2005 APS March Meeting
Monday–Friday, March 21–25, 2005;
Los Angeles, CA
Session P15: Focus Session: Relaxation and Phonons in Nanostructures
11:15 AM–1:15 PM,
Wednesday, March 23, 2005
LACC
Room: 405
Sponsoring
Unit:
FIAP
Chair: Fran Wise, Cornell University
Abstract ID: BAPS.2005.MAR.P15.4
Abstract: P15.00004 : Effects of Band Structure on the Electronic and Optical Properties of Semiconductor Nanocrystals: Lead Selenide vs. Cadmium Selenide*
11:51 AM–12:27 PM
Preview Abstract
Abstract
Author:
Richard Schaller
(Los Alamos National Laboratory)
PbSe and CdSe nanocrystals (NCs) can be synthesized with high
monodispersity, have a size-tunable band gap, and can exhibit near unity
photoluminescence quantum yields. These two materials have significant
differences with respect to bulk band gap, crystal structure, Bohr radius,
and carrier effective masses that result in very distinct energy structures.
Here we perform a side-by-side comparison of the optical and electronic
properties of these two materials in both the single and multiexciton
regimes for NCs of comparable sizes. Femtosecond transient absorption (TA)
spectroscopy is used to study inter- and intraband relaxation of
photo-generated carriers including various types of Auger-relaxation
processes. We have discovered that for PbSe NCs, photo-generated single
excitons with sufficient energy in excess of the band gap are able to relax
by producing multiple excitons (carrier multiplication). In carrier
multiplication, intraband excess energy is transferred to a valence band
electron that is excited into the conduction band, resulting in the
formation of two or more excitons per initially photo-excited exciton [Phys.
Rev. Lett. 2004, v.92, 186601/1-4]. We have found that this effect of
multiexciton generation, which has never been found to occur with
significant efficiency in bulk semiconductors, can occur with up to 100{\%}
efficiency in PbSe NCs depending upon the absorbed photon energy and occurs
at wavelengths that are relevant to solar energy conversion. This process,
which is an enabler of Generation III solar cells, has the potential to
considerably increase the power conversion efficiency of NC-based
photovoltaics. Pump-power dependent TA studies performed with a probe pulse
tuned to near the photoluminescence maximum have revealed that both PbSe NCs
and CdSe NCs can exhibit optical gain and, when incorporated into high
optical quality sol-gel waveguides, are capable of producing size-tunable
amplified spontaneous emission [J. Phys. Chem. B 2003, v.107, 13765-8].
Similar TA studies performed with a probe pulse tuned to the band-edge
absorption feature reveal that differences in crystal structure cause the
two materials to have significantly different exciton degeneracy, which
directly results in different thresholds for gain.
*Acknowledgement: Victor I. Klimov, Jeffrey M. Pietryga, and Melissa A. Petruska
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2005.MAR.P15.4