Lecture 9: Materials characterisation - Synthetic

Question 1

1. What are the differences between TEM and SEM?

Answer 1

Type of electrons SEM : Scattered, scanning electrons TEM: transmitted electrons Image SEM: 3D surface TEM: 2D projection image of inner structure

Question 2

2. How is contrast obtained in a TEM?

Answer 2

The contrast between two adjacent areas in a TEM image can be defined as the difference in the electron densities in image plane

Question 3

3. What are bright field and dark field micrographs acquired in a TEM?

Answer 3

If using the Objective aperture, the central beam is selected (the rest of the signal is blocked) and the bright field image (BF image) is obtained. If we allow the signal from the diffracted beam, the dark field image (DF image) is received. (Note: this is carried out in bright field image)

Question 4

4. What is the main difference between the images obtained in an SEM in the secondary electrons mode and in the backscattered electrons mode?

Answer 4

They occur due to elastic collisions of electrons with atoms, which causes a change in the electrons’ trajectory.

In contrast, secondary electrons originate from the surface or the near-surface regions of the sample. They occur due to inelastic interactions between the primary electron beam and the sample and contain lower energy than the backscattered electrons. Secondary electrons are very beneficial for the inspection of the topography of the sample’s surface

Question 5

5. How can you analyze the elemental composition of a sample inside an electron microscope?

Answer 5

x-ray diffraction?

Question 6

6. Explain how a focused ions beam microscope works.

Answer 6

FIB works like SEM by focusing a beam of ions at the surface which leaves the surface as either secondary ions or neutral atoms. The primary beam also produces secondary electrons. The signal from the sputtered ions or secondary electrons is collected to form an image.

Question 7

7. Explain how an SPM works.

Answer 7

SPM - Scanning Probe Microscopy

is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen.

Question 8

8. Explain what kind of information you can obtain with UV-Vis, IR, Raman, ICP and Mass spectrometry.

Answer 8

UV/Vis spectroscopy

• Molecules containing bonding and non-bonding electrons (n-electrons) can absorb energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb.
• Use: is routinely used in analytical chemistry for the quantitative determination of different analytes. UV/Vis can be applied to determine the kinetics or rate constant of a chemical reaction.

IR spectroscopy -

• The frequency of the absorbed radiation matches the frequency of the bond or group that vibrates.
• The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling. Tells you about the functional group.

Raman

• It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the vis, near IR, or near UV range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down.
• The shift in energy gives information about the functional groups - “the vibrational modes”

Inductively Coupled Plasma

• is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.
• Detection of amounts of chemical elements, and burns it.

Mass spectrometry

• is an analytical technique that ionizes chemical species and sorts the ions into a spectrum based on their mass-to-charge ratio.

Question 9

9. What kind of information can you obtain with X-Ray diffraction, TGA and DSC?

Answer 9

X-Ray diffraction-is a nondestructive technique that provides detailed information about the crystallographic structure, chemical composition, and physical properties of materials.

TGA - Thermogravimetric analysis or thermal gravimetricanalysis is a method of thermal analysis in which the mass of a sample is measured over time as the temperature changes

Differential scanning calorimetry, or DSC, is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature.

Question 10

10. Explain how chromatography works.

Answer 10

Chromatography is a laboratory technique for the separation of a mixture. The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase. The various constituents of the mixture travel at different speeds, causing them to separate.