Yb3+-sensitized upconversion and downshifting luminescence in Nd3+ ions through energy migration

Qi Zhu ac, Tianying Sun ac, Mei Nog Chung a, Xinwen Sun b, Yao Xiao b, Xvsheng Qiao *b and Feng Wang *ac
aDepartment Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, China. E-mail: fwang24@cityu.edu.hk
bState Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: qiaoxus@zju.edu.cn
cCity Universities of Hong Kong Shenzhen Research Institute, Shenzhen 518057, China

Received 18th January 2018 , Accepted 7th February 2018

First published on 12th February 2018


A core–shell–shell nanostructure composed of NaGdF4:Yb/Tm@NaGdF4:Nd@NaYF4 is developed to realize Yb3+-sensitized upconversion and downshifting luminescence in Nd3+ ions. The unusual photon conversion property stems from a gadolinium sublattice mediated Yb3+ → Tm3+ → Gd3+ → Nd3+ energy transfer pathway. The energy transfer processes are investigated by varying the dopant concentration and distribution, in conjunction with time decay measurements.


The research on lanthanide-doped luminescent nanoparticles has been rapidly growing in the past few decades on account of their high photochemical stability, sharp emission bandwidths, and large Stokes shifts.1,2 These attractive merits promote applications in diverse fields including bioimaging, solid-state lasers, and photovoltaic devices.3–5 In order to realize the full potential of lanthanide-doped nanoparticles for technological applications, considerable efforts have been devoted to tuning the optical properties of these nanoparticles by exploring new compositions and structures.

The combination of core–shell nanostructural engineering and energy migration provides a versatile method for creating novel luminescence processes.6 By incorporating a set of lanthanide ions into Gd- and Yb-based core–shell nanoparticles, tuning upconversion emissions were realized in a wide collection of lanthanide (Eu3+, Tb3+, Sm3+, Dy3+, and Ce3+) and transition metal (Mn2+) ions without long-lived intermediary energy states.7–12 Through the use of a gadolinium-based nanostructure, Ce3+-sensitized quantum cutting in Nd3+ and Yb3+ ions has also been demonstrated recently.13 Despite the encouraging achievements, no approach is established to realize energy migration-mediated emission in Nd3+ ions by near infrared excitation.

Herein, we report the observation and mechanistic investigation of Yb3+-sensitized upconversion and downshifting emission in Nd3+ ions by taking advantage of gadolinium sublattice mediated energy migration in a NaGdF4:Yb/Tm@NaGdF4:Nd@NaYF4 nanostructure (Scheme 1). We investigate the effects of the dopant concentration and distribution on the luminescence processes by means of steady-state and time-resolved spectroscopic measurements.


image file: c8dt00218e-s1.tif
Scheme 1 Schematic of the core–shell–shell nanostructure and proposed energy transfer for the Yb3+-sensitized upconversion and downshifting luminescence in Nd3+ activators.

Our design employs NaGdF4:Yb/Tm@NaGdF4:Nd layers to realize the spectral conversion processes, which makes use of a gadolinium sublattice mediated Yb3+ → Tm3+ → Gd3+ → Nd3+ energy transfer pathway. In addition, an inert NaYF4 protection shell is employed to eliminate the surface quenching of excitation energies stored in Nd3+ and Gd3+ ions. The synthesis of the core–shell–shell nanoparticles was adapted from a literature method,8 which consists of the growth of NaGdF4:Yb/Tm core nanoparticles followed by the successive deposition of the NaGdF4:Nd inner shell and NaYF4 outermost shell layers. Fig. 1a shows the transmission electron microscopy (TEM) images of the nanoparticles at different stages of the synthesis, which demonstrate a highly uniform size and morphology. The steady increase in the particle size along with the layer-by-layer growth is in good accordance with a previous report and indicates the successful shell coating. The high-angle annular dark-field (HAADF) scanning TEM image (inset of Fig. 1a) clearly reveals the core–shell structure, based on different Z contrast between the NaGdF4 and NaYF4 layers. In addition, the X-ray diffraction (XRD) patterns of the corresponding samples all display close resemblance to that of the β-NaGdF4 crystal (JCPDS file number 27-0699), substantiating the high crystallinity of the nanoparticles with a single hexagonal phase (Fig. 1b).


image file: c8dt00218e-f1.tif
Fig. 1 (a) TEM images and (b) XRD patterns of NaGdF4:Yb/Tm@NaGdF4:Nd(0.5%)@NaYF4 nanoparticles at different stages of the synthesis. (c) Emission spectra of the NaGdF4:Yb/Tm@NaGdF4:Nd (0 and 0.5%)@NaYF4 nanoparticles under excitation of a 980 nm CW diode laser. Inset: Time decay curves of Gd3+ at 311 nm in the corresponding samples under excitation at 980 nm.

Fig. 1c displays the photoluminescence spectra of the NaGdF4:Yb/Tm@NaGdF4:Nd (0.5%)@NaYF4 and the NaGdF4:Yb/Tm@NaGdF4@NaYF4 nanoparticles by excitation at 980 nm. Without Nd3+ dopants, the nanoparticles show characteristic emissions peaks that can be attributed to the 6P7/28S7/2 (311 nm) transition of Gd3+ and the 1I63F4 (345 nm), 1D63H6 (361 nm), 1D63F4 (451 nm), and 1G43H6 (475 nm) transitions of Tm3+. This observation is in good agreement with previous studies and confirms an efficient Yb3+ → Tm3+ → Gd3+ energy transfer process.14 After adding Nd3+ dopants in the inner shell layer, sharp emission peaks in the UV and NIR regions emerge along with a decrease in Gd3+ emissions, indicating an energy transfer from Gd3+ to Nd3+. The time decay studies reveal that the Gd3+ lifetime is shortened by the Nd3+ dopants (inset of Fig. 1c). In contrast, the Tm3+ lifetimes were essentially not affected by the Nd3+ ions (Fig. S1, ESI). Therefore, we can confirm that Nd3+ received the excitation energy from Gd3+ as designed.

The emission process in the core–shell–shell nanoparticles is strongly dependent on the dopant concentration of Nd3+. We have assessed a series of samples doped with varying amounts of Nd3+ ions (0%, 0.2%, 0.5% and 5%) and observed a maximum upconversion emission at a dopant concentration of 0.5% (Fig. 2a). Theoretically, a higher dopant concentration of Nd3+ favors the emission process by enhancing the capture of excitation energy stored in the gadolinium sublattice.8 The drop in the emission intensity at elevated Nd3+ concentrations is thus ascribed to concentration quenching resulting from cross-relaxation between Nd3+ ions (Fig. 2b). The time decay measurements detect a steady decrease in the Nd3+ lifetime as the dopant concentration increases (Fig. 2c), confirming the concentration quenching effect. Notably, the downshifting emission in the NIR region is also subjected to concentration quenching (Fig. S2, ESI), which is consistent with previous studies.13


image file: c8dt00218e-f2.tif
Fig. 2 (a) Emission spectra of NaGdF4:Yb/Tm@NaGdF4:Nd@NaYF4 nanoparticles as a function of Nd3+ concentrations (0%, 0.2%, 0.5% and 5%). (b) Simplified energy level diagram showing cross-relaxation between Nd3+ dopants. (c) Comparison of the time decay curves of Nd3+ at 382 nm in the NaGdF4:Yb/Tm@NaGdF4:Nd (0.2%, 0.5% and 5%)@NaYF4 nanoparticles. The samples were all excited at 980 nm.

The protection shell of NaYF4 is essential for achieving the Nd3+ emissions. As shown in Fig. 3a, Nd3+ emissions were markedly attenuated in the absence of the outermost NaYF4 shell. The results are attributed to the exposure of Nd3+ and Gd3+ ions to surface quenchers such as surface defects and passivating ligands, which are known to consume excitation energies (inset of Fig. 3a).15–19 Due to fast energy migration through the gadolinium sublattice, excitation energy is readily trapped by surface quenchers and dissipated, leading to reduced energy delivery to Nd3+ activators.8 Furthermore, the excited Nd3+ ions are also subjected to surface quenching. The excited states of Nd3+ are highly susceptible to high-energy surface oscillators because of the small energy gap in the energy level structure (Fig. S3, ESI).


image file: c8dt00218e-f3.tif
Fig. 3 (a) Comparison of the emission spectra of NaGdF4:Yb/Tm@NaGdF4:Nd(0.5%) core–shell and NaGdF4:Yb/Tm@NaGdF4:Nd(0.5%)@NaYF4 core–shell–shell nanoparticles. Inset: Schematics of suppressed surface quenching by the protection shell. (b) Emission spectrum of NaGdF4:Yb/Tm/Nd@NaYF4 nanoparticles with all the lanthanide ions homogeneously doped in the core layer. Inset: Schematic illustration of back-energy-transfer from Nd3+ to Yb3+ that results in the quenching of Nd3+ emissions.

The spatial separation of Yb/Tm and Nd3+ is also indispensable for establishing the desired energy transfer pathway and giving rise to the expected emission processes. When Nd3+ and Yb/Tm are homogeneously incorporated into the core layer, Nd3+ emissions were hardly detected (Fig. 3b). This observation is ascribed to the back energy transfer from Nd3+ to Yb3+ (inset of Fig. 3b). As reported by several independent groups,13,20–22 depletion of the excitation energy of Nd3+ by the surrounding Yb3+ ions is highly efficient (Fig. S4, ESI).

Conclusions

To conclude, we have demonstrated a novel upconversion and downshifting luminescence process by using a NaGdF4:Yb/Tm@NaGdF4:Nd@NaYF4 nanostructure. The core–shell–shell design eliminates deleterious interactions between Nd and Yb/Tm. Furthermore, the gadolinium sublattice enables a Yb3+ → Tm3+ → Gd3+ → Nd3+ energy cascade, leading to characteristic Nd3+ emissions by excitation into Yb3+ sensitizers. The improved control and understanding of energy transfer as demonstrated here should enhance our ability in the rational design of spectral converters using lanthanide ions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21573185 and 51332008), the Research Grants Council of Hong Kong (CityU 11208215), and the City University of Hong Kong (project 7004650).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8dt00218e

This journal is © The Royal Society of Chemistry 2018