Exam Microscopic Measurement Techniques 4T th of April, 2008

Transcription

1 Exam Microscopic Measurement Techniques 4T th of April, 2008 Name / Initials: Ident. #: Education: This exam consists of 5 questions. Questions and sub questions will be rewarded with the amount of points as indicated (in total 100 points can be earned). In addition to these 100 points, bonus points are rewarded for visiting the three lab sessions ('lenses', 'microscopy', and 'lab market'), using the following formula: 1 lab session attended: 1 bonus point 2 lab sessions attended: 3 bonus points all 3 lab sessions attended: 6 bonus points Good luck! Question 1: Scanning electron microscopy (25pt) a) Above are given two SEM-s of oil in water. Which one is taken with SE-contrast and which one with BSE-contrast and why? What can you say about 'structure' of the sample? (4pt) b) Below are shown two SEM-s, recorded using a Secondary Electron detector, of a new nanoporous membrane suitable for virus filtration with good dimensional stability under high pressures; figure (a) show a top view of the membrane, while figure (c) shows a cross-sectional view. On the basis of these s, explain the three different contrast mechanisms in Secondary Electron mode. Also give the reason why the membrane appears perfectly flat in figure (a), but quite rough in figure (c). (8pt) (Adv. Funct. Mater. April 2008, 18, pp.1 7).

2 Nitrogen Nitrogen / 30% Hydrogen Carbon Dioxide c) Above are given three SEM-s, recorded in ESEM-mode, of a micro-system. The only difference between the s is the gas that was let in the vacuum chamber to create the s; for each the used gas is indicated at the top-left corner of the. Reason on the basis of the working principle of SEM-imaging in ESEM-mode, what must be the main difference between the three gasses that can explain the differences in contrast/brightness observed. (5pt) - What do you think would happen if you would use water as the inlet gas for creating your ESEM-? (+1 bonus point) (a) (b) (c) (d) d) Above are given four SEM-s (figures (a)-(d)) of the same section of a functional device with different microscopic components. The s are recorded using the following four contrast modes (in random order): BSE-compositional contrast, BSE-topographic contrast (A-B), BSE-topographic contrast (B-A), and SE-contrast. Reason which is recorded with which contrast mode and explain why. In addition, describe the different materials used in the functional device? (8pt) Question 2: Optical Microscopy and Diffraction (30pt) a) The figure below shows a 1D-(line)-diffraction pattern at a detector screen. The diffraction is created by a 1D-array of point sources, all spaced at equal distance from its neighbors, and the detector is placed in the 'far-field', i.e. Fraunhofer diffraction; position 0 denoted the optical axis. How many point sources were used to create this diffraction pattern? Explain why. (5pt) Light intensity 0 Position at detector screen

3 b) The figure below (figure (a) and (b)) denotes two different (grey) sheets both containing 6 holes positioned in a 2 x 3 array; the distances of the holes (black dots) in x- and y-direction, for situation (a) and (b), are x a, y a, x b, and y b, respectively, with x a > y b > x b > y a. Draw for both sheets, in a qualitative manner, the 'far-field' (Fraunhofer) diffraction pattern that is created behind the sheet, if the sheet is illuminated by a planar light wave, i.e. each holes become a point source of in-phase light. Note, it is not necessary to do any calculations, or to get the absolute scale of the diffraction pattern correct. It is only necessary to draw the correct relative distances between the diffraction peaks, in x- and y-direction and in comparison between situation (a) and (b). Give a to-the-point, bullet-wise explanation of the choices you made. (9pt) x a x b y a y b (a) y x (b) y x c) "Bright field" optical microscopy can be interpreted as the result of a double diffraction process. Use the description of the double diffraction process, and the diffraction pattern formed at the back focal, to deduce the relationship between: - the smallest detail on the object that can be resolved d, - the wavelength λ, - the index of refraction n of the medium between the lens and sample, - and the capturing angle θ of the lens. (8pt) bright field polarization contrast d) Above are shown a "bright field" and a "polarization contrast" microscopic of an (unstained) polymer fiber. It is clear that bright field yields little contrast for this specimen, whereas the sample is visible in good detail using polarization contrast. In the polarization, the core of the fiber has a reddish color. Explain the reason for this reddish color on the basis of the working principle of polarization contrast. (4pt) e) Name two other optical microscopy techniques that can be use to visualize this polymer fiber with good contrast. For each of the two techniques, explain in maximum two sentences what is the working principle to yield the good contrast. (4pt) Question 3: Measurement of surface topography (25pt) A microscopist uses the AFM in the Multi-Scale lab, in tapping mode, to measure the shape of polystyrene particles on a substrate, which should have a size of approximately 2 µm, see figures (a)-(c) where the height ranges with grey value from black to white: (a) (b) (c) a) As a general question, give a concise, bullet-wise explanation of all the basic components of Scanning Probe Microscopes (SPM). In addition, explain briefly the physical principle of AFM tapping mode that provides the contrast in the figures. (5pt)

4 b) The microscopist first measures the topography shown in figure (a) above. However, he suspects that the is flawed by some sort of artifact. Therefore, he measures the same sample two times more, using two different tips (figs. (b) and (c)). - What is the artifact that the microscopist suspect, and how is this artifact caused? - Which of the three s (if any) shows the real of the polystyrene particles, and why do you think so? - What additional information must be available from these measurements to confirm the shape/size of the particles. (8pt) c) What other common surface topography techniques besides AFM (that are also available in the Multi-Scale lab) could the microscopist use to measure the shape/size of the polystyrene particles? - What are the disadvantages of these techniques? (7pt) d) Nevertheless, the microscopist sticks to her AFM and uses it in contact mode to measure a polymer sample, see below (again the height increases with grey value ranging from black to white). This topography is also flawed by a serious artifact. Please give a concise description/explanation of the cause of this artifact. - What kind of polymer sample is measured here? (5pt) Question 5: Ray diagrams (20pt) A student inspects an object using a (confocal) magnifying glass. The figure below shows the magnifying glass with its two focal points (marked by 'f lens ') and the eye of the student with one of its focal point (marked by 'f eye '). a) Back-track the position of the object in the figure below. Draw three real rays from the object to the at the retina. Draw all real rays with solid lines, and all imaginary rays with dashed lines. Note: do not copy the figure, but present your answer by drawing in this figure below. (8pt) f lens f eye f lens

5 b) If the object is moved further away from the lens, will the object at the retina increase or decrease? Explain why. (4pt) c) Next, the student uses a simple microscope to look at an object. This is shown in the ray diagram below, which depicts the object in the Object, the intermediate in the Intermediate, the virtual in the Virtual, the final in the Final, the object lens with its focal points marked by 'f 1 ', the ocular lens with its focal points marked by 'f 2 ', and the eye lens with its focal points marked by 'f eye '. Also shown are two rays coming from the object (dashed blue arrows). Ray trace the path of these rays towards the at the retina. Note: do not copy the figure below, but present your answer by drawing in this figure below. (8pt) f 1 f 2 f 2 f 1 f eye f eye Virtual Object Intermediate Final