Chapter 5 - Visualizing atomically thin crystals Flashcards
Give examples of methods using electrons as both probe and signal.
SEM, TEM, AES, EELS
Give an example of a method using electrons as probe, and generated x-rays as signal.
EDX (Energy Dispersive X-Ray Spectroscopy)
Give examples of methods using ions as probe, and electrons as signal.
HIM (Helium Ion Microscopy)
Give examples of methods using photons in the X-ray regime as probe, and electrons as signal.
XPS
Give examples of methods using photons in the visible regime as probe and signal.
Optical microscopy, elliopsometry, interferometry.
Give an example of a method using photons in the visible regime as a probe, and phonons as signal.
Raman spectroscopy.
Explain the basic principles of AFM.
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.
What are some advantages of AFM?
- 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)
Explain the basic principles of STM.
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)
If you want to examine the morphology or surface of a sample, which techniques would be best suited?
- SEM / HIM
- STM
- AFM
- Optical microscopy
- RHEED / LEED
If you want to examine the structure of a sample, which techniques would be best suited?
- (HR)TEM / STEM
- RHEED / LEED
- XRD
- STM / STS
If you want to examine the chemical composition and the chemical bonds of a sample, which techniques would be best suited?
- 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)
Describe the main differences in the mechanisms of XES, XAS and XPS.
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.
What does the XAS spectrum show?
The absorption as a function of incoming photon energy.
What are observable in NEXAFS spectra?
Rydberg states, continuum and unfilled orbitals.
What are the requirements for NEXAFS?
- Highly monochromatic X-rays
- X-rays with tunable energies
- Sufficient intensity (high brilliance)
- High degree of polarization
In other words, a synchrotron source.
How can NEXAFS be used to determine the orientations of for example adsorbed molecules?
The absorption is highly dependent on the polarization of the incoming photons.
What is the interaction volume of electrons in matter dependent on?
The atomic number of the sample: higher Z, smaller interaction volume.
The energy of the electrons: higher E, larger interaction volume.
What is the typical energy range of electrons in TEM?
80-400keV (200keV those in NanoLab)
What different types of detectors exists in SEM?
Everhart-Thornley Detector (detects secondary electrons) and in-lens detectors (for example energy selective BSE detector, or in-lens SE)
How is the magnification in SEM dependent on composition?
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).
What three types of electron guns are there?
W-filament, LaB6 and FEG.
How does a FEG work?
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).
Name an advantage of FEG over thermionic gun.
Energy dispersion is much lower (five-fold).
Can the yield of secondary electrons surpass the number of primary electrons?
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.
What is the difference between escape depths in metals and semiconductors?
Electrons are scattered more in metals than in semiconductors, so the escape depth is much lower.
