NANO LETTERS
Observation of Size Dependence in Multicolor Upconversion in Single Yb3+, Er3+ Codoped NaYF4 Nanocrystals
2009 Vol. 9, No. 6 2477-2481
Stefan Schietinger,*,† Leonardo de S. Menezes,‡ Bjo¨rn Lauritzen,†,§ and Oliver Benson† Nano-Optics, Institute of Physics, Humboldt-UniVersität zu Berlin, HausVogteiplatz 5-7, D-10117 Berlin, Germany, and Departamento de Física, UniVersidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil Received April 20, 2009; Revised Manuscript Received May 11, 2009
ABSTRACT In this Letter we report on the investigation of the upconversion emission of single NaYF4 nanocrystals codoped with Yb3+ and Er3+. Single nanocrystals on a coverslip are excited with continuous wave laser light at 973 nm in a confocal setup and the upconversion fluorescence is analyzed with a spectrometer. With the help of an atomic force microscope the size of the nanocrystals is simultaneously determined. A strong size-dependence of the spectral properties of the upconversion signal of individual nanocrystals is observed. We attribute this to a differing number of available phonons in the individual crystals for multiphonon relaxation processes, depending on their size. We believe that this result provides a new strategy in the synthesis of upconversion nanoparticles with different spectral properties by changing only their size as it is well-known from the case of semiconductor quantum dots.
Upconversion (UC) is the process of converting photons of lower energy, typically infrared or near-infrared (NIR) light, into photons with higher energies. A well-known example is the conversion of NIR into green visible light in materials with nonlinear polarizability by second harmonic generation and is widely used in simple laser pointers as well as in highpower pump lasers.1 While this process can be efficient in the high-power and coherent regime, a wide range of applications demands high conversion rates at low intensities and incoherent radiation. An efficient UC mechanism is present in materials doped with rare-earth ions, the so-called photon upconversion. The main difference compared to parametric processes or two photon absorption is the use of a real metastable intermediate state. With this advantage in performance, a wide range of possible applications spans from laser materials,1 nanoscale biolabels,2,3 over lighting and display technology,4,5 and NIR quantum counters6 to the increase of solar-cell efficiency.7 Among the lanthanide-doped host materials, NaYF4 sensitized by Yb3+ ions has been reported to be the best UC material8 allowing, for example, biolabeling with moderate excitation intensities in the NIR. The high yield of the photon UC process in these particles is attributed to the mechanism * Corresponding author,
[email protected]. † Humboldt-Universität zu Berlin. ‡ Departamento de Física, Universidade Federal de Pernambuco. § Current address: Group of Applied Physics, University of Geneva, CH-1211 Geneva 4, Switzerland. 10.1021/nl901253t CCC: $40.75 Published on Web 05/21/2009
2009 American Chemical Society
of energy transfer (ET) UC. While the fundamental processes of excitation via multiple ET from an excited Yb3+ ion to the luminescent rare earth ions seem to be well understood,9-11 some of the UC luminescence characteristics are still under investigation. Recently, an unusual power and time dependence in the luminescence of Ho3+/Yb3+ codoped yttria (Y2O3) nanocrystals (NCs) was reported by Yang et al.12 Because of the outstanding UC performance in NaYF4, strong efforts were made to improve and facilitate the synthesis of NaYF4 nanoparticles.2,13-20 The possibility to fine-tune the spectral emission properties in these NCs opens the door to more complex, multiplexed labeling. This finetuning can be achieved by using different host/activator combinations, by controlling the concentration of the embedded ions, or, within some restrictions, by changing the morphology of the particles.18,19,21 However, studies on codoped NaYF4 NCs so far were based on ensemble measurements and therefore did not provide any information on the influence of the specific size of a single NC on its optical properties. In this Letter we report on the observation of size-dependence of the UC emission of single Yb3+/Er3+ codoped NaYF4 NCs. A size-dependence of the UC emission in Yb3+/Er3+ doped Y2O3 has been observed in an ensemble measurement and was assigned to surface effects:22 for three different NC sizes a decrease of the ratio between the intensities of green and red emission bands (GRR) was observed with a decrease of
Figure 1. (a) Schematic of the setup: An AFM on top of a homemade confocal microscope allows direct size determination together with optical characterization. With the avalanche photodiode (APD) it is possible to detect weak fluorescence from small NCs. A telescope with a moveable lens in the detection path corrects the chromatic aberration. (b) Typical scan over two well-separated particles.
their size. The authors attributed this behavior to high energetic vibrations of bound OH- and CO32- groups that can effectively bridge the energetic gaps between different levels which results in the enhanced population of the redemitting level. Another size-dependent effect is via phonon confinement, and its implication for the luminescence dynamics in Y2O2S has been investigated.23 While in all these experiments the size of the nanoparticles was only determined as an estimated ensemble average, in the present work we measure the size of the individual nanoparticles directly with an atomic force microscope (AFM) to gain full information about the relation of optical UC properties and size. We study in detail the GRR and find a dramatic influence of the size of the nanoparticle. In previous ensemble studies on these crystals this dependence could not be revealed which we attributed to rather broad size distributions of the ensembles investigated. Moreover, the GRR size dependence we observe is contrary to that observered in Yb3+/Er3+ doped Y2O3.22 Our experimental setup consits of a homemade inverted confocal microscope with a commercial AFM mounted on top (see Figure 1a). While the confocal setup allows optical investigation on a single NC level, the AFM (Nanowizard I, JPK Instruments) provides information on its size. In the confocal microscope (objective, Olympus Plan Apo, 60×, N.A.)1.4, oil immersion) the NCs are excited with 973 nm continuous wave light from a Ti:sapphire laser, pumped by a diode-pumped, frequency-doubled Nd:YVO4 (Coherent Mira 900 and Coherent Verdi V10, respectively). On the detection side, the excitation light is filtered out by a Bragg filter (Linos Calfex). The high chromatic aberration originating from a large difference between excitation and detection wavelength is corrected by an adjustable telescope in the detection path (see Figure 1a). For the detection of the UC fluorescence we use an avalanche photodiode (PerkinElmer, SPCM) and an EMCCD camera (Andor iXon). The fluorescence light is analyzed by a spectrometer (Princeton Instruments Acton 2500i with Andor iDus camera). The investigated cubic R-phase NaYF4 NCs were synthesized as described in ref 13 with a dopant percentage of 2%/ 20% Er3+/Yb3+ to achieve highest UC efficiency. In hexagonal β-phase NaYF4 NCs, up to 50% of the absorbed 2478
photons take part in the upconversion process,11 while in the cubic phase the efficiency is reported to about 1 order of magnitude smaller.8 All the measurements were done at room temperature and under normal atmospheric conditions. In a first step we disperse the NCs in solution of toluene on a coverslip by spin-coating. By scanning the sample single NCs are located via detection of fluorescence with the avalanche photodiode. Because of the long lifetimes of the excited Er3+ levels, typical count rates of small NCs are in the range of a few 100 s-1. To guarantee that only a single NC is in the focus area, an additional scan with the AFM is performed. A typical scan of two well-separated particles is shown in Figure 1b. The UC fluorescence spectrum of an individual NC is then measured with the spectrometer. A typical result is plotted in Figure 2a. The integration times per spectrum were between 15 and 120 s. The excitation powers were up to 2.00 mW at the entrance pupil of the objective which corresponds to an excitation intensity of about 6.25 × 105 W cm-2. Figure 2b shows the relevant energy levels in the Yb3+/Er3+ ion system hosted by NaYF4. Also indicated are the most relevant transitions that occur: energy transfers, cross-relaxation, as well as radiative and nonradiative processes. Infrared photons are absorbed by the strong transition from the ground state 2F7/2 to the first excited state 2 F5/2 in the Yb3+ ions. In successive steps, energy transfers carry these excitations over to an Er3+ ion nearby. In the Er3+ ion nonradiative processes populate the radiating states. For a detailed discussion of these processes see ref 11. The transitions from the levels 2H9/2, 2H11/2, 4S3/2, and 4F9/2 to the ground state 4I15/2 are responsible for blue (∼410 nm), green (∼520 and ∼550 nm), and red (∼670 nm) emissions, respectively. From power-dependent measurements on small clusters of NCs (data not shown here) we find that in the excitation density regime we work in there is also a threephoton process involved in the green emission, changing the shape of this band on the lower energy side around 575 nm. We attribute this to a transition in the erbium ions from the 2 H9/2 directly to the first excited state, 2H9/2 f 4I13/2. In Figure 3 the spectra of two individual particles are presented. In both cases the excitation power is 0.25 mW (blue curves) and 1.00 mW (red curves), as measured at the entrance of the microscope objective. The insets show the corresponding Nano Lett., Vol. 9, No. 6, 2009
Figure 2. (a) Typical spectrum of a single NaYF4NC with a height of 47 nm. Emission bands corresponding to the transitions 2H9/2 f 4I15/2 (∼410 nm), 2H11/2 f 4I15/2 (∼520 nm), 4S3/2 f 4I15/2 (∼550 nm), 4F9/2 f 4I15/2 (∼660 nm), and 4F7/2 f 4I13/2 (∼700 nm) can be seen. (b) Relevant excitation and luminescence scheme in the Yb3+/Er3+ system: energy transfer (dashed), radiative (full), multiphonon processes (dotted), and cross-relaxation (curly lines).
Figure 3. Spectra of two particles with a height of 47.1 nm (a) and 5.6 nm (b): Excitation power at the entrance of the microscope objective for both particles are 0.25 mW (blue curves) and 1.00 mW (red curves), respectively. The insets show the AFM scans to determine the size of the particles. The bigger particle has a much higher emission intensity in the red.
AFM scans of the particles. The increase of the GRR with increasing power is well-known,11 but a main difference is apparent for both excitation intensities: The amount of green emission (from 500 to 600 nm) in relation to red emission (from 620 to 720 nm) is much smaller for the larger particle compared to the smaller one. This is in contradiction to the previous result reported on codoped Y2O3, where an opposite size dependence of the GRR was observed.22 To get further insight of the process, we investigate four individual NCs in more detail by means of their UC spectra as a function of their sizes. We use the height of the particle as a parameter of the particle size. As is typical in AFM studies, the height can be determined very precisely compared to the lateral extension, as the latter is a convolution of the particle’s shape with the shape of the AFM tip. In Figure 4a, the integrated total intensity of emission between 400 and 750 nm is shown as a function of the particle height h. The overall intensity increases with increasing height with a nonlinear dependence as one would Nano Lett., Vol. 9, No. 6, 2009
expect, since the volume and therefore the number of emitting ions scales with ∝h3. The GRR for the particles as a function of size is shown in Figure 4b. Green (red) emission was integrated from 500 to 600 nm (620 to 720 nm). With decreasing NC size the green share in the total emission increases and even dominates for particles smaller than ∼10 nm. In contrast to the explanation applying to the codoped Y2O3 particles,22 this demonstrates that surface-group related mechanisms do not enhance the nonradiative relaxation processes in the NaYF4 nanoparticles, even for very small sizes. We suggest that the change in the GRR is due to a size-related bottleneck for nonradiative phonon relaxation processes. An influence of phonon confinement on the optical properties of Er3+ codoped yttria NCs has been reported recently. The size confinement of the less energetic acoustic phonons gives rise to discrete modes in the density of phonon states. This leads to restricted phonon relaxation within the crystal field levels and enhanced emission from the upper 2479
Figure 4. (a) Accumulated emitted UC intensity versus the height of the particle. With increasing NC height the emission increases nonlinearly, as expected. (b) Intensity ratio of green to red emissions (GRR) versus the height. With decreasing size the green emission becomes more and more pronounced. The error made in height determination is below 1 nm. The relative error for the emission intensities is estimated to be 15% and the resulting maximal error indicated by the error bars.
crystal field levels at low temperatures23 and a change in the ET between adjacent Er3+ ions.24 The nonradiative relaxation between two electronic levels in lanthanides can take place via multi optical phonon relaxation. The related relaxation rate knr follows the energy gap law25 knr ∝ exp (-βg), where β is a specific material constant and g the reduced energy gap in units of the effective vibrational mode pωeff. It is well-known that the confinement effect of optical phonons26 has to be considered to explain the interaction with charge carriers in semiconductor nanoparticles to understand their Raman spectra. Due to the phonon confinement, the selection rule q ) 0 for the phonon quasi-momentum is lifted.27,28 This confinement effect has to be taken into account if the dimension of the particle is below 20 lattice constants. This condition is fulfilled by the smaller particles investigated here as the lattice constant of the cubic NaYF4 nanocrystals is 5.485 Å.13 Hence, we suggest that a similar modification of phonon modes strongly influences the optical phonon-mediated processes in the NaYF4 NCs. The spatial distribution of phonon mode has also to be taken into account. As the phonon mode reduces at the particle boundaries, for ions near the surface the coupling to these confined phonon modes is reduced and results in an overall reduction of the multiphonon processes in the NCs. Therefore, the higher GRR for smaller particles can be explained qualitatively: to populate the red-emitting 4F9/2 level (see Figure 2b) an energy gap of approximately 5000 cm-1 has to be bridged by multiphonon emission, while for green emission (2H11/2, 4S3/2) the gap is around 2000 cm-1. In the case of the smaller particles not enough phonons are present to efficiently populate the red-emitting level. While this phonon bottleneck can also reduce the excitation rates as well as it slows down the population of the relevant emitting levels, it should also enhance the overall quantum efficiency because of the suppression of nonradiative relaxation mediated by phonons. For confirming these conjectures, it is necessary to perform measurements on the dynamics of the UC signals on the single particle level, a problem which is to be addressed in future studies. This measurements could also include single-particle absorption measurements to 2480
determine the absolute energy efficiency.29,30 A more advanced model which predicts the modified phonon density of states and thus of the UC processes quantitatively goes beyond the scope of this paper. Such a model could be confirmed by Raman spectroscopy besides the results of the above-mentioned time-dependent and efficiency measurements. In summary, we have shown that upconversion of NaYF4NCs changes dramatically with size. We exclude that surface effects can be responsible for this observation and attribute this behavior to a change of phonon-mediated processes in the emitting ions. Both the confinement effect on the spatial distribution of phonon modes and a reduced spectral density of phonon states may contribute to the more efficient fluorescence in the green spectral range with decreasing size of the NCs. We believe that the observed size effect can open a new route to tailor optical properties of rare earth doped NCs by changing their size, similar as widely used in semiconductor quantum dots. Acknowledgment. Samples were synthesized in the group of Prof. Hans-Ulrich Gu¨del, University of Bern, whom we thank also for helpful discussion. Financial support was provided by the DFG SFB 448. L. de S. Menezes acknowledges financial support from Rede Nanofoton/CNPq (Brazilian agency) and the Alexander von Humboldt Foundation. References (1) Scheps, R. Prog. Quantum Electron. 1996, 20, 271–358. (2) Yi, G.; Lu, H.; Zhao, S.; Ge, Y.; Yang, W.; Chen, D.; Guo, L. Nano Lett. 2004, 4, 2191–2196. (3) Lim, S.; Riehn, R.; Ryu, W.; Khanarian, N.; Tung, C.; Tank, D.; Austin, R. Nano Lett. 2006, 6, 169–174. (4) Phillips, M.; Hehlen, M.; Nguyen, K.; Sheldon, J.; Cockroft, N. Upconversion phosphors: Recent advances and new applications. Physics and Chemistry of Luminescent Materials: Proceedings of the Eighth International Symposium, 2000. (5) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (6) Joubert, M. Opt. Mater. 1999, 11, 181–203. (7) Shalav, A.; Richards, B.; Trupke, T.; Kra¨mer, K.; Gu¨del, H. Appl. Phys. Lett. 2005, 86, 013505. (8) Kra¨mer, K.; Biner, D.; Frei, G.; Gu¨del, H.; Hehlen, M.; Lu¨thi, S. Chem. Mater. 2004, 16, 1244–1251. Nano Lett., Vol. 9, No. 6, 2009
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