Nanoscience, nanotechnology, and modeling

nanotechnology are currently quite popular in both the scientific and the general press. These intermediate-length structures are intriguing because generally in the nanometer region, almost all physical and chemical properties of systems become size-dependent. For example, although the color of a piece of gold remains golden as it reduces from inches to millimeters to microns, the color changes substantially in the regime of nanometers. Similarly, the melting points of such particles change as they enter the nanoscale, where the surface energies become comparable to the bulk energies. Because properties at the nanoscale are size-dependent, nanoscale science and engineering offer an entirely new design motif for developing advanced materials and their applications. From the point of view of computation, nanostructures are of interest in two quite different directions. As a computational challenge, they are quite striking: ordinarily, we define nanostructures as having characteristic dimensions between one and 100 nm. Given that atoms have characteristic sizes between 0.1 and 0.3 nm, this suggests that nanoscale structures will contain between 103 and 1010 atoms. Modeling the behavior of such structures is then a substantial computational undertaking. The second reason that nanostructures are of interest to the computational community is that the tools with which we compute will almost certainly involve nanoscale structures. As Moore’s law is extended beyond the next 10 years, functional devices will certainly operate below 100 nm. Design rules for transport based on simple Ohmic behavior and digital off/on field-effect transistor function will then become dubious, and we will need entirely new design schemes to deal with the idiosyncrasies of nanoscale structures. Such components as molecular switches, nanotube connectors, crossbar arrays, magnetic nanodot memories, and electronic paper embody the challenge and promise of nanomaterials in computing. This issue of CiSE contains three articles that pinpoint particular important avenues in the modeling of nanostructures. All three point out the difficulty that scaling imposes as we move from the characteristic size of small molecules (containing a few atoms) to true nanostructures containing more than 105 atoms. Modeling methods thus require a judicious integration of high-accuracy quantum-mechanical treatments for small structures ranging from approximate quantum dynamics for medium-length-scale materials, to classical molecular electronics for large-scale materials, through a form of continuum mechanics for macroscopic structures. Be-