Design for Brighter Photon Upconversion Emissions via Energy Level

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Design for Brighter Photon Upconversion Emissions via Energy Level Overlap of Lanthanide Ions Xingwen Cheng, Huan Ge, Yang Wei, Kun Zhang, Wenhong Su, Jimin Zhou, Lisha Yin, Qiuqiang Zhan, Su Jing, and Ling Huang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04988 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Design for Brighter Photon Upconversion Emissions via Energy Level Overlap of Lanthanide Ions Xingwen Cheng,†,§ Huan Ge,†,§ Yang Wei,† Kun Zhang,† Wenhong Su,† Jimin Zhou,‡ Lisha Yin,† Qiuqiang Zhan,# Su Jing,*‡ and Ling Huang*† †

Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for

Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816 China ‡

School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816

China #

Centre for Optical and Electromagnetic Research, Guangdong Provincial Key Laboratory

of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangzhou, China Corresponding Author *Email: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The perfect energy level overlap of 2H11/2, 4S3/2, and 4F9/2 in Er3+ ions with those of 5F3, 5

F4/5S2 and 5F5 in adjacently co-doped Ho3+ ions allows efficient inter-energy transfer.

Therefore, in addition to routine activators, Er3+ or Ho3+ can further act as sensitizers to transfer the upconverted energy to nearby Ho3+ or Er3+, resulting in enhanced upconversion luminescence due to the emission overlap. Proper co-doping of Er3+/Ho3+ or Ho3+/Er3+ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er3+ or Ho3+doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF4:Er/Ho@LiYF4 nanoparticles and Ho3+-sensitized Er3+ upconversion emissions excited by 1150 nm laser was sequentially obtained. With great enhancement compared with that of singly Ho3+ doped counterparts, this work demonstrates the generality and rationality of our design strategy. KEYWORDS energy level overlap, inter-energy transfer, enhancement, upconversion emission, lanthanide


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Although lanthanide-doped upconversion nanomaterials possess great potential in temperature probing, deep-tissue bioimaging, theranostics, three-dimensional display, superresolution nanoscopy, and lasing,1-13 the inherently low photon upconversion efficiency has severely limited their practical applications. Recently, considerable efforts have been devoted towards enhanced upconversion luminescence in approaches such as surface plasmon coupling, design of varied host lattice structures with optimal crystal parameters, formation of nanoclusters, energy transfer modulation, fabrication of core-shell nanostructures, as well as organic dye sensitization.14-18 For example, Zhao obtained 197-fold enhanced red-to-green intensity ratio by co-doping Mn2+ into α-phase NaYF4:Yb/Er nanoparticles,19 while Han realized red emissions from α-NaYbF4:Er(2 mol%)@CaF2 nanoparticles that are 15 times stronger than those of routinely designed β-NaYF4:Yb/Er(20/2 mol%)@β-NaYF4 nanoparticles.20 However, fabrication of the above-described nanomaterials usually requires complicated structural design, multi-step chemical reactions, and tedious synthesis operation. Therefore strategies that can enhance the upconversion emissions of nanomaterials with simplified photon upconversion designs and easy accessibility are highly desired. Because of the perfect energy level overlap of Er3+ located at 2H11/2 (19256 cm-1), 4S3/2 (18462 cm-1), and 4F9/2 (15245 cm-1) with those of 5F3 (20673 cm-1), 5F4/5S2 (18612 cm-1/18354 cm-1), and 5

F5 (15519 cm-1) of Ho3+, respectively,21,

22

inter-energy transfer or inter-sensitization

between optimally co-doped Er3+ and Ho3+ ions might occur under proper excitation such that enhanced upconversion emissions originated from the summed emissions of both Ho3+ and Er3+ can be obtained (Scheme 1).


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Herein we report a facile design where in addition to the intrinsic upconversion emission pathways that already exist in singly Er3+-doped host materials, a portion of the energy accumulated at the 2H11/2 and 4F9/2 energy levels of properly over-doped Er3+ might be simultaneously transferred to those of 5F4/5S2 and 5F5 of optimally co-doped Ho3+. Thus emissions at 544/550 and 648 nm are generated in Ho3+ when the energy relaxes to the ground level of 5I8, which overlap with those of Er3+ at 525/544 and 658 nm (Scheme 1a), respectively. On the other hand, optimally over-doped Ho3+ ions can also transfer energy accumulated at the 5F3 and 5F5 energy levels to those of 2H11/2 and 4F9/2 of adjacently codoped Er3+ ions under proper excitation, and emissions at 525/544 and 658 nm are generated with relaxation to the ground level of 4I15/2, which overlap with those of Ho3+ at 544/550 and 648 nm (Scheme 1b), respectively. Thus, enhanced upconversion emissions compared with those of singly Er3+ or Ho3+-doped host materials can be routinely obtained owing to dual contributions from Er3+/Ho3+ or Ho3+/Er3+. Moreover, the characteristic emission at 1172 nm from co-doped Ho3+ ions falls within the second near-infrared window (NIR-II, 1000-1700 nm), which brings extra value to this material design strategy to satisfy multi-purpose requirements, especially in the areas of deep-tissue bioimaging, photodynamic therapy, and optical communication.23-25

Results and Discussion To prove this concept, NaYF4:Er nanoparticles co-doped with and without Ho3+ were synthesized.26-28 The negligible ionic radius difference between Er3+ (0.089 nm) and Ho3+ (0.090 nm) allows arbitrary co-doping with each other, and a small variation of the doping concentration of Er3+ and Ho3+ does not obviously change either the size (Figure S1a-l and


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S2a-f in Supporting Information, SI) or crystal structure of the final products (Figure S3, SI). 29

Comparison of the upconversion luminescence intensities of NaYF4:Er and NaYF4:Er/Ho nanomaterials suggests the optimal molar concentration for singly doped Er3+ is 5 mol% (Figure 1a and Figure S1, SI), which increases to 10 mol% if 0.5 mol% Ho3+ ions are co-doped (Figure 1b and Figure S2, SI). This observation implies that co-doping of Ho3+ ions could increase the highest-possible luminescence quenching concentration of Er3+ from 5 mol% to 10 mol%, which further leads us to deduce that the first 5 mol% doped Er3+ ions might work as routine activators similar to those in singly Er3+-doped host materials, whereas the second 5 mol% doped Er3+ ions could work as sensitizers and transfer the upconverted excitation energy to adjacently co-doped 0.5 mol% Ho3+ due to the energy level overlap (Scheme 1a). Under 1532 nm laser excitation, emissions at 544/550 and 648 nm are generated when the energy at 5F4/5S2 and 5F5 levels of Ho3+ relaxes to the ground level of 5I8, which overlap with the emissions at 525/544 and 658 nm generated by the first 5 mol% doped Er3+, respectively. Since Ho3+ has no absorption at 1532 nm, the upconversion luminescence emission can only be sensitized by nearby Er3+ ions. Compared with singly Er3+-doped nanoparticles (Figure S4a and S4d, SI), the green and red emission intensities was enhanced 1.7 and 41 times by co-doping of Ho3+ (Figure S4b and S4e, SI), which further increased to 8.4 and 149 times (Figure 2a), respectively, after coating with a 2 nm NaYF4 inert layer (Figure S4c and S4f, SI).30, 31 Such an obvious emission intensity increase can be easily visualized by the naked eye (inset in Figure 2a). Singly Er3+-doped NaYF4 nanoparticles show green-dominant upconversion luminescence (Sample 1) while co-doping of Ho3+


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results in red-dominant emissions (Sample 2), which is further enhanced after NaYF4 shell coating (Sample 3). It is worth noting that phonon-assisted energy transfer from the 4I13/2 (6610 cm-1) energy level of Er3+ to that of 5I7 (5116 cm-1) in Ho3+ might also occur (Figure 2b),32-35 which is subsequently excited to the 5F5 level of Ho3+ through a two-photon upconversion process. Red emission at 648 nm after relaxation to 5I8 is then generated and overlaps with that of Er3+ at 658 nm, which is a three-photon upconversion process (Figure 2c). The limited enhancement (1.7 times) in the green emission at 544 nm (Figure 2a) might be due to the possible energy relaxation from 5F4/5S2 to 5I7 levels of Ho3+, which generates emission at 751 nm (Figure S1, SI). This is also reflected by the small lifetime prolongment (from 239.8 to 259.2 μs, Figure 2d) after Ho3+ co-doping.36 In contrast, the direct relaxation from the 5F5 to 5

I8 energy levels of co-doped Ho3+ ions significantly increases the overall lifetime of the 658

nm emission from 249.1 to 482.1 μs (Figure 2e), which agrees well with the 41 times intensification of the red upconversion emission. Because Er3+ ions have absorption at both 1532 and 980 nm,37 we further infer that such energy level overlap should also generate enhanced emissions under 980 nm laser excitation. Indeed, compared with nanoparticles doped only with Er3+ (10 mol%, Figure S5), a ~60-fold red upconversion emission enhancement in optimally 0.5 mol% Ho3+ co-doped NaYF4:Er nanoparticles (Figure S6, SI) was obtained and the emission color changed from green- to red-dominant (Samples 1 and 2 in Figure 3a). Moreover, an even more significant 199-fold red upconversion emission enhancement was obtained after a 2 nm NaYF4 layer coating (Sample 3 in Figure 3a). Meanwhile, the green emission at 544 nm was also enhanced by factors of 15 and 20 (Figure 3a) without and with NaYF4 layer coating, respectively.


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Similarly, the limited green emission enhancement is probably also due to the phononassisted 751 nm emission (Figure S5, SI).36 The photon upconversion and energy transfer pathways that cause the enhanced luminescence emissions under 980 nm laser excitation follow mechanisms (Figure 3b) similar to those at 1532 nm, as suggested by the power-density dependence and decay curves shown in Figure 3c-e.34 However, it should be noted that although 980 nm laser excitation causes enhanced upconversion emissions to a greater extent, the absolute upconversion emission intensity is still weaker than that of 1532 nm (Figure S7) because Er3+ has larger absorption cross-section constant at 1532 (1.7×10-21 cm-2) than at 980 nm (0.75×10-21 cm2 37

). Similar upconversion emission enhancement observed in other samples such as LiYF4

(Figure S8), NaGdF4 (Figure S9), and NaLuF4 (Figure S10) doped with Er3+ (10%) and Er3+/Ho3+ (10/0.5%) further proves the generality of this emission enhancement design strategy. Comparison of representative samples with strong red upconversion emissions indicates that the red emission from LiYF4:Er/Ho (Sample 4, Figure 4) remains record-strong under 1532 nm excitation, which is 23 times that of NaYF4:Er/Ho(10/0.5 mol%) (Sample 3, Figure 4). Although the absorption cross-section constant of Yb3+ at 980 nm (1.2×10-20 cm-2) is approximately one order of magnitude larger of that of Er3+ at 1532 nm, the emission of Sample 4 under 1532 nm excitation is still 34 (Sample 2) and 88 (Sample 1) times of those of NaYF4:Yb/Er/Mn(18/2/30 mol%) in Figure S11 (SI) and NaYF4:Er/Ho(10/0.5 mol%), respectively, under 980 nm excitation.


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Despite the fact that the record-strong red upconversion emission of αNaYF4:Yb/Er(80/2 mol%)@CaF2 under 980 nm excitation is ~6.8 times that of LiYF4:Er/Ho(10% mol)@LiYF4 under 1532 nm excitation (Figure S12), they actually have similar absolute red emission intensities because the absorption cross-section constant of Yb3+ at 980 nm is ~7 times that of Er3+ at 1532 nm.20 Thus, LiYF4:Er/Ho@LiYF4 has the strongest red upconversion emission noted thus far under 1532 nm excitation, which not only complements those previously reported NIR-I excitation sources at 980 and 808 nm, but also extends the excitation source to the NIR-II window for bioimaging or deep tissue thermodynamic therapy.23-25 Further studies have indicated that co-doping of Ho3+ with Er3+ can also result in emission in the NIR-II region under excitations at both 1532 and 980 nm. Under 1532 nm excitation (Figure 5a), the energy in 4I13/2 of Er3+ could transfer to 5I7 of Ho3+ through phononassisted energy transfer and get to the 5I5 of Ho3+ by absorbing one photon.32 The energy in 5

I5 of Ho3+ nonradiatively relaxes to the 5I6 and get to the ground state of 5I8 following the

NIR-II emission at 1172 nm (Figure 5b).32 Alternatively, the 4I11/2 of Er3+ transfers energy to 5

I6 of Ho3+ (Figure 5c) under 980 nm excitation,33 generating emission at 1172 nm when

relaxing to 5I8, as seen in both NaYF4:Er/Ho and NaYF4:Er/Ho@NaYF4 nanoparticles with enhancement factors of 1.56 and 2.23 (Figure 5b, d), respectively. Different from Er3+, Ho3+ has no absorption at 1532 nm, thus observation of the characteristic emission of Ho3+ at 1172 nm directly implies that Er3+ ions transfer the absorbed excitation energy to nearby Ho3+ ions. The optimal concentration of Er3+ is proved to be 10 mol%, whereas that of Ho3+ is 0.5 mol% for enhanced 1172 nm emission (Figure S13, SI). Similarly, when excited at 980 nm, the 1172 nm emission of optimally co-doped


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Ho3+ ions sensitized by properly over-doped Er3+ in NaYF4:Er/Ho (Figure S14, SI) also appears, which is further enhanced in core-shelled nanostructures (Figure 5d). Thus, combination of NIR-II emission and NIR-II excitation, i.e., integration of NIR-II emission at 1172 nm generated via energy transfer from 4I13/2 level of Er3+ to 5I7 level of co-doped Ho3+ (Figure 5) under NIR-II excitation at 1532 nm, is principally superior to the physical combination of visible light emission under NIR-I excitation in the above applications.23-25 Following the same design strategy, overlap of the 5F3 and 5F5 energy levels in Ho3+ with those of 2H11/2 and 4F9/2 in Er3+ also facilitates energy transfer from Ho3+ to Er3+ (Scheme 1b) such that properly over-doped Ho3+ can also sensitize optimally co-doped Er3+, generating upconversion emissions that overlap with those of Ho3+ (Figure S15, SI). Because Er3+ has no absorption at 1150 nm and except for the characteristic emission peaks of Ho3+ at 544, 550, and 648 nm in NaYF4:Ho nanoparticles, the newly appeared characteristic peaks of Er3+ at 525, 544, 658, and 664 nm (Figure 6) straightforwardly suggest the energy transfer from Ho3+ to Er3+. Obviously, co-doping of Er3+ with optimally over-doped Ho3+ also results in enhanced upconversion emissions, as seen in their respective optical images (inset in Figure 6). It should be emphasized that observation of Ho3+ ions working as both activators and as sensitizers under 1150 nm excitation has further widened the scope of photon upconversion luminescence explorations.

Conclusion In summary, we have proposed a general approach towards enhanced upconversion emissions via energy level overlap between co-doped lanthanide ions in which additional upconverted photons are generated due to the elevated highest-possible doping


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concentration. This facile and generally applicable design strategy offers more options for construction of nanomaterials with greatly enhanced upconversion luminescence without the addition of tedious synthesis procedures, especially if transition metal ions are combined for enrichment and fine-tuning of the upconversion luminescence. Moreover, the integration of NIR-II excitation and NIR-II emission into one host material facilitates even wider applications of such nanomaterials in biomedical applications, which are ongoing projects in our lab.

Experimental Section Materials Er(CH3CO2)3·4H2O (99.9%), Ho(CH3CO2)3·xH2O (99.9%), Y(CH3CO2)3·xH2O (99.9%), 1octadecene (ODE, 90%), and oleic acid (OA, 90%) were purchased from Sigma-Aldrich. NH4F (96+%) and NaOH (96+%) were purchased from Nanjing Wanqing Chemical Glassware Instrument. Trifluoroacetic acid (99%, Acros), ethanol (C2H6O, 99.9%), cyclohexane (C6H12, 99.5%), the corresponding lanthanide oxides (Y2O3, Yb2O3, Ho2O3, Er2O3), CaO, lanthanide nitrate (Y(NO3)3, Yb(NO3)3, Er(NO3)3) and manganese(II) chloride tetrahydrate (MnCl2•4H2O) were purchased from Sinopharm Chemical Reagent, Co., Ltd. (Beijing, China). All reagents and solvents were used as received without further purification. Synthesis of NaYF4:Er/Ho nanoparticles In a typical experiment, 0.4 mmol of Y(CH3CO2)3, Er(CH3CO2)3, and Ho(CH3CO2)3 water solutions at varied ratios were added into a 50 mL flask containing a mixture of 1-octadecene (7 mL) and oleic acid (3 mL). The solution was heated to 150 °C for 1 h and cooled down to


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room temperature. Subsequently, a 6 mL methanol solution of NH4F (1.6 mmol) and NaOH (1 mmol) was added to the flask, and the resulting mixture was stirred for 30 min. After removal of methanol by evaporation, the solution was heated to 290 °C under argon environment for 1.5 h and subsequently cooled down to room temperature. The nanoparticles were precipitated by addition of ethanol, collected by centrifugation, washed several times with cyclohexane and ethanol, and finally dispersed in cyclohexane. Synthesis of NaYF4:Er/Ho@NaYF4 nanoparticles In a typical experiment, water solution containing 0.2 mmol Y(CH3CO2)3 was added into a 100 mL flask containing a mixture of 1-octadecene (7 mL) and oleic acid (3 mL). The solution was heated to 150 °C for 1 h and cooled down to room temperature. Subsequently, a 6 mL methanol solution of NH4F (1.6 mmol) and NaOH (1 mmol) and the pre-formed core nanoparticles dispersed in cyclohexane were added to the flask, and the resulting mixture was stirred for 30 min. After removal of methanol by evaporation, the solution was heated to 290 °C under argon environment for 1.5 h and cooled down to room temperature. The nanoparticles were precipitated by addition of ethanol, collected by centrifugation, washed several times with cyclohexane and ethanol, and finally dispersed in cyclohexane. Synthesis of Ln(CF3COO)3 and Ca(CF3COO)2 In a typical process, lanthanide oxide, CaO, and trifluoroacetic acid at a certain ratio were added into a 100 mL flask under continuous stirring. The solution was refluxed at 90 °C until becoming transparent. After removal of insoluble substances via filtration, Ln(CF3COO)3 and Ca(CF3COO)2 were obtained by drying the filtrate at 85 °C for 24 h.


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Synthesis of LiYF4:Er and LiYF4:Er/Ho nanoparticles 0.8 mmol of Y(CF3COO)3, Er(CF3COO)3, and Ho(CF3COO)3 at varied ratios and 10 mL OA and 10 mL ODE were added into a 100 mL flask and heated to 125 °C for 30 min under N2 atmosphere. After heating to 320 °C for 40 min, the solution was naturally cooled down to room temperature and nanoparticles were precipitated by addition of ethanol and collected by centrifugation, which were washed several times using water and ethanol and finally dispersed in cyclohexane. Synthesis of LiYF4:Er/Ho@LiYF4 nanoparticles 0.4 mmol of Y(CF3COO)3, 10 mL OA and 10 mL ODE and 2 mL LiYF4:Er/Ho nanoparticles were added into a 100 mL flask and heated to 125 °C for 30 min under N2 atmosphere. The solution was heated to 320 °C for 40 min. After the solution was naturally cooled down to room temperature, the nanoparticles were precipitated by addition of ethanol and collected by centrifugation. After washing several times with water and ethanol, the product was finally dispersed in cyclohexane. Characterization Transmission electron microscopy (TEM) measurements were performed on a Hitachi 7700 transmission electron microscope at an acceleration voltage of 100 kV. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/max 2550 X-ray diffractometer with Cu Kα radiation (λ= 1.5406 Å). Upconversion luminescence spectra were measured using a Fluorolog-3 spectrometer (Horiba) under excitation at 1532 and 980 nm. The decay curves were detected on a Fluorolog-3 spectrometer (Horiba) equipped with pulsed 1532 and 980


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nm lasers as the excitation source. All spectra were recorded under either 1532 or 980 nm continuous-wave laser excitation at room temperature. For the part of 1150 nm excitation experiments, laser-scanning microscopic imaging and upconversion spectra were performed based on an inverted multi-photon scanning microscope coupled with the 1150 nm light source (excitation power: 500 W) output from an optical parameter oscillator pumped by a tunable Ti:Sapphire laser (Coherent Mira). Luminescence spectra for analysis were collected by a fiber spectrometer (QE65Pro, Ocean Optics).


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Figure 1. Upconversion luminescence spectra and optical images of (a) NaYF4:Er nanoparticles as a function of Er3+ doping concentration and those of (b) NaYF4:Er/Ho when co-doped with 0.5 mol% Ho3+. Both spectra were collected under 1532 nm laser excitation.


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Figure 2. (a) Upconversion luminescence spectra and optical images of 1: NaYF4:Er(10 mol%), 2: NaYF4:Er/Ho(10/0.5 mol%), and 3: NaYF4:Er/Ho(10/0.5 mol%)@NaYF4 nanoparticles. (b) Proposed energy transfer pathways from Er3+ to co-doped Ho3+. The black solid arrows, dashed, wavy, and colorful solid arrows represent photon excitation, multiphonon relaxation, energy transfer, and emission processes, respectively. (c) Powerdensity dependence of Er3+ emissions at 544 and 658 nm in NaYF4:Er/Ho(10/0.5 mol%) nanoparticles. Luminescence decay curves of (d) NaYF4:Er(10 mol%) and (e)


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NaYF4:Er/Ho(10/0.5 mol%). All of the spectra were collected under 1532 nm laser excitation.


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Figure 3. (a) Upconversion luminescence spectra and optical images of 1: NaYF4:Er(10 mol%), 2: NaYF4:Er/Ho(10/0.5 mol%), and 3: NaYF4:Er/Ho(10/0.5 mol%)@NaYF4 nanoparticles. (b) Proposed energy transfer pathways from Er3+ to co-doped Ho3+. The black solid arrows, dashed, wavy, and colorful solid arrows represent photon excitation, multiphonon relaxation, energy transfer, and emission processes, respectively. (c) Powerdensity dependence of Er3+ emissions at 544 and 658 nm in NaYF4:Er/Ho(10/0.5 mol%)


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nanoparticles. Luminescence decay curves of (d) NaYF4:Er(10 mol%) and (e) NaYF4:Er/Ho(10/0.5 mol%). All of the spectra were collected under 980 nm laser excitation.


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Figure 4. Upconversion luminescence spectra of 1: NaYF4:Er/Ho(10/0.5 mol%) and 2: αNaYF4:Yb/Er/Mn(18/2/30 Upconversion

mol%)

luminescence

nanoparticles

spectra

of

3:

under

980

nm

NaYF4:Er/Ho(10/0.5

laser mol%)

excitation. and

4:

LiYF4:Er/Ho(10/0.5 mol%) nanoparticles under 1532 nm laser excitation. Inset: Optical images of samples 1-4.


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Figure 5. (a) Proposed energy transfer pathways between Er3+ and Ho3+ under 1532 nm laser excitation. The black solid arrows, dashed, wavy, and red solid arrows represent photon excitation, multiphonon relaxation, energy transfer, and emission processes, respectively. (b) NIR-II emission spectra of NaYF4:Er/Ho and NaYF4:Er/Ho@NaYF4 nanoparticles. All spectra were recorded under 1532 nm continuous-wave laser excitation at room temperature. (c) Proposed energy transfer processes between Er3+ and Ho3+ under 980 nm laser excitation. The solid black arrows, dashed, wavy, and red solid arrows represent photon excitation, multiphonon relaxation, energy transfer, and emission processes, respectively. (d) NIR-II emission spectra of NaYF4:Er/Ho and NaYF4:Er/Ho@NaYF4 nanoparticles. All spectra were recorded under 980 nm continuous-wave laser excitation at room temperature.


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Figure 6. (a) Upconversion luminescence spectra and optical images of NaYF4:Ho(1 mol%) and NaYF4:Ho/Er(1/2 mol%) nanoparticles under 1150 nm laser excitation. Inset shows optical images with improved emission intensity after Er3+ co-doping.


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Scheme 1. Schematic illustrations of the energy transfer pathways between (a) Er3+/Ho3+ and (b) Ho3+/Er3+ under 1532 and 1150 nm laser excitation, respectively.


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Author Contributions §

X. C. and H. G. contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21671101 and 61675071) and the Guangdong Provincial Science Fund for Distinguished Young Scholars (2018B030306015), the Pearl River Nova Program of Guangzhou (201710010010). ASSOCIATED CONTENT The XRD pattern, TEM images, size contribution, upconversion luminescence spectra, and upconversion luminescence mechanism of prepared nanoparticles are provided. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.


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Table of Contents

Taking advantage of the energy level overlap in Er3+ with those of adjacently co-doped Ho3+, a facile and general design strategy is proposed whereby except as routine activators, properly over-doped Er3+ or Ho3+ can also sensitize co-doped Ho3+ or Er3+ for additional upconversion emissions such that enhanced luminescence emissions in lanthanide-doped upconversion nanomaterials are readily obtained.


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Design for Brighter Photon Upconversion Emissions via Energy Level

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