TEM bright field imaging of thick specimens: nodes in Thon ring patterns

Area of Science:

• Electron microscopy
• Materials science
• Image analysis

Background:

• Transmission electron microscopy (TEM) depth of field limits imaging of thick objects.
• Image projection deviates from true 3D structure when object thickness exceeds depth of field.
• This deviation is observable in the image's power spectrum.

Purpose of the Study:

• To experimentally demonstrate the effect of object thickness on TEM power spectra.
• To establish a method for determining object thickness from power spectrum features.
• To improve contrast transfer function (CTF) correction for thicker samples.

Main Methods:

• Experimentally imaged carbon foils of varying thicknesses at different acceleration voltages (40-300 kV).
• Determined foil thicknesses using electron tomography.
• Analyzed power spectra to observe Thon ring patterns and their modulation by object thickness.

Main Results:

• Observed a sinc function envelope attenuating Thon ring amplitudes in thick objects.
• Identified "nodes" (zero amplitude points) in the power spectrum.
• Demonstrated that node positions shift to lower spatial frequencies with increased thickness and decreased acceleration voltage.
• Confirmed that object thickness can be derived from node positions.

Conclusions:

• Thick object Thon ring patterns are a sum of power spectra from thin, independently scattering slices.
• Object thickness can be quantitatively determined from power spectrum node positions.
• This method aids in more reliable CTF correction for thicker samples and lower acceleration voltages in TEM.