Simon Salleh Atri is a PhD student at the Tel Aviv University, Faculty of Exact Sciences, where he also achieved his bachelor’s (2016-2019) and master’s (2019-2021) degree in physics. As a Laboratory and Research Assistant to Dr. Moshe Ben Shalom, he focused on polarization in graphene polytypes and Electrical-induced stack switching in MoS2 and graphene polytypes. His current research is aimed at the study of the unique properties that arise when Van der Walls layered materials are stacked into different polytypes (recently called Van der Walls Polytypes) and why this occurs.

1. Please summarize the research you do and explain why it is significant?

Multi-layer graphene can be stacked in different polytypes, each with unique properties as shown in trilayer rhombohedral graphene, where orbital-magnetism and superconductivity were found. In the four-layer case, there are three possible polytypes, Bernal (ABAB), Rhombohedral (ABCA), and ABCB, from which only the latter has a polar axis and a non-centrosymmetric unit cell, the necessary conditions for the existence of electrical polarization.
In this project we show that electrical polarization indeed arises in this polytype, study its behavior under electrostatic doping, and use theoretical tools to understand its source.
Contrary to other ferroelectric and polar materials, this multilayer is composed of only one kind of atom (Carbon), thus the study of electrical polarization here helps to understand fundamentally the existence of polarization and the spatial distribution of electrons within the crystal (or why would polarization arise in such a system). Additionally, we observe for the first time that the addition of charge carriers (holes in this case) increases the value of the polarization rather than decreasing it, as would be normally expected.

2. How might your research be used?

My research focuses mainly on the fundamental properties of the different polytypes of multi-layer graphene. Understanding why the properties change and how electron correlations depend on the crystal stacking is important in terms of fundamental physics. Additionally, learning how to switch between polytypes is appealing from a technological point of view, since it will allow one to choose a desired property just by switching the stack of the multilayer.

3. Why is the Park AFM important for your research?

Two-dimensional layered materials have by definition a very small thickness compared to their spanning area; Park AFM becomes a natural tool for their study since it allows one to measure different quantities over the sample area, including, topography, piezo, magnetic, and electrostatic response.
When studying Van der Walls polytypes, apart from learning how those properties change with the stacking sequence, measuring and mapping those quantities helps us to characterize and understand which polytype we are dealing with before further studies, it is then an essential step in our research.

4. What features of Park AFM are the most beneficial and why?

We work with NX10 and the HIVAC system:

- User-friendly interface that allows scanning in a fast way
- Versatility: it is easy to change the operating mode, and there are many to choose
- Real non-contact mode: Allows longer tip life and avoids damaging the sample, which is very important for us when dealing with quasi-equilibrium polytypes that can disappear with contact or tapping mode
- Sideband KPFM: allows high spatial resolution and a low noise KP signal, which was essential to measure the electrical polarization of ABCB graphene, it allowed us to see a 3 mV step between polytypes
- Working in a high vacuum with HIVAC is essential to measure electrostatic properties in our materials.