2D CHARM: MBE growth and advanced characterization of large-scale van der Waals heterostructures

The concept of van der Waals (vdW) heterostructures has proven to be a promising route toward developing two-dimensional (2D) metamaterials with hybrid properties. Stacking different 2D crystals promotes synergistic effects that give rise to new functionalities. To date, a wide range of exotic properties in vdW heterostructures has been explored, primarily in structures assembled by stacking exfoliated flakes. A major obstacle to their integration into applications lies in their limited size and compatibility with production processes. Any practical application requires a scalable approach to material fabrication. Therefore, the large-area realization of single-crystalline vdW heterostructures is a crucial step on the path toward their ultimate applications.

Within the 2D-CHARM project, we will develop the synthesis of vdW heterostructures using molecular beam epitaxy (MBE). These heterostructures will consist of the 2D insulator hexagonal boron nitride (hBN) and 2D ferroelectric materials (2D-FEs, either SnSe or In₂Se₃). Such 2D-FE/hBN heterostructures are promising building blocks for a variety of applications, including ultracompact non-volatile ferroelectric memories and artificial multiferroics.

The development of growth processes will be strongly supported by advanced characterization techniques that provide comprehensive structural information. Our analytical approach will span length scales from the millimeter down to the atomic level, employing state-of-the-art techniques based on scanning transmission electron microscopy (STEM). In particular, four-dimensional (4D) STEM will be used to obtain complete structural information, which will be combined with results from other conventional techniques such as X-ray diffraction. In addition, 4D-STEM enables phase-retrieval methods that provide access to atomic-scale strain as well as local electrostatic fields at the atomic level.

Applying atomic-resolution 4D-STEM to 2D-FE/hBN heterostructures will therefore make it possible to uncover the relationship between precise structural properties and the associated local polarizations. These will also be investigated using piezoresponse force microscopy and Kelvin probe force microscopy in appropriate configurations.

The ultimate goal of this project is the realization of large-area 2D-FE/hBN vdW heterostructures using MBE. To achieve this objective, we will focus on three challenges: (i) the growth of high-quality, large-area hBN layers; (ii) controlled growth of 2D-FEs on the synthesized hBN; and (iii) a fundamental understanding of the properties of 2D-FE/hBN heterostructures. Within the project, we will bring together the complementary expertise of the partners. In particular, feedback between MBE growth and advanced characterization will enable the development of hBN and 2D-FEs by MBE.

Funded by the ANR-DFG French-German Collaboration for Joint Projects - Project number 545820811