First detailed look at how charge transfer distorts a molecule’s structure

Charge transfer is highly important in most areas of chemistry, including photosynthesis and other processes in living things. A SLAC X-ray laser study reveals how it works in a molecule whose lopsided response to light has puzzled scientists for nearly a decade.

When light hits certain molecules, it dislodges electrons that then move from one location to another, creating areas of positive and negative charge. This “charge transfer” is highly important in many areas of chemistry, in biological processes like photosynthesis and in technologies like semiconductor devices and solar cells.

Even though theories have been developed to explain and predict how charge transfer works, they have been validated only indirectly because of the difficulty of observing how a molecule’s structure responds to charge movements with the required atomic resolution and on the required ultrafast time scales.

In a new study, a research team led by scientists from Brown University, the Department of Energy’s SLAC National Accelerator Laboratory and the University of Edinburgh used SLAC’s X-ray free-electron laser to make the first direct observations of molecular structures associated with charge transfer in gas molecules hit with light.

Molecules of this gas, called N,N′-dimethylpiperazine or DMP, are normally symmetric, with a nitrogen atom at each end. Light can knock an electron out of a nitrogen atom, leaving a positively charged ion known as a “charge center.”

Read more on the SLAC website

Image: In experiments with SLAC’s X-ray free-electron laser, scientists knocked electrons out of a molecule known as DMP to make the first detailed observations of how a process called charge transfer affects its molecular structure. Left: DMP is normally symmetric. Center: When a pulse of light knocks an electron out of one of its nitrogen atoms (blue spheres), it leaves a positively charged ion known as a charge center, shown in pink. This creates a charge imbalance that shifts the positions of atoms. Right: But within three trillionths of a second, the charge redistributes itself between the two nitrogen atoms until it evens out and the molecule becomes symmetric again.

Credit: Greg Stewart/ SLAC National Accelerator Laboratory

Fe Cations Control the Plasmon Evolution in CuFeS2 Nanocrystals

Research on the synthesis of CuFeS2, an exciting semiconductor, outlines a method to verify its phase purity and investigate its properties.

Plasmonic semiconductor nanocrystals have become an appealing avenue for researching nanoscale plasmonic effects due to their wide spectral range (visible to infrared) and great tunability compared to traditional precious metal nanocrystals. CuFeS2 is an exciting semiconductor that has a prominent plasmon absorption band in the visible range (∼498 nm). In this work, the researchers determined the origin of the plasmonic behaviour in CuFeS2 by characterizing the nucleation and growth stages of the reaction through a series of ex situ and in situ probes (e.g., X-ray absorption spectroscopy and X-ray emission spectroscopy). They showed that the plasmon formation is driven by band structure modification from Fe(II) incorporation into the nanocrystals. Mixed oxidation state of Cu(I)/Cu(II) and Fe(II)/Fe(III) was observed.  Using these results, the researchers proposed a reaction mechanism for synthesis of CuFeS2 and outlined a method to verify the phase purity of the material.

Read more on the CHESS website