Acquire a Spectrum
- Find the general region of interest at low magnification; once located, increase the magnification. Ensure that you choose an area with suitable thickness. A thickness map is ideal for this determination.
A typical TEM image of BN flakes on a lacy grid. Low magnification is used to identify ROI (left), and increased magnification is used further to identify ROI and sample thickness (right).
- Reduce the spot size and condenser aperture to typical values for microanalysis.
- Put the beam on the feature of interest; select a region that appears highly electron transparent.
- Select the Diffraction mode on the TEM, then set the camera length to 5 – 20 cm to give a collection angle in the region of ~20 mrad.
- Make sure the center spot (disc) is in the center of the screen.
- Lift the screen, then click the View button on the EELS palette to start a continuous view of the low-loss spectrum.
- Click on the EELS status text to control the dispersion, aperture, and energy offset. You can choose the energy offset by an element from the periodic table icon.
Note: Generally choose the smallest dispersion (largest eV/ch value) that still gives the required energy resolution. Assume at least 3 channels are needed to define the energy resolution (e.g., 0.5 eV/ch will yield at best 0.5 x 3 = 1.5 eV resolution). For systems with adjustable apertures, choose Hi-SNR (largest aperture) for mapping and general EELS, and choose Hi-Res for low-loss EELS and ELNES studies where the best energy resolution is needed.
- Select Auto in the Exposure field of the EELS palette to use the auto exposure function.
- If you prefer Manual mode, disable Auto and enter the Exposure time. Alternately, with the live EELS window frontmost, hitting the up/down arrow keys will double/half the current Exposure time.
- Choose among the four EELS detector settings (SNR, View, HDR, and User) as needed for the experiment.
- SNR uses the lowest noise settings and is ideal for weak signals.
- View provides a fast refresh of the spectrum with a good dynamic range.
- HDR is preferred when viewing strong signals and delivers the cleanest spectra.
- User enables custom camera settings; click on the Setup icon to access and change the user settings.
Note: View and Capture use the same camera settings.
- Use the Align ZLP button to adjust the ZLP to 0 eV.
- Select the Thickness button in the EELS Processing palette to estimate the thickness of your region of interest.
