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Chemical structure 9

1. CORRELATION BETWEEN THE

CHEMICAL STRUCTURE AND THE

MATERIAL PROPERTIES

1.1. The atom

The Atom (gr. a-tomos = indivisible) is the smallest particle still

characterizing a chemical element.

Chemical element, or element, is a type of atom which is defined by its

atomic number Z, that is given by the number of protons in its nucleus.One atom consists of:

- nucleus (dense) – formed by positively charged protons and

electrically neutral neutrons;

- larger electron clouds consisting of negatively charged electrons.

Figure 1. Schematic representation of the atom

++

+++

nucleus (+)

electron cloud (-)


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10 GENERAL CHEMISTRY

An atom is electrically neutral; it has the same number of protons and

electrons. The number of protons in an atom defines the chemical element to which

it belongs, while the number of neutrons determines the isotope of the element.

Isotopes are any of the several different forms of an element, each of then

having different atomic mass (mass number A). Isotopes of an element have nuclei

with the same number of protons (the same atomic number Z but different numbers

of neutrons. Therefore, isotopes have different mass numbers, which give the total

number of nucleons (the total number of protons plus neutrons)

Ex: 12C, 13C, 14C, 131I, 238U (isotopes of carbon (C), iodine (I) and uranium

(U))

The proton (gr. proton = first) is a subatomic particle with an electric

charge of one positive fundamental unit (+1 a.u. (atomic unit) = 1.6x10-19

C).

The electron is a subatomic particle with a negative electric charge of

– 1a.u.

The atom is characterized by the atomic number Z and the mass number A.

The difference A-Z represents the number of neutrons.

The periodic law (D. I. Mendeleev 1869)

The elements, if arranged according to their atomic mass, exhibit an

apparent periodicity of properties – periodic table of elements.

The periodic table is formed from 8 principal groups and 8 secondary

groups and the elements are placed according to their atomic number.

Among the 8 principal groups there are the group of alkali metals (Ist

principal group Li, Na, K, Rb, Cs, Fr), the group of alkaline earth metals (IInd

principal group: Be, Mg, Ca, Si, Ba, Ra), the halogens group (VIIth

, F, Cl, Br, I),

the noble gases group, gr. VIIIth (He, Ne, Ar, Kr, Xe, Rn).

In the secondary group the transition metals are placed (Ti, V, Fe, Co, Cu,

Cr, Ag, Au, Pt, Pd, etc). The rare earth elements form the lanthanide (lanthanoide)

and actinide (actinoide) series.


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Figure 2. Periodic table

The most of substances in nature are combinations of identical or different

atoms. These combinations are the results of the interactions between atoms.

Interactions between atoms are named chemical bonds.

Why the interactions appear? The study of the electronic configuration of

the atoms from periodic table shows that noble gases – characterized by a

remarkable chemical stability – have on the external shell (valence shell) 8 coupled

electrons (excepting He – 2e-). The atoms of the other elements have less than 8

electrons on the valence shell. They are unstable and therefore they interact with

other atoms in order to form maximal stable configuration. In this way chemical

bonds are formed and stable complex systems results (elemental substance or

chemical compounds).

The chemical bond was described using different theories:

− electronic theory which establish that there are two types of chemical

bonds namely:


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ionic bond (W. Kassel - 1916) based on transfer of electrons

between atoms and

covalent bond (G.N. Lewis - 1916) based on sharing the

electrons between the atoms

− quantum theory which affirms that the chemical bond is unitary.

According to the quantum theory the ionic bond and covalent bond are

only apparently different. Each of them are two limit states of the unique way of

interaction of atoms, the covalent way.

It is possible to establish a chemical bond between atoms due to the

concentration of the positive charges in the atoms into a very small volume on one

side and the dispersion of the electronic clouds into a big volume on the other side.

In these conditions some mutual influences could exist between the nuclear field of

one atom and the electronic cloud of the other atom. When the interatomic

distances diminish the changes in the distribution of valence electronic clouds take

place, resulting in the formation of chemical bond. When sharing these electrons

there is a maximum density of the electronic cloud between the two nuclei and the

chemical bond is called covalent or homeopolar. When the electronic cloud is

concentrated around one of the atoms, this atom becomes a negative ion (anion)

and the atom with a deficit in electrons becomes a positive ion (cation). These ions

attract each other by electrostatic forces, and the chemical bond is called ionic or

heteropolar.

The quantum theory proved that a chemical bond between two atoms

cannot have a pure covalent character or a pure ionic character. It is possible to

affirm that the chemical bond is dominant ionic or covalent. A representative

example is considered the ionic compound CsF which is 93% ionic and 7%

covalent. In this way in the covalent compounds the chemical bond could be

partially ionic (covalent polar) with the increasing of electronegativity difference

between atoms. Covalent bond could be delocalized on many atoms. When the

electronic cloud is delocalized on a very large number of atoms belonging to a


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metallic lattice, the chemical bond is called metallic bond. In this case the electrons

move easily from one atom to another.

Special polarization interactions through van der Waals forces, hydrogen

bond, dipole-dipole bond could be established in case of atoms with stable

electronic configuration, that are without chemical affinity (e.g. noble gases) as

well as of molecules as stable structure units. Such interactions are not purely

chemical bonds because the deformation of the electronic cloud does not involve

any changes in the electronic configuration of the atoms. These bonds assure the

stability and some physical properties of molecular compounds: boiling

temperature, melting temperature.

Every type of chemical bonds determines some characteristics of chemical

substances:

• covalent bonds from substances with atomic lattice lead to rigid

structures with high hardness and high physical thermal constants;

• ionic bond leads to ionic lattice, a structure with salt character and ionic

conductibility in melt state or in solutions;

• metallic bond leads to lattice with electric conductivity and metallic

character (aspect, color, etc.);

• van der Waals bonds put together the particles into a weak lattice which

can be destroyed with a minimum energy.

In majority of the solid structures the types of chemical bonds coexist and

the dominant component determines the specific features of the substance.

1.2. Ionic bond

By chemical combination, the atoms modify their valence shells in order to

realize a stable electronic configuration, structure of the near noble gas.

Ex: NaCl


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- Na- Ist principal group (1 electron in the last level 1s22s22p63s1) – can lose

one electron:

+− →− Nae Na

- Cl – VIIth principal group (7 electrons on the last level 1s22s22p63s23p5) –

can accept one electron:

−− →+ CleCl 1

The ions formed in this way are not isolated. They attract each other by

electrostatic forces until a minim distance at which a repulsion force between

electronic clouds occurs:

−••

••

+••

•• ⎥⎦⎤

⎢⎣⎡→⋅+⋅ ::: Cl NaCl Na

The elements forming Ist and II

nd principal groups have a few electrons on

the valence level. They can be easily lost giving positive ions (cations) while the

elements from VIIth

and VIth

principal groups accept easily these electrons resulting

in negative ions (anions). In this way ionic bonds are formed. Examples: NaCl, KI,CsF, MgCl2, MgS, MgO, Na2O, K 2O, CaO.

Factors which determine the ionic bond formation:

- electronegativity (Pauling) – the capacity of an atom, belonging to one

molecule, to attract electrons to it. The electronegativity increases in period

from Ist group to VII

th group and decreases in groups up to bottom;

- ionization energy (ionization potential) – the energy used to remove one

or many electrons from an isolated (gas state) atom. In every group, the

ionization energy decreases up to bottom and in period the ionization

energy increases from left to right. Thus the highest ionization energy

exhibit the noble gases, proving that the external level is completely

occupied with electrons so they are very stable.

- affinity for the electrons – the energy resulting when one atom accepts

one electron in its valence level, becoming in this way a negative ion.

Halogens have the highest affinity, while the alkali metals have no electron


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affinity. The bivalent anion formation O2-

, S2-

requires some energy. This

phenomenon could be explained by the repulsion of the second electron by

the first already present in the molecule.

- Lattice energy – the energy produced when an ionic crystalline lattice is

formed. An ionic crystalline lattice is a regular geometric construction

formed by the electrostatic attraction. It arranges, around a positive ion,

negative ions which will attract other positive ions. The energy produced

depends on the number of ions which are involved in the construction of

ionic lattice.

- Solvating energy – the energy produced when one ionic compound is

solving. The formed ions are attached to the water (or other solvent)

molecules by many weak bonds. This process is an exothermic step. The

solvating energy increases with the valence and decreases with the ion

radius.

- Atom dimensions - atoms with small dimensions have a higher tendency

to form covalent bonds than the atoms with similar electronic structure, but

larger.

Examples:

Li (Ist group; small atom) – forms also covalent bonds (LiH);

Na (Ist group; bigger atom than Li) – forms only ionic bonds;

B (IIIrd

group; small atom) – forms only covalent bonds;

Al (IIIrd

group; bigger atom than B) – forms easily ionic bonds.

The tendency to form covalent bonds increases from left to right along the

periods and from bottom to up in the groups. The ionic bond is characterized by

high melting and boiling points. Also the ionic compounds exhibit larger electrical

conductivities.


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1.3. Covalent bond

The covalent bond is a form of chemical bonding that is characterized by

sharing of pairs of electrons between atoms in order to create an octet (doublet)

configuration around each atom in the formed molecule:

2

2

Cl ClClClClClCl

H H H H H H H

−→⋅+⋅

−→⋅+⋅••

••

••

••

••

••

••

••:::::

:

In quantum mechanics theory the covalent bond is formed by overlapping

the electronic orbitals of two atoms which are bonded. Thus, when two atoms are

close enough their atomic orbitals overlap and form molecular orbitals:

- one of low energy called bonding orbital;

- another of the high energy called anti-bonding orbital.

These orbitals are common to both atoms forming a covalent bond. By

overlapping two atomic orbitals of s type, or one s orbital with one p orbital, or 2

atomic orbitals of p type (along the common symmetry axes) a simple covalent

bond of type is formed (figure 3).

The overlap of p atomic orbitals, when they are oriented parallel, leads to

type covalent bond (figure 3).

σ and π bonds are hybrid bonds, forming hybrid orbitals.

Hybridization – is the concept of mixing atomic orbitals to form new

orbitals suitable for the qualitative description of atomic bonding properties

(figures 4 and 5).

Hybridization: - sp3 one s orbital with 3 p orbitals

- sp2 one s orbital with 2 p orbitals

- sp one s orbital with one p orbital


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Figure 3. The simple covalent bond of and type


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Figure 4. Hybridization and the σ and type covalent bonds

2s

2px 2py 2pz 2pz 2pz2py

fundamental state sp3 hibridization sp2 hibridization sp hibridization

Energy

(ex. CH4) (ex. H2C=CH2) (ex. HC CH)

sp3 sp3 sp3 sp3

sp2 sp2 sp2

sp sp

Figure 5. Electronic configurations of carbon in different hybridization states


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The covalent bond has the following characteristics: rigid, oriented in

space according to well established angles; stable. This is why substances with

covalent bonds (except diamante and graphite) have low melting points; low

hardness, large thermal and electric resistance, they are good isolators (they have

no charge carrier, they do not pass the current).

1.4. Metallic bond

Metallic bond is the bond between the atoms within metals. This bond

involves sharing and delocalization of free electrons on the metallic lattice.

Metallic bond is the electrostatic attraction between the metal nuclei or

ions and delocalized electrons, also called conduction electrons. This is why atoms

or layers are allowed to slide past each other resulting in the characteristic

properties of malleability and ductility.

Metal atoms typically contain fewer electrons in their valence shell relative

to their period or energy level. These electrons can be easily lost by the atoms and

therefore become delocalized and form a sea of electrons surrounding a giant

lattice of positive ions.

Metallic bond characteristics are: non – polar type; strength; malleability;

ductility; conduction of heat and electricity; luster.

1.5. Intermolecular bond

1.5.1 Van der Waals bonds (forces)

The van der Waals bond is a cohesion force of physical nature (electrostatic)

which exists between the molecules with stable electronic configuration, i.e. molecules with

no need to share or to transfer electrons. It is a weaker bond with low bonding energy.


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There are three types of van der Waals forces:

a) dispersion forces (Fritz London – 1929) or London interactions. These

forces have universal character being present in all microparticles. London

forces can be exhibited by nonpolar molecules because electron density

moves probabilistically around a molecule. There is a high probability that

the electron density will not be evenly distributed throughout a nonpolar

molecule. When an uneven distribution occurs, a temporary multipole is

created. This multipole may interact with other nearby multipoles.

Between the nonpolar molecules only the dispersion forces could exists.

b) orientation forces or Keesom interactions or dipole – dipole interactions.

They exist between the polar molecules which exhibit permanent dipole

moment1. Under the action of these forces the molecules orient according

to the electrostatic attraction, forming oriented arrangements. The stability

of these arrangements increases when the temperature decreases.

Increasing the temperature, the thermal movement is increasing, disturbing

the dipoles orientation.

c) Induction forces or Debye interactions exist between polar and nonpolar

molecules as consequence of polarizing action of the permanent dipoles

from polar molecules which determines dipoles in nonpolar molecules. The

induced dipole disappears when the electric field is off, and the induced

dipole – dipole interactions are weaker than dipole – dipole interactions.

Within polar molecules all three types of interactions are usually present.

The participation weight depends on two molecular properties: polarity – given by the

dipole moment (µ) and the polarizability – given by the deformability of bond or molecule.

In small and high polar molecules most important are orientation forces

(dipole – dipole): H2O, HF while in big and deformable molecules important are

dispersed forces. In nonpolar molecules – there are only dispersed forces.

1 Dipole moment is the measured polarity of a polar covalent bond. It is defined as the

product magnitude of charge on the atoms and the distance between the two bonded atoms


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The energy of van der Waals bonds determine all the properties which

depend on cohesion energy: volatility, solubility, viscosity, plasticity.

1.5.2. Hydrogen bond

Substances having in their molecules high electronegative atoms (HF, H2O,

NH3) exhibit some anomalies in physical properties with respect to similar

substances of the following elements of periodic table (HCl, H2S, PH3). They have

high melting points, densities, vaporization heats. This kind of anomalies appears

also in alcohols, organic acids, amines and they can be explained only by

association of the molecules (HF)n, (H2O)n, (NH3)n through weaker bonds than

covalent bonds, but stronger than van der Waals forces. This kind of bonds

established through hydrogen atoms and determined by molecular associations are

called hydrogen bond or hydrogen bridge bonds.

1.6. Dispersed systems

A system formed of one substance in which there is another substance as

small particles is called dispersed system. A dispersed system is formed of

dispersion environment and dispersed phase. Both can be found in one of the three

aggregation states: gas, liquid, solid and in particles of different dimensions.

The dispersed systems are classified according to:

a) aggregation state of the dispersion environment and of the dispersed phase

(see table 1)

b) the size of particles of dispersed phase:

- big dispersions (10-3 – 10-6m)

- colloidal dispersions (10-6 – 10-9m: micro and nanodispersions)

- molecular dispersions (10-9 – 10-10m)


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Table 1 Dispersed Systems classified after aggregation state

Dispersion

medium

Dispersed

substance

Examples

G Air

L FogG

S Smoke, aerosols, dust, smog

G CO2, HCl solution in water

L Solutions (H2O + HNO3), emulsions, gelL

SSolutions (H2O + soluble subst.)

Dispersed systems, colloids

G N2, H2, dispersed in Fe, Pt, Pd, etc.

L Inclusions in oresS

S Solid solutions, alloys

G – gas, L – liquid, S – solid

1.6.1 Big dispersions

These are heterogeneous systems in which the particles are visible with the

eye, the magnifying or the glass microscope. They are unstable and have the

tendency to separate in the phases. Bigger the particles and the specific weight are,

higher is the tendency to separate.

Depending on the medium and dispersed phase aggregation state there are:

• emulsions – the medium and the dispersed phase are immiscible (ex: water

in oil, mayonnaise)

• suspensions – solids dispersed in liquid (sand in water, pharmaceutical

suspensions)

• aerosols – liquid or solid particles dispersed in gases (fog, foam)


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1.6.2. Colloidal dispersions or micro and nanodispersions

The particles are too small to be seen with a microscope and they pass

through the usual filters. The colloid particles are bigger than molecules or ions

from the usual solutions. That is why they diffuse much slower than molecules or

ions from usual solutions and they do not cross through some membranes,

membranes through which the solutions pass easily. In other words the colloidal

solutions do not dialysis through membranes. (Colloid – greek Kolla = glue –

Thomas Graham 1861).

Solid suspensions are usually called sols. At any sol there is a disperse

medium, corresponding to the solvent from a usual solution and the dispersed

phase corresponding to the solute. The amorphous phase isolated from a sol is

called gel.

The colloidal systems are classified as follows:

a) After the aggregation state of dispersed phase and the dispersion medium

(table 2):

b) After the mutual character of dispersion medium and dispersed phase

- liophob colloids – dispersion colloids or suspenoids do not exhibit

important interactions between dispersion medium and dispersed phase

due to the fact that the chemical bonds are different (water/oil,

metal/colloids). So the dispersion medium do not moisten the dispersed

particles (dispersion medium water ⇒ hydrophobs colloids)

- liophil – They exhibit interactions between the disperse medium and

the dispersed phase due to the fact that the chemical bonds are of the

same type. So the dispersion medium moistens the particle surface

(dispersion medium water ⇒ hydrophilic colloids) ex: oil – benzene.

The liophil colloids can be:

• association colloids – the union of normal molecules by van der

Waals forces ⇒ emulsions (tensioactive agents (surfactants), fatty


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acids, superior alcohols – in water formed by stirring colloidal

dispersions)

• macromolecular colloids – the molecules dissolved in a solvent

are molecularly dispersed, but the size of these molecules is

between 10-6

and 10-9

m. (“water glass”, soluble proteins –

albumins, carbohydrates – amidon)

Colloidal properties depend on dispersion degree:

- Electrical properties: unilateral migration of the colloids under the

action of electric field to one or another of electrodes. This

phenomenon is called electrophoresis.

- Optical properties:

o the color of liophobs colloids is characteristic and this color can be

different for the same sol depending on the light (direct or

reflected)

o Tyndal effect – specific to colloidal systems; it is a lighted cone

when the colloid is traversed by a beam of light.

- Surface properties

o sorption – appears at very high separation surfaces between

colloidal particles and dispersion medium:

adsorption – mobile particles are attached on the colloidal

particles surface;

absorption – mobile particles go inside the colloidal

particles;

o coagulation – means the increase of aggregates, which leads to the

phase separation;

o peptization – means the obtaining of liophob sol from a coagulated

aggregate;

o jellification – it is a step in coagulation process in which the

system pass in the gel phase. Sol-gel transformation determines


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disappearance of some liquid properties (letting out) and

appearance of some solid properties (rigidity, elasticity, plasticity).

Practical importance – examples:

- defectoscopy with magnetic powders uses iron powders of iron

dispersed in organic medium

- fabrication of electric contacts for high current intensities and voltages

(over 1000A and 1000V); one uses finely dispersed alloy powders (W-

Cu, Ag – Ni, W – Ag, Mo – Ag) in graphite paste;

- decoration: colloidal gold for ceramic decoration;

- enhanced oil recovery using microemulsions;

- cosmetic applications: skin care products

- agriculture: mocrocolloidal aqueous emulsions are used in order to

increase the efficacy of insecticides;

- food industry: emulsions like mayonnaise, creams, yoghourt etc;

- pharmaceutical industry: different suspensions used as drugs

1.6.3. Sol – gel process

The sol – gel process is a wet chemical technique for the fabrication of

materials (typically a metaloxide) starting from a chemical solution containing

colloidal precursors (sol). Typical precursors are metal alkoxides and metal

chlorids, which undergo hydrolysis and polycondensation reactions to form a

colloid, a system composed of solid particles (size ranging from 1 nm to 1 µm)

immersed in a solvent. The sol evolves then to the formation of an inorganic

network containing a liquid phase (gel). Formation of a metal oxide involves

connection of the metal centres with oxo (M – O – M) or hydroxo (M – OH – M)

bridges, thus generating metal – oxo or metal – hydroxo polymers in solution. The

drying process serves to remove the liquid phase from the gel thus forming a


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porous material. Then a thermal treatment (firing) may be performed in order to

favour further polycondensation and enhance mechanical properties.

The precursor sol can be either deposited on a substrate to form a film (e.g.

by dip – coating or spin – coating), cast into a suitable mould with the desired

shape (e.g. to obtain monolithic ceramics, glasses, fibres, membranes, aerogels) or

used to synthesize powders (e.g. microspheres, nanospheres). The sol – gel

approach is interesting since it is a cheap and low temperature technique that

allows the fine control on the products chemical composition, as even small

amounts of dopants, such as organic dyes and rare earth metals can be introduced

in the sol resulting in a finely dispersed product. It can be used in ceramics

manufacturing process, as an investment casting material, or as a mean of

producing very thin films of metal oxides for various purposes. Sol – gel derived

materials have diverse applications in optics, electronics, energy, space, biosensors,

medicine and separation technology.

1.7. Semiconductors

1.7.1 History and importance of semiconductors

Semiconductors are materials having a conductivity ranging between

conductors (generally metals) and nonconductors or insulators (such as most of

ceramics). Semiconductors can be pure elements, such as silicon or germanium, or

compounds such as gallium arsenide or cadmium selenide. In a process called

doping, small amounts of impurities are added to pure semiconductors causing

large changes in the conductivity of the materials.

Due to their wide pallet of industrial applications in the fabrication of

electronic devices, semiconductors play an important role in our life. It is difficult

to imagine life without electronic devices. There would be no radios, no TV’s, no

computers, no video games, no cell phones and rather poor medical diagnostic


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equipment. Even many electronic devices could be fabricated by using vacuum

tube technology, the developments in semiconductor technology during the past 50

years have made electronic devices smaller, faster and more reliable.

Think for a minute to all your encounters with electronic devices. How

many of the following have you seen or used in the last twenty four hours ? Each

of them has important components that have been manufactured based on

electronic materials: microwave oven, radio, watch, computer, calculator,

diagnostic equipment, car, electronic balance, television, CD player, lights,

telephone, clock, security devices, video games, air conditioner, musical greeting

cards, refrigerator, etc.

Fields such as communication, computers, medicine, basic sciences and

engineering, use extensively electronics.

Semiconductors historical timeline

1600 – William Gilbert is the first person who used the term electricity;

1824 – John Jacob Berzelius isolates and identifies the silicon;

1823 – Michael Faraday discovers that electrical resistivity decreases as

temperature increases in silver sulphide. This is the first investigation on a

semiconductor.

1873 – William Smith discovers the photoconductivity of selenium. Modern

copier machine take advantage of this property.

1927 – Arnold Sommerfeld and Felix Bloch apply quantum mechanics to solids.

This allows scientists to explain the conduction of electricity in semiconductors.

1943 – Karl Lark –Horowitz uses high quality germanium to make diode detectors

used by Allied army to bomb Germany;

1947 – Schockley, Brattain and Bardeen invent the transistor. The semiconductor

electronics industry is born !

1958 – Robert Noyce, founder of Intel Corporation develops a planar process for

making semiconductors called monolithic IC technology;


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1962 – W. P. Dumke shows that semiconductors such as GaAs can be used to

make lasers. This opens the field of optoelectronics;

1970 – The first charge coupled devices (CCD’s) are made, greatly increasing the

memory capacity of silicon chips;

1980’s – The explosion in use of personal computers stimulates a similar boom in

electronic industry;

1993 – Gallium nitride light emitting diodes are made which can produce blue

light. Possible applications are flat screen display, screens and high density

memory storage.

2000’s – Organic and smart biomaterials start to be used in electronic devices

fabrication.

1.7.2. Principles of semiconductors

All materials have electrical properties that allow them to be organized into

three broad categories: conductors, insulators and semiconductors. Metals (pure

elements and alloys) are typically conductors of electricity. There are thousands of

kilometres of aluminium and copper wires which transport electricity throughout

the world. A relatively small number of nonmetallic substances can be also

classified as conductors. Also, a very few ceramic compounds exhibit the unusual

property of superconductivity at low temperature of liquid nitrogen (77 K) or

below. The non-metallic elements and their compounds fall into the class of

electrical insulators. Most ceramics and plastics do not conduct electricity under

ordinary circumstances. Plastic coatings are frequently found covering copper

wires to protect the user from the shock and keep devices from short circuiting.

The third group of materials, the semiconductors, as it follows from their name, fall

somewhere midway between conductors and insulators.

Although pure elements such as silicon play an important role in many

semiconductor devices, it is most often utilized by adding very small but controlled


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amounts of impurities in order to alter their properties. Silicon based materials

dominate in the semiconductor industry and in electronic devices.

Figure 6 shows the elements which could be found in the semiconductor

materials. Pure semiconductors are made by elements which belong to the IVth

group: Si, Ge, Sn. As impurities elements from Vth

or IIIrd

groups are used. Others

compound semiconductors can be formed by combining elements from IIIrd

and Vth

principal groups or from IInd

secondary group and VIth principal group.

3A 4A 5A 6A

2B Al Si P S

Zn Ga Ge As Se

Cd In Sn Sb Te

Hg

Figure 6. Elements which can be found in semiconductor materials

As it was already mentioned, an important step in understanding how the

semiconductors are working was the application of quantum mechanics to solids.

When an electric field is applied, electrons may flow through a material if there are

empty states in valence shells of the atoms that make up the material. An electron

will not easily transfer itself between atoms if there is no a vacant state of similar

energy in the receiving atom for it to occupy. A single atom has electrons localized

around itself. An atomic orbital of one atom may overlap with an atomic orbital of

another atom forming two molecular orbitals. One, called the bonding molecular

orbital, is of low energy and the other with higher energy is called the anti –

bonding molecular orbital. As more and more atoms assemble to form a solid, the

number of bonding and anti – bonding orbitals of about the same energy increases,

and they begin to take on the characteristics of the energy band. The energy

differences between orbitals within a band energy are small. Electrons can move

freely among these orbitals within an energy band as long as the orbitals are not


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completely occupied. The highest occupied energy band is called valence band.

However, there is a region that separates the valence band from the conduction

band with no orbitals. Electrons are not allowed to have these energies. In

insulators, this energy gap is relatively large and in semiconductors, the energy gap

is intermediate (figure 7).

Figure 7. Principle of semiconductor

Atoms that form metallic conductors have many partially and fully

unoccupied levels with similar energies: a large number of mobile charge carriers

are able to move across the material when an electrical potential (voltage) is

applied. In a semiconductor or insulator, the valence band is completely filled with

electrons in bonding states so the conduction cannot occur. There are no vacant

levels of similar energy on neighbouring atoms. At absolute zero, its anti – bonding

states (the conduction band) are completely empty. There are no electrons there to

conduct electricity. This is why insulators cannot conduct. In case of

semiconductors, as temperature increases, electrons in valence band acquire

enough energy to be promoted across the energy gap into the conduction band.

When this occurs, these promoted electrons can move and conduct electricity. The

smaller the energy gap is, the easier for electrons is to move to the conduction

band.

Metals

Valence band

Energy Gap

E N

E

RG

Y

Semiconductors Insulators

Conduction band


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Heat energy in a semiconductor increases the number of electrons

promoted into otherwise empty conduction band. The vacancies (or holes) created

in this process allow mobility of electrons in the valence band through the material.

At high temperatures semiconductors are relatively good conductors because there

is a relatively large number of electrons in the conduction band and

correspondingly, a relatively large number of holes in the valence band available

for the electron movement. At low temperature the semiconductors are insulators

since the number of electrons in the conduction band and the number of holes in

the valence band are diminished. At absolute zero, a semiconductor will have no

electrons in the conduction band.

There are two types of semiconductors:

- Intrinsic semiconductors in which the charge carriers come from the

chemical bonds;

- Extrinsic semiconductors, where some impurity (another element) has

been intentionally added in the solid state in order to increase the

conductivity. The properties of an extrinsic semiconductor are

governed by the presence of these impurities.

The process in which some impurities are added into a solid is called

doping. Doping can produce two types of semiconductors depending on the

element added. If the element used for doping has at least one more valence

electron than the host semiconductor, then an n-type (negative type)

semiconductor is created (for example As – 5 electrons added to a silicon crystal –

4 electrons). With silicon or other Group 4A elements, any member of Group 5A

(except nitrogen) could be used in order to form an n-type semiconductor. If the

semiconductor is doped with an element having at least one electron less than the

host material, then a p-type (positive type) semiconductor is formed (Al – 3

electrons added to silicon – 4 electrons). Similarly, any member of principal IIIrd

Group could dope a host semiconductor from 4A Group and show the same effect.

The solid will have a “positive” hole in its electronic structure that would move in


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opposite direction of the electron flow. Thus a p-type semiconductor would be

formed.

Doping is done in the range of ppm (parts per millions) concentrations

(mg/kg), but may be up to a few g/kg. A semiconductor doped to several g/kg level

has a conductivity close to that of a poor metal.

The building block of most semiconductor devices involves combining p-

type and n-type regions into p-n junctions. Between the electronic devices that

function using combinations of p-n junctions are:

o Diodes – p-n junction applications that act as a rectifier for

converting alternating current to direct current. This is due to the

ability of a diode to allow current flow in one direction but not in

the other.

o Solar cells are p-n junction devices which use sunlight to create

electrical energy. It is the energy of the sun’s photons that causes

the electrons to be promoted into the conduction bands and carry

the current. However, the current derived from the solar cell is

small. It takes many energy solar cells to produce enough current.

This is why nowadays the solar cells are use more for individual,

isolated applications than for industrial applications.

o Transistors which, unlike diodes, contain p-n-p or n-p-n

junctions. Because of this, a transistor can be used in a circuit to

amplify a small voltage or current to higher values or function as

an on-off switch. Transistors are of two main types: bipolar

junction transistor (BJT) and field effect transistor (FET).

When selecting a semiconductor material for electronic applications, a

number of factors must be considered. Of primary importance is the inherent band

gap size (the gap energy). Furthermore, the ordinary chemical and physical

properties of the host material and its compounds play important roles as well.


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1.8. Liquid crystals

Usually there are three aggregation states of matter that can be considered:

solid, liquid and gaseous, as shown in figure 8. However in the middle of 19th

centaury the German scientists observed a bizarre behaviour of a nerve fiber in

polarized light. In 1878 another German scientist, Otto Lehmann, made an

observation that some compounds when heated pass from a clear crystalline phase

through a “milky” phase to again a clear, transparent phase (isotropic liquid). He

interpreted it as an incomplete transition from the crystalline to the liquid phase.

Only ten years later, another German scientist, F. Reinitzer, remarked that this

“intermediate” phase is another state of the matter and he called it Liquid Crystal

(Fluessige Kristalle). This name was later disputed, particularly by French

scientists, which contributed enormously to the knowledge on liquid crystals and

which proposed another name: soft matter (matière molle). In fact, as it was shown

later, a lot of different, stable phases may occur between solid and liquid.

Solid LC Liquid Gas

Temperature

Figure 8. States of the matter with increasing temperature

Liquid crystalline phases (called also mesophases) occur only in some

molecular materials, such as fats (eg butter), soaps, detergents, surftactants etc.

Molecules having property of forming a liquid crystal are called mesogens. It is

important that the molecule contains in its structure an aliphatic chain –(CH2)n. A

sufficiently long chain, determined by the value of n is required to provide liquid

crystalline phase. The formation of an LC phase may be well understood

considering two molecules: heptane and cyanobiphenyle (cf. Fig. 9). Heptane is


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liquid between -91°C and 98.4°C, while cyanobiphenyle is solid up to 88°C.

Neither heptane (Fig. 9a) nor cyanobiphenyle (Fig. 9b) exhibits a liquide

crystalline phase. Molecule made by binding chemically heptane to

cyanobiphenyle, known under the commercial name 7CB (cf. Fig. 9c), exhibit a

nematic LC phase between 28.5°C and 42°C.

(a)

(b)

(c)

Figure 9. Phase transitions of heptane (a), cyanobiphenyle (b) and liquid crystal (c) 7CB

Crystal Isotropic liquid

88°C

C N

Temp

CH3

C

H2

CH

2

C

H2

CH

2

CH3

C

H2

Crystal Isotropic liquid Gas

-91°C 98.4°C

Temp

CrystalIsotropic liquid

28.5°C 42°C

CH3

C

H2

CH

2

C

H2

CH

2

CH

2

C

H2

C N

NLC

Temp


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The mesogen molecules, depending on their shape, are classified as

follows:

(i) calamitic, they have a quasi 1D rod like shape;

(ii) discotic, planar, 2 D;

(iii) conique, 3D;

Molecules (ii) and (iii) form columnar liquid crystals.

The transition between the crystal phase, with a perfect orientation of

molecules and isotropic phase, with a random distribution of molecular axes may

go through different states characterized by different degrees of orientation of

molecules. Depending on the degree of orientation the following basic phases exist:

(a) Nematic (N), with mesogen orientation shown schematically in figure 10a.

Molecules exhibit relatively large distribution of molecular axes. The

director vector is perpendicular to the mesogen layers.

(b) Smectic A (SA) (cf. Fig. 10b). In this case the mesogens are arranged in

layers with a much higher degree of orientation than in nematics. The

director vector is also perpendicular to the mesogen layers, but exhibits

much more narrow distribution.

(c). Smectic C (SC) (cf. Fig. 10c). Here the director vector is tilted from the

normal to layers.

Some of mesogens, such as fatty acids, their sells (soaps) soluble in water,

form so called lyotrope liquid crystals. The other, not containing water molecules

are called thermotrope liquid crystals.

Important notion in description of liquid crystal is the director vector n. It

gives the direction of the maximum of the distribution of molecular axes, or in

other words, the maximum probability of finding a molecular axis.


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a b cFigure 10. Schematic presentation of the arrangements of mesogenes in nematic (a),

smectic A (b) and smectic C (c) liquid crystals; n represents the director vector of liquidcrystal

Another interesting class of mesophases are cholesteric liquid crystals,

composed from chiral rod-like type molecules, and forming a chiral structure (Fig.

11). The mesogens are arranged in successive planes, in which they exhibit a high

degree of orientation. The direction vector rotates from one layer to another. The

length of the structure corresponding to the rotation of director vector by 2π is

called pitch.

Figure 11. Schematic presentation of a choleteric liquid crystal

Polymer liquid crystals

Interesting class of mesophases represent liquid crystal polymers. These

are polymers which either spontaneously exhibit a liquid crystal phase such as

polystyrene/polymethyl methacrylate (Fig. 12) or polymers which are chemically

functionalized with mesogens (e.g. side chain liquid crystal polymer).

Nematic

N

n

A

n

S

Smectic A

nz θ

SC

Smectic C


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Figure 12. Chemical structure of a copolymer

Practical interest of liquid crystals arises fundamentally from their

interaction with electric and magnetic fields. Relatively low electric fields change

easily their direction leading to an important variation of the index of refraction. It

finds application in electro-optic modulators. Another, greatly developed

application is in LC displays. They use the ability of orientation mesogenes of

nematic LC in the direction of the applied electric field. A very thin layer of liquid

crystals is placed between two transparent electrodes and crossed polarizers. The

mesogenes are attached to the substrate (by appropriate brushing) to follow this

orientation. Consequently the light polarization rotates by 90 degrees, and pass

through the LC cell. The pixel is “transparent”. By applying electric field all

mesogenes will turn in direction parallel to it, thus perpendicular to the electrodes

plane. The cell becomes to be opaque. Light does not pass through. In practice the

LC screens are formed from a lot of pixels, each pixel is addressed independently,

so a picture can be obtained. Colour LC screens are made from pixels, containing

at least 3 subpixels, each of them being equipped with filters, passing one of the

three colors: blue, red, green. If there is no voltage on e.g. red pixel only, the red

color will pass. Other colors are obtained by combination of these three

fundamental shades. In practice one uses a mixture of several liquid crystals what

gives a larger operation wavelength than when using only one of them.

As it was already mentioned the liquid crystals (LC) are complex highly


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correlated molecular systems where crystalline and liquid properties are

intertwined with various electronics and optical processes to offer a rather rich

ground for fundamental and practical pursuits. Based on recent development of

reconfigurable nano-particle network, a new class of so-called nano-dispersed

supra-nonlinear liquid crystalline material system have emerged as a highly

promisingly material for advanced next generation opto-electronics devices. By

dispersing such supra-nonlinear liquid crystal [SNLC] in nanoparticle network, one

could anticipate even more dramatic and revolutionary improvement in the already

impressive performance characteristics of SNLC. The nanoparticle network could

be dielectric, conductive, semiconductive or even magnetic, and most importantly,

it is capable of memory mode that is reconfigurable and electronically switchable.

Nano-dispersed supra-nonlinear liquid crystalline material system [NDSNLC] will

push the operation characteristics of these optoelectronic devices to unprecedented

laser power regime [microWatt to nanoWatt], spectral bandwidth [visible - IR] and

temporal [dynamic - storage mode].

Curs Chimie Cap1

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