Mesoscopic Systems

The prefix "meso-" means "in between" or "intermediate." Mesoscopic systems are those that are larger than atoms and yet very much smaller than the largescale everyday objects that we can see and touch. They are a thousand to a hundred thousand times smaller than the diameter of a human hair and they range in size from several hundred nanometers or billionths of a meter. That is why materials composed of mesoscopic parts are also known as nanostructures and the technology based on these systems is referred to as nanotechnology. Since one nanometer is less than the width of ten atoms in a row, mesoscopic systems are made up of less than a thousand atoms. Mesoscopic or nanoscale systems behave very differently from large-scale objects and they often have unusual physical and chemical properties. This makes them extremely interesting to scientists and engineers who hope that, by manipulating systems on the nanoscale, they can make computers, sensors and other devices that have exactly the properties they want.

There are many examples of mesoscopic systems in nature. The molecules in our bodies that break down the foods in our stomachs and intestines, and the molecules that carry oxygen from the lungs to other parts of the body, are nothing but nanoscale machines. Artificial nanostructures, however, have been studied and fabricated only in the last few decades. Multilayered nanostructures, made up of thin mesoscopic layers of different materials, have been investigated since the 1970s and have been used in devices such as high-efficiency lasers and light-emitting diodes (LEDs). More recently, researchers have begun studying and synthesizing atom clusters which are balls and tubes made up of ten to a thousand atoms. These clusters are sometimes put together to create new materials with novel properties. The properties depend on the size of the cluster. Copper, assembled from nanoscale clusters five to seven nanometers in diameter, is five times harder than ordinary copper. Brittle ceramics become ductile when they are synthesized from clusters with sizes less than fifteen nanometers. Cadmium selenide clusters of different sizes appear to have different colors.

Mesoscopic systems, particularly multilayered materials, are created and studied by a variety of well-established methods. One way of producing thin layers is to deposit atoms, from a chemical vapor or a beam of molecules, on to a base of some other material in an airless evacuated chamber. This is somewhat like spraying a thin layer of paint on a wall. Clusters of atoms are produced by chemical methods. An important tool in making and studying mesoscopic systems is the scanning tunneling microscope or STM. The STM creates an image of a surface at the atomic level by measuring the current which flows as the needle of the STM touches each atom. The STM can actually pick up individual atoms and move them around. In 1990, a group of researchers created a stir by writing "IBM" with thirty five precisely placed xenon atoms on the surface of a nickel crystal.

The study of mesoscopic systems provides scientists with a picture of how the behavior of a material changes as it grows from a few atoms to large visible and tangible objects. This information will be useful in fabricating minuscule machines when nanotechnology finally becomes a reality in the coming decades. The first steps are already being taken. Researchers have used the STM to create a 'switch', that could be used in computers, with a single atom which moves back and forth rapidly between two positions. Biologists are trying to manipulate large organic molecules such as proteins which could be used in the future for tasks such as the repair of damaged organs. Including wide-ranging applications in the computer industry, the possibilities of nanotechnology seem endless. Independent researcher Eric Drexler predicts that sometime in the future we may have minute machines that will repair clogged arteries, libraries that will fit into our pockets, and clothing that will change shape, color, and texture according to need.