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 zn

LIST OF FIGURESoctahedral symmetry: Ligands for tetrahedral symmetry octagonal symmetry field in different symmetryThe separation of energy levels of some transition metal level separati

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

LE THI THAO VIEN

Synthesis and properties of undoped and transition

metal (Mn 2+ , Cr 3+ ) doped Zn2SiO4

DOCTORAL DISSERTATION ON MATERIAL SCIENCES

HANOI – 2020

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

LE THI THAO VIEN

Synthesis and properties of undoped and transition

metal (Mn 2+ , Cr 3+ ) doped Zn2SiO4

Majors: Material Sciences

Code: 9440122

DOCTORAL DISSERTATION ON MATERIAL SCIENCES

ADVISORS:

HANOI – 2020

Although my name is on the cover of this dissertation, many people were ofgreat importance to this research I want to take a moment to extend my gratitude tothe involved

The first, I would like to express my sincerest thanks to my supervisor, Prof.Pham Thanh Huy, excellence and estimable teacher, for all of his supports Hisdedication to science has been encouraging me so much, protected me from theconfusion since I started studying and researching at the Advanced Institute forScience Technology (AIST)

This dissertation was carried out at AIST, together with several researchgroups researches I had garnered variable information from these seminars withfree discussions coming from all of our group members Possibly just as important

as the practical aid was the friendly, cooperative atmosphere at AIST; it made meenjoy virtually every second of working on my dissertation I wish to thankAssociate prof Dao Xuan Viet; Dr Nguyen Tu; Dr Nguyen Duy Hung, and all of

my teammates for their friendships with kind-hearts and unconditional assistance.The last few months weren’t easy, and I want to thank all my dearest friends,who helped me get back on track when I lost my laptop and found many difficulties

in life Without your care, understanding, and motivational speeches, this thesiswould no doubt look different and not for the better Your friendship makes merealize what a lucky person I am

For the last, more than I can say, I would like to express manifest thanks to myhusband and two children for always being by my side, putting their truth in meduring my duration at AIST

Lastly, I want to mention my father, mother, my parents-in-law, and twosisters, and thank them for making me the person that I have become

Le Thi Thao Vien

ii

LIST OF FIGURES viii

LIST OF TABLES xiv

BRIEF INTRODUCTION 1

Chapter 1 INTRODUCTION 8

1.1 Background of Luminescence 8

1.1.1 Luminescence 8

1.1.2 Optical quenching 9

1.1.3 Electroluminescence 9

1.1.4 Thermoluminescence 10

1.2 Background of Transition Metal (TM) ions in the crystal field………10

1.2.1 Transition metals………10

1.2.2 The effect of crystal fields on the separation of TM ions……… 11

1.2.3 Tanabe-Sugano diagrams 15

1.2.4 Energy levels of Mn2+ ion in a crystal field 18

1.2.5 Energy levels of Cr3+ ion in a crystal field 20

1.3 Literature review of transition metal (Mn2+, Cr3+) doped Zn2SiO4 and Zn2SnO4 phosphors 22

1.3.1 Structure and optical properties of Zn2SiO4: Mn2+……… 22

1.3.2 Structure and optical properties of Zn2SnO4, Zn2SnO4:Mn2+ ……… 24

1.4 Phosphor-based LEDs 26

1.4.1 LED 26

1.4.2 Phosphor-based LEDs 27

1.4.3 LED application in agricultural lighting 30

Chapter 2 EXPERIMENTAL TECHNICS 32

2.1 Introduction 32

2.2 Synthesis of Zn2SiO4, Zn2SiO4:Mn2+, Zn2SnO4, Zn2SnO4:Mn2+, Zn2SnO4:Cr3+, Zn2SnO4:Cr3+, Al3+ 33

2.2.1 Materials 33

2.2.2 Synthesis of Zn2SiO4 33

2.2.3 Synthesis of Zn2SiO4: Mn2+ 34

2.2.4 Synthesis of Zn2SnO4 34

2.2.5 Synthesis of Zn2SnO4:Mn2+ 34

2.2.6 Synthesis of Zn2SnO4:Cr3+ and Zn2SnO4:Cr3+, Al3+ 34

2.2.7 Mechanical milling 35

iii

2.3 Techincal methods 35

2.3.1 Structural characterisation 35

2.3.2 Photoluminescent characterization 30

2.4 LED package process 43

2.4.1 Die bonding 44

2.4.2 Wire Bonding 45

2.4.3 Phosphor Dosing 45

2.4.4 Dispensing 46

2.4.5 Curing 47

2.4.6 Testing 47

Chapter 3 STRUCTURE AND OPTICAL PROPERTIES OF Zn2SiO4 AND Zn2SiO4:Mn2+ PHOSPHORS 48

3.1 Introduction 48

3.2 Structure and optical properties of Zn2SiO4 phosphors 49

3.2.1 X-ray diffraction of Zn2SiO4 49

3.2.2 Phosphor morphology of Zn2SiO4 50

3.2.3 Vibrational analysis: Raman spectra of Zn2SiO4 51

3.3 Structure and optical properties of Zn2SiO4:Mn2+ phosphors 55

3.3.1 X-ray diffraction of Zn2SiO4:Mn2+ 55

3.3.2 Phosphor morphology of Zn2SiO4:Mn2+ 57

3.3.3 Vibrational analysis of Zn2SiO4:Mn2+ 58

3.3.4 Optical properties of Zn2SiO4:Mn2+ 61

3.3.5 Thermoluminescence (TL) properties and Decay time of Mn2+ doped Zn2SiO4 64

3.3.6 Application of Mn2+ doped Zn2SiO4 on UV LED 66

3.4 Conclusion 67

Chapter 4 STRUCTURE AND OPTICAL PROPERTIES OF Zn2SnO4 AND Zn2SnO4:Mn2+ PHOSPHORS 68

4.1 Introduction 68

4.2 Structural and optical properties of Zn2SnO4 phosphors 69

4.2.1 X-ray diffraction of Zn2SnO4 69

4.2.2 Optical properties of Zn2SnO4 74

4.3 Structural and optical properties of Zn2SnO4:Mn2+ 80

4.3.1 X-ray diffraction of Zn2SnO4:Mn2+ 80

4.3.2 Phosphor morphology of Zn2SnO4:Mn2+ 84

4.3.3 Optical properties of Zn2SnO4:Mn2+ 84

iv

4.3.4 Decay time of 5%Mn2+ doped Zn2SnO4 89

4.3.5 Temperature-dependent PL and internal quantum efficiency of Zn2SnO4:5%Mn2+ phosphors 91

4.3.6 Application of un-doped and Mn2+ doped Zn2SnO4 on LED 92

4.4 Conclusion 93

Chapter 5 OPTICAL PROPERTIES OF Zn2SnO4:Cr3+ AND Zn2SnO4:Cr3+, Al3+ FOR PLANT CULTIVATION LED 95

5.1 Introduction 95

5.2 Structural and optical properties of Zn2SnO4:Cr3+ phosphors 97

5.2.1 X-ray diffraction of Zn2SnO4:Cr3+ 97

5.2.2 Phosphor morphology of Zn2SnO4:Cr3+ 100

5.2.3 Optical properties of Zn2SnO4:Cr3+ 101

5.2.4 Application of the prepared phosphor for fabricating infrared LEDs 105 5.3 Structural and optical properties of Zn2SnO4:Cr3+, Al3+ phosphors 106 5.3.1 X-ray diffraction and FESEM of Zn2SnO4:Cr3+,Al3+ 106

5.3.2 Crystal field analysis 109

5.3.3 The effect of Al3+ on optical properties of ZTO: Cr3+ 111

5.3.4 Application of the prepared phosphor 116

5.4 Conclusion 117

CONCLUSIONS AND FUTURE WORKS 120

PUBLICATIONS 123

RELATED PUBLICATIONS 124

REFERENCES 125

v

Field emission scanning electron MicroscopePhotoluminescence excitation

UltravioletHalf-Width at half-maximumInfra-red

Transition MetalElectroluminescenceNon – bridging oxygen hole centersRed, Green and Blue

Fourier – transform infrared spectroscopyHigh – energy planetary ball mill

Advanced Institute for Science and Technology

Joint committee on powder diffraction standards

Full width at half maximum

vi

Correlated color temperatureBrurstein – Moss

White light-emitting diodeQuantum efficiency

Atomic orbitals

vii

LIST OF FIGURES

octahedral symmetry: Ligands for tetrahedral symmetry

octagonal symmetry

field in different symmetryThe separation of energy levels of some transition metal

level separation of Cr3+ ions when take into account thespin-orbit interaction L-S (with B = 918 cm-1) (b)

Energy level diagram for the d2 configuration (From

Theory and its Applications, Syokabo, Tokyo, 1969 (inJapanese)

Energy level diagram for the d3 configuration (From

Theory and its Applications, Syokabo, Tokyo, 1969 (inJapanese)

Energy level diagram for the d5 configuration (From

Theory and its Applications, Syokabo, Tokyo, 1969 (inJapanese)

Figure 1.9 Tanabe–Sugano diagram for the Mn

2+

in Zn2SiO4 crystal

19field

3+

electron

21configuration in the octahedral crystal field C/B = 4.7

(a) The number of SiO4− units that are connected togetherFigure 1.11 by sharing the oxygen atoms and (b) Structure of the23

Willemite -Zn2SiO4

viii

structure (C) Tandem structure (D) Pixelated structure.

(E) Down-conversion white LEDs

Three principal white-lighting strategies (a) A three-LEDstrategy with red, green, and blue (RGB) LED chips (b)Figure 1.14 A three-phosphor strategy with a UV LED and RGB 27

phosphors (c) A blue LED with a yellow down-converting phosphor

mechanical milling

microscope

fluorescence

spectra

containing type LED package (b)

of 1:2 after high-energy planetary ball milling for 40 hours

ix

and annealing at different temperatures for 2 hours in airenvironment.

FESEM and EDS images of Zn2SiO4 powder with theweight ratio of 1:2 after high-energy planetary ball millingFigure 3.2 for 40 hours (a) and annealing at 500 C (b); 900 C (c); 51

Gaussian Fitted of PL spectrum (b)

of Zn2SiO4

PL spectra of Zn2SiO4 with different ratio of ZnO:SiO2 atFigure 3.6 (0.5:2 (a); (1:2 (b); (2:1(c) and (2:2(d)) calcinated at 1250 54

CXRD patterns of 5 %wt Mn2+-doped Zn2SiO4 powdersFigure 3.7 after ball-milling for 40 hours without and with annealing 55

at different temperatures in the range of 500 – 1350 C inair

Figure 3.8 XRD patterns of Zn2SiO4:x%Mn

2+

(x=0-8) samples after

57milling followed by the annealing in air at 1250 C

FESEM images of 5 wt % Mn2+ doped ZnO/SiO2 powdersafter milling for 40 hours (a), the milled sample and

Figure 3.9 annealed at different temperatures for 2 hours in air: 500 58

hours in air: 500 °C (b), 900 °C (c), 1150 °C (d), 1200 °C(e), 1250 °C (f) , and 1300 °C (g)

Raman spectra of 5 wt% Mn2+-doped ZnO/SiO2 powders after milling for

40 hours (a), the samples milled for 40 Figure 3.11 hours followed byannealing at different temperatures of 500 °C (b), 900 °C (c), 1150 °C (d),

1200 °C (e), 1250 °C(f) , 1300 °C (g), for 2 hours in air

nm (curve 1) and 525 nm (curve 2); and (b)

temperatures in the range of 500-1350 C

PL spectra of Zn2SiO4:x%Mn2+ (x=0-8) samples after

milling followed by the annealing in air at 1250 C The

Figure 3.14

inset displays the dependence of the PL intensity on Mn2+

doping concentrations

and 235 oC (a) and the decay curve of the Zn2SiO4:5%wtFigure 3.17

Mn2+ phosphor (b) The decay curve of Zn2SiO4 sample is shown in the inset of Fig 3.17b

The CIE chromaticity coordinates of a green LED device fabricated bycoating HEBM-SLS Zn2SiO4:5wt%Mn2+ Figure 3.18 phosphor on 270

nm UV-LED under drive current of 60 mA and the image of actual green

LED device taken bydigital camera

XRD patterns of ZnO/SnO2 powder after milling for 60

xi

FESEM images of Zn2SnO4 powders after ball-milling for

60 hours without or with annealing in air at differenttemperatures in the range of 600-1200 C

The grain-size distribution of the as-obtained phosphor aftercalcinating at different temperatures (a) 900 C; (b) 1000 C;(c) 1100 C and (d) 1200 C

UV-Vis spectra of ZnO-SnO2 powders after ball-milling for

60 hours, followed by annealing at different temperatures inair: (a) 900 °C, (b) 1000 °C and (c) 1100 °C

PL spectra of milled-samples for 60 hours, followed by (a)without and (b) with annealing at 1000 ˚C in air

PL spectra of Zn2SnO4 powders after ball-milling for 60hours without or with annealing in air at differenttemperatures in the range of 600-1200 C

Deconvoluted photoluminescence spectra of the milled

Zn2SnO4 powder annealed at 1000 ˚C in airEnergy level diagram showing some of the principal defectlevels in Zn2SnO4 Zn2SnO4 has an assumed band gap of 3.7eV

Thermal analysis of as-milled powder (a) and XRD pattern

of Zn2SnO4:5%Mn2+powder without annealing andannealed at various temperature(b)

EDS spectra of Zn2SnO4:Mn2+ annealed at 1000 C (a) and

1100 C (b)Powder XRD pattern of ZTO dop with different %concentration of Mn2+ions

FESEM images of the as-milled powder (a) and the samplesannealed at 700 C (b), 800 C (c), 900 C (d), 1000 C (e),

1100 C (f)UV-Vis spectra of the un-doped and Mn2+doped Zn2SnO4 at

1000 C

PL spectra of 5 %wt Mn2+ doped Zn2SnO4, powders aftermilling for 40 hours followed by the annealing at differenttemperatures

73

74

75

76777880

81828384

8586

xii

Figure 4.16 PLE (a) and PL (b) spectra of the sample Zn2SnO4 doped 87

with different concentration of Mn2+ calcinated at 1000 °C

PL and PLE spectra of Zn2SnO4:Mn2+ The insert: PhotosFigure 4.17 of the phosphor under nature and UV light (a) and Tanabe 88

– Sugano diagram of the 3d5 configuration of Mn2+ ions

in the Zn2SnO4 crystal field (b)Figure 4.18 Decay curve of Zn2SnO4:5%Mn

2

(black line) and its

89double exponential fitted (red line)

The temperature-dependent PL spectra of the phosphor

The insert: Integrated PL intensity as sample temperatureFigure 4.19 (a) The PL spectra of 430 nm light source, scattered light 90

from excitation source and luminescence light from thephosphor

light and YuJi LEDCIE 2015-10° xy color chromaticity coordinates of thespectral 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+ phosphorsFigure 4.22 (b) The insets in (a) are the electroluminescence spectrum 93

(1) and the image captured by a digital camera (2) of thegreen-emitting LED The insets in (b) are theelectroluminescence spectrum (1) and the image captured

by a digital camera (2) of the white-emitting LEDFigure 5.1 XRD patterns of Zn2SnO4:3%Cr

3+

powder un-annealed

97and annealed in the range of 900-1200 °C in air

Figure 5.2 XRD patterns of Zn2SnO4:x%Cr

3+

(x=0-6%) powder

99annealed at 1100 °C in air

Figure 5.3 FESEM of Zn2SnO4:3%Cr

3+

powder un-annealed and

100annealed in the range of 900-1200 °C in air

Figure 5.4 PL spectra of the Zn2SnO4: Cr

3+

obtained at different

101annealing temperature

Figure 5.5 PL spectra of the Zn2SnO4: xCr

3+

(x=1-6%) obtained at

102

1100 C

PLE spectra of the Zn2SnO4: xCr3+ (x=1-6%) obtained at

Figure 5.6

1100 CFigure 5.7 Tanabe – Sugano diagram for Cr3+ doped ZTO phosphor

PL spectra of LED device using 460 Blue LED Chip

FESEM image and EDS spectra of pure ZTO (a, d), ZTO:

Figure 5.10 Cr3+ (b, e); ZTO: Cr3+, Al3+ (c, f) calcinated at 1100 °C in

air

Tanabe – Sugano diagram for Cr3+ and Cr3+, Al3+ in ZTOFigure 5.11

phosphorUV-vis spectra of the prepared phosphor The inset is theFigure 5.12 estimated band gap of the ZTO: 3%Cr3+ and ZTO:3%Cr3+,

x%Al3+(x=0.2,0.4,0.6,0.8) phosphorOptical band gap shifts as a function of carrier

Figure 5.13

concentrationPhotoluminescence excitation spectra λemi. = 730 nm) ofFigure 5.14 pure ZTO (the black line), ZTO: Cr3+ (the blue line); ZTO:

Cr3+, Al3+ (the red line) calcinated at 1100 °C in air Photoluminescence spectra (λexci.=460 nm) of pure ZTOFigure 5.15 (the black line), ZTO: Cr3+ (the blue line); ZTO: Cr3+,Al3+

(the red line) calcinated at 1100 °C in airDecay time curve of pure ZTO (the black line), ZTO: Cr3+

Figure 5.16 (the blue line); ZTO: Cr3+, Al3+ (the red line) monitored at

105

107

108109111

112

113

114115

116

xiv

LIST OF TABLES

configuration in terms of the Racah parameters B and Crelated to the energy of the ground term 6S

configuration in terms of the Racah parameters B and Crelated to the energy of the ground term 4F

Table 1.4 Splitting of LS terms of the Cr3+ ions in the octahedral or 20

tetrahedral crystal fields

lattice

the most important LED phosphors

ZnO/SiO2 as-milled sample, and the 40h milled samplesannealed at the different temperatures in air, calculatedfrom the FWHM of the peaks at 2 =35.9 (101) for thesample annealing at T ≤ 900 C and 2 =33.8 (410) forthe sample annealing at higher temperatures

evaluated using R Chen method

at different temperatures in airTable 4.2 The crystal sizes of Zn2SnO4 particles calculated by 71

Debye-Scherrer’s equation

Mn2+ (3-7% Mn) of the samples treated at 1000 C

temperature (CCT); Colour rendering index (CRI);

xv

Luminous efficacy of radiation (LER); R9 and CIE coordinates(x,y)) of LED covered phosphor.

Table 5.1 The crystal sizes of Zn2SnO4:3%Cr3+ particles calculated 98

in ZTO host sample

xvi

Chapter 1 BRIEF INTRODUCTION

1 Essentials of research project

Currently, white light-emitting diodes (WLEDs) have replaced traditional lightsources owing to their high luminous efficiency, environment friendliness, longlifetime, energy-savings, and compact size [1–3] A WLED is usually created bythree 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 chipwith single phased Y3Al5O12: Ce3+ yellow or mixed green and red phosphors [4,5].Because the first approach shows lots of disadvantages, such as complicatedelectrics, high cost and mismatched aging properties (different thermal and drivingbehaviors), etc., then the two following fabrication methods making use ofphosphors have become the primary trend in the academic researches and practicalapplications In these two methods, phosphors -based WLEDs are considered as one

of the essential factors in determining the quality of WLEDs Thus, synthesis anddevelopment of phosphors with different emission colors and low costs are beingexplored and developed for lighting Also, this is evaluated as the most critical andurgent challenges in the lighting field

In general, the phosphors, namely luminescence materials, are constructed by amatrix (crystalline host) doped with an activator (luminescent center) To be used as ahigh-quality solid-state light source, phosphors – based LEDs should contain thefollowing essential characteristics Firstly, the excitation spectrum should match wellwith the pumping LED chips and shows a broad absorption in the n – UV region (360

– 420 nm) or blue region (420 – 480 nm) Secondly, the emission spectrum of LEDchips combined with the emission spectrum of phosphors produce a pure whiteemission with a specific color rendering index (CRI) and corresponding colortemperature (CCT); thirdly, efficient luminescence with high quantum efficiency.Also, they should have excellent physical, chemical, and thermal properties.Concerning these factors, the choice of material used for LED phosphors willinclude 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 based

on the understandings of the crystal and local structures, so that the PL spectrum can betailored The optical characteristics of phosphor materials are mainly affected by thehost structure and coordination environment around the activator ion Presently, themost widely used host materials for WLED phosphors are aluminates (Y3Al5O12,CaY2Al4SiO12, BaMgAl10O17, CaAl12O19, Sr2MgAl22O36,…), silicates (A2SiO4 (A = Ca,

Sr, Ba), Ca3Sc2Si3O12, A3B2C3O12, M5(Si3O9)2, and Ca3Si2O7,…), Sulfides(Y2CaSr)F4S2, (Y,Gd)FS, Y2(Ca,Sr)F4S2, CaSrLaGa3S6O,…), borates (LiSr4(BO3)3,

NaSr4(BO3)3, NaSrBO3,Na3SrB5O10, NaSrB5O9, and NaBa4(BO3)3, MM’4(BO3)3 (M

= Li, M’= Sr; M = Na, M = Sr, Ba), phosphates (LiSrPO4, KSrPO4, KBaPO4,

Ca3(PO4)2, Sr8MgLa(PO4)7, ), nitrides (BaLi2Al2Si2N6, CaAlSiN3, Ca(LiAl3N4)2, ),fluorides, chlorides, and so on For instance, the aluminate phosphors have drawn muchattention due to high quantum conversion efficiency and wide excitation range

The most famous phosphate phosphors are Y3Al5O12: Ce3+ and its ramification.And the first phosphor-converted-WLED was fabricated by combining the yellow-

emitting Y3Al5O12: Ce3+ (YAG: Ce) phosphor with a blue-emitting InGaN chip [6,7].Firstly reported by Seshadri’s group, Sr2.975-xBaxCe0.025AlO4F (SBAF:Ce3+) wasanother new phosphor in the aluminate family [8] Emitting efficiently green light

under 400 nm excitation, Mn4+ doped aluminates were another promising system todiscover line-type red-emitting phosphors, such as CaAl12O19: Mn4+ [9],

Sr2MgAl22O36: Mn4+ [10], SrMgAl10O17:Mn4+ [11], and so on BaMgAl10O17 is a

well-known aluminate phosphor host, and Wang et al reported that BaMgAl10O17:

Mn4+, Mg2+ phosphors emit narrow-band (FWHM =30 nm) in the red region (peaking

at 660 nm), attaining a high color purity and excellent color stability against heat [12].Besides, rare-earth-activated silicates are widely used as WLEDs phosphors becausethey have versatile chemical compositions and crystal structures, luminescencetunability, high quantum efficency(QE), low cost, and so on [13,14] Among them, Zn-Si-O and Zn-Sn-O compounds with a wide band gap, have attracted much attention ofscientists because of their interesting optical properties which can be used

in WLEDs [15,16] For instance, Eu3+ and Ca2+ co-doped Zn2SnO4 were synthesizedusing a hydrothermal method, and its emission was in the red band, as reported by

Krishna Sagar et al in 2018 [17] In 2016, Dimitrievska et al synthesized Eu3+

doped Zn2SnO4 by solid-state reaction method The prepared phosphor showedseveral emission bands, including narrow bands of magnetic dipole emission at 595

nm and electric dipole emission at 615 nm [18] Li et al [19] reported near-infrared

luminescence of Zn2SnO4: Cr3+, Eu2+ Besides, Mn2+-doped -Zn2SiO4 phosphorexhibited a single intense green emission band centered at 525 nm due to theelectronic transitions 4T1-6A1 of Mn2+ was reported by Ming Zhang et al.[20].

Secondly, selecting suitable activator ions for doping in the crystal host asluminescence centers also essential The reason is that they play an important role in

PL tuning and luminescence optimization Nowadays, the synthesis and searching ofphosphors for the WLED application are 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 WLDEs of these phosphors have been limited Metaltransition ions, such as Mn2+, Mn4+, Cr3+, and so on, are less expensive andenvironment-friendly Hence, recently eco-friendly phosphors based on nonrare

2

- earth receive increasing interest in the field of WLEDs Among them, Mn ions,which have been widely used in phosphor materials, are common non-rare earth

luminescence centers The luminescence behavior of Mn2+ ion-doped materials hasbeen thoroughly investigated As an example, Liu’s group reported narrow-band red-

emitting fluoride phosphor KNaSiF6: Mn2+ for warm WLEDs [21] Wang’s groupreported the HF-free hydrothermal synthesis of K2SiF6: Mn2+ phosphors [22] Mn2+ions doped ZnAl2O4, Zn2SiO4, Zn2GeO4, MgGa2O4, CaCl2, CdSiO3, NaCaPO4 havebeen studied as phosphors in fluorescent lamps and WLEDs [23-27] According tothese reports, the low energy levels of Mn2+ ions strongly depend on the co-valencyinteraction with the host crystal or the crystal field In a suitable host lattice, theemission color of the Mn2+ 4T1→6A1 transition is tunable from blue to red Forexample, ZnAl2O4: Mn phosphors produced by the sol-gel method, providing green

and red emissions, were reported by D Zhang et al.[23] and Q Zhou et al gave a

review of synthesis, luminescence and applications in WLEDs of Mn2+ and Mn4+ redphosphors [28] They suggested that the crystals with co-existing Mn2+ and Mn4+ ionsemit green and red light and have the potential to be used as red-green-emitting

phosphors for blue LEDs Like Mn4+ ions, Cr3+ ions activated phosphors show strongabsorption in the blue region (400-480 nm) and strong emission in the infrared range

(700-740 nm) The Cr3+ activated phosphors were studied the long afterglow

properties [29–34]

Third, the most important factor is that activators are doped in crystal hostlattice to produce suitable emission and excitation spectra that match an LEDapplication When activators are doped to host lattice, their local coordinationenvironment will be affected by the host crystal field, which makes the change inphosphor properties such as excitation and emission wavelengths, luminescenceefficiency, and resistance to thermal quenching effects The doping of Mn2+ or Cr3+

to ZnO-SiO2/SnO2 have been synthesized and reported by many previous works[20,26,30,35–53] Based on the similar ionic radii and oxidation states of Zn, Snand Mn, Cr (0.60 Å for Zn2+ and 0.66 Å for Mn2+), Mn2+, Cr3+ ions may havesubstituted the Zn2+ or Sn4+ sites in the ZnO-SiO2/SnO2 matrix and make them bepromising phosphors for applications in phosphor-converted WLEDs However,most studies above focus on the gas-sensitive [54], phosphorescent [29,56], orphotocatalyst properties [57–61] of materials There are only a small number ofreports that focus on the optical properties of these materials Furthermore, theapplication of these phosphors on the WLED has not been much regarded

Beside, phosphors based on metal transition are mainly doping Mn4+ ions into hostlattice to supplement the red zone to increase CRI in the WLED application [4,9–11,21] Whereas, the green light-emitting phosphors produced from ZnO-SiO2/SnO2

doped Mn2+ with high color purity can compensate for the missing green light inthe spectrum of LED generated from Blue chip and YAG yellow powder Or thephosphors combined with red and blue powder also help improve the CRI ofWLED So that, these problems have been much regarded.

In addition to general lighting, LED is also used for some specialized lightingsuch as LED lighting to stimulate flowering of plants In particular, Cr3+ dopedZnO-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, theuse of this material in this purpose has not been studied

In VietNam, to the best of our knowledge, phosphors based on manganese orchromium activated Zinc silicate and Tin stannate have not been reported yet

In this work, we focus on the synthesis and properties of un-doped andtransition metal (Mn2+, Cr3+) doped Zn2SiO4 and Zn2SnO4 phosphors

2 The goal of the research project

The thesis includes some primary 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 ballmilling, followed by annealing in the air or a reducing atmosphere

Study the effects of the annealing temperature and the doped concentration onthe structural and optical properties of fabricated materials systems

Evaluate the applicability of produced phosphors through the evaluation of theLED devices' parameters fabricated by directly coating the produced phosphors onthe ultraviolet or blue LED chips

3 Research method

The primary research method of the dissertation is the experimental method Inthis dissertation, all samples were fabricated by high high-energy planetary ballmilling combined with annealing in the air or a reducing atmosphere at theAdvanced Institute for Science and Technology (AIST) The crystal structure,surface morphology, particle size, chemical composition, and optical properties ofthe produced phosphors are investigated using modern analytical techniques such asFESEM (FESEM – Jeol, JSM-7600F equipped with an energy dispersivespectroscope (EDS)) at the Advanced Institute for Science and Technology (AIST),X-ray ((XRD) using CuK radiation (Bruker D8 Advance) in the 2θ = 10o – 70orange at Can Tho university), FTIR (Perkin Elmer Spectrum GX spectrometer at 2

cm-1 resolutions at the Viet Nam national university), Raman (Horiba Jobin Yvon

4

LabRAM HR-800 spectrometer using He–Ne laser (632.8 nm) with a power density

of 215 W/cm2 at the Viet Nam national university) , and PL, PLE spectra (Nanologspectrometer (Horiba Jobin Yvon) equipped with a 450 W xenon lamp as anexcitation source at the Advanced Institute for Science and Technology (AIST)).The TL glow curves were recorded after 90Sr β-irradiation by using a TL-Reader(Harshaw-3500) with heating range of 50 to 450 C and at heating rate of 2 C.s-1.Decay curves were measured by Cary Eclipse Fluorescence Spectrophotometer(Agilent) Internal Quantum efficiency of the phosphor, luminous efficiency andchromaticity coordinates of LED devices were measured by a LED testing system(Gamma Scientific) at the Advanced Institute for Science and Technology (AIST)

4 New contributions of the dissertation

Three groups of materials: Zn2SiO4 and Zn2SiO4: Mn2+; Zn2SnO4 and Zn2SnO4:

Mn2+; Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al 3+ have been successfully fabricated

by high energy planetary ball milling followed by annealing, which temperatures200-300 °C lower than those synthesized by the conventional solid-phase method

Zn2SiO4 phosphor represents a broadband emission spectrum peaking at 735

nm in the infrared region - an effective excitation wavelength for phytochromes, so

it has potential for application in specialized LEDs for agricultural lighting

Zn2SiO4: Mn2+ phosphor gives green emission spectrum with high color purity( 85%) when measured on LEDs fabricated by coating Zn2SiO4: Mn2+ powder onthe UV LED chip

A new infrared emission peak at 684 nm has been first time observed in theemission spectrum of Zn2SnO4 phosphor - capable of being used for specializedLED in agriculture

Zn2SnO4: Mn2+ phosphor has been first time synthesized, shows stronglyabsorbing 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 ablue LED chip

Zn2SnO4: Cr3+ material was first studied for their fluorescence properties Theemission spectrum gives a broadband peaking at 740 nm in the infrared region Theexcitation spectrum shows strong absorption of blue light (460 nm) The resultindicates that Zn2SnO4: Cr3+ phosphor can be used in specialized LEDs to stimulateflowering

Zn2SnO4: Cr3+, Al3+ phosphor has a broad photoluminescence spectrum peaking at

730 nm in the infrared region The excitation spectrum shows strong absorption of

blue light (450 nm) The excitation and emission spectra are 10 nm blue shiftcompared to those of Zn2SnO4: Cr3+ The cause of the peak excitation and emissionshifted is because of the Burstein–Moss shift.

❖ The scientific significance:

The dissertation has introduced an effective method of fabricating luminescentmaterials by combining the traditional solid-state reaction method and the highenergy ball milling method

The results of research on transition metal doped Zn2SiO4 and Zn2SnO4

phosphors have been presented systematically in this dissertation - a new researchtrend on environmentally friendly, non-rare earth phosphors Therefore, thedissertation 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 practicalproblem, which is to synthesize new types of environmentally friendly non-rareearth phosphors used in WLED or specialized LEDs for Agricultural lighting

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 arebeing tested on the application of phosphor-converted LED models This is animportant technological step that the results obtained can help evaluate the practicalapplicability of phosphor-converted LEDs in this material system

6 The structure of the dissertation

Apart from the introduction and conclusion, the content consists of 5 chapters

as follow:

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 arepresented

Chapter 2 Experimental techniques:

In this chapter, experimental methods used to synthesize and characterizematerials are presented

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

In this chapter, firstly, the synthesis conditions of Zn2SiO4 and Zn2SiO4: Mn2+phosphors are estimated, and after those optical properties of Zn2SiO4 and Zn2SiO4:

Mn2+ phosphors are studied in detail

6

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

In this chapter, firstly, the conditional synthesis of Zn2SnO4 and Zn2SnO4:

Mn2+ phosphors are estimated, and then the optical properties of Zn2SnO4 and

Zn2SnO4: Mn2+ phosphors are studied in detail

Chapter 5 Structure and Optical properties of Zn2SnO4: Cr3+ and Zn2SnO4:

Cr3+, Al3+ plant cultivation LED

In this chapter, firstly, the synthesis conditions of Zn2SnO4 and Zn2SnO4: Cr3+phosphors are estimated and then optical properties of Zn2SnO4 and Zn2SnO4: Cr3+phosphors are studied in detailed Also, the effect of Al3+ on structural and opticalproperties of Zn2SnO4: Cr3+ is studied in detail

If, based on the lifetime of radiation, luminescence can be divided intofluorescence and phosphorescence When is called the time when the material emitsradiation after stopping stimulation (or called the lifetime of radiation), it isclassified as:

The luminescence process having <10-8 s is called the fluorescent process

The luminescence process with > 10-8 s is called the phosphorescent process

So, fluorescence is a process of luminescence that the emission happens almostsimultaneously with the absorption of energy by matter and shuts off immediatelyafter stopping the stimulation The phosphorescence is a very long process ofluminescence, in which the lifetime of the radiation can be delay hours afterfinishing the stimulation Furthermore, the lifetime is characterized by the delaybetween the energy absorption process and time to radiation reaches maximumintensity Phosphors can be excited by either optical or other excitation, such aselectrical energy, chemical energy, mechanical energy, etc If semiconductors givelight emissions by absorbing incident light, this is call photoluminescence (PL) Inthe mechanism of PL, electrons in the valence band of a semiconductor absorbphotons from light To be absorbed, the photons should have energy either equals orhigher than the energy band gap of the semiconductor, but still too low to causephotoionization, leaving an unoccupied state (or hole) in the valence band Theprocess is called the absorption process These excited electrons generated byoptical excitation will return to the ground state, accompanied by emitting photons,which is called the emission process Excitation and emission processes are themain mechanics in photoluminescence

8

1.1.2 Optical quenching

There are many reasons for the decrease in the luminous intensity of materials,

in which large concentrations of impurities can also cause some from externalphysical, chemical, and environmental agents such as temperature, humidity, etc orfluorescent depletion

The luminous intensity of the material is very sensitive to the concentration ofluminescent centers (or doped concentrations) in the matrix Typically, when theconcentration of luminescent centers is not too large, the concentration of dopingincreases gradually, the intensity of luminescence increases with the increase ofoptical centers When the concentration of the emitting centers reaches a certainthreshold value (critical value), the further increase in doping concentration willreduce the luminous intensity of the fluorescent powder This phenomenon is calledconcentration quenching

The cause of optical quenching due to concentration is the effect of transmittingenergy between impurity ions without photon emission when they have highconcentrations This phenomenon is created by one of three mechanisms: chargeexchange interaction, radiation reabsorption, and multi-electrode interaction.According to Blasse equation [62], the critical RC distance between the opticalcenters of optical quenching phenomenon due to the concentration is determined:

3

1/3

4

Where X C is the concentration of the activation center that starts the quenching

phenomenon, N is the cation number in the unit cell, and V is the volume of the base

cell

The charge exchange interaction usually takes place when the RC distancereaches a small value, ≈ 5 A ° [62] The reabsorption process may occur if materialscontain the luminescent center and sensitive light centers, and there must be anoverlap of the excitation spectra with the emission centers of the luminescentcenters The first two mechanisms are often difficult to occur in concentrationquenching Multi-electrode interactions, according to Dexter's theory [63], oftenplay a significant role in explaining the energy transfer process that reducesfluorescence when the concentration is doping high enough

1.1.3 Electroluminescence

Electroluminescence (EL) is a light-emitting phenomenon under electricalexcitation It is also considered as an electro-optic conversion process in which aphosphor emits light in response to the injection of electric current In the mechanism

of EL, holes, and electrons are separately injected into the valence and conduction

bands of the semiconductor The recombination of holes and electrons will releasetheir energy as photons-light The phenomena are often happening in inorganicsemiconductors, which have direct band gaps [e.g., the III–V compounds GaAs andindium phosphide (InP)], where holes and electrons in their valence and conductionbands, respectively, are considered as free carriers.

1.1.4 Thermoluminescence

Thermoluminescence (TL) is the phenomenon of light emission of asemiconductor when it is heated at a constant rate from room temperature to sometemperature after irradiated at low temperatures (room temperature or liquidnitrogen ) by ionizing radiation such as UV, X-ray, …The TL spectrum is called

“glow-curve,” and the luminescence emitted is a function of temperature Typically,the distinct peaks occurring at different temperatures in the glove - curve relate tothe electron traps present in the sample [64]

The TL process can be summarized as follows:

(i) The material must be a semiconductor with a large band gap

(ii) Before heating, the material absorbs energy during irradiation, and

luminescence occurs when heated

(iii) When the sample irradiated by ionizing radiation is heated from roomtemperature to some temperature, electrons in traps are excited to conduction band,and some of these reach luminescence centers (L); if so, light (i.e., TL) is emitted as aresult of the process of recombination into these centers

The Zn2SiO4 and Zn2SiO4: Mn phosphors show good thermoluminescenceproperties [43,65,66]

1.2 Background of Transition Metal (TM) ions in the crystal field1.2.1 Transition metals

TM ions are formed from atoms in the fourth period of the periodic table; frombeyond the calcium atom (element 20 in the periodic table), with electronicconfiguration (Ar)4s2, up to the zinc atom (element 30), with electronicconfiguration (Ar)3d104s2 and some atoms in the fifth period from Z = 39 (Ytri) →

Z = 48 (Cadimi); period 6 from Z = 57 (Lanthan) → Z = 80 (mercury); and the 7cycle elements starting from Z = 89 (Actini) In general TM shows many differentoxidation numbers For example manganese (Mn, Z = 25, configuration1s22s22p63s23p63d54s2) has six oxidation states: +2, +3, +4, +5, +6 and + 7

Transition metal (TM) ions in the lattice often create magnetic properties for thematerial but are also used as optically active dopants in host lattices Normally, TMions have an electronic configuration that is not filled in the outermost 3d subclass

10

Thus, they have an electronic configuration 1s22s22p63s23p63dn, where n (1<n<10)denotes the number of 3d electrons The optical properties of transition metal ions arerelated to the shifts between the electron levels of the d layer that are not yet filled andare not protected by the outer layer, so are strongly influenced by the crystal field.These electrons are responsible for the optical transitions When TM doped into hostlattice, 3d electrons interact strongly with the crystal field, which makes separation ofion energy level Based on the separation of energy levels, the emission and absorptionregion of the transition metal ion extends from the UV region to the visible region Thischaracteristic offers great potential for lighting applications of TM In fact, in TM ions,the 3d orbitals are large radius, but they are not unshielded by outer shells so thatstrong ion–lattice coupling often occurs in TM ions As a result, the spectra of TM ionsshow both broad (S > 0) and sharp (S ≈ 0) bands [67–69] Crystal field theory is one ofthe useful methods to explain the interaction of 3d electrons and the crystal field withall these changes in anions (ligands) surrounding the metal ion as point electriccharges Furthermore, if the theory is extended to take into consideration the overlap ofelectron orbitals of the metal ion and ligands, it is called ligand field theory Forinstance, in 1930, Bethe and Van Vleck used crystalline field theory to explain thecolor emission of transition metal salt crystals successfully In 1951, chemists also usedthis theory and demonstrated the spectral of complexes, and the crystal field theory waswidely used.

1.2.2 The effect of crystal fields on the separation of TM ions

Figure 1.1 Shapes of d orbitals and ligand positions: Ligands for octahedral

The remaining three atomic orbitals (AO) d (t2g), which direct between ligands,leading to interactions between these AOs and ligands, are weaker Thus the energy

of the three AO t2g increases lower than that of the AO, eg As a result, under the

ligand field, the energy of five AO d are separated into two different levels, inwhich eg, having higher energy than t2g [69] The different energy between twolevels eg, and t2g is called the field separation parameter o = Eeg – Et2g (Figure 1.2)

In the case of tetrahedral coordination, the separation of AO ions d is opposite tothe above There are three AOs that are closer to the ligands, so the energy of theseAOs is received greater than that of the remaining two AO d, corresponding to theenergy level of AO t2g higher than the energy of the two AO, eg T = Et2g – Eeg ; T

T = 4 o/9The higher the charge of the central ions, the stronger the ability to attract ligandstowards it, increasing the electron repulsion interaction between ligands and d ions.The higher the central ion radius, the more deformed the AOs will be, and the ligandscan approach the central ion, causing greater separation of AO d energy levels, theseparation parameter crystal field will increase Also, the smaller the ligand and the

12

larger the negative charge, the easier it is to approach the central ion, causing agreater separation of the crystal field [69].

Figure 1.3 The separation of AO d of the central ion by the crystal field in different

symmetry [69]

The crystal field theory successfully explains the separation of energy levelsdue to the electrostatic interaction of transition metal ions However, the emissionspectrum of d ions in the host lattice is quite complicated Additional theories from

a quantum mechanical perspective are needed to explain the absorption andemission spectra of transition metal-doped fluorescent powders [69] In an atom,each electron not only moves around the nucleus in defined orbits but also performsits own spin For atoms, according to quantum mechanics:

⃗ ⃗⃗

Where ⃗ is total momentum Moment of an atom; ⃗⃗ is Orbital Moment of the atom; and ⃗ is the spin Moment of the atom.

If only considering the electrostatic interaction among the electrons in the atom,the total momentum Moment of the atom is always constant The atom will beseparated into different energy states corresponding to the 2S + 1L However, it isnecessary to take into account the spin-orbit interaction of electrons Meanwhile, theenergy levels with the same number of values of L and S are separated into energylevels with different values of J (J will receive the values of L + S, L + S-1, | L-S |).Thus, with the given L and S values, the energy level will be split into (2S + 1) levels if

L > S or (2L + 1) if L < S Each of these levels is degraded (2J + 1) by the spatialquantum of the total momentum moment vector The energy levels of the atom

13

will be denoted: 2S + 1LJ The sub-levels of the atom will be denoted as S, P, D, F,

G, H, I, corresponding to the values of L = 0, 1, 2, 3, 4, 5,

Figure 1.4 The energy level separation of Cr 3+ ions when take into account the

spin-orbit interaction (L-S) (with B = 918 cm-1) (a ) and internuclear sepatation(b) [69]

Figure 1.4 illustrates the separation of energy levels, in which Dq is also thecrystal field separation parameter (Δ = 10Dq) and B is the electrostatic repulsionparameter The field separation parameters of some metal ions are evaluated asfollows: Mn2+ Ni2+ Co2+ Fe2+ V2+ Fe3+ Cr3+ V3+ Co3+ Mn4+[69].

Thus, the electrostatic interaction, the spin-orbit interaction, and the effect ofthe crystal field lead to the split energy levels for metal ions d, which cause theattractive optical properties of the transition metal-doped fluorescent powder

Table 1.1 Correlation of Reduced Representations [69]

Figure 1.5 3d level splitting caused by the crystal field [69]

For the above procedures, group theory may be utilized based on the symmetry ofthe geometric arrangement of the central ion and ligands This is based on the fact that

a crystal field having a certain symmetry is invariant When the coordinates aretransformed by elemental symmetry operations that belong to a point group associatedwith the symmetry; all terms other than the crystal field in the Hamiltonian forelectrons are also not changed in form by the elemental symmetry operations Inaddition, electron wave functions can be used as the basis of a representative matrix forthe symmetry operations, and the eigenvalues (energies) of the Hamiltonian can becharacterized by the reduced representations Particularly when the Hamiltonianincludes the inter-electron electrostatic and spin-orbit interactions in a multi-electronsystem, group theory is useful for obtaining energy level splitting and wavefunctions,calculating level energies, and predicting the selection rule for transitions betweenenergy levels Wavefunctions for the t2 and e orbitals are the basis for the reducedrepresentations T2g and Eg, respectively, in the Oh group [67–69]

1.2.3 Tanabe-Sugano diagrams

It is clear that when 3d transition metal ions are incorporated in a crystal, they areaffected by surrounding anions and crystal field The two physicists named Tanabe andSugano gave the solutions of calculating the interaction of the electron configurationfor the d2 to d8 configurations in an octahedral crystal field, as presented in Tanabe-Sugano diagrams Take the case of the 3d2 electron configuration; this Tanabe-Suganodiagram is shown in Figure 1.6 The Tanabe-Sugano diagram has an essential role inthe analysis of the absorption and emission spectra of d ions in the host lattice Inwhich the level energies (E) from the ground level are plotted against

the crystal field energy (Dq), both in units of B Normally, the value of Dq/B is in therange 4.2 to 4.9 for free ions obtained from the experimental spectra are used Note thatthe diagram for dn is the same as that for d10–n for Dq = 0 because of the configurationinteraction for n electrons occupying 10 d orbitals in the same manner as that for (10–n)holes Besides, the electric charge of electrons is opposite with that for holes, so thesign of the Dq value for electrons becomes opposite for holes Also, the diagram for dn

in the tetrahedral field can be used for the case of d10–n in the octahedral field Take theoptical absorption spectra for [M (H2O)6] n+ complex ions of 3d metals for an example

It can be well explained by the Tanabe-Sugano diagrams which the two empiricalparameters of Dq and B were calculated about 1000 cm-1 According to experiment, the

Dq values for metal ions are in the order [67–69]:

Figure 1.6 Energy level diagram for the d 2 configuration [69]

According to Tanabe and Sugano, d3 and d5 configurations have the lowestexcited levels (light-emitting levels) located in the visible spectral region On theTanabe-Sugano diagram, the point at Dq/B = 2.2 is considered as a boundary point,

so that on the right of this point is the strong crystal field, and on the left of thispoint is the weak crystal filed For d3 configuration, such as Cr3+, as shown in Fig.1.7, the light-emitting levels are 2E(2G) and 4T2(4F) above and below the crossovervalue of Dq/B ~ 2.2, respectively, corresponding to the luminescence bands fromthese two levels for Cr3+ depending on the crystal field strength of host materials

16

Figure 1.7 Energy level diagram for the d 3 configuration [69]

Figure 1.8 Energy level diagram for the d 5 configuration [69]

For d5 configuration, such as Mn2+, as shown in Figure 1.8, the 4T1(4G) is thelowest excited level, which is located in the visible region at the weak crystal field ofDq/B < 1.5 So, in the case of this configuration, Mn2+ ion has the smallest Dq valueamong transition metal ions, and it is a suitable activator for green- to red-emitting

phosphors As can be seen from Fig 1.8, the dependence of the 2E(2G) states on Dq

is almost parallel to that of the ground level so that the emission light does notdepend significantly on the crystal field strength of different host materials or thetemperature and the spectral band may be a sharp line On the other hand, the curves

of the 4T2(4F) for d3 and 4T1(4G) for d5 have steep slopes when plotted against Dq,which showed that the position of the emitting bands would depend strongly on hostmaterials, resulting their bandwidths may be broadband

1.2.4 Energy levels of Mn 2+ ion in a crystal field

Table 1.2 Energies of the electrostatic terms of the d 5 electron configuration in terms of

the Racah parameters B and C related to the energy of the ground term 6 S [69]

of which +2, +4 are the two most stable states In this section, we will focus on thespectroscopic properties of the Mn2+ ion, which has five d-electrons in the unfilled 3d

shell, and its electronic configuration is 1s22s22p63s23p63d5 As can be known that,with a configuration of 5 electrons distributed on five AO 3d, there are 252 possibilities

to place these electrons among five 3d-orbitals (including two spin orientations).Specifically, they are 16 LS terms including: spin-sextet 6S, four spin-quartets 4P, 4D,

4F, and 4G, and eleven spin-doublets 2S, 2P, 2D(1), 2D(2), 2D(3), 2F(1), 2F(2), 2G(1), 2G(2),

2H, and 2I Meanwhile, upon the effect of the crystal field, the atom

18

will be separated into different energy states corresponding to the 2S + 1L, where S isthe total spin of the particular configuration, and L is its total orbital momentum (L

= 0, 1, 2, 3, 4, and 5 for the S, P, D, F, G, and H terms, respectively) Thus, with thesame value of the L given, the energy level will be split into (2S + 1)(2L + 1)different terms and is equal to 6 for the 6S and 2P terms, 12 for the 4P term, 20 forthe 4D term, 28 for the 4F term, 36 for the 4G term, 2 for the 2S term, 10 for the 2Dterms, 14 for the 2F terms, 18 for the 2G terms, 22 for the 2H term and 26 for the 2Iterm, in which the ground state is the orbital singlet 6S, so it cannot be split by acrystal field Besides, all absorption and emission transitions of the free Mn2+ ionterms are spin-forbidden, resulting in their low intensity However, upon theCoulomb interaction among these d electrons, these terms have different energies.These terms are called Racah parameters B and C, having the dimensions of energy.Table 1.2 gives the energies of all free ion terms

As mentioned above, the Tanabe-Sugano (T-S) diagram has an important role inthe analysis of the absorption and emission spectra of d5 ions in the host lattice inwhich the crystal field splitting, including the energy intervals between the splitenergy levels, are calculated

Figure 1.9 Tanabe–Sugano diagram for the Mn 2+ in Zn 2 SiO 4 crystal field [69]

The T-S diagram for the Mn2+ ion in the Zn2SiO4 crystal field configuration isshown in Fig 1.9 As seen from the Figure 1.9, when the value of crystal field islarger than 2.2 (Dq/B ~ 3), it means that d5 configuration is in the strong crystalfields, there is a change in the ground state: instead of the spin-sextet 6A1, the spin-