Lecture 7

Question 1

what are the seven crystal systems ?

Answer 1

cubic
tetragonal
orthorhombic
monoclinic
triclinic
trigonal
hexagonal

Question 2

what does polymorphism mean ?

Answer 2

elements can have a different crystal system depending on the temperature

Question 3

what do the diffractograms of single crystals, polycrystalline and amorphous materials look like ?

Answer 3

single -> regularly spaced dots
poly -> concentric circles
amorphous -> faded circle

Question 4

explain the formation of a diffraction pattern (several peaks)

Answer 4

the peaks are constructive interference between 2 reflective waves (x-ray beams) on 2 consecutive planes.

Several peaks because multiple reflection planes in a lattice

Question 5

explain the principle of an XRD measurement

Answer 5

we shine x-ray beams on a specimen at a certain angle -> we detect the reflected waves if they are coherent (constructive interference)

Question 6

list the components (5) of an XRD system and explain function

Answer 6

• x-ray tube = source of x-rays
• x-ray optics = condition the beam before it hits the sample
• goniometer = platform that holds and moves the sample, optics, detector and/or tube
• x-ray optics = condition the beam after it encountered the sample
• detector = count the number of scattered x-rays

Question 7

Bragg equation ?

Answer 7

2dsin(theta) = n*lambda

d is the spacing between the planes
n is the order of diffraction
lambda is the wavelength

Question 8

how is the diffractogram influenced by : lattice paramater, crystallite size, atomic number

Answer 8

• increase lattice parameter = angles decrease
• increase crystallite size = same angles but higher intensity, if very small -> tends to amorphous (scherrer equation)
• intensity is proportional to atomic number

Question 9

describe the functionality of a SEM, what can we see ?

Answer 9

Interaction of fast electrons with a sample :
- backscattered electrons : atomic number contrast (heavy = bright), a bit deeper than surface
- secondary electrons : topography contrast, come from surface
- photons : for EDX, the deepest

Question 10

demonstrate usefulness of BSE and EDX detectors

Answer 10

we can combine the info from BSE and EDX to map a whole surface : see where the heavy / light elements are, see each individual elements

Question 11

explain creation of an EDX mapping

Answer 11

when shooting electrons to a sample, there is a photon released -> creates energy peaks depending on the element.
We can map a surface that way, to find out where each element is

Question 12

what is the depth of info in SEM/EDX ?

Answer 12

the depth is around 0.5 to 2 micro meters.

Question 13

explain the photoelectric effect, minimum frequency ?

Answer 13

shine light on a surface (= emits photons) -> if the photons have a high enough energy, they will kick out electrons from the sample.

The minimum frequency / max wavelength is element dependent

Question 14

explain the principle of UV and X-Ray photoelectron spectroscopy

Answer 14

we shine light of a sample, and depending on the kinetic energy of the resulting electrons, we can find their binding energy
h*f = Ek + Eb (+work)

UV -> electrons in valence band
X-ray -> electrons in core levels

Question 15

describe how XPS spectra are aquired and interpreted

Answer 15

We get the binding energy of the electrons from the surface of the material.
It also gives info on what kind of bonds are present in the material (electron transfers change the bonding energy).

Question 16

tell about the surface sensitivity of XPS

Answer 16

it only goes about 100 angstrom deep.
The x-ray beam can penetrate further but the deep electrons won’t have the energy to emerge from the surface.

Question 17

discuss the idea of angle resolved XPS measurement

Answer 17

the smaller the angle between the detector and sample, the smaller the information depth (because electrons have to travel a longer distance).
Grazing angle -> only top layer.

Question 18

in what condition is XPS perfomed ?

Answer 18

ultra high vacuum because electrons can’t travel in air

Question 19

how does the applied voltage affect the current in photoelectric effect ?

Answer 19

A negative voltage will slow down the elctrons (lower current).
If too negative, current is zero, then it increases until the saturation current. After saturation, an increase in voltage will not change the current.