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NANOCRYSTALS'
FLUORESCENCE
Light emission in the whole visible range is available from organic
fluorescent dyes. Depending on the colour sought, one has to choose
from a collection of thousands of different molecules.
The alternative is semiconductor nanocrystals, the emission colour
(read "wavelength") of which could extend from blue (480 nm) to red
(>650 nm) in the case of CdSe or InP, depending only on the nanocrystals' size.
One synthesis route and surface chemistry can therefore deal with all
the colours. Characterizing, through spectroscopy measurements, the
spectrum of light emitted by semiconductor nanocrystals allows us to
give feedback on the syntheses trials to optimize the nanocrystals'
structure.
Further insight will be gained from excitation spectroscopy techniques
and from time-resolved fluorescence measurements.
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FLUORESCENCE QUANTUM
YIELD (Q.Y.)
This parameter is of
paramount importance, as it measures the ratio of the number of emitted
photons to the number of absorbed, excitation photons. It characterizes
the efficiency with which the dye converts the excitation light into
light of a well-defined colour.
Usual fluorescent dyes are organic molecules, the quantum yield of
which is of the order of tens of percent, up to 100%.
As an example, quinine sulfate, which is often used as a fluorescence
standard (and can be found in small amounts in tonic beverages),
presents a quantum yield of about 50%, shining a bright turquoise light
under UV lamps.
The fluorescence quantum yield of semiconductor nanocrystals virtually
reaches 100% for CdSe-based core/shell systems, these upper values
being measured on CdSe nanocrystals that
emit yellow-green light.
In order to quickly get precise and reliable values, part of our work
was devoted to the design of a new method and apparatus for the
measurement of the quantum yield.
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SINGLE-PARTICLE
FLUORESCENCE
When semiconductor nanocrystals are
taken individually, their photon emission does not follow usual
photon
statistics.
On the short time scale (t < 100 ns), the
probability that one single nanocrystal emits two photons at the same
time is zero (X. Brokmann, E. Giacobino, M. Dahan, and J. P. Hermier,
Appl. Phys. Lett. 85(5), 712-4 (2004)). That makes them good candidates
for single-photon emitters.
The picture on the right represents a fluorescence microscopy
set-up, used to record the photon emission from single nanocrystals.
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On longer time scales (t > 100 ns), a semiconductor nanocrystal
presents a succession of time intervals during which it emits light
("on-state") or not ("off-state"). The statistics of these intervals'
length is a power law for both on- and off-states. It is described in
the literature as "fluorescence blinking", although this "blinking"
word could induce some idea of periodicity, which is completely
lacking.
When ones measures the fluorescence intensity, the succession of these
on- and off-states for a few nanocrystals translates in an intensity
distribution which is multimodal, as can be seen in the figure on the
left.
Collaboration: Nguyen Quang Liem, Thuy Ung
(Vietnamese Academy of Science & Technology, Hanoi) |
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