Nanotechnology: the physics of the very small

January 26/February 2, 2005
The laws of physics behave differently at very small scales. At the nanoscale, electrons travel more quickly through wires, transistors can mete out electrons one at a time, objects stick to each other, and light can bend matter.

Going ballistic

Researchers have made semiconductor nanowires as narrow as a few nanometers, gold nanowires about half a nanometer wide, and carbon nanotubes just six atoms across. The structures can be used as electrical wires, but with a key advantage over ordinary wires.

Though electrons travel from point to point at the speed of light, they rarely travel through metal and semiconductor crystals in a straight line. Electrons scatter in all directions as they bounce off a wireís boundaries and impurities.

At the nanoscale, wires have negligible impurities and closely spaced walls, leading electrons to travel more or less straight through, or ballistically. The result is electron transit times that are hundreds of times faster than for ordinary wires.

One at a time

Transistors that are small enough can take advantage of quantum effects to control the flow of electricity at the rate of one electron at a time. Ordinary transistors have a semiconductor channel, source and drain electrodes that move electrons into and out of the channel, and a gate electrode that changes the channelís electrical conductance in order to control the flow of electricity through the device.

Single-electron transistors contain a tiny reservoir, or island, rather than a channel. The island can hold a set number of electrons at a time, and barriers, or junctions, between the electrodes and the island block electrons from moving on or off the island. When a voltage is applied to the transistorís gate electrode, the junctionsí resistance to the electrons is weakened but not removed entirely.

The exact position of an electron, like that of all quantum particles, is a matter of probability. Physicists describe electrons as clouds, and an electron has a certain probability of being at any given point in its cloud. Electron clouds fluctuate, and if an electronís cloud extends beyond a barrier that would otherwise block the electron, at some point the probability of the electron being at a point beyond the barrier is high enough that the electron simply appears there. The phenomenon, quantum tunneling, is widely used in electronics.

Because the negatively-charged electrons repel each other, adding an electron through tunneling when the island is at its maximum capacity forces another off. This forms a sort of turnstile that assures that electrons will pass through the transistor one at a time.

Come closer, but not too close

At the nanoscale, the force of attraction between molecules ó the van der Waals force ó becomes a major player. It draws molecules close together, but it also keeps them from coming into contact. This makes it possible to stick carbon nanotubes together while also allowing the inner tubes of multiwall carbon nanotubes to slide telescope-style without any friction.

Dipole molecules are electrically unbalanced, meaning their constituent atoms are configured so that the distribution of electrons makes one end of the molecule electrically positive and the other negative. Dipole molecules are infinitesimal magnets, and the force of attraction between them is very strong. The van der Waals force is the sum of the electrostatic attraction and repulsion between individual electrically balanced, or neutral, molecules. The positive and negative charges within neutral molecules, while balanced on average, vary over time because the distribution of electrons fluctuates. For the brief instant that a moleculeís electron distribution is uneven, the molecule becomes a dipole.

The distribution of electrons also shifts in response to the electrons of nearby molecules. A neutral molecule that is momentarily a dipole can induce a nearby molecule to also become a dipole because like charges repel each other and unlike charges attract each other. The two moleculesí electron distributions become synchronized and the molecules are drawn together. The effect also applies to large numbers of molecules.

Neutral molecules are drawn together until their electron clouds nearly meet. Beyond that point the van der Waals force becomes repulsive because the negatively-charged electron clouds repel each other.

Sunburned shape shifters

The right kind of light, usually ultraviolet, causes certain molecules to change shape. In bulk, the molecules form rubber-like materials that visibly contract and expand under alternating light conditions.

Researchers have recently demonstrated the effect in individual polymer molecules. Polymers are long and chain-like. The shape-changing polymers have side groups, which are collections of atoms that attach to the side of the chain. One or more of the side groups can be shifted from one side of the chain to the other. Photon energy of the right wavelength changes the moleculeís chemical bonds, causing the shift, and light of a different wavelength shifts the side group back.

When the side groups are all on one side of the molecule, known as the trans configuration, the molecule is relatively straight. When the side groups are on alternating sides of the molecule, the cis configuration, the molecule is bent, and therefore shorter. This light-induced shape change produces a mechanical force that can be harnessed to do work.



Page One

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Nanotechnology: the physics of the very small

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