nl302517z_si_001.pdf (404.51 kB)

Download file# Loosening Quantum Confinement: Observation of Real Conductivity Caused by Hole Polarons in Semiconductor Nanocrystals Smaller than the Bohr Radius

journal contribution

posted on 12.09.2012, 00:00 by Ronald Ulbricht, Joep J. H. Pijpers, Esther Groeneveld, Rolf Koole, Celso
de Mello Donega, Daniel Vanmaekelbergh, Christophe Delerue, Guy Allan, Mischa BonnWe report on the gradual evolution of the conductivity
of spherical
CdTe nanocrystals of increasing size from the regime of strong quantum
confinement with truly discrete energy levels to the regime of weak
confinement with closely spaced hole states. We use the high-frequency
(terahertz) real and imaginary conductivities of optically injected
carriers in the nanocrystals to report on the degree of quantum confinement.
For the smaller CdTe nanocrystals (3 nm < radius < 5 nm), the
complex terahertz conductivity is purely imaginary. For nanocrystals
with radii exceeding 5 nm, we observe the onset of real conductivity,
which is attributed to the increasingly smaller separation between
the hole states. Remarkably, this onset occurs for a nanocrystal radius
significantly smaller than the bulk exciton Bohr radius

*a*_{B}∼ 7 nm and cannot be explained by purely electronic transitions between hole states, as evidenced by tight-binding calculations. The real-valued conductivity observed in the larger nanocrystals can be explained by the emergence of mixed carrier-phonon, that is, polaron, states due to hole transitions that become resonant with, and couple strongly to, optical phonon modes for larger QDs. These polaron states possess larger oscillator strengths and broader absorption, and thereby give rise to enhanced real conductivity within the nanocrystals despite the confinement.