Skoltech and MIPT scientists have predicted and experimentally confirmed the existence of exotic hexagonal thin films of NaCl on a diamond surface. These films could be used as gate dielectrics for field effect transistors in electric vehicles and telecommunication equipment.


Graphene, the famous two-dimensional carbon, was experimentally prepared and characterized in 2004 by future Nobel laureates Andre Geim and Konstantin Novoselov, and scientists have since started looking into other 2D materials with interesting properties. Among these are silicene, stanene and borophene -- monolayers of silicon, tin, and boron, respectively -- as well as 2D layers of MoS2, CuO, and other compounds.

Skoltech PhD student Kseniya Tikhomirova, Dr. Alexander Kvashnin of Skoltech and Professor Artem R. Oganov of Skoltech and MIPT together with their colleagues built on previous studies of NaCl thin films to hypothesize the existence of an unusual nanometer-thick hexagonal NaCl film on the (110) surface of diamond.

"Initially we decided to perform only a computational study of the formation of new 2D structures on different substrates, driven by the hypothesis that if a substrate interacts strongly with the NaCl thin film, one can expect major changes in the structure of the thin film. Indeed, we obtained very interesting results and predicted the formation of a hexagonal NaCl film on the diamond substrate, and decided to perform experiments. Thanks to our colleagues who performed the experiments, we synthesized this hexagonal NaCl, which proves our theory," said Kseniya Tikhomirova, the first author of the paper.

Researchers used USPEX at first, the evolutionary algorithm developed by Oganov and his students, to predict structures with the lowest energy based only on the chemical elements involved. After predicting the existence of the hexagonal NaCl film, they confirmed this by performing experimental synthesis and characterization by XRD (X-ray diffraction) and SAED (selected area electron diffraction) measurements. The average thickness of the NaCl film was about 6 nanometers -- a thicker film would revert from hexagonal to cubic structure, which is typical for table salt.

Scientists believe that because of strong binding to the diamond substrate and a wide bandgap, hexagonal NaCl could work well as a gate dielectric in diamond FETs -- field-effect transistors that have a potential use in electric vehicles, radars, and telecommunication equipment. Now these FETs typically use hexagonal boron nitride, which has similar bandgap but much weaker binding to the substrate.

"Our results show that the field of 2D materials is still very young, and scientists have discovered only a small portion of possible materials with intriguing properties. We have a long-standing story starting in 2014 when we described the way cubic NaCl thin films can be split into hexagonal graphene-like layers. This shows that this simple and common compound, seemingly well-studied, hides many interesting phenomena, especially in nanoscale. This work is our first step towards the search for new materials like NaCl but having better stability (lower solubility, higher thermal stability, and so on) which then can be effectively used in many applications in electronics," said Alexander Kvashnin, senior research scientist at Skoltech.

This work moves us nearer to understanding how to control the appearance and the properties of two-dimensional materials using a substrate. The research has opened the door to more 2D materials with potential applications in electronics and beyond.

Other institutions involved in this research include the RAS Emanuel Institute of Biochemical Physics, Institute of Nanotechnologies of Microelectronics of the Russian Academy of Sciences, and Higher School of Economics.