Upconversion Luminescence Properties of Y 2 O 3 : Yb , Er and Y 2 O 2 S : Yb , Er Nanoparticles Prepared by Complex Precipitation

The Yb, Er doped Y 2 O 3 and Y 2 O 2 S upconversion nanophosphors were prepared by the direct complex precipitation method with the mixed solution of NH 4 HCO 3 and NH 3 ⋅H 2 O as the complex precipitant. The precipitate of Re(OH) x (CO 3 ) y calcined at 900C in air presents the pure Y 2 O 3 with cubic structure, and the calculated crystalline size is about 26 nm, while the Y 2 O 2 S:Yb, Er nanocrystals were obtained by annealing the same precipitate at 900C but in the atmosphere of N 2 gas containing sulfur vapor. The obtained sample presents the pure hexagonal structure of Y 2 O 2 S 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 Yb in Er doped Y 2 O 3 as well as the Yb, Er codoped Y 2 O 2 S nanocrystals.The high-efficient red and yellow upconversion emissions were both observed by naked eyes in day time corresponding to the Y 2 O 3 :Yb, Er and Y 2 O 2 S:Yb, Er phosphors, respectively. Thus the upconversion nanoparticles combining its high efficient emission would pave the way for ideal fluorescence probes in biological applications.


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 Y 2 O 3 and Y 2 O 2 S materials have attracted much interest as host materials for its excellent chemical stability, insoluble in water, high melting point, and low phonon energy [7][8][9][10][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(NH 2 ) 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 Yb 3+ , Er 3+ codoped Y 2 O 3 and Y 2 O 2 S 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.

Experiment Principles.
In the complex precipitant solution of NH 4 HCO 3 and NH 3 ⋅H 2 O, the equilibrium balances in the solution are shown as follows: When the rare earth ions join in the reaction, Here  1 ,  2 , and  3 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 HCO 3 − both possess the properties of weak electrolytes, so that the quantity of the OH − and CO 3 2− ions is little in the solution.When the complex precipitant solution of NH 4 HCO 3 and NH 3 ⋅H 2 O was dropped into the mixed solution of Y 3+ , Yb 3+ , and Er 3+ ions, in the small area around the complex precipitants, the precipitants ionize out OH − and CO 3 2− ions slowly according to the reversible reactions listed above.Then there is always little change of the OH − and CO 3 2− 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 CO 3 2− 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 (Re(OH)  (CO 3 )  , where the Re represents Y, Yb, and Er) were formed by reacting with the OH − , CO   and NH 3 ⋅H 2 O were 3 and 1 mol/L, respectively.The nitrate solutions of rare earth (Y 2 O 3 , Yb 2 O 3 , and Er 2 O 3 powers (99.99%)) were weighed on the stoichiometry and mixed thoroughly, where the doped concentrations of the activator Er 3+ were 0.8 mol%, and concentrations of the sensitizer Yb 3+ 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 Re(OH)  (CO 3 )  were centrifuged and washed by water, ethanol, and acetone, respectively.The Re(OH)  (CO 3 )  precursor was finally dried at 60 ∘ C.
Y 2 O 3 :Yb, Er nanophosphors were obtained by annealing the Re(OH)  (CO 3 )  precursor in air at 900 ∘ C for 1 h.While Y 2 O 2 S:Yb, Er nanophosphor was obtained by calcining the same Re(OH)  (CO 3 )  sample at 900 ∘ C for 1 h in the atmosphere of N 2 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 N 2 gas.

Characterizations. X-ray diffraction (XRD) patterns of
the samples were recorded on Hitachi DMAX-3A diffractometer equipped with Co  ( = 0.15406 nm) radiation.The morphology of the sample was characterized by TEM (Tecnai G 2 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).

Structure and Morphology.
Figure 1 shows the XRD patterns of the samples that were calcined at 900 ∘ C in air and in N 2 gas containing sulfur vapor, respectively.The doping concentrations of Yb 3+ and Er 3+ ions of the two samples were and Er 3+ have no influence on the crystal structure.And the as-prepared Re(OH)  (CO 3 )  powder obtained by the complex precipitation can be sulfurized thoroughly using the simple method of flowing N 2 gas containing the sulfur vapor.According to the Scherrer formula [17],  = / cos , the calculated crystallite size of theY 2 O 3 :Yb, Er and Y 2 O 2 S:Yb, Er particles was 26 and 29 nm, respectively.
For the observation of nanoparticles morphology, we loaded the Y 2 O 2 S: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.

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  play an important role.Since the Yb 3+ ion has a much higher absorption cross-section than that of Er 3+ ion at 980 nm [20], the roles that the Er 3+ ion plays in the population of the excitation levels decrease with the increasing of the Yb 3+ concentration.
As shown in the energy level diagram (Figure 4), the upconversion green and red emission mechanisms of the Y 2 O 3 :Yb, Er samples are analyzed as follows.
It is well known that the increase of concentration of Yb 3+ in Er 3+ doped nanocrystals should greatly promote the ET from Yb 3+ ( 2 F 5/2 ) to Er 3+ ( 4 I 11/2 ) [21].Thus more Er 3+ will be excited to the 4 I 11/2 level by the ET0, and correspondingly more Er 3+ will be excited to the 4 F 7/2 state with the increase of the population of 4 I 11/2 through ET2 as shown in Figure 4.According to (i) (see [22]), where  is the cross-relaxation probability,  0 ,  1 are the population densities of the two states which are involved in the cross-relaxation process.Therefore, the cross-relaxation probability between the 4 I 11/2 and 4 F 7/2 levels increased greatly with the increase of Yb 3+ concentration.This directly contributes to the population of the 4 F 9/2 level to a large extent.Additionally, the population increase of the 4 I 11/2 state will also result in the decay of Er 3+ ions from 4 I 11/2 to 4 I 13/2 level; thus the process of ET1 is enhanced due to the increasing of population densities of Er 3+ ( 4 I 13/2 ) and Yb 3+ ( 2 F 5/2 ) ions.This also contributes to the population of 4 F 9/2 level.Therefore, the occurrence probabilities of channels ( 3) and ( 4) both increase with the increase of Yb 3+ concentration, which consequently result in the enhancement of population of 4 F 9/2 level.As observed in Figure 3, the relative intensity ratio   /  increases gradually with the Yb 3+ 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, where  vis is the intensity of the upconversion emission,   is pump power, and  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 Y  Figure 6 shows the upconversion spectra of Y 2 O 3 :Yb, Er and Y 2 O 2 S:Yb, Er nanocrystal samples by the 980 nm LD excitation, and the doping concentrations of Yb 3+ and Er 3+ 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 4 F 9/2 → 4 I 15/2 transition of Er 3+ , there are still some differences between them.The maximum of red emission peak of the Y 2 O 3 :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 Y 2 O 2 S: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.
The phosphors of Y 2 O 3 and Y 2 O 2 S come from the same precursor Re(OH)  (CO 3 )  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 Y 2 O 3 and Y 2 O 2 S are 597 and 520 cm −1 , respectively [24,25], Y 2 O 3 obviously possesses much higher phonon energy.So the probability of phononassisted nonradiative relaxation of 4 S 3/2 → 4 F 9/2 and 4 I 11/2 → 4 I 13/2 is much larger in Y 2 O 3 host, which lead to the more effectively bypass or quench the green-light emitting states, as shown in Figure 6.As compared with the Y 2 O 3 :Yb, Er samples, in the spectrum of Y 2 O 2 S: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 Er 3+ 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 Eu 3+ doped oxysulfide system, that is, the emission peak arising from the 5 D 0 → 7 F 2 transition of Eu 3+ 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 Er 3+ ion may lead to the differences in the peak width and splitting mode.

Conclusions
The Y 2 O 3 :Yb, Er and Y 2 O 2 S: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 NH 4 HCO 3 and NH 3 ⋅H 2 O as the complex precipitant, and the two samples present the pure cubic and hexagonal structure, respectively.For the Y 2 O 3 :Yb, Er sample, when keeping the Er 3+ concentration a constant (0.8 mol%), the ratio   /  has increased greatly with the increase of Yb 3+ concentration from 0 to 11 mol%.This could be attributed to the ET between rare earth ions and the crossrelaxation 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 Y 2 O 3 :8% Yb, 0.8% Er and Y 2 O 2 S: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.
The certain amount of NH 4 HCO 3 powders was dissolved into the NH 3 ⋅H 2 O solution and then stirred vigorously to obtain the complex precipitant solution of 70 mL, and concentrations of the NH 4 HCO 3 Figure 4: Schematic energy level diagram of Yb 3+ and Er 3+ ions under excitation of 980 nm LD.
[23]3 :0.8%Er,Y 2 O 3 :1% Yb, 0.8% Er, and Y 2 O 3 :11% Yb, 0.8% Er, as plotted in Figure5.For the sample of Y 2 O 3 :Er, the slope corresponding to the emission intensities of the transitions 2 H 11/2 / 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 is both approximately 2. This indicates that generation of these transitions is predominantly due to two-photon absorption by Er 3+ .For the sample of Y 2 O 3 :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 Yb 3+ to Er 3+[23].For the sample of Y 2 O 3 :11% Yb, 0.8% Er, the slope values are even smaller than that in the Y 2 O 3 :1% Yb, 0.8% Er sample, which implies that the upconversion mechanism must include more crossrelaxation and the ET processes from Yb 3+ to Er 3+ .These results further confirm the upconversion mechanism stated above.