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Copyright © 2014 by American Scientific Publishers All rights reserved. Printed in the United States of America

Science of Advanced Materials Vol. 6, pp. 1–8, 2014 (www.aspbs.com/sam)

Doping Triple Lanthanum Ions in GdPO4 Nanocrystals Through Multiple Synthesis Routes and Their Dual Mode Spectrum Conversion Behaviour Vineet Kumar, Sukhvir Singh, and Santa Chawla∗ CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India

ABSTRACT

KEYWORDS: Lanthanum Ion, Co-Precipitation Method, Sol Gel Method, Photoluminescence, Dual Excitation Nanophosphor, Spectrum Conversion.

1. INTRODUCTION Lanthanum group comprise light emitting rare earth (RE) ions and RE doped luminescent inorganic phosphors and their nanoparticles have engrossed great attention in applications such as fluorescent lighting, white light emitting diodes (WLEDs), field-emission displays (FEDs), plasma display panels (PDPs), luminescent solar collector (LSC) materials, medical diagnostics, sensors, security applications etc.1–6 Most lighting and display require phosphors which give Stokes shifted down conversion (DC) emission. Anti Stokes shifted upconversion (UC) emitters that can absorb infra red (IR) light and emit in the visible are important materials for in vivo biological imaging as well as solar spectrum modifier.7 A very important application of nanophosphor that is emerging in recent times is solar spectrum conversion for enhancing solar cell efficiency. Due to limited absorption ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: XX Xxxx XXXX Accepted: XX Xxxx XXXX

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range of silicon solar cells (550 to 1100 nm) on account of its bandgap (1.15 eV), most of the terrestrial solar energy (∼300–2400 nm) in the UV and IR remains unutilized by solar cells. Suitable phosphors, when integrated with solar cells can convert solar UV and IR radiation in the visible range which can be absorbed by silicon for photo carrier generation and thus reduce photon losses arising due to thermalization and transmission losses. For a nanophosphor to be suitable for solar spectrum conversion the excitation range should be in the UV (above 300 nm)/IR, the emission must be in the high spectral response region of solar cell. Most reports discuss the possibility of using either down conversion (UV to visible) and/or upconversion (IR to visible) phosphors that can be used on the front or rear surface of solar cell respectively. Use of individual DC and UC phosphor for solar spectrum conversion necessitate a bifacial solar cell. Since IR light transmitted through appreciable Silicon wafer thickness and fabricated surface electrodes is insignificant to excite multiphoton UC process in phosphor, a novel solution is to employ a single nanophosphor that can be dually excited by both UV

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doi:10.1166/sam.2014.1997

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Doping multiple lanthanum ions in crystal of nanometer dimensions to achieve desired properties is an arduous task. This has been achieved in an important lanthanum phosphate (GdPO4 ) via three different synthesis routes including soft chemical processes such as room temperature co precipitation and pechini type sol–gel method and a facile solid state diffusion reaction. All the methods produce well crystalline nanoparticles with dimensions ranging from few nm to tens of nm but with different morphology. The three lanthanum ions Ho3+ , Yb3+ , Eu3+ introduce up and down conversion luminescence in GdPO4 nanocrystals making them simultaneously excitable by UV and IR light and emit in the visible. The concept of dual excitation nanophosphor is quite unique for applications such as security ink and solar spectrum conversion for efficient energy harvesting. Intense orange red emission from Eu3+ (5 D0 →7 FJ transitions) under broad UV excitation range (250 nm–400 nm) and predominant red emission from Ho3+ (640–654 nm, 5 F5 –5 I8 ) through two to three photon UC process under IR (980 nm) excitation in GdPO4 :Ho3+ , Yb3+ , Eu3+ suggest a general approach to constructing a new class of luminescent materials with simultaneous up and down conversion emissions by controlled manipulation of energy transfer within a nanoscopic region.

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Doping Triple Lanthanum Ions in GdPO4 Nanocrystals Through Multiple Synthesis Routes

and IR solar radiation and emit in the visible region with high luminescence yield. Lanthanum orthophosphates are excellent host materials for doping multiple rare earth ion emitters because of their large band gap (> 6 eV) that can accommodate discrete 4f levels of RE ions. Towards this goal, we have developed nano particles of triple lanthanum ion doped GdPO4 : Ho3+ , Yb3+ , Eu3+ as rare earth doped GdPO4 phosphors have shown high luminescence yield8 9 and their excitation range in the UV extends up to 400 nm. Moreover, the colloidal solution of such dual excitation nanophosphor can be used as security ink in currencies that can be detected both by UV and IR lamp. For DC emission, rare earth ion Eu3+ is chosen as it is an excellent emitter in orange-deep red. For UC emission Ho3+ is chosen as it can emit in green/red region with Yb3+ ions as sensitizer to augment the UC process since absorption cross section of Yb3+ for IR radiation is much higher than Ho3+ . The unique energy level structure arising from the 4f inner shell configuration of the trivalent lanthanide ions (Ho3+ , Yb3+ , Eu3+ ) gives a variety of options for efficient up- and down conversion. The concept of dual excitation and dual emission (DE2 ) has been studied by a number of researchers.10–17 Tunable dual excitation (both UV and IR) and dual emission has been reported for YVO4 doped with Er3+ , Yb3+ , Eu3+ ,18 in NaGdF4 : Tm, Yb/NaGdF4 : Eu core/shell structure,19 in Gd2 O3 :Er,Yb/Eu/DBM)3 Phen organic complex20 in GdPO4 :Yb3+ , Tb3+21 BaGd2 (MoO4 4 :Eu3+ , Er3+ , Yb3+ .22 Though dual emission properties were reported by using rare earth organic complexes, fluorescent dye systems,11 12 23 24 but excitation was not by UV and IR radiation. For UC phosphors, Yb3+ is commonly used as sensitizer in hosts such as NaYF4 and NaGdF4 with emitters Er3+ , Tm3+ , Ho3+ .25–28 For our chosen material system lanthanide orthophosphates (LnPO4 , Ln = Y, La, Gd, Lu), Ln3+ have an empty or half-filled or fully filled 4f electron shell with a stable structure. LnPO4 also present high thermal and chemical stability. Thus, LnPO4 are considered as excellent host matrixes for luminescent materials and RE ions-doped LnPO4 phosphors and their nanoparticles have been investigated intensively.29–31 In the present work, we report the synthesis through various routes, of monoclinic phase nanoparticles of dual excitation GdPO4 :Ho3+ , Yb3+ , Eu3+ for the first time, capable of producing intense visible emission via both UC and DC processes. Successful simultaneous doping of triple rare earth ions in monophasic GdPO4 crystals of few nanometers dimensions synthesized through three different routes and their extended excitation range up to 400 nm in the ultraviolet and appreciable luminescence yield under both UV and IR excitation are the highlights of the present work.

2. EXPERIMENTAL DETAILS 2.1. Chemicals and Synthesis We prepared triple rare earth ion doped GdPO4 by three different methods, including solid state reaction (SSR), 2

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Co-precipitation (CPP) and Pechini-type Sol–Gel (SG) method. Analytical grade reagents were used as reactants without further purification. Gadolinium (III) oxide (Gd2 O3 , Specpure, 99.99%), holmium (III) oxide (Ho2 O3 , Specpure, 99.99%), ytterbium oxide (Yb2 O3 , SigmaAldrich, 99.99%) and ammonium dihydrogen phosphate (NH4 2 HPO4 , Specpure, 99.999%) were used as precursors. Nitric acid (Merck, p. a.) and ammonia solution (Merck, p. a.), ethylene glycol (EG, molecular weight = 82.7, A. R.), ethanol (A. R.) were used throughout the experiment in CPP and sol gel synthesis. Controlled reaction at high temperature in the solid state was adopted to synthesize a reference sample. The precursors used were Gd2 O3 , (NH4 2 HPO4 , Eu2 O3 , Ho2 O3 and Yb2 O3 . The stoichiometric amount of precursor materials were thoroughly mixed in agate mortar packed in an alumina boat and fired at 1200 C for 2 hours in air atmosphere. For co-precipitation synthesis the stoichiometric quantities of all the precursors (Gd079 PO4 :Ho001 Yb015 Eu005 ) were dissolved in minimal amount of diluted nitric acid (HNO3 ) separately to make a clear solution of nitrate salts of the precursors. NH4 OH and H2 O2 in the volume ratio 3:1 was added drop wise into the solution till a pH of the solution reached 8 when ultrafine particles of GdPO4 doped with RE ions started forming and were precipitated out in centrifuge. The white body colour of the precipitate confirmed the formation of GdPO4 NPs.32 The precipitate was collected by centrifuging and washed repeatedly with DI water and ethanol in ultrasonic bath to remove soluble as well as surface bound impurities and water molecules from the surface of nanoparticles. NP’s were washed with DI water and then with ethanol to dehydrate the surfacebound water molecules, as OH– is a known quencher of luminescence. Finally, the precipitates were dried in an oven at 40 C for 12 h. The third method that we adopted for synthesis of Gd079 PO4 :Ho001 Yb015 Eu005 nanoparticles is Pechini-type SG method, adopted from Xia et al.33 The stoichiometric amount of precursors i.e., Gd2 O3 , Eu2 O3 and (NH4 2 HPO4 Ho2 O3 and Yb2 O3 were dissolved in diluted HNO3 (A. R.) under vigorous stirring at 80 C for 15 minutes to make metal nitrate salts, then a water–ethanol (Vw /Ve = 1:7) solution was added. Citric acid (3 mol) was added as chelating agent of metal ions, these chelates undergo polyesterification when heated with poly hydroxyl alcohol such as ethylene glycol (6 mol) at a temperature of 150 C to form a polymeric precursor resin. The cations are expected to be dispersed uniformly throughout the polymeric resin. The transparent solution after stirring for 15–20 minutes turned to slightly brown sol which was further heated at 150 C for an hour, at this temperature the polycondensation of ethylene glycol and citric acid starts, resulting in polymer citrate gel formation. The final product was obtained as white powder which were then washed Sci. Adv. Mater., 6, 1–8, 2014

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2.2. Characterization X-ray powder diffraction patterns of the nanoparticles in powder form were recorded in the 2 range of 15–60 on a Rigaku MiniFlex Diffractometer using Cu K radiation. HRTEM images were recorded with a FEI TECNAI F 30 TWIN, TECNAI transmission electron microscope, using an accelerating voltage of 300 kV. The photoluminescence (PL) excitation, emission spectra and time resolved decay of luminescence were measured using combined steady state fluorescence and lifetime spectrometer of Edinburgh Instruments FLSP920 with Xe lamp as excitation source. Up conversion luminescence was measured employing a power tunable 980 nm diode laser (MDL-N-980-6W) coupled with optical fiber, as excitation source. Luminescence decay measurements were carried out using excitation of microsecond pulsed Xe lamp and employing time correlated single photon counting (TCSPC) technique. Confocal fluorescence microscopy was done using WITec confocal microscope system alpha 300 M+ using 375 nm and 980 nm diode lasers as excitation source for down and up conversion fluorescence measurements respectively.

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and dried at 150 C for 12 hr. The sol gel synthesis was employed as it can ensure uniformity of composition and lessen segregation of particular metal ions. When the sample was calcined at higher temperature (400 C), oxidation and pyrolysis of the polymer matrix begin, which lead to the formation of X-ray amorphous oxide and/or carbonate precursor. Further heating of this precursor results in the formation of the required material with a high degree of homogeneity and dispersion. Part of samples prepared by CPP, ACP and SG method were annealed at 1000 C to investigate the effect on crystallinity and luminescence efficiency. Pellet of powder sample was made for confocal measurement.

3. RESULTS AND DISCUSSION 3.1. Phase Formation Phase characterization of triple rare earth ion doped GdPO4 synthesized by all three methods through powder X-ray diffraction (XRD) (Fig. 1) shows interesting relation to crystalline phase formation with the synthesis method. Nanoparticles synthesized in alkaline environment by CPP method showed formation of hexagonal gadolinium phosphate hydrate (GdPO4 · H2 O) (JCPDS 39-0232, inset of Fig. 1) as per the following chemical reaction.6 Gd3+ + NO− 3 + x · H2 O + NH4 OH + NH4 2 HPO4 → GdPO4 · xH2 O + NH4 NO3 + H3 PO4

(1)

All the XRD peaks have been indexed as per JCPDS card no. 39-0232 and the position of smaller peaks at (113) and (203) are indicated by arrows. Most reports suggest that at synthesis temperature lower than 150 C, LnPO4 crystallizes in hexagonal phase and annealing/synthesis at around Sci. Adv. Mater., 6, 1–8, 2014

Fig. 1. (a) XRD patterns of annealed (1000 C) GdPO4 :Ho3+ , Yb3+ , Eu3+ samples by pechini type sol–gel, CPP and SSR and inset shows the XRD of as prepared samples by CPP and SG method; (b) EDAX pattern of annealed samples synthesized by Pechini type sol–gel method and CPP method indicating peaks arising from dopants.

900 C, leads to phase transformation/crystallization in monoclinic phase.21 In striking contrast, as synthesized sample prepared by pechini type SG method at 150 C for 12 hr was found to be monoclinic and monophasic (JCPDS No. 32-0386, inset of Fig. 1). As synthesized samples were further annealed at 1000 C and the hexagonal phase synthesized by CPP method is completely transformed to monoclinic phase due to removal of H2 O from GdPO4 (Fig. 1), also the unreacted Holmium oxide peak disappears indicating effective substutional doping during annealing process. As synthesized sample by pechini type SG crystallized in 3

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monoclinic phase and crystallinity of monoclinic phase further improved by annealing. Samples synthesized by solid state reaction at 1200 C exhibited well crystalline monoclinic phase and XRD pattern as shown in Figure 1 clearly indicates that all the diffraction peaks of the GdPO4 :Ho3+ , Yb3+ , Eu3+ samples can be directly indexed to the monoclinic phase of GdPO4 (JCPDS No. 32-0386). Lanthanum phosphates (LnPO4 ) usually crystallize either in the tetragonal zircon, hexagonal or monoclinic monazite structure21 34 for both bulk and nanomaterials depending upon the ionic radii of the lanthanide. LnPO4 type structure,21 at low temperature (900 C) La–Dy phosphates crystallize in monoclinic phase. Our results also show that Gd phosphate can crystallize in hexagonal or monoclinic structure as ionic radii of Gd3+ is covered in any of these structures. The average crystallite size has been estimated by using Scherer’s formula {D = k/ cos } and tabulated in Table I. EDAX pattern of annealed samples synthesized by Pechini type sol gel method and CPP method indicating peaks arising from dopants is shown in Figure 1(b). 3.2. Morphology The morphology and crystallinity of all the synthesized and annealed samples were investigated by TEM and HRTEM (Fig. 2). Figures 2(i) and (ii) shows HRTEM and TEM images of as prepared and annealed GdPO4 :Ho3+ , Yb3+ , Eu3+ respectively, synthesized via CPP. The well crystalline nano rod with diameter about 6 nm clearly exhibit lattice fringes corresponding to (210) lattice planes of hexagonal (JCPDS 39-0232) phase in as synthesized samples (CPP method) supported by FFT (Fig. 2(i) and inset). However, as can be seen in the HRTEM picture, lattice strain exists along the boundaries of the nano rod. Upon annealing, the nanoparticles grow in size but retain the cylindrical shape (Fig. 2(ii)) with very good crystallinity in monoclinic phase (JCPDS No. 32-0386) as revealed by selected area electron diffraction (SAED) pattern (inset, Fig. 2(ii)) which correspond well with XRD result. As synthesized nanoparticles by pechini type sol gel method, on the other hand show particles size of 5 nm and above with well formed lattice planes (Fig. 2(iii)) with interplanar spacing 0.304 nm which corresponds to the Table I. Average crystallite size calculated using debye-scherer equation. GdPO4 :Ho3+ , Yb3+ , Eu3 SSR CPP (as prepared) CPP (annealed at 1000 C SG (as prepared) SG (annealed at 1000 C

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2 (degrees) 29.64 29.52 29.64 29.52 29.64

hkl 1 1 1 1 1

2 1 2 1 2

0 0 0 0 0

Average size (nm) 15 12 59 9 74

Fig. 2. HRTEM and TEM images of as prepared and annealed GdPO4 :Ho3+ , Yb3+ , Eu3+ respectively, (i) and (ii) synthesized via CPP, (iii) and (iv) by S-G method, (v) and (vi) HRTEM and TEM images of GdPO4 :Ho3+ , Yb3+ , Eu3+ sample prepared via SSR method.

(120) plane of the GdPO4 monoclinic phase that is also supported by XRD result. Upon annealing at 1000 C, the particles tend to become rounded with bigger size and show beautiful diffraction pattern with sharp diffraction spots (Fig. 2(iv) and inset) that conform to XRD results, suggesting well crystalline grains. HRTEM and TEM images of samples synthesized by solid state reaction method showed well crystalline rounded particle of size ∼13 nm with majority of particle with (120) planes of monoclinic phase as shown on HRTEM and TEM images (Figs. 2(v) and (vi)). Formation of nanoparticles of tens of nanometer dimensions by high temperature solid state reaction is a very good achievement since very well crystalline nanoparticles with desirable optical properties could be achieved by this method that produces large yield. It is also observed that large amount of rare earth dopant ions in substitutional positions of Gd3+ do not interfere with the crystalline phase formation indicated by their XRD and HRTEM results which do not show any precipitated phase of the rare earth ions. Sci. Adv. Mater., 6, 1–8, 2014

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Fig. 3. (a) DC PL emission spectra (inset, PL excitation spectra), (b) UC PL emission spectra of GdPO4 :Ho3+ , Yb3+ , Eu3+ samples prepared by all three synthesis protocol. The background in (a) and (b) are actual photographs of the nanoparticles under UV and IR (980 nm) light.

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Figure 5 shows the time resolved luminescence decay of GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles synthesized by all three methods and depicts natural logarithm of 5 D0 →7 F1 (592 nm) emission intensity against time elapsed since excitation by microsecond pulse of 395 nm light. The luminescence decay involving f –f transitions between states of the same parity is in the millisecond range due to long excited state lifetime. The decay curves could be well fitted with the following exponential decay function, It = I0 + A1 exp−t/ 1 + A2 exp−t/ 2 , where I0 is the initial emission intensity at t = 0 and is the decay time (1/e decay time i.e., time to decay to 37% of original intensity). The time decay study revealed a shortening in the lifetime of GdPO4 :Ho3+ , Yb3+ , Eu3+ emission at 592 nm depending on synthesis method as calculated average decay times are 1.216, 1.169 and 0.699 millisecond for

Fig. 4. Confocal fluorescence image of SSR sample and corresponding confocal fluorescence spectra under UV (375 nm) excitation (a) and (b); under IR (980 nm) excitation (c) and (d) respectively.

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3.3. Down Conversion Luminescence The DC photoluminescence excitation spectra (PLE) of the prepared Ho, Yb, Eu activated GdPO4 phosphor at 592 nm emission is shown in the inset of Figure 3(a) and exhibit six excitation peaks at 260, 273, 311, 360, 381, and 395 nm. The PL excitation spectrum is broad at 255–270 nm which could be ascribed to the O2− → Eu3+ charge transfer band (CTB) and arises due to electron delocalization from an oxygen 2p orbital to an empty 4f orbital of europium ion.35–38 The sharp peak at 274 nm is due to the Gd3+8 S7/2 →6 IJ transition and clearly suggests Gd3+ to Eu3+ energy transfer.39 Peaks in the range of 300–320 nm are from the Gd3+8 S7/2 →6 PJ transition.29 30 32 40 41 The f –f transition of Eu3+ 42 43 manifest as sharp excitation lines (350–400 nm) and the highest excitation peak at 395 nm is due to transition between 7 F0 –5 L6 level of Eu3+ and have maximum probability in GdPO4 host. The PL emission spectrum (exc = 395 nm) consists of the signature Eu3+ emission peaks around 592 nm (orange), 618 nm (red), 699 nm (deep red) which can be ascribed to 5 D0 →7 F1 , 5 D0 →7 F2 and 5 D0 →7 F4 transitions respectively of Eu3+ ion (Fig. 3(a)). The actual photograph of the powder sample comprising GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles under UV light is shown as inset in Figure 3(a). The emission intensity of orange emission due to 5 D0 →7 F1 magnetic dipole transition is much more compared to 5 D0 →7 F2 electric dipole transition implying that the inversion symmetry of Gd site is high in well crystalline GdPO4 so that Eu3+ substituted in Gd site has forbidden electric dipole transitions.44 Such strong emission under an extended UV excitation range in rare earth red emitting GdPO4 is very important for terrestrial solar spectrum conversion from UV to visible as well as security ink for counterfeit currency applications. Confocal fluorescence mapping of selected area (SSR sample) under 375 nm diode laser excitation (Fig. 4(a)) and corresponding high resolution emission spectra (Fig. 4(b)) exhibits resolved emission from stark shifted sublevels corresponding to 5 D0 →7 FJ transitions of Eu3+ .


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converted to one photon of higher energy according to the expression (2) IUC ∝ Iin

Fig. 5. Luminescence decay curves under UV excitation of all three samples (GdPO4 :Ho3+ , Yb3+ , Eu3+ ) prepared via different route. Table II. Luminescence decay parameters of 592 nm emission under UV (395 nm) excitation.

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GdPO4 :Ho3+ , Yb3+ , Eu3+ SSR CPP (annealed at 1000 C S-G (annealed at 1000 C

1 (m sec) (Rel. %)

2 (m sec) (Rel. %)

av (m sec)

0.248 (14%) 0.239 (6%) 0.144 (6%)

1.371 (86%) 1.230 (94%) 0.732 (94%)

1.216 1.169 0.699

Where IUC = UC emission intensity and Ii = excitation IR light intensity and n is the number of photons needed to populate the emitting Ho3+ states. To gain insight into the mechanism of the observed up conversion process, the intensity of the 980 nm diode laser was varied and the upconversion luminescence emission spectra were measured. The integrated emission intensity of both green and red emission of GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles synthesized by all the three processes was measured and plotted against the laser pump power in a logarithmic scale (Figs. 6(a) and (b)) which shows slope between 2.12 to 2.74 suggesting two to three photon process being operative. Among the three possible UC processes namely ground state absorption/excited state absorption (GSA/ESA), ground state absorption/energy transfer upconversion (GSA/ETU) and photon avalanche process (PA), the PA mechanism can be ruled out as no power threshold was observed. GSA/ESA UC occurs when photons are absorbed sequentially within a single ion raising its energy to a higher energy state. In the UC phosphor,

SSR, CPP and SG samples respectively (Table II). Since the magnetic dipole transition 5 D0 →7 F1 (592 nm) in the monoclinic phase is considered for all the samples, the difference in decay time could be due to non radiative pathways in the photo physical process. The photoluminescence decay time of the phosphor ( ) is affected by the radiative transition rate (Ar ), non-radiative transition rate due to multi-phonon relaxation (Anr ), and energy transfer rate (Pt ), which can be expressed by 1/ = Ar + Anr + Pt . For samples synthesized by wet chemistry, the surface states providing non radiative recombination would be far too many compared to solid state synthesized samples that lead to lesser luminescence yield and a shorter decay time. 3.4. Up Conversion Luminescence The UC emission spectrum (exc = 980 nm) from GdPO4 :Ho3+ , Yb3+ , Eu3+ nanocrystals prepared by CPP, pechini type sol–gel method and SSR method is shown in Figure 3(b). Nanocrystals prepared by CPP, S-G method showed up conversion luminescence though higher luminescence yield was observed for sample prepared by SSR method. There is both green (539 nm, 5 S2 , F4 –5 I8 ) and red (640–654 nm, 5 F5 –5 I8 ) UC emission from Ho3+ in GdPO4 :Ho3+ , Yb3+ , Eu3+ . The photograph of the sample under IR laser (980 nm) excitation is shown as inset in Figure 3(b). The confocal fluorescence map and high resolution confocal UC emission spectra for SSR sample is shown in Figures 4(c) and (d) which clearly exhibits the transitions between stark split sublevels of Ho3+ . UC is a multi photon non-linear process where n photons of smaller energy are absorbed by the material and 6

Fig. 6. Dependence of green and red UC emission intensity on pump power of the NIR laser (980 nm) of GdPO4 :Ho3+ , Yb3+ , Eu3+ samples prepared by three different synthesis processes. Sci. Adv. Mater., 6, 1–8, 2014

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Fig. 7. Energy level diagram showing excitation and energy transfer processes leading to both DC and UC luminescence emission from Eu3+ and Ho3+ ions in GdPO4 . Sci. Adv. Mater., 6, 1–8, 2014

depicted in the energy level diagram (Fig. 7). Therefore more than two photon absorption and subsequent energy transfer leads to green and red visible upconversion emission. 3.5. Application Potential of Dual Excitation, Dual Emission Nanophosphor A schematic arrangement of the lanthanide ions doped GdPO4 nanoparticle and its dual excitation and dual emission process is shown in Figure 8(a). The developed GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles were made into a colloidal solution and used as ink to write the letters NPL which when put under UV light glows in orange red. Under 980 nm excitation, the letters glow in a mixture of red and green but could not be photographed in full due to small laser beam size. Thin film of such phosphor nanoparticles placed above a Si solar cell can simultaneously use the UV and Infrared region of sun energy and emit in the visible region to generate extra e–h pairs (Fig. 8(c), inset). The up and down converted emission spectrum is shown in context to terrestrial solar spectrum (Fig. 8(c)) which immediately brings into focus the importance of such dual excitation phosphor materials for better energy harvesting.

Fig. 8. Schematic representation of stokes and anti stokes shifted emission inside a GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticle, (b) the developed GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles used as ink to write the letters NPL by hand, that glows in orange red under UV light, (c) Terrestrial solar spectrum (AM1.5), maximum solar energy utilized by Silicon solar cell (blue curve) and PL excitation spectra (dark blue line curve in UV region), DC emission spectra (blue line curve), UC emission spectra(black line curve) of GdPO4 :Ho3+ , Yb3+ , Eu3+ nanoparticles. PL excitation range in UV and IR covers the region unutilized by Silicon solar cell and PL emission spectra covers the maximum utilization region of Silicon and the arrow indicates how the spectrum conversion from UV and IR by nanophosphor can harness the unutilized region of solar spectrum towards enhanced visible photon input to solar cell. The inset in (c) shows schematically that a thin film of dual excitation nanophosphor on top of Si solar cell can improve solar cell performance through enhanced carrier generation.

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Yb3+ is co doped since Yb3+ ions have much higher excitation cross section than the emitter rare earth ion Ho3+ and since Yb3+ concentration is much higher, much higher population of excited Yb3+ ions will be on the 2 F5/2 state. Such UC phosphors utilize the ETU mechanism with Yb3+ acting as the sensitizer (around 980–1000 nm) and Ho3+ as the activator resulting in visible luminescence. Upon irradiation by IR laser, two Yb3+ ions resonantly absorb two 980 nm photons corresponding to 2 F7/2 to 2 F5/2 transition of Yb3+ followed by transfer of energy from one Yb3+ ion to the second Yb3+ ion (Fig. 7). The excited second Yb3+ ion non-radiatively transfers the energy to neighboring Ho3+ ions exciting it from 5 I8 ground state to intermediate excited state 5 I6 . This process may be followed by interaction of another excited Yb3+ ion which elevates the excited Ho3+ ion from 5 I6 state into the emitting 5 S2 level as 980 nm photon energy matches with the level spacing of 5 I6 −5 S2 transition.45 Ho3+ ions in the 5 S2 ,5 F4 levels can relax to ground state 5 I8 by emission of green photons. The red UC emission from Ho3+ occurs due to transition from excited 5 F5 level to ground 5 I8 level. The red emitting 5 F5 level can be populated either due to direct excited state absorption of one 980 nm photon from 5 I7 which is used as a bridge level that is populated by multiphonon relaxation from 5 I6 or through phonon relaxation from 5 S2 , 5 F4 levels. The non radiative relaxation process is dependent on the host whose maximum phonon energies could determine the multiphonon relaxation process. It is generally accepted that if the energy gap between two levels is more than 5 times the highest phonon energy, non radiative relaxation is less probable. Oxide hosts have phonon energy on the order of 600 cm−1 . The 5 S2 level is separated from the 5 F5 level by 2660 cm−1 and 5 F5 level can be populated by multiphon relaxation from the 5 S2 level. Also, population of 5 F5 level can occur through excited state absorption from 5 I7 level or energy transfer process46 47 as


Such emission characteristics well fit the maximum spectral response region of Si solar cells.

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4. CONCLUSIONS In summary, high-quality dual excitation, dual emission GdPO4 :Ho3+ , Yb3+ , Eu3+ nanocrystals have been synthesized using three different routes i.e., SSR, co precipitation and pechini type sol–gel approach, and their photoluminescence properties at room temperature have been studied. The nanocrystals can be produced via simple procedures, and the rare earth dopant ions Eu3+ and Ho3+ emit strong characteristic luminescence under UV and IR excitation respectively. The results clearly indicate that nanoparticles of GdPO4 :Ho3+ , Yb3+ , Eu3+ exhibit simultaneous excitation by UV and IR and show orange-red emission at 592 and 618 nm from Eu3+ ions under UV excitation (250 nm– 400 nm) and green (539 nm) and red (654 nm) emission from Ho3+ ions under IR excitation. Such dual excitation nanophosphor has great potential as security ink for counterfeit applications that can be detected both by UV and IR light. Two dimensional conformal transparent layer of such a single nanophosphor on a silicon solar cell can simultaneously convert solar UV/IR to visible region where spectral response of solar cell is maximum. 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. Acknowledgments: Authors gratefully acknowledge the research grant under TAPSUN program of CSIR to carry out the work.

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Sci. Adv. Mater., 6, 1–8, 2014