Operating modes, electron microscopes

As noted earlier, most electron diffraction studies are perfonned in a mode of operation of a transmission electron microscope. The electrons are emitted themiionically from a hot cathode and accelerated by the electric field of a conventional electron gun. Because of the very strong interactions between electrons and matter, significant diffracted intensities can also be observed from the molecules of a gas. Again, the source of electrons is a conventional electron gun. 

The scanning transmission electron microscope (STEM) combines the two modes of operation. Here, the scanning coils are used to illuminate a small area of... 

The operational modes of three types of electron microscope are shown schematically in Figure 7.3. 

HRTEM images were obtained on a Jeol JEM2010 ultra high-resolution analytical electron microscope equipped with an emission cathode operated at 200KV in bright field mode. The samples were examined with a magnification of 400,000. The ex situ treated samples were supported holey carbon coated copper grids for the experiment. 

This chapter describes briefly the basic construction and characteristics of the modern transmission electron microscope and discusses its principal modes of operation. Because the electron microscope is an analogue of the optical (or light) microscope, we also consider briefly the basic features of the optical microscope this will also provide a link with our earlier discussion of the optical principles of image formation by a lens. 

The detailed interpretation of electron microscope images produced using any of the operating modes discussed in this chapter requires as complete an understanding as possible of the diffraction process. The next two chapters develop and explain as simply as possible the current theories of electron diffraction by crystals in order to provide a basis for the interpretation of images of crystal defects (such as dislocations, stacking faults, and twins) and of lattice images. 

Resolution in the STEM is limited by the probe diameter, which is about 1 nm in equipment dedicated to this operating mode, at the cost of using a cold field emission gun requiring an ultravacuum. Because of the high-precision optics and the point-by-point image formation principle, the STEM combines the advantages of scanning electron microscope analysis with resolution performance levels similar to the transmission electron microscope. 

Morphology was studied by AFM (Solver P47) in semicontact mode using a 10 nm radius tip. Philips CM 20 transmission electron microscope operating at 200 keV was used to analyze the surface and cross-section of the samples thinned by our standard ion beam milling procedure [4]. 

Etching was performed by dipping the samples into the 4% aqueous solution of hydrofluoric acid at room temperature. After etching the samples were washed in distilled water and then probed by scanning electron microscope Hitachi S-806 and atomic force microscope NTEGRA Prima operating in a contact mode. 

Figure 3.84. Voxel obtained by irradiation of a mixed urethane acrylate oligomer and a urethane acrylate monomer in the presence of a mixture of benzoyl cyclohexanol and morpholino phenyl amino ketones at 780 nm using a 150-fs pulsed Ti sapphire mode-locked laser operating at 76 MHz, where light was focused by a 1.4 numerical aperture objective lens (a) scanning electronic microscopic images of the voxel and (b) longitudinal and lateral voxel size as function of the exposure time. (From Ref. [580] with permission of the American Institute of Physics.)...

The principle involves using X-rays (beam of electrons) to image atomic structures. The very short wavelengths allow much higher resolution than with a standard optical microscope. There are several types of electron microscope which allow different types of image to be formed. They operate in either transmission mode (electrons pass through the sample) or reflection mode (electrons are reflected from the surface)... 

Electron microscopic measurements were made on film samples embedded in Spurr s low viscosity medium and microtomed into sections which were thin supported on Cu grids. Measurements were made on a Phillips Model 401 LS TEM operated at 100 KV in the bright field mode at calibrated magnifications up to 500,000x. At his magnification the instrument is capable of resolving features that are about 3.5 A in diameter. 

---