Lewis B. Leder

Characteristic Energy Losses of Electrons in Carbon

The values reported for the characteristic energy losses of electrons in carbon vary by as much as 50%. In an attempt to resolve this discrepancy the electron energy losses have been remeasured for evaporated carbon and natural graphite, and it is found that there is a large difference for these two forms of carbon. Electron diffraction patterns of evaporated carbon show it to be highly amorphous. Annealing of the films causes growth of the crystallites, and also an increase of the energy loss toward the loss values for graphite. It is shown by calculation that the difference in the energy losses for the two forms is due to a difference in density, and that annealing increases the density of the evaporated carbon and, therefore, the energy loss value.

Characteristic Energy Losses of Electrons in Solids

Publisher Summary This chapter provides an overview of the wealth of information acquired on the characteristic energy losses of electrons and describes the methods and instruments used in the measurements, which show that the observed characteristic energy losses are, in first approximation at least, independent of the methods and instruments used in their observation. It is shown that the instrumentation used in measurements done in recent times use the Mollenstedt method. Where photographic plates have been used for detection, there would be some doubt as to the accuracy of intensity and width measurements even though attempts were made to calibrate the photographic plates. In this respect the early measurements of Rudberg, Haworth, and Farnsworth using Faraday cage and electrometer and the recent work of Marton and Leder using photomultiplier should be more dependable. The use of the electron microscope allows easy inspection of the specimen through observation of its image projected by the objective lens on to a fluorescent screen in the slit plane. This inspection is very useful, because thin film specimens frequently burst or curl up under the action of the intense primary beam, and from a practical viewpoint it is a great advantage to be able to discard a damaged specimen or to select an undamaged area. This chapter also reviews experimental data, qualitative interpretation, and theoretical problems related to the observations.

Improved Electrical Differentiation of Retarding Potential Measurements

An improved system for obtaining differential spectra from retarding potential measurements by the superposition of a small ac signal on the retarding voltage has been developed. The principal improvements are the choice of the point of injection of the modulation signal, the use of a low impedance preamplifier to improve the accuracy of the energy scale, and the use of a synchronous detector to improve the stability and reduce the effect of noise. The energy profile of a 15‐kev electron beam and the characteristic energy loss spectra in collodion and aluminum have been measured. The quality of these measurements illustrates the over‐all performance of the present system.

Liquid Crystal Properties of Some Steryl Chlorides

The liquid crystalline properties of cholestanyl, cholesteryl, campesteryl, sitosteryl, and stigmasteryl chlorides were studied by measuring their behavior in mixtures with cholesteryl nonanoate. Of the five chlorides only cholesteryl chloride shows an obvious mesophase temperature range and microscopic texture, but all of the chloride–nonanoate mixtures are cholesteric over a wide range of compositions. The decrease in effective pitch observed as the 17β side chain is modified appears to correspond directly to the complexity of the modification. We conclude that all of the chlorides studied are liquid crystalline despite the fact that the ordinary tests for mesomorphism do not give this indication.

Field‐Emission Study of Carbon Monoxide on Tantalum

Adsorbed carbon monoxide shows three states of binding on tantalum. The weakest of these is desorbed at temperatures above 125°K with no observable surface migration. The bonding with the surface is of the van der Waals type. This is in contrast with the second state, which is desorbed above 650°K. The third state is dissociated before desorption to give the oxygen‐on‐tantalum pattern. Surface migration involving the second adsorbed state occurs with an activation energy of 38 kcal. The work function of a carbon monoxide‐covered tantalum surface, obtained by spreading the carbon monoxide on a shadowed tip at 40°K, is 0.8 eV greater than that of the corresponding clean tantalum. Fowler—Nordheim plots of clean and shadowed tips show that at least under these conditions, the infinite‐field extrapolation of logi/V2 is proportional to the log of the emitting areas.

Field‐Emission Observations of Carbon on Tantalum

The carbon—tantalum system, observed with field emission, shows the temperature effects on the solubility and precipitation of the carbide phase. Carbon, once deposited on tantalum, cannot be removed by high‐temperature treatment, as in the case of tungsten. With low‐carbon contamination the 334 planes appear as dark areas, just as for carbon on tungsten. At temperatures in the region of 950°K, platelets, presumably Ta2C, form and migrate to the [111] zones of the emitter single‐crystal tip. The energy of activation of this surface migration process is 54 kcal.

Characteristic energy losses of electrons

It has been recognized for more than 20 years (1) that one of the most important factors in the formation of electron microscope images is electron scattering. A number of papers have appeared (2) based on accepted theories of elastic and inelastic scattering. In order to obtain numerical results, approximations were made and in some cases the results were in fair agreement with experiments (3). We now believe that this agreement was only fair, due to the fact that the theoretical treatments were essentially based on statistical considerations where an average energy loss was used for the inelastic part of the scattering. During the last few years, quite an effort has been made, both experimental and theoretical, toward a better exploration of the inelastic process.As a result, we are now in a much better position to say something more definite about the factors contributing to the inelastic part of the electron scattering process and its role in image formation in electron microscopy.

Electron Diffraction Structure Analysis and the Investigation of Semiconducting Materials

Publisher Summary This chapter presents a study of electron diffraction structure analysis and investigation of semiconducting materials. Besides being used to investigate surface structure and surface phenomena, electron-diffraction method is broadly used for determination of the atomic structure of solids and has proven to be a valuable supplement to X-ray structure analysis. Several special scattering effects of electrons by atoms and crystals make it highly effective for the solution of many special problems in physics and engineering. Electron-diffraction analysis of crystal structure reveals new perspectives. The essential advantage of the electron-diffraction method is the possibility of determining the positions of light atoms in the presence of heavy ones: hydrogen in the presence of carbon, nitrogen, oxygen, fluorine, and even silicon; nitrogen in the presence of iron, molybdenum, and so on. Structural investigation helps in making precise determinations of the nature of chemical bonding and of the concentration of components in phases of various structures and aids in the solution of a number of other questions. This chapter elaborates state of the electron-diffraction method of investigating solids at the present time. Investigation of the structure of semiconducting phases is also discussed in the chapter.