74
P. Lecoq
sion, enhanced stability compared to organic dyes, and the fluorescence is tunable
from the UV to the near-infrared spectral range (300–3000 nm) by nanocrystal size
and material composition.
A novel route towards the realization of ultrafast timing resolution is possible with the use of colloidal CdSe nanosheets (CQwells) [24], a new class of
two-dimensional materials. CQwells are solution-processed analogs to epitaxial
quantum wells (Qwells). However, being synthesized in solution, they can be
deposited on any substrate with arbitrary geometrical configuration. Further, a large
dielectric mismatch between the inorganic CdSe CQwells and the surrounding
organic environment results in much stronger quantum confinement than in epitaxial
Qwells. This mismatch combined with very little dielectric screening due to the
1.5 nm CQwell thickness results in strongly enhanced exciton and biexciton binding
energies of 132 and 30 meV, respectively, making both populations stable at room
temperature.
The strong electron and hole confinement in one dimension and free motion
in the plane has several important consequences, including strict momentum
conservations rules (in contrast to quantum dots) and a giant oscillator strength
transition. Momentum conservation in CQwells limits the available states for
Auger transitions, reducing the recombination rate of this nonradiative channel. In
addition to the enhanced exciton and biexciton binding energies, a giant oscillator
transition results in radiative lifetimes that are significantly shorter than in bulk
CdSe (~400 and ~100 ps, respectively). All of these properties contribute to the
ultralow threshold stimulated emission (or superluminescence) with sub-ps decay
time that has been observed with these CQwells (Fig. 3.15). Such systems could find
Fig. 3.15 Time-resolved spectral decay under femtosecond excitation (a) Streak image showing
the spectral decay of exciton (X) and biexciton (XX) emission from CdSe CQwells. (b) Stimulated
emission at an ultralow excitation fluence of F 0 = 6 μJ/cm 2 , with characteristic spectral narrowing
and lifetime shortening. From Ref [24]
P. Lecoq
sion, enhanced stability compared to organic dyes, and the fluorescence is tunable
from the UV to the near-infrared spectral range (300–3000 nm) by nanocrystal size
and material composition.
A novel route towards the realization of ultrafast timing resolution is possible with the use of colloidal CdSe nanosheets (CQwells) [24], a new class of
two-dimensional materials. CQwells are solution-processed analogs to epitaxial
quantum wells (Qwells). However, being synthesized in solution, they can be
deposited on any substrate with arbitrary geometrical configuration. Further, a large
dielectric mismatch between the inorganic CdSe CQwells and the surrounding
organic environment results in much stronger quantum confinement than in epitaxial
Qwells. This mismatch combined with very little dielectric screening due to the
1.5 nm CQwell thickness results in strongly enhanced exciton and biexciton binding
energies of 132 and 30 meV, respectively, making both populations stable at room
temperature.
The strong electron and hole confinement in one dimension and free motion
in the plane has several important consequences, including strict momentum
conservations rules (in contrast to quantum dots) and a giant oscillator strength
transition. Momentum conservation in CQwells limits the available states for
Auger transitions, reducing the recombination rate of this nonradiative channel. In
addition to the enhanced exciton and biexciton binding energies, a giant oscillator
transition results in radiative lifetimes that are significantly shorter than in bulk
CdSe (~400 and ~100 ps, respectively). All of these properties contribute to the
ultralow threshold stimulated emission (or superluminescence) with sub-ps decay
time that has been observed with these CQwells (Fig. 3.15). Such systems could find
Fig. 3.15 Time-resolved spectral decay under femtosecond excitation (a) Streak image showing
the spectral decay of exciton (X) and biexciton (XX) emission from CdSe CQwells. (b) Stimulated
emission at an ultralow excitation fluence of F 0 = 6 μJ/cm 2 , with characteristic spectral narrowing
and lifetime shortening. From Ref [24]