Dynamics on the Nanoscale

"Time-domain ab initio studies of quantum dots and carbon nanotubes"

Device miniaturization requires an understanding of the dynamical response of materials on the nanometer scale. A great deal of experimental and theoretical work has been devoted to characterizing the excitation,charge, spin, and vibrational dynamics in a variety of novel materials,including carbon nanotubes, quantum dots, conducting polymers, inorganicsemiconductors and molecular chromophores. We have developedstate-of-the-art non-adiabatic molecular dynamics techniques andimplemented them within time-dependent density functional theory in orderto model the ultrafast photoinduced processes in these materials at theatomistic level, and in real time.

The electron-phonon interactions in carbon nanotubes (CNT) determine theresponse times of optical switches and logic gates, the extent of heatingand energy loss in CNT wires and field-effect transistors, and even asuperconductivity mechanism. Our ab initio studies of CNTs directly mimicthe experimental data and reveal a number of unexpected features,including the fast intrinsic intraband relaxation and electron-holerecombination, the importance of defects, the dependence of the relaxationrate on the excitation energy and intensity, and a detailed understandingof the role of active phonon modes.

Quantum dots (QD) are quasi-zero dimensional structures with a uniquecombination of molecular and bulk properties. As a result, QDs exhibit newphysical properties such as carrier multiplication, which has thepotential to greatly increase the efficiency of solar cells. Theelectron-phonon and Auger relaxation in QDs compete with carriermultiplication. Our detailed studies of the competing processes in PbSeQDs rationalize why carrier multiplication was first observed in thismaterial.