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Colloidal core/shell heterostructures with alloy composition: Synthesis, theoretical electronic band structure, and magneto-optical characterization
Prof. Efrat Lifshitz, Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute, Technion, Haifa, 32000, Israel
Two decades of research were devoted to explore the electronic properties of semiconductor nanocyrstals by varying their size, shape and surface coverage. The option to engineer the properties is of a significant importance for the materials’ implementation as absorbers or emitters in photovoltaic cells, light emitting diodes, optical switches, gain devices, photodetectors, thermoelectric, spintronics devices and biological platforms. However, some of these applications impose constrains on the size and shape, say, incorporating them into biological membrane or a mesoporous substrate, while retaining the demand for a suitable emission color. These restrictions can be overcome by new strategies gaining property control using: (a) alloyed ternary or quaternary compounds, when a ternary material is comprised of two different cations/anions with a common anion/cation, while quaternary includes four elements. In all, the elements can be either distributed homogeneously or exhibit a graded composition along a selective direction; (b) core/shell heterostructures, comprised of a semiconductor core (sphere or rod shape), covered by a shell, of another semiconductor, when the band-edge off-set at the core/shell interface, can be tuned from a type-I (when shell ban-edge is rapping that of the core), through qausi-type-II, to a type-II (when, band-edge of the constituents are staggered) alignment. Moreover, one of the constituents (core or shell) may have alloyed composition.
The present work demonstrates a progressive effort in the synthesis of alloyed colloidal quantum dots (QDs) or nanorods (NRs), by employing an effective high-temperature synthetic strategy with balancing of precursors’ reactivity. Unique alloyed core/shell heterostructures, such as PbSe/PbSexS1-x,1 CdTe/CdTexSe1-x, and CdSe/CdSexS1-x/CdS,2 were developed, offering better crystallographic and dielectric match at the dot/shell or rod/shell interface, regulating carriers’ delocalization and/or charge separation by tunability of the band off-set. Theoretical description of the electronic band structure of alloyed core, dot/shell or a rod/shell nanocrystals, using a k*p model, covered a wide physical aspects, including an effective mass anisotropy, dielectric variation between the constituents, a sharp or a smooth off-set at the core/shell interface and electron-hole Coulomb interactions, laid a ground for tailoring heterostructures with the desired composition and optical properties. Representative theoretical plot of a charge distribution between a dot and a shell in a QD is shown in Fig. 1.4 Engineering of a band-edge, as well as the remote energy states of alloyed PbSexS1-x QD is illustrated in Fig. 2. This figure reveals a few interesting aspects, related to grouping of states nearly into min-bands with a degeneracy that depends on composition (x) and even some level crossing. The physical explanation will be discussed in the talk.4
Experimental evidences investigating the mentioned heterostructures showed a relatively high emission quantum yield, chemical stability, and in a few cases (e.g., CdTe/CdTexSe1-x/CdSe) an option to stabilize an emission intensity, pronounced as a blinking free behavior when measuring a single QD.2 Further on, significant knowledge on carriers’ localization was gained by the use of a magneto-optical methodology, e.g., optically detected magnetic resonance (ODMR) spectroscopy, supplying a magnetic resonance identification of a carrier and its surrounding, phenomenological g-factor, electron-hole exchange interaction and crystal anisotropy.3

1. (a) Brumer M., et al., Adv. Funct. Mater., 2005, 15, 1111; (b) Maikov G., et al., ACSNano, 2010, 4, 6547.
2. (a) Osovsky R., et al. Phys. Rev. Lett. 2009, 102, 197401, (b) Wang F. et al. Nature 2009, 459, 686-689.
3. (a) Lifshitz E., et al., Annual Review of Physical Chemistry, 2004, 55, 509; (b) unpublished results.
4. R. Vaxenburg, E. Lifshitz, submitted (2011)