Chapter 5 - Visualizing atomically thin crystals

Question 1

Give examples of methods using electrons as both probe and signal.

Answer 1

SEM, TEM, AES, EELS

Question 2

Give an example of a method using electrons as probe, and generated x-rays as signal.

Answer 2

EDX (Energy Dispersive X-Ray Spectroscopy)

Question 3

Give examples of methods using ions as probe, and electrons as signal.

Answer 3

HIM (Helium Ion Microscopy)

Question 4

Give examples of methods using photons in the X-ray regime as probe, and electrons as signal.

Answer 4

XPS

Question 5

Give examples of methods using photons in the visible regime as probe and signal.

Answer 5

Optical microscopy, elliopsometry, interferometry.

Question 6

Give an example of a method using photons in the visible regime as a probe, and phonons as signal.

Answer 6

Raman spectroscopy.

Question 7

Explain the basic principles of AFM.

Answer 7

AFM - Atomic Force Microscopy:

• scans a tip on a cantilever over the surface.
• cantilever is deflected due to the atomic forces between the tip and the sample (draw force diagram).
• deflection measured by shining a laser on the cantilever
• can be operated in contact, non-contact and tapping mode.

Question 8

What are some advantages of AFM?

Answer 8

• We can get sub-nm resolution even at room temperature and ambient conditions.
• Works on every surface (even molecules, if stable)
• Can obtain atomic resolution with sophisticated equipment (in vacuum and low T)

Question 9

Explain the basic principles of STM.

Answer 9

STM - Scanning Tunneling Microscopy:

• scans a tip over a conducting surface.
• measures the current tunneling through the barrier between the tip and the sample.
• tunneling current very dependent on the distance between the tip and the sample.
• gives a picture of the local density of state, rather than topology, and can probe different bands depending on the voltage applied (unoccupied or occupied - depending on direction of current)

Question 10

If you want to examine the morphology or surface of a sample, which techniques would be best suited?

Answer 10

• SEM / HIM
• STM
• AFM
• Optical microscopy
• RHEED / LEED

Question 11

If you want to examine the structure of a sample, which techniques would be best suited?

Answer 11

• (HR)TEM / STEM
• RHEED / LEED
• XRD
• STM / STS

Question 12

If you want to examine the chemical composition and the chemical bonds of a sample, which techniques would be best suited?

Answer 12

• Secondary Ion Mass Spectroscopy (SIMS)
• Particle-induced X-ray Emission (PIXE)
• Electron Energy Loss Spectroscopy (EELS)
• Auger Electron Spectroscopy (AES)
• Energy Dispersive X-ray Spectroscopy (EDX)
• (Wavelength dispersive X-ray spectroscopy (WDX))
• X-ray Emission Spectroscopy (XES)
• X-ray Absorption Spectroscopy (XAS)
• X-ray Photoelectron Spectroscopy (XPS)
• Near Edge X-Ray Absorption Fune Structure Spectroscopy (NEXAFS)

Question 13

Describe the main differences in the mechanisms of XES, XAS and XPS.

Answer 13

XES: looks at the x-rays emitted after species being excited.
XAS: looks at the absorption of x-rays that are used to excite electrons
XPS: looks at the energy of the photoelectrons generated when irradiating a sample with x-rays.

Question 14

What does the XAS spectrum show?

Answer 14

The absorption as a function of incoming photon energy.

Question 15

What are observable in NEXAFS spectra?

Answer 15

Rydberg states, continuum and unfilled orbitals.

Question 16

What are the requirements for NEXAFS?

Answer 16

• Highly monochromatic X-rays
• X-rays with tunable energies
• Sufficient intensity (high brilliance)
• High degree of polarization

In other words, a synchrotron source.

Question 17

How can NEXAFS be used to determine the orientations of for example adsorbed molecules?

Answer 17

The absorption is highly dependent on the polarization of the incoming photons.

Question 18

What is the interaction volume of electrons in matter dependent on?

Answer 18

The atomic number of the sample: higher Z, smaller interaction volume.
The energy of the electrons: higher E, larger interaction volume.

Question 19

What is the typical energy range of electrons in TEM?

Answer 19

80-400keV (200keV those in NanoLab)

Question 20

What different types of detectors exists in SEM?

Answer 20

Everhart-Thornley Detector (detects secondary electrons) and in-lens detectors (for example energy selective BSE detector, or in-lens SE)

Question 21

How is the magnification in SEM dependent on composition?

Answer 21

Specimens with a high atomic number produce high yield of electrons and achieve a higher useful magnification than low Z samples (such as carbon or plastics).

Question 22

What three types of electron guns are there?

Answer 22

W-filament, LaB6 and FEG.

Question 23

How does a FEG work?

Answer 23

Field Effect Gun - uses quantum tunneling. The high field causes the potential barrier to narrow, and the electrons can tunnel through. The current density is very high (compared to thermionic gun).

Question 24

Name an advantage of FEG over thermionic gun.

Answer 24

Energy dispersion is much lower (five-fold).

Question 25

Can the yield of secondary electrons surpass the number of primary electrons?

Answer 25

Yes, at low enough energies the number of SE generated per PE can be above 1. At higher energies they are generated further down in the bulk, and does not escape.

Question 26

What is the difference between escape depths in metals and semiconductors?

Answer 26

Electrons are scattered more in metals than in semiconductors, so the escape depth is much lower.