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We report intense upconversion photoluminescence (PL) in colloidal LiYF4:Er3+ nanocrystals under excitation with telecom-wavelength at 1490 nm. The intensities of two- and three-photon anti-Stokes upconversion PL bands are higher than or comparable to that of the Stokes emission under excitation with low power density in the range of 5–120 W/cm2. The quantum yield of the upconversion PL was measured to be as high as ~1.2±0.1%, which is almost 4 times higher than the highest upconversion PL quantum yield reported up to date for lanthanide-doped nanocrystals in 100 nm sized hexagonal NaYF4:Yb3+20%, Er3+2% using excitation at ~980 nm. Power dependence study revealed that the intensities of all PL bands have linear dependence on the excitation power density, which was explained by saturation effects in the intermediate energy states.
Lanthanide-doped nanocrystals can convert the absorbed light with lower energy, usually in the near infrared (NIR) range, into the photoluminescence (PL) emission with higher energy in the ultraviolet, visible, or NIR ranges.1–3 Because of the existence of real intermediate energy levels in lanthanide ions,2 this upconversion process is much more efficient than the conventional multiphoton absorption induced fluorescence where the intermediate levels are virtual.4 An efficient energy upconversion in nanocrystals is highly desirable for numerous applications, such as displays,5,6 PL imaging,7–14 photodynamic therapy,15–19 etc. Lanthanide-doped upconversion nanocrystals are also excellent candidates for improving solar cell efficiency by harvesting energy in the NIR range due to their ladder-like system of energy levels.20–22
Despite recent progresses in the synthesis and design of lanthanide-doped nanocrystals,23–27 it is still of great challenge to obtain an efficient upconversion PL since the intensity of anti-Stokes upconversion PL is usually much lower than that of the Stokes emission.3 Furthermore, upconversion PL generally show a higher power dependence on the excitation intensity, thus requiring an intense source to generate it. Fluoride nanocrystals are known to possess host lattice of low phonon energy, which results in decreased nonradiative relaxations and, in turn, in relatively high upconversion PL efficiency.28–30 Although optical properties of trivalent lanthanide ions have been widely studied in several fluoride matrices in the bulk, investigations on their corresponding dispersible nanocrystals are mostly restricted to NaYF4.31–35 Nanocrystals of NaYF4 (10–100 nm sized) codoped with Yb3+ and Er3+ ions, are now considered to be among the most efficient upconversion nanocrystals, exhibiting visible emission under excitation at ~ 980 nm, with the upconversion PL quantum yield of ~0.005–0.3%.36–38 An efficient conversion of telecomm-wavelength (1.3–1.55 μm) into visible or NIR light is of particular importance due to their potential uses in frequency upconversion lasing,39,40 photovoltaics,20,41 IR quantum counter,1 detection of surface plasmons,42 displays and sensors.43 The Er3+ ions are well suitable for these endeavors owing to their absorption at ~1500 nm, which corresponds to the 4I13/2 energy level.1 However, a lack of efficient sensitizers (like Yb3+ ions at ~980 nm) and a suitable host matrix for upconversion of ~1.5 μm has limited the study of this technologically important process.
The LiYF4 can be considered as a suitable fluoride host lattice for the upconversion processes under excitation at ~1.5 μm, since Er3+ ions have long-lived intermediate states in this low-phonon lattice (e.g., 4I13/2 with a characteristic lifetime of ~10 ms).44–46 These intermediate excited energy states with long lifetime can act as energy reservoirs when populating higher energy levels. In this case, upconversion might work by a more effective one-photon mechanism, leading to efficient upconversion PL generation under lower power excitation.47–49 Despite these unique features, investigations of the upconversion PL in Er3+-doped LiYF4 under excitation at ~1.5 μm are surprisingly limited. Most of the studies on Er3+-doped LiYF4 are devoted to laser output at ~2.7–3.0 μm or green/red upconversion under excitation at 980 nm.44–46 Using LiYF4 single crystal doped with Er3+, the upconversion laser action at cryogenic temperatures in the visible range (~0.55 μm) under excitation at ~1.5 μm was reported two decades ago,50,51 but was not noticeably utilized since then. On the other hand, investigations of lanthanide-doped colloidal nanocrystals of LiYF4 are extremely limited and only involve upconversion of ~980 nm excitation to the visible range.52–55
In this work, we report intense upconversion PL in colloidal LiYF4:Er3+ nanocrystals using excitation at the telecom-wavelength of 1490 nm. Under excitation with low power density, the intensities of the anti-Stokes PL emission bands (three-photon induced PL bands peaked at 550 and 670 nm and two-photon induced emission with maximum at ~970 nm) are comparable to and higher than the one-photon induced Stokes emission at ~1.5 μm.
The colloidal LiYF4:Er3+ nanocrystals were synthesized via modification from a thermolysis procedure.7 As one can see in Figure 1A, the resulting nanocrystals are of spindle shape and monodisperse. They have a uniform aspect ratio (length/breadth) of about 1.9; the average length was evaluated to be about 85 nm. A high-resolution TEM (HRTEM) image (Supporting Information, Figure S1) distinguishes the lattice arrangement of the atoms in one individual nanoparticle, indicating high crystallinity of the prepared nanoparticles. A selected area electron diffraction pattern in Figure 1B demonstrates the formation of tetragonal phase of the nanocrystals and is indexed to the corresponding (hkl) planes of the standard JCPDS 85-0806 LiYF4 structure. The prepared LiYF4:Er3+ nanocrystals form transparent and stable colloidal suspension in chloroform (Figure 1C). These colloidal nanocrystals produce upconversion PL under excitation at 1490 nm with unfocused laser beam of 4 W/cm2, which appears bright green to the naked eye, as one can see in Figure 1D (See also Supporting Information, Figure S2).
The PL spectrum of colloidal LiYF4 nanocrystals doped with 10% Er3+ ions under laser excitation at 1490 nm are shown in Figure 2. The shape of PL spectra remains almost the same in a wide range of excitation power density due to a close to linear dependence of all PL bands (Figure 3). Four upconversion PL bands are clearly resolved; they have maxima at 550, 670, 800, and 970 nm, which correspond to transitions from the 2H11/2/4S3/2, 4F9/2, 4I9/2, and 4I11/2 to the ground 4I15/2 state of Er3+, respectively.20 The long wavelength edge of ~800 nm band might also be contributed by the 4S3/2→4I13/2 transition (Figure 4). In addition to the upconversion PL, the Stokes radiation at 1500 nm was also displayed which corresponds to the transition 4I13/2 →4I15/2 of the Er3+ ions. The intensities of the anti-Stokes emission generated by multiphoton processes are usually tens times lower than those of Stokes emission for low power density excitation, due to their inefficient generation mechanism. For example, three-photon blue emissions at 480 nm in hexagonal bulk NaYF4: 0.3% Tm3+, 25% Yb3+ crystals (the most efficient blue emitter) is about 46 times less than the Stokes emission under 980 nm excitation of 80 W/cm2.38 It is striking that the anti-Stokes, two-photon induced emission at ~975 nm is more intense than the Stokes emission at 1500 nm, while the three-photon induced upconversion PL at 550 and 670 nm are comparable to that, suggesting an extremely efficient upconversion mechanism here. An upconversion quantum yield (UCQY) were measured to be about 0.13±0.02% and 0.07±0.02% for three-photon green and red upconversion PL, while for the two-photon NIR emission at 800 and 970 nm they were evaluated to be about 0.05±0.01% and 0.95±0.05% under 1490 nm excitation of 150 W/cm2 (Supporting Information, Part II). The total quantum yield for the visible and the NIR emission is 1.2±0.1%, which is almost 4 times higher than the highest UCQY reported in the literature for the upconversion nanocrystals (100 nm sized hexagonal NaYF4:20%Yb3+/2%Er3+ nanocrystals under 980 nm excitation with 150 W/cm2).36
The effect of the Er3+ ion concentration on the upconversion emission bands was also investigated (Figure 2, inset). As one can see, the intensity of upconversion PL increases slightly with the increase in the content of Er3+ ions from 5% to 20%, and remains nearly unchanged for Er3+ concentrations of 20% and 30% ions, due to the concentration quenching effect.1 The PL spectra normalized to the Er3+ content show that the nanocrystal LiYF4:Er3+ with 10% Er3+ has the highest upconversion efficiency (Supporting Information, Figure S6). It should be noted that the ratio of the intensities of the green band at ~550 nm and the red one at ~670 nm (green-to-red ratio, GRR) is tuned by the Er3+ ion concentration; an increase in the content of Er3+ ions induces a decrease in GRR. This feature enables us to manipulate the color output of the colloidal nanocrystals LiYF4:Er3+ by adjusting the Er3+ ion concentration.
Figure 3 shows the measured pump power dependences of all PL bands for the colloidal LiYF4 nanocrystals doped with 10% of Er3+ ions. The number of photons required to populate the upper emitting state can be obtained from the equation IPL Pn, where IPL is the PL intensity, P is the pump laser power, and n is the number of the laser photons.47–49 It is known that when upconversion PL is excited by sequential absorption and energy transfer upconversion of n photons, its dependence on the absorbed pump power P decreases from Pn down to P1 as long as the upconversion rate well exceeds the decay rate for the intermediate states. This applies for the highest energy electronic state that is excited through upconversion, while lower-lying states (excited by upconversion with m photons, where m<n) were predicted to have a power dependence even less than P1.48,49
As shown in Figure 3, the dependence of intensity of the upconversion PL bands on the laser power for low excitation power density range of 5–120 W/cm2 is overall close to linear, with lesser slope for PL arising from lower energy states. This result explains the high intensity of anti-Stokes emission versus the Stokes one (Figure 2), since an inefficient multiphoton upconversion mechanism has been switched to an efficient one-photon-like mechanism. The power dependences of the PL emission were also measured for excitation power density lower than 5 W/cm2 (range of ~0.3–4.5 W/cm2; Supporting Information, Figure S7). It was found that although the slope value for power dependence of Stokes emission at 1500 nm remains the same as in Figure 3, those for three-photon (peaks at 550 and 669 nm) and two-photon (850 and 1000 nm) emissions increase as compared to the corresponding power dependencies shown in Figure 3. It should be noted that a power density as high as 150 W/cm2 had to be used in 100 nm sized hexagonal NaYF4:20%Yb3+/2%Er3+ nanocrystals to achieve the onset saturation under 980 nm excitation.36 In our case, saturation occurs at much lower power density, which also illustrates the high efficiency of upconversion in the colloidal LiYF4:Er3+ nanocrystals under 1490 nm excitation. It is worth noting that the upconversion emission of the sample under study was visible to the naked eye at the excitation power as low as ~0.3 W/cm2. To further clarify the upconversion mechanism, we have measured the PL decays at 1535, 1005, 548, and 667 nm (which correspond to the 4I13/2, 4I11/2, 4S3/2, and 4F9/2 states of Er3+ ions) (Supporting information, Figures S8–S11). The average lifetimes of 4I13/2 and 4I11/2 states were evaluated to be about 5.4–10 ms and 1.5–2.4 ms in colloidal LiYF4:Er3+ nanocrystals, respectively; while lifetimes of ~56–136 μs and ~63–120 μs were measured for emission from the 4S3/2, and 4F9/2 states under direct excitation using 532 nm. These short lifetimes of emission from the upper states under direct excitation, along with long lifetimes of the intermediate states, suggest that the long-lived intermediate energy states are important to realize high efficiency of upconversion owing to their strong reservoir capability and low energy losses.47
Figure 4 shows the energy levels of Er3+ ions as well as the proposed upconversion pathways under laser excitation with 1490 nm.56 First, laser photons directly excite the Er3+ ions from the ground 4I15/2 state to the long-lived 4I13/2 state which results in Stokes emission at ~1500 nm. Different processes can then lead to the population of higher excited Er3+ states, which rely either on multistep excited state absorption or energy transfer upconversion mechanisms. Since the concentration of Er3+ ions in all synthesized LiYF4 nanocrystals is high enough, it is plausible that energy transfer upconversion is the dominant mechanism due to the strong multipolar ion-ion interactions induced by the short ion-ion distance.1 With the assistance of lattice phonons, the Er3+ ion at the 4I13/2 state can be consecutively promoted to the 4I9/2 and the 2H11/2/4S3/2 states via energy transfers from Er3+ ions at the excited 4I13/2 state. Radiative decay from the 2H11/2/4S3/2 state to the ground state generates an intense three-photon green emission with maxima at ~520/550 nm. Subsequently, nonradiative relaxations from this state populate the 4F9/2 state, from which strong three-photon red emissions with peak at ~670 nm are produced. Similarly, the radiative decay from the 4I9/2 to the ground state generates a weak two-photon emission at ~800 nm, while nonradiative relaxation from this state populates the 4I11/2 state, from which a two-photon radiation with maximum at 970 nm is emitted. We would like to emphasize that the radiative decay from 4I11/2 to 4I13/2 can generate emission at ~2.7–3 μm, which is known to be utilized for laser output in bulk crystals of LiYF4:Er3+.44–46 The 2.7–3 μm range is beyond the detection range of our spectroscopy setup, but we assume that the intensity of the 4I11/2→4I13/2 PL in LiYF4:Er3+ nanocrystals can be estimated from the branch ratio of the 4I11/2→4I13/2 transition in the LiYF4 crystal, which was found to be about 2/3 times the intensity of the 970 nm peaked emission (4I11/2 →4I15/2 transition).45 The cross relaxation process 2H11/2/4S3/2+4I11/2 → 24F9/2 (Figure 4) might play an important role in tuning the GRR.57 An increase in the Er3+ ion concentration enforces the Er3+-Er3+ interaction due to the shorter ion-ion distance, and, thus, leads to higher cross relaxation rates. The increased cross relaxation rates will cause a decrease in the GRR with the increase of Er3+ ions, which is in good agreement with the observation shown in the inset of Figure 2. It should be noted that since the lifetime in the intermediate states of Er3+ ions is long enough (See Supporting information, Figures S8 and S9), upconversion rates in these states might be able to readily exceed their radiative decay rates and induce serious saturation effects even under low energy light excitation.47 This important feature causes a change of the multi-photon mechanism of population of the upper energy states into one-photon-like mechanism, thereby leading to intense upconversion PL in colloidal LiYF4:Er3+ nanocrystals (Figures 2 and and33).
To summarize, we demonstrate highly efficient upconversion PL in colloidal LiYF4:Er3+ nanocrystals under excitation with telecom-wavelength at 1490 nm. The intensities of two- (970 nm peak) and three-photon (maxima at ~550 and ~670 nm) anti-Stokes upconversion PL bands are higher than or comparable to that of the Stokes emission (at ~1500 nm) under low power excitation. The total upconversion PL quantum yield under excitation with 1490 nm was measured to be as high as 1.2±0.1%, which is almost 4 times higher than the UCQY for the most efficient upconversion nanocrystals reported up to date (100 nm sized hexagonal NaYF4:20%Yb3+/2%Er3+ nanocrystals under excitation at 980 nm). In addition, the intensities of all PL bands were shown to depend linearly on the excitation power in low power density range (5–120 W/cm2), which was explained by saturation effects in the intermediate energy states. This work might evoke wide interests in biophotonic and photonic applications of these nanocrystals.
Nanocrystals of LiYF4 doped with 5, 10, 15, 20, and 30% of Er3+ ions were synthesized by thermal decomposition of lanthanide and lithium trifluoroacetate precursors in the presence of oleic acid coordinating ligands and noncoordinating 1-octadecene solvent molecules. All chemicals used in the synthesis were purchased from Sigma-Aldrich and used as received. In the case of LiYF4:Er3+ 10% nanocrystals, 0.166g (2.25 mmol) Li2CO3, 0.048 g Er2O3 (0.125 mmol), 0.254 g Y2O3 (1.125 mmol) were dissolved in 50% concentrated trifluoroacetic acid at 95 °C in a three-necked 100 mL flask. Then, the solutions were evaporated to dryness under the argon gas purge. Next, 30 mL oleic acid (90%, technical grade) and 30 mL 1-octadecene (90%, technical grade) were added into the three-necked flask. The resulting solution was then heated at 120°C with magnetic stirring for 30 minutes to remove water and oxygen. The brown solution was then heated to 330 °C at a rate of about 8 °C per min under argon gas protection, and kept at this temperature under vigorous stirring for about 1 h. A syringe needle was used to let the argon gas out during the synthesis. The mixture was cooled to room temperature and precipitated by acetone in an ultrasonic bath and collected by centrifugation at 11000 rpm for 10 min. The precipitate was washed with ethanol for several times, and the nanocrystals were dispersed in 10 mL of chloroform for further characterizations.
The size and morphology of the nanocrystals were characterized by TEM microscopy using a JEM-2010 microscope at an acceleration voltage of 200 KV. Upconversion PL spectra in the range of 480–1300 nm were recorded using an EPP 2000 fiber spectrofluorometer (Stellar Net Inc.). Synchronously, a SPEX 270M spectrometer (Jobin Yvon), equipped with an InGaAs TE-cooled photodiode (Electro- Optical Systems, Inc.), was used to record PL spectra in the range of 900–1700 nm. The two PL spectra were matched at the overlapped 970 nm emissions to present the whole spectra of 480–1700 nm. All PL spectra have been corrected for the spectral sensitivity of the spectral systems using as a reference the black body emission of a tungsten bulb with color temperature of about 2900 K.58 Laser excitation of 971 nm or 1490 nm was provided by a tunable optical parametric oscillator (OPO, Levante Emerald, APE, Germany) pumped by the second harmonic of Nd:YVO4 laser (High Q Laser, Australia) emitting at 532 nm with a pulse width of 6 ps and a repetition rate of 76 MHz. The emission signal of the sample in the cuvette was collected at 90° relative to the excitation light. Absorption spectra of nanocrystals colloidal suspension were acquired using a Shimadzu 3600 UV–Visible-NIR spectrophotometer. The PL decay profiles at 1500 nm and 1005 nm were recorded at the Infinium oscilloscope (Hewlett-Packard) coupled to a thermoelectric cooled NIR photomultiplier tube (Hamamatsu, H10330-45 NIR-PMT) which was attached to the second output of the SPEX 270M spectrometer. The laser diode from QPhotonics operating at 975 nm was used in a pulsed mode as an excitation source. The decays of the emissions at 548 nm and at 667 nm were recorded at the Infinium oscilloscope (Hewlett-Packard) coupled to the PMT of Fluorolog-3.11 Jobin Yvon spectrofluorometer. A second harmonic (532 nm) from a nanosecond pulsed Nd:YAG laser (Lotis TII, Belarus) operating at 20 Hz was used as the excitation source in this case. Photographic images of UC nanocrystals colloidal were taken by a digital camera (Lumix DMC-Fx520, Japan) without any filter or image processing.
This work was supported by grants from the National Institutes of Health (R01CA119358 and RO1CA104492), the Swedish Energy Agency (project 32076-1), and the John R. Oishei Foundation. We thank Drs. A. Kuzmin and G. He for useful discussions.
Supporting Information Available: HRTEM image of single LiYF4 nanocrystal doped with 10% of Er3+ ions; photographic images of the upconversion PL under excitation at 1490 nm with unfocused laser beam; details of the upconversion quantum yield measurement; dependences of PL intensity on power density in the range of ~0.3–4.5 W/cm2; PL spectra and decays for LiYF4 nanocrystals doped with different concentrations of Er3+ ions (5%, 10%, 15%, 20%, 30%). This material is available free of charge via the Internet at http://pubs.acs.org.