Microscopy, Exotic

What is a Microscope for?

A microscope is, essentially, a tool for creating a visual image of a sample that contains information that is finer grained than information available to the naked eye. Microscopes can do other things as well, of course—some microscopes generate aural “images,” some manipulate the sample while imaging it, and so on. But the gathering and amplification of fine-grained information, and conversion of that information into something perceived by the microscopist's eye, are the fundamental traits of a microscope. This general definition helps explain why the development of new microscopes is such a thriving part of nanoscience. A naive understanding of microscopy would see the development of new types as purely a matter of increasing resolution—a drive to ever-more fine-grained images. However, by defining the purpose of microscopy as capturing information, we can see that microscopists will pursue new instrumentation so long as it yields new kinds of data about a sample, even if that data is not at a high resolution.

Field Emission

Until the 1930s, “microscope” meant exclusively an optical microscope, usually one in which light is focused on a sample using glass and flint lenses. In the 1930s, though, it became possible to focus electrons on a sample, leading to the scanning electron microscope (SEM) and transmission electron microscope (TEM). For the next 50 years, optical and electron microscopy would dominate microscope development. Electron microscopy delivered higher resolution, while optical microscopy offered greater convenience and a more secure theoretical understanding. Both technologies had drawbacks, though. For instance, both gave only rudimentary information about the chemical species present in a sample—a major hindrance to the adoption of microscopes by chemists. Both damaged and distorted samples, especially biological materials—optical microscopy by requiring chemical dyes (“stains”) to increase contrast, [Page 420]electron microscopy by requiring that samples be kept in vacuum and coated in gold.

At the same time that the SEM and TEM were developed, a third electron microscope emerged which did not use electron optical elements to focus an electron beam onto a sample. Instead, the field emission microscope (FEM) used the sample as the source of an electron beam. Invented by Erwin Müller in 1936, the FEM requires a metal sample that is carved into a very sharp point, placed in high vacuum, and kept at a high voltage. Under these conditions, electrons will be “emitted” from near the sample tip (i.e., they will quantum-mechani-cally “tunnel” out of the sample and then move ballistically through the vacuum). The voltage near the tip will vary slightly depending on features of the sample surface such as grain boundaries. Thus, if a fluorescent screen is placed in the path of the emitted electrons, they will form an image of those surface features. In this way, FEM gives fine-grained information about the sample, but without any focusing element.

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