chế tạo vật liệu điện môi họ srtimo (m = fe, co, ni) và nghiên cứu một số tính chất của chúng bản tóm tắt tiếng anh

In many reports about groups of dielectric materials doped with transitional metals M, structure, electric and magnetic properties, Raman spectrum at room temperature have been focused o

INTRODUCTION

In recent years, perovskite structure compounds, especially ABO3 (A = Sr, Ba,

Pb, Ca and B = Ti, Zr) have been paid attention and researched popularly because of their great applications in technology and practicality ABO3 materials have interesting characters, such as optical, ferroelectric and piezoelectric responses and others Therefore, these materials have been applied

to make capacitor, rheostat, photoelectrodes, ferroelectric storage, gas sensor

In group of ABO3 materials, one of the most researched materials is dielectric Strontium titanate, SrTiO3 (STO), especially after their ferroelectric responses were investigated Because of high dielectric constant, which increases as freezing and has low short-wave loss, this material is applied in devices with high frequency, short-wave, even at low temperature There are many researches on STO focusing on Ti or Sr doping or replacing with metal ions to investigate the distortion of perfect cubic structure that causes interesting physical phenomena

In the report about doping Sr in SrTiO3, it was shown that replacing metallic ions for Sr position caused the suppression of paraelectric state Substitution of Bi for Sr leads to the occurrence of several polarization modes and phase transition to ferroelectric behavior La doping in STO materials strongly suppresses the paraelectric state, without the occurrence of intrinsic polarization modes, except for polarization effects related to oxygen vacancies SrTiO3 doped with transition metal M have been researched excitingly by many authors Recently, in application as sensor, Fe doped STO with high concentration has been synthesized successfully and applied as transport emission level This material carries required stability and transport properties

at relatively high temperatures Most investigation of Fe doped STO focus on effects of Fe on structure, size of grains, impedance spectroscopy and Raman spectra at room temperature

As we know, STO is material with high dielectric constant (at room temperature, ε = 300) Ti ion exists at 3d0 state, so this material does not have magnetic characters Lately, ferroelectric properties of doped STO with magnetic ions have been discovered and it is hoped that this response can be applied in spintronics When investigating Co substituted TiO2 , Matsumoto et al found ferromagnetic properties of the material at the room temperature, which introduced new research approaches on oxide materials with Ti Then, many researches have been carried out with good results However, the origin of ferroelectric in these materials has not been explained thoroughly and there are

many opposite opinions For example, with Co substituted STO, ferromagnetic properties occur in bulk materials with high Co content, but does not occur in thin film materials

In many reports about groups of dielectric materials doped with transitional metals M, structure, electric and magnetic properties, Raman spectrum at room temperature have been focused on research, while optical responses and Raman at low temperature have been hardly researched There have several studies on Raman scattering spectroscopy it low temperature but

do not systematic, specially on the effect of transitional metals Fe, Co, Ni on electromagnetic responses and optical responses of SrTi1-xMxO3

STO materials doped with transition metal (Fe, Co, Ni) are not only interesting and complicated research object on material science, but also promising ones in application in Spin electronics, Diluted Magnetic Semiconductor (DMS) Basing on practical situation and research condition such as experimental devices, references, research ability and research groups

in Vietnam and abroad the following study and solutions to unsolved problems are feasible and may give good results

Therefore, we chose the topic of thesis: "Preparation of SrTi 1-x M x O 3 (M

= Fe, Co, Ni) system and investigation some their properties"

The purpose of thesis is: (i) Preparation of SrTi1-xMxO3 (M = Fe, Co, Ni) systems by sol-gel and Pulsed Laser Deposition (PLD) method (ii) Investigating effects of substituted content on their structural, ferroelectric and optical properties

Research methods: Experimental method with data analysis was used to

investigate the effects of the substitution on the structure as well as properties of materials We used polycrystalline samples made by sol-gel and PLD methods

in the laboratory of Center for Nano Science and Technology, Hanoi National University of Education Structure morphology and components of samples were examined by X-ray diffraction, Scanning Electron Microscopic (SEM), Atomic Force Microscope (AFM) and Energy Dispersive Spectra (EDS) Impedance measurement was performed by Le-Croy using Lab-View 8.0 in the Center for Nano Science and Technology, Hanoi National University of Education Raman scattering spectroscopy measurement at low temperature which used in Ewha University, Korea was carried out on spectrometer device T6400, using activate laser of 514 nm in 10-300 K Besides that, measurement

of magnetic, Raman scattering spectroscopy at room temperature, absorption spectra were also performed by devices having high accuracy at various laboratories in Vietnam Exciting source of both Raman was Ar laser of 514

nm Magnetic measurement was used by DMS 880 (Digital Measurement System Inc), basing on rules of vibrating sample magnetometer with sensitivity

of 10-5 emu at Material Science Center of University of Science Vietnam National University Absorption spectra of samples were measured on Jasco

670 UV at laboratory of Physics Department of Hanoi National University of Education Diagram of energy and density of state were calculated by Material Studio

The thesis includes: overview about perovskite Strontium titanate

(SrTiO3), experimental methods, results of researches on effects of Fe, Co, Ni substitution on structure, electromagnetic and optical properties of SrTi1-xMxO3samples synthesized by Sol-gel and PLD method

Composition of the thesis: the thesis consists of 140 pages, including introduction, 5 chapters of content, conclusion and references The detailed composition as follow:

Introduction

Chapter 1: Overview on SrTiO3 materials

Chapter 2: Experimental methods

Chapter 3: The effects of Fe, Co, Ni substitution on structure of SrTi

1.1 Crystal structure of SrTiO 3 materials

Strontium titanate SrTiO3 (STO) is one of the important compounds in the group of perovskite ABO3 At the room temperature, STO materials have cubic structure, with crystal space of Pm3m ( 1

h

O ) and lattice constant of 3.905 Å Corner positions of cubic are Sr cations, center of 6 sites is oxygen anion, center of the cubic is Ti cation Ion Sr2+ has coordination number of 12, radius

of rSr 2 += 1.44 Å Ion Ti4+ has coordination number of 6, radius of rTi 4 + = 0.605

Å Ion O2- has coordination number of 8, radius of rO2−= 1.42 Å Figure 1.1 is

perovskite at room temperature At the low temperature, the materials show phase transition from cubic structure

into tetragonal one of I4/mcm (105 K) In

the stoichiometric composition, ratio

Sr/Ti = 1, O/Sr = 3, STO is dielectric

with band gap energy of 3.2 eV State

2p of oxygen predominates at peaks of

valence band and 3d state of Ti

predominates on conduction band STO

show both covalent bond and ionic

bond Hybridization between 2p state of

oxygen and 3d state of Ti presents

covalent bond and between ion Sr2+ and

O2- presents ionic bond

The important character of STO structure is existence of octahedral TiO6

in basic cells In the perfect state, octahedral TiO6 has 90o angle and the length

of 6 bonds is 1.952 Å The distance of ion O2- and ion Sr2+ in each site of the cubic is 2.769 Å However, in the distortion state, depending on the chemical component of materials, crystal structure is not the cubic, the bond distance is not homogeneous and physical properties of the materials are also effected

1.2 Properties of SrTiO 3 materials

1.2.1 Electromagnetic properties of SrTiO 3 materials

Dielectric properties of STO used to be

investigated by impedance spectroscopy

measurement Impedance spectroscopy is

more general than impedance because it

includes phase shift between electric voltage

and current Normally, vector quantity is

presented by relation Z( ) Z ω = ' + jZ ", in which

Z’ is the real part and Z’’ is the imaginary

part

On the complex plane, impedance

diagram is presented as figure 1.2 with:

'

Z = Z cos( ) θ , Z " = Z sin( ) θ , 1 ''

'

Z tan Z

 , Z = (Z '2 + Z ) ''2 21

θ is the angle between impedance Z and the real part Z’

Theoretically, dependence expression of the real and imaginary part is semi-circle having center on the material axis Practically, due to different

Sr

Ti

O

restoration time, the semicircle

can be distortion having center

under the material axis

X Guo et al investigated

impedance spectroscopy of

single crystal and crystal of

STO The result for single is 2

semicircles with the

contribution of grain and grain

boundary (figure 1.4a), for

electrodes, at the medium by

grain boundary From the cross

point of these semicircles with

material axis, we can define

resistance of grain, grain

boundary and electrodes

It is known that in the perovskite ABO3 material at B sites are ions of transition metal Cations B with d orbit are the condition that magnetic moment and magnetic order exist For dielectric materials SrTiO3, ion Ti4+ haven’t electronic orbit d (do), so there is not magnetic properties in the pure STO The magnetic properties occur only when replacing or doping metal ions for ion

Sr2+, Ti4+ ion O2-

1.2.2 Optical properties of SrTiO 3 materials

For the optical properties of SrTiO3 materials, it was often focused on Raman scattering spectroscopy Theoretically, correlation method can be used

to calculate Raman and infrared active modes in STO crystal The results show that in this material, mode 3F1u is active infrared and F2u is inactive Raman and infrared Optical phonons were also investigated in many reports Oscillation modes which are typical of 1st Raman scattering are: TO1 mode at around 90

cm-1, TO2-LO1 band at around 170 cm-1, TO3-LO2 mode is inactively optical one (266 cm-1), mode TO4at 545 cm-1, LO4-A2g at 795 cm-1 Oscillation modes for 2nd Raman scattering are between 200-400 and 600-800 cm-1 The Raman

Figure 1.4 Impedance spectroscopy

STO crystal at 773 K in Ar

scattering spectroscopy at low temperature indicate that in STO, there appears phase transition from cubic to tetragonal structure at 105-110 K

For perovskite ABO3 materials having B site with ions of transition metal

of d group, elements of d and oxygen define properties of materials Basing on estimation of energy band, it can be seen that orbital s, p of A have no influence

on width of covalent band ABO3

From diagram of reduced energy of STO (figure 1.10) K V Benthem et

al said that absorbing edge is in accordance with shift from 2p of oxygen and 4p

of Strontium to 3d of Titanium At near Fermi level, there is hybridization of p and d 3d state affects the conduction band and 2p of oxygen in the valence band The width of band gap energy is around 3.2 eV, which means that 2p of oxygen at peaks of valence band to 3d of Ti t2g and eg in conduction zone Bonding of Sr and TiO6 is strong ionic bonding, while covalent bonding of Ti and O is the result of 2p (O) and 3d (Ti)

1.3 The effects of substitution on the structure and properties of SrTiO 3 1.3.1 The substitution at site A

1.3.2 The substitution at site B

1.4 Chemical defects of SrTiO 3 in replacing donor and acceptor

1.4.1 Chemical defects

1.4.2 Defect chemistry of donor doped SrTiO 3

1.4.3 Defect chemistry of undoped and acceptor doped SrTiO 3

energy level for STO

Figure 1.11 Density of

state of STO

1.5 Effect of processing parameters on the microstructural and electrical properties of the STO crystal

1.5.1 Stoichiometric and nonstoichiometric composition of STO

1.5.2 Sintering temperature

1.5.3 Partial pressure during sintering

Chapter 2 EXPERIMENTAL METHODS

2.1 Method of synthesized samples

In this thesis, we have synthesized the following systems and investigated their structure, electromagnetic, optical and properties of these following systems:

Systems was synthesized by sol-gel method SrTi1-xMxO3 (x = 0.0; 0.1; 0.2; 0.3; 0.4 and 0.5), including SrTi1-xFexO3, SrTi1-xCoxO3, SrTi1-xNixO3

Systems SrTi1-xMxO3 films was synthesized by PLD with different contents, including SrTi1-xFexO3 films (x = 0.0; 0.1; 0.2), SrTi1-xCoxO3 films (x

= 0.0; 0.1; 0.2; 0.3; 0.4), SrTi1-xNixO3 films (x = 0.0; 0.1; 0.2; 0.3)

2.1.1 Preparation of targets by solid phase reaction

2.1.2 Preparation of samples by sol-gel method

2.1.3 Preparation of samples by PLD method

2.2 Analysis of structure and components of samples

2.2.1 X-ray diffraction method (XRD)

2.2.2 Technique of scanning electron microscopic (SEM)

2.2.3 Atomic force microscope (AFM)

2.2.4 Analysis of component by energy dispersive spectra (EDS)

2.3 Impedance spectroscopy measurement

2.4 Magnetic measurement

2.5 Raman scattering spectroscopy measurement

2.6 Absorption spectra measurement

Chapter 3 THE EFEECT OF TRANSITION METAL M (Fe, Co, Ni)

SUBSTITUTION ON STRUSTURE OF SrTi 1-x M x O 3 MATERIALS

3.1 The effects of transition metal M on structure of SrTi 1-x M x O 3 synthesized by sol-gel method

3.1.1 Diagram of X-ray diffraction of SrTi 1-x M x O 3 samples

Results of investigation structure of SrTi1-xMxO3 by X-Ray diffraction are

presented in figure 3.1

On diagram of 3 systems samples, we see that diffraction peaks occurring

at angles of about 32, 40, 46, 52, 57, 68o By comparing the diagram of X-ray diffraction pattern of pure sample with x = 0.0 with standard JCPDS 35-374 code, these peaks are in accordance with group of planes: (100), (110), (111), (200), (210), (211) và (220)

Figure 3.1a presents

diagram of X-ray diffraction of

SrTi1-xFexO3 samples When Fe

content increases, diffraction lines

change For example, peaks of 2

-theta at 22 and 52o disappear

when substituted content reaches

to x = 0.2 Especially, position of

diffraction peaks shifts

considerably when Fe content

increases The reason for shift

may be related to the doped of Fe

in Ti4+ in lattice cells It was

known that, in octahedral, ionic

radius of Sr2+ and Ti4+ are 1.44 Å

and 0.605 Å successively Ion Fe

with different oxidation state has

different ionic radius In this

thesis, our result indicates that

lattice constant of SrTi1-xFexO3

decreases when Fe content

increases Therefore, it is

estimated that Fe3+ (LS) or ion

Fe4+ having smaller ionic radius

substituted for ion Ti4+ in lattice

cells, leading to decrease of lattice

constant With these Fe content

and heating temperature, with x =

0.2; 0.3; 0.4; 0.5, on diagram,

peaks correlating with 2θ of 27.3o occur and they are TiO2 peaks of Rutile, with space group of P4m/mmm In order to limit Rutile, in careation of samples,

we can replece Ti(OC3H7)4 with crude Ti, because when Ti(OC3H7)4 is diluted

in water, amorphous phase TiO2 often occurs

Figure 3.1 X-ray diffraction diagram of

SrTi 1-x M x O 3 synthesized by sol-gel method: (a) SrTi 1-x Fe x O 3 , (b): SrTi 1-x Co x O 3 , (c): SrTi 1-

x Ni x O 3 Symbols presents: TiO 2 (*), TiO (♥),

Figure 3.1b present diagram of X-ray diffraction of SrTi1-xCoxO3 samples

by sol-gel method method The peaks shift at right low Co content (x = 0.1; 0.2) and expand when Co content rises (x = 0.3; 0.4; 0.5) Especially, at angle of lager 2θ, diffraction peaks expand and unbalance Therefore, it is estimated that when Co content is higher, structural phase can be changed The results of lattice constants of SrTi1-xCoxO3 indicate the value decreases when Co content increases We know that ion Co can exist in many states of oxygen such as:

Co2+, Co3+, Co4+ with different ionic radius Maybe ion Co4+ or Co3+ (LS) with smaller ionic radius than Ti4+ substituted for ion Ti4+ in crystal cells, which causes decrease in cell's size and lattice constant when Co content changes

Figure 3.1c present diagram of X-ray diffraction of SrTi1-xNixO3 samples, which shows that when substitute Ni content is low, (x = 0.1), the sample is pure and has suitable structure with pure STO When Ni content increases to x

= 0.2 and x = 0.3, contaminant phase TiO occurs (*) If Co content increases to

x = 0.4 and x = 0.5, other phases such as Ti3O5 (♦), TiO2 (♥), Ni (♠) occur Besides that, intensity of diffraction line also decreases and diffraction peaks shift to lager 2θ Therefore, lattice constant and size of lattice cell decrease The reason for peak shifting and constant changing may be related to substitution ion Ni for Ti4+ in lattice cells

According to experimental condition, in substitution ion Ni2+ for Ti4+ in SrTi1-xNixO3, if Ni2+ has radius of 0.69 Å, size of cell and lattice constant will increase We know that, like Fe and Co, ion Ni can exist in many oxidation states In octahedral crystal, with coordination number of 6, ion Ni3+ (HS) has radius of 0.6 Å, Ni3+ (LS) of 0.56 Å and ion Ni4+ only exist in HS with radius

of 0.48 Å It means that in doped with Ni in lattice cells, oxidation states of

Ni3+ và Ni4+ predominate

3.1.2 SEM images of SrTi 1-x M x O 3 synthesized by sol-gel method

SEM images of SrTi1-xFexO3 samples show that grain size of Fe

substituted samples is relative homogeneous and suitable to grain size of pure STO when Fe content increases to x = 0.3 When Fe content increases to x = 0.4; 0.5, grain size decreases to about 10-20 nm SEM images of SrTi1-xCoxO3indicate that when Co content reaches to x ≥ 0.2, grain size decreases to 10-20

nm For SrTi1-xNixO3 samples, even when Ni content Ni reaches to x ≥ 0.1 grain size decreases considerably to only 10 nm

We see that size of crystal grain calculated by formula of Debye-Scherer

is bigger than estimated size from SEM images The reason is that in calcinations at high temperature, grains accumulate which lead to increase in size

3.1.3 Measurement results of energy dispersive spectra (EDS) of SrTi

1-x Fe x O 3 samples synthesized by sol-gel method method

Figure 3.6 presents EDS of SrTi1-xFexO3 samples Figure 3.6a shows that only peaks which correspond with Sr, Ti, O occur When substituting Fe for a part of Ti, we see EDS of samples as on figure 3.2 (b-g) Besides, spectrum line

of Fe also occurs at different energy level When Fe content is of x = 0.1; 0.2, spectrum lines which are typical of Fe occur at about 0.7 and 6.2 keV When Fe content is of x = 0.3; 0.4; 0.5, there is also another spectrum line at around 7.1 keV In substitution Fe, intensity of spectrum peaks of Ti tend to decrease gradually and spectrum peaks of Fe tend to increase This result is suitable to initial estimation, because when Fe content increases gradually, (from 0 to 50%), Ti content decreases ( between 100 and 50%)

3.2 Effects of transition metal ions M on structure of SrTi 1-x M x O 3 material synthesized by PLD method

3.2.1 Diagram of X-ray diffraction of SrTi 1-x M x O 3 samples synthesized by PLD method

Figure 3.7 present diagram of X-ray diffraction of SrTi1-xMxO3 samples synthesized by PLD Like SrTi1-xMxO3 samples synthesized by sol-gel method,

structure of this samples are cubic of Pm3m On the diagram, diffraction peaks of pure STO film have high intensity at 2θ of about 22, 32, 40, 50o which correspond with Muller index (100), (110), (111), (210) When substitute element and its content is different, intensity as well as diffraction peaks also change Figure 3.7 show the XRD of Fe doped STO samples Diagram presents

diffraction lines correspond with 2θ of 22 and 32o When Co substitutes in STO, in the diagram, diffraction lines correspond with 2θ of 22 and 40o, and for

Ni, they are 2θ of 22 and 52o Besides that, position of diffraction peaks shifts considerably to large 2θ when substitute content increases The reason for peak shifting (change in lattice constant) may be related to ions' substitution of Fe,

Co, Ni in Ti4+ of cells Constant decreases sharply in accordance with content

of ion M, which indicates that ion Fe3+ (LS) replaced Ti4+ in SrTi1-xFexO3 films, ion Co4+ or Co3+ (LS) replaced ion Ti4+ in SrTi1-xCoxO3 films, ion Ni4+ or Ni3+(LS) replaced ion Ti4+ in SrTi1-xNixO3 films

Figure 3.6 Energy dispersive spectra of SrTi 1-x Fe x O 3 samples

(x = 0.0 ÷ 0.5) synthesized by sol-gel method

3.2.2 Atomic Force Microscope (AFM) of SrTi 1-x Fe x O 3 films synthesized by PLD method

From AFM of SrTi

1-xFexO3 films (x = 0 ÷ 0.3), we

can observe surface

morphology and estimate grain

size Results indicate that

lattice models accumulating on

layer Si (100) have averagely

structure of two SrTi1-xMxO3

systems synthesized by sol-gel

and PLD method, we have

some following comments:

For both samples, lattice

constant decreases according

to substitute content, which

means that ions of transition

metal such as Fe, Co, Ni at

different oxidation states

replaced in ion Ti4+ in cells

In the diagram of X-ray

diffraction of SrTi1-xMxO3

samples, all diffraction peaks

that are typical of STO occur,

and in diagram of SrTi1-xMxO3

films, only some peaks occur

The reason for this

phenomenon is that when we

irradiate X-ray on SrTi1-xMxO3

samples, X-ray will diffract to

all directions, and on SrTi

1-xMxO3 films, X-ray diffract to

1 priority direction- direction of layer

Figure 3.7 Diagram of X-ray diffraction