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Introduction to Materials and Classification of Low Dimensional Materials

R. John Bosco Balaguru Professor

School of Electrical & Electronics Engineering SASTRA University

B. G. Jeyaprakash

Assistant Professor School of Electrical & Electronics Engineering

SASTRA University



Table of Content

1. INTRODUCTION TO MATERIALS................................................................3 1.1 HISTORICAL OUTLOOK........................................................................................................3 1.2 GENERAL CLASSIFICATION OF MATERIALS..................................................................3 1.3 STRUCTURE OF MATERIALS...............................................................................................4 1.4 AMORPHOUS SOLIDS……………………………………………………………………....5 1.5 CRYSTALLINE SOLIDS……………………………………………………………………..6 1.6 SOLIDIFICATION………………………………………………………………………….…6

2. CLASSIFICATION OF LOW DIMENSIONAL MATERIALS....................7

2.1 BASIC PROPERTIES OF LOW DIMENSIONAL

SEMICONDUCTOR NANOSTRUCTURES............................................................................9 2.2 WHY PROPERTIES OF NANOMATERIALS ARE DIFFERENT.......................................10

2.2.1 Increase in surface area to volume………………………………………………….10 2.2.2 Quantum confinement……………………………………………………………….10

2.3 INFLUENCE OF PHYSICAL DIMENSION ON DIFFERENT PROPERTIES....................11

3.3.1 Structural properties………………………………………………………………..11 3.3.2 Thermal properties…………………………………………………………………..12 3.3.3 Chemical properties…………………………………………………………………12 3.3.4 Mechanical properties………………………………………………………………12 3.3.5 Magnetic properties…………………………………………………………………12 3.3.6 Optical properties…………………………………………………………………...13 3.3.7 Electronic properties………………………………………………………………..14

2.4 COMPARISION OF BULK AND NANOSTRUCURED SEMICONDUCTING MATERIAL........................................................................................15

3. QUIZ AND ASSIGNMENT..............................................................................17

3.1 SOLUTIONS............................................................................................................................17

4. REFERENCES..................................................................................................18



1 Introduction to Materials

This lecture provides you about the fundamentals of solid materials, its classification and general properties 1.1 Historical Outlook

Materials are more

important in our everyday lives. Different materials in various forms were used in transportation, housing, clothing, communication, recreation, and food production.Historically, the development and advancement of societies have been intimately tied to the member’s ability to produce and deploy materials to fill their needs. In fact, early civilizations have been designated by the level of their materials development as Stone Age, Bronze Age and Iron Age.

The earliest humans utilized stone, wood, clay, skins that occur naturally for their needs. With time they discovered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Furthermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. 1.2 General classification of Materials

There are thousands of materials available for various applications. Most of the

materials fall into one of three classes such as metallic, ceramic and polymer. This classification is based on the atomic bonding forces of a particular material. Also, these materials can be combined to form a composite material to have unique properties than its constituent. Within each of these classifications, materials are usually organized into



groups based on their chemical composition or certain physical or mechanical properties. Composite materials are often grouped by the types of materials combined or the way the materials are arranged together. Table 1 shows a common classification of materials.

• Metals

Ferrous metals and alloys (irons, carbon steels, alloys steels, stainless steels)

• Nonferrous metals and alloys (Al, Cu, Ni, Mg, Ti, super alloys

• Polymer

• Thermoplastics

• Thermoset plastics Elastomers

• Ceramics

• Glass

• Graphite Diamond

• Composites

• Reinforced plastics

• Metal-matrix composites Concrete

Table 1 General Classification of materials

1.3 Structure of Materials

•

We know all the matter is made up of atoms. From the periodic table, it can be seen that there are about 114 different kinds of atoms. These 114 atoms form thousands of different substances ranging from the air we breathe to the metal used to support all buildings. Metals behave differently than ceramics, and ceramics behave differently than polymers. The properties of matter depend on the type of atoms and how they are bonded together. The three most common major classification of structural, listed generally in increasing size, are:

Atomic structure, which includes features that cannot be seen, such as the types of bonding between the atoms, and the way the atoms are arranged.



•

Microstructure, which includes features that can be seen using a microscope, but seldom with the naked eye.

•

Macrostructure, which includes features that can be seen with the naked eye)

The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and optical properties. The microstructure and macrostructure can also affect these properties but they generally have a larger effect on mechanical properties and on the rate of chemical reaction. The properties of a material offer clues as to the structure of the material. The strength of metals suggests that these atoms are held together by strong bonds. In solids, the way the atoms or molecules arrange themselves contributes to the appearance and the properties of the materials.

Atoms can be gathered together as an aggregate through a number of different

processes, including condensation, pressurization, chemical reaction, electrodeposition, and melting. 1.4 Amorphous Solids

A solid substance with

its atoms held apart at equilibrium spacing, but with no long-range periodicity in atom location in its structure is an amorphous solid. Examples of amorphous solids are glass and some types of plastic. They are sometimes described as supercooled liquids because their molecules are arranged in a random manner somewhat as in the liquid state. For example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline structure. When the sand is melted and the liquid is cooled rapidly enough to avoid crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a sharp phase change from solid to liquid at a definite melting point, but rather soften gradually when they are heated. The physical properties of amorphous solids are identical in all directions along any axis so they are said to have isotropic properties, which will be discussed in more detail later



1.5 Crystalline Solids More than 90% of naturally occurring and artificially prepared solids are

crystalline. Minerals, sand, clay, limestone, metals, carbon (diamond and graphite), salts (NaCl, KCl etc.), all have crystalline structures. A crystal is a regular, repeating arrangement of atoms or molecules. The majority of solids, including all metals, adopt a crystalline arrangement because the amount of stabilization achieved by anchoring interactions between neighbouring particles is at its greatest when the particles adopt regular (rather than random) arrangements. In the crystalline arrangement, the particles pack efficiently together to minimize the total intermolecular energy.

The regular repeating pattern that the atoms arrange in is called the crystalline lattice. The scanning tunnelling microscope (STM) makes it possible to image the electron cloud associated individual atoms at the surface of a material. 1.6 Solidification

The crystallization of a large amount of material from a single point of nucleation results in a single crystal. The moment a crystal begins to grow is known as nucleation and the point where it occurs is the nucleation point. At the solidification temperature, atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals. The final sizes of the individual crystals depend on the number of nucleation points.

Fig. 1. a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope

In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal without smooth faces because its growth was impeded by contact with another grain or a boundary surface. The interface formed between grains is called a



grain boundary. The atoms between the grains (at the grain boundaries) have no crystalline structure and are said to be disordered and are shown in Fig. 1.

Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains. Rapid cooling generally results in more nucleation points and smaller grains (a fine grain structure). Slow cooling generally results in larger grains which will have lower strength, hardness and ductility.

2 Classification of Low Dimensional Materials

• Solids can hold their own shape unless something happens to them.

This lecture provides you about the classification of low dimensional materials (nanomaterials) and its properties

Anything made up of matter with one or more substances is termed as material. A

material’s state can be solid, liquid or gas and thus the building blocks may be atoms, ions or molecules and its basic properties is shown in Table 2. Also the materials from one state to other state can be changed by changing temperature or pressure, and is shown in Fig. 2.

• Liquid flow and take the shape of their container. • Gases are usually invisible and spread out to fill up spaces. • Plasma are invisible and spread out to fill up space

Materials can be classified according to their state, size, shape, texture, colours, flexibility, strength, hardness, malleable and whether they are a good conductor or bad conductor of heat/electricity.

Table 2 Various states of a materials



Fig. 2.Change in materials state and its basic properties Solid state materials with reduced dimension in one or two or three directions are

recognized as low dimensional materials. Based on reduced dimension, the low dimensional materials are in generally classified as 2D, 1D and 0D, and are shown in Fig. 3. Bulk materials is called as 3D materials, Due to reduced dimension, electron motion in the materials is restricted either in one or two or three directions.

Fig. 3. Schematic representation of various forms of low dimensional materials

2.1. Basic Properties of Low Dimensional Semiconductor Nanostructures



The properties of any solids materials depend on the chemical composition, atomic structure and the size of a solid in one, two or three dimensions. For example, Hardness and optical reflection of carbon changes when it transform from diamond to graphite (Fig. 4 (a&b)). Both are made up of carbon atoms, but the atom arrangements and bond between them differs. Fig. 4. (a) Diamond (b) Graphite

Also the change in microstructure (i.e. arrangement of atoms / molecules / ions) size equivalent to a few inter-atomic spacing in one, two or three dimensions forming building blocks of solid materials makes change in the properties. For example, colour changes of gold at nanoscale. The materials synthesis with new properties by means of the controlled microstructure size and shape on the atomic level has become important to achieve new properties with enhanced device performance.

The materials microstructure with well-defined boundaries having a characteristics length scale of the order of 1-10nm, where quantum size effect can be expected are termed as nanostructured materials or simply nanomaterials. It should be noted that, the material need not be so small; it can be a large surface whose thickness is in the scale of nanometers or a long wire whose diameter is in the scale of nanometers or a particle whose diameter is in the scale of nanometers. The microstructure of nanometer-size crystallites with different crystallographic orientations are always far away from thermodynamic equilibrium, because the boundary atom and the grain atoms both have different nature. However nanostructured materials synthesized from supramolecular chemistry are in thermodynamic equilibrium state.



2.2 Why properties of nanomaterials are different?

The following two factors make the nanomaterials to have considerably different properties that its bulk one.

1. Increase in relative surface area

2. Quantum confinement effect

2.2.1 Increase in surface area to volume

Nanomaterials have a relatively greater surface area when compared to the same volume or mass of the same material in bulk form. For example, consider cube of 1m3 volume (Fig. 5), it has surface area of 6m2. If this cube of same volume is divided into eight small cubes, then the total surface area increases to 12m2

. Further dividing cube leads to increase in surface area. This is illustrated in the following figure. Also if the size of nanomaterials decreases, a greater proportion of atoms are found at the surface compared to those inside. This makes materials more chemically reactive.

Fig.5. Increase in surface area of cube

2.2.2 Quantum Confinement

In nanomaterials, the electronic energy levels are not continuous as in the bulk but are discrete. This is due to the confinement of the electronic wave function in one, two or three physical dimensions of the materials and accordingly it can be classified as



1. 1D confinement – Thin film, Quantum well 2. 2D confinement – Nanotubes, Nanorod, Quantum wire 3. 3D confinement - Precipitates, Colloids, quantum dots

2.3 Influence of phyical dimension on different properties ? As seen in the previous topics, due to reduced physical size in one, two or three

dimension of a materials, leads to change in surface area and electron confinment, makes the change in materials properties. The following section deals about different properties of nanomaterials which differs from its bulk one.

2.3.1 Structural properties

Crystal structure of nanomaterials may or maynot same as its bulk one, but has different lattice parameters. For example, gold and aluminium nanoparticles of size with few nanometers are icosahedral rather than face-centered cubic in bulk (fig.6). Indium of size less than 6.5nm is face-centered cubic rather than face-centered tetrahedral for size greater than 6.5nm. Also, the inter atomic spacing in nanomaterials decreases than bulk due to long range electrostatic forces and the short range core-core repulsion. For example, decrease in aluminum separation to 2.81Å from 2.86Å and the binding energy also decreases to 2.77eV from 3.39v.

Fig. 6. Crystal structure of (a) bulk (FCC) (b) nano (Icosahedral) Gold



2.3.2 Thermal properties

The large increase in surface energy and the change in inter atomic spacing as a function of size has marked effect on melting properties of nanomaterials. For example, the melting point of gold nanoparticle shown in Fig.7 decreases rapidly as size reduces.

Fig. 7. Schematic diagram of the variation in melting point of gold nanoparticle as a function of particle size.

2.3.3 Chemical properties

The ionisation energy is generally higher for small atomic cluster than for the corresponding bulk one. Nanoscale structures have very high surface area to volume ratio and potentially different crystallographic structures which may be lead to a radical alteration in chemical reactivity. Catalysis using finely divided nanoscale system can increase the rate, selectivity and effeciency of chemical reactions.

2.3.4 Mechanical properties

The presence of defects in any materials will alters the mechanical proeprties of it. In nanomaterials, defects are high and increases due to non-thermal equilibrium and hence mechanical properties changes. Some novel nanostructures which are very different from bulk structure in terms of atomic structural arrangement, will show very different mechanical properties. For examples single and multiwalled carbon nanotube show high mechanical strength and high elastic limit which inturn lead to considerable flexibility. 2.3.5 Magnetic properties Magnetic nanomaterials has multifunction applications, including ferrofluids, colour imaging, bioprocessing, refrigeration as well as high storage density magnetic memory



media. Due to large proportion of surface atoms which have a different local environment leads different magnetic coupling with neighbouring atoms and in turns differs in magnetic properties than its bulk one. Ferromagnetic materials in bulk form has multiple magnetic domains, whereas nanoparticle often has one domain as shown in Fig.8 and exhibit superparamagnetism phenomena. Also Giant magnetoresistance (GMR shown in Fig. 9) is a phenomenon observed in nanoscale multilayer consisting of a strong ferromagnet(e.g, Fe , Co) and a weaker magnetic or non-magnetic buffer(e.g, Cr,Cu). It is usually employed in data storage and sensing.

Fig. 8. Superparmagnetism in nanomaterials

Fig. 9. GMR materials are made from alternating layers of magnetic and non- magnetic metals that are nanometers in thickness.

2.3.6 Optical properties

In bulk materials, optical emission and absorption depends on transition between valence band and conduction band. Large changes in optical properties such as colour is seen in low dimensional semiconductor and metal. For example, Colloidal solutions of gold nanoparticle have deep red colour which progressively more yellow as a particle size increases (Fig.10) as a result of surface plasmon resonance occuring in low dimensional materials. Semiconductor nanoparticles in the form of quantum dots shows size dependent behaviour in the frequency and intensity of light emission as well as



modified non linear optical properties and enhanced gain for certain emission energy or wavelength. Other properties which may affected by reduced dimensionality include photocatalysis, photoconductivity, photoemission and electroluminescence.

Fig. 10. The diameter of gold nanoparticle determines the colour

2.3.7 Electronic properties The electonic properties changes occurs in the low dimensional material are related

to the wave like property of the electron and scaracity of the scattering centres. As the size of the system becomes comparable with the de Broglie wavelength of the electrons, the discrete nature of the energy states becomes occur as shown in Fig. 11. In certain cases, conduction material become insulator below the critical length scale.



Fig. 10. Density of states low dimensional materials

In macroscopic system , electronic trsnsport is determined by scattering with phonons , impurities or other carriers . However, if the system become sufficiently small, all scattering centres will disappears and the electronic transport become purely ballastic transport. Conduction in highly confined structure such as quantum dot is very sensitive to the presence of other charge carriers and hence the charge state of the dot known as coulomb blockade effect. This results in conduction processes involving single electrons which in turn require a minimum amount of energy to operate the switch, transisitor, or memeory element.

2.4 Comparison of bulk and nanostructured Semiconducting material Property/ Phenomenon

Specific properties

Bulk state Nano scale

Structure confinement No confinement Confine in zero, one ,two and three dimension

Surface area It is small as compare to its volume

Collective surface area can be enormous

Surface to volume ratio(S/V)

(S/V) is small. Because it become insignificant as object becomes larger.

Approaches 1, when all atoms are surface atoms

Lattice spacing

Constant and characteristics of bulk

Lattice spacing is altered. spacing near surface contracts due to rearrangement and ion vacancies are larger

Atom co-ordination

Co-ordination saturated except at surface where it is negligible

Co-ordination is unsaturated at surface and volume

Electron orbitals

Continuous over the breadth of the material

It is not continuous

Quantum mechanics

Quantum mechanics applies at the bulk level and it is called “ bath tub waves “

Nanomaterial exist at Quantum – classical interface

Electromagnetic property

Radiation : Absorption

Black body radiation. Absorption Emission

Influenced by the Bohr radius



Emission bands are broader Optical response

1. Metal reflects with partial absorption of light

2. Micron size particle scatter the light

3. High order Plasmon resonance is delocalized

1. Size dependent absorption and emission

2. Environmental dependent

3. Localized surface Plasmon resonance

Optical constant (n,k,α)

Bulk values are valid in micron size particles

Bulk optical constants no longer valid below 10 to 20 nm

Band gap Bulk values are valid Band gap is size dependent. Ex: Gold behaves as a semiconductor in nano particle form (<5nm)

Electrical conduction

Electrical conduction are continuous and obeys ohms law

Electrical conduction are not continuous and not obey the ohms law.

Thermodynamic properties

Melting point Metal have high melting point

Metal nanoparticle have low melting point

Surface tension

Independent of size Dependent of size

Chemical bond

Most of the bulk material ionic , covalent and metallic bonds are predominant

Intermolecular forces are important. Hydrogen bond and van der walls force dominant rather than ionic or covalent or metallic bond

Entropy In bulk state , it links between macro and micro scale

In nanoscale , the entropy is very high and it cannot be measured as that of bulk one .

Solubility Large particle have limited solubility

In smaller particle have enhanced solubility . It is important in targeted drug delivery system

Magnetic properties

Magnetic effect

Bulk ferromagnetic material usually forms multiple magnetic domains.

Small magnetic nanoparticle consist of only one domain and exhibit a super paramagnetism

GMR Not possible in bulk It is observed in nanoscale Electronic Energy level Continuous energy level Discrete energy level



properties Density of states

Directly proportional to the E

2D=constant 1D α E1/2 -1/2

0D- different for individual energy levels .

Electronic transport

Diffusive type of transport

Ballistic Transport

Coulomb blockade effect

It is never occurs in bulk material

It is occurs in nanoscale material

3 Quiz and Assignment

1. Materials can be classified into three types. They are metal, _________ and ceramic

2. Ceramic are _______ and metals are ductile 3.

4.

The _______________ microscope makes it possible to image the electron cloud associated individual atoms at the surface of a material

5. The moment a crystal begins to grow is known as _________

6. Hardness and optical reflection of carbon changes when it transform from _______to graphite

Rapid cooling generally results in more nucleation points and ________ grains. Slow cooling generally results in _________ grains which will have lower strength, hardness and ductility

7. Colour of gold can be changed at ______ scale 8. The microstructure of nanometer-size crystallites with different crystallographic

orientations are always far away from _______________ equilibrium 9. The two factors make the nanomaterials to have considerably different properties

that its bulk is: (1). Increase in relative surface area and (2). _________ 10. If the size of nanomaterials ________, a greater proportion of atoms are found at

the surface compared to those inside. This makes materials more chemically reactive.

11. 1D confinement materials can be called either quantum well or _______ 12. In general, the inter atomic spacing in nanomaterials ________ than bulk due to

long range electrostatic forces and the short range core-core repulsion 13. The ionisation energy is generally ______ for nanoparticles than for the

corresponding bulk one 14. Ferromagnetic materials in bulk form has multiple magnetic domains, whereas

nanoparticle often has one domain as shown in figure and exhibit _________ phenomena.

15. In nanomaterials, all scattering centres will disappears and the electronic transport become purely _________transport.



3.1 Solutions

1. Polymer 2. brittle 3. 4. nucleation

Scanning Tunneling

5. 6. diamond

smaller, larger

7. nano 8. thermodynamic 9. Quantum confinement effect 10. decreases 11. Thin films 12. decreases 13. higher 14. superparamagnetism 15. Ballastic

4 References [1] William D. Callister, Jr. , Fundamentals of Materials Science and Engineering-An

Interactive e-text, John Wiley & Sons, Inc. , 2001.

[2] H. Hosono, Y. Mishima, H. Takezoe, K.J.D. Mackenzie, Nanomaterials from

Research to Applications, Elsevier Inc., 2006.

[3] Guozhong Cao, Nanostructures & Nanomaterials Synthesis, Properties &

Applications, Imperial College Press, 2004.

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Introduction to Materials and Classification of Low ...nptel.ac.in/courses/115106076/Module 5/Module 5.pdf · Introduction to Materials and Classification of Low ... arrangement of