[Todos] HOY, 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.
>
>