Nghiên cứu chế tạo và tính chất của vật liệu zn2sio4 và zn2sno4 không pha tạp và pha tạp các ion kim loại chuyển tiếp (mn2+, cr3+) (synthesis and properties of undoped and transition metal (mn2+, cr3+) doped zn2sio4a

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY Le Thi Thao Vien Synthesis and properties of undoped and transition metal (Mn2+, Cr3+) doped Zn2SiO4 and Zn2SnO4 phosphors Majors: Materials Science Code: 9440122 SUMMARY OF DOCTORAL DISSERTATION ON MATERIALS SCIENCE Hanoi – 2020 The dissertation was completed at Hanoi University of Science and Technology Advisors: 1: Prof Dr Pham Thanh Huy 2: Dr Nguyen Thi Khoi Reviewer 1: Reviewer 2: Reviewer 3: The dissertation is evaluated by the Board of Doctoral Thesis Evaluation Council at Hanoi University of Technology at …… h, date … month … year ……… The dissertation can be found the libraries: Ta Quang Buu Library – Hanoi of science and technology Vietnam National Library A INTRODUCTION Essentials of research project Currently, white light-emitting diode (WLED) has replaced traditional light sources such as incandescent and fluorescent lamps owing to the high luminous efficiency, environment friendliness, long lifetime, energysavings, and compact size A WLED is usually created by three methods: (1) combination of monochromatic red, green, and blue LED chips; (2) coating a UV LED chip with red, green, blue and (3) coating a blue LED chip with single phased Y3Al5O12: Ce3+ yellow or mixed green and red phosphors In the above methods of manufacturing LED, the technique of combining monochromatic LED chips has many outstanding advantages, but this method is quite complicated, and high manufacturing costs, etc., The two following methods of manufacturing LED, phosphors-based LEDs, are quite simple, easy to adjust the color, and broad-spectrum In these methods, phosphors -based WLEDs are considered as one of the most important factors in determining the quality of WLEDs Thus, synthesis and development of phosphors with different emission colors and low costs are being explored and developed for lighting Also, this is evaluated as the most critical and urgent challenges in the lighting field In general, phosphors, namely luminescence materials, are constructed by a matrix (crystalline host) doped with an activator (luminescent center Concerning these factors, the material used for LED phosphors will include the two steps: (1) investigation and evaluation of different host phosphors and (2) the selection of suitable activators First, regarding host material, a suitable crystal structure should be selected due to the understandings of the crystal and local structures, wide bandgap to easily doping so that the PL spectrum can be tailored The optical characteristics of phosphor materials are mainly affected by the host structure and coordination environment around the activator ion Secondly, selecting suitable activator ions for doping in the crystal host as luminescence centers also essential The reason is that they play an important role in PL tuning and luminescence optimization Nowadays, the synthesis and searching of phosphors for the WLED application is mainly based on rare-earth-doped phosphors It is well known that the synthesis of rare earth phosphors is costly and even toxic due to the synthesis of (oxy) nitrides performed at high temperature and high pressure Thus, the applications in WLEDs of these phosphors have been limited Metal transition ions, such as Mn2+, Mn4+, Cr3+, and so on, are less expensive and environment-friendly Hence, recently eco-friendly phosphors based on nonrare - earth receive increasing interest in the field of white light-emitting diodes (WLEDs) Third, the most important factor is that activators are doped in crystal host lattice to produce suitable emission and excitation spectra that match an LED application When activators are doped to host lattice, their local coordination environment will be affected by the host crystal field, which makes the change in phosphor properties such as excitation and emission wavelengths, luminescence efficiency, and resistance to thermal quenching effects The doping of Mn2+ or Cr3+ to ZnO-SiO2/SnO2 has been synthesized and studied their properties, which reported by many previous works Based on the similar ionic radii and oxidation states of Zn, Sn and Mn, Cr (0.60 Å for Zn2+ and 0.66 Å for Mn2+), Mn2+, Cr3+ ions may have substituted the Zn2+ or Sn4+ sites in the ZnO-SiO2/SnO2 matrix and make them be promising phosphors for applications in phosphor-converted WLEDs However, most studies above focus on the gas-sensitive, phosphorescent, or photocatalyst properties of materials There are only a small number of reports that focus on the optical properties of these materials Furthermore, the application of these phosphors on the WLED has not been much regarded Beside, phosphors based on metal transition are mainly doping Mn 4+ ions into host lattice to supplement the red zone to increase CRI in the WLED application Whereas, the green light-emitting phosphors produced from ZnO-SiO2/SnO2 doped Mn2+ with high color purity can compensate for the missing green light in the spectrum of LED generated from Blue chip and YAG yellow powder Or the phosphors combined with red and blue powder also help improve the CRI of WLED However, this problem has not been much regarded In addition, LED lighting can be used to promoting flowering is also being interested in scientists In particular, Cr3+ doped ZnO-SnO2 phosphor for emission spectra in the far-red region is likely to be coated to blue Chip to create a device for applications in specialized lighting However, the use of this material in this purpose has not been studied In this work, we focus on synthesis and properties of undoped and transition metal (Mn2+, Cr3+) doped Zn2SiO4 and Zn2SnO4 phosphors The goal of the research project The thesis includes some main purposes of research as follows: - Study the synthesis process of Zn2SiO4, Zn2SiO4: Mn2+, Zn2SnO4, Zn2SnO4: Mn2+, Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ by high energy planetary ball milling, followed by annealing in the air or a reducing atmosphere - Study the effects of the annealing temperature and the doped concentration on the structural and optical properties of fabricated materials systems - Evaluate the applicability of produced phosphors through the evaluation of the LED devices' parameters fabricated by directly coating the produced phosphors on the ultraviolet or blue LED chips Research method - The primary research method of the dissertation is the experimental method In this dissertation, all samples were fabricated by high highenergy planetary ball milling combination with annealing in the air or a reducing atmosphere The crystal structure, surface morphology, particle size, chemical composition, and optical properties of the produced phosphors are investigated using modern analytical techniques such as SEM, X-ray, EDX, UV-Vis, FTIR, Raman, and PL, PLE spectra, etc The dissertation also uses LED packaging and evaluation techniques at the AIST Institute New contributions of the dissertation - We have successfully fabricated three groups of material systems: Zn2SiO4 and Zn2SiO4: Mn2+; Zn2SnO4 and Zn2SnO4: Mn2+; Zn2SnO4: Cr3+ and 4: Cr3+, Al3+ by planetary ball milling combined with annealing at low temperatures The results lower than 200-300 C compared to the conventional solid-phase method - Zn2SiO4 phosphor has emission spectrum in the infrared region with a peak of 735 nm, which is an effective excitation wavelength for phytochromes, so it has potential for application in specialized LEDs for agricultural lighting - Zn2SiO4: Mn2+ phosphor gives green emission spectra with high color purity ( 85%) when measured on LEDs fabricated by coating Zn2SiO4: Mn2+ powder on the UV LED chip - A new infrared emission peak at 684 nm was first found in the emission spectrum of Zn2SnO4 phosphor - capable of being used for specialized LED in agriculture - Zn2SnO4: Mn2+ phosphor was first synthesized, shows strongly absorbing excitation light in the blue region (444 nm), and gives emission in the green area (523 nm), thus having potential for application in fabricating green LED using blue LED chip - Zn2SnO4: Cr3+ material was first studied for their fluorescence properties The emission spectrum has a broad band in the infrared region with the peak of 740 nm and strong absorption of blue light - capable of being used in specialized LEDs to stimulate flowering - Zn2SnO4: Cr3+, Al3+ phosphor has a broad PL spectrum in the infrared region with the peaks of 730 nm, capable of being used in specialized LED fabrication applications for rustic lighting The excitation and emission spectra are 10 nm blue shift compared to those of Zn2SnO4: Cr3+ The cause of the peak excitation and emission shift is because of the Burstein–Moss shift The scientific and practical significance of the dissertation ❖ The scientific significance: - The dissertation has introduced an effective method of manufacturing luminescent materials by combining the traditional solid-state reaction method and the high energy ball milling method - The results of research on transition metal doped Zn 2SiO4 and Zn2SnO4 phosphors have been presented systematically in this dissertation - a new research trend on environmentally friendly, non-rare earth phosphors Therefore, the dissertation can be used as a useful reference for further research in this area ❖ The practical significance: - The objective of the dissertation research is to solve a specific practical problem, which is to synthesize new types of environmentally friendly nonrare earth phosphors used in WLED or specialized LEDs for Agriculture - The three groups of phosphors produced in the dissertation are Zn2SiO4 and Zn2SiO4: Mn2+ , Zn2SnO4 and Zn2SnO4: Mn2+, and Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ They are systematically studied to evaluate the structural properties, morphology, dimensions, chemical composition, optical properties Also, they are being tested the application of phosphorconverted LED models, and this is an important technological step that the results obtained can help evaluate the practical applicability of phosphorconverted LEDs in this material system The structure of the dissertation The content of the dissertation consists of chapters as follow: - Chapter Introduction - Chapter Experimental techniques - Chapter Optical properties of Zn2SiO4 and Zn2SiO4: Mn2+ phosphors - Chapter Optical properties of Zn2SnO4 and Zn2SnO4: Mn2+ phosphors - Chapter Optical properties of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ phosphors B CONTENTS Chapter INTRODUCTION In this chapter, the theoretical knowledge of luminescence, the background of TM ions in crystal field, and literature review of TM doped ZnO-SiO2/SnO2 are presented Chapter EXPERIMENTAL TECHNICS Figure 2.1 Synthesis Zn2SiO4 powder process The high-energy planetary ball mill method is one of the effective techniques to mix well and reduce the particle size of the powdery material The source material is crushed by the impact of the balls (made of high hardness material) when they are placed in a closed chamber and centrifuged at a very high speed The energy generated by the impact during the crushing process helps to break the bonds on the surface of the material; the large particles break down into smaller particles and, at the same time, are mixed, which helps for the solid phase reaction to occur more smoothly and more evenly So, in this dissertation, all samples are synthesized by combining the high energy planetary ball milling method with the traditional solid-state reaction In particular, the source materials are mixed well before being put into annealing at high temperatures in the atmosphere This method helps overcome some of the disadvantages of solid-state reaction methods such as the source materials are more uniformly mixed, and the reaction temperature is lower due to the smaller size of the source material The crystalline structure of the samples was analysed by powder XRD (XRD-Bruker D8 Advance) using CuK radiation (=1.5406 Å) operated at 40 mA tube current The XRD patterns were collected in the range of 20°  2θ  60° with a step of 0.05° The surface morphology and average size of particles were observed by FE-SEM (JSM-7600F, Jeol) Chemical bonds were investigated by FTIR spectra using a Perkin Elmer Spectrum GX spectrometer at cm-1 resolutions Raman spectra were recorded with a Horiba Jobin Yvon LabRAM HR-800 spectrometer using He–Ne laser (632.8 nm) with a power density of 215 W/cm2 The optical properties of all samples were investigated by using a PL spectrophotometer (Nanolog, Horiba Jobin Yvon) equipped with a 450 W xenon discharge lamp as an excitation source The TL glow curves were recorded after 90Sr βirradiation by using a TL-Reader (Harshaw-3500) with a heating range of 50 to 450 C and a heating rate of C.s-1 Chapter OPTICAL PROPERTIES OF Zn2SiO4 AND Zn2SiO4: Mn2+ PHOSPHORS 3.1 Introduction In chapter 3, the dissertation raises the problem of researching to synthesize a single-phase of Zn2SiO4 willemite by a high-energy planetary ball mill at a low annealing temperature (≤1250 C) The effect of annealing temperature and doping concentration on the optical properties of Zn 2SiO4 and Zn2SiO4: Mn2+ materials have been studied in detail To evaluate the applicability of the produced phosphors, they have been tested for use in green LED by coating the produced phosphors directly on the ultraviolet LED chips The results obtained show that LED devices emit respectively in the infrared (~ 735 nm) and green (~ 523 nm) (with high color purity ~ 85%) so that the Zn2SiO4 and Zn2SiO4: Mn2+ phosphors have the potential to be used as a fluorescent powder for LED applications for agriculture and WLED 3.2 Structural and optical properties of Zn2SiO4 phosphors 3.2.1 X-ray diffraction of Zn2SiO4 The results of X-ray diffraction patterns of figure 3.1 show that, at the annealing temperature of 1250 C, the produced sample is single-phase Willemite Zn2SiO4 And as can be seen from the figure, all patterns display sharp and well-defined diffraction peaks, which characterized the fine structure of Zn 2SiO4 Figure 3.1 XRD patterns of ZnO-SiO2 powder with weight ratio of 1:2 after high-energy planetary ball milling for 40 hours and annealing at different temperature for hours in air environment 3.2.2 Phosphor morphology of Zn2SiO4 Figure 3.2 FESEM and EDS images of ZnO-SiO2 powder (a) and annealing at 500 C (b); 900 C (c); 1000 C (d); 1150 C (e); 1250 C (f), 1300 C (g) and 1350 C (h) oC for hours in air environnent As shown in the figure 3.2, the particle size increases and reaches the average size of ~1 to 1.5 m after annealing at 1250 C for hours in an air environment 3.2.3 Vibrational analysis: F-TIR of Zn2SiO4 The Raman spectra (figure 3.3) contain vibrational modes at 348, 397, 868, 903, and 947 cm-1, which correspond to the surface of the siloxance group (the Si–O– Si linkage) and characteristics of Zn2SiO4 material Figure 3.3 Raman spectra of ZnO-SiO2 powder (with weight ratio of 1:2) after high-energy planetary ball milling for 40 hours (a) and annealing at 900 C (b), 1250 C (c), and 1350 C (d) for hours in air environment 3.2.4 Optical properties of Zn2SiO4 Survey results on the dependence of the photoluminescent spectra of Zn2SiO4 material on the annealing temperature in Figure 3.4 show that when annealed at high temperature (1150, 1250 and 1350 C), the PL spectrum of the Zn2SiO4 phosphor consists of two main emission bands with the peaks at 525 nm and 735 nm The 735 nm emission band has an asymmetric shape, extending towards the long wavelength and can be analyzed into two emission bands with the peaks of 730 and 760 nm, respectively The origin of the two emission peaks is explained by the NBOHs interface defects of the electrons at 2px and 2py orbitals Figure 3.4 PL spectra of ZnO-SiO2 and after annealing at different temperatures for hours in air environment (a) and Gaussian Fitted of PL spectrum (b) 3.3 Structural and optical properties of Zn2SiO4:Mn2+ 3.3.1 X-ray diffraction of Zn2SiO4:Mn2+ The XRD data (figure 3.7) indicates that the crystallinity of the sample was enhanced upon increasing the temperature to 1250 C, but then reduce at the higher annealing temperature (1300 and 1350 ) The XRD patterns of the Zn2SiO4: x%Mn2+ (x=0-8) samples after milling for 40 hours, followed by annealing in air at 1250 °C are shown in Fig 3.8 The result indicates that all patterns display sharp and well-defined diffraction peaks, which characterized the willemite structure of -Zn2SiO4 Figure 3.7 XRD patterns of ZnO/SiO2:5%Mn2+ powders annealing at different temperatures Figure 3.8 XRD patterns of Zn2SiO4:x%Mn2+ (x=0-8) samples annealed in air at 1250 C 3.3.2 Phosphor morphology of Zn2SiO4: Mn2+ Similar to the results obtained for Zn2SiO4 material, the FESEM image results of Mn-doped Zn2SiO4 phosphor in figure 3.9 show that the particle size increased rapidly when the sample was annealed at a temperature of 1150-1250 C The average size is about micrometers (2-5 µm) when annealed at 1250 C At higher temperatures, samples tend to agglomerate into large clumps The EDS spectra show that the main components of the sample include O, Zn, Si, and Mn-doped elements with a percentage of % atoms that are entirely consistent with the ratio of the initial compositing Figure 3.9 FESEM images of wt % Mn2+ doped ZnO/SiO2 powders after milling for 40 hours (a), the milled sample and annealed at different temperatures for hours in air 3.3.3 Vibrational analysis of Zn2SiO4: Mn2+ The FTIR spectra of the 5% Mn-doped ZnO/SiO2 powder milled for 40 hours and the milled powders annealed at different temperatures are shown in Fig 3.10 The results of the FTIR spectrum show the the peaks centered The results of the TL glow curve of Zn2SiO4: 5% Mn2+ reveals two peaks at 158 °C and 235 °C (see Fig 3.15b) and linear response with a dose up to 25 minutes of β-ray exposure time (see Fig 3.15a) This result shows that produced materials have a high potential application of the Mn 2+ doped Zn2SiO4 in TLD The thermoluminescence emission spectra at 158 °C and 235 °C (figure 3.16a) and the decay time at 158 C (figure 3.15b) show that the sample emits a strong emission at 525 nm with the electronic lifetime of 10.5 ms This value is much longer than that of the undoped sample (~ 1000 ns) (inset fig.3.16b) Figure 3.16 Thermoluminescence emission spectra measured at 158 and 235 oC (a) and the decay curve of the Zn2SiO4:5%wt Mn2+ phosphor (b) 3.3.6 Testing the application of Zn 2SiO4: Mn2+ phosphor in fabricating the phosphor-converted LED The applicability of fabricated Zn2SiO4: Mn2+ materials was evaluated by coating the colloidal silicon solution containing Zn 2SiO4: 5% Mn2+ onto 270 nm UV LED chip The results of figure 3.17 show that under the current of 60 mA, LED device emits green light with a strong intensity (insert in Figure 3.17) with coordinates color (x; y) of (0.2477; 0.6829) Figure 3.17 Electroluminescence spectrum (a) and the CIE coordinate plot of the prototype green-emitting LED under drive current of 60 mA (b) The inset of Fig 12b is the digital image of the actual green-emitting LED 11 3.3 Conclusion - Near-infrared Zn2SiO4 and green Zn2SiO4: Mn2+ phosphor have been produced successfully by the high-energy ball milling technique followed by annealing temperature of 1250 C in air - Zn2SiO4 phosphor emits a broad spectrum in the infrared (maximum at 735 nm) due to the overlap of two emission bands with the maximum of 730 and 760 nm, respectively The origin of these two emission peaks is explained by the non-bridging oxygen defects in the Zn2SiO4 lattice The Mn2+-doped Zn2SiO4 phosphor emits an intense green band at 525 nm - The TL glove curve of Zn2SiO4: 5%Mn2+ shows a strong peak at 158 °C and a shoulder at 235 °C, and displays linear dose-response with β-ray exposure time which indicates the phosphor could be useful for the dosimetric application - A green LED device was fabricated by using a 270 nm UV LED chip combined with 5% Mn2+-doped Zn2SiO4 phosphor, which provides 525 nm green light with CIE chromaticity coordinates of (0.2477; 0.6829) and the color purity of nealy 85% Chapter OPTICAL PROPERTIES OF Zn2SnO4 AND Zn2SnO4:Mn2+ PHOSPHORS 4.1 Introduction In chapter 4, the dissertation raises the study of synthesis single-phase spinel Zn2SnO4 and Zn2SnO4: Mn2+ phosphors by the high-energy planetary ball milling at a low annealing temperature (≤1000 C) The crystal structure, morphology, particle size distribution, and optical properties of Zn2SnO4 and Zn2SnO4: Mn2+ were also investigated in detail The applicability of the produced Zn2SnO4: 5% Mn2+ phosphor was tested for use in fabricating green LED by coating it directly on blue LED chips (450 nm) Besides, to evaluate the ability to increase the color rendering index (CRI) of white LEDs, the produced Zn2SnO4: 5% Mn2+ powder was mixed with the red phosphor Zn2SnO4: 3% Cr3+, 0.6% Al3+ and the mixture was then coated onto blue LED chip (450 nm) The results show that the green LED has color coordinates (x = 0.2419; y = 0.3953) and the white LED has color coordinates x = 0.3783 and y = 0.3520, the color temperature is 3858 K and the color rendering index is 91 Zn 2SnO4: Mn2+ material system can be used as a phosphor for the WLED application 4.2 Structural and optical properties of ZnO/SnO2 phosphors 4.2.1 X-ray diffraction of ZnO/SnO2 phosphors 12 Figure 4.1 XRD patterns of ZnO/SnO2 powder after milling for 60 hours and annealing in the range of 600-1200 °C Figure 4.2 XRD photograph of the sample with different ZnO and SnO2 ratio The results of X-ray diffraction patterns given in figure 4.1 show that the crystallization of the sample increases as the annealing temperature increases to 1000 C, however, when the temperature is further increased to 1100 and 1200 C, the phase ZnO reappeared The results of X-ray diffraction patterns in Figure 4.2 show that singlephase Zn2SnO4 formed at the ratio of ZnO: SnO2 is 2:1 4.2.2 FESEM images and grain-size distribution of ZnO/SnO2 phosphor As can be seen from the FESEM image of Zn2SnO4 in the figure 4.3, the particle size increases sharply when further increasing the annealing temperature to 1000 °C and above After annealing at 1200 °C, the average particle size is in the range of 4.0-5.0 μm Figure 4.3 FESEM images of ZnO/SnO2 powders after ball-milling for 60 hours without or with annealing in air at different temperatures Figure 4.4 The grain-size distribution of the as-obtained phosphor after calcinating at different temperatures The grain-size distribution of the samples is measured, and their results are shown in Figure 4.4 It can be seen that the average particle size strongly increased with increasing the annealing temperature from 900 to 1200 °C 13 4.2.1 Optical properties of ZnO/SnO2 The results of calculating the bandgap of Zn2SnO4 material in Figure 4.5 is about 3.74 eV at annealing temperature at 1000 C When the annealing temperature increases to 1100 C, the optical band of the produced phosphor decreases from 3.74 eV to 3.67 eV The results of the PL of ZnO, SnO2, and Zn2SnO4 materials at 1000 C in figure 4.6b show that for Zn2SnO4 materials, the emission spectrum has a broad in the of range of 450 - 800 nm, asymmetry, and observing new emission peaks at 684 nm The emission peak in the far red at 684 nm is likely to be used in specialized agricultural lighting and has not been observed by previous studies In addition, an intriguing far-red emission peaking at 684 nm, which has never been explicitly reported in the works of literature, has been observed As shown in the PL spectra Zn2SnO4 (figure 4.7), the PL intensity first increases to reach the maximum value at 1000 C and then decreases with the further increment of annealing temperature Because the phase-pure Zn2SnO4 is obtained at 900 C and 1000 C, the new far-red emission is highly attributed to defects in Zn2SnO4 Figure 4.5 UV-Vis spectra of ZnO-SnO2 powders after ball-milling for 60 hours, followed by annealing at different temperatures in air Figure 4.6 PL spectra of milled-samples for 60 hours, followed by (a) without and (b) with annealing at 1000 ˚C in air 14 The band emission centered at 684 nm could be attributed to recombination of an electron trapped at the tin/zinc interstitial (Zn i/Sni) states and a hole confined at the VO++ state On the other hand, it could be originated from a recombination of an electron from the V O* state with a hole represented at the tin/zinc defect (VZn/VSn) states Figure 4.7 PL spectra of ZnO/SnO2 powders after ball-milling for 60 hours without or with annealing in air at different temperatures Figure 4.8 Deconvoluted photoluminescence spectra of the milled Zn2SnO4 powder annealed at 1000 ˚C in air 4.3 Structural and optical properties of Zn2SnO4:Mn2+ 4.3.1 TG–DTA spectra and X-ray diffraction of Zn2SnO4: Mn2+ phosphor Figure 4.10 Thermal analysis of as-milled powder (a) and XRD pattern of Zn2SnO4:5%Mn2+powder without annealing and annealed at various temperature(b) From TG–DTA results (figure 4.10a), sintering temperatures were chosen from 700 °C to 1100 °C for 2h to prepare the Zn 2SnO4: 5%Mn2+ phosphors The results of X-ray diffraction patterns of Zn2SnO4: Mn2+ given in figure 4.10b show that the intensity of XRD peaks corresponding to Zn2SnO4 increases with the enhancement of the annealing temperature from 800 C to 1000 C Further increase of the annealing temperature to 1100 C gives, beside diffraction peaks of Zn2SnO4, the appearance of other peaks corresponding to the ZnO phase The intensity reduction and the reappearance of ZnO at 1100 C may be due to the evaporation of SnO2, which leads to a shortage of Sn in the ZTO 15 The effect of Mn2+ doped concentration on the formation of crystalline phases in figure 4.12 shows that the samples doped with 3, and % concentration of Mn2+, the X-ray diffraction diagram only observe the diffraction peaks characteristic for single-phase spinel Zn2SnO4 while for the sample with and % of Mn2+ ions, the two impurity – related diffraction peaks which assigned to (220) and (510) plane of MnO phase (JCPDS-card no 44-0141) were observed Figure 4.12 Powder XRD pattern of ZTO doped with different % concentration of Mn2+ions 4.3.2 Optical properties of Zn2SnO4: Mn2+ phosphor The PLE spectrum of the Zn2SnO4: Mn2+ (figure 4.17a) shows five peaks at about 359 nm, 380 nm, 424 nm, 435 nm and 444 nm which are due to transitions of 6A1(6S) →4E(4D), 6A1(6S) →4T2(4D), 6A1(6S) →4E(4G), 6A1(6S) →4T2(4G) and 6A1(6S) →4T1(4G), respectively PL spectrum of the Zn2SnO4: Mn2+ (figure 4.17b) shows a broadband emission peaked at 523 nm which is assigned to forbidden transitions of 4T1(4G) →6A1(6S) of Mn2+ ions on the tetrahedral sites in the host lattice The calculating result on the crystal field value around the Mn2+ ion in the Zn2SnO4 lattice from the Tanabe-Sugano diagram in Figure 4.17b gives the value of Dq / B 1.2 Figure 4.17 PL and PLE spectra of Zn2SnO4:5%Mn2+ (a) and Tanabe – Sugano diagram of 3d5configuration of Mn2+ion in Zn2SnO4 crystal field (b) 16 4.4 Testing the application of Zn2SnO4: 5%Mn2+ phosphor in fabricating the phosphor-converted LED Figure 4.21 CIE 2015-10° xy color chromaticity coordinates of the spectral emission from a blue LED Chip coated by Zn2SnO4:5%Mn2+ phosphor (a) and by a mixture of green Zn2SnO4:5%Mn2+ and red Zn2SnO4:3%Cr3+ phosphors A device based on a blue LED chip (450 nm, SemiLEDs) coated by the prepared Zn2SnO4: 5%Mn green phosphor With the device driven by a current of 30 mA, its CIE x, y chromaticity coordinates (x = 0.2419; y = 0.3953) are shown on the CIE chromatic diagram (Fig 4.21a) An actual light capture of the LED is shown in the inset of figure 4.21a Further, by mixing Zn2SnO4: 5%Mn with the synthesized Zn2SnO4: 3%Cr3+,0.6%Al phosphor (emission peak located at 700 nm) to coat on a blue LED chip, a warm WLED with chromaticity coordinates x = 0.3783 and y = 0.3520 (see Fig 4.21b) was achieved It has a correlated color temperature of 3858 K and a color rending index of 91 4.5 Conclusion - The single phase of the infrared phosphor Zn 2SnO4 and single-phase of green phosphor Zn2SnO4: Mn2+ have been successfully synthesized by high energy planetary ball milling combined with annealing at 1000 C in air and reduced gas atmosphere - The Zn2SnO4 phosphor gives emission in the range of 450-800 nm with a peak of 684 nm This is the first time the red emission peak at 684 nm has been clearly observed for Zn2SnO4 phosphor Literally, there are possibly two potential mechanisms for this emission: (1) recombination of a deeply trapped electron (VO* state) with a deeply trapped hole (VSn/VZn), or (2) recombination of a shallowly/deeply trapped hole (V O++) with a deeply trapped electron (Zni/Sni) - Zn2SnO4: Mn2+ phosphor was the first synthesis giving green emission spectra with a peak of 523 nm Excitation photoluminescence spectra (PLE) of Zn2SnO4: 5% Mn2+ show a strong absorption peak characteristic of Mn2+ 17 ions at 444 nm, so it has a high potential for application in green and white LEDs using the blue LED chip - LED device fabricated by coating Zn2SnO4: 5% Mn2+ powder on blue LED chip with color coordinates (x = 0.2419; y = 0.3953) Besides, when using Zn2SnO4: 5% Mn2+ powder mixed with Zn2SnO4 red powder: 3% Cr3+, 0.6% Al3+ and covered 450 nm blue LED chip for white LED device with color coordinates x = 0.3783 and y = 0.3520, color temperature is 3858 K and color rendering index is 91 Chapter OPTICAL PROPERTIES OF Zn2SnO4:Cr3+ AND Zn2SnO4:Cr3+, Al3+ 5.1 Introduction In chapter 5, the dissertation raises the problem of synthesizing singlephase of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ phosphors by high energy planetary ball milling at a low annealing temperature (≤1100 C) The crystal structure, morphology and optical properties of Zn 2SnO4:Cr3+ and Zn2SnO4: Cr3+, Al3+ systems have also been studied in detail The effect of Al3+ ion doping on optical properties of Zn 2SnO4:Cr3+ materials were also considered Besides, to evaluate applicability, Zn2SnO4: 3%Cr3+ and Zn2SnO4: 3%Cr3+,0.06% Al3+ phosphors have been tested for fabricating infrared emission LEDs by coating directly on the blue LED chips (460 nm) The obtained results show that the LEDs have an efficiency of 6.6% and 16.3%, respectively The Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ phosphors are capable of being applied as powders for agriculture-specific LEDs 5.2 Structural and optical properties of Zn2SnO4:Cr3+ 5.2.1 X-ray diffraction of Zn2SnO4:Cr3+ Figure 5.1 XRD patterns Zn2SnO4:3%Cr3+ un-annealed annealed different temperature of and Figure 5.2 XRD patterns of Zn2SnO4:x%Cr3+ (x=0-6%) annealed at 1100 °C in air The results of the X-ray diffraction diagram of Zn2SnO4: 3%Cr3+ (figure 5.1) shows that the intensity of diffraction peaks increases with increasing annealing temperature from 500 C-1100 C Further increasing the 18 annealing temperature to 1200 C, the ZnO phase reappeared When Cr3+ doped Zn2SnO4 with different concentrations from to 6%, the analysis results of figure 5.2 show that there was no significant change in the X-ray diffraction diagram 5.2.2 Optical properties of Zn2SnO4: Cr3+ Figure 5.4 PL spectra of the Zn2SnO4: Cr3+ obtained at different annealing temperature Figure 5.5 PL spectra of the Zn2SnO4: xCr3+ (x=1-6%) obtained at 1100C The PL spectra of Zn2SnO4: 3%Cr3+ un-annealed and annealed different temperature (figure 5.3) show the emission in the infrared region with a peak at 740 nm The PL intensity of this emission band increases with the annealing temperature up to 1100 C By further increasing the annealing temperature to 1200 C, the PL intensity begins to reduce Figure 5.5 represents the photoluminescence spectra of Zn2SnO4: x% Cr3+ (x = 1-6) It shows that the PL intensity increases with the Cr3+ doping concentration in the range from to % and decreases when the doping concentration exceeds % Figure 5.6 PLE spectra of the Zn2SnO4: xCr3+ (x=1-6%) obtained at 1100C Figure 5.7 Tanabe – Sugano diagram for Cr3+ doped ZTO phosphor Figure 5.6 shows the excitation photoluminescence spectra of Cr3+ doped ZTO; besides a strong UV absorption in the range 280-325 nm, there is two broad absorption band in the visible range at 460 nm and 630 nm 19 The calculating result of the crystal field value of the Cr3+ ion in the Zn2SnO4 lattice in Figure 5.7 gives the Dq/B value of  2.2 5.2.3 Testing the application of Zn2SnO4: 3%Cr3+ phosphor in fabricating the phosphor-converted LED Figure 5.8 PL spectra of LED device using 460 Blue LED Chip coated with ZTO:3%Cr3+ The inset shows the image captured by a digital camera The applicability of the produced Zn2SnO4:Cr3+ material was evaluated by coating the silicon solution containing Zn2SnO4: 3% Cr3+ onto a 460 nm blue LED chip As shown in figure 5.8, the results show that under the effect of electric current of 60 mA, LED fabrication gives infrared emission of Zn2SnO4: 3%Cr3+ and blue emission of LED chip-the direct light of LED as shown in the inset in figure 5.8 The efficiency of the fabricated LED is about 6.6% 5.3 Structural and optical properties of Zn2SnO4:Cr3+, Al3+ 5.3.1 X-ray diffraction and FESEM of Zn2SnO4: Cr3+, Al3+ Figure 5.9 XRD patterns of ZTO: 3%Cr3+ (a) and ZTO: Cr3+, x%Al3+(x =0.2, 0.4, 0.6, 0.8) annealed at 1100 °C in air Figure 5.10 FESEM image of and EDS spectra of pure ZTO (a, d), ZTO: Cr3+ (b,e); ZTO: Cr3+,Al3+ (c,f) calcinated at 1100 °C in air The results of the X-ray diffraction diagram of Zn2SnO4: Cr3+, Al3+ (Figure 5.9) show that there is no significant change in the X-ray diffraction diagram and no observed of any phase of Cr3+ ions or Al3+ 20 The results of FESEM image of Zn2SnO4: Cr3+, Al3+ phosphor with varying Cr3+ concentration (0.2-0.8% mol) (figure 5.10) shows that the particle size decreases with increasing Al3+ ion concentration The average size is about 0.5-1.0 μm when the concentration of Al3+ reaches 0.6% mol 5.3.2 Crystal field analysis Figure 5.11 Tanabe – Sugano diagram for 3%Cr3+ and 3%Cr3+,0.6%Al3+ in ZTO phosphor The results of calculating the crystal field (Dq/B) of the Zn 2SnO4: 3% Cr3+, x% Al3+ (Figure 5.11) show that when the Al3+ ion co-doped into the Zn2SnO4: Cr3+, the value of the Dq/B increased up 5.3.3 The effect of Al3+ on optical properties of ZTO: Cr3+ Figure 5.12 UV-vis spectra of the prepared phosphor The inset is the estimated band gap of the ZTO: 3%Cr3+ and ZTO:3%Cr3+, x%Al Figure 5.13 Optical band gap shifts as a function of carrier concentration The results of calculating the band gap of the Zn2SnO4: 3% Cr3+, x% Al3+ sample (Figure 5.12) shows that when the concentration of Al 3+ ions increased from 0.2 -0.8% mol, the band gap of the material increased The widening of the optical bandgap when Cr3+, Al3+ co-doped ZTO may be influenced by the change in carrier concentration and can be explained by Burstein – Moss shift effect 21 As shown in figure 5.13, the carrier concentration increases linearly with the increase of the band gap of Zn2SnO4: 3% Cr3+, x% Al3+ sample (x = 0.2 – 0.8%) The results of the excitation spectra and emission spectra of Zn 2SnO4: 3% Cr3+, x% Al3+ phosphors (Figure 5.14 and 5.15) show that when Al 3+ co-doped Zn2SnO4: Cr3+, the absorption peak and emission peak is shifted the shorter wavelength compared to those of Zn 2SnO4: Cr3+ material The cause of the blue shift absorption and emission is due to the Burstein - Moss effect and the strengthen of the crystal field around the Cr3+ ion Figure 5.14 PL spectra of the ZTO: 3%Cr3+ and ZTO:3%Cr3+,x%Al3+phosphor calcinated at 1100 °C Figure 5.15 PL spectra of the ZTO: 3%Cr3+ and ZTO:3%Cr3+,x%Al3+ phosphor calcinated at 1100 °C 5.3.4 Testing the application of Zn2SnO4: 3%Cr3+, 0.6Al3+ phosphor in fabricating the phosphor-converted LED Figure 5.17 PL spectra of LED device using 460 Blue LED Chip coated with ZTO:3%Cr 3+, 0.6%Al3+ The inset shows the image captured by a digital camera The applicability of the produced Zn2SnO4: Cr3+, Al3+ material was evaluated by coating the silicon solution containing Zn 2SnO4: 3% Cr3+,0.6%Al3+ onto a 460 nm blue LED chip As shown in figure 5.17, the 22 results show that LED fabrication gives infrared emission of Zn 2SnO4: 3%Cr3+ and blue emission of LED chip-the direct light of LED as shown in the inset of figure 5.17 The efficiency of the fabricated LED is about 16.3% 5.4 Conclusion - The single-phase of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ phosphors have been successfully synthesized by high-energy planetary ball milling combined with annealing at the low temperature of 1100 C in air - The Zn2SnO4: Cr3+ material was first investigated for the properties of phosphor-converted LEDs The excitation spectra of Zn2SnO4: Cr3+ phosphors show strong absorption in blue and red areas with the peaks at 460 nm and 630 nm The emission spectra of this material give emission in the infrared region with a peak at 740 nm - When Al3+ ions co-doped Zn2SnO4: Cr3+ material, the excitation spectrum and the emission spectrum show blue shift due to the Burstein – Moss effect and the crystal field around the Cr3+ ion increased from 2.24 to 2.71 Excitation spectra and emission spectra of Zn 2SnO4: Cr3+, Al3+ materials give strong absorption in blue and red regions with the peaks at 450 nm and 620 nm and emission in infrared areas with a peak at 730 nm - LED device was fabricated using blue LED Chip combined with the obtained ZTO: Cr3+ or ZTO: Cr3+, Al3+ have the corresponding energy conversion efficiency of 6.6% and 16.3%, respectively The infrared phosphors Zn2SnO4: 3%Cr3+ and Zn2SnO4: 3%Cr3+, 0.6% Al3+ show the potential for the application in manufacturing specialized LED for agricultural lighting C CONCLUSIONS The dissertation has some results, as follow: - Three groups of materials: Zn2SiO4 and Zn2SiO4: Mn2+; Zn2SnO4 and Zn2SnO4: Mn2+; Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ have successfully fabricated by high energy planetary ball milling combined with annealing, which temperatures 200-300 °C lower than those synthesized by the conventional solid-phase method - The Zn2SiO4 gives the PL spectrum with a broadbands centered at 735 nm This broadband was fitted into two Gaussian peaks at 730 nm and 760 nm The origin of the two peaks is ascribed to the NBOHs of the unpaired electron on 2px or 2py orbitals The Mn2+-doped Zn2SiO4 phosphor emits an intense green band at 525 nm The TL glove curve of Zn2SiO4: 5%Mn2+ shows a strong peak at 158 °C and a shoulder at 235 °C, and displays linear dose-response with β-ray exposure time which indicates the phosphor could 23 be useful for the dosimetric application A green LED device was fabricated by using a 270 nm UV LED chip combined with 5% Mn 2+-doped Zn2SiO4 phosphor, which provides 525 nm green light with CIE chromaticity coordinates of (0.2477; 0.6829) and the color purity of nealy 85% - The Zn2SnO4 gives the PL spectrum with a new emission peaking at 684 nm Literally, there are possibly two potential mechanisms for this emission: (1) recombination of a deeply trapped electron (V O* state) with a deeply trapped hole (VSn/VZn), or (2) recombination of a shallowly/deeply trapped hole (VO++) with a deeply trapped electron (Zni/Sni) Zn2SnO4:Mn2+ green phosphors have been fist time synthesized successfully by high energy ball milling and calcination in a reducing gas atmosphere, giving green emission spectra with a peak of 523 nm Excitation photoluminescence spectra (PLE) of Zn2SnO4: 5% Mn2+ show a strong absorption peak characteristic of Mn2+ ions at 445 nm It has a high potential for application in green and white LEDs using the blue LED chip The results of the quenching temperature indicate that green phosphor Zn2SnO4: Mn2+ has excellent thermal stability With increasing temperature further to 200 °C, the emission intensity remains at 62 % of the initial value (at 25 °C) so that the prepared phosphor shows excellent promise for LED application Besides, the internal quantum efficiency of the Zn2SnO4: 5%Mn2+ is about 40%, similar value, which has been achieved in previous reports about Mn2+ doped oxide phosphors The prepared green phosphor Zn2SnO4:5%Mn2+ coated on a blue LED chip shows a strong blue absorption and emits an intense green band peaking at 523 nm Further, a warm WLED could be fabricated by coating a mixture of green phosphor (Zn2SnO4:5%Mn2+) and red phosphor (Zn2SnO4:3%Cr3+,0.6%Al) on a blue LED chip, achieving a low color temperature (3858 K) and a high color rending index (91) - Zn2SnO4:3%Cr3+ phosphor has an excitation spectrum showing strong absorption in blue and red areas with the peaks at 460 nm and 630 nm The emission spectrum of this material gives emission in the infrared region with a peak at 740 nm The excitation spectrum of Zn 2SnO4: 3%Cr3+, 0.6%Al3+ phosphor provides strong absorption in blue and red areas with the peaks at 450 nm and 620 nm The emission spectrum of this material (peak at 730 nm) is blue-shifted 10 nm compared to those peaks of Zn2SnO4: 3%Cr3+ LED device was fabricated using blue LED Chip combined with the obtained ZTO:3%Cr3+ or ZTO:3%Cr3+,0.6%Al3+ phosphors and both show near-infrared light output powder of nearly 30 mW, and 35 mW, the corresponding energy conversion efficiency of 6.6% and 16.3%, respectively 24 PUBLICATIONS L.T.T Vien, Nguyen Tu, Manh Trung Tran, Nguyen Van Du, D.H Nguyen, D X Viet, N.V Quang, D.Q Trung, P.T Huy (2020), “A new far-red emission from Zn2SnO4 powder synthesized by modified solid state reaction method”, Optical materials, vol 100, 109670, pp 1-9 L T T Vien, N Tu, D X Viet, D D Anh, D H Nguyen and P T Huy, (2020), “Mn2+-doped Zn2SnO4 green phosphor for WLED applications”, Journal of Luminescence, vol 227, 117522, pp 1-9 L.T.T Vien, Nguyen Tu, T.T Phuong, N.T Tuan, N.V Quang, H Van Bui, Anh-Tuan Duong, D.Q Trung and P.T Huy (2019), “Facile synthesis of single phase α-Zn2SiO4:Mn2+ phosphor via high-energy planetary ball milling and postannealing method”, Journal of Luminescence, vol 215, 116612 pp 1-8 L.T.T Vien, Nguyen Tu, N.Tri Tuan, N.D Hung, D.X Viet, N.T Khoi and P.T Huy (2018), “Near infrared-emitting Zn2SiO4 powders produced by high-energy planetary ball milling technique”, Vietnam Journal of Science and Technology, vol 56, pp 212-218 L.T.T Viễn, T.T Phương, N Tư, N.T Tuấn, N.T Khôi P.T Huy (2017), “Nghiên cứu chế tạo khảo sát tính chất quang vật liệu Zn2SiO4 khơng pha tạp pha tạp Mn2+ chế tạo phương pháp nghiền bi hành tinh lượng cao”, Proceeding of the tenth national conference on solid state physics and material sciences (SPMS-2017), pp 594-598 L.T.T Viễn, N.V Quang, N T, N.T Tuấn, N.T Khơi P.T Huy (2017), “Khảo sát tính chất quang vật liệu tổ hợp ZnO-SnO2 chế tạo phương pháp nghiền bi hành tinh lượng cao”, Proceeding of the tenth national conference on solid state physics and material sciences (SPMS-2017), pp 588-593 ... Huy (2017), ? ?Nghiên cứu chế tạo khảo sát tính chất quang vật liệu Zn2SiO4 khơng pha tạp pha tạp Mn2+ chế tạo phương pháp nghiền bi hành tinh lượng cao”, Proceeding of the tenth national conference... Chapter Optical properties of Zn2SiO4 and Zn2SiO4: Mn2+ phosphors - Chapter Optical properties of Zn2SnO4 and Zn2SnO4: Mn2+ phosphors - Chapter Optical properties of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+,... of Dq / B 1.2 Figure 4.17 PL and PLE spectra of Zn2SnO4: 5%Mn2+ (a) and Tanabe – Sugano diagram of 3d5configuration of Mn2 +ion in Zn2SnO4 crystal field (b) 16 4.4 Testing the application of Zn2SnO4: