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Upconversion enhancement in Yb3+,Tm3+:BaY2F8 quasi-nanoparticles
Journal of Luminescence 132 (2012) 2268–2274
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Upconversion enhancement in Yb3 þ ,Tm3 þ :BaY2F8 quasi-nanoparticles Alessandra Toncellia,b,n, Banafshe Ahmadib, Fabio Marchettic a b c
NEST Istituto Nanoscienze-CNR, Italy Dipartimento di Fisica, Universita di Pisa Largo B. Pontecorvo 3, 56127 Pisa, Italy Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2011 Received in revised form 29 March 2012 Accepted 4 April 2012 Available online 13 April 2012
Yb3 þ Tm3 þ :BaY2F8 single crystals have been milled to quasi-nanometric size. A complete characterization of the quasi-nanoparticles has been compared with that of a bulk crystal with the same composition. The emission spectra did not show any difference as for the shape and relative intensity of the various peaks within each band, but the infrared lifetime of the quasi-nanoparticles is significantly longer than that of the bulk crystal. In agreement with other literature results we observed a strong increase of the upconverted luminescence intensity in the quasi-nanoparticles. An explanation is given as the effect of radiation trapping of the pumping radiation that increases the effective pump intensity in the volume of the quasi-nanoparticles with respect to the bulk crystal. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystals Rare earth Spectroscopy Upconversion enhancement
1. Introduction In recent years a still increasing scientific interest has been devoted to fluorescent nanoparticles (NPs) for their potential applications in a number of fields, such as photovoltaic conversion, scintillation, high resolution imaging and biophotonics. Various types of NPs have been investigated, such as Quantum Dots, magnetic NPs and other fluorescent NPs mainly for biological applications, but the quest for the ‘perfect’ bioprobe is still open. In this respect rare earth-doped fluoride nanocrystals (NCs) have recently attracted much interest because of their unique spectral features [1,2]. Their main peculiarities consist in the long lifetimes of the emitting levels and in the possibility to convert the pump radiation either to longer-wavelength or shorterwavelength emission through the so-called up-conversion and cross-relaxation nonlinear processes [3]. Moreover fluoride NCs are usually considered non-toxic and the large choice of dopants and doping levels permits to tailor the emission features for the desired application from UV to the mid-infrared region. In particular some rare earths like Yb, Tm, Er, and Ho have energy levels that can absorb and/or emit radiation in the 1-mm region and this can permit a deep penetration in the tissues. Moreover when pumped in the infrared with low-cost cw diode lasers some dopants can emit at a wavelength shorter than the pump (even in
n Corresponding author at: Dipartimento di Fisica, Universita di Pisa Largo B. Pontecorvo 3, 56127 Pisa, Italy. Tel.: þ39 050 2214556; fax: þ39 050 2214333. E-mail address: [email protected] (A. Toncelli).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.04.001
the visible region) thus completely eliminating the problem of cellular autofluorescence. Many different fluoride crystals have been investigated in nano-sized shape by many researchers including our group, such as YF3, LaF3, BaYF5, NaLaF4, NaGaF4, and NaYF4 [4–12] with interesting up-conversion emissions, but the choice of the host material is limited by the possibility of obtaining it by chemical synthesis. Many more fluoride crystals are well known as bulk materials, and each of them has thermo-optical properties that affect the emission features of rare earth dopants. For this reason it would be extremely desirable to extend this characterization to the whole class of fluoride crystals. Moreover chemical synthesis of these materials involves the handling of toxic or dangerous solvents or surfactants that make the scalability of the process difficult and can affect the emission features of the NPs. For these reasons we propose an alternative method to obtain high quality NCs by mechanical milling of macroscopic crystals of a variety of compositions with a simple and easily scalable process. Moreover this approach permits us to compare the spectral features of the NCs with those of a bulk crystal of the same composition, and this gives a deep insight in the physical processes that lead to the extremely interesting fluorescence of these materials. In this work we present a complete characterization of the spectroscopic features of 5%Yb, 1%Tm:BaY2F8 quasi-NCs (qNCs) in comparison with those of a bulk crystal with the same composition. BaY2F8 (BaYF) is a well known laser host material, in fact efficient and sometimes widely tunable laser emission has been obtained from this crystal when doped with a variety of rare earth ions (for example Nd, Tm, Ho, Yb, etc.) [13–17], but has never
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been obtained in nanometric size by chemical synthesis. Therefore we present a complete spectroscopic characterization of this material in the wavelength region from visible to near infrared. Moreover the comparison with the bulk crystal evidences an increase of the relative population of the up-converted levels with respect to the bulk material. An explanation is proposed. In fact NCs sometimes show an anomalous upconversion behaviour: they usually show a decrease of the upconversion efficiency with decreasing size (measured as surface to volume ratio) [18–20] and mainly ascribed to surface quenching centres that are also responsible for a shortening of lifetimes, but in few other cases an increase of upconversion efficiency is observed, instead [8,21,22]. No theoretical interpretation of this interesting effect has been given, on the contrary it is extremely important to understand the behaviour of the NPs in comparison with bulk crystals in order to assess the potentialities of NPs for various applications.
2. Experimental We grew a bulk 5%Yb, 1%Tm:BaY2F8 crystal with Czochralski technique and prepared qNCs of the same composition through mechanical milling. 2.1. Crystal growth BaY2F8 has monoclinic structure, with unit cell parameters ˚ b¼10.505 A, ˚ c ¼4.260 A, ˚ and angle g between the a ¼6.972 A, a- and c-axes of 99.761. The symmetry group is C2/m. The rareearth dopants enter substitutionally the Y sites. The crystal growth facility consists of a home-made computercontrolled Czochralski furnace with conventional resistive heating and optical automatic diameter control system. The furnace is equipped with an evacuation system that can reach an ultimate pressure limit of about 10 7 mbar. The growth process takes place in high purity Argon atmosphere. The starting materials were proper amounts of 5N fluoride powders supplied by AC materials (Tarpon Springs, FL, USA). The dopants were added to the pure BaY2F8 melt in trivalent form (YbF3 and TmF3), and a proper amount of BaF2 was added for compensation. The growth temperature was in the range 992–983 1C, the pull rate was 0.5 mm/h, and the rotation rate was 5 rpm. The seed was a piece of undoped BaYF crystal oriented along the a crystallographic axis. The bulk sample was obtained with a standard cutting and lapping procedure for optical measurements.
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molecules (locally injected through a micro-nozzle) is employed to partially neutralize the electron charging on the insulating sample. In order to verify the nature and the purity of phases present in the qNCs used for this study, its XRD pattern has been collected on a no-background sample holder. The powder diffraction pattern has been collected in the 2y range 10–601 at steps of 0.021 using a Philips PW1050/25 Bragg-Brentano diffractometer. The measurements have been performed at room temperature using a Cu source powered by 40 kV and 20 mA and a graphite ˚ The monochromator on the secondary beam (Ka, l ¼ 1:54184 A). diffracted beam has been measured using a scintillation counter integrating for 10 s/step. Room temperature fluorescence spectra were obtained by exciting the sample with a diode laser at 972 nm with maximum input power P¼270 mW. The pump beam was focused onto the sample with a 10 cm focal length lens. qNCs were deposited on a glass substrate and inclined at about 451 with respect both to the pumping beam and to the detected fluorescence. The bulk sample was mounted either with the input face perpendicular to the pump beam and the fluorescence detected at normal angle or at about 451 with respect both to the pumping beam and to the detected fluorescence, that is in the same experimental conditions used for the qNCs. In all cases the fluorescence signal was detected perpendicularly to the pump laser direction. Particular care was taken to assure that the pumping conditions were identical both for the bulk sample and for the qNCs. The luminescence was chopped and focused by a 7.5 mm focal length lens on the input slit of a Acton monochromator with 25 cm focal length equipped with a suitable grating in the various spectral regions. The signal was detected by a liquid nitrogen cooled InSb detector or a photomultiplier equipped with a S20 cathode, fed into pre-amplifiers, processed by a lock-in amplifier and stored on a PC. The acquired spectra were normalized for the optical response of the system using a black body source at 3000 K. For infrared lifetime measurements the set-up was similar but the pump beam was mechanically chopped and the signal from the detector was amplified and sent to a digital oscilloscope connected to a computer. The response time of the system was 100 ms. For the visible lifetime measurements the pump laser was a doubled Ti:Sapphire laser tuned to the maximum absorption of the sample at around 460 nm. The laser had a 30 ns pulse duration and 10 Hz repetition frequency and the response time of the system was 1 ms.
3. Results and discussion 2.2. qNCs preparation We prepared a polycrystalline mass with mm-sized grains using exactly the same composition and thermal treatment of the Czochralski grown crystal. The starting materials were loaded into a crucible and heated in a vacuum-tight furnace in high purity Ar atmosphere up to complete melting. The melt was then left at high temperature for several days (a typical time-scale for Czochralski crystal-growth) and then slowly cooled down. We obtained a polycrystalline mass made of mm-sized grains. The grains were then pestled and subsequently dry milled for some hours. Separation of the nanometric component was performed through standard centrifugation technique.
The morphology of the qNCs was inspected with SEM. A good uniformity of shape and dimension were observed in the powder sample, the average dimension of the qNCs is around 200– 300 nm, as shown in Fig. 1, even if a few smaller and larger grains are present. X-ray analysis confirmed the single crystal character of the obtained qNCs. The powder diffraction pattern is shown by a black line in Fig. 2. The pattern drawn by a red line has been calculated from the known structure of BaYF [23]. The good agreement in line positions suggests that the doped sample maintains the crystal structure of pure BaYF [24], different phases being present only at trace level. The differences in line intensities may probably be ascribed to preferential orientation effects.
2.3. Characterization 3.1. Emission spectra Scanning Electron Microscope (SEM) imaging is obtained with an Ultra Plus system (ZEISS), employing charge-compensation imaging technique: electron-induced ionization of nitrogen
Rare earths in crystals posses a series of discrete energy levels that can absorb and emit radiation. Due to their long lifetime
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these levels act as energy sinks that can either transfer this energy to other levels or emit it at a later time. Therefore the excitation energy can be transferred to higher lying or lower lying levels through a bilinear process before being emitted. In Fig. 3 we show the first excited levels of the 4f configuration of Tm3 þ and Yb3 þ ions together with the most probable processes involved.
Fig. 1. SEM image of the Yb3 þ Tm3 þ :BaY2F8 qNCs.
Fig. 2. Measured (black line) and calculated (red line) powder X-ray diffraction patterns for BaY2F8. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
After pumping both the bulk crystal and the qNCs in the 2F5/2 Yb3 þ level at around 970 nm we observed emission from five different emitting levels: 2F5/2 of Yb3 þ and 3F4, 3H4, 3F3 and 1G4 of Tm3 þ , as reported in Fig. 4a–f. Up-converted emissions are particularly interesting for biological applications. In particular the emission at around 800 nm lies in the transmission window of tissues, and this can be exploited for deep-tissue imaging because the pump is also in the same window and no cellular autofluorescence is expected with this pumping scheme. Moreover visible emissions are very interesting, too, because high sensitive detectors are of standard use in this wavelength region. The assignment of the various bands was performed according to Ref. [25]. No emission from levels higher than 1G4 was observed at our pumping level. In all cases the shape and relative intensity of the various peaks within each band is similar in the qNCs and in the bulk crystal and small differences can be explained by the different geometries of the samples and from the fact that the crystal matrix is strongly anisotropic (the qNCs correspond to an average of all the possible orientations, and the bulk crystal, although non-polarized, is an average of only two possible orientations), but big differences in the overall intensity of the various bands with respect to the others exist. In particular the up-converted emissions from 1G4 (Fig. 4a and b) in the qNCs are more intense than in the bulk crystal, while the emissions from 3F3, 3H4, and 3F4 are less intense. Although an absolute comparison of the emission intensities in the two samples is meaningless, the comparison of the emission intensities from different levels is connected to the different population of the multiplets and can give information about the energy transfer processes inside the material. At a first sight it appears that in the qNCs the ratio of the population of 1G4 with respect to that of the lower levels is much higher than in the bulk crystal in the same pumping conditions. This is particularly clear from Fig. 4b where two bands arising from 1G4 and 3F3 are present simultaneously and are acquired with the same set-up. In the bulk crystal (black line) the signal from 3F3 is higher than that from 1G4, and in the qNC (red line) it is the opposite. We calculated the ratio of the integral signal of the various bands in the crystal and in the qNCs. Integrals are all expressed with respect to the integral of the absorbing band (2F5/2) because its population should be directly connected to the pump power. For the qNCs we subtracted the contribution of the strong scattering line at the laser wavelength around 970 nm which is evident in Fig. 4d. The results are summarized in Fig. 5. Taken the population of 2F5/2 as a reference, we observe that the population of 3F4 in the crystal is a factor of
Fig. 3. Energy level scheme of Tm3 þ and Yb3 þ and main energy transfer processes.
A. Toncelli et al. / Journal of Luminescence 132 (2012) 2268–2274
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Fig. 4. (a) 1 G4 -3 H6 Tm3 þ emission. (b) 1 G4 -3 F4 and 3 F3 -3 H6 Tm3 þ emissions. (c) 3 H4 -3 H6 Tm3 þ emission. (d) 2 F4=2 -2 F7=2 Yb3 þ emission. (e) 3 H4 -3 F4 Tm3 þ emission. (f) 3 F4 -3 H6 Tm3 þ emission. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
two higher than in the qNCs, while the population of the levels higher than the pump is lower. In particular the populations of 3 H4 and 3F3 are a factor of three higher in the qNC than in the crystal, and that of1G4 is a factor of 15 higher. This is a strong evidence of the fact that the upconversion process is much more efficient in the qNCs than in the crystal in the same pumping conditions. This observation is in contrast with what usually
observed for various-size nanoparticles which show a decrease of upconversion efficiency with increasing the surface to volume ratio (therefore for diminishing particle dimension) and a threshold dimension around 70 nm above which the spectral properties become similar to those of the bulk material [18]. This implies that the explanation in our case must be different from that given in other cases (mainly surface quenching effects) but whether it is
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index of the material and by that of the surrounding medium (air in our case) according to this formula: neff ðxÞ ¼ xngrain þð1xÞnmed
ð1Þ
where x is the ‘filling factor’, that is the fraction of the sample volume occupied by grains (ngrain is the refractive index of the material, nmed is the refractive index of the medium, note that 0 ox o1). The effective refractive index affects the lifetime of a multiplet when it replaces the usual refractive index in the expression of electric dipole transition probability: 2
tR p
1 l0 f ðEDÞ ½1=3ðn2 þ2Þ2 n
ð2Þ
where f ðEDÞ is the oscillator strength for the electronic dipole transition and l0 is the wavelength in vacuum. Given the measured values of the lifetime in the bulk crystal and the qNCs, we obtain a filling factor of about 0.85. Fig. 5. Integral signal of the various bands with respect to that of 2F5/2.
Table 1 Lifetimes of the Tm levels. Level
Crystal lifetime (ms)
qNCs lifetime (ms)
3
15 500 7500 8707 50 6507 50 6507 50
18 100 7500 8607 50 7007 50 6607 50
F4 H4 F3 1 G4 3 3
to be ascribed to different energy-transfer processes, different efficiency of the same processes, or any other reason is the subject of the following sections. 3.2. Decay-time We measured the decay time of the main multiplets observed both in the crystal and in the qNCs. The decay of 3F4 is exponential in both samples, but all the others are not. For non-exponential decays the lifetime is derived as the fit of the exponential tail of the decay at long times. The results are summarized in Table 1. Usually the presence of defects or quenching centres in a material tends to de-populate the energy levels involved, therefore the main effect is a shortening of the decay time. In our case all lifetimes measured in the qNCs are in agreement or longer than those in the bulk crystal. This means that there is no evidence of possible quenching of the fluorescence from internal defects introduced by the milling process or surface quenching centres, unlike what sometimes happens with NCs obtained by chemical synthesis [19,20]. If we compare our lifetime results with those already in the literature for this compound [25] we can see that some discrepancies arise, but they can be ascribed to the different analysis of non-exponential decays and/or to the different experimental setup used because in our case the pump was a pulsed laser and Noginov et al. used a square-pulsed excitation. As for the 3F4 decay, the difference between 15.5 ms measured in the crystal and 18.1 ms in the qNCs is definitely larger than the experimental error. In other cases longer decay times have been observed in powder samples than in crystals [19,26,27] and these discrepancies have been ascribed to the well known ‘effective refractive index’ effect [28,29]. According to the proposed model the lifetime in powder samples changes because of a change of the ‘effective refractive index’ seen by the emitting centres. In fact if the grains are smaller than the emitted wavelength, the ‘effective refractive index’ is determined both by the refractive
3.3. Upconversion In order to investigate the reason of the differences in the emission intensities of the various bands we measured the emission intensity of the emitted signal as a function of pump power. It is well known that this is a powerful tool to identify the energy transfer path of the excitation inside a material because in bilogarithmic scale the emitted signal should have a linear dependence on the pump power with a slope that is connected to the number of photons (n) involved in the population of that particular level. In formula I0 ¼Pn where I0 is the emitted intensity and P is the pump power. In real cases n is rarely an integer number because some loss processes are usually present and can lower this number by a certain amount that usually increases at increasing the pump power. In our case all the observed intensities both in the bulk crystal and in the qNCs showed an almost linear dependence with the pump power in bilogarithmic scale, as expected, and the results of the linear fit are reported in Table 2 together with a comparison with literature results. As an example we show in Fig. 6 the bilogarithmic plot of one of these dependances. Although no direct comparison of the pump density between our results and Ref. [25] can be made because we could not measure the dimension of the diode laser beam in the focus, our slopes are usually intermediate between the high-density and low-density regimes of Ref. [25] (which span five decades of pump density), therefore we can infer that our results are compatible with the energy transfer path proposed [30–33] (see Fig. 3) and refer to an intermediate pump regime with respect to that of Ref. [25]. In particular the 3F4 metastable level is populated by a one-photon process, the 3H4 and 3F3 by two-photon processes, and 1G4 by a three-photon process. As already mentioned we did not observe any emission by higher lying levels (neither in the crystal nor in the qNCs) which should be populated by fourphoton processes, probably because of the low pump density used in our experiment. Moreover results in the crystal are either in agreement or slightly higher than in the qNCs. This slight Table 2 Slopes of the I vs. P plots. Level
Crystal slope
qNCs slope
Ref. [25] slope low excitation
Ref. [25] slope high excitation
3
0.88 0.89 1.70 1.59 2.35
0.88 0.93 1.53 1.60 2.17 (2.29)
0.7 0.5 1
1 1 1.5
1.8
3
F4 F5/2 H4 3 F3 1 G4 2 3
A. Toncelli et al. / Journal of Luminescence 132 (2012) 2268–2274
Fig. 6. I vs. P plot for the 1G4 level.
discrepancy could be due to two possible reasons: stronger lossprocesses in the qNCs or a higher pump density in the qNCs. The first hypothesis is disproved by the higher population of 1G4 with respect to the lower lying levels (3F3 in particular) and by the lifetime results that did not evidence any shortening of the decay times, but the second seems to be in contrast to the fact that particular care was taken in order to assure exactly the same pumping conditions. After careful observation of the powerdependence we observed that the slope for the 1G4 level of the qNCs is not perfectly linear, therefore we fitted again the results of the low-pump-power side of the curve, till the beginning of the slight non-linearity and the result is reported in Table 2 in parenthesis. Actually this low-pump density result is in better agreement with the crystal and confirms the second hypothesis. A possible explanation of this is that once a photon enters a grain of nanopowder it is trapped inside the particle by the total internal reflection effect, unlike what happens in the crystal that has all surfaces polished. As a result a photon travels the path inside a single NC grain several times before escaping, and this increases the effective power density inside the grain. To verify this point we acquired a series of emission spectra in the 620–720 nm region as a function of the pump power. This region is covered by only one detector and the spectra were taken for the crystal and the qNCs in the same experimental conditions. As this region contains the emission from two different levels (1G4 and 3F3) we calculated the ratio of the integral intensity of the emission from the two levels as a function of the pump power for the crystal and qNCs. In both cases the bilogarithmic plot of the results is linear, but the data points for the qNCs are shifted to lower-pump-power, as we can see in Fig. 7. This linear dependence can be written as LogðIÞ ¼ LogðAÞ þ B LogðPÞ where I is the ratio of the intensity of the two bands, P is the pump density and A and B are the fitting parameters. Assuming that the energy transfer processes in this pumping regime are the same in the crystal and the qNCs (assumption that is verified by the fact that the slope is the same and the lifetimes agree) we can ascribe the shifting to a different ‘effective pumping density’ inside the samples. In other words we assume the two samples behave in the same way provided that the pump density for the qNCs is replaced by the effective pump density P 0 ¼ aP. The comparison of the two fitting lines permits to estimate the value of a. In our case a ¼ 3:8 7 1 (the error comes from the error of the parameters of the fit), this means that a photon that enters a grain is reflected in average 2.8 times ( 71) at the grain boundary
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Fig. 7. Integral signal of the 1G4 band with respect to that of 3F3 vs. pump power.
before escaping. If we consider a simple model where a photon hits the boundary of a grain at a random angle it has a probability p to exit the grain and 1p to be scattered back inside the particle. As the refractive index of BaYF is about 1.53, this corresponds to a total internal reflection angle of 40.81, therefore p¼0.453. In this model the average number of reflections a photon undergoes before escaping is 1.7, which is in reasonably good agreement with the experimental estimation. This simple model explains the stronger up-converted luminescence in qNCs with respect to a bulk crystal, and could be applicable to any kind of NPs surrounded by a lower-refractive index medium, therefore it could be helpful in interpreting some of the anomalous behaviour of NPs emissions reported in the literature [8,21,22]. For example this effect is expected to be amplified in oxide NCs because the refractive index of oxide materials is usually higher than fluorides and ranges from 1.8 to 2 which corresponds to a total internal reflection angle from 33.71 to 301. In these cases we obtain p ¼0.375 and p¼ 0.333, respectively, and the total number of internal reflections before escaping is 2 and 2.3, respectively.
4. Conclusion We presented a method for producing qNPs that does not involve the use of dangerous chemical solvents or surfactants. It is very simple and easily scalable and is capable of producing large quantities of NPs with nanometric dimensions with a great variety of compositions. Although the dimensions of the qNCs presented in this work are in the 200–300 nm range, we recently produced NPs with dimensions smaller than 20 nm with the same technique, therefore the technique is capable of producing NCs with dimensions suitable for applications. We have also shown a complete spectroscopic characterization of a new type of qNCs, namely 5%Yb, 1%Tm:BaY2F8 that can be pumped at around 970 nm and show a series of emission bands in the visible region (460–490 nm, 630–670 nm, 670–720 nm) and in the near infrared (760–820 nm, 900–1100 nm, 1460–1500 nm, 1600–200 nm). Some of these bands are interesting for various applications, but most of all for biolabelling even in deep tissues. Moreover the variety of compositions and of dopants available permits to tailor the emission for the specific application, also for multicolour imaging.
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The comparison of the emission features of the qNCs and the bulk crystal has shown that in the qNCs no new loss mechanisms for the excitation take place with respect to the bulk crystal, instead the behaviour is compatible with a higher pump density inside the grains that leads to a higher population of the upconverted levels in the qNCs with respect to the absorbing level. This effect somehow amplifies the up-converting properties of the material, and explains why NP are such efficient up-converting materials. A simple model explains this effect and it can be easily applied to any other dielectric material in nanometric scale. These results help in understanding the behaviour of the excitation inside nanoscaled materials with important effects on the emission features of qNCs with respect of bulk materials. Moreover, even if the experiment has been performed on qNCs of around 200 nm in size, the authors believe the same effect can influence the emission of NPs of different compositions and/or different sizes. A possible generalization to oxide materials is given. A careful analysis of this effect as a function of the NP dimensions is currently being carried out.
Acknowledgement The authors wish to thank Mrs. I. Grassini for helping in the preparation of the samples P. Pingue for SEM measurements and M. Tonelli for helpful discussions. References [1] F. Vetrone, J.A. Capobianco, Int. J. Nanotechnol. 5 (2008) 1306. ¨ [2] M. Haase, H. Schafer, Angew. Chem. Int. Ed. 50 (2011) 5808. [3] F. Wang, Y. Han, C.S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, X. Liu, Nature 463 (2010) 1061. [4] F. Pelle , M. Dhaouadi, L. Michely, P. Aschehoug, A. Toncelli, M. Tonelli, S. Veronesi, Phys. Chem. Chem. Phys. 13 (2011) 17453, http://dx.doi.org/ 10.1039/c1cp20725c.
[5] L. Wang, Y. Zhang, Y. Zhu, Nano Res. 3 (2010) 317. [6] J. Sun, J. Xian, Z. Xia, H. Du, J. Rare Earths 28 (2010) 219. [7] G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, L.H. Guo, Nano Lett. 4 (2004) 2191. [8] R. Yan, Y. Li, Adv. Funct. Mater. 15 (2005) 763. [9] S. Heer, K. Kompe, H.U. Gudel, M. Haase, Adv. Mater. 16 (2004) 2102. [10] H.X. Mai, Y.W. Zhang, R. Si, Z.G. Yan, L.D. Sun, L.P. You, C.H. Yan, J. Am. Chem. Soc. 128 (2006) 6426. [11] L. Gao, X. Ge, Z. Chai, G. Xu, X. Wang, C. Wang, Nano Res. 2 (2009) 565. [12] X. Yu, M. Li, M. Xie, L. Chen, Y. Li, Q. Wang, Nano Res. 3 (2010) 51. [13] A. Agnesi, A. Guandalini, A. Tomaselli, E. Sani, A. Toncelli, M. Tonelli, Opt. Lett. 29 (2004) 1638. [14] N. Coluccelli, G. Galzerano, D. Parisi, A. Toncelli, M. Tonelli, P. Laporta, Opt. Exp. 14 (2006) 1518. [15] F. Cornacchia, D. Parisi, C. Bernardini, A. Toncelli, M. Tonelli, Opt. Exp. 12 (2004) 1982. [16] D. Gatti, G. Galzerano, A. Toncelli, M. Tonelli, P. Laporta, Appl. Phys. B 86 (2007) 269. [17] A. Agnesi, F. Pizio, G. Reali, A. Toncelli, M. Tonelli, H.P. Jenssen, Opt. Commun. 281 (2008) 6094. [18] H.X. Mai, Y.W. Zhang, L.D. Sun, C.H. Yan, J. Phys. Chem. C 111 (2007) 13721. [19] J. Amami, D. Hreniak, Y. Guyot, W. Zhao, G. Boulon, J. Lumin. 130 (2010) 603. [20] J. Shan, M. Uddi, R. Wei, N. Yao, Y. Ju, J. Phys. Chem. C 114 (2010) 2452. [21] G.Y. Chen, H.J. Liang, H.C. Liu, G. Somesfalean, Z.G. Zhang, J. Appl. Phys. 105 (2009) 114315. [22] J.A. Capobianco, F. Vetrone, T. D’Alesio, G. Tessari, A. Speghini, M. Bettinelli, Phys. Chem. Chem. Phys. 2 (2000) 3203. [23] W. Kraus, G. Nolze, PowderCell, rel. 2.3, Federal Institute for Materials Research and Testing, Berlin, Germany, 1999. [24] L.H. Guilbert, J.Y. Gesland, A. Bulou, R. Retoux, Mater. Res. Bull. 28 (1993) 923. [25] M.A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, H.P. Jenssen, J. Opt. Soc. Am. B 14 (1997) 2126. [26] R.I. Zakharchenya, A.A. Kaplyanskii, A.B. Kulinkin, R.S. Meltzer, S.P. Feofilov, Phys. Solid State 45 (2003) 2209. [27] D. Hreniak, W. Strek, P. Gluchowski, M. Bettinelli, A. Speghini, Appl. Phys. B 91 (2008) 89. [28] H.P. Christensen, D.R. Gabbe, H.P. Jenssen, Phys. Rev. B 25 (1982) 1467. [29] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012. [30] B.M. Antipenko, A.A. Mak, O.B. Raba, K.B. Seiranyan, T.V. Uvarova, Sov. J. Quant. Electron. 13 (1983) 558. [31] B.M. Antipenko, R.V. Dumbravyanu, Yu.E. Perlin, O.B. Raba, L.K. Sukhareva, Opt. Spectrosc. 59 (1985) 3. [32] X.X. Zhang, P. Hong, M. Bass, B.H.T. Chai, Phys. Rev. B 51 (1995) 9298. [33] F. Auzel, C. R. Acad. Sci. Paris B 263 (1966) 819.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Upconversion enhancement in Yb3 þ ,Tm3 þ :BaY2F8 quasi-nanoparticles Alessandra Toncellia,b,n, Banafshe Ahmadib, Fabio Marchettic a b c
NEST Istituto Nanoscienze-CNR, Italy Dipartimento di Fisica, Universita di Pisa Largo B. Pontecorvo 3, 56127 Pisa, Italy Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2011 Received in revised form 29 March 2012 Accepted 4 April 2012 Available online 13 April 2012
Yb3 þ Tm3 þ :BaY2F8 single crystals have been milled to quasi-nanometric size. A complete characterization of the quasi-nanoparticles has been compared with that of a bulk crystal with the same composition. The emission spectra did not show any difference as for the shape and relative intensity of the various peaks within each band, but the infrared lifetime of the quasi-nanoparticles is significantly longer than that of the bulk crystal. In agreement with other literature results we observed a strong increase of the upconverted luminescence intensity in the quasi-nanoparticles. An explanation is given as the effect of radiation trapping of the pumping radiation that increases the effective pump intensity in the volume of the quasi-nanoparticles with respect to the bulk crystal. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocrystals Rare earth Spectroscopy Upconversion enhancement
1. Introduction In recent years a still increasing scientific interest has been devoted to fluorescent nanoparticles (NPs) for their potential applications in a number of fields, such as photovoltaic conversion, scintillation, high resolution imaging and biophotonics. Various types of NPs have been investigated, such as Quantum Dots, magnetic NPs and other fluorescent NPs mainly for biological applications, but the quest for the ‘perfect’ bioprobe is still open. In this respect rare earth-doped fluoride nanocrystals (NCs) have recently attracted much interest because of their unique spectral features [1,2]. Their main peculiarities consist in the long lifetimes of the emitting levels and in the possibility to convert the pump radiation either to longer-wavelength or shorterwavelength emission through the so-called up-conversion and cross-relaxation nonlinear processes [3]. Moreover fluoride NCs are usually considered non-toxic and the large choice of dopants and doping levels permits to tailor the emission features for the desired application from UV to the mid-infrared region. In particular some rare earths like Yb, Tm, Er, and Ho have energy levels that can absorb and/or emit radiation in the 1-mm region and this can permit a deep penetration in the tissues. Moreover when pumped in the infrared with low-cost cw diode lasers some dopants can emit at a wavelength shorter than the pump (even in
n Corresponding author at: Dipartimento di Fisica, Universita di Pisa Largo B. Pontecorvo 3, 56127 Pisa, Italy. Tel.: þ39 050 2214556; fax: þ39 050 2214333. E-mail address: [email protected] (A. Toncelli).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.04.001
the visible region) thus completely eliminating the problem of cellular autofluorescence. Many different fluoride crystals have been investigated in nano-sized shape by many researchers including our group, such as YF3, LaF3, BaYF5, NaLaF4, NaGaF4, and NaYF4 [4–12] with interesting up-conversion emissions, but the choice of the host material is limited by the possibility of obtaining it by chemical synthesis. Many more fluoride crystals are well known as bulk materials, and each of them has thermo-optical properties that affect the emission features of rare earth dopants. For this reason it would be extremely desirable to extend this characterization to the whole class of fluoride crystals. Moreover chemical synthesis of these materials involves the handling of toxic or dangerous solvents or surfactants that make the scalability of the process difficult and can affect the emission features of the NPs. For these reasons we propose an alternative method to obtain high quality NCs by mechanical milling of macroscopic crystals of a variety of compositions with a simple and easily scalable process. Moreover this approach permits us to compare the spectral features of the NCs with those of a bulk crystal of the same composition, and this gives a deep insight in the physical processes that lead to the extremely interesting fluorescence of these materials. In this work we present a complete characterization of the spectroscopic features of 5%Yb, 1%Tm:BaY2F8 quasi-NCs (qNCs) in comparison with those of a bulk crystal with the same composition. BaY2F8 (BaYF) is a well known laser host material, in fact efficient and sometimes widely tunable laser emission has been obtained from this crystal when doped with a variety of rare earth ions (for example Nd, Tm, Ho, Yb, etc.) [13–17], but has never
A. Toncelli et al. / Journal of Luminescence 132 (2012) 2268–2274
been obtained in nanometric size by chemical synthesis. Therefore we present a complete spectroscopic characterization of this material in the wavelength region from visible to near infrared. Moreover the comparison with the bulk crystal evidences an increase of the relative population of the up-converted levels with respect to the bulk material. An explanation is proposed. In fact NCs sometimes show an anomalous upconversion behaviour: they usually show a decrease of the upconversion efficiency with decreasing size (measured as surface to volume ratio) [18–20] and mainly ascribed to surface quenching centres that are also responsible for a shortening of lifetimes, but in few other cases an increase of upconversion efficiency is observed, instead [8,21,22]. No theoretical interpretation of this interesting effect has been given, on the contrary it is extremely important to understand the behaviour of the NPs in comparison with bulk crystals in order to assess the potentialities of NPs for various applications.
2. Experimental We grew a bulk 5%Yb, 1%Tm:BaY2F8 crystal with Czochralski technique and prepared qNCs of the same composition through mechanical milling. 2.1. Crystal growth BaY2F8 has monoclinic structure, with unit cell parameters ˚ b¼10.505 A, ˚ c ¼4.260 A, ˚ and angle g between the a ¼6.972 A, a- and c-axes of 99.761. The symmetry group is C2/m. The rareearth dopants enter substitutionally the Y sites. The crystal growth facility consists of a home-made computercontrolled Czochralski furnace with conventional resistive heating and optical automatic diameter control system. The furnace is equipped with an evacuation system that can reach an ultimate pressure limit of about 10 7 mbar. The growth process takes place in high purity Argon atmosphere. The starting materials were proper amounts of 5N fluoride powders supplied by AC materials (Tarpon Springs, FL, USA). The dopants were added to the pure BaY2F8 melt in trivalent form (YbF3 and TmF3), and a proper amount of BaF2 was added for compensation. The growth temperature was in the range 992–983 1C, the pull rate was 0.5 mm/h, and the rotation rate was 5 rpm. The seed was a piece of undoped BaYF crystal oriented along the a crystallographic axis. The bulk sample was obtained with a standard cutting and lapping procedure for optical measurements.
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molecules (locally injected through a micro-nozzle) is employed to partially neutralize the electron charging on the insulating sample. In order to verify the nature and the purity of phases present in the qNCs used for this study, its XRD pattern has been collected on a no-background sample holder. The powder diffraction pattern has been collected in the 2y range 10–601 at steps of 0.021 using a Philips PW1050/25 Bragg-Brentano diffractometer. The measurements have been performed at room temperature using a Cu source powered by 40 kV and 20 mA and a graphite ˚ The monochromator on the secondary beam (Ka, l ¼ 1:54184 A). diffracted beam has been measured using a scintillation counter integrating for 10 s/step. Room temperature fluorescence spectra were obtained by exciting the sample with a diode laser at 972 nm with maximum input power P¼270 mW. The pump beam was focused onto the sample with a 10 cm focal length lens. qNCs were deposited on a glass substrate and inclined at about 451 with respect both to the pumping beam and to the detected fluorescence. The bulk sample was mounted either with the input face perpendicular to the pump beam and the fluorescence detected at normal angle or at about 451 with respect both to the pumping beam and to the detected fluorescence, that is in the same experimental conditions used for the qNCs. In all cases the fluorescence signal was detected perpendicularly to the pump laser direction. Particular care was taken to assure that the pumping conditions were identical both for the bulk sample and for the qNCs. The luminescence was chopped and focused by a 7.5 mm focal length lens on the input slit of a Acton monochromator with 25 cm focal length equipped with a suitable grating in the various spectral regions. The signal was detected by a liquid nitrogen cooled InSb detector or a photomultiplier equipped with a S20 cathode, fed into pre-amplifiers, processed by a lock-in amplifier and stored on a PC. The acquired spectra were normalized for the optical response of the system using a black body source at 3000 K. For infrared lifetime measurements the set-up was similar but the pump beam was mechanically chopped and the signal from the detector was amplified and sent to a digital oscilloscope connected to a computer. The response time of the system was 100 ms. For the visible lifetime measurements the pump laser was a doubled Ti:Sapphire laser tuned to the maximum absorption of the sample at around 460 nm. The laser had a 30 ns pulse duration and 10 Hz repetition frequency and the response time of the system was 1 ms.
3. Results and discussion 2.2. qNCs preparation We prepared a polycrystalline mass with mm-sized grains using exactly the same composition and thermal treatment of the Czochralski grown crystal. The starting materials were loaded into a crucible and heated in a vacuum-tight furnace in high purity Ar atmosphere up to complete melting. The melt was then left at high temperature for several days (a typical time-scale for Czochralski crystal-growth) and then slowly cooled down. We obtained a polycrystalline mass made of mm-sized grains. The grains were then pestled and subsequently dry milled for some hours. Separation of the nanometric component was performed through standard centrifugation technique.
The morphology of the qNCs was inspected with SEM. A good uniformity of shape and dimension were observed in the powder sample, the average dimension of the qNCs is around 200– 300 nm, as shown in Fig. 1, even if a few smaller and larger grains are present. X-ray analysis confirmed the single crystal character of the obtained qNCs. The powder diffraction pattern is shown by a black line in Fig. 2. The pattern drawn by a red line has been calculated from the known structure of BaYF [23]. The good agreement in line positions suggests that the doped sample maintains the crystal structure of pure BaYF [24], different phases being present only at trace level. The differences in line intensities may probably be ascribed to preferential orientation effects.
2.3. Characterization 3.1. Emission spectra Scanning Electron Microscope (SEM) imaging is obtained with an Ultra Plus system (ZEISS), employing charge-compensation imaging technique: electron-induced ionization of nitrogen
Rare earths in crystals posses a series of discrete energy levels that can absorb and emit radiation. Due to their long lifetime
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these levels act as energy sinks that can either transfer this energy to other levels or emit it at a later time. Therefore the excitation energy can be transferred to higher lying or lower lying levels through a bilinear process before being emitted. In Fig. 3 we show the first excited levels of the 4f configuration of Tm3 þ and Yb3 þ ions together with the most probable processes involved.
Fig. 1. SEM image of the Yb3 þ Tm3 þ :BaY2F8 qNCs.
Fig. 2. Measured (black line) and calculated (red line) powder X-ray diffraction patterns for BaY2F8. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
After pumping both the bulk crystal and the qNCs in the 2F5/2 Yb3 þ level at around 970 nm we observed emission from five different emitting levels: 2F5/2 of Yb3 þ and 3F4, 3H4, 3F3 and 1G4 of Tm3 þ , as reported in Fig. 4a–f. Up-converted emissions are particularly interesting for biological applications. In particular the emission at around 800 nm lies in the transmission window of tissues, and this can be exploited for deep-tissue imaging because the pump is also in the same window and no cellular autofluorescence is expected with this pumping scheme. Moreover visible emissions are very interesting, too, because high sensitive detectors are of standard use in this wavelength region. The assignment of the various bands was performed according to Ref. [25]. No emission from levels higher than 1G4 was observed at our pumping level. In all cases the shape and relative intensity of the various peaks within each band is similar in the qNCs and in the bulk crystal and small differences can be explained by the different geometries of the samples and from the fact that the crystal matrix is strongly anisotropic (the qNCs correspond to an average of all the possible orientations, and the bulk crystal, although non-polarized, is an average of only two possible orientations), but big differences in the overall intensity of the various bands with respect to the others exist. In particular the up-converted emissions from 1G4 (Fig. 4a and b) in the qNCs are more intense than in the bulk crystal, while the emissions from 3F3, 3H4, and 3F4 are less intense. Although an absolute comparison of the emission intensities in the two samples is meaningless, the comparison of the emission intensities from different levels is connected to the different population of the multiplets and can give information about the energy transfer processes inside the material. At a first sight it appears that in the qNCs the ratio of the population of 1G4 with respect to that of the lower levels is much higher than in the bulk crystal in the same pumping conditions. This is particularly clear from Fig. 4b where two bands arising from 1G4 and 3F3 are present simultaneously and are acquired with the same set-up. In the bulk crystal (black line) the signal from 3F3 is higher than that from 1G4, and in the qNC (red line) it is the opposite. We calculated the ratio of the integral signal of the various bands in the crystal and in the qNCs. Integrals are all expressed with respect to the integral of the absorbing band (2F5/2) because its population should be directly connected to the pump power. For the qNCs we subtracted the contribution of the strong scattering line at the laser wavelength around 970 nm which is evident in Fig. 4d. The results are summarized in Fig. 5. Taken the population of 2F5/2 as a reference, we observe that the population of 3F4 in the crystal is a factor of
Fig. 3. Energy level scheme of Tm3 þ and Yb3 þ and main energy transfer processes.
A. Toncelli et al. / Journal of Luminescence 132 (2012) 2268–2274
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Fig. 4. (a) 1 G4 -3 H6 Tm3 þ emission. (b) 1 G4 -3 F4 and 3 F3 -3 H6 Tm3 þ emissions. (c) 3 H4 -3 H6 Tm3 þ emission. (d) 2 F4=2 -2 F7=2 Yb3 þ emission. (e) 3 H4 -3 F4 Tm3 þ emission. (f) 3 F4 -3 H6 Tm3 þ emission. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
two higher than in the qNCs, while the population of the levels higher than the pump is lower. In particular the populations of 3 H4 and 3F3 are a factor of three higher in the qNC than in the crystal, and that of1G4 is a factor of 15 higher. This is a strong evidence of the fact that the upconversion process is much more efficient in the qNCs than in the crystal in the same pumping conditions. This observation is in contrast with what usually
observed for various-size nanoparticles which show a decrease of upconversion efficiency with increasing the surface to volume ratio (therefore for diminishing particle dimension) and a threshold dimension around 70 nm above which the spectral properties become similar to those of the bulk material [18]. This implies that the explanation in our case must be different from that given in other cases (mainly surface quenching effects) but whether it is
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index of the material and by that of the surrounding medium (air in our case) according to this formula: neff ðxÞ ¼ xngrain þð1xÞnmed
ð1Þ
where x is the ‘filling factor’, that is the fraction of the sample volume occupied by grains (ngrain is the refractive index of the material, nmed is the refractive index of the medium, note that 0 ox o1). The effective refractive index affects the lifetime of a multiplet when it replaces the usual refractive index in the expression of electric dipole transition probability: 2
tR p
1 l0 f ðEDÞ ½1=3ðn2 þ2Þ2 n
ð2Þ
where f ðEDÞ is the oscillator strength for the electronic dipole transition and l0 is the wavelength in vacuum. Given the measured values of the lifetime in the bulk crystal and the qNCs, we obtain a filling factor of about 0.85. Fig. 5. Integral signal of the various bands with respect to that of 2F5/2.
Table 1 Lifetimes of the Tm levels. Level
Crystal lifetime (ms)
qNCs lifetime (ms)
3
15 500 7500 8707 50 6507 50 6507 50
18 100 7500 8607 50 7007 50 6607 50
F4 H4 F3 1 G4 3 3
to be ascribed to different energy-transfer processes, different efficiency of the same processes, or any other reason is the subject of the following sections. 3.2. Decay-time We measured the decay time of the main multiplets observed both in the crystal and in the qNCs. The decay of 3F4 is exponential in both samples, but all the others are not. For non-exponential decays the lifetime is derived as the fit of the exponential tail of the decay at long times. The results are summarized in Table 1. Usually the presence of defects or quenching centres in a material tends to de-populate the energy levels involved, therefore the main effect is a shortening of the decay time. In our case all lifetimes measured in the qNCs are in agreement or longer than those in the bulk crystal. This means that there is no evidence of possible quenching of the fluorescence from internal defects introduced by the milling process or surface quenching centres, unlike what sometimes happens with NCs obtained by chemical synthesis [19,20]. If we compare our lifetime results with those already in the literature for this compound [25] we can see that some discrepancies arise, but they can be ascribed to the different analysis of non-exponential decays and/or to the different experimental setup used because in our case the pump was a pulsed laser and Noginov et al. used a square-pulsed excitation. As for the 3F4 decay, the difference between 15.5 ms measured in the crystal and 18.1 ms in the qNCs is definitely larger than the experimental error. In other cases longer decay times have been observed in powder samples than in crystals [19,26,27] and these discrepancies have been ascribed to the well known ‘effective refractive index’ effect [28,29]. According to the proposed model the lifetime in powder samples changes because of a change of the ‘effective refractive index’ seen by the emitting centres. In fact if the grains are smaller than the emitted wavelength, the ‘effective refractive index’ is determined both by the refractive
3.3. Upconversion In order to investigate the reason of the differences in the emission intensities of the various bands we measured the emission intensity of the emitted signal as a function of pump power. It is well known that this is a powerful tool to identify the energy transfer path of the excitation inside a material because in bilogarithmic scale the emitted signal should have a linear dependence on the pump power with a slope that is connected to the number of photons (n) involved in the population of that particular level. In formula I0 ¼Pn where I0 is the emitted intensity and P is the pump power. In real cases n is rarely an integer number because some loss processes are usually present and can lower this number by a certain amount that usually increases at increasing the pump power. In our case all the observed intensities both in the bulk crystal and in the qNCs showed an almost linear dependence with the pump power in bilogarithmic scale, as expected, and the results of the linear fit are reported in Table 2 together with a comparison with literature results. As an example we show in Fig. 6 the bilogarithmic plot of one of these dependances. Although no direct comparison of the pump density between our results and Ref. [25] can be made because we could not measure the dimension of the diode laser beam in the focus, our slopes are usually intermediate between the high-density and low-density regimes of Ref. [25] (which span five decades of pump density), therefore we can infer that our results are compatible with the energy transfer path proposed [30–33] (see Fig. 3) and refer to an intermediate pump regime with respect to that of Ref. [25]. In particular the 3F4 metastable level is populated by a one-photon process, the 3H4 and 3F3 by two-photon processes, and 1G4 by a three-photon process. As already mentioned we did not observe any emission by higher lying levels (neither in the crystal nor in the qNCs) which should be populated by fourphoton processes, probably because of the low pump density used in our experiment. Moreover results in the crystal are either in agreement or slightly higher than in the qNCs. This slight Table 2 Slopes of the I vs. P plots. Level
Crystal slope
qNCs slope
Ref. [25] slope low excitation
Ref. [25] slope high excitation
3
0.88 0.89 1.70 1.59 2.35
0.88 0.93 1.53 1.60 2.17 (2.29)
0.7 0.5 1
1 1 1.5
1.8
3
F4 F5/2 H4 3 F3 1 G4 2 3
A. Toncelli et al. / Journal of Luminescence 132 (2012) 2268–2274
Fig. 6. I vs. P plot for the 1G4 level.
discrepancy could be due to two possible reasons: stronger lossprocesses in the qNCs or a higher pump density in the qNCs. The first hypothesis is disproved by the higher population of 1G4 with respect to the lower lying levels (3F3 in particular) and by the lifetime results that did not evidence any shortening of the decay times, but the second seems to be in contrast to the fact that particular care was taken in order to assure exactly the same pumping conditions. After careful observation of the powerdependence we observed that the slope for the 1G4 level of the qNCs is not perfectly linear, therefore we fitted again the results of the low-pump-power side of the curve, till the beginning of the slight non-linearity and the result is reported in Table 2 in parenthesis. Actually this low-pump density result is in better agreement with the crystal and confirms the second hypothesis. A possible explanation of this is that once a photon enters a grain of nanopowder it is trapped inside the particle by the total internal reflection effect, unlike what happens in the crystal that has all surfaces polished. As a result a photon travels the path inside a single NC grain several times before escaping, and this increases the effective power density inside the grain. To verify this point we acquired a series of emission spectra in the 620–720 nm region as a function of the pump power. This region is covered by only one detector and the spectra were taken for the crystal and the qNCs in the same experimental conditions. As this region contains the emission from two different levels (1G4 and 3F3) we calculated the ratio of the integral intensity of the emission from the two levels as a function of the pump power for the crystal and qNCs. In both cases the bilogarithmic plot of the results is linear, but the data points for the qNCs are shifted to lower-pump-power, as we can see in Fig. 7. This linear dependence can be written as LogðIÞ ¼ LogðAÞ þ B LogðPÞ where I is the ratio of the intensity of the two bands, P is the pump density and A and B are the fitting parameters. Assuming that the energy transfer processes in this pumping regime are the same in the crystal and the qNCs (assumption that is verified by the fact that the slope is the same and the lifetimes agree) we can ascribe the shifting to a different ‘effective pumping density’ inside the samples. In other words we assume the two samples behave in the same way provided that the pump density for the qNCs is replaced by the effective pump density P 0 ¼ aP. The comparison of the two fitting lines permits to estimate the value of a. In our case a ¼ 3:8 7 1 (the error comes from the error of the parameters of the fit), this means that a photon that enters a grain is reflected in average 2.8 times ( 71) at the grain boundary
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Fig. 7. Integral signal of the 1G4 band with respect to that of 3F3 vs. pump power.
before escaping. If we consider a simple model where a photon hits the boundary of a grain at a random angle it has a probability p to exit the grain and 1p to be scattered back inside the particle. As the refractive index of BaYF is about 1.53, this corresponds to a total internal reflection angle of 40.81, therefore p¼0.453. In this model the average number of reflections a photon undergoes before escaping is 1.7, which is in reasonably good agreement with the experimental estimation. This simple model explains the stronger up-converted luminescence in qNCs with respect to a bulk crystal, and could be applicable to any kind of NPs surrounded by a lower-refractive index medium, therefore it could be helpful in interpreting some of the anomalous behaviour of NPs emissions reported in the literature [8,21,22]. For example this effect is expected to be amplified in oxide NCs because the refractive index of oxide materials is usually higher than fluorides and ranges from 1.8 to 2 which corresponds to a total internal reflection angle from 33.71 to 301. In these cases we obtain p ¼0.375 and p¼ 0.333, respectively, and the total number of internal reflections before escaping is 2 and 2.3, respectively.
4. Conclusion We presented a method for producing qNPs that does not involve the use of dangerous chemical solvents or surfactants. It is very simple and easily scalable and is capable of producing large quantities of NPs with nanometric dimensions with a great variety of compositions. Although the dimensions of the qNCs presented in this work are in the 200–300 nm range, we recently produced NPs with dimensions smaller than 20 nm with the same technique, therefore the technique is capable of producing NCs with dimensions suitable for applications. We have also shown a complete spectroscopic characterization of a new type of qNCs, namely 5%Yb, 1%Tm:BaY2F8 that can be pumped at around 970 nm and show a series of emission bands in the visible region (460–490 nm, 630–670 nm, 670–720 nm) and in the near infrared (760–820 nm, 900–1100 nm, 1460–1500 nm, 1600–200 nm). Some of these bands are interesting for various applications, but most of all for biolabelling even in deep tissues. Moreover the variety of compositions and of dopants available permits to tailor the emission for the specific application, also for multicolour imaging.
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The comparison of the emission features of the qNCs and the bulk crystal has shown that in the qNCs no new loss mechanisms for the excitation take place with respect to the bulk crystal, instead the behaviour is compatible with a higher pump density inside the grains that leads to a higher population of the upconverted levels in the qNCs with respect to the absorbing level. This effect somehow amplifies the up-converting properties of the material, and explains why NP are such efficient up-converting materials. A simple model explains this effect and it can be easily applied to any other dielectric material in nanometric scale. These results help in understanding the behaviour of the excitation inside nanoscaled materials with important effects on the emission features of qNCs with respect of bulk materials. Moreover, even if the experiment has been performed on qNCs of around 200 nm in size, the authors believe the same effect can influence the emission of NPs of different compositions and/or different sizes. A possible generalization to oxide materials is given. A careful analysis of this effect as a function of the NP dimensions is currently being carried out.
Acknowledgement The authors wish to thank Mrs. I. Grassini for helping in the preparation of the samples P. Pingue for SEM measurements and M. Tonelli for helpful discussions. References [1] F. Vetrone, J.A. Capobianco, Int. J. Nanotechnol. 5 (2008) 1306. ¨ [2] M. Haase, H. Schafer, Angew. Chem. Int. Ed. 50 (2011) 5808. [3] F. Wang, Y. Han, C.S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, X. Liu, Nature 463 (2010) 1061. [4] F. Pelle , M. Dhaouadi, L. Michely, P. Aschehoug, A. Toncelli, M. Tonelli, S. Veronesi, Phys. Chem. Chem. Phys. 13 (2011) 17453, http://dx.doi.org/ 10.1039/c1cp20725c.
[5] L. Wang, Y. Zhang, Y. Zhu, Nano Res. 3 (2010) 317. [6] J. Sun, J. Xian, Z. Xia, H. Du, J. Rare Earths 28 (2010) 219. [7] G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen, L.H. Guo, Nano Lett. 4 (2004) 2191. [8] R. Yan, Y. Li, Adv. Funct. Mater. 15 (2005) 763. [9] S. Heer, K. Kompe, H.U. Gudel, M. Haase, Adv. Mater. 16 (2004) 2102. [10] H.X. Mai, Y.W. Zhang, R. Si, Z.G. Yan, L.D. Sun, L.P. You, C.H. Yan, J. Am. Chem. Soc. 128 (2006) 6426. [11] L. Gao, X. Ge, Z. Chai, G. Xu, X. Wang, C. Wang, Nano Res. 2 (2009) 565. [12] X. Yu, M. Li, M. Xie, L. Chen, Y. Li, Q. Wang, Nano Res. 3 (2010) 51. [13] A. Agnesi, A. Guandalini, A. Tomaselli, E. Sani, A. Toncelli, M. Tonelli, Opt. Lett. 29 (2004) 1638. [14] N. Coluccelli, G. Galzerano, D. Parisi, A. Toncelli, M. Tonelli, P. Laporta, Opt. Exp. 14 (2006) 1518. [15] F. Cornacchia, D. Parisi, C. Bernardini, A. Toncelli, M. Tonelli, Opt. Exp. 12 (2004) 1982. [16] D. Gatti, G. Galzerano, A. Toncelli, M. Tonelli, P. Laporta, Appl. Phys. B 86 (2007) 269. [17] A. Agnesi, F. Pizio, G. Reali, A. Toncelli, M. Tonelli, H.P. Jenssen, Opt. Commun. 281 (2008) 6094. [18] H.X. Mai, Y.W. Zhang, L.D. Sun, C.H. Yan, J. Phys. Chem. C 111 (2007) 13721. [19] J. Amami, D. Hreniak, Y. Guyot, W. Zhao, G. Boulon, J. Lumin. 130 (2010) 603. [20] J. Shan, M. Uddi, R. Wei, N. Yao, Y. Ju, J. Phys. Chem. C 114 (2010) 2452. [21] G.Y. Chen, H.J. Liang, H.C. Liu, G. Somesfalean, Z.G. Zhang, J. Appl. Phys. 105 (2009) 114315. [22] J.A. Capobianco, F. Vetrone, T. D’Alesio, G. Tessari, A. Speghini, M. Bettinelli, Phys. Chem. Chem. Phys. 2 (2000) 3203. [23] W. Kraus, G. Nolze, PowderCell, rel. 2.3, Federal Institute for Materials Research and Testing, Berlin, Germany, 1999. [24] L.H. Guilbert, J.Y. Gesland, A. Bulou, R. Retoux, Mater. Res. Bull. 28 (1993) 923. [25] M.A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, H.P. Jenssen, J. Opt. Soc. Am. B 14 (1997) 2126. [26] R.I. Zakharchenya, A.A. Kaplyanskii, A.B. Kulinkin, R.S. Meltzer, S.P. Feofilov, Phys. Solid State 45 (2003) 2209. [27] D. Hreniak, W. Strek, P. Gluchowski, M. Bettinelli, A. Speghini, Appl. Phys. B 91 (2008) 89. [28] H.P. Christensen, D.R. Gabbe, H.P. Jenssen, Phys. Rev. B 25 (1982) 1467. [29] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012. [30] B.M. Antipenko, A.A. Mak, O.B. Raba, K.B. Seiranyan, T.V. Uvarova, Sov. J. Quant. Electron. 13 (1983) 558. [31] B.M. Antipenko, R.V. Dumbravyanu, Yu.E. Perlin, O.B. Raba, L.K. Sukhareva, Opt. Spectrosc. 59 (1985) 3. [32] X.X. Zhang, P. Hong, M. Bass, B.H.T. Chai, Phys. Rev. B 51 (1995) 9298. [33] F. Auzel, C. R. Acad. Sci. Paris B 263 (1966) 819.