Chapter 12 Solids and Modern Materials

Compound semiconductors maintain the same _______ valence electron count as ___________ semiconductors- ______ per atom.

-AVERAGE -ELEMENTAL -4

Examples of molecular solids ...

-Ar -H2O -CO2

The large difference in physical properties of graphite and diamond - both of which are pure _________ - arise from differences in their ___________________ and _____________.

-CARBON -3-dimentional structure and bonding.

Gold is typically alloyed with...

-SILVER -COPPER

Examples of face-centered cubic metals include...

-aluminium -lead -copper -silver -gold

In pure Si, all of the valence-band molecular orbitals are filled and all of the conduction-band molecular orbitals are empty. Because phosphorus has five valence electrons but silicon has only four, the "extra" electrons that come with the dopant phosphorus atoms are forces to occupy the ___________ _______.

-conduction band

Examples of body-centered cubic metals include...

-iron -chromium -Sodium -Tungsten

The doped material is called an ___ _____ semiconductor, __ signifying that the number of negativity charged electrons in the conduction band has increased.

-n-type -n

Typically, the interstitial element is a _______ that makes covalent bonds to the neighboring _________ atomsl

-non-metal -metal

How many atoms does the primitive cubic contains

1

There are 7 possible shapes for a 3D unit cell: ( primitive lattice)

1. Cubic 2. Tetragonal 3. Orthorhombic 4. Rhombohedral 5. Hexagonal 6. Monoclinic 7. Triclinic

An __________ is a material that contains more than one element and has the characteristic properties of a metal.

ALLOY.

Polymers characteristics

Are normally stronger and have higher melting points than molecular solids, more flexible then metallic, ionic, or covalent-network solids.

___________ consist of atoms held together in large networks by covalent bonds. Because covalent bonds are much stronger than intermolecular forces, these solids are much harder and have higher melting points than molecular solids.

COVALENT-NETWORK SOLIDS

Responsible for the unique properties of semiconductors as well as materials like diamond.

Covalent-network solids

Covalent-network solids

Covalent-network solids solids are held together by an extended network of covalent bonds. Resulting in materials are that extremely hard.

Will GaP have a larger or smaller band gap than ZnS?

The size of the band gap depends on the vertical and horizontal positions of the elements in the periodic table. Thus, we would expect the electronegativity difference to be larger for ZnS, which should result in ZnS having a larger band gap than GaP.

The small repeating unit found in crystalline solids

UNIT cell

Unlike metals, in semiconductors an energy gap develops between the filled and empty states, much like the energy gap between bonding and antibonding orbitals. The band that forms from bonding molecular orbitals is called the ....

VALENCE BAND.

Molecular solids consist of atoms or neutral molecules held together by: - - -

dipole-dipole forces Dispersion forces Hydrogen bonds

Because the valence electron in ionic compounds are confined to the anions, rather than being delocalized, ionic compounds are typically ______________.

electrical insulators.

classification of solids

- Metallic solid -Ionic solids -covalent network solids -Molecular solids -Polymers -Nano materials

Silicon and germanium lay directly below ________ in the periodic table, the significance of this is...

-CARBON -Like carbon, each of these elements has four valence electrons, just the right number to satisfy the octet rule by forming single covalent bonds with four neighbors. Hence, silicon and germanium, as well as the gray form of tin, crystallize with the same infinite network of covalent bonds as diamond.

In a semiconductor, the valence band is filled with ___________ and the conduction band is ___________.

-ELECTRONS -EMPTY

Semiconductors can be divided into two classes: - -

-ELEMENTAL semiconductors which contain only one type of atom -COMPOUND semiconductors, which contain two or more elements.

The presence of the extra bonds provided by the interstitial component causes the metal lattice to become ___________, ___________, and less_________.

-Harder, stronger, and less DUCTILE.

As we move down the periodic tables, bond distances _________, which __________ orbital overlap.

-INCREASE -DECREASES This decrease in overlap reduces the energy difference between the top of the valence band and the bottom of the conduction band. As a result, the band gap decreases on going from diamond (5.5eV), to silicon (1.11eV) to germanium (0.67 eV) to gray tin (0.08 eV).

The band gap of a compound semiconductor tends to _______ as the difference in group number ______.

-INCREASE -INCREASE

As the difference in electronegativity of the elements _________, the bonding becomes more _______ and the band gap _________.

-INCREASES -POLAR -INCREASE

In the heaviest group 4A element, _______, the band gap collapses altogether. As a result, lead has the structure and properties of a ________.

-LEAD -METAL

Most metals are __________, which means that they can be hamered into thin sheets, and ______, which means that they can be drawn into wires. These properties indicate that the atoms are capable of slipping past one another. ______ and ________ -network solids do not exhibit such behavior; they are typically brittle.

-MALLEABLE -DUCTILE -Ionic and Covalent-Network

In diamond, each carbon atom is bonded to four other carbon atoms, forming a ________ In graphite, sheets are held together by _______ forces.

-Tetrahedron -Dispersion

Name the five basic 2-dimensional lattice shapes:

1. Oblique lattice (most general) 2. Square lattice y=90 3. Rectangular lattice y=90 4. Hexagonal lattice y=120 5. Rhombic lattice

Alloys can be divided into 4 categories:

1. Substitutional alloys 2. interstitial alloys (both of the above are considered are both homogeneous mixtures in which components are dispersed randomly and uniformly.) 3. heterogeneous alloys, 4. intermetallic compounds.

Substitutional alloys are formed when the two metallic components have similar atomic radii and chemical-bonding characteristics. For example, silver and gold form such an alloy over the entire range of possible compositions. When two metals differ in radii by more than about ____%, solubility is greatly more limited.

15%

In the semiconductor community, energies are given in electron volts (eV) 1eV = ...

1eV = 1.602 x 10^-19 J

How many atoms does the body centered cubit unit cell contains

2

How many atoms does the face centered cubic unit cell contains

3

The elemental semiconductors all come from group ______.

4A

how many types of two dimensional lattice

5

____________ and ___________ , two allotropes of carbon, are two of the most familiar covalent-network solids. Other examples are silicon, germanium, quartz (SiO2), silicon carbide (SiC), and baron nitride (BN). In all cases, the bonding between atoms is either completely covalent or more covalent than ionic.

Diamond and Graphite

For example, Eg = 0.67 in Ge, but Eg =1.43 eV in GaAs. If we increase the difference in group number to four, as in ZnSe (groups 2b and 6A), the band gap _________________

Increases to 2.70 eV. This progression is a result of the transition from pure covalent bonding in elemental semiconductors to polar covalent bonding in compound semiconductors.

Metals conduct electricity extremely well. Many solids, however, conduct electricty somewhat, but nowhere near as well as metals, which is why such materials are called _____________.

SEMICONDUCTORS

Most important iron alloy is.....

SS

When atoms of the solute in a solid solution occupy positions normally occupied by a solvent atom, we have a ___________________ alloy.

SUBSTITUTIONAL

Two example of semiconductors are _______ and _______.

Silicon and germanium

Band gaps greater than ~_______ are so large that the material not a semiconductor; it is an __________ and does not conduct electricity.

~3.5 eV INSULATOR

Examples of metals with primative cubic structure:

they are rare, radioactive element polonium is an example.

What needs to happen for crystal structure and the lattice points to have identical patterns?

this happens in solids which have atoms that are identical, in other words, ELEMENTS can form structures of this type.

T or F. Ionic solids do not conduct electricity well and are brittle

true

Hexagonal close packing ...

When the fourth layer lays directly over the first layer, leading to the ABAB stacking pattern which is called this.

Solids in which atoms are arranged in an orderly repeating pattern are called...

crystalline solids

___________ solids (from the greek word without form) lack the order found in crystalline solids. These solids are similar to strutures of liquids but do not have the freedom of motion. Examples are rubber, glass, obsidian (volcanic glass).

Amorphous solids

Which of these statements is false? a. as you go down column 4A in the periodic table, the elemental solids become more electrically conducting b. As you go down column 4A in the periodic table, the band gaps of the elemental solids decrease. c. The valence electron count for a compound semiconductor averages out to four per atom. d. Band gap energies of semiconductors range from ~0.1-3.5 eV. e. In general, the more polar the bonds are in compound semiconductors, the smaller the band gap.

B.

Examples of alloys:

Bronze = copper + tin Brass = Copper + zinc

When you place an additional lattice point in a specific location in the unit cell, you create

CENTERED lattices

The band that forms the antibonding orbitals is called the ...

CONDUCTION BAND.

Polymers

Contained long chains of atoms (usually carbon) where the atoms within a given chain are connected by covalent bonds and adjacent chains are held to one another largely by weaker intermolecular forces.

Consider what happens when a few phosphorus atoms (known as ______) replace silicon atoms in a silicon crystal.

DOPANTS

The electrical conductivity of a semiconductor is influences by the presence of small numbers of impurity atoms. The process of adding controlled amounts of impurity atoms to a material is known as _______.

DOPING

These solids usually have flat surfaces or faces that make definite angles with one another. Examples are sodium chloride, quartz, and diamond.

crystalline solids

The crystaline structure of graphene illustrates two important characteristics of crystals....

First, we see that no atoms lie on the lattice points. Thus, to build up a structure you must know the location and orientation of the atoms in the motif with respect to the lattice points. Second, we will see that bonds can be formed between atoms in neighboring unit cells and the bonds between atoms need not be parallel to the lattice vectors.

Will GaP have a larger or smaller band gap than ZaN?

For both GaP and GaN the more electropositive element is gallium. SO we need only compare the positions of the more electronegative elements, P and N. Nitrogen is located above phosphorous in group 5A. Therefore, we would expect GaN to have a larger band gap than GaP.

In most crystals, the atoms are not exactly coincident with the lattice points. Instead, a group of atoms, called a _______, is associated with each lattice point.

MOTIF

Metallic Solids

Held together by "delocalized sea" of collectively shared valence electrons.

The different types of atoms in an __________ compound are ordered rather than randomly distributed. This leads to better structural stability and higher melting points. Negatively, they tend to be more brittle than substitutional alloys.

INTERMETALLIC

When the solute atoms occupy interstitial positions in the "holes" between solvent atoms, we have an _____________ alloy.

INTERSTITIAL

_______ compounds tend to be brittle because when stress is applied, there is a shift so that the alignmnet becomes cation-cation, anion-anion. The resulting repulsive interaction causes the planes to split away. (carving of gemstones).

Ionic solids/o

NANOMATERIALS

NANOMATERIALS Are solids in which the dimensions of individual crystals have been reduced to the order of 1-100nm,

In a heterogeneous alloy, the components are ...

NOT dispersed uniformly. In general, the properties rely on both the composition and the manner in which the solid is formed from the molten mixture.

Two classes of solids that do not fall into the other categories:

POLYMERS and NANOMATERIALS

When you place a lattice point at each corner of a unit cell, you get a _________________ lattice

PRIMITIVE

Intermetalic compounds have definite...

PROPERTIES and their composition cannot be varied.

Structures of metallic solids: 1. 2. 3.

Primative cubic structure Body-centered cubic structure Face-centered cubic structure

The centered rectangular lattice is another name for the ______

RHOMBIC lattice

When the spheres in the third layer do not lay directly over the first or second layer. When this sequence repeats itself, you get ABCABCABC results known as ...

cubic close packing.

There are _____ atoms in the primitive hexagonal unit cell, one from each layer.

TWO, neither atom sits directly on the lattice point

Responsible for the fact that most metals are relatively strong without being brittle.

The "delocalized sea" that holds together metallic solids.

This form of bonding allows metals to conduct electricity...

The "delocalized sea" that holds together metallic solids.

Diamond is strong because ...

The carbon atoms are sp3-hybridized and held together by strong carbon-carbon single covalent bonds. The strength and directionality of these bonds make diamond the hardest known material.

What is the coordination number?

The number of atoms immediately surrounding a given atom in a crystal structure.

lattice vectors

The vectors, a, b , that define a crystal lattice. The position of any lattice point in a crystal can be represented by summing integer multiples of the lattice vectors

These two bands are separated by the energy _______ __________

_BAND GAP -Eg

Molecular orbital model

a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.

solid solution

a solid homogeneous mixture

The structure of a crystalline solid is defined by a.) b.)

a.) the size and shape of the unit cell b.) the locations of atoms within the unit cell

MOLECULAR SOLIDS

are held together by the intermolecular forces we studied in ch. 11. Because these forces are relatively weak, they tend to be soft and have low melting points.

The crystal structures of many metals are simple and can be generated by placing a single ...

atom on each lattice point.

When atomic s and p orbitals overlap, they form...

bonding molecular orbitals and antibonding molecular orbitals. Each pair of s orbitals overlaps to give one bonding and one antibonding molecular orbital, whereas the p orbitals overlap to give three bonding and three antibonding molecular orbitals.

Geometrical pattern of points on which the unit cells are arranged is called a _____

crystal lattice (it is an abstract, not real) Imaginary.

structures of solids

crystalline and amorphous

A Face-centered cubbic lattice has...

one lattice point at the center of each of the six faces of the unit cell in addition to the lattice points at the eight corners.

A body-centered cubic lattice has ...

one lattice point at the center of the unit cell in addition to the lattice points at the eight corners.

electron sea model

represents the way electrons exist in metals

For example, in gallium arsenide, GaAs, each Ga atom contributes three electrons and each As atom contributes five, which averages out to four per atom- the same number as in silicon or germanium. Hence, GaAs is a ___________.

semiconductor

Ionic Solids

solids are held together by the mutual electrostatic attraction between cations and anions. Differences between ionic and metallic bonding make the electrical and mechanicals properties of these solids very different from those of metals.

metallic solids

solids that are composed of metal atoms

The most efficient way to pack one layer of equal-sized spheres is to ...

surround each sphere by six neighbors.

The physical properties as well as the structures of solids are dictated by.......

the type of bonds that hold the atoms in place