Shahriar Muhammad Nahid - Ferroelectricity at the 2D Limit

What are 2D materials good for? To answer this question, we must delve into their distinct properties that set them apart from conventional materials for any specific application.

So, in my doctoral study, I am exploring how the unique properties of 2D materials affect the electrical polarization and domain walls in 2D ferroelectrics. While the list of distinct properties of 2D materials is fairly extensive, the most relevant properties (of course, in my personal opinion) to ferroelectricity are the enhanced flexibility, lower dimensionality, heterogeneous integration ability, and the van der Waals bonding along the out-of-plane direction.

Enhanced Flexibility

Bend Induced Domain Walls in α-In2Se3

Ferroelectric materials are sensitive to mechanical deformation. Understanding the interplay between mechanical deformation and polarization at the ultimate limit of strain gradient is important for emerging applications such as nano-transducers or sensors, microrobotics, micro or nano electromechanical systems, and wearable electronics. However, in all the previous studies, the applied strain gradient varies between 105 to 107 m-1 as conventional ferroelectrics undergo fracture beyond that.

2D materials, due to the van der Waals bonding along the out-of-plane, bends to the smallest possible radius of curvature of 1-10 nm, corresponding to a strain gradient of 108 to 109 m-1. We utilize this enhanced flexibility of 2D materials and mechanically bend a 2D ferroelectric α-In2Se3 to the ultimate limit of strain gradient. We combine scanning probe microscopy, electron microscopy, and density functional theory to understand the coupling between polarization and strain gradient. At this highest limit of strain gradient, we observe a bending angle dependent generation of ferroelectric domain wall. When the bending angle is above 330, a wedge disclination forms, which separates the two up and down domains of α-In2Se3. We use patterned substrates to precisely control the domain wall formation.

This study establish opportunities for polarization manipulation and domain wall engineering in deformable ferroelectrics, which will be useful for wearable sensors and flexible electronics.

Reference:

*Han, E., *Nahid, S.M., Rakib, T., Nolan, G., Ferrari, P.F., Hossain, M.A., Schleife, A., Nam, S., Ertekin, E., van der Zande, A.M. and Huang, P.Y., 2023. Bend-Induced Ferroelectric Domain Walls in α-In2Se3. ACS nano, 17(8), pp.7881-7888. (* equal contribution)

Lower Dimensionality

Depolarization Field Driven Photovoltaic Effect in α-In2Se3

Ferroelectric photovoltaic effect (FPVE) is a unique characteristic of ferroelectrics originating from the broken symmetry of the crystal structure. For over five decades, we've known about this phenomenon, yet there's still disagreement about its underlying mechanism. At the nanoscale, the FPVE can generate from different mechanisms, such as the depolarization field, asymmetric Schottky barrier at the interface, asymmetric transmission of non-thermalized electrons, or non-linear optical responses. Unraveling the dominant mechanism in a system is important to identify the design principles of the photovoltaic cell and define the fundamental limit of the power conversion efficiency.

The key research question is to understand the FPVE mechanism of an ideal band gap 2D ferroelectric α-In2Se3. In this endeavor, I fabricate few-layer graphene/α-In2Se3/few-layer graphene heterostructures and use scanning photocurrent-photovoltage microscopy, and piezoelectric force microscopy to identify the underlying photovoltaic mechanism of this system. The direction of the photocurrent and its scaling with the α-In2Se3 thickness confirms the depolarization field as the underlying mechanism. This depolarization field is generated from the unscreened polarization charges at the surface and facilitated by the finite density of states of few-layer graphene.

Reference:

Nahid, S.M., Nam, and van der Zande, A.M. 2024. Depolarization Field Induced Photovoltaic Effect in Graphene/α-In2Se3/Graphene Heterostructures. ACS Nano, (just accepted)

Heterogeneous Integration Ability

Van der Waals Bonding along Out-of-Plane