Combustion synthesis and characterization of Er³⁺-doped and Er³⁺, Yb³⁺-codoped YVO₄ nanophosphors oriented for luminescent biolabeling applications

In recent years studies on synthesis, optical properties and applications of rare-earth doped nanomaterials have been attracting much interest [1–6]. In particular, for many novel applications such as biological labels or security labels, properties of phosphors need to be optimized at the nanoscale [5–8]. Among these luminescent materials, upconversion luminescent materials can convert near-infrared radiation into visible radiation by emitting a higher-energy photon after absorbing multiple lower-energy photons. It was reported that efficient upconversion luminescent materials with grain size small enough could be used as new luminescent markers for the detection of biomolecules. For biological applications, upconversion luminescent nanomaterials exhibit many advantages over conventional organic dye markers and quantum dots, which are excited in the UV or blue spectral region, such as high chemical stability, low toxicity, less harmful to cells and living organisms, high light penetration depth in tissues, weak autofluorescence from cells or tissues, low background light and high sensitivity for detection [5, 8, 9].

In addition, the availability of low-cost near-infrared laser diodes (980 nm) has also stimulated research and applications in the area of upconversion luminescent materials. Materials doped with ^{Er 3+} ions and/or codoped with ^{Yb 3+} ions have been widely studied as ^{Er 3+} ions possess a favorable metastable energy level with longer lifetime excited states, and ^{Yb 3+} ions have a large absorption cross-section around 980 nm and can efficiently transfer the excitation energy to ^{Er 3+} ions [9].

^{Eu 3+}-activated yttrium orthovanadate (^YVO ₄), a well-known rare-earth (RE) doped inorganic luminescent material, has been used as an important commercial red phosphor since 1964 owing to its high luminescence efficiency on electron-beam excitation. Recently, considerable efforts have been devoted to developing different synthesis methods for the preparation of luminescent colloidal ^YVO ₄:^{RE 3+} (^RE=^Er, Sm, Yb, etc) nanoparticles [4, 10, 11]. For example, PL and upconversion luminescence of ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} nanomaterials have been investigated [10, 11]. Only a few studies focusing on the synthesis of ^YVO ₄:^{RE 3+} via a combustion method have been reported [12]. However, to our knowledge, no report on the synthesis and properties of nano-sized ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} via a combustion method is available yet.

In this paper we present the synthesis and optical properties of ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} nanomaterials synthesized via a combustion method.

^YVO ₄:^{Er 3+} nanomaterials were synthesized by a combustion method from ^Y(^NO ₃)₃, ^Er(^NO ₃)₃, ^Yb(^NO ₃)₃, ^NH ₄ ^VO ₃ and urea (^H ₂ ^NCONH ₂). In this study, yttrium oxide (99.99%), erbium oxide (99.99%), ytterbium oxide (99.99%), ^NH ₄ ^VO ₃ (99.8%), nitric acid and urea (99%) were used as the starting raw materials. ^Y(^NO ₃)₃, ^Er(^NO ₃)₃ and ^Yb(^NO ₃)₃ stock solutions were prepared by dissolving ^Y ₂ ^O ₃, ^Er ₂ ^O ₃ and ^Yb ₂ ^O ₃ in nitric acid and diluting with deionized water. Aqueous solutions of ^Y(^NO ₃)₃ and ^RE(^NO ₃)₃ were mixed to appropriate molar ratios in a glass beaker and then a suitable amount of urea was added. The mixture was dissolved by magnetic stirring until a uniform solution was achieved. The solution was then concentrated by heating at 80 °^C until any excess free water evaporated. Finally, the solid residue was preheated at 500 °^C in a furnace for 1 h and then calcined at temperatures in the range of 550–900 °^C for 1 h.

Micro-sized ^YVO ₄:^{Er 3+} materials were also synthesized by conventional solid state reaction at 1100 °^C for 3 h to be used as references. Stoichiometric amounts of ^Y ₂ ^O ₃ (99.99%), ^Er ₂ ^O ₃ (99.99%) and ^NH ₄ ^VO ₃ (99.8%) were mixed and finely grinded in an agate mortar. This mixture was prefired for 3 h at 900 °^C, then regrinded and calcined at 1100 °^C for 3 h in air.

The morphology and structure of the prepared samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Photoluminescent (PL) and photoluminescent excitation (PLE) spectra were obtained at room temperature using a Spex Fluorolog-3 spectrophotometer with 450 W Xe light sources. Emission and upconversion emission spectra of ^YVO ₄:^{Er 3+} nanoparticles were also obtained by using an iHR 550 system under 266 and 980 nm excitation (by a diode laser).

The XRD patterns of the resulting products ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+}, were indexed to tetragonal ^YVO ₄ phase without any other crystalline phase being observed (JCPDS card 17-341). Figure 1 displays the XRD patterns of ^YVO ₄:^{Er 3+} samples annealed at 550–900 °^C for 1 h. The formation of ^YVO ₄ phase was completely obtained at temperature as low as 550 °^C and for calcination time as short as 1 h by using a combustion method. This is quite an impressive advantage of the preparative route using a combustion method in comparison with a solid-state reaction method. For the latter case, only a small amount of ^YVO ₄ phase could be obtained after calcination at 900 °^C for 3 h and it is necessary to carry out further heat treatment process at 1100 °^C for 3 h to obtain the single phase of the ^YVO ₄ host material (results are not shown here).

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From figure 1, the broadening of diffraction peaks clearly indicates that small nanocrystals are present in the samples. In addition, the diffraction peak widths of samples calcined at different temperatures ranging from 550 to 900 °^C were almost the same. This revealed that the crystal size of the prepared samples only slightly depends on calcination temperature in the range from 550 to 900 °^C. The crystal size of the sample can be estimated by Scherrer's formula: , where D is the average diameter of the grains, λ is the wavelength of the x-ray (λ=0.1541 ^nm), β and θ are the full width at half maximum (FWHM) of the XRD lines and the Bragg angle, respectively. The average particle size was estimated to be 19 and 20 nm for the samples heated at 550 and 900 °^C, respectively.

Similar to the XRD results, SEM images show that no significant effect of calcination temperature on the crystal size of ^YVO ₄:^{Er 3+} nanomaterials calcined at 550–900 °^C for 1 h was found with a narrow particle size distribution from 20 to 30 nm. Figure 2 presents TEM images of ^YVO ₄:^{Er 3+} samples calcined at 550 and 900 °^C for 1 h. One can see from figure 2 that the average particle size of ^YVO ₄:^{Er 3+} nanoparticles can range from 20 to 30 nm. XRD patterns of ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} calcined at 700 °^C for 1 h do not show any significant difference between samples doped with low doping concentration (0.5 at.% Er) and with high concentration (10 at.% Er or 2 at.% Er and 10 at.%Yb).

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To investigate the optical properties of the prepared materials, their PLE and PL spectra were measured. Figure 3 shows the PLE spectrum of the ^YVO ₄:^{Er 3+} sample prepared by a combustion method. Six excitation peaks at 367, 379.5, 408, 452.5, 490 and 522 nm in the PLE spectrum are attributed to transitions from ground state to excited states of ^{Er 3+} ions: ^{4 I} _15/2 →^{2 K} _15/2, ^{4 I} _15/2 →^{4 G} _11/2, ^{4 I} _15/2 →^{2 H} _9/2, ^{4 I} _15/2 →^{4 F} _{5/2^, 3/2}, ^{4 I} _15/2 →^{4 F} _7/2 and ^{4 I} _15/2 →^{2 H} _11/2, respectively [5, 13, 14]. In addition, the PLE spectrum consists of a broad intense band at around 310 nm which is contributed by the ^VO ₄ ^3- group in the host matrix [9, 15, 16]. This observation indicates that the emission of ^{Er 3+} ions occurs via energy transfers from the excited ^VO ₄ ^3- group.

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PL spectra of ^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} under 266 nm excitation are presented in figure 4. Upon 266 nm excitation, which is the host absorption, two strong green emissions centered around 524 and 552 nm were observed in the PL spectra of the ^YVO ₄:^{Er 3+} nanoparticles. Emissions at 520–535 and 543–560 nm correspond to the ^{2 H} _11/2—^{4 I} _15/2 and ^{4 S} _11/2—^{4 I} _15/2 transitions of ^{Er 3+} ions, respectively [2, 10, 11]. Very weak emission in the red region of 650–675 nm corresponding to the ^{4 F} _9/2 →^{4 I} _15/2 transitions of ^{Er 3+} ion was also detected. The explanation is that the vanadate groups absorbed UV excitation and transferred energy to the erbium ions, and the emissions in the green and red regions could then be observed. The origin of these emissions is due to the electronic transitions of ^{Er 3+}. As a result, it is quite understandable that PL intensity under 266 nm excitation was not significantly affected by the addition of ^{Yb 3+} (figure 4). Figure 5 shows PL spectra of ^YVO ₄:^{Er 3+} nanoparticles under 266 nm excitation with ^{Er 3+} concentration of 0.5, 1, 2, 5 and 10 at.%. The emission intensity increased with ^{Er 3+} atomic concentration and reached a maximum value with 5 at.% ^{Er 3+}.

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As mentioned above, the crystal size of the prepared samples was almost unchanged for the calcination temperature range of 550–900 °^C but the difference in PL intensity of the ^YVO ₄:^{Er 3+} samples was clearly seen. Figure 6 shows the PL spectra and PL intensity of ^YVO ₄:^{Er 3+} (2 at.%) nanoparticles versus calcination temperature. It is shown that PL intensity increases with calcination temperature.

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The above emission is contributed by the ^VO ₄ ^3- group in the host matrix by UV excitation where the energy of emission is lower than that of excitation. From the PLE spectrum, it is seen that ^{Er 3+} ions can directly absorb energy at 367, 379.5, 408, 452.5, 490 and 522 nm to populate in ^{2 K} _15/2, ^{4 G} _11/2, ^{2 H} _9/2, ^{4 F} _{5/2^, 3/2}, ^{4 F} _7/2 and ^{2 H} _11/2 states. After nonradiative relaxation from these states to ^{2 H} _11/2 and ^{4 S} _11/2 states, green emission can be observed. It is very interesting that ^YVO ₄:^{Er 3+} nanoparticles can absorb lower energy (at 980 nm) than ^{2 H} _11/2 and ^{4 S} _11/2 states and provide a green emission, which is called the upconversion luminescent process. Figure 7 shows the upconversion luminescent spectrum of ^YVO ₄:^{Er 3+} nanoparticles under 980 nm laser excitation. The sharp peaks in the green region of 520–560 nm correspond to the (^{2 H} _11/2, ^{4 S} _3/2) →^{4 I} _15/2 transitions of ^{Er 3+}  [10, 11]. In particular, the intensity of these peaks emitted from nano-sized samples is higher than that from micro-sized ones. The upconversion luminescent process in ^{Er 3+} ions of our prepared ^YVO ₄:^{Er 3+} samples can be explained by using an energy diagram as shown in figure 7 [10, 11, 14, 17]. For the excited-state absorption (ESA) process, a 980 nm photon from the laser excites the ^{Er 3+} ion from the ground-state ^{4 I} _15/2 to the excited state ^{4 I} _11/2. After the first-level excitation, the same laser pumps the excited atom from the ^{4 I} _11/2 to the ^{4 F} _7/2 level [10, 16]. For the energy transfer upconversion (ETU) process, two excited ^{Er 3+} ions at ^{4 I} _11/2 interact each with other, one is de-excited to ^{4 I} _15/2, and another is excited to the ^{4 F} _7/2 level [10, 13]. Subsequent nonradiative relaxation populates the ^{2 H} _11/2 and ^{4 S} _3/2 levels. Bright green emission is observed owing to the ^{2 H} _11/2, ^{4 S} _3/2→^{4 I} _15/2 transitions.

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^YVO ₄:^{Er 3+} and ^YVO ₄:^{Er 3+}, ^{Yb 3+} nanomaterials were successfully synthesized via a combustion method at 550–900 °^C for 1 h. The average size of the nanoparticles is about 20–30 nm in diameter and almost unchanged with calcination temperatures in the range of 550–900 °^C. ^YVO ₄:^{Er 3+} nanoparticles emit a green color corresponding to the emission transitions of ^{Er 3+} ions under the excitation of both infrared (980 nm) and UV (266 nm) radiation. It is very interesting for further applications such as luminescent biolabeling.

The present research was supported by grants from National Foundations for Science and Technology Development (NAFOSTED) code 103.03.81.09. Part of the work was done in the National Key Laboratory for Electronic Materials and Devices, Institute of Materials Science (VAST).