6. Nanoscale Materials and Assembly

There is a great deal of interest in the production of nanometer-scale particles. Matter discretized at such lengthy scales can behave quite differently from the bulk material. For example, polycrystalline metals become harder as the size of the individual crystal grains is reduced, a phenomenon known as the Hall-Petch effect. This is considered to arise from the obstruction of the movement of dislocations-the main deformation mechanism-by the proliferation of grain boundaries. When the grains are just ~100 nm across, however, a new strengthening mechanism may come into operation: the grains are too small even to allow dislocations to be nucleated. Nanoscale grain size in ceramics, meanwhile, can induce the opposite effect of enhanced plasticity, leading even to "superplastic" deformation where the ceramic deforms like a plastic. This property, potentially useful for the forming of ceramic components, is thought to arise from sliding at grain boundaries, lubricated by the formation of fluids at the interface.

Ultrafine particles of metals can serve as highly active catalysts, and there is some indication that when the number of atoms in each particle is less than 100, the selectivity of the catalyst can become highly dependent on the particle size. So such catalysts might be tailored to order by control of the particle size.

Nanoscale particles of titania have been used in a new solar cell that is efficient and cheaper than conventional cells made from silicon. One can regard these devices as a genuine "integrated chemical system," assembled from molecular, supramolecular and nanoscale components. The titania nanoparticles, deposited as a thin film on the back electrode, are coated with dye molecules which release an electron when they absorb a photon. The semiconducting titania conveys the electron to the electrode. In this way, the tasks of generation and conduction of charged particles are performed by separate entities (unlike the case of silicon solar cells), reducing the chance that oppositely charged particles will recombine and re-emit the absorbed energy. Meanwhile, the high surface area of the nanoparticle film enhances the light-harvesting efficiency. The circuit is completed by an electrolyte carrying an electron donor to replenish the oxidized dye molecules.

Much of the excitement about nanoscale particles stems from the way that they interact with light. Semiconductors will absorb light at wavelengths corresponding to their bandgap-the difference in energy between the conduction and valence electronic bands. They may emit light of the same wavelength when negative and positive charge carriers (electrons and holes respectively) are injected into the material; these recombine across the bandgap with the release of a photon. So light-emitting semiconductors can mediate between light and electricity, and thus serve as the fabric of optoelectronic technologies, which transmit and process information using a combination of electricity and photons. The attraction of nanoscale engineering here is that the "color" of the semiconductors-the wavelength at which they absorb and emit-can be tuned by altering the particle size.

This is a quantum-mechanical effect: the classic "particle-in-a-box" system, in which the energy states (of, e.g., the conduction and valence electrons) are determined by the dimensions of the box. For this reason, the particles are sometimes called quantum dots or Q-particles. One illustration of the potential of nanoscale particles in this area, is the creation of a light-emitting diode in which the emitting material is a thin film of cadmium selenide particles just a few nanometers across.

There are now several methods available for making nanoscale metal and semiconductor particles with well-defined sizes. One of the most common utilizes the "compartmentalization" principle mentioned in the discussion of biomimetics. The particles are precipitated from a saturated aqueous solution of salts inside organic aggregates called reverse micelles. These are roughly spherical structures formed from surfactants in a non-polar solvent, in which they will cluster with their water-soluble heads pointing inwards and the hydrophobic tails directed outwards like the spines of a porcupine. The interior of the reverse micelles can accommodate a small reservoir of water, and it is in this that the crystalline material precipitates. The size of the resulting particles is then determined by the size of the micelles.

Some prospective applications of these optically active nanoparticles require that they be organized into ordered arrays. Two-dimensional arrays of quantum dots might, for example, act as memory elements, optically addressable by laser beam. Under the right conditions, the nanoparticles can condense spontaneously onto a surface with an ordered hexagonal packing, if they are all the same size. Even more strikingly, a mixture of two particle sizes can form single-layer films, in which the two types of particle alternate regularly along the rows-a demonstration that interparticle forces and packing constraints can do a lot of hard work for us.

Figure 10. Quantum Dots-Nanometer-scale Crystals of (in this case)
Metals-of two Different Sizes will Self-organize into Lattices in
which the Two Types of Particle Alternate

This kind of self-organization of quantum dots is also manifested in a quite different synthetic approach, in which the dots are formed by deposition of the constituent elements from the vapor phase. The synthesis of atomically thin films by vapor deposition is a standard technique in semiconductor technology, and generally it results in the formation, atomic layer by layer, of smooth films. But if the deposited atoms can move about on the surface of the substrate, they can in some cases, congregate into small clusters or islands. The effects of the strain induced by the mismatch between the lattice spacing of the crystalline substrate and that of the islands can then give rise to an effective repulsion between islands, resulting in their forming with more or less constant size and separation: as an ordered array of dots or stripes (quantum wires).

Figure 11. An Array of Self-organized Quantum Dots Formed by
Chemical Vapor Deposition into a Surface

But what if more complicated arrangements of the dots is needed, e.g., if they must be grouped in threes, or if dots of different composition need to be adjacent to one another? Techniques for linking nanoscale particles of metals or semiconductors into clusters of well-defined shape or size make use of small molecules that serve as bridging groups. Specificity in the linking process may be achieved through molecular recognition: in effect, onto one particle is grafted a lock, and onto another, a key. Silver nanoparticles have been assembled into linked arrays by grafting onto their surface the protein molecule streptavidin and the small molecule called biotin, which binds very tightly to streptavidin. The two appendages then snap together. More flexibility can be achieved by using as the linking unit "sticky-ended" strands of DNA. In this way, a particular type of nanoparticle can be programmed to adhere to one other kind of individual and no other. Here, we see the use of the informational aspect of biological interactions as a constructional tool for nanoscale materials synthesis.

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