INFLUENCE OF ANNEALING TEMPERATURES ON STRUCTURAL AND OPTICAL PROPERTIES OF ION Er3+
DOPED ZnO/SiO2 NANOCOMPOSITE FILMS
Le Thi Thu Hien1,*, Le My Phuong2, Le Thi Hue3
1 Department of Basic Science, Hung Yen University of Technology and Education
2 Viet Nam Maritime University, Hai Phong, Viet Nam
3 Nguyen Mong Tuan High School, Rung Thong Town, Dong Son District, Thanh Hoa Province
* Corresponding author: [email protected] Received: 15/02/2021
Revised: 15/04/2021
Accepted of publication: 05/06/2021 Abstract:
In this study, we report the influence of annealing temperature on morphology, microstructures, and optical properties of ion Er3+ doped ZnO–SiO2 nanocomposites thin films. Thin layers of ZnO–SiO2 nanocomposites doped with Er3+ were prepared by sol-gel method and spin-coating process. The structure and morphology of the thin films as a function of annealing temperature were studied and analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). Upon annealing temperatures at 700 oC, we observed emission peaks at around 1540 nm assigned for the 4I13/2 -4I15/2 transition of Er3+ ions. Energy transfer processes upon photon absorptions within ZnO nanocrystals and Eu3+ ions are discussed via photoluminescence spectrum in the range of excitation wavelength 325 nm.
Keywords: ZnO/SiO2 nanocomposite, Rare earth dopant, Sol–gel, Energy transfer, Erbium.
1. Introduction
In recent years, optical telecommunications has become an important technology due to some advantages such as carrying big amounts of information, small sizes, and available materials [1]. However, optical communication signals during transmission can be decreased due to absorption or scattering ..., which makes the output signal lower than the signal input. Therefore, there is a need to fabricate materials for optical amplifiers that emission wavelengths coinciding with optical windows with low attenuation in the waveguide [2].
Silica doped with lanthanide elements material:
erbium, europium, yttrium, cerium, neodymium ... are considered fundamental materials in the fabrication of waveguides and waveguides signal enhancers due to their ability to emission in the near-infrared and visible region [3, 4]. Er3+ doped SiO2 have potential optical telecommunication applications given their capability to radiate and amplify optical signals over the 1.54 μm wavelength range [5, 6]. This ability stems from relaxation within the splitting 4f electronic shell of
rare-earth ions. However, Er3+-doped SiO2 material has certain disadvantages, such as small absorption cross-sections of approximately 10-21 cm2 [7], sharp absorption peaks are difficult to effectively excited directly [8]. Therefore, the emission efficiency in the near-infrared region of the rare earth ions is low lead to difficulty for application in fabricating optical waveguides. Co-doping with semiconductor crystals with a large absorption cross-section is an effective solution for enhancing the emission of rare-earth ions [3, 9]. Semiconductors can increase the emission of rare earth ions by absorbing the energy of excitation photons and energy transfer to the rare-earth ions.
Nanostructured ZnO has attracted increasing interest in the past decade due to its novel properties and availability. Zinc oxide is an II-VI semiconductor with a direct bandgap Eg ~ 3.4 eV at room temperature and a large exciton binding energy of 60 meV [9]. There are many publications proving the presence of ZnO and rare-earth co-doped silica, which are effectively stimulating the emission of rare-earth ions [10-12]. However, the low energy
transfer efficiency between ZnO and rare-earth ions is a challenge for scientists. Therefore, more in- depth studies are needed to understand this material.
In this work, the thin film contains ZnO crystals dispersed in the SiO2 matrix doped with Er3+ ions.
The influence of temperature on the morphology, structure, and optical properties of the material is investigated. The mechanism for transferring energy from Er3+-doped ZnO-SiO2 nanocomposites.
2. Experimentals
The sol-gel and spin-coating method was used to prepare of ion Er3+ doped ZnO-SiO2 nanocomposites thin films (Figure 1). Tetraethyl orthorsilicate (TEOS)Si(OC2H5)4 was mixed with C2H5OH, HNO3, and H2O to prepare sol A. The solution was heated to 70 °C, stirred for 2 h, and cooled to room temperature. Zn(CH3COO)2.2H2O and C2H5OH were mixed with diethanolamine
(DEA), stirred for 2 h at 70 °C, and cooled to room temperature to prepare sol B. Subsequently, the SiO2 and ZnO sols were slowly mixed and stirred for 1 h to form a homogeneous mixture. Next, Er(NO3)3.5H2O dissolved in C2H5OH was added to the mixture. The mixture was then continuously stirred for 20 h at room temperature. Consequently, the spin coating was conducted to deposit 30 layers of the final mixture on Si substrates. The sample was annealed from 600 °C to 1000 °C for 2 min to complete gelation and to create thin films after the deposition of each layer. Thin films were heated in air for 3 h at 600 °C-1000 °C in the final stage.
Fluorescence experiments were performed by using a HORIBA Robin Yvon NanoLog SpectroFluorometer (USA). A Xenon lamp (450 W) was used as the excitation source. X-ray diffraction measurements were performed at room temperature
Figure 1. Diagram synthesis of Er3+ doped SiO2/ZnO nanocomposites
by using a Bruker D8 Advance (Germany) apparatus with CuKα wavelength λ = 0.154 nm. Scanning electron microscopy (SEM) images and energy- dispersive X-ray spectroscopy (EDS) results were acquired by employing S4800-Hitachi (Japan) and JSM-7600F-Jeol (Japan) SEM systems.
3. Results and discussions
The crystallinity of the prepared materials after heat treatment was investigated by using a series of samples with a fixed Er3+ doping concentration of 0.3 at.% and the molar ratio of SiO2:ZnO = 95:5. Figure 2 shows the X-ray diffraction (XRD) diagrams of samples annealed at 700 °C, 800 °C, 900 °C, and 1000 °C. As shown in the figure, samples have a low crystallization temperature. Diffraction peaks ascribed to the hexagonal crystal structure of the ZnO Zincite phase, reported in JCPDS File Card No.05-0664, can be observed in the XRD patterns of samples annealed at temperatures exceeding 800 °C. The co-existence of two crystal phases of ZnO Zincite and Zn2SiO4 Willemite reported in JCPDS File Card No.08-0492, was determined in films annealed at high temperatures. Broad peaks centered at 34.4° and 36.3° correspond to (002) and (011) ZnO Zincite crystal planes. Intense diffraction peaks centered at 21.6°, 22.1°, 25.5°, 31.5°, 34°, and 38.8°correspond to (12-1), (030), (220), (113), (140), and (223), respectively, of the Zn2SiO4 Willemite crystal planes [7, 9, 13].
Figure 2. XRD patterns of Er3+ -doped SiO2/ZnO nanocomposites with a SiO2:ZnO:Er3+ 95:5:0.3M ratio after heat treatment at different temperatures
(700 °C–1000 °C). Diffraction peak positions of ZnO Zincite (●) and Zn2SiO4 Willemite( ) are
taken from JCPDS File Card No.05-0664
Figure 3 shows the FESEM images of Er3+- doped ZnO/SiO2 nanocomposites with molar ratios of SiO2:ZnO:Er3+ = 95:5:0.3 annealed at 700 °C.
The bright dots were identified as nanoparticles of ZnO, distributed quite homogeneously in the films, and the crystal size is about 10-20 nm. The EDX spectrum shows the percentage concentration
% of the chemical elements was quite clear, the percentage of O and Si is up to 54 and 42.7%, while the percentage of Zn and Er were low at 3 and 0.3%.
The compositions of the samples were in accordance with the desired composition as confirmed through EDS analysis.
Figure 3. FESEM images of Er3+-doped SiO2/ZnO nanocomposites with a SiO2:ZnO:Er3+ = 95:5:0.3 M ratio, annealed at 700 °C. The inset shows EDS
spectrum (above)
Heat treatment influences the optical properties of the prepared samples. Upon excitation wavelength at 325 nm, we saw emission bands at approximately 1.54 μm is characteristic of radiative transitions from the first excited state (4I13/2) to the ground state (4I15/2) of the 4f electron shell of optically active Er3+ ions (Fig. 4). The position of the characteristic emission peak of the Er3+ ion at 1538 nm didn’t change with different annealing temperatures. This peak intensity increased with increasing temperature and reached a maximum at 700 °C. The temperature increased to 800, 900
oC, the new Zn - O - Si bonding states appeared, forming the Zn2SiO4 phases, the contact interface between ZnO crystal with SiO2 decreased. This is an important cause to reduce the efficiency of indirect energy transfer capacity that Tao Lin confirmed in a statement about ZnO/SiO2: Eu3+ [7].
Figure 4. PL spectra of Er3+ -doped SiO2/ZnO nanocomposites with a SiO2:ZnO:Er3+= 95:5:0.3
M ratio annealed at 600°C–1000 °C, excitation wavelength at 325 nm
The fluorescence of the material was measured at low temperatures with an excitation wavelength of 325 nm. Figure 5 shows a diagram of the fluorescence spectrum of the thin films which were measured at a temperature between 10 K and 300 K. Similar to the fluorescence spectrum at room temperature, emission peak related to the 4I13/2-
4I15/2 of Er3+ ions had the strongest intensity. The emission peak expands due to increased phonon oscillation from the Er3 + ion pulsed crystal field when the temperature increased from 10 K to 300 K. The decrease in emission intensity when the temperature increased from 10 K to 300 K was thought to be due to the fluorescence quenching phenomenon in the thin films.
As shown in the PL spectrum, when the samples were excited at 325 nm, the characteristic emission of Er3+ ion around 1538 nm was observed. It demonstrates that the mechanism of indirect energy transfer from ZnO to rare-earth ions increasing the emission of rare-earth ions.
The mechanism to account for the energy transfer behavior in the SiO2 thin films co-doped with ZnO nanoparticles and Er3+ ions is summarized in Figure 6. ZnO nanoparticles are pumped by incident photons and generate electron and hole pairs in the ZnO nanoparticles. The photo-excited electrons were in the excited state a short time then most of them quickly recombine with the hole in the valence region and emit photons to return to the ground state. Many electrons in the recovery process may
jump back to energy levels corresponding to defects such as oxygen vacancies ... resulting in wideband emissions concentrated in the 400 nm band, while levels the energy at 4G11/2 of the elemental ion Er3+
has a wavelength of about 378 nm. The distance between these energies and those of elemental Er3+
ions is small enough that the energy of the ZnO particles is transferred to the Er3 + ions as the Er3+
ions in the 4G11/2 excited state. Then the electrons to decay non-emitting to energy levels 4F7/2, 4S3/2, from 4S3/2 - 4I9/2, 4I9/2 -4I13/2 [19]. Electrons to relax from energy level 4I13/2 to energy level 4I15/2, which emitted a characteristic 1540 nm wavelength.
Figure 5. PL spectra of Er3+-doped SiO2/ZnO nanocomposites with a SiO2:ZnO:Er3+ = 95:5:0.3 M ratio annealed at low temperature from 10 K to 300 K 4. Conclusion
In conclusion, SiO2 thin films co-doped with Er3+ and ZnO nanoparticles were prepared by sol-gel technique. The films containing ZnO crystals were obtained as revealed by the XRD measurements. The energy transfer efficiency is influenced by the temperatures. It was shown that the strongest luminescence enhancement occurred under annealing temperature of 700 °C. The optical quenching of Er3+-related PL intensity at high annealing temperature is assumed to come from a coexistence of hexagonal structure of ZnO and willemite structure of Zn2SiO4. The mechanism of indirect energy transfer from ZnO to Er3+ ions increasing the emission of rare-earth ions.
Acknowledgements
We gratefully acknowledge that this work was financially supported by the Project UTEHY.L.2020.35.
Figure 6. Schematic diagrams illustrating the mechanism of energy transfer process between ZnO nanoparticles and Er3+ ions
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ẢNH HƯỞNG CỦA NHIỆT ĐỘ LÊN CẤU TRÚC VÀ TÍNH CHẤT QUANG CỦA VẬT LIỆU NANOCOMPOSITE ZnO/SiO2 PHA TẠP ION Er3+
Tóm tắt:
Trong nghiên cứu này, chúng tôi nghiên cứu ảnh hưởng của nhiệt độ ủ lên hình thái, cấu trúc, và tính chất quang của màng mỏng vật liệu nanocomposite ZnO/SiO2:Er3+. Màng mỏng được chế tạo bằng phương pháp sol-gel kết hợp quá trình quay phủ. Thông qua các phép đo nhiễu xạ tia X (XRD) và hiển vi điện tử quét phát xạ trường (FESEM), hình thái và cấu trúc của vật liệu đã được nghiên cứu và được xem là một hàm phụ thuộc vào nhiệt độ ủ. Ở nhiệt độ ủ 700 oC, cường độ đỉnh phát xạ đặc trưng của ion Er3+ tại 1540 nm tương ứng với quá trình chuyển mức từ 4I13/2 -4I15/2 đạt giá trị lớn nhất. Cơ chế truyền năng lượng gián tiếp từ từ các tinh thể nano ZnO sang ion đất hiếm Er3+ được thảo luận thông qua phổ huỳnh quang vật liệu khi kích thích ở bước sóng 325 nm.
Từ khóa: ZnO/SiO2 nanocomposite, pha tạp đất hiếm, sol-gel, truyền năng lượng, Erbium.
