Correlated Structural and Optical Characterization during Van der Waals Epitaxy of PbI2 on Graphene

TL;DR

This study explores the real-time structural and optical evolution of PbI2 layers grown on graphene via Molecular Beam Epitaxy, revealing strain effects and charge transfer phenomena crucial for optoelectronic applications.

Contribution

It provides the first detailed correlation between structural and optical properties during van der Waals epitaxy of PbI2 on graphene, including strain relaxation and charge transfer insights.

Findings

Layer-by-layer growth with van der Waals epitaxy observed.

Significant modifications in properties of the first monolayer, including tensile strain.

Charge transfer from graphene to PbI2 evidenced by energy shifts.

Abstract

Van der Waals heterostructures of 2D layered materials have gained much attention due to their flexible electronic properties, which make them promising candidates for energy, sensing, catalytic, and biomedical applications. Lead iodide (PbI2), a 2D layered semiconductor material belonging to the metal halide family, shows a thickness-dependent band gap with an indirect-to-direct transition above one monolayer. It has emerged as an excellent candidate for photodetectors and is a key component in metal halide perovskites solar cells. In the current work, we investigated the growth dynamics and the real-time correlation between structural and optical properties of PbI2 layers deposited on graphene/SiC(0001) by Molecular Beam Epitaxy. The structural and optical properties are probed respectively by Grazing Incidence Fast Atom Diffraction and Surface Differential Reflectance Spectroscopy. The growth proceeds layer-by-layer in a van der Waals-like epitaxy, with the zigzag direction of PbI2 parallel to the armchair direction of graphene. Both techniques bring evidence of significant modifications of the structural, electronic, and optical properties of the first PbI2 monolayer, characterized by a 1% tensile strain that relaxes over 3 to 5 monolayers. For a single monolayer, Angle-Resolved Photoemission Spectroscopy reveals a charge transfer from graphene to PbI2, demonstrated by an energy shift of the order of 50 meV in the graphene band structure.