Fabrication of dual excitation dual emission phosphor with plasmonic enhancement of fluorescence for...

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ISSN 1144-0546

1144-0546(2010)34:12;1-1

www.rsc.org/njc Volume 34 | Number 12 | December 2010 | Pages 2685–3016

New Journal of Chemistry An international journal of the chemical sciences

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http://dx.doi.org/10.1039/c3nj00889d

http://pubs.rsc.org/en/journals/journal/NJ

Fabrication of dual excitation dual emission phosphor with

plasmonic enhancement of fluorescence for simultaneous conversion

of solar UV and IR to visible radiation

Santa Chawla*, M. Parvaz, Vineet Kumar and Zubair Buch

Luminescent Materials Group, CSIR-National Physical Laboratory, Dr.K.S.Krishnan Road, New

Delhi – 110012, India

Dual excitation phosphor with simultaneous conversion capability of solar UV and IR radiation to red and

green light is developed

Page 1 of 25 New Journal of Chemistry

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View Article OnlineDOI: 10.1039/C3NJ00889D


1

Fabrication of dual excitation dual emission phosphor with plasmonic enhancement of

fluorescence for simultaneous conversion of solar UV and IR to visible radiation

Santa Chawla*, M. Parvaz, Vineet Kumar and Zubair Buch

Luminescent Materials Group, CSIR-National Physical Laboratory, Dr.K.S.Krishnan Road, New

Delhi – 110012, India

Abstract

A dual excitation, dual emission phosphor has been fabricated by simultaneous doping of

lanthanide ions Er3+

, Yb3+

, Eu3+

in a highly efficient host YVO4. YVO4 doped with Er3+

, Yb3+

,

Eu3+

showed dual excitation i.e., simultaneously excitable by UV and IR radiation and dual

emission i.e., fluorescence in bright red under UV excitation and intense green under IR

excitation. Red DC fluorescence arises due to 5

D0-7FJ transitions of Eu

3+. UC emission spectra

indicate that Yb3+

not only sensitize Er3+

to emit predominantly in the green but also Eu3+

to

produce signature red emission. A direct assembly of Ag nanoparticles (NPs) and YVO4: Er3+

,

Yb3+

, Eu3+

on a suitable substrate showed enhancement of fluorescence of Eu3+

red emission

under UV excitation. Such studies indicate that two dimensional conformal transparent layer of

Ag NP-phosphor combine on a silicon solar cell may be used as DC and UC solar spectrum

converter from UV/IR to visible region where spectral response of Si is high.

Keywords: Dual excitation phosphor; surface plasmon resonance; confocal fluorescence; solar

spectrum conversion

*Corresponding author, e-mail:[email protected]

Page 2 of 25New Journal of Chemistry

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1. Introduction

Luminescence phosphors are usually categorized as Stokes shifted down conversion (DC)

and anti – Stokes shifted upconversion (UC) emitters. Rare earth lanthanide ions with specific

energy level spacing and high radiative recombination rate is chosen as activators in phosphors

amongst which Eu3+

is an excellent UV to red down shifter and Er3+

as IR to green/red up

converter with Yb3+

as sensitizer that resonantly absorbs IR radiation and efficiently transfer to

emitter Er3+

. DC and UC phosphors are prepared separately by choosing an appropriate host that

can absorb UV efficiently and transfer energy to activator ion (DC) or have low phonon energy

for enhanced radiative recombination from Er3+

levels. UV excitable phosphors with very high

luminescence efficiency have specific use in industries such as lighting and display. Efficient up

conversion phosphors are not too many due to multiphoton absorption and single photon

emission process and mostly fluorides hosts are chosen due to their low phonon energy.[1-2]

A

very important new application of phosphors is for enhancing solar energy conversion efficiency

that requires conversion of hitherto unutilized solar UV and IR radiation, through appropriate

phosphors, to visible region where spectral sensitivity of available solar cells are maximum. For

such application, a single phosphor host doped with appropriate lanthanide activators that can

perform the role of dual excitation and dual emission can prove to be ideal. Further,

nanoparticles of such highly efficient lanthanide doped luminescent material can be used to make

thin transparent layer to be used with solar cells to improve efficiency through enhanced light

input. Lanthanide doped inorganic nanophosphors have definite advantage over organic

fluorescent dyes and quantum dots in terms of sharp and narrow emission, long decay time, low

toxicity and negligible photo bleaching of f-f transitions of Ln3+

emitters. Such materials are also

very useful for biological applications.


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In this regard we have chosen YVO4 as host material and rare earth i.e. europium, erbium,

ytterbium as light emitting activators. Individual down conversion (DC) and up conversion (UC)

emission have been reported for YVO4:Eu3+

and YVO4:Er3+

, Yb3+

respectively, but there is no

report for both DC and UC dual mode luminescence from YVO4 doped with identical set of

lanthanide ions. YVO4 is an ideal UV sensitizer due to efficient host (VO43-

) UV absorption and

subsequent energy transfer to rare earth activator ion with high luminescence efficiency. [3-4]

YVO4:Eu3+

has proved to be suitable for various applications such as white light emitting diodes

(WLEDs), field emission display panels, in color television and cathode ray tubes (CRT’s).[5]

Individual DC and UC emission have been reported for YVO4 doped with Eu, Sm etc. [6-7]

and

Er, Yb.[8]

But there is no report for both DC and UC dual mode luminescence, to the best of our

knowledge, from YVO4 doped with identical set of lanthanide ions. In the present study, YVO4

doped with Er3+

, Yb3+

, Eu3+

showed tunable dual excitation (simultaneously excitable by UV and

IR radiation) and dual emission (under UV excitation bright red and under IR light intense green

light) phosphor characteristics. Both UV and IR mode excitation has been reported for NaGdF4:

Tm, Yb/ NaGdF4: Eu core/shell structure, [9]

in Gd2O3:Er,Yb/Eu/DBM)3 Phen organic complex

[10] in GdPO4:Yb

3+,Tb

3+,[11]

BaGd2(MoO4)4:Eu3+

,Er3+

,Yb3+

. [12]

Dual emission has been achieved

by using rare earth organic complexes, fluorescent dye systems [13-16]

but UV and IR were not

used for their excitation. UC luminescence is usually observed in Yb3+

sensitized hosts such as

NaYF4 and NaGdF4 with emitters Er3+

, Tm3+

, Ho3+

. [17-20]

The IR resonant energy levels of Yb3+

efficiently absorb two IR photons and transfer energy to the ladder like energy levels of emitter

ions Er3+

/Tm3+

/Ho3+

. Due to large energy gap between the Yb3+

levels (2F5/2) to Eu

3+ emitting

levels (5DJ), direct sensitization of Eu

3+ ions for UC emission has less probability.


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For clean solar photovoltaic power, efficiency enhancement of commercial solar cells is

very important. As silicon solar cells can effectively absorb in the spectral range 550- 1100 nm

(hν ~ Eg - 2Eg), most of the terrestrial solar energy ( ~ 300 - 2400nm) in the UV and IR remains

unutilized. Solar spectrum conversion by employing suitable phosphors to harness the unutilized

part of solar spectrum is a promising solution to reduce photon losses. The suitability of a

phosphor system for solar spectrum modification depends on the excitation range of phosphor

system in the UV and IR. Also, the emission spectra under UV and IR excitation should cover

the high spectral response region of solar cell, the luminescence yield must be high and the

phosphor system should be excitable by low light intensity. Both down conversion (DC) and

upconversion (UC) phosphors can be used on the front and back faces of solar cells respectively

to convert solar ultraviolet (UV) and transmitted infra red (IR) energy to visible region for

effective absorption. Use of both DC and UC phosphors need a bifacial cell and technological

difficulties in solar cell-phosphor layer integration poses a challenge as transmitted IR light is

insignificant due to appreciable Si wafer thickness and fabricated electrodes on surfaces.

Towards this goal, a dual excitation phosphor that can absorb both UV and IR solar radiation and

emit in the visible region with high luminescence yield has been developed. In this regard, we

have chosen the material system (YVO4: Er3+

,Yb3+

,Eu3+

) as rare earth doped YVO4 phosphors

have shown high luminescence yield.[21-22]

To further augment the luminescence yield,

plasmonic enhancement of fluorescence has been accomplished by making a phosphor-Ag

nanoparticle (NP) layer.


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2. Experimental

2.1 Synthesis of phosphor

Yttrium oxide (Y2O3,99.99%), Vanadium pentoxide (V2O5,CDH Analytical Reagent

99.5%) was used for host and Ytterbium oxide (Yb2O3, 99.99%, Aldrich), Erbium oxide (Er2O3,

99.99%, Aldrich), Eu2O3(99.95%, Aldrich) as activators precursors and nitric acid (Merck,

99.9%) ethanol (Merck), hydrochloric acid, and dichloromethane (Merck) were used without

any further purification, for the synthesis of multi ion doped yttrium Vanadate (YVO4). For

synthesis by solid state reaction (SSR) method, the starting materials Y2O3 (75 mol%), V2O5 (100

mol%), Er2O3(2.0 mol%),Yb2O3(18 mol%), Eu2O3(5 mol%) were thoroughly mixed, packed in

an alumina boat and fired at 1300oC for 2 hours in air atmosphere. The resultant product was

ground in mortar pestle to obtain fine powder. YVO4: Er (2%), Yb(18%) sample was also

prepared by identical method to examine the difference in UC characteristics due to doping of

Eu3+

ions. For synthesis of nanoparticles of YVO4:Er3+

, Yb3+

, Eu3+

, co-precipitation (CPP)

method was employed using above mention precursors in same stoichiometric proportions.

Nitrate solutions of precursors were obtained by dissolving the oxides in minimum amount of

HNO3. NH4OH and H2O2 in the volume ratio 3:1 was added drop wise into the solution till the

pH of the solution was 8 when ultrafine particles of YVO4 doped with rare earth started forming

and were precipitated out in centrifuge. The powder sample was washed repeatedly with DI

water and then with ethanol to dehydrate the surface bound water molecules as OH- is a known

quencher of luminescence. Finally the precipitates were dried in an oven at 400C for 12 hrs.


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2.2 Synthesis of spherical silver nanoparticles

Silver nanoparticles were prepared by reducing AgNO3 by D-Glucose, using PVA as

capping agent. NaOH was used for maintaining high pH and for fast reduction of AgNO3,

yielding mono dispersed Ag NPs. UV-Vis absorption measurement of Ag colloidal solution

showed maximum absorptions at 400nm and the color of the colloidal solution was straw yellow.

No oxidation, confirmed by change in color was observed from the Ag colloidal solution over

few months from the date of synthesis.

2.3 Phosphor – Ag NP combination

The prepared colloidal solution of Ag NPs was directly mixed with YVO4:Er3+

, Yb3+

,

Eu3+

(synthesized by SSR) dispersed in ethanol and both solutions were homogenized by

thorough sonifcation. A direct assembly of homogenized composite solution of phosphor

particles and Ag NPs were deposited as thin films on microscopic cover slips by spin coating.

The prepared thin film was examined under confocal fluorescence microscope. A thin film of

only YVO4:Er3+

, Yb3+

, Eu3+

particles were also prepared in a similar way and examined under

confocal fluorescence microscope.

2.4 Material Characterization

The X-ray diffraction (XRD) pattern was recorded using a Rigaku MiniFlex

Diffractometer with Cu-Kα radiation. The morphology of developed nanoparticles was examined

with transmission electron microscopy (TEM) model no. JEOL JEM-1011. The

photoluminescence (PL) excitation, emission spectra and time resolved decay of luminescence

were recorded using combined steady state fluorescence and lifetime spectrometer of Edinburgh


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Instruments (FLSP920) with Xe lamp as excitation source for DC and IR (980 nm, 400 mW) for

UC measurement. Decay measurements were performed using excitation of microsecond pulsed

Xe lamp and employing time correlated single photon counting (TCSPC) technique. Confocal

fluorescence imaging was done using a WITec confocal Raman microscope (alpha300RS) with

UV laser and IR laser (980 nm, 2 mw) for DC & UC measurement respectively.

3. Results and discussion

3.1 Structure and morphology

The triple rare earth doped yttrium vanadate particles were synthesized by SSR method at

high temperature as well as nanoparticles by CPP method at room temperature. XRD analysis

was carried out for determining phase composition and the crystallite size of YVO4:Er3+

, Yb3+

,

Eu3

particles. Fig. 1 shows the XRD pattern of YVO4:Er3+

, Yb3+

, Eu3

prepared by SSR (a) and

CPP method (b) indicating monophasic zircon type orthovanadate tetragonal structure of pure

YVO4 phase (JCPDS card 17-0341) without any precipitated phase. The average crystallite size

was calculated based on Scherrer’s relation. The increase in particle size from bulk to nano can

be easily discerned by the change in peak width in XRD pattern of samples prepared by SSR and

CPP method. The well developed morphology of synthesized YVO4:Er3+

, Yb3+

, Eu3+

nanoparticles prepared by co-precipitation method are shown in the TEM images (Fig. 2a & b)

that showed hexagonal morphology with average particle size of 30-50 nm. The particles

prepared by SSR method exhibit well formed rounded morphology (Fig. 2 c & d) with average

particle size about 1 µm. The thin film prepared by direct assembly of YVO4:Er3+

, Yb3+

, Eu3+

particles and Ag nanoparticles were examined by TEM and the micrograph showed black portion

corresponding to Ag NPs and transparent particles correspond to YVO4 particles.


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3.2 Up conversion luminescence

The UC PL emission spectra of YVO4:Er3+

, Yb3+

, Eu3+

and YVO4:Er3+

, Yb3+

prepared by

SSR method is shown in Fig. 3(a), the background bright green spot is the photograph of the

powder phosphor under IR (980nm) illumination. To validate comparison, the measurements

were performed under identical experimental conditions and normalized against sample weight.

The UC PL in the visible range for both Er3+

, Yb3+

, Eu3+

and Er3+

, Yb3+

doped YVO4 powder

phosphor under 980nm diode laser excitation exhibit three distinct Er3+

emission bands in green

(523-531nm, 543-552nm) and red (652 – 669nm). The prominent green emission band between

543-552 nm has two peaks at 544nm, 552 nm. The level space 4I15/2→

4I11/2 of the Er

3+ ion and

2F7/2→

2F5/2 of the Yb

3+ ion resonantly match the energy of 980nm laser photons, but due to much

higher excitation cross section and concentration of Yb3+

ions compared to Er3+

ions, the

predominant IR excitation mechanism is through ground state excitation of two Yb3+

ions to 2F5/2

level by absorption of two IR photons and energy transfer to Er3+

ions populating the 4I11/2 level

(Fig.6). Other Er3+

levels can be populated through energy transfer to 4F7/2 level followed by

multiphonon relaxation. Green emission at 523 nm is due 4F7/2 -

4I15/2 and strong green peaks at

544 nm and at 552 nm are assigned to the 2H11/2→

4I15/2 and

4S3/2→

4I15/2 transitions respectively.

Much smaller red emission was observed between 652 – 669nm originating from the 4F9/2→

4I15/2

to 4I11/2→

4I15/2 transitions of Er

3+. Due to addition of Eu

3+ dopant in YVO4: Er

3+, Yb

3+; there is

significant enhancement of UC emission from the Er3+

related peaks and a set of new

emission peaks related to Eu3+

transitions appeared under IR (980nm) excitation. Such

enhancement of Er3+

emission in triple activated Y2O3:Er,Yb,Eu has also been reported.21

A

comparison of the UC PL emission spectra of YVO4:Er3+

,Yb3+

and YVO4:Er3+

,Yb3+

, Eu3+

(Fig.3a) synthesized by SSR method clearly show the additional UC emission peaks in triple


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activated YVO4:Er3+

, Yb3+

, Eu3+

in the red arising from Eu3+

at 593 nm (5D0-

7F1), at 615-618nm

(5D0-

7F2), 697-704nm (

5D0-

7F4) and a small peak at 720nm (

5D0-

7F6).

The UC emission spectra from YVO4:Er3+

,Yb3+

, Eu3+

nanoparticles prepared by CPP

method (Fig.4a), showed much weaker red emission from Er3+

(650-665nm, 4F9/2 –

4I15/2) and

Eu3+

UC emission at 615nm (5D0-

7F2) and a sharp intense peak at 720nm (

5D0-

7F6). Since in NPs,

Eu3+

may occupy surface sites without inversion symmetry, 5D0-

7F6 transition becomes

prominent. Considerably less luminescence in NP’s is due to inefficient incorporation of high

concentration of activators (Er3+

,Yb3+

) ions near room temperature synthesis.

3.3 Down conversion luminescence

The DC emission spectra of YVO4: Er3+

, Yb3+

, Eu3+

nano and bulk phosphor material is

shown in Figure 3(b). The fluorescence spectrum showed major peaks at 618 nm and 698 nm

when excited at 300 nm arising due to 5D0-

7F2 and

5D0-

7F4 transitions of Eu

3+ ions occupying Y

3+

sites. The electric dipole transitions of Eu3+

(ΔJ = ±2, ±4) are hypersensitive to site symmetry and

the effect can be seen with very intense 5D0-

7F2 and

5D0-

7F4 transitions. It was clearly indicated

that under UV excitation, the same sample gives red color emission which confirms the down

conversion of the UV wavelength to visible wavelength. The background bright orange red spot

shows the actual photograph of YVO4: Er3+

, Yb3+

, Eu3+

(SSR) under UV excitation. The

photoluminescence excitation (PLE) spectra of YVO4:Er3+

, Yb3+

, Eu3+

nanoparticles at 618 nm

Eu3+

emission is shown as inset in Fig.4(b). The spectrum shows a broad band from 250 to 350

nm due to the charge transfer absorption of the VO43-

in the host YVO4 and sensitizing the

activator Eu3+

ion. [21]

The broad charge transfer band clearly shows the efficient host (YVO4) to


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activator (Eu3+

) charge transfer process. The other sharp excitation peaks are due to direct f-f

transitions of Eu3+

ion e.g., peak at 394 nm is due to direct excitation of Eu3+

levels 7

F0 – 5L6.

Dual excitation (UV and IR) and dual emission (green and red) of YVO4:Er3+

, Yb3+

, Eu3+

is thus

clearly established.

3.4 Time resolved decay of luminescence

Time resolved luminescence decay of YVO4:Er3+

, Yb3+

, Eu3+

nano and bulk phosphor

were measured to investigate the dynamics of photo physical process related to bound exciton

and their effect on particles with nanometer dimensions. The decay was recorded for 5D0-

7F2

transition at 618nm emission under 300nm excitation by a time correlated single photon counting

technique using a microsecond pulsed xenon flash lamp as the source of excitation. The decay

curves for YVO4:Er3+

, Yb3+

, Eu3+

powder samples prepared by SSR and CPP method are shown

in Fig.4b. It can be clearly seen that decay becomes much faster for nanoparticles prepared by

CPP method compared to micron sized phosphor particles prepared by SSR method. The decay

curves of the YVO4:Er3+

, Yb3+

, Eu3+

prepared with solid state reaction method have single

exponential decay and nanophosphor prepared with co-precipitation method exhibited multi-

exponential decay and have been fitted into exponential equation, the estimated decay

parameters are listed in Table 1. Due to large surface to volume ratio of NPs compared to micron

sized particles, there would be surface states providing non radiative pathways as well as some of

the Eu3+

ions may be located on the surface sites without inversion symmetry leading to faster

decay.

3.5 Plasmonic enhancement of fluorescence and confocal fluorescence measurements

Confocal UC fluorescence image and confocal UC fluorescence spectrum of YVO4:Er3+

,

Yb3+

, Eu3+

particles under 980 nm (2mW) laser excitation is shown in Fig. 5a(i) & 5a. The


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spectra clearly show the green emission from Er3+

as well as distinct sharp signature peaks from

Eu3+

in the red region. Corresponding high resolution confocal UC fluorescence spectra from

Er3+

ion arising from transitions between Er3+

Stark split levels are clearly indicated in Fig.5 a(ii)

and the inset shows the intense green UC emission of powder sample under very low power

(2mW) 980 nm diode laser excitation.

The graph in Figure 5b shows fluorescence spectrum taken for YVO4:Er3+

, Yb3+

, Eu3+

and Ag NP composite thin films under UV laser excitation. The spectra shows the signature Eu3+

red emission due to UV excitation of YVO4:Er3+

,Yb3+

, Eu3+

and Ag NP combine. The optical

(Fig.5b(i)), confocal fluorescence image (Fig.5b(ii)) and corresponding fluorescence spectra at

the respective marked places as shown on Fig.5b clearly indicate fluorescence enhancement at

places where Ag NPs are present which is manifested by dark region in optical image and

corresponding bright fluorescent confocal image. [24]

Enhancement of red emission up to 40

times has been observed under UV excitation. The UV-vis absorption spectra of Ag NPs reveal

the maximum absorbance peak at 400nm (Fig.5b(iii)), which corresponds to the localized

Surface Plasmon Resonance (LSPR) due to dipolar oscillation of spherical Ag nanoparticles.

Because of the small size of Ag NPs, their presence is observed as dark region in the optical

image and due to enhancement of fluorescence from phosphor in proximity of Ag NPs, the

corresponding region in confocal fluorescence image is bright. The graph clearly shows increase

in the fluorescence intensity in regions where Ag NPs are present in proximity of phosphor

particles as can be seen by comparing the bright and dark region in optical image and

corresponding dark and bright fluorescent confocal image with the fluorescence spectra recorded

in different regions. This increase in fluorescence can be accredited to the plasmonic effect of

silver nanoparticles which create highly localized electric field around Ag NPs when illuminated


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by UV light. Phosphor particles in proximity of Ag NPs experience enhanced excitation field due

to LSPR and these highly localized fields are responsible for enhancement of the fluorescence

emission from phosphors placed in the near-field of Ag NPs. [25]

Fluorescence enhancement in

YVO4: Eu3+

phosphor in close proximity of Ag nanofilm has also been reported through

enhancement of the electric dipole transitions of Eu3+

ion. [26]

3.6 Mechanism of dual UC emission from Eu3+

and Er3+

The schematic diagram of dual mode excitation, energy transfer leading to UC and DC

dual emission in a YVO4:Er3+

, Yb3+

, Eu3+

particle is shown in Fig.6(a). Fig.6(c) shows the

proposed energy level diagram for DC and UC luminescence through energy transfer between

the host and lanthanide dopants. For UV excitation (DC luminescence), the energy is absorbed

by the host YVO4, mainly due to various intra- and intermolecular transition of VO43-

, followed

by relaxation through phonons to the bottom of the excitation band from where the energy is

transferred to neighboring Eu3+

ions to the 5DJ levels. Very efficient radiative transition from the

lowest 5D0 excited level to lower

7FJ levels give rise to sharp emission peaks in the red spectral

region (Fig.3b). DC fluorescence enhancement happens through energy transfer from Ag NP to

Eu3+

levels and is indicated in the energy level diagram (Fig.6c). UC excitation in Eu3+

occurs

through cooperative energy transfer from two Yb3+

ions (Fig.6c).

For IR excitation at 980nm and UC luminescence from Er3+

, two photons are

simultaneously resonantly absorbed by the 2F7/2→

2F5/2 of the Yb

3+ ion. One of the Yb

3+ ions

imparts the energy to the other Yb3+

ion that comes back to the ground 2F7/2 state. The excited

Yb3+

ion successfully transfer the energy to various ladder like levels of Er3+

ion from where non

radiative relaxation and radiative transition (2H11/2→

4I15/2 and

4S3/2→

4I15/2) take place giving rise


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to predominant green emission. UC in Er3+

, Yb3+

system by ground state absorption

(GSA)/energy transfer upconversion (ETU) is shown in Fig.6c. UC emission corresponding to

Er3+

ions increases with simultaneous doping of Eu3+

ion in YVO4. This is in contrast with the

observation of NaGdF4: Tm, Yb, Eu [9]

where UC PL intensity decreases with Eu concentration

due to unwanted cross relaxation between Tm and Eu ions. Characteristic emission peaks

corresponding to 5D0 -

7FJ transitions of Eu

3+ ions in the UC spectra of YVO4:Er

3+, Yb

3+, Eu

3+,

clearly indicate that energy transfer from Yb3+

sensitizer to emitter Eu3+

takes place thus

populating 5DJ levels of Eu

3+ leading to red UC emission. The likely mechanism for UC emission

from Eu3+

ions is cooperative energy transfer between a pair of Yb3+

ions and one Eu3+

ion as

depicted in Fig.6c. A pair of Yb3+

ions resonantly absorb two 980nm photons to reach 2F5/2 level

and simultaneously transfer their energy to the emitter Eu3+

ion in the neighborhood to excite it

to the 5DJ levels. Subsequent nonradiative relaxation to the lowest

5D0 level and radiative

recombination to 7FJ levels produce red UC fluorescence which is clearly seen in the inset of

Fig.3a and confocal fluorescence spectra (Fig.5a). Such Yb3+

sensitized UC emission from Eu3+

and Tb3+

ions have been reported for Y2O3:Eu3+

,Yb3+ [16]

GdPO4:Tb3+

,Yb3+

.[11]

Synthesized dual excitation phosphor YVO4:Er3+

, Yb3+

, Eu3+

have potential as solar

spectrum converter and Fig. 6(b) clearly shows how a single phosphor can harness solar UV and

IR region by spectral conversion to visible region for maximum photovoltaic utilization.

4. Conclusions

Monophasic YVO4 doped with three trivalent rare earth ions Er3+

, Yb3+

, Eu3+

was

prepared via low temperature co-precipitation method with particle size ranging between 30-50

nm, and high temperature solid state reaction method with average particle size about 1 µ. Y0.75%


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VO4:Er3+

0.02%, Yb3+

0.18%, Eu3+

0.05% was the optimum concentration used. The developed material

showed intense green upconversion fluorescence under low intensity IR (980nm, 2mW)

excitation and bright orange red emission under UV (250- 395nm) excitation. A coupled system

comprising Ag nanoparticles and YVO4:Er3+

, Yb3+

, Eu3+

phosphor particles embedded on a

substrate showed enhancement of red emission up to 40 times under UV excitation relative to

only YVO4:Er3+

, Yb3+

, Eu3 phosphor. Enhancement of DC fluorescence from Eu

3+ occur due to

excitation enhancement by generated near field of Ag NPs and this was confirmed through

confocal fluorescence imaging and spectroscopy.

The concept of producing dual excitation and dual emission through trivalent rare earth

doped phosphor material for enhancing solar energy conversion i.e. using infrared (IR) and UV

light for intense visible emission via upconversion (UC) and downconversion (DC) is introduced

in this work. Such studies indicate that a single phosphor may be used as DC and UC solar

spectrum converter from UV/IR to visible region where spectral response of solar cell is

maximum. Two dimensional conformal transparent layer of Ag NP-phosphor combine on a

silicon solar cell has potential to improve the performance of solar cells without altering the

existing solar cell device structure. The dual excitation property can be used to harvest sunlight

more efficiently as single phosphor layer can convert both hitherto unutilized UV and IR light to

visible radiation for efficient photovoltaics.

Acknowledgements

Authors gratefully acknowledge the research grant under TAPSUN program of CSIR to carry out

the work. We thank Hitesh Mamgain of WITec Instruments, Germany, for confocal

measurements.


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References

[1] J.F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K.W. Kramer, C.

Reinhard, H.U. Gudel, Optical Materials 2005, 27, 1111-1130.

[2] F. Vetrone, R. nacche, V. Mahalingam, C.G. Morgan, and J.A. Capobinaco, Adv. Funct.

Mater. 2009, 19, 1-6.

[3] R. Jagannathan, R. Lakshminarayanan, and R.P. Rao, Bulletin of Electrochemistry, 1990, 6,

318-319.

[4] http://www.lamptech.co.uk/Spec%20Sheets/Sylvania%20H175-DX.htm

[5] L.Chen, Y. Liu,Y, K. Huang, Material Research Bulletin, 2006, 41, 158-166.

[6] K.Riwotzki, and M.Haase, J. Phys. Chem. B 1998, 102, 10129-10135.

[7] Z.Hou, P.Yang,C. Li, Chem. Mater. 2008, 20, 6686–6696.

[8] V.Nguyen, T.K.C.Tran, and D.V.Nguyen, Adv. Nat. Sci.: Nanosci. Nanotechnol. 2011, 22

045011.

[9] Y. Liu,D. Tu,H. Zhu,R. Li,W. Luo, and Xueyuan, Chen Advanced Mat. 2010, 223266-3271.

[10] S.K. Singh, A.K. Singh, and S.B. Rai, Nanotechnology, 2011, 22, 275703.

[11] T. Grzyb, A. Gruszeczka, R.J.Wiglusz, Z.Sniadecki, B.Idzikowski, S.Lis, J.Mater.chem.

2012, 22 , 22989.

[12] J.Sun, Wei Zhang, W.Zhang, H.Du, Materials Research Bulletin, 2012,47,3, 786-789.

[13] C. Guo, Jie Yu,X. Ding,Z. Ren, and J.Bai, J.Electrochem.Soc 2011, 158, 42-46.

[14] A.Ajayaghosh, C.Vijayakumar, V.Praveen, S.S.Babu, R.Varghese, J. Am. Chem. Soc. 2006,

128, 7174.

[15] T.A.Fayed,M.K. Awad, Chem. Phys. 2004, 303, 317.

[16] J.Shan, X.Qin,N. Yao, and Y.Ju, Nanotechnology, 2007, 18, 445607.


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ishe

d on

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9/20

13 1

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:26.



16

[17] J.Zhang, C. Mi,H. Wu,H. Huang, C.Mao, and S.Xua, Anal Biochem. 2012, 15, 673–679.

[18] J. Ryu,H.Y. Park, K.Kim, H.Kim, J.H.Yoo, M. Kang, J. Phys. Chem. C, 2010, 114, 21077–

21082.

[19] Q.Cheng, J.Sui, and W.Cai, Nanoscale, 2012, 4, 779-784.

[20] H.Wang, C.K.Duan, and P.A.Tanner, J. Phys. Chem. C 2008, 112, 16651-16654.

[21] W.L. Wanmaker, A. Bril, J.W. Vrugt, J. Broos, Philips Res. Rep. 1966, 21, 270.

[22] R. C. Ropp, Journal of the optical society of America, 1976, 57, 10.

[23] A.Huignard, T.Gacoin, and J.P. Boilot, Chem. Mater. 2000,12, 1090.

[24] W.Feng, L.Sun, & C.Yan,. Chem. Commun., 2009, 4393–4395.

[25] F.Song, Q.Wang, S.Lin, J.Liu, H.Zhao, C.Zhang,C. Ming, E.Y.B.Pun, Optics Express,

2011, 19, 7001.

[26] C.Jang, S.M. Lee, K.C.Choi, Optics Express, 2012, 20, 2143.


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Table 1.

Decay parameters of YVO4:Er3+

, Yb3+

, Eu3+

nano and bulk phosphor

Sample name

(Emission nm)

τ1 (µs)

(Rel.%)

τ2 (µs)

(Rel.%)

τ3 (µs)

(Rel.%)

YVO4: Er+3

,Yb+3

,Eu+3

[SSR] 74.06

(7%)

309.54

(93%)

-

YVO4: Er+3

,Yb+3

,Eu+3

[CPP] 0.76

(44%)

14.09

(31%)

300.97

(25%)


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Figure captions

Figure 1 XRD pattern of YVO4:Er3+

,Yb3+

, Eu3+

powder obtained by (a) SSR and (b) CPP

method

Figure 2 (a) & (b) TEM images of YVO4:Er3+

, Yb3+

, Eu3+

prepared by co-precipitation method

showing separated individual particles and (c) & (d) SEM images of YVO4:Er3+

, Yb3+

, Eu3+

prepared by SSR method.

Figure 3 (a) UC PL emission spectra under 980 nm IR laser excitation of YVO4:Er3+

,Yb3+

, Eu3+

and YVO4:Er3+

,Yb3+

powder obtained by SSR, inset shows the appearance of Eu3+

UC emission

peaks and enhanced Er3+

emission due to Eu3+

codoping; (b) DC PL emission spectra under

UV excitation of YVO4:Er3+

,Yb3+

, Eu3+

nano and bulk powder. The backgrounds show the

actual PL images of the developed phosphor under (a) IR and (b) UV excitation.

Figure 4 (a) UC PL Emission spectra of YVO4: Er+3

, Yb+3

,Eu+3

nanoparticles synthesizes by CPP

method under 980 nm IR excitation. (b) Time resolved PL decay at 300 nm excitation and

618nm emission of YVO4:Er3+

, Yb3+

, Eu3+

nano and bulk phosphor, decay is much faster for

nanoparticles, inset shows PL excitation spectra at 618nm emission.

Figure.5 ‘color online’ (a) Confocal upconversion fluorescence spectrum of YVO4:Er3+

, Yb3+

,

Eu3+

particles under 980 nm (2mW) laser excitation at brightest and less bright spot, inset (i)

shows the confocal upconversion fluorescence image, inset (ii) shows high resolution confocal

UC fluorescence spectra from Er ion and the inset shows the intense green UC emission of

powder sample under very low power (2mW) 980 nm diode laser excitation; (b) inset (i) optical

image, inset (ii) confocal fluorescence image and confocal fluorescence emission spectra at

marked places under UV laser excitation of Ag NP phosphor film, showing enhancement in

fluorescence in regions dark (optical image) and corresponding bright (confocal fluorescence

image) due to presence of Ag nanoparticles, inset (iii) shows the measured UV-Vis absorption

spectra of Ag NPs.

Figure 6 ‘color online’ Schematic diagram of (a) dual mode excitation, energy transfer leading to

UC and DC dual emission in a YVO4:Er3+

, Yb3+

, Eu3+

particle; (b) possible utilization of solar

UV and IR radiation through spectral conversion by dual excitation dual emission YVO4:Er3+

,

Yb3+

, Eu3+

phosphor, green region denotes the solar spectral region that is utilized maximum by

Si solar cell for photovoltaic conversion; (c) proposed energy level diagram for DC and UC

luminescence, DC excitation and fluorescence enhancement through energy transfer from host

(VO4) and Ag NP to Eu3+

levels respectively is indicated. UC excitation in Eu3+

occurs through

cooperative energy transfer from two Yb3+

ions. UC in Er3+

, Yb3+

system by GSA/ETU is also

shown.


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Figure 1


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Figure 2


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Figure3


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Figure 4


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Figure 5


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Figure 6


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Fabrication of dual excitation dual emission phosphor with plasmonic enhancement of fluorescence for...

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