1. Introduction

Upconversion materials have received substantial attention due to their promising applications ranging from nanophosphors, three-dimensional displays, bioimaging and bioprobes in recent years [1–12]. Upconversion of lanthanides holds many advantages including large anti-Stokes shift, narrow emission profiles, superior photo stability and chemical durability [13–15]. To date, the energy transfer upconversion (ETU) is the most efficient method for photon upconversion from nanosized materials, upon which emissions from Er³⁺, Tm³⁺, Ho³⁺ were obtained [1,13–15]. By taking advantage of energy migration in core-shell structure, the upconversion from Tb³⁺ and Eu³⁺ was also realized by using the energy migration upconversion [16]. However, achieving upconversion from more lanthanides remains challenging because of the complex electron configuration, which makes the upconversion lanthanides limited in Er³⁺, Tm³⁺, Ho³⁺, Eu³⁺, Tb³⁺, Dy³⁺ and Sm³⁺. We recently discovered that the precise control of energy transfer at the core-shell interfacial area is a promising approach for upconversion of nanosized materials, and the upconverted emissions from the afore-mentioned lanthanides were realized [17,18]. Also, such interfacial energy transfer shows merit in obtaining more efficient upconversion than the way of energy migration upconversion in core-shell nanostructures [19]. Therefore, this progress would allow for the possibility of realizing upconverison from Nd³⁺ which was rarely studied previously and usually used as the sensitizer with 808 nm excitation [10,20–23].

Here, we report the realization of upconversion from Nd³⁺ through using the Gd³⁺-mediated interfacial energy transfer (IET) mechanism in NaYbF₄:Tm/Gd@NaYF₄:Nd core-shell nanocrystals. The Gd³⁺ is selected as the energy donor, which can be effectively activated by the Yb-Tm couple under 980 nm irradiation, leading to the activation and emission of Nd³⁺ through the Gd³⁺-mediated IET, as illustrated in Fig. 1. It was found that the spatial separation of lanthanide ions can effectively minimize the luminescence quenching due to the serious interactions between them. This design presents a novel way to realize the upconversion of lanthanides that have no existed intermediate states, and more importantly, it may help the investigation of new upconversion mechanism for lanthanides and promote the frontier applications.

 

Fig. 1 Mechanistic illustration of the proposed Gd-mediated interfacial energy transfer (IET) for the NIR upconversion of Nd³⁺ in the NaYbF₄:Tm/Gd@NaYF₄:Nd core-shell nanostructure. The right panel shows the detail of the energy transportation processes under 980 nm excitation through IET. MPR stands for multi-phonon relaxation.

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2. Experimental section

The core, core-shell and core-shell-shell nanoparticles were synthesized using the coprecipitation chemical method. Typically, for the NaYbF₄:Tm/Gd (1/50 mol%) core seeds, to a 50-mL flask containing oleic acid (4 mL) and 1-octadecene (6 mL) was added a water solution (2 mL) containing 0.4 mmol lanthanide acetates (Yb:Tm:Gd = 49:1:50 mol%). The resulting mixture was heated at 150 °C for 1 h to form lanthanide oleate complexes and then cooled down to room temperature. Subsequently, a methanol solution containing NaOH (1 mmol) and NH₄F (1.6 mmol) was added and stirred at 50 °C for 0.5 h, and then heated at 290 °C under an argon flow for 1.5 h before cooling down to room temperature. The resulting core nanocrystals were collected by centrifugation, washed with ethanol, and finally dispersed in cyclohexane. Other core seeds were prepared using the same procedures except for the precursor. The core-shell (or core-shell-shell) nanocrystals were prepared by a 2-step (or 3-step) co-precipitation method using the pre-synthesized core (or core-shell) nanocrystals as seeds for growing the shell layer, and collected with dispersion in cyclohexane. The chemical procedure of the ligands exchange for the as-prepared samples is realized by centrifugation at 16500 rpm for 20 min after ultrasonication for 15 min in a 2-mL HCl solution (0.1 M), and the resulting products were washed and re-dispersed in deionized water.

The transmission electron microscopy (TEM) measurements and element mapping were carried out on a field emission transmission electron microscope (JEOL Model JEM-2100F). The powder X-ray diffraction (XRD) data was recorded on a Rigaku Smart Lab X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The upconversion emission spectra were recorded by an Edinburgh FLS920 spectrometer equipped with a 980-nm laser diode. The upconversion emission photographs were taken by a digital camera.

3. Results and discussion

Figure 2(a) shows the XRD pattern of the as-synthesized NaYbF₄:Tm/Gd(1/50 mol%) core and NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(10 mol%) core-shell nanocrystals, exhibiting hexagonal phase of the lattice. The TEM images of these as-synthesized core and core-shell nanocrystals are shown in Figs. 2(b) and 2(c), exhibiting uniform sphere morphology. The size increases from 11.7 to 18.1 nm for the core seeds after epitaxially growing the NaYF₄:Nd shell layer. The high-resolution TEM image (Fig. 2(d)) was also measured for a single core-shell nanocrystal showing clear lattice fringes confirming the good crystallinity of the core-shell nanostructures. The lattice fringes distance was obtained to be 0.52 nm, corresponding to the d-spacing of the (100) lattice planes of hexagonal structure, which is in agreement with the Fourier transform pattern (Fig. 2(e)) and the XRD result (Fig. 2(a)). The element mapping measurement was performed, see Figs. 2(f)-2(k), and clear contrasts in the distribution of each element further confirmed the success in synthesizing the core-shell nanostructure.

 

Fig. 2 (a) X-ray diffraction data and (b,c) TEM images of NaYbF₄:Tm/Gd(1/50 mol%) core and NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(10 mol%) core-shell nanoparticles. (d) HRTEM image and (e) the corresponding Fourier transform diffraction pattern of the core-shell sample in (a). (f-k) Element mappings of Gd, Yb, F, Nd and Y for the core-shell sample in (a).

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Figure 3(a) shows the near-infrared upconversion spectra from the NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd core-shell nanocrystals under 980 nm excitation. Typical emission band of Nd³⁺ at 861 nm due to the ⁴F_3/2→⁴I_9/2 transition was observed. The infrared emission bands at 1058 and 1343 nm were also observed (Fig. 3(b)) and they are due to the ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 transitions, respectively. With the increase of Nd³⁺ in the shell layer, the emission exhibits an initial increment in luminescence intensity and then a gradual decline and the maximum intensity was obtained for the samples with Nd³⁺ doping of 10 mol%. The decline of Nd³⁺ emissions in luminescence intensity at high doping concentrations may be ascribed to the cross-relaxation process [⁴F_3/2; ⁴I_9/2]→[⁴I_15/2; ⁴I_15/2] (see Fig. 3(c)), upon which the energy at the ⁴F_3/2 emitting level is nonradiatively released to the intermediate level instead of a radiative decay to the ground state or other terminate states. This concentration quenching effect was widely observed in a set of Nd³⁺-doped materials [11,24,25].

 

Fig. 3 (a,b) Near-infrared emission spectra from NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(x mol%) core-shell nanoparticles. (c) Cross relaxation process [⁴F_3/2; ⁴I_9/2]→[⁴I_15/2; ⁴I_15/2] for quenching the Nd³⁺ emission. (d) Comparative upconversion emission spectra from core-shell nanoparticles with and without Gd³⁺ in the core. (e) Intensity dependence of Nd³⁺ emission at 861 nm on pump power. (f) Visible upconversion emission spectra obtained from (a) samples. Note that all the emission spectra were measured under 980 nm excitation.

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To further check the role of Gd³⁺ in the core-shell structure, we synthesized the NaYbF₄:Tm/Y(1/50 mol%)@NaYF₄:Nd core-shell sample without doping of Gd³⁺ in the core, and measured its emission spectrum (Fig. 3(d)). Unfortunately, no emission of Nd³⁺ was observed, confirming that the activation of Nd³⁺ is due to the Gd³⁺-mediated IET approach instead of the energy transfer from Yb³⁺ or Tm³⁺ in the core. This result is also confirmed by the emission intensity dependence on pump power with a slope value of 4.21 for the 861 nm emission (Fig. 3(e)) because the donor Gd³⁺ is activated by the Yb-Tm couple through a 5-photon upconversion (Fig. 1). Aiming to give a more in-depth understanding of the upconversion mechanism, we measured the upconversion spectra at visible region, as shown in Fig. 3(f). Typical upconverted emissions of Tm³⁺ from the core were recorded, and they are from the ¹I₆→³F₄ (345 nm), ¹D₂→³H₆ (361 nm), ¹D₂→³F₄ (450 nm), ¹G₄→³H₆ (476 nm), ¹G₄→³F₄ (645 nm), and ³H₄→³H₆ (801 nm) transitions, respectively. However, no visible emissions from Nd³⁺ were recorded might be due to the multi-phonon relaxation (see Fig. 1), indicating that the upconversion emissions of Nd³⁺ at the infrared wavelength region are dominant in the release of the radiative energy through IET.

Next, we studied the detail of interfacial energy transfer from Gd³⁺ in the core to the Nd³⁺ in the shell layer. Because the cyclohexane solution is also active at the ultraviolet wavelength region which cause an error in the measurement, we thus removed the surface oleic ligands for all the samples. As shown in Fig. 4(a), intense emission of Gd³⁺ at 311 nm due to ⁶P_7/2→⁸S_7/2 transition was observed and it presents a rapid decrease in emission intensity as the Nd³⁺ concentration in the shell increases, indicating an efficient energy transfer channel from Gd³⁺ to Nd³⁺ at the core-shell interfacial area (inset in Fig. 4(a)). It should be noted that there is no obvious change in the infrared emission spectra for the samples after ligands exchange (Fig. 4(b)).

 

Fig. 4 (a) Ultraviolet upconversion emission spectra of Gd³⁺ from the NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(0-20 mol%) core-shell nanoparticles. (b) Upconversion emission spectra from NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(10 mol%) nanoparticles with and without the ligand exchange. (c) Infrared emission spectra from the NaYbF₄:Tm/Gd(1/50 mol%)@NaXF₄:Nd(10 mol%; X = Lu,Gd,Y,La) core-shell nanoparticles. All the spectra were measured under 980 nm excitation.

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On the other hand, it is of very importance for an in-depth investigation of the Gd³⁺-mediated upconversion because it was claimed to be due to the energy migration upconversion in the NaGdF₄@NaGdF₄ core-shell system [16]. To further verify the role of Gd³⁺ in the core-shell nanostructure, we prepared a set of core-shell samples with different shell matrix compositions, namely NaYbF₄:Tm/Gd(1/50 mol%)@NaXF₄:Nd (X = Lu, Gd, La, Y; 10 mol%) core-shell nanocrystals. The intense upconversion of Nd³⁺ in these samples is clearly observed under 980 nm excitation (Fig. 4(c)), meaning that IET is a general and shell matrix-independent process. Therefore, the IET is the dominant process in the achievement of Nd³⁺ upconversion. Similar results were observed for the upconversions from Eu³⁺ and Tb³⁺ in the NaGdF₄@NaGdF₄ core-shell system [17].

In order to further enhance the near-infrared upconversion emission, we propose a core-shell-shell strategy to increase the absorbing ability of 980 nm irradiation by incorporating the sensitizer Yb³⁺ into the NaYF₄:Nd active shell layer together with an outside protective NaYF₄ layer as shown in Fig. 5(a). This would allow more excitation energy to be absorbed and transferred to the Yb³⁺ in the core area through energy migration, subsequently leading to the activation of more Gd³⁺ through the Yb-Tm coupled system. To check this hypothesis, we prepared the NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd/Yb@NaYF₄ core-shell-shell samples using the three-step coprecipitation method. Figure 5(b) shows the upconversion emission spectra under 980 nm excitation. With the presence of Yb³⁺ in the interlayer, there is indeed an increase of the emission intensity, and the optimized Yb³⁺ concentration is found to be 10 mol%. A higher dopant concentration would lead to a decrease of the emission intensity, which may be due to the deleterious interaction between Nd³⁺ and Yb³⁺ because efficient Nd-to-Yb energy transfer across the core-shell interface easily occurred for the Nd-Yb coupled NaYF₄:Nd@NaYF₄:Yb system (Fig. 5(c)). This result implies that back energy transfer from Nd³⁺ to Yb³⁺ is a primary quenching channel for the samples with high doping of Yb³⁺.

 

Fig. 5 (a) Schematic of proposed core-shell-shell structure for enhancing upconversion of Nd³⁺ by introducing Yb³⁺ into the innerlayer through energy accumulation. (b) Comparison of the NIR upconversion of Nd³⁺ from the NaYbF₄:Tm/Gd(1/50 mol%)@NaYF₄:Nd(20 mol%)@NaYF₄, NaYbF₄:Nd(20 mol%)@NaYF₄, NaYbF₄:Tb(30 mol%)@NaYF₄:Nd(20 mol%) control samples under 980 nm excitation. (c) Emission spectra from NaYF₄:Nd@NaYF₄:Yb core-shell nanoparticle under 808 nm excitation.

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4. Conclusion

Efficient upconversion of Nd³⁺ has been achieved using the Gd³⁺-mediated interfacial energy transfer strategy in a core-shell nanostructure. Such upconversion shows an independent feature on the host materials, suggesting its general applicability. Efficient Gd-to-Nd energy transfer was confirmed by the rapid decline of Gd³⁺ emission at 311 nm. The upconversion of Nd³⁺ can be further enhanced by taking advantage of the energy migration in the Yb³⁺ sublattice in a designed core-shell-shell structure. These findings present a new mechanistic strategy for the upconversion of lanthanides in particular for those without intermediate states under 980 nm excitation. More importantly, they would contribute to the search of new upconversion mechanism together with new-type lanthanide-based upconversion materials.