Eu 2+ phosphors for white LED

Since the blue emission bands related to Eu 2þ ions is sensitive to the host lattice environment, the change of the blue emission band with sintering temperature may indicate the change [r]

(1)

Original article

Synthesis and optical properties of red/blue-emitting

Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors for white LED

Tong Thi Hao Tama,b, Nguyen Duy Hunga, Nguyen Thi Kim Liena,

Nguyen Duc Trung Kiena, Pham Thanh Huya,*

aAdvanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hanoi 10000, Viet Nam bNational Economics University (NEU), No 207 Giai Phong Street, Hanoi 10000, Viet Nam

a r t i c l e i n f o

Article history: Received June 2016 Received in revised form 12 June 2016

Accepted 12 June 2016 Available online 18 June 2016 Keywords:

Sr2MgSi2O7:Eu3ỵ/Eu2ỵ

Photoluminescence

Red and blue emitting phosphor

a b s t r a c t

Phosphor-converted white light emitting diodes (white LEDs) have received great attention in recent years since they have several excellent features such as high lumen output, low power consumption, long lifetime and environmentally friendly In this work, we report the co-precipitation synthesis of red/blue Sr2MgSi2O7:Eu3ỵ/Eu2ỵphosphors with various Eu doping concentration The results show that the

ob-tained Sr2MgSi2O7:Eu3ỵ/Eu2ỵphosphors have good crystallinity and emit strong red (Sr2MgSi2O7:Eu3ỵ)

and blue (Sr2MgSi2O7:Eu2ỵ) emissions under near UV light excitation The sharp emission peaks at 577,

590, 612, 653, and 701 nm corresponded to the typical5D

0/7Fj(jẳ 0,1,2,3,4) transitions of Eu3ỵ, and

the blue emission peaking at 460 nm is attributed to the typical 4f65d1-4f7transition of Eu2ỵin the same Sr2MgSi2O7host lattice Both phosphors can be well excited in the wavelength range of 260e400 nm

where the near UV-LED is well matched The above results suggest that the Sr2MgSi2O7:Eu3ỵ/Eu2ỵ

phosphors are promising red/blue-emitting phosphors for the application in near UV pumped phosphor-converted white LEDs

1 Introduction

Phosphors are widely used in solid-state lighting, especially for the phosphor-converted light emitting diode (white LED) in which yellow light-emitting phosphor (such as YAG:Ce3ỵ) are pumped by GaN chips to generate white light[1] On the one side, the current white LEDs show several advantages over incandescent and ﬂuo-rescent lamps including low operating voltage, low energy con-sumption, long lifetime… However, on the other side, this kind of white LED shows relatively low color-rendering index (CRI), and high color temperature due to lack of a red-light emitting compo-nent [2,3] So far, one solution to these problems has been to fabricate a white LED with high color rendering by combining red, green and blue emitting tricolor phosphors pumped by a near UV-LED[4,5] Therefore, extensive efforts have been made to develop new blue and red phosphors with light luminous efﬁciency, good color, and high CRI[6]

Recently, the alkaline earth silicates based-phosphors (alker-manite phosphors) have been reported as one of the most essential luminescent materials due to their excellent thermal and chemical stability and high brightness Particularly, Sr2MgSi2O7 is a good

candidate for UV-LED application since it has a rigid tetragonal structure and strong absorption band in UV region[7,8] It is well known that Europium (Eu) is the most common rare earth to be used as an activator in phosphors Eu3ỵion is a preferable activator for red phosphors with sharp emission peaks in the red region (from 570 to 700 nm) caused by the5D0/7Fj(J¼ 0, 1, 2, 3, 4)

transitions of the trivalent state, while Eu2ỵis the most frequently used activator in the blue phosphors and its emission usually consist of a broad band due to transitions from the 4f65d to the 4f7 ground state Additionally, Eu2ỵion can emit light from the UV to the infrared with broad band emitting luminescence on different host matrices since the involved 5d orbital of Eu2ỵion is external and strongly inﬂuenced by the crystal ﬁeld[9,10]

Until now, the phosphors based on Sr2MgSi2O7host lattice were

prepared by different methods such as solid-state reaction, hy-drothermal, solegel methods or combustion processing with ul-trasonic dispersion technique [11e14] Among the various synthesis methods, the co-precipitation method is known to

* Corresponding author Tel.: ỵ84 36230435; fax: ỵ84 36230293 E-mail address:huy.phamthanh@hust.edu.vn(P.T Huy)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2016.06.009

(2)

produce phosphor powders with uniform, narrow size distribution, and homogeneous distribution of the activator ions [15] It is important to note that in most of the previous research, to syn-thesis the Sr2MgSi2O7:Euỵ2phosphor, the precursor powders were

normal sintered in reduced gas environment in a one-step syn-thesis process, therefore only the blue emitting phosphors can be obtained

In this work, we present the results of our study on red/blue phosphors based on Eu-doped Sr2MgSi2O7 prepared by

co-precipitation method Initially, Eu3ỵ-doped phosphor was synthe-sized as a red emitting phosphor, and its structure and luminescent properties were investigated as a function of the sintering tem-perature Lately, Eu2ỵdoped phosphor was obtained by reducing the corresponding Eu3ỵ phosphor in forming gas environment Moreover, the inuence of Eu3ỵ doping concentration on the

luminescent properties of the phosphors was also investigated Experimental

The Sr2MgSi2O7:Eu3ỵ phosphor was synthesized by a

co-precipitation reaction In this reaction, nitrate salts Sr(NO3)2,

Mg(NO3)2$6H2O, tetraethylorthosilicate (C2H5O)4Si (TEOS), and

europium oxide Eu2O3were used as precursors All these chemicals

were of analytic grade The raw materials were weighed according to the nominal composition of Sr2-xMgSi2O7:xEu3ỵ(xẳ 0.02, 0.03,

and 0.04) Sr(NO3)2.4H2O and Mg(NO3)2$6H2O were mixed in

distilled water A stoichiometric amount of TEOS and Eu2O3was

also dissolved in ethanol and HNO3, respectively The solutions

were stirred until the solution became transparent, after which they were mixed and continuously stirred for h Subsequently, an appropriate amount of NH4OH was added to the solution to enable

precipitation The precipitate and solution were continuously stir-red to obtain a white viscous gel Then, centrifugal force was applied to allow the resulting precursor to be separated The separated precursor was washed with DI water for several times After drying at 200C for 24 h, the dry powder was calcined in air at various temperatures for h to receive the Sr2-xMgSi2O7:xEu3ỵ

phosphors To produce the Sr2-xMgSi2O7:xEu2ỵ phosphors, the

corresponding Sr2-xMgSi2O7:xEu3ỵphosphor was subjected to ion

reduction in the mixture of H2/N2(10%/90%) gas at different

tem-peratures for h

The phase purity of the phosphors was identified by X-ray diffraction (XRD) pattern Measurements were carried out on a D8/ Advance-Bruker diffractometer with CuKaradiation (l¼ 1.5403 Å) The scan rate was kept at s/step at a scattering angle range of 20e70 The Raman spectra were recorded on a Horiba Jobin Yvon LabRAM HR-800 spectrometer using HeeNe laser (632.8 nm) with a power density of 215 W/cm2 A high-resolution mode of 1.2 cm1 was used Morphology was taken with a JSM-7600F (Jeol Co., Japan) field emission scanning electron microscope (FESEM) PL and PLE were measured on a NANO LOG spectroﬂuorometer PL and PLE spectra were obtained by using a 450 W xenon light source with a spectral resolution of about nm

3 Results and discussion

Fig 1shows XRD patterns of the product sintered at 900, 1100, 1200, and 1300C for h

It can be seen that until temperature of 1200 C, the main crystalline phase in the powder is Si2SiO4, beside the Sr2MgSi2O7

and Sr3MgSi2O8 phases with smaller portion Contents of the

desired Sr2MgSi2O7 phase and the secondary Sr3MgSi2O8 phase

increase with increasing the sintering temperature At the tem-perature of 1300C, intensity of the diffraction peaks related to the Sr2SiO4phase decreased abruptly, in contrast to the strong increase

of the Sr2MgSi2O7phase These results indicate that the presence

and dominance of the Sr2MgSi2O7phase can only be obtained in the

sample sintered at 1300C Our results are similar to those reported by Kwon et al., in which the Sr2SiO4 phase in the Sr2MgSi2O7

phosphor (synthesized by a conventional solid-state reaction method) disappeared only after sintering at 1300C or higher[16] The XRD patternFig 1also conﬁrms the tetragonal structure of the Sr2MgSi2O7host lattice The main phase Sr2MgSi2O7has tetragonal

crystal structure Sr2ỵion in this crystal structure occupies a unique position (position symmetry Cs) with eight neighboring O2ions

and the SreO distance is 2.662 Å in average When Eu is doped into Sr2MgSi2O7, Eu ions are expected to replace the position of Sr2ỵin

the crystal network because of the excellent compatibility ionic radius of Eu3ỵand Sr2ỵ, 1.25 and 1.26 Å, respectively[17e19]

To evaluate the possibility of replacing the Eu3ỵ ions on the position of Sr ions in the host lattice, Raman spectra measurement of the doped 4% Eu3ỵand undoped Sr2MgSi2O7host were carried

out The Raman spectra taken at room temperature are shown in Fig For the host lattice (curve a), Raman peaks are observed at 901, 652, 315, 220, 201 and 153 cm1 The peaks correspond to the stretching vibrations of the SieO and SieSi bonds of the Si2O7

group[20] For the doped Sr2MgSi2O7:4%Eu3ỵsample, the Raman

spectrum (curve b) is similar to that of the undoped Sr2MgSi2O7

sample, no other peaks were found This result implies that the Eu3ỵdopant ion was not substituted on the Si4ỵsite, and did not change the unite cell volume and SiOSi angle, instead they were incorporated in to host lattice by replacing the Sr2ỵsites

The morphology of the phosphors was characterized by FESEM Fig 3show FESEM images of the as-received phosphor (dry pow-der) (a) and the Sr2MgSi2O7:Eu3ỵphosphor sintered at 1300C for

3 h (b) The dry powder show clusters of particles with variety shapes and sizes, whereas the Sr2MgSi2O7:Eu3ỵphosphor exhibits

needle-like shape particles with an average length of about micron The chemical composition of the Sr2MgSi2O7:3%Eu3ỵ

phosphor has been measured using energy dispersive X-ray spec-troscopy (EDS) The result of the EDS analysis is shown inFig 3(c) which is representing the composition of the phosphor powder studies

Fig illustrates the photoluminescence (PL) spectra of Sr2MgSi2O7:4%Eu3ỵ phosphor samples sintered at different

tem-peratures in the range of 900e1300C Under the near UV

excita-tion of 360 nm, a broad blue emission band centered around 430e470 nm and several sharp lines in the orangeered region

Fig XRD patterns of Sr2MgSi2O7:Eu3ỵphosphors sintered at 900, 1100, 1200, and

(3)

peaking at about 577, 590, 612, 653, and 701 nm The sharp red emission lines should be ascribed to the transitions within the 4f6 conguration of Eu3ỵ These lines corresponds to the5D0 /7F0, 5D

0/7F1,5D0/7F2,5D0/7F3and5D0/7F4transitions of Eu3ỵ,

respectively[21] Here, the emission line at 612 nm is attributed to the electric dipole transition (5D0 / 7F2), while the emission

around 590 nm is assigned to the magnetic dipole transition (5D0/7F1), which is sensitive to site symmetry According to the

parity selection rule, when the Eu3ỵions are located at the site with an inversion symmetric center, the 5D0 / 7F1 magnetic dipole

transition is permitted, which results in orangeered emission around 590 nm In the other case, if the Eu3ỵions located at the site without an inversion symmetric center, because the opposite parity 5d conﬁguration is mixed into 4f conﬁguration, the parity selection rule is able to lifted, and fef forbidden transition is partially allowed, the hypersensitive5D

0/7F2electric dipole transition will

be permitted, which results in red emission around 612 nm [13,21,22] Thus, the observation of the strongest emission peak at 612 nm in our phosphor may indicate that Eu3ỵions mainly occupy non-inversion symmetric center in the host lattice For the broad blue emission band, it is known that Eu2ỵpresents a broad emis-sion band peaking at around 460 nm due to the 4f65d1 to 4f7 transition of Eu2ỵ(8S7/2-7Fj, jẳ 0, 1, 2, 3, and 4) Since no reduction

process has been carried out with the phosphor, it is quite possible that during the high temperature sintering, a small amount of Eu3ỵ ions were reduced to Eu2ỵ ions that leads to the blue emission [23e26] Longer sintering time and higher temperature can enhance this ion transformation It can also be seen fromFig 4that while the PL intensity of the red emissions increased with increasing sintering temperature, the peak position and the shape of the blue band change arbitrary with increasing temperature Since the blue emission bands related to Eu2ỵions is sensitive to the host lattice environment, the change of the blue emission band with sintering temperature may indicate the change of the crys-talline phases in the sample as observed from XRD results In our opinion, the increase of the PL intensity of the red emission is related to the higher content of the Sr2MgSi2O7 phase upon

increasing temperature from 900 to 1300C

Fig 5shows the excitation spectrum (PLE monitored at 612 nm) of the Sr2MgSi2O7:Eu3ỵphosphor The PLE spectrum covers a wide

region between 350 and 600 nm revealing that the phosphor can be excited by near UV at 360, 381, 393 and 463 nm Such excitation wavelengths are well matched with near UV-LED excitation wavelength, indicating a great potential for white LED application Further, the strong excitation peak at 463 nm points out that the Sr2MgSi2O7:Eu3ỵphosphor can also be used in the blue LED

pum-ped white LED

To investigate the effect of Eu3ỵ doping concentration on PL intensity of Sr2MgSi2O7phosphors synthesized by co-precipitation

method, the emission spectra of the phosphors at various Eu3ỵ concentrations (xẳ 0.02, 0.03, and 0.04) are presented inFig The emission intensity increases until x¼ 0.03 and then decreases as a result of enhanced dipoleedipole interaction This optimal Eu3ỵ concentration is lower than that reported in the literature [17]

Fig Raman spectra of the Sr2MgSi2O7(a) and Eu3ỵ-doped Sr2MgSi2O7(b) phosphors

sintered at 1300C in air ambient for h

Fig FESEM images of the as-received powder (dry powder) (a) and the Sr2MgSi2O7:3%Eu3ỵphosphor powder sintered at 1300C in air ambient for h (b) and

(4)

The PL and PLE spectra of Sr2MgSi2O7:Eu2ỵphosphors sintered

at 1300C for h and reduced at 1300 for h are shown inFig The PL spectra show a broad emission band in the blue peaking at 462 nm under the excitation wavelength of 360 nm This emission is due to the 4f65d1e4f7transition of Eu2ỵions in the host lattice.

Also, the PLE spectrum monitored at 460 nm is shown inFig It is shown that the blue emission band can be efﬁciently excited by both UV and near UV excitation source from 260 to 415 nm Thus, the Sr2MgSi2O7:Eu2ỵphosphor obtained in this work matches well

the excitation wavelength of the near UV LED chip Conclusions

Sr2MgSi2O7:Eu3ỵ/Eu2ỵ phosphors were prepared by the

co-precipitation method followed by sintering at 1300C for h in air ambient (Sr2MgSi2O7:Eu3ỵ) and reduced at 1300C for h in

forming gas environment (Sr2MgSi2O7:Eu2ỵ) The Sr2MgSi2O7:Eu3ỵ

phosphor shows strong red emission peaking at 612 nm that can be excited by both near UV (360, 381, 393 nm) and blue (463 nm) LED

The Sr2MgSi2O7:Eu2ỵphosphor emits strong blue light peaking at

462 nm and can be excited by both UV and near UV-LED These results suggested that the Sr2MgSi2O7:Eu3ỵ/Eu2ỵphosphors have

high potential for phosphor-converted white LED application Acknowledgments

This work was supported by the National Program on Technol-ogy Innovation, project number DM.06.DN/13 This paper is dedi-cated to PETER BROMMERe a former physicist of the University of Amsterdam and good friend of the Vietnamese physicists References

[1] S Lee, S Seo, Optimization of yttrium aluminum garnet: Ce3ỵphosphors for white light-emitting diodes by combinatorial chemistry method, J Electrochem Soc 149 (2002) J85eJ88,http://dx.doi.org/10.1149/1.1511755 [2] Y.X Pan, M.M Wu, Q.J Su, Tailored photoluminescence of YAG: Ce3ỵ phos-phor through various methods, J Phys Chem Solids 65 (2004) 845e850,

http://dx.doi.org/10.1016/j.jpcs.2003.08.018

[3] H.S Jang, W.B Im, D.C Lee, D.C Jeo, S.S Kim, Enhancement of red spectral emission intensity of Y3Al5O12:Ce3ỵ phosphor via Pr co-doping and Tb

Fig Photoluminescence spectra of Sr2MgSi2O7:4%Eu3ỵpowders sintered at different

temperatures from 900 to 1300C in air ambient for h under 360 nm near UV excitation

Fig Excitation spectrum (PLE) of Sr2MgSi2O7:Eu3ỵphosphor (monitor for 612 nm

emission line)

Fig PL spectra of Sr2MgSi2O7:xEu3ỵphosphors with different Eu3ỵdoping

con-centration (xẳ 0.02, 0.03 and 0.04) under optimum excitation wavelength of 393 nm

Fig PL (lex¼ 360 nm) and PLE spectra of Sr2MgSi2O7:Eu2ỵphosphor sintered at

1300C in air ambient for h and reduced at 1300C for h in forming gas of 10%H2/

(5)

substitution for the application to white LEDs, J Lumin 126 (2007) 371e377,

http://dx.doi.org/10.1016/j.jlumin.2006.08.093

[4] H.L Liu, D.W He, F Shen, Luminescence properties of green-emitting phos-phor(Ba1-xSrx)SiO4:Eu2ỵfor white LEDS, J Rare Earths 24 (2006) 121e124 http://dx.doi.org/1002-0721(2006)01-0121-04

[5] K.Y Jung, J.H Kim, Y.C Kang, Luminescence enhancement of Eu-doped cal-cium magnesium silicate blue phosphor for UV-LED application, J Lumin 129 (2009) 615e619,http://dx.doi.org/10.1016/j.jlumin.2009.01.001

[6] S.K Jong, K.K Ae, H.P Yun, Luminescent and thermal properties of full-color emitting X3MgSi2O8:Eu2ỵ,Mn2ỵ(Xẳ Ba, Sr, Ca) phosphors for white LED,

J Lumin 122e123 (2007) 583e586, http://dx.doi.org/10.1016/ j.jlumin.2006.01.231

[7] J.K Park, M.A Lim, C.H Kim, H.D Park, White light-emitting diodes of GaN-based Sr2SiO4:Eu and the luminescent properties, Appl Phys Lett 82 (2003)

683e685,http://dx.doi.org/10.1063/1.1544055

[8] J.K Sheu, S.J Chang, C.H Kuo, Y.K Su, L.W Wu, Y.C Lin, W.C Lai, J.M Tsai, G.C Chi, R.K Wu, White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors, IEEE Photonics Technol Lett 15 (2003) 18e20,http://dx.doi.org/10.1109/LPT.2002.805852

[9] T Justel, H Nikol, C Ronda, New development in theﬁeld of luminescence materials for lighting and displays, Angew Chem Int Ed 37 (1998) 3084e3103 http://dx.doi.org/10.1002/(SICI)1521-3773(19981204)37: 22<3084::AID-ANIE3084>3.0.CO;2-W

[10] W.B Im, J.H Kang, D.C Lee, S Lee, D.Y Jeon, Y.C Kang, K.Y Jung, Origin of PL intensity increase of CaMgSi2O6:Eu2ỵphosphor after baking process for PDPs

application, Solid State Commun 133 (2005) 197e201, http://dx.doi.org/ 10.1016/j.ssc.2004.10.016

[11] M Zhang, J Wang, W Ding, Q Zhang, Q Su, Luminescence properties of M2MgSi2O7:Eu2ỵ(Mẳ Ca, Sr) phosphors and their effects on yellow and blue

LEDs for solid-state lighting, Opt Mater 30 (2007) 571e578, http:// dx.doi.org/10.1016/j.optmat.2007.01.008

[12] Y Xu, D Chen, Combustion synthesis and photoluminescence of Sr2MgSi2O7:

Eu,Dy long lasting phosphor nanoparticles, Ceram Int 34 (2008) 2117e2120,

http://dx.doi.org/10.1016/j.ceramint.2007.08.012

[13] Y Zhai, Z You, Y Liu, Y Sun, Q Ji, Properties of red-emitting phosphors Sr2MgSi2O7:Eu3ỵprepared by gel-combustion method assisted by microwave,

J Rare Earth 30 (2012) 114e117,http://dx.doi.org/10.1016/S1002-0721(12) 60005-2

[14] J Wan, Y Yao, G Tang, Hydrothermal synthesis and size-enhanced chroma-ticity of Sr2ZnSi2O7:Eu3ỵ nanoparticles, J Nanosci Nanotechnol (2008)

1449e1453,http://dx.doi.org/10.1166/jnn.2008.364

[15] Q.Y Zhang, X.Y Huang, Recent progress in quantum cutting phosphors, Prog Mater Sci 55 (2010) 353e427, http://dx.doi.org/10.1016/ j.pmatsci.2009.10.001

[16] K.H Kwon, W.B Im, D.Y Jeon, Energy transfer in Sr2MgSi2O7:Eu2ỵphosphors

in nano scale and their application to solid state lighting with excellent color rendering, J Nanosci Nanotechnol 13 (2013) 4079e4083,http://dx.doi.org/ 10.1166/jnn.2013.6999

[17] H Wu, Y Hu, Y Wang, F Kang, Z Mou, Investigation on Eu3ỵdoped Sr2MgSi2O7

red-emitting phosphors for white-light-emitting diodes, Opt Laser Technol 43 (2011) 1104e1110,http://dx.doi.org/10.1016/j.optlastec.2011.02.006 [18] T Laamanen, Defects in Persistent Luminescent Materials (Ph.D thesis),

University of Turku, 2011

[19] J H€ols€a, J Niittykoski, M Kirm, T Laamanen, M Lastusaari, P Novak, J Raud, Synchrotron radiation study of the M2MgSi2O7:Eu2ỵ, persistent luminescence

materials, ECS Trans (2008) 1e10,http://dx.doi.org/10.1149/1.2938743 [20] J Hanuza, M Ptak, M Maczka, K Hermanowicz, J Lorenc, A.A Kaminskii,

Polarized IR and Raman spectra of Ca2MgSi2O7, Ca2ZnSi2O7and Sr2MgSi2O7

single crystals: temperature-dependent studies of commensurate to incom-mensurate and incomincom-mensurate to normal phase transitions, J Solid State Chem 191 (2012) 90e101,http://dx.doi.org/10.1016/j.jssc.2012.02.051 [21] S Yao, L Xue, Y Yan, Investigation of the electronic structure and

photo-luminescence properties of Eu3ỵin Sr2Mg1-xZnxSi2O7(0 x 1), J Electroceram

26 (2011) 112e115,http://dx.doi.org/10.1007/s11434-011-4946-5

[22] T Kano,“Luminescence center of rare-earth ions,” in Phosphor Handbook, W M Yen, S Shionoya, and H Yamamoto, Eds.,chapter 3.3, CRC Press, Boca Raton, Fla, USA, 2206

[23] Z Xia, J Sun, L Liao, H Du, Phase structure and temperature dependent luminescence properties of Sr2LiSiO4F:Eu2ỵand Sr2MgSi2O7:Eu2ỵphosphors,

J Rare Earths 28 (2010) 874e877,http://dx.doi.org/10.1016/S1002-0721(09) 60228-3

[24] A.A Setlur, A.M Srivastava, H.L Pham, Charge creation, trapping, and long phosphorescence in Sr2MgSi2O7:Eu2ỵ,RE3ỵ, J Appl Phys 103 (2008)http:// dx.doi.org/10.1063/1.2844473, 053513

[25] H Zhang, N Terasaki, H Yamada, C.N Xu, Blue light emission from stress-activated Sr2MgSi2O7:Eu, Int J Mod Phy B 23 (2009) 1028e1033,http:// dx.doi.org/10.1142/S0217979209060415

[26] F.B Hermi, H Jukka, H.J Jorma, Optical energy storage properties of Sr2MgSi2O7:Eu2ỵ, R3ỵpersistent luminescence materials, Therm Anal

Định dạng
Số trang	5
Dung lượng	1,28 MB