Transmission electron microscope images of CdSe colloidal nanoplatelets. Different lateral shapes (a-d) can be obtained using different precursors. Scale bars: b) 20 nm, others, 10 nm. Image: Benoit Dubertret |
Significant
advances in the application of colloidal structures as light emitters
and lasers may soon be realized following the discovery of very fast
fluorescence emission rates in colloidal nanoplatelets. These
nanoplatelets combine the best characteristics of two domains: the wide
tunability of the absorption and photoluminescence of nanocrystals and
the short decay time of excitons in quantum wells.
This
discovery, which was announced by scientists at the Naval Research
Laboratory and Laboratoire de Physique et d’Etude des Matéiaux, UMR8213
du CNRS, ESPCI, suggests that the nanoplatelets are an important, novel
material for constructing tunable light-emitting diodes, low-threshold
lasers, and photo-voltaic solar cells. The complete findings of the
study are published on line in the October 23, 2011, issue of the
journal Nature Materials.
Nanoplatelets
are a new class of optical materials that are essentially atomically
flat, quasi-2D colloidal CdSe, CdS, and CdTe layers with well-defined
thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have
electronic properties of two-dimensional quantum wells formed by
molecular beam epitaxy, and their thickness-dependent absorption and
emission spectra are completely controlled by the layer thickness. The
very high spatial confinement of carriers in these colloidal structures,
practically inaccessible in epitaxial quantum wells, combined with
opportunities to create very thin, flat layers (down to 1.5 nm) of the
semiconductors makes the band gap of this material tunable over a 1.4 eV
range. The widely tunable absorption band edge, which is controlled
primarily by the nanoplatelet thickness, results in widely tunable
emission spectra.
Strong
enhancement of electron-hole Coulomb interaction due to the small
dielectric constant of the surrounding media is another property of
colloidal nanoplatelets that exists in neither spherical colloidal
nanocrystals nor in epitaxial quantum wells. This phenomenon
significantly decreases the radius of excitons and shortens their
radiative decay time. In addition, nanoplatelet shape affects the
strength of the exciton coupling with emitted photons because the
tangential component of the photon electric field does not change its
value when it penetrates through the surface of the flat nanoplatelets.
This also shortens the fluorescent decay time in these structures.
Finally,
the ground exciton states in quasi-2D nanoplatelets can have a giant
oscillator strength transition connected with the exciton center of mass
coherent motion. The giant oscillator strength transition is a quantum
mechanical phenomenon that may be described as coherent excitation of
the volume, which is significantly larger than the volume of the
exciton. The phenomenon was predicted 50 years ago by Rashba. The giant
oscillator strength transition of the ground exciton state enhances the
absorption cross-section and shortens significantly the exciton
radiative decay time. In the case of two-dimensional structures, the
enhancement is proportional to the ratio of the area of the exciton
coherent motion to the square of the exciton Bohr radius.
The
research teams at Laboratoire de Physique et d’Etude des Matéiaux and
NRL found that at room temperatures, the fluorescence lifetime of CdSe
nanoplatelets is shorter than that of CdSe nanocrystals with similar
quantum yield and emission wavelength. Importantly, the fluorescence
lifetime of nanoplatelets decreases with temperature, whereas their
emission intensity increases. Such a temperature dependence of the
fluorescence lifetime is a unique signature of the giant oscillator
strength transition, which previously was observed only in quantum wells
at helium temperatures. At 6 K the radiative decay time becomes shorter
than 1 ns, which is two orders of magnitude less than for spherical
CdSe nanocrystals. This makes the nanoplatelets the fastest colloidal
fuorescent emitters known and strongly suggests that they show a giant
oscillator strength transition.
Future
efforts will be focused on optimization of these nanoplatelet
structures with a goal to eliminate the nonradiative processes connected
with the surface. The growth of core-shell nanoplatelets would further
extend the properties and applications of the materials presented here
and would pave the way for the synthesis of colloidal,
multiple-quantum-well structures. Such structures should enable
researchers to take full advantage of the observed shortening of the
radiative decay time and tunability, and point the way to future
breakthroughs in photonics, lasers, and other optical applications of
nanoplatelets.
The
research described above was conducted by Dr. S. Ithurria, Dr. M. D.
Tessier, Dr. B. Mahler, Dr. R. P. S. M. Lobo, and Dr. B. Dubertret from
Laboratoire de Physique et d’Etude des Matéiaux, UMR8213 du CNRS, ESPCI;
and Dr. Alexander Efros from NRL’s Material Science and Technology
Division.