Synthesis of monodispersed YbF₃:Er³⁺ nanoplates with rhombus shapes

Abstract

Novel monodispersed YbF₃:Er³⁺ nanoplates with rhombus shapes are synthesized via a thermolysis process in this study. The morphology and luminescence properties have been investigated using transmission electron microscopy (TEM) and emission spectroscopy.

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Controlling inorganic nanocrystals (NCs) with well-defined shapes and architectures is always an important route to finely tune their properties, because the properties of the materials closely relate to geometrical factors such as morphology, dimensionality, and size.^1–3 In the past decade, lots of effort has been made to fabricate a variety of inorganic crystals to enhance their performance. Among them, the wet chemical method has been proved to be one of the most effective and convenient approaches in preparing various inorganic materials with diverse controllable morphologies and architectures in terms of low cost and large scale production.

In recent years, research on lanthanide (Ln³⁺) ions doped fluoride crystals is rapidly increasing due to their wide applications in solar cell, flat panel display, fluorescence imaging, drug deliver.^4–8 The Ln³⁺ ions can undergo a process known as up-conversion (UC) in which they convert low energy radiation such as near infrared (NIR) or IR light to high energy radiation including visible or ultraviolet (UV) light.^9,10 Typically in UC fluoride crystals, hexagonal (β) phase NaREF₄ (RE = rare earth ions) crystals are identified as the ideal UC luminescence matrices owing to their low phonon energy, high Ln³⁺ ion solubility and high photochemical stability.^11–13 Besides β-NaREF₄, orthorhombic REF₃ crystals have been proved to be another efficient high order UC luminescence materials.^14–16 Furthermore, YbF₃:Er shows strong down-conversion (DC) properties when excited with UV light.^17,18

Here, Er³⁺ doped YbF₃ rhombus nanoplates (NPs) are first synthesized with a high temperature thermolysis process. Fig. 1a presents a TEM image of as-synthesized YbF₃:10% Er³⁺ NPs. It is found that almost all the nanoplates show clear rhombus shape and size of as-synthesized NPs is well dispersed. The length and width of NPs are 12.2 ± 2.1 nm and 10.9 ± 1.8 nm, respectively. The corresponding XRD and electron diffraction patterns (shown as S4 and S5†) demonstrate that as-synthesized nanoplate is YbF₃ {Pnma} orthorhombic crystal structure. The diffraction band of as-synthesized nanoplates is consistent with the JCPDS 49-1805 file of crystal YbF₃. Furthermore, from the high resolution TEM image of Fig. 1b, well crystalline lattice structures can be observed. Most of nanoplates grow along (101) with a lattice distance of 0.360 nm. The shape of YbF₃ nanoplate can be affected by the amount of Er³⁺ and oleic acid. Fig. 2a presents a TEM image of YbF₃:50% Er³⁺ nanoplates. As shown in Fig. 2a, instead of well disperse single rhombus plate, short belts consisted by 2–4 single rhombus plates are observed for YbF₃:50% Er³⁺ plate. Fig. 2b is a HRTEM image of an aligned YbF₃:50% Er³⁺ plate standing on the edge array. The distance between two standing plate is about 2.5 nm, and the thickness of nanoplate is ca. 2.8 nm. The analysis results from HRTEM image of plate edge demonstrate that the edge grow along (111) and (020) directions. While, when the amount of Er³⁺ is up to 90%, as-synthesized nanoplates are well disperse single rhombus plates (see S9†). Furthermore, the shape or structure can be affected by the amount of oleic acid. For example, for YbF₃:10% Er³⁺ discussed above, with same synthesis conditions, when volume of oleic acid is reduced from 3.2 ml to 1.6 ml, similar structure to that of YbF₃: 50% Er³⁺ nanoplate are also observed. Currently, we attribute the formation of short belt of YbF₃:10% to the overgrowth of nanoplate. With less oleic acid, several nanoplates grow and band together to form large nanoplate belt. Further experiments and analyses are being done to explore the effect of Er³⁺ amount and oleic acid on the structure of Er³⁺ doped YbF₃ nanoparticles.

Fig. 1 TEM (a) and HRTEM (b) images of as-synthesized YbF₃:10% Er³⁺ nanoplates.

Fig. 2 (a) TEM image of as-synthesized YbF₃:50% Er³⁺ nanoplates, (b) HRTEM image of an aligned YbF₃:50% Er³⁺ plate standing on the edge array. (c) HRETM image of an aligned YbF₃:50% Er³⁺ plate standing edge.

For the photovoltaic efficiency of semiconductor solar cells, the thermalization and sub-band gap losses with UV and NIR are the major bottleneck. Up-conversion can generate one high energy photon from two or more incident low energy (sub-band gap) photons, whereas the down-conversion can generate more than one low-energy (sub-band gap) photons for each incident high-energy photon. In this communication, we report the synergistic effect of UV down-conversion and NIR up-conversion of as-synthesized YbF₃:10% Er³⁺ nanoplates.

Fig. 3 presents the absorption and emission spectra of as-synthesized YbF₃:10% Er³⁺ NPs. It is well known that (Er³⁺, Yb³⁺) is one of the efficient up-conversion couples. Efficient up-conversion has been reported for this couple in many host lattices and it is used in the efficient detection of near 1000 nm IR radiation. After excitation to the ²F_5/2 level of Yb³⁺, two sequential energy transfer steps excite the Er³⁺ ion from the ⁴I_15/2 ground state to the ⁴I_11/2 excited state and from the ⁴I_11/2 excited state to the higher energy ⁴F_7/2 excited state. Visible emission may be observed from the ⁴F_7/2 state or, after relaxation, from the lower energy ²H_11/2 and ⁴S_3/2 states. Fig. 3b is the UC emission spectra of as-synthesized YbF₃:10% Er³⁺ NPs upon an excitation wavelength of 980 nm. The weak violet emission centred at 430 nm is attributed to the ²H_9/2–⁴I_15/2 transition of Er³⁺ ions. The obvious green emission peak at 542 nm is assigned to the ⁴S_3/2–⁴I_15/2 transition of the Er³⁺ ions, while the red emission peak at 610 nm is attributed to the Er^{3+ 4}F_9/2–⁴I_15/2 transition. Furthermore, (Er³⁺, Yb³⁺) couple is an attractive DC materials for use in combination with c-Si solar cells.^19–21 The choice of Yb³⁺ as the emitting acceptor is due to the favourable energy of the ²F_5/2 excited state and the fact that the Yb³⁺ has no other 4f excited states that can interfere with the down-conversion process. Fig. 3c shows a DC emission spectrum of as synthesized YbF₃:10% Er³⁺ NPs excited at 369 nm. The strong emission at 480 nm is attributed to the ⁴F_7/2–⁴I_15/2 transition of the Er³⁺ ions.

Fig. 3 Absorption spectrum (a), up-conversion (b) and down-conversion (c) Emission of as-synthesized YbF₃:10% Er³⁺ nanoplates (in b, the laser power of 980 nm laser diode is 1 W).

Conclusions

In this study, monodisperse Er³⁺ doped YbF₃ nanoplates with regular rhombus shape are synthesized by a thermolysis process. The as-synthesized YbF₃:10% Er³⁺ nanoplates are small and its size is less than 15 nm. The thickness of plate is about 2.8 nm. All the nanoplates are well dispersed and have a good crystalline. The analysis results demonstrate that the structure of plate can be affected by the amounts of doped Er³⁺ ion and oleic acid reagent. Moreover, the as-synthesized YbF₃:10% Er³⁺ nanoplates show strong up-conversion and down-conversion emission properties.

Acknowledgements

This work is funded by National Key Basic Research Program (973 Program) of China (no. 2012CB933301) and Tianjin Research Program of Applied Basic & Cutting-edge Technologies (no. 09JCYBJC27200).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed materials synthesis method and characterization data. See DOI: 10.1039/c4ra13271h

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