The game-changing applications of graphene, a one-dimensional material with a zero bandgap, have spurred the search for two-dimensional (2D) materials with larger bandgaps and tunable thermal and optoelectronic properties. In a surprising discovery along the path to characterizing the candidate material tungsten diselenide (WSe₂), researchers from the University of Connecticut (UConn) and Los Alamos National Laboratory (LANL) have found that the optical properties of this 2D semiconductor are influenced by its isotopic form. As they report in a recent issue of Nano Letters, this suggests that the material’s properties can be intrinsically tuned by controlling the ratio of isotopes.

WSe₂ is one of the transition metal dichalcogenides (TMD), a group of atomically thin materials that have a transition metal layer sandwiched between two chalcogen layers. TMDs are emerging as leading candidates in the search for next-generation component materials because of their high spin-orbit coupling, strong interaction with light, and other appealing optoelectronic properties.

“[2D TMDs] have potential applications in energy conversion, electronics, and quantum computing based on unique properties that arise out of their low dimensionality,” says Michael Pettes, a scientist at LANL. Realizing these applications requires a thorough characterization of the optical, electronic, and vibrational properties of TMDs and methods for precisely tuning them. Researchers have tuned the properties of TMDs by external forces, such as the application of tensile strain or electric fields, but until now there have been no reported means of intrinsically tuning the materials.

In this new research, Pettes and Wei Wu, a then-graduate student at UConn, synthesized two bilayer samples of WSe₂—the naturally abundant (NA) form that includes five isotopes of tungsten and six of selenium, ^NAW^NASe₂, and the isotopically pure form ¹⁸⁶W⁸⁰Se₂. The single-crystal samples were grown on silicon substrates by chemical vapor deposition (CVD) under identical conditions.

Using x-ray diffraction and temperature-dependent Raman and photoluminescence spectroscopy, Pettes and Wu worked with UConn’s Mayra Daniela Morales-Acosta to characterize the optical, electronic, and vibrational properties of the samples over the temperature range of 4–300 K.

WSe₂ has an indirect bandgap, so its electronic transitions involve the emission or absorption of phonons (atomic vibrations). Since phonon energies depend on atomic mass, the researchers anticipated a difference in vibrational spectra. However, they predicted no measurable difference in electronic band structure or optical emission because the samples differed only in the number of neutral particles (the definition of an isotope).

To their surprise, the optical spectrum of the isotopically pure sample was blue-shifted by ~4 meV across the full temperature range. “[W]hile we fully expected to see a large change in the vibrational spectra as we changed the atomic masses, we were very surprised that we also saw changes in the light emission spectra,” Pettes says. “The effect is so unexpected and computationally difficult that theorists usually don’t consider isotopic make-up when modeling these properties,” he says.

To confirm that their results reflected an isotope effect and not a difference in crystalline quality, the researchers collaborated with LANL’s Yongqiang Wang to characterize the sample quality and precisely measure the atomic ratios using ion beam analysis. The results revealed that the two samples had the same high-quality crystal structure and the expected atomic ratios.

“This is the first report of an isotope effect in a TMD,” says Joan Redwing, an expert on the synthesis and characterization of 2D optoelectronic materials at The Pennsylvania State University. “Looking at isotopically pure materials has been an important area of research for a number of years because the presence of a range of different isotopes can impact the electronic properties and vibrational properties of materials, and other properties like thermal conductivity,” she says. Redwing notes that the researchers not only performed the study but also synthesized an isotopically pure TMD. “This is an impressive contribution,” she says.

Pettes and his team propose that the optical shift is due to a slight difference in phonon energy, stemming from the difference in atomic masses, that effectively shifts the optical bandgap energy. The researchers are now looking at isotopically purified materials with ultrafast spectroscopy to better understand the transition dynamics. Their discovery suggests that isotope enrichment could be a means to modifying the optoelectronic properties of other 2D materials with similar structures and will likely spur experimental and theoretical research in this direction.

Read the abstract in Nano Letters.