A.V. Crewe

A scanning microscope with 5 A resolution.

Abstract We describe a scanning electron microscope using a field emission source which has achieved 5 A point resolution using transmitted electrons in thin specimens. Details of the design and construction of the instrument are given together with examples of its performance on objects of biological interest. From this work it appears that scanning microscopes are capable of the same performance as conventional electron microscopes and they possess some advantages over those instruments.

A Simple Scanning Electron Microscope

A simple scanning microscope has been built which uses a field emission electron gun alone, without the aid of auxiliary lenses. The design and operation of the microscope are described and the calculated performance is compared with experiment. Resolution of 100 A has been obtained and is shown in transmission electron micrographs. The probe current is of the order of 10−10 to 10−11 A, a value which is high enough to allow micrographs to be taken with scan times of 10 sec.

A High Resolution Electron Spectrometer for Use in Transmission Scanning Electron Microscopy

The design and construction of a double focusing uniform field magnetic electron spectrometer are described. The pole pieces of the magnet are curved in an effort to correct all median plane aberrations and thereby increase the collecting power. The spectrometer has been installed on a scanning microscope which uses a field emission gun to produce a 100 A probe. Energy analysis of the transmitted electrons as well as filtered diffraction patterns can be obtained from areas as small as 100 A in diameter. In addition, the area under study can be observed directly in the scanning transmission mode, using electrons of any selected energy loss. A resolution of the spectrometer of 2 ppm at an electron energy of 20 keV is shown and the measured second order aberration coefficient is <5% of that of an equivalent straight edge sector magnet. Also, the solid angle for collection of electrons is about an order of magnitude greater than that of the equivalent straight edge magnet at any given resolution. Experimental results in all modes of operation are presented.

On the optimum resolution for a corrected STEM

Abstract In choosing the operating parameter for a STEM, we need to take into account various uncertainties and instabilities. This paper examines this question for a normal STEM, which is limited by third order spherical aberration and more particularly for a corrected STEM, which is limited by fifth order aberrations.

Scanning transmission electron microscopy of extracellular annelid hemoglobins

Micrographs of negatively stained extracellular hemoglobins of Lumbricus terrestris and Arenicola marina obtained with the scanning transmission electron microscope show that their appearance is consistent with a model of two hexagonal tiers of pentagonally shaped subunits. The dimensions are similar: a vertex-to-vertex diameter of 300±8 A and a height of 200±11 A. The diameter of each subunit is about 110 A. These dimensions are markedly larger than those reported previously by other investigators. However, these values are similar to those recently obtained by X-ray diffraction of wet crystals of Lumbricus hemoglobin.

Optimization of small electron probes

Abstract The size of an electron probe is determined by many different factors including diffraction, lens aberrations, space charge and defocus. In many applications it is desirable to optimize the probe to achieve the smallest size (or, equivalently, the best resolution). This is particularly difficult to do when the effect of diffraction is significant. We have noted that many of the published approaches to this problem are based upon false premises and give erroneous results. In this paper we attempt to correct at least some of theserrors, although it is undoubtedly true that some remain.

Scanning transmission electron microscopy.

The scanning transmission electron microscope is of quite recent origin, and it is only in the last few years that it has been shown that this instrument is capable of giving the same high resolution as the conventional electron microscope. In this article we examine the conditions necessary for the achievement of high resolution and also the various modes of contrast which can be obtained from this instrument. Finally, we suggest other ways in which the microscope can be used in future investigations.

The molecular size of Myxicola infundibulum chlorocruorin and its subunits

The molecular shape and size of the extracellular chlorocruorin of Myxicola infundibulum was determined using scanning transmission electron microscopy and its dissociation in the presence of sodium dodecyl sulfate (SDS) was investigated using polyacrylamide gel electrophoresis. The shape of the chlorocruorin is that of a two-tiered hexagon with a vertex-to-vertex diameter of 29.0-29.5 nm and a height of 19.0-19.7 nm: it appears to be smaller by 5-10% relative to several annelid extracellular hemoglobins examined by scanning transmission electron microscopy. The quaternary structure of the chlorocruorin appears to be sensitive to Ca(II) concentration; dissociation fragments of the whole molecule were observed, consisting of octamers an dimers of one-twelfth subunits. The unreduced chlorocruorin dissociated into two subunits with estimated molecular masses of 23 000 (1) and 60 000 (2); the reduced chlorocruorin dissociated into subunits with estimated molecular masses of 13 000 (I), 14 000 (II) and 30 000 (III). SDS-polyacrylamide gel electrophoresis of reduced subunits 1 and 2 showed that subunit 1 corresponded to subunit III and that subunit 2 dissociated to subunits I and II. Densitometry of the polyacrylamide gels indicates that 85-90% of the Myxicola chlorocruorin consists of disulfide-bonded tetramers of polypeptide chains of about 15 000. Such a pattern of subunit aggregation has not been observed previously in annelid extracellular hemoglobins and chlorocruorins.

The use of backscattered electrons for imaging purposes in a scanning electron microscope.

It is shown that the use of a very large detector for backscattered electrons can provide a signal comparable to that obtained in the secondary emission mode and that the resolution available is also comparable. A technique is described whereby the difference is taken between these two signals, and the use of this difference signal can significantly enhance surface details. It is believed that the use of such a signal should ultimately prove advantageous in increasing resolution.

Some Chicago Aberrations

This article presents my reminiscences of the work in Chicago on the correction or reduction of lens aberrations. Studies began in the early 1960s and extended over a period of almost 40 years, although it was never the primary focus of the work of the laboratory. The account is almost entirely based on my own memory, which is not a very reliable instrument. It is not intended to be a review and is more accurately describable as a personal recollection.