Ferroelectric resistive switching behavior in two-dimensional materials/BiFeO3 hetero-junctions

Multi‐Length Engineering of (K, Na)NbO3 Films for Lead‐Free Piezoelectric Acoustic Sensors with High Sensitivity

With increasing concerns about noise pollution, the pursuit of highly dependable piezoelectric acoustic sensors for real‐time noise monitoring has come to the forefront of scientific research. Lead‐based perovskite piezoelectric films, exemplified by lead zirconate titanate Pb(Zr,Ti)O3 (PZT), surpass traditional piezoelectric materials such as ZnO and AlN in their piezoelectric properties, promising substantial advancements in next‐generation acoustic sensor technologies. However, the toxic nature of lead in PZT materials poses formidable environmental and human health risks. In an unprecedented breakthrough, it presents the pioneering development of an environmentally benign lead‐free piezoelectric Micro‐Electro‐Mechanical System (MEMS) acoustic sensor based on potassium sodium niobate (K,Na)NbO3 (KNN) film. High‐quality <001> textured 3 µm‐thick KNN film is successfully integrated into commercially used Si substrate, rendering exceptional piezoelectricity (transverse piezoelectric coefficients e31* of ≈8.5 C m−2) with satisfactory thermal stability. The atomic‐scale Z‐contrast imaging and piezoresponse force microscopy characterizations reveal that the outstanding piezoresponse originates from the local coexistence of multiple phases and the enhancement of extrinsic piezoelectric contributions from in‐plane polarization anisotropy. Finite element simulation is employed to design the triangular cantilever structure and annular diaphragm structure, each corresponding to different operating bandwidths. The resultant MEMS acoustic sensors stand out with outstanding acoustic performance (the high sensitivity and expansive receiving field of view), which are attributed to the microstructural engineering at multi‐length scales for the excellent piezoelectric properties of KNN film. These features enable sensitive acoustic monitoring in various environments, including large‐scale power grids and urban traffic.

Ferroelectric-Optoelectronic Hybrid System for Photodetection

Photodetectors (PDs), as functional devices based on photon-to-electron conversion, are an indispensable component for the next-generation Internet of Things system. The research of advanced and efficient PDs that meet the diverse demands is becoming a major task. Ferroelectric materials can develop a unique spontaneous polarization due to the symmetry-breaking of the unit cell, which is switchable under an external electric field. Ferroelectric polarization field has the intrinsic characteristics of non-volatilization and rewritability. Introducing ferroelectrics to effectively manipulate the band bending and carrier transport can be non-destructive and controllable in the ferroelectric-optoelectronic hybrid systems. Hence, ferroelectric integration offers a promising strategy for high-performance photoelectric detection. This paper reviews the fundamentals of optoelectronic and ferroelectric materials, and their interactions in hybrid photodetection systems. The first section introduces the characteristics and applications of typical optoelectronic and ferroelectric materials. Then, the interplay mechanisms, modulation effects, and typical device structures of ferroelectric-optoelectronic hybrid systems are discussed. Finally, in summary and perspective section, the progress of ferroelectrics integrated PDs is summed up and the challenges of ferroelectrics in the field of optoelectronics are considered.

Multilevel resistive switching memory in lead-free double perovskite La[Formula: see text]NiFeO[Formula: see text] films

Dense and flat La[Formula: see text]NiFeO[Formula: see text] (LNFO) films were fabricated on the indium tin oxide-coated glass (ITO/glass) substrate by sol-gel method. The bipolar resistive switching behavior (BRS) could be maintained in 100 cycles and remained after 30 days, indicating that the LNFO-based RS device owned good memory stability. Surprisingly, the multilevel RS characteristics were firstly observed in the Au/LNFO/ITO/glass device. The high resistance states (HRSs) and low resistance state (LRS) with the maximum ratio of [Formula: see text] 500 could be remained stably in 900 s and 130 cycles, demonstrating the fine retention and endurance ability of this LNFO-based RS device. The BRS behavior of Au/LNFO/ITO/glass devices primarily obeyed the SCLC mechanism controlled by oxygen vacancies (OVs) dispersed in the LNFO layer. Under the external electric field, injected electrons were captured or discharged by OVs during trapping or detrapping process in the LNFO layer. Thus, the resistive state switched between HRS and LRS reversibly. Moreover, the modulation of Schottky-like barrier formed at the Au/LNFO interface was contributed to the resistive states switchover. It was related to the change in OVs located at the dissipative region near the Au/LNFO interface. The multilevel RS ability of LNFO-based devices in this work provides an opportunity for researching deeply on the high density RS memory in lead-free double perovskite oxides-based devices.

Tunable Molecular Electrodes for Bistable Polarization Screening

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance-hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO₃ ) coated with resonant para-aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene-electode capacitors.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Tunable valley and spin splitting in 2H-VSe2/BiFeO3(111) triferroic heterostructures

The spin and valley degrees of freedom in monolayer transition metal dichalcogenides have potential applications in spintronics and valleytronics. However, nonvolatile control on the valley and spin degrees of freedom of two-dimensional ferromagnetic materials by multiferroic materials has been rarely reported. Here, the electronic structure of monolayer 2H-VSe2/BiFeO3(111) triferroic heterostructures has been investigated by first-principles calculations. It is found that the V magnetic moment, spin and valley splitting of monolayer VSe2 can be affected by the BiFeO3(111) substrate with ferroelectric polarization and G-type antiferromagnetic order. Particularly, the reversed orientation of ferroelectric polarization and magnetic order of the BiFeO3(111) substrate can modulate the magnitude of spin and valley splitting, and change the spin splitting direction and the spin-dependent valley state in the valence band of monolayer VSe2. The coupling among ferroelectrics, magnetism and ferrovalley is realized in 2H-VSe2/BiFeO3(111) triferroic heterostructures. These results provide a new platform for multiferroic regulation in spintronics and valleytronics, which can enrich the diversity for high-performance devices based on two dimensional multiferroic heterostructures.

Resistive switching behavior in α-In2Se3 nanoflakes modulated by ferroelectric polarization and interface defects

Resistive switching devices based on ferroelectric two-dimensional (2D) van der Waals (vdW) nanomaterials may display simple structures, high density, high speed, and low power consumption, and can be used in flexible electronics and highly integrated devices. However, only a few studies about the in-plane (IP) resistive switching behavior of ferroelectric 2D vdW nanomaterials have been reported because it is very hard to achieve asymmetric barriers only by tuning the IP polarization directions when the electrodes of the planar device are all of the same type. In the current work, we developed a planar device based on an α-In₂Se₃ nanoflake, in which the IP/OOP (out-of-plane) polarization, free carriers and oxygen vacancies in SiO₂ contribute to the resistive switching behavior of the device. This behavior of the device was shown to be affected by exposure to light, and the photoelectric performance was also investigated when the device was in the OFF state. The demonstration of this planar resistive switching device may promote the further development of resistive devices based on 2D vdW nanomaterials, and provide great inspiration for the development of new kinds of transistors.

A planar device based on an α-In₂Se₃ nanoflake, in which the in-plane/out-of-plane polarization, free carriers and oxygen vacancies in SiO₂ contribute to the resistive switching behavior of the device.