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Center for Nonlinear Studies |
Carbon nanotubes are one-atom-thick layer of graphite rolled into a tube forming rigid long cylinders with very tiny diameters. Depending on a way of rolling (chirality), a nanotube can be a metal or a semiconductor. The unique mechanical, electronic, and optical properties of nanotubes make them very attractive materials for a number of technological applications. In our research we focus on electronic and optical properties of semiconducting nanotubes. The absorption of the light photon excites an electron in a nanotube to its higher electronic state, while a new positively charged particle, a hole, is created on the initial state of the exited electron. In contrast to bulk macrosize crystals, an electron and a hole photoexcited in nanotubes strongly interact with each other so that they behave like one particle, called an exciton. In addition, the photo-excitation may slightly change atoms positions in a tube, as well. By numerical simulations, we investigate what happens with electrons, holes, excitons, and position of carbon atoms in nanotubes with different diameters upon such excitation. Knowledge of how strongly optically exited electrons and holes are bound together (excitonic effects) and how sensitive they are to the tube geometry and atomic oscillations (vibrational effects) is vitally important for correct description of photoinduced dynamics of carbon nanotubes. The existence of strong excitonic and vibrational effects in carbon nanotube materials was realized only recently (2-3 years ago). Theoretical studies of these phenomena using accurate quantum-chemical methods are complicated and involve significant computational efforts. Subsequently, existing theoretical investigations focus only on one class of effects: either excitonic or vibrational. We use novel approaches and codes developed at Los Alamos National Laboratory, which make possible simulation of exciton-vibrational dynamics in very large molecular systems, while retaining the necessary quantitative accuracy. This allows us to model in detail excited state phenomena in carbon nanotubes of different diameter and chirality. The results of our extensive simulations are in quantitative agreement with available spectroscopic experimental data and show intricate details of excited state dynamics in carbon nanotubes. In particular, the first time, it is shown that photoexcitations lead to substantial corrugation of the tube surface. The change of the nanotube shape forces electron and hole to stay closer to each other (to be more localized in space) forming so-called self-trapped exciton. We found this effect to be more pronounced in tubes with larger radius, which provide more mechanical flexibility for atoms. Our results allow for better understanding of photoinduced electronic dynamics in nanotube materials, guide design of new experimental probes, and, ultimately, may lead to new nanotechnological implications, such as optical and conductance switches, quantum wires, logic gates, miniature field-effect transitions and lasers.