Scanning Transmission Electron Microscopy at High Resolution

Abstract

We have shown that a scanning transmission electron microscope with a high brightness field emission source is capable of obtaining better than 3 A resolution using 30 to 40 keV electrons. Elastic dark field images of single atoms of uranium and mercury are shown which demonstrate this fact as determined by a modified Rayleigh criterion. Point-to-point micrograph resolution between 2.5 and 3.0 A is found in dark field images of micro-crystallites of uranium and thorium compounds. Furthermore, adequate contrast is available to observe single atoms as light as silver.The ability to obtain high resolution images of very small objects is of considerable importance in the biological and material sciences. The only instrument heretofore available for this purpose has been the conventional transmission electron microscope (CTEM).Invented in the 1930's, the basic design of this instrument is an electron analog of the light microscope. The performance of the CTEM has steadily improved during the last few decades, and it is now limited primarily by diffraction effects and the large aberrations inherent in electromagnetic lenses.For the moment, we will ignore the effects of chromatic aberration. This is justifiable if the electron energy is high enough. Then one can minimize the combined effect of diffraction and spherical aberration to calculate the objective aperture which gives the best instrumental resolution. In this case, a hypothetical point source of scattered electrons in the specimen plane would be imaged as a modified Airy disc whose radius to the first intensity minimum is given by a = 0.43 Cs1/4X" [1] where C, is the coefficient of spherical aberration (generally of the order of 1 mm in high quality objective lenses) and X is the wavelength of the incident electron (0.037 A for 100-keV electrons) (1). The image of two incoherent point objects separated by this distance 6 will have an intensity minimum between them of 0.75 of the peak intensity and will be resolved according to the modified Rayleigh criterion of Scherzer (1). In order to obtain this resolution, the objective aperture should subtend a half-angle at the specimen given by (2) a(pt = (4X/Cs) 1/4.

Chromatin architecture: investigation of a subunit of chromatin by dark field electron microscopy.

“…All specimens were examined at 30 keV in dark field using the high-resolution scanning transmission electron microscope developed in the laboratory of Dr. A. V. Crewe (15)(16)(17). Microscope operating conditions were as described before, except that we chose not to outgas the specimen by baking (17).…”

“…Microscope operating conditions were as described before, except that we chose not to outgas the specimen by baking (17). The elastically and inelastically scattered electron signals were usually simultaneously stored in digital form directly on magnetic tape, using a 512 X 512 picture element format (16,17). At low instrumental magnification each picture element represented 15.4 A at the specimen; at high magnification each picture element represented 4.96 A.…”

“…We have eliminated several sources of artifacts by observing unfixed, unstained specimens in the high-resolution scanning transmission electron microscope (15)(16)(17). On the basis Abbreviation: H1, histone 1 (very lysine-rich).…”

Biological Scanning Transmission Electron Microscopy: Imaging and Single Molecule Mass Determination

“…The high contrast delivered by the dark-field detector system [15] can also be exploited in negative or positive stain microscopy [16]. Here the STEM yields clear single-shot images that are free from phase contrast fringes.…”

“…Further, as electron energy loss spectroscopy [11][12] or energy dispersive X-ray spectroscopy [13] can be performed while imaging, the STEM also offers the possibility of determining the distribution of specific elements in protein complexes. Indeed the feasibility of mapping single atoms bound to protein molecules has recently been demonstrated [14].The high contrast delivered by the dark-field detector system [15] can also be exploited in negative or positive stain microscopy [16]. Here the STEM yields clear single-shot images that are free from phase contrast fringes.…”