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Author: AndyExtance 2019-02-14T19:00:00+00:00

CdS nanocrystals reversibly transform between "wurtzite" and "sphalerite" forms in one step

Scientists in the United States and Israel have captured an unprecedented transformation in inorganic clusters—a change similar to the isomerization of organic compounds. Uri Banin of the Hebrew University of Jerusalem said that CdS clusters represent the "ultimate bridge" between nanocrystals and molecules. "We now know that if the inorganic material is reduced to a small enough size, it can act like an organic molecule," adds Richard Robinson, a Barning collaborator at Cornell University. "The impact is huge."

In bulk crystalline materials, structural changes occur through phase transitions, which start from a nucleation event and then propagate through the material. At the other end of the length scale, small molecule isomerization occurs in a coherent step. The Cd37S20 cluster studied by Banin and Robinson exists between these two extremes and has the characteristics of both. "It is worth noting that [cluster] reversibly transforms between two "isomers" or "phases" in a consistent manner," Baning told Chemical World.

Source: © Curtis B. Williamson et al./Science/AAAS

The isomerization of small organic molecules and the phase transition of a large number of inorganic solids are at both ends of the length scale and have different mechanisms. Inorganic clusters are somewhere in between and have two changing characteristics. 

The discovery was made during Robinson’s visit to Barin in Jerusalem, and his doctoral student Curtis Williamson sent samples of their new cluster. In Israel, scientists discovered that these star clusters absorb ultraviolet light of a different wavelength than expected. Robinson said that after confirming that it was indeed the correct sample, they realized that the sample had changed during the flight, which "caused a series of problems and experiments."

Back at Cornell University, Robinson and Tobias Hanrath's team created a high-purity cluster that absorbs at 324nm, which they designated as α-Cd37S20. After exposing the thin films of these clusters to methanol vapor, the peak weakened and a different narrow absorption peak appeared at 313nm, which they named β-Cd37S20. Heating to remove the methanol and restore the structure to its original shape, scientists repeated this change many times to measure the conversion rate. 

Source: © Curtis B. Williamson et al./Science/AAAS

When α-Cd37S20 adsorbs methanol, it will be transformed into β-Cd37S20, and the adsorption spectrum will change accordingly. The conversion during alcohol adsorption and desorption is reversible (illustration)

The researchers used X-ray total scattering to analyze the crystal structure of the isomers and used advanced pair distribution function analysis to accurately determine the positions of the atoms. They showed that the α-isomer has a "wurtzite-like" structure, while the β-isomer has a "sphalerite-like" structure. Hanrath pointed out that although these clusters are not completely similar to the bulk phase, this structural change is like the phase change between the hexagonal and cubic crystal forms of CdS. 

The scientists also showed that methanol triggers isomerization by changing the arrangement of surface carboxylate ligands. This, the first-order kinetics, and the lack of any intermediate states between the α and β forms, indicate that the change occurs in a single, coherent, isomerization-like step, which Hanras calls the "revelation."

"This is a wonderful chemical reaction," said Brand Cossaert of the University of Washington. "This phenomenon lies between two well-known worlds: solid-solid transformation and molecular isomerization." Cossaert said the team provided "convincing evidence that there are coherent structural differences in inorganic nanoclusters. Structured".

Robinson expects to find similar single-step transitions in other semiconductor and metal clusters, because the initiation of the surface may be related to other material systems. He warned that applications are difficult to foresee, but said they may be used as switches and sensors between two states. In addition, this transformation can help produce larger nanocrystals. Robinson added: "Although it is quite challenging, it would be tempting to solve this transition directly in time." "We expect this to happen on a time scale of 100fs."

CB Williamson et al. Science, 2019, DOI: 10.1126/science.aau9464

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