Enhanced Red Emission in Er³⁺-Sensitized NaLuF₄ Upconversion Crystals via Energy Trapping

Abstract

Luminescence efficiency of trivalent lanthanide-doped upconversion (UC) materials is significantly limited by luminescence concentration quenching. In this work, red UC emission is dramatically enhanced in Er 3+sensitized NaLuF 4 UC crystals through energy trapping under multiple excitation wavelengths. Cross-relaxation quenching and the energy migration to internal lattice defects are simultaneously suppressed by confining the excitation energy in the Er 3+ activator after introducing the Tm 3+ or Ho 3+ energy trapping center. The enhanced red UC emission (Er 3+ : 660 nm) mainly comes from the effective excitation energy confinement by Tm 3+ and Ho 3+ trapping centers through an easy energy transfer between Er 3+ and Tm 3+ /Ho 3+ : 4 I 11/2 (Er 3+ ) → 3 H 5 (Tm 3+ ) → 4 I 13/2 (Er 3+ ) and 4 I 11/2 (Er 3+ ) → 5 I 6 (Ho 3+ ) → 4 I 13/2 (Er 3+ ). It is found that the confining efficiency of excitation energy in Er 3+ -sensitized NaLuF 4 crystals is higher than that in Yb 3+ /Er 3+ cosensitized NaLuF 4 crystals, and the luminescence efficiency of Er 3+ -sensitized NaLuF 4 crystals is much higher than that of Er 3+ -based host sensitization UC crystals (NaErF 4 ). Moreover, Er 3+ -sensitized UC particles can be efficiently excited by three different wavelengths (808, 980, and 1532 nm), indicating huge advantages for applications in bioimaging, anticounterfeiting, and solar cells.

Second near-infrared upconversion and downconversion luminescence in a core–multishell nanophotoswitch

“…30 However, excessive doping of the Ho 3+ trapping center may increase the nonradiation loss, resulting in the decline of the emission intensity. 21 A similar result is obtained under 1532 nm NIR-II excitation (Fig. S3, ESI†).…”

“…17–19 Specifically, for photodynamic therapy (PDT), chemical reactions can be induced by ultraviolet (UV)–blue emissions to kill tumor cells, 18,19 and red emission (600–700 nm) can be used for real-time imaging owing to its deep tissue penetration. 20–22 Kong's group reported a core–multishell UC nanophotoswitch, with which imaging-guided “off–on” therapy could be realized by UV–blue and red emissions under 980 and 808 nm excitations. 19 Li et al demonstrated mesoporous silica nanospheres with CaF 2 :Yb,Er UC nanocrystals entrapped in their porous structure, and they found that the coating of the thin MnO 2 layer may enhance the red-to-green ratio and red emission intensity, resulting in an efficient energy transfer to the loaded photosensitizer Chlorin e6 (Ce6), which favors the PDT.…”

Dual-Wavelength Excited Intense Red Upconversion Luminescence from Er3+-Sensitized Y2O3 Nanocrystals Fabricated by Spray Flame Synthesis

“…Furthermore, we also demonstrated the possible mechanism of ET and BET processes, non-radiative transitions, and UC emissions under 808 nm excitation, as depicted in Figure 7 d. Unlike the processes under 980 nm excitation, firstly, the electrons in the ground state of Er 3+ were excited to the 4 I 9/2 state by GSA after absorbing an 808 nm photon, then reached the 4 I 11/2 state through a non-radiative transition. After that, the emitting states ( 2 H 11/2 , 4 S 3/2 and 4 F 9/2 ) could be populated by ETU, CR and BET processes, which were elaborated in Figure 5 [ 51 ]. In addition, the power dependence tendencies of the UCL in Y 2 O 3 :Er 3+ (8 mol%) and Y 2 O 3 :Er 3+ /Tm 3+ (8/1 mol%) UCNPs are given in Figure S4a,b , which indicates that all the UC emissions also exhibited a two-photon absorption process under 808 nm excitation.…”

Remarkably Enhanced Red Upconversion Emission in β-NaLuF4:Er,Tm Microcrystals via Ion Exchange

“…The R/G ratio before the ion exchange is much larger than that after the ion exchange as the doping concentration of Tm 3+ is less than 1% (Figure e). It might due to the fact that the higher concentration of the OH – vibrations before the ion exchange can greatly depopulate the 4 I 11/2 excited state to the 4 I 13/2 excited state and further populate the 4 F 9/2 state by energy pumping with another 980 nm photon. ,, However, as the doping concentration of Tm 3+ is above 1%, the R/G ratio before the ion exchange increases quite smoothly while the R/G ratio after the ion exchange further increases sharply with the increase of Tm 3+ , and thus, the R/G ratio is obviously larger than that after the ion exchange. This is due to the fact that the OH – removal after the ion exchange effectively suppresses multiphonon nonradiative relaxation and accelerates the energy transfer between Er 3+ and Tm 3+ with the increase of Tm 3+ , which results in the enhanced R/G ratio .…”

“…Er 3+ -enriched upconversion materials usually demonstrate a high R/G ratio via the intense cross-relaxation among Er 3+ activators. − In contrast to the conventional Yb 3+ -sensitized upconversion crystals, Er 3+ acts as both the sensitizer and activator. , Er 3+ -sensitized upconversion materials can be efficiently excited by multiband excitation in the near-infrared region (∼800, ∼980, and ∼1530 nm), which is essential for extending their applications . However, a high doping level of Er 3+ always causes a severe concentration quenching effect owing to fast energy migration from the sensitizers to the internal OH – defects and surface defects. − An appropriate doping of Tm 3+ into the Er 3+ -enriched crystals not only favors the population of the 4 F 9/2 state and lifts the R/G ratio via the energy transfer between Er 3+ and Tm 3+ but also maximally minimizes this luminescence quenching effect as trapping excitation energies. ,, However, the main quenching pathway is still energy transfer to internal OH – defects because the internal OH – is easy to be introduced into the crystal lattice by substituting F – ions. ,, A commonly employed method for OH – removal is high-temperature annealing, but the morphology and size of the products are uncontrollable. − Recently, the ion-exchange method is an attractive way to improve upconversion luminescence and preserve the crystal structure and monodispersed morphology well. − A great amount of Na + and F – is essential for the ion exchange process, and the concentration of OH – defects can be greatly reduced by the replacement of F – ions.…”