Pharmacists from Rice University and the University of Texas at Austin have discovered that it is not always better when it comes to supporting cells that accept charges on semiconductor nanocrystals, Other connections are less favorable for hybrid nanomaterials.
Combinations of organic and inorganic materials in hybrid nanomaterials can be designed to uniquely capture, detect, reflect or control light. Interest in these materials is high and the pace of scientific literature on them has increased more than tenfold in the past 20 years. For example, they can improve the efficiency of solar power systems by capturing energy from longer wavelengths of sunlight – such as infrared – that traditional photovoltaics miss.
To create those materials, chemists combine semiconductor nanocrystals that capture light with molecules that act as ligands, which attach to the surface of the semiconductor and transport electrons away from the nanocrystals.
“The most studied nanocrystal systems exhibit a large amount of receptors that are attached to semiconductor crystals,” said Rice chemist Peter Rossky, who co-authored a recent paper in the Journal of the American Chemical Society. “In general, people try to maximize the surface area of the acceptors because they expect that the rate of electron transfer increases and continues to increase the surface-acceptor .”
A few published experiments have shown that electron transfer initially increases with surface concentration, then decreases if the surface concentration increases. Rossky and collaborator Sean Roberts, an assistant professor of chemistry at UT Austin, knew that the molecular orbitals of ligands can interact in ways that can affect charge transfer, and they expected it to have time to pack more ligands. above the surface of the crystal will make such a relationship.
Rossky and Roberts are faculty researchers at the Rice-based Center for Adapting Flaws into Features (CAFF), an interdisciplinary university program supported by the National Science Foundation (NSF) that seeks to use microscopic chemical defects in materials to create materials that – create new things. , insurance and electronics.
To test their hypothesis, Rossky, Roberts and their CAFF colleagues carefully studied hybrids containing sulfide nanocrystals and various combinations of a commonly studied aromatic compound called perylene diimide (PDI). The experiments showed that the continuous increase in PDI and the surface of the nanocrystals eventually produced a significant decrease in the electron transfer rate.
Rossky says the key to that behavior is the effect of ligand-ligand bonding between PDI molecules on the geometry of the PDI aggregates on the crystal surface. Assembling the evidence showing the impact of these cumulative effects requires the expertise of each research group and the careful use of spectroscopic studies, electronic structure modeling and molecular dynamics simulations.
Roberts said, “Our results highlight the importance of considering ligand-ligand interactions when designing nanocrystalline hybrid materials for separation. We have shown that ligand aggregation can reduce electron transfer in some cases. But surprisingly, our computer model predicts that ligand stacking can also accelerate electron transfer in other cases.
