Size-free tunability of electronic structure of colloidal quantum dots, when introducing alloy composition, in
core and cor/shell heterostructures
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
Fig. 1: Plot of relative spatial density distribution of a hole
(red) and an electron (blue) between a core and a shell in
core), through qausi-type-II, to a
PbSe/PbS QD (view of one quarter of a sphere)
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 hightemperature synthetic strategy with balancing of precursors’ reactivity. Unique
R =2nm
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
PbS
PbSe
dielectric match at the dot/shell or rod/shell interface, regulating carriers’
Fig. 2: Plot of the energy of electronic states
delocalization and/or charge separation by tunability of the band off-set.
versus composition, x, in PbSexS1-x QD with
average diameter of 2 nm
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
1.5
Energy (eV)
1.0
0.5
-0.5
-1.0
-1.5
-2.0
c
-2.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
x (composition)
1.
2.
3.
4.
(a) Brumer M., et al., Adv. Funct. Mater., 2005, 15, 1111; (b) Maikov G., et al., ACSNano, 2010, 4, 6547.
(a) Osovsky R., et al. Phys. Rev. Lett. 2009, 102, 197401, (b) Wang F. et al. Nature 2009, 459, 686-689.
(a) Lifshitz E., et al., Annual Review of Physical Chemistry, 2004, 55, 509; (b) unpublished results.
R. Vaxenburg, E. Lifshitz, submitted (2011)