Emergent functionalities in van der Waals ferroelectrics: photovoltaics, electromechanical coupling, and domain wall conductivity

Ferroelectrics are useful for a broad range of applications, including memory, sensing, actuation, energy harvesting, and storage. With the growing technological demand of miniaturization and integration in electronic devices, thin-film ferroelectrics have become highly sought after. However, owing to interfacial and surface discontinuities and intrinsic thickness limit, conventional thin-film ferroelectrics pose challenges in probing their fundamental properties at the nanoscale. In contrast, van der Waals ferroelectrics, lacking chemical bonds in the out-of-plane direction, offer stable ferroelectricity down to the monolayer limit and enable easy integration with dissimilar materials. Leveraging the unique characteristics of van der Waals ferroelectrics, this dissertation investigates their emerging functionalities in previously unexplored physical regimes. Using α-In2Se3 as a prototype, this dissertation will focus on three specific characteristics of van der Waals ferroelectrics and their effects on ferroelectricity related phenomena. First, the reduced dimensionality of these materials allows probing of the underlying mechanism of the ferroelectric photovoltaic effect (FPVE) and the associated scaling laws with thickness. We fabricate vertical few-layer graphene/α-In2Se3/graphene heterostructures and employ scanning photocurrent and photovoltage microscopy to understand the thickness-dependent FPVE response. The short-circuit photocurrent is antiparallel to the ferroelectric polarization and decays exponentially with increasing thickness. These observations confirm the depolarization field, originating from the unscreened bound charges due to the finite density of states in semimetal few-layer graphene, as the underlying mechanism of FPVE in this system. The maximum depolarization field reaches 547 kV/cm (518-572 kV/cm with 95 % confidence interval) in 18 nm α-In2Se3, around 63 % of that in a completely unscreened flake. These results demonstrate the importance of the depolarization field at the nanoscale and define design principles for harnessing FPVE at reduced dimension. Second, we utilize the enhanced mechanical flexibility of these materials to study the stability of ferroelectric polarization at the ultimate limit of strain gradient exceeding 10^8 /m. At this limit, bent α-In2Se3 produces two classes of structures: arcs, which form at bending angles below 33°, and kinks, which form above 33°. Arcs preserve the original polarization of the material while kinks form ferroelectric domain walls via disclination. Geometric and energetic constraints set the conditions for the kink formation. Finally, we utilize this phenomenon to pattern ferroelectric polarization using controlled bending over templated substrates. These results describe the electromechanical coupling of α-In2Se3 at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning. Finally, we utilized the heterostructure formation ability of van der Waals ferroelectrics to generate highly conductive charged domain walls. We demonstrate that stacking two oppositely polarized α-In2Se3 domains leads to the formation of lateral and electrically accessible head-head (H-H) charged domain wall. The controlled formation along with lateral geometry and electrical accessibility of these artificial charged domain walls allows us to address the longstanding challenge of understanding the mechanism of electrical transport in these systems. At room temperature, H-H charged domain walls show metal-like conductivity and mobility which are more than 3 and 2 orders of magnitude higher than single domain α-In2Se3. They also exhibit a temperature-dependent metal-to-semiconductor transition. A combination of hysteresis analysis, mobility scaling with temperature, magneto-transport, and scanning tunneling electron microscopy (STEM) imaging confirms that transport is dominated by disorders in both the inter-domain separation and the atomic structure of the interfacial layer. These disorders have a characteristic length scale of 50-70 nm and energy barriers of 64.6-74.2 meV. The artificial H-H CDWs show the lowest room-temperature resistance of < 3.1 kΩ which is 2-9 orders of magnitude smaller than CDWs in thin-film ferroelectrics. The study lays the groundwork for precise control and understanding of the electronic properties of ferroelectric domain walls. These results also resolve longstanding challenges posed by high charged domain wall resistance, opening opportunities for gigahertz memory and neuromorphic computing. Overall, this dissertation underscores the potential of van der Waals ferroelectrics for both fundamental understanding of ferroelectric physics as well as practical device applications. The emerging functionalities, as demonstrated here, will allow further development of thin-film ferroelectric-based nanoelectronics including energy harvesters, memory, interconnects, and memristors.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Nahid, Shahriar Muhammad

Contributors dc:contributor

• van der Zande, Arend
• Nam, SungWoo
• Huang, Pinshane
• Ertekin, Elif

• Van Der Waals Ferroelectric
• Α-in2se3
• Depolarization Field
• Charged Domain Walls
• Mechanical Switching

Statement dc:rights

• Copyright 2025 Shahriar Muhammad Nahid

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129821

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129821