Both disulphides are unique semiconductor structures with high application potential in modern electronics and optoelectronics. In their article, physicists from WUT described the process of placing a single layer of carbon atoms (graphene) onto them. This scenario may occur, for instance, when connecting graphene electrodes to thin semiconductor layers of MoS₂ or WS₂.

A glimpse into new ideas at the nanoscale

“We have gathered extensive information on the expected changes in thermal expansion (and consequently, in the geometry of the entire structure) when covering a thin-film semiconductor, MoS₂ or WS₂, with a single layer of graphene,” says Konrad Wilczyński, PhD, from the Faculty of Physics, the lead author of the publication. “We have learned about the impact of interactions between individual layers of this two-layer ‘sandwich’ on the equilibrium geometry of the structure and its internal vibrations, also as a function of temperature. The strength of this interaction largely depends on how well these layers adhere to each other and at what angle. This opens up broad possibilities for engineering these interactions when designing future nanodevices.”

Quantum-mechanical simulations

At the core of these studies is the use of simulations based on fundamental physics at the microscopic level, which is essentially “observing” each atom and its valence electrons individually. Once a virtual model of the examined structure is created, interactions between any pair of its atoms can be studied, allowing for the calculation of crystal lattice vibrations.

By employing more advanced theoretical models, it is also possible to investigate temperature effects, in this case thermal expansion and the dependence of lattice vibrations on their amplitude. Our team also conducted precise spectroscopic measurements to experimentally study these relationships “on a living patient”. This is the result of two years of work, during which a technology for stacking any two-dimensional layers was developed. Much to the authors' satisfaction, both approaches led to remarkably similar results.

“We used several original computational tricks to enable the consideration of all relevant temperature effects, even for such complex structures as MoS₂ and WS₂ covered with graphene,” emphasizes dr Wilczyński. “Without these newly developed methods, we would have been forced – like other research groups – to make excessive simplifications, ultimately losing the excellent agreement between simulation results and reality.”

More challenges on the horizon

The conducted research is just the beginning of what can still be explored in the field of multilayer two-dimensional structures. The unique combination of expertise in sample fabrication and theoretical modelling opens the door to studying increasingly sophisticated samples. As dr Wilczyński notes, one of the remaining challenges is the possibility of covering thin layers with an insulating material such as hexagonal boron nitride (hBN), which is just as important as graphene.

Additionally, there are still cases to explore where thin layers are stacked at different angles, potentially leading to unexpected physical properties in the entire structure. Furthermore, understanding crystal lattice vibrations is a crucial step toward the future engineering of thermal conductivity in such structures.

Please refer to the article: Phonon Anharmonicity and Thermal Expansion in Two-Dimensional Graphene/MoS₂ and Graphene/WS₂ Heterostructures: DFT and Raman Study (K. Wilczyński, A.P. Gertych, M. Zdrojek) (IF = 8.3).

The project was funded by the Young PW grant, led by Konrad Wilczyński, PhD. Young PW is a competition conducted as part of the Excellence Initiative – Research University programme at the Warsaw University of Technology.