Tm3+ nanocrystals and highly efficient multicolor up-conversion luminescence

Accepted Manuscript Title: Synthesis of Orthorhombic K2 YF5 : Yb3+ , Er3+ /Tm3+ Nanocrystals and Highly Efficient Multicolor Up-conversion Luminescence Authors: Xian Wang, Gejihu De, Yuanyuan Liu PII: DOI: Reference:

S0025-5408(18)32510-8 https://doi.org/10.1016/j.materresbull.2018.10.038 MRB 10250

To appear in:

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Received date: Revised date: Accepted date:

10-8-2018 24-10-2018 24-10-2018

Please cite this article as: Wang X, De G, Liu Y, Synthesis of Orthorhombic K2 YF5 : Yb3+ , Er3+ /Tm3+ Nanocrystals and Highly Efficient Multicolor Up-conversion Luminescence, Materials Research Bulletin (2018), https://doi.org/10.1016/j.materresbull.2018.10.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of Orthorhombic K2YF5: Yb3+, Er3+/Tm3+ Nanocrystals and Highly Efficient Multicolor Up-conversion Luminescence

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Xian Wang1, Gejihu De1,2,3, Yuanyuan Liu1

1 College of Chemistry and Environment Science, Inner Mongolia Normal University,

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Hohhot 010022, People’s Republic of China

2 Physics and Chemistry of Functional Materials, Inner Mongolia Normal University,

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Hohhot 010022, People’s Republic of China

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3 State Key Laboratory on Integrated Optoelectronics, Jilin University, Jilin 130012,

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E-mail: [email protected]

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People’s Republic of China

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Graphical Abstracts The K2YF5 nanocrystals were synthesized via solvothermal method in a mixture of oleic acid and 1-Hexanol, and the precursors were prepared by hydrothermal route. The effect of the molar ratio of KF-to-RE (RE=Y, Yb, Er) on

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the phase transition and size-controlled of the nanoparticles were studied. Besides, the multicolor output of samples was achieved througth adjusting the concentration

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of Yb3+ in the K2YF5:Yb3+/Er3+.

Highlights

system.

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1) The K2YF5:Yb3+/Er3+ nanoparticles were synthesized in oleic acid-1-Hexanol

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2) The size-controlled of the nanoparticles were studied by changing reaction time.

3) The optimal doping concentration of Yb3+, Er3+/Tm3+ in the K2YF5 was found.

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4) The tuning of multicolor upconversion of orthorhombic K2YF5 NPs was achieved.

Abstract In our work, the orthorhombic K2YF5 nanocrystals were successfully 2

synthesized via solvothermal method in a mixture of oleic acid and 1-Hexanol. The up-conversion (UC) luminescent properties of orthorhombic K2YF5:Yb3+, Er3+/Tm3+ nanoparticles (NPs) were studied, and the multicolor output, including green, yellowgreen, yellow, pink, and blue, was achieved by adjusting the concentrations of Yb3+

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sensitizer ion and kinds of activator ions. Meanwhile, the optimal doping concentrations of Yb3+, Er3+/Tm3+ which is a condition that green or blue emissions

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intensity can reach a maximum were found under 980 nm laser diode (LD) excitation. Furthermore, the fluorescence lifetimes of 4S3/2 excited state of Er3+ and 1G4 excited

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state of Tm3+ in the K2YF5:Yb3+, Er3+/Tm3+ were tested, which showed that the

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fluorescence lifetime can reach 1.14 ms and 0.57 ms at the optimal doping

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concentrations of Yb3+/Er3+ (30%/3%) and Yb3+/Tm3+ (25%/0.2%) respectively,

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indicating that the K2YF5 nanomaterial owns extremely strong UC emission intensity

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and long fluorescence lifetime. Indubitably, the outstanding optical properties will make K2YF5 NPs have potential applications in the fields of 3D displays, optical

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communications, phosphors, and so on.

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Key words: Rare earth; Nanoparticles; Upconversion luminescence; Multicolor output

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1. Introduction In recent years, the research of lanthanide (Ln3+)-doped upconverting

nanoparticles (UCNPs) has been paid extensive attention because of their potential use in the areas of lighting [1], three-dimensional displays [2], solid-state lasers [3], optical communication [4], solar cells [5-7], biological imaging and biomarkers [83

10] and so on. The UC photoluminescence, an anti-Stokes phenomenon that is a nonlinear process, was proposed firstly by a French scientist who is Auzel, which refers to a process that can convert low-energy photos to high-energy ones via the consecutive absorption of multi-photon or energy transfer pathway [11-12].

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Specifically, sensitizer ions with simple energy level structure and long lifetime in excited state obtain energy through low-energy excitation, then the sensitizer ions

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deliver the excitation energy to adjacent activator ions with multiple energy level

structure, thereby achieving the greatly efficient high-energy emissions by a series of

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complicated nonradiative relaxation processes and excitation progress [13-14], which

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has been regarded as the most efficient anti-Stokes process resulting from their greatly

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distinct 4f shell electronic configuration [15-17]. Consequently, compared with the

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conventional fluorescent materials, such as semiconducting quantum dots and organic

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dyes [18], the UCNPs have more favorable optical properties such as large antiStokes shifts, long excited-state lifetimes, narrow emission bandwidth, and high

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conversion efficiency [19-21].

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For the moment, the Ln3+-doped UC luminescent materials such as rare-earth oxide, fluorides, chlorides, phosphates, vanadates etc. have been widely investigated [22-23], among of which, the Ln3+-doped fluorides, by reason of their characteristics

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of low phonon energies, low non-radiation attenuation rate, high radiative emissivity and high chemical stability, are considered as the best hosts for performing multicolor UC luminescence and extremely favored by the researchers [24-26]. Although the hexagonal NaYF4 is now considered to be one of the most efficient Ln3+ hosts, it is 4

essential to explore and investigate other complex fluorides with improved UC luminescence properties. As a class of excellent efficient host materials for UC emissions, potassium yttrium fluorides, including hexagonal KYF4, tetragonal KY3F10 and orthorhombic K2YF5, have attracted increasing attention of researchers,

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among which the K2YF5 crystals have received special concern because they possess high efficiency of up-conversion luminescence that results from their unique

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orthorhombic crystal structure [27]. To date, many efforts have been devoted to the

investigation of synthesis and luminescence properties on the orthorhombic K2YF5

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crystals. For instance, Wang’s group reported the hydrothermal synthesis and

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spectroscopic properties of Tm3+ ions in K2YF5 crystal [28]. Méndez-Ramos’s group

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synthesized Yb3+-Er3+ doped K2YF5 fluoride crystal using hydrothermal method and

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investigated the luminescence properties by increasing the concentration of Yb3+ [29].

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Loiko’s group reported the growth under hydrothermal conditions and detailed spectroscopic study by doping different concentrations of Er3+ of K2YF5 crystals and

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achieved the multicolor output from yellow-green to yellow [30]. However, the

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K2YF5 crystals synthesized which have been reported using hydrothermal methods all belong to microcrystals. To the best our knowledge, there are no reports as for the hydrothermal synthesis and controlled tuning of multicolor UC emissions of Ln3+-

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K2YF5 at the nanoscale. In this work, we reported a facile and general solvothermal method for the synthesis of orthorhombic K2YF5 nanoscrystals with various well-defined shapes by briefly adjusting the molar ratio of KF/RE (RE=Y, Yb, Er, Tm) and reaction 5

temperature. In addition, UC luminescent properties of the orthorhombic K2YF5 NPs doped with the different lanthanide doping ions (Yb3+/Er3+, Yb3+/Tm3+) were discussed and the optimal concentrations of Yb3+/Er3+ and Yb3+/Tm3+ were found. Meanwhile, the multicolor output of orthorhombic K2YF5 NPs was achieved,

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including blue, green, yellow-green, yellow and pink. Furthermore, the fluorescence lifetime decay curves of K2YF5 NPs at the optimal concentrations of

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30%Yb3+/3%Er3+ and 25%Yb3+/0.2%Tm3+ were measured. This paper will lay the

foundation for the research of tunable multicolor UC emissions and synthesis of other

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phase potassium yttrium fluoride at the nanoscale.

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2. Experimental Section

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2.1 Materials

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Yttrium oxide (Y2O3, 99.99%), ytterbium oxide (Yb2O3, 99.99%), erbium oxide

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(Er2O3, 99.99%), thulium oxide (Tm2O3, 99.99%), oleic acid (OA, chemical grade), Trifluoroacetic acid (CF3COOH, chemical grade, 99%) and n-Hexanol (C6H14O, 98%)

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were all obtained from Sinopharm Chemical Reagent Beijing Co Ltd. Potassium

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fluoride (KF, 99%), absolute ethanol (CH3CH2OH, 99.7%) and cyclohexane (C6H12, 99.5%) were supplied by Kemeng Chemical Industry and Trade limited Corporation,

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respectively. All the chemicals were used as received without any further purification.

2.2 Synthesis of lanthanide trifluoroacetate precursors The lanthanide trifluoroacetate precursors were prepared via the facile

hydrothermal routes reported by our group previously [31]. Specifically, 3 mmol lanthanide oxides, ultrapure water and trifluoroacetic acid were measured in 6

proportion, then heated in a 50 mL autoclave at 120 ℃ for 24 h. After the system was naturally cooled to room temperature, 1 mmol precursor solution was transferred to a 100 mL three-neck flask, and dried to remove redundant water and trifluoroacetic acid. Finally, the white powder was obtained. The volume ratio of H2O/CF3COOH

2.3 Synthesis of products K2YF5: Yb3+, Er3+/Tm3+ NPs

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was 9:2 for 1 mmol precursor throughout this work.

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The K2YF5: Yb3+, Er3+ NPs were synthesized by a facile solvents method using

oleic acid with high boiling point as the ligand and n-Hexanol as the cosurfactant. In

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a typical synthetic procedure, 1 mmol of metal trifluoroacetates, 4 mmol of KF and

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30 mL solvent (22.5 mL OA/7.5 mL n-Hexanol) were brought to a 100 mL three-neck

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flask at room temperature, then the slurry was stirred violently for several hours to

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form an transparent solution. Subsequently, the clear solution was transferred to a 50

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mL Teflon-lined autoclave, which was then tightly sealed and heated at 200 ℃ for 48 h followed by cooling down to room temperature naturally. Next, an excess amount

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of ethanol was added into the resultant mixture at room temperature to make more

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samples precipitate, then the samples were isolated via centrifugation with cyclohexane/ethanol (1:4 v/v) for several times and dried in a vacuum drying oven at

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60 °C for 12 h.

Synthesis of other Yb3+/Tm3+-doped K2YF5 NPs was the same as the process as

described above, except that the metal trifluoroacetates used were different from the K2YF5: Yb3+, Er3+ NPs.

2.4 Characterization 7

Power X-ray diffraction (XRD) analysis was performed on a Rigaku Ultima IV x-ray diffractometer with CuKα radiation (λ = 1.5406 Å, 200 mA and 40 kV). The sizes and shapes of NCs were characterized using transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) with

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accelerating voltages of 200 kV. The up-conversion fluorescence spectra of NCs were measured on a Hitachi F-4600 fluorescence spectrophotometer (λ = 400～700 nm,

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PMT voltage: 700 V, slit width: 5.0 nm) with an external 980 nm laser diode as

excitation source. The UC luminescent decay curves analysis were carried out using

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a lifetime fluorescence spectrometer (DeltaFlex TCSPC system, Horiba scientific in

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Scotland) with a tunable pulse laser (OKI-Syn0101, wavelength: 980 nm, output: 100

room temperature.

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3. Results and discussion

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Hz, pulse width 200 ns, slit width 16 nm). All the measurements were performed at

3.1 Effects of KF/Y3+ molar ratio, reaction temperature and time on

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the synthesis of orthorhombic K2YF5 NPs

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The effects of KF/Y3+ molar ratio, reaction temperature and time on the synthesis of orthorhombic K2YF5 NPs were studied. XRD patterns of samples obtained at different molar ratios of KF to Y3+ are shown in the Fig. 1, indicating that the

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diffraction peaks of the samples synthesized could be readily assigned to the tetragonal phase of KY3F10 (JCPDS No. 27-0465) at molar ratios of KF to Y3+ below 4. The crystal structure of the tetragonal phase was determined with lattice parameters of a=b=8.161 Å, c=11.539 Å, and space group of Pnma (62). Nevertheless, when the 8

molar ratio of KF to Y3+ was increased to 4, all the reflections of samples were characteristic of a pure orthorhombic phase (space group: Pna21(33), a=10.791 Å, b=6.607 Å, c=7.263 Å) of K2YF5, which could be well indexed to the pure orthorhombic K2YF5 (JCPDS No. 72-2387), demonstrating that the relatively high

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molar ratio of KF to Y3+ is more favorable for the formation of orthorhombic K2YF5 NPs, more specifically, only the molar ratio of KF to Y3+ reaches to 4, pure

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orthorhombic K2YF5 NPs can be obtained in the conditions of 200 ℃ for 48 h. Consequently, the KF/Y3+ molar ratio of 4 was used throughout this study for

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synthesizing pure orthorhombic K2YF5 NPs.

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Fig. 1 XRD patterns of samples prepared at different molar ratios of KF to Y3+: (a) 1:1; (b) 2:1; (c) 2.5:1; (d) 4:1, and the standard data of orthorhombic K 2YF5 (JCPDS No. 72-2387) and tetragonal K3YF10 (JCPDS No. 27-0465) given as references. Other synthesis conditions: 22.5 mL OA/7.5 mL HA; 200 ℃ for 48 h; Yb3+/Er3+ (20%/2%).

Fig. 2 shows the XRD data of the products synthesized at different reaction 9

temperatures of 140 ℃, 160 ℃, 180 ℃, 200 ℃, and 220 ℃, respectively. It could be concluded that when the experiment was performed at lower temperatures, including 140 ℃ and 160 ℃, all X-ray diffraction peaks of the samples obtained were closely matched with the standard pattern of tetragonal KY3F10 (JCPDS No. 27-0465).

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However, when the reaction temperature was increased to 180 ℃, the diffraction peaks of samples can be mainly coincided with the standard data of orthorhombic K2YF5,

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but it still contained few diffraction peaks of tetragonal KY3F10, as shown in the asterisk-marked position. However, along with the further rise of temperature up to

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200 ℃ or above, the pure orthorhombic K2YF5 NPs with well-defined diffraction

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prepare high quality orthorhombic K2YF5 NPs.

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peaks can be obtained. Therefore, the 200 ℃ was used throughout this research to

Fig. 2 XRD patterns of products synthesized at different temperatures: (a) 140 ℃; (b) 160 ℃; (c) 180 ℃; (d) 200 ℃; (e) 220 ℃, and the standard data of orthorhombic K2YF5 (JCPDS No. 72-2387) and tetragonal K3YF10 (JCPDS No. 27-0465) given as references. Other synthesis 10

conditions: 22.5 mL OA/7.5 mL HA; KF-to-Y3+ of 4; 48 h; 20%Yb3+/2%Er3+.

As we know, the effective collision between the molecules or atoms of the reactants is the prerequisite for occurrence of a chemical reaction, and the direct influencing factors of such an effective collision include the reaction temperature and

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the monomer concentration of the reactants in the system. For the above reaction: when the reaction temperature is relatively low, the product is tetragonal phase

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KY3F10. However, as the reaction temperature increases, the time of effective

collisions and the number of effective ions (K+) in the system increase, thereby

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accelerating the reaction rate, therefore, the product of the reaction is orthorhombic

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phase K2YF5. When the reaction temperature is constant, the product is tetragonal

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phase KY3F10 at lower KF concentration, but with the KF concentration in the system

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increasing, the time of effective collisions and the number of effective ions increase,

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and thereby the reaction rate increases, which results in accelerating the nucleation rate of the reaction. And compared with the low KF concentration, with the growth

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of nanomaterials, the energy barriers of the system is further reduced, promoting the

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material to undergo a phase transition, resulting in the product being the orthorhombic phase K2YF5 [32-33]. Fig. 3 displays the XRD patterns of samples synthesized at different times of 6

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h, 12 h, 24 h, 48 h, 72 h, respectively, from which we can observe that when the experiment was performed at a short reaction time of 6h, the diffraction peaks of samples were almost both indexed to the standard pattern of tetragonal KY3F10 (JCPDS No. 27-0465), also containing few diffraction peaks of orthorhombic K2YF5 11

(JCPDS No.72-2387) that were located in around 15.714o. Nevertheless, with the further increase in reaction time from 6h to 12h, the diffraction peaks of samples can be mainly assigned to the standard pattern of orthorhombic K2YF5. As the reaction time was extended to 24 h, almost all diffraction peaks of the as-prepared samples

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were consistent with values in the standard cards of orthorhombic K2YF5 (JCPDS No.72-2387) except the asterisk-marked position. Upon a further extension of time

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from 48 h to 72 h, the diffraction peaks of samples were in good accordance with the

standard cards of orthorhombic K2YF5, implying that the pure orthorhombic K2YF5

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NPs were synthesized successfully.

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Fig. 3 XRD patterns of samples synthesized at different times: (a) 6 h; (b) 12 h; (c) 24 h; (d) 48 h; (e) 72 h, and the standard data of orthorhombic K2YF5 (JCPDS No. 72-2387) given as a reference. Other synthesis conditions: 22.5 mL OA/7.5 mL HA; KF-to-Y3+ of 4; 200 ℃; 20%Yb3+/2%Er3+.

On the basis of the above findings, we can draw a conclusion that the relatively 12

higher KF/Y3+ molar ratio, higher reaction temperature and comparatively longer reaction time are more conducive to the formation of pure and high crystalline K2YF5 NPs of orthorhombic phase. Hence, the 200 ℃, 4 mmol KF, and 48 h were used throughout subsequent experiment for investigating the UC properties of K2YF5 NPs.

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Here, it should be pointed out that the reaction temperature and the ratio of KF to Y3+ play a more significant role in the phase transition of the materials compared to the

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reaction time.

Fig. 4 shows the TEM and HRTEM images of the as-obtained

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K2YF5:20%Yb3+/2%Er3+ NPs for different times. Here, it should be pointed out that

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the images in Fig. 4a-c and Fig. 4g-i were obtained by the TEM called FEI Tecnai

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JEM-2100F, and the images in Fig. 4d-f were measured by TEM called GZ F20. The

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results showed that when the reaction was performed at 12 h, the orthorhombic

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K2YF5:20%Yb3+/2%Er3+ NPs which were nearly spherical in shape with the average size of 15.7 nm were obtained. The lattice fringes observed in the HRTEM images

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indicated that the individual particles were highly crystalline, and the FFT analysis

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indicated lattice fringes with an observed d-spacing of 0.305 nm, which was in good agreement with the lattice spacing of the (112) planes of orthorhombic phase K2YF5 (Fig. 4a-c and Fig. 4j). When the reaction time was increased to 24 h, the sample was

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composed of spherical NPs with the average size of 16.8 nm, and the determined interplanar distance of 0.330 nm between the adjacent lattice planes corresponded to the (020) plane of orthorhombic K2YF5 (Fig. 4d-f and Fig. 4k). When the reaction time was further increased to 48 h, the nanospheres with the average size of 52.1 nm 13

were formed, and the inserts indicated that orthorhombic K2YF5 NPs were enclosed by (020) facets. Meanwhile, there was accompanied with the self-assembly phenomenon (Fig. 4g-i and Fig. 4l). Based on the above results, we think that plenty of KOA can be generated at the relatively high concentration of KF, and the mutual

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effect between excess KOA as the surfactant and 1-Hexanol as the cosurfactant promotes the small NPs to self-assemble. Therefore, with the reaction time extending,

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the self-assembly phenomenon aggravates as well.

Fig. 4 TEM and HRTEM images of the as-synthesized K2YF5:20%Yb3+/2%Er3+ NPs at different times: (a-c) 12 h, (d-f) 24 h, (g-i) 48 h. The corresponding size distribution 14

histograms (j) 12 h, (k) 24 h, (l) 48 h.

3.2 Multicolor up-conversion luminescent properties of the K2YF5: Yb3+, Ln3+ (Ln=Er, Tm) NPs Fig. 5A shows the UC luminescence spectra of the orthorhombic K2YF5:

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x%Yb3+, 2%Er3+ NPs with different concentrations of Yb3+ ion (3%-78%). The emission peaks of Er3+ centered at 524 nm, 546 nm and 652 nm corresponding to the

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emission from 2H11/2 and 4S3/2 levels to the 4I15/2 ground state and transition from the

F9/2 → 4I15/2 state respectively for all ten samples can be observed. Fig.5B shows the

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corresponding peak area of green and red emission bands and the green to red

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intensity ratio (GRR), which indicates that when the doped Yb3+ concentration

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changes from 3% to 30%, the peak areas of all emission bands gradually become

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larger. Nevertheless, upon a further increase of doped Yb3+ concentration from 50%

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to 78%, the peak areas of green emission bands are decreased and the peak areas of red emission bands still increase, which indicates that when the doped Yb3+

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concentration is 3%-30%, the green emission is the dominating color while the doped

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Yb3+ concentration increases to 50% or above, the color is given priority to with red emission. The line chart of peak area ratio of the green region to the red region demonstrates that the peak area ratio decreases from 5.46 to 0.19 when the doped

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Yb3+ concentrations increase from 3% to 78%, which results in multicolor output of the samples. Fig. 5C presents the corresponding luminescence photographs of K2YF5: Yb3+, Er3+ NPs with different Yb3+ concentration. As shown, the output colors of the samples vary from green (3%-10%) to yellow-green (15%-18%) then to yellow (20%15

30%) and pink (50%-78%) with the increase of Yb3+ concentration. The above research results are consistent with the reported work of Yb3+-Er3+ doped β-KYF4 and β-NaYF4 [34-35]. Based on the above discusses, the UC mechanism in K2YF5:Yb3+, Er3+ NPs with different Yb3+ can be explained by the follow theory. The gradual

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increase of Yb3+ ions dopants into the K2YF5 host matrix will result in the decrease of the Yb-Er inter-atomic distance and hence facilitate the back-energy-transfer from

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Yb3+ to Er3+, and therefore the population in excited levels of 2H11/2 and 4S3/2 are

suppressed, which results in the decrease of green (2H11/2, 4S3/2 → 4I15/2) emissions.

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Meanwhile, the energy transfer from Yb3+ to Er3+ results in the saturation of the 4I13/2

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(Er3+) state and then the excited Yb3+ ions transfer its energy to Er3+ ions through the

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energy-transfer process 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), which

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emission (4F9/2 → 4I5/2) [36-37].

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can directly populate the 4F9/2 level, thereby resulting in the enhancement of red

Fig. 5 The UC luminescence properties of K2YF5:Yb3+, Er3+ NPs with different Yb3+ doping concentration. (A) Room-temperature UC luminescence spectra of the orthorhombic K2YF5: 16

x%Yb3+, 2%Er3+ NPs (x=3, 5, 7, 10, 15, 18, 20, 30, 50, 78) synthesized under 980 nm LD excitation; (B) the peak area of the green and red emissions and green to red intensity ratio (GRR); (C) luminescence photographs of the orthorhombic K2YF5: Yb3+, Er3+ NPs dispersed in cyclohexane solution excited with a 980 nm diode laser of 2.5 W cm-2.

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In order to find the optimal doping concentration of Yb3+ and Er3+ when the green emissions intensity can research a maximum, the luminescent properties of

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K2YF5: 30%Yb3+, x%Er3+ NPs (x=0.2, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0) were investigated,

as shown in Fig. 6. Fig. 6A shows that the emission peaks have obvious change with

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the Er3+ concentration increasing from 0.2% to 5%. In order to further reveal the

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relationship between the doping concentration and luminescence intensity, the

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emission peaks centered at 510 nm and 546 nm are analyzed using the correlation

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integral method. Fig. 6B displays that the calculated peak area of green region at

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different Er3+ concentrations, which demonstrates that relative peak intensity of the green light emissions firstly increases and then decreases, and reaches a maximum at

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the doped Er3+ concentration of 3%. Therefore, it can be concluded that the optimal

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doping concentrations of Yb3+ and Er3+ are 30% and 3%, respectively. Fig. 6C exhibits the corresponding luminescence photographs of K2YF5:Yb3+, Er3+ NPs with different Er3+ concentrations, showing that all samples can emit yellow light, and

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samples emit the strongest light at the Er3+ concentration of 3%, which is consistent with the data investigated by UC luminescence spectra. Here, it should be pointed out that the above result is ascribed to the abundant energy level structure of Er3+ ions. Specifically, with the Er3+ concentration increasing, the chance of match between 17

energy levels in Er3+ ions will increase, which causes the appearance of crossrelaxation mechanisms such as 2H11/2 + 2I15/2 → 4I9/2 + 4I13/2, as shown in Fig. 8, resulting in significant depopulation of the upper excited states. Therefore, when the Er3+ concentration reaches to a certain degree, the concentration quenching appears

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and luminescence intensity decreases [38].

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Fig. 6 The UC luminescence properties of K2YF5:Yb3+, Er3+ NPs with different Er3+ doping concentration. (A) Room-temperature UC luminescence spectra of the orthorhombic K2YF5:

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30%Yb3+, x%Er3+ NPs (x=0.2, 0.5, 1.0, 2.0, 3.0, 5.0) synthesized under 980 nm LD excitation; (B) calculated peak area of green region as a function of Er3+ concentration; (C) luminescence

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photographs of the orthorhombic K2YF5: Yb3+, Er3+ NPs dispersed in cyclohexane solution excited with a 980 nm diode laser of 2.5 W cm-2.

In order to find the optimal doping concentrations of Yb3+ and Tm3+, the luminescent properties of K2YF5 doped with the different concentrations of Yb3+ and Tm3+ were investigated, as shown in Fig. 7. The UC spectra of K2YF5:20%Yb3+, 18

x%Tm3+ (x=0.1, 0.2, 0.3, 0.4, 0.5) NPs is exhibited in Fig. 7A, from which the strong blue emission bands centered at 477 nm ascribed to the transition of 1G4 → 3H6 and one weak red emission band centered at 655 nm attributed to the emission of 1G4 → 3

F4 can be observed for the all samples. As the content of Tm3+ increases, the intensity

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of strong blue emission firstly climbs up and then declines, and the luminescence intensity is the strongest at the Tm3+concentration of 0.2%, as shown in Fig. 7B. The

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corresponding luminescence photographs are exhibited in Fig 7C, from which we can

see that all samples can emit bright blue light and the sample doped with the

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Tm3+concentration of 0.2% has the brightest light, which is identical with the result

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of UC spectra. Fig. 7D and Fig. 7E display the UC spectra and calculated peak area

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of blue emission bands of K2YF5: x%Yb3+, 0.2%Tm3+ (x=10, 15, 18, 20, 25, 30) NPs

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separately, from which it can be seen that with the increase of Yb3+ concentration, the

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intensity of blue emissions presents an remarkable increase when the Yb3+ concentration increases from 10% to 25% and reaches the maximum at the Yb3+

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concentration of 25%. However, with the increase of Yb3+ concentration up to 30%,

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the intensity of blue emission declines sharply. Fig. 7F shows the corresponding luminescence photographs of K2YF5:Yb3+/Tm3+ NPs with the different Tm3+. Obviously, the K2YF5:25%Yb3+/0.2%Tm3+ sample emits the brightest light, which is

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consistent with the result of Fig. 7D and Fig. 7E. Subsequently, one can draw a conclusion that the optimal doping concentrations of Yb3+ and Tm3+ are 25% and 0.2% respectively in Yb3+/Tm3+ doped K2YF5. Based on the above discussion, the change of output color in Fig. 7C is because the Tm3+ ion owns abundant energy level in 19

which there are more matching chances between two energy levels, therefore, the concentration quenching appears when the concentration rises to be a certain degree. In addition, the change rule of blue light in Fig. 7F can be explained as follows. With the increase of doped Yb3+ concentration in K2YF5:Yb3+,Tm3+, the back-energy-

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transfer from Yb3+ to Tm3+ will be facilitated. Consequently, the population of Tm3+ ions in the excited levels of 1G4 may be suppressed, thereby leading to the decrease

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in 1G4 → 3H6 blue emission [35].

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Fig. 7 (A) Room-temperature UC luminescence spectra of the orthorhombic K2YF5: 20%Yb3+, x%Tm3+ NPs (x=0.1, 0.2, 0.3, 0.4, 0.5) synthesized under 980 nm LD excitation; (B) calculated peak area of blue region as a function of Tm3+ concentration; (C) the corresponding luminescence photographs of K2YF5:Yb3+/Tm3+ with different Tm3+ concentrations; (D) Room-temperature UC luminescence spectra of the orthorhombic K2YF5: 20

x%Yb3+, 0.2%Tm3+ NPs (x=10, 15, 18, 20, 25, 30) synthesized under 980 nm LD excitation; (E) calculated peak area of blue region as a function of Yb3+ concentration; (F) the corresponding luminescence photographs of K2YF5:Yb3+/Tm3+ NPs with different Yb3+ concentrations.

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The proposed UC mechanism in the Yb3+/Er3+ and Yb3+/Tm3+ doped K2YF5 NPs is displayed in Fig. 8. We can see that the green and red emissions in the Yb3+/Er3+

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doped K2YF5 NPs all need a two photon process to populate the 2H11/2/4S3/2 or 4F9/2 level while the blue and red emissions in the Yb3+/Tm3+ doped K2YF5 NPs all require

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a three photon process to populate the 1G4 level. The specific luminescence

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mechanism is explained as follows:

F7/2 level to the excited state level (2F5/2) in the energy level of Yb3+ after sensitizer

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2

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In the Yb3+/Er3+ co-doped system, there first is a transition from ground state

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ion (Yb3+) absorbing a 980 nm photon, subsequently, the excited Yb3+ ions return to the ground state through non-radiative transition, which may transfer the energy to a

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adjacent Er3+ ion, resulting in the transition from 4I15/2 to the 4I11/2 state of Er3+ ion,

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then the Er3+ ions in the 4I11/2 state jump into a higher excited state level (4F7/2) through capturing the energy of the second excited Yb3+ ion under 980-nm excitation. Finally, the Er3+ ions in the 4F7/2 state relax to the 2H11/2 and 4S3/2 level via non-radiative

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relaxation, and then back to the ground state level (4I15/2) by radiation transition, producing a green upconversion emission (523 nm, 546 nm). Meanwhile, the Er3+ ions (4I15/2) absorb a 980 nm photon in the same way and then jump to the 4I11/2 level, concuquently, they can reach to the 4I13/2 level by non-radiative relaxation, then the 21

Er3+ ions jump to the 4F9/2 level after absorbing a another 980 nm photon in a similarly way, eventually, they return to the the ground level (4I15/2) from excited state level (4F9/2) by radiation transition, emitting a red upconversion emission (652 nm) [12]. For the Yb3+/Tm3+ co-doped system, a 980-nm photon excites an sensitizer Yb3+

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ion from the ground state 2F7/2 to the excited state 2F5/2, which may transfer the energy to a nearby Tm3+ ion, promoting the Tm3+ ion from 3H6 to the 3H5 state, then the 3H5

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relaxes rapidly to the 3F4 level, and if the latter is already populated, the Tm3+ ion in

the 3F4 state can be excited to a higher 3F2 excited state through capturing the energy

F3 level. Subsequently, the third excited Yb3+ ion is captured to induce the transition

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3

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of the second excited Yb3+ ion under 980-nm excitation, then the 3F2 relaxes to the

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of Tm3+ ion from 3F3 to the 1G4, then the Tm3+ ion in the energy level of 1G4 returns

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to the level of 3H6 and 3F4 via radiative transition, which can produce a blue emission

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(477 nm) and red emission (659 nm), respectively [39-40].

Fig. 8 The energy level diagram of Yb3+/Er3+ and Yb3+/Tm3+ doped K2YF5 samples under 22

near-infrared (980 nm) excitation.

Fig. 9a shows the UC luminescent lifetimes (τ) of the 4S3/2 (544 nm) state of Er3+ and 1

G4

(477

nm)

state

of

Tm3+

in

the

K2YF5:30%Yb3+/3%Er3+

and

K2YF5:25%Yb3+/0.2%Tm3+, respectively. The decay curves of samples are fitted with

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double-exponential function and cubic exponential function, respectively. The calculations demonstrate that the average fluorescence lifetime of 4S3/2 state of Er3+

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and 1G4 state of Tm3+ at optimal doping concentration can reach 1.14 ms and 0.57 ms, respectively. Fig. 9b exhibits the rise time of 4S3/2 (546 nm) state of Er3+ in the and

1

G4

(477

nm)

state

of

Tm3+

in

the

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K2YF5:30%Yb3+/3%Er3+

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K2YF5:25%Yb3+/0.2%Tm3+, respectively. Moreover, the rise time curves of 4S3/2 (544

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nm) state of Er3+ and 1G4 (477 nm) state of Tm3+ in the K2YF5:30%Yb3+/3%Er3+ and

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K2YF5:25%Yb3+/0.2%Tm3+ are fitted with quartic exponential function and quantic

4

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exponential function, respectively. The calculations indicate that the Er3+ ions from I15/2 state to 4S3/2 level require 0.50 ns while the Tm3+ ions from 3H6 state to 1G4 need

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19.9 ms. The results aboved testify that the orthorhombic K2YF5 nanomaterial has

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long fluorescence lifetime and strong UC emission intensity and further demonstrate that it has potential applications in the fields of three-dimensional displays, optical

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communications, phosphors and so on.

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the

K2YF5:30%Yb3+/3%Er3+

and

1

G4

(477

nm)

K2YF5:25%Yb3+/0.2%Tm3+.

state

of

Tm3+

in

the

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Conclusion

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Fig. 9 (a) Photoluminescence decay and (b) rise time curves of 4S3/2 (546 nm) state of Er3+ in

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In short, the orthorhombic K2YF5 NPs and microrods were successfully

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synthesized by the facile hydrothermal method using oleic acid and 1-Hexanol, and the effect of the KF/Y3+ molar ratio, reaction temperature and time on the synthesis

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of orthorhombic K2YF5 NPs was discussed. The multicolor output, including green,

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yellow-green, yellow, pink, and blue, was achieved. Meanwhile, the optimal doping concentrations of Yb3+/Er3+ and Yb3+/Tm3+ in the K2YF5 NPs, 30%/3% and

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25%/0.2%, were found, which will build the foundation for the synthesis and multicolor output of other rare earth compounds, and will lay the foundation for their

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application in the diverse fields.

Acknowledgements This work was supported by the national natural science foundation of China (grant No. 2126016), the Science and Technology Innovation Guidance Project, Inner Mongolia, China (000-21090179), the Talents Project Inner Mongolia (CYYC: 5026) 24

and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (China No.IOSKL2013KF08).

References [1] S. Sivakumar, F. C. van Veggel, M. Raudsepp, Bright white light through up-

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conversion of a single NIR source from sol-gel-derived thin film made with Ln3+doped LaF3 nanoparticles, J. Chem. Inform. 127 (2005) 12464-12465.

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[2] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, A three-color, solid-state, three-dimensional display, Science 273 (1996) 1185-1189.

U

[3] R. Scheps, Upconversion laser processes, J. Progress in Quantum Electronics 20

N

(1996) 271-358.

A

[4] W. Denk, J. H. Strickler, W. W. Webb, C. R. F. Inc, Two-photon laser scanning

M

microscopy, Science 248 (1990) 73-76.

ED

[5] X. M. Bian, Q. F. Shi, P. Huang, et al. Near-infrared luminescence and energy transfer mechanism in K2YF5: Nd3+, Yb3+, Mater. Res. Bull. 110 (2019) 102-106.

PT

[6] H. Zhang, Q. S. Zhang, Y. Q. Lv, et al. Upconversion Er-doped TiO2 Nanorod

CC E

Arrays for Perovskite Solar Cells and the Performance Improvement, Mater. Res. Bull. 106 (2018) 346-352.

A

[7] S. H. Choi, H. Song, I. K. Park, J. H. Yum, S. S. Kim, Synthesis of size-controlled CdSe quantum dots and characterization of CdSe–conjugated polymer blends for hybrid solar cells, Journal of Photochemistry and Photobiology A Chemistry 179 (2006) 135-141. [8] F. V. D. Rijke, H. Zijlmans, S. Li, T. Vail, A. K. Raap, Up-converting phosphor 25

reporters for nucleic acid microarrays, Nat. Biotechnol. 19 (2001) 273-276. [9] S. Sivakumar, F. C. van Veggel, P. S. May, Near-infrared (NIR) to red and green up-conversion

emission

from

silica

sol-gel

thin

films

made

with

La(0.45)Yb(0.50)Er(0.05)F(3) nanoparticles, hetero-looping-enhanced energy transfer

IP T

(Hetero-LEET): a new up-conversion process, J. Am. Chem. Soc. 129 (2007) 620-625.

SC R

[10] S. Gai, C. Li, P. Yang, J. Lin, Recent progress in rare earth micro/nanocrystals:

soft chemical synthesis, luminescent properties, and biomedical applications,

U

Chem. Rev. 114 (2014) 2343-2389.

N

[11] F. Auzel, Compteur quantique par transfert d’ energie deux ions de terres rares,

A

M. C. R. Acad. Sci. (Paris) 262 (1966) 819-821.

M

[12] F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids,

ED

Chem. Rev. 104 (2004) 139-173.

[13] F. Wang, X. Liu, Multicolor tuning of lanthanide-doped nanoparticles by single

PT

wavelength excitation, Acc. Chem. Res. 47 (2014) 1378-1385.

CC E

[14] Q. Liu, B. Yin, T. Yang, Y. Yang, Z. Shen, A general strategy for biocompatible, high-effective upconversion nanocapsules based on triplet–triplet annihilation, J. Am. Chem. Soc. 135 (2013) 5029-5037.

A

[15] S. Schietinger, T. Aichele, H. Q. Wang, Plasmon-enhanced upconversion in single NaYF4:Yb3+/Er3+ codoped nanocrystals, Nano Lett. 10 (2009) 134-138. [16] F. Wang, Y. Han, C. S. Lim, Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping, Nature 463 (2010) 1061-1065. 26

[17] H. H. Gorris, O. S. Wolfbeis, Photon-upconverting nanoparticles for optical encoding and multiplexing of cells, biomolecules, and microspheres, Angew. Chem. Int. Ed. 52 (2013) 3584-3600. [18] W. Denk, J. H. Strickler, W. W. Webb, C. R. F. Inc, Two-photon laser scanning

IP T

microscopy, Science 248 (1990) 73-76. [19] S. B. Wu, X. F. Wu, Y. X. Liu, et al. Au/NaGdF4: Yb3+, Er3+ hybrid fluorescent

SC R

system for rapid detection of ethanol, Mater. Res. Bull. 109 (2019) 155-159.

[20] F. Wang, X. G. Liu, Recent advances in the chemistry of lanthanide-doped

U

upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976-989.

N

[21] S. Heer, K. Kömpe, H. U. Güdel, Highly efficient multicolour upconversion

M

Mater. 16 (2004) 2102-2105.

A

emission in transparent colloids of lanthanide-doped NaYF4 nanocrystals, Adv.

ED

[22] Y. Yang, C. Mi, F. Jiao, X. Su, X. Li, L. Liu, J. Zhang, F. Yu, Y. Liu and Y. Mai, A novel multifunctional upconversion phosphor: Yb3+/Er3+ codoped

PT

La2S3, J. Am. Ceram. Soc. 97 (2014) 1769-1775.

CC E

[23] A. Yin, Y. Zhang, L. Sun and C. Yan, Colloidal synthesis and blue based multicolor upconversion emissions of size and composition controlled

A

monodisperse hexagonal NaYF4: Yb, Tm nanocrystals, Nanoscale 2 (2010) 953959.

[24] M. Y. Ding, C. H. Lu, L. H. Cao, W. J. Huang, Y. R. Ni, Z. Z. Xu, Molten salt synthesis of tetragonal LiYF4:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystals with multicolor upconversion luminescence, CrystEngComm 15 (2013) 6015-6021. 27

[25] K. W. Krämer, D. Biner, G. Frei, H. U. Güdel, M. P. Hehlen, S. R. Lüthi, Hexagonal sodium yttrium fluoride based green and blue emitting upconversion phosphors, Chem. Mater. 16 (2004) 1244-1251. [26] X. Zhang, P. Yang, C. Li, D. Wang, J. Xu, S. Gai, J. Lin, Facile and mass

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production synthesis of β-NaYF4:Yb3+, Er3+/Tm3+ 1D microstructures with multicolor up-conversion luminescence, Chem. Commun. 47 (2011) 12143-

SC R

12145

[27] Y. A. Kharitonov, Y. A. Gorbunov, B. A. Maksimov, Crystal structure of

U

potassium yttrium fluoride K2YF5, Kristallografiya 28 (1983) 1031-1032.

N

[28] D. Wang, Y. Guo, Q. Wang, Judd-Ofelt analysis of spectroscopic properties of

A

Tm3+ ions in K2YF5 crystal, J. Alloy. Comp. 474 (2009) 23-25.

M

[29] J. Méndez-Ramos, P. Acosta-Mora, J. C. Ruiz-Morales, N. M. Khaidukov, Role

ED

of the Yb3+ concentration in the high efficient UV-blue up-conversion emission from hydrothermally grown Yb3+/Er3+-doped K2YF5 crystals, J. Alloy. Comp.

PT

575 (2013) 263-267.

CC E

[30] P. A. Loiko, N. M. Khaidukov, J. Méndez-Ramos, Up-and down-conversion emissions from Er3+ doped K2YF5 and K2YbF5 crystals, J. Lumin. 170 (2016) 17.

A

[31] S. T. Liu, G. De, Y. S. Xu, X. Wang, et al. Size, Phase-Controlled Synthesis, The Nucleation and Growth Mechanisms of NaYF4: Yb/Er Nanocrystals, J. Rare Earth 10 (2018) 1060-1066. [32] Y. Wei, F. Q. Lu, X. R. Zhang, D. P. Chen, Synthesis and characterization of 28

efficient near-infrared upconversion Yb and Tm codoped NaYF4 nanocrystal reporter, J. Alloys Compd. 427 (2007) 333-340. [33] G. S. Yi, H. C. Lu, S. Y. Zhao, Y. Ge, W. J. Yang, D. P. Chen, L. H. Guo, Synthesis, characterization, and biological application of size-controlled nanocrystalline

IP T

NaYF4 :Yb, Er infrared-to-visible up-conversion phosphors, Nano Lett. 4 (2004) 2191-2196.

SC R

[34] B. Zhou, Y. F. Wang, D. L. Xia, Colloidal β-KYF4:Yb3+, Er3+/Tm3+ nanocrystals:

tunable multicolor up-conversion luminescence from UV to NIR regions, Rsc.

U

Adv. 5 (2015) 66807-66814.

N

[35] F. Wang, X. G. Liu, Upconversion multicolor fine-tuning: visible to near-infrared

M

(2008) 5642-5643.

A

emission from lanthanide-doped NaYF4 nanoparticles, J. Am. Chem. Soc. 130

ED

[36] F. Wang, X. G. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976-989.

PT

[37] N. Niu, P. Yang, F. He, X. Zhang, S. Gai, Tunable multicolor and bright white

CC E

emission of one-dimensional NaLuF4:Yb3+,Ln3+ (Ln = Er, Tm, Ho, Er/Tm, Tm/Ho) microstructures J. Mater. Chem. 22 (2012) 10889-10899.

A

[38] F. Vetrone, J. C. Boyer, J. A. Capobianco, Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+,Yb3+ nanocrystals, J. Appl. Phys. 96 (2004) 661-667. [39] C. Z. Zhao, X. G. Kong, X. M. Liu, et al. Li+ ion doping: an approach for improving the crystallinity and upconversion emissions of NaYF4:Yb3+, Tm3+ 29

nanoparticles, Nanoscale 5 (2013) 8084-8089. [40] L. L. Wang, W. P. Qin, Z. Y. Liu, D. Zhao, Improved 800 nm emission of Tm3+ sensitized by Yb3+ and Ho3+ in β-NaYF₄ nanocrystals under 980 nm excitation,

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Opt. Express 20 (2012) 7602-7607.

30