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Unformatted text preview: Exciton Recombination Dynamics in CdSe Nanowires: Bimolecular to Three-Carrier Auger Kinetics Istva ´n Robel, ²,‡ Bruce A. Bunker, ‡ Prashant V. Kamat,* ,²,§, | and Masaru Kuno* ,²,§ Radiation Laboratory, Department of Chemistry and Biochemistry, Department of Chemical and Biomolecular Engineering, and Department of Physics, Uni V ersity of Notre Dame, Notre Dame, Indiana 46556 Received January 26, 2006; Revised Manuscript Received April 8, 2006 ABSTRACT Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron- hole pair densities above 10 19 cm- 3 the dominant process for carrier cooling is the “bimolecular” Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton- exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron- hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime. In recent years it has become possible to tailor the size, shape, and dimensionality of semiconductor nanostructures with ever-growing precision. 1 Studies of the electronic and optical properties of these systems have provided classic examples of quantum-confinement effects in reduced geometries. 2,3 Understanding charge carrier dynamics in semiconductors with limited spatial degrees of freedom is therefore of fundamental scientific interest with practical implications in the design of optoelectronic and photovoltaic devices. The ultrafast relaxation of charge carriers in semiconductor quantum dots (QDs) has been extensively studied 4 revealing the role of particle size 5 and excitation density 5- 7 in carrier cooling. It has been assumed that due to large confinement- induced energies, both the electron and hole are separately confined within the QD. 8 At high excitation densities many carriers are present in each nanocrystal and de-excitation occurs through multicarrier processes. 5- 7 An example of such an Auger-mediated recombination includes a scenario where one electron- hole (e- h) pair gives up its energy to another carrier. The rate of this three-charge process (also referred to as a CHCC mechanism 9 ) is thus cubic in carrier density. Recent experiments with nanorods 10,11 (NRs) have ex- tended these studies showing the effects of shape on the electronic properties of low-dimensional semiconductors....
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This note was uploaded on 09/30/2010 for the course MANAGEMENT MG2432 taught by Professor Muhammadadeel during the Spring '10 term at École Normale Supérieure.
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