arXiv subjects

Yuanyuan Ni

Publications and source records attributed to Yuanyuan Ni.

4 recordsLinked to original sources

arXiv2026

Proton-Transfer Ferroelectrics with Exceptional Switching Endurance

Reliable organic ferroelectrics for memory applications require extreme endurance under repeated electrical switching. Here we demonstrate exceptional fatigue resistance in highly crystalline 2-methylbenzimidazole (MBI) films grown by low-temperature deposition followed by restrained crystallization (LDRC) in a simple Pt/MBI/Pt capacitor geometry. Switching kinetics analyzed using the Kolmogorov-Avrami-Ishibashi (KAI) model reveal characteristic millisecond switching times and quasi-one-dimensional domain growth associated with proton transfer along hydrogen-bond chains. Guided by these kinetics, we implemented a stringent fatigue protocol designed to maximize switching stress, involving bipolar switching at approximately 2Ec with 5 ms pulses, well beyond the characteristic switching time, for continuous operation over approximately 2 weeks. The remanent polarization exhibits only a minor wake-up (+10% within the first 10^4 cycles) and ultimately returns to approximately its initial value after 10^8 cycles, with testing limited by experimental duration rather than device failure. This robust endurance is achieved in an unengineered structure and contrasts with polymer ferroelectrics such as P(VDF-TrFE), where comparable performance typically relies on interfacial engineering. The combination of LDRC-enabled high crystallinity and localized proton-transfer switching, which introduces minimal structural perturbation during polarization reversal, enables this outstanding fatigue tolerance and highlights MBI as a simple, fluorine-free platform for durable organic ferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2024

Spherulite-enhanced Macroscopic Polarization in Molecular Ferroelectric Films from Vacuum Deposition

Proton-transfer type molecular ferroelectrics hold great application potential due to their large spontaneous polarizations, high Curie temperatures, and small switching fields. However, it is puzzling that preparation of quasi-2D films with macroscopic ferroelectric behaviors has only been reported in few molecular ferroelectrics. To resolve this puzzle, we studied the effect of microstructures on macroscopic ferroelectric properties of 5,6-Dichloro-2-methylbenzimidazole (DC-MBI) films grown using low-temperature deposition followed by restrained crystallization (LDRC) method. We revealed a competition between dense spherulites and porous microstructures containing randomly oriented nanograins in as-grown films. Post-growth annealing at moderate temperature promotes the formation of spherulites which leads to macroscopic ferroelectric polarization switching. These results highlight microstructure density as a critical factor for macroscopic ferroelectric properties, potentially resolving the puzzle for absence of macroscopic ferroelectric behavior in molecules ferroelectric films. We expect the approach for enhancing microstructure density offered in this work to greatly advance fabrication of quasi-2D molecular ferroelectrics films and to unlock their potential in device applications.

cond-mat.mes-hall↗

arXiv2023

Hydrogen-induced tunable remanent polarization in a perovskite nickelate

Materials with field-tunable polarization are of broad interest to condensed matter sciences and solid-state device technologies. Here, using hydrogen (H) donor doping, we modify the room temperature metallic phase of a perovskite nickelate NdNiO3 into an insulating phase with both metastable dipolar polarization and space-charge polarization. We then demonstrate transient negative differential capacitance in thin film capacitors. The space-charge polarization caused by long-range movement and trapping of protons dominates when the electric field exceeds the threshold value. First-principles calculations suggest the polarization originates from the polar structure created by H doping. We find that polarization decays within ~1 second which is an interesting temporal regime for neuromorphic computing hardware design, and we implement the transient characteristics in a neural network to demonstrate unsupervised learning. These discoveries open new avenues for designing novel ferroelectric materials and electrets using light-ion doping.

cond-mat.str-el↗

arXiv2021

Textured organic ferroelectric films from physical vapor deposition and amorphous-to-crystalline transition

Crystallization is a key for ferroelectricity which is a collective behavior of microscopic electric dipoles. On the other hand, uncontrolled crystallization leads to uneven morphology and random crystal orientations, which undermines the application potential of ferroelectric thin films. In this work, we introduce a film fabrication method of low-temperature physical vapor deposition followed by restrained crystallization, with electrical properties monitored in real-time by in situ measurements. This method was adopted to fabricate films of 2-methylbenzimidazole (MBI), whose molecule crystals are proton-transfer type biaxial ferroelectrics and tend to grow into a hedgehog-shaped spherulites morphology. The in situ measurements confirm that the crystallization, corresponding to a clear transition of physical properties, occurs dominantly during post-deposition warming. This enables the fabrication of micron-thick films in disk-shaped morphology with one polarization axis aligned along the out-of-plane direction, while the measured spontaneous polarization and coercive field are comparable to the single-crystal values. These results mark an important advancement of film growth that is expected to benefit widely the fabrication of molecular materials films whose functional properties hinge on crystallization to achieve desirable morphology and crystallinity.

cond-mat.mtrl-sci↗