Polarization-Modified Upconversion ... - ACS Publications

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The Journal of Physical Chemistry

Polarization-modified Upconversion Luminescence in Er-Doped Single-Crystal Perovskite PbTiO3 Nanofibers Siyu Gong†, Ming Li†, Zhaohui Ren*†, Xin Yang†, Xiang Li†, Ge Shen†, Gaorong Han*† †

State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Cyrus

Tang Center for Sensor Materials and Application, Zhejiang University, Hangzhou P.R. China

ACS Paragon Plus Environment

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ABSTRACT: Understanding the energy transition which influenced by doping ions and host materials in the upconversion (UC) physical processes is of vital importance for further optimizing performance and extending applications of UC materials. In this work, we have selected 4 % Er-doped perovskite PbTiO3 (PTO) nanofibers as a model system to explore the effects of tetragonality and polarization on UC photoluminescence (PL) properties. By means of in-situ X-ray diffraction, the tetragonality and polarization of these nanofibers have been determined to gradually decrease with an increasing of the temperature from 50 K to 300 K, leading to an obvious enhancements in UC green band emission of 523 nm (about 43 times) and red band emission of 656 nm (about 8 times), in contrast to the decreased green and red UC intensities in Er-doped BaTiO3 or PTO particles. Moreover, the significant enhancement in the intensity ratio of green to red band has been achieved from 0.17 to 0.86, indicating that the emission enhancement is highly wavelengthdependent. On the basis of in-situ UC decay curves from 50 K to 300 K, the UC lifetimes of 4S3/2 and 4F9/2 level have been derived to be 128.04±0.47 µs and 278.10±1.07 µs at 50 K, and the values are kept basically as the temperature increased. The observed UC phenomena in Er-doped perovskite PTO nanofibers can be ascribed to an assisted effect of the low-energy E(1TO) phonon on the UC process. Such phonon energy can be easily tailored by the tetragonality and polarization and thus a modification of UC emissions in Er-doped PTO nanofibers has been achieved. The findings in this work could provide new insights into the understanding to UC process in perovskite oxides and offer an opportunity to tune UC emission by an external field, such as electric field in addition to temperature. KEYWORDS: perovskite, upconversion, tetragonality, polarization, phonon


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INTRODUCTION

In recent years, upconversion (UC) materials doped with rare-earth (RE) ions have attracted intense interest due to their outstanding properties1,2 and practical applications, such as all-solid compact lasers, three-dimensional displays, UC phosphors, photovoltaic devices, solar cells and biomedical imaging.2-9 To further optimize the performance and extend the applications of UC materials, a thorough understanding the energy transition determined by doping ions and host materials during the UC physical processes is highly desirable. Generally, the color and efficiency of the UC materials can be tailored by changing the doping ions and/or host materials. For instance, the four-color emissions nanocrystals without the use of any color filter have been realized by changing the doping ions (Tm3+, Ho3+, Er3+, and Yb3+), respectively.10 To obtain the high efficiency multicolor UC emission, Yb3+ ions is intentionally added to improve the absorbing excitation radiation process in lanthanide-doped nanocrystals.11 Furthermore, increasing the dopant concentration leads to a relative increase in the UC emission intensity of the UC materials.6,12,13 On the other hand, the host material which provides the crystal-field around trivalent RE ions in matrices is also essential in obtaining high UC efficiency and controllable UC emission via influencing the 4f-4f transition probabilities of the dopant ions.11,14 Perovskite oxides are continuously being explored as host materials due to their excellent physical properties, low lattice phonon energies and adjustable crystal structure.15-24 Considerable efforts have been devoted to introducing RE ions (e.g. Eu3+, Tb3+, Pr3+ and Er3+) into perovskite oxide host matrix to fabricate and design UC materials or optical devices.18-24 However, the previous studies have mostly focused on the synthesis method and characterization on UC PL properties of RE ions doped perovskite oxides. Only a few researches were designed to investigate the effect of host matrix on UC photoluminescence (PL) properties, including the


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crystal-field effects caused by different perovsktie oxide host matrix and different site substitutions of dopant RE ions.24-29 As discussed, the effect of crystal-field on the UC emission properties can be ascribed to the various symmetries around dopant ions, that is, low-symmetry hosts typically exert a crystal-field containing more uneven components around the dopant RE ions which enhance 4f-4f transition probabilities of the dopant RE ions,14 leading to an increase in the UC efficiency. In particular, a low voltage electric field induced enhancement and modification of UC PL has been designed and achieved in Yb/Er co-doped BaTiO3 thin film, which is on the basis of the tunable structure symmetry and lattice distortion of perovskite ferroelectric oxides with an external electric field.29 Nevertheless, the effects of tetragonality and polarization on UC PL properties in perovskite oxide host matrix remain unclear. As a typical perovskite ferroelectric oxide,15,16 the high tetragonality and spontaneous polarization of PbTiO3 (PTO) could be changed with the temperatures and thus the crystal field.30 In our recent work, single-crystal PTO nanofibers has been prepared and determined to grow along [001] (the c axis),31 exhibiting obvious tetragonality and spontaneous polarization along this axis31,32. When doped with Er3+ ions, the nanofibers demonstrated the typical UC emission of green and red bands.28 Furthermore, it was revealed that the doping Er3+ ions probably occupy Ti4+ sites in Er-doped perovskite PTO nanofibers,28 which makes the crystalfield around the Er3+ ions in such an ideal system more easily tailored. Hence, it is of great interest to take advantage of the properties of PTO host nanofiber to modulate the UC PL emission of the the dopant Er3+ ions in the nanofiber when its tetragonality and polarization vary as the temperature changes. Herein, by means of in-situ X-ray diffraction and in-situ UC measurement under infrared excitation at 980 nm at different temperatures, we chose 4 % Erdoped perovskite PTO nanofibers as a model system to explore the effects of tetragonality and


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polarization on the UC PL properties. It is interesting to find that the UC emission intensity of green and red band is strongly enhanced as the decreased tetragonality and polarization in 4 % Er-doped perovskite PTO nanofibers. Furthermore, the in-situ UC decay curves reveal that the lifetimes of 4S3/2 and 4F9/2 level are about 128.04±0.47 µs and 278.10±1.07 µs at 50 K and virtually remain unchanged when the measuring temperature increased from 50 K to 300 K. The observed UC phenomena in Er-doped perovskite PTO nanofibers can be ascribed to the assisted effect of the low-energy E(1TO) phonon in the UC process. EXPERIMENTAL SECTION The 4 % Er-doped perovskite PbTiO3 (PTO) single nanofibers were synthesized by a hydrothermal and solid state phase transformation method which have been mentioned in our previous studies.28 In-situ X-ray diffraction (XRD) was used to determine the crystal structure of the prepared sample. The XRD measurements were performed using an Empyrean XRD analyzer (Cu Kα) and a PIXcel3D detector (PANalytical Company) at 50 K, 100 K, 150 K, 200 K, 250 K and 300 K. Each XRD pattern of the sample was measured in the 2θ range from 15° to 120° at a scan speed of 0.02 °/s. The in-situ UC emission spectra and UC lifetime of the sample were recorded with the fluorescence spectrophotometer (Edinburgh FLSP920), using a 980 nm laser as excitation source. The Raman spectra of 4 % Er-doped PTO nanofibers were acquired by using a Renishaw InVia Raman microscope at room temperature. A 785 nm laser was used as excitation with the maximum power available of 50 mW and the power of the laser focused at the sample could be adjusted by opening or closing an iris shutter. All Raman data acquisition and processing were carried out by using the WiRE 3.0 software supplied by Renishaw.


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RESULTS AND DISCUSSION The results of in-situ low temperature X-ray diffraction patterns of 4 % Er-doped perovskite PbTiO3 (PTO) nanofibers at decreased temperatures from 300 K to 50 K are shown in Figure 1. By indexing, all of the diffraction patterns of the sample measured at 300 K are consistent with the tetragonal perovskite-structured PTO with lattice constants of a=3.914(2)Å and c=4.158(9) Å, matching well with the literature values of a=3.899(3) Å and c=4.153(2) Å (Joint Committee on Powder Diffraction Standards [JCPDS] Card No. 06-0452).31 As previously reported, the Er3+ ions are effectively doped and incorporated into the crystal lattices of single-crystal tetragonal perovskite-structured PTO host matrix, without any impurity phases.28 In 4 % Er-doped perovskite PTO nanofibers, the TiO6 octahedron pairs share the corners of one another forming a three-dimensional network structure and the substitution of Er3+ for Ti4+ at B site leads to the expansion of crystal lattices in perovskite PTO nanofibers.28 In the process of decreasing the temperature, 4 % Er-doped perovskite PTO nanofibers retain the tetragonal structure down to 50 K, no sign to phase transitions was found at low temperature. As shown in Figure 1b, the (001) and (101) peaks of 4 % Er-doped perovskite PTO gradually shift to higher angle with increasing the measuring temperature from 50 K to 300 K, reflecting the variations of the crystal structure in the host PTO nanofibers. The lattice parameters and refined XRD patterns of 4 % Er-doped PTO nanofibers using the Rietveld refinement by MAUD software are shown in Table1 and Figure S1, respectively.


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Temperature

a

c

c/a

Lattice vol. (Å3)

50 K 100 K

3.905(0) 3.905(4)

4.170(5) 4.172(6)

1.068(0) 1.068(4)

63.6(0) 63.6(4)

150 K

3.906(6)

4.171(8)

1.067(9)

63.6(7)

200 K

3.908(7)

4.168(8)

1.066(5)

63.6(9)

250 K

3.911(3)

4.164(6)

1.064(8)

63.7(1)

300 K

3.914(2)

4.158(9)

1.062(5)

63.7(2)

Table 1. Lattice parameters and lattice volumes obtained from the XRD studies and the corresponding structural refinements.

•

•

•

(001)

•

• •• •

40 50 2θ (deg) 50 K 100 K 150 K 200 K 250 K 300 K

60

70

(101)

Intensity (a.u.)

30

300K 250K 200K 150K 100K 50K

PbTiO3 (111)

•

•

• Tetragonal Perovskite

•

20

b

(110)

(001) (100)

Intensity (a.u.)

•

(002) (200) (102) (201) (210) (112) (211)

(101)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21.0 21.3 21.6 21.9 2θ (deg)

80 50 K 100 K 150 K 200 K 250 K 300 K

31.2 31.5 31.8 2θ (deg)

Figure 1. (a) In-situ low temperature X-ray diffraction patterns of 4 % Er-doped perovskite PTO nanofibers at different temperatures. (b) The enlarged (001) and (101) diffraction peak for the patterns.


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a

3.920 3.915

a

4.175

c

4.170

3.910

a

4.165

3.905

4.160

3.900

4.155 50

100

150

200

Temperature (K)

250

300 64.0

c/a Lattice Volume

1.068

63.8 1.064

63.6

1.060

63.4 50

100

150

200

250

300

3

Temperature (K)

Lattice Volume (Å )

b c/a

c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 2. The refined values of (a) the length of a and c lattice parameters; (b) c/a value and lattice volume at each temperature for the 4 % Er-doped perovskite PTO nanofibers. The refined values of the a and c axis lattice parameters at each temperature of the 4 % Erdoped perovskite PTO nanofibers are shown in Figure 2 and Table 1. The a axis lattice parameter increases from 50 K to 300 K, whereas c-parameter decreases gradually. Consequently, the c/a ratio decreases very fast from 1.068(0) at 50 K to 1.062(5) at 300 K. The changes of c/a of 4 % Er-doped perovskite PTO nanofibers are shown in Figure 2(b). For tetragonal ferroelectric perovskites, it is known that the spontaneous polarization (PS) can be estimated from the tetragonal distortion of the lattice, in addition, the spontaneous strain (c/a-1) is proportional to the square of PS(PS2).33 And thus, the decreased spontaneous polarization is commensurate with the corresponding decreased tetragonality and is in line with the accepted strong strain–polarization coupling model in such systems.34 Therefore, the spontaneous polarization of 4 % Er-doped perovskite PTO nanofibers decreases as the increased temperature from 50 K to 300 K. In addition, as shown in Figure 2(b), the lattice volume expands when the measuring temperature increases from 50 K to 300 K.


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Figure 3. (a) Schematic view of the crystal structures of single-crystal perovskite PTO nanofibers in a projection along the crystallographic b axis (red sphere-O; shallow blue sphereTi, gray sphere-Pb). (b) The lengths of Ti-O bonds in 4 % Er-doped perovskite nanofibers at different measuring temperatures (dS for small and dL for large values, respectively). It has been proved that a linear relation exists between the spontaneous polarization and the displacement of Ti from the centre of the oxygen octahedron in PTO.35 Figure 3(a) shows the schematic view of the crystal structure of single-crystal perovskite PTO nanofiber in a projection along b axis. In the tetragonal perovskite PTO nanofibers, the position of Ti atoms (blue) and O atoms (red) displace along the c axis direction by a distance δTi and δO with respect to the position of centrosymmetry, that is, the positions at the intersection points between the dashed lines and between the dashed and solid lines. The displacement of Ti can also be characterized by means of the Ti-O bond length dS (S for small) and dL (L for large) which parallel to the c axis. Figure 3(b) shows the variations of Ti-O bond lengths at different measuring temperatures, which are derived from the in-situ low-temperature XRD measurement by using the MAUD and


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Crystal Maker softwares. The difference between the values of dL and dS substantially indicate the variations of Ti-O bond lengths at different temperatures. The changes of the atomic displacements at different temperatures essentially show the same behavior as the c axis lattice parameters and c/a value, implying the variations in tetragonality and spontaneous polarization of the host PTO nanofibers matrix. The spontaneous polarization, which arises as a result of the separation of the charge centre of cations from that of anions, can be investigated by measuring the atomic positions.34,36 The difference value between dL and dS (dL- dS) could be used to investigate the relative shifts between Ti and O atoms, which implying the intensity of sportaneous polarization of the host PTO nanofibers matrix. As shown in Figure 3(c), the value of (dL- dS) at lower temperature is much bigger than that at higher temperatures, indicating a decreased spontaneous polarization of the host PTO nanofibers matrix with increasing the measuring temperatures from 50 K to 300 K. The decreased tetragonality and spontaneous polarization of the host perovskite nanofibers can directly influence the chemical environment around the dopant Er3+ ions and probably further lead to the variations in the UC emission properties.


10

H11/2 I15/2

5

1x10

0

b

550 600 650 Wavelength (nm) 2

50K 100K 150K 200K 250K 300K

4

4

F9/2 I15/2

5 0

d

e

515

520

525

530

535

Wavelength (nm)

640

660

680

Wavelength (nm)

ESA

ET

I13/2

4

I15/2

3+

Er 80.0k

523 nm 656 nm

40.0k 0.0 50

RED

4

120.0k

Enhancement Factor

Intensity (a.u.)

H11/2 I15/2

GREEN

4

700

Intensity (a.u.)

500

10

CR

4

F9/2 I15/2

4

550 nm

4 2

ET

656 nm

5

2x10

ESA

15

F7/2 H11/2 S3/2 4 F9/2 4 I9/2 4 I11/2 2 4

523 nm

5

3x10

4

20

GSA

50K 100K 150K 200K 250K 300K

25

980 nm

5

4x10

c

980 nm

4

S3/2 I15/2

-1

4

3

a Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy (10 cm )

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100 150 200 250 Temperature (K)

300

0.8 0.4 0.0

Ratio of Green (523 nm) to Red (656 nm)

50

100 150 200 250 Temperature (K)

300

Figure 4. (a) Temperature dependence of the UC PL emission of 4 % Er-doped perovskite PTO nanofibers excited at 980 nm. (b) The enlarged UC PL emission spectra of 4 % Er-doped perovskite PTO nanofibers around 523 nm and 656 nm. (c) Energy level diagram of Er3+ and possible mechanisms of UC process under excitation at 980 nm. (d) The enhanced degree of UC emission intensities at 523 nm and 656 nm of 4 % Er-doped perovskite PTO nanofibers. (e) The UC emission intensity ratio of 523 nm to 656 nm at different temperatures. The UC emission spectra of 4 % Er-doped perovskite PTO nanofibers under infrared excitation (980 nm) at different temperatures are shown in Figure 4(a). The strong UC green emission centered at 523/555 nm as well as the red UC emission around 656 nm of 4 % Er-doped perovskite PTO nanofibers shown in Figure 4(a) are corresponding to the characteristic UC PL


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of Er3+ions.37,38 The pumping mechanism of the UC processes under the excitation of 980 nm is explained by the energy level diagram shown in Figure 4 (c).39,40 Under the excitation of 980 nm, the Er3+ ion is excited from the ground state 4I15/2 to 4F7/2 state by the ground-state absorption (GSA) process and the excited state absorption (ESA) process. By multi-phonon relaxation, the Er3+ ions at 4F7/2 level could decay non-radiatively to 2H11/2, 4S3/2 or 4F9/2 levels, and then return back to the ground state 4I15/2, producing the green and red emissions. The enlarged UC PL emission spectra of 4 % Er-doped perovskite PTO nanofibers around 523 nm and 656 nm are shown in Figure 4(b). Obviously, the UC emission intensity of Er-doped perovskite PTO nanofibers can be obviously enhanced with the increased temperature, in contrast to the decreased green and red UC intensities in Er-doped PTO particles (Figure S5(b)) and Er-doped BaTiO3 powders25. In addition, Figure 4(d) and Figure 4(e) show the significantly enhancement in the UC PL emission intensity ratio of green (523 nm) to red band (656 nm) from 0.17 to 0.86 when the measuring temperature increases from 50 k to 300 k. Therefore, the enhancement degree of the UC PL emission intensity at 523 nm is much higher than that at 656 nm, indicating that the emission enhancement is highly wavelength-dependent.


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Figure 5. Schematic illustration of the crystal lattice of 4 % Er-doped PTO nanofibers at (a) 300 K and (b) 50 K. When decreasing the measuring temperature to 50 K, the Ti4+ and O2- ions are shifted in the same direction (original positions at 300 K indicated by dotted circles), assuming the Pb2+ ions to be rigid. A large number of previous studies have indicated that the UC PL of dopant ions could be influenced by the crystal symmetry of the host materials.11,26-29,41 To provide an in-depth explanation to the effects of tetragonality and polarization on UC PL of Er-doped PTO nanofibers, a schematic illustration of the crystal lattice of 4 % Er-doped PTO nanofibers is shown in Figure 5. The prototypical perovskite PTO nanofiber used as the host material is non-


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centrosymmetric in tetragonal phase with the space group P4mm at room temperature.28 The Ti4+ ions shift related to the center of the negatively charged TiO6 octahedron, causing the separation of the charge center of cations from that of anions, producing the spontaneous polarization.34 Based on the reported facts, the centrosymmetric crystal field arises from reducing tetragonality in the tetragonal (ferroelectric) phase,26 and the dopant Er3+ ions occupy Ti4+ sites in 4 % Erdoped perovskite PTO nanofibers.28 We can assume that the decreased tetragonality and spontaneous polarization of PTO nanofibers host lead to the variations in the vibrations mode of the phonon, and thus cause the changes in the UC emissions by the interaction between phonons

200

400 600 800 -1 Raman shift (cm )

1000

4 % Er-PTO E(1TO)

Intensity (a.u.)

750 E(3LO)

651 A1(3TO)

4% Er-PTO PTO 505 E(3TO)

437 E(2LO)+A1(2LO)

b 352 A1(2TO)

287 E(4TO+4LO)

216 E(2TO)

151 A1(1TO)

a

85 E(1TO)

and Er3+ ions.

Nomalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 80 100 120 -1 Raman shift (cm )

Figure 6. (a) Room-temperature Raman spectra acquired for an individual perovskite PTO nanofiber and 4 % Er-doped perovskite PTO nanofiber. (b) The enlarged E(1TO) Raman peak of 4 % Er-doped perovskite PTO nanofiber. The room temperature Raman spectra of an individual perovskite PTO nanofiber and 4 % Erdoped perovskite PTO nanofiber are displayed in Figure 6(a), within the frequency range of 50-


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1000 cm-1. All the Raman modes of perovskite PTO nanofiber are consistent with those of the reported single crystal PTO.42,43 The Raman spectrum of the PTO nanofiber exhibits three small peaks located at 151 cm-1, 362 cm-1 and 650 cm-1, which are attributed to the A1(1TO), A1(2TO) and A1(3TO) phonons, respectively. Simultaneously, the intense peaks at 85 cm-1, 216 cm-1 and 505 cm-1 are corresponding to the E(1TO), E(2TO) and E(3TO) phonons, respectively. In addition, the peaks around 130 cm-1 and 437 cm-1 correspond to E(1LO) and E(2LO)+ A1(2LO) phonons. In the previous work, the temperature dependence of the ferroelectric soft mode ωF in ABO3 perovskite ferroelectric oxides could be approximately described by ωF2 =A(T-Tc).44 According to the low temperature Raman measurement of PTO at 10 K and 200 K, a continuous frequency increase of the Raman shift of PTO is detected with the decreased temperature.45 This result could indicate that the phonon energy of Er-doped PTO increases with the decreased temperature. The phonons play an important role in the UC process, and thus the variation of phonon energy of Er-doped PTO nanofibers could lead to the variation in the UC emission. The effect of low-energy phonons on the UC emission of Er-doped PTO nanofibers are discussed in the following parts. As reported, the excitation spectrum of the 18200 cm-1 (549.5 nm) UC emission in a NaYF4: 10 % Er3+ sample has been recorded at 12 K by Suyver, et al..46 The peaks observed in the excitation spectrum correspond to electronic and vibronic excitations of the 4I11/2 multiplet of Er3+ ions.46,47 The 4I11/2 multiplet are identified ranging from 10244 cm-1 to 10370 cm-1 with the most efficient excitation wavelength at about 10310 cm-1. The excitation wavelength chosen in the UC measurement of 4 % Er-doped perovskite PTO nanofibers is 980 nm (10204 cm-1), which having a lower energy than the lowest excitation energy of 4I11/2 multiplet (10244 cm-1). Therefore, one mayargue that the ground-state absorption (GSA) process from ground state 4I15/2


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to 4I11/2 energy level should be assisted by the low-energy phonons of the perovskite PTO host. Figure 7(b) shows the schematic 4I11/2 multiplet energy-level diagram of Er3+ as well as the phonon-assisted energy transfer during the UC process. Under the excitation of 980 nm, onephoton energy (10204 cm-1) is not enough to excite Er3+ ions from ground state 4I15/2 to lowest energy-level 4I11/2|0〉 energy level. Thus, the E(1TO) phonon energy ∆E (85 cm-1) will efficiently transfer to the Er3+ ion bumping the Er3+ ion to 4I11/2|1〉 energy-level with the cooperation of one 980 nm photon.

Figure 7. (a) Schematic illustration of the UC PL process (red sphere-O; shallow blue sphere-Ti, purple sphere-Er). (b) Schematic energy transition diagram showing the probable energy transition process between phonon energy and Er3+ ions. The schematic illustration of the UC PL process of 4 % Er-doped perovskite PTO nanofibers under infrared excitation (980 nm) is shown in Figure 7(a). A probable mechanism was proposed to explain the polarization dependence of Er3+ UC process in Er-doped PTO nanofibers. Because efficient energy transfer is possible from the E(1TO) phonon energy ∆E to Er3+ ions in the


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ground-state absorption (GSA) process, the UC process of Er3+ ions could be influenced by the phonon energy of PTO 4 % Er-doped perovskite PTO nanofibers. In the previous analysis, when the temperature decreased from 300 K to 50 K, the increase in tetragonality and spontaneous polarization of the 4 % Er-doped single-crystal perovskite PTO nanofibers leads to the increased E(1TO) phonon energy of Er-doped PTO nanofibers. For example, the E(1TO) phonon energy ∆E′ of Er-doped PTO nanofibers (at 50 K) is much higher than that at 300 K.43 The energy level width of the Er3+ 4I11/2 multiplet is about 130 cm-1 and the most efficient energy level range is about 60 cm-1.44 After excited by a 980 nm (10204 cm-1) laser, the higher E(1TO) phonon energy might not be suitable to assist the ground-state absorption (GSA) process. Therefore, the enhancement of the UC PL emission can be ascribed directly to the more efficient energy transfer process from the E(1TO) phonon energy ∆E to Er3+ ion in the GSA process, which induced by the higher polarization of PTO nanofiber host. After the GSA process in Er-doped PTO nanofibers, the number of excited Er3+ ions at 4I11/2 level at 300 K is larger than that at 50 K. In addition to the excited state absorption (ESA) process from 4I11/2 state to 4F7/2 state, some of the excited Er3+ ions at 4I11/2 level could relax to the 4I15/2 level non-radiatively and transfer the energy to another neighboring Er3+ ion at 4I11/2 level through the energy transition upconversion (ETU) process, bumping the neighboring ion to 4F7/2 level. As shown in Figure 7(b), the 2H11/2 or 4

S3/2 level is closer to 4F7/2 level compared to 4F9/2 level. The Er3+ ions at 4F7/2 level could decay

non-radiatively to the 2H11/2 or 4S3/2 level more efficiently than to the 4F9/2 levels by multiphonon relaxation. Consequently, the observed increase of the UC intensity ratio of green (523 nm) to red band (656 nm) could be attributed to the different multiphonon transition probability from 4F7/2 level to the 2H11/2, 4S3/2 and 4F9/2 levels, indicating the highly wavelength-dependent enhancement in UC PL emissions.


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Compared to that of Er-doped BaTiO3 (BTO) powders,25 the lowest phonon (A1(TO1)) energy of BTO powders are at about 180 cm-1, and thus are supposed to be not suitable to assist the excitation photon (980 nm), bumping the Er3+ ion to 4I11/2|1〉 energy level in the GSA process. The SEM images of Er-doped perovskite PTO particles with different Er concentration are shown in Figure S5. It is obvious that the growth direction of Er-doped PTO particlesis not along [001] (the c axis). As for Er-doped PTO particles, the Er3+ ions occupying Ti site in Er-doped PTO particles have not been arranged along the c axis. The free arrangement of Er3+ ions in Erdoped PTO particles might not be favorable for the polarization-modified ETU process between the neighboring Er3+ ions, which is not beneficial for enhancing in UC emission intensity of Erdoped PTO particles. Therefore, the enhancement of UC PL emission intensities around 523 nm (about 43 times) and 656 nm (about 8 times) in Er-doped PTO nanofibers are different to that in Er-doped BaTiO3 or PTO particles.


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Intensity (a.u.)

a

1.0

140

50 K 100 K 150 K 200 K 250 K 300 K

0.8 0.6

130

120

0.4 0.2

4

50

100 150 200 250 Temperature (K)

S3/2

4

500

1000 Time (µs)

300

I15/2

0.0 0

1500

2000

290

1.0

50 K 100 K 150 K 200 K 250 K 300 K

0.8 0.6

280 Time (µ s)

b Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Time (µ s)

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270 260 250

0.4 4

F9/2

0.2

50

100 150 200 250 Temperature (K)

300

4

I15/2

0.0 0

500

1000 Time (µs)

1500

2000

Figure 8. Upconversion decay curves of the (a) 4S3/2 and (b) 4F9/2 emissions of the 4 % Er-doped PTO nanofibers at different temperatures. The insets show the variations of calculated values of upconversion lifetime of 4S3/2 and the 4F9/2 emissions at different temperatures. The UC decay curves at 552 nm and 669 nm of 4 % Er-doped PTO nanofibers were measured at different temperatures under 980 nm laser excitation and the results are presented in Figure 9. The measured UC decay curves were fitted to the equation as I(t) = A*exp(-t/τ1), where τ is the life time of emitting level. As shown in Figure S1 and Figure S2, the decay data is found to fit well with mono-exponential fitting having goodness of fit R2

Polarization-Modified Upconversion ... - ACS Publications

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