structure and properties of doped nano-barium titanate

STRUCTURE AND PROPERTIES OF DOPED NANO-BARIUM TITANATE

Thesis submitted to the

VISVA-BHARATI UNIVERSITY, SANTINIKETAN

For the Award of Degree of

DOCTOR OF PHILOSOPHY (SCIENCE) IN

PHYSICS

BY

PUSPENDU BARIK

Department of Physics Visva-Bharati University Santiniketan-731235 West Bengal, INDIA

Dedicated To My Family

Certificate This is to certify that the thesis entitled “Structure and Properties of Doped Nano-Barium Titanate” submitted by Puspendu Barik in the Department of Physics, Visva-Bharati, Santiniketan, WB, India, for the award of the degree of Doctor of Philosophy (Science) is a record of bona fide research work carried out by him under my supervision. The results embodied in this thesis have not been submitted for the award of any other degree/diploma anywhere.

(Dr. Tapas Kumar Kundu) (Supervisor) Department of Physics Siksha-Bhavana (Institute of Science) Visva-Bharati University Santiniketan-731235, WB INDIA

The thesis entitled “Structure and Properties of Doped Nano-Barium Titanate” describes the work done by me under the supervision of Dr. Tapas Kumar Kundu, Department of Physics, Visva-Bharati University. This is being submitted for the fulfilment of requirements for the degree of Doctor of Philosophy (Science) of VisvaBharati University. I hereby declare that no portion of the work reported here has been submitted at this University or anywhere else by me or any other person for the award of any degree. Date: Place:

(Puspendu Barik) Department of Physics Siksha-Bhavana (Institute of Science) Visva-Bharati Santiniketan-731235

Acknowledgement I would like to thank my supervisor, Dr. Tapas Kumar Kundu (Department of Physics, Visva-Bharati University, Santiniketan, Birbhum, West Bengal, INDIA). Working with my supervisor proved to be successful and productive. I am indebted to Dr. Kundu for his continuous guidance, constructive comments, technical and moral support during the course of this study. My thinking has immeasurably sharpened by having so many invaluable discussions with Dr. Kundu. His support and invaluable advice greatly appreciated. It’s been my good fortune to be his student. I would like to express my everlasting feeling of gratefulness to him. This project would not have been possible without much assistance from Teachers and researchers at Visva-Bharati University as well as excellent research environment provided by Visva-Bharati University. I wish to express my deepest gratitude to my fellow colleagues from Dr. Atanu Jana, Mr. Santanu Mishra, Mr. Nantu Karak and others. I greatly appreciate the precious co-operations, inspirations and discussions of those individuals who created such a delightful and productive work environment. I have cherished my friends inside and outside Santiniketan. It was their constant inspiration and love that has been brightened my student life during this course of study. I also would like to acknowledge the financial supports from both DST-FIST and VisvaBharati University for the scholarship provided during this study. Finally, I wish to dedicate this work to my family, especially my aunts and parents. I am very grateful for their consistent encouragement, support and understanding during my study in Santiniketan. All of them have guided and changed my life with brighter and prosperous future. None of this work would be possible, without their love and inspiration, which have been my source of happiness and encouragement.

Date:

(Puspendu Barik)

Place:

Department of Physics Siksha-Bhavana (Institute of Science) Visva-Bharati Santiniketan-731235 i

Table of Content

Table of Content

I

Acknowledgement

i

Table of Content

ii

List of Tables

vi

List of Figures

viii

List of Publications

xii

Abbreviation

xiii

Introduction, Review and Goal

1

1.1. Nanotechnology: A 21st-Century Technology

2

1.1.1. Nanoparticles

3

1.1.2. Properties of Matter Change at the Nanoscale

3

1.1.3. Why Do These Properties Change at the Nanoscale?

4

1.1.3.1. Volume to Surface Area

5

1.1.3.2. Size-dependent properties

5

1.2. Future Applications of Nanotechnology

6

1.3. Background of Perovskite structure

7

1.4. Ferroelectric Materials

8

1.4.1. Classification of Ferroelectric Material

9

1.4.2. Properties of Ferroelectric materials

9

1.4.2.1. Hysteresis Loop and Polarization Switching

9

1.4.2.2. Dielectric and Susceptibility

10

1.4.2.3. Domains

11

1.4.2.4. Ferroelectric Phase Transitions and Curie-Weiss Law

12

1.5. History of Barium Titanate (BT)

12

1.5.1. Structural phase transitions in BT

13

1.5.2. Synthesis of nanoscale BT

14

1.5.3. Characterization of BT nanoparticles

16

1.5.4. Ferroelectricity in BT

18

1.5.5. Dielectric Properties

20

1.6. Luminescence

21

1.6.1. What is luminescence?

21

1.6.1.1. Special Features of Photoluminescence spectroscopy

ii

24

Table of Content

1.6.1.2. Uses of Photoluminescence

II

24

1.6.2. Perovskites as a Wide Band-Gap Semiconductor

24

1.6.3. Bulk electronic structure of BT

25

1.6.4. Background of the luminescent emission of BT

26

1.7. Statement of the problem

29

1.8. Objective

29

1.9. Organization of the thesis

30

1.10. References

31

Experimental

Procedures

and

Characterization

36

Techniques 2.1. Introduction

37

2.2. Synthesis of nanomaterial using Sol-gel method

38

2.3. Measurement techniques

39

2.3.1. Microstructure

39

2.3.1.1. X-ray diffractogram

39

2.3.1.2. Scanning Electron Microscope (SEM)

40

2.3.1.3. High Resolution Transmission Electron Microscope

40

(HRTEM) 2.3.2. Physical Properties

41

2.3.2.1. Electron paramagnetic resonance (EPR)

41

2.3.2.2. Dielectric Properties

41

2.3.2.2.1. Calcinations

41

2.3.2.2.2. Nucleation and growth

41

2.3.2.2.3. Pellet Preparation

42

2.3.2.2.4. Sintering

42

2.3.2.2.5. Electroding

42

2.3.2.2.6. Dielectric Permittivity

43

2.3.3. Optical Properties

III

43

Light Emission from Ferroelectric Barium Titanate

45

Nanocrystals 3.1.

Introduction

46

3.2.

Experimental

47

3.3.

Results and discussion

47 iii

Table of Content

IV

3.3.1. X-ray diffraction and microstructure

47

3.3.2. Paramagnetic vacancies or interstitials

50

3.4

Light emission

52

3.5.

Conclusions

57

3.6.

References

58

Photoluminescence in Fe3+ Ion Doped Barium

60

Titanate Nanoparticles

V

4.1.

Introduction

61

4.2.

Experimental

62

4.3.

Results and discussions

63

4.3.1. Structural properties

63

4.3.2. Electron paramagnetic resonance

65

4.3.3. Optical Properties

67

4.4.

Conclusions

71

4.1.

References

72

Photoluminescence in Ce and Ni ion Doped Barium

74

Titanate Nanoparticles 5.1.

Introduction

75

5.2.

Experimental Procedure

75

5.2.1. Sol-gel Synthesis

75

5.2.2. Characterization

76

5.3.

VI

Results and Discussion

76


76

5.3.2. Paramagnetic vacancies or interstitials

78

5.3.3. Light emission

80

5.4.

Conclusion

83

5.5.

References

84

Dielectric

Properties

of

Barium

Titanate

85

Nanoparticles in a Composite Structure 6.1.

Introduction

86

6.2.


87

6.3.

Results and Discussions

88

iv

Table of Content

6.3.1. Structural properties

88

6.3.2. Dielectric properties

90

6.3.2.1. Relative Permittivity

90

6.3.2.2. Dielectric loss

93

6.3.2.3. Curie-Weiss fitting

94

6.3.3. Electric Polarization

VII

95

6.4.

Conclusion

96

6.5.

References

97

Structure and Dielectric Behaviour of Co-doped

98

Nano Barium Titanate Nanoparticles 7.1.

Introduction

99

7.2.


101

7.2.1. Sol-gel Synthesis

101

7.2.2. Characterization

101

7.3.

VIII

Results and Discussion

102


102

7.3.2. Dielectric Properties

109

7.4.

Conclusions

112

7.5.

References

114

Conclusion and Future Work

117

8.1.

Conclusion

117

8.2.

Future Work

119

v

List of Tables

List of Tables No.

Caption

Page

Table 1.1

The properties of bulk BT

14

Table 1.2

Methods for wet-chemical synthesis of BT powders

15

Table 1.3

Summary of the characterization techniques

17

Table 1.4

Important Events in Ferro-electricity

19

Table 1.5

Important Events in Perovskite Era (Early BT Period)

19

Table 1.6

The various types of luminescence and mode of excitation

22

Table 3.1

Crystal structures, volume fractions (), average D-values, and

48

-values in o-BT and t-BT processed in selective conditions Table 3.2

Band positions, bandwidths, and relative intensities in individual

55

bands after deconvolution of the observed light emission in t-BT nanocrystals Table 4.1

Heat-treatment schedule, crystal structure, and average D value

62

Table 4.2

Observed and calculated dhkl values in t-BT and o-BT

64

polymorphs in a Nano composite structure. Table 4.3

Intensity of EPR spectra and g-value for 2% Fe doped BT

67

nanopowders after heating a sol-gel precursor at (a) 400 C, (b) 600 C, and (c) 700 C for 2 hr in air. Table 4.4

Band positions, bandwidths, and relative intensities in individual

70

bands after de-convolution of the observed light emission for 2% Fe doped BT nanopowders after heating a sol-gel precursor at (a) 400 C, (b) 600 C, and (c) 700 C for 2 h in air. Table 5.1

Heat-treatment schedule, crystal structure, and average D value

77

Table 6.1

The heat treatment schedule of the sample

88

Table 6.2

Assignments of observed dhkl values in o-BT-II and t-BT

89

polymorphs in a nanocoposite structure of sample Table 6.3

Ferroelectric properties in typical perovskites

91

Table 6.4

Temperature dependent ferroelectric properties of BT prepared

92

here. Table 6.5

The temperature dependence of parameters derived from Curie–

vi

95

List of Tables

Weiss law fitting. Table 6.6

Ferroelectric properties of BT.

Table 7.1

Observed dhkl values in t-BT and o-BT polymorphs in a

96 103

nanocomposite structure. Table 7.2

Assignments of observed dhkl values in t-BT and o-BT

104

polymorphs in a 0.6 mole % Co doped BT structure. Table 7.3

The average values of particle size (D), lattice volume V, lattice

105

number z and density ρ in both the phases of o-BT(II) and t- BT in nanocomposite structure. Table 7.4

The lattice parameters, surface areas S0, and volume fraction (φ) of o-BT(II) phase after heating a polymer template at 750 oC for 2 h in air.

vii

105

List of Figures

List of Figures No. Figure 1.1

Caption The crystal structure of perovskite oxides with ABO3

Page 8

formula unit. Figure 1.2

A Classification scheme for the 32 crystallographic point

8

groups Figure 1.3

A typical hysteresis loop

10

Figure 1.4

The phase transition sequence in perovskites

13

Figure 1.5

(a) The oxygen ions are at face centers, Ba+2 ions are at cube

18

corners and Ti+4 is at cube center in cubic BT. (b) In tetragonal BT, the Ti+4 is off-center and the unit cell has a net polarization. Figure 1.6

Energy-level representations of TL and OSL processes. (i)

21

Ionization due to exposure to nuclear radiation with trapping of electrons and holes at defects T and L, respectively. (ii) Storage of radiation energy during time, which is dependent on the energy depth E of the trap below the conduction band. (iii) By heating or shining light onto the sample, electrons are evicted from the electron traps and some of these reach luminescence centers (L); if so, light (i.e. TL or OSL) is emitted as a result of the process of recombining into these centers. Figure 1.7

The possible physical effects resulting from interaction of

23

light with matter. Figure 3.1

XRD pattern for a BT nanopowder after heating a sol-gel

48

o

precursor at 400 C for 2 h in air. The peaks (211)* and (101) refer to o-BT and t-BT phases respectively. Figure 3.2

XRD patterns for BT nanopowders after heating a sol-gel

49

precursor at (a) 600 oC and (b) 750 oC for 2 h in air. A closeup in the inset compares the shift in (101) peak in the two powders Figure 3.3

HRSEM images showing t-BT nanocrystals of thin laminates viii

50

List of Figures

(heated at 750 oC for 2 h in air). A model laminate in the right describes L, W and δ-edges. Figure 3.4

EPR spectra for BT nanopowders after heating a sol-gel o

o

51

o

precursor at (a) 400 C, (b) 600 C, and (c) 750 C for 2 h in air. A sharp band-I is superposed over a broad band-II (dotted curve ABC) in spectra (b) and (c). Figure 3.5

Emission spectra for BT nanopowders after heating a sol-gel

52

precursor at (a) 400 oC, (b) 600 oC, and (c) 750 oC for 2 h in air. Bandgroup ‘B’ develops at the expense of bandgroup ‘A’ in sample (c). Figure 3.6

A deconvolution of the light emission into five bands in t-BT

54

nanocrystals (heated at 750 oC for 2 h in air). Figure 3.7

Schematic view of the energy band diagram proposed for t-

57

BT nanocrystallites (heated at 750 C for 2 h in air) as per the emission spectrum Figure 4.1

XRD patterns for 2% Fe doped BT nanopowders after

63

heating a sol-gel precursor at (a) 400 oC and (b) 600 oC (c) 700 oC for 2 hr in air. A close-up in the inset compares the shift in (101) peak of sample (b) and (c). Figure 4.2

A typical (a) TEM image and (b) Diffraction pattern taken

65

from specimen ‘c’ heat-treated at 700 oC for 2hr. The diffraction rings in the pattern (from center to edge) can be indexed as the (101), (111), and (002) peaks of a tetragonal (P4mm) BT phase. Figure 4.3

EPR spectra for 2% Fe doped BT nanopowders after heating

66

a sol-gel precursor at (a) 400 C, (b) 600 C, and (c) 700 C for 2 hr in air. Figure 4.4

Photoluminescence

spectra

for

2%

Fe

doped

BT

68

o

nanopowders after heating a sol-gel precursor at (a) 400 C, (b) 600 oC, and (c) 700 oC for 2 hr in air. Figure 4.5

Photoluminescence emission spectra of the specimen heated

69

at 700 oC for 2 hr fitted with Gaussian curves. Figure 5.1(a)

X-ray diffraction patterns showing controlled t-BT to o-BT

ix

77

List of Figures

phase transformation in a nano composite structure after doping with 1 mole % Ce for different calcination temperature. Figure 5.1(b)

X-ray diffraction patterns showing controlled t-BT to o-BT

78

phase transformation in a nano composite structure after doping with 1 mole % Ni for different calcination temperature. Figure 5.2(a)

EPR spectra for 1 % Ce doped BT nanopowders after

79

heating a sol-gel precursor at (a) 700 C, (b) 800 C, and (c) 1000 C for 2 hr in air. Figure 5.2(b)

EPR spectra for 1% Ni doped BT nanopowders after heating

79

a sol-gel precursor at (a) 700 C, (b) 800 C, and (c) 1000 C for 2 hr in air. Figure 5.3(a)

Photoluminescence

spectra

for

1%

Ce

doped

BT

80

nanopowders after heating a sol-gel precursor at 400 -1000 o

C for 2 hr in air.

Figure 5.3(b)

Photoluminescence

spectra

for

1%

Ni

doped

BT

81

nanopowders after heating a sol-gel precursor at 400 -1000 o

C for 2 hr in air.

Figure 5.4

Comparison of photoluminescence spectra for 1% Ce and Ni

82

doped BT nanopowders after heating a sol-gel precursor at (a) 400 oC, (b) 600 oC, (c) 700 oC, (d) 800 oC and (e) 1000 o

C for 2 hr in air. Comparison of photoluminescence peak

intensities with annealing temperature for three most intense peaks centred at (f) 421 nm, (g) 486 nm, and (h) 528 nm. Figure 6.1

A typical XRD pattern for BT-II sample in the form of pellet

88

sintered at 1000 °C for 2 h in air. Figure 6.2

Dielectric constant Vs. Temperature at frequencies ranging

90

from 1 kHz to 1 MHz. of BT-II. Figure 6.3

Dielectric constant Vs. Temperature at frequencies ranging

91

from 10 kHz to 1 MHz. of BT-III. Figure 6.4

The variation of εr (At 100 kHz) with temperature of the BTII nanocomposite for determination of the ∆W value. x

91

List of Figures

Figure 6.5

Dielectric constant Vs. Temperature at frequencies 100 kHz

92

of all samples. Figure 6.6

The variation of dielectric loss with temperature of BT

93

samples at 100 kHz frequency. Figure 6.7

Variation of 1/εr Vs. T for all samples to fit Curie-Weiss law.

94

Figure 6.8

Polarization-field hysteresis loops of (a) BT-I, (b) BT-II and

95

(c) BT-III samples at room temperature. Figure 7.1

X-ray diffraction patterns showing controlled t- BT to o-BT

102

phase transformation in a nano composite structure after doping with (b)1.6 mole % Co ion, comparing with (a) Undoped BT. Figure 7.2

(a) Bright field Transmission Electron Micrographs of 0.6

107

mole % Co ion doped BT nanocrystals and (b) Selected Area Diffraction patterns of 0.6 mole % Co ion doped BT nanopowders. Figure 7.3

A close-up of view of the XRD patterns in the range 2θ = 21 o

108

– 26o showing shifts in (100)t, (111) and (102) peaks of o-

BT specimens with (b) 0.3 mole % Co ion (c) 0.6 mole % Co ion and (d) 1.6 Co mole % Co ion, comparing with (a) Undoped BT after heating at 750 oC for 2 h in air. Figure 7.4

Variation of dielectric constants with temperature obtained at

109

a signal frequency of 10 kHz. Figure 7.5

Variation of dielectric loss with temperature obtained from the specimens at 10 kHz signal frequency.

xi

110

List of Publications

LIST OF PUBLICATIONS 1. T. K. Kundu, S. Mishra, N. Karak, P. Barik, “Effect of Ti4+ ions doping on microstructure and dc resistivity of nickel ferrites”, Journal of Physics and Chemistry of Solids, 73, 2012, pp. 579–583. 2. P. Barik, A. Jana and T. K. Kundu, “Influence of Co – ion doping on tetragonal – orthorhombic polymorphic transformation and dielectric behavior in BaTiO3 nanoparticles”, Journal of American Ceramic Society, 94, 2011, pp. 2119– 2125. 3. T. K. Kundu, N. Karak, P. Barik and S. Saha, “Optical properties of ZnO nanoparticles prepared by chemical method using polyvinyl alcohol (PVA) as capping agent”. International Journal of Soft Computing and Engineering, 1, 2011, pp. 19-24. 4. P. Barik, T. K. Kundu and S. Ram, “Light emission from ferroelectric barium titanate nanocrystals”, Philosophical Magazine Letters, 89, 2009, pp. 545–555. 5. T. K. Kundu, A. Jana and P. Barik, “Doped barium titanate nanoparticles”, Bulletin of Material Science, 31, 2008, pp. 501–505.

PAPERS PRESENTED IN INTERNATIONAL/NATIONAL CONFERENCES 6. “Optical and Electrical Properties of CdS Quantum Dots embedded in Barium Titanate Matrix”, P. Barik and T. K. Kundu, 13th National Symposium in Chemistry (NSC-13), 4th-6th February, 2011. 7. “Photoluminescence of Doped Nano Barium Titanate”, T. K. Kundu and P. Barik, International Conference on Nanotechnology & Medical Sciences (ICNAMS-2010), 21th-23th October, 2010. 8. “Doped BaTiO3 nanoparticles”, T. K. Kundu, A. Jana and P. Barik, Review and Coordination meeting on Nanoscience and Nanotechnology held at ARCIHyderabad (INDIA), 2007.

xii

Abbreviation A-E ac AES AFM BT CET DFWM dc EDS/EDX eV ESA ESCA EPR ESR EELS

LEEM

Alternating Current Auger Electron Spectroscopy Atomic Force Microscopy Barium Titanate Cooperative Energy Transfer Degenerate Four Wave Mixing Direct Current Energy-Dispersive XRay Spectroscopy Electron Volt Excited State Absorption Electron Spectroscopy For Chemical Analysis Electron Paramagnetic Resonance Electron Spin Resonance Electron Energy Loss Spectroscopy

LEELS MOCVD NIR NNI NMR NSOM OSL

P-T ps PL PT RHEED ST STEM

F-J SEM FTIR HRTEM

HRSEM

IR

Low-Energy Electron Microscopy Low Energy Electron Loss Spectroscopy Metal Organic Chemical Vapor Deposition Near Infrared National Nanotechnology Initiative Nuclear Magnetic Resonance Near-Field Scanning Optical Microscope Optically Stimulated Luminescence

Fourier Transform Infrared Spectroscopy High-Resolution Transmission Electron Microscopy High-Resolution Scanning Electron Microscopy Infrared Spectroscopy

SERS STM TEM TL

Picosecond Photoluminescence Lead Titanate Reflection High-Energy Electron Diffraction Strontium Titanate Scanning Transmission Electron Microscopy Scanning Electron Microscopy Surface Enhanced Raman Spectroscopy Scanning Tunnelling Microscope Transmission Electron Microscopy Thermoluminescence U-Z

K-O XPS KT nm nc-BT μm LEED

Potassium Tantalate Nanometer Nanocrystalline-Barium Titanate Micrometer Low-Energy Electron Diffraction

XRD YAG

xiii

X-Ray Photoelectron Spectroscopy X-ray Diffraction Yttrium Aluminium Garnet

Structure and Properties of Doped Nano-Barium Titanate

Chapter - I Introduction, Review and Goal

Chapter-I:


Chapter-I Introduction, Review and Goal 1.1. Nanotechnology: A 21st-Century Technology The prefix, ―nano,‖ (Nano is derived from the Greek word meaning ―dwarf‖) is used to indicate a tool, an enterprise, a particle, a phenomenon, a project, or a manufactured item operating on or concerned with a scale at one billionth of a meter. Reference is often made to a lecture given by Richard Feynman [1] in 1959 at Caltech (where he was working at that time). Entitled ―There’s Plenty of Room at the Bottom‖, he envisaged machines making the components for smaller machines (a familiar enough operation at the macroscale), themselves capable of making the components for yet smaller machines, and simply continuing the operation until the atomic realm was reached. Nanotechnology is the ability to observe, measure, manipulate and manufacture things at the nanometres scale, the size of atoms and molecules. The word ―nanotechnology‖ was first introduced in the late 1970s. While many definitions for nanotechnology exist, most groups use the National Nanotechnology Initiative (NNI) definition. The NNI calls something ―nanotechnology‖ only if it involves all of the following:  Research and

technology development

at

the

atomic,

molecular,

or

macromolecular levels, in the length scale of approximately 1 to 100-nanometer range.  Creating and using structures, devices, and systems that have novel properties and functions because of their small and/or intermediate size.  Ability to control or manipulate on the atomic scale. The word ―nanotechnology‖ was coined by Norio Taniguchi in 1983 to describe the lower limit of this process [2]. Current ultrahigh-precision engineering is able to achieve surface finishes with a roughness of a few nanometers. This trend is mirrored by relentless miniaturization in the semiconductor processing industry. Ten years ago the focus was in the micrometer domain. Smaller features were described as decimal

2

Chapter-I:


fractions of a micrometer. Now the description, and the realization, is in terms of tens of nanometers.

1.1.1. Nanoparticles A nanoparticle is a quasi-zero-dimensional nano-object in which all characteristic linear dimensions are of the same order of magnitude (not more than 100 nm). Nanoparticles can basically differ in their properties from larger particles. Nanoparticles with a clearly ordered arrangement of atoms (or ions) are called nanocrystallites. Nanoparticles with a clear-cut discontinuity of the system of electronic energy levels are often referred to as `quantum dots' or `artificial atoms'. Nanoparticles mostly rank as passive nanostructures. At present, they represent almost the only part of nanotechnology with commercial significance. However, it is sometimes questioned whether they can truly represent nanotechnology because they are not new. For example, the Flemish glassmaker John Utynam was granted a patent in 1449 in England for making stained glass incorporating nanoparticulate gold; the Swiss medical doctor and chemist von Hohenheim (Paracelsus) prepared and administered gold nanoparticles to patients suffering from certain ailments in the early 16th century. The fabrication of nanoparticles by chemical means seems to have been well established by the middle of the 19th century (e.g., Thomas Graham‘s method for making ferric hydroxide nanoparticles).

1.1.2. Properties of Matter Change at the Nanoscale The properties of matter depend on size. The physical, chemical, and biological properties of matter significantly differ at the nanoscale when compared to the larger quantities of the same material. This is due to the difference in surface area per unit of volume at the nanoscale. For a given substance, increasing the number of nanoscale particles also increases the proportion of atoms on the surface compared with the number of internal atoms. Atoms at the surface often behave differently from those located in the interior. Atoms at the surface have a higher energy state, which means they are more likely to react with particles of neighbouring substances. The result is that chemical reactions can take place between atoms and molecules at surfaces acting as miniature chemical reactors. By modifying materials at the nanoscale other properties such as magnetism, hardness, electrical and heat conductivity can be changed substantially. These changes arise from confining electrons in nanometer-sized structures. As one 3

Chapter-I:


example, electrons (subatomic particles) do not flow in streams as they do in ordinary electrical wires. At the nanoscale, electrons act like waves. When electrons act as waves, they can pass through insulation that blocks flowing electrons. Here some of examples, which show changes at the nanoscale:  At the macro scale, gold metal is shiny yellow as you noticed in jewelry. If you break up the gold, to particle 100 nanometers wide, it still looks shiny yellow. But, when you break down the particle of gold to 30 nanometers across, the gold appears bright red. As the particle of gold gets even smaller than 30 nanometers, it looks purple and when it is a little bit smaller it will appear brownish in color. Color can change in some other substances, like gold, at the nanoscale.  At the macro scale, a sheet of aluminium is harmless. However, when the particles of aluminium are cut down in size to 20 to 30 nanometers, the metal can explode.  The bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper.  Ferroelectric materials smaller than 10 nm can switch their magnetization direction using room temperature thermal energy, thus making them useless for memory storage.  Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid.

1.1.3. Why Do These Properties Change at the Nanoscale? Using new tools that allow us to see and manipulate small groups of molecules whose size in the nanoscale, scientists have now discovered that these tiny amounts of a given substance often exhibit different properties and behaviors than larger particles of

4

Chapter-I:


the same substance! When we look at nanosized particles of substances, there are some distinct reasons that change from macroscale objects.  Due to the small mass of the particles, gravitational forces are negligible. Instead electromagnetic forces are dominant in determining the behavior of atoms and molecules.  At nanoscale sizes, we need to use quantum mechanical descriptions of particle motion and energy transfer instead of the classical mechanical descriptions.  Nanosized particles have a very large surface area to volume ratio.  At this size, the influences of random molecular motion play a much greater role than they do at the macroscale.

1.1.3.1. Volume to Surface Area Nanomaterials have a large proportion of surface atoms, and the surface of any material is where reactions happen. Because of nanoparticles‘ huge surface area and thus very high surface activity, nanotechnologists can potentially use much less material. The amount of surface area also allows a fast reaction with less time. Therefore, many properties can be altered at the nanoscale. That‘s the power of nanotechnology. The powders are amorphous, crystalline or show a metastable or an unexpected phase, the reason for which is far from being clear. Due to the small sizes, any surface coating of the nanoparticles strongly influences the properties of the particles as a whole. Studies have shown that the crystallization behaviour of nanoscale silicon particles is quite different from micron-sized powders or thin films. It was observed that tiny polycrystallites are formed in every nanoparticle, even at moderately high temperatures.

1.1.3.2. Size-dependent properties Roughly two kinds of "nanostructure induced effects" can be distinguished: first, the size effect, in particular, the quantum size effects, where the normal bulk electronic structure is replaced by a series of discrete electronic levels and, secondly, the surface or interface induced effect, which is important because of the enormously increased specific surface in particle systems. While the size effect is mainly considered to describe physical properties, the surface or interface induced effect plays an eminent role for 5

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chemical processing, in particular in connection with heterogeneous catalysis. Experimental evidence of the quantum size effect in small particles has been provided by different methods, while the surface induced effect could be evidenced by measurement of thermodynamic properties like vapour pressure, specific heat, thermal conductivity and melting point of small metallic particles. Both types of size effects have also been clearly separated in the optical properties of metal cluster composites. Very small semiconductor

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