In recent years, isotope effects have received increasing attention in materials science and engineering due to the direct effect of isotope alteration on phonons. However, the impact of isotopic mass on the optoelectronic features of two-dimensional semiconductors remains a mystery because of measurement uncertainties.
(Photo: Wikimedia Commons/ K 27)
Challenges in Isotopic Engineering
Electronic devices and systems become more sophisticated due to advances in semiconductors. For this reason, scientists have investigated ways to improve semiconductor compounds to influence how they carry electrical current. One possible approach is utilizing isotopes to change materials' technological, physical, and chemical properties.
Isotopes refer to atoms with the same number of protons but different numbers of neutrons. Traditionally, isotope engineering has focused on improving bulk materials that have uniform characteristics in 3D. However, previous studies have advanced the boundary of isotope engineering where current is limited to two dimensions inside flat crystals. The 2D materials were found to be more promising because having only a few atoms thick allows precise control over their electronic behavior.
Utilizing the Potential of Tweaking Isotopes
At the Department of Energy's Oak Ridge National Laboratory (ORNL), scientists have demonstrated that small changes in the isotopic content of thin semiconductor materials can affect their optical and electronic properties. Their findings are described in the paper "Anomalous isotope effect on the optical bandgap in a monolayer transition metal dichalcogenide semiconductor."
Led by Yiling Yu, the experts grew isotopically pure 2D crystals of thin molybdenum disulfide using molybdenum atoms of various masses. They noticed small shifts in the color of light released by the crystals through photoexcitation.
The research team observed a surprising isotope effect in the optoelectronic properties of an individual layer of molybdenum disulfide when the heavier isotope of molybdenum in the crystal was substituted. This opens up opportunities to engineer two-dimensional optoelectronic devices for solar cells, microelectronics, photodetectors, and even next-generation computing technologies.
The researchers knew that the crystal vibrations must be scattering the optical excitations in the confined dimensions of the ultrathin crystals. They just discovered how much scattering changes the optical bandgap to the red end of the light spectrum for isotopes with heavier masses.
The optical bandgap describes the minimum energy required to make a material absorb or emit light. By adjusting the optical bandgap, the experts can make semiconductors absorb or release various light colors, and such a feature is essential in designing new devices.
This study demonstrates that even a slight change of isotope masses in atomically thin two-dimensional semiconductor materials can influence their optical and electronic properties. This recent work was unprecedented in that the scientists synthesized a two-dimensional material with two isotopes of the same element but with different masses.
The research team plans to work with scientists at the High Flux Isotope Reactor and the Isotope Science and Engineering Directorate at ORNL. These facilities can offer highly enriched isotope precursors to grow various isotopically pure two-dimensional materials. They will also allow the team to investigate further the isotope effect on spin properties used in spin electronics and quantum emissions.
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