Texas chemists learn dangers of adding too many change-acceptor molecules to semiconducting nanocrystals

A new study has found that adding too many charge-acceptor molecules to the surface of semiconducting nanocrystals can be detrimental.

Chemists with Rice University and the University of Texas (UT) at Austin led a joint effort in a study that revealed packing charge-acceptor molecules on the face of semiconducting nanocrystals can diminish electron transfer rates, according to a news release from Rice University.

This revelation goes against the idea that raising the number of charge acceptors on the surface of a semiconductor crystal enhances the rate of electron transfer.

The researchers found that at a certain point packing more ligands onto a crystal’s surface can give rise to interactions that slow down electron transfer rates. 

Team members created hybrid nanomaterials by combining nanocrystals of light-capturing semiconductors with charge acceptor molecules that act as ligands, attaching to the semiconductor’s surface and transporting electrons away from the nanocrystals.

The chemists then methodically studied hybrid materials encompassing lead sulfide nanocrystals and contrasting strengths of the organic dye, perylene diimide (PDI). What they found was that enhancing the strength of PDI on the surface of nanocrystals drops electron transfer rates.

Rice chemist Peter Rossky co-authored the study with Sean Roberts, an associate professor of chemistry at UT Austin. They realized the ligand-ligand connections between PDI molecules have on the geometries of PDI aggregates on crystal surfaces.

"The most-studied nanocrystal systems feature high concentrations of charge acceptors that are bound directly to the semiconducting crystals," Rossky said. "Generally people try to maximize the surface concentration of charge acceptors because they expect the rate of electron transfer to continuously increase with surface-acceptor concentration."

Rossky and Roberts are researchers for the Center for Adapting Flaws into Features (CAFF), a multi-university program backed by the National Science Foundation, which aims to exploit microscopic chemical defects in materials to make innovative catalysts, coatings and electronics.

Roberts said that their results demonstrate the importance of considering ligand-ligand interactions when designing light-activated hybrid nanocrystal materials for charge separation. 

While ligand aggregation can slow electron transfer in some circumstances, their computational models predict that it can also speed electron transfer in other circumstances. Interest in these hybrid nanomaterials is high, and the pace of scientific publication about them has grown more than tenfold over the past 20 years.