Microscopy 101
Figure 1: Two focus conditions for a Au film on a carbon support film. A) Mini- mum contrast condition for the amorphous carbon support film. Note the ghost diffraction images of the Au grains noted by the yellow circles. B) Minimum con- trast for a Au crystal film with the difference in height of 37 nm. Note the minimum contrast of the same grains within the yellow circles in (A) and the increased con- trast of the amorphous support film. The inserted FFT shows the Thon rings from the carbon support film.
center) should all be set, and the condenser aperture should be aligned.
Note: It is good practice to eliminate hysteresis in the lenses prior to doing the alignment, if your microscope has the capability of doing so. Before I begin my session with the alignment procedure, I switch the microscope to low-mag mode and then back to mag mode and press the standard focus. Tis turns the objective lens off and then back on and sets the lens to the pre-determined standard focus strength. Next, I turn the brightness knob fully counterclockwise and then fully clockwise before bringing the beam to crossover and centering it. Ten I center the condenser aperture.
a. Center the region of interest where the diffraction pattern will be acquired. With the standard focus set and at a mag- nification above 40 kX, (200 kX is better), raise/lower the sample to get minimum contrast. If using a live fast Fourier transform (FFT) from a digital camera to help with astig- matism, the innermost dark ring of the Ton rings from an amorphous sample will go to a maximum. For a crystalline
42
Figure 2: A) Lower camera exposure, condense the beam, and center the illumination. B) Spread the illumination to view the area of interest. C) Insert desired SAD aperture and center it.
sample, look for the point where all of the diffracted images coalesce with the bright-field image as shown in Figure 1.
b. Notes i. With a perfect lens, all of the diffracted images will coalesce, but because of spherical aberration, higher- index reflection images may not coalesce well with lower-index ones.
ii. If a crystalline material is on a support film, focus on the crystalline material, not on the amorphous
www.microscopy-today.com • 2020 March
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76
