Theme
Crystalline materials with nanometer-scale physical dimensions often display different
properties than bulk crystals of the same compounds, because they are smaller than
characteristic length scales for light absorption and scattering, excited electronic
states, and charge transport (conductivity). This is most readily seen in the size-dependent
absorption and emission spectra of fluorescent semiconductor nanocrystal “quantum
dots.”
Our group develops surface-sensitive metrics, purification strategies, and synthetic
steps for QDs and other colloidal nanocrystals that permit increasingly precise and
sophisticated control of the resulting physical and chemical properties. We are also
interested in the transport of matter, charge, and energy within nanoscale systems
and across interfaces. We use microfabrication, optoelectronic measurements, and functional
imaging techniques to characterize these transport processes. An ultimate goal of
our work is to improve the performance of QD solar cells and other optoelectronic
devices based on nanostructured materials, and to advancing the biomedical applications
of nanocrystal-based imaging and therapeutic agents. These themes are explored in
three main project areas.
Purification & metrics for sequential chemistry of shell growth and ligand exchange with colloidal semiconductor nanocrystals
Nanocrystal quantum dots (QDs) are soluble, nanometer-scale particles composed of
semiconductor materials. QDs can have size-tunable absorption and fluorescence due
to quantum confinement of states available to electronics within them. Quantum dots
are now ubiquitous in fluorescent backlights for flat panel TVs, computer monitors,
and mobile devices because their narrow emission spectra allow the rendering of highly
saturated colors. However, only a limited understanding exists of many details of
the chemical and physical properties of colloidal quantum dots. A key challenge in
this regard is that colloidal nanocrystals (NCs) such as QDs are complex assemblies
of a crystalline core and an interfacial ligand layer that, given time, may exchange
matter with the solution and other NCs.
Our group has emphasized gel permeation chromatography (GPC) as a general approach
to purification of NCs in anhydrous solvents, by separating the NCs from small molecule
impurities and weakly bound ligands on the basis of size. We are using these purified
QDs to investigate the role of surface ligands in controlling QD brightness and decay
rate dispersion. We are also conducting quantitative investigations of ligand binding
to nanocrystal surfaces, using purified NCs as a well-defined initial state.
We also investigate nanocrystal growth processes, including work on selective ionic
layer adhesion and reaction (SILAR) and related processes for the formation of high
quality core/shell QDs.
Biomedical applications of nanoparticles with well-defined surface chemistry
Nanoparticles are of interest for a variety of applications in bioimaging, such as
the use of QDs as labels and sensors in fluorescence microscopy, and as therapeutics,
such the use of nanoparticle carriers to overcome limitations of pharmacokinetics
and off-target adverse effects in delivery of drugs to combat cancer and heart disease.
Key requirements for biomedical applications of nanoparticles are a high degree of
solubility and colloidal stability in water, control of hydrodynamic size, elimination
of non-specific binding, and the ability to append specific targeting groups. Additionally,
advantageous physical properties such as fluorescence or magnetism of the core nanocrystal
must be maintained.
For quantum dots used in bioimaging, the exchange of native hydrophobic ligands for
hydrophilic ligands is a key strategy by which to achieve these requirements. We have
developed a family of methacrylate-based polymeric imidazole ligands (MA-PILs) that
possess multiple imidazole groups that can anchor the ligand to the surface of chalcogenide
QDs. The GPC purification and shell growth expertise developed within the group contribute
to a highly reliable process for formation of QDs with low non-specific binding to
cells and low acute toxicity. We have used these QDs to label the surfaces of enveloped
viruses and track their infection of target cells.
Transport processes in low-dimensional and assembled materials
We use electron and optical microscopy, spectroscopy, and electronic transport measurements
to explore the role of the surface in dictating the properties of semiconductor nanostructures:
recently, this has focused on assembled colloidal nanocrystal films.
Nanocrystals, including QDs and extended structures such as nanoplatelets and nanorods,
are of interest for solar energy capture in photovoltaic and photocatalytic systems
as they can absorb sunlight at energies above their bandgaps, can be deposited over
large areas on diverse and inexpensive substrates, and exhibit large junction areas
that could increase the rate at which absorbed light is captured as separated charges.
NCs are also of interest for building infrared detection capabilities into large-area
and flexible devices, and NC-crystalline hybrid architectures.
In addition to work on nanoscale semiconductors made in our lab, we investigate transport
processes in a variety of other material systems in collaborative efforts. These systems
include host-guest interactions in self-assembled macrocycle fibers (collaboration
with Prof. Linda S. Shimizu) and energy transfer processes between donor and acceptor
chromophores organized in metal-organic frameworks (collaboration with Prof. Natalia
B. Shustova). We have also used our scanning photocurrent microscopy system to investigate
epitaxial graphene and wide-bandgap carbide, nitride, and oxide-based single crystal
semiconductor devices in collaborations with Profs. MVS Chandrashekhar and Asif Khan
in USC’s Department of Electrical Engineering and Computing.