, Z. Xia, M.S. Molokeev, Q. Liu, Synthesis, crystal structure, and enhanced luminescence of garnet-type Ca3Ga2Ge3O12:Cr3+ by codoping Bi3+, J. Am. Ceram. Soc. 98 (2015) 1870-1876. [27] A.L. Patterson, The scherrer formula for X-Ray Particle size determination, Phys. Rev. 56 (1939) 978-982. [28] S.K. Hussain, G. Nagaraju, E. Pavitra, G. Seeta Rama Raju, J.S. Yu, La(OH)3:Eu3+ and La2O3:Eu3+ nanorod bundles: growth mechanism and luminescence properties, CrystEngComm 17 (2015) 9431-9442. [29] D. Huang, Y. Zhou, W. Xu, Z. Yang, Z. Liu, M. Hong, Y. Lin, J. Yu, Photoluminescence properties of M3+ (M3+ = Bi3+, Sm3+) activated Na5Eu(WO4)4 red-emitting phosphors for white LEDs, J. Alloys Compd. 554 (2013) 312-318. [30] K. Li, H. Lian, M. Shang, J. Lin, A novel greenish yellow-orange red Ba3Y4O9:Bi3+,Eu3+ phosphor with efficient energy transfer for UV-LEDs, Dalton Trans. 44 (2015) 20542-20550. [31] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep 24 (1969) 131-144. [32] S.J. Gwak, P. Arunkumar, W.B. Im, A new blue-Emitting oxohalide phosphor Sr4OCl6:Eu2+ for thermally stable, efficient white-light-emitting devices under near-UV, J. Phys. Chem. C 118 (2014) 2686-2692.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[33] F. Kang, Y. Zhang, M. Peng, Controlling the energy transfer via multi luminescent centers to achieve white light/tunable emissions in a single-phased X2-Type Y2SiO5: Eu3+, Bi3+ phosphor for ultraviolet converted LEDs, Inorg. Chem. 54 (2015) 1462-1473. [34] Y. Tian, B. Chen, B. Tian, R. Hua, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, Y. Zheng, T. Yu, L. Huang, Q. Meng, Concentration-dependent luminescence and energy transfer of flower-like Y2(MoO4)3:Dy3+ phosphor, J. Alloys Compd. 509 (2011) 6096-6101. [35] Y. Tian, B. Chen, R. Hua, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, Y. Zheng, T. Yu, Optical transition, electron-phonon coupling and fluorescent quenching of La2(MoO4)3:Eu3+ phosphor, J. Appl. Phys. 109 (2011) 053511. [36] Y.I. Jeon, L.K. Bharat, J.S. Yu, Eu3+ ions co-doped CLPO: Dy3+ single phase white-light emitting phosphors for near UV-based white LEDs, J. Alloys Compd. 649 (2015) 531-536. [37] V. Sivakumar, U.V. Varadaraju, A promising orange-red phosphor under near UV excitation, Electrochem. Solid-State Lett. 9 (2006) H35-H38. [38] Z. Huo, C. Chen, D. Chu, H. Li, Y. Li, Systematic synthesis of lanthanide phosphate nanocrystals, Chem. - Eur. J. 13 (2007) 7708-7714. [39] X. Liu, Q. Wang, Z. Gao, J. Sun, J. Shen, Fabrication of lanthanide phosphate nanocrystals with well-controlled morphologies by layer-by-layer adsorption and reaction method at room temperature, Cryst. Growth Des. 9 (2009) 3707-3713. [40] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836850. [41] G.A. Kumar, P. Biju, G. Jose, N. Unnikrishnan, Static energy transfer for Mn2+:Pr3+ system in phosphate glasses, Mater. Chem. Phys. 60 (1999) 247-255. [42] P. Paulose, G. Jose, V. Thomas, N. Unnikrishnan, M. Warrier, Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass, J. Phys. Chem. Solids 64 (2003) 841-846. [43] D. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (1954) 1063-1070. [44] Y.-w. Tan, C. Shi, Ce3+→ Eu2+ energy transfer in BaLiF3 phosphor, J. Phys. Chem. Solids 60 (1999) 1805-1810. [45] H. Zhu, C.C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R.-S. Liu, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014). [46] C.-H. Huang, W.-R. Liu, T.-M. Chen, Single-phased white-light phosphors Ca9Gd(PO4)7: Eu2+, Mn2+ under near-ultraviolet excitation, J. Phys. Chem. C 114 (2010) 18698-18701. [47] R. Van Deun, D. Ndagsi, J. Liu, I. Van Driessche, K. Van Hecke, A.M. Kaczmarek, Dopant and excitation wavelength dependent color-tunable white light-emitting Ln3+:Y2WO6 materials (Ln3+= Sm, Eu, Tb, Dy), Dalton Trans. 44 (2015) 15022-15030.
ACCEPTED MANUSCRIPT
Table
1
Energy
transfer
efficiencies
(η)
and
CIE
chromaticity
values
for
SYGO:0.02Bi3+/yEu3+ (y = 0 to 0.07 mol) phosphors. η from Bi3+ to Eu3+ (%) 3.66
SYGO:0.02Bi3+/0.01Eu3+
9.46
SYGO:0.02Bi3+/0.02Eu3+
16.52
SYGO:0.02Bi3+/0.03Eu3+
19.44
(0.1277, 0.1091)
(0.1390, 0.1199)
(0.1505, 0.1235)
M AN U
SYGO:0.02Bi3+/0.005Eu3+
RI PT
SYGO:0.02Bi3+
CIE values (x,y)
SYGO:0.02Bi3+/0.04Eu3+ SYGO:0.02Bi3+/0.05Eu3+ SYGO:0.02Bi3+/0.06Eu3+
EP AC C
(0.2011, 0.1568)
24.17
(0.2490, 0.1815)
33.72
(0.2868, 0.2098)
48.51
(0.2938, 0.2067)
62.90
(0.2963, 0.2074)
TE D
SYGO:0.02Bi3+/0.07Eu3+
(0.1781, 0.1449)
SC
Phosphors
the
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 1. (a) XRD patterns of the SYGO samples at different annealing temperatures and (b) XRD patterns of SYGO:0.05Eu3+, SYGO:0.02Bi3+ and SYGO:0.02Bi3+/0.05Eu3+ phosphor samples annealed at 1200 °C.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 2. (a) FE-SEM image, (b) EDS spectrum and (c-f) elemental mappings for SYGO host
AC C
EP
TE D
lattice.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Fig. 3. PLE and PL emission spectra of the SYGO:0.02Bi3+ phosphor.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4. PL emission spectra of SYGO:Bi3+ phosphors as a function of Bi3+ ion concentration under 374 nm excitation wavelength. The inset shows the relationship of log (C) versus log (I/C)
AC C
EP
for the SYGO:Bi3+ phosphors.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 5. PLE (λem =594 nm) and PL emission (λex = 238 and 393 nm) spectra of the
AC C
ion concentration.
EP
SYGO:0.05Eu3+ phosphor sample. Inset shows the PL emission intensity as a function of Eu3+
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 6. (a) PLE and (b) PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor at different emission and excitation wavelengths.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
TE D
Fig. 7. PL emission spectra of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04,
AC C
EP
0.05, 0.06 and 0.07) phosphors under 374 nm excitation wavelength.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 8. Energy transfer efficiency graph of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07) phosphors under 374 nm excitation wavelength. Inset represents
AC C
EP
the schematic energy level diagram for the energy transfer mechanism from the Bi3+ to Eu3+ ions.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 9. Decay curves of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.02, 0.03, 0.05, and 0.07) phosphors monitored at 467 nm emission and 374 nm excitation wavelengths.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 10. (a) Temperature-dependent PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor under 374 nm excitation wavelength and (b) the corresponding plot of ln (I0/I-1) versus 1/kT.
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 11. CIE chromaticity diagram for the (1) SYGO:0.02Bi3+, (2) SYGO:0.02Bi3+/0.005Eu3+, (3) SYGO:0.02Bi3+/0.01Eu3+, (4) SYGO:0.02Bi3+/0.02Eu3+, (5) SYGO:0.02Bi3+/0.03Eu3+, (6) SYGO:0.02Bi3+/0.04Eu3+, (7) SYGO:0.02Bi3+/0.05Eu3+, (8) SYGO:0.02Bi3+/0.06Eu3+, and (9) SYGO:0.02Bi3+/0.07Eu3+ phosphors.
ACCEPTED MANUSCRIPT
Highlights
EP
TE D
M AN U
SC
RI PT
Sr3Y2Ge3O12:Bi3+/Eu3+ phosphors were prepared by a pechini type sol-gel method. The morphological properties of the Sr3Y2Ge3O12 host lattice were studied. The energy transfer mechanism between Bi3+ and Eu3+ was explained. Temperature-dependent and quantum yield were measured for the optimized phosphor.
AC C
• • • •
S0925-8388(17)30399-7
DOI:
10.1016/j.jallcom.2017.01.345
Reference:
JALCOM 40712
To appear in:
Journal of Alloys and Compounds
Received Date: 28 November 2016 Revised Date:
29 January 2017
Accepted Date: 30 January 2017
Please cite this article as: S.K. Hussain, L.K. Bharat, D.H. Kim, J.S. Yu, Facile pechini synthesis of 3+ 3+ Sr3Y2Ge3O12:Bi /Eu phosphors with tunable emissions and energy transfer for WLEDs, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.01.345. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Graphical abstract:
ACCEPTED MANUSCRIPT
Facile pechini synthesis of Sr3Y2Ge3O12:Bi3+/Eu3+ phosphors with tunable emissions and energy transfer for WLEDs
RI PT
Sk. Khaja Hussain, L. Krishna Bharat, Dong Hyun Kim and Jae Su Yu*
*Corresponding author
M AN U
E-mail: [email protected] Phone: +82-31-201-3820; FAX: +82-206-2820.
SC
Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Republic of Korea.
Abstract
AC C
EP
TE D
A series of Sr3Y2Ge3O12 (SYGO), SYGO:Bi3+, SYGO:Eu3+ and SYGO:Bi3+/Eu3+ phosphors were synthesized by a facile pechini type sol-gel method. X-ray diffraction patterns exhibited a characteristic cubic lattice structure with a space group of la3(230) after annealing at 1200 °C for 8 h. The morphological properties of the SYGO host lattice were studied by a field-emission scanning electron microscope, which displayed nearly spherical-shaped particles. Under ultraviolet (UV) or near-UV excitations, the SYGO:Bi3+ phosphors showed the blue emission of Bi3+ (3p1 → 1s0) at 467 nm and SYGO:Eu3+ phosphors exhibited orange red emission (5D0 → 7F1) at 594 nm, respectively. The energy transfer process was investigated in between the Bi3+ and Eu3+ ions by the wide spectral overlap of Bi3+ emission spectrum and Eu3+ excitation spectrum. The efficient energy transfer phenomenon from Bi3+ to Eu3+ was studied by adjusting the Eu3+ ion concentration in SYGO:Bi3+/Eu3+ phosphors and it was strongly confirmed by their luminescence decay curves. The critical distance was calculated for the energy transfer process from Bi3+ to Eu3+ due to the concentration quenching effect and it was further demonstrated to be a resonance type of quadrupole-quadrupole interactions. The obtained photoluminescence results revealed tunable emissions from blue to cool white, depending on the Eu3+ ion concentration. Furthermore, thermal properties and quantum yield of the optimized cool white-light emitting SYGO:0.02Bi3+/0.05Eu3+ phosphor sample were studied and it exhibited good thermal stability with 20.5% quantum yield. Commission International de I’Eclairage chromaticity coordinates were calculated for all the prepared samples. Therefore, all these results indicate that the SYGO:0.02Bi3+/0.05Eu3+ is a promising phosphor for the application of white light-emitting diodes. Keywords: Garnet phosphors; Energy transfer; Tunable emissions; White-light emitting diodes
ACCEPTED MANUSCRIPT
1. Introduction White light-emitting diodes (WLEDs) as a next-generation lighting source have appeared
RI PT
to be the most promising in a typical form of solid-state lighting owing to their superior properties such as low power consumption, high brightness, long working lifetime, durable energy saving capability, and eco-friendly feature with mercury-free properties and less thermal radiations [1-3]. Generally, available WLEDs in the market are based on blue light-emitting
SC
InGaN chips in the wavelength range of 450-470 nm, coated with yellow emitting Y3Al5O12:Ce3+
M AN U
(YAG:Ce3+) phosphors, which is one of the widely used techniques [4-6]. The YAG:Ce3+ based WLEDs have some merits such as cost effectiveness, single-chip implementation and circuit flexibility. However, they suffer from low color-rendering index (CRI) which is < 80 due to the deficiency of the green and red color emitting phosphor components in the lower energy region, high correlated color temperature, and thermal quenching at elevated temperatures[7, 8].
TE D
Consequently, ultraviolet (UV) or near-UV (NUV) LEDs coated with novel single-phase tricolor-emitting phosphors are considered as an alternative for getting the white-light emission
EP
with higher CRI (> 90) values.[7, 9, 10]
Recently, increasing attention has been focused on tunable emissions from a single
AC C
component with energy transfer process mechanism from sensitizer to acceptor in many host lattices by doping with the co-activators such as Ce3+/Mn2+[11], Ce3+/Eu2+, Ce3+/Tb3+[12], Ce3+/Tb3+/Mn2+[13], and Eu2+/Tb3+/Mn2+[14, 15]. The tunable emissions occur due to their broadband emission spectra with higher intensities. Besides, Bi3+/Eu3+ have been introduced and co-doped into distinct host lattices to obtain the tunable emissions including white light from a single composition[16-18]. It is well known that Eu3+ ions doped phosphors exhibit pure red or orange-red emissions owing to their 5D0 → 7F2 electric dipole and 5D0 → 7F1 magnetic dipole
ACCEPTED MANUSCRIPT
transitions of Eu3+ ions, respectively. The incorporation of Bi3+ sensitizer enhances the emission intensities into various host lattices such as tungstates [19], silicates [16], molybdates [20], and vanadates [21, 22], exhibiting tunable blue-green emissions along with the red emissions in
RI PT
between 400 to 700 nm wavelength due to the characteristic 1S0 → 3P1 electronic transition under NUV excitation[16]. The energy transfer process from Bi3+ to Eu3+ ions and the white-light emission in a single phase composition have been widely investigated in many phosphor
SC
materials [16-18, 23-25]. In this manner, we made an attempt to enhance the emission intensities and study the energy transfer process from Bi3+ to Eu3+ ions in a novel germanium garnet-type
M AN U
metal oxide phosphor.
The trivalent rare earth (RE3+) ions doped garnet-type A3B2X3O12 (A = Ca, Sr, B= Y, Ga, In, Sc, Al and X = Si, Ge) phosphors such as Ca3Sc2Si3O12 (CSSO) and YAG have been investigated as good host lattice materials for luminescent properties. Sr3Y2Ge3O12 (SYGO) is
TE D
also a garnet-type phosphor which is analogous to CSSO silicate phosphor including the cubic structure with the space group of la3(230) [26]. It could be expected that Sr3Y2Ge3O12 phosphors exhibit excellent photoluminescence (PL) properties by activating with the RE3+ ions
EP
because Si and Ge belong to the same elemental group in the periodic table. Thus, we selected
AC C
Sr3Y2Ge3O12 as the garnet host lattice material to obtain the white-light emission under UV/NUV excitation. To our knowledge, until now, there have been no reports on the Sr3Y2Ge3O12 host lattice co-doped with Bi3+ and Eu3+ ions. In this work, we reported the synthesis of Bi3+ and/or Eu3+ single- and co-doped Sr3Y2Ge3O12 phosphor materials by a facile pechini type sol-gel method which is one of the finest synthetic methods to prepare the phosphor materials with definite morphology. The effect of annealing temperature was investigated on the pure phase formation of Sr3Y2Ge3O12 host lattice. The PL properties of the Sr3Y2Ge3O12:Bi3+/Eu3+
ACCEPTED MANUSCRIPT
phosphors were investigated in details. Also, the thermal properties and quantum yield were studied for the optimized phosphor sample. Tunable emissions with cool white-light are obtained
RI PT
based on the concentration of the Bi3+ and Eu3+ ions in SYGO:Bi3+/Eu3+ phosphors. 2. Experimental procedures
A series of Sr3Y2Ge3O12 host lattice, Sr3Y2-xGe3O12:x (Bi3+ or Eu3+) (x = 0.005, to 0.07
SC
mol) single-doped and Sr3Y1.98-yGe3O12:0.02Bi3+/yEu3+ (y = 0.005 to 0.07 mol) co-doped phosphor samples were synthesized by a facile pechini type sol-gel method using the
M AN U
stoichiometric amounts of high-purity chemicals (Sigma-Aldrich Co.) such as strontium nitrate (Sr(NO3)2), yttrium nitrate hexahydrate (Y(NO3)3 . 6H2O), germanium oxide (GeO2), europium nitrate pentahydrate (Eu(NO3)3) . 5H2O), bismuth nitrate pentahydrate (Bi(NO3)3 . 5H2O), citric acid (HCO(COOH)(CH2COOH)2), and poly (ethylene glycol) (PEG) ((C2H4O)n . H2O). At first,
TE D
in a 50 mL beaker, 3 mol of germanium oxide was dissolved in 10 mL of concentrated nitric acid. After 10 min, 20 mL of de-ionized (DI) water was added to get complete germanium nitrate solution (solution I). The solution II was prepared by mixing the stoichiometric amounts of
EP
strontium nitrate, yttrium nitrate hexahydrate, europium nitrate pentahydrate/bismuth nitrate pentahydrate in 200 mL of DI water. After 30 min of stirring, the solution I was added slowly to
AC C
the solution II. A 16 mol of complexing agent citric acid (2:1 ratio of complexing agent and metal ions) was added to the above solution mixture and maintained stirring for 15 min. At last, 1 g of PEG was added as a cross-linking agent. The supposed citric acid in the solution not only makes a stable metal complex network but also produces polyesterification with the added PEG via polymeric resin process to minimize the segregation in between the metal ions. Later, the beaker was covered with a polyethylene cap and the stirring was continued for 1 h for complete homogenization process of all the reactants. Furthermore, the stable metal complex mixture
ACCEPTED MANUSCRIPT
solution was heated on hot plate at 85 °C for about 2 h with magnetic stirring. After progressive reaction time, the magnetic bead and the cap were removed, and then the solution slowly evaporated, which results in the formation of brownish wet gel. The wet gel was dried in an oven
RI PT
at 120 °C for 1 day in ambient atmosphere, which produces a white colored porous solid matrix called as a xerogel. The precursor xerogel was calcined at 500 °C for 5 h to form black colored flakes. The polymeric resins at a moderate temperature of 500 °C created pure phase
SC
multicomponent metal oxides. Finally, for characterization, the obtained samples were annealed
M AN U
in a muffle furnace at 1200 °C for 8 h.
The X-ray diffraction (XRD) patterns of the all the prepared Sr3Y2Ge3O12 (hereafter referred as SYGO), SYGO:Bi3+ and SYGO:Bi3+/Eu3+ phosphor samples annealed at 1200 °C were characterized on a Mac Science M18XHF-SRA X-ray powder diffractometer. The morphological properties of the SYGO host lattice were measured by using a high-resolution
TE D
field-emission scanning electron microscope (HR FE-SEM: CARL ZEISS, MERLIN). The PL properties of the synthesized samples were studied by using a fluorescence spectrometer (FluoroMate FS-2, Scinco, South Korea) with xenon lamp as an excitation source and the
EP
luminescence decay lifetimes were recorded on a Photon Technology International (PTI, USA)
AC C
phosphorimeter attachment to the main system with a Xe flash lamp of 25 W power. The quantum yield was characterized by using a Hamamatsu Photonics C9920-02 system with an integrating sphere. Furthermore, temperature-dependent PL spectra (30 to 210 °C) were recorded on the same fluorescence spectrometer (FluoroMate FS-2, Scinco, South Korea) equipped with a thermocouple in a temperature controlled chamber (NOVA ST540). 3. Results and discussion 3.1. XRD and elemental analyses of SYGO and SYGO:Bi3+/Eu3+ phosphors
ACCEPTED MANUSCRIPT
Fig. 1(a) shows the XRD patterns of the synthesized SYGO samples and the SYGO samples annealed at different temperatures in the range of 1000 to1300 °C. The SYGO phosphor at 1000 °C exhibited an intermediate phase with major diffraction peak. As the annealing
RI PT
temperature increased to 1100 °C, the corresponding diffraction pattern peak intensities significantly increased, which indicates the good crystallinity of the SYGO samples with increased crystallite sizes. At 1200 °C, all the diffraction peaks are attributed to the pure cubic
SC
crystal phase of garnet with a space group of la3(230) and are in good agreement with the available standard JCPDS # 85-2410. By increasing the annealing temperature to 1300 °C, the
M AN U
SYGO sample revealed the formation of Y2Ge2O7 (JCPDS # 38-0288) impurity peaks. Thus, we confirmed that 1200 °C is an optimum temperature for pure phase formation of the SYGO host lattice. Commonly, the crystallite sizes of the metal oxides can be calculated by the Scherrer equation [27, 28]:
.
TE D
D =
(1)
Here, D is the average grain size or crystallite size, k (0.9) is a shape factor, β is the full width at
EP
half maximum (FWHM), λ is wavelength (1.5406 Å) and θ is the diffraction angle of the noticed peaks, respectively. The major diffraction peaks were used to calculate the crystallite sizes of the
AC C
SYGO samples prepared at 1000, 1100, 1200 and 1300 °C annealing temperatures, yielding the average crystallite sizes of 57, 69.5, 79.4 and 75.8 nm, respectively. Fig. 1(b) shows the XRD patterns of the SYGO:0.05 mol Eu3+ (i.e., SYGO:0.05Eu3+), SYGO:0.02Bi3+ and SYGO:0.02Bi3+/0.05Eu3+ phosphors, respectively. Any impurity peaks or other significant changes were not identified by the optimized single and co-doping of Eu3+ or Bi3+ ions in the current phase form of SYGO host lattice, which demonstrates that it has good stability for the accommodation of dopant ions. The lattice constant a of the SYGO host lattice (a = 13.04),
ACCEPTED MANUSCRIPT
SYGO:0.02Bi3+ phosphor (a = 12.97), SYGO:0.05Eu3+ phosphor (a = 12.96), and SYGO:0.02Bi3+/0.05Eu3+ phosphor (a = 12.98) were calculated based on the formula 1/d2 = (h2+k2+l2)/a2 for cubic structures. The obtained lattice constant values were in good agreement
RI PT
with the values reported in JCPDS # 85-2410 (a = 13.08). Fig. 2 (a) shows the FE-SEM images of the SYGO host lattice annealed at 1200 °C. The FE-SEM images revealed that the particles were nearly spherical in shape with non-identical sizes. The morphology of the SYGO host
SC
lattice materials was not effected distinctly for single- and co-doping of Eu3+ or Bi3+ ions as shown in Fig. S1. The spherical-shaped particles are more desirable, exhibiting better
M AN U
photoluminescent properties due to their lower scattering of light and higher packing density. Additionally, it is also noticed that the FE-SEM image displays particle sizes of >1 µm which are different from the calculated average crystallite size for the SYGO host lattice (~80 nm), because a particle may be composed of one or more crystallites. Fig. 2(b) shows the energy-dispersive X-
TE D
ray spectroscopy (EDS) spectrum of the SYGO host lattice. The EDS spectrum established that all the elements were homogenously distributed in the host lattice and no elements other than the Sr, Y, Ge and O were presented, which further supports the XRD results. Fig. 2(c-f) displays the
lattice.
EP
color mappings of the all the elements in a single spherical-shaped particle of the SYGO host
AC C
3.2. PL properties of SYGO:Bi3+ or Eu3+ single-doped phosphors In general, Bi3+ ion consists of ns2-type of luminescence center.[29] By Seitz models, the
Bi3+ ion is assigned with 6S2 electronic configuration from the ground state to the first 6S6P excited state. The ground state 6S2 electronic configuration of Bi3+ ion is ascribed to the 1S0 electronic transition and the first excited state is composed of three triplet state 3P0, 3P1, 3P2 and one 1P1 singlet state with increasing order of excitation energy. The transitions of 1S0 → 3P1, 1S0
ACCEPTED MANUSCRIPT
→ 3P2 and 1S0 → 1P1 can be noticeable in optical absorption spectroscopy [18, 30]. Due to the existence of the spin-orbital coupling in between the 3P1 and 1P1 excited states, the absorption bands of 1S0 → 3P1 and 1S0 → 3P2 conflict by partial allowance. The 1S0 → 3P2 absorption band is
RI PT
completely spin-forbidden, while the 1P1 → 1S0 transition may occur in the UV region. At the same time, the 3P1 → 1S0 absorption band is commonly observed in the NUV or visible region [18]. Fig. 3 shows the PL excitation (PLE) and PL emission spectra of the SYGO:0.02Bi3+
SC
phosphor monitored by the corresponding emission and excitation wavelengths, respectively. The PLE spectrum of the SYGO:0.02Bi3+ phosphor exhibited a broadband ranging from 225 to
M AN U
450 nm with the band maxima at 374 nm. The PL emission spectrum revealed a broadband blue emission from 425 to 545 nm with an origination at 467 nm due to the 3P1 → 1S0 electronic transition of the Bi3+ ions, exhibiting the FWHM of 40 nm. Therefore, the presence of the broad blue emission of Bi3+ ions with higher FWHM value can be favorable for the formation of cool
TE D
white- light by combining with a red component.
Fig. 4 shows the PL emission spectra of the Bi3+ single-doped SYGO phosphors under 374 nm excitation wavelength. From the spectra, the PL profile of all the prepared phosphor
EP
samples seems to be identical with each other. A series (0.005 to 0.05 mol) of Bi3+ ions were doped into the SYGO host lattice, and the blue emission intensity increased up to the 0.02 mol
AC C
concentration and then it started to decrease monotonously with further increase of Bi3+ ion concentration, which is attributed to the concentration quenching effect. Thus, the optimum concentration was found to be 0.02 mol. The concentration quenching occurred due to the heavier energy transfer between the Bi3+ ions in the SYGO host lattice. By increasing the Bi3+ ion concentration above its optimum level, the distance between the Bi3+-Bi3+ ions was decreased, which may promote the energy transfer, resulting in the decrease of PL emission intensity. The
ACCEPTED MANUSCRIPT
critical distance (Rc) is used to calculate the distance between the Bi3+ ions using the Blasse expression [31, 32]:
" #
!,
(2)
RI PT
R ≈ 2
where V and N are the volume and number of host cations in the unit cell, respectively, and χc is the total concentration of the Bi3+ ions. For the SYGO host lattice, V = 2241.46 Å3, N = 5 and χc
SC
= 0.02 (optimum concentration for Bi3+ ions). Therefore, the calculated critical distance value
M AN U
from the Eq. (2) is Rc = 34.98 Å. Usually, the exchange interaction is preferable for the energy transfer process when the critical distance value is restricted to about 5 to 8 Å [33]. But, the above calculated Rc value exceeds 8 Å, indicating the absence of the exchange interactions in this mechanism. Consequently, electric multipolar interaction grants to the non-radiative concentration quenching mechanism in between the two nearest Bi3+ atoms, which was found by
TE D
Huang’s theoretical expression [34, 35]: (
log ' * = K − ' * log(C),
(3)
EP
)
where K is the activator constant, I is the luminescence intensity, C is the substituted mole
AC C
fraction of the Bi3+ ions at the optimum levels and θ is the electric multipolar parameter, and it is equal to 6, 8 and 10, which corresponds to the dipole-dipole, dipole-quadrupole and quadrupolequadrupole interactions, respectively. The inset of Fig. 4 represents the linear relationship between the log(I/C) versus log(C) of Bi3+ ions and is fitted by a straight line with a slope of 1.93. Thus, θ value was found to be 10.14 and it is very close to 10. Therefore, the quadrupolequadrupole interaction commands the energy transfer process between the Bi3+ ions.
ACCEPTED MANUSCRIPT
Fig. 5 represents the PLE and PL emission spectra of the SYGO:0.05Eu3+ phosphor under their corresponding emission and excitation wavelengths, respectively. The PLE spectrum showed a broad charge transfer band (CTB) of Eu3+-O2- from 210 to 310 nm with band maxima
7
RI PT
at 238 nm and also exhibited the f-f transitions at 380, 393, 463 and 527 nm which correspond to F0 → 5G2, 7F0 → 5L6, 7F0 → 5D2, and 7F0 → 5D1 characteristic electronic transitions, respectively.
Under 238 nm UV excitation, the SYGO:0.05Eu3+ phosphor sample exhibited sharp emission
SC
lines from 500 to 750 nm, corresponding to the 5D0 → 7FJ (J = 0, 1, 2, 3 and 4) electronic transitions of Eu3+ ions. The peaks at 587 and 594 nm are assigned with the 5D0 → 7F0 and 5D0
M AN U
→ 7F1 magnetic-dipole transitions and an intense sharp emission peak occurred at 594 nm. The remaining emission peaks were observed at 610, 654 and 706 nm with the corresponding 5D0 → 7
F2, 5D0 → 7F3 and 5D0 → 7F4 electron transitions, respectively [36, 37]. It is noticeable that the
both emission spectra are not similar at 238 and 393 nm excitation wavelengths. Upon the 238
TE D
nm excitation, an orange-red emission peak at 594 nm is predominant with the 5D0 → 7F1 magnetic-dipole transition, which describes that Eu3+ ions occupied lattice sites with inversion centrosymmetry. Under characteristic 393 nm excitation wavelength, the intense red emission
EP
exhibited with the corresponding 5D0 → 7F2 electric-dipole transition due to the lack of inversion symmetry. When excited at 238 nm, more Eu3+ ions were present in the inversion
AC C
centrosymmety sites which enhanced the 5D0 → 7F1 magnetic-dipole transition, showing an orange red emission as the principal product. Conversely, under 393 nm excitation, the less number of Eu3+ ions appeared in the inversion site, more will be the 5D0 → 7F2 electric-dipole transition and exhibited red luminescence. Thus, the 5D0 → 7F1 magnetic-dipole transition showed stronger orange emission than the red emissive 5D0 → 7F2 electric-dipole transition [38, 39]. The valence state and ionic radius of the added Eu3+ ions are relevant to occupy the Y3+ site
ACCEPTED MANUSCRIPT
in the SYGO host lattice, leading to an intense orange-red emission at 594 nm in the emission spectra. The inset of Fig. 5 shows the PL emission intensity of SYGO:Eu3+ as a function of Eu3+ ion concentration under 238 nm excitation. As the Eu3+ ion concentration increased up to 0.05
RI PT
mol, the orange emission intensity increased and then decreased with further increasing the Eu3+ ion concentration due to the concentration quenching effect. Thus, the optimum Eu3+ ion concentration was found to be 0.05 mol in the SYGO host lattice. Therefore, the SYGO:0.05Eu3+
SC
phosphor can be utilized as a probable orange emissive phosphor material for WLEDs. The spectral overlap occurred between the sharp excitation lines of the Eu3+ ions and the broadband
M AN U
blue emissive Bi3+ ions doped SYGO host lattice as shown in Fig. S2, indicating that the resonance type of energy transfer mechanism happened from Bi3+ to Eu3+ ions according to the Dexter proposal [40]. Consequently, Bi3+ ions are incorporated into the SYGO:0.05Eu3+ phosphor and are expected to act as a sensitizer to enhance the absorption of Eu3+ ions in the
TE D
NUV region and transfer the energy to Eu3+ ions.
3.3. PL properties of SYGO:Bi3+/ Eu3+ phosphors and energy transfer mechanism from Bi3+
EP
to Eu3+ ions.
Fig. 6(a) and (b) shows the PLE and PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+
AC C
phosphor sample monitored at their corresponding emission and excitation wavelengths, respectively. The co-doped SYGO:0.02Bi3+/0.05Eu3+ phosphor sample revealed a broadband blue emission along with a strong orange emission when monitored at 374 nm excitation wavelength. Under 332 and 393 nm excitations, similar emission results were observed with different intensities as shown in Fig. 6(b). By monitoring at 594 nm emission wavelength, the PLE spectrum showed the broadband from 215 to 475 nm with band maxima at 380 nm due to the spectral overlap of the f-f transitions of Eu3+ ions and the excitation spectrum of the Bi3+ (Fig.
ACCEPTED MANUSCRIPT
6(a)). The excitation spectrum consists of the CTB of Eu3+-O2- ions, the Bi3+ excitation band and the characteristic excitation band of Eu3+ ions centered at 237, 374 and 393 nm, respectively. From the spectrum, Bi3+ (374 nm) excitation band exhibited stronger excitation intensity than the
RI PT
CTB of Eu3+-O2- ions (237 nm), which demonstrates that the effective energy transfer occurred from Bi3+ to Eu3+ ions. When monitored at 467 nm emission wavelength, the PLE spectrum of SYGO:0.02Bi3+/0.05Eu3+ phosphor is similar to that of the Bi3+ single-doped SYGO phosphor.
SC
Consequently, the PLE spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor suits with the NUV
M AN U
(360-410 nm) chip and can be potentially applicable for WLEDs.
Fig. 7 shows the PL emission spectra of SYGO:0.02Bi3+/yEu3+ (y = 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mol) phosphors under 374 nm excitation wavelength. When the Eu3+ ion concentration increased, the orange emission intensity gradually increased at 594 nm and the blue emission intensity at 467 nm monotonically decreased due to the efficient energy
TE D
transfer from Bi3+ to Eu3+ ions. The emission intensity of Eu3+ ions increased up to y = 0.05 mol, and then it decreased above y = 0.05 mol due to the general concentration quenching effect. Thus, y = 0.05 mol represents the optimum concentration to the co-doped SYGO:0.02Bi3+/yEu3+
EP
phosphors. The obtained results give one more endorsement to the existence of energy transfer from Bi3+ to Eu3+ ions. Additionally, the percentage of energy transfer efficiency from the Bi3+
AC C
sensitizer to the Eu3+ acceptor can be estimated by the following expression [41, 42]:
η/ = 1 −
(1
(12
.
(4)
Here, ηT is the energy transfer efficiency, Is and Iso are the corresponding emission intensities with and without dopant ions in the SYGO host lattice, respectively. Therefore, with the increase of Eu3+ ion concentration in SYGO:0.02Bi3+/yEu3+ phosphors, the efficient energy transfer from
ACCEPTED MANUSCRIPT
Bi3+ to Eu3+ ions gradually increased and the maximum energy transfer efficiency was found to be 62.90%, as shown in Fig. 8. The possible schematic energy transfer mechanism from Bi3+ to Eu3+ ions was depicted in the inset of Fig. 8. From the schematic diagram, the 1S0 electrons of
RI PT
the Bi3+ ions absorb the excitation energy from the ground state and enter into the 3P1 excited state. The only some of the excited electrons come back to the ground state via the relaxation process to produce the blue broadband luminescence and the remaining electrons are utilized for
SC
the energy transfer mechanism with the Eu3+ ions. In addition, Eu3+ ions can also be excited from the ground state to the 5G2 excited state and then the energy will descend to the first 5D0 excited
M AN U
state. Therefore, the possible Eu3+ emission lines were obtained with their corresponding electronic transitions.
Typically, the energy transfer from the sensitizer to activator happens through the resonance-type mechanism and it is a favor to the exchange interactions and multipolar
TE D
interactions. The both exchange and multipolar interactions strongly depend on the distance between the both the sensitizer and activator ions at higher concentrations. The critical distance between the Bi3+ and Eu3+ ions was determined using the Eq. (2), where the χc is treated as the
EP
critical concentration of the dopant ions at which the luminescence intensity of Bi3+ is half of that
AC C
sample without the existence of Eu3+ ions and it is 0.07. Thus, the Rc value for the energy transfer was determined to be 23.04 Å. The obtained Rc value exceeds the required exchange interaction value (5 to 8 Å). This result also excludes the exchange interaction mechanism. Hence, in this case, the energy transfer between the Bi3+ and Eu3+ ions is mainly dependent on the multipolar interactions. By the below given equation, the multipolar interactions can be identified on account of Dexter’s energy transfer proposal:[43, 44]
ACCEPTED MANUSCRIPT
312 31
:
∝ C(56789) # ,
(5)
where ηso and ηs are the PL quantum efficiencies of the Bi3+ sensitizer without and with the
RI PT
presence of the Eu3+ activator ions, respectively. C represents the sum of the concentrations of the Bi3+ and Eu3+ ions and α is analogous to θ in the Eq. (3). Nevertheless, ηso/ηs is strenuous to determine and it can be approximately estimated by taking Iso/ Is instead of ηso/ ηs. Iso and Is are
SC
the relative PL emission intensities of the Bi3+ sensitizer without and with the presence of the Eu3+ activator ions, respectively. Thus, the above equation can be modified as follows: :
M AN U
(12 (1
∝ C(56789) # .
(6)
The relationship between the Iso/ Is and C(Bi+Eu) at different α (6, 8 and 10) values was depicted in Fig. S3. It is noticed that the fitting in Fig. S3(c) is more linear when compared with Fig. S3(a)
TE D
and (b) and the corresponding α value for the linearly fitted graph is 10. Therefore, quadrupolequadrupole interactions are mainly responsible for the energy transfer mechanism from Bi3+ to Eu3+ ions. Furthermore, the luminescence decay curves were measured for SYGO:0.02Bi3+/yEu3+
EP
(y = 0, 0.005, 0.01, 0.03, 0.05, and 0.07 mol) phosphors under 374 nm excitation and 467 nm emission wavelengths, respectively, as shown in Fig 9. The decay profiles of all the phosphors
AC C
were well fitted to a second-order exponential function. The calculated average lifetime values of SYGO:0.02Bi3+/yEu3+ (y = 0, 0.005, 0.01, 0.03, 0.05, and 0.07 mol) phosphor samples were 13.40, 12.66, 12.14, 8.94, 4.67 and 3.11 µs. From the decay curves, it is clear that the lifetime values of the phosphors decreased from 13.40 to 3.11 µs with increasing the Eu3+ ion concentration. The monotonous decrease of lifetime values under 467 nm emission wavelength confirms the existence of energy transfer from Bi3+ to Eu3+ ions [16, 17].
ACCEPTED MANUSCRIPT
3.4. Thermal properties of the SYGO:0.02Bi3+/0.05Eu3+ phosphor For better WLEDs practical applications, the thermal PL property is one of the most
RI PT
popular and essential parameters for phosphors. Fig. 10(a) represents the temperature-dependent PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor sample under 374 nm excitation in the temperature range of 30 to 210 °C (step of 20 °C). From the spectra, it is noticeable that the luminescence intensity, position and shape gradually changed with increasing the operating
SC
temperature. It is clear that the reason for the decrease of emission intensity is ascribed to the
M AN U
thermal quenching effect. The relative emission intensities of Bi3+ and Eu3+ ions at 150 °C maintained to about 59 and 38%, respectively. The decline rate of emission intensities of Bi3+ and Eu3+ ions in the SYGO:0.02Bi3+/0.05Eu3+ phosphor is completely different due to the energy transfer process from Bi3+ to Eu3+ ions. According to Arrhenius equation, the activation energy (Ea) for the Bi3+ emission in the SYGO:0.02Bi3+/0.05Eu3+ phosphor is estimated by the
TE D
following expression [45, 46]:
(
8
ln ' <* = ln A − > , (
/
(7)
EP
where I0 and I are the luminescence intensities of the optimized SYGO:0.02Bi3+/0.05Eu3+
AC C
phosphor at room temperature and applied temperatures, respectively. A is an Arrhenius’s constant, k is a Boltzmann’s constant (8.617 × 10-5 eV/K). The corresponding plots of ln (I0/I – 1) versus 1/kT could be fit to a straight line with the slope of -0.3033 as shown in Fig. 10(b). The calculated Ea value can be inferred to be 0.3033 eV. Therefore, the higher Ea value means that the prepared SYGO:0.02Bi3+/0.05Eu3+ phosphor has good thermal properties. Fig. S4 shows the shift of Commission International de I’Eclairage (CIE) chromaticity coordinates from (0.2863, 0.2094) to (0.2175, 0.2003) when the temperature increased from 30 to 210 °C. The calculated
ACCEPTED MANUSCRIPT
CIE values at different temperatures are presented in the inset of Fig. S4. The color tunability of the SYGO:0.02Bi3+/0.05Eu3+ phosphor can also assess the application in LEDs.
RI PT
3.5. Quantum yield and CIE chromaticity coordinates Moreover, quantum yield (ηQE) is another important parameter to estimate the efficiency
sphere method based on the following formula [47]: @ A B
@ 8C D@ 8B
,
(8)
M AN U
η?8 =
SC
of the optimized SYGO:0.02Bi3+/0.05Eu3+ phosphor and it was determined by an integrated
where LS represents the emission spectrum of the optimized sample, ES is the spectrum of light which is used for the exciting the sample and ER is the spectrum of excitation light in the absence of the sample, respectively. Accordingly, from the measurement, the estimated quantum yield of
TE D
the optimized SYGO:0.02Bi3+/0.05Eu3+ phosphor under 374 nm excitation wavelength is about 20.5%.
Fig. 11 shows the calculated CIE chromaticity coordinates for the SYGO:0.02Bi3+/yEu3+
EP
(y = 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mol) phosphors. The CIE chromaticity coordinates varied from (0.1277, 0.1091) to (0.2868, 0.2098) corresponding to y = 0 to 0.05 mol,
AC C
locating in the blue and cool white regions, respectively. Accordingly, tunable emissions are possible from blue to cool white under NUV excitation and these emissions occurred due to the energy transfer process from Bi3+ to Eu3+ ions by adjusting the Eu3+ ion concentration in the SYGO host lattice. The calculated energy transfer efficiencies and CIE chromaticity coordinate values are listed in Table 1. 4. Conclusions
ACCEPTED MANUSCRIPT
Bi3+, Eu3+ single-doped and Bi3+/ Eu3+ ions co-doped SYGO phosphors were successfully synthesized by the facile pechini type sol-gel process. After annealing at 1200 °C, the XRD patterns confirmed the cubic structure with a space group of la3(230). The FE-SEM image
RI PT
showed a spherical-shaped morphology for the SYGO host lattice. The SYGO:Bi3+ phosphor exhibited a broadband blue emission at 467 nm with higher FWHM value of 40 nm due to the allowance of the 3P1 → 1S0 transition under 374 nm excitation wavelength. By supplying the
SC
different concentrations of orange emissive Eu3+ ions into the optimized SYGO:0.02Bi3+ phosphor, the emission color was tuned from blue to cool white due to the higher energy transfer
M AN U
process from Bi3+ to Eu3+ ions. The energy transfer process was explained using the schematic energy level diagram, by calculating the energy transfer efficiencies and the spectral overlap of the SYGO:0.02Bi3+ emission and SYGO:0.05Eu3+ excitation. This is further strongly confirmed by the variation of decay times in the SYGO:Bi3+/Eu3+ phosphors. The CIE chromaticity
TE D
coordinate were calculated to the SYGO:Bi3+/Eu3+ phosphors and are well tuned from (0.1277, 0.1091) for the SYGO:0.02Bi3+ phosphor to (0.2963, 0.2074) for the SYGO:0.02Bi3+/0.07Eu3+ phosphor. A cool white-light emission was produced at (0.2868, 0.2098) CIE values for the
EP
SYGO:0.02Bi3+/0.05Eu3+ phosphor with the quantum yield of 20.5%. The optimized phosphor showed good thermal properties with high activation energy. Therefore, from the obtained results,
AC C
we are able to suggest that the single-phased color-tunable emissions from SYGO:Bi3+/Eu3+ phosphors have potential applications in WLEDs under NUV excitations. Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No. 2015R1A5A1037656).
ACCEPTED MANUSCRIPT
References
AC C
EP
TE D
M AN U
SC
RI PT
[1] H. Liu, Y. Luo, Z. Mao, L. Liao, Z. Xia, A novel single-composition trichromatic whiteemitting Sr3.5Y6.5O2(PO4)1.5(SiO4)4.5:Ce3+/Tb3+/Mn2+ phosphor:synthesis, luminescent properties and applications for white LEDs, J. Mater. Chem. C 2 (2014) 1619-1627. [2] G. Li, Y. Zhang, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Single-composition trichromatic white-emitting Ca4Y6(SiO4)6O:Ce3+/Mn2+/Tb3+ phosphor:luminescence and energy transfer, ACS Appl. Mater. Interfaces 4 (2012) 296-305. [3] M. Dalal, V.B. Taxak, J. Dalal, A. Khatkar, S. Chahar, R. Devi, S.P. Khatkar, Crystal structure and Judd-Ofelt properties of a novel color tunable blue-white-red Ba5Zn4Y8O21:Eu3+ nanophosphor for near-ultraviolet based WLEDs, J. Alloys Compd. 698 (2017) 662-672. [4] L. Wu, Y. Zhang, M. Gui, P. Lu, L. Zhao, S. Tian, Y. Kong, J. Xu, Luminescence and energy transfer of a color tunable phosphor: Dy3+-, Tm3+-, and Eu3+-coactivated KSr4(BO3)3 for warm white UV LEDs, J. Mater. Chem. 22 (2012) 6463-6470. [5] D. Wang, J. Fan, M. Shang, K. Li, Y. Zhang, H. Lian, J. Lin, Pechini-type sol–gel synthesis and multicolor-tunable emission properties of GdY (MoO4)3:RE3+(RE=Eu, Dy, Sm, Tb) phosphors, Opt. Mater. 51 (2016) 162-170. [6] Z.-w. Zhang, J.-w. Hou, J. Li, X.-y. Wang, X.-y. Zhu, H.-x. Qi, R.-j. Lv, D.-j. Wang, Tunable luminescence and energy transfer properties of LiSrPO4: Ce3+, Tb3+, Mn2+ phosphors, J. Alloys Compd., 682 (2016) 557-564. [7] Y. Zhang, W. Gong, J. Yu, Y. Lin, G. Ning, Tunable white-light emission via energy transfer in single-phase LiGd(WO4)2:Re3+(Re = Tm, Tb, Dy, Eu) phosphors for UV-excited WLEDs, RSC Adv. 5 (2015) 96272-96280. [8] Z.-B. Tang, C.-L. Xu, X.-R. Wei, X.-G. Zhang, Y.-B. Chen, Improved photoluminescence intensity and thermal stability brought by increasing Eu3+ content in KBaY1−xEuxSi2O7 solidsolution phosphors, J. Alloys Compd. 695 (2017) 2745-2750. [9] L.K. Bharat, P. Du, J.S. Yu, Long-wave UV-A radiation excited warm white-light emitting NaGdTiO4: Tm3+/Dy3+/Eu3+ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties, J. Alloys Compd. 666 (2016) 270-278. [10] M. Zhang, Y. Liang, R. Tang, D. Yu, M. Tong, Q. Wang, Y. Zhu, X. Wu, G. Li, Highly efficient Sr3Y2(Si3O9)2:Ce3+,Tb3+/Mn2+/Eu2+ phosphors for white LEDs: structure refinement, color tuning and energy transfer, RSC Adv. 4 (2014) 40626-40637. [11] G. Li, D. Geng, M. Shang, C. Peng, Z. Cheng, J. Lin, Tunable luminescence of Ce3+/Mn2+coactivated Ca2Gd8(SiO4)6O2 through energy transfer and modulation of excitation: potential single-phase white/yellow-emitting phosphors, J. Mater. Chem. 21 (2011) 13334-13344. [12] Z. Jiang, Y. Wang, Near-UV Excitable White Sr2Al2GeO7 :Ce3 +, Tb3 + phosphor for light emitting diodes, Electrochem. Solid-State Lett. 13 (2010) J68-J70. [13] W. Lü, N. Guo, Y. Jia, Q. Zhao, W. Lv, M. Jiao, B. Shao, H. You, Tunable Color of Ce3+/Tb3+/Mn2+-Coactivated CaScAlSiO6 via energy transfer:A single-component red/whiteemitting phosphor, Inorg. Chem. 52 (2013) 3007-3012. [14] W. Lü, Z. Hao, X. Zhang, Y. Luo, X. Wang, J. Zhang, Tunable full-color emitting BaMg2Al6Si9O30:Eu2+, Tb3+, Mn2+ phosphors based on energy transfer, Inorg. Chem. 50 (2011) 7846-7851. [15] X. Chen, P. Dai, X. Zhang, C. Li, S. Lu, X. Wang, Y. Jia, Y. Liu, A highly efficient white light (Sr3,Ca,Ba)(PO4)3Cl:Eu2+, Tb3+, Mn2+ phosphor via dual energy transfers for white lightemitting diodes, Inorg. Chem. 53 (2014) 3441-3448.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[16] K. Li, J. Fan, M. Shang, H. Lian, J. Lin, Sr2Y8(SiO4)6O2:Bi3+/Eu3+: a single-component white-emitting phosphor via energy transfer for UV w-LEDs, J. Mater. Chem. C 3 (2015) 99899998. [17] H. Zhou, Y. Jin, M. Jiang, Q. Wang, X. Jiang, A single-phased tunable emission phosphor MgY2Si3O10: Eu3+, Bi3+ with efficient energy transfer for white LEDs, Dalton Trans. 44 (2015) 1102-1109. [18] H. Zhou, Q. Wang, Y. Jin, Temperature dependence of energy transfer in tunable white light-emitting phosphor BaY2Si3O10:Bi3+,Eu3+ for near UV LEDs, J. Mater. Chem. C 3 (2015) 11151-11162. [19] M. Thomas, P.P. Rao, S.P.K. Mahesh, L.S. Kumari, P. Koshy, Luminescence properties of Eu3+, Bi3+ coactivated CaLaNbWO8 red phosphors under near UV and blue excitations, Phys. Status Solidi A 208 (2011) 2170-2175. [20] H. Xie, F. Li, H. Xi, R. Tian, X. Wang, Luminescent properties of sol–gel processed redemitting phosphor Eu3+, Bi3+ co-doped (Ca,Sr)(Mo,W)O4, J. Mater. Sci.: Mater. Electron. 26 (2015) 23-31. [21] L. Chen, K.-J. Chen, C.-C. Lin, C.-I. Chu, S.-F. Hu, M.-H. Lee, R.-S. Liu, Combinatorial approach to the development of a single mass YVO4:Bi3+,Eu3+ phosphor with red and green dual colors for high color rendering white light-emitting diodes, J. Comb. Chem. 12 (2010) 587-594. [22] J. Su, X. Mi, J. Sun, L. Yang, C. Hui, L. Lu, Z. Bai, X. Zhang, Tunable luminescence and energy transfer properties in YVO4: Bi3+, Eu3+ phosphors, J. Mater. Sci. 52 (2017) 782-792. [23] M. Noh, S.-H. Cho, S. Park, Tunable luminescence in Bi3+ and Eu3+ co-doped Sr3AlO4F oxyfluorides phosphors, J. Lumin. 161 (2015) 343-346. [24] W. Ran, L. Wang, L. Tan, D. Qu, J. Shi, Remote Control Effect of Li+, Na+, K+ ions on the super energy transfer process in ZnMoO4:Eu3+, Bi3+ phosphors, Sci. Rep. 6 (2016) 27657. [25] R. Cao, T. Fu, D. Peng, C. Cao, W. Ruan, X. Yu, Synthesis, energy transfer and tunable emission properties of SrSb2O6:Eu3+, Bi3+ phosphor, Spectrochim. Acta, Part A 169 (2016) 192196. [26] C. Liu, Z. Xia, M.S. Molokeev, Q. Liu, Synthesis, crystal structure, and enhanced luminescence of garnet-type Ca3Ga2Ge3O12:Cr3+ by codoping Bi3+, J. Am. Ceram. Soc. 98 (2015) 1870-1876. [27] A.L. Patterson, The scherrer formula for X-Ray Particle size determination, Phys. Rev. 56 (1939) 978-982. [28] S.K. Hussain, G. Nagaraju, E. Pavitra, G. Seeta Rama Raju, J.S. Yu, La(OH)3:Eu3+ and La2O3:Eu3+ nanorod bundles: growth mechanism and luminescence properties, CrystEngComm 17 (2015) 9431-9442. [29] D. Huang, Y. Zhou, W. Xu, Z. Yang, Z. Liu, M. Hong, Y. Lin, J. Yu, Photoluminescence properties of M3+ (M3+ = Bi3+, Sm3+) activated Na5Eu(WO4)4 red-emitting phosphors for white LEDs, J. Alloys Compd. 554 (2013) 312-318. [30] K. Li, H. Lian, M. Shang, J. Lin, A novel greenish yellow-orange red Ba3Y4O9:Bi3+,Eu3+ phosphor with efficient energy transfer for UV-LEDs, Dalton Trans. 44 (2015) 20542-20550. [31] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep 24 (1969) 131-144. [32] S.J. Gwak, P. Arunkumar, W.B. Im, A new blue-Emitting oxohalide phosphor Sr4OCl6:Eu2+ for thermally stable, efficient white-light-emitting devices under near-UV, J. Phys. Chem. C 118 (2014) 2686-2692.
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
[33] F. Kang, Y. Zhang, M. Peng, Controlling the energy transfer via multi luminescent centers to achieve white light/tunable emissions in a single-phased X2-Type Y2SiO5: Eu3+, Bi3+ phosphor for ultraviolet converted LEDs, Inorg. Chem. 54 (2015) 1462-1473. [34] Y. Tian, B. Chen, B. Tian, R. Hua, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, Y. Zheng, T. Yu, L. Huang, Q. Meng, Concentration-dependent luminescence and energy transfer of flower-like Y2(MoO4)3:Dy3+ phosphor, J. Alloys Compd. 509 (2011) 6096-6101. [35] Y. Tian, B. Chen, R. Hua, J. Sun, L. Cheng, H. Zhong, X. Li, J. Zhang, Y. Zheng, T. Yu, Optical transition, electron-phonon coupling and fluorescent quenching of La2(MoO4)3:Eu3+ phosphor, J. Appl. Phys. 109 (2011) 053511. [36] Y.I. Jeon, L.K. Bharat, J.S. Yu, Eu3+ ions co-doped CLPO: Dy3+ single phase white-light emitting phosphors for near UV-based white LEDs, J. Alloys Compd. 649 (2015) 531-536. [37] V. Sivakumar, U.V. Varadaraju, A promising orange-red phosphor under near UV excitation, Electrochem. Solid-State Lett. 9 (2006) H35-H38. [38] Z. Huo, C. Chen, D. Chu, H. Li, Y. Li, Systematic synthesis of lanthanide phosphate nanocrystals, Chem. - Eur. J. 13 (2007) 7708-7714. [39] X. Liu, Q. Wang, Z. Gao, J. Sun, J. Shen, Fabrication of lanthanide phosphate nanocrystals with well-controlled morphologies by layer-by-layer adsorption and reaction method at room temperature, Cryst. Growth Des. 9 (2009) 3707-3713. [40] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836850. [41] G.A. Kumar, P. Biju, G. Jose, N. Unnikrishnan, Static energy transfer for Mn2+:Pr3+ system in phosphate glasses, Mater. Chem. Phys. 60 (1999) 247-255. [42] P. Paulose, G. Jose, V. Thomas, N. Unnikrishnan, M. Warrier, Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass, J. Phys. Chem. Solids 64 (2003) 841-846. [43] D. Dexter, J.H. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (1954) 1063-1070. [44] Y.-w. Tan, C. Shi, Ce3+→ Eu2+ energy transfer in BaLiF3 phosphor, J. Phys. Chem. Solids 60 (1999) 1805-1810. [45] H. Zhu, C.C. Lin, W. Luo, S. Shu, Z. Liu, Y. Liu, J. Kong, E. Ma, Y. Cao, R.-S. Liu, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014). [46] C.-H. Huang, W.-R. Liu, T.-M. Chen, Single-phased white-light phosphors Ca9Gd(PO4)7: Eu2+, Mn2+ under near-ultraviolet excitation, J. Phys. Chem. C 114 (2010) 18698-18701. [47] R. Van Deun, D. Ndagsi, J. Liu, I. Van Driessche, K. Van Hecke, A.M. Kaczmarek, Dopant and excitation wavelength dependent color-tunable white light-emitting Ln3+:Y2WO6 materials (Ln3+= Sm, Eu, Tb, Dy), Dalton Trans. 44 (2015) 15022-15030.
ACCEPTED MANUSCRIPT
Table
1
Energy
transfer
efficiencies
(η)
and
CIE
chromaticity
values
for
SYGO:0.02Bi3+/yEu3+ (y = 0 to 0.07 mol) phosphors. η from Bi3+ to Eu3+ (%) 3.66
SYGO:0.02Bi3+/0.01Eu3+
9.46
SYGO:0.02Bi3+/0.02Eu3+
16.52
SYGO:0.02Bi3+/0.03Eu3+
19.44
(0.1277, 0.1091)
(0.1390, 0.1199)
(0.1505, 0.1235)
M AN U
SYGO:0.02Bi3+/0.005Eu3+
RI PT
SYGO:0.02Bi3+
CIE values (x,y)
SYGO:0.02Bi3+/0.04Eu3+ SYGO:0.02Bi3+/0.05Eu3+ SYGO:0.02Bi3+/0.06Eu3+
EP AC C
(0.2011, 0.1568)
24.17
(0.2490, 0.1815)
33.72
(0.2868, 0.2098)
48.51
(0.2938, 0.2067)
62.90
(0.2963, 0.2074)
TE D
SYGO:0.02Bi3+/0.07Eu3+
(0.1781, 0.1449)
SC
Phosphors
the
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 1. (a) XRD patterns of the SYGO samples at different annealing temperatures and (b) XRD patterns of SYGO:0.05Eu3+, SYGO:0.02Bi3+ and SYGO:0.02Bi3+/0.05Eu3+ phosphor samples annealed at 1200 °C.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 2. (a) FE-SEM image, (b) EDS spectrum and (c-f) elemental mappings for SYGO host
AC C
EP
TE D
lattice.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
Fig. 3. PLE and PL emission spectra of the SYGO:0.02Bi3+ phosphor.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4. PL emission spectra of SYGO:Bi3+ phosphors as a function of Bi3+ ion concentration under 374 nm excitation wavelength. The inset shows the relationship of log (C) versus log (I/C)
AC C
EP
for the SYGO:Bi3+ phosphors.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 5. PLE (λem =594 nm) and PL emission (λex = 238 and 393 nm) spectra of the
AC C
ion concentration.
EP
SYGO:0.05Eu3+ phosphor sample. Inset shows the PL emission intensity as a function of Eu3+
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 6. (a) PLE and (b) PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor at different emission and excitation wavelengths.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
TE D
Fig. 7. PL emission spectra of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04,
AC C
EP
0.05, 0.06 and 0.07) phosphors under 374 nm excitation wavelength.
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 8. Energy transfer efficiency graph of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07) phosphors under 374 nm excitation wavelength. Inset represents
AC C
EP
the schematic energy level diagram for the energy transfer mechanism from the Bi3+ to Eu3+ ions.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 9. Decay curves of SYGO:0.02Bi3+/yEu3+ (y = 0.00, 0.02, 0.03, 0.05, and 0.07) phosphors monitored at 467 nm emission and 374 nm excitation wavelengths.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 10. (a) Temperature-dependent PL emission spectra of the SYGO:0.02Bi3+/0.05Eu3+ phosphor under 374 nm excitation wavelength and (b) the corresponding plot of ln (I0/I-1) versus 1/kT.
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 11. CIE chromaticity diagram for the (1) SYGO:0.02Bi3+, (2) SYGO:0.02Bi3+/0.005Eu3+, (3) SYGO:0.02Bi3+/0.01Eu3+, (4) SYGO:0.02Bi3+/0.02Eu3+, (5) SYGO:0.02Bi3+/0.03Eu3+, (6) SYGO:0.02Bi3+/0.04Eu3+, (7) SYGO:0.02Bi3+/0.05Eu3+, (8) SYGO:0.02Bi3+/0.06Eu3+, and (9) SYGO:0.02Bi3+/0.07Eu3+ phosphors.
ACCEPTED MANUSCRIPT
Highlights
EP
TE D
M AN U
SC
RI PT
Sr3Y2Ge3O12:Bi3+/Eu3+ phosphors were prepared by a pechini type sol-gel method. The morphological properties of the Sr3Y2Ge3O12 host lattice were studied. The energy transfer mechanism between Bi3+ and Eu3+ was explained. Temperature-dependent and quantum yield were measured for the optimized phosphor.
AC C
• • • •