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Our research focuses on the preparation of
inorganic colloidal nanocrystals of various compositions, shapes, and
coatings, their organization on surfaces to form ordered structures, and
studies of their physical properties.
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One
important goal in “nano-electronics” is the ability to wire nanoscale
electronic components (molecules, nanocrystals) in dense arrays. For this
purpose we are developing techniques to obtain ordered arrays of metal
nanorods or nanowires on surfaces by wet chemical methods employing
self-assembly ideas.
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In
another set of projects we are producing two-dimensional ordered arrays of
magnetite (Fe3O4) nanocrystals. These arrays serve us
as for various interesting physical studies:
1.
Magnetite has very interesting electronic properties – it is ferrimagnetic and
is half-metallic at room temperature. That is, it is supposed to be a good
spin-polarized conductor. The bulk material shows a metal-insulator phase
transition around 120K and we were able to observe this phenomenon around 100K
in 5 nm nanocrystals. We were also able to obtain very large magnetoresistance
effects for electrons traversing through several layers of nanocrystals.
2.
The magnetic nanoparticle arrays are good model systems to study a system of
interacting dipoles as function of temperature and external field. There are
interesting physical issues to be studied such as phase transitions, dynamics of
switching of the magnetization vector of the nanocrystals and the effect of
array dimensionality on these issues.
We are currently looking at the temperature dependent noise in the tunneling
current in such arrays, both in lithographically fabricated and in scanning
tunneling microscopy tunnel-junctions. These noise characteristics reflect the
magnetization switching dynamics in the arrays.
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absorption and CD (circular
dichroism) in chiral molecules, such as bio-molecules using surface plasmon
excitations in metal nanostructures. As part of this we are studying also
the appearance of CD signal in the optical response of noble metal
nanostructures themselves, which is a signature of chirality. There are many
open questions regarding the mechanism that leads to this CD signal. |
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We
are studying the controversial subject of defect induced magnetism in
diamagnetic nanocrystals. We were able to control the defect concentration
in hafnium dioxide nanorods by tuning
the synthesis conditions to the level of developing ferromagnetism in part
of the nanorods. More high-resolution electron microscopy combined with
simulations should be able to tell us about the exact nature of the defects
leading to ferromagnetism in this material, which would be very useful for
theoreticians trying to model this form of magnetism.
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We have developed a
wet-chemical technique for the deposition of high aspect ratio gold/silver
nanowire mesh films by depositing the growth solution on a substrate and
letting the nanowire grow while the film is drying. This forms a new type of
transparent conductor films which we hope to exploit for various
applications. |
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We have recently began
working on ferroelectric nanocrystals. We produce colloidal barium titanate
nanocubes in the size range of 20-100 nm and use Electron Holography to
study their internal electric polarization in collaboration with the group
of Prof. Hannes Lichte from Dresden University. We are able to observe size
dependent polarization fields as well as other interesting effects in these
nanocubes. |
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