American researchers have demonstrated that small changes in the isotopic composition of thin semiconductor materials can influence their optical and electronic properties. This discovery potentially opens the way to new advanced designs using these semiconductors.
The importance of semiconductors in modern electronics
Semiconductors play a crucial role in the constant evolution of electronic devices and systems, which become more advanced and sophisticated every day. For decades, researchers have studied ways to improve semiconductor compounds to influence the way they conduct electrical current. One approach is to use isotopes to modify the physical, chemical and technological properties of materials.
Isotopes are members of a family of elements that all have the same number of protons but a different number of neutrons, and therefore a different mass. Isotopic engineering has traditionally focused on improving so-called materials.in bulk”, which exhibit uniform properties in three dimensions (3D).
A new frontier in 2D isotope materials engineering
However, new research led by the U.S. Department of Energy’s Oak Ridge National Laboratory has advanced the frontier of isotope engineering where current is confined in two dimensions (2D) within flat crystals and where a layer is only a few atoms thick. These 2D materials are promising because their ultrathin nature could allow precise control of their electronic properties.
Kai Xiao, a scientist at ORNL, explains: “We observed a surprising isotope effect in the optoelectronic properties of a single layer of molybdenum disulfide when we substituted a heavier isotope of molybdenum into the crystal, an effect that opens opportunities for designing 2D optoelectronic devices for microelectronics, solar cells, photodetectors and even next-generation computing technologies.»
An unexpected isotope effect in molybdenum disulfide crystals
Yiling Yu, a member of Kai Xiao’s research team, grew isotopically pure 2D crystals of atomically thin molybdenum disulfide using molybdenum atoms of different masses. Yu noticed small shifts in the color of the light emitted by the crystals under photoexcitation, that is, under stimulation by light.
“Unexpectedly, the light from molybdenum disulfide containing the heavier molybdenum atoms was shifted more toward the red end of the spectrum, which is the opposite of the shift one would expect for bulk materials“, Kai Xiao clarified. This red shift indicates a change in the electronic structure or optical properties of the material.
Understanding exciton diffusion mechanisms in ultrathin crystals
Xiao and his team, working with theorists Volodymyr Turkowski and Talat Rahman of the University of Central Florida, knew that phonons, or crystal vibrations, must disperse excitons, or optical excitations, in unexpected ways within the confined dimensions of these ultra-thin crystals. They discovered how this diffusion shifts the optical bandgap towards the red end of the light spectrum for heavier isotopes.
There “optical bandgap» refers to the minimum amount of energy required for a material to absorb or emit light. By tuning the bandgap, researchers can make semiconductors absorb or emit different colors of light, and this tuning ability is essential for designing new devices.
Synthesis of 2D crystals with two isotopes of the same element
ORNL’s Alex Puretzky described how different crystals grown on a substrate can exhibit small shifts in emitted color, caused by regional stress in the substrate. To prove the anomalous isotope effect and measure its magnitude to compare with theoretical predictions, Yu grew molybdenum disulfide crystals with two isotopes of molybdenum in a single crystal.
“Our work was unprecedented in that we synthesized a 2D material with two isotopes of the same element but with different masses, and we joined the isotopes laterally in a controlled and stepwise manner into a single single-layer crystal.“, added Kai Xiao. “This allowed us to observe the intrinsic anomalous isotope effect on the optical properties in the 2D material without the interference caused by an inhomogeneous sample.»
Prospects for new applications thanks to isotopic engineering
The study demonstrated that even a small change in isotopic mass in atomically thin 2D semiconductor materials can influence optical and electronic properties – a finding that provides an important basis for further research.
Kai Xiao explains: “Previously, it was thought that making devices such as photovoltaics and photodetectors required combining two different semiconductor materials to create junctions to trap excitons and separate their charges. But in reality, we can use the same material and simply change its isotopes to create isotope junctions in order to trap the excitons. This research also tells us that through isotope engineering we can tune optical and electronic properties to design new applications.»
Illustration caption: It is surprising to note that changing the isotopic masses of molybdenum in a single layer of semiconductor molybdenum disulfide changes the color of the light emitted when the layer is illuminated. The study revealed the potential of isotope engineering to design new technologies in 2D materials. Credit: Chris Rouleau/ORNL, US Dept. of Energy
Article: “Anomalous isotope effect on the optical bandgap in a monolayer transition metal dichalcogenide semiconductor” – DOi: 10.1126/sciadv.adj0758
