# Upconversion Luminescence Properties of Y2O3:Yb, Er and Y2O2S:Yb, Er Nanoparticles Prepared by Complex Precipitation

Authors: Ying Tian; Yao Fu; Mingming Xing; Xixian Luo
Journal: Journal of Nanomaterials
 (2015)

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## Abstract

The Yb3+, Er3+ doped Y2O3 and Y2O2S upconversion nanophosphors were prepared by the direct complex precipitation method with the mixed solution of NH4HCO3 and NH3·H2O as the complex precipitant. The precipitate of ReOHxCO3y calcined at 900°C in air presents the pure Y2O3 with cubic structure, and the calculated crystalline size is about 26 nm, while the Y2O2S:Yb, Er nanocrystals were obtained by annealing the same precipitate at 900°C but in the atmosphere of N2 gas containing sulfur vapor. The obtained sample presents the pure hexagonal structure of Y2O2S with calculated crystalline size of 29 nm. According to the transmission electronic microscopy (TEM), the nanophosphors exhibit uniform quasispherical shape and size about 30 nm. By using the 980 nm excitation laser, the properties of upconversion luminescence and energy transfer processes were studied in detail for the different concentration of Yb3+ in Er3+ doped Y2O3 as well as the Yb3+, Er3+ codoped Y2O2S nanocrystals. The high-efficient red and yellow upconversion emissions were both observed by naked eyes in day time corresponding to the Y2O3:Yb, Er and Y2O2S:Yb, Er phosphors, respectively. Thus the upconversion nanoparticles combining its high efficient emission would pave the way for ideal fluorescence probes in biological applications.


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## Body

## 1. Introduction

An upconversion process is one that takes multiple photons of lower energy and converts them to one photon of higher energy. There has been considerable research on the upconversion phosphor since it was found by Professor Auzel [1] for the first time and put forward being used in infrared detection and short-wave laser. More recently, people have found that if the upconversion phosphors can realize the nanocrystalline, combining its unique characteristics of luminescence, such as lack of background phosphorescence, no photobleaching during assay, and narrow emission bands [2, 3], it will have enormous potential applications in the areas of biomedical diagnosis [4], the anticounterfeiter [5], and display screen and X-ray imaging [6]. In particular, the Y2O3 and Y2O2S materials have attracted much interest as host materials for its excellent chemical stability, insoluble in water, high melting point, and low phonon energy [7–11]. Therefore, the high luminescence efficiency can be achieved in those systems via suitable selection of rare earth doping ions and excitation routes.Many synthesis techniques of rare earth oxide and oxysulfide nanomaterials have been studied, such as precipitation method [12, 13], combustion synthesis [14], and emulsion liquid membrane system [15]. Among them, the precipitation method has become a promising chemical preparation rout owing to its advantages of simple process without complex equipment, easy doping, and low production cost. In particular, the homogeneous precipitation method has drawn a great attention in the fields of nanomaterial synthesis. Because the precipitants sedimentate out homogeneously and slowly in the solution, which accordingly conduce to the homogeneous formation of precipitates. And the obtained nanoparticles are uniform and fine in size [13, 16]. The urea (CO(NH2)2) as the precipitant has been most commonly used in the homogeneous precipitation method. However, the hydrolysis of urea is very slow process and need certain temperature. Moreover, the low concentration of urea is a necessity in order to obtain uniform nanoparticles; thus the big amount of reaction solution causes lots of difficulties in the posterior processes, such as filtration.In the current work, the nanocrystalline upconversion phosphors were prepared by the direct complex precipitation method using the ammonium water and ammonium bicarbonate as the complex precipitants. This novel method makes use of the principal of homogeneous precipitation method but successfully overcomes the disadvantages of the urea homogeneous precipitation route. The Yb3+, Er3+ codoped Y2O3 and Y2O2S upconversion nanophosphors are obtained with homogeneous morphology and meanwhile exhibit high efficient upconversion luminescence. The luminescence properties and mechanisms are discussed in detail for different host materials and doping concentrations.


## 2. Materials and Methods

### 2.1. Experiment Principles

In the complex precipitant solution of NH4HCO3 and NH3·H2O, the equilibrium balances in the solution are shown as follows:(1)NH4HCO3⟷NH4++HCO3-(2)HCO3-⟷H++CO32-K1=4.8×10-11(3)NH3·H2O⟷NH4++OH-K2=1.79×10-5(4)H2O⟷H++OH-K3=1×10-14When the rare earth ions join in the reaction,(5)RE3++yCO32-+xOH-+nH2O⟶REOHxCO3y·H2O↓HereK1, K2, and K3 are the equilibrium constants of the three relevant reactions, respectively. Since the ammonium bicarbonate is strong electrolyte, the first reaction is a complete reaction. While the ammonium water and HCO3- both possess the properties of weak electrolytes, so that the quantity of the OH− and CO32- ions is little in the solution. When the complex precipitant solution of NH4HCO3 and NH3·H2O was dropped into the mixed solution of Y3+, Yb3+, and Er3+ ions, in the small area around the complex precipitants, the precipitants ionize out OH− and CO32- ions slowly according to the reversible reactions listed above. Then there is always little change of the OH− and CO32- concentrations within the small area, and the reversible reaction goes towards the positive direction with the formation of the precipitates. Namely, the concentration of the OH− and CO32- within the small area is correlative with the formation of precipitates. Therefore, the precipitation process is a local homogeneous process. When introducing the rare earth ions, the precipitates of carbonate (ReOHxCO3y, where the Re represents Y, Yb, and Er) were formed by reacting with the OH−, CO32- ions simultaneously.


### 2.2. Materials Preparation

The certain amount of NH4HCO3 powders was dissolved into the NH3·H2O solution and then stirred vigorously to obtain the complex precipitant solution of 70 mL, and concentrations of the NH4HCO3 and NH3·H2O were 3 and 1 mol/L, respectively. The nitrate solutions of rare earth (Y2O3, Yb2O3, and Er2O3 powers (99.99%)) were weighed on the stoichiometry and mixed thoroughly, where the doped concentrations of the activator Er3+ were 0.8 mol%, and concentrations of the sensitizer Yb3+ were changed from 0 to 11 mol%. Then the complex precipitant solution was slowly dropped into the nitrate solution of rare earth that was vigorously stirred. The mixed solution was continuously stirred for 30 mins and then stayed for 2 hs. The resultant white precipitates of ReOHxCO3y were centrifuged and washed by water, ethanol, and acetone, respectively. The ReOHxCO3y precursor was finally dried at 60°C.Y2O3:Yb, Er nanophosphors were obtained by annealing the ReOHxCO3y precursor in air at 900°C for 1 h. While Y2O2S:Yb, Er nanophosphor was obtained by calcining the same ReOHxCO3y sample at 900°C for 1 h in the atmosphere of N2 gas containing sulphur vapour, the sulphur vapour was generated by heating a sulfur powder at 400°C and then flowed into the tube by the N2 gas.


### 2.3. Characterizations

X-ray diffraction (XRD) patterns of the samples were recorded on Hitachi DMAX-3A diffractometer equipped withCoKα (λ=0.15406 nm) radiation. The morphology of the sample was characterized by TEM (Tecnai G2 20). The Hitachi F-4500 fluorescence spectrophotometer was utilized to measure the upconversion luminescence spectra. The excitation source was 980 nm laser diode (LD). The power of the LD was measured by the laser power meter (LPE-1 type).


## 2.1. Experiment Principles

In the complex precipitant solution of NH4HCO3 and NH3·H2O, the equilibrium balances in the solution are shown as follows:(1)NH4HCO3⟷NH4++HCO3-(2)HCO3-⟷H++CO32-K1=4.8×10-11(3)NH3·H2O⟷NH4++OH-K2=1.79×10-5(4)H2O⟷H++OH-K3=1×10-14When the rare earth ions join in the reaction,(5)RE3++yCO32-+xOH-+nH2O⟶REOHxCO3y·H2O↓HereK1, K2, and K3 are the equilibrium constants of the three relevant reactions, respectively. Since the ammonium bicarbonate is strong electrolyte, the first reaction is a complete reaction. While the ammonium water and HCO3- both possess the properties of weak electrolytes, so that the quantity of the OH− and CO32- ions is little in the solution. When the complex precipitant solution of NH4HCO3 and NH3·H2O was dropped into the mixed solution of Y3+, Yb3+, and Er3+ ions, in the small area around the complex precipitants, the precipitants ionize out OH− and CO32- ions slowly according to the reversible reactions listed above. Then there is always little change of the OH− and CO32- concentrations within the small area, and the reversible reaction goes towards the positive direction with the formation of the precipitates. Namely, the concentration of the OH− and CO32- within the small area is correlative with the formation of precipitates. Therefore, the precipitation process is a local homogeneous process. When introducing the rare earth ions, the precipitates of carbonate (ReOHxCO3y, where the Re represents Y, Yb, and Er) were formed by reacting with the OH−, CO32- ions simultaneously.


## 2.2. Materials Preparation

The certain amount of NH4HCO3 powders was dissolved into the NH3·H2O solution and then stirred vigorously to obtain the complex precipitant solution of 70 mL, and concentrations of the NH4HCO3 and NH3·H2O were 3 and 1 mol/L, respectively. The nitrate solutions of rare earth (Y2O3, Yb2O3, and Er2O3 powers (99.99%)) were weighed on the stoichiometry and mixed thoroughly, where the doped concentrations of the activator Er3+ were 0.8 mol%, and concentrations of the sensitizer Yb3+ were changed from 0 to 11 mol%. Then the complex precipitant solution was slowly dropped into the nitrate solution of rare earth that was vigorously stirred. The mixed solution was continuously stirred for 30 mins and then stayed for 2 hs. The resultant white precipitates of ReOHxCO3y were centrifuged and washed by water, ethanol, and acetone, respectively. The ReOHxCO3y precursor was finally dried at 60°C.Y2O3:Yb, Er nanophosphors were obtained by annealing the ReOHxCO3y precursor in air at 900°C for 1 h. While Y2O2S:Yb, Er nanophosphor was obtained by calcining the same ReOHxCO3y sample at 900°C for 1 h in the atmosphere of N2 gas containing sulphur vapour, the sulphur vapour was generated by heating a sulfur powder at 400°C and then flowed into the tube by the N2 gas.


## 2.3. Characterizations

X-ray diffraction (XRD) patterns of the samples were recorded on Hitachi DMAX-3A diffractometer equipped withCoKα (λ=0.15406 nm) radiation. The morphology of the sample was characterized by TEM (Tecnai G2 20). The Hitachi F-4500 fluorescence spectrophotometer was utilized to measure the upconversion luminescence spectra. The excitation source was 980 nm laser diode (LD). The power of the LD was measured by the laser power meter (LPE-1 type).


## 3. Results and Discussions

### 3.1. Structure and Morphology

Figure1 shows the XRD patterns of the samples that were calcined at 900°C in air and in N2 gas containing sulfur vapor, respectively. The doping concentrations of Yb3+ and Er3+ ions of the two samples were both 8 and 0.8%. The results show that the sample calcined in air presents the pure Y2O3 phases with cubic structure, while the sample calcined in N2 gas containing sulfur vapor exhibits the hexagonal structure of Y2O2S. Both of the patterns present pure Y2O3 and Y2O2S crystal structures without other phase. This result implies that the ions of Yb3+ and Er3+ have no influence on the crystal structure. And the as-prepared ReOHxCO3y powder obtained by the complex precipitation can be sulfurized thoroughly using the simple method of flowing N2 gas containing the sulfur vapor. According to the Scherrer formula [17], D=Kλ/βcos⁡θ, the calculated crystallite size of theY2O3:Yb, Er and Y2O2S:Yb, Er particles was 26 and 29 nm, respectively.Figure 1
XRD patterns of (a) Y2O3:Yb, Er and (b) Y2O2S:Yb, Er nanocrystals calcined at 900°C (∗ and ⚪ denote the characteristic peaks corresponding to Y2O3 and Y2O2S, resp.).For the observation of nanoparticles morphology, we loaded the Y2O2S:Yb, Er sample dispersed with ethanol on a TEM grid mesh. As shown in the TEM image (Figure 2), the obtained particles are in the quasispherical shape. Although some nanoparticles aggregate and form large particles, there are many well-dispersed nanoparticles around the agglomerates. Thus, it is feasible to observe the homogeneity of the particles as well as to evaluate the mean particle size. The assessed mean particle size from TEM image is about 30 nm, which exhibits a fairly good agreement with the calculated crystalline particle size on the basis of XRD measurement. In addition, the uniformity of the nanoparticles in both shape and size is achieved as shown in Figure 2, thereby indicating the promising advantages of the simple complex precipitation method.Figure 2
TEM image of the Y2O2S:Yb, Er phosphor annealed at 900°C.


### 3.2. Upconversion Luminescence Properties

Despite the ground bulk upconversion phosphors exhibit high luminescent efficiency, the big size and uneven morphology prevent their utilizations in the fields of biological applications as well as high-resolution displays. As suggested in the TEM measurements, the upconversion phosphors synthesized by the complex precipitation method reach to the high requirements of nanosize and homogeneity in both shape and size distribution. These features are ideal for the biological applications. Meanwhile, the very bright upconversion luminescence of the nanophosphors is observed by naked eyes when excited by LD with power as low as 5 mW. Figure3 presents the upconversion emission spectra of the Er3+ (0.8 mol%), Yb3+ (0–11 mol%) codoped Y2O3 nanophorsphors by the excitation of 980 nm LD. The green and red emission bands at about 563 and 660 nm are assigned to the 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 intra-4f transitions of Er3+, respectively. Upconversion luminescence is generally composed of ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), multiphonon relaxation, cross relaxation, and radiative transition [18]. Figure 3   (i) shows the upconversion luminescence spectrum of Y2O3:Er3+ (Er: 0.8 mol%) sample. The excitation wavelength matches the absorption transition between the ground state 4I15/2 and excited level 4I11/2. After the first-level excitation, two mechanisms are considered as follows [19]. One route is the simple ESA, where the same energy photons are absorbed by the excited state Er3+ ion from the 4I11/2 to the 4F7/2 level. The other route is ETU (energy transfer upconversion), where a mutual interaction between two excited Er3+ (4I11/2) may occur, with the results of one ion deexcited to 4I15/2 while the other ion is excited to 4F7/2 level. The above two upconversion mechanisms can both populate the 4F7/2 state which then decays nonradiatively to the 2H11/2/4S3/2 and 4F9/2 levels, subsequently emitting green and red luminescence when returning to the ground state 4I15/2, respectively. Due to the low doping concentration of Er3+ ion in the Y2O3:Er3+ sample, it was considered that the probability of occurring the first ESA mechanism is much more than the ETU mechanism between two Er3+ ions.Figure 3
Upconversion spectra of the Er3+ (0.8 mol%), Yb3+ (0–11 mol%) codoped Y2O3 samples calcined at 900°C when excited by 980 nm LD.Figure3 (ii)–(v) show the upconversion spectra of different concentration of Yb3+ in Er3+ doped Y2O3 nanocrystals under 980 nm LD excitation. It can be seen from the spectra that the relative intensity ratio Ir/Ig (Ir and Ig represent the red and green emission, resp.) rapidly increases with increasing the Yb3+ concentration, where the Yb3+ ions obviously play an important role. Since the Yb3+ ion has a much higher absorption cross-section than that of Er3+ ion at 980 nm [20], the roles that the Er3+ ion plays in the population of the excitation levels decrease with the increasing of the Yb3+ concentration.As shown in the energy level diagram (Figure4), the upconversion green and red emission mechanisms of the Y2O3:Yb, Er samples are analyzed as follows.Figure 4
Schematic energy level diagram of Yb3+ and Er3+ ions under excitation of 980 nm LD.The upconversion channel of green luminescence (1) is as follows:(1)
Yb3+F5/22+Er3+I15/24→Yb3+F7/22+Er3+I11/24;Er3+I11/24+Yb3+F5/22→Yb3+F7/22+Er3+F7/24~·~ErS3/24/H11/22 (~·~  denotes the process of multiphonons relaxation).The4F7/2 state of Er3+ is populated via the twice energy transfer of channel (1), after nonradiative relaxation to the 4S3/2/4H11/2 levels and then returns to the ground state emitting the upconversion green luminescence via radiation transition.The upconversion channels of red luminescence (2), (3), and (4) are as follows:(2)
after the population of 4S3/2 level by channel (1),Er(S3/24)~·~Er(F9/24) occur;(3)
Er3+F7/24+Er3+I11/24→2Er3+F9/24;(4)
Yb3+F5/22+Er3+I15/24→Yb3+F7/22+Er3+I11/24~·~Er3+I13/24;Yb3+F5/22+Er3+I13/24→Yb3+F7/22+Er3+F9/24.One can see that, three channels are involved in populating the excited state 4F9/2 of Er3+ ion, that is, multiphonons relaxation from 4S3/2 level, cross-relaxation process between Er3+ ions, and the 4I13/2 level (Er3+) reabsorbing the energy from excited state 2F5/2 (Yb3+) to further populate the 4F9/2 level via the process of ET1, as shown in Figure 4.It is well known that the increase of concentration of Yb3+ in Er3+ doped nanocrystals should greatly promote the ET from Yb3+ (2F5/2) to Er3+ (4I11/2) [21]. Thus more Er3+ will be excited to the 4I11/2 level by the ET0, and correspondingly more Er3+ will be excited to the 4F7/2 state with the increase of the population of 4I11/2 through ET2 as shown in Figure 4. According to i (see [22]),iP∝N0N1,where P is the cross-relaxation probability, N0, N1 are the population densities of the two states which are involved in the cross-relaxation process. Therefore, the cross-relaxation probability between the 4I11/2 and 4F7/2 levels increased greatly with the increase of Yb3+ concentration. This directly contributes to the population of the 4F9/2 level to a large extent. Additionally, the population increase of the 4I11/2 state will also result in the decay of Er3+ ions from 4I11/2 to 4I13/2 level; thus the process of ET1 is enhanced due to the increasing of population densities of Er3+ (4I13/2) and Yb3+ (2F5/2) ions. This also contributes to the population of 4F9/2 level. Therefore, the occurrence probabilities of channels (3) and (4) both increase with the increase of Yb3+ concentration, which consequently result in the enhancement of population of 4F9/2 level. As observed in Figure 3, the relative intensity ratio Ir/Ig increases gradually with the Yb3+ concentration increasing from 0 to 11%.The pump mechanism can be studied by the relationship between the upconversion emission intensity and pump power; that is,iiIvis∝Ipn,where Ivis is the intensity of the upconversion emission, Ip is pump power, and n is the number of pump-photons required to populate the emitting state [23]. In order to confirm the upconversion mechanism discussed above, the intensity of green (563 nm) and red (660 nm) emission is measured as a function of pump power in three samples of Y2O3:0.8% Er, Y2O3:1% Yb, 0.8% Er, and Y2O3:11% Yb, 0.8% Er, as plotted in Figure 5. For the sample of Y2O3:Er, the slope corresponding to the emission intensities of the transitions 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 is both approximately 2. This indicates that generation of these transitions is predominantly due to two-photon absorption by Er3+. For the sample of Y2O3:1% Yb, 0.8% Er, the slope values are slightly smaller than 2, which shows the upconversion mechanism is two-photon process but including some cross-relaxation and the ET processes from Yb3+ to Er3+ [23]. For the sample of Y2O3:11% Yb, 0.8% Er, the slope values are even smaller than that in the Y2O3:1% Yb, 0.8% Er sample, which implies that the upconversion mechanism must include more cross-relaxation and the ET processes from Yb3+ to Er3+. These results further confirm the upconversion mechanism stated above.Figure 5
The upconversion emission intensity as a function of pump power for the three samples: (a) Y2O3:0.8% Er, (b) Y2O3:1% Yb, 0.8% Er, and (c) Y2O3:11% Yb, 0.8% Er.
(a)
(b)
(c)Figure6 shows the upconversion spectra of Y2O3:Yb, Er and Y2O2S:Yb, Er nanocrystal samples by the 980 nm LD excitation, and the doping concentrations of Yb3+ and Er3+ in the two samples are the same about 8 and 0.8%, respectively. Although the main peaks in the spectra are both red emission arising 4F9/2→4I15/2 transition of Er3+, there are still some differences between them. The maximum of red emission peak of the Y2O3:Yb, Er sample is located at 660 nm; moreover, the green luminescence is almost quenched comparing with the intensity of red emission. When excited by LD only with the power of 5 mW, the upconversion red luminescence can be seen by naked eyes in the daytime. While in the spectrum of Y2O2S:Yb, Er sample, the maximum of red emission peak (668 nm) shifts to longer wavelength about 8nm when compared with that of yttrium oxide sample, and the green emission is enhanced to a certain extent. Similarly, under the excitation of 980 nm LD with power of 5 mW, the upconversion luminescence also can be seen in the daytime but appearing yellow to the naked eyes.Figure 6
The upconversion spectra of Er3+ (0.8 mol%), Yb3+ (8 mol%) codoped Y2O2S and Y2O3 samples under the 980 nm excitation.The phosphors of Y2O3 and Y2O2S come from the same precursor ReOHxCO3y sample, and the heat treatment, doping concentration, and measure conditions are all the same, so it is believed that the obvious differences between the two spectra must be related to the different host. As the intrinsic phonon energies of Y2O3 and Y2O2S are 597 and 520 cm−1, respectively [24, 25], Y2O3 obviously possesses much higher phonon energy. So the probability of phonon-assisted nonradiative relaxation of 4S3/2→4F9/2 and 4I11/2→4I13/2 is much larger in Y2O3 host, which lead to the more effectively bypass or quench the green-light emitting states, as shown in Figure 6.As compared with the Y2O3:Yb, Er samples, in the spectrum of Y2O2S:Yb, Er nanocrystals, the red shift of the main peak may be caused by the nephelauxetic effect in the system composed by the sulphured compounds [16]. Since the Er3+ ion is surrounded by the sulfur in the oxysulfide, this increases the covalent interaction in the system and shifts the transitions to smaller energies compared with the yttrium oxide. Pires has observed similar phenomenon in the Eu3+ doped oxysulfide system, that is, the emission peak arising from the 5D0→7F2 transition of Eu3+ red shifts 14 nm in the oxysulfide compared with the oxide. The differences in the peak shape may be correlative to the crystal structure of the host crystal. When changing the crystal structure from cubic into hexagonal structure (from oxide to oxysulfide), the variation of the crystal-field surrounding the Er3+ ion may lead to the differences in the peak width and splitting mode.


## 3.1. Structure and Morphology

Figure1 shows the XRD patterns of the samples that were calcined at 900°C in air and in N2 gas containing sulfur vapor, respectively. The doping concentrations of Yb3+ and Er3+ ions of the two samples were both 8 and 0.8%. The results show that the sample calcined in air presents the pure Y2O3 phases with cubic structure, while the sample calcined in N2 gas containing sulfur vapor exhibits the hexagonal structure of Y2O2S. Both of the patterns present pure Y2O3 and Y2O2S crystal structures without other phase. This result implies that the ions of Yb3+ and Er3+ have no influence on the crystal structure. And the as-prepared ReOHxCO3y powder obtained by the complex precipitation can be sulfurized thoroughly using the simple method of flowing N2 gas containing the sulfur vapor. According to the Scherrer formula [17], D=Kλ/βcos⁡θ, the calculated crystallite size of theY2O3:Yb, Er and Y2O2S:Yb, Er particles was 26 and 29 nm, respectively.Figure 1
XRD patterns of (a) Y2O3:Yb, Er and (b) Y2O2S:Yb, Er nanocrystals calcined at 900°C (∗ and ⚪ denote the characteristic peaks corresponding to Y2O3 and Y2O2S, resp.).For the observation of nanoparticles morphology, we loaded the Y2O2S:Yb, Er sample dispersed with ethanol on a TEM grid mesh. As shown in the TEM image (Figure 2), the obtained particles are in the quasispherical shape. Although some nanoparticles aggregate and form large particles, there are many well-dispersed nanoparticles around the agglomerates. Thus, it is feasible to observe the homogeneity of the particles as well as to evaluate the mean particle size. The assessed mean particle size from TEM image is about 30 nm, which exhibits a fairly good agreement with the calculated crystalline particle size on the basis of XRD measurement. In addition, the uniformity of the nanoparticles in both shape and size is achieved as shown in Figure 2, thereby indicating the promising advantages of the simple complex precipitation method.Figure 2
TEM image of the Y2O2S:Yb, Er phosphor annealed at 900°C.


## 3.2. Upconversion Luminescence Properties

Despite the ground bulk upconversion phosphors exhibit high luminescent efficiency, the big size and uneven morphology prevent their utilizations in the fields of biological applications as well as high-resolution displays. As suggested in the TEM measurements, the upconversion phosphors synthesized by the complex precipitation method reach to the high requirements of nanosize and homogeneity in both shape and size distribution. These features are ideal for the biological applications. Meanwhile, the very bright upconversion luminescence of the nanophosphors is observed by naked eyes when excited by LD with power as low as 5 mW. Figure3 presents the upconversion emission spectra of the Er3+ (0.8 mol%), Yb3+ (0–11 mol%) codoped Y2O3 nanophorsphors by the excitation of 980 nm LD. The green and red emission bands at about 563 and 660 nm are assigned to the 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 intra-4f transitions of Er3+, respectively. Upconversion luminescence is generally composed of ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), multiphonon relaxation, cross relaxation, and radiative transition [18]. Figure 3   (i) shows the upconversion luminescence spectrum of Y2O3:Er3+ (Er: 0.8 mol%) sample. The excitation wavelength matches the absorption transition between the ground state 4I15/2 and excited level 4I11/2. After the first-level excitation, two mechanisms are considered as follows [19]. One route is the simple ESA, where the same energy photons are absorbed by the excited state Er3+ ion from the 4I11/2 to the 4F7/2 level. The other route is ETU (energy transfer upconversion), where a mutual interaction between two excited Er3+ (4I11/2) may occur, with the results of one ion deexcited to 4I15/2 while the other ion is excited to 4F7/2 level. The above two upconversion mechanisms can both populate the 4F7/2 state which then decays nonradiatively to the 2H11/2/4S3/2 and 4F9/2 levels, subsequently emitting green and red luminescence when returning to the ground state 4I15/2, respectively. Due to the low doping concentration of Er3+ ion in the Y2O3:Er3+ sample, it was considered that the probability of occurring the first ESA mechanism is much more than the ETU mechanism between two Er3+ ions.Figure 3
Upconversion spectra of the Er3+ (0.8 mol%), Yb3+ (0–11 mol%) codoped Y2O3 samples calcined at 900°C when excited by 980 nm LD.Figure3 (ii)–(v) show the upconversion spectra of different concentration of Yb3+ in Er3+ doped Y2O3 nanocrystals under 980 nm LD excitation. It can be seen from the spectra that the relative intensity ratio Ir/Ig (Ir and Ig represent the red and green emission, resp.) rapidly increases with increasing the Yb3+ concentration, where the Yb3+ ions obviously play an important role. Since the Yb3+ ion has a much higher absorption cross-section than that of Er3+ ion at 980 nm [20], the roles that the Er3+ ion plays in the population of the excitation levels decrease with the increasing of the Yb3+ concentration.As shown in the energy level diagram (Figure4), the upconversion green and red emission mechanisms of the Y2O3:Yb, Er samples are analyzed as follows.Figure 4
Schematic energy level diagram of Yb3+ and Er3+ ions under excitation of 980 nm LD.The upconversion channel of green luminescence (1) is as follows:(1)
Yb3+F5/22+Er3+I15/24→Yb3+F7/22+Er3+I11/24;Er3+I11/24+Yb3+F5/22→Yb3+F7/22+Er3+F7/24~·~ErS3/24/H11/22 (~·~  denotes the process of multiphonons relaxation).The4F7/2 state of Er3+ is populated via the twice energy transfer of channel (1), after nonradiative relaxation to the 4S3/2/4H11/2 levels and then returns to the ground state emitting the upconversion green luminescence via radiation transition.The upconversion channels of red luminescence (2), (3), and (4) are as follows:(2)
after the population of 4S3/2 level by channel (1),Er(S3/24)~·~Er(F9/24) occur;(3)
Er3+F7/24+Er3+I11/24→2Er3+F9/24;(4)
Yb3+F5/22+Er3+I15/24→Yb3+F7/22+Er3+I11/24~·~Er3+I13/24;Yb3+F5/22+Er3+I13/24→Yb3+F7/22+Er3+F9/24.One can see that, three channels are involved in populating the excited state 4F9/2 of Er3+ ion, that is, multiphonons relaxation from 4S3/2 level, cross-relaxation process between Er3+ ions, and the 4I13/2 level (Er3+) reabsorbing the energy from excited state 2F5/2 (Yb3+) to further populate the 4F9/2 level via the process of ET1, as shown in Figure 4.It is well known that the increase of concentration of Yb3+ in Er3+ doped nanocrystals should greatly promote the ET from Yb3+ (2F5/2) to Er3+ (4I11/2) [21]. Thus more Er3+ will be excited to the 4I11/2 level by the ET0, and correspondingly more Er3+ will be excited to the 4F7/2 state with the increase of the population of 4I11/2 through ET2 as shown in Figure 4. According to i (see [22]),iP∝N0N1,where P is the cross-relaxation probability, N0, N1 are the population densities of the two states which are involved in the cross-relaxation process. Therefore, the cross-relaxation probability between the 4I11/2 and 4F7/2 levels increased greatly with the increase of Yb3+ concentration. This directly contributes to the population of the 4F9/2 level to a large extent. Additionally, the population increase of the 4I11/2 state will also result in the decay of Er3+ ions from 4I11/2 to 4I13/2 level; thus the process of ET1 is enhanced due to the increasing of population densities of Er3+ (4I13/2) and Yb3+ (2F5/2) ions. This also contributes to the population of 4F9/2 level. Therefore, the occurrence probabilities of channels (3) and (4) both increase with the increase of Yb3+ concentration, which consequently result in the enhancement of population of 4F9/2 level. As observed in Figure 3, the relative intensity ratio Ir/Ig increases gradually with the Yb3+ concentration increasing from 0 to 11%.The pump mechanism can be studied by the relationship between the upconversion emission intensity and pump power; that is,iiIvis∝Ipn,where Ivis is the intensity of the upconversion emission, Ip is pump power, and n is the number of pump-photons required to populate the emitting state [23]. In order to confirm the upconversion mechanism discussed above, the intensity of green (563 nm) and red (660 nm) emission is measured as a function of pump power in three samples of Y2O3:0.8% Er, Y2O3:1% Yb, 0.8% Er, and Y2O3:11% Yb, 0.8% Er, as plotted in Figure 5. For the sample of Y2O3:Er, the slope corresponding to the emission intensities of the transitions 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 is both approximately 2. This indicates that generation of these transitions is predominantly due to two-photon absorption by Er3+. For the sample of Y2O3:1% Yb, 0.8% Er, the slope values are slightly smaller than 2, which shows the upconversion mechanism is two-photon process but including some cross-relaxation and the ET processes from Yb3+ to Er3+ [23]. For the sample of Y2O3:11% Yb, 0.8% Er, the slope values are even smaller than that in the Y2O3:1% Yb, 0.8% Er sample, which implies that the upconversion mechanism must include more cross-relaxation and the ET processes from Yb3+ to Er3+. These results further confirm the upconversion mechanism stated above.Figure 5
The upconversion emission intensity as a function of pump power for the three samples: (a) Y2O3:0.8% Er, (b) Y2O3:1% Yb, 0.8% Er, and (c) Y2O3:11% Yb, 0.8% Er.
(a)
(b)
(c)Figure6 shows the upconversion spectra of Y2O3:Yb, Er and Y2O2S:Yb, Er nanocrystal samples by the 980 nm LD excitation, and the doping concentrations of Yb3+ and Er3+ in the two samples are the same about 8 and 0.8%, respectively. Although the main peaks in the spectra are both red emission arising 4F9/2→4I15/2 transition of Er3+, there are still some differences between them. The maximum of red emission peak of the Y2O3:Yb, Er sample is located at 660 nm; moreover, the green luminescence is almost quenched comparing with the intensity of red emission. When excited by LD only with the power of 5 mW, the upconversion red luminescence can be seen by naked eyes in the daytime. While in the spectrum of Y2O2S:Yb, Er sample, the maximum of red emission peak (668 nm) shifts to longer wavelength about 8nm when compared with that of yttrium oxide sample, and the green emission is enhanced to a certain extent. Similarly, under the excitation of 980 nm LD with power of 5 mW, the upconversion luminescence also can be seen in the daytime but appearing yellow to the naked eyes.Figure 6
The upconversion spectra of Er3+ (0.8 mol%), Yb3+ (8 mol%) codoped Y2O2S and Y2O3 samples under the 980 nm excitation.The phosphors of Y2O3 and Y2O2S come from the same precursor ReOHxCO3y sample, and the heat treatment, doping concentration, and measure conditions are all the same, so it is believed that the obvious differences between the two spectra must be related to the different host. As the intrinsic phonon energies of Y2O3 and Y2O2S are 597 and 520 cm−1, respectively [24, 25], Y2O3 obviously possesses much higher phonon energy. So the probability of phonon-assisted nonradiative relaxation of 4S3/2→4F9/2 and 4I11/2→4I13/2 is much larger in Y2O3 host, which lead to the more effectively bypass or quench the green-light emitting states, as shown in Figure 6.As compared with the Y2O3:Yb, Er samples, in the spectrum of Y2O2S:Yb, Er nanocrystals, the red shift of the main peak may be caused by the nephelauxetic effect in the system composed by the sulphured compounds [16]. Since the Er3+ ion is surrounded by the sulfur in the oxysulfide, this increases the covalent interaction in the system and shifts the transitions to smaller energies compared with the yttrium oxide. Pires has observed similar phenomenon in the Eu3+ doped oxysulfide system, that is, the emission peak arising from the 5D0→7F2 transition of Eu3+ red shifts 14 nm in the oxysulfide compared with the oxide. The differences in the peak shape may be correlative to the crystal structure of the host crystal. When changing the crystal structure from cubic into hexagonal structure (from oxide to oxysulfide), the variation of the crystal-field surrounding the Er3+ ion may lead to the differences in the peak width and splitting mode.


## 4. Conclusions

The Y2O3:Yb, Er and Y2O2S:Yb, Er upconversion phosphor nanoparticles with crystallite size of 26 and 29 nm have been successfully prepared by the complex precipitation method by using the mixed solution of NH4HCO3 and NH3·H2O as the complex precipitant, and the two samples present the pure cubic and hexagonal structure, respectively. For the Y2O3:Yb, Er sample, when keeping the Er3+ concentration a constant (0.8 mol%), the ratio Ir/Ig has increased greatly with the increase of Yb3+ concentration from 0 to 11 mol%. This could be attributed to the ET between rare earth ions and the cross-relaxation processes. In addition, the upconversion emission spectra of the two samples before and after sulphuration are different from each other. This may be correlated to the crystal structure and the intrinsic properties of different host crystals. It is worthy pointing out that the high efficient upconversion luminescence from the Y2O3:8% Yb, 0.8% Er and Y2O2S:Yb 8%, 0.8% Er phosphors can be seen by naked eyes in daytime when excited by the 980 nm LD with power as low as 5 mW. Therefore, these up-converting phosphor nanoparticles, with excellent luminescence properties, may become ideal fluorescence probes in the biochip technology and may be utilized as the anticounterfeiter and display screen materials.



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*Source: 573253-2015-03-04.xml*