Overview
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parts. Thus, a conventional MOSFET (metal oxide semiconductor field effect transistor) will no longer be able to control the flow of electrons as its size reaches the sub-50-nm regime. At these dimensions, electron transport in n- and p-doped contacts is affected by the quantum mechanical probability that electrons simply tunnel through the interface. These tunneling processes will begin to dominate in the nanometer size regime, causing errors in electronic data storage and manipulation. A second problem inherent in any nanoscale device is that chemical heterogeneities will influence device properties such as turn-on voltage. Defects, size dispersity, and variable dopant densities, normally of little concern in macroscale devices, can cause fluctuations in electronic function and make device reproducibility unlikely on the nanoscale. In fact, even a single pentagon-heptagon defect in a single-walled carbon tube can change I–V response (51). These seemingly insurmountable obstacles to fabricating nanoscale electrical devices in many ways form the genesis of research into new methods for synthesizing size monodisperse and chemically tailorable metal nanoparticles. Establishing basic nanoparticle size and surface chemistry-electronic function relationships in these materials is at the forefront of current nanoscale electronics research. The identification of novel electronic behaviors and device applications which capitalize on quantum effects is expected to follow from fundamental structure-function determinations. These are discussed in more detail below. One electronic behavior observed in nanoscale objects is single-electron tunneling—the correlated transfer of electrons one-by-one through the object. Single-electron tunneling was first hypothesized in the early 1950s (52), a time when many physicists pondered how the electronic properties of a material (e.g., a metal wire) would change as material dimensions were reduced to the micron or nanometer scale. Gorter and others argued that, provided the energy to charge a metal with a single electron, e/ 2C (e is electron charge, C is metal capacitance), was larger than kT, electrons would be forced to flow through the metal in discrete integer amounts rather than in fluid-like quantities normally associated with transport in macroscopic materials. Further reasoning led to the prediction that current-voltage (I–V) curves of a nanoscopic metal should be distinctly nonohmic; that is, current steps should appear corresponding to the transport of 1e , 2e , 3e , etc., currents through the metal (Fig. 3A). In fact, these predictions turned out to be true, although it was not until the late 1980s that well-defined single-electron tunneling steps were observed experimentally. Even then, enthusiasm for single-electron devices was tempered by the fact that these initial experiments were performed on relatively large metal islands (micron sized) prepared with photolithography or metal evaporation (Fig. 3B) (53). Thus, in order to satisfy the requirement e/ 2C kT, it was necessary to cool the microstructures to below 1 K. Herein lies perhaps the greatest obstacle to implementing single-electron devices: to avoid thermally induced tunneling