Shell Nanocrystals

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C: Physical Processes in Nanomaterials and Nanostructures

Energy Loss Mechanism of Upconversion Core/Shell Nanocrystals Yanqing Hu, Qiyue Shao, Yan Dong, and Jianqing Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07176 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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The Journal of Physical Chemistry

Energy Loss Mechanism of Upconversion Core/Shell Nanocrystals

Yanqing Hu,† Qiyue Shao,*, † Yan Dong,† 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 Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, People’s Republic of China

*Corresponding author Email address: [email protected] (Q. Shao)

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Abstract: Small-sized upconversion nanocrystals (< 10 nm) show a quite low luminescent efficiency. Even if these nanocrystals are coated by 2 nm thick inert-shell, the core/shell nanocrystals still exhibit weak upconversion luminescence. The involved energy loss mechanism is under debate. Here, we have demonstrated that the major contribution to the low upconversion efficiency is ascribed to an overtone vibrational energy transfer from an electronic transition of Yb3+ excited state to overtone transitions of deactivating group vibrations by dipole-dipole coupling. The maximum coupling distance reaches ~11 nm. Moreover, we firstly find that an ultra-thick inert-shell (> 11 nm) is not beneficial for upconversion luminescence due to a strong scattering effect. A novel lifetime model is proposed to precisely descript the decay times of Yb3+ 2F5/2 state and Er3+ 4S3/2 state as a function of inert-shell thickness. Based on an insight into the luminescence loss, we design β-Na91%YbF4:9%Er@NaGdF4 nanocrystals with 11.2 nm inert-shell to completely block the surface quenching and achieve more efficient and stronger upconversion emission.

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1. Introduction Upconversion nanocrystals (UCNCs) doped with lanthanides can absorb near-infrared (NIR) photons and emit visible photons.1 Comparing with semiconductor quantum dots and organic fluorophores, these NCs have the unique optical characteristics of sharp emission bandwidth and high photochemical stability. Moreover, NIR excitation source has remarkable penetration depth in biological tissue and minimal autofluorescence background. Along with the excellently optical properties of NCs and the competitively inherent advantages of excitation source, these UCNCs were broadly applied, including biologic imaging,2,3 photothermal therapy,4,5 drug delivery,6 temperature sensing,7 anticounterfeiting,8 display and photovoltaic technologies.9,10 To apply better to the field of life sciences, recent studies have focused on the synthesis of small-sized NC (< 10 nm) and have made great progress.11-15 However, the small-sized NCs exhibit a much weaker upconversion luminescence (UCL) in comparison with the corresponding bulk materials. Even 2 nm thick inert-shell is encapsulated outside the core, the UCL efficiency of core/shell NCs is two orders of magnitude lower than that of bulk materials.15 The involved energy loss mechanism is still unclear. Inert-shell is commonly used to suppress the surface-related quenching and improve the UC efficiency.16 Meanwhile, UC efficiency generally increases with the increased shell thickness (ST).17-19 The highest UC quantum yield values of core/shell NCs are very similar with that of UC phosphor powders. The ST-dependent UC efficiency enhancement implies that underlying energy loss still exists in the inert-shell NCs. Rabouw et al. proposed that the energy loss for Yb3+/Er3+ codoped nanocrystals in solvents originated from Förster resonance energy transfer (FRET) from emitting state of Er3+ ions to vibrational modes of surrounding ligand and solvent molecules.20 Huang et al. hold the view that the energy loss should also take into account the overtone 3



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vibrational transition-induced Yb3+ excited state quenching.21 Rabouw et al. proposed that Huang’s theory needs to be proved experimentally.22 The energy loss mechanism is under debate, and there is no model to fit well both the lifetimes of Yb3+ and Er3+. Yb3+ excited state energy is distributed via three routes: energy transfer upconversion, downshifting luminescence, and nonradiative transition.23 For the investigation of nonradiative transition, analysis of UCL thermal behavior is an effective

approach.

In

this

work,

we

synthesized

a

series

of

NaGdF4:20%Yb/2%Er@NaGdF4 core/shell NCs with various shell thicknesses. UCL thermal behaviors in various atmospheres confirm a crucial factor resulting in the low UC efficiency. Furthermore, the energy loss mechanism is revealed by the overlap between the emission spectrum of Yb3+and the absorption spectra of water and toluene. Meanwhile, the effective energy transfer distance is determined experimentally by analyzing transient spectra. A novel lifetime equation is proposed to preciously descript the decay times of Yb3+ 2F5/2 and Er3+ 4S3/2 state as a function of inert-shell thickness. The energy loss mechanism guides the design of highly efficient UC core/shell NCs.

2. Experimental section 2.1 Nanocrystal synthesis. Hexagonal-phase NaGdF4:20%Yb/2%Er NCs were synthesized by a coprecipitation method.24 Hexagonal-phase Na91YbF4:9%Er nanocrystals were synthesized by modifying a protocol previously reported.15 Core/shell NCs were synthesized by a successive layer-by-layer strategy.25 Detailed synthesis procedures of all samples are provided in the Supporting Information. 2.2 Characterization. Low-resolution transmission electron microscopy (TEM) images were taken with a Tecnai G2 microscope. XRD analysis was carried out on a Shimadzu XD-3A X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The luminescence spectra were obtained with a portable spectrometer (Maya2000Pro, 4


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Ocean Optics Co.), in conjunction with a continuous 975 nm diode laser as the excitation source. The temperature-dependent UCL spectra of solid-state nanocrystals were measured in air with the aid of a temperature-controlled heating cell. The absorption spectra were obtained by a Cary 5000 UV-vis-NIR spectrophotometer. Time-dependent Yb3+ 2F5/2 and Er3+ 4S3/2 emissions were investigated by a FLS1000 fluorescence spectrometer (Edinburgh Instruments) upon excitation with a pulsed 975 nm laser.

3. Results and discussion 3.1 Morphological analysis. Hexagonal-NaGdF4:20%Yb/2%Er core nanocrystals were synthesized in an oleic acid/octadecene solvent mixture, and β-NaGdF4 inert shell were prepared by modifying the layer-by-layer method (see Supporting Information for details). In our system, Yb3+ acts as sensitizers for Er3+. The calculated mean sizes of the core and other seven core/shell samples are 5.7 nm, 8.0 nm, 11.8 nm, 16.6 nm, 19.6 nm, 28.3 nm, 32.1 nm and 41.1 nm (Figure S1). The corresponding ST is 0 nm (core), 1.1 nm, 3.0 nm, 5.4 nm, 6.9 nm, 11.3 nm, 13.2 nm and 17.7 nm, respectively (Figure 1). ST is controlled by adjusting the molar ratio of Ln3+ ions in shell to ones in core. Xray diffraction (XRD) analysis confirms that the as-synthesized NCs show pure hexagonal phase (Figure S2).

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Figure 1. TEM images of nanocrystals with shell thickness (ST) of (a) 0 nm (core), (b) 1.1 nm, (c) 3.0 nm, (d) 5.4 nm, (e) 6.9 nm, (f) 11.3 nm, (g) 13.2 nm and (h) 17.7 nm (scale bar: 50 nm).

3.2. Shell thickness-dependent UCL. The UCL spectra of all solid state NCs in air were collected with a portable spectrometer under the same light path (Figure 2a). The mass of each tested sample is 0.2 g. The solid powder samples were put into a copper sample cell, which is 10 mm in diameter and 2 mm in depth. In order to obtain accurate data, the integrated intensities of spectra (480−700 nm) were determined by averaging the values of repeated measurements (five times) under the same light path, as shown in figure 2b. Although a thin β-NaGdF4 inert-shell (~1.1 nm) can passivate surface defects of active-core, the UCL intensity of core/shell NCs is almost as weak as that of core-only. For a thicker inert-shell (1.1 < ST ≤ 6.9 nm), the UCL intensity goes up with ST increasing and exhibits a linear dependence on the ST. However, we did not observe further UCL enhancement or saturated UCL for ST above 6.9 nm. The UCL intensity remarkably declines with ST above 6.9 nm. It is worth mentioning that when the integrated intensity is converted into the luminescence intensity per unit effective volume (Supporting Information, section B), the peak value appears at ST = 11.3 nm (Figure 2c). In addition, the UCL spectra of NCs in toluene were collected by using a colorimetric dish under the same light path. The colloidal concentration of each sample 6


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in toluene is 10 mg/mL. In toluene, the ST-dependent UCL behaviors is almost identical with that in air except that the measured peak value is at ST = 5.4 nm (Figure 2d−f and S3). Inexplicably, obtaining the maximum UCL intensity requires such a thick inertshell.

Figure 2. UCL spectra, integrated intensities, and unit effective volume intensities of NCs (a−c) in air and (d−f) in toluene. The mass of the measured solid samples in air is 0.2 g, and the toluene colloid concentration is 10 mg/mL. Noting that the power density is 4 W/cm2 for measurement in air and 20 W/cm2 for measurement in toluene, respectively.

3.3. Energy loss mechanism. To clarify the origin of the weak emission of 1.1 nm thick inert-shell NCs, we measured temperature-dependent UCL in air. Strikingly, the core/shell NCs exhibit temperature-dependent UCL enhancement. The green and red emission intensities increase to 7.7- and 5.1-fold as temperature reaches 150 oC, respectively (Figure 3a). It seems that something in air suppresses the UCL potential of core/shell NCs. Besides, we also measured temperature-dependent UCL in Ar, Ar/H2O and Ar/D2O atmospheres (Figure S4). For the measurement in Ar, the heating cell containing solid state NCs were placed into an atmosphere-protected chamber for temperature-dependent spectral measurements. Because NCs was transferred in air, 7



samples were kept at 150 oC for 10 min under Ar flow to remove the surface volatile molecules before testing. A quartz sheet doesn’t need to be tightly capped. After temperature decreased to 30 oC, the samples began to be measured. When the highpurity Ar gas was delivered into the measuring chamber through a bubbling bottle containing D2O or H2O, the temperature-dependent UCL spectra in Ar/D2O or Ar/H2O atmospheres can be obtained. Compared to the temperature-dependent UCL in air, the UCL intensity in Ar decreases due to thermal quenching (Figure 3b). It is notable that, in Ar/H2O atmosphere, the core/shell NCs also exhibit temperature-dependent UCL enhancement as similar as that in air (Figure 3c). These results indicate that H2O molecules are a critical factor resulting in the weak UCL at room temperature and the strong UCL at high temperature. The temperature-dependent UCL enhancement is attributed to the gradually-attenuated H2O quenching effect.26 The temperaturedependent UCL decline in Ar/D2O atmosphere further confirms the H2O quenching effect (Figure 3d), because the vibration modes of O−D can’t effectively quench Yb3+ excited state energy.27 These results suggest that the 1.1 nm inert-shell is not enough thick to prevent the surface quenching effect. To prove an electronic-to-vibrational energy transfer from Yb3+ to their surrounding environment, Yb3+ 2F5/2 emission spectrum at 1050 nm as well as the absorption spectra of toluene and water were measured, as shown in Figure 3e. The absorption spectrum of toluene shows vibrational overtones at 1142 nm, which originates from aromatic and methyl C–H stretching mode at ~3070 cm-1.28,29 The absorption spectrum has partial overlap with the Yb3+ 2F5/2 emission spectrum, and there is a close distance between Yb3+ ions and C–H groups (~ 1.1 nm). The two factors satisfy the law of FRET,30 so an overtone vibrational energy transfer (OVET) occurs from an electronic transition of Yb3+ to the C–H vibrational overtones of toluene molecules by dipole-dipole coupling. 8


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In water, the O–H stretching vibrational overtones exhibit a broad absorption (900– 1300 nm), which has much better spectral overlap with emission spectrum of Yb3+. This implies that H2O quenching effect is much stronger than toluene quenching. As a result, Yb3+ 2F5/2 state can be significantly quenched by overtone transitions of O–H vibrations of H2O molecules or C–H vibrations of toluene molecules. Meanwhile, there is also resonance energy transfer from an electronic transition of Er3+ to O–H or C–H vibration,20 but the effect on the luminescence dynamics is less significant due to the relatively low ratio of Er3+ ions and the nonlinear optical processes. The absorption spectrum of oleic acid (OA) was also measured, and there was almost no spectral overlap between Yb3+ emission and OA absorption (Figure S5). This result indicates that OA ligands on the surface of NCs hardly quench the Yb3+ excited state energy.

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Figure 3. Temperature-dependent UCL intensities of NaGdF4:20%Yb/2%Er@NaGdF4 core/shell NCs with the 1.1 nm thick shell in (a) air, (b) Ar, (c) Ar/H2O, and (d) Ar/D2O atmospheres (power density: 1.6 W/cm2). Noting that the intensity is normalized to that at 30 oC, respectively. (e) Yb3+ 2F

5/2

emission spectrum at 1050 nm as well as the absorption spectra of H2O and toluene.

As shown in Figure 2b, the maximum UCL intensity occurs at the ST of 6.9 nm in air. To clarify why the strongest UCL requires such a thick inert-shell, we tested the emission spectra of all samples at elevated temperature (Figure S6). For the ST from 0 to 5.4 nm, the UCL enhancement factor is greater than 1 but appears almost constant at ST of 5.4 nm (Figure 4a). The constant UCL at elevated temperature indicates that both H2O quenching effect and thermal quenching effect have been in equilibrium. In 10


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contrast, for the ST from 6.9 to 17.7 nm, the UCL enhancement factor is less than 1, suggesting that the thermal quenching effect is dominant (Figure 4b). Thus, when the dominant surface quenching effect becomes subordinate, the UCL intensity is maximum. In toluene, the maximum UCL intensity occurs at the ST of 5.4 nm (Figure 2e). However, after the integrated intensities are converted into unit effective volume luminescence (UEVL) intensity, the maximum intensities occur at ST = 11.3 nm in all cases (Figure 2c and f). It can be inferred that the maximum coupling distance of dipoledipole is ~11 nm. To verify this point, decay curves of Yb3+ 2F5/2 emission and Er3+ 4S

3/2

emission were measured, as shown in Figures 4c and S7a. The two types of

lifetimes tend to saturate at ST = 11.3 nm. Hence, the electronic-to-vibrational energy transfer is a long-range quenching behavior, and the effective coupling distance is 0– 11 nm. It is worthwhile mentioning that the UEVL intensity decreases rapidly at ST > 11.3 nm (Figure 2c and f). Although the ultra-thick inert-shell can excellently suppress surface quenching effect, it doesn’t benefit to upconversion emission. The STdependent UCL decline may be caused by the laser scattering. To verify this postulation, we measured the absorption spectra of all samples. The absorption intensity at 975 nm decreases with ST increasing (Figure S8a). The absorption peak of core-only is most obvious, whereas that of NCs with above 6.9 nm thick shell has almost no absorption peak. The unit effective volume absorption intensity increases at ST = 0 ~ 6.9 nm, but decreases at ST > 6.9 nm (Figure S8b), suggesting that scatting effect also exists in the absorption process. As a result, the excessively thick inert-shell reduces the absorption of Yb3+ ions in core to NIR photons. It has been demonstrated that Yb3+ excited state can be directly quenched by the 11



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overtone transitions of O–H or C–H vibrations. The lifetimes of the Yb3+ 2F5/2 energy level are closely related to coupling distance, and can be described by the following equation (Supporting Information, Section C) 𝜏𝑖

𝜏𝑚 = 1+

8𝜋𝜏𝑖𝜅2|𝜇𝑒𝑙|2|𝜇𝑣𝑖𝑏|2 (𝑟 + 𝑅)2 ℏ𝛾𝑟20𝑛4

(1)

𝑅6

Where 𝜏𝑚 and 𝜏𝑖 are the measured lifetime and the intrinsic lifetime, respectively, 𝜅2 = 2/3 is a geometric factor, 𝜇𝑒𝑙 and 𝜇𝑣𝑖𝑏 are the electronic dipole moment and the vibrational transition dipole moment, respectively, ℏ is the reduced Planck constant, 𝛾 ―1 is the density of states, 𝑟0 is the effective radius of a quencher, n represents the refractive index between nanocrystals and molecules, and R is the inert-shell thickness (coupling distance), respectively. To verify a plausibility, we fitted the decay times of Yb3+ as a function of inert-shell thickness with Equation 1. The 𝜏𝑖 and

8𝜋𝜏𝑖𝜅2|𝜇𝑒𝑙|2|𝜇𝑣𝑖𝑏|2 ℏ𝛾𝑟20𝑛4

are fitting parameters. Figure 4d shows excellent agreement between measured data and fits. The fitting intrinsic lifetime (𝜏𝑖) of Yb3+ 2F5/2 state is 1.72 ms, which matches well to the lifetime of bulk materials (𝜏𝑏𝑢𝑙𝑘 = 1.64 ms, Figure S9a). Besides, the model also fits well the Er3+ 4S3/2 decay time, and the 𝜏𝑖 is 0.73 ms (Figure S7b), which is good agreement with that of corresponding bulk materials (0.89 ms, Figure S9b). Thus, Equation 1 describes precisely the decay times as a functions of shell thickness.

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Figure 4. (a-b) Temperature-dependent UCL intensities of core-only and core/shell NCs with various inert-shell thickness (power density: 1.6 W/cm2). Noting that the intensity is normalized to that at 30 oC, respectively. (c) Time-dependent Yb3+ 2F5/2 emissions of all samples after 975 nm excitation. (d) Decay times of Yb3+ 2F5/2 emission from Figure 4c are fitted with Equation 1. The coefficient of determination R′2 is 0.99, and the fitting intrinsic lifetime 𝜏𝑖 is 1.72 ms.

The thinner or thicker inert-shell is not favorable for the UC emission of core/shell NCs. On the one hand, although the thin inert-shell would reduce the scattering effect, overtone transitions of O–H or C–H vibrations seriously quench Yb3+ excited state energy due to the close coupling distance. On the other hand, despite the thick inertshell can separate better the Yb3+ ions in the core from the surface quenchers, the scatting seriously affect the absorption of Yb3+ ions to NIR photons (Figure 5a). It has been proved that the maximum coupling distance is ~11 nm for OVET. After the surface quenching effect is completely blocked, the 20%Yb/2%Er co-doped core/shell nanocrystals have exerted their full UCL potential. To break through this bottleneck 13



and obtain stronger UCL, a core/shell structure NC was fabricated with high concentration of sensitizers and activators. Firstly, small-sized Na91%YbF4:9%Er core-only nanocrystals (~6.0 nm) were synthesized by a previously reported method.15 And then the 7.5 nm and the 11.2 nm thick inert-shells were coated on the core by the layer-by-layer method, respectively (Figure S10).25 The as-synthesized core/shell NCs are named as 91Yb/9Er@Gd(7.5) and 91Yb/9Er@Gd(11.2), correspondingly. Compared to the NaGdF4:20%Yb/2%Er@NaGdF4 NCs with 6.9 nm thick inert-shell (20Yb/2Er@Gd(6.9)), the 91Yb/9Er@Gd(7.5) nanocrystals show a lower UCL although coated by the almost thick inert-shell. The 7.5 nm inert-shell is not enough thick to suppress the OVET, leading to a highly efficient surface quenching due to the rich Yb3+ ions in core. However, the integrated UCL intensity of 91Yb/9Er@Gd(11.2) NCs is 2.2-fold as strong as that of 20Yb/2Er@Gd(6.9). This result further confirms that the effective coupling distance is 0–11 nm for OVET by dipole-dipole interaction.

Figure 5. (a) Schematic illustration of energy loss mechanism of core/shell NCs. Noting that energy 14


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transfer is abbreviated as ET. (b) UCL intensities of 91Yb/9Er@Gd(11.2) and 91Yb/9Er@Gd(7.5) as well as 20Yb/2Er@Gd(6.9) NCs. The mass of the measured solid nanocrystals is 0.2 g.

For surface quenching ， the dipole-dipole coupling efficiency depends on the distance, spectral overlap, sensitizer concentration, and surface-to-volume ratio of core. The optimum shell thickness, where the maximum UCL intensity occurs (e.g. Figure 2b and e), varies directly with the coupling efficiency. However, the critical shell thickness, where the decay time tends to be saturated, is ~11 nm in all cases due to the fact that the dipole-dipole interaction occurs in the range of 0−11 nm. For example, Homann et al. have reported that the Yb3+ decay times of 45 nm core/shell NCs (core: 23 nm, shell thickness: 11 nm) are very similar with that of the corresponding upconversion phosphor powder.17 This result also suggests that the maximum coupling distance is 11 nm for core/shell NCs with a large-sized core.

4. Conclusions In conclusion, we have experimentally and theoretically revealed the energy loss mechanism of upconversion core/shell NCs. The low UCL efficiency of thin inert-shell NCs is mainly ascribed to Yb3+ 2F5/2 state quenching. The energy loss is induced by dipole-dipole coupling from electronic transition of Yb3+ excited state to overtone transitions of deactivating group vibrations, i.e., O–H stretch of water molecules or C– H stretch of toluene molecules. This surface quenching is a long-range OVET behavior, and the effective coupling distance is 0–11 nm. For inert-shell thickness above 11 nm, the UCL of core/shell NCs decreases seriously due to the scattering effect. Importantly, the deduced lifetime equation not only accurately describes the distance dependence of Yb3+ 2F5/2 state lifetime, but also does that of Er3+ 4S3/2 state lifetime. These results provide a powerful solution for designing a high efficient UCL nanocrystals with high 15



concentration of sensitizers and activators.

Associated Content Supporting Information Detailed synthesis procedures of core/shell nanocrystals, methods of calculation, deduction processes of the lifetime equation, and supplementary figures including more XRD patterns, UCL spectra, transient spectra, absorption spectra and TEM images. The supporting Information is available free of charge on the …

Author Information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the Natural Science Foundation of Jiangsu Province of China (BK20160073) and the Fundamental Research Funds for the Central Universities (2242019K40061).

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