Oxide glass used as inorganic template for fluorescent fluoride nanoparticles synthesis

Optical Materials 28 (2006) 1401–1404 www.elsevier.com/locate/optmat

Oxide glass used as inorganic template for fluorescent fluoride nanoparticles synthesis Michel Mortier a

a,*

, Gilles Patriarche

b

Laboratoire de Chimie Applique´e de l’Etat Solide, CNRS-UMR 7574, Ecole Nationale Supe´rieure de Chimie de Paris, CNRS-UPR20 Marcoussis, France b Laboratoire de Photonique et Nanostructures, CNRS-UPR20 Marcoussis, France Received 22 April 2005; accepted 2 July 2005 Available online 21 September 2005

Abstract We report an original way to synthesise single-crystal PbF2 nanoparticles by selective chemical attack of a bulk nanocomposite oxyfluoride glass–ceramic. Free of impurities and homogeneously doped with Er3+ ions, the particles are of narrow size dispersion around 15 nm and weakly aggregated. The nanocrystallites emit a very intense green and blue up conversion fluorescence after infrared excitation. The doping level and the size of the particles is finely driven through the precursor glass–ceramic synthesis and composition. Ó 2005 Elsevier B.V. All rights reserved. PACS: 81.05. t; 42.70. 4; 81.05.Ys Keywords: Nanoparticle; Synthesis; Luminescent; Fluoride

1. Introduction Fluorescent inorganic particles of different chemical compositions, pure metal oxides, semiconductor quantum-dots, are currently prepared in engineered organic nanoscale reactors [1] such as microemulsion or micellar systems where the soft colloidal template allows the size and shape control of the inorganic nanocrystals [2]. However, rare-earth ions doped nanocrystallites obtained by such methods have poorly reproducible broad size distribution, often irregular shape and poor crystal quality [3–6], variable doping between particles [4] and frequent organic impurities [4–6]. Here, we report the synthesis of single-crystal PbF2 particles free of impurities, doped with Er3+ ions by selective chemical attack of a bulk nanocomposite glass–ceramic. Indeed, we are currently synthesising transparent oxyfluo-

*

Corresponding author. E-mail address: [email protected] (M. Mortier).

0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.07.008

ride glass–ceramic samples either doped or codoped with rare-earth ions for optical applications [7–11]. Glass–ceramic bulk sample is obtained by the partial and controlled devitrification of an oxyfluoride glass (GeO2–PbO–PbF2) doped with LnF3, where Ln = Er, Yb or Ce for instance. Nanoparticle growth is ensured inside the glass by thermally induced inhomogeneous nucleation of a unique phase, cubic Pb1 xLnxF2+x solid solution [12] isotype to cubic b-PbF2. For the sake of simplicity, this phase is called erbium doped PbF2 in this paper. For the PbF2 content of 10% used in this study, the homogeneous nucleation of the non-doped crystallites can not be reached and would necessitate a larger PbF2 content but it was not our goal. Indeed, erbium induced inhomogeneous nucleation is much more interesting to better control the nucleation process and ensure the crystallisation of the PbF2 phase around the rareearth ions. The resulting glass–ceramic consists in an oxide glassy matrix (GeO2PbO) with monodisperse spherical PbF2 nanocrystallites embedded, typical of a nanocomposite material. The size of the crystallites, between 8 and 50 nm, is adjusted by the PbF2 and LnF3 relative content

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in the initial melt [7]. A larger PbF2 content with regard to PbO induces bigger particles. For a constant PbF2 content, the increase of the LnF3 doping produces more numerous seeds and then more numerous but smaller crystallites. The time and temperature of the thermal treatments are not used as adjustable parameters to design the nanophase because we research a complete segregation of the rareearth ions into the crystallites which is only effective after ending the crystallisation of PbF2. So, time and temperature were chosen according to the thermal properties of the as melted glass to avoid the crystallisation of the oxide matrix [7–11]. The crystalline state of the bulk sample can be checked by powder X-ray diffraction (XRD) indicating the mean size of the crystallites and the lattice parameter of the Pb1 xLnxF2+x phase [12,13] also reflecting the local Ln3+ concentration x. 2. Experimental 2.1. Sample preparation A glass sample, of molar composition (48.55GeO2: 38.83PbO:9.71PbF2:2.91ErF3) was obtained from high purity (>3 N) commercial raw materials following our standard method [7–11]. The powders were melted in a covered platinum crucible at 1050 °C for 20 min in the air. The resulting melt was poured on a copper plate and quenched with another plate quickly deposited on the sample. The glassy sample of 2 mm thickness was then annealed 100 °C below the glass transition temperature (340 °C) to release the constrains induced by the quenching process. Finally, the glass–ceramic sample was obtained after 10 h reheating at 360 °C in air of the glass piece. Then, we use the unique ability of hydrofluoric acid to dissolve almost all inorganic oxides to attack the oxide network of the glassy matrix (GeO2–PbO) embedding the PbF2 nanophase. The selective chemical attack is performed in teflon container with hydrofluoric acid (73%) on bulk sample up to complete dissolution (8 days for 3 g sample). At the end of the chemical dissolution, the hydrofluoric acid is diluted 10 times with water before washing with ethanol and drying around 90 °C in a furnace for 5 h.

tation at 975 nm. The signal was detected with a CCD camera placed at the exit slit of a 0.25 m monochromator with a resolution better than 1.5 nm after unfocused excitation with 1 mJ and 8 ns pulse from an optical parametric oscillator at 975 nm. 3. Results and discussion We obtained a large number of crystallites weakly aggregated as observed with transmission electron microscopy TEM (Fig. 1). The crystallites have been observed in high resolution HR-TEM, as shown in Fig. 2. The {1 1 1} and {2 0 0} atomic planes are observed in many particles showing a perfect regularity. The interatomic distance was measured on the selected area electron diffraction ˚ for the (SAED) indicating a mean value of 5.67 ± 0.05 A lattice cell parameter of the cubic erbium doped PbF2 and reflecting a local doping level higher than 30% when ˚ ). In compared with pure PbF2 lattice parameter (5.94 A Figs. 1 and 2, it is possible to observe a 2 nm thick amorphous film around the PbF2 particles of a mean diameter equal to 15 nm resulting in a core/shell structure. Energy dispersive X-ray microanalysis (EDX) has been made globally on few particles with their amorphous shell (Table 1). Taking into account the PbF2–ErF3 content of the core particles, the EDX analysis tend to indicate that this thin shell correspond to an oxyfluoride glassy film rich in lead and poor in germanium, with F gradient down to the fully oxide external surface (Fig. 3) resulting from the crystal growth inside the glass and explaining the resistance of this film to hydrofluoric attack. This natural film prevent the fluoride particles from any further surface pollution or evolution. The incorporation of the whole Er content inside the PbF2 core particle corresponds to a high local content which is in agreement with the starting composition of

2.2. Measurements Transmission electron microscopy was realised on powder after weak ultrasonic dispersion in ethanol without any grinding and also without centrifugation to reflect the true size distribution and aggregation level. The powder dispersed in ethanol was placed onto a holey carbon film and naturally dried before introduction in a Philipps 200 kV CM20 microscope equipped with an energy dispersive X-ray (EDX) analyser which enabled the microanalysis of the samples with a detection limit of 0.01 at.%. The fluorescence spectrum was recorded on a quasi monolayer of nanoparticles, deposited on a double side adhesive film stuck on a microscope slide, after pulsed exci-

Fig. 1. TEM image of the erbium doped PbF2 particles after dispersion in ethanol. The light grey background is the carbon film.

M. Mortier, G. Patriarche / Optical Materials 28 (2006) 1401–1404

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Intensity (arbitrary units)

4

S 3 /2

4

4

G1 1 /2

2

360

2

H 9 /2

400

4

F9 /2

H 1 1 /2

F5 /2

440

480

520

560

600

640

680

Wavelength (nm) Fig. 2. High resolution TEM image of the particles. The {1 1 1} and {2 0 0} atomic planes are observed. The light grey back background is the carbon film when the grey zone surrounding the crystallised core is an amorphous shell.

Fig. 4. Fluorescence spectrum of a quasi monolayer of particles arising from the electronic levels of erbium indicated on the figure down to the 4 I15/2 ground level after excitation at 975 nm at room temperature.

Table 1 Average result of EDX analysis of the nanoparticles in at.% Pb

Er

Ge

F

O

30.3 ± 2

4.8 ± 0.6

2.4 ± 2

40.6 ± 4

21.9 ± 2

2

G11/2 H9/2 4

4 2

F7/2 4 4

4

O

S 3/2

4

4

I13/2

4

I15/2

S3/2

I11/2

4

a -

F7/2

4

I 9/2

I11/2

0.97 µm

2-

4

H 11/2

F3/2

F5/2

F9/2

4

F

4 2

4

c

G 9/2

I15/2

b 3+

Fig. 3. Scheme of the core (PbF2 + ErF3) in white and shell structure (PbO + GeO2) in grey of the particles and the associated composition profiles (F in bold line, O2 in dotted line) deduced from the XRD, SAED and EDX results.

Fig. 5. Energy level diagram of Er ions and transition schemes for (a) two- and (b) three-photons ETU processes. The bold arrows correspond to the exciting photon around 0.97 lm. The dotted arrows correspond to non-radiative processes. The vertical arrows down to the ground level correspond to the emission lines observed on Fig. 4 and arising from the levels labelled in bold character. The levels labelled on the left side of the level diagram are involved in the ETU process.

the glass and the strongly reduced parameter cell of PbF2 measured with SAED and also observed with XRD of the glass–ceramic samples [13] before dissolution of the matrix. The reduction of the lattice parameter of the Pb1 xErxF2+x solid solution can be predicted by using a rough model accounting for a charge compensation associated to the substitution of Pb2+ ions by Er3+ ions [12,13]. The nanoparticles have shown (Fig. 4) a very efficient two-photons (Fig. 5(a)) red and green emission and weaker three-photons (Fig. 5(b)) UV and blue up conversion fluorescence [14] after infra red excitation (Fig. 5(a) and (b))

when similar fluorescence measurements on nanoparticles generally necessitate a thick pressed disk of particles [6,15]. A higher up conversion rate is observed with regard to erbium doped oxide nanoparticles through the observation of the three-photons blue fluorescence which is not observed in oxide particles [6]. This three-photons up conversion process is weakly probable and very sensitive to impurity traces and to high phonon energies. Indeed, many intermediate levels must offer a quite long lifetime in order to allow the multiphoton processes. The

r

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4

I11/2 level is the intermediate excited state involved in the energy transfer up-conversion (ETU) processes for twophotons schemes (Fig. 5(a)) associated to the red emission from 4F9/2 level and green emission arising from 4S3/2 and 2 H11/2 levels [14]. For the three-photons blue emission arising from the 4F5/2 level and the UV emission arising from the 2H9/2 and 4G11/2 levels, two intermediate levels are involved, i.e. the 4I11/2 and 4S3/2 levels (Fig. 5(b)). In fact, the second energy transfer corresponds to the absorption of the 4F7/2 level whose lifetime is too short to allow any ETU and the ion relax down to the 4S3/2 level to permit the three-photon process. A small energy gap, DE, between an excited energy level and the immediately below level corresponds to a higher probability of non-radiative transitions as described with the energy gap law kNR = b exp[ aDE] [16]. The gap of the levels involved in the ETU processes are equal to: DE(4I11/2 4I13/2) = 3800 cm 1, DE(4S3/2 4F9/2) = 3200 cm 1 and DE(4F7/2 2 H11/2) = 1300 cm 1. The very small gap below the 4F7/2 level explains the purely non-radiative character of this level towards the 2H11/2 and 4S3/2 levels. The three-photons ETU suffers from both the strong sensitivity of the 4I11/2 and 4S3/2 levels to non-radiative processes and explains the lack of three photons ETU in oxide compounds [6]. The whole ETU emission spectrum ranging up to UV wavelength has been clearly observed under unfocused low power excitation and with an experimental set-up which was not specially designed for thin film study proving the high quality of the nanoparticles. The low phonon energy of PbF2 (336 cm 1 [9]) and the high quantum yield of fluoride compounds with regard to oxide ones [14] is a great advantage with nanoparticle embedding only few thousands of emitting centers. The high crystalline quality of the doped nanocrystal phase enhances also strongly the fluorescence intensity thanks to the corresponding narrow inhomogeneous linewidth of erbium ions observed in the fluorescence spectra (Fig. 4). When it is easily observed on a quasi monolayer of our particles, such a strong upconversion spectrum over the whole visible range needs ytterbium codoping to be observed in the recent results with porous silicon [17]. Also, the close proximity of the erbium ions inside the nanocrystals due to their high concentration favours the ETU processes. 4. Conclusion

The particles emit a very intense visible non-linear up conversion fluorescence [14] over the whole visible range after infra red excitation and should be seen as a white light source [17]. Without any bleaching compared to organic dyes, with a toxicity lower than most of the semiconductor quantum-dots such as CdTe, CdSe and with a permanent fluorescence compared to their blinking emission [18], such particles should be used as an alternative for many applications. Embedded in a natural thin surface oxidised film, these particles should be easily surface functionalised for many purposes such as biological labelling. They should also be used as a single fluorescent nanosource and/or nanodetector for near field optical microscopy and spectroscopy experiments [19] and nanothermometry [15]. Thanks to the segregation process, that we master for various ions at the same time into the nanophase, it is possible to make doped or codoped compounds, up to three different ions up today, with a perfectly controlled doping [11]. This easy rare-earth ion codoping allows many energy transfer schemes such as upconversion or downconversion by photon cutting and also the ability to modify the fluorescence lifetime [11,14], demonstrating a strong versatility. Acknowledgements The authors acknowledge Professor Marc Leblanc for helpful discussion and Lionel Aigouy and Yannick De Wilde for stimulating collaboration through the program ‘‘Nano-Objet Individuel’’ supported by the CNRS. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

We have obtained weakly aggregated Pb1 xErxF2+x nanocrystallites, of narrow size dispersion around 15 nm, regular spherical shape and perfect crystal lattice as observed by high resolution transmission electron microscopy. The erbium doping level and particle size are adjusted through the composition of the oxyfluoride parent glass (GeO2–PbO–PbF2–ErF3) and the erbium induced inhomogeneous nucleation of the PbF2 cubic phase nanocrystallites inside the glass [7–11] which is used as an inorganic template before selective dissolution of the oxide matrix (GeO2–PbO) with hydrofluoric acid.

[13] [14] [15] [16] [17] [18] [19]

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