Research led by scientists at the Department of Energy’s Oak Ridge National Laboratory has shown that small changes in the isotopic content of thin semiconductor materials can impact their optical and electronic properties. This opens the door to new and advanced designs with the semiconductors. Semiconductors play a crucial role in developing electronic devices, and researchers have long sought ways to enhance semiconductor compounds to influence how they conduct electricity. Isotope engineering, which involves changing the physical, chemical, and technological properties of materials by using isotopes, is one approach to achieve this.
Isotopes are variants of an element with the same number of protons but different numbers of neutrons and masses. Traditionally, isotope engineering focused on enhancing bulk materials with uniform properties in three dimensions. However, new research by ORNL has advanced isotope engineering to 2D materials, where current is confined in flat crystals only a few atoms thick. ORNL scientist Kai Xiao stated that they observed an isotope effect in the optoelectronic properties of molybdenum disulfide when a heavier isotope of molybdenum was substituted in the crystal. This effect presents opportunities to engineer 2D optoelectronic devices for various applications like microelectronics, solar cells, and photodetectors.
A member of Xiao’s team, Yiling Yu, grew isotopically pure 2D crystals of molybdenum disulfide using molybdenum atoms of different masses. Yu noticed small shifts in the color of light emitted by the crystals under light stimulation. The light from the molybdenum disulfide with heavier molybdenum atoms displayed a shift towards the red end of the spectrum, contrary to what was expected for bulk materials. Through collaborations with theorists at the University of Central Florida, Xiao and his team identified how phonons scatter excitons in unexpected ways in the confined dimensions of ultrathin crystals, leading to the red shift in optical bandgap for heavier isotopes.
ORNL’s Alex Puretzky demonstrated how different crystals grown on a substrate can exhibit small shifts in emitted color due to regional strain in the substrate. To validate the anomalous isotope effect and measure its magnitude, Yu grew molybdenum disulfide crystals with two molybdenum isotopes in one crystal. This groundbreaking approach allowed the team to investigate the intrinsic anomalous isotope effect on the optical properties in 2D materials without interference from an inhomogeneous sample. The study revealed that even a slight change in isotope masses in atomically thin 2D semiconductor materials can influence their optical and electronic properties significantly, laying the foundation for further research.
The research suggests that by changing the isotopes of a semiconductor material, it is possible to tune its optical and electronic properties to design new applications without needing to combine multiple materials to create junctions. The ability to create isotopic junctions within the same material could have implications for the development of devices like photovoltaics and photodetectors. To further explore the isotope effect on spin properties for applications in spin electronics and quantum emission, Xiao and his team plan to collaborate with experts at ORNL’s High Flux Isotope Reactor and Isotope Science and Engineering Directorate.
This study was supported by the DOE’s Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, and was conducted at the Center for Nanophase Materials Sciences (CNMS) at ORNL, an Office of Science user facility. The CNMS supported various measurements in the research, such as TOF-SIMS, STEM, and optical spectroscopy. The availability of highly enriched isotope precursors from facilities like the High Flux Isotope Reactor and Isotope Science and Engineering Directorate will enable the team to grow different isotopically pure 2D materials for further investigation into isotope effects on spin properties and other applications.
