Bridging Size Scales with Self-Assembling Supramolecular Materials
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Scales with SelfAssembling Supramolecular Materials
Peter Kazmaier and Naveen Chopra Introduction:There’s Plenty of Room at the Bottom The notion of micromachines made up of miniature gears and motors may seem like a fairy tale. In 1959, Richard Feynman delivered a lecture entitled “There’s Plenty of Room at the Bottom,”1 a bold prediction of what was to come in the future, where one could fit the entire Encyclopaedia Britannica onto the head of a pin or use ions focused through a microscope lens in reverse to etch away silica to create patterns on a submicron scale. In retrospect, Feynman’s hypotheses were amazingly accurate. He was describing the techniques of microlithography and the manipulation of atoms by scanning tunneling electron microscopy, methods that are commonplace today. Advances in biochemistry have revealed the enormous complexity of “simple” organisms, leading one to conclude that “micromachines” (of a complexity that a visionary giant such as Feynman had foreseen) have long been operating quite happily without any assistance from chemists! The Holy Grail of micromachines is a device that can accurately mimic the complex processes of living organisms. Nanotechnology, MEMS (microelectromechanical systems), and molecular selfassembly are small steps in that direction. MEMS devices are micron-sized machines that move, flip, bend, or undergo other mechanical transformations. Two approaches may be envisaged for making devices on a continually shrinking length scale: the top-down approach and the bottom-up approach. Let’s begin with the
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top-down approach: What are the limits? What are the smallest features that we can create using mechanical and optical processes?
Continually Shrinking Length Scales Feynman’s concept of a top-down approach was to build the smallest pair of hands possible and then use those hands to build even smaller features. In this approach, a size reduction of about 400 would take a human being to the scale of an ant (from 1.75 m to 4 mm). A second
similar reduction (from 4 m to 11 m) would take one to the size of a bacterium, while a third reduction (from 11 m to 27 nm) would take one to the probable limit of lithography. Although this is an intriguing and ingenious approach, another approach has proven to be more readily accessible. Figure 1 compares the length scales under consideration, along with the accessible wavelengths of light. Even in 1959, photography had made enormous strides in resolution, and this really paved the way for Feynman’s “magic hands.”
The Role of Photography A typical color photograph contains 35 Mbits of information. Photographic film consists of silver halide crystals dispersed in a gelatin emulsion. When exposed to light, the silver halide grains create a latent image on the exposed emulsion. The film is developed by treating it with various reducing agents to reduce the exposed silver ions to elemental silver, resulting in the formation of a negative image. The resolution of the images created via the photographic emulsion is affected by th
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