Ultrasensitive Polarized Up-Conversion of Tm3+–Yb3+ Doped β

Letter pubs.acs.org/NanoLett

Ultrasensitive Polarized Up-Conversion of Tm3+−Yb3+ Doped β‑NaYF4 Single Nanorod Jiajia Zhou,† Gengxu Chen,‡ E Wu,‡ Gang Bi,§ Botao Wu,‡ Yu Teng,† Shifeng Zhou,† and Jianrong Qiu*,†,∥ †

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China § Institute of Information and Electronics, Zhejiang University City College, Hangzhou, 310015, China ∥ State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: Up-conversion luminescence in rare earth ions (REs) doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth. However, there have been no reports on REs doped nanoparticles to investigate their polarized energy transfer upconversion, especially for single particle. Herein, the polarized energy transfer up-conversion from REs doped fluoride nanorods is demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, for example, sharp energy level split and singlet-to-triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, are observed upon excitation with 980 nm linearly polarized laser. Furthermore, nanorods with the controllable aspect ratio and symmetry are fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by density functional theory calculations. Taking advantage of the REs based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from REs doped fluoride single nanorod or nanorod array. KEYWORDS: Polarized up-conversion, single rod, rare earth ions, β-NaYF4

U

powders and large single crystals of NaYF4 with conventional techniques though adjusting the molar ratio of the starting materials NaF and YF3 very carefully according to the appropriate phase diagrams, owing to the incongruent melt and extreme atmosphere condition.10,11 Alternatively, benefiting from the conveniences to obtain pure phase, controllable particle morphology, and the moderate reaction condition of wet chemistry method, UC from active REs sensitized by Yb3+ ions in nano- and microstructural β-NaYF4 crystals has been widely investigated.12−23 However, to the best of our knowledge there’s relatively little effort on the single nano- or

p-conversion (UC), which relies on converting photons of lower energy into photons of higher energies, is a rapidly developing field of research on rare-earth ions (REs) doped optical materials. Considerable efforts have been devoted to UC for a wide range of possible applications spanning from laser materials, nanoscale biolabels, over lighting and display technology, and NIR quantum counters to the increase of solarcell efficiency,1−7owing to the superior features of the complex energy level structure of REs, their exceptional photostability, long lifetimes of the excited electronic states, and high flexibility in choosing the dopant species and host material. The optical transitions of REs are sensitive to their local coordination, and the emission intensity of REs doped compounds strongly depends on the crystal structure. Worthy of mention is the fact that β-NaYF4 appears to be the most efficient material for UC.8,9 However, it is hard to obtain the pure hexagonal © 2013 American Chemical Society

Received: March 5, 2013 Revised: April 7, 2013 Published: April 23, 2013 2241

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microparticle spectroscopy of couple ion-based polarized energy transfer (ET) UC, while polarized UC of single rare earth ion has received attention in very recent years.24,25 Micropolarized spectroscopy that focuses on the single particle fluorescence has attracted great attention in semiconductor system, not only for their fundamental scientific importance but also because of their potential applications, ranging from polarized light-emitting diodes (LED) and flat panel displays to photodetectors.26−29 For example, sizedependence of polarization anisotropy was observed in InAs quantumwires, CdSe nanobelts, GaN nanorods, InP nanowires, and so forth, and different polarization anisotropy theories including quantum size effect and dielectric confinement of optical electric field were proposed.27,30−32 This intrinsic anisotropy was used to create polarization sensitive nanoscaled photodetectors, which are useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and highresolution detectors.27 In contrast to single emission peaks observed for semiconductor system, the REs-doped UC particles generally show a distinct set of sharp emission peaks arising from f−f orbital electronic transitions. The multiplepeak patterns should provide spectroscopic fingerprints, which are particularly useful for accurate interpretation in the event of overlapping emission spectra, thus possibly lending themselves to detailed theoretical analysis. In particular, there exists the interesting possibility of using well-known semiconductor single particle spectroscopy techniques to probe REs doped particle isolated in room temperature micro/nanoenvironments. Here we demonstrate micropolarized optical detection of UC based on REs doped inorganic dielectric system, namely, Tm3+−Yb3+ ion couple active β-NaYF4 single nanorod. Unique luminescent phenomena, for example, sharp energy level split and singlet-to-triplet transitions at room temperature (RT), multiple discrete fluorescence intensity periodic variation with polarized direction have been observed upon excitation with 980 nm linearly polarized laser. Simultaneously, based on the comparative experiments with samples with different aspect ratio/symmetry, possible mechanism of UC polarization anisotropy from REs doped β-NaYF4 and the factors affecting the polarization degree have been discussed. β-NaYF4 rods were synthesized using hydrothermal method (for details see the Supporting Information) at initial doping concentrations of 2 mol % Tm3+−18 mol % Yb3+. The products were composed of uniformly rods with diameters of about 170 nm and lengths of about 1550 nm, as shown by scanning electron microscopy (SEM) photos in Figure 1a,b. Highresolution transmission electron microscopy (HRTEM) image with crystal lattice shows interplanar spacings of 0.290 and 0.297 nm corresponding to the (101) and (110) planes of βNaYF4, respectively (Figure 1c). X-ray diffraction (XRD) patterns show representative reflections for a hexagonal phase β-NaYF4 (PDF no. 16-0334) with space group P63/m (Figure 1d).33 Such highly crystalline rods are resulted from the preferential growth along the [001] direction (c-axis) identical with the long axis of the rods.33 A home-built scanning confocal optical microscope system was employed for microscopic studies of β-NaYF4/Tm3+−Yb3+ particles spin-coated on a glass slide. The schematic diagram of the scanning confocal microscope system is shown in Supporting Information Figure S1. The 980 nm linearly polarized laser was used as the excitation source, which passed through a half-wave-plate and a high reflective mirror at 980

Figure 1. (a,b) SEM photos, (c) HRTEM photo, and (d) XRD pattern of β-NaYF4/Tm3+−Yb3+ rods.

nm, and then was focused with a high numerical aperture microscope objective lens (NA = 1.40, 100×, oil immersion) to a spot diameter of about 200 nm. Emission spectra were collected by the same microscope objective, sent to the detection part after spatial and spectral filtering, and then detected by a spectrometer equipped with a liquid nitrogen cooled CCD detector. No change in the emission intensity was observed after several hours of continuous illumination showing absolute photostability of β-NaYF4/Tm3+−Yb3+ particle. We began with the spectral scanning of Tm3+−Yb3+ UC from a single rod upon 100 mW linealy polarized laser excitation at 980 nm. A number of sharp (down to 10 nm) peaks exactly match the transitions of Tm3+: 1D2→3F4, 1G4→3H6, 1G4→3F4, 3 F3→3H6, 1D2→3F3, 1G4→3H5, and 3H4→3H6, as shown in Figure 2a. Among the spectrum, the sharp peak at 768 nm is carefully identified by comparison between directly excitation of Tm3+ to 1G4 and 3F3 levels (Supporting Information Figure S2), and hence it is attributed to 1G4→3H5 transition rather than a split of 3H4→3H6 transition. Besides, it is found that all the transition intensities are comparable, while 3H4→3H6 transition owns 100 times enhancement comparative to other transitions for powder state measurement (Supporting Information Figure S3a−c). This could be understood by the excitation power dependent intensity study. As shown in Figure 2b, the power dependencies do not follow simple power law (I ∝ Pn) for multiphoton UC, where I is the emission intensity, P is the excitation laser power, and n is the number of photons.34 The luminescence intensity of 3F3→3H6, 1G4→3F4, 1D2→3F4 transitions increases linearly with increasing excitation power at first and then becomes saturated at high excitation power, while the luminescence intensity of 3H4→3H6 transition almost remains constant. These imply that 3H4→3H6 transition is in the saturation regime while the rest are in the transition state from linear increase to saturation when the excitation power approaches 100 mW. In limitation of detecting the UC from single rod at very low excitation power, we probed the linear regime for analysis of multiphoton UC mechanism in powder state (Supporting Information Figure S3d). The fitted slopes reveal that the excited states 1G4 and 3H4 are populated via three and two-photon processes, respectively, which are schematically shown by ET (1,2,3) in Figure 2c.35 It should be mentioned that the 3F3→3H6 transition also involves threephoton process owing to the cross relaxation of Tm3+/1G4 + 2242

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Figure 2. (a) Emission spectrum of UC from a β-NaYF4/Tm3+−Yb3+ single rod upon excitation with 980 nm linearly polarized diode laser (100 mW). (b) Normalized peak intensities of UC from β-NaYF4/Tm3+−Yb3+ single rod as a function of excitation power at 980 nm. The real peak intensities were obtained based on peak deconvolution method. The peak intensity of each transition under different excitation power was normalized through dividing by the luminescence intensity in the case of minimum power excitation. (c) Energy level diagram of the energy transfer UC from Yb3+ to Tm3+, accompany with the crystal field interaction related schematic splitting levels of Tm3+.

Figure 3. Emission spectra of UC from β-NaYF4/Tm3+−Yb3+ single rod in the transitions of Tm3+: (a) 1G4→3F4, (b) 3F3→3H6, 1D2→3F3, (c) 1 G4→3H5, and (d) 3H4→3H6, respectively. (e−h) The dependence of the corresponding spectra on the emission polarization angle (φem).

Tm3+/3F4 →Tm3+/3H4 + Tm3+/3F2 when the concentration of Tm3+ up to 2 mol %.36,37 Besides, the crystal field splitting energy levels 1D2 and 1G4 of Tm3+, and singlet-to-triplet transitions including 1D2→3F3, 1G4→3H5 which are normally assumed weak since they are often at least partially forbidden, are clearly observed at room temperature (RT). They are unable to be observed for the powder state in ordinary detecting system even at 10K (Supporting Information Figure S3a,b) and thus exemplifies the ultrasensitive spectroscopic fingerprints effect of REs based micropolarized UC at RT. Inspired by these crystal field involved optical transitions in micropolarized UC condition, we further considered that the transition dipoles between any states of Γm↔Γn symmetry

representations are oriented in relation to the axes of the local coordination polyhedron of luminescence center based on the transition selection rule, which implies the occurrence of some polarization dependent UC phenomena.24 In an attempt to probe the polarization dependent UC phenomena, we added a half-wave-plate before emission collection in the system. By rotating the half-wave-plate before the polarizer, polarized UC emission was checked as a function of the polarization angle. We defined the polarization state with most intensive emission as 0°. In Figure 3a−d, polarization angle at 90 and 180° were first set to detect the UC emission spectra of Tm3+−Yb3+ couple upon excitation with 980 nm linearly polarized diode laser, where intensity variation indicates 2243

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Figure 4. (a) SEM photo of β-NaYF4/Tm3+−Yb3+ rods with 30 mol % Gd3+ doping. (b) SEM photo of β-NaYF4/Tm3+−Yb3+ disks. Polar plots of the UC peak intensity as a function of the emission polarization angle, which is corresponding to the transitions of Tm3+: (c) 1G4→3F4, (d) 3 F3→3H6, (e) 1D2→3F3, (f) 1G4→3H5, and (g) 3H4→3H6. The peak intensities were obtained based on peak deconvolution method. (h) The composite effect of two orthogonal linearly polarized emissions.

anisotropy. However, different polarization dependence appears in case of Gd3+ doping, where two kinds of intensity variation states could be observed for each transition. This may be due to the composite effect of Tm3+ ions located in two sites with different local symmetry. To verify the local symmetry changing of Tm3+ after Gd3+ doping, we performed first principles calculations based on density functional theory (DFT) to simulate the crystal structure of β-NaYF4 (Figure 5a) and the bonding structure

the polarization anisotropy of these REs doped UC rods. Subsequently, fine rotation of φem was necessary to find the periodic dependence of emission intensity versus φem at each peak in Figure 3e−h. What’s surprising here is the high polarization degree (ρ = (Imax − Imin)/(Imax + Imin), Imax and Imin present the maxium and minimum emission intensities, respectivly) of 1G4→3H5, 3H4→3H6 transitions, which approaches to be 1. Besides, the sharp peak of 1G4→3H5 transition vanishes while others approach to the most intensive state; in short, it owns counter-cyclical dependence with polarization. To identify the local symmetry in β-NaYF4 crystal field as the dominant role that is responsible for the polarization anisotropy, the β-NaYF 4 rods with 30 mol % Y 3 + (RY3+, 9‑coordination = 1.08 Å) ion-radius increscent replacement by Gd3+ (RGd3+, 9‑coordination = 1.11 Å) was synthesized (Figure 4a).38,39 In the micropolarized spectroscopy of semiconductor system, aspect ratio is a critical factor which affects the polarization property of particle.40 In order to rule out the effect of aspect ratio coming along with the extra doping,18 comparative β-NaYF4 disks with only aspect ratio variation from ∼9 to ∼0.6 was introduced (Figure 1b and Figure 4b). Polar plots of the UC peak intensity as a function of the emission polarization angle are shown in Figure 4c−g, which is corresponding to the transitions of Tm3+: (c) 1G4→3F4, (d) 3 F3→3H6, (e) 1D2→3F3, (f) 1G4→3H5, and (g) 3H4→3H6. All of the peak intensities in β-NaYF4 single rod and β-NaYF4 single disk could be fitted well with the I = A(1 + 2 cos2 φem) + B function.41 The perfect splayed fitting contours of (f) 1 G4→3H5, (g) 3H4→3H6 transitions indicate their linearly polarized characteristic, and the orthogonal orientation for (f) 1 G4→3H5 transition relative to others’ indicates its countercyclical polarization anisotropy, which is in accordance with Figure 3. Relatively, the fitting contours of (c) 1G4→3F4, (d) 3 F3→3H6, (e) 1D2→3F3 transitions, which are without jugular full shrink, could be regarded as the composite effect of two orthogonal linearly polarized emissions, as shown in Figure 3h.42 For the single rod and single disk, almost the same fitting contours in each transitions could be found, which implies the neglectable effect of the aspect ratio factor on the polarization

Figure 5. (a) Schematic presentation of β-NaREF4 structure. (b−d) Tm3+−F− bonding structure in β-NaYF4 before (b) and after (c) Gd3+ doping.

between Tm3+ and F− (Figure 5b−d). Given a detailed insight on the crystall structure of β-NaYF4, there are two cation site for REs: one is fully occupied by RE3+, whereas the other shows occupational disorder involving a 1:1 ratio of Na+ and RE3+, which are coordinated by 9 F− ions forming tricapped trigonal prisms with crystallographic C3h symmetry.43 From Figure 4b,c and the listed table (d), Tm3+−F− bonding length and angle changing are clearly observed between with and without Gd3+ doping, which may magnify the local symmetry difference in two REs sites. However, for quantitative mechanism analysis, more systematic experiments are necessary. These findings indicate that various RE-doped nanoparticles can be used for 2244

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precise bioimaging by detecting their polarization dependent UC emissions. In conclusion, Tm3+−Yb3+ codoped β-NaYF4 with different aspect ratios and symmetries were synthsized for new insight of polarized up-conversion. Unique luminescent phenomena, for example, sharp energy level split and singlet to triplet transitions at room temperature (RT), multiple discrete fluorescence intensity periodic variation with polarized direction have been observed upon excitation with 980 nm linearly polarized laser. Furthermore, the rods with controllable aspect ratio and symmetry were fabricated for analysis of the mechanism of polarization anisotropy from rare earth ions doped β-NaYF4. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by DFT calculations. Because of the abundant energy level structures and Laporte-forbidden f−f electronic transitions of REs, these sharp emission peaks can be easily distinguished compared to the broad emission of semiconductor. These important findings lead to the potential application in microscopic multi-information transportation system by using polarized up-conversion single rod or rod array, which should have more transinformation content and higher identifiability than that of photoluminescent RE complex films.44

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, Figure S1, Figure S2, and Figure S3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 50872123, 50802083, 51072054, and 51102209), National Basic Research Program of China (2011CB808100). G.B. acknowledges the financial support from National Natural Science Foundation of China (Grant 61275108) and Natural Science Foundation of Zhejiang Province in China (Grant Y111049).

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