We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF 3 and BaLaF 5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na + for La 3+ ions (97% of total Na + codopant) in the crystal system, and interstitial Na + (3% of detected Na +) and La 3+ (3% of detected La 3+) present in (Na)LaF 3 and only direct substitution of Na + for Ba 2+ in Ba(Na)LaF 5. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08–0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E. 3+) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5–100 W/cm 2 range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm 3+ 3F 3 and 3H 4 states was observed and noted as a key initial condition for effective population of the 1D 2 and 1I 6 UV emitting states, via Tm-Tm cross-relaxation.
References
[1]
Xu, C.F.; Ma, M.; Yang, L.W.; Zeng, S.J.; Yang, Q.B. Lanthanide doping-facilitated growth of ultrasmall monodisperse Ba2LaF7 nanocrystals with excellent photoluminescence. J. Colloid Interface Sci. 2012, 368, 49–55, doi:10.1016/j.jcis.2011.10.072.
[2]
Wang, F.; Liu, X. Upconversion multicolor fine-tuning: Visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles. J. Am. Chem. Soc. 2008, 130, 5642–5643, doi:10.1021/ja800868a.
[3]
Wang, F.; Liu, X. Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals. Chem. Soc. Rev. 2009, 38, 976–989, doi:10.1039/b809132n.
[4]
Yi, G.-S.; Chow, G.-M. Colloidal LaF3:Yb,Er, LaF3:Yb,Ho and LaF3:Yb,Tm nanocrystals with multicolor upconversion fluorescence. J. Mater. Chem. 2005, 15, 4460–4464, doi:10.1039/b508240d.
[5]
Chen, G.; Shen, J.; Ohulchanskyy, T.Y.; Patel, N.J.; Kutikov, A.; Li, Z.; Song, J.; Pandey, R.K.; ？gren, H.; Prasad, P.N. (α-NaYbF4:Tm3+)/CaF2 core/shell nanoparticles with efficient near-infrared to near-infrared upconversion for high-contrast deep tissue bioimaging. ACS Nano 2012, 6, 8280–8287, doi:10.1021/nn302972r.
[6]
Dong, C.; Pichaandi, J.; Regier, T.; van Veggel, F.C.J.M. Nonstatistical dopant distribution of Ln3+-doped NaGdF4 nanoparticles. J. Phys. Chem. C 2011, 115, 15950–15958.
Pichaandi, J.; Boyer, J.-C.; Delaney, K.R.; van Veggel, F.C.J.M. Two-photon upconversion laser (scanning and wide-field) microscopy using Ln3+-doped NaYF4 upconverting nanocrystals: A critical evaluation of their performance and potential in bioimaging. J. Phys. Chem. C 2011, 115, 19054–19064.
[9]
Chatterjee, D.K.; Fong, L.S.; Zhang, Y. Nanoparticles in photodynamic therapy: An emerging paradigm. Adv. Drug Deliv. Rev. 2008, 60, 1627–1637, doi:10.1016/j.addr.2008.08.003.
[10]
Yang, Y.; Velmurugan, B.; Liu, X.; Xing, B. NIR photoresponsive crosslinked upconverting nanocarriers toward selective intracellular drug release. Small 2013, 9, 2937–2944, doi:10.1002/smll.201201765.
[11]
Yang, Y.; Shao, Q.; Deng, R.; Wang, C.; Teng, X.; Cheng, K.; Cheng, Z.; Huang, L.; Liu, Z.; Liu, X.; et al. In vitro and in vivo uncaging and bioluminescence imaging by using photocaged upconversion nanoparticles. Angew. Chem. 2012, 51, 3125–3129, doi:10.1002/anie.201107919.
[12]
Auzel, F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev. 2004, 104, 139–173, doi:10.1021/cr020357g.
[13]
Suyver, J.F.; Grimm, J.; van Veen, M.K.; Biner, D.; Kr？mer, K.W.; Güdel, H.U. Upconversion spectroscopy and properties of NaYF4 doped with Er3+, Tm3+ and/or Yb3+. J. Lumin. 2006, 117, 1–12, doi:10.1016/j.jlumin.2005.03.011.
[14]
Huang, T.C.; Hsieh, W.F. Er-Yb codoped ferroelectrics for controlling visible upconversion emissions. J. Fluoresc. 2009, 19, 511–516, doi:10.1007/s10895-008-0440-0.
[15]
Mai, H.-X.; Zhang, Y.-W.; Sun, L.-D.; Yan, C.-H. Highly efficient multicolor up-conversion emissions and their mechanisms of monodisperse NaYF4:Yb,Er core and core/shell-structured nanocrystals. J. Phys.Chem. C 2007, 111, 13721–13729, doi:10.1021/jp073920d.
[16]
Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning upconversion through energy migration in core–shell nanoparticles. Nat. Mater. 2011, 10, 968–973, doi:10.1038/nmat3149.
[17]
Zheng, K.; Qin, W.; Wang, G.; Wei, G.; Zhang, D.; Wang, L.; Kim, R.; Liu, N.; Ding, F.; Xue, X. Upconversion luminescence properties of Yb3+, Gd3+ and Tm3+ co-doped NaYF4 microcrystals synthesized by the hydrothermal method. J. Nanosci. Nanotechnol. 2010, 10, 1920–1923, doi:10.1166/jnn.2010.2077.
[18]
Wang, Y.; Liu, K.; Liu, X.; Dohnalova？, K.I.; Gregorkiewicz, T.; Kong, X.; Aalders, M.C.G.; Buma, W.J.; Zhang, H. Critical shell thickness of core/shell upconversion luminescence nanoplatform for FRET application. J. Phys. Chem. Lett. 2011, 2, 2083–2088, doi:10.1021/jz200922f.
[19]
Cheng, Q.; Sui, J.; Cai, W. Enhanced upconversion emission in Yb3+ and Er3+ codoped NaGdF4 nanocrystals by introducing Li+ ions. Nanoscale 2012, 4, 779–784, doi:10.1039/c1nr11365h.
[20]
Huang, Q.; Yu, J.; Ma, E.; Lin, K. Synthesis and characterization of highly efficient near-infrared upconversion Sc3+/Er3+/Yb3+ tridoped NaYF4. J. Phys. Chem. C 2010, 114, 4719–4724, doi:10.1021/jp908645h.
[21]
Chen, D.; Yu, Y.; Huang, F.; Wang, Y. Phase transition from hexagonal LnF3 (Ln = La, Ce, Pr) to cubic Ln0.8M0.2F2.8 (M = Ca, Sr, Ba) nanocrystals with enhanced upconversion induced by alkaline-earth doping. Chem. Commun. 2011, 47, 2601–2603, doi:10.1039/c0cc04846a.
[22]
Chen, G.Y.; Liu, H.C.; Somesfalean, G.; Sheng, Y.Q.; Liang, H.J.; Zhang, Z.G.; Sun, Q.; Wang, F.P. Enhancement of the upconversion radiation in Y2O3:Er3+ nanocrystals by codoping with Li+ ions. Appl. Phys. Lett. 2008, 92, 96–100.
[23]
Wang, C.; Tao, H.; Cheng, L.; Liu, Z. Near-infrared light induced in vivo photodynamic therapy of cancer based on upconversion nanoparticles. Biomaterials 2011, 32, 6145–6154.
[24]
Xiong, L.; Yang, T.; Yang, Y.; Xu, C.; Li, F. Long-term in vivo biodistribution imaging and toxicity of polyacrylic acid-coated upconversion nanophosphors. Biomaterials 2010, 31, 7078–7085, doi:10.1016/j.biomaterials.2010.05.065.
[25]
Jalil, R.A.; Zhang, Y. Biocompatibility of silica coated NaYF4 upconversion fluorescent nanocrystals. Biomaterials 2008, 29, 4122–4128, doi:10.1016/j.biomaterials.2008.07.012.
[26]
Li, C.; Lin, J. Rare earth fluoride nano-/microcrystals: Synthesis, surface modification and application. J. Mater. Chem. 2010, 20, 6831–6847, doi:10.1039/c0jm00031k.
[27]
Hu, H.; Xiong, L.; Zhou, J.; Li, F.; Cao, T.; Huang, C. Multimodal-luminescence core-shell nanocomposites for targeted imaging of tumor cells. Chemistry 2009, 15, 3577–3584, doi:10.1002/chem.200802261.
[28]
Wang, L.; Yan, R.; Huo, Z.; Wang, L.; Zeng, J.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. Fluorescence resonant energy transfer biosensor based on upconversion-luminescent nanoparticles. Angew. Chem. 2005, 117, 6208–6211, doi:10.1002/ange.200501907.
[29]
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, doi:10.1002/adma.200901174.
[30]
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, doi:10.1364/OE.16.011907.
[31]
Balakrishnaiah, R.; Kim, D.W.; Yi, S.S.; Jang, K.; Lee, H.S.; Ahn, C.W.; Jeong, J.H. Infrared to ultraviolet frequency upconversion studies of Tm3+ ions in a YbLiF4 crystal. J. Korean Phys. Soc. 2008, 53, 791–795, doi:10.3938/jkps.53.791.
[32]
Shan, J.; Ju, Y. A single-step synthesis and the kinetic mechanism for monodisperse and hexagonal-phase NaYF4:Yb,Er upconversion nanophosphors. Nanotechnology 2009, 20, 275603:1–275603:13.
[33]
Chen, X.Y.; Zhuang, H.Z.; Liu, G.K.; Li, S.; Niedbala, R.S. Confinement on energy transfer between luminescent centers in nanocrystals. J. Appl. Phys. 2003, 94, 5559–5565, doi:10.1063/1.1614865.
[34]
Wang, Y.; Qin Wei, P.; Di, W.-H.; Zhang Ji, S.; Cao, C.-Y. Infrared-to-visible and infrared-to-violet upconversion fluorescence of rare earth doped LaF3 nanocrystals. Chin. Phys. B 2008, 17, 3300–3305, doi:10.1088/1674-1056/17/9/026.
[35]
Yi, G.-S.; Chow, G.-M. Water-soluble NaYF4:Yb,Er(Tm)/NaYF4/polymer core/shell/shell nanoparticles with significant enhancement of upconversion fluorescence. Chem. Mater. 2007, 19, 341–343, doi:10.1021/cm062447y.
[36]
Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Luo, W.; Chen, X. A strategy to achieve efficient dual-mode luminescence of Eu3+ in lanthanides doped multifunctional NaGdF4 nanocrystals. Adv. Mater. 2010, 22, 3266–3271, doi:10.1002/adma.201000128.
[37]
Chen, G.; Yang, C.; Prasad, P.N. Nanophotonics and nanochemistry: Controlling the excitation dynamics for frequency up- and down-conversion in lanthanide-doped nanoparticles. Acc. Chem. Res. 2013, 46, 1474–1486, doi:10.1021/ar300270y.
[38]
Zhang, Q.; Zhang, Q.-M. Synthesis and photoluminescent properties of α-NaYF4:Nd/α- NaYF4 core/shell nanostructure with enhanced near infrared (NIR) emission. Mater. Lett. 2009, 63, 376–378, doi:10.1016/j.matlet.2008.10.064.
[39]
Qu, Y.; Li, M.; Zhang, L.; Zhao, L. Preparation and characterization of upconversion luminescent LaF3:Yb3+, Er3+/LaF3 core/shell nanocrystals. Appl. Surf. Sci. 2011, 258, 34–37, doi:10.1016/j.apsusc.2011.07.144.
[40]
Bai, Y.; Wang, Y.; Yang, K.; Zhang, X.; Song, Y.; Wang, C.H. Enhanced upconverted photoluminescence in Er3+ and Yb3+ codoped ZnO nanocrystals with and without Li+ ions. Opt. Commun. 2008, 281, 5448–5452, doi:10.1016/j.optcom.2008.07.041.
[41]
Chen, G.Y.; Liu, H.C.; Liang, H.J.; Somesfalean, G.; Zhang, Z.G. Enhanced multiphoton ultraviolet and blue upconversion emissions in Y2O3:Er3+ nanocrystals by codoping with Li+ ions. Solid State Commun. 2008, 148, 96–100, doi:10.1016/j.ssc.2008.08.001.
[42]
Awan, S.U.; Hasanain, S.K.; Bertino, M.F.; Jaffari, G.H. Ferromagnetism in Li doped ZnO nanoparticles: The role of interstitial Li. J. Appl. Phys. 2012, 112, 103924:1–103924:9.
[43]
Cates, E.L.; Wilkinson, A.P.; Kim, J.-H. Delineating mechanisms of upconversion enhancement by Li+ codoping in Y2SiO5:Pr3+. J. Phys. Chem. C 2012, 116, 12772–12778, doi:10.1021/jp302515t.
[44]
Baikie, T.; Ng, G.M.; Madhavi, S.; Pramana, S.S.; Blake, K.; Elcombe, M.; White, T.J. The crystal chemistry of the alkaline-earth apatites A10(PO4)6CuxOy(H)z (A = Ca, Sr and Ba). Dalton Trans. 2009, 34, 6722–6726.
[45]
Haase, M.; Schafer, H. Upconverting nanoparticles. Angew. Chem. 2011, 50, 5808–5829, doi:10.1002/anie.201005159.
[46]
Mai, H.-X.; Zhang, Y.-W.; Sun, L.-D.; Yan, C.-H. Size- and phase-controlled synthesis of monodisperse NaYF4:Yb,Er nanocrystals from a unique delayed nucleation pathway monitored with upconversion spectroscopy. J. Phys. Chem. C 2007, 111, 13730–13739, doi:10.1021/jp073919e.
[47]
Pollnau, M.; Gamelin, D.R.; Lüthi, S.R.; Güdel, H.U.; Hehlen, M.P. Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B 2000, 61, 3337–3346, doi:10.1103/PhysRevB.61.3337.
[48]
Suyver, J.; Aebischer, A.; García-Revilla, S.; Gerner, P.; Güdel, H. Anomalous power dependence of sensitized upconversion luminescence. Phys. Rev. B 2005, 71, 125123:1–125123:9.
[49]
Renero-Lecuna, C.; Marti？n-Rodri？guez, R.; Valiente, R.; Gonza？lez, J.; Rodri？guez, F.; Kr？mer, K.W.; Güdel, H.U. Origin of the high upconversion green luminescence efficiency in β-NaYF4:2% Er3+, 20% Yb3+. Chem. Mater. 2011, 23, 3442–3448, doi:10.1021/cm2004227.
