Size Dependent Absorption Anisotropy Measurements of CdSe Nanocrystals: Symmetry Assignments for the Lowest Exciton State

Semiconducting nanocrystals smaller in size than the bulk Bohr exciton display quantum confinement, leading to a size dependent energy gap and concentration of oscillator strength into a few transitions. Given these properties these materials are being investigated for potential device applications. CdSe nanocrystals have already been implemented in tunable light emitting diodes. 1 We have performed absorption anisotropy experiments on CdSe crystals ranging in diameter from 30-50A with the goal of elucidating the symmetry of the lowest energy optical transition and the earliest events of carrier evolution. The optical anisotropy is defined as R(t)=I ‖ (t)-I ⊥ (t)/K(t), where K(t) is the total population decay (I ‖ + 2I ⊥ ). The initial value of the anisotropy, R(0), is indicative of the nature of the optical transition. For example, if the transition is spherically degenerate, as for C 60 , R(0) is expected to be 0.0. A linear dipole transition has R(0)=0.4 and for a degenerate planar transition R(0)=0.15. 2 The decay in the anisotropy may contain contributions from rotational diffusion, rotation of the transition dipole, as in an isomerization reaction, or population transfer between two excited state levels with transition dipoles of different directions. High time resolution measurements performed on nanocrystals thus give us insight into the shape of the optical transition and the earliest dynamics of the excited carriers. We’ve discovered that a competition exists between the intrinsic wurtzite crystal field and the nonspherical confinement in the crystal 3 which dictates the symmetry of the first exciton state. The crystals are thus intrinsically dichroic, a phenomenon which may be exploited in device applications.