Lecture 2

Question 1

Characteristics of gases and liquids

Answer 1

Gases : density 10^19 cm^-3, free mean path large compared to size

Liquid: density 10^22 cm^-3, stronger interactions, mean free path comparable to size

Question 2

Differences between amorphous and crystalline solid

Answer 2

Amorphous: short range order, microscopically stable
Crytsalline: 10^23 cm^-3, most sable (lowest energy), gain of energy by crystal formation

Question 3

Do perfect crystals exist ?

Answer 3

No -> defects (type, density, distribution)

Question 4

How can we visualize crystalline solids ?

Answer 4

Direct : scanning tunneling microscopy or TEM.
Indirect : x-ray diffraction

Question 5

Some general features of TEM

Answer 5

strong interactions bewteen probe and condensed matter -> very thin samples and vacuum conditions -> very expensive

Question 6

features of STM, what is it sensitive to ?

Answer 6

Works under ambient conditions, sample and tip need to be conductive for current to pass, current depends exponentially (negative) on distance.

Sensitive to local electronic structure of the sample

Question 7

Why do we have a specific lattice constant ?

Answer 7

equilibrium of repulsive and attractive forces (LOOK AT GRAPHS ON SLIDE 9)

Question 8

Describe the crystalline structure (components)

Answer 8

basis (building block) + lattice (mathematical points)
Translation vectors.
Unit cell volume : a*bxc

Question 9

What are primitive lattice vectors?

Answer 9

map any lattice point to all other points

Question 10

2 conditions for the choice of lattice

Answer 10

• primitive lattice should ba as small as possible, angles of 90 or 120
• system of 3 axes should be right-handed

Question 11

What are P, I, F and C unit cells ?

Answer 11

primitve, bcc, fcc, one side fc

Question 12

How many crystal systems and lattice types do we have in 3d ? Why do we use those bravais lattices ?

Answer 12

7 systems and 14 lattices (SLIDE 13)
We use them because not always advantageous to take the primitive unit cell.

Question 13

What is the formula for packing fraction ?

Answer 13

Voccupied/Vcell

Question 14

How many lattices in 2D?

Answer 14

5 (SEE SLIDE 15)

Question 15

What planes form the crystal’s surface ?

Answer 15

The one with small indices, because it means high point densities = minimal surface tension

Question 16

Why is the surface important to consider ?

Answer 16

Because it exhibits a different arrangement of atoms than the bulk

Question 17

In what order (mm, micro, nano, ..) does the surface dominate ?

Answer 17

nano range

Question 18

What does texture describe ?

Answer 18

orientation distribution of crystals within a polycrystalline aggregate
(describes the non-random distribution of crystallographic orientations within a polycrystalline material or single crystal. In other words, it characterizes how the crystal lattice planes or axes are oriented with respect to a particular direction or surface within the material.)