Preparation, growth mechanism and upconversion and near infrared

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Preparation, growth mechanism and upconversion and near infrared photoluminescence properties of convex-lens like NaYF4 microcrystals doped with various rare earth ions excited at 808 nm Yingjin Ma, Zhengwen Yang, Hailu Zhang, Jianbei Qiu, and Zhiguo Song Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01667 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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Crystal Growth & Design

Preparation, growth mechanism and upconversion and near infrared photoluminescence properties of convex-lens like NaYF4 microcrystals doped with various rare earth ions excited at 808 nm Yingjin Ma, Zhengwen Yang*, Hailu Zhang, Jianbei Qiu*, Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China *Corresponding Author: E-mails: [email protected], [email protected]

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ABSTRACT: Preparation of rare earth ions doped photoluminescence materials with controlled morphology was desired to fulfill the requirement of different applications. In the work, the convex-lens like NaYF4 microcrystals doped with various rare earth ions were prepared by adjusting preparation parameters including the reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene and types of doping ions. Possible growth mechanism of convex-lens like NaYF4 microcrystals is proposed based on reaction time and temperature dependent morphology evolution. The formation of micro convex-lens includes the three processes of NaYF4 nanoparticles self-assemble, dissolution–nucleation and regrowth. Doping-ions dependent near infrared and upconversion luminescence properties of convex-lens like NaYF4 microcrystals were investigated excited at the 808 nm. The visible upconversion luminescence was observed in the Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens like NaYF4 microcrystals, and near infrared luminescence was obtained in the Nd3+, Nd3+/Er3+, Yb3+/Er3+, Nd3+/Yb3+, Nd3+/Yb3+/Er3+ doped NaYF4 convex-lens like NaYF4 microcrystals. The Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped convex-lens like NaYF4 microcrystals exhibit various upconversion luminescence mechanisms. The energy transfer of the Er3+→Yb3+ and the Nd3+→Er3+ was observed in the Yb3+/Er3+ and Nd3+/Er3+ doped convex-lens like NaYF4 microcrystals, respectively. The UC emission of Nd3+/Yb3+/Er3+ doped convex-lens like NaYF4 microcrystals is from the energy transfer mechanisms of Nd3+→Yb3+→Er3+. 1. Introduction Very recently, rare earth (RE) ions doped luminescent materials have attracted much attention due to their special optical properties and extensive applications in the fields of three-dimensional display, biological imaging and detection, drug delivery, solar cells and photodynamic therapy [1-7]. Among the various RE ions doped luminescent materials, the RE fluoride compounds (NaREF4) upconversion (UC)


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materials excited at the 980 nm exhibited the higher UC luminescence efficiency due to its lower phonon energy, which were extensively reported [8-10]. However, the 980 nm excitation light is overlapped with the absorption of water molecules in biological issues, which can result in the overheating of biological issues. Thus the significant cell death and tissue damage occurred under the excitation of 980 nm. In contrast to the NaREF4 UC materials upon the 980 nm excitation, the NaREF4 UC and near infrared (NIR) luminescent materials excited at the 808 nm is more suitable for the biological applications due to avoiding the overheating of biological issues caused by 980 nm excitation light[11-13]. Therefore, the NaREF4 phosphor with the UC and NIR luminescence excited at the 808 nm is very important for their applications. At present, the UC and NIR luminescence NaREF4 phosphor tri-doped with Nd3+, Yb3+ and Er3+ were extensively investigated under the excitation of 808 nm, however, few investigations were carried out on the UC luminescence Er3+ single-doped and Yb3+ and Er3+ co-doped NaREF4 phosphor upon the 808 nm excitation. It is well-known that the photoluminescence properties of the NaREF4 phosphors are dependent on the types of dopants and co-dopants, and morphology (shape and size). For example, it was reported that the sensor sensitivities of Er3+ and Yb3+ co-doped NaREF4 optical thermometers were influenced by their morphologies[14, 15]. The fine modification of UC luminescence color from Er3+ and Yb3+ co-doped NaREF4 can be obtained by the precise control of size and morphology of UC nanocrystals [16-20]. Therefore, the synthesis of rare earth ions doped the NaREF4 phosphors with precise morphology is particularly significant for their applications. Up to now, the hexagonal phase NaYF4 crystals with various sizes and morphologies such as nanoparticles, nano-rods and nano-plates were prepared [21-26]. Very recently, the Yb3+, Er3+ doped NaYF4 micro-plates were prepared, however, the



research was focused on the UC luminescence property of NaYF4 nano and micro-plate in the previous work, and the formation mechanisms of NaYF4 micro-plates was not investigated in detail [17, 27, 28]. In contrast to other morphology material, the convex-lens like materials exhibited the optical focusing property at the infrared wavelengths [29, 30], which may result in the UC emission enhancement due to the enhancement of focusing near infrared excitation. In this work, the convex-lens like NaYF4 microcrystals dopes with various RE ions were prepared by the co-precipitation method using oleic acid as the surfactants. The influence of reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene and types of doping ions on the morphology, size and phase structure of convex-lens like NaYF4 microcrystals was systematically studied. Possible growth mechanism of convex-lens like NaYF4 microcrystals was proposed based on the reaction time and temperature dependent morphology evolution. In addition, the NIR and UC luminescence properties and mechanisms of convex-lens like NaYF4 microcrystals doped with various RE ions were investigated excited at the 808 nm. The UC luminescence mechanisms of convex-lens like NaYF4 microcrystals doped with the Er3+, Nd3+/Er3+, Yb3+/Er3+ or Nd3+/Yb3+/Er3+ were different under the excitation of 808 nm. 2. Experimental High purity (99.99%) Y2O3, Yb2O3, Er2O3, Nd2O3, analytical reagent grade oleic acid (OA), 1-octadecylene (ODE), NH4F and NaOH were used to prepare the convex-lens like NaYF4 microcrystals as raw materials without further purification. The convex-lens like NaYF4 microcrystals doped with different RE ions of Nd3+, Er3+, Yb3+, Nd3+/Er3+, Nd3+/Yb3+, Yb3+/Er3+ and Nd3+/Yb3+/Er3+ were prepared by co-precipitation method. In order to obtain the influence of reaction time, reaction


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temperature, NaOH concentration, ratio of OA to ODE and types of doping ions on the morphology, size and phase structure of NaYF4 microcrystals, the various preparation parameters were used, as listed in the Table I. The S1-S18 samples were prepared following by the similar procedure except for changing the preparation parameters listed in the Table I. As an example, the preparation of S5 sample was described as follows. Firstly, The RECl3 (RE =Y3+, Yb3+, Er3+ and Nd3+) compounds were prepared by dissolving the corresponding RE2O3 compounds in hot HCl solution. The obtained YCl3, YbCl3, NdCl3 and ErCl3 were dissolved in the deionized water to form the 0.5 mol/L solution, respectively. The 1.72 mL YCl3, 200 µL YbCl3 and 40 µL NdCl3, 40 µL ErCl3 solution were added into the mixture of 8 mL oleic acid and 12 mL ODE in a 100 mL three-necked flask. The total volume of OA and ODE was about 20 mL and the ration of OA to ODE was the 4/6. The mixture was heated to 150 ℃ for 60 min to remove deionized water under the argon shield and continues stirring. After cooling to the room temperature, the 2 mL NaOH and 6 mL NH4F methanol solutions were introduced into the 3-neck flask, then it was heated at the 80℃ for 1 h to remove the methanol of solutions. After removing methanol, the solution was heated at the 305℃ for 80 min under the argon shield. The resulting solution was centrifuged five times with the mixture solution of 2 mL cyclohexane and 6 mL ethanol, and the NaYF4:Yb3+, Nd3+, Er3+ products were prepared. The crystal phases of the samples were examined by the X-ray diffraction diffractmeter quipped with Cu Kα radiation. The morphology and size of products were observed by a transmission electron microscopy (TEM, JEOL 2100), a field emission scanning electron microscopy (FESEM, QUANTA 650) and a scanning probe microscopy(SPM-9600). The UC luminescence spectra and NIR luminescence spectra of the samples excited at the 808 nm were collected with a HITACHI F-7000



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spectrophotometer and an Edinburgh FLS980 spectrophotometer, respectively. The refractive index of products was measured by using a WAY-1S refractometer. Table I. the detailed experimental parameters, average diameter of NaYF4 products Sample

OA/ODE

NaOH

Time

Temperature

Types of doping ions

Diameter

ratio

(mmol)

(min)

(℃)

S1

4/6

5

10

305

2%Nd3+/10%Yb3+/2%Er3+

11

S2

4/6

5

30

305

2%Nd3+/10%Yb3+/2%Er3+

21/309

S3

4/6

5

50

305

2%Nd3+/10%Yb3+/2%Er3+

419

S4

4/6

5

65

305

2%Nd3+/10%Yb3+/2%Er3+

470

S5

4/6

5

80

305

2%Nd3+/10%Yb3+/2%Er3+

719

S6

4/6

5

80

275

2%Nd3+/10%Yb3+/2%Er3+

10

S7

4/6

5

80

290

2%Nd3+/10%Yb3+/2%Er3+

17/615

S8

4/6

5

80

320

2%Nd3+/10%Yb3+/2%Er3+

660

S9

3/7

5

80

305

2%Nd3+/10%Yb3+/2%Er3+

1216

S10

5/5

5

80

305

2%Nd3+/10%Yb3+/2%Er3+

516

S11

4/6

3.75

80

305

2%Nd3+/10%Yb3+/2%Er3+

750

S12

4/6

6.25

80

305

2%Nd3+/10%Yb3+/2%Er3+

739

S13

4/6

5

80

305

2%Nd3+/10%Yb3+

399

S14

4/6

5

80

305

2%Nd3+/2%Er3+

325

S15

4/6

5

80

305

10%Yb3+/2%Er3+

721

S16

4/6

5

80

305

2%Nd3+

305

S17

4/6

5

80

305

2%Er3+

342

S18

4/6

5

80

305

10%Yb3+

542

(nm)

3. Results and discussion Preparation parameters will influence the structure, morphology and size of rare earth ions doped NaYF4 products. The effect of the ratio of OA to ODE, NaOH concentration, reaction time, reaction temperature and types of doping ions on the morphology, size and structure of NaYF4:Nd3+, Yb3+, Er3+ UC materials were studied


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in details. 3.1 Influence of reaction time and temperature on the morphology of NaYF4:Nd3+, Yb3+, Er3+ products The influence of reaction time on crystal structure and morphology of NaYF4:Nd3+,Yb3+,Er3+

products

was

investigated.

The

XRD

patterns

of

NaYF4:Nd3+,Yb3+,Er3+ products prepared at the 305℃ for 10, 30, 50, 65 and 80 min are showed in the Figure 1(a). The product prepared at the 10 min consists of pure cubic phase NaYF4 (JCPDS No. 77-2042). Further increasing the reaction time to 30 min, the product consists of the cubic and hexagonal phase NaYF4 (JCPDS No. 16-0334). The XRD peaks of products prepared at the different reaction times from 50 to 80 min are well-indexed to the standard hexagonal NaYF4, exhibiting the formation of pure hexagonal phase NaYF4. However, the relative intensity of diffraction peaks is not in agreement with these of the standard PDF card. In contrast to the standard diffraction peaks, the relative intense (002) diffraction peak was observed in the all hexagonal NaYF4 products, which suggested that the NaYF4:Nd3+, Yb3+, Er3+ products were grown preferentially along the (002) planes. The diffraction intensity of (002) plane of hexagonal NaYF4 products was increased with the increasing of reaction time, which suggested the larger NaYF4 products may be obtained due to their further growth. The XRD results suggested that there is a phase transformation for the NaYF4 products, and the unstable cubic phase can be transformed to stable hexagonal phase with the increasing of reaction time. The representative TEM images of NaYF4 products obtained under different reaction times were obtained, as shown in the Figure 1(b)-(f). It is noted that the reaction time plays a role on controlling the morphology of NaYF4:Nd3+, Yb3+, Er3+ products. At the reaction time of 10 min, the final products were composed of the non-uniform nanoparticles with an average



diameter of 11 nm ranging from 24 to 5 nm, as seen from the Figure 1(b). With increasing reaction time to 30 min, the final products were consisted of some hexagonal micro-plates deposited on its surface with a lot of nanoparticles, as shown in the Figure 1(c). With further increasing reaction time to 50 min, all the nanoparticles completely disappeared, the plate like NaYF4 microcrystals were formed by the further growth of NaYF4:Nd3+, Yb3+, Er3+ nanoparticles, which was demonstrated by the TEM images of corresponding lateral shapes of products given in the inset of Figure 1 and the FESEM image in the Figure 1(g). The FESEM image of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 80 min was shown in the Figure 1(h), exhibiting the convex-lens like NaYF4 microcrystals with the center thickness and thin edge after further growth of hexagonal micro-plates like NaYF4:Nd3+,Yb3+,Er3+, as shown in the TEM images of corresponding lateral shapes of products given in the inset of Figure 1. The AFM image of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 80 min was measured, as shown in the Figure 1(i), which further demonstrated that the formation of convex-lens like NaYF4 microcrystals at the 80 min. The reaction time has an influence on the size of NaYF4:Nd3+, Yb3+, Er3+ products, as listed in Table I. The sizes of NaYF4:Nd3+, Yb3+, Er3+ products were increased with the increasing of reaction time. The insert image of Figure 1(c) exhibited the SAED pattern of NaYF4 micro-plates on its surface with a lot of nanoparticles. The ordered bright spots from the hexagonal phase NaYF4 and diffraction rings from the cubic phase NaYF4 were observed, which agrees well with the results measured by the XRD analysis. Figure 1(j) exhibited the SAED pattern (inset) and HRTEM image of thin edge of convex-lens like NaYF4 microcrystals prepared at 80 min. The distances between adjacent lattice fringes are both about 0.51 nm, which correspond to the d spacing of (100) or (010) plane of hexagonal NaYF4 product. The HRTEM image


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Figure 1 the XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ products prepared at different reaction time; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 10 (b), 30 (c), 50 (d), 65 (e) and 80 min (f); the insert images in (c) are the enlargement TEM image and SAED pattern of NaYF4 micro plate; the inserts in the (d)-(f) are lateral shape of NaYF4 products; the FESEM images of NaYF4 products prepared at the 50 (g) and 80 min (h); the AFM image (i) of NaYF4 products prepared at 80 min, the insert in (i) is the curve of profile; (j) the HRTEM image and SAED pattern (inset) of convex-lens like NaYF4 microcrystals prepared at the 80 min.



demonstrated that the convex-lens like NaYF4 microcrystals were grown along [100] and [010] orientation, namely (002) plane [31]. The highly ordered bright spots suggested that the convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals are the single crystalline. The refractive index of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the 10, 50 and 80 min were measured, receptively, which was about 1.4270, 1.4277 and 1.4279. It can be seen that the refractive index of NaYF4:Nd3+, Yb3+, Er3+ products with the various morphologies has slight difference. To deeply certify the morphology of convex-lens like NaYF4 microcrystals, a diagram of electromagnetic field in two-dimensional space was simulated by a finite element solver, as shown in Figure 2. In the simulation, the structural parameters are consistent with the experiment results exhibited in the Figure 1. The red spot means the source of excitation under the 808 nm and the black line of convex-lens means the convex-lens like NaYF4 microcrystals. The refractive index of the NaYF4 microcrystals is set to 1.4279, and the surrounding medium is air. Figure 2(a) showed the amplitude image of simulation magnetic field. It is clearly seen that when a monochromatic point source with a wavelength of 808 nm is positioned the 600 nm away from the surface at the left side of the NaYF4 microcrystals, the spherical wave of source with the excitation of 808 nm is converted to plane wave. We can see the distribution of transverse magnetic (TM) polarization with magnetic field along z direction, as shown in Figure 2(b), and the beam is almost free from divergence in propagation. The simulation demonstrates the NaYF4 microcrystals can also focus light, which means the morphology of NaYF4 microcrystals are the convex-lens according to the reciprocity of optics [32].


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Figure 2 Electromagnetic field simulation of NaYF4 microcrystals based on the structural parameters exhibited in the Figure 1; amplitude of magnetic field (a) and (b) the distribution of transverse magnetic(TM) polarization with magnetic field

With the increasing of the reaction time, the NaYF4:Nd3+, Yb3+, Er3+ products suffered from the transformation from the cubic to hexagonal phase, and the morphologies changed from the nanoparticles to micro plates then to micro convex-lens. The formation mechanism of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was inferred from the morphology changing of the products exhibited in the Figure 1. A simple schematic illustration for the formation process of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals is exhibited in the Figure 3. The formation of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals includes the three process of self-assemble of NaYF4:Nd3+, Yb3+, Er3+ nanoparticles, dissolution–nucleation and regrowth, as shown in Figure 1. For the formation of NaYF4 nanoparticles, the combination of Y3+and F- form the [YF4]-, and the reaction between Na+ and [YF4]cause the fast nucleation of NaYF4 [33], as exhibited in the Figure 3(a). The isotropic growth of cubic NaYF4:Nd3+, Yb3+, Er3+ products occurred due to its isotropic crystal structures. Thus the small NaYF4:Nd3+, Yb3+, Er3+ nanoparticles in the cubic phase



were formed at the 10 min reaction time, as exhibited in the Figure 3(b). At the 30 min, some NaYF4:Nd3+, Yb3+, Er3+ nanoparticles were self-assembled due to their high surface energy [33]. Some nanoparticles transformed from the cubic to hexagonal phase because the cubic NaYF4:Nd3+, Yb3+, Er3+ seeds are unstable, and the interfaces between transformed nanoparticles were obviously observed, as shown in Figure 1(c). The further growth of hexagonal NaYF4:Nd3+, Yb3+, Er3+ results in the formation of micro plates like NaYF4, as shown in the Figure 1(d) and Figure 3. After the formation of plates like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, some nano-crystals were attached to their surfaces. Moreover, most nanocrystals were aggregated at the centers of plates like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, as shown in Figure 1(c) and Figure 3(e). With the time increasing to 50 min, the cubic nanoparticles disappeared completely, as shown in the TEM images of Figure 1, which suggested that the further dissolution and regrowth of nanocrystals on the surfaces of micro-plates take place, resulting in the formation of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals.

Figure 3 the schematic illustration for the formation process of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals; the blue balls and pink sheets in Figure 3 (a)-(e) represent NaYF4:Nd3+, Yb3+, Er3+ nanocrystals and micro-plates, respectively; the pink sheets in Figure 3(f) represents convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals.

The reaction temperature of co-precipitation method is an influence important factor for the NaYF4:Nd3+, Yb3+, Er3+ preparation. The NaYF4:Nd3+, Yb3+, Er3+ was


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prepared under the various reaction temperatures. Figure 4 shows the XRD pattern of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 275, 290 and 320 ℃. It is noted that when the reaction temperature is the 275 ℃ , the XRD diffraction peaks of NaYF4:Nd3+, Yb3+, Er3+ products are matched well with the standard cubic NaYF4. As the temperature increased to 290 ℃, the XRD diffraction peaks of hexagonal and cubic NaYF4:Nd3+, Yb3+, Er3+ phase in the XRD pattern were observed, which indicated that the mixed NaYF4:Nd3+, Yb3+, Er3+ phases of cubic and hexagonal co-exist in the as-prepared products. Further increasing temperature to 305 (as shown in the Figure 1(a)) and 320 ℃, all the diffraction peaks are consistent with pure hexagonal phase of NaYF4, indicated that the hexagonal NaYF4:Nd3+, Yb3+, Er3+ phase was prepared. The TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the various temperatures were measured, as shown in the Figure 4. It can be seen from Figure 4(b) that the NaYF4:Nd3+, Yb3+, Er3+ products prepared under the 275 ℃ consists of a large number of uneven nanoparticles with an diameter of 10 nm. When the preparation temperature was 290 ℃, the product morphology is composed of a lot of nanoparticles with a diameter of 17 nm and a few convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with a diameter of 615 nm. As the reaction temperature increases to the 305 ℃, the nanoparticles structure completely disappears and the convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with uniform size were obtained, as showed in Figure 1(f). The average diameter of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was about the 719 nm. With the further increasing of reaction temperature to 320 ℃ , the

660 nm convex-lens like

NaYF4:Nd3+, Yb3+, Er3+ microcrystals with smoother surface were observed. The formation mechanisms of convex-lens like NaYF4 microcrystals prepared at



the various temperatures is similar with that of samples prepared at different reaction time. Both dissolution–nucleation and regrowth are responsible for the convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals prepared at the various temperatures. The small NaYF4:Nd3+, Yb3+, Er3+ nanoparticles in the cubic phase were formed at the low reaction temperature (275℃), as exhibited in the TEM image and the XRD pattern of Figure 4(a). With further increasing reaction temperature to 305℃, only hexagonal convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals were observed. However, the regrowth of crystals is not complete, and few smaller bulges were observed on the surface of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals, which resulted in that the surface of micro convex-lens was not smooth, as exhibited in the TEM image of Figure 1(f). With further increasing reaction temperature to 320℃, the hexagonal convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with smooth surface was

Figure 4 the XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ crystals prepared by the different reaction temperature; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 275℃ (b), 290℃ (c), 320℃ (d); the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at 305℃ was shown in the Figure 1.


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observed, which suggested that the regrowth process is very complete. In addition, some stripes were observed on the surface of hexagonal convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The distribution of nanocrystals is not uniform, and some nanocrystals were aggregated at the interface of hexagonal convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. When they were dissolved and re-grown, resulting in the formation of hexagonal convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals with some stripes, as shown in Figure S1. Some nanoparticles and stripes without complete dissolution and growth were observed at the interface of hexagonal micro convex-lens, as exhibited in the TEM image of Figure 4(c), supporting the mechanisms of growth of micro convex-lens and formation of stripes.

3.2 Influence of ratio of oleic acid to 1-octadecene on morphology of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals Oleic acid as surfactant has an influence on the morphology of NaYF4:Nd3+, Yb3+, Er3+ products. The convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals were prepared with different the ratio of oleic acid (OA) to 1-octadecene (ODE), as exhibited in the table I. The XRD patterns of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals prepared by the 3/7 and 5/5 ratio of OA to ODE were is showed in the Figure 5(a). Based on the Figure 1 (XRD pattern of sample prepared by the 4/6 ratio of OA to ODE) and Figure 5, all the diffraction peaks of the three samples are corresponding to the pure hexagonal NaYF4, exhibiting high purity and good crystallinity. The quite intense (002) diffraction peak was observed in the all samples, and the (002) diffraction peak intensity was decreased while increasing the volume of OA. Figure 5(b) and (c) exhibited the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared at the different ratio of OA to ODE, and the corresponding FESEM images were given in the inset of Figure 5(b) and (c), respectively. It is noted from the



TEM and FESEM images that the NaYF4:Nd3+, Yb3+, Er3+ products are made up of the hexagonal convex-lens like microcrystals with the thickness center and thin edge. Based on the TEM and FESEM images, the diameter of convex-lens like microcrystals were measured, as listed in the table I. When the ratio of OA to ODE is the 3/7, 4/6 and 5/5, the average diameter from corner to corner is about 1216, 719 and 516 nm, respectively. It is concluded that the ratio of OA to ODE influences greatly the size of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The diameter of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals was decreased with the increasing of the ratio of OA to ODE. For the NaYF4:Nd3+, Yb3+, Er3+ products prepared by the present hydrothermal method, the crystal growth and selfassembly co-exist[33]. For the crystal growth process, a large number of OA provided the fewer opportunities than small number of OA for the adhesion of monomers on the surfaces of nucleation seeds. Thus the OA as a capping ligand limit the growth of NaYF4 nanoparticles, resulting in the formation of small convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals. The previous investigations demonstrated that the OA as a capping ligand suppressed the growth of NaYF4 nanoparticles, which is consistent with the present results [34]. Additionally, the self-assemble of NaYF4 nanocrystals was influenced by the organic chelating agent such as Na3Cit [33]. The OA has the cross-linking and crystal growth depressant role as the Na3Cit. The increasing of OA content can lower the surface energy of the NaYF4 nanocrystals, limiting their self-assemble. Thus the size of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals reduces with the increasing of OA content.


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Figure 5 the XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ products prepared by the various ratio of oleic acid to 1-octadecene; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared by different ratio of oleic acid to 1-octadecene: (b) 3/7and (c) 5/5; the insert is the corresponding FESEM images of NaYF4:Nd3+, Yb3+, Er3+ products.

3.3 Influence of NaOH concentration on morphology of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals To investigate the effect of NaOH concentration on the crystal structures and morphologies of NaYF4:Nd3+, Yb3+, Er3+ micro convex-lens, the NaOH with the 3.75, 5 and 6.25 mmol NaOH were used to synthesize the NaYF4:Nd3+, Yb3+, Er3+ micro convex-lens while maintaining other experimental conditions at fixed values. Figure 6(a) shows the XRD patterns of NaYF4:Nd3+, Yb3+, Er3+ products prepared by the 3.75 and 6.25 mmol NaOH. Based on the Figure 1 (NaYF4:Nd3+, Yb3+, Er3+ products prepared by the 5 mmol NaOH) and Figure 6, the XRD patterns of NaYF4:Nd3+, Yb3+, Er3+ products prepared by the 3.75 5 and 6.25 mmol NaOH are matched well with standard hexagonal NaYF4. The relative intense (002) diffraction peak was observed in the all samples, which suggested that the NaYF4:Nd3+, Yb3+, Er3+ products were grown preferentially along the (002) planes. The TEM images and FESEM images of NaYF4:Nd3+, Yb3+, Er3+ products prepared by the 3.75 and 6.25 mmol NaOH are exhibited in Figure 6(b) and (c), respectively. From the TEM and FESEM images of Figure 1 (NaYF4:Nd3+, Yb3+, Er3+ products prepared by the 5 mmol NaOH) and 6, the morphology of products prepared by the 3.75, 5 and 6.25 mmol NaOH possess the hexagon convex-lens. The average diameter of convex-lens like NaYF4:Nd3+, Yb3+,



Er3+ microcrystals were measured from TEM and FESEM images, as shown in the Table I. The micro convex-lens has an average diameter of about 730 nm. The XRD and the TEM demonstrated that the NaOH concentration has slight effect on the morphology and size of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals.

Figure 6 the XRD patterns (a) of NaYF4:Nd3+, Yb3+, Er3+ micro convex-lens prepared by the different NaOH concentration; the TEM images of NaYF4:Nd3+, Yb3+, Er3+ micro convex-lens prepared by the 3.75 (b) and 6.25 mmol (c) NaOH concentration; the insert in (b) and (c) is the corresponding FESEM images of NaYF4:Nd3+, Yb3+, Er3+ micro convex-lens.

3.4 Influence of types of doping ions on the morphology of convex-lens like NaYF4 microcrystals The Nd3+, Er3+ and Yb3+ single doped NaYF4 and Nd3+/Yb3+, Nd3+/Er3+ and Yb3+/Er3+ co-doped NaYF4 products were prepared with the corresponding preparation conditions as listed in the Table I. Figure 7(a) exhibited the XRD patterns of NaYF4 products doped with the different kinds of rare earth ions. All the diffraction peaks correspond to the standard hexagonal NaYF4 structure and there are no other impure peaks observed, which implied that the different types of doping ions have no influence on the NaYF4 crystal structure. Figure 7(b)-(g) illustrate the TEM images of NaYF4 doped with the various kinks of rare earth ions. It is obvious that all samples possess the similar morphologies of hexagonal convex-lens like NaYF4 microcrystals regarding of the types of doping ions. However, the types of doping ions have an influence on the size of convex-lens like NaYF4 microcrystals. The diameter of convex-lens like NaYF4 microcrystals calculated by the TEM and


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FESEM images were listed in the table I. It is clearly that the different types of doping ions in the NaYF4 products change the size of micro convex-lens.

Figure 7. the XRD patterns of NaYF4 products doped with the different rare earth ions (a); the TEM images of NaYF4 products doped with Nd3+(b), Er3+(c) , Yb3+(d), Nd3+/Yb3+(e) , Nd3+/Er3+(f), Yb3+/Er3+(g).

3.5 Upconversion photoluminescence property of NaYF4:Nd3+, Yb3+, Er3+ with various morphologies excited at the 808 nm The dependence of morphology and structure of NaYF4:Nd3+, Yb3+, Er3+ products on the UC luminescence property was investigated excited at the 808 nm, as shown in the Figure 8. It is noted that the nanoparticles, micro plates and convex-lens like NaYF4:Nd3+, Yb3+, Er3+ products were prepared at the reaction time of 10, 50 and 80 min, respectively, exhibiting the typical UC emissions from the Er3+. The 523 and 548 nm green UC emissions were attributed to the 2H11/2→2I15/2 and 4S3/2→2I15/2 transitions of Er3+, respectively, and the 660 nm red UC emission was from



4

F9/2→4I15/2 transition in the all samples. It can be seen from the UC luminescence

photos of samples shown in the inset of Figure 8 that the UC emission of the convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals is more intense than that of the plate and nanoparticle like NaYF4:Nd3+, Yb3+, Er3+ products. It is well known that the size of NaYF4 products has an important influence on its UC emission intensity, and the UC emission intensity decreased with the decreasing of the size. As shown in the table I, the size of convex-lens likes NaYF4:Nd3+, Yb3+, Er3+ microcrystals is larger than that of the plate and nanoparticle like NaYF4:Nd3+, Yb3+, Er3+ products, resulting in its more intensive UC emission [35-37], as shown in the Figure 8. In addition, it was reported that the lens like materials exhibited the optical focusing property at the infrared wavelengths [29, 30], which may cause the focusing of 808 nm infrared excitation around the convex-lens likes NaYF4:Nd3+, Yb3+, Er3+ microcrystals, as shown in Figure 2. Therefore, the more intense UC emission for convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals may be attributed to the enhancement of excitation fields.

Figure 8 the UC emission spectra of the nanoparticles, micro plates and convex-lens like NaYF4:Nd3+, Yb3+, Er3+ products upon the 808 nm excitation; the insert is the UC emission photos of samples.


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3.6

Upconversion and near infrared photoluminescence property and

mechanisms of convex-lens like NaYF4:Nd3+, Yb3+, Er3+ microcrystals doped with the various rare earth ions excited at the 808 nm The visible UC emission spectra of convex-lens like NaYF4 microcrystals doped with the various rare earth ions were measured under the excitation of 808 nm. No visible UC emissions was observed for the convex-lens like NaYF4 microcrystals doped with the Nd3+,Yb3+ or Nd3+/Yb3+ (the Figure was not exhibited). But the convex-lens like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+ or Nd3+/Yb3+/Er3+ present the visible UC emission from Er3+ ions, as shown in the Figure 9(a). It can be seen from the UC emission photos of samples shown in the inset of Figure 9(a) that the convex-lens like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+ or Nd3+/Yb3+/Er3+ exhibited the various UC emission behaviors. The convex-lens like NaYF4 microcrystals doped with the Er3+ or Yb3+/Er3+ have the green UC emission color, exhibiting the 523 and 548 nm UC emissions of Er3+. No obvious 660 nm red UC emission peak was observed. For the convex-lens like NaYF4 microcrystals doped with the Nd3+/Er3+, the intense 660 nm red UC emission peak and weak 523 and 548 nm green UC emission peaks of Er3+ were observed, and the UC emission color of sample is red, as shown in the inset of Figure 9(a). For the convex-lens like NaYF4 microcrystals tri-doped with the Nd3+/Yb3+/Er3+, the 660 nm red UC emission and 523 and 548 nm green UC emission is very intense in contrast to NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+ and Nd3+/Er3+, exhibiting the yellow UC emission color. The convex-lens like NaYF4 microcrystals doped with the various rare earth ions exhibited the various UC emission behaviors, which may be caused by the various UC mechanisms. The UC emission mechanisms can be inferred by dependence of pump



power (P) of excitation light on the UC intensity (I) of NaYF4 microcrystals. The relationship between the I and P could be expressed as the logI=nlogP, where n is the number of absorbed photons required for emitting one UC visible photon. Based on the logI=nlogP equation, a straight line with slope n could be obtained. The slopes of 524, 545 and 660 nm UC emissions are shown in the Figure S2 of supporting information, which indicated that both green and red UC emissions were from the two-photon process. The UC emission mechanism of convex-lens like NaYF4 microcrystals doped with the Er3+, Yb3+/Er3+, Nd3+/Er3+ or Nd3+/Yb3+/Er3+ was presented in the Figure 9. For the Er3+ ions single-doped NaYF4 convex-lens, the successive transition from the 4I15/2 to 4I9/2 to 2H9/2 state of the Er3+ ions take place under the excitation of 808 nm. The Er3+ ions in the 2H9/2 state relaxed to the 2H11/2 and 4S3/2 states. The transition from the 2H11/2/4S3/2 states to the ground state leads to the green UC emission, as shown in the Figure 9(c). For the Yb3+/Er3+ co-doped NaYF4 micro convex-lens, the Yb3+ ions cannot absorb the 808 excitation light, the UC emission mechanism is similar with that of the Er3+ ions single-doped NaYF4 micro convex-lens. For the Nd3+/Er3+ co-doped NaYF4 micro convex-lens, the red UC emission was observed beside the green UC emissions in contrast to the the Yb3+/Er3+ co-doped and Er3+ single-doped NaYF4 micro convex-lens, which suggested that the Nd3+ doping has an influence on the UC mechanism of Nd3+/Er3+ co-doped NaYF4 micro convex-lens. The UC mechanism of Nd3+/Er3+ co-doped NaYF4 micro convex-lens was proposed in the Figure 9(d). The Nd3+ ion has the larger absorption cross section at the 808 nm than the Er3+ ion. The Nd3+ ions at the excited 4F3/2 state transfer their part energy to the adjacent Er3+ ions, causing the transition from the ground state to the 4I11/2 excited state of Er3+ ions. The energy transfer processes from the Nd3+ at the excited 4F3/2 state to the Er3+ ions at 4I11/2 excited state populated the


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4

F7/2 state of the Er3+ ions. The 2H11/2 and 4S3/2 states of the Er3+ ions were populated

by the non-radiative relaxation from the 4F7/2 state of the Er3+ ions, resulting in the green UC emission. In addition, the population of 4I13/2 excited state of Er3+ ions was obtained by the non-radiative relaxation from the 4I11/2 state of the Er3+ ions. The 4F9/2 state of the Er3+ ions.was populated by the energy transfer from the Nd3+ at the excited 4F3/2 state to the Er3+ ions at 4I13/2, resulting in the red UC emission. As shown in the Figure 9(a), the UC emission of Nd3+/Yb3+/Er3+ tri-doped NaYF4 micro convex-lens was enhanced significantly in contrast to that of Nd3+/ Er3+ doped NaYF4 micro convex-lens, which suggested that the Yb3+ ions plays an important role in the enhancement of UC emission of Nd3+/Yb3+/Er3+ tri-doped NaYF4 micro convex-lens. The UC emission of Nd3+/Yb3+/Er3+ doped NaYF4 micro convex-lens is from the energy transfer mechanisms of Nd3+→Yb3+→Er3+, as shown in the Figure 9(e). The NIR spectra of convex-lens like NaYF4 microcrystals doped with the various rare earth ions were measured under the excitation of 808 nm, as shown in the Figure 9(b). The 880, 1060 and 1336 nm NIR emission emissions of Nd3+ were observed in the Nd3+ single-doped or Nd3+/Er3+ co-doped convex-lens like NaYF4 microcrystals, which are attributed to the 4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions of Nd3+, respectively. For the Nd3+/Yb3+, Yb3+/Er3+ and Yb3+/Nd3+/Er3+ co-doped convex-lens like NaYF4 microcrystals, the 978 nm NIR emission was assigned to the transition from 2F7/2 to 2F5/2 of Yb3+ ions, as shown in the inset of Figure 9(b). No near infrared emission was observed in the Er3+ or Yb3+ single-doped NaYF4 micro convex-lens (the Figure was not exhibited). The weak 978 nm near infrared emission of Yb3+ was observed in the Yb3+/Er3+ co-doped NaYF4 micro convex-lens. The result suggested that the energy transfer from the Er3+ ions to the Yb3+ ions occurred under the excitation of 808 nm, resulting in the weak 978 nm near infrared emission of Yb3+.



The energy transfer mechanism from the Er3+ ions to the Yb3+ ions was shown in Figure 9(c). The non-radiative relaxation from the 4I9/2 populated the 4I11/2 state of the Er3+ ions. The energy transfer from the Er3+ to the Yb3+ take place by the 4I11/2 (Er3+)+ 2

F7/2 (Yb3+)→4I15/2 (Er3+)+ 2F5/2 (Yb3+) process, populating the 2F5/2 state of Yb3+.

Thus the weak 978 nm near infrared emission of Yb3+ was observed due to the transition from the 2F5/2 to the 2F7/2.

Figure 9 the UC (a) and NIR (b) emission spectra of NaYF4 samples with different doping ions upon the 808 nm excitation; the insert in (a) and (b) is the UC emission photos of samples and enlargement image of NIR emission, respectively; the UC and NIR emission mechanism of NaYF4 doped with Er3+(c), Yb3+/Er3+(c), Nd3+/Er3+(d) and Nd3+/Yb3+/Er3+(e).

Conclusions In the work, the influence of reaction time, reaction temperature, NaOH concentration, ratio of oleic acid to 1-octadecene and types of doping ions on the morphology, size and phase structure of NaYF4 micrometer convex lens were systematically studied. Reaction temperature and times have an influence on the morphology of NaYF4 products. Increasing reaction temperature and time, cubic phase NaYF4 nanoparticles


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evolved into hexagonal phase NaYF4 micrometer convex lens. Besides the changing of size, the morphology of NaYF4 micrometer convex lens is independent of the NaOH concentration, ratio of oleic acid to 1-octadecene and types of doping ions. Possible growth mechanism of NaYF4 micrometer convex lens is proposed based on reaction time and temperature dependent morphology evolution. Doping-ions dependent near infrared and upconversion luminescence properties of NaYF4 micrometer convex lens were investigated excited at the 808 nm. Excited at the 808 nm, visible upconversion luminescence was observed in the Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped NaYF4 micrometer convex, and near infrared luminescence was obtained in the Nd3+, Nd3+/Er3+, Yb3+/Er3+, Nd3+/Yb3+, Nd3+/Yb3+/Er3+ doped NaYF4 micrometer convex lens. The upconversion luminescence mechanism of Er3+, Yb3+/Er3+, Nd3+/Er3+, Nd3+/Yb3+/Er3+ doped NaYF4 micrometer convex lens was discussed based on the energy level scheme, exhibiting various upconversion luminescence mechanisms. Acknowledgments：： This work was supported by the National Natural Science Foundation of China (51762029, 11674137), and the Applied basic research key program of Yunnan Province. References [1]

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The morphology of NaYF4 was changed from the nanoparticles to micro plates then to micro convex-lens. A possible growth mechanism of NaYF4 micro convex-lens is proposed based on the experimental results.


Preparation, growth mechanism and upconversion and near infrared

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