Charge Transfer in Metal-Molecule Heterostructures
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What is nanoscale? Albert Einstein's doctoral research, using experimental data on the diffusion of sugar in water, showed that each single sugar molecule measures about a nanometer in diameter, a billionth of a meter. One nanometer is the approximate width of 10 hydrogen atoms laid side by side. It is one thousandth the length of a typical bacterium (1 µm, 10− 6 m), one millionth the size of a pin head, one billionth the length of Michael Jordan's well-muscled legs (∼ 1 m). While the exact definition of “nanotechnology” is somewhat imprecise, the term generally implies devices or structures with at least two characteristic dimensions in the range of 1–100 nm. Along with biomedical research and defense—fighting cancer and building missile shields—nanotechnology has become a visible and energized discipline in science and technology. It spans fields from condensed matter physics, engineering, molecular biology to large swath of chemistry. Key themes within this area include 1) the ability to understand, and ultimately to control, important properties of materials and structures at the nanometer scale; and 2) improved understanding of how nanostructured elements interact with the external environment.
In contrast to the relatively recent recognition of nanotechnology as a field, charge transfer has been known for decades. It is a very important process in chemistry, biology, and physics in semiconductor devices and in electronics. Our respiration, photosysnthesis, many biological processes, and thin films of tunnel-diode involve electron or charge transfer, in some cases even at a large distance. In both respiration and photosynthesis, the primary action of the energy source (combustion of substrate by oxygen in respiration and absorption of light by chlorophyll or bacteriochlorophyll in photosynthesis) is to move charges or electrons, a long distance, in an electron-transport chain. Charge transfer in photosynthetic reaction centers and in protein–protein electron-transfer complexes such as hemoglobin can even occur at liquid helium temperature. The measurement of electrical currents, I, tunneling through insulating layers, as in the invention of Esaki tunnel-diode, is a classic example of charge transfer in semiconductor devices. In this chapter, results about charge transfer at the nanoscale from the electrostatic surface potential (ESP) measurements of self-assembled monolayers (SAMs) are presented. A brief overview of current theoretical understanding including our ongoing calculations is included.