jl cef3-ppo manuscript

7232019 JL CeF3-PPO Manuscript

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a matrix The nanocomposites show enhancement in the lumines-cence due to FRET energy transfer from the nanoparticle to theorganic 1047298uor

In the last decade rare earth 1047298uoride nanoparticles were exten-sively studied for various applications [17ndash21] Among the rare earth1047298uorides cerium 1047298uoride (CeF3) nanoparticles have been of parti-cular interest due to the Ce3thorn emission CeF3 nanoparticles weresynthesized using different techniques from hydrothermal [22]

solvothermal [23] extraction method [24] microwave [25] sonica-tion assisted method [26] reverse micelles route [27] microemulsion[2829] to wet chemistry [19] First identi1047297ed as a scintillator in 1989CeF3 is known as an appealing scintillating material due to its highdensity and short decay time [30] However the emission of CeF3 lieswithin UV range which is not suitable for direct detection byphotomultiplier tube and photodiode Therefore a wavelength shifteris needed to transfer the emission energy from CeF3 to higherwavelength For this purpose 25-diphenyloxazole (PPO) is selecteddue to its appropriate luminescent properties Furthermore PPO hasalready been widely used as a 1047298uor in plastic scintillators Here wehave chosen PVT as an organic polymer matrix that has been widelyused as a base for plastic scintillator due to its ease of fabrication andlow cost We have synthesized CeF3 nanoparticles coated with oleicacid and embedded these nanoparticles into the polymer matrixalong with PPO to make nanocomposite scintillators Enhancementin the luminescence of PPO has been observed under ultraviolet (UV)and X-ray excitations

2 Experimental

21 Chemicals

Cerium (III) nitrate hexahydrate (999 trace metal basis)sodium 1047298uoride (ACS reagent Z99) 25-diphenyloxazole (99)and benzoyl peroxide (97) were purchased from Sigma-AldrichOleic acid (90) and 4-methylstyrene (98thorn stab with 0135-di-tert-butylcatechol) were purchased from Alfa-Aesar

22 Synthesis of cerium 1047298uoride nanoparticles

In this work CeF3 nanoparticles were synthesized via a pre-cipitation reaction Brie1047298y 30 mmol sodium 1047298uoride (NaF) wasdissolved into 80 mL of DI water and mixed with 80 mL of ethanolsolution containing 15 mL oleic acid The mixture was heated to80 1C under vigorous stirring with purging of argon gas 10 mmol of Ce(NO3)3 6H2O was then dissolved into 60 mL of DI water andadded into the above mixture dropwise The reaction was kept as80 1C for 4 h and then allowed to cool in the air The precipitateswere collected by centrifugation and washing with ethanol for threetimes and then dried at 50 1C for 12 h under vacuum

23 Preparation of CeF 3 PPOPVT nanocomposites

After removal of the polymerization inhibitor from PVT mono-mers using a silica column benzoyl peroxide (as the free radicalinitiator) (01 wt) and PPO (05 wt) were dissolved in PVT mono-mer solution and subjected to ultrasonication for 20 min Thendifferent concentrations (wt) of CeF3 nanoparticles were added intothe above mixture and further sonicated for 45 min During the 1047297rst45 min of the incubation process at 75 1C the CeF3 nanoparticle-contained PVT monomer solution was agitated with vortex mixturefor several times to ensure a good dispersion of CeF3 nanoparticles inthe monomer After these treatments the polymerization of PVTmonomers has been suf 1047297ciently initiated so that the enhancedviscosity would prevent CeF3 nanoparticles from aggregation The

composite was then allowed to be incubated at 75 1C for 90 h and

then cooled down to room temperature Free-standing CeF3PPOPVTcomposites were then obtained after removing the glass container

24 Instrumentation

The HRTEM images of the particles were obtained using a JEOL JEM-2100 electron microscope with accelerating voltage of 20 0 kVOptical absorption was recorded with a SHIMADZU UV-2450

spectrophotometer Photoluminescence emission (PL) and excita-tion (PLE) were taken on a SHIMADZU RF-5301 PC Spectro1047298uo-rometer X-ray Excited Optical Luminescence (XEOL) wasmeasured in a light-proof X-ray cabinet equipped with optic 1047297berconnection to an outside detector X-ray irradiation (90 kV p and5 mA) was performed using a Faxitron RX-650 X-ray cabinet(Faxitron X-ray Corp IL USA) The luminescence spectra wererecorded using a QE65000 spectrometer (Ocean Optics IncDunedin FL) connected to the X-ray chamber using a 600 mm corediameter P600-2-UV ndashvis 1047297ber optic (Ocean Optics Inc DunedinFL) XRD were recorded using a Siemens Kristallo1047298ex 810 D-500X-ray diffractrometer operating at 40 kV voltage and 30 mAcurrent with a radiation beam of λfrac1415406 Aring

3 Results and discussion

The XRD pattern is shown in Fig 1 illustrating the hexagonalphase of crystalline CeF3 Fig 2(A) shows the TEM images of CeF3nanoparticles The histogram shown in Fig 2(B) indicates that theaverage size of as-prepared CeF3 nanoparticles is about 12 nm Thepresence of oleic acid on the nanoparticle surfaces not onlysigni1047297cantly reduces the size of CeF3 nanoparticles but alsoenables the uniform dispersion of the nanoparticles into thepolymer matrix It was utilized to con1047297ne the growth of CeF3crystals so that small particle sizes can be achieved for minimizinglight scattering

Fig 3 shows photoluminescence emission (PL) and excitation(PLE) of CeF3 nanoparticles along with the absorption spectrum of

PPOPVT CeF3 nanoparticle has an excitation peak at 290 nm andan emission peak at 330 nm CeF3 exhibits several excitationbands and the low energy one is centered about 250 nm whensaturation effect is discarded The emission of CeF3 crystals isusually centered at 305 nm which is attributed to the 5dndash4f transition of the Ce3thorn ion [3132] The emission at 330 nm in ourCeF3 nanoparticles is mainly from surface states as pointed byDujardin et al [33]

20 30 40 50 60 70 80

0

100

200

300

400

500

600

700

800

C o u n t s

2theta(degree)

1 1 0

1 1 1

1 1 2

3 0 0

1 1 3

3 0 2

2 2 1

2 2 3

Fig 1 XRD pattern of the as synthesized CeF3 nanoparticles

S Sahi et al Journal of Luminescence 159 (2015) 105ndash109106



The emission in CeF3 nanoparticle overlaps with the absorption

of PPOPVT and ful1047297ll the condition for FRET It is expected that theenergy can be transferred from CeF3 nanoparticles to PPOPVT dueto this FRET overlapping The lifetime of 330 nm emission of CeF3 is 30 ns [3031] and that of PPO is 16 ns [34] as reported byvarious authors These lifetimes also ful1047297ll the criteria that thedonor lifetime must be suf 1047297ciently longer than that of acceptor forFRET to occur When being excited at 290 nm PPOPVT shows anemission band peaking at around 390 nm To elucidate theenhancement effect of CeF3 doping on the emission of PPOPVTcomposites different concentrations of CeF3 nanoparticles areloaded into PVT matrix while the concentration of PPO was keptconstant at 05 wt Nanocomposites loaded with CeF3 nanoparti-cles exhibited brighter emissions under UV-light as compared tothe unloaded sample as can be seen in Fig 4 The room

temperature photoluminescence spectra of nanocompositesexcited at 290 nm are shown in Fig 5 It is clear that CeF3nanoparticles signi1047297cantly enhanced the photoluminescence of PPOPVT composites This enhancement in the emission intensityof nanocomposites loaded with CeF3 nanoparticles is due to theenergy transfer from CeF3 to PPO in PVT matrix as expected Alsothere is no shift in PPO emission position in the nanocompositesIn addition the photoluminescence spectra of CeF3PPOPVTcomposites do not show any emission from CeF3 nanoparticleswhen being excited at 290 nm wavelength which further sup-ported the argument that the enhancement in photoluminescenceis due to the FRET from CeF3 nanoparticles to PPOPVT composite

The mechanism of energy transfer can be explained using theenergy level diagram as shown in Fig 6 Upon irradiation with UV or

X-ray excitons are generated on CeF3 nanocrystals Since the

emission of CeF3 overlaps perfectly with the absorption of PPO inPVT excitions on the nanoparticles can transfer their energy to PPOin PVT matrix as long as the distance between nanoparticles and1047298uor molecules reaches the critical radius as required by FRETprinciples [35] Once excitons transfer their energy to PPO theywould recombine to give luminescence Enhancement in PL is morethan 3 times for the CeF3 loading concentration of 10 wt and then

decreases to about 28 times for 15 wt This decrease in PL with

Fig 2 (A) HR-TEM image and (B) size distribution of CeF3 nanoparticles

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12

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CeF3 PLE

Wavelength(nm)

A b s o r b a n c e ( a

u )

PPOPVT absorption

CeF3 PL

0

50

100

150

200

250

300

350

400

P L i n t e n s i t y ( a u )

Fig 3 Photoluminescence emission (PL) and excitation (PLE) of as synthesizedCeF3 nanoparticles and absorption spectrum of PPOPVT

Fig 4 Picture of CeF3 loaded (0 wt 5 wt and 10 wt from left to right) PPOPVTcomposite under normal light (top panel) and under UV light (bottom panel)

300 350 400 450 500 550

0

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100

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250

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400

450

500

I n t e n s i t y ( a u )

Wavelength(nm)

0 CeF3

5 CeF3

8 CeF3

10 CeF3

15 CeF3

Fig 5 Photoluminescence of PPOPVT with different loading concentrations of CeF3 nanoparticles

S Sahi et al Journal of Luminescence 159 (2015) 105ndash109 107



higher loading concentration might be due to the increased popula-tion of the non-radiative triplet state or the loss of transparency of the nanocomposites at higher nanoparticle loading concentrationsAs shown in Fig 4 the transparency of the nanocompositesdecreases with increasing nanoparticle loading concentration

Since the energy transfer mechanism is similar in case of both UV and X-ray excitations we have used X-ray as an excitation source toprovide further evidences of energy transfer in the nanocompositesWe have employed 90 kV p X-ray as an excitation source to measurethe X-ray Excited Optical Luminescence (XEOL) of the nanocompo-sites Fig 7 shows the XEOL of nanocomposites with differentnanoparticle loading concentrations Again enhanced luminescencewas observed for nanocomposites loaded with CeF3 nanoparticles

under X-ray excitation This further strengthens the energy transferevidence Also no CeF3 emission is seen under X-ray excitation whichis due to the strong absorption of PPOPVT composite in the region of CeF3 emission In case of X-ray excitation the peak is red shifted by10 nm in all the nanocomposites and enhancement is 25 times for theloading concentration of 10 wt compared to only PPOPVT compo-site as shown in Fig 7 Again at a higher nanoparticle loadingconcentration (15 wt) the enhancement of XEOL decreased to21 times as compared to the one without nanoparticle Thus energytransfer from CeF3 nanoparticle to PPOPVT is observed when thenanocomposites are excited by both UV and X-ray sources

It should be pointed out that in addition to energy transfer theincrease of the stopping power by doping CeF3 nanoparticles intoPPOPVT and the escape of charges from CeF3 nanoparticles also

contribute to the luminescence enhancement For the same

reasons we believe the quenching of CeF3 emission is due toenergy transfer self-absorption charge escape as well as thesurface modi1047297cation when doped into PPOPVT Energy transferis a complex process and more studies are necessary to prove theenergy transfer and mechanism The 1047298uorescence resonanceenergy transfer (FRET) process involves a pair of dissimilar1047298uorophores in close proximity typically less than 10 nm apartThe shorter-wavelength 1047298uorophore (donor) is directly excited

with incident light and then transfers energy nonradiatively tothe longer-wavelength 1047298uorophore (acceptor) which then emitslight In this case the excitation wavelength is chosen for max-imum absorption by the donor but the emitted light is character-istic of the acceptor The magnitude of the FRET signal is extremelydependent on the separation between the 1047298uorophores and socan be used to monitor binding events and conformationalchanges In the case of PPOPVT by CeF3 we believe the energytransfer is simply due to the absorption of the emission energyfrom CeF3 nanoparticles by PPOPVT organic scintillators How-ever energy transfer is not the only reason for the luminescenceenhancement or the quenching of CeF3 luminescence Morestudies are needed in order to understand the optical processesoccurred in these materials

4 Conclusion

CeF3 nanoparticles were synthesized and their emission is at330 nm which is mainly from surface states Enhanced luminescenceis observed in PPOPVT scintillators when embedded with CeF3nanoparticles Enhancement in the PL is more than 3 times for10 wt of CeF3 loading whereas enhancement is 25 times whenX-ray is used as excitation source for the same nanoparticles loadingconcentration The luminescence enhancement in PPOPVT by CeF3nanoparticles is attributed to the energy transfer the increase of thestopping power by doping CeF3 nanoparticles as well as the escapeof charges from CeF3 nanoparticles The observations provide anew method to improve PPOPVT organic scintillators for radiation

detection

Acknowledgment

We would like to acknowledge the support from the startupfunds from UTA the NSF (CBET-0736172) and DHS-DNDO ARIprogram (2011-DN-077-ARI053-02 3 4 amp5)

References

[1] P Lecoq J Lumin 60ndash61 (1994) 948[2] CWE van Eijk Nucl Instrum Methods Phys Res Sect AmdashAccel Spectrom

Detect Assoc Equip 509 (1ndash3) (2003) 17

[3] CL Melcher Industrial applications of scintillators in F DeNotaristefaniP Lecoq M Schneegans (Eds) Heavy Scintillators for Scienti1047297c and IndustrialApplicationsmdashCrystal 20001993 pp 75ndash84

[4] CWE van Eijk Nucl Instrum Methods Phys Res Sect AmdashAccel SpectromDetect Assoc Equip 460 (1) (2001) 1

[5] BL Rupert et al Europhys Lett 97 (2) (2012) 22002[6] HM Hamada et al Nucl Instrum Methods Phys Res Sect AmdashAccel

Spectrom Detect Assoc Equip 422 (1ndash3) (1999) 148[7] EA McKigney et al Nucl Instrum Methods Phys Res Sect AmdashAccel

Spectrom Detect Assoc Equip 579 (1) (2007) 15[8] VG Vasilrsquochenko AS Solovrsquoev Instrum Exp Tech 46 (6) (2003) 758[9] Z Kang et al J Lumin 131 (10) (2011) 2140

[10] H Zhong et al Nanotechnology 19 (2008) 50[11] RK Feller et al J Mater Chem 21 (15) (2011) 5716 [12] W Cai et al J Mater Chem C 1 (10) (2013) 1970[13] SE Letant TF Wang Nano Lett 6 (12) (2006) 2877 [14] M Yao et al J Appl Phys 113 (2013) 1 [15] M Hossu et al Appl Phys Lett 100 (2012) 1 [16] S Sahi W Chen Radiat Meas 59 (0) (2013) 139

[17] Y Liu et al J Appl Phys 103 (6) (2008) 063105

FRET

4f

5d

Visible

light

CeF3

PPO

S0

S1

Fig 6 Mechanism of 1047298uorescence resonance energy transfer (FRET) from CeF3to PPO

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0

20

40

60

80

100

120

140

160

C o u n t s

Wavelength(nm)

0 CeF3

5 CeF3

8 CeF3

10 CeF3

15 CeF3

Fig 7 XEOL of PPOPVT with different loading concentrations of CeF3nanoparticles




[18] Y Liu et al Appl Phys Lett 92 (2008) 4[19] Z Sun et al J Nanosci Nanotechnol 9 (11) (2009) 6283[20] Y Liu Q Ju X Chen Rev Nanosci Nanotechnol 1 (3) (2012) 163[21] LG Jacobsohn et al J Nanomater 2011 (2011) (Article ID 523638) [22] C Li et al J Phys Chem C 112 (39) (2008) 15602 [23] X Qu et al J Solid State Chem 184 (2) (2011) 246 [24] L Wang et al Mater Res Bull 43 (8ndash9) (2008) 2220[25] L Ma et al Mater Lett 64 (14) (2010) 1559[26] D Arun Kumar et al Solid State Sci 14 (5) (2012) 626 [27] H Zhang et al J Colloid Interface Sci 302 (2) (2006) 509

[28] Q Sunqing D Junxiu C Guoxu Wear 230 (1) (1999) 35 [29] S Qiu J Dong G Chen Powder Technol 113 (1ndash2) (2000) 9[30] WW Moses SE Derenzo IEEE Trans Nucl Sci 36 (1) (1989) 173[31] WW Moses et al J Lumin 59 (1ndash2) (1994) 89[32] CWE vanEijk Nucl Instrum Methods Phys Res Sect AmdashAccel Spectrom

Detect Assoc Equip 392 (1ndash3) (1997) 285[33] C Dujardin et al IEEE Trans Nucl Sci 57 (3) (2010) 1348 [34] R Luchowski Chem Phys Lett 501 (4ndash6) (2011) 572[35] M-C Chirio-Lebrun M Prats Biochem Educ 26 (4) (1998) 320




a matrix The nanocomposites show enhancement in the lumines-cence due to FRET energy transfer from the nanoparticle to theorganic 1047298uor

In the last decade rare earth 1047298uoride nanoparticles were exten-sively studied for various applications [17ndash21] Among the rare earth1047298uorides cerium 1047298uoride (CeF3) nanoparticles have been of parti-cular interest due to the Ce3thorn emission CeF3 nanoparticles weresynthesized using different techniques from hydrothermal [22]

solvothermal [23] extraction method [24] microwave [25] sonica-tion assisted method [26] reverse micelles route [27] microemulsion[2829] to wet chemistry [19] First identi1047297ed as a scintillator in 1989CeF3 is known as an appealing scintillating material due to its highdensity and short decay time [30] However the emission of CeF3 lieswithin UV range which is not suitable for direct detection byphotomultiplier tube and photodiode Therefore a wavelength shifteris needed to transfer the emission energy from CeF3 to higherwavelength For this purpose 25-diphenyloxazole (PPO) is selecteddue to its appropriate luminescent properties Furthermore PPO hasalready been widely used as a 1047298uor in plastic scintillators Here wehave chosen PVT as an organic polymer matrix that has been widelyused as a base for plastic scintillator due to its ease of fabrication andlow cost We have synthesized CeF3 nanoparticles coated with oleicacid and embedded these nanoparticles into the polymer matrixalong with PPO to make nanocomposite scintillators Enhancementin the luminescence of PPO has been observed under ultraviolet (UV)and X-ray excitations

2 Experimental

21 Chemicals

Cerium (III) nitrate hexahydrate (999 trace metal basis)sodium 1047298uoride (ACS reagent Z99) 25-diphenyloxazole (99)and benzoyl peroxide (97) were purchased from Sigma-AldrichOleic acid (90) and 4-methylstyrene (98thorn stab with 0135-di-tert-butylcatechol) were purchased from Alfa-Aesar

22 Synthesis of cerium 1047298uoride nanoparticles

In this work CeF3 nanoparticles were synthesized via a pre-cipitation reaction Brie1047298y 30 mmol sodium 1047298uoride (NaF) wasdissolved into 80 mL of DI water and mixed with 80 mL of ethanolsolution containing 15 mL oleic acid The mixture was heated to80 1C under vigorous stirring with purging of argon gas 10 mmol of Ce(NO3)3 6H2O was then dissolved into 60 mL of DI water andadded into the above mixture dropwise The reaction was kept as80 1C for 4 h and then allowed to cool in the air The precipitateswere collected by centrifugation and washing with ethanol for threetimes and then dried at 50 1C for 12 h under vacuum

23 Preparation of CeF 3 PPOPVT nanocomposites

After removal of the polymerization inhibitor from PVT mono-mers using a silica column benzoyl peroxide (as the free radicalinitiator) (01 wt) and PPO (05 wt) were dissolved in PVT mono-mer solution and subjected to ultrasonication for 20 min Thendifferent concentrations (wt) of CeF3 nanoparticles were added intothe above mixture and further sonicated for 45 min During the 1047297rst45 min of the incubation process at 75 1C the CeF3 nanoparticle-contained PVT monomer solution was agitated with vortex mixturefor several times to ensure a good dispersion of CeF3 nanoparticles inthe monomer After these treatments the polymerization of PVTmonomers has been suf 1047297ciently initiated so that the enhancedviscosity would prevent CeF3 nanoparticles from aggregation The

composite was then allowed to be incubated at 75 1C for 90 h and

then cooled down to room temperature Free-standing CeF3PPOPVTcomposites were then obtained after removing the glass container

24 Instrumentation

The HRTEM images of the particles were obtained using a JEOL JEM-2100 electron microscope with accelerating voltage of 20 0 kVOptical absorption was recorded with a SHIMADZU UV-2450

spectrophotometer Photoluminescence emission (PL) and excita-tion (PLE) were taken on a SHIMADZU RF-5301 PC Spectro1047298uo-rometer X-ray Excited Optical Luminescence (XEOL) wasmeasured in a light-proof X-ray cabinet equipped with optic 1047297berconnection to an outside detector X-ray irradiation (90 kV p and5 mA) was performed using a Faxitron RX-650 X-ray cabinet(Faxitron X-ray Corp IL USA) The luminescence spectra wererecorded using a QE65000 spectrometer (Ocean Optics IncDunedin FL) connected to the X-ray chamber using a 600 mm corediameter P600-2-UV ndashvis 1047297ber optic (Ocean Optics Inc DunedinFL) XRD were recorded using a Siemens Kristallo1047298ex 810 D-500X-ray diffractrometer operating at 40 kV voltage and 30 mAcurrent with a radiation beam of λfrac1415406 Aring

3 Results and discussion

The XRD pattern is shown in Fig 1 illustrating the hexagonalphase of crystalline CeF3 Fig 2(A) shows the TEM images of CeF3nanoparticles The histogram shown in Fig 2(B) indicates that theaverage size of as-prepared CeF3 nanoparticles is about 12 nm Thepresence of oleic acid on the nanoparticle surfaces not onlysigni1047297cantly reduces the size of CeF3 nanoparticles but alsoenables the uniform dispersion of the nanoparticles into thepolymer matrix It was utilized to con1047297ne the growth of CeF3crystals so that small particle sizes can be achieved for minimizinglight scattering

Fig 3 shows photoluminescence emission (PL) and excitation(PLE) of CeF3 nanoparticles along with the absorption spectrum of

PPOPVT CeF3 nanoparticle has an excitation peak at 290 nm andan emission peak at 330 nm CeF3 exhibits several excitationbands and the low energy one is centered about 250 nm whensaturation effect is discarded The emission of CeF3 crystals isusually centered at 305 nm which is attributed to the 5dndash4f transition of the Ce3thorn ion [3132] The emission at 330 nm in ourCeF3 nanoparticles is mainly from surface states as pointed byDujardin et al [33]

20 30 40 50 60 70 80

0

100

200

300

400

500

600

700

800

C o u n t s

2theta(degree)

1 1 0

1 1 1

1 1 2

3 0 0

1 1 3

3 0 2

2 2 1

2 2 3

Fig 1 XRD pattern of the as synthesized CeF3 nanoparticles












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CeF3 PLE

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u )

PPOPVT absorption

CeF3 PL

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4 Conclusion


detection

Acknowledgment


References










FRET

4f

5d

Visible

light

CeF3

PPO

S0

S1


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15 CeF3



















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u )

PPOPVT absorption

CeF3 PL

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400




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0 CeF3

5 CeF3

8 CeF3

10 CeF3

15 CeF3












4 Conclusion


detection

Acknowledgment


References










FRET

4f

5d

Visible

light

CeF3

PPO

S0

S1


350 400 450 500 550 600

0

20

40

60

80

100

120

140

160

C o u n t s

Wavelength(nm)

0 CeF3

5 CeF3

8 CeF3

10 CeF3

15 CeF3


















4 Conclusion


detection

Acknowledgment


References










FRET

4f

5d

Visible

light

CeF3

PPO

S0

S1


350 400 450 500 550 600

0

20

40

60

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0 CeF3

5 CeF3

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15 CeF3









jl cef3-ppo manuscript

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