While previous high pressure optical and vibrational studies
on CdSe quantum dots (QDs) have observed no evidence for size
dependent scaling laws in the experimental pressure behavior,
we have found that low pressure studies exhibit anomalous pressure
dependencies that can be analyzed in two specific size regimes
(2r < ao and 2r > ao) corresponding to the limits of strong
and weak confinement of the exciton. By using photoluminescence
and resonance Raman spectroscopies, we find optical and vibrational
coefficients that significantly deviate from reported bulk values
(82 meV GPa -1 and 3.51 cm -1 GPa -1 , respectively) when 2r
< ao. At the largest QD sizes studied (2r = 80 Å),
bulk-like pressure dependence is observed. We believe this arises
from changes in the electron-electron and electron-phonon (el-ph)
coupling due to polaronic-type defects in the QD lattice. Analyses
of longitudinal optical mode Grüneisen parameters show
the smallest dots studied are extremely covalent in nature,
where el-ph coupling is highest, which may explain the observed
anomalies. A pressure dependent model is proposed which argues
that polaronic de-coupling occurs following a 1/r scaling law
arising from changes in electronic coupling to defect centers
in these dimensionally restricted materials.
Interested in learning more?
Contact Artjay
Javier
or read the published
articles:
"Pressure Induced Electronic Coupling
in CdSe Semiconductor Quantum Dots." Meulenberg, R.W.;
Strouse, G.F. Phys. Rev. B, 66, 035317 (2002).
[ view article - PDF ]
"Low Pressure Band Tuning in Wurtzite
CdSe Quantum Dots." Meulenberg, R.W.; Offen, H.W.; Strouse,
G.F. Mat. Res. Soc. Symp. Proc., 636, D9.46, 1-7 (2001). [view
article - PDF ]
The research effort on 2-dimensional self-assembled nano-crystalline
films is achieved via ‘evaporative assembly’; similar
to efforts in other groups on Ag and Au nano-dots. The nano-crystalline
CdSe materials can be surface exchanged with hexadecylamines
or hexadecyl thiols, producing materials that readily assemble
into hexagonally packed regions in the 2-d films, as illustrated
in the TEM image. We are currently probing the assembly dynamics
of these materials and have begun to investigate energy transfer
between large and small nano-crystals imbedded in the evaporated
film. A recent effort that shows promise is the assembly of
these materials using electrostatic interactions. Capping of
a 4.0 nm CdSe nano-crystal with an acid terminated alkyl thiol
and a 6.0 nm CdSe nano-crystal with amine functionality has
produced very regular films. We are currently using TEM imaging
to probe the structure of the assembled films. The investigation
of charge transport in these assembled films is ongoing.
In a recent study in our lab, we have initiated a study of organic
polymers as molecular scaffolding for CdSe nano-composites.
These polymer based nanocomposites exhibit efficient energy
transfer from the organc oligomers to the nanomaterial. Energy
transfer appears to be dominated by a dipole-dipole exchange
mechanism in which the size dependent nature of this energy
transfer process is clearly evident in the loss of quantum efficiency
for transfer above 5 nm. The quenching of the polymer by appended
nanoscale CdSe via directed energy transfer proceeds with near
unit quantum efficiency in the region where the spectral overlap
<J-integral> is favorable. At 5 nm the nature of the directed
energy transfer from the polymer to CdSe can be modulated by
photolysis. The observation of a photo-initiated switch in the
efficiency of energy transfer coupled to recent crystal structures
of the oligomers suggest that structural reorientation of the
composite may lead to the opening of transverse optical energy
transfer along a pi-pi stacking direction. We are developing
a series of oligomerically linked (n = 1, 3, 5) systems based
on PAO, phenylethynylene, appended to CdSe. The PAO is linked
to the CdSe through a benzylic thiol bond. The size dependent
nature of energy transfer allows fine control over energy transport
from the surface of the CdSe.
Interested in learning
more?
Contact Artjay
Javier
or read the published articles:
"Energy Transport in
CdSe Nanocrystals Assembled with Molecular Wires." Javier,
A.; Yun, C.S.; Sorena, J.; Strouse, G.F.
J. Phys. Chem. B, 107, 435-442
(2003).
[ view article
- PDF ]
|
