Dark Field Detector Research Articles

Digital Image Processing of STEM Images of Tobacco Mosaic Virus (TMV)

Low radiation doses are necessary to prevent undue specimen damage when recording electron micrographs. Unfortunately, such a requirement results in low signal to noise in the image which may limit the effective resolution. Digital image processing techniques have been used to increase the effective signal to noise and enhance scanning transmission electron micrographs of TMV. The original micrographs were taken on the University of Chicago STEM [1] using the annular dark field detector signal and processed on computer facilities at Cornell University. The specimen consists of negatively stained (1% uranyl acetate) particles of TMV (intact virus) as described in [2]. The electron probe diameter was ˜2.5Å and the total dose to the specimen was ˜11/Å2.

Observation of Biological Specimens Using Dark Field STEM

Unstained biological specimens were observed in the dark field STEM mode using a JEM-100CX TEMSCAN equipped with an annular type dark field detector. It was confirmed, by comparing the micrographs obtained in the above mode with micrographs obtained in the dark field CTEM mode, that the former were superior in regard to observable detail. In electron microscopy, it is a well known fact that the observation of unstained biological specimens consisting of light elements such as carbon, nitrogen, oxygen, etc., is extremely difficult due to the small scattering factor involved. In general, although negative or positive staining by heavy metallic elements is feasible for such specimens, the possibility of structural, morphological and chemistry related (e.g., bonding) degradation, as a result of such staining, is always present. Accordingly, freeze etching and freeze sectioning, in spite of the complicated preparation procedure involved, are becoming quite popular in that a specimen so treated, becomes highly resistant to degradation resultant upon staining. A further popular means for surmounting the above problems relating to specimen degradation, poor contrast, etc. is the so-called “dark field method”, the salient feature of which is that it is simple to utilize and dispenses with the problems involved in the freeze etching and freeze sectioning techniques. The main reason why dark field STEM is more effective than dark field CTEM is that, in the case of former, there are no lenses below the specimen, thus permitting thicker specimens to be observed without having to take into consideration lens chromatic aberration caused by inelastic scattering. Another reason Is that increased space availability allows a large angle acceptance detector to be used, thereby enhancing the collection efficiency1) of the elastically scattered electrons which in turn ensures a high contrast image and low-dose specimen observation.

Some progress in the use of a scanning transmission electron microscope for the observation of biomacromolecules

A new scanning transmission electron microscope equipped with an efficient dark-field detector is used to visualize biological specimens chosen among those best suitable for high resolution conventional electron microscopy. Micrographs of stacked-disk aggregates of tobacco mosaic virus protein, type I pili of Escherichia coli , DNA, and chromatin beads are presented. They compare favorably with the best images obtained to date with conventional electron microscopy.

Optimum Detector Arrangements in the Stem for Thick Objects

Radiation damage is the main obstacle in achieving high resolution images of biological objects. Therefore it is desirable to indicate that imaging method which yields maximum information about a specific object for a given number N0 of incident electrons. The amount of useful information is determined by the signal to noise ratio. Applying difference techniques the contrast in the STEM can be varied arbitrarily (l). Although the subjective impression of the image quality may be improved by such a procedure the amount of information available will not be changed. Most of the present day STEMs operate in the dark field mode using an annular detector which collects all electrons scattered out of the illumination cone. The corresponding incoherent bright field image is obtained from the electrons which pass through the hole of the dark field detector.

Asymmetrical dark field detectors in the stem

Although the annular dark field detector in the STEM collects a large fraction of the elastically scattered electrons, it does not use all the information carried by these electrons because it ignores the distribution of electrons over its surface. This information can be obtained by dividing the detector into sections. This paper calculates the number of electrons scattered by a single atom onto each side of an annular dark field detector split in half by a line passing through its center. This scattering problem is unusual becuase the incident wave is not a plane wave in the STEM. The sum and difference signals are calculated for a single atom from its scattering amplitude for a STEM limited by primary or secondary spherical aberration operating at 10 kV, 70kV, or 100 kV. All the cases calculated show that a single thorium atom should give an observable difference signal which is very sensitive to the focal conditions of the STEM.