Upconversion Luminescence Properties of Gd₂O₃: Er³⁺ Nanospheres and Gd₂O₃: Er³⁺@Silica Nanocomposites

Abstract

The upconversion luminescence (UCL) of Gd₂O₃: Er³⁺ monodisperse nanospheres, synthesized by multistep chemical method, and nanoparticles, produced by combustion synthesis, are presented for comparison. The UCL of nanospheres Gd₂O₃: Er³⁺ and Gd₂O₃: Er³⁺@Silica have shown strong red or green emission under excitation at 976 nm or 980 nm by a diode laser with a remarkable increase of the bright red color in the nanospheres. In the core/shell structured nanospheres Gd₂O₃: Er³⁺@Silica, we find a higher intensity of upcoversion emission, increased stability and better dispersion capability in solvents and water. The UCL intensities of green and red color while 976 nm or 980 nm excitation were dependent on the diode laser power. The slope values of Er³⁺ ion’s transitions ²H_11/2–⁴I_15/2, ⁴S_3/2–⁴I_15/2 and ⁴F_9/2–⁴I_15/2 in the silica-coated nanospheres are 1.99, 1.62 and 1.66 respectively, which properly indicate the two-photon mechanism of upconversion emission. The synthesized Gd₂O₃ nanospheres codoped with Yb and Er show strong UCL intensity than those of Gd₂O₃: Er³⁺. The obtained Gd₂O₃: Er³⁺ nanospheres and nanoparticles, as well as Gd₂O₃: Yb³⁺, Er³⁺ and its silica-coated versions, are promising materials to develop with potential application in high technology and biomedicine.

HRTEM image of nanospheres Gd₂O₃: Yb³⁺ 18%, Er³⁺ 2.0%. @Silica after heating at 650°C (a); TEM image of Gd₂O₃:Er³⁺ 2.0%@ Silica after heating at 650°C (b); UCL spectra of Gd₂O₃:2.0%Er@silica, λ_exc 980 nm excited powers of 105, 260 and 400 mW (c); Log UCL band of Gd₂O₃:Er³⁺2.0%@Silica versus Log laser pump power (λ_exc 980 nm) (d).

1. Introduction

Gd₂O₃: Er³⁺ high efficient monodispersive oxide nanospherical phosphors (NSPs) have been successfully prepared using wet chemical multistep methods. The UCL characteristics and mechanisms have received significant attention due to fundamental nanoscience and a variety of applications of Gd₂O₃: Er³⁺ NSPs and also Gd₂O₃: Er³⁺@Silica nanoparticles (NPs) in optoelectronic displays, security technology and biomedicine. Firstly, Gd₂O₃ exhibit high chemical and thermal stability, low phonon energy (600 cm⁻¹), and is an efficient host matrix for doping lanthanide ions, especially ion Er³⁺.¹⁾ On the other hand, the core/shell structures of spherical Gd₂O₃: Ln³⁺ NSP codoped with Yb and Er enable higher intensity and have different applications in the near-infrared region, such as simultaneous bioimaging and drug delivery in vivo or in vitro medical media.²⁾ The energy transfer process from the host network to an activator in Gd₂O₃ is provided Er³⁺ down-conversion luminescence in green, red, infrared spectral regions. While the excitation in the near-infrared region can be used upconversion luminescence for enhancing the efficiency of solar energy conversion.^3,4) Furthermore, NSPs Gd₂O₃: Ln³⁺ possesses excellent optical properties, such as fluorescence, long lifetime, sharp emission, photostability. In liquid media, gadolinium oxides with a spherical shape and a narrow size distribution that creates dynamic high mobility. The energy levels of Er³⁺ ion in Gd₂O₃ matrix, the energy transfers in Er³⁺ doped Gd₂O₃ nanoparticles, as well as the UCL mechanism and color tunability of Gd₂O₃:Er³⁺ are the subjects of many investigations.^5,6) Electroluminescence in near-infrared from metal oxide semiconductor devices with erbium-doped gadolinium oxide on silicon was studied previously.⁷⁾ The influence of structure on the photoluminescent behavior from a thin film of Er³⁺ and Yb³⁺ doped Gd₂O₃ on silicon substrate was presented.⁸⁾ Energy transfer in Gd₂O₃: Er NPs as a down-conversion layer was investigated for the improvement of solar cell efficiency.⁹⁾ Our group studied lanthanide ions doped nano oxides Gd₂O₃: Eu³⁺ and Gd₂O₃: Eu³⁺@Silica fabricated by multistep synthesize.¹⁰⁾ The preparation of mesoporous silica-coated Gd₂O₃: Eu³⁺ nanoparticles used in cell imaging and as drug delivery agents were reported.¹¹⁾ We have also previously studied the UCL processes of Gd₂O₃: Er³⁺ monoclinic crystal and Gd₂O₃: Er³⁺@Silica nano phosphors fabricated by EDTA combustion synthesis method.¹²⁾

In the last years, there are studies of the UCL spectra of Er³⁺ in gadolinium oxide.^13,14) Some laboratories reported Gd₂O₃ nanoparticles codoped Yb and Er.^15–17) The optical properties of Gd₂O₃:Eu³⁺ or Gd₂O₃: Er³⁺ by EDTA assisted combustion method at temperatures as low as 350°C and with a very short reaction time 5 minutes were studied by our research group. Coating the surface of the phosphors with silica endows the coated phosphor with hydrophilic characteristics. It makes the phosphor disperse better in different solvents, even in water. Then the coated phosphors can be easily functionalized to develop high-resolution optical imaging in biomedical diagnostics and fabricate a photonic bandgap structure.

The preliminary results on the preparation and upconversion properties of nanospheres Gd₂O₃: Er³⁺ and Gd₂O₃: Er³⁺@Silica were previously reported.²⁰⁾ However, it is necessary to modify the synthesis method with an appropriate processing and annealing condition after heating at high temperatures to control the size, shape and suspensibility of the nanophosphors in solvent and water. Furthermore, the synthesis of upconversion Gd₂O₃ nanospheres codoped with Yb and Er exhibiting higher intensity is also required.

In this work, the fabrication, morphology, and upconversion luminescence characterization of Gd₂O₃: Er³⁺ and Gd₂O₃: Yb³⁺, Er³⁺ nanospheres and nanocomposite as well as the core/shell structured Gd₂O₃: Er³⁺@Silica will be presented. In this paper, we will compare the our new results to the previous reports from our group and international literature. The structure, composition, construction and optical mechanism of UCL have been investigated and discussed to provide more insight into the optical transition process and find a higher intensity of upconversion luminescence.

2. Experimental Procedure

2.1 Materials

The raw chemicals Gd (NO₃)₃.6H₂O 99.9% Sigma Aldrich, Er (NO₃)₃.5H₂O, Yb (NO₃)₃.5H₂O 99.9% Sigma Aldrich, Tetraorthoxysilane TEOS [(C₂H₅O)₄ Si] 99% Sigma Aldrich, urea [CO (NH₂)₂] 99.8% Prolabo, HNO₃ NH₄OH 28%, absolute C₂H₅OH were used without further purification.

2.2 Synthesis of Gd₂O₃: Er³⁺, Gd₂O₃: Yb³⁺, Er³⁺ nanospheres and Gd₂O₃: Er³⁺@Silica nanocomposites

Gd₂O₃: Er³⁺ nanospheres have been prepared by a modified synthesis method using urea as the soft space template and then heating treatment to converse intermediate into oxides while retaining their shape and structure.¹⁰⁾ In the multistep synthesis method, urea [CO (NH₂)₂] possesses two functions. The first is to serve as a space template compound. The second is to act as a precipitation agent since it can self-decomposes into OH⁻ and CO₃²⁻ at a temperature of about 85–93°C; this change in the reaction condition is one of the major modifications in this work. The Gd₂O₃ codoped with 18%Yb³⁺ and 2.0% Er³⁺ has been successfully synthesized using this modified method.

The synthesis of Gd₂O₃: Er³⁺ nanosphere coated with a silica out layer was prepared by sol-gel technology. Typical fine powder Gd₂O₃: Er³⁺were well-dispersed in a mixed solution of 20–30 ml of absolute C₂H₅OH and 80–100 ml of DI water by CLEANER UCP 20 ultrasonication for 1 h. 150–200 µl TEOS in 10 ml ethanol was slowly added to the above solution and was followed by the addition of NH₄OH (28%) to keep the pH value at 7.4–7.6. The solution was mixed using a magnetic bath and warmed up to 60–70°C for 1 h. This coating reaction conducted with different TEOS/Gd₂O₃: Er³⁺ rate. The temperature of the reaction was then remained for 3.0–3.5 hours. The coated Gd₂O₃: Er³⁺ and Gd₂O₃: Er³⁺, Yb³⁺ nanomaterials were prepared by this sol-gel coating procedure exhibit better the unique properties in protecting, enhancing the intensity of luminescence, and improving dispensability in solvents and water.

When the synthesis reaction with urea was completed, the suspension was quickly cooled down to room temperature. It then was centrifuged to separate the final product by a UNIVERSAL 320. The obtained material was washed three times with ethanol and DI water, dried at 70°C for 24 h, 105°C for 2 h and 150°C for 2 h. In these works, the concentrations of Er³⁺ ion were 1, 1.8, 2.0 and 2.5 mol%. The urea/total RE (Gd³⁺+Er³⁺) ratio 25/1 was selected according to our previous experience about the desired size and shape of the oxide. The higher reaction temperature above 85°C was observed to produce a strong intensity of upconversion emission. The samples were annealed by step-by-step procedure at 200°C for 2 h, 400°C for 2 h, 600°C for 2 h and 650°C for 3 h, resulting in the formation of Gd₂O₃: Er³⁺ perfect spheres.¹⁰⁾

2.3 Characterization

The morphology and shape of nanospheres and nanocomposite of Rare Earth oxides were investigated using high-resolution transmission electron microscopy, JEM 2100, Japan and FESEM Hitachi 4800, Japan, which are in the Institute of Materials Science. The structure of all the samples was recorded by XRD Model Brucker D8-4800. Fourier Transform Infrared spectrometer (FTIR, Model Fourier NEXUS 670) in the Institute of Tropical Technology. Luminescence spectra were measured on NANOLOG 320 in Advanced Institute of Science and Technology, Hanoi University of Science and Technology as well as in the Institute of Low Temperature and Structure Research, Wroclaw, Poland using iHR 550 (Jobin- Yvon) NANOLOG 320 equipment. In the case of the UCL spectra, it was measured using a 980 nm laser diode (LD) system for excitation using different powers from 100 mW to 500 mW and a 976 nm laser diode, with excitation powers from 130 mW to 980 mW.

3. Results and Discussions

3.1 Structure and morphology

The HRTEM image of nanospheres Gd₂O₃: Er³⁺@silica is shown in Fig. 1 and the TEM of Gd₂O₃: Er³⁺@silica is presented in Fig. 2. These synthesized samples have a perfectly spherical shape with a smooth surface and were mostly monodispersed. The size range of 150 nm to 200 nm of the spheres after annealing at a high temperature of 650°C for 3 h could be reached.

Fig. 1

HRTEM image of Gd₂O₃:Er³⁺2.0%@Silica, after heating at 650°C.

Fig. 2

TEM image of Gd₂O₃: Er³⁺2.0%@ Silica, after heating at 650°C.

By controlled synthesis conditions and a careful heat treatment regime, we can obtain monodisperse perfect nanospheres without cracks, deformation, agglomeration and sintering, although these can be achieved after heating at a high temperature of 650°C or more.

Figure 2 shows the TEM image of Gd₂O₃: Er³⁺ 2.0%@silica coated by 200 µl TEOS per 150 mg fine powder Gd₂O₃:Er³⁺ 2.0%. The spherical core Gd₂O₃: RE³⁺ has been homogeneously coated with a silica layer from the TEM images of nanospheres Gd₂O₃:2.0% Er after heating at 650°C for 3 h. The XRD pattern shows the cubic phase of Gd₂O₃ (JCPDS Card No. 43-1014) space group la-3 and the unit space a = 10.79 Å; these are presented in Fig. 3. The crystalline phase of oxide Gd₂O₃ depends on the synthesis method. The combustion synthesis in using EDTA produces the monoclinic form.¹²⁾ However, the multistep chemical synthesis with urea as the soft template and decomposition agent can fabricate the cubic one. The symmetric property and solid character of the host matrix of Gd₂O₃ has a great influence on the luminescent behavior of lanthanide ions such as Eu³⁺ or Er³⁺. The synthesis method, preparation condition for the wet chemical reaction and the critical heating treatment regime greatly affect the quality and behavior of the nanophosphors.^1,10,27) For the case of Gd₂O₃: Er³⁺ the apropos annealing temperature is 650°C or higher. The integration of material composition, surface treatment and heat treatment of a nano phosphor to toward developing an application is essential.

Fig. 3

XRD patterns of Gd₂O₃:Er³⁺2.0% after heating at 650°C.

3.2 Chemical composition

The FTIR spectra of Gd₂O₃:2% Er³⁺@ silica composites appear at 455 and 545 cm⁻¹, which corresponds to the Gd–O vibration of Gd₂O₃. The peak at 1633 cm⁻¹ corresponds to the Er–O vibration. The band of the Si–O–Si group was seen at 1082 cm⁻¹, confirming the formation of Gd₂O₃: Er³⁺@ silica structure. The band at approximately 3400 cm⁻¹ is from the OH groups: the surface absorbed H₂O and hydroxide molecules.²⁰⁾

3.3 Optical properties

The photoluminescence excitation spectrum (PLE) of Gd₂O₃: Er³⁺ was measured.

3.3.1 Optical properties of Gd₂O₃: Er³⁺ and Gd₂O₃: Er³⁺, Yb³⁺ nanospheres

The photoluminescence excitation spectrum (PLE) of Gd₂O₃: Er³⁺2.0% nanoparticles, annealed at 700°C for 3 h and monitored at λ_em.562 nm indicates that one can use the wavelengths of 275 nm or 379 nm for the sufficient excitation of the Gd₂O₃: Er³⁺.²⁰⁾ The decay time of Gd₂O₃: Er³⁺ 2.0%, annealed 650°C was measured by about milliseconds.

Figure 4: PL of Gd₂O₃: Er³⁺ nanoparticles excited by 379 nm²⁰⁾ has a high and sharp peak in the green region at 562 nm.

Fig. 4

PL spectra of Gd₂O₃:Er³⁺, λ_exc 379 nm.

Figure 5 presents the photoluminescence spectra of Gd₂O₃: 2.0%Er³⁺@silica nanoparticles coated by TEOS. The TEOS/Gd₂O₃:Er³⁺ rates are 2, 4, 6 mg/60 mg in coating. Their luminescence spectra can be seen as curves 1, 2 and 3 in black, red, green color, respectively. The intensity of the luminescence of the Er³⁺ activator increases when the TEOS/Gd₂O₃: Er³⁺ rate is higher. This means that the silica out layer protects the core nanophosphor and decreases the quenching site on the surface of nanophosphor. Moreover, the silica out layer is hydrophilic, enabling an easier dispersion of the nanophosphors in solvents or water.

Fig. 5

PL spectra of Gd₂O₃:Er³⁺@Silica with different TEOS of 2, 4, 6 mg/60 mg nanoparticles, from 1–3, λ_exc 275 nm.

UCL green and red of the Er³⁺ optical transitions were studied. The green one consisting of peaks at 521 nm and 537 nm could be assigned to the transition from ²H_11/2 to ⁴I_15/2 energy state. Figure 6 shows the upconversion spectra of Gd₂O₃: Er³⁺2.0% @silica, excited by the 976 nm diode laser with a power range from 700 to 900 mW. One can see a robust red color region, in which some sharp peaks at 654, 661 and 682 nm arising from the transition of energy state ⁴F_9/2 to ⁴I_15/2 can be seen clearly. The upconversion spectra of Gd₂O₃: Er³⁺ coated with TEOS was measured and excited by a 980 nm laser diode with an excitation power range from 100 to 400 mW; results are presented in Fig. 7. The intensity rate of red to the green color band nearly remained. At the same time, the excitation power of the laser diode was changed. It indicates that the excitation by laser at 976 nm with the low power could produce the pure red color from upconversion emission.

Fig. 6

UCL spectra of Gd₂O₃: Er³⁺ 2%@Silica, 976 nm excited powers of 693, 749 and 876 mW.

Fig. 7

UCL spectra of Gd₂O₃: Er³⁺2.0%@Silica, λ_exc 976 nm excited powers of 105, 260 and 400 mW.

The upconversion luminescence spectra of the probe Gd₂O₃: Er³⁺2.0% @Silica, excited by a 976 nm diode laser in the power range from about 250 mW to more than 900 mW have been investigated to estimate the emission property and stability of this nano phosphor in laboratory conditions. Strong luminescence intensity is observed.

Figure 8 shows the UCL spectra of Gd₂O₃: Er³⁺2.0%@silica with excitation at 980 nm, using different excitation powers. The excitation powers were from 360 mW to 450 mW. As seen in Fig. 8, the UCL spectra obtained are similar to those excited at 976 nm. However, the intensity of the upconversion luminescence was much lower, and the intensity rate of the band to the green one excited at 980 nm was much lower than in the case of the excitation at 976 nm. It was enabled to tune the color of UCL in using an excitation source with different wavelengths in near-infrared.

Fig. 8

UCL spectra of Gd₂O₃:Er³⁺2.0% @Silica, λ_exc 980 nm, excited powers of 360 mW (a), 400 mW (b) and 450 mW (c).

The UCL spectra of the nanospheres @ silica show very sharp lines. This means that the fine Gd₂O₃ host matrix crystallizes in cubic form after heating at 650°C for several hours.

To obtain more insight on the mechanism behind the observed UCL, the upconversed luminescence intensities of the three transitions ³H_11/2; ⁴S_3/2; ⁴F_9/2 to ⁴I_15/2 of Er³⁺ in Gd₂O₃: Er³⁺ have been measured as a function of the power of the excitation laser. The effect of laser power on the intensities of UCL bands of Gd₂O₃: Er³⁺ is shown in Fig. 9. The values of the photon number of the three UCL bands are 1.99 for ²H_11/2 to ⁴I_15/2, 1.62 for ⁴S_3/2 to ⁴I_15/2 and 1.66 for ⁴F_9/2 to ⁴I_15/2. The n value of the 525 nm emission is 1.99, indicating the two-photon UC process in which two near-infrared excitation photons are absorbed to emit one. The upconversion luminescence mechanisms were discussed to understand the multiphoton process of UCL in many host nanomaterials in various nano sizes and shape.^2,9,20) It is important because of its possible applications in different fields, such as cell imaging and drug delivery, color-tunability in the display, and designing core/shell nano-architectures for anti-counterfeiting inks and latent fingerprint recognition.¹⁹⁾ At present, in our group, the synthesized nanospheres Gd₂O₃ containing the coactivator Yb and Er have been produced to enhance UCL intensity and to tune the color between the green and red regions.

Fig. 9

Log UCL band of Gd₂O₃: Er³⁺ 2.0%@Silica versus Log laser pump power (λ_exc 980 nm).

Figure 10 shows the HRTEM image of the synthesized core/shell structured nanospheres Gd₂O₃: Yb³⁺ 18%, Er³⁺ 2.0%. The preliminary investigation indicates that the core/shell nanospheres enhance the intensity of the upconversion luminescence under excitation by a diode laser in the near infrared region (Fig. 11). The suspension of the treated Gd₂O₃: Yb³⁺, Er³⁺ nanospheres in water (after heating at 650°C) by using an ultrasound system at room temperature was stable for several days.

Fig. 10

HRTEM of nanospheres Gd₂O₃: Yb³⁺ 18%, Er³⁺ 2.0%@Silica, after heating at 650°C.

Fig. 11

UCL of Gd₂O₃: Yb³⁺ 18%, Er³⁺ 2.0%@Silica, λ_exc 976 nm with different powers 1–255 mW, 2–693 mW, 3–750 mW, 4–982 mW.

In the coactivated Gd₂O₃: Yb³⁺ 18%, Er³⁺ 2.0% the upconversion emission is enhanced upon photon absorption of the ground state ²F_7/2 of Yb³⁺ ions. After that, the absorption energy from Yb³⁺ ions could transfer to the excited intermediate state ⁴I_11/2 level by another pump photon. From this energy level, non-radiative relaxation to different lower states occurs. Then there are transitions of three excited states ²H_11/2, ⁴S_3/2, and ⁴F_7/2 to the ground state ⁴I_15/2 of Er³⁺ ions attributed to the upconversion spectra shown in Fig. 11. Figure 12(a) and 12(b) present upconversion emission optical process of Gd₂O₃: Er³⁺ nanospheres and Gd₂O₃: Yb³⁺, Er³⁺ nanospheres when excited by 976 nm diode laser.

Fig. 12

(a) Energy level diagram of Er³⁺ ions in Gd₂O₃:Er³⁺@Silica, λ_exc 976 nm, Upconversion process. (b) Energy level diagram of Gd₂O₃: Yb³⁺, Er³⁺@Silica, λ_exc 976 nm, Upconversion process.

The research, development and potential application of the upconversion luminescence of nano phosphors in a variety of host compounds have been undertaken intensively in some groups.^18,21,26) These nanospheres and nanoparticles containing Rare Earth ions would open interesting necessary investigations and promising applications in some breakthrough technology. Shortly, research will tend to nano oxides Gd₂O₃ doped with RE ions (RE = Yb, Nd, Er, Tm and Eu) in the form of nanospheres and their nanocomposites as a platform for the photonics, energy conversion, bioimaging in medicine, and sensing environment.^22–25) In this research topic, the HRTEM is important to control and evaluate of size, shape and multilayered structure of nanostructured spheres.²⁷⁾ In the field of Rare Earth (nano), the oxide is as potential antibacterial materials when used in a suitable polymer reported.²⁸⁾ The splitting of the spectrum of Gd₂O₃: Yb³⁺, Er³⁺ nanospheres indicates the poly-nano-crystalline nature of nanospheres product at several hundred nm, and it is also related to the crystal symmetry features.²⁹⁾ The mechanism of the upconversion emission of NaYF₄: Yb³⁺, Er³⁺ was studied in detail in our research group.³⁰⁾

4. Conclusions

The typical characteristics and mechanisms of the upconversion luminescence Gd₂O₃: Er³⁺ nanospheres with high monodispersed and size from 150 nm to 200 nm and Gd₂O₃: Er³⁺@Silica under 976 and 980 nm excitation by diode laser were demonstrated. After treatment at a high temperature of 650°C by the controlled annealing procedure, the fabricated nanospheres Gd₂O₃: Er³⁺ are in single and perfect spheres without cracks and sintering and could be well-dispersed in solvents and water. The decay time of the UCL of Gd₂O₃: Er³⁺ is on the scale of milliseconds. The mechanism of upconversion luminescence is properly two-photon in both Gd₂O₃: Er³⁺ nanospheres and in nanoparticles. Extreme upconversion intensities of both Gd₂O₃: Er³⁺ and Gd₂O₃: Er³⁺@Silica nanospheres with Er³⁺ concentrations were observed from 1.8 to 2.0%. The energy transfer from Yb³⁺ to Er³⁺ in Gd₂O₃ nanospheres drastically enhanced the intensity of upconversion emission upon excitation at 976 nm or 980 nm. These upconversion luminescence nanomaterials could be developed for application in security technology, solar energy conversion and especially promising for bioimaging and developing probes for biology and medicine.

Acknowledgments

This work is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.03-2019.07. The authors are grateful to Duy Tan University and Institute of Materials Science, Vietnam Academy of Science and Technology in support this research.

REFERENCES

• 1)   Y.A.  Kuznetsova,  A.F.  Zatsepin,  V.A.  Mashkovtsev and  V.N.  Rychkov: J. Phys.: Conf. Ser. 917 (2017) 052015.
• 2)   K.  Zheng,  Z.  Liu,  Y.  Liu,  W.  Song and  W.  Qin: J. Appl. Phys. 114 (2013) 183109.
• 3)   X.  Chen,  E.  Ma and  G.  Liu: J. Phys. Chem. C 111 (2007) 10404–10411.
• 4)   G.  Boopathi,  S.G.  Raj,  G.R.  Kumar and  R.  Mohan: Procedia Mater. Sci. 6 (2014) 1436–1443.
• 5)   Y.A.  Kuznetsova and  A.F.  Zatsepin: J. Phys.: Conf. Ser. 917 (2017) 062001.
• 6)   L.Q.  Minh,  T.  Endo,  T.T.  Huong,  N.T.  Huong,  L.T.K.  Giang,  L.D.  Tuyen,  D.X.  Loc and  T.K.  Anh: Trans. Mater. Res. Soc. Japan 35 (2010) 417–422.
• 7)   C.  Jin,  J.  Liu and  J.  Sun: Appl. Phys. Lett. 114 (2019) 211102.
• 8)   R.K.  Tamrakar and  K.  Upadhyay: Mater. Technol. 34 (2019) 313–323.
• 9)   V.  Danny and  L.  Zhiqun: Sci. Adv. Mater. 3 (2011) 26–40.
• 10)   T.K.  Anh,  N.T.  Huong,  P.T.  Lien,  D.K.  Tung,  V.D.  Tu,  N.D.  Van,  W.  Strek and  L.Q.  Minh: Mater. Sci. Eng. B 241 (2019) 1–8.
• 11)   W.  Song,  W.  Di and  W.  Qin: Dalton Trans. 45 (2016) 7443–7449.
• 12)   L.Q.  Minh,  T.K.  Anh,  N.D.  Hung,  P.T.M.  Chau,  N.T.Q.  Hai,  H.V.  Tuyen,  V.T.T.  Ha,  V.D.  Tu and  W.  Strek: J. Rare Earths 37 (2019) 1126–1131.
• 13)   R.K.  Tamrakar,  D.P.  Bisen,  K.  Upadhyay and  I.P.  Sahu: J. Alloy. Compd. 655 (2016) 423–432.
• 14)   M.  Pokhrel,  G.A.  Kumar and  D.K.  Sardar: J. Mater. Chem. A 1 (2013) 11595.
• 15)   E.  Hemmer,  H.  Takeshita,  T.  Yamano,  T.  Fujiki,  Y.  Kohl,  K.  Love,  N.  Venkatachalam and  H.  Hyodo: J. Mater. Sci. Mater. Med. 23 (2012) 2399–2412.
• 16)   Y.  Yang,  J.  Gu,  R.  Yang,  Q.  Shang and  J.  Yang: Nanosci. Nanometrol. 2(2) (2016) 41–45.
• 17)   R.K.  Tamrakar,  D.P.  Bisen,  K.  Upadthyay and  N.  Brame: Superlattices Microstruct. 81 (2015) 34–48.
• 18)   W.  Zheng,  B.  Sun,  Y.  Li,  R.  Wang and  Y.  Xu: Mater. Chem. Front. (2019) 1–16.
• 19)   J.  Xu,  B.  Zhang,  L.  Jia,  Y.  Fan,  R.  Chen,  T.  Zhu and  B.Z.  Liu: ACS Appl. Mater. Interfaces 11 (2019) 35294–35304.
• 20)   N.T.Q.  Hai,  T.K.  Anh,  P.T.M.  Chau,  V.T.T.  Ha,  H.V.  Tuyen,  T.T.  Huong,  H.T.  Phuong and  Q.L.  Minh: J. Mater. Sci. Mater. Electron. 31 (2020) 3354–3360.
• 21)   H.T.  Khuyen,  T.T.  Huong,  D.K.  Tung,  P.T.  Thu,  N.T.  Binh,  L.Q.  Minh,  T.K.  Anh,  L.N.  Diep,  N.T.H.  Lien and  P.A.  Tuan: J. Electron. Mater. 45 (2016) 4400–4406.
• 22)   V.D.  Tu,  H.V.  Chia,  N.  Nababan,  L.Q.  Minh,  S.W.  Kno,  L.C.  Chau,  C.C.  Ting,  H.C.  Kan and  C.C.  Hsu: ACS Photonics 5 (2018) 3263.
• 23)   H.T.  Phuong,  T.T.  Huong,  D.T.  Thao,  H.T.  Khuyen,  P.T.  Lien,  N.T.  Huong,  L.T.  Vinh and  L.Q.  Minh: J. Rare Earths 37 (2019) 1183–1187.
• 24)   B.  Purohit,  Y.  Guyot,  D.  Amans,  M.-F.  Joubert,  B.  Mahler,  S.  Mishra,  S.  Daniele,  C.  Dujardin and  G.  Ledoux: ACS Photonics 6 (2019) 3126–3131.
• 25)   H.  Zhao,  C.  Liu,  Z.  Gu,  L.  Dong,  F.  Li,  C.  Yao and  D.  Yang: ACS Nano Lett. 20 (2020) 252–260.
• 26)   H.  Li,  S.  Song,  W.  Wang and  K.  Chen: Dalton Trans. 44 (2015) 16081–16090.
• 27)   H.  Sasaki,  G.  Saito,  N.  Sakaguchi,  R.  Ota and  H.  Takahashi: Mater. Trans. 60 (2019) 688–692.
• 28)   J.A.M.  Torresm,  A.L.  Marure,  M.G.  Hernandez,  G.B.  Islas and  M.A.D.  Sanchez: Mater. Trans. 59 (2018) 1915–1919.
• 29)   E.  Antic-Fidancev,  J.  Hölsä and  M.  Lastusaari: J. Phys. Condens. Matter 15 (2003) 863–876.
• 30)   L.T.K.  Giang,  T.K.  Anh,  N.T.  Binh,  L.Q.  Minh,  L.  Marciniak and  W.  Lojkowski: Int. J. Nanotechnol. 12 (2015) 538–547.