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We report the realization of photon upconversion through interfacial energy transfer in Yb3+/Tb3+ coupled core-shell nanostructure. The spatial separation of sensitizer and activator at different layers through the core-shell structure enables the enhancement of photon emission by minimizing the non-radiative decays of the activator. Furthermore, the energy migration among Tb3+ ions within the core matrix lattice effectively facilitates the energy transfer to a much farther distance, resulting in efficient photon upconversion by the Tb3+-mediated interfacial energy transfer. Our result offers a novel and widespread approach for achieving photon upconversion of lanthanide-doped nanocrystals that would help develop a new class of efficient upconverting materials.
© 2017 Optical Society of America
1. Introduction
Featuring by the short-wavelength photon emission under the long-wavelength photon excitation, photon upconversion has received substantial attention due to their great potential applications in solid-state lasers, optical data storage, displays, photovoltaics, biological imaging, and so on [1–10]. This is mainly due to the excellent emissive properties of trivalent lanthanides as a result of unique 4f electronic configuration, which enables abundant emission bands ranging from ultraviolet to visible and near-infrared spectral regions under low-energy infrared photon excitation [11–14]. Intense photon upconversion has been obtained in classes of materials especially in nano-sized crystals, greatly contributing to their frontier applications [1]. Typical upconversion systems include Er3+ (and Ho3+) for the green and red emissions, and Tm3+ for the blue emission under 980 nm excitation with the sensitization of Yb3+ [11]. Recently, excitation sources at other wavelengths have been explored with good results reached. For instance, laser sources working at around 800 nm was successfully used to excite the upconverting emission of lanthanides through using Nd3+ as the co-sensitizer in a core-shell nanosctructure [15–20]. This is an important progress for the biological application because the water absorption at around 800 nm wavelength is much lower than that at 980 nm, and thus the heat effect due to the excitation lasing irradiation can be effectively reduced.
However, the intense upconverting emissions of Er3+, Tm3+ or Ho3+ is mainly realized by the way of energy transfer upconversion (ETU) and/or the excited-state absorption (ESA) [11,12], and ETU (or ESA) does not work well for other lanthanide rare earth ions (e.g., Eu3+) because they cannot absorb the infrared excitation energy directly [11]. Recently, through using core-shell structure design and spatial separation of sensitizer system and activator, a new photon upconversion process containing ETU, energy transfer (ET) and energy migration (EM) was reported by Wang et al [21]. Using this upconversion scheme, upconverting emissions from lanthanide ions such as Tb3+ have been obtained under conventionally used 980 nm lasing source. However, the emission from such system shows a close dependence on the Gd3+-constituted nanocrystals, and the multiple 5-photon upconversion by the Yb3+/Tm3+ couple is also a limitation because it requires a high pump power density for an efficient energy management [11,22].
In this work, we report the photon upconversion of Eu3+ in the Yb3+/Tb3+ coupled core-shell nanostructure through a way of interfacial energy transfer (see Fig. 1). The use of NaYbF4 as host matrix allows the effective activation of Tb3+ by the cooperative sensitization approach, offering a possible candidate for modifying the photon emission as an energy donor under 980 nm irradiation. The photon upconversion obtained through this mechanistic scheme exhibits stable yellow emission at different pump powers, and importantly, the energy migration among Tb3+ ions gives an additional improvement of photon upconversion. Our result provides a platform for further investigating the interactions between lanthanide ions, contributing to the development of new classes of upconverting materials.
Fig. 1 Schematic of a core-shell nanostructure design with interfacial area (left panel) and detail of Tb3+-mediated interfacial energy transfer upconversion (right panel). CS, ET and IETU represent cooperative sensitization, energy transfer and interfacial energy transfer upconversion, respectively. NIR and VIS represent near-infrared excitation and visible emission photons, respectively.
2. Results and discussion
The samples of NaYbF4:Tb(30 mol%) core and NaYbF4:Tb(30 mol%)@NaXF4:Eu (X = Lu, Y, Gd and La) core-shell nanocrystals were synthesized using a co-precipitation method [23], which has been well developed during past years in particular for the synthesis of high-quality core-shell nanostructures [24–26]. As shown in Fig. 2(a), the as-synthesized NaYbF4:Tb core nanoparticles have a sphere morphology with an average diameter of 14.5 nm. After coating the shell layer, its size grows to a value of 21.7 nm with a shell thickness around 3.6 nm. The as-synthesized core and core-shell samples are in hexagonal phase based on the X-ray diffraction (XRD) line profiles (Fig. 2(b)), implying an epitaxial growth route during the co-precipitation approach. The core-shell nanostructure is also indicated by the contrast in the enlarged transmission electron microscope (TEM) image of finally synthesized nanoparticle samples (Fig. 2(a)). The observation of sharpening XRD diffraction peaks for the samples after coating the shell layer is in agreement with the increase in size for the core-shell nanoparticles (Fig. 2(b)).
Fig. 2 (a) TEM images, high-resolution TEM images, selected area electron diffraction (SAED) pattern, and (b) XRD patterns of as-synthesized NaYbF4:Tb core and NaYbF4:Tb@NaLuF4:Eu core-shell nanoparticles. (c) Upconversion emission spectra of NaYbF4:Tb@NaXF4:Eu(X = Y, Gd, Lu, La; Eu3+: 30 mol%) core-shell nanoparticles. (d,e) A comparison of emission spectra from NaYbF4:Tb@NaLuF4:Eu, NaYbF4:Tb@NaLuF4 and NaYbF4@NaLuF4:Eu core-shell nanocrystals. Inset of (d) shows the emission photographs of the former two samples. Note that all emission spectra were measured at 980 nm excitation.
The upconversion emission spectra of the core-shell samples measured at 980 nm excitation are shown in Fig. 2(c). The characteristic emission of Eu3+ was clearly observed, and the peaks at 580, 615 and 695 nm are due to the optical transitions from 5D0 excited state to 7F1, 7F2 and 7F4 states, respectively. The emission bands recorded at 490, 546, 585 and 620 nm come from the Tb3+-related transitions (e.g., 5D4 → 7F5 transition at 546 nm). Notably, no emission with regard to Eu3+ is recorded for the core-shell samples without doping of Eu3+ in the shell layer (Fig. 2(d)). Considering that the activator Eu3+ in the shell layer may also be excited by Yb3+ in the core as displayed in Yb3+-Eu3+ coupled systems (e.g., through a way of cooperative sensitization) [27–29], it is needed to further check the role of Yb3+ during the course of interfacial energy transfer. As a control, we designed and synthesized a core-shell sample with only Yb3+ in the core area (the sample NaYbF4@NaLuF4:Eu) using the co-precipitation method. The Eu3+ emission intensity from this sample is extremely reduced in contrast to the NaYbF4:Tb@NaLuF4:Eu core-shell nanoparticles (see Fig. 2(e)), meaning that the possibility of interfacial energy transfer from Yb3+ to Eu3+ at the core/shell interfacial area can be excluded. Therefore, the occurrence of Tb3+-mediated interfacial energy transfer is confirmed, which plays a critical role in realizing the activation and thereafter the emission of Eu3+ from the shell layer under 980 nm excitation.
To further understand the Tb3+-mediated interfacial energy transfer, we measured the decay curves of Tb3+ at monitoring wavelength of 546 and 415 nm, corresponding to the optical transitions from respectively 5D4 and 5D3 states to the ground state. As displayed in Fig. 3(a), the decay curves of the core-shell sample with Eu3+ in the shell layer show a decrease in lifetime value compared to that without Eu3+-doping (i.e., the sample NaYbF4:Tb@NaLuF4). This result means that the energy transfer from both 5D4 and 5D3 states have contribution to the Eu3+ emission by the way of interfacial energy transfer. However, it is noticed that the lifetime of Tb3+ at its 5D4 state is much longer than that at 5D3 state, and the emission intensity of transitions from 5D4 to the ground state is also much stronger than that from the upper 5D3 state (see Fig. 3(b)). The rise time shown in Fig. 3(a) confirmed the dominance of ETU rather than ESA for the population of 5D3 state of Tb3+ under 980 nm excitation [11]. Together, these results indicated that the activation of Eu3+ in the shell layer is mainly due to the interfacial energy transfer from Tb3+ at its 5D4 state, and the interfacial energy transfer efficiency (η) is further obtained to be 9.3% by the equation η = 1−τ/τ0, where τ ( = 4.054 ms) and τ0 ( = 4.471 ms) are the lifetime values of core-shell samples with and without Eu3+ in the shell layer. The total energy transfer pathways within the interfacial area is schematically illustrated in Fig. 3(c).
Fig. 3 (a) Decay curves of Tb3+ emissions at 546 and 415 nm for the NaYbF4:Tb@NaLuF4:Eu and NaYbF4:Tb@NaLuF4 core-shell samples upon a pulse 980-nm excitation. (b) A comparison of upconversion emission spectra of the two samples in (a). (c) Proposed possible interfacial energy transfer (IET) pathways in activation of Eu3+ in the shell under 980 nm excitation. Note that CSU stands for cooperative sensitization upconversion.
In general, the energy transfer between lanthanide ions exhibits a sensitive dependence on their separation, for instance, the energy transfer probability shows a −6 orders of magnitude to the donor-acceptor separation under dipole-dipole interaction [11,30–32]. This means that the energy transfer from Tb3+ to Eu3+ would be confined in a narrow region close to the interface. Indeed, the emission of Eu3+ in the shell layer is easily prohibited by growing an optically inactive thin inter-layer between the sensitizing NaYbF4:Tb core and the emissive NaLuF4:Eu shell layer (Fig. 4(a)). A further study of the relation between the energy transfer efficiency and interlayer thickness would contribute to deep understanding of the spatial scope of interfacial energy transfer. It should be pointed out that when the Tb3+ and Eu3+ ions were simultaneously co-doped together in the core (e.g., the sample NaYbF4:Tb,Eu@NaLuF4), the emission intensity is markedly reduced than the core-shell sample with a spatial separation of Tb3+ and Eu3+ at different layers (Fig. 4(b)). This gives a strategy to control the non-radiative interactions among lanthanide ions especially when they are doped in a very high concentration. Meanwhile, it is noticed that the emission intensity of Tb3+ from NaYbF4:Tb@NaLuF4:Eu is markedly enhanced in contrast to the NaYbF4:Tb core only nanocrystals, being similar to the emission spectrum obtained from NaYbF4:Tb@NaLuF4 core-shell sample (Fig. 4(b)). This implies that the Eu3+-doped shell layer is also capable of preventing the Tb3+ energy donor in the core from surface quenchers [33–36], which is of critical importance for the activation of Tb3+ as well as the subsequent realization of an efficient interfacial energy transfer. The total emission color can be gradually tuned from green to yellow by increasing the content of Eu3+ in the shell layer, and more importantly, such yellow emission from the sample with fixed Eu3+ concentration exhibits an independence feature on pump power, as shown in Figs. 4(c) and 4(d).
Fig. 4 (a) Upconversion emission spectra of NaYbF4:Tb@NaLuF4:Eu core-shell and NaYbF4:Tb@NaLuF4@NaLuF4:Eu core-shell-shell nanocrystals. Inset, the schematic of IET confining in a narrow layer close to the core/shell interface. (b) Upconversion emission spectra of core NaYbF4:Tb, and core-shell NaYbF4:Tb@NaLuF4:Eu and NaYbF4:Tb/Eu@NaLuF4 samples. (c) Emission photographs and (d) upconversion emission spectra of NaYbF4:Tb@NaGdF4:Eu with increasing concentration of Eu3+ from 0 to 30 mol%. Left inset of (d) shows the detail of color change in the CIE(x,y) chromaticity diagram. Right inset of (d) shows the Eu-to-Tb emission intensity ratio as a function of pump power. (e) Proposed energy migration among Tb3+ ions for activating the Eu3+ in the shell layer through using the Tb3+ ions far away from the interfacial area. (f) Upconversion emission spectra obtained from NaYbF4:Tb@NaYbF4:Tb@NaLuF4:Eu and NaYbF4@NaYbF4:Tb@NaLuF4:Eu tri-layer nanoparticles. Note that all emission spectra were measured under 980 nm excitation.
We have demonstrated that the Tb3+-to-Eu3+ interfacial energy transfer is mainly confined in a narrow core/shell interfacial area, and the Tb3+ ions in the core far away from the interface cannot directly activate the Eu3+ in the shell layer. Considering the fact that Tb3+ has a long lifetime at its 5D4 state and it also shows a much low dependence on concentration effect, the energy migration among Tb3+ (5D4) ions may occur, which could help to accumulate the energy to the interfacial area and thereafter contribute to enhancement of the photon upconversion of Eu3+, as schematically illustrated in Fig. 4(e). To validate our hypothesis, we designed two classes of core-shell-shell structures by using the nanoparticles with and without doping of Tb3+ as core seeds, and prepared the tri-layer NaYbF4@NaYbF4:Tb@NaLuF4:Eu and NaYbF4:Tb@NaYbF4:Tb@NaLuF4:Eu samples. Their emission spectra are plotted in Fig. 4(f), showing an obvious decline in the Eu3+ emission intensity for the sample without doping of Tb3+ in the core. This result clearly indicates the occurrence of energy migration among Tb3+ ions, and more importantly, it favors to obtain more efficient photon upconversion of Eu3+. Therefore, we can conclude that the Tb3+ dopants in the core area have played dual roles of donor and migrator in the present system, resulting in efficient interfacial energy transfer upconversion.
3. Conclusions
In summary, photon upconversion through the interfacial energy transfer in a core-shell structure has been demonstrated, which is almost independent on the composition of shell layer. The emission color can be gradually tuned from green to yellow by increasing the concentration of Eu3+ dopant in the shell layer, and the resultant yellow emission shows a stable generation at different pump powers, offering a potential stable yellow-emitting nanophosphor material. It is further found that the energy migration effect among Tb3+ ions locating within core area allows an accumulation of more energy at the core/shell interfacial area, resulting in improved photon upconversion. The result discovered in this work would bring an active contribution to the research of lanthanide-based upconversion materials, which show great potential in a broad scope of frontier applications.
Funding
The “One-Hundred Young Talents Program” (220413145) and “One-Hundred Talents Program” of Guangdong University of Technology (GDUT).
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