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Science of Advanced MaterialsVol 4 pp 623ndash630 2012(wwwaspbscomsam)

Synthesis and Upconversion Spectroscopy of Yb ErDoped M2O2S (M= La Gd Y) PhosphorsG A Kumar1lowast Madhab Pokhrel1 Alejandrina Martinez2 and D K Sardar1

1Department of Physics and Astronomy University of Texas at San Antonio San Antonio Texas 78249 USA2Centro De Investigaciones En Optica Leon Gto Mexico 37150

ABSTRACT

Yb and Er doped M2O2S (M= Y Gd La) phosphor was synthesized by solid state flux fusion method and theirup conversion spectral properties were studied as a function different Yb concentrations The solid state fluxfusion results in well crystallized hexagonal shaped phosphor particles of average size 4ndash6 m Upconversionspectral studies shows that all the compositions are stronger in green emission with the green emission intensity17 times than the red in composition Gd2O2SYb(8)Er(1) Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theinternal upconversion efficiency for the green emission bands was calculated to be 74 62 100 respectivelyin Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1) La2O2SYb(8)Er(1) Mechanisms of up conversion by two photon andenergy transfer processes are interpreted and explained The x y color coordinates are measured and thecolor tunability was analyzed as a function of the 980 nm excitation power Results shows that all phosphoroffers power dependent color tuning properties where the emission color can be tuned from 490 nm to 550 nmby simply changing the 980 nm excitation power from 10ndash50 mW

KEYWORDS Upconversion Phosphor Two Photon Absorption Energy Transfer Energy Conversion

1 INTRODUCTION

Up conversion is the spectroscopic process where a lowenergy photon of higher wavelength is converted into oneor more high energy photons of lower wavelength Spec-troscopically this is happening through the multiphotonabsorption process where a low energy photon is beingabsorbed to the higher state by multiphoton absorption Inorder to have efficient two photon absorption the mate-rial should have higher multiphoton absorption coefficientand that enables the application of several organic nonlin-ear materials as potential candidates for efficient up con-version However it was found that up conversion canalso occur efficiently in several trivalent rare earth dopedmaterials Here the up conversion is mainly happeningthrough multiphoton absorption through real excited statesthrough the process of excited state absorption and otherenergy transfer processes Since the processes is happeningthrough the real states the up conversion emission could beobserved even at low excitation laser power and that is anadded advantage of rare earth doped up conversion phos-phors At present rare earth doped up conversion phosphorfind a big market in the Photonics industry The appli-cation of these phosphors could found in several areas

lowastAuthor to whom correspondence should be addressedEmail mail-akgshyahoocomReceived 1 September 2011Accepted 26 September 2011

such as security solid state lasers1 IR detection medicalimaging therapy23 solid state displays (2D 3D)4 fiberoptic amplifiers5 The up conversion efficiency and spec-tral range of the material depends on the suitable selec-tion of the host-dopant combination In trivalent rare earthssince the absorption energy levels spread over a wide rangeof the electromagnetic spectrum very wide spectral exci-tation and emission is possible At present trivalent ErHo and Tm are considered as the potential upconvertorsfor Red Green and Blue emission Though considerableamount of work has been done on this topic in single crys-tals only limited work has been done in the area of ceramicpowder phosphor that shows efficient upconversionOne of the key requirements for efficient IR to visi-

ble up conversion is that the host material possess a lowphonon spectral mode which permits halides and heavymetal combination as the best materials for efficient upconversion However since many of the halides are airsensitive as well as toxic many of them could not findlarge scale industrial applications Chalcogenides such asS Se Te etc are also found to be potential candidatesfor up conversion though the phonon frequency is lit-tle higher than halides Among Chalcogenides rare earthoxysulfide possesses several excellent properties such aschemical stability low toxicity and can be easily mass pro-duced at low cost It has average phonon energies of about520 cmminus16 For example Y2O2S Yb Er is one of thebest mass produced commercial up conversion phosphors

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LEIt was found that the upconversion brightness of Y2O2SYb Er is 65 times to that of Y2O3 Yb Er7 Yocomet al8 demonstrated that Y2O2S Yb Er exhibited 82light outputs to that of fluoride In this work a solid-stateflux fusion method was adopted to prepare fine M2O2SYb Er (M= Y Gd La) phosphors with hexagonal shapenarrow size distribution and high luminescence efficiencyThe up conversion optical characteristic of green emitterphosphors are investigated for wide range of dopant com-positions and optimum compositions have been exploredfor the highest light outputCrystal structure of La2O2S is shown in Figure 1 The

symmetry is trigonal and the space group is P3macrm1 Thereis one formula unit per unit cell The structure is veryclosely related to the A-type rare-earth oxide structure thedifference being that one of the three oxygen sites is occu-pied by a sulfur atom Atom positions in La2O2S usinglattice vector units areplusmn (13 23 u) for two metal atomswith usim028 plusmn1323 v for two oxygen atoms withvsim063 and (0 0 0) for a sulfur atom Each metal atomseems to be bonded to four oxygen atoms and three sul-fur atoms to form a seven coordinated geometry with theoxygen and the metal in the same plane

2 EXPERIMENTAL DETAILS

A high temperature solid state flux fusion method wasused for the phosphor synthesis The starting materials areM2O3 Yb2O3 Er2O3 (M=Y Gd La Sigma Aldrich all99999) S (powder) and flux Na2CO3 K3PO4 (SigmaAldrich 9999) Two different sets of samples were pre-pared with different Yb Er ratio In the first series thetotal Yb+Er molar ratio was fixed at 10 mol viz (010)

Fig 1 Crystal structure of La2O2S showing the location of La (Pink)O (red) and S (yellow) atoms Both La and O are in the same plane

(19) (28) (37) (55) (91) (9505) In the second setthe Er concentration was kept constant at 1 mol whileYb was varied and the Yb Er ratios were viz (01) (11)(21) (31) (51) (91) (81) Both S and Na2CO3 were30 to 50 wt and K3PO4 was 20 wt of the total weightThe starting chemicals were thoroughly mixed using agatemortar and then heated in a muffle furnace at 1150 C for60 min When the furnace was cooled down the sampleswere taken out and washed with distilled water 6 times andfinally with a mild hydrochloric acid The washed pow-der was subsequently dried and sieved The particle sizeis analyzed by Coulter LS230 light scattering particle sizeanalyzer X-ray powder diffraction is performed at 40 kVand 30 mA in the parallel beam method using a RIGAKUAltima IV X-ray diffractometer with Cu K The morphol-ogy of the samples was observed using a Hitachi S5500field emission scanning transmission electron microscope(FE-STEM) operated at 30 kV in secondary electron modeThe up conversion spectra are recorded by the USB2000Ocean optics spectrometer with a power tunable 980 nmlaser diode (BWTek 980) and optical fiber For comparisonall samples were equally weighed and packed in cuvettesand excited with 20 mW of laser power For CIE colorcoordinates measurements the spectrometer was used inthe emissive color mode at 2 degree observer angle

3 RESULTS AND DISCUSSION

31 Phase and Morphology

A typical XRD pattern obtained for the La2O2S YbEr(LOS) was shown in Figure 2 The peak positions areexactly matching with the hexagonal phase of oxy sulphide(JCPDS Card No 26-1422) The XRD results reveal thatthe well-crystallized La2O2S Yb Er sample is in hexago-nal structure with cell parameters a = b= 03852 nm c=06567 nm According to the dynamic light scattering mea-surements (Fig 3) the average particle size was estimated

20 40 60 800

1000

2000

3000

4000

5000

6000

Inte

nsity

(co

unts

)

2θ

Fig 2 XRD pattern of the La2O2S YbEr sample

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LE

0 5 10 15 20 25 300

1

2

3

4

5

6

7

Y2O2SGd2O2S

Vol

ume

()

Size (μm)

Oxide Precursor

La2O2S

Fig 3 Particle size distribution from dynamic light scatteringmeasurements

to be 6176 m with a FWHM of 136 m SEM micro-graph obtained from different locations shows that thematerial mostly crystallized in hexagonal shape with meanparticle length 666plusmn058 m and width 267plusmn01 mFor Y2O2S YbEr (YOS) and Gd2O2S YbEr (GOS) theaverage particle size obtained from light scattering mea-surements are respectively 38 m (FWHM of 49 m)and 615 m (FWHM of 89 m) A typical SEM micro-graph of the La2O2S YbEr sample is shown in Figure 4(c)in comparison with the SEM images of Gd2O2S YbEr andY2O2S YbEr SEM images show that the average size andparticle morphology almost remains the same in Y2O2SYbEr and La2O2S YbEr whereas in Gd2O2S YbEr severalparticles are rod-like It should be noted that the primaryparticle size of M2O3 was reduced from 10 m to 38 mby flux fusion method SEM micrograph obtained fromdifferent locations shows that the material mostly crystal-lized in hexagonal shape Analysis of the elemental com-position by EDS shows the wt of various elements insidethe composition M2O2S Yb (2) Er (1) as M = 425Yb= 253 Er = 105 and S= 746 which is almostin agreement with the starting composition M = 422Yb= 23 and Er = 074The growth mechanism of M2O2S phosphor in flux

fusion process is well understood9 The formation ofM2O2S phosphor particle follows the chemical formula

M2O3+RE2O3+Flux S+Na2CO3+K3PO4

rarrM2O2S RE+flux residues Na2Sx +Na2SO4

+gaseous products HS+SO4+CO2+O

M= GdYLaRE = YbEr

The oxysulphide particles are grown on the M2O2 nucleithrough the sulfurization process on the M2O3 by themolten Na2Sx flux and the process was shown in Figure 5

Fig 4 SEM micrograph of oxysulphide phosphor powder prepared bythe flux fusion method (a) Gd2O2S YbEr (b) Y2O2S YbEr (c) La2O2SYbEr

Since the M2O3 particles are dissolved in the molten Na2Sx

flux the original precursor M2O3 particle size and distri-bution are not maintained and the dissolved particle growinto M2O2S particle Assuming the solubility and mobilityof the M2O2S in the Na2Sx flux solution are very smallthe dissolution of M2O3 subsequent nucleation and growthof M2O2S seed crystal may be restricted to a small vol-ume covering one particle As the temperature increasesall of the M2O3 particles dissolve suddenly in a volume V1

of the flux converting them into M2O2S If the solubilityof M2O2S in the flux solution is less the flux solution theregion V1 is quickly saturated instantly with M2O2S andthis leads formation of the first M2O2S nucleation in this

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LEM2O2S seed crystal

V3V2V1

M2O3

Na2Sx flux

Fig 5 Crystal growth mechanism of M2O2S phosphor M=Gd Y La

saturated solution As the flux temperature increases fewM2O2S particles are dissolved in the molten flux solutionof volume V2(V2 gt V1) and thereby creating a saturatedregion of M2O2S in volume V2 and the process continuesto different flux volume thereby increasing the size of theM2O2S particles The rate of particle growth is given by

d

dt= A exp

(minusE

kT

)(1)

Where A is a constant for a particular flux k the Boltz-mann constant T the temperature in K and E is the acti-vation energy The particle size and shape thus dependson various factors such as the firing temperature time andtype of fluxes10

32 Upconversion

Figure 6 shows a 980 nm excited upconveresion inLa2O2S Yb (3) Er (7) in comparison with Gd2O2SYb(8)Er(1) and Y2O2SYb(9)Er(1)

a As usual all samplesshow green and red emissions The fluorescence branchingratios of the green and red up conversion bands are respec-tively 485 and 388 In the case of YOS and GOSthey are respectively 495 (G) 483 (R) and 567(G) 432 (R) However the GR intensity ratios in GOSYOS and LOS are respectively 131 201 and 205 Inall samples the up conversion in these samples are sointense that even with lt15 W excitation the emissioncan be seen with naked eye Further the emission couldbe recorded with 700 W excitation power Because ofthe extreme brightness of the emission nearly 20 mW ofexcitation power was used in all experiments The insetof Figure 5(b) shows a photograph of the green upconver-sion emission under 20 mW excitation at 980 nmThe up conversion process in Yb Er system is well

understood in several materials1112 and is briefly explainedin the following energy level diagram shown in Figure 7Emission bands were observed at 410 nm 492 nm533 nm 549 nm 6698 nm and 822 nm and are assigned tothe 2H92 rarr 4I152

4F72 rarr 4I1522H112 rarr 4I152

4S32 rarr4I152

4F92 rarr 4I152 and 4I92 rarr 4I152 transitions respec-tively The appearance of both the blue green and redemission bands can be explained on the basis of variousprocesses such as two photon absorption (TPA) excited

aCompositions based on highest emission intensity

400 500 600 700 800

0

1000

2000

3000

4000

5000

6000gosyoslos

Wavelength (nm)

Inte

nsity

(a

u)

Fig 6 Upconversion emission spectrum of the oxysulphide phosphorunder 980 nm laser excitation with 20 mW excitation power

state absorption (ESA) and energy transfer (ET) TPA is anonlinear process where the visible photons are created bythe simultaneous absorption of two photons and mediatedthrough a real or virtual intermediate level TPA is relevantwhen excitation light sources being used have high powerthat is sufficient enough to create virtual intermediate lev-els in materials with high TPA cross section In the caseof trivalent Er3+ TPA can occur through the real statesbecause of the presence of such matching energy levelsWhen the 4I112 level is excited by 980 nm directly throughEr3+ or through Yb3+ excitation photons part of the exci-tation energy at the 4I112 level relaxes non-radiatively tothe 4I132 level giving rise to the 4I132 rarr 4I152 emissionAt the same time the populated 4I112 level is excited to the4F72 state by ESA of a second photon From 4F72 level

0

5000

10000

15000

20000

25000

ET

Er3+

MPA

980

nm

980

nm76

7 nm

822

nm

669

nm

548

nm

528

nm

492

nm

409

nm

2F72

2F52

4F32

2H112

4F52

4F72

Ene

rgy

(cm

ndash1)

2H92

4S32

4F92

4I112

4I92

4I132

4I152

MPA ESA

ET

Yb3+

Fig 7 Energy level diagram of Yb Er system showing possible exci-tation and de excitation mechanism under 980 nm MPA-Multiphotonabsorption ET-Energy Transfer

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LE

10 100

104

105

106

Inte

nsity

(a

u)

Pup Power (mW)

670 nm

slope138

528 nm 550 nm 670 nm

Fig 8 Dependence of the excitation power on the emission intensityof green and red emission

another 980 nm photon can be absorbed to an equally sim-ilar excited level As explained earlier these excited stateprocesses can equally happen under high photon densitythrough nonlinear absorption processes such as two pho-ton or three photon absorption After these excited stateprocesses de excitation accumulates electron in differentexcited states and the intensity of emission depends on theelectron density at that particular level as well as the non-radiative contribution to the emission band The popula-tion accumulated in level 2H92 gives the 410 nm emissionby radiatively decaying to 4I152 Since the efficiency ofthis third order process is less the emission intensity if the410 nm band is comparatively weak On the other handthe two photon processes that populate the 4F72 level isso efficient that it can give very efficient green emissiontransition from 2H112 due to the fast multiphonon non-radiative decay of 4F72 level to

2H112 Because of severalclosely spaced levels in between 2H112 and 4S32 multi-phonon relaxation results in decaying the 2H112 level to4S32 yielding the strongest emission band at 549 nm Thedecay time of the 4S32 level is measured to be 295 swhich is sufficient to make this as a meta stable level forthe green emission Further since the energy gap between4S32 and 4F92 is 2800 cmminus1 multiphonon relaxation isunlikely to happen and hence the 4S32 rarr 4F92 transitioncould be radiativeb Part of the population decaying to level4F92 can result in the red emission at 6698 nm throughthe 4F92 rarr 4I152 transition Finally fractional populationaccumulated in 4I92 level give rise to the 822 nm emis-sion through 4I92 rarr 4I152 transition The major processthat populate the 4F92 level of Er3+ is the energy trans-fer process from Yb according to the equation 2F52(Yb

3+)4I132(Er

3+)rarr 2F72(Yb3+)+ 4F92(Er

3+)In order to establish the presence of these non linear

processes the pump power dependence of the emission

bWe observed 32 m emission through the 4S32 rarr 4F92 transition

bands on the emission intensity was measured and the rela-tionship obtained is plotted in Figure 8 The slope obtainedfor 533 549 and 6698 nm emission bands are respectively138 145 and 141 which show that two photon processescontribute to these emissions It is interesting to note thatthe present material shows upconversion under 980 nm

0 2 4 6 8 105000

10000

15000

20000

25000

30000

35000

40000

Yb concentration (mol)

Er 1mol Green

Red

0 2 4 6 8 10

1times104

2times104

3times104

Yb concentration (mol)

Green

Red

Er = 1 mol

0 2 4 6 8 100

1times104

2times104

3times104

4times104

5times104

6times104

Yb concentration (mol)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Green

Red

Er = 1 mol

(a)

(b)

(c)

Fig 9 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er concentration at 1 mol (a) Gd2O2S YbEr(b) Y2O2S YbEr (c) La2O2S YbEr

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LE

0 2 4 6 100

1times104

4times104

5times104

6times104

Er concentration (mol )

Yb concentration (mol )

0246810

Green

Red

0 2 4 6 8 10

18times104

24times104

30times104

Er concentration (mol)

2468

Yb concentration (mol)

Yb+Er=10 molGreen

Red

10 0

0 2 4 6 8

2times104

3times104

4times1042468

Er concentration (mol)(a)

(b)

(c)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Yb concentration (mol)

Yb+Er=10 mol

Green

Red

10

1times104

12times104

8

3times104

2times104

Fig 10 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er+Yb concentration at 10 mol (a) Gd2O2SYbEr (b) Y2O2S YbEr (c) La2O2S YbEr

excitation from Xe lamp this corroborates the fact that inthis composition ESA also contributes to the upconversionprocess The fact that the material shows intense upconver-sion under low power excitation shows that the efficiencyof upconversion in this material is comparatively higherFurther under high power excitation the material showsalmost linear or saturation in intensity which is probablydue to the excitation population saturation at high excita-tion powerIn order to understand the functional dependence of the

red and green emission intensity on the Yb concentrationstwo sets of samples were prepared in the first set the YbEr concentration was kept fixed at 10 mol by propor-tionally changing Yb and Er concentrations In the secondset the Yb concentration was varied from 0 to 9 mol bykeeping the Er constant at 1 mol Results of both exper-iments are presented in Figures 9(a)ndash(c) and 10(a)ndash(c) forGOS YOS and LOS The green and red emission intensityvariations in these two experimental are entirely differentin each of the material In the first sets of experiments withEr 1 mol both green and red emission intensity peaks at7 mol of Yb in GOS and LOS whereas in YOS up to9 mol of Yb both emission intensity goes on increasingOn comparing the three hosts the green emission is moreintense in GOS followed by LOS and YOS In the secondsets of experiments with Er+Yb = 10 mol both greenand red emission intensity decreases at 7 mol 8 mol4 mol of Yb respectively in GOSYOS and LOS In LOSafter 8 mol of Yb the emission again seems to increasesComparison of the green emission intensity shows thatLOS is the brightestThe internal quantum efficiency (Int) of the upcon-

version band can be evaluated from the ratio of the flu-orescence to radiative decay time13 Fluorescence decaycurves obtained for the 555 nm emission band areshown in Figure 11 The fluorescence lifetime obtainedfor the 555 nm emissions are respectively 260 163 s

00

02

04

06

08

10

252015

Inte

nsity

(a

u)

Time (ms)

LOS YOS GOS

05 10

Fig 11 Fluorescence decay curve of the green emission band in GOSYOS and LOS at 8 mol Yb and 1 mol Er dopant concentrations

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

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LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

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Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

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LE

0 5 10 15 20 25 300

1

2

3

4

5

6

7

Y2O2SGd2O2S

Vol

ume

()

Size (μm)

Oxide Precursor

La2O2S

Fig 3 Particle size distribution from dynamic light scatteringmeasurements

to be 6176 m with a FWHM of 136 m SEM micro-graph obtained from different locations shows that thematerial mostly crystallized in hexagonal shape with meanparticle length 666plusmn058 m and width 267plusmn01 mFor Y2O2S YbEr (YOS) and Gd2O2S YbEr (GOS) theaverage particle size obtained from light scattering mea-surements are respectively 38 m (FWHM of 49 m)and 615 m (FWHM of 89 m) A typical SEM micro-graph of the La2O2S YbEr sample is shown in Figure 4(c)in comparison with the SEM images of Gd2O2S YbEr andY2O2S YbEr SEM images show that the average size andparticle morphology almost remains the same in Y2O2SYbEr and La2O2S YbEr whereas in Gd2O2S YbEr severalparticles are rod-like It should be noted that the primaryparticle size of M2O3 was reduced from 10 m to 38 mby flux fusion method SEM micrograph obtained fromdifferent locations shows that the material mostly crystal-lized in hexagonal shape Analysis of the elemental com-position by EDS shows the wt of various elements insidethe composition M2O2S Yb (2) Er (1) as M = 425Yb= 253 Er = 105 and S= 746 which is almostin agreement with the starting composition M = 422Yb= 23 and Er = 074The growth mechanism of M2O2S phosphor in flux

fusion process is well understood9 The formation ofM2O2S phosphor particle follows the chemical formula

M2O3+RE2O3+Flux S+Na2CO3+K3PO4

rarrM2O2S RE+flux residues Na2Sx +Na2SO4

+gaseous products HS+SO4+CO2+O

M= GdYLaRE = YbEr

The oxysulphide particles are grown on the M2O2 nucleithrough the sulfurization process on the M2O3 by themolten Na2Sx flux and the process was shown in Figure 5

Fig 4 SEM micrograph of oxysulphide phosphor powder prepared bythe flux fusion method (a) Gd2O2S YbEr (b) Y2O2S YbEr (c) La2O2SYbEr

Since the M2O3 particles are dissolved in the molten Na2Sx

flux the original precursor M2O3 particle size and distri-bution are not maintained and the dissolved particle growinto M2O2S particle Assuming the solubility and mobilityof the M2O2S in the Na2Sx flux solution are very smallthe dissolution of M2O3 subsequent nucleation and growthof M2O2S seed crystal may be restricted to a small vol-ume covering one particle As the temperature increasesall of the M2O3 particles dissolve suddenly in a volume V1

of the flux converting them into M2O2S If the solubilityof M2O2S in the flux solution is less the flux solution theregion V1 is quickly saturated instantly with M2O2S andthis leads formation of the first M2O2S nucleation in this

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LEM2O2S seed crystal

V3V2V1

M2O3

Na2Sx flux

Fig 5 Crystal growth mechanism of M2O2S phosphor M=Gd Y La

saturated solution As the flux temperature increases fewM2O2S particles are dissolved in the molten flux solutionof volume V2(V2 gt V1) and thereby creating a saturatedregion of M2O2S in volume V2 and the process continuesto different flux volume thereby increasing the size of theM2O2S particles The rate of particle growth is given by

d

dt= A exp

(minusE

kT

)(1)

Where A is a constant for a particular flux k the Boltz-mann constant T the temperature in K and E is the acti-vation energy The particle size and shape thus dependson various factors such as the firing temperature time andtype of fluxes10

32 Upconversion

Figure 6 shows a 980 nm excited upconveresion inLa2O2S Yb (3) Er (7) in comparison with Gd2O2SYb(8)Er(1) and Y2O2SYb(9)Er(1)

a As usual all samplesshow green and red emissions The fluorescence branchingratios of the green and red up conversion bands are respec-tively 485 and 388 In the case of YOS and GOSthey are respectively 495 (G) 483 (R) and 567(G) 432 (R) However the GR intensity ratios in GOSYOS and LOS are respectively 131 201 and 205 Inall samples the up conversion in these samples are sointense that even with lt15 W excitation the emissioncan be seen with naked eye Further the emission couldbe recorded with 700 W excitation power Because ofthe extreme brightness of the emission nearly 20 mW ofexcitation power was used in all experiments The insetof Figure 5(b) shows a photograph of the green upconver-sion emission under 20 mW excitation at 980 nmThe up conversion process in Yb Er system is well

understood in several materials1112 and is briefly explainedin the following energy level diagram shown in Figure 7Emission bands were observed at 410 nm 492 nm533 nm 549 nm 6698 nm and 822 nm and are assigned tothe 2H92 rarr 4I152

4F72 rarr 4I1522H112 rarr 4I152

4S32 rarr4I152

4F92 rarr 4I152 and 4I92 rarr 4I152 transitions respec-tively The appearance of both the blue green and redemission bands can be explained on the basis of variousprocesses such as two photon absorption (TPA) excited

aCompositions based on highest emission intensity

400 500 600 700 800

0

1000

2000

3000

4000

5000

6000gosyoslos

Wavelength (nm)

Inte

nsity

(a

u)

Fig 6 Upconversion emission spectrum of the oxysulphide phosphorunder 980 nm laser excitation with 20 mW excitation power

state absorption (ESA) and energy transfer (ET) TPA is anonlinear process where the visible photons are created bythe simultaneous absorption of two photons and mediatedthrough a real or virtual intermediate level TPA is relevantwhen excitation light sources being used have high powerthat is sufficient enough to create virtual intermediate lev-els in materials with high TPA cross section In the caseof trivalent Er3+ TPA can occur through the real statesbecause of the presence of such matching energy levelsWhen the 4I112 level is excited by 980 nm directly throughEr3+ or through Yb3+ excitation photons part of the exci-tation energy at the 4I112 level relaxes non-radiatively tothe 4I132 level giving rise to the 4I132 rarr 4I152 emissionAt the same time the populated 4I112 level is excited to the4F72 state by ESA of a second photon From 4F72 level

0

5000

10000

15000

20000

25000

ET

Er3+

MPA

980

nm

980

nm76

7 nm

822

nm

669

nm

548

nm

528

nm

492

nm

409

nm

2F72

2F52

4F32

2H112

4F52

4F72

Ene

rgy

(cm

ndash1)

2H92

4S32

4F92

4I112

4I92

4I132

4I152

MPA ESA

ET

Yb3+

Fig 7 Energy level diagram of Yb Er system showing possible exci-tation and de excitation mechanism under 980 nm MPA-Multiphotonabsorption ET-Energy Transfer

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LE

10 100

104

105

106

Inte

nsity

(a

u)

Pup Power (mW)

670 nm

slope138

528 nm 550 nm 670 nm

Fig 8 Dependence of the excitation power on the emission intensityof green and red emission

another 980 nm photon can be absorbed to an equally sim-ilar excited level As explained earlier these excited stateprocesses can equally happen under high photon densitythrough nonlinear absorption processes such as two pho-ton or three photon absorption After these excited stateprocesses de excitation accumulates electron in differentexcited states and the intensity of emission depends on theelectron density at that particular level as well as the non-radiative contribution to the emission band The popula-tion accumulated in level 2H92 gives the 410 nm emissionby radiatively decaying to 4I152 Since the efficiency ofthis third order process is less the emission intensity if the410 nm band is comparatively weak On the other handthe two photon processes that populate the 4F72 level isso efficient that it can give very efficient green emissiontransition from 2H112 due to the fast multiphonon non-radiative decay of 4F72 level to

2H112 Because of severalclosely spaced levels in between 2H112 and 4S32 multi-phonon relaxation results in decaying the 2H112 level to4S32 yielding the strongest emission band at 549 nm Thedecay time of the 4S32 level is measured to be 295 swhich is sufficient to make this as a meta stable level forthe green emission Further since the energy gap between4S32 and 4F92 is 2800 cmminus1 multiphonon relaxation isunlikely to happen and hence the 4S32 rarr 4F92 transitioncould be radiativeb Part of the population decaying to level4F92 can result in the red emission at 6698 nm throughthe 4F92 rarr 4I152 transition Finally fractional populationaccumulated in 4I92 level give rise to the 822 nm emis-sion through 4I92 rarr 4I152 transition The major processthat populate the 4F92 level of Er3+ is the energy trans-fer process from Yb according to the equation 2F52(Yb

3+)4I132(Er

3+)rarr 2F72(Yb3+)+ 4F92(Er

3+)In order to establish the presence of these non linear

processes the pump power dependence of the emission

bWe observed 32 m emission through the 4S32 rarr 4F92 transition

bands on the emission intensity was measured and the rela-tionship obtained is plotted in Figure 8 The slope obtainedfor 533 549 and 6698 nm emission bands are respectively138 145 and 141 which show that two photon processescontribute to these emissions It is interesting to note thatthe present material shows upconversion under 980 nm

0 2 4 6 8 105000

10000

15000

20000

25000

30000

35000

40000

Yb concentration (mol)

Er 1mol Green

Red

0 2 4 6 8 10

1times104

2times104

3times104

Yb concentration (mol)

Green

Red

Er = 1 mol

0 2 4 6 8 100

1times104

2times104

3times104

4times104

5times104

6times104

Yb concentration (mol)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Green

Red

Er = 1 mol

(a)

(b)

(c)

Fig 9 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er concentration at 1 mol (a) Gd2O2S YbEr(b) Y2O2S YbEr (c) La2O2S YbEr

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 2 4 6 100

1times104

4times104

5times104

6times104

Er concentration (mol )

Yb concentration (mol )

0246810

Green

Red

0 2 4 6 8 10

18times104

24times104

30times104

Er concentration (mol)

2468

Yb concentration (mol)

Yb+Er=10 molGreen

Red

10 0

0 2 4 6 8

2times104

3times104

4times1042468

Er concentration (mol)(a)

(b)

(c)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Yb concentration (mol)

Yb+Er=10 mol

Green

Red

10

1times104

12times104

8

3times104

2times104

Fig 10 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er+Yb concentration at 10 mol (a) Gd2O2SYbEr (b) Y2O2S YbEr (c) La2O2S YbEr

excitation from Xe lamp this corroborates the fact that inthis composition ESA also contributes to the upconversionprocess The fact that the material shows intense upconver-sion under low power excitation shows that the efficiencyof upconversion in this material is comparatively higherFurther under high power excitation the material showsalmost linear or saturation in intensity which is probablydue to the excitation population saturation at high excita-tion powerIn order to understand the functional dependence of the

red and green emission intensity on the Yb concentrationstwo sets of samples were prepared in the first set the YbEr concentration was kept fixed at 10 mol by propor-tionally changing Yb and Er concentrations In the secondset the Yb concentration was varied from 0 to 9 mol bykeeping the Er constant at 1 mol Results of both exper-iments are presented in Figures 9(a)ndash(c) and 10(a)ndash(c) forGOS YOS and LOS The green and red emission intensityvariations in these two experimental are entirely differentin each of the material In the first sets of experiments withEr 1 mol both green and red emission intensity peaks at7 mol of Yb in GOS and LOS whereas in YOS up to9 mol of Yb both emission intensity goes on increasingOn comparing the three hosts the green emission is moreintense in GOS followed by LOS and YOS In the secondsets of experiments with Er+Yb = 10 mol both greenand red emission intensity decreases at 7 mol 8 mol4 mol of Yb respectively in GOSYOS and LOS In LOSafter 8 mol of Yb the emission again seems to increasesComparison of the green emission intensity shows thatLOS is the brightestThe internal quantum efficiency (Int) of the upcon-

version band can be evaluated from the ratio of the flu-orescence to radiative decay time13 Fluorescence decaycurves obtained for the 555 nm emission band areshown in Figure 11 The fluorescence lifetime obtainedfor the 555 nm emissions are respectively 260 163 s

00

02

04

06

08

10

252015

Inte

nsity

(a

u)

Time (ms)

LOS YOS GOS

05 10

Fig 11 Fluorescence decay curve of the green emission band in GOSYOS and LOS at 8 mol Yb and 1 mol Er dopant concentrations

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

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LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LEM2O2S seed crystal

V3V2V1

M2O3

Na2Sx flux

Fig 5 Crystal growth mechanism of M2O2S phosphor M=Gd Y La

saturated solution As the flux temperature increases fewM2O2S particles are dissolved in the molten flux solutionof volume V2(V2 gt V1) and thereby creating a saturatedregion of M2O2S in volume V2 and the process continuesto different flux volume thereby increasing the size of theM2O2S particles The rate of particle growth is given by

d

dt= A exp

(minusE

kT

)(1)

Where A is a constant for a particular flux k the Boltz-mann constant T the temperature in K and E is the acti-vation energy The particle size and shape thus dependson various factors such as the firing temperature time andtype of fluxes10

32 Upconversion

Figure 6 shows a 980 nm excited upconveresion inLa2O2S Yb (3) Er (7) in comparison with Gd2O2SYb(8)Er(1) and Y2O2SYb(9)Er(1)

a As usual all samplesshow green and red emissions The fluorescence branchingratios of the green and red up conversion bands are respec-tively 485 and 388 In the case of YOS and GOSthey are respectively 495 (G) 483 (R) and 567(G) 432 (R) However the GR intensity ratios in GOSYOS and LOS are respectively 131 201 and 205 Inall samples the up conversion in these samples are sointense that even with lt15 W excitation the emissioncan be seen with naked eye Further the emission couldbe recorded with 700 W excitation power Because ofthe extreme brightness of the emission nearly 20 mW ofexcitation power was used in all experiments The insetof Figure 5(b) shows a photograph of the green upconver-sion emission under 20 mW excitation at 980 nmThe up conversion process in Yb Er system is well

understood in several materials1112 and is briefly explainedin the following energy level diagram shown in Figure 7Emission bands were observed at 410 nm 492 nm533 nm 549 nm 6698 nm and 822 nm and are assigned tothe 2H92 rarr 4I152

4F72 rarr 4I1522H112 rarr 4I152

4S32 rarr4I152

4F92 rarr 4I152 and 4I92 rarr 4I152 transitions respec-tively The appearance of both the blue green and redemission bands can be explained on the basis of variousprocesses such as two photon absorption (TPA) excited

aCompositions based on highest emission intensity

400 500 600 700 800

0

1000

2000

3000

4000

5000

6000gosyoslos

Wavelength (nm)

Inte

nsity

(a

u)

Fig 6 Upconversion emission spectrum of the oxysulphide phosphorunder 980 nm laser excitation with 20 mW excitation power

state absorption (ESA) and energy transfer (ET) TPA is anonlinear process where the visible photons are created bythe simultaneous absorption of two photons and mediatedthrough a real or virtual intermediate level TPA is relevantwhen excitation light sources being used have high powerthat is sufficient enough to create virtual intermediate lev-els in materials with high TPA cross section In the caseof trivalent Er3+ TPA can occur through the real statesbecause of the presence of such matching energy levelsWhen the 4I112 level is excited by 980 nm directly throughEr3+ or through Yb3+ excitation photons part of the exci-tation energy at the 4I112 level relaxes non-radiatively tothe 4I132 level giving rise to the 4I132 rarr 4I152 emissionAt the same time the populated 4I112 level is excited to the4F72 state by ESA of a second photon From 4F72 level

0

5000

10000

15000

20000

25000

ET

Er3+

MPA

980

nm

980

nm76

7 nm

822

nm

669

nm

548

nm

528

nm

492

nm

409

nm

2F72

2F52

4F32

2H112

4F52

4F72

Ene

rgy

(cm

ndash1)

2H92

4S32

4F92

4I112

4I92

4I132

4I152

MPA ESA

ET

Yb3+

Fig 7 Energy level diagram of Yb Er system showing possible exci-tation and de excitation mechanism under 980 nm MPA-Multiphotonabsorption ET-Energy Transfer

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LE

10 100

104

105

106

Inte

nsity

(a

u)

Pup Power (mW)

670 nm

slope138

528 nm 550 nm 670 nm

Fig 8 Dependence of the excitation power on the emission intensityof green and red emission

another 980 nm photon can be absorbed to an equally sim-ilar excited level As explained earlier these excited stateprocesses can equally happen under high photon densitythrough nonlinear absorption processes such as two pho-ton or three photon absorption After these excited stateprocesses de excitation accumulates electron in differentexcited states and the intensity of emission depends on theelectron density at that particular level as well as the non-radiative contribution to the emission band The popula-tion accumulated in level 2H92 gives the 410 nm emissionby radiatively decaying to 4I152 Since the efficiency ofthis third order process is less the emission intensity if the410 nm band is comparatively weak On the other handthe two photon processes that populate the 4F72 level isso efficient that it can give very efficient green emissiontransition from 2H112 due to the fast multiphonon non-radiative decay of 4F72 level to

2H112 Because of severalclosely spaced levels in between 2H112 and 4S32 multi-phonon relaxation results in decaying the 2H112 level to4S32 yielding the strongest emission band at 549 nm Thedecay time of the 4S32 level is measured to be 295 swhich is sufficient to make this as a meta stable level forthe green emission Further since the energy gap between4S32 and 4F92 is 2800 cmminus1 multiphonon relaxation isunlikely to happen and hence the 4S32 rarr 4F92 transitioncould be radiativeb Part of the population decaying to level4F92 can result in the red emission at 6698 nm throughthe 4F92 rarr 4I152 transition Finally fractional populationaccumulated in 4I92 level give rise to the 822 nm emis-sion through 4I92 rarr 4I152 transition The major processthat populate the 4F92 level of Er3+ is the energy trans-fer process from Yb according to the equation 2F52(Yb

3+)4I132(Er

3+)rarr 2F72(Yb3+)+ 4F92(Er

3+)In order to establish the presence of these non linear

processes the pump power dependence of the emission

bWe observed 32 m emission through the 4S32 rarr 4F92 transition

bands on the emission intensity was measured and the rela-tionship obtained is plotted in Figure 8 The slope obtainedfor 533 549 and 6698 nm emission bands are respectively138 145 and 141 which show that two photon processescontribute to these emissions It is interesting to note thatthe present material shows upconversion under 980 nm

0 2 4 6 8 105000

10000

15000

20000

25000

30000

35000

40000

Yb concentration (mol)

Er 1mol Green

Red

0 2 4 6 8 10

1times104

2times104

3times104

Yb concentration (mol)

Green

Red

Er = 1 mol

0 2 4 6 8 100

1times104

2times104

3times104

4times104

5times104

6times104

Yb concentration (mol)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Green

Red

Er = 1 mol

(a)

(b)

(c)

Fig 9 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er concentration at 1 mol (a) Gd2O2S YbEr(b) Y2O2S YbEr (c) La2O2S YbEr

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 2 4 6 100

1times104

4times104

5times104

6times104

Er concentration (mol )

Yb concentration (mol )

0246810

Green

Red

0 2 4 6 8 10

18times104

24times104

30times104

Er concentration (mol)

2468

Yb concentration (mol)

Yb+Er=10 molGreen

Red

10 0

0 2 4 6 8

2times104

3times104

4times1042468

Er concentration (mol)(a)

(b)

(c)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Yb concentration (mol)

Yb+Er=10 mol

Green

Red

10

1times104

12times104

8

3times104

2times104

Fig 10 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er+Yb concentration at 10 mol (a) Gd2O2SYbEr (b) Y2O2S YbEr (c) La2O2S YbEr

excitation from Xe lamp this corroborates the fact that inthis composition ESA also contributes to the upconversionprocess The fact that the material shows intense upconver-sion under low power excitation shows that the efficiencyof upconversion in this material is comparatively higherFurther under high power excitation the material showsalmost linear or saturation in intensity which is probablydue to the excitation population saturation at high excita-tion powerIn order to understand the functional dependence of the

red and green emission intensity on the Yb concentrationstwo sets of samples were prepared in the first set the YbEr concentration was kept fixed at 10 mol by propor-tionally changing Yb and Er concentrations In the secondset the Yb concentration was varied from 0 to 9 mol bykeeping the Er constant at 1 mol Results of both exper-iments are presented in Figures 9(a)ndash(c) and 10(a)ndash(c) forGOS YOS and LOS The green and red emission intensityvariations in these two experimental are entirely differentin each of the material In the first sets of experiments withEr 1 mol both green and red emission intensity peaks at7 mol of Yb in GOS and LOS whereas in YOS up to9 mol of Yb both emission intensity goes on increasingOn comparing the three hosts the green emission is moreintense in GOS followed by LOS and YOS In the secondsets of experiments with Er+Yb = 10 mol both greenand red emission intensity decreases at 7 mol 8 mol4 mol of Yb respectively in GOSYOS and LOS In LOSafter 8 mol of Yb the emission again seems to increasesComparison of the green emission intensity shows thatLOS is the brightestThe internal quantum efficiency (Int) of the upcon-

version band can be evaluated from the ratio of the flu-orescence to radiative decay time13 Fluorescence decaycurves obtained for the 555 nm emission band areshown in Figure 11 The fluorescence lifetime obtainedfor the 555 nm emissions are respectively 260 163 s

00

02

04

06

08

10

252015

Inte

nsity

(a

u)

Time (ms)

LOS YOS GOS

05 10

Fig 11 Fluorescence decay curve of the green emission band in GOSYOS and LOS at 8 mol Yb and 1 mol Er dopant concentrations

628 Sci Adv Mater 4 623ndash630 2012

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LE

10 100

104

105

106

Inte

nsity

(a

u)

Pup Power (mW)

670 nm

slope138

528 nm 550 nm 670 nm

Fig 8 Dependence of the excitation power on the emission intensityof green and red emission

another 980 nm photon can be absorbed to an equally sim-ilar excited level As explained earlier these excited stateprocesses can equally happen under high photon densitythrough nonlinear absorption processes such as two pho-ton or three photon absorption After these excited stateprocesses de excitation accumulates electron in differentexcited states and the intensity of emission depends on theelectron density at that particular level as well as the non-radiative contribution to the emission band The popula-tion accumulated in level 2H92 gives the 410 nm emissionby radiatively decaying to 4I152 Since the efficiency ofthis third order process is less the emission intensity if the410 nm band is comparatively weak On the other handthe two photon processes that populate the 4F72 level isso efficient that it can give very efficient green emissiontransition from 2H112 due to the fast multiphonon non-radiative decay of 4F72 level to

2H112 Because of severalclosely spaced levels in between 2H112 and 4S32 multi-phonon relaxation results in decaying the 2H112 level to4S32 yielding the strongest emission band at 549 nm Thedecay time of the 4S32 level is measured to be 295 swhich is sufficient to make this as a meta stable level forthe green emission Further since the energy gap between4S32 and 4F92 is 2800 cmminus1 multiphonon relaxation isunlikely to happen and hence the 4S32 rarr 4F92 transitioncould be radiativeb Part of the population decaying to level4F92 can result in the red emission at 6698 nm throughthe 4F92 rarr 4I152 transition Finally fractional populationaccumulated in 4I92 level give rise to the 822 nm emis-sion through 4I92 rarr 4I152 transition The major processthat populate the 4F92 level of Er3+ is the energy trans-fer process from Yb according to the equation 2F52(Yb

3+)4I132(Er

3+)rarr 2F72(Yb3+)+ 4F92(Er

3+)In order to establish the presence of these non linear

processes the pump power dependence of the emission

bWe observed 32 m emission through the 4S32 rarr 4F92 transition

bands on the emission intensity was measured and the rela-tionship obtained is plotted in Figure 8 The slope obtainedfor 533 549 and 6698 nm emission bands are respectively138 145 and 141 which show that two photon processescontribute to these emissions It is interesting to note thatthe present material shows upconversion under 980 nm

0 2 4 6 8 105000

10000

15000

20000

25000

30000

35000

40000

Yb concentration (mol)

Er 1mol Green

Red

0 2 4 6 8 10

1times104

2times104

3times104

Yb concentration (mol)

Green

Red

Er = 1 mol

0 2 4 6 8 100

1times104

2times104

3times104

4times104

5times104

6times104

Yb concentration (mol)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Green

Red

Er = 1 mol

(a)

(b)

(c)

Fig 9 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er concentration at 1 mol (a) Gd2O2S YbEr(b) Y2O2S YbEr (c) La2O2S YbEr

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Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 2 4 6 100

1times104

4times104

5times104

6times104

Er concentration (mol )

Yb concentration (mol )

0246810

Green

Red

0 2 4 6 8 10

18times104

24times104

30times104

Er concentration (mol)

2468

Yb concentration (mol)

Yb+Er=10 molGreen

Red

10 0

0 2 4 6 8

2times104

3times104

4times1042468

Er concentration (mol)(a)

(b)

(c)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Yb concentration (mol)

Yb+Er=10 mol

Green

Red

10

1times104

12times104

8

3times104

2times104

Fig 10 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er+Yb concentration at 10 mol (a) Gd2O2SYbEr (b) Y2O2S YbEr (c) La2O2S YbEr

excitation from Xe lamp this corroborates the fact that inthis composition ESA also contributes to the upconversionprocess The fact that the material shows intense upconver-sion under low power excitation shows that the efficiencyof upconversion in this material is comparatively higherFurther under high power excitation the material showsalmost linear or saturation in intensity which is probablydue to the excitation population saturation at high excita-tion powerIn order to understand the functional dependence of the

red and green emission intensity on the Yb concentrationstwo sets of samples were prepared in the first set the YbEr concentration was kept fixed at 10 mol by propor-tionally changing Yb and Er concentrations In the secondset the Yb concentration was varied from 0 to 9 mol bykeeping the Er constant at 1 mol Results of both exper-iments are presented in Figures 9(a)ndash(c) and 10(a)ndash(c) forGOS YOS and LOS The green and red emission intensityvariations in these two experimental are entirely differentin each of the material In the first sets of experiments withEr 1 mol both green and red emission intensity peaks at7 mol of Yb in GOS and LOS whereas in YOS up to9 mol of Yb both emission intensity goes on increasingOn comparing the three hosts the green emission is moreintense in GOS followed by LOS and YOS In the secondsets of experiments with Er+Yb = 10 mol both greenand red emission intensity decreases at 7 mol 8 mol4 mol of Yb respectively in GOSYOS and LOS In LOSafter 8 mol of Yb the emission again seems to increasesComparison of the green emission intensity shows thatLOS is the brightestThe internal quantum efficiency (Int) of the upcon-

version band can be evaluated from the ratio of the flu-orescence to radiative decay time13 Fluorescence decaycurves obtained for the 555 nm emission band areshown in Figure 11 The fluorescence lifetime obtainedfor the 555 nm emissions are respectively 260 163 s

00

02

04

06

08

10

252015

Inte

nsity

(a

u)

Time (ms)

LOS YOS GOS

05 10

Fig 11 Fluorescence decay curve of the green emission band in GOSYOS and LOS at 8 mol Yb and 1 mol Er dopant concentrations

628 Sci Adv Mater 4 623ndash630 2012

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

Sci Adv Mater 4 623ndash630 2012 629

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

630 Sci Adv Mater 4 623ndash630 2012

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 2 4 6 100

1times104

4times104

5times104

6times104

Er concentration (mol )

Yb concentration (mol )

0246810

Green

Red

0 2 4 6 8 10

18times104

24times104

30times104

Er concentration (mol)

2468

Yb concentration (mol)

Yb+Er=10 molGreen

Red

10 0

0 2 4 6 8

2times104

3times104

4times1042468

Er concentration (mol)(a)

(b)

(c)

Inte

nsity

(co

unts

)In

tens

ity (

coun

ts)

Inte

nsity

(co

unts

)

Yb concentration (mol)

Yb+Er=10 mol

Green

Red

10

1times104

12times104

8

3times104

2times104

Fig 10 Variation of Green and Red emission intensity as a function ofYb concentration at fixed Er+Yb concentration at 10 mol (a) Gd2O2SYbEr (b) Y2O2S YbEr (c) La2O2S YbEr

excitation from Xe lamp this corroborates the fact that inthis composition ESA also contributes to the upconversionprocess The fact that the material shows intense upconver-sion under low power excitation shows that the efficiencyof upconversion in this material is comparatively higherFurther under high power excitation the material showsalmost linear or saturation in intensity which is probablydue to the excitation population saturation at high excita-tion powerIn order to understand the functional dependence of the

red and green emission intensity on the Yb concentrationstwo sets of samples were prepared in the first set the YbEr concentration was kept fixed at 10 mol by propor-tionally changing Yb and Er concentrations In the secondset the Yb concentration was varied from 0 to 9 mol bykeeping the Er constant at 1 mol Results of both exper-iments are presented in Figures 9(a)ndash(c) and 10(a)ndash(c) forGOS YOS and LOS The green and red emission intensityvariations in these two experimental are entirely differentin each of the material In the first sets of experiments withEr 1 mol both green and red emission intensity peaks at7 mol of Yb in GOS and LOS whereas in YOS up to9 mol of Yb both emission intensity goes on increasingOn comparing the three hosts the green emission is moreintense in GOS followed by LOS and YOS In the secondsets of experiments with Er+Yb = 10 mol both greenand red emission intensity decreases at 7 mol 8 mol4 mol of Yb respectively in GOSYOS and LOS In LOSafter 8 mol of Yb the emission again seems to increasesComparison of the green emission intensity shows thatLOS is the brightestThe internal quantum efficiency (Int) of the upcon-

version band can be evaluated from the ratio of the flu-orescence to radiative decay time13 Fluorescence decaycurves obtained for the 555 nm emission band areshown in Figure 11 The fluorescence lifetime obtainedfor the 555 nm emissions are respectively 260 163 s

00

02

04

06

08

10

252015

Inte

nsity

(a

u)

Time (ms)

LOS YOS GOS

05 10

Fig 11 Fluorescence decay curve of the green emission band in GOSYOS and LOS at 8 mol Yb and 1 mol Er dopant concentrations

628 Sci Adv Mater 4 623ndash630 2012

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Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

Sci Adv Mater 4 623ndash630 2012 629

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

630 Sci Adv Mater 4 623ndash630 2012

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Kumar et al Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) PhosphorsARTIC

LETable I CIE (x y z) (X Y Z) color coordinates color temperature (CCT) and color purity of La2O2S Yb (8) Er (1) for different excitation power

Power Dominant CCT(mW) color (nm) X Y Z x y z (K) Purity

12 4987 13156 18512 12319 02991 04209 02801 6531 034718 5122 69674 104056 26308 03483 05202 01315 5222 026524 5466 20726 313992 42296 03678 05572 00751 5396 050830 5544 299893 43133 61047 03785 05444 00771 4730 048840 5488 337453 479412 79799 03763 05347 00890 4754 041250 5494 374987 499362 95472 03867 05149 00984 4516 034360 5390 378279 506588 111218 03798 05086 01117 4620 027470 5295 381879 513644 128241 03730 05017 01253 4725 022678 5165 385275 521782 144344 03664 04963 01373 4836 021280 5109 388687 528549 160749 03606 04903 01491 4940 021196 5143 392762 537260 179801 03539 04841 01620 5064 0218102 5060 395734 544454 193267 03491 04804 01705 5159 0226114 5081 400202 557390 213671 03417 04759 01824 5317 0241126 5001 404270 564049 233950 03363 04692 01946 5436 0251

and 295 in Gd2O2SYb(8)Er(1) Y2O2SYb(8)Er(1)La2O2SYb(8)Er(1) Following Judd-Ofelt1415 model wecalculated radiative decay times of 353 264 and 295 sand this yield an internal quantum yield of 74 617 and100 in GOS YOS and LOSThe excitation saturation at comparatively low power

excitation is particularly useful in several Photonics appli-cations such as solar energy converter or laser amplifierwhere high power excitation was required to achieve thedesired energy conversion With comparatively low powerexcitation efficient upconversion phosphor layers it wouldbe possible to enhance the energy conversion efficiency ofthe Si solar cells The same concept applies to the devel-opment of low pump threshold solid state lasers ceramiclasers and fiber amplifiers created with similar phosphorcompositions As a continuation of this work we are inthe process of measuring the external quantum efficiencyof the 9801550 nm to visible and NIR upconversion effi-ciency in these materials as a function of the compositionswith the intension of using them our current solar cellproject

33 CIE Color Coordinates Analysis

To measure the color of the visible emission that thehuman eye perceives the Commission internationale delrsquoeacuteclairage (CIE) coordinates (x y z) were calculatedfrom the (X Y Z) tristimulus values as follows16

x = X

X+Y +Z y = Y

X+Y +Z z= Z

X+Y +Z(2)

where the X Y and Z are related to the spectral powerdistribution function P13 as

X=intP13x13d13Y =

intP13y13d13Z=

intP13z13d13

(3)where is the equivalent wavelength of the monochromaticlight and x y z are color matching functions

In addition using the color coordinates calculatedabove the correlated color temperatures (CCT) of themixed upconversion fluorescence corresponding to differ-ent excitation powers were given by the McCamy empiri-cal formula17

CCT =minus437n3+3601n2minus6861n+551431 (4)

where n = x minus xey minus ye is the inverse slope lineand xe = 03320 and ye = 01858 The color coordinatesand CCT under different excitation powers are listed inTable I and the color coordinates are marked in Figure 12for the brightest compositions in YOS GOS and LOSThe (x y) coordinates obtained for YOS GOS and LOSare respectively (03541 05591) (03678 05507) and

Fig 12 CIE color coordinate plot showing the x y coordinates of theemission in M2O2S Yb (8) Er (1) (M = Y Gd and La) The emissioncolor was marked by the white arrow

Sci Adv Mater 4 623ndash630 2012 629

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

630 Sci Adv Mater 4 623ndash630 2012

Delivered by Ingenta toUniversity of Texas at San Antonio Library

IP 129115241Mon 13 Aug 2012 070958

Synthesis and Upconversion Spectroscopy of Yb Er Doped M2O2S (M= La Gd Y) Phosphors Kumar et alARTIC

LE

0 30 60 90 120

500

520

540

560E

mis

sion

Col

or

980 nm Pump Power (mW)

LOSYOSGOS

Fig 13 Power dependent emission color tuning in oxysulphidephosphor

(03867 05149) It was found that the nature of powerdependence of both x and y coordinates are almost thesame in all the 3 hostsThe color tuning range of the three phosphors are shown

in Figure 13 where it can be seen that the emission colorcan be easily tuned from 490 nm to 550 nm by changingthe excitation power from 10ndash50 mW The curve showsthat LOS is sharper in green compared to GOS and YOSThe color power dependent color tuning possibility of

the present phosphor material could find variety of appli-cations such as tunable upconversion ceramic lasers colortunable phosphor coatings multicolor biomedical imag-ing 3D display infrared activated LED design as well asin polymer fiber lasers and planar waveguides Recentlywe successfully made nanoparticles of several oxysulphideupconversion phosphors and currently investigating theoptical properties of these nanoparticles in several fluo-ropolymers with the intension of making optical fibersand planar waveguide structures for low pump thresholdupconversion laser applications

4 CONCLUSIONS

Using the solid state flux fusion method hexagonal shapedYb Er doped M2O2S (M = Y Gd La) phosphors weresynthesized and its upconversion spectroscopic propertieswere studies for range of Yb and Er dopant concentrationsAnalysis shows that all the compositions are stronger ingreen emission with the green emission intensity 2 timesstronger than the red in composition Gd2O2 SYb(8)Er(1)

Y2O2SYb(9)Er(1) La2O2SYb(3)Er(7) (All mol) Theupconversion efficiency in this material is so high that evenwith less than a 15 W of 980 nm excitation one couldsee the green upconversion with naked eye The internalquantum efficiency obtained for the green emission was74 62 100 respectively for the GOS YOS and LOTphosphor The low power excitation upconversion mecha-nism in this material offers several potential applicationssuch as solar energy conversion and low threshold upcon-version lasers Further the material offers power depen-dent color tuning properties where the emission color canbe tuned from 490 nm to 550 nm by simply changingthe 980 nm excitation power from 10ndash50 mW Upconver-sion color tunability by changing the pump power inten-sity offers several diverse Photonics applications such asnovel three-dimensional solid-state display upconversionLED lighting biomedical multi-color imaging lasers etc

Acknowledgments This work was supported by theNational Science Foundation Partnerships for Researchand Education in Materials (PREM) Grant No DMR-0934218 Authors are thankful to R C Dennis for theassistance in SEM imaging

References and Notes

1 L F Johnson and H Guggenheim Appl Phys Lett 19 44 (1971)2 S F Lim R Riehn W S Ryu N Khanarian C Tung D Tank

and R H Austin Nano Lett 6 169 (2006)3 K Kuningas T Rantanen T Ukonaho T Loumlvgren and T Soukka

Anal Chem 77 7348 (2005)4 E Downing L Hesselink J Ralston and R Macfarlane Science

273 1185 (1996)5 P A Tick N F Borrelli L K Cornelius and M A Newhouse

J Appl Phys 11 6367 (1995)6 G F J Garlick and C L Richards J Lumin 9 432 (1974)7 X-X Luo and W-H Cao Sci China Ser B Chem 50 505 (2007)8 P N Yocom J P Wittke and I Ladany Metall Trans A

Phys Metall Mater Sci 2 763 (1971)9 L Ozawa J Electroch Sooc 124 413 (1977)10 C L Lo J G Duh B S Chiou C C Peng and L Ozawa Mat

Chem Phys 71 179 (2001)11 P C Becker N A Olsson and J R Simpson Erbium Doped

Fiber Amplifiers-Fundamentals and Technology Academic PressNew York (1999)

12 M J M Diggonet (ed) Rare Earth Doped Fiber Lasers and Ampli-fiers Marcel Dekker Inc NY (1993)

13 A A Kaminskii Laser Crystals Their Physics and PropertiesSpringer Verlag Berlin (1981)

14 B R Judd Phys Rev 127 750 (1962)15 G S Ofelt J Chem Phys 37 511 (1962)16 httpenwikipediaorgwikiCIE_1931_color_space17 C S McCamy Color Res Appl 17 142 (1992)

630 Sci Adv Mater 4 623ndash630 2012