Preparation and Assembly of Energy-Related Materials

Semiconductor nanocrystals with various shapes such as, quantum dots, quantum wires, quantum platelets, and quantum rods have shown promise in developing photovoltaic devices and light-emitting diodes. The success of these nanocrystals in modern technology has prompted nanoscience researches to design effective colloidal synthetic methods for gram-scale production of high quality nanocrystals. Commonly used hot injection methods produce high quality nanocrystals, however, quantity of synthesized nanocrystals is small.  This obstacle can be solved by synthesizing nanocrystals at low temperature, with lower reactor volume, and using highly reactive precursors. Our group has been actively working to optimize these experimental conditions for the synthesis of high quality nanocrystals in gram scale quantity.

Surface passivating ligands are ubiquitous in colloidal nanocrystal synthesis. Apart from preventing nanocrystals from aggregation, surface passivating ligands control nanocrystal photoluminescence, carrier mobility, and charge transport properties and thus modulate the efficiency of photovoltaic devices. We have been investigating experimentally ligand-induced manipulation of optoelectronic properties of chalcogenide semiconductor nanocrystals by controlling delocalization of their exciton wave functions. Our recent work showed an unprecedentedly large and controllable change in the optical band gap (up to 107 nm, 610 meV) of ultrasmall CdSe nanocrystals by passivating their surface with exciton delocalizing conjugated ligands (e.g., phenyldithiocarbamates, PDTCs and metal-carboxylate). Simultaneously, we conduct density functional theory (DFT) calculations to support our experimental results. We are pursuing the hypothesis that the chemical nature (mode of binding, binding head groups, extend of pi-conjugation, etc.) of the surface passivating ligands controls the exciton delocalization phenomena of chalocogenide nanocrystals.

Over the last five years, hybrid organometal halide perovskites have received substantial attention as low-cost, direct band-gap semiconductors for fabrication of a new generation of solar cells and light-emitting diodes. In this context, solar cell fabrication with either thin-films or bulk single crystals of organometal halide perovskites has demonstrated certified power conversion efficiency as high as 22.1% (in 2016). Because of the success of efficient thin film metal chalcogenide nanocrystal-based device fabrication, we expect that anisotropically-shaped, quantum confined hybrid perovskite nanocrystals will provide unique optoelectronic and charge transport properties, thus resulting in highly efficient solar cells fabrication. At present manipulation of the shape of perovskite nanocrystals is done mostly by trial-and-error experimental approaches. Our long-term research objective is to advance our scientific understanding of structure-property relationships in hybrid perovskite nanocrystals by investigation how anisotropically-shaped nanostructures can be grown in solution by programmable manipulation of reaction conditions (e.g., surface ligand chemistry, solvent, and temperature). Furthermore, we are also exploring the unique surface chemistry that is expected to assemble nanocrystals into ordered superlattices.