Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles

Author’s Accepted Manuscript Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles Jun Dong, Jie Zhang, Qingyan Han, Xing Zhao, Xuewen Yan, Jihong Liu, Haibo Ge, Wei Gao www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31618-1 https://doi.org/10.1016/j.jlumin.2018.11.041 LUMIN16111

To appear in: Journal of Luminescence Received date: 3 September 2018 Revised date: 17 November 2018 Accepted date: 20 November 2018 Cite this article as: Jun Dong, Jie Zhang, Qingyan Han, Xing Zhao, Xuewen Yan, Jihong Liu, Haibo Ge and Wei Gao, Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.11.041 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 galley proof before it is published in its final citable 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.

Tuning and enhancing the red upconversion emission of Er3+ in LiYF4 nanoparticles Jun Dong, Jie Zhang, Qingyan Han, Xing Zhao, Xuewen Yan, Jihong Liu, Haibo Ge, Wei Gao*

School of Electronic Engineering, Xi’an University of Posts & Telecommunications, Xi'an, 710121, China;

*Corresponding author: Wei Gao

E-mail: [email protected]

Abstract The tetragonal-phased LiYF4:Yb3+/Er3+ nanoparticles with different core-shell nanostructures have been successfully synthesized by a modified solution-based method. The upconversion emissions of LiYF4: Yb3+/Er3+ nanoparticles were precisely tuned from green to red with the increase in Yb3+ concentrations under 980 nm excitation. The corresponding intensity ratios of the red and green emissions (R/G ratios) were enhanced from 0.06 to 5.33. However, the overall integral upconversion intensity of LiYF4:Yb3+/Er3+ nanoparticles clearly decreased when the Yb3+ doping concentrations were increased. For the enhancement of the red upconversion intensity and expansion of their application, two kinds of core-shell nanostructures, namely, LiYbF4:Er3+@LiYF4 and LiYbF4:Er3+@LiYbF4, were constructed through epitaxial growth at similar experimental conditions. The LiYbF4:Er3+@LiYF4 core-shell nanostructures exhibited stronger upconversion emission and higher R/G ratios than the LiYbF4:Er3+ and LiYbF4:Er3+@LiYbF4 nanoparticles. The highest R/G ratios

increased to 6.27 in LiYbF4:Er3+@LiYF4 core-shell nanostructures, increasing 104-fold. The upconversion regulation and enhancing mechanism were also discussed according to the emission spectra and fluorescence lifetimes.

Graphical Abstract

Keywords: LiYF4 nanoparticles, upconversion, core/shell nanostructures, cross

relaxation

1. Introduction Lanthanide (Ln3+)-doped fluoride upconversion luminescent (UCL) nano- and micro-materials have been applied in a variety of disciplines, including optical imaging, biosensors, information storage, solar cell, and anticounterfeit technology, owing to their unique optical properties, such as weak autofluorescence background, long life expectancy, narrow emission bands, strong penetration ability for near infrared light, and low cytotoxicity [1–7]. Many fluoride AREF4 (A = alkali or alkaline earth metal, RE = Ln3+ rare earth ion), such as NaYF4, NaGdF4 and LaF3 UC nanoparticles (NPs) have been successfully synthesized through different methods, and their UC emission properties were also comprehensively studied according to different ways of excitation [8–10]. The hexagonal-phased NaYF4 has been recognized as one of the most efficient host materials for UC emission [11]. Similarly, the pure tetragonal-phased LiYF4 crystals are considered an ideal host matrix and can emit strong ultraviolet (UV), visible light (VIS), and near-infrared light (NIR) because of their low photon energy [12–14]. Prasad et al. found that the total UC quantum yield of LiYF4:Er3+ nanocrystals under 1490 nm excitation is approximately 4 times higher than that of NaYF4:Yb3+/Er3+ nanocrystals under 980-nm excitation [15]. Qin et al. found that LiYF4:18%Yb, 2%Er NCs exhibit a stronger emission than NaYF4:18%Yb, 2%Er at around 1530 nm under the excitation of a 980 nm diode laser [16].

A variety of approaches have been reported to improve and tune the UC emission

of AREF4 UCNPs. These approaches include co-doping different ions [17], changing the surface ligands [18], and constructing the core–shell (CS) structure [19]. Enhancing UC emission intensity and obtaining a pure UC emission peak have become a huge challenge for researchers, especially in single red UC emission. As the “optical window” of biological tissue is located at 600–1100 nm, selecting excitation and emission wavelengths within the “optical window” are much more important for improving the penetration depth and detection sensitivity and reducing light damage. Red UC emission in Er3+, Ho3+, or Tm3+ co-doping UC crystals can be achieved by increasing Yb3+ concentrations, introducing Ce3+, Mn2+, and Pb2+, and attaching gold NPs at the surface [20–24]. However, to our best knowledge, few reports have been done to enhance the red UC emission of Er3+ in LiYF4 NPs. In the current work, we first attempt to tune the UC emission of Er3+ in pure tetragonal-phased LiYF4:Yb3+/Er3+ UCNPs by increasing the Yb3+ ion concentrations. The LiYbF4:Er3+@LiYF4 and LiYbF4:Er3+@LiYbF4 core-shell (CS) nanostructures were obtained through epitaxial growth method. The UC emission spectra with different emission intensities were obtained for the exploration of the optical properties and mechanisms of UC emission. The strong red UC emission in the LiYbF4:Er3+@LiYF4 CS NPs will have great superiority in the biological field and color display application due to their small size and high emission intensity.

2. Experimental section 2.1. Materials

All lanthanide chlorides (YCl3·6H2O, YbCl3·6H2O, ErCl3·6H2O,), 1-octadecene (C18H36, ODE) were purchased from Shanghai Macklin Biochemical Limited Corporation. Oleic acid (C18H34O2, OA) was purchased from Alfa Aesar. Lithium hydroxide (LiOH), ammonium fluoride (NH4F), methanol (CH3OH), and ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Limited Corporation. All of them were analytical grade. All the chemicals in this study were used as starting materials without further purification. 2.2. Synthesis of the LiYF4: Yb3+/Er3+ UC nanoparticles The tetragonal-phased LiYF4: Yb3+/Er3+ UCNPs doping with different Yb3+ concentrations were synthesized through solution-based method as previously reported [25]. In a typical synthesis, the corresponding lanthanide chlorides (RECl3) (RE=Y, Yb, Er) in accordance with the mass percent of 2 mmol LiYF4: x%Yb3+,2%Er3+ (x=5,10,20,40,60,80,100) were first taken in a 100 mL four-necked flask, and then 30 mL of ODE and 12 mL of OA were added. The mixed solution was vigorously stirred and heated to 160 °C for 1 h to form a clear yellow solution under a gentle argon flow. Subsequently, when the mixture’s temperature was cooled down to 90 °C, 30 mL CH3OH solution of NH4F (8 mmol) and LiOH (5 mmol) was added to the flask. After the mixture was reacted at 30 °C for 1h. Next, the solution was heated to 60 °C and kept again for 1 h. Then the temperature was raised to 108 °C for 1min in order to remove of the methanol by evaporation. The vacuum was taken for about 20 min to remove oxygen and water to form a transparent solution. The solution was heated up to 310 °C for 1 h and finally cooled to room temperature. The samples were

precipitated by addition of cyclohexane, collected by centrifugation, washed with cyclohexane and ethanol for several times, dried in a dry oven at 60 °C for 12 h. Half of prepared UCNPs was dispersed in 4 mL cyclohexane. The LiYbF4:Er3+@LiYF4, LiYbF4:Er3+@LiYbF4 CS NPs were also synthesized by above method. The detailed experiment process is in the Supporting information.

2.2. Measurements and characterization

The phase and structure of the UCNPs were identified by X-ray diffraction (XRD) patterns recorded the D/Max2550 diffraction meter with Cu Kα (40 kV, 40 mA) irradiation (λ= 0.15406 nm) in 2θ ranging from 10o to 70o with a scan rate of 8 per minute. The morphology and size of the particles were measured by transmission electron microscopy (TEM) analysis. TEM images of the UCNPs were prepared by placing several drops of dispersed UC nanocrystals cyclohexane solution dried on carbon-coated copper grids. For spectroscopic measurements, the Ti sapphire femtosecond laser (Mira-900) was employed as excitation sources (wavelength: 700 nm-1000 nm). The UC emission signals were detected and collected by a spectrometer (SP2750i) with a spectral resolution of 0.008 nm. The fluorescence lifetime of the UC emission was measured by a Boxcar (SRS), oscilloscope (Tektronix) and photomultiplier tube (PMT, ACTON). All of the spectroscopic measurements were carried out at room temperature.

3. Results and discussions 3.1. Structure and morphology

Fig.1 shows the XRD patterns of LiYF4:Yb3+/Er3+ UC NPs co-doping with different Yb3+ concentrations, LiYbF4:2%Er3+@LiYF4, and LiYbF4:2%Er3+@LiYbF4 CS NPs. The main diffraction peak positions of all samples were consistent with the standard pattern LiYF4 (JCPDS No. 81-2254), indicating that all the samples had a tetragonal-phased crystal structure [26]. The diffraction peaks of the samples slightly shifted to the large angle side after the Yb3+ concentration increased, which indicates that the unit cell of LiYF4 diminishes due to the substitution of Y3+ (r=0.089 nm) ions with smaller Yb3+ (r=0.086 nm) ions in the LiYF4 host lattice [27–28]. The diffraction peak of the LiYbF4:2%Er3+@LiYF4 or LiYbF4:2%Er3+@LiYbF4 CS nanostructures was narrower than that of the naked core NPs after the shell growth. The results showed that the crystallinity of the CS NPs improved, and no impure peak was detected in these NPs after the increase in Yb3+ concentrations and formation of CS NPs. The TEM images of the prepared LiYF4:Yb3+/Er3+ NPs are provided in Fig. 2(a–d). All the NPs were uniform bipyramidal structures with enhanced particle-size distribution, and their sizes were approximately 20 nm. The shapes of the samples were not influenced by the introduction of different Yb3+ concentrations possibly because Y3+ and Yb3+ ions have the same ionic radius [27]. Fig. 2 (e–f) present the TEM images of the LiYbF4:2%Er3+@LiYF4, LiYbF4:2%Er3+@LiYbF4 CS NPs produced through the epitaxial growth method. The TEM images show an increase in CS particle size for LiYbF4:2%Er3+@LiYF4, and the LiYbF4:2%Er3+@LiYbF4 CS nanostructures were obtained by coating the CS NPs with 5 nm and 8 nm LiYF4 and

LiYbF4 shells, respectively (Fig.2 (e) and (f)). Comparing the results with the size of LiYbF4:2%Er3+ core-only NPs, we confirmed the existence of the shell structure. The corresponding high-resolution TEM (HRTEM) images of LiYbF4: 2%Er3+@LiYF4 and LiYbF4:2%Er3+@LiYbF4 CS NPs are shown in the Fig.2 (g) and Fig.2 (h) for verifing the result. The corresponded d spacing of 0.46 nm for the (101) plane and 0.30 nm for the (112) plane of tetragonal-phase LiYbF4 (JCPDS No 71-1211) verify the high crystallinity of the UC NPs that is shown in inset of Fig.1(h). Moreover, the CS structures with the clear crystal fringes are shown in the Fig.2 (g) and Fig.2 (h), further indicates that the LiYF4 and LiYbF4 layer have successfully coated onto LiYbF4: 2%Er3+ core NPs. 3.2. UC luminescence properties Fig. 3(a) shows a collection of UC emission spectra of LiYF4:x%Yb3+/2%Er3+ (5– 98%), LiYbF4:2%Er3+@LiYF4, and LiYbF4:2%Er3+@LiYbF4 CS nanostructures under the excitation of a 980 nm NIR laser. The representative UC green emission bands (523–575 nm) and the red emission bands (625–675 nm) were clearly observed; these bands corresponded to the 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 transitions for Er3+ in all the samples [29]. The UC emission intensity of LiYF4:Yb3+/Er3+ was enhanced at first and then decreased with the increase in Yb3+ ion concentrations (Fig. 3(b)). The strongest UC emission intensity was obtained after the Yb3+ concentration reached 20%. The R/G ratio of Er3+ also obviously increased from 0.06 to 5.33 after the Yb3+ ion concentrations increased from 5% to 98%. However, a different phenomena of UC emission intensity and R/G ratio was observed in the

LiYbF4:2%Er3+@LiYbF4 and LiYbF4: 2%Er3+@LiYF4 CS NPs. Compared with the relative

UC

emission

intensity

of

the

LiYbF4:2%Er3+

core

NPs,

the

LiYbF4:2%Er3+@LiYbF4 CS nanoparticle emission intensity slightly decreased. Conversely, the UC emission of LiYbF4:2%Er3+@LiYF4 CS NPs significantly increased, and the R/G ratio of the CS NPs was enhanced to 6.27. The Commission International de I’Eclairage (CIE) coordinates of the LiYF4:Yb3+, Er3+ NPs co-doping with different Yb3+ ion concentrations (5–98%) and the LiYbF4:2%Er3+@LiYbF4, LiYbF4:2%Er3+@LiYF4 CS NPs are shown in Fig. 4. The emission color varied from green to red with the corresponding changes in the CIE coordinates from (0.2223, 0.7231) to (0.5701, 0.3564). UC output color from green to red in LiYF4:Yb3+, Er3+ NPs can be controlled by adjusting the Yb3+ ion concentration, and the red UC emission can be also tuned by building CS nanostructures. The calculated CIE Chromaticity coordinates (x, y) of the corresponding nanoparticle samples are shown in Table 1. The results demonstrate that the output color of nanoparticles can be adjusted by controlling Yb3+ doping concentrations and building core-shell structures.

3.3. UC mechanisms The UC emission mechanisms of Yb3+ and Er3+ ions co-doping LiYF4:Yb3+/Er3+ and CS NPs were first discussed according to energy level diagrams shown in Fig. 5. For the core LiYF4:Yb3+/Er3+ NPs, the UC fluorescence emission mainly derived from the Er3+ ions. As Yb3+ ions have a larger absorption cross-section for infrared light and

longer excited state lifetimes than Er3+ ions, the Er3+ main population pathway depends on the energy transfer from Yb3+ ions to Er3+ under 980 nm excitation. When the Yb3+ ions were excited, the obtained energy immediately transferred from the Yb3+ to Er3+ ions. Subsequently, the nearby Er3+ ions were excited to the corresponding 4

I11/2, 4F9/2, and 4F7/2 excited levels, resulting in the green (2H11/2, 4S3/2→4I15/2) and red

(4F9/2→4I15/2) UC fluorescence emissions of Er3+. Generally speaking, the Yb3+ and Er3+ co-doping system usually exhibit intense green UC emission accompanied by weak red UC emission. However, after the Yb3+ ion concentrations increased by more than 20%, the red UC emission was enhanced and the green UC emission decreased. The highest R/G was obtained in the LiYF4:98%Yb3+/2%Er3+ NPs. As shown in Fig. 5, when the excited state 4F9/2 radiatively decayed to 4I15/2 ground states, red UC emission was detected, which mainly originated from the 4F9/2 excited state. Thus, the first task for enhancing red UC emission intensity is to increase the population of the 4F9/2 level. Two dominating processes are present to achieve the population; one is from the 2H11/2/4S3/2 states to the 4F9/2 state through a direct nonradiative transition process, and the other is from the 4I13/2 state to the 4F9/2 state through the ET processes. The 4I13/2 state can be directly populated by a nonradiative transition process from 4I11/2 to 4I13/2 state. The energy gaps of 2H11/2/4S3/2→ 4F9/2 and 4I11/2→4I13/2 are about 3217 and 3609 cm−1, respectively. Thus, the two nonradiative relaxation processes are ineffective according to the multi-phonon nonradiative relaxation process [30]. Furthermore, the Yb-Er system usually show strong green UC emission and extremely weak red ones at low-doped

concentrations. Nevertheless, when the Yb3+ concentrations increased by more than 20%, different UC emission phenomena and increase in red UC emission were observed. To understand the red UC emission enhancement mechanism, the LiYF4:Yb3+/Er3+ NPs was excited with diverse excitation power densities, and the excitation power density dependencies of the UC emission were investigated according to I up  P n (Iup is the UC emission intensity, P is excitation pump power, and n is the number of pump photons) under the unsaturated condition shown in Fig. 6 [31]. The value for n can be obtained from the slope of the line in the plot of log(P) versus log(Iup) (Table 2). In the LiYF4:Yb3+/Er3+ NPs, the slopes of n deviated from 2 when the Yb3+ ion concentrations increased to 98%, this phenomenon was mainly attributed to the two cross-relaxation processes CR1:[ 4 S 3/2 → 4 F 9/2 : 4 I13/2 → 4 I11/2 ], CR2:[ 4 S 3/2 → 4 I9/2 : 4

I15/2→4I13/2] and energy back transfer from Er3+ (2H11/2/4S3/2) to Yb3+ (2F5/2) under

high Yb3+ ion-doped concentrations, which was theoretically described by rate equations [31–32]. Similar experimental phenomena were also observed in NaYF4:Yb3+/Er3+ and NaYF4:Yb3+/Tm3+ crystals [33]. As shown in Fig. 3(a), when the Yb3+ ion doped concentrations increased to 98%, the red UC emission intensity increased, but the overall UC emission intensity decreased. For the enhancement of red UC emission intensity, CS construction nanocrystals, such as LiYbF4:2%Er3+@LiYbF4 and LiYbF4:2%Er3+@LiYF4 CS, were obtained. As is well known, using the CS architecture is one of the most effective

approaches for improving UC efficiency, preventing the transportation of excitation energy to surface-quenching centers, and recovering surface luminescent centers for the suppression of nonradiative decay of activators in the core [34]. However, the UC emission intensity of the core LiYbF4:2%Er3+ NPs decrease when covering the LiYbF4 shell. When the core LiYbF4:2%Er3+ NPs were covered with the LiYF4 shell, the UC emission intensity of LiYbF4:2%Er3+@LiYF4 CS was enhanced two fold. In order to further prove the red UC emission enhancement, the green and red UC emission dynamic curves of 2H11/2/4S3/2→4I15/2 (545 nm) transition and 4F9/2→4I15/2 (650 nm) for Er3+ ions in LiYF4:x%Yb3+/2%Er3+(5–98%), LiYbF4:2%Er3+@LiYF4, and LiYbF4: 2%Er3+@ LiYbF4 CS NCs were measured under 980 nm pulse laser excitation as shown in Fig. 7. Each decay curve can be fitted into a biexponential exponential functional It  A1 exp( t / 1 )  A2 exp( t /  2 ) , where It is the luminescence intensity, A1 and A2 are constants, t is the time,τ1 andτ2 are the short and long lifetimes for exponential components, respectively. The average lifetimes can be defined as:

A t  A2t2   11 A1t1  A2t2 2

2

(1)

Table 3 shows the calculated luminescence lifetimes of the samples under 980 nm excitation according to equation (1). When the Yb3+ ions concentrations increase from 5% to 98%, the luminescence lifetimes of green and red UC emission first prolong and then change short in Fig.7(a) and (b). As for UC emission, the long lifetimes usually means strong UC emission [9]. Thus, this result indicates that the

addition of Yb3+ ions in the LiYF4:Yb3+/Er3+ effectively varies the population of excited states of Er3+. Compared with the LiYbF4: 2%Er3+ core NPs, the obvious prolonged lifetimes are experimentally observed for the LiYbF4:2%Er3+@LiYF4 CS NPs, and shortened lifetimes are noted in the LiYbF4: 2%Er3+@LiYbF4 CS NPs. As we know, the experimental lifetimes can be determined by three factors, such as the theoretical lifetime, the nonradiative transition rate and the energy transfer rate, that can be described as 1   1  RAD  WNR  WET [35]. Thus, it can easily explain the different experimental phenomenon in the LiYbF4:2%Er3+@LiYF4 and LiYbF4: 2%Er3+@ LiYbF4 CS NPs. The longer lifetimes of red and green UC emission in the LiYbF4:2%Er3+ @LiYF4 CS NPs, which is due to the decreased of the nonradiative transition rate, that is for preventing the transportation of excitation energy to surface-quenching centers [34]. The enhancement in UC emission intensity was mainly attributed to the protection provided by the LiYF4 shell to the luminescence center against nonradiative decay. However, the shorter lifetimes of red and green UC emission in the LiYbF4:2%Er3+ @LiYbF4 CS NPs, which owing to the deleterious energy transfer from Yb3+/Er3+in the core to Yb3+ in the shell [36]. Thus, building appropriate CS nanostructure is an effective way for tuning UC emission intensity, and has no significant influence on the characteristic of the host materials . Fig. 8 shows the LiYbF4:2%Er3+@LiYbF4 and LiYbF4:2%Er3+@LiYF4 CS NPs pumping power dependence properties. We found that the UC emission intensities of the LiYbF4:2%Er3+@LiYbF4 and LiYbF4:2%Er3+@LiYF4 CS NPs were all enhanced at 5–120 mW excitation energy. The corresponding R/G ratio was changed from 0 to

6.36 and from 4.9 to 5.7 in LiYbF4:2%Er3+@LiYbF4 and LiYbF4:2%Er3+@LiYF4 CS NPs, respectively. The increased R/G ratio in the CS NPs can be attributed to the occurrence of CR between Er3+ ions at high excitation power [32]. Therefore, a high red UC emission intensity in LiYbF4:Er3+ NPs can be obtained by building CS structures and increasing excitation power. Similar to NaYF4 nanocrystals, this kind of red UC emission nanocrystals has potential applications in biological imaging, diagnosis, and treatment.

4. Conclusion Tetragonal-phased LiYF4:Yb3+/Er3+ UC NPs were successfully synthesized through a modified solution-based method. By doping different Yb3+ ion concentrations, we tuned the UC emission from green to red in LiYF4:Yb3+/Er3+ UC NPs under 980 nm excitation. The corresponding R/G ratio was enhanced by about 54 times by increasing the Yb3+ ion concentrations to 100%. However, the UC emission intensities decreased because of the two cross-relaxation processes CR1: [4S3/2→4F9/2: 4

I13/2→4I11/2], CR2: [4S3/2→4I9/2: 4I15/2→4I13/2] and energy back transfer from Er3+

(2H11/2/4S3/2) to Yb3+ (2F5/2) under high Yb3+ ion doped concentrations. The different spectrum phenomena have been proved according to the emission spectra and fluorescent lifetimes. High UC emission intensities and R/G ratio were obtained by building LiYbF4:Er3+@LiYF4 CS nanostructures. The enhancement of UC emission intensity was mainly attributed to the protection provided by the LiYF4 shell to the luminescence center against nonradiative decay. These NPs have potential applications in biological imaging, diagnosis, and treatment.

Acknowledgment This work was supported by the National Science Foundation of China (11604262), Shaanxi Provincial Research Plan for Young Scientific and Technological New Stars (Program No.2019KJXX-31), the New Star Team of Xi'an University of Posts & Telecommunications, the Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2018JM1052 and 2018JQ1070).The project was supported by the Open Research Fund of Key Laboratory of Spectral Imaging Technology, Chinese Academy of Sciences (LSIT201810D), the Research Plan of Xi’an University of Posts & Telecommunications(CXJJ2017001).

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Figure

Fig. 1 XRD patterns of LiYF4: x%Yb3+, 2%Er3+ NPs co-doping with different Yb3+ concentrations and LiYbF4:2%Er3+@LiYF4, LiYbF4:2%Er3+@LiYbF4 CS NPs.

Fig. 2 TEM images of the LiYF4:Yb3+, 2%Er3+ UC NPs co-doping with different Yb3+ concentrations and CS NPs ( (a-b) LiYF4:x%Yb3+/2%Er3+(x=5%, 20%, 60%, 98%), (e) LiYbF4:2%Er3+@LiYF4 and (f) LiYbF4:2%Er3+@LiYbF4.). The (g) and (h) are the corresponding HRTEM images for (e) and (f).

Fig. 3 LiYF4: x%Yb3+, 2%Er3+ NCs co-doping with different Yb3+ concentrations and LiYbF4:2%Er3+@LiYF4, LiYbF4:2%Er3+@LiYbF4 CS NPs under 980 nm excitation: (a) the UC emission spectra, (b) relative emission intensity, (c) R/G ratio.

Fig. 4

CIE chromaticity diagram for the LiYF4: x%Yb3+, 2%Er3+ NPs co-doping

with different Yb3+ concentrations (a:5%, b:10%, c:20%, d:40%, e:60%, f:80%, g:98%) and CS NPs (h: LiYbF4:2%Er3+@LiYbF4, i:LiYbF4:2%Er3+@LiYF4) under 980 nm laser excitation.

Fig. 5 Energy level diagrams of Yb3+ and Er3+, showing possible energy transfer processes in the LiYbF4:2%Er3+@LiYbF4 CS NCs.

Fig. 6 Pump power dependences of LiYF4:20%Yb3+/2%Er3+ and LiYbF4:2%Er3+ NCs.

Fig.7 Decay profiles of red (654 nm) and green (540 nm) UC emission of LiYF4: x%Yb3+, 2%Er3+ (x=5%, 20% and 98%)NPs and LiYbF4:2%Er3+@LiYF4, LiYbF4: 2%Er3+@LiYbF4 CS NPs under 980 nm excitation.

Fig. 8 UC emission spectra and R/G ratio of LiYbF4:2%Er3+@LiYbF4 and LiYbF4:2%Er3+@LiYF4 CS NPs under different excitation power from 20mW to 120mW.

Table 1 The calculated CIE chromaticity coordinate (x, y) of LiYF4: x%Yb3+/2%Er3+ NPs with doping different Yb3+ ion concentrations and LiYbF4:2%Er3+@ LiYbF4, LiYbF4:2%Er3+@LiYF4 CS NPs. CIE chromaticity coordinate Samples y

x

a: LiYF 4 :5%Yb 3+ /2%Er 3+

0.2223

0.7231

b: LiYF 4 :10%Yb 3+ /2%Er 3+

0.2402

0.7132

c: LiYF 4 :20%Yb 3+ /2%Er 3+

0.2473

0.6841

d: LiYF 4 :40%Yb 3+ /2%Er 3+

0.3226

0.6217

e: LiYF 4 :60%Yb 3+ /2%Er 3+

0.3726

0.5446

f: LiYF 4 :80%Yb 3+ /2%Er 3+

0.4717

0.4521

g: LiYF 4 :98%Yb 3+ /2%Er 3+

0.5201

0.3908

h: LiYbF 4 :2%Er 3+ @LiYbF 4

0.5381

0.3978

i: LiYbF 4 :2%Er 3+ @LiYF 4

0.5701

0.3564

Table 2 The n value of the laser photons required for green and red UC emission of LiYF4: x%Yb3+/2%Er3+ NPs with doping different Yb3+ ion concentrations.

Samples

green(n)

red(n)

a: LiYF4:5%Yb3+/2%Er3+

n=1.98±0.03

n=1.96±0.02

b: LiYF4:10%Yb3+/2%Er3+

n=1.96±0.02

n=1.94±0.03

c: LiYF4:20%Yb3+/2%Er3+

n=1.88±0.01

n=1.86±0.05

d: LiYF4:40%Yb3+/2%Er3+

n=1.78±0.04

n=1.75±0.02

e: LiYF4:60%Yb3+/2%Er3+

n=1.74±0.03

n=1.70±0.04

f: LiYF4:80%Yb3+/2%Er3+

n=1.70±0.02

n=1.66±0.03

g: LiYF4:98%Yb3+/2%Er3+

n=1.68±0.02

n=1.64±0.02

Table 3 Luminescence lifetimes of LiYF4:20%Yb3+/2%Er3+ NPs with different Yb3+

concentration and LiYbF4:2%Er3+@LiYbF4, LiYbF4:2%Er3+@LiYF4 CS NPs under 980 nm excitation at 545 nm and 650 nm.

Lifetime (μs) Samples

540nm

650 nm a: LiYF4 :5%Yb3+/2%Er 3+

193.7±2.2

96.2±3.5

b: LiYF4:10%Yb3+/2%Er3+

256.5±4.2

127.3±2.2

c: LiYF4:20%Yb3+/2%Er3+

394.2±3.5

206.5±2.1

d: LiYF4:40%Yb3+/2%Er3+

273.4±1.4

187.3±1.9

e: LiYF4:60%Yb3+/2%Er3+

206.2±4.1

168.4±2.5

f: LiYF4 :80%Yb3+/2%Er3+

154.3±3.2

154.6±3.2

g: LiYF4 :98%Yb 3+ /2%Er 3+

89.7±2.4

146.2±2.3

h: LiYbF4 :2%Er 3+ @LiYbF 4

63.8±2.7

121.3±1.5

i: LiYbF4:2%Er3+@LiYF4

123.7±2.2

356.1±2.7