Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Mechanistic Investigations on the Dramatic Thermally Induced Luminescence Enhancement in Upconversion Nanocrystals Yanqing Hu,† Qiyue Shao,*,† Peigen Zhang,† Yan Dong,† Feng Fang,† and Jianqing Jiang†,‡ †
School of Materials Science and Engineering, Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, People’s Republic of China ‡ School of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, People’s Republic of China
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S Supporting Information *
ABSTRACT: Luminescent bulk materials generally suffer from thermal quenching, while upconversion nanocrystals (UCNCs) have recently been found to show increase of dramatic emission at elevated temperatures. A deep understanding on this quite different light−heat interaction at the nanoscale is important both scientifically and technologically. Herein, temperature-dependent upconversion luminescence (UCL) is investigated for UCNCs with various sizes, activators (Ho3+, Tm3+, Er3+), and core/shell structures. An anomalous UCL enhancement with increasing temperature is found for UCNCs with larger surface/volume ratios (SVRs). Moreover, this UCL increase shows a pronounced dependence on the SVRs, activators, emitting levels, and measuring environments. Substantial evidence confirms that the thermally induced UCL increase is primarily due to the temperature-dependent quenching effect of surface-adsorbed H2O molecules, instead of the previously proposed surface phonon-assisted mechanism. Temperature-dependent spectral investigations also show that the energy-loss process of Yb3+-sensitized UCNCs is largely due to the deactivation of Yb3+ ions caused by surface quenchers, rather than the direct quenching to activators. UCNCs with an active shell (doped with Yb3+) exhibit similar thermally induced UCL increase, due to energy migration to the surface over the Yb−Yb internet. It implies that active-core/active-shell UCNCs are susceptible to surface quenchers and would be unsuitable for applications in aqueous environments.
1. INTRODUCTION Lanthanide-doped upconversion nanocrystals (UCNCs) are highly desired for applications in life science,1−4 safety markers,5−7 and photovoltaics,8 due to their unique antiStokes emission characteristics upon near-infrared (NIR) excitation. Among various UCNCs, hexagonal NaYF4:Yb/Er is considered as one of the most efficient UC nanomaterials. Despite over 10 years of development, the upconversion luminescence (UCL) efficiency of NaYF4-based UCNCs is still much lower than the corresponding bulk materials.9,10 Even in the case of surface-protected core/shell UCNCs, especially for small ones, the UCL efficiency is still lower than that of their bulk counterparts.11 Therefore, the deep understanding on UCL quenching pathways at the nanoscale is necessary to further guide the design and synthesis of more efficient UCNCs. In luminescent materials, radiative transition (i.e., luminescence) directly competes with nonradiative transition (i.e., heat generation). The relative probability between them determines the quantum yield of luminescent materials. In addition to crystalline phase, particle size, doping concentration, surface modification, etc., temperature is another key factor that can © XXXX American Chemical Society
affect both radiative and nonradiative transition processes in UCNCs. Thus, temperature-dependent luminescence investigations provide a useful approach to identify the dominant energy-loss pathways in UCNCs. In general, the interplay behavior between light and heat in luminescence materials is described by the well-known thermal quenching effect. However, it fails to explain the complex temperature dependences of UCL. For instance, it was found that the UCL of NaYF4:Yb/Er bulk materials or nanomaterials exhibited an anomalous increase with the temperature increasing over 10−200 K.12−14 This phenomenon has been well documented in the literature and attributed to the increased population of the 2F5/2 |1> crystal-field level of Yb3+ ions at elevated temperatures, thus leading to more efficient energy transfer from Yb3+ to Er3+.13,14 For both bulk UC materials and large UCNCs, a normal thermal quenching was observed with further increased temperatures above room temperature (RT).13−15 Surprisingly, our group found that Received: August 14, 2018 Revised: October 16, 2018 Published: October 18, 2018 A
DOI: 10.1021/acs.jpcc.8b07899 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. (a1−d1) TEM (or SEM) images, (a2−d2) temperature-dependent UCL spectra, and (a3−d3) normalized intensities of NaGdF4:20%Yb/ 2%Ho small-NPs (column a1−3), NaYF4:20%Yb/2%Ho short-NRs (column b1−3), NaYF4:20%Yb/2%Ho large-NPs (column c1−3), and NaYF4:20% Yb/2%Ho long-MRs (column d1−3). Small-NPs: ∼7 nm; short-NRs: ∼17 × 44 nm2; large-NPs: ∼28 nm; long-MRs: ∼190 × 1600 nm2. Laser power density: 1.6 W/cm2. Int. I: integrated intensity including green and red bands.
istics, UCNCs can be used to design fluorescence nanothermometers.21−23 Temperature-sensing applications based on UCNCs have been demonstrated in both living cells and microfluidics.24,25 Utilizing both the luminescent temperature dependences and photothermal conversion capability of UCNCs, our group designed the nanoparticle (NP)/nanowire composite UC nanosystems and core/shell-structured UCNCs doped with Ho3+ and Tm3+ ions.26,27 Both of them showed obvious color shifts upon irradiation by the NIR laser and could be used to produce more secure anticounterfeiting patterns.26,27 Therefore, a deep understanding on the luminescent thermal behavior of UCNCs would be beneficial for further improvements of their practical performances in specific application fields. In this work, temperature-dependent emission properties of UCNCs with various sizes and activators were investigated. The influences of measuring atmospheres and core/shell structures on their luminescent thermal behavior were discussed. We presented bulk of spectral analysis confirming that the thermally enhanced UCL of small UCNCs actually originated from temperature-dependent surface quenching effects, instead of the surface phonon-assisted upconversion. In addition, the main quenching pathways of core-only and core/shell UCNCs were discussed. The results not only illuminate the unique light−heat interplay mechanism in UC
small NaYF4:Yb/Er UCNCs (3 nm) to effectively reduce the surface quenching, which is in agreement with the reported results in the literature.36,37 As an alternative means, one-pot heating-up (OPH) method has also been widely used to synthesize core/shell UCNCs.35,38 However, it is found that the core is only partially covered by the shell using the OPH method.35 We synthesized NaGdF4:Yb/Tm@NaGdF4 active-core/inert-shell UCNC (shell thickness ∼3.5 nm, Figure S24) via an OPH method and recorded their temperature-dependent UCL spectra. As shown in Figure 9b, an obvious UCL increase at elevated temperatures was found for the OPH-synthesized
that more excitation energy can be transferred to activators, which is in favor of the UC emissions from higher levels. Therefore, the UCL at shorter wavelengths is more obviously enhanced than that at longer wavelengths with increasing temperature. 3.4. Temperature-Dependent UCL of Core/Shell NCs. To shed more light on the understanding about the thermal behavior of UCL, temperature-dependent UC emissions of active-core/inert-shell UCNCs were also investigated. NaGdF4:Yb/Er@NaGdF4 core/shell UCNCs (size ∼14 nm, Figure 8a) have a core diameter of ∼7 nm and a shell thickness of ∼3.5 nm. Unlike core-only NCs, the UCL intensity of activecore/inert-shell UCNCs shows a continuous decline with increasing temperature (Figure 8b,c). It indicates that the inert shell can greatly suppress the influences of surface-adsorbed moisture on the emissive core (Figure 9a). The thermal quenching of Yb3+ or Er3+ ions results in the UCL decline at higher temperatures. This result further confirms that the UCL increase of core-only NCs at elevated temperatures is due to temperature-dependent surface quenching effects. Small UCNCs have extremely large SVRs and their UCL properties are highly susceptible to the surface quenching. Growth of the inert shell around the emissive core is an effective strategy to suppress surface quenching processes. In this case, the completeness of the inert shell is essential to insulate the core from the environment influence. In general, the characterization of the shell completeness needs complex and costly electron microscopy or elemental analysis G
DOI: 10.1021/acs.jpcc.8b07899 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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core to the surface would result in the decreased UCL intensity. To validate the above hypothesis, the UCL properties of NaGdF4:Yb/Er@NaGdF4 active-core/inert-shell and NaGdF4:Yb/Er@NaGdF4:Yb active-core/active-shell UCNCs were compared in various measuring environments. Both of them have an average diameter of ∼14 nm and a shell thickness of ∼3.5 nm. When dispersed in cyclohexane, active-core/activeshell UCNCs show higher emission intensity than the activecore/inert-shell ones (Figure 11a). By contrast, in solid state, the UCL intensity of active-core/active-shell UCNCs is significantly lower than that of the active-core/inert-shell ones (Figure 11b). The Yb3+ ions in the active shell can play two roles: amplifying the absorption to the excitation energy and dissipating the absorbed energy through energy migration to the surface. The relatively strong UCL of active-core/activeshell NCs dispersed in cyclohexane can be attributed to the extra absorption to the pump light by the active shell and subsequent energy transfer to the active core. For solid-state UCNCs, the quenching effect of surface-adsorbed moisture is strong. The energy migration to the surface over Yb3+−Yb3+ internet reduce the overall energy transferred to the emitting ions and induces the decreased UCL of solid-state active-core/ active-shell UCNCs (Figure 9a). We also transferred two types of core/shell UCNCs into the CO-520/ethanol/water (50:40:10 vol %) solution, where the quenching effect of water molecules was much stronger. Further decreased emission intensity was detected for active-core/active-shell UCNCs (Figure 11c). This direct data comparison further confirms that the active-core/active-shell UCNCs are more susceptible to the surface quenching and may be not suitable for applications in aqueous system. Although great advances have been achieved, the UCL efficiencies of core/shell NCs (inert-shell) are still lower than those of the corresponding bulk materials. For instance, Haase group synthesized ∼10 nm β-NaYF4:Yb/Er@NaYF4 UCNCs with the inert-shell thickness of ∼2.5 nm and found that they were ∼500 times less efficient than the bulk counterparts.11 The low UCL efficiency of active-core/inert-shell UCNCs is quite surprising because the inert shell with sufficient thickness should be able to passivate the surface defects and insulate the active core from the environmental effects. From this perspective, the UCL efficiency of UCNCs protected by the inert shell should be comparable to that of the corresponding bulk materials, especially taking into account the fact that highbrightness core/shell semiconductor quantum dots have been
core/shell UCNCs. It shows that the inert shell grown by the OPH method is incomplete and fails to shield the emission core from the quenching effect of surface-adsorbed moisture. These results indicate that the temperature dependences of UCL provide a simple criterion for verifying the completeness of the inert shell. The optically active shell is also widely employed to further improve the UCL properties or to integrate functionality on UCNCs.39−41 We synthesized NaGdF4:Yb/Er@NaGdF4:20% Yb3+ active-core/active-shell UCNCs (Figure S25, size ∼14 nm, shell thickness ∼3.5 nm) and investigated their temperature-dependent UCL intensities. Unlike active-core/inertshell samples, NaGdF4:Yb/Er@NaGdF4:Yb3+ active-core/ active-shell UCNCs show the same thermal behavior of UCL as core-only ones, of which the integrated UCL intensity at 150 °C increases by a factor of ∼2.7 in comparison to that at 30 °C (Figure 10). This result indicates that the emission core
Figure 10. Temperature-dependent (a) UCL spectra and (b) integrated intensities of NaGdF4:20%Yb/2%Er@NaGdF4:20%Yb active-core/active-shell UCNCs under 975 nm excitation in air.
in active-core/active-shell UCNCs is still accessible to the quenching of surface-adsorbed moisture. It can be attributed to the energy migration among Yb3+ ions in both the active core and the active shell, which allows the absorbed energy to reach the surface and be subsequently quenched by OH vibrations (Figure 9a). From this perspective, the active-core/active-shell structure may be not a good choice for UCNCs used in solid state or in aqueous system because the coupling of the emissive
Figure 11. Room-temperature UCL spectra of NaGdF4:0.2Yb/0.02Er@NaGdF4 active-core/inert-shell and NaGdF4:20%Yb/2%Er@NaGdF4:20% Yb active-core/active-shell UCNCs in different environments: (a) dispersed in cyclohexane (10 mg/mL); (b) solid-state powder in air; and (c) dispersed in a solution of CO-520/ethanol/water (50:40:10 vol %, 10 mg/mL). The insets give the corresponding luminescent images of core/shell UCNCs under the 975 nm laser irradiation. Laser power density: 4.7 W/cm2. H
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The Journal of Physical Chemistry C well proved with the quantum yields larger than 90%.42,43 It signifies that there are still unique and hidden energy-loss pathways combating with the UCL process in lanthanidedoped UCNCs with active-core/inert-shell structures. With reference to Figure 2, the emissions of Ho3+ ions are much more sensitive to temperature than those of Er3+ and Tm3+ ions. It was surprisingly found that NaGdF4:Yb/Ho@ NaGdF4 active-core/inert-shell UCNCs (shell thickness ∼3.5 nm) still showed a UCL increase at higher temperatures (Figure S26). It is distinct from Er3+- and Tm3+-activated active-core/inert-shell UCNCs with the same shell thickness, in which a typical thermal quenching behavior occurs (Figures 8 and 9). The thermally induced UCL increase of NaGdF4:Yb/ Ho@NaGdF4 UCNCs indicates that the Ho3+ emission in the core is still affected by the quenching of surface-adsorbed moisture, even under the protection of the inert shell. One possible reason may be the unintentional incorporation of Yb3+ ions during the inert-shell growth. Recently, Hudry et al. have reported direct evidence of significant cation intermixing in active-core/inert-shell UCNCs by local chemical analysis.44 The dissolution of the starting active cores, as well as the cation exchanges between the core NCs and the shell precursor solution45,46 do exist during the inert-shell growth process. In this case, a chemically “pure” inert shell is questionable because the incorporation of optically active elements into the inert shell is almost unavoidable. The presence of trace levels of Yb3+ ions in the inert shell results in the same thermally induced emission increase of active-core/inert-shell NCs as that of the active-shell samples, due to energy migration to the surface over the Yb3+−Yb3+ internet (Figure 9a). Recently, Rabouw et al. have ascribed residual quenching in active-core/inert-shell UCNCs to OH− defects incorporated during the NC synthesis.47 However, the quenching of OH − defects incorporated into the NC lattice will not induce the UCL increase at elevated temperatures in a reversible way. Noting that the energy migration over Yb3+−Yb3+ internet is very efficient and the action distance is long-ranged (>20 nm).37,48 The Yb3+−Yb3+ internet formed in both the active core and the inert shell makes the entire emission core susceptible to the surface quenchers, including surface defects and surface-bound ligands. This is possibly the main cause of the lower UCL efficiency of active-core/inert-shell UCNCs. Further studies are necessary to suppress this additional quenching pathway, either by the improved synthesis strategy or by the rational core/shell design.
dynamics investigations also indicate that the surface quenching effect to Yb3+ ions is much stronger than that to activator ions, which is mainly responsible for the energy-loss process in Yb-sensitized UCNCs. Due to the energy migration to the surface over the Yb−Yb internet, the UCL of activecore/active-shell UCNCs still can be affected by surface quenchers and show a similar thermally induced increase, implying their limited applications in aqueous environment. Albeit largely suppressed, the energy-loss process through the Yb−Yb internet still occurs to some extent for active-core/ inert-shell UCNCs. It is due to the unintentional incorporation of Yb3+ ions into the inert-shell during the NC synthesis and may be responsible for the decreased UCL efficiency of small active-core/inert-shell UCNCs. The findings not only clarify the origin of the intriguing thermally induced UCL increase in small UCNCs, but also contribute to the deep understanding of the energy-loss mechanism in core-only or core/shell UCNCs.
4. CONCLUSIONS In this work, temperature-dependent UCL of Yb/Er (or Ho, Tm) codoped UCNCs with various sizes was investigated and the unique light−heat interplay mechanism in small UCNCs was unraveled. Unlike luminescent bulk materials, small UCNCs show an anomalous thermally induced UCL enhancement in solid state. Moreover, this UCL increase phenomenon depends strongly on the SVRs of NCs and becomes more significant for UCNCs with larger SVRs. Taking into account the temperature-dependent characteristics of the Yb3+ DCL intensity and lifetime, as well as the influences of environmental atmosphere and core/shell structures, this thermally induced UCL enhancement can be attributed to the temperature-dependent quenching effect of surface-adsorbed moisture. At elevated temperatures, the release of H2O molecules from the NC surface leads to decreased surface quenching and increased UCL. Temperature-dependent UCL and DCL
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07899.
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Detailed synthesis procedures of nanocrystal; rate equation analysis; and supporting figures including more TEM images, XRD patterns, upconversion/ downconversion luminescence spectra, and decay curves (Figures S1−S26) (PDF)
AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Qiyue Shao: 0000-0002-1485-3288 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Jiangsu Province of China (BK20160073), the Jiangsu Key R&D Program (BE2015102), and the Fundamental Research Funds for the Central Universities (2242018K40107). REFERENCES
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DOI: 10.1021/acs.jpcc.8b07899 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.8b07899 J. Phys. Chem. C XXXX, XXX, XXX−XXX
