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 for 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, with recent work focusing on selective ionic layer adhesion and reaction (SILAR) and related processes for the formation of high quality core/shell QDs. Our work has emphasized the role of precursor conversion to surface-bound equivalents in eliminating undesired cross-reaction between the shell precursors.
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 such as NWs and assembled colloidal nanocrystal films.
Both NCs and one-dimensional nanowires (NWs) 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 on diverse and inexpensive substrates, and exhibit large junction areas that could increase the rate at which absorbed light is captured as separated charges. Semiconductor NWs are synthesized in our lab by using a catalyst nanoparticle to direct growth in only one crystal direction – and can be synthesized “dry” using vapor-phase precursors. The solvent-free vapor-liquid-solid (VLS) growth method provides the thermal budget to achieve low bulk defect densities, so that carrier mobilities can approach bulk values; this can decrease the series resistance of solar cells. However, compared to colloidal NCs, less attention has been given to the surface chemistry of compound semiconductor NWs. Recently, we have demonstrated the MA-PIL ligands can associate to the surface of previously unmodified, VLS-grown CdS NWs.
In addition to work on NCs and NWs grown 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). A recently-completed scanning photocurrent microscope creates opportunities for collaboration in the investigation of optoelectronic devices.