Chemists have well-tested methods to synthesize materials, but those methods do not provide access to every crystalline structure that is theoretically possible. Now scientists have demonstrated a new synthesis method that allows them to create materials they couldn’t make before, and discovered that some of those materials have desirable electronic and optical properties.
The researchers developed a flux, or high-temperature solvent, of hydroxide and halide molten salts that allow reactions to take place at lower temperatures than other processes. Their mixture contained lithium hydroxide and lithium chloride, and they used it to synthesize chalcogenides with two different structures containing strontium, silver, lithium, and selenium. Chalcogenides are compounds containing negatively charged sulfur, selenium or tellurium. Chalcogenides of silver are of particular interest because they have useful thermoelectric properties, but no version containing the three elements strontium, silver, and selenium has been made with traditional synthesis methods, and few other related compounds have been created.
The researchers say that making these materials at lower temperatures may allow them to form stable crystalline structures that are not accessible via higher temperatures. The research team found that, by changing the ratio of hydroxide to halide, they could tune how basic the solvent was. That in turn changed the final structure of the compound. Increasing the lithium hydroxide ratio increased the amount of lithium replacing the silver. Increasing the temperature of the reaction had a similar effect, so the scientists now have two ways to fine-tune the final product.
The crystals they created turned out to be photoluminescent direct bandgap semiconductors. Direct bandgap semiconductors are desirable for their efficient optical absorption and photoluminescence. They are sought after in optoelectronics, solar cells, and LEDs for their robust optical properties. Increasing the lithium makes the bandgap wider, and the ability to select the bandgap gives the researchers control over the electronic properties of the crystal. The ability to tune electronic and optical properties of the material could allow researchers to create better photodetectors, solar cells, or light-emitting diodes.
To see the different crystalline structures in their compounds and measure the lithium ratio, the researchers performed single-crystal X-ray diffraction at the National Science Foundation’s ChemMatCARS, Beamline 15-ID-D at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. The high-intensity beam from the APS was required because lithium diffracts X-rays only weakly. They also gathered complementary, though lower-resolution, data using a lab-based X-ray source, and compared data from both sources on two of their samples.
In the future, researchers would like to study the reaction further by running it in front of the APS beams to see how the structure evolves. They have performed similar studies on other materials, and seen that the elements form soluble structures or other intermediates, and then proceeded in stages until they create the desired compound. Overall, the group has published reports of more than 60 new materials. They say their work could lead to the synthesis of many more undiscovered compounds.
Going in, the researchers did not know their compounds would be direct bandgap semiconductors with photoluminescence – they were simply trying to understand the reaction pathway of the materials and where it would lead. The fact that the materials they created had interesting and potentially useful properties was a bonus. The two compounds they created are structurally related in a way that predicts the existence of similar compounds. That may allow scientists to build previously unknown semiconductor materials with properties that may prove to be technologically valuable. – Neil Savage
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See: X. Zhou1, B. Wilfong2, X. Chen1, C. Laing3, I.R. Pandey3, Y-P. Chen4, Y-S Chen4, D-Y Chung1, M.G. Kanatzidis1,3, “Sr(Ag1−xLix)2Se2 and [Sr3Se2][(Ag1−xLix)2Se2] Tunable Direct Band Gap Semiconductors,” Angewandte Chemie International Edition 62 14 e202301191 (2023)
Author affiliations: 1Argonne National Laboratory; 2United States Naval Academy; 3Northwestern University; 4University of Chicago.
This work was supported by the U.S. Department of Energy Officeof Science Basic Energy Sciences, Materials Sciences and Engineering Division. The use of the Advanced Photon Source (APS) and Center for Nanoscale Materials (CNM) at Argonne National Laboratory, an Office of Science user facility, was supported by the U.S. Department of Energy Office of Science under Contract No. DE-AC02-06CH11357. NSF’s ChemMatCARS Sector 15 at APS was supported by the Division of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1834750. The authors acknowledge the University of Maryland supercomputing resources made available for conducting the band calculations reported in this paper.
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