2024 Lecture 5 - Surface Analysis - Exercises Anwsers.pdf

Lecture 5 - Analytical Techniques II: Surface Analysis Exercises Scanning Electron Microscope (SEM) & Transmission Electron Microscope (TEM) 1. How are the X-rays generated when doing SEM-EDX? Because we bombard the sample with electrons, x-rays are generated by a similar mechanism as...

Lecture 5 - Analytical Techniques II: Surface Analysis Exercises Scanning Electron Microscope (SEM) & Transmission Electron Microscope (TEM) 1. How are the X-rays generated when doing SEM-EDX? Because we bombard the sample with electrons, x-rays are generated by a similar mechanism as that in XRF. First, the energy transferred to the atomic electron removes it from the inner electron shell, leaving behind a hole. Second, its position is filled by another electron from a higher energy shell, and the X-ray with characteristic wavelength is released. 2. Describe the main difference between SEM and TEM? Transmission electron microscopy (TEM) creates an image using transmitted electrons that are passing through a thin sample (e.g. 100 nm). This requires a thin sample slice, requiring skilled operators for sample preparation. TEM however does enable studying internal structure/composition of a sample. In scanning electron microscopy (SEM), back-scatter electrons and/or secondary electrons are used to create an image of the sample. No complex sample preparation is required, however for non-conductive samples, a thin layer coating or carbon or gold should be applied. 3. What is the difference in images recorded from the secondary electrons and the back scatter electrons? Why is this relevant? SEs are emitted near the surface of specimens and provide surface information on tissues and cells. SEs that are produced in deeper regions of the samples are absorbed because of their low energy. BSEs can escape from deeper regions of specimens and contribute to the depth images of tissues and cells owing to their high energy. The production of BSEs is heavily dependent on the mean atomic number of the specimen, the higher the atomic number, the brighter the material appears in the image. This allows the detection of composition information. 4. What types of samples would a TEM and SEM be used to view? TEM is the choice when you want to get information from the inner structure, while SEM is preferred when surface information is required. 5. Which technique provides a higher spatial resolution? SEM resolution is limited to ~0.5 nm, while with the recent development in aberration- corrected TEMs, images with spatial resolution of even less than 50 pm have been reported. However, its advised to remain cautious when comparing features such spatial resolution or sensitivity. This is due to the spatial resolution being dependent on the performance of the instrument (including electron optics) and on the imaging mode used. In addition, spatial resolution also depends on the specimen/object measured, particularly its thickness and radiation sensitivity. 6. What are two problems researchers encounter when using electron microscopes to view live cells and tissues? Vacuum – As electrons are easily scattered by other molecules in the air, samples must be analyzed in a vacuum. This means that live specimens cannot be studied by  this technique. This means that biological interactions cannot be properly observed, which limits the applications of electron microscopy in biological research. Conductivity – Typically, live cells and tissues are non-conductive and require a thin conductive coating to be analyzed by electron microscopes. Black and white images – Only black and white images can be produced by an electron microscope. Images must be falsely colorized. X-ray Photoelectron Spectroscopy (XPS) 7. What is the binding energy in XPS and why is this relevant for XPS? The binding energy is the energy of an electron attracted to a nucleus. When electrons are in orbitals farther from the nucleus, less energy is required to eject them, so the binding is lower for higher orbitals. Electrons contained in different subshells (s,p,d,etc.) have different energies as well. Therefore, Chemical shifts in XPS spectra can be observed when an element enters a different bound state, which results in changes in the binding energy of core electrons. A primary carbon would have a slightly different binding energy than a carboxyl carbon for example. In general, an increased oxidation state (removal of valence electrons) increases the Binding Energy and addition of valence electrons decreases the Binding Energy. Thus the binding energy in XPS is relevant because it provides additional depth in information on the sample (e.g. oxidative states). 8. Describe the differences between the information that can be obtained from XPS and XRF? XPS can provide additional information on the oxidative state, which cannot be measured with XRF. 9. For which type of samples/questions would you XPS be a viable option? Samples/questions for which detailed information on the surfaces is required, the most common example is Catalyst due to the importance of understanding the oxidative state. But you will also see XPS used in the analysis of Glass, coatings, adhesives, lubricants, thin films, microcircuits etc. Atomic Force Microscopy (AFM) 10. Name two advantages and two disadvantages of AFM compared to electron microscopy Advantage Disadvantage Direct 3D data as topography is AFM is significantly slower measured in the z-direction rather than a 2D image Greater level of detail of the surface AFM can only study small sample sizes AFM does not require high vacuum SEM has a large depth of field compared to AFM AFM can measure in liquids, EM cannot No compositional information can be obtained like in SEM-EDX AFM cannot measure internal structures like TEM could do 11. What type of chemical information could be obtained using CFM that is not possible with AFM?  Surface morphology by using the interaction forces between the chemically- functionalized tip and the surface functional groups or molecules of the sample (e.g. chemical-composition heterogeneity of polymers). Some examples of interaction between the tip and substrate include: hydrogen bond formation, charge-transfer complex formation, metal-ligand complexation, dimerization, host-guest complexation. X-Ray Fluorescence 12. Is XRF a quantitative or qualitative analysis technique? XRF is a quantitative technique, The response for any element is directly related to the concentration of that element within the sampling volume. This is assuming that interaction between elements are accounted for and that standards with a similar matrix can be obtained. Hence, more elements will be available for detection. 13. Why is XRF typically performed in a vacuum? X-Rays are absorbed and lose intensity under normal atmospheric conditions. As a result, using a vacuum lowers the detection limits and extends the range of analysis towards lower atomic numbers. This is specifically necessary for lighter elements as air will absorb their tube radiation and thus not reach the detector. 14. What is the advantage of wavelength dispersive over energy dispersive XRF? WD-XRF has a resolution and sensitivity advantage in comparison to ED-XRF. This is especially useful in the low atomic numbers (