Researchers from Rice University and the University of Texas at Austin have found that adding too many charge-acceptor molecules to the surface of semiconducting nanocrystals can be detrimental. Hybrid nanomaterials, which are made of a combination of organic and inorganic components, have the potential to capture, detect, convert, or control light in special ways. These materials have gained a lot of interest in recent years, and the number of scientific publications about them has increased significantly over the past two decades.
One potential application for these materials is to improve the efficiency of solar power systems by harvesting energy from wavelengths of sunlight that traditional photovoltaic panels cannot capture, such as infrared.
To create hybrid nanomaterials, chemists combine nanocrystals of light-absorbing semiconductors with "charge acceptor" molecules that attach to the surface of the semiconductor and facilitate the transport of electrons away from the nanocrystals.
These charge-acceptor molecules function as ligands, which are substances that bind to a central atom or molecule to form a coordination complex. The resulting materials can be tailored to have specific properties and can be used in various applications, such as capturing, detecting, converting, or controlling light.
'Less is More'
To Peter Rossky, a chemist at Rice University and co-author of a recent study published in the Journal of the American Chemical Society, the most widely studied nanocrystal systems typically have high concentrations of charge acceptors bound directly to the semiconducting crystals. It is commonly believed that increasing the surface concentration of charge acceptors will result in a continuous increase in electron transfer rate. However, the research conducted by Rossky and his colleagues suggests that this may not always be the case and that there may be an optimal concentration of charge acceptors, beyond which adding more does not result in further improvements.
Rossky and his colleague Sean Roberts, an associate professor of chemistry at UT Austin, previous studies have suggested that the rate of electron transfer initially increases as the surface concentration of charge acceptors increases but then decreases if the concentration is increased further. Rossky and Roberts believed that the molecular orbitals of the ligands could interact in a way that would affect charge transfer and that there might be a point at which adding more ligands to the surface of a crystal would lead to these interactions. They conducted a study to investigate this possibility and discovered that there is indeed an optimal concentration of charge acceptors, beyond which adding more does not result in further improvements in electron transfer.
The two chemists are co-principal investigators at the Center for Adapting Flaws into Features (CAFF), a multi-university research program based at Rice University and supported by the National Science Foundation (NSF). CAFF aims to use microscopic chemical defects in materials to develop innovative catalysts, coatings, and electronics. CAFF brings together researchers from various institutions to collaborate on research projects to find new and innovative applications for materials with defects or imperfections.
Testing the Hypothesis
To test their hypothesis, Rossky, Roberts, and their colleagues at CAFF conducted a series of experiments in which they systematically studied hybrid materials containing lead sulfide nanocrystals and varying concentrations of perylene diimide (PDI). This organic dye has been widely studied in the field. The experiments showed that as the concentration of PDI on the surface of the nanocrystals increased, the rate of electron transfer initially increased but then sharply decreased beyond a certain point. This suggests that there is an optimal concentration of PDI beyond which adding more does not result in further improvements in electron transfer.
The scientists explained that the way influenced the behavior of the hybrid nanomaterials and that interactions between PDI molecules affected the geometry of PDI aggregates on the surface of the crystal. To understand these effects and how they impacted electron transfer, the researchers conducted a series of experiments using spectroscopic techniques, electronic structure calculations, and molecular dynamics simulations.
They found that while ligand aggregation can sometimes slow electron transfer, it can increase the electron transfer rate in other cases. These findings demonstrate the importance of considering ligand-ligand interactions when designing hybrid nanocrystal materials for charge separation. Rossky is the Harry C. and Olga K. Wiess Chair in Natural Sciences at Rice University and holds faculty positions in chemistry, chemical, and biomolecular engineering.
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