Intense upconversion red emission from Gd-doped NaErF4:Tm-based core/shell/shell nanocrystals under 980 and 800 nm near infrared light excitations

Jung Eun Choi ab, Donghwan Kim bc and Ho Seong Jang *ad
aMaterials Architecturing Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. E-mail:
bDepartment of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
cKU-KIST Graduate School of Energy and Environment (Green School), Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
dDivision of Nano & Information Technology, KIST School, Korea University of Science and Technology (UST), Seoul 02792, Republic of Korea

Received 13th November 2018 , Accepted 19th December 2018

First published on 20th December 2018

Bright and single band red-emitting NaErF4:Tm,Gd-based upconversion nanophosphors (UCNPs) were successfully synthesized by the formation of a core/intermediate shell/active shell structure. The NaErF4:Tm,Gd/NaYF4:Ca,Yb/NaYF4:Nd,Yb core/shell/shell UCNPs exhibit intense red light under both 980 and 800 nm near infrared light excitations.

Among various luminescent materials such as organic dyes, quantum dots, and lanthanide ion-doped inorganic phosphors, upconversion nanophosphors (UCNPs) have attracted many researchers’ attention due to their unique optical properties.1–3 They show anti-Stokes shift luminescence through which they absorb near infrared (NIR) light (e.g., ∼980 nm and ∼800 nm) and emit visible light.4–6 The NIR light is advantageous for bio-imaging because it does not induce autofluorescence from biomolecules and it makes clear fluorescence imaging with a high signal-to-noise ratio possible.7 In addition, it was reported that the upconversion (UC) mechanism via energy transfer from sensitizer ions to activator ions is much more efficient than the two-photon absorption process.8 Thanks to this high efficiency of UCNPs, a relatively cheap diode laser can be utilized as an excitation source.9 Due to the efforts of many researchers, the efficiencies of the UCNPs have been improved and several kinds of efficient UCNPs were reported.10–13 Recently, Chen's group reported highly efficient LiLuF4-based blue- and green-emitting core/shell (C/S) UCNPs.14 Compared with blue- and green-emitting UCNPs, red-emitting UCNPs are beneficial for in vivo imaging application because red light lies in the optically transparent window of a biological system.15,16 Indeed, red light can penetrate deeper into the tissue than green light.17 Previously, Liu's group reported single-band red-emitting KMnF3:Yb,Er UCNPs17 and Gao's group reported strong red-emitting YOF:Yb,Er/YOF UCNPs.18 These previously reported red-emitting UCNPs exhibit strong red emission under 980 nm excitation. However, 980 nm NIR light may not be the best excitation source since the 980 nm NIR light causes heating problems.19,20 Recently, our group and several research groups reported red-emitting NaGd(Y)F4:Yb,Ho,Ce-based nanophosphors which can be excited with an 800 nm NIR light.21–23 The Yb,Ho,Ce-doped UCNPs emit red light by the cross relaxation process between Ho3+ and Ce3+ and the cross relaxation may be a stumbling block in achieving strong luminescence.23,24

More recently, NaErF4-based red-emitting UCNPs were reported.25–28 Because Er3+ ions absorb ∼980 nm NIR light and ∼800 nm NIR light via4I15/24I11/2 and 4I15/24I9/2 transitions, respectively, NaErF4-based UCNPs emit red light under both 980 and 800 nm excitations.25–28 However, NaErF4 showed much lower absorbance at ∼800 nm than at ∼980 nm28 and realization of strong red UC luminescence (UCL) under 800 nm excitation is still challenging.

In this study, we report strong red-emitting NaErF4-based UCNPs, which have small and uniform size, under 980 and 800 nm excitations. When a small amount of Tm3+ is doped into NaErF4 host nanocrystals, red light purity was increased and red emission intensity was enhanced by confinement of excitation energy through Tm3+-mediated energy trapping.28 According to Liu's group, 0.5% Tm3+ doping into the NaErF4 host was the optimal Tm3+ doping condition for strong UCL.28 Thus, 0.5% Tm3+ was doped into NaErF4 and Gd3+ was co-doped to synthesize uniform small sized NaErF4-based UCNPs. Finally, red luminescence was enhanced by the formation of a core/shell/shell (C/S/S) structure where an intermediate shell and a Nd3+-doped active shell were grown for shielding luminescence quenching and increasing the absorption of 800 nm NIR light, respectively.

Fig. 1 shows the transmission electron microscopy (TEM) images of Gd-substituted NaErF4:Tm(0.5%) UCNPs. Under our experimental conditions, NaErF4:Tm(0.5%) UCNPs exhibited a relatively large size of ∼31.9 nm. However, when Gd3+ ions were doped into the NaErF4:Tm(0.5%) UCNPs, the particle size decreased. According to Liu's group, larger ion doping into the NaYF4-based UCNPs induces smaller particle size due to an increase of surface charge density of the surface of the UCNPs.29 Similarly, since the Gd3+ ion is larger than the Er3+ ion, Er3+ substitution with Gd3+ ions can cause a decrease of the size of NaErF4.29 Indeed, 10% Gd3+ substitution resulted in a decrease of the size of the NaErF4:Tm(0.5%) UCNPs (Fig. 1b). While 10% Gd-substituted NaErF4:Tm(0.5%) UCNPs exhibited uniform size and shape, higher concentration of Gd-substituted NaErF4:Tm(0.5%) UCNPs showed wide size distribution, as shown in Fig. 1c and Fig. S1 (ESI). This trend in the monodispersity of the UCNPs’ size is consistent with the previously reported results.30 The synthesis of polydispersed UCNPs can be explained as follows. As the concentration of Gd3+ increased in NaErF4:Tm(0.5%),Gd, Ostwald ripening of the UCNPs became dominant and polydispersity in the size distribution of the UCNPs increased.31

image file: c8cc09031a-f1.tif
Fig. 1 TEM images of NaErF4:Tm(0.5%),Gd UCNPs substituted with various Gd3+ concentrations of (a) 0%, (b) 10%, and (c) 30%.

Thus, NaErF4:Tm(0.5%),Gd(10%) (NaErF4:Tm,Gd) UCNPs with small and uniform size were used as cores for shell growth. The TEM images and size distributions of the synthesized C/S UCNPs are shown in Fig. S2 (ESI). Recently, it was reported that inert shell formation of the NaErF4-based cores significantly enhances UCL from the Er3+ ions by efficiently preventing surface quenching.25,28 As shown in Fig. 2a and b, the NaErF4:Tm,Gd/NaYF4 UCNPs exhibited a strong emission peak at the red spectral region due to 4F9/24I15/2 electronic transition in Er3+ ions under 980 and 800 nm excitations.28 When Tm3+ is doped into the host lattice, it prevents migration of excited energy to defect sites in the lattice and it contributes to the enhancement of Er3+ emission intensity.28 As shown in the absorption spectrum, Er3+ ions absorb ∼980 nm NIR light and the electron in the ground 4I15/2 state is excited to the 4I11/2 state (Fig. 2c). The absorbed energy can be transferred to adjacent Er3+ ions. In addition, the absorbed energy can be trapped at the 3H5 level of Tm3+ and the trapped energy is back-transferred from the 3H5 level of Tm3+ to the 4I13/2 level of Er3+, as shown in Fig. 2d.28 After that, 4I13/24F9/2 transition occurs by absorbing NIR light at 980 nm and 4F9/24I15/2 transition results in strong red UCL from NaErF4:Tm,Gd/NaYF4 UCNPs.28 The photoluminescence (PL) intensity of the C/S UCNPs under the condition of 980 nm excitation was stronger than that under the condition of 800 nm illumination. The weak red UCL from the C/S UCNPs under 800 nm excitation is partly attributed to a much lower absorbance at around 800 nm than in the 980 nm NIR region (Fig. 2c). Previously, Liu's group and Yan's group reported that geometric separation between activator ions and Nd3+ ions is an effective way to realize strong UCL from the UCNPs under 800 nm excitation.19,20 In their studies, activator ions were doped into the core and sensitizer Nd3+ ions were doped into the shell. In this study, a Nd3+-doped NaYF4 active shell was also grown on the NaErF4:Tm,Gd UCNPs and Nd3+ was separated from the core by the formation of a C/S structure. As shown in Fig. 2c, the growth of the NaYF4:Nd(20%) active shell largely enhanced the absorbance of the UCNPs in an ∼800 nm NIR region. However, the UCL from the NaErF4:Tm,Gd/NaYF4:Nd(20%) C/S UCNPs was not enhanced but significantly quenched, regardless of the excitation wavelength (Fig. 2a and b). This UCL quenching can be explained as follows. In NaErF4:Tm,Gd, activator Er3+ ions constitute the host crystal and Er3+ ions are exposed to the surface of the NaErF4:Tm,Gd core UCNPs. Thus, when the NaYF4:Nd(20%) shell was grown on the NaErF4:Tm,Gd core, Er3+ ions and Nd3+ ions became close and the excitation energy can be easily transferred from the Er3+ ions in the core to the Nd3+ ions in the shell (Fig. 2d). Thus, Nd3+ ions act as quenching centres and Er3+ luminescence is quenched. On the other hand, Ca2+-doping into the NaYF4 shell further enhanced UCL from Er3+ ions without an increase of absorption of 800 nm NIR light. According to Zhang et al., Ca2+-doping into Y3+ sites induces F vacancies for the charge neutrality in the host lattice, resulting in a uniform size distribution of the UCNPs by diffusion controlled growth and a decrease of crystal symmetry.12 The lower crystal symmetry affects the local symmetry of crystal field and, in turn, f–f transition probability, which enhances the UCL intensity.12,32 The NaErF4:Tm,Gd/NaYF4:Ca(20%) C/S UCNPs showed ∼10% and ∼20% higher PL intensity than NaErF4:Tm,Gd/NaYF4 C/S UCNPs under 980 and 800 nm excitations, respectively. Although Ca2+-doping into the shell enhanced UCL under 980 and 800 nm excitations, enhancement of red UCL under 800 nm excitation was not significant. Thus, 800 nm NIR light should be largely absorbed by the UCNPs and then the absorbed energy should be transferred to activator Er3+ ions without back energy transfer from Er3+ ions to sensitizer ions for a large increase of UCL intensity under 800 nm excitation.

image file: c8cc09031a-f2.tif
Fig. 2 PL spectra of (i) NaErF4:Tm,Gd core, (ii) NaErF4:Tm,Gd/NaYF4 C/S, (iii) NaErF4:Tm,Gd/NaYF4:Nd(20%) C/S, and (iv) NaErF4:Tm,Gd/NaYF4:Ca(20%) C/S UCNPs under (a) 980 and (b) 800 nm excitations (laser power = 2 W). (c) Absorption spectra of (i) NaErF4:Tm,Gd core, (ii) NaErF4:Tm,Gd/NaYF4 C/S, (iii) NaErF4:Tm,Gd/NaYF4:Nd(20%) C/S, and (iv) NaErF4:Tm,Gd/NaYF4:Ca(20%) C/S UCNPs. (d) Schematic energy level diagram showing the UC mechanism of NaErF4:Tm,Gd/NaYF4:Nd(20%) C/S UCNPs.

To avoid back energy transfer from Er3+ ions to Nd3+ ions, insertion of the intermediate shell between the core and Nd3+-doped shell is effective.33 However, to our knowledge, the intermediate shell has not been applied to NaErF4-based UCNPs. Thus, in this study, the Ca2+-doped NaYF4 shell was used as an intermediate shell between the NaErF4:Tm,Gd core and the Nd3+-doped NaYF4 shell. Yb3+ was co-doped into the Ca2+-doped NaYF4 intermediate shell because the energy transfer from the sensitizer Nd3+ to Er3+ can be bridged via Yb3+-mediated energy migration (Fig. S3, ESI).33Fig. 3 and Fig. S4 (ESI) show the TEM images of the NaErF4:Tm,Gd core UCNPs, the NaErF4:Tm,Gd/NaYF4:Ca(20%),Yb(10%) (NaErF4:Tm,Gd/NaYF4:Ca,Yb) C/S UCNPs, and the NaErF4:Tm,Gd/NaYF4:Ca,Yb/NaYF4:Nd(40%),Yb(10%) (NaErF4:Tm,Gd/NaYF4:Ca,Yb/NaYF4:Nd,Yb) C/S/S UCNPs. An increase in particle size can be the first evidence of the formation of the intermediate shell and the outermost shell. As shown in the insets of Fig. 3a–c, the core, C/S, and C/S/S UCNPs exhibited clear lattice fringes, indicating high crystallinity and no lattice mismatches, indicating epitaxial growth of the first and the second shell (also see Fig. S5, ESI). Nonetheless, we can distinguish core and shell regions in the scanning transmission electron microscopy (STEM) image shown in Fig. 3d, due to brightness contrast resulting from the large difference between atomic numbers of Er (Z = 68) in the core and Y (Z = 39) in the first shell. However, the first shell cannot be distinguished from the second shell in the STEM image because each shell contains high concentration of Y (70% for the first shell and 50% for the second shell) and there was little difference in image brightness. The formation of the C/S/S structure was confirmed by the energy dispersive X-ray spectroscopy (EDS) analysis (Fig. 3e and Fig. S6, ESI). The C/S/S structure can be visualized by EDS spectrum imaging. The EDS map from Er is clearly confined in the core region, whereas the EDS map from Ca spans a wider region than the core and its signal is strong in the shell region. The EDS map from Nd spans the particle and it showed a strong signal in the outer shell region. The composite EDS map was generated by superposing Er Lα, Ca Kα, and Nd Lα maps, as shown in Fig. 3e and Fig. S6 (ESI). The C/S/S structure is apparently observed in the composite EDS map in which signals from Er, Ca, and Nd are strong in the core, the first shell, and the second shell, respectively (Fig. 3e). The X-ray diffraction (XRD) patterns of the UCNPs with core, C/S, and C/S/S structures were well matched with the reference XRD pattern of NaErF4 (JCPDS card No. 27-0689), as shown in Fig. 3f. It indicates the formation of single hexagonal phase NaErF4:Tm,Gd-based core, C/S, and C/S/S UCNPs without impurities. The selected area electron diffraction pattern of the C/S/S UCNPs also supports the formation of the hexagonal phase (Fig. S7, ESI).

image file: c8cc09031a-f3.tif
Fig. 3 TEM images of the (a) NaErF4:Tm,Gd core, (b) NaErF4:Tm,Gd/NaYF4:Ca,Yb C/S, and (c) NaErF4:Tm,Gd/NaYF4:Ca,Yb/NaYF4:Nd,Yb C/S/S UCNPs, respectively. Insets show high resolution TEM (HR-TEM) images of the corresponding core, C/S, and C/S/S UCNPs, respectively. (d) STEM image of the C/S/S UCNPs. (e) EDS map produced by superposing the Er Lα (red), Ca Kα (green), and Nd Lα (blue) maps from the C/S/S UCNPs. (f) XRD patterns of the core, C/S, and C/S/S UCNPs.

Fig. 4 exhibits the optical properties of the UCNPs with core, C/S, and C/S/S structures under 980 and 800 nm excitation. As shown in Fig. 4a, there was little change in absorption spectra after growth of the intermediate shell on the NaErF4:Tm,Gd core UCNPs. A slight increase of absorption peak at ∼980 nm was observed due to doping of Yb3+ ions into the intermediate shell. However, strong absorption peaks were additionally observed at 574, 740, 794, and 865 nm due to 4f–4f transitions (4I9/24G5/2, 4F7/2, 4F5/2, and 4F3/2) in Nd3+ ions after the formation of the Nd-doped outer shell.19 The strong absorption peaks were observed at ∼800 nm for NaErF4:Tm,Gd/NaYF4:Nd(20%) UCNPs and NaErF4:Tm,Gd/NaYF:Ca,Yb/NaYF4:Nd,Yb UCNPs as shown in Fig. 2c and 4a. However, these two UCNPs showed totally different PL properties. The Nd3+-doped C/S UCNPs showed complete luminescence quenching (Fig. 2a and b). In contrast, when the intermediate shell was formed between the core and the Nd3+-doped shell, back energy transfer from Er3+ to Nd3+ can be effectively blocked as shown in Fig. 4b and Fig. S8 (ESI), whereas, the excited energy can be efficiently transferred from Nd3+ to Er3+via Yb3+-mediated bridging in the intermediate shell.33 Under 980 nm excitation, NaErF4:Tm,Gd/NaYF4:Ca,Yb C/S UCNPs showed ∼560-fold PL enhancement compared with the core. Furthermore, the NaYF4:Nd(40%),Yb(10%) active-shell grown C/S/S UCNPs exhibited additional PL enhancement (∼800-fold enhancement compared with the core). When we measured the absolute UC quantum yield (QY) of the C/S/S UCNPs, they showed a high UC QY value of 1.9% under 980 nm excitation (Table S1, ESI). The NaErF4:Tm,Gd/NaYF4:Ca,Yb C/S UCNPs also showed a strong PL peak in the red spectral region under 800 nm excitation. It is noted that an enhancement factor could not be obtained because the NaErF4:Tm,Gd core UCNPs showed nearly no PL peak under 800 nm excitation (Fig. 4d). Under the condition of 800 nm excitation, the growth of the outermost NaYF4:Nd(40%),Yb(10%) active shell significantly enhanced red UCL and the C/S/S UCNPs exhibited ∼3.4 times higher UCL intensity compared with the C/S UCNPs (Fig. 4d). As a result, the C/S/S UCNPs showed strong red UCL under 980 and 800 nm excitations (Fig. 4, insets). The absolute UC QY of the C/S/S UCNPs was measured to be 0.06% under 800 nm NIR light irradiation (Table S2, ESI). This lower UC QY under 800 nm excitation than that under 980 nm excitation is attributed to large NIR light absorption by Nd3+ ions although strong red light is emitted from the C/S/S UCNPs under 800 nm NIR light. In addition, when we compared the C/S/S UCNPs with the NaErF4:Tm,Gd/NaYF4:Ca(20%) C/S UCNPs, the C/S/S UCNPs showed ∼1.5 times and ∼3.2 times higher PL intensities than the C/S UCNPs under 980 and 800 nm excitation, respectively. This study indicates that the core/intermediate shell/active shell structure should be applied to NaErF4:Tm,Gd-based UCNPs for strong UCL under 800 nm excitation. In addition, the C/S/S UCNPs showed stable UCL against long-term laser irradiation (Fig. S9 and S10, ESI).

image file: c8cc09031a-f4.tif
Fig. 4 (a) Absorption spectra of the core, C/S, and C/S/S UCNPs. (Transitions from Yb3+, Nd3+, and Er3+ are denoted in brown, violet, and red, respectively.) (b) Schematic energy level diagram of C/S/S UCNPs showing the UC mechanism under NIR light. PL spectra of core, C/S, and C/S/S UCNPs under (c) 980 and (d) 800 nm excitations. Insets show photographs showing the UC luminescence from (i) core, (ii) C/S, and (iii) C/S/S UCNPs under 980 and 800 nm excitations, respectively (laser power = 2 W).

In summary, intense red-emitting NaErF4:Tm,Gd/NaYF4:Ca,Yb/NaYF4:Nd,Yb C/S/S UCNPs were synthesized. By doping Gd3+ into the host nanocrystals, small and uniform sized NaErF4-based core UCNPs were synthesized. To enhance the absorption under ∼800 nm NIR light, a Nd3+-doped active shell was grown and an intermediate shell was inserted between the core and the active shell to hinder the quenching of Er3+ luminescence via Er3+ → Nd3+ back energy transfer. The formation of a C/S/S structure was confirmed by EDS analysis. The NaErF4:Tm,Gd-based C/S/S UCNPs exhibited ∼800-fold enhancement of red UCL compared with the core UCNPs under 980 nm excitation. Moreover, the C/S/S UCNPs showed intense red emission under 800 nm NIR light and exhibited 3.4-fold PL enhancement compared with the C/S UCNPs. The intense red-emitting C/S/S UCNPs can have potential for bio-imaging applications, in particular in vivo imaging after further studies on surface modification and functionalization.

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A2B5A03023239) and the Future Key Technology program of the KIST (Project No. 2E28020). We acknowledge A-Ra Hong for her help with TEM measurements.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available: Experimental section, HR-TEM images, size distributions, EDS spectrum, etc. See DOI: 10.1039/c8cc09031a

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