Context: A typical dust particle (0.1mm across) might contain 10¹⁶ atoms. A typical grain in a crystal (10 microns across) might contain 10^13-14 atoms. The self-organised semiconductor quantum dots contain perhaps 10000 atoms. At this size, they show the Coulomb blockade, since adding an extra electron to one already present can need more than kT at room temperature. And then there are the dots of perhaps 200 atoms (typically CdS or CdSe; these dots can be made in many ways, including wet chemistry and even by persuading yeasts to help, as was shown in UCL?s Bioengineering Department). At this few-hundred atom size, the excitons show a size-dependent blue shift: in principle, the properties can be tuned by size. Their potential is for photonics, rather than microelectronics. As an example, their polarisability (contribution to the refractive index) is different in the excited state. So one could imagine switching a phonon beam very fast indeed (perhaps in picoseconds) by exciting the dot with a laser, then de-exciting it by stimulated emission. But this shows complications?

Current activities: Our interest is in modelling and understanding the dynamics of dots in their excited states. What happens in that first few picoseconds, and why? To mention some of the issues, (i) there can be major differences between a dot of 200 atoms and one of 201 atoms (the N+1 problem), especially associated with the electric fields in the dot; (ii) the dot will want to change its size and shape in the excited state and this can shift the emission so as to make de-excitation harder; (iii) the relaxation processes can leave quite a lot of vibrational energy in the dot which (depending on its environment) may remain for a significant time; (iv) the dot can lose electrons (exciting two excitons would make an Auger process possible, for example) and this has a major effect of the dot.

People involved (including external collaborators)

Dr Mike Burt (BT Laboratories, Martlesham Heath), with his co-workers Drs Steve Kershaw and Mike Harrison, Professor Brian Ridley FRS, and links to Professor Horst Weller, Dr Alex Eychmüller, Dr Herwig Doellefeld (Hamburg), and Dr Alf Mews (Mainz).

Dr Andrew Fisher and Dr P Thornton Greenland (UCL, collaborators with Marshall Stoneham on an EPSRC ROPA on decoherence in quantum computing).

Recent (and relevant) papers

370. Excited state dynamics in quantum dots
A M Stoneham 1998 J Pakistan Society for Semiconductor Science and Technology (special issue in memoriam Professor Ed Lightowlers) 7(4) 135-138

381 Excitation Dynamics and Dephasing in Quantum Dots
A M Stoneham & B McKinnon 1998 J Phys Cond Matt 10 (34) 7665-7677

30. Paramagnetic Relaxation in Small Crystals.
A M Stoneham 1965 Sol. St. Comm. 3, 71-73.

Cross-links Dot dynamics introduces issues of coherence and decoherence in at least two senses: in the sense of quantum computing (quantum coherence), and in the sense of coherent control of excited state chemistry (vibrational coherence). Dots are similar in size to molecules, and there are parallel processes in some of our conducting polymer studies.