Unusually enhancing high-order photon avalanche upconversion of layered BiOCl:Er3 + semiconductor poly-crystals via Li+ ion intercalation doping

doi:10.1016/j.matdes.2016.05.080

Materials & Design

Volume 105, 5 September 2016, Pages 290-295

https://doi.org/10.1016/j.matdes.2016.05.080 Get rights and content

Highlights

•
Photon avalanche upconversion of BiOCl:Er^3 + poly-crystals increases greatly by doping Li⁺ ions.
•
Upconversion enhancement is due to increasing excitation field.
•
Intercalation doping of Li⁺ in layered BiOCl crystals improves its internal electric field.
•
Excitation field of Er^3 + ions increases due to enhanced internal electric field.

Abstract

Efficient upconversion (UC) enhancement of lanthanide doped phosphors is highly desirable due to great application, but it might face severe challenges, especially for poly-crystal materials due to having relatively fewer defects and with less flexibility in material design. Here we present BiOCl semiconductor poly-crystals with two-dimensional structure, in which approximate 100-fold UC luminescence enhancement of Er^3 + ion dopants can be readily achieved via intercalation doping of Li⁺ ions. As Li⁺ ions were incorporated into the interlayer of BiOCl crystals, unique internal electric field (IEF) in BiOCl that trigger unusual photon avalanche (PA) UC phenomena accompanying with excitation field enhancement of Er^3 + ions can be improved significantly, leading to the excellent UC enhancement. These results may pave a novel way for constructing greatly efficient UC poly-crystal materials as well as enable an improved understanding for material structure to design efficient photonic materials.

Graphical abstract

Introduction

Investigations on rare earth (RE) doped upconversion (UC) materials have been greatly intensified for a variety of potential applications such as three-dimensional displays, solid-state lasers, infrared detection, and so on [1], [2], [3], [4]. However, up to now, low efficiency continues to be one of the factors limiting the practical use of the UC phosphors. It, therefore, attracts great interest of researchers to further significantly increase the UC radiations of RE activated materials in the past decade, and considerable efforts, such as approaches based upon tailoring surface plasmon resonance and photonic bandgap [5], [6], or different synthesis routes together with new complementary techniques for material characterization have been devoted to this area. Notably, most of the efforts, at present, have been focused on the nano-materials rather than the bulk or poly-crystal materials [7], [8]. Besides the possible difference in application, one reason is likely that nano-sized UC materials generally exhibit one order of less efficiency than the corresponding bulk ones due to having great defects and impurities [1]. It might offer higher possibility for the nano-sized materials to get remarkable UC improvement. On the other hand, the bulk or poly-crystal ones have relatively less flexibility of material synthesis and design [9], [10], [11], which greatly limited the use of many methods. This implies that the UC enhancement for bulk or poly-crystal UC materials might have less potential and face more severe challenge than the ones in nanoscale. For instance, reports on the enhancement for nano UC materials could present nearly two orders of magnitude increases in emission intensity [12], but that for bulk or poly-crystal UC materials rarely reached 10 fold.

Compared with common multi-photons UC phenomena, the special one which that has high-order dependence on input light intensity provide an opportunity to realize significant UC enhancement of RE activated bulk or poly-crystal materials. Because utilizing the nonlinear effect of multi-photon process, it could be supposed that the methods to improve the efficiency of general UC materials might become more effectively for the special one with higher upconverting photon numbers. However, the enhancement for UC phenomena defined as or similar to a photon avalanche (PA) process, which generally exhibits much high-order power dependence over the threshold, has rarely received attention up to date, whether in nano or bulk UC materials [3], [12], [13]. It is likely because that the PA UC emission of RE ions usually is an infrequent process with the requirement for strict construction conditions, such as high pumping intensity, existence of a long lifetime intermediate state and some efficient cross-relaxation (CR) processes [14]. Therefore, although considered as relatively efficient one among three basic UC mechanisms, the PA emission behavior has seldom been discovered in UC materials, no more than its luminescence enhancement [3], [15].

Very recently, we reported a hetero-looping enhanced PA UC process of Er^3 + singly-doped BiOCl poly-crystals at room temperature [16]. However, we found that Yb^3 + ion dopants, typical sensitizer in Er^3 + ions doped UC materials due to greater adsorption at 980 nm band, not only degenerated the PA properties but also decreased the UC emission intensity [17]. The reason by which Yb^3 + ion dopants degenerate PA emission is related to special role of Yb^3 + ion on the internal electric field (IEF) of BiOCl and actually may have several mechanisms being seemingly dependent on the specific material. However, we interestingly found that the unique layered structure of BiOCl offers opportunity to archive efficient UC enhancement for RE ions doped poly-crystals. In this work, we utilize co-doping of Li⁺ ions which in general acts as doping lattice modifiers to improve UC emission of RE ions, to realize a surprising two orders and even an over 100 fold of enhancement for the UC emission of Er^3 + doped BiOCl ploy-crystals. Importantly, the enhancing role of Li⁺ ion dopants on the PA UC emission of Er^3 + ions was not attributable to that of traditional lattice modifier, such as increasing radiative lifetime of RE ions or host crystalline improvement [18], [19], [20], [21]. It is mainly due to efficient enhancement of IEF in layered BiOCl crystals causing by in intercalation doping of Li⁺ ions, which induce significantly increased excitation field of Er^3 + ions accompanying with the special multi-photons effect of PA phenomena.

Section snippets

Experimental

Samples of BiOCl:4% Er^3 + co-doped Li⁺ ions were prepared by solid state reaction. The mixture ratio of the reactants was: Bi_{(1 − x − 0.04)}Er_0.04Li_xOCl (x = 0, 5, 10, 30%) (molar fraction, the same below). NH₄Cl (A.R.), Bi₂O₃ (99.99%), Er₂O₃ (99.99%), Li₂CO₃ (99.99%) were used as the starting materials. The weighed raw materials were thoroughly mixed in an agate mortar and then placed in a corundum crucible. Then, the powders were sintered at 500 °C for 3 h in air. Some excessive NH₄Cl (20%) is

Results and discussion

Fig. 1(a) shows the XRD patterns of the BiOCl:4% Er^3 + poly-crystals co-doped with 0, 5, and 10% of Li⁺ ions. All the samples can be well assigned to the standard pattern of tetragonal structure BiOCl (JCPDS: No. 06-2649) and no other impurity phase was detected. To reveal subtle differences caused by Li⁺ doping, a selected region of diffraction peaks was magnified (Fig. 1(b)). It exhibits that the main diffraction peaks gradually shift toward smaller angles as the Li⁺ ion dopant concentration

Conclusion

The UC emission of Er^3 + doped BiOCl poly-crystals can be improved efficiently by doping Li⁺ ion but the reason is not ascribed to the radiative rate increase of Er^3 + ion. It is involved in not only the special multi-photons property of PA emission but also the increasing IEF, which might be caused by Li⁺ intercalation in layered BiOCl semiconductor crystals. The finding may pave the way for efficiently enhancing the emission of poly-crystal UC phosphors and offer a novel insight into the

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Nos. 61465006 and 61265007) and the Reserve talents project of Yunnan Province (2015HB013).

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In this work, we report a new optical properties and modification methods for the rare-earth (RE) ion doped upconversion (UC) materials. Here, The Yb³⁺- Eu³⁺ co-doped BiOCl layered micrometer crystals were synthesized via the traditional solid state reaction. The experiment results show that due to efficient photocarrier space separation role of IEF, a band-to-band multiphoton excitation process of Yb³⁺ ions and the subsequent photopolarization role have been triggered inside BiOCl crystals via a commercial 980 nm laser. Then, it combines the photopolarization electric field (PPF) of the laser beam to generate a multiplexed tunable photopolarization on the Eu³⁺ ions. Consequently, the uponverting Stark splitting and emission intensity ratio of electrical dipole transitions (EDTs) of Eu³⁺ ions could be tailored simultaneously, either by pumping power or Yb³⁺ ion doping level, but showing an opposite trend. This research provides new insights into the interaction between the light and materials as well as the UC tailored method of lanthanide ions, which may inspire the design of opto-electric multifunctional devices.
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Under 980 nm excitation, bright visible upconversion (UC) photon avalanche (PA) luminescence as well as 1560 nm near infrared (NIR) emission with looping behavior have been generated from Er³⁺-doped BiOCl nanosheets synthesized by solvent-thermal method. On tuning the surfactant addition, solvent content and pH value in synthesis, the cycle numbers for the UC and NIR PA processes could be modified accordingly. Comprehensive investigation indicated that the PA behaviors were initiated by the intense Raman vibration models due to Er³⁺ doping, which could be greatly intensified and modify the PA behaviors consequently when the synthesis conditions change the orientation of BiOCl nanosheets.
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Exploring a new tuning way to facilely realize single-band red emission in trivalent rare-earth ions (RE³⁺) doped upconversion (UC) materials is still desirable. In this work, the intense single-band red emission is achieved by co-doping only Ho³⁺ in the BiOCl: Er³⁺ under 1550 nm excitation. In the BiOCl layered host, co-doping Ho³⁺ can further enhance the red emission and simultaneously suppress the green emission of Er³⁺, and thus obviously improve the red-to-green (R/G) ratio. It is found that Ho³⁺ does not serve as energy trapping through the ⁵I₆ state as in traditional UC materials but acts as ET bridge (⁴S_3/2, ²H_11/2 (Er³⁺) → ⁵F₄, ⁵S₂ (Ho³⁺) → ⁴F_9/2 (Er³⁺)). The tuning mechanism of Ho³⁺ is discussed in detail and further confirms through a comparative experiment. Our research gives an unusual perspective to tune the UC behavior of Er³⁺ through co-doping Ho³⁺, which might be inspiring for achievement of single-band red UC emission.

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¹: These authors made equal contribution to this work.

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Unusually enhancing high-order photon avalanche upconversion of layered BiOCl:Er3 + semiconductor poly-crystals via Li+ ion intercalation doping

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgments

Chem. Eng. J.

Opt. Mater.

J. Lumin.

J. Lumin.

Mat. Res. Bull.

Upconversion nanoparticles: design, nanochemistry, and applications in theranostics

Chem. Rev.

Lanthanide-doped luminescent nanoprobes: controlled synthesis, optical spectroscopy, and bioapplications

Chem. Soc. Rev.

Upconversion and anti-stokes processes with f and d ions in solids

Chem. Rev.

Combinatorial approaches for developing upconverting nanomaterials: high-throughput screening, modeling, and applications

Chem. Soc. Rev.

Effect of photonic bandgap on upconversion emission in YbPO4: Er inverse opal photonic crystals

Appl. Opt.

Ag nanowires enhanced upconversion emission of NaYF4:Yb,Er nanocrystals via a direct assembly method

Chem. Commun.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

Chem. Soc. Rev.

Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms

Environ. Sci: Nano

Controlled synthesis of rare earth nanostructures

J. Mater. Chem.

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Nat. Nanotechnol.

Upconversion emission enhancement in Yb3 +/Er3 +-codoped Y2O3 nanocrystals by tridoping with Li+ ions

J. Phys. Chem. C

Near-infrared (NIR) to red and green up-conversion emission from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles, hetero-looping-enhanced energy transfer (Hetero-LEET): a new up-conversion process

J. Am. Chem. Soc.

Unusually enhancing high-order photon avalanche upconversion of layered BiOCl:Er^3 + semiconductor poly-crystals via Li⁺ ion intercalation doping

Effect of photonic bandgap on upconversion emission in YbPO₄: Er inverse opal photonic crystals

Ag nanowires enhanced upconversion emission of NaYF₄:Yb,Er nanocrystals via a direct assembly method

Upconversion emission enhancement in Yb^3 +/Er^3 +-codoped Y₂O₃ nanocrystals by tridoping with Li⁺ ions

Near-infrared (NIR) to red and green up-conversion emission from silica sol-gel thin films made with La_0.45Yb_0.50Er_0.05F₃ nanoparticles, hetero-looping-enhanced energy transfer (Hetero-LEET): a new up-conversion process