Synthesizing Alloyed CdSeTe Quantum
Dots - a Description of the Experimental Process
The Results that Emerged from this
Studying Alloyed Quantum Dots Using
Absorption and Photoluminescence Spectroscopy
Explaining the Behaviour of Quantum
Dots When Examined with Absorption and Photoluminescence Spectroscopy
The optical properties of quantum
dots (QDs) have potential applications in
optoelectronics, biosensing and biolabeling,
memory devices, and sources of laser light. For example, alloyed CdSeTe QDs are shown herein to possess
a nonlinear change in their photoluminescence spectra, correlated to
size and composition, as monitored by the versatile Spex® FluoroMax® spectrofluorometer. The QDs' emission wavelength can be as high as 850 nm, which may
be useful for imaging deeper into living tissue than visible light can
Synthesizing Alloyed CdSeTe Quantum Dots
- a Description of the Experimental Process
The procedure for synthesizing alloyed
CdSeTe QDs (2.7-8.6 nm diameter)
from pure CdO, Se shot, and Te powder in tri-n-octylphosphine oxide and hexadecylamine is in the ‘Journal of Physical Chemistry’
1996). The nanoparticles were purified by
precipitation and centrifugation, then stored at room temperature. Absorption
spectra were monitored on a Shimadzu spectrophotometer (slit = 1.0 nm).
Fendler et al's method for
finding the absorption onset and band-gap energies was used with the
absorption data. Photoluminescence spectra were recorded using a Spex® FluoroMax® spectrofluorometer. The emission
spectra were performed with an excitation wavelength of 475 nm and slit-widths
of 2.0 nm. All spectra were corrected for the detector's wavelength-dependent
The Results that
Emerged from this Experiment
QDs in layered solutions (CCL4 below; water, above) under ambient
and UV light are shown in Figure 1. QDs coated with tri-n-octyl phosphine oxide
remain in the organic layer, while those coated with mercaptoacetic acid are
in the aqueous layer.
Figure 1. QDs coated with tri-n-octyl phosphine oxide
(tri) and mercaptoacetic acid (mer) under (a) ambient and (b) ultraviolet illumination.
The upper layer is water; the lower layer is CCL4.
Alloyed Quantum Dots Using Absorption and Photoluminescence Spectroscopy
A range of alloyed QDs were examined via absorption and photoluminescence spectroscopy
(see figure 2, below). Comparative literature values for bulk alloys
are included. The data reveal resolved electronic transitions, plus
fluorescence emission at the band-edge. Note the unexpected depression
in band-gap for all nanoparticle sizes at
about 60% Te. The generally successful Vegard's
law for thin-film and bulk alloys is linear:
= xEA + (1 - x)EB
where x = mole fraction, and EA, EB and Ealloy
are the band-gaps for pure materials A, B and alloy of A and B respectively.
Vegard's law, however, is only a first approximation, and others have found
this "optical bowing" in bulk CdSeTe, so this effect is not solely caused by
Figure 2. Composition versus absorption and emission
energies for CdSe1-xTex nanoparticles. (a) Absorption
and photoluminescence of CDSe0.34Te0.66 QDs; (b) absorption-energy
onset related to Te content; (c) emission peak-wavelength versus Te content.
the Behaviour of Quantum Dots When Examined with Absorption and Photoluminescence
Zunger et al suggest the observed effects arise because of: (a) the various
ionic sizes in the alloy; (b) the various electronegativities of these ions; and (c) the binary structures
of these ions have various lattice constants. Relaxation of the ionic
bonds to equilibrium positions may lead to local order in the structure
and a larger than expected reduction in the band-gap.
Particle size and composition can
control quantum confinement. These QDs may
be useful for molecular imaging in living systems, because of their
near-IR and far-red fluorescence, where deep-tissue imaging is necessary,
away from blood and water light-absorption QDs
also provide absorption coefficients an order-of-magnitude larger than
typical organic dyes. The ultra-sensitive Spex® FluoroMax® spectrofluorometer is useful in
a wide array of research related to nanostructures and materials science
for the future.
Note: A complete set of references can be found by referring
to the original document.
Source: “Photoluminescence Spectroscopy of Quantum Dots, monitored
by the Spex FluoroMax” - Application note by Horiba Scientific.
For more information on this source please visit Horiba