ScienceWeek

CONDENSED MATTER: SUPERLATTICES: NANOCRYSTALS AND QUANTUM DOTS

The following points are made by F.X. Redl et al (Nature 2003 423:968):

1) Recent advances in strategies for synthesizing nanoparticles -- such as semiconductor quantum dots(1), magnets and noble-metal clusters(2) -- have enabled the precise control of composition, size, shape(3), crystal structure(4), and surface chemistry. The distinct properties of the resulting nanometer-scale building blocks can be harnessed in assemblies with new collective properties(2,5), which can be further engineered by controlling interparticle spacing and by material processing.

2) Nanocrystal (NC) superlattices have attracted increasing attention since they were first reported in 1989. Studies of colloidal microspheres (natural and synthetic) have shown that mixtures of such spheres with selected size ratios can co-crystallize into binary AB, AB2, AB5 or AB13 colloidal crystals.(5), ordered monolayers displaying the binary AB2 or AB composition, or a trilayer consistent with the AB5 structure.

3) The study of the authors was motivated by the emerging concept of metamaterials -- materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi-component nanocrystal assemblies have usually resulted in amorphous or short-range-ordered materials, because of non-directional forces or insufficient mobility during assembly.

4) The authors report the self-assembly of PbSe semiconductor quantum dots and Fe2O3 magnetic nanocrystals into precisely ordered three-dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB13 or AB2 superlattices with potentially tunable optical and magnetic properties. The authors suggest this synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical and mechanical stimuli.

References (abridged):

1. Trindade, T., O'Brien, P. & Pickett, N. L. Nanocrystalline semiconductors: Synthesis, properties, and perspectives. Chem. Mater. 13, 3843-3858 (2001)

2. Murray, C. B., Kagan, C. R. & Bawendi, M. G. Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annu. Rev. Mater. Sci. 30, 545-610 (2000)

3. Manna, L., Scher, E. C. & Alivisatos, A. P. Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals. J. Am. Chem. Soc. 122, 12700-12706 (2000)

4. Sun, S. & Murray, C. B. Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices. J. Appl. Phys. 85, 4325-4330 (1999)

5. Brust, M. & Kiely, C. J. Some recent advances in nanostructure preparation from gold and silver particles: A short topical review. Colloids Surf. A 202, 175-186 (2002)

Nature http://www.nature.com/nature

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ON QUANTUM DOTS

The following points are made by D. Gammon and D.G. Steel (Physics Today 2002 October):

1) Atomic physics progressed rapidly at the beginning of the last century, thanks, in large part, to optical spectroscopy. Quantization and spin were discovered through optical studies, as were other fundamental atomic properties. With the advent of the laser, physicists learned how to manipulate atomic wavefunctions by applying coherent optical fields. More discoveries followed. Now, at the beginning of the new century, optical techniques are being used to explore a new scientific frontier: the atom-like entities known as quantum dots (QDs).

2) Measuring 1-100 nm across, QDs are semiconductor structures in which the electron wavefunction is confined in all three dimensions by the potential energy barriers that form the QD's boundaries. A QD's electronic response, like that of a single atom, is manifest in its discrete energy spectrum, which appears when electron hole pairs are excited. Although the wavefunction of a QD electron, and its corresponding hole, extends over many thousands of lattice atoms, the pair -- termed an 'exciton' --behaves in a quantized and coherent fashion. The coherence is relatively easy to detect and control optically -- for two reasons. First, the superposition of the ground and excited states dephases more slowly in QDs than in higher-dimensional semiconductor structures. Second, QDs have large dipole moments (50-100 times larger than those of atoms). Thanks to these advantages, it is possible to probe and manipulate the wavefunction of a single QD.

3) QDs possess another attractive property. Their size, shape, and composition can all be tailored to create a variety of desired properties. These "artificial atoms" can, in turn, be positioned and assembled into complexes that serve as new materials. [Researchers} who work on QDs anticipate that a host of complex, customized QD-based materials will become available.

Physics Today http://www.physicstoday.org

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ON THE PRODUCTION OF QUANTUM DOTS BY ETCHING

The following points are made by Richard Turton (citation below):

1) What length scales are required to produce a quantum dot? In a quantum well the wavelike properties of the electrons only become apparent for layer widths of about a hundredth of a micron. To produce a quantum dot we need to achieve similar length scales along the other two dimensions. Using present techniques it is relatively easy to reduce the lateral dimensions of the islands to about a tenth of a micron. With great effort further reductions can be achieved, but producing a structure one hundredth of a micron across presents an immense technical challenge.

2) Fortunately nature is on our side. Suppose that we etch two parallel channels through the layered materials to leave a ridge approximately one-tenth of a micron across. In the etching process the atoms on the vertical surfaces of the ridge are disturbed so that the properties of the material are changed considerably. In fact, surfaces in general have quite different properties to bulk material. Let us consider an example using a crystal of silicon. We know that each silicon atom tends to bond with four neighboring atoms to obtain a full complement of electrons in its outer shell. However, atoms at a surface cannot always find another four atoms with which to bond. This means that surface atoms have a tendency to hold on to any nearby free electrons in order to obtain a full shell. This process alone tends to remove conduction electrons from the surface layers. In addition, the accumulation of negative charges trapped by these surface atoms creates an electric field which repels other electrons from this region.

3) As a result we find that the only free electrons in our tiny ridge-like structure are confined to an even smaller channel in the center. It is rather like a coaxial cable in which the central copper core is surrounded by an insulating layer. For a ridge which is a tenth of a micron across, the electrons are confined to a region which is only about a hundredth of a micron wide. This is just the right length scale required to observe the wavelike nature of the electron.

4) So although at first sight the dimensions appear to be far too large, we can observe lateral quantum confinement in these structures. In this case we have created a quantum wire, since the electrons are still free to travel along the length of the ridge. From this stage the construction of a quantum dot is relatively straightforward. If we etch another set of similarly spaced grooves at right-angles to the first set, then the surface effects in these directions are sufficient to cause the electrons to be trapped in a tiny box. Using these techniques it is possible to pattern a relatively large area to produce a regular grid of quantum dots.

Adapted from: Richard Turton: The Quantum Dot: A journey into the Future of Microelectronics. Oxford University Press 1996, p.147 More information at: http://www.amazon.com/exec/obidos/ASIN/0195109597/scienceweek

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