[Todos] Jueves 23 de febrero - 13 hs., Seminarios DQIAQF - INQUIMAE

[Graciela A. González]

Seminarios DQIAQF - INQUIMAE,  jueves 23 de febrero - 13 hs.

Aula de Seminarios INQUIMAE - DQIAQF
Facultad de Ciencias Exactas y Naturales
Ciudad Universitaria - Pab. 2  -  Piso 3


SIZE-FREE ELECTRONIC TUNEABILITY, THERMAL AND CHEMICAL STABILITY,
BLINKING-FREE MULTIPLE EXCITONS IN ALLOYED COLLOIDAL QUANTUM DOTS AND RODS
Efrat Lifshitz, Roman Vaxenberg, Evgeny Tilchin, Georgy Maikov, Diana
Yanover, Anna Brusilevsky, Dikla Ahituv, Alina Amel, Aldona Sashchiuk

Prof. Efrat Lifshitz,
Matwei Gunsbuourgh Academic Chair
Schulich Faculty of Chemistry
Russell Berrie Nanotechnology Institute
Solid State Institute
Technion
Haifa, 32000, Israel

This work introduces a new class of heterostructure quantum dots (QDs) and
quantum rods (QRs), comprised of PbSe or CdTe cores (spheres or rods),
coated by PbSe or CdSe shells, respectively, when, either the core or the
shell has an alloy composition with a general chemical formula PbSexS1-x
or CdTexSe1-x (when 0<x<1), and when x is distributed homogenously or as
graded component across the nanocrystal.  This work describes the unique
properties of the mentioned heterostructures and their benefits in a few
technological applications. Electronic band-structure calculations (using
k*p model) included the influence of anisotropy of electron and hole
effective masses (particularly in lead chalcogenides), smooth or sharp
core/shell interface, shell-thickness/core-radius size ratio, adjustable
composition (x) either at the core, at the shell or in both. Those
calculations led to the following conclusions: (1) the band-gap energy and
 the band-extreme chemical potential can be tuned by a change in x; (2)
the energy of remote states and their density of states can be adjusted by
x and the internal division between core and shell; (3) the mentioned
nanocrystals have a core/shell quasi-type-II alignment, when one carrier
is partially delocalized over the entire heterostructure, and the degree
of delocalization, exchange and Coulomb interactions are controlled by the
value of x and the shell-thickness/core-radius ratio; (4) pure PbSe
nanocrystals and their relevant heterostructures, showed partial charge
separation, dominated by the anisotropy of effective mass, when addition
of a shell further increase this separation. The discovered properties
render understanding of current scientific issues discussed in the
literature, such as the origin of the long lifetime in lead chacogenides
nanocrystals, preferred multiple exciton generation in PbSe QDs, efficient
charge separation in QDs and QRs for photovoltaic applications, Auger
relaxations rates and long lived multiple excitons in colloidal
nanocrystals.  A few experimental evidences exhibit additional properties
of the discussed heterostructures: (1) the continuous-wave and transient
photoluminescence measurements over a wide temperature range (1.4 - 300 K)
revealed a spectral shift of absorption and emission bands with a change
in x in accordance of the theoretical work, an increase of the emission
quantum yield when alloy components were included due to a decrease of
stoichiometric defects, alleviation of a dark exciton recombination (a
reduction of an exchange interaction) and reduction of the band-edge
temperature coefficient (dE/dT) viz, induction of thermal stability; (2)
micro-photoluminescence spectra of single core/shell QDs involving alloy
component, showed a unique spectral stability (blinking-free) and
well-resolved single and multiple exciton emission bands, all showed
patterned dependence on the existence of an external magnetic field and
selective linear or circular polarization, with/without temporal
resolution.  Those experiments revealed information about the angular
momentum of the remote states and recombination processes of multiple
excitons.