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

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

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MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

Le Thi Thao Vien

Synthesis and properties of undoped and

Majors: Materials Science

Code: 9440122

SUMMARY OF DOCTORAL DISSERTATION ON

MATERIALS SCIENCE

Hanoi – 2020

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The dissertation was completed at Hanoi University of

Science and Technology

Advisors:

1: Prof Dr Pham Thanh Huy

2: Dr Nguyen Thi Khoi

The dissertation can be found the libraries:

1 Ta Quang Buu Library – Hanoi of science and technology

2 Vietnam National Library

A INTRODUCTION

1 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, energy-savings, 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

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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 Mn4+ 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 (Mn 2+ , Cr 3+ ) doped Zn 2 SiO 4 and Zn 2 SnO 4 phosphors

2 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

3 Research method

- The primary research method of the dissertation is the experimental method In this dissertation, all samples were fabricated by high high-energy 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

4 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

- 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

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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

5 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 Zn2SiO4 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 non-rare 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 phosphor-converted LED models, and this is an important technological step that the results obtained can help evaluate the practical applicability of phosphor-converted LEDs in this material system

6 The structure of the dissertation

The content of the dissertation consists of 5 chapters as follow:

- Chapter 1 Introduction

- Chapter 2 Experimental techniques

- Chapter 3 Optical properties of Zn2SiO4 and Zn2SiO4: Mn2+ phosphors

- Chapter 4 Optical properties of Zn2SnO4 and Zn2SnO4: Mn2+ phosphors

- Chapter 5 Optical properties of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+

phosphors

B CONTENTS Chapter 1 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 2 EXPERIMENTAL TECHNICS

Figure 2.1 Synthesis Zn 2 SiO 4 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 2 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 β-irradiation by using a TL-Reader (Harshaw-3500) with a heating range of

50 to 450 C and a heating rate of 2 C.s-1

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 Zn 2 SiO 4 phosphors 3.2.1 X-ray diffraction of Zn 2 SiO 4

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 Zn2SiO4

Figure 3.1 XRD patterns of ZnO-SiO 2 powder with weight ratio of 1:2 after high-energy planetary ball milling for 40 hours and annealing

at different temperature for 2 hours in air environment

3.2.2 Phosphor morphology of Zn 2 SiO 4

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 2 hours in an air environment 3.2.3 Vibrational analysis: F-TIR of Zn 2 SiO 4

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-SiO 2 powder (with weight ratio of 1:2) after high-energy planetary ball milling for 40 hours (a) and

3.2.4 Optical properties of Zn 2 SiO 4

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-SiO 2 and after annealing at different temperatures for 2 hours in air environment (a) and Gaussian Fitted of PL spectrum (b)

3.3 Structural and optical properties of Zn 2 SiO 4 :Mn 2+

3.3.1 X-ray diffraction of Zn 2 SiO 4 :Mn 2+

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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

different temperatures

Figure 3.8 XRD patterns of Zn 2 SiO 4 :x%Mn 2+

3.3.2 Phosphor morphology of Zn 2 SiO 4 : Mn 2+

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

40 hours (a), the milled sample and annealed at different temperatures for 2

hours in air

3.3.3 Vibrational analysis of Zn 2 SiO 4 : Mn 2+

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

at 462 and 586 cm-1 are related to symmetric stretching vibrations of ZnO4

group and the absorption peaks at 880 cm-1 are due to asymmetric stretching vibrations of SiO4 group The results of Raman scattering spectra in Figure 3.11 show the appearance of the oscillation mode typical for the symmetric knife of the siloxane group (Si – O – Si bond) in -Zn2SiO4 crystal at the frequency of 865 cm-1 Thus, FTIR and Raman results have shown the formation of the -Zn2SiO4 phase at a relatively low temperature (900 C) when it is produced by high energy planetary ball mill in combination with

annealing in the air environment

Figure 3.10 FTIR spectra of 5 wt% Mn 2+

annealed at different temperatures

Figure 3.11 Ramann spectra of 5 wt% Mn 2+

annealed at different temperature

3.3.4 Optical properties of Zn 2 SiO 4 :Mn 2+

Figure 3.12 is the photoluminescence (PL) and excitation photoluminescence (PLE) spectra of the Zn2SiO4: Mn2+ phosphor Compared with the PL and PLE spectra of the undoped sample (Zn2SiO4), the PL spectrum of the doped sample reveals an intensive green emission band peaking at around 525 nm This is the well-known green emission band of α-Zn2SiO4: Mn2+ and is originated from the electronic transitions

4T1-6A1 in Mn2+ ions PLE spectra show that besides the strong absorption

in the region of 250-300 nm, in the PLE spectrum of Zn2SiO4: Mn2+ samples also appeared weak absorption peaks in the 350-460 nm area at 357, 379,

421 and 433 nm This is an experimental demonstration showing the ability

to stimulate the sample of Zn2SiO4: Mn2+ with UV LED and blue LED excitation sources

The effect of the PL intensity on annealing temperature (for 5% doped samples) and doped concentrations (from 2 to 8%), as shown in figure 3.13 and 3.14, show the peak PL at 525 nm, which increases with increasing annealing temperature and reach maximum intensity at 1250 C

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Figure 3.12 (a) PLE spectra measured at maxima of the emission at 740 nm (curve 1) and 525 nm

doped with 5% Mn

At higher temperatures, the PL intensity decreases Similarly, the PL intensity increases with increasing doped Mn2+ concentration and reaches a maximum at 5% doped concentration The PL intensity decreases at higher concentrations of Mn2+ doped

Figure 3.13 PL spectra of Mn 2+ doped

Figure 3.14 PL spectra of Zn 2 SiO 4 :x%Mn 2+

3.3.5 Thermoluminescence (TL) properties and decay time

of Mn 2+ doped Zn 2 SiO 4

Figure 3.15 TL spectra of Zn 2 SiO 4 :5%Mn 2+ under various β-ray exposure time(a) and decomposition of glow curve into individual peaks (b)

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 Mn2+ 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 o C (a) and the

3.3.6 Testing the application of Zn 2 SiO 4 : Mn 2+ phosphor in fabricating the phosphor-converted LED

The applicability of fabricated Zn2SiO4: Mn2+ materials was evaluated by coating the colloidal silicon solution containing Zn2SiO4: 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.