MOSHE HARATS
Welcome
In 2004, the thinnest material known to date was discovered – graphene. Graphene is a material that consists of carbon atoms arranged in a honeycomb lattice. Its uniqueness lies in the fact that the atoms are arranged in two dimensions so that the honeycomb layer is only one atom thick. This was the first two-dimensional material presented to the world.
What does two-dimensionality mean? After all, the atoms themselves are still three-dimensional and live in a three-dimensional world! The definition of a two-dimensional material stems from the fact that electrons can only move within the layer of the two-dimensional material and, on the other hand, are prevented from moving in the third dimension, outside the layer.
In a different project, we looked into the role of the defects in a particular transition-metal dichalcogenide, MoS 2 . Unlike traditional methods of investigation of the defects using STM or TEM measurements, we looked into the photoluminescence of the defects, manifested in energy bands visible at low temperatures. We assembled a unique experimental setup where we could cool down the sample down to cryogenic temperatures (4K) and deposit gases such as O 2 to test the influence of the gases on the emission related to the defects. Our findings revealed the contribution of the intrinsic nature of the defects that are shallow Sulfur vacancies, and the contribution of the gas molecules that passivated the Sulfur vacancies and doped the material.
Since then, the field has developed and there are hundreds of two-dimensional materials. Our laboratory focuses on advanced optical measurements of two-dimensional materials such as semiconductors, under elastic strains.
The obvious question arises – how can a two-dimensional material be elastically stretched?
After all, it is not possible to grab a material with thickness of a single atom with tweezers and stretch it? To this end, part of the laboratory's work is developing new methods of elastic
stretching of two-dimensional materials.
Research in two-dimensional materials is moving to the next stage – connecting different two-dimensional materials on top of each other. This can be seen as a Lego game with an additional degree of freedom that does not exist in regular Lego – the angle of rotation between the honeycomb of one material compared to the honeycomb of the next layer of the next material in line. By controlling this angle, they were able to demonstrate interesting phenomena, including the creation of single photons used for secure quantum communication, as well as superconductivity in graphene layers, for which the Wolf Prize in Physics was awarded in 2020.
Recent Work
In my recent work I was able to construct a home-built all-electrical AFM with optical access. This experimental tool was useful as I was able to test for the first time the phenomenon of exciton funneling in non-uniform strained monolayers of TMDCs. Surprisingly, I discovered that not only funneling is negligible, a different effect is more dominant – the conversion of the neutral excitons into negatively charged trions. This effect, termed as “mechanical gating”, shows that we can change the charging of the material my mechanical means only – non-uniform strain – and without any external electric field as is usually done by electrostatic gating of the material. Mechanical gating has been shown to occur in any kind of non-uniform strain profile such as in air-pressurized membranes.
