Passivation of lanthanide surface sites in sub-10 nm NaYF4:Eu3+ nanocrystals
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We examined in detail the optical properties of NaYF4:Eu3+ nanocrystals of ~9 nm in diameter. For such small nanocrystals roughly 17 % of Y3+ ions occupy surface sites and can be efficiently substituted by optically active Eu3+ ions. In order to determine the influence of surface Eu3+ on the optical properties of the whole nanocrystal, small β-NaYF4:Eu3+ nanocrystals with homogenous size distribution were prepared using trioctylphosphine oxide as a coordinating solvent. In order to passivate the surface sites, a thin β-NaYF4 shell was further deposited on nanocrystals core and the optical properties were investigated. For this purpose absorption, photoluminescence, photoluminescence excitation, and photoluminescence decays were recorded and analyzed. The optical characteristics of surface Eu3+ significantly diminish for surface passivated nanocrystals. We calculated the increase of quantum yield to the value of 64 % when NaYF4:Eu3+ core was capped by undoped shell. The optical spectroscopy techniques were shown to be sufficient in determination of surface passivation of nanocrystals with high surface to volume ratio.
KeywordsNaYF4 Europium Surface site Passivation Nanocrystal
Recently, various groups have reported the preparation of sodium yttrium fluorides (NaYF4) nanocrystals (NCs) in either α or β phase (Wang et al. 2010a). Yan’s group initially reported the first use of trifluoroacetate salts as precursors for one step synthesis of NaYF4 NCs (Mai et al. 2006). Since yttrium can be simply substituted by other lanthanide ions, the material has received considerable attention as a nanocrystalline matrix for doping by lanthanide ions. The additional advantages of the β-NaYF4 matrix arise from a wide energy bandgap (~8 eV) (Chong et al. 2007) and a very low phonon energy (~360 cm−1 ≈ 45 meV) (Li et al. 2007; Haase and Schäfer 2011), as compared to oxide NCs (Güdel and Pollnau 2000). Thus, the photoluminescence quantum yield (QY) in this material is high, since non-radiative relaxation of excited electrons diminishes. The reduced non-radiative relaxation rate is also responsible for the intense emission from upper excited states of lanthanide ions in a low concentration (Li et al. 2009). The same reasons also account for an efficient up-conversion emission for co-doped NaYF4:Yb3+, Er3+ NCs (Li et al. 2007). Nowadays, NCs of the β phase NaYF4 doped with various lanthanide ions are well-known phosphors which have found use in a variety of applications, especially as efficient markers in biological systems (Chatterjee et al. 2010).
In the search for the best capping ligands for fluoride NCs, Shan et al. proposed the use of trioctylphosphine oxide (TOPO) as a coordinating solvent in the reaction from trifluoroacetate precursors (Shan et al. 2007). Since TOPO has a higher boiling point than the previously used solvents (oleylamine, oleic acid, or octadecene) (Mai et al. 2006; Shan et al. 2007; Boyer et al. 2006), the resulting particles were more monodispersed, crystallized in the pure hexagonal phase and have found to be small enough (~10 nm) to be used as biological labels.
Small size of NCs is a requirement in bioapplications but the large number of surface atoms could have several implications. In the case of NaYF4, the fraction of surface Y3+ reaches 17 % for 8.7 nm sized NCs. For doped NCs, the percentage of doping ions on the surface can even be higher as dopants are often pushed out from the NC core (Podhorodecki et al. 2009). This leads to a decrease in QY, because of an increase in the efficiency of non-radiative relaxation by interaction of the optically active ions with surface defects as well as with organic ligands (Heer et al. 2004). Yi et al. ascribed this interaction to a significant reduction of lanthanide (Ln) emission efficiency in NCs compared to the bulk matrix (at least one order of magnitude) (Yi and Chow 2006b; Mialon et al. 2010).
In order to preserve the high PL QY, Yi et al. initially reported the synthesis of a core/shell structure composed of NaYF4:Yb3+, Er3+/NaYF4 and NaYF4:Yb3+, Tm3+/NaYF4, and showed reduced interactions of lanthanides with surface defects, ligands, and solvent (Yi and Chow 2006a). As a result, the enhancement of PL intensity increased up to 29 times when a shell was deposited on up-converting NaYF4:Yb3+, Tm3+ core NCs. Even bigger increase, up to 450 times, was reported by Wang et al. for shell covered small NCs (10 nm) (Wang et al. 2010b). Additionally, more complex shell compositions (e.g., NaYF4:Yb3+,Er3+/NaGdF4:Yb3+) were also proposed to further increase the observed PL intensity (Guo et al. 2010). However up till now, only the PL intensity was investigated in the above-mentioned studies. Moreover, the work only considered up-conversion PL and did not take advantage of crystal phase and/or oxygen-sensitive Eu3+ ions.
In order to correctly interpret an increase in PL intensity of colloidal NCs, its concentration has to be evaluated precisely. There is often lack of information on how the NCs concentration was calculated. To the best of our knowledge, none of the studies related to Ln doped NCs show the absorption spectra, due to difficulties associated with weak absorbing NCs:Ln3+. Evaluation of NCs concentration based on NCs mass is even more problematic due to differences in mass of core and core/shell NC as well as different mass contribution from surface passivated ligands. Thus, the calculation of lanthanide doped NCs concentration is a nontrivial task, leading to a significant experimental error. Therefore, the impact of the shell formation only on PL intensity can over-estimate the obtained results.
The lack of structural information brings additional uncertainty to the discussion on the origin of enhanced PL intensity. A crucial current limitation is that transmission electron microscopy (TEM) methods are generally unable to differentiate between the core and the shell structure of NaYF4/NaLnF4 NCs due to the similarity of the lattice parameters resulting in small contrast in TEM images (Abel et al. 2011; Liu et al. 2010; Cheng et al. 2010). On the other hand, techniques such as Electron Energy Loss Spectroscopy (EELS), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) provide presence and distribution of ions inside NCs. Veggel and co-workers recently carried out detailed XPS studies to determine the formation of lanthanide based core/shell particles (Abel et al. 2009). In another study, the authors have used EELS and EDS techniques and concluded that a complete and uniform shell growth does not occur on all of the NCs and an improvement in the synthesis of core/shell structure is required (Abel et al. 2011). Despite the encouraging results presented by Veggel’s group, more widely available and user-friendly techniques and protocols are required to distinguish in a routine manner the formation of shell around NC core when doped with lanthanide ions.
Synthesis of NaYF4:Eu3+ hexagonal nanocrystals
Sodium trifluoroacetate Na(CF3COO) (98 %), yttrium(III) trifluoroacetate hydrate Y(CF3COO)3·xH2O (99 %), europium(III) trifluoroacetate trihydrate Eu(CF3COO)3·3H2O (98 %), and trioctylphosphine oxide (TOPO, 90 %) were purchased from Sigma Aldrich Ltd. All chemicals were used as received. For the synthesis of β-NaYF4 nanocrystals doped with Eu3+ ions, a single step co-thermolysis method was utilized as described previously (Podhorodecki et al. 2012a). A mixture of 1.25 mmol Na(CF3COO) (0.170 g), 0.485 mmol Y(CF3COO)3 (0.207 g), and 0.039 mmol Eu(CF3COO)3 (0.022 g) was dissolved in 10 g TOPO (26 mmol) and heated up to 120 °C under vacuum for 30 min in a standard Schlenk line technique. For high quality hexagonal NCs, the growth temperature was increased to 350 °C under N2 within 10 min and grown for 60 min. For the precipitation of NCs, resulting solution was cooled to about 70 °C and excess dry ethanol was added followed by the centrifugation to collect the product.
Synthesis of NaYF4:Eu3+/NaYF4 core/shell hexagonal nanocrystals
In order to passivated NC surface, the few monolayers of undoped NaYF4 were deposited on the NaYF4:Eu3+ NCs forming core/shell like structure. It was done in a two step approach. The shell precursors solution containing Na(CF3COO) (42.5 mg, 0.312 mmol) and Y(CF3COO)3 (51.2 mg, 0.122 mmol) dissolved in TOPO (5 g, 13 mmol) were degassed under vacuum at 120 °C. After 30 min, the precursor solution was added dropwise to NaYF4:Eu3+ NCs remained at 350 °C in a 3-neck flask after the first step of synthesis. The NaYF4 shell was deposited on NaYF4:Eu3+ core within 30 min.
Structural and optical characterization
Samples for TEM experiment were prepared by evaporating a dilute toluene solution of the nanoparticles onto carbon coated copper grids (S166-3, Agar Scientific), and a Philips Tecnai transmission electron microscope was used to obtain TEM images of the nanoparticles.
T is domain size to be determined, λ is x-ray wavelength, B is width of the diffraction peak in interest, and θ is the angle of the corresponding diffraction peak.
The PLE and PL spectra were measured using xenon lamp (450 W) coupled with a monochromator (Jobin–Yvon TRIAX 180) as an excitation source. The PL signal was recorded by CCD spectrometer (HR4000 Ocean Optics) and divided by the light source characteristic. The absorption spectra were measured on JASCO V-570 spectrophotometer. Photon Technology International Inc. systems equipped with flash xenon lamp and strobe detector both coupled with monochromators were used to observe the PL decays. PL quantum yield (PL QY) was calculated based on the experimentally determined PL decay time and calculated radiative PL decay time using the equation originally given by Verhoeven et al. (Werts et al. 2002).
Results and discussion
Crystal structure, shape, and size of the nanocrystals
Eu3+ concentration in a core NCs is expected to be 5 %, which equals the precursor EuTFA3 used in the synthesis. Our recent work demonstrates that the intended Eu3+/Y3+ precursors ratio corresponds well with the final composition of Eu3+ doped NaYF4 NCs (Podhorodecki et al. 2012b). The level of doping was chosen to observe intense emission from the 5D0 and 5D1 levels.(Chong et al. 2007; Podhorodecki et al. 2012b).
β-NaYF4 crystallizes in a hexagonal structure in which Eu3+ ions can occupy three sites. The two main sites are of C3h symmetry, which should induce a strong asymmetric splitting of the 7FJ multiplet due to axial character of their symmetry. It is also possible that Eu3+ substitutes Na+ at the 2h site with C3 symmetry. However, this situation is less probable, especially in a matrix with low doping percentage, and will not be considered in the present work. (Zakaria et al. 1997). Thus, if Eu3+ occupy any other sites than C3h, they can be related to ions localized on the NCs surface.
On the XRD spectra there were no diffraction peaks corresponding to residual Ln2O3 phase. Moreover, the CT band totally disappeared for surface passivated sample, what suggests that all Eu–O bonds are efficiently broken. It confirms that the CT band is due to interaction of Eu3+ on the NCs surface and oxygen from the P=O group of TOPO surface ligands. The absorption spectra can be used in a simple way to prove the passivation of surface sites in Eu3+ doped fluoride NCs.
The photoluminescence excitation (PLE) experiment is much more sensitive to the optical properties for weakly absorbing materials. Hence, the PLE spectra corresponding to the direct excitation of the Eu3+ ions in NaYF4:Eu3+ and NaYF4:Eu3+/NaYF4 NCs were recorded.
The PLE spectra of β-NaYF4:Eu3+ and β-NaYF4:Eu3+/β-NaYF4 samples are quite similar. Due to the low symmetry of the crystal field of β-NaYF4:Eu3+ NCs, the 5HJ and 5FJ terms are split into separate energy levels: 285, 297, 307 and 317 nm. After the shell formation, the excitation band at 307 nm disappears, which can be associated with the reduction of Eu3+ on the surface site.
In the PL spectra given in Fig. 5, many well-resolved emission bands corresponding to the electronic and magnetic dipole transitions from 5D1 and 5D0 states to 7Fj manifolds can be distinguished. The PL peak positions of all the observed bands are consistent for both samples. The green emission from upper excited state (5D1) is possible due to the limited phonon assisted relaxation rate in the high quality β-NaYF4 matrix. It indicates that the emission from 5D1 and 5D0 states is competitive.
The surface passivation causes an increase in the intensity ratio of the electric dipole transitions from the lowest and upper excited state (5D1–7F3/5D0–7F2). Two possible phenomena could cause this observation. First, when β-NaYF4 shell was formed, some of Eu3+ ions may diffuse from the core to the shell (Dong and van Veggel 2008). The separation between the ions increases and the probability of cross-relaxation processes diminishes. As only a thin layer of shell is applied (~1.5 nm from XRD, ~1.9 nm form TEM), the diffused Eu3+ ions would, in fact, again occupy the surface sites. Hence another mechanism takes place in this case. Excited Eu3+ ions located on the surface sites interact with higher energy surface phonons and/or ligand molecules, and the nonradiative relaxation from 5D1 state is more prominent due to multi-phonon transitions. A thin β-NaYF4 shell successfully passivates the surface active sites and separates Eu3+ ions from TOPO and solvent. As a result, the green emission from 5D1 of Eu3+ state of core/shell NCs increased, which is an important evidence for the shell formation. Moreover, this explanation is consistent with conclusions derived from absorption measurements.
The characteristic FWHM parameters of the magnetic and electric dipole transitions presented in the PL spectra (Fig. 5) are reduced by 1 nm when the shell is formed (5.86–4.87 nm and 5.63–4.65 nm for 5D0–7F1 and 5D0–7F2 transitions, respectively). This observation is yet another indication that the number of Eu3+ occupied sites was reduced by the surface passivation.
PL decay times (τ), disorder parameters (β), and QYs of NaYF4:Eu3+ and NaYF4:Eu3+/NaYF4 samples
5D1–7F2 (556 nm)
5D0–7F2 (616 nm)
NaYF4:Eu3+ (5 %)
1,325 ± 73
0.835 ± 0.014
−434 ± 62
6,170 ± 170
NaYF4:Eu3+ (5 %)/NaYF4
1,623 ± 32
0.912 ± 0.005
−828 ± 52
7,288 ± 150
NaYF4:Eu3+ (2 %)
2,686 ± 38
0.828 ± 0.025
−1,017 ± 120
7,782 ± 140
Recently, the cation exchange was suggested to form a gradient alloy structure, when the core/shell fluoride nanocrystals were expected (Dong et al. 2011). The authors concluded that this mechanism can significantly deplete the Eu3+ concentration in a NC core when undoped shell is intended to be deposited. As a result, any changes in the optical properties due to a reduction of the ion–ion interaction are incorrectly interpreted as a proof of shell formation.
In order to exclude the cation exchange as a possible mechanism for improvement in the optical properties of our core/shell structure, we evoke our results of PL decay times of NaYF4 NCs doped with 2 % of Eu3+. The doping level was chosen to be 2.5 times lower than the original concentration, because the volume of core/shell NCs increases two times compared to core ones. For 2 % sample, in a comparison with 5 % core sample, the τPL decay time will increase due to bigger inter ionic distances, which are supposed to reduce the efficiency of Eu3+–Eu3+ cross relaxation. The determined value of PL decay times (τPL) are 7,782 and 2,682 μs for emission from 5D0 and 5D1, respectively. Both of them significantly exceed the values obtained for core as well core/shell sample (Table 1). These results indicate that during shell formation the cation diffuse was not sufficient to separate Eu3+ ions efficiently, thus the observed changes of PL spectra have to arise directly from surface passiation.
ε is the dielectric constant of NaYF4 matrix and cyclohexane equal 2.477 and 2.023, respectively (Lage et al. 2005), AMD,0 is the spontaneous emission probability for 5D0–7F1 transition in a vacuum (14.65 s−1), and (ITOT/IMD) is the ratio of the total Eu3+ emission spectrum to the area of the 5D0–7F1 band (Vela et al. 2008). The calculated values of radiative lifetime are τR = 10.87 ms (NaYF4:Eu3+) and τR = 11.39 ms (NaYF4:Eu3+NaYF4). Taking values of τPL from PL decay experiments, the PL QY was calculated to be 57 % for the core and 64 % for the core/shell. This observation implies that the shell formation results in an increase in the efficiency of Eu3+ luminescence.
In our investigations of Eu3+ doped NaYF4 NCs, we observe a significant absorption band in the UV range. We attribute this to the Eu–O charge-transfer state, which arises from the interaction between oxygen (from trioctylphosphine oxide) and Eu3+ at the nanocrystal surface. For NCs smaller than 10 nm, due to their high surface to volume ratio, charge transfer becomes an important absorption mechanism. We have shown that the surface of NaYF4:Eu3+ NCs can be successfully passivated by a thin layer of undoped matrix. This was confirmed by the absence of CT band in the absorption spectra. Moreover, the surface passivation modifies crystal field influence on the significant fraction of Eu3+ ions. This was observed as changes in PL and PLE characteristic features, ED/MD ratio and peaks broadenings, which are discussed in details. The change in the emission lifetime is determined as well, which is used to calculate an increase in the PL emission efficiency up to a value of 64 % for surface passivated NaYF4:Eu3+/NaYF4 NCs.
This work was supported by grant from The National Center for Research and Development (Lider/13/14/L-2/10/NCBiR/2011). M.B. acknowledges financial support from fellowship co-financed by European Union within European Social Fund. M.A. would like to thank Center of Research Excellence in Renewable Energy, KFUPM, for the support.
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