Probing Interaction Distance of Surface Quenchers in Lanthanide

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

C: Physical Processes in Nanomaterials and Nanostructures

Probing Interaction Distance of Surface Quenchers in Lanthanide Doped Upconversion Core-Shell Nanoparticles Denghao Li, Xiaofeng Liu, and Jianrong Qiu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02707 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18 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

The Journal of Physical Chemistry

Probing Interaction Distance of Surface Quenchers in Lanthanide Doped Upconversion Core-Shell Nanoparticles Denghao Li†, Xiaofeng Liu*† and Jianrong Qiu*§ †

School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China

§

State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and

Engineering, Zhejiang University, Hangzhou 310027, China

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 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

Abstract: Core-shell structures which employ an optically inert shell to physically separate the emitting core from the surface quenchers are often designed to optimize the emission efficiency of nanoscale emitters. However, it remains unclear at what distance the effect of different surface quenchers, such as defects and adsorbed moieties, can be completely screened by the shell. Here, in a model upconversion system, we examine the interaction distance of surface quenchers in core-shell nanoparticles by using upconversion spectroscopy. Steady state as well as time-resolved spectra shows that the quenching effect of surface adsorbed hydroxyl (OH-) group diminishes at a distance (shell thickness) of 3.5 nm in diameter and 8.0 nm in length, which is larger than that for oleate-capped counterparts. With the increase of pumping density, the interaction distance of the surface quenchers does not apparently change, while saturation of the surface related states notably reduces the optimal shell thickness for surface passivation.


2

Page 2 of 18

Page 3 of 18 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


Introduction In recent years, lanthanide-doped nanocrystals have been widely studied for a range of applications such as photovoltaic solar energy conversion, anti-counterfeiting, high contrast 3D bioimaging and photodynamic.1,2,3 In addition, comparing to conventional downshifting luminescence (DSL) bioprobes like organic dyes and quantum dots (QDs), lanthanide-doped upconversion nanoparticles (UCNPs) possess superior physicochemical features, such as large anti-Stokes shifts, high resistance to photobleaching, low autofluorescence background, low toxicity and high penetration depth, and thus are regarded as a new generation of luminescent bioprobes.4,5,6 Rare-earth fluorides, and in particular NaYF4, are one of the most efficient types of host materials and the high-quality NaYF4 nanoparticles can be synthesized by co-precipitation of the lanthanide fluorides with long-chain hydrocarbons (e.g., 1-octadecene) and unsaturated fatty acids, typically oleic acid.7 However, these nanoparticles have to be rendered water-dispersible in order to be applied in vitro and in vivo directly since biological systems contain a large pool of water.7,8 To this end, a variety of surface modification strategies like ligand exchange, ligand oxidation, ligand removal, silanization, layer-by-layer assembly and amphiphilic polymer coating have been carried out to replace the hydrophobic surfaces of the nanoparticles in recent years.9,10,11,12,13 Among these surface treatment methods, ligand removal of oleic acid (OA) capped on nanoparticles with acid treatment is more convenient way which may be used in large quantity compared to the other methods.14 However, surface functionalization with hydrophilic groups at the same time introduces new quenching pathways due to the presence of hydroxyl groups which strongly limit upconversion efficiency. An epitaxial grown shell onto the native particles can separate the emitter core from these surface moieties and thus results in enhanced


3


emission. This strategy is often applied for nanoscale emitters including quantum dots and UCNPs,15,16 while the interaction distances of various surface quenchers under different excitation conditions remain to be evaluated in a precise fashion. Herein, in a model upconversion system, Yb3+ and Tm3+ doped β-NaYF4 core-shell nanoparticle, we investigate the interaction distance of surface quenchers by upconversion spectroscopy. Two series of core-shell UCNPs capped by OA and OH have been synthesized and examined. The results indicate that the interaction distances are dependent on the type of surface quenchers but independent of the power density. The results could be of particular importance for designing efficient upconversion nanoparticles for bioimaging applications. Results & Discussion The size and shape evolution of the as-synthesized core-shell nanoparticles are first examined by transmission electron microscopy (TEM), as shown in Fig.1(a-f). The shell growth process can be controlled precisely through adding different amount of shell precursor into core solution, as listed in Table S1. After epitaxial growth of NaYF4 shell on the core nanoparticles, the as-synthesized NaYF4:Yb,Tm@Na-YF4 core-shell nanoparticles are still well dispersible in different solvents with a rather focused size distribution. Different from an isotropic growth process, the length is larger than the diameter so the final core-shell has a rod-like shape (Fig.1 (g)). In addition, the ratio between diameter and length is about 0.8, as indicated by the size distribution shown in Fig.1 (g). Similar shape evolution has been reported previously,17 which is mainly determined by the ration of oleate anions (OA-) to molecules (OAH) in reaction process.18 We calculated the lattice constants of three samples from the XRD patterns (shown in Fig. S4): CS-0: a=b=5.9676 Å, c=3.5129 Å, α=β=90 o. γ= 120 o; CS-4-OA: a=b=5.9920 Å, c=3.5044 Å, α=β=90 o. γ= 120 o; CS-4-OH: a=b=5.9848 Å, c=3.5040 Å, α=β=90 o. γ= 120 o.


4

Page 4 of 18

Page 5 of 18 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


There’s no obvious change in lattice constant change when the nanocrystal was coated or OAwas replaced by OH-. Fourier Transform Infrared Spectroscopy (FTIR) spectra were recorded to confirm that OAand OH- were successfully coated onto the surface of core-shell nanoparticles. Fig. S1 (a) shows the process of ligand replacement on the UCNP surface. We chose CS-0 as an example to examine the surface characteristics by FTIR spectra, as shown in Fig.S1 (b). It is obvious that OA- coated CS-0 (KBr+CS-0-OA) presents characteristic bands of oleate acid in the FTIR spectrum. Two bands at 1465 and 1564 cm-1 originate from the asymmetric and the symmetric stretches of -COO-, respectively. The band at 1712 cm-1 is attributed to stretching vibration of C=O and the 2850 and 2930 cm-1 bands both correspond to the stretching vibration of -CH2- . As for the OH- coated CS-0 (KBr+CS-0-OH), all the characteristic bands of oleate acid disappear and instead two strong absorption peaks at 1640 and 3200 - 3700 cm-1 appear, which are attributed to the vibration of OH-. These results indicate that the UNCPs are successfully coated by OA- and OH- by the applied method. To probing the interaction distance of OA- and OH- adsorbed on the surface of UNCPs with different NaYF4 shell thickness, we measured the upconversion emission spectra ranging from 300 to 500 nm, as shown in Fig 2 (a) and Fig 2 (b). Upon 980 nm laser excitation, all the samples exhibit both UV and blue emission, which are assigned to 1I6→3F4 (345 nm), 1D2→3H6 (360 nm), 1

D2→3F4 (450 nm) and 1G4→3H6 (474 nm) transitions of Tm3+, respectively. The UC intensity

increases firstly and then decreases with the increase of shell thickness in both OA and OH adsorbed UCNPs. The enhanced UC intensity can be explained by the suppression of surface quenching through physical separation by the optically inert shell.19,20 A further increase in shell thickness beyond a certain threshold leads to a decrease in the emission intensity. This may be


5


related with the decrease in the average RE concentration in the core-shell UCNPs. Fig.2 (c) shows the integrated emission intensity for the 1D2→3H6, 1D2→3F4 and 1G4→3H6 transitions versus shell thickness calculated from Fig.2 (a) and Fig.2 (b). Impressively, for OH- coated UCNPs, CS-3 has the strongest UC intensity, while CS-2 shows the strongest UC intensity among the OA- coated UCNPs. In addition, the emission intensity just keeps stable with time for the examined samples which is showed in Fig. S5. To understand the photophysical dynamics of the system, we measured the time-resolved emission spectra for the core-shell UCNPs, as shown in Fig. 3. As expected, the lifetimes at 450 and 474 nm increases as the shell growth because surface quenching processes are passivated more effectively at for thicker shells, which is similar to the results reported previously.21,22 The lifetime becomes saturated when the shell thickness increases to 1.6 nm in diameter and 5.0 nm in length for OA- coated UCNPs. In comparison, the saturation of lifetime occurs when the thickness is 3.5 nm in diameter and 8.0 nm in length for OH- coated UCNPs. The result indicated that a thicker shell is needed to fully eliminate the surface quenching in OH- coated UCNPs which is consistent with the UC emission spectra. Interestingly, for both 450 nm and 474 nm emissions, the saturated lifetime value of OH- coated UCNPs is slightly higher than that of OAcoated UCNPs, suggesting again the interaction distance of OH is longer than that of OA. We believe that OH- plays an important role in affecting the transitions involving energy levels of 1

D2 and 1G4 that ultimately affect the emission intensities at 450 and 474 nm. Fig. S2 shows the

energy level diagram of Yb3+-Tm3+ system. In Tm3+ and Yb3+ co-doped system, Yb3+ can successively transfer energy to Tm3+ to populate the 3H5, 3F3/3F2 and 1G4 levels.23,24 However, the 1

D2 level of Tm3+ cannot be populated by the fourth photon absorbed by Yb3+ via energy transfer

to the 1G4 due to the large energy mismatch (~3500 cm-1) between them.25 Instead, 1D2 level can


6

Page 6 of 18

Page 7 of 18 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


be populated through the cross-relaxation processes 3F3→3H6: 3F3→1D2 (Tm3+).26,27 For energy levels of 3H5 and 3F4, the 3H5 level can be populated by absorbing a 980 photon and after that by a multi-phonon relaxation process the 3F4 level will be populated. The multi-phonon relaxation rate can be expressed by the Miyakawa–Dexter equation as shown below 28 Wp = Wp(0)exp ቀ α = ln ቄ

௣

ିఈ∆୉

୥ሾ௡(௧)ାଵሿ

‫≈݌‬

ቁ

(1)

ቅ−1

(2)

ℏఠ

∆ா

ℏఠ

(3)

where Wp is the multi-phonon relaxation rate, g is the electron-phonon coupling strength, ∆‫ ܧ‬is the energy gap to the next lower level, ℏ߱ is the phonon energy and the p is phonon numbers needed in the multi-phonon relaxation. For samples coated with OH-, because of high-energy vibrational modes (3200-3700 cm-1) of OH- group, the transition between 3H5 and 3F4 which is about 3000 cm-1 could be strongly coupled with phonon-assisted process. According to equation (1), the 3H5 levels can be quickly depopulated to 3F4 in the presence of OH- comparing to the intrinsic phonons in NaYF4 (~350 cm-1). As a result, more 3F4 levels would be populated which will ultimately affect the higher energy states including 3H5, 3F3/3F2 and 1G4. To confirm the mechanism, we measure the lifetime of 3H4 levels which are responsible for the 804 nm emission. From Fig.S3, a longer lifetime is observed in OH- coated samples when the shell thickness exceeds 3.5 nm in diameter and 8.0 nm in length. A longer lifetime of 3H4(3F3) implies a high probability for the cross-relaxation processes 3F3→3H6: 3F3→1D2. Therefore, the lifetime for the 1

D2 and 1G4 levels of the OH- coated samples is slightly longer than that of OA- coated samples. To have a closer insight into the upconversion behavior of this two series of samples, the

enhancement (defined by the ratio of emission intensity of the core-shell particles to that of the core-only particle) in the integrated emission intensity of 1D2→3H6, 1D2→3F4 and 1G4→3H6


7


transitions are plotted against shell thickness, as shown in Fig. 4. Fig.4 (a) shows the UC emission spectra of samples with different shell thicknesses under different power densities. The enhancement factor is defined by the ratio of the integrated emission intensity of the core-shell UCNPs to that of the core-only (CS-0) sample (Fig. 4 (b)). The intensity enhancement shows a maximum value as the shell gets thicker. Interestingly, the positions at which the enhancement factor reaches maximum values are different for the OH- and OA- coated core-shell UCNPs, obviously. For OA- coated samples, surface defects are believed to be the main factor to quench energy; therefore a relatively thin shell may lead to a huge UC emission enhancement in low power density (3.32 W/cm2). When the power density increases, on one hand, the surface defects get saturated and are no longer the main factor for quenching. At high excitation densities, more Tm3+ ions are excited and be responsible for the emission intensity. As a result, CS-2 has a maximum intensity enhancement when power density is higher. Unlike the OA- coated samples, in addition to the surface defects, the OH- on the surface has a stronger quenching effect because its high vibrational energy. When the size of particle is small, the high specific surface area leads to a high density of OH- groups and the enhancement is low. As the particle becomes larger and the shell becomes thicker, a relative lower density of OH- groups can be achieved and the enhancement factor increases. So a thicker shell is needed to suppress the quenching effect of OH- groups. A saturation of enhancement was observed when the power density reaches a threshold value, as observed in an early report.29,30 Conclusions We have examined the upconversion spectra of lanthanide doped NaYF4 core-shell UCNPs with different shell thickness and covered with different moieties and the unraveled the interaction distance of the surface quenchers. The results indicate that OH- groups accelerate


8

Page 8 of 18

Page 9 of 18 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


multi-phonon relaxation process of 3H5→3F4 and ultimately affect the transitions involving higher energy states including 3H5, 3F3/3F2, 1G4 and 1D2, and the cross-relaxation process involving 1D2 level. Due to the longer interaction distance of OH- groups compared with OA-, a thicker NaYF4 shell is required to screen the surface quenching effect, while the requirement for surface passivation by the shell is mitigated at higher excitation density in which the surface states are saturated. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org . Detailed experimental section is given. Corresponding Author *E-mail: [email protected] & [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NO. 11504323, 61775192 and 51772270). The authors thank the open fund of the State Key Laboratory of High Field laser physics of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science. References (1) Lian, H. Z.; Hou, Z. Y.; Shang, M. M.; Geng, D. L.; Zhang, Y.; Lin, J. Rare Earth Ions Doped Phosphors for Improving Efficiencies of Solar Cells. Energy 2013, 57, 270-283. (2) Zhou, B.; Shi, B. Y.; Jin, D. Y.; Liu, X. G. Controlling Upconversion Nanocrystals for


9


Page 10 of 18

Emerging Applications. Nat. Nanotechnol 2015, 10, 924-936. (3) Dong, H.; Du, S. R.; Zheng, X. Y.; Lyu, G. M.; Sun, L. D.; Li, L. D.; Zhang, P. Z.; Zhang, C.; Yan, C. H. Lanthanide Nanoparticles: From Design toward Bioimaging and Therapy. Chem. Rev. 2015, 115, 10725-10815. (4) Liu, B.; Chen, Y. Y.; Li, C. X.; He, F.; Hou, Z. Y.; Huang, S. S.; Zhu, H. M.; Chen, X. Y. Lin, J. Poly(Acrylic Acid) Modification of Nd3+-Sensitized Upconversion Nanophosphors for Highly Efficient UCL Imaging and pH-Responsive Drug Delivery. Adv. Funct. Mater. 2015, 25, 4717-4729. (5) Wu, X.; Zhang, Y.; Takle, K.; Bilsel, O.; Li, Z.; Lee, H.; Zhang, Z.; Li, D.; Fan, W.; Duan, C., et al. Dye-Sensitized Core/Active Shell Upconversion Nanoparticles for Optogenetics and Bioimaging Applications. Acs Nano 2016, 10, 1060-1066. (6) Wolfbeis, O. S. An Overview of Nanoparticles Commonly Used in Fluorescent Bioimaging. Chem. Soc. Rev. 2015, 44, 4743-4768. (7) Chatteriee, D. K.; Rufalhah, A. J.; Zhang, Y. Upconversion Fluorescence Imaging of Cells and Small Animals Using Lanthanide Doped Nanocrystals. Biomaterials 2008, 29, 937-943. (8) Hao, S.; Yang, L.; Qiu, H.; Fan, R.; Yang, C.; Chen, G. Heterogeneous Core/Shell Fluoride Nanocrystals with Enhanced Upconversion Photoluminescence for in Vivo Bioimaging. Nanoscale 2015, 7, 10775-10780. (9) Wang, F.; Deng, R. R.; Liu, X. G. Preparation of Core-Shell NaGdF4 Nanoparticles Doped with Luminescent Lanthanide Ions to Be Used as Upconversion-Based Probes. Nat. Protoc. 2014, 9, 1634-1644. (10) Chen, G. Y.; Ohulchanskyy, T. Y.; Law, W. C.; Agren, H.; Prasad, P. N. Monodisperse NaYbF4:

Tm3+/NaGdF4

Core/Shell

Nanocrystals

with


10

near-Infrared

to

near-Infrared

Page 11 of 18 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


Upconversion Photoluminescence and MagneticResonance Properties. Nanoscale 2011, 3, 2003-2008. (11) Zhou, H. P.; Xu, C. H.; Sun, W.; Yan, C. H. Clean and Flexible Modification Strategy for Carboxyl/ Aldehyde-Functionalized Upconversion Nanoparticles and Their Optical Applications. Adv. Funct. Mater. 2009, 19, 3892-3900. (12) Sivakumar, S.; Diamente, P. R.; van Veggel, F. C. Silica-coated Ln(3+)-Doped LaF3 Nanoparticles as Robust Down- and Upconverting Biolabels. Chem-Eur. J. 2006, 12, 5878-5884. (13) Jiang, G. C.; Pichaandi, J.; Johnson, N. J. J.; Burke, R. D.; van Veggel, F. C. J. M. An Effective Polymer Cross-Linking Strategy To Obtain Stable Dispersions of Upconverting NaYF4 Nanoparticles in Buffers and Biological Growth Media for Biolabeling Applications. Langmuir 2012, 28, 3239-3247. (14) Bogdan, N.; Vetrone, F.; Ozin, G. A.; Capobianco, J. A. Synthesis of Ligand-Free Colloidally Stable Water Dispersible Brightly Luminescent Lanthanide-Doped Upconverting Nanoparticles. Nano Lett. 2011, 11, 835-840. (15) Mahmoud, W. E. Synthesis and Characterization of 2a-3shpa decorated ZnS@CdS Core-Shell Heterostructure Nanowires as a Fluorescence Probe for Antimony Ions Detection. Sensor. Actuat. B-Chem. 2017, 238, 1001-1007. (16) Johnson, N. J. J.; van Veggel, F. C. J. M. Sodium Lanthanide Fluoride Core-Shell Nanocrystals: A general Perspective on Epitaxial Shell Growth. Nano Res. 2013, 6, 547-561. (17) Yang, Y.; Zhu, Y. B.; Zhou, J. J.; Wang, F.; Qu, J. R. Integrated Strategy for High Luminescence Intensity of Upconversion Nanocrystals. Acs Photonics 2017, 4, 1930-1936. (18) Liu, D. M.; Xu, X. X.; Du, Y.; Qin, X.; Zhang, Y. H.; Ma, C. S.; Wen, S. H.; Ren, W.; Goldys, E. M.; Piper, J. A., et al. Three-dimensional Controlled Growth of Monodisperse Sub-50


11


Page 12 of 18

Nm Heterogeneous Nanocrystals. Nat. Commun. 2016, 7,10254. (19) Wang, F.; Wang, J. A.; Liu, X. G. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem.-Int. Edit. 2010, 49, 7456-7460. (20) Su, Q. Q.; Han, S. Y.; Xie, X. J.; Zhu, H. M.; Chen, H. Y.; Chen, C. K.; Liu, R. S.; Chen, X. Y.; Wang, F.; Liu, X. G. The Effect of Surface Coating on Energy Migration-Mediated Upconversion. J. Am. Chem. Soc. 2012, 134, 20849-20857. (21) Zhang, F.; Che, R. C.; Li, X. M.; Yao, C.; Yang, J. P.; Shen, D. K.; Hu, P.; Li, W.; Zhao, D. Y. Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties. Nano Lett. 2012, 12, 2852-2858. (22)

Chen,

D.

Q.;

Huang,

P.

Highly

intense

Upconversion

Luminescence

in

Yb/Er:NaGdF4@NaYF4 Core-Shell Nanocrystals with Complete Shell Enclosure of the Core. Dalton T. 2014, 43, 11299-11304. (23) Zhou, J. J.; Chen, G. X.; Zhu, Y. B.; Huo, L. L.; Mao, W.; Zou, D. N.; Sun, X. W.; Wu, E.; Zeng, H. P.; Zhang, J. J., et al. Intense Multiphoton Upconversion of Yb3+-Tm3+ Doped Beta-NaYF4 Individual Nanocrystals by Saturation Excitation. J. Mater. Chem. C 2015, 3, 364-369. (24) Mahalingam, V.; Vetrone, F.; Naccache, R.; Speghini, A.; Capobianco, J. A. Colloidal Tm3+/Yb3+-Doped LiYF4 Nanocrystals: Multiple Luminescence Spanning the UV to NIR Regions via Low-Energy Excitation. Adv. Mater. 2009, 21, 4025-4028. (25) Qin, G. S.; Qin, W. P.; Wu, C. F.; Huang, S. H.; Zhang, J. S.; Lu, S. Z.; Zhao, D.; Liu, H. Q. Enhancement of Ultraviolet Upconversion in Yb3+ and Tm3+ Codoped Amorphous Fluoride Film


12

Page 13 of 18 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


Prepared by Pulsed Laser Deposition. J. Appl. Phys. 2003, 93, 4328-4330. (26) Thrash, R. J.; Johnson, L. F. Up-Conversion Laser-Emission from Yb3+-Sensitized Tm3+ in BaY2F8. J. Opt. Soc. Am. B. 1994, 11, 881-885. (27) Noginov, M. A.; Curley, M.; Venkateswarlu, P.; Williams, A.; Jenssen, H. P. Excitation Scheme for the Upper Energy Levels in a Tm:Yb:BaY2F8 Laser Crystal. J. Opt. Soc. Am. B. 1997, 14, 2126-2136. (28) De, G. J.; Qin, W. P.; Zhang, J. S.; Wang, Y.; Cao, C. Y.; Cui, Y. Effect of OH- on the Upconversion Luminescent Efficiency of Y2O3 : Yb3+, Er3+ Nanostructures. Solid State Commun 2006, 137, 483-487. (29) Wang, G.; Qin, W.; Wang, L.; Wei, G.; Zhu, P.; Kim, R. Intense Ultraviolet Upconversion Luminescence from Hexagonal NaYF4 : Yb3+/Tm3+ Microcrystals. Opt. Express 2008, 16, 11907-11914. (30) Quintanilla, M.; Cantarelli, I. X.; Pedroni, M.; Speghini, A.; Vetrone, F. Intense Ultraviolet Upconversion in Water Dispersible SrF2: Tm3+, Yb3+ Nanoparticles: The Effect of the Environment on Light Emissions. J. Mater. Chem. C 2015, 3, 3108-3113.


13


Figure 1. Structural characterization of the as-synthesized β-NaYF4 nanocrystals. (a-e) TEM image of Core β-NaYF4: 20 Yb3+,0.5 Tm3+ nanocrystals and a series of β-NaYF4: 20 Yb3+,0.5 Tm3+ @NaYF4 Core@Shell Structure (scale bar 20 nm). (f) The mean diameter and length shell thickness of samples. (g) Size histograms representing the dimension distribution of these nanocrystals in length and diameter.


14

Page 14 of 18

Page 15 of 18 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


Figure 2. Upconversion emission spectra of the nanocrystals attached to OA(a) and OH(b) with different shell layer thickness.(c) Integrated emission intensity for 1D2→3H6, 1D2→3F4 and 1

G4→3H6 transitions versus different shell thickness calculated from fig.3(a) and fig.3(b). All

samples were excited with a 980 nm diode laser at a power density of 14.5 W cm-1


15


Figure 3. UC emission decay curves of Tm3+: 1D2→3F4(450 nm), 1G4 →3H6(474 nm) transitions for samples with different shell thickness under excitation at 980 nm. (a-b) Decay curves of 450 nm emission of UCNPs coated by OA and OH. (c-d) Decay curves of 450 nm emission of UCNPs coated by OA and OH.


16

Page 16 of 18

Page 17 of 18 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


Figure 4. Integrated emission intensity enhancement of 1D2→3H6, 1D2→3F4 and 1G4→3H6 transitions under excitation at 980 nm. (a) The UC emission spectrum of samples with a series of shell thickness under different power density. (b) Intensity enhancement ratios versus different shell thickness under different power density.


17


TOC graphics


18

Page 18 of 18

Probing Interaction Distance of Surface Quenchers in Lanthanide

Recommend Documents