Imaging is about communicating. Through images, it is possible to convey information about a subject in an exceedingly efficient manner. We are all familiar with the nightly news reports and how a few visual images can rapidly convey a deep impression far beyond that of the accompanying spoken word. The worlds of art and photography are based on images and their power to invoke a sense of beauty and awe in our minds. Both of these aspects are also important for imaging in materials science, although the beauty and awe we wish to convey concerns our understanding of the behavior of materials or the laws of physics and not the aesthetic appeal itself.

An image can convey an insight into a materials problem which is unrivaled in depth and breadth. In many cases, it can reveal aspects of the problem which were completely unforeseen. An image may even reveal the key to the entire problem and so provide a fundamental advance in our understanding, in contrast to the incremental advances which usually result from more or better data. It is this theme which links the articles in this issue of the MRS BULLETIN. Covering semiconductors to cement, the atomic scale to the macroscopic, the theme in common is the power of the image to unravel the story. Often this involves changing our preconceived ideas on the subject (images provide constant proof that “truth is stranger than fiction”), but this is what major advances in research and development are all about.

Atomic-resolution techniques for surface characterization are often limited in their ability to probe individual atoms or molecules. In the last few years, scanning tunneling microscopy (STM) has emerged as a powerful probe for studying the atomic-scale structure of surfaces. One of the greatest features of STM is that it is applicable to a wide variety of problems, restricted only by the condition that the material to be studied must be a conductor. Although STM has been applied to a wide variety of materials science problems, the greatest concentration of STM work has been in the study of semiconductor surfaces. This results not only because semiconductor surfaces exhibit complicated and interesting reconstructions of the atoms in the outermost surface layers, but also because many important properties of semiconductor surfaces and interfaces are dominated by defects and other surface imperfections with atomic dimensions.

The transmission electron microscope (TEM) has had a major impact on materials science in the last five decades, despite the fact that it is necessary to prepare thin samples in order to use the technique. The primary reason for this effectiveness is the ability to access both real space and diffraction data in the same instrument, and to filter in one and observe the effect in the other. This is possible because of the wave nature of electrons and the existence of effective magnetic lenses for focusing. Abbe showed that any lens has the ability to Fourier transform its input wavefield in its focal plane, and to provide a second Fourier transform in the image plane. This is schematically shown in Figure 1. A crystalline object will diffract only in certain directions, with Bragg angles (θB) depending on the inverse of the interplanar spacing. The diffraction pattern is a series of spots in the Fourier, or focal, plane of the lens. A filter placed in the focal plane serves to limit the resolution by limiting the bandwidth of the image, but it also can serve to select certain parts of the Fourier spectrum in the image. The simplest examples of this, as used in optical microscopy, are bright-field and dark-field imaging. In the former the un-scattered beam is allowed to reach the image, in the latter it is not.

The structural and compositional integrity of interfaces between semiconductor multilayers can profoundly influence the optical and electronic properties of epitaxially grown heterostructures. Understanding the atomic-scale interfacial structure and chemistry is therefore essential to correctly relate electrical measurements to theoretical models and to correlate such effects with growth conditions. High-resolution electron microscopy (HREM) has played a pivotal role in this process, providing important information on interface commensurability and revealing the presence and nature of defects.

More recently, significant advances have been made in applying HREM to the difficult problem of chemical composition mapping in systems where no structural change occurs across the interfaces. The basis of such methods involves using the objective lens as a bandpass filter and tuning in on a specific range of spatial frequencies to form a chemically sensitive interference pattern. By using a suitable low-index zone axis and choosing an optimum range of specimen thickness, the patterns can indeed be extremely sensitive to the strength and periodicities of the projected potential.

Scanning electron microscopes offer several unique advantages and they have evolved into complex integrated instruments that often incorporate several important accessories. Their principle advantage stems from the method of constructing an image from a highly focused electron beam that scans across the surface of a specimen. The beam generates backscattered electrons and excites secondary electrons and x-rays in a highly localized “spot.” These signals can be detected, and the results of the analysis are displayed as a specific intensity on a screen at a position that represents the position of the electron spot. As with a television image, after a given period, information about the entire field of view is displayed on the screen, resulting in a complete image. If the specimen is thin, the same type of information can be gathered from the transmitted electrons, and a scanning transmission image is thus constructed.

The scanning electron microscope is highly versatile and widely used. The quality of the image is related to its resolution and contrast, which, in turn, depend on the diameter of the focused beam as well as its energy and current. Because electron lenses have inherently high aberrations, the usable aperture angles are much smaller than in a light microscope and, therefore, the electron beam remains focused over a relatively large distance, giving these instruments a very large depth of focus.

Scanning electron microscopes are versatile in their ability to detect and analyze a lot of information. As a result, modern versions of these instruments are equipped with a number of detectors. Developments are sometimes related to placing the detectors in a geometrically attractive position close to the specimen.

A little more than 20 years ago we learned how to direct photons, electrons, or ions a few keV or lower energy onto a surface and measure the energy or mass distribution of backscattered or emitted species. By arranging conditions so that the bombarding or detected species is strongly attenuated by matter, the information obtained is restricted to not more than the outermost several atomic layers. We then learned how to infer the composition, chemical state, and (sometimes) the molecular structure of the surface from these spectra. So began the era of “alphabet soup.” The ensuing decades brought several things:

■ The commercial availability of major surface spectroscopies. Even if not totally user friendly, they aren't outright hostile and work for most people on most days. Instrument makers seeking increased sales volume will continue to improve their ease of operation and reliability.

■ The scrutiny of most materials and structures. There is reliable literature about the very large majority of applications. We know where to begin interpretation and what are the major pitfalls. We have little excuse for the egregious errors we sometimes still make.

■ Many good research groups. When something really new comes along (e.g., the high temperature superconductors), the correct understanding of the data is worked out quickly enough to use it as a tool, guiding research and development.

■ The seeming reluctance, outside certain specialized areas, of the materials community to integrate surface spectroscopies into other than basic research. Expense is often cited as a barrier, but this is hardly credible when the daily cost of top-notch spectroscopy isn't more than twice typical industrial researcher internal billine rates.