white light emission of eu3+/ag co-doped y2si2o7

JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014, P. 779

Foundation item: Project supported by the National Natural Science Foundation of China (11074232, 11274288, 21002097, 11304300), the National Basic Research Program of China (2011CB932801, 2012CB933702), and Ministry of Education of China (20123402110034)

* Corresponding author: ZHANG Zengming (E-mail: [email protected]; Tel.: +86-551-63607671)

DOI: 10.1016/S1002-0721(14)60140-X

White light emission of Eu3+/Ag co-doped Y2Si2O7

DENG Yuhang (邓宇航)1, SONG Wenshen (宋文申)2, DONG Weile (董伟乐)2, DAI Rucheng (代如成)3, WANG Zhongping (王中平)3, ZHANG Zengming (张增明)3,*, DING Zejun (丁泽军)1,2 (1. Hefei National Laboratory for Physical Sciences at Microscale, Hefei 230026, China; 2. Department of Physics, University of Science and Technology of China, Hefei 230026, China; 3. The Center of Physical Experiments, University of Science and Technology of China, Hefei 230026, China)

Received 25 September 2013; revised 24 April 2014

Abstract: The Eu3+/Ag co-doped rare earth disilicate Y2Si2O7 microcrystal was synthesized by sol-gel method. Through controlling the thermal treatment process of Y2Si2O7:Eu3+/Ag precursor, various phases (amorphous, α, β, γ, δ) were prepared. White light emis-sion was observed under UV light excitation in the samples heavily doped with Ag. The white light was realized by combining the intense red emission of Eu3+, the green emission attributed to the very small molecule-like, non-plasmonic Ag particles (ML-Ag-particles), and the blue emission due to Ag ions. Results demonstrated that Eu3+/Ag co-doped Y2Si2O7 microcrystal could be potentially applied as white light emission phosphors for UV LED chips.

Keywords: Y2Si2O7; Eu3+ ion fluorescence; ML-Ag-particle; white light emission; rare earths

Recently, white light emitting diodes (WLEDs) have attracted significant attention due to their advantages such as higher efficiency, less energy consumption, and longer lifetime over traditional lighting techniques[1]. As the next generation of solid-state lighting, WLED can be applied as devices indicators, backlights, automobile headlights and general illumination[2]. Commercial WLEDs are usually produced by combining blue LED chips and yellow-emitting phosphor materials or inte-grating LEDs with different colors. Traditional WLEDs suffer from a color deviation and have poor white light performance, originating from the difference between in-dividual degradation rates of chips and phosphors coated on chips[2]. Consequently, it is important to develop sin-gle-component phosphors which are capable of emitting light covering full visible light band under the excitation of UV chips.

Rare earth (RE) ion doped materials are widely used as the monochromatic sources because of their strong emis-sion intensities, high thermal and chemical stabilities and low production costs. YVO4:Eu3+ [3], YBO3:Tb3+ [4] and NaSrBO3:Ce3+ [5] have been developed as red, green and blue phosphors, respectively. Besides some researches focused on tuning relative amount of RE ions corre-sponding to different emission bands in host materials[6,7], many other attempts are made to realize white light emission by, such as surface modification[8], and inte-grating materials containing RE ions with semiconductor quantum dots[9] or even noble metals[2,10].

Originally, doping noble metals, such as silver and gold, was used as a means to improve the fluorescence performance of RE ions due to surface plasmon reso-nance[11,12]. The noble metal nanoparticles nearby fluo-rescent centers at suitable separations can magnify the fluorescence intensity hundreds times[13]. By the effort to optimize metal-enhanced fluorescence, people have dis-covered the emission originating from molecule-like, non-plasmonic noble metal articles (ML-particles)[2,10,14]. The combination of RE emission peaks and these emis-sion bands displays excellent white light emission per-formance.

In this work, Eu3+/Ag co-doped Y2Si2O7 microcrystals with different phases were synthesized via a sol-gel method, followed by different thermal treatments. Y2Si2O7 can be potentially used as the high temperature ceramic. Doped with rare earth elements, it is an inter-esting material for luminescent application[15]. Spectro-scopic measurements revealed that under 325 nm UV light excitation, white light emission was realized owing to the mixture of red emission from Eu3+ ions, green emission from ML-Ag-particles, and blue emission from Ag+. Meanwhile, because the excitation bands of Eu3+ ions and ML-Ag-particles fall into different wavelength ranges, it is convenient to switch emission colors be-tween white and red by selecting proper excitation lights. The present research thus provided a way to develop sin-gle-component WLED phosphors; it may also raise in-terest in studying the fluorescence mechanism of ML-

780 JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014

Ag-particles.

1 Experimental

1.1 Preparation and thermal treatment

Samples were synthesized by a sol-gel method[16]. Firstly, Eu3+ doped Y2Si2O7 (Y2Si2O7:Eu3+, 1 mol.% and 10 mol.%) dry gel was synthesized. To get Eu3+/Ag co- doped Y2Si2O7 (Y2Si2O7:Eu3+/Ag), different amount of AgNO3 (purity>99.95%) dissolved in water were added to the Y2Si2O7:Eu3+ dry gel (1 mol.% Eu). In about 12 g dry gel, 0.17 g AgNO3 in aqueous solution was added to the dry gel to reform a uniform sol. In another 9 g dry gel, 0.3 g AgNO3 in aqueous solution was mixed. Two sols turned into gels again after 80 °C water bath and into dry gels further at 100 °C. We named two dry gels as “EuAg1” and “EuAg2”. The dry gel of Y2Si2O7 doped with Ag alone was also synthesized by using the similar method to prepare Y2Si2O7:Eu3+/Ag. 9 g dry gel of Y2Si2O7 was mixed with 0.3 g AgNO3 in aqueous solu-tion and then heated to reform a gel. We named the gel as “Ag”.

The as-prepared precursors and dry gels were heated at different temperatures for different time intervals. Final samples were named in the way of “species-thermal treatment temperature-thermal treatment time”. For ex-ample, after heating dry gel “EuAg1” at 1200 °C for 6 h, we got sample “EuAg1-1200-6”. Dry gel “Ag” heated at 1200 °C for 6 h was named as “Ag-1200- 6”. Precursors of Y2Si2O7:Eu3+ (1% or 10%) series after thermal treat-ments were named as “Eu1-900-2” or “Eu10-1200-6”, etc.

1.2 Characterization and spectroscopic measurement

X-ray diffraction (XRD) patterns were obtained from a rotating anode X-ray diffractometer (Rigaku, TTR III) under Cu Kα radiation. The morphologies of samples were inspected by a scanning electron microscope (SEM, JEOL JSM-6700F). Raman spectra and fluorescence spectra excited by lasers were collected by LabRAM HR (HORIBA Jobin Yvon); high temperature spectra were acquired with the help of LinkamTS1500 heating stage. Emission and excitation spectra were measured by a Fluorolog-3 spectrofluorometer (HORIBA).

2 Results and discussion

2.1 XRD analysis

Y2Si2O7 has five (or perhaps six) phases (y, α, β, γ, δ and z)[17]. Among these structures, α, β, γ and δ phases were classified according to their increasing stability with temperature, following the sequence[18]

1535 C1225 C 1445 C ° ° °α β γ δ⎯⎯⎯→ ⎯⎯⎯→

In order to obtain different phases, precursors and dry gels were heated at 900, 1350, 1400 and 1500 °C to get Eu10-900-2, Eu1-900-2, Eu10-1350-4, Eu10-1500-4, Eu1-1400-6 and Eu1-1500-6. Their XRD patterns are shown in Fig. 1. By comparing XRD patterns with JCPDS cards, Eu10-1350-4, Eu10-1500-4, Eu1-1400-6 and Eu1-1500-6 could be attributed to α, δ, β and γ phase of Y2Si2O7:Eu3+. Despite the doping of Eu atoms into Y2Si2O7 matrix, XRD patterns are found to be consistent with standard cards well, which indicates that Eu atoms occupy the Y-sites. It should also be noticed that keeping precursors of Y2Si2O7:10% Eu at 1350 or 1500 °C for 4 h could induce the formation of α or δ phase, rather than β or γ phase for pure Y2Si2O7

[18]. This demonstrates the ef-fect of heavy Eu doping on the phase transition of Y2Si2O7. It is reported that[19], the occurrence of α phase Y2Si2O7:6.5% Eu at 1250 °C was attributed to the Eu doping, because larger ionic radius of rare earth elements (Eu3+: 0.095 nm, Y3+: 0.089 nm) stabilizes α phase up to higher temperature. Eu2Si2O7 owns two polymorphs. The high temperature phase was obtained from solid state re-action of the oxides at 1600 °C and the low temperature phase was obtained by slowly cooling from 1600 to 1450 °C for 100 h. The low temperature phase of Eu2Si2O7 is isostructural with δ phase Y2Si2O7 while the high tem-perature phase is related to orthorhombic high tempera-ture phase of the disilicate group of the larger lanthanides, La→Sm[20]. It has been pointed out that[18], for Ln2Si2O7, Ln=Ho, Dy, Tb, Gd (the rare earth elements with ionic radius bigger than Y), α phase directly transforms into δ phase at about 1400 °C. Those two facts may explain why only α phase Y2Si2O7:10%Eu was formed in the temperature range from 1200 to 1350 °C, and Y2Si2O7: 10%Eu transforms into δ phase directly at 1500 °C, which is inconsistent with the phase transition sequence of pure Y2Si2O7.

Fig. 1 XRD patterns of amorphous (a), α, β, γ and δ phase (b) of

Y2Si2O7:Eu3+

DENG Yuhang et al., White light emission of Eu3+/Ag co-doped Y2Si2O7 781

For Y2Si2O7:Eu3+/Ag samples, XRD patterns reflected that their structures were influenced by the doping of Ag, especially in the case of the heavy doping EuAg2 series. Fig. 2 presents XRD patterns for EuAg1-900-2, EuAg1- 1500-8, EuAg2-1200-6, EuAg2-1400-6 and EuAg2- 1500-6. As shown in Fig. 2(a), there are both broad bands and sharp peaks (characteristic peaks of y-phase Y2Si2O7) in XRD pattern for EuAg1-900-2, unlike Eu1-900-2 sample without Ag doping. EuAg1-1500-8 could be identified as γ phase. For EuAg2-1200-6, except for peaks which are due to α phase (major phase) and y phase (minor phase), there are still some peaks that are hard to attribute; perhaps it is owing to the lattice distor-tion caused by Ag doping. XRD patterns of EuAg2- 1400-6 and EuAg2-1500-6 are similar, both demonstrat-ing peaks of β phase. Again, impurity peaks appear. XRD pattern of silver crystal (fcc, JCPDS 04-0783) cannot be observed in all three EuAg2 samples, though the nominal mass fraction of Ag in Y2Si2O7:Eu3+/Ag was

calculated to be higher than 8%. This fact indicated that Ag in Y2Si2O7:Eu3+ is not in the form of crystal. This is-sue will also be discussed afterwards.

In a word, amorphous, α, δ, β and γ phases of Y2Si2O7:Eu3+ were obtained via controlling thermal treatment processes. XRD patterns of Y2Si2O7:Eu3+/Ag samples demonstrated that the influence is brought about by doping Ag on crystallization of Y2Si2O7:Eu3+.

2.2 Morphology and size

Fig. 3 shows the SEM photographs of Eu1-1200-4 and Eu10-1400-6. The particle sizes of both kinds of Y2Si2O7:Eu3+ are several micrometers and particle shapes are irregular. Figs. 3(b) and 3(d), the enlarged photo-graphs of some particles shown in Figs. 3(a) and (c) re-spectively, show clear grain boundaries between indi-vidual smaller nanocrystals, suggesting that in the proc-ess of thermal treatment at high temperature, precursor of Y2Si2O7:Eu3+ is turned into Y2Si2O7:Eu3+ nanocrystals

Fig. 2 XRD patterns of Y2Si2O7:Eu3+/Ag samples

(a) EuAg1 series; (b) EuAg2 series

Fig. 3 SEM photographs of Eu1-1200-4 (a, b) and Eu10-1400-6 (c, d)


firstly, then followed by the subsequent crystal growth. The average size of individual crystals in Fig. 3(d) is bigger than that in Fig. 3(b), since higher temperature and longer thermal treatment time promote crystal growth.

2.3 Raman spectra analysis and temperature-induced crystallization

Raman spectra excited by 514.5 nm laser of some phases of Y2Si2O7:Eu3+ are present in Fig. 4. Most Ra-man peak positions of different phases in the present work match well with the previous report[21] (see Table 1). Some additional peaks come from impurity phases.

In XRD patterns of Eu10-900-2 and Eu1-900-2 (Fig. 1(a)) there are two broad bands and some peaks, indicat-ing the existence of amorphous phase. High temperature (up to 1400 °C) Raman spectra of Eu1-900-2 are dis-played in Fig. 5. Only the broad low-frequency shoulder and very broad scattering bands, which were indicatives of the amorphous nature of materials, were observed in the range from room temperature to 1050 °C. When tem-perature reached 1100 °C, many sharp peaks arose, showing the formation of crystals. Therefore the onset temperature of crystallization was between 1050 and 1100 °C. For spectra at 1100 °C, because main peaks at 416, 566, 888, 941 cm–1 could be attributed to α phase, we can conclude that Y2Si2O7 experienced a phase tran-

sition from amorphous phase to α phase from 1050 to 1100 °C. This result was supported by researches on the onset temperature of the crystallization process by means of ex situ high temperature XRD and differential thermal analysis (DTA). Li et al.[16] found that Y2Si2O7 precursor gradually demonstrated characteristic peaks of α phase Y2Si2O7 when annealing temperature reached to 1000 °C. Parmentier et al.[17] attributed the exothermic peak at 1060 °C in the DTA performed on the xerogel of Y2Si2O7 to the crystallization of α phase.

There are still some peaks that do not belong to α phase, for example the peak at 126, 151, and 1010 cm–1. These peaks appearing at 1100 °C maintain up to 1300 °C, until the strong thermal radiation of the sample at 1400 °C makes Raman signal too weak to discern. The sample was then cooled after heating to 1400 °C. At last, Raman spectra at room temperature was obtained, in which Raman modes of α phase: 227, 249, 333, 446, 977, 389 and 593 cm–1, and of β phase: 147, 270, 487, 911 and 1044 were observed.

As shown in Fig. 5, α phase is the main phase even at 1300 °C, which seems to be somewhat contradictory to the conclusion[18] that α transits into β phase at 1225 °C. This phenomenon was also found by Parmentier et al.[17]: at temperature higher than 1225 °C and for short iso-thermal heat treatment α was the main phase; this phe-nomenon was considered to indicate that α phase

Fig. 4 Raman spectra of α, β, γ and δ phase of Y2Si2O7:Eu3+


Table 1 Wavenumbers (cm–1) of Raman modes of different

phases of Y2Si2O7:Eu3+

αa αb βa βb γa γb δa δb

156 153 115 108 108 108 173 172 148 115 117 115 113 182 179 181 186 119 124 200 199 184 124 125 129 212 211 222 129 129 132 132 219 218 268 140 140 140 229 229 284 286 145 154 154 240 239 345 148 158 245 360 362 156 171 169 253 250 365 182 182 177 176 261 259 404 191 191 186 279 277 419 419 209 209 197 195 288 287 427 258 258 208 205 299 298 453 279 279 219 217 313 469 333 333 230 228 320 317 485 486 339 340 238 235 332 520 521 371 371 240 362 361 552 408 409 250 248 367 614 420 420 259 256 380 643 433 270 267 388 387 660 659 452 277 400 399 672 482 482 283 419 418 774 490 489 322 320 453 452 794 514 515 331 329

438 848 574 574 352 350 469 468 854 611 376 377 491 491 868 634 390 389 503 504 874 661 662 420 416 519 516 916 917 901 900 434 433 527 935 913 913 449 541 541 950 950 945 945 477 474 549 548 1005 1020 484 560 559 1047 1057 507 505 568 567 1088 1086 527 523 585 585 556 554 593 597 592 685 681 611 609 728 727 633 633

794 635 813 662

828 680 860 703 869 869 745

888 780 902 900 847 845

918 908 906 943 942 949 967 966 956 954

979 978 975 973 1017 978 1027 1026 1013 1011

1022 1047 1045

a Data taken from Ref. [21]; b The present work

Fig. 5 High temperature Raman spectra of Y2Si2O7:10%Eu

was a preferential crystallization from the amorphous state even at the temperatures outside its range of thermal stability. It was also because of the low rate of recon-struction of the Y2Si2O7 crystalline structure. When tem-perature of the sample returned to room temperature, Raman modes of both α and β phases appeared, suggest-ing that the formation of β phase still happened at high temperature region, in spite of the low rate of phase transformation.

2.4 Spectroscopic properties

Emission spectra of some Y2Si2O7:Eu3+ samples are presented in Fig. 6(a) by 325 nm excited laser. The emis-sion bands of Eu3+ ions can be attributed to 5D0→7F0 (578 nm), 5D0→7F1 (592 nm), 5D0→7F2 (617 nm), 5D0→7F3 (654 nm) and 5D0→7F4 (704 nm) transitions, respectively. No transitions from higher energy levels (5D1, 5D2) were observed because the multiphoton re-laxation due to the vibration of silicate groups (usually with frequency at about 1000 cm–1) was able to bridge

Fig. 6 Emission spectra (λex=325 nm) of different phases of

Y2Si2O7:Eu3+ (a) and excitation and emission spectra of Y2Si2O7 without doping (b)


the gaps between 5D1, 5D2 and 5D0 levels of Eu3+ effec-tively[22]. Compared with amorphous Eu1-900-2, other four crystalline samples demonstrate sharper emission peaks due to the strong crystal field effect coming from their good crystallinity. 5D0→7F2 transition is electric di-pole type, the intensity of which increases drastically with the deviation from the centrosymmetry of the site occupied by doping RE ions[23]. The intensity ratio of 5D0→7F2 to 5D0→7F1 can be used to judge the degree in which doping ions deviate from symmetric center[23,24]. As can be seen in Fig. 6(a), for all four crystalline sam-ples, integrated area of 5D0→7F2 peaks is bigger than that of 5D0→7F1, indicating that Eu3+ ions mainly occupy asymmetric sites. Besides the visible light emission, broad near infrared (NIR) emission bands were observed in samples after thermal treatments with T≥1400 °C. These bands should not be attributed to Eu3+ ions, since they also appeared in Y2Si2O7 without doping as seen in Fig. 6(b). Keeping Y2Si2O7 at 1300, 1400 and 1500 °C for several hours could generate NIR bands centered at about 820 nm while 1200 °C could not result in this emission. Excitation band of those NIR emissions is at 308 nm. The NIR emission bands were also found in Y2Si2O7: Eu3+/Ag and Y2Si2O7:Ag series samples. This NIR emission may come from minor silicon nanocrystals in Y2Si2O7 due to high temperature annealing since there are many reports pointing out that silicon nanocrystals (with diameter several nanometers) can emit NIR light at the range from 800 to 900 nm[25–27].

Figs. 7 and 8 give emission spectra (325 nm excitation) of Y2Si2O7:Eu3+/Ag series samples. Bright white light luminescence emitted from EuAg1-900-2 and EuAg2- 1200-6 were observed. Spectra demonstrate that the white light comes from the mixture of red emission

Fig. 7 Emission spectra (λex=325 nm) of Eu and Ag co-doped

samples: EuAg1 series

Fig. 8 Emission spectra of Eu and Ag co-doped samples:

EuAg2 series peaks from Eu3+ ions and the broad emission band cen-tered at about 510 nm. These green emission bands can be ascribed to the doping of Ag, since Y2Si2O7:Ag sam-ples emitted similar bands under the excitation of 325 nm laser as shown in Fig. 9. Excitation spectra of EuAg2 series are displayed in Fig. 10. Excitation spectrum obtained by monitoring the 5D0→7F2 transition of Eu3+ (612 nm) includes sharp lines 7F0, 1→5D4 at 324 nm, 7F0, 1→5GJ, 5L7 at 363 nm, 7F0→5L6 at 395 nm, 7F0, 1→5D3 at 416 nm, 7F0→5D2 at 466 nm, and 7F0→5D1 at 534 nm[28,29]. The very high edge from 240 nm to 280 nm belongs to the O2–-Eu3+ charge trans-fer (CT) band[2,28]. Spectrum monitoring the green band (510 nm) have a broad band centered at 330 nm, which also appears in excitation spectra monitoring λem=612 nm. The additional excitation band in spectra suggests there is an energy transfer from the unknown luminescent center with 510 nm emission to the Eu3+ ions. In other materials doped with Ag and Eu3+, for exam-

Fig. 9 Emission spectra (λex=325 nm) of Y2Si2O7:Ag samples


Fig. 10 Excitation spectra of EuAg2 series

ple soda-lime silicate glass[10], H3BO3-BaF2 glass[14] and oxyfluoride glass[2], white light emission was also found. In these systems, the green emission band is owing to the ML-Ag-particles. The present work supposed that in the Y2Si2O7:Eu3+ particles Ag exists in the form of ML-Ag-particles, which contribute to the emission band centered at 514 nm. Firstly, no silver XRD peaks were found in the XRD pattern of Y2Si2O7:Eu3+/Ag crystal, though the mass fraction of Ag in the crystal was as high as 8%, the amount that usually can be detected by XRD characterization. So, the silver atoms should not exist in the crystalline form. Secondly, the excitation band of green emission lies in the range from 310 to 366 nm, the range in which molecule-like silver species absorb light[10]. Finally, the green emission band at 514 nm is often related to ML-Ag-particles[10,14]. As far as we know, there is no report on the fluorescence of Ag nanocrystal, which shows surface plasmon resonance effect. The fact that Ag nanocrystal has very small possibility to emit light and that no crystalline Ag exists in Y2Si2O7:Eu3+ lead to the conclusion that ML-Ag-particles emit the green light. As for the small blue emission band at about 410 nm appearing in the emission spectra of EuAg2- 1200-6 and Y2Si2O7:Ag series, it is assigned to the 1S0→ 3D1 transition of isolated Ag+ ions[14,30–32]. Blue emission from Ag+ ion, green emission from ML-Ag-particle and red emission from Eu3+ ion contribute jointly to the white light emission of Y2Si2O7:Eu3+/Ag microcrystals.

3 Conclusions

Amorphous, α, β, γ and δ phase of Y2Si2O7:Eu3+ mi-crocrystals were prepared. Raman spectra determined the onset of crystallization temperature of amorphous Y2Si2O7: Eu3+ lay in the range from 1050 to 1100 °C. Under UV light excitation, besides emissions from 5D0→7FJ (J=0, 1, 2, 3, 4) transitions of Eu3+ ion, Y2Si2O7:Eu3+/Ag rendered additional emission broad

bands at 514 and 410 nm. These green and blue bands complemented the red emission of Eu3+ to make Y2Si2O7:Eu3+/Ag emit bright white light. Excitation spectra and XRD patterns indicated that the green band was due to ML-Ag-particles formed in the host materials and the blue band resulted from the 1S0→3D1 transition of isolated Ag+ ions.

Because the excitation band of ML-Ag-particles was in the range from 310 to 366 nm, commercial UV LED chips could be used to excite Y2Si2O7:Eu3+/Ag to gener-ate white light. This made Y2Si2O7:Eu3+/Ag a candidate phosphor for the next generation of WLED. It was also possible to choose white or red emission conveniently by selecting proper excitation lights. The present research might provide a platform to design novel WLED phos-phors, and also attract researchers’ interest in studying ML-Ag-particles further.

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white light emission of eu3+/ag co-doped y2si2o7

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