Research Themes
Our group is actively developing a research portfolio broadly
focused on single molecule energy and charge transfer studies of nanomaterials
with potential in solar energy conversion and biosensing. This includes hybrid
organic/inorganic nanomaterials, biological/inorganic nanostructures and
photonic (conjugated) polymers. We use self-assembly methods to connect various
donor and acceptor components and to obtain nanostructures with controlled
optical behavior. We also develop ultrasensitive nanoscale optical imaging
and probing methods for charge and energy transfer studies, and with single
molecule sensitivity. These methods are described in the
instrumentation page and can be accessed
through the BNL-CFN science user program
A. Hybrid Nanomaterials for Solar Energy Conversion
Photoinduced charge transfer and energy transfer are amongst the most important
processes occurring in hybrid photovoltaic solar cells. Nanoscale
characterization and control of such processes is an important research topic in our group.
We use self-assembly methods to create quantum dot-based hybrid nanomaterials
with controlled behavior, in particular charge transfer, energy transfer and plasmon-assisted
emission. Through self-assembly we regulate molecular parameters
such as intercomponent distance, bandgap or metal nanoparticle size
to control the magnitude and fluctuation of various light-induced processes .
Some examples highlighting recent results are shown below.
- Qdot-bridge-fullerene hybrids
We have self-assembled donor-bridge-acceptor dimers based on CdSe/ZnS Qdot and fullerene
with varying bridge length and bandgap and demonstrated control of electron
transfer rate and of fluctuations of electron transfer rate at the single
molecule level. With excellent, size-dependent light absorption properties
conferred by the incorporated Qdots, these inorganic/organic hybrids are promising power generating
nanounits for molecular electronics.
(see
related article in Angew.Chem.Intl.Ed.).
Using the same donor-bridge-acceptor hybrid system where the fullerene actc as a
well defined and well positioned charge trap towards the Qdots, we demonstrated
the effect of external charge traps on the blinking dynamics of isolated Qdots.
(see
related article in Small).
- Qdot/Conjugated Polymer hybrids
We have self-assembled a series of Qdot/conjugated polymer hybrids using
core/shell Qdots with varying shell thickness. In such systems, photoinduced
hole transfer, a desired photovoltaic process for PV applications but
detrimental for LED or biosensing, can be controlled by varying the thickness of
the Qdot shell, in this case ZnS. (see
related article in ACS Nano). Collaboration with Maye's group at
Syracuse U.
By linking individual Qdots with gold nanoparticles via DNA
linkers, we demonstrated the ability to enhance or quench the photoluminescence intensity of
Qdots when part of such hybrid dimers and due to plasmon-exciton interactions. The method of linking the nanoparticles (developed by Gang's group at BNL) involves a surface-based
stepwise DNA self assembly procedure to avoid formation of nanoclusters with
uncontrollable stoichiometry and it can produce Qdot-DNA-gold hybrid dimers with controllable interparticle
distance. Properly engineered interparticle
distances could lead to a photoluminescence enhancement of a single Qdot
of around 5 times on average, and up to 20 times maximal. (see
related article in ChemComm). Collaboration with Gang's group at
BNL.
B. Hybrid Nanomaterials for Biosensing
We have strong interests in the development of biosensing platforms
based on biological/organic and biological/inorganic hybrids that can achieve
high sensitivity, towards single analyte (biomolecule) level. We have so far
focused on developing protein/Qdot conjugates with multivalent binding
capability as detailed in
related article in Small and on developing label free DNA sensors
based on cationic conjugated polymers.
C. Structure-Function Relationship in
Conjugated Polymers.
Conjugated polymers are cheap materials with high promise in
photovoltaic, light emitting diode and biosensing applications. The electronic and optical properties of
conjugated
polymers are strongly dependent on the polymer chain structure visualized here
as conformation and aggregation state. Control and manipulation of the
conjugated polymer
structure with the aim of improving their optoelectronic properties is another
research topic of our group and several
recent results are presented below.
-Transparent Thin Films with Microporous Structure for Solar Light
Harvesting.
We developed a self-assembly method based on the breath-figure
technique to fabricate highly transparent thin films based on blends of
conjugated polymers and fullerene (Collaboration with H.Wang at LANL). These
films exhibit exhibit highly efficient charge transfer
and charge transport while at the same time high transparency. The obtained thin films show structural regularity over
areas up to 1mm2 and consist of hexagons with tunable size, 1-10μm, and with
most polymer materials concentrated in the hexagonal frame. (see
related article in Chemistry of Materials). Since our first
demonstration, we have been able to apply the deposition method to a variety of
polymer/fullerene blends and to scale up the method to the level of integration
onto photovoltaic device, an effort we currently pursue in our group.
-Control of Polymer Chain Conformation and Optical Properties of
Nonionic Conjugated Polymers.
Water soluble nonionic conjugated polymers are explored in organic light
emitting diodes due to their environmental friendly processing. By tuning
polymer/solvent interaction we demonstrated control of chain conformation, film
morphology, and photophysical properties in the case of a nonionic water-soluble
conjugated polymer. We used a combination of optical and scanning probe
microscopy methods to probe chain conformation down to the level of a single
polymer entity.
Depending on solvent polarity, we could master the polymer to exhibit extended,
coiled, and collapsed chain conformations in solutions, which lead to distinct
morphology and optical properties in solid films. (see
related article in J.Phys.Chem B).
-Side Chain Length-Dependent Optoelectronic Properties
of Ethylene Glycol Substituted PPVs
We recently demonstrated a series of water-soluble,
fluorescent, conjugated polymers with varying side chain length (ethylene glycol
repeat units) imposing water solubility and concomitantly leading to a
side-chain-dependent conformation and solvent-dependent photoluminescence
quantum efficiency. An increase in the ethylene glycol repeat units on the
polymer side chain structure results in changes in chain packing, with
crystallinity evolving from semicrystalline to liquid crystalline to completely
amorphous. At the same time, an increase in the length of the side chain leads
to changes in the polymer−olvent interaction as manifested in the photophysical
properties of these polymers. These novel polymers exhibit two glass transition
temperatures, which can be readily rationalized by differences in microstructure
when casted from hydrophobic and hydrophilic solvents. The potential of having a
nanoscaled domain structure and stabilizing two electrons on a polymer chain
signifies the potential of these polymers in fabricating electronic and
photovoltaic devices.(see
related article in ACS Applied Materials and Interfaces).
Collaboration with H.L.Wang at LANL.
Last Modified: June 28, 2012
