Jiuxiang Dai

Enormous emergent phenomena are discovered from oxide heterointerfaces, due to the reconstruction of charge, lattice and spin degrees of freedom. This work reports a new approach to manipulate the interfacial electronic state of complex oxides through the oxygen ionic evolution, demonstrating a direct correlation between oxygen ionic migration across the interface and interface polyhedral stacking. Such control changes dramatically the magnetic and electronic states of adjacent oxide layer.

Abstract

Complex oxide heterointerfaces and heterostructures have demonstrated enormous emergent phenomena over the last decades, attributed to the reconstructions of mis-matched crystalline structure, polarity, and spin ordering across the heterointerfaces. This work employs the heterostructures of La_0.7Sr_0.3MnO₃ and CaFeO_2.5 as model system to demonstrate an interface-specific oxygen migration/reconstruction across the interfaces due to the mismatched chemical potential, which dramatically influences the ferromagnetic and electronic states of La_0.7Sr_0.3MnO₃ layer. Specifically, the alternative stacking of octahedral (O_h ) and tetrahedral (T_d ) layers in CaFeO_2.5 are used to form two distinct heterointerfaces, namely the O_h-T_d and the O_h-O_h interfaces with the adjacent La_0.7Sr_0.3MnO₃ layer. Interestingly, the oxygen ion migrates toward opposite directions across the interface for these two cases, in which the CaFeO_2.5 layer acts as an “oxygen pump” and manipulates the oxygen contents of its adjacent La_0.7Sr_0.3MnO₃ layers. Such manipulation leads to a dramatically changed ferromagnetic transition temperature for the heterostructure with the O_h-T_d and O_h -O_h interface. This work establishes a feasible and efficient strategy to control the oxygen ionic distribution through atomic-scale interface design and opens up new opportunities to exploit emergent states at the complex oxide heterostructures through selective oxygen ion evolution.

A universal interface gain mechanism is introduced that significantly amplifies the impact of interface contact barriers on ferroelectric tunneling electro-resistance (TER). Reversing the polarization state allows the interface to reversibly switch between Ohmic contact and Schottky contact, thereby achieving a giant TER at the nanoscale. This breakthrough overcomes thickness limits in high TER, enabling ultrathin memory devices.

Abstract

Ultrathin ferroelectric tunnel junctions (FTJs) hold considerable promise for next-generation, high-speed, low-power, and high-density nonvolatile memory applications. Achieving a substantial tunneling electro-resistance (TER) remains a challenge as the ferroelectric layer is thinned to nanoscale dimensions, often resulting in a diminished or lost polarization. An innovative interface barrier gain mechanism is introduced, employing interface electronic state modulation to precisely control the size of an additional interface barrier. This strategy lessens the dependency of the tunneling barrier on ferroelectric polarization strength, facilitating a remarkable TER even at ferroelectric thicknesses as minimal as ≈1 nm. The focus is on composite FTJs using In₂Se₃/MTe₂ (M = Mo, W), where the inclusion of an MTe₂ monolayer disrupts the asymmetric electrode configuration. The weak ferroelectric polarization reversal of the In₂Se₃ monolayer effectively modulates the electronic state coupling at the In₂Se₃/MTe₂ interface. This modulation leads to variations in the width and height of the Schottky barrier at the heterojunction-electrode interface corresponding with the ferroelectric polarization reversal, establishing a beneficial Ohmic contact in the “on” state and resulting in an exponential TER increase up to 5.4 × 10⁶%. This work introduces a universal mechanism to overcome the thickness limitations traditionally associated with enhancing TER, marking a significant advancement in the development of ultrathin ferroelectric nonvolatile devices.

Nature Chemistry, Published online: 21 August 2024; doi:10.1038/s41557-024-01595-w

Publication date: November 2024

Source: Materials Today, Volume 80

Author(s): Pengfei Liu, Li-ping Feng, Xiaodong Zhang, Yulong Yang, Xiaoqi Zheng, Xitong Wang

npj 2D Materials and Applications, Published online: 21 August 2024; doi:10.1038/s41699-024-00489-2

This study uncovers the pivotal role of sulfur evaporation rates in directing the growth modes of MoS₂ on sapphire. Low sulfur levels preserve O/Al-terminated step edges, fostering atomic-edge epitaxy, while high sulfur levels favor S-terminated edges, promoting van der Waals epitaxy. The findings highlight that van der Waals epitaxy achieves superior MoS₂ alignment (≈99%) on a 2 in. wafer, offering new insights into precise 2D material fabrication.

Abstract

Epitaxial growth of 2D transition metal dichalcogenides (TMDCs) on sapphire substrates has been recognized as a pivotal method for producing wafer-scale single-crystal films. Both step-edges and symmetry of substrate surfaces have been proposed as controlling factors. However, the underlying fundamental still remains elusive. In this work, through the molybdenum disulfide (MoS₂) growth on C/M sapphire, it is demonstrated that controlling the sulfur evaporation rate is crucial for dictating the switch between atomic-edge guided epitaxy and van der Waals epitaxy. Low-concentration sulfur condition preserves O/Al-terminated step edges, fostering atomic-edge epitaxy, while high-concentration sulfur leads to S-terminated edges, preferring van der Waals epitaxy. These experiments reveal that on a 2 in. wafer, the van der Waals epitaxy mechanism achieves better control in MoS₂ alignment (≈99%) compared to the step edge mechanism (<85%). These findings shed light on the nuanced role of atomic-level thermodynamics in controlling nucleation modes of TMDCs, thereby providing a pathway for the precise fabrication of single-crystal 2D materials on a wafer scale.

High-quality WSe₂ nanoscrolls with uniform crystalline orientation are achieved by applying ethanol droplets to vapor deposition-grown bilayer WSe₂ nanosheets. Raman spectroscopy studies demonstrate that nanoscrolls exhibit strong anisotropic optical properties. The photodetectors based on the nanoscrolls demonstrate competitive overall performance, strong polarization-sensitive detection capabilities, and excellent polarization-sensing imaging capabilities. These studies indicate that WSe₂ nanoscrolls are promising candidates for high-performance and polarization-sensitive photodetectors.

Abstract

The intrinsic low-symmetry crystal structures or external geometries of low-dimensional materials are crucial for polarization-sensitive photodetection. However, these inherently anisotropic materials are limited in variety, and their anisotropy is confined to specific crystal directions. Transforming 2D semiconductors, such as WSe₂, from isotropic 2D nanosheets into anisotropic 1D nanoscrolls expands their application in polarization photodetection. Despite this considerable potential, research on polarization photodetection based on nanoscrolls remains scarce. Here, the uniform crystalline orientation of WSe₂ nanoscrolls is achieved conveniently and efficiently by applying ethanol droplets to vapor deposition-grown bilayer WSe₂ nanosheets. Angle-resolved polarized Raman spectroscopy of WSe₂ nanoscrolls demonstrates vibrational anisotropy. Photodetectors based on these nanoscrolls show competitive overall performance with a broadband detection range from 405 to 808 nm, a competitive on/off ratio of ≈900, a high detectivity of 3.4 × 10⁸ Jones, and a fast response speed of ≈30 ms. Additionally, WSe₂ nanoscroll-based photodetectors exhibit strong polarization-sensitive detection with a maximum dichroic ratio of 1.5. More interestingly, due to high photosensitivity, the WSe₂ nanoscroll detectors can easily record sequential puppy images. This work reveals the potential of WSe₂ nanoscrolls as excellent polarization-sensitive photodetectors and provides new insights into the development of high-performance optoelectronic devices.

The in situ van der Waals (vdW) epitaxial SnSe₂/SnSe heterostructures have an atomically sharp interface and specific orientation. On the basis of this structure, the tunable band alignments covering type-II and type-III are naturally constructed due to the robust valley polarization effect of anisotropic SnSe, suggesting promising applications in valley(opto)tronic devices.

Abstract

2D van der Waals (vdW) layered semiconductor vertical heterostructures with controllable band alignment are highly desired for nanodevice applications including photodetection and photovoltaics. However, current 2D vdW heterostructures are mainly obtained via mechanical exfoliation and stacking process, intrinsically limiting the yield and reproducibility, hardly achieving large-area with specific orientation. Here, large-area vdW-epitaxial SnSe₂/SnSe heterostructures are obtained by annealing layered SnSe. These in situ Raman analyses reveal the optimized annealing conditions for the phase transition of SnSe to SnSe₂. The spherical aberration-corrected transmission electron microscopy investigations demonstrate that layered SnSe₂ epitaxially forms on SnSe surface with atomically sharp interface and specific orientation. Optical characterizations and theoretical calculations reveal valley polarization of the heterostructures that originate from SnSe, suggesting a naturally adjustable band alignment between type-II and type-III, only relying on the polarization angle of incident lights. This work not only offers a unique and accessible approach to obtaining large-area SnSe₂/SnSe heterostructures with new insight into the formation mechanism of vdW heterostructures, but also opens the intriguing optical applications based on valleytronic nanoheterostructures.

Ferroelectricity, band topology, and superconductivity are respectively local, global, and macroscopic properties of quantum materials, and understanding their mutual couplings offers unique opportunities for exploring rich physics and enhanced functionalities. This review summarize the ferroelectricity-tuned band topology, superconductivity, superconducting diode effect and topological superconductivity in 2D materials and related heterostructures from both theoretical and experimental sides. Perspectives on this vibrant area are given as well.

Abstract

Ferroelectricity, band topology, and superconductivity are respectively local, global, and macroscopic properties of quantum materials, and understanding their mutual couplings offers unique opportunities for exploring rich physics and enhanced functionalities. In this mini-review, the attempt is to highlight some of the latest advances in this vibrant area, focusing in particular on ferroelectricity-tuned superconductivity and band topology in 2D materials and related heterostructures. First, results from predictive studies of the delicate couplings between ferroelectricity and topology or superconductivity based on first-principles calculations and phenomenological modeling are presented, with ferroelectricity-enabled topological superconductivity as an appealing objective. Next, the latest advances on experimental studies of ferroelectricity-tuned superconductivity based on different 2D materials or van der Waals heterostructures are covered. Finally, as perspectives, schemes are outlined that may allow to materialize new types of 2D systems that simultaneously harbor ferroelectricity and superconductivity, or that may lead to enhanced ferroelectric superconductivity, ferroelectric topological superconductivity, and new types of superconducting devices such as superconducting diodes.

A red-emissive CsPb(Br_0.25I_0.75)₃-PMMA film with high photoluminescence quantum yield and favorable reversible thermal photoluminescence property below 160 °C is reported, which is used for a self-powered temperature detection system displaying temperature data in real time.

Abstract

It is still a challenge to prepare the red-emissive perovskite-polymer composite with high photoluminescence quantum yield (PLQY) and reversible thermal photoluminescence (RTPL), which is key for improving the thermal tolerance of luminescent solar concentrators (LSCs). In this work, the red-emissive (661 nm) CsPb(Br_0.25I_0.75)₃-PMMA composite with high PLQY of 85.96% and favorable RTPL below 160 °C is reported via a strategy of the defect passivation by halide ligand additives. The composite film possesses excellent long-term stability in air, water, and high temperature environments. Then, a self-powered temperature detection system, which consists of LSCs and a temperature detection module based on the RTPL composite film, is fabricated. On one hand, RTPL composite makes LSCs of superior thermal tolerance even after 100 cycles of temperature between room temperature and 160 °C. On the other hand, RTPL composite in the opaque box of the temperature detection module can give rise to the signal light without the interference of ambient light. As a result, the self-powered temperature detection system achieves the quantitative temperature data display at different environment lights for 24 h in a day.

In this work, the synthesis of high-quality 2D non-layered FeS nanosheets on SiO₂/Si substrates by molecular sieves-assisted chemical vapor deposition (CVD) method is realized. The thickness of ultrathin FeS nanosheets can be as low as 2.3 nm. The inverted symmetry broken structure is confirmed by angle-resolved polarized Raman and SHG characterizations. The synthesized FeS nanosheets exhibit exceptional metallic behavior, with conductivity up to 1.63 × 10⁶ S m⁻¹ at 300 K for an 8 nm thick sample. This work provides a significant contribution to the synthesis and characterization of 2D non-layered Fe-based materials.

Abstract

Fe-based 2D materials exhibit rich chemical compositions and structures, which may imply many unique physical properties and promising applications. However, achieving controllable preparation of ultrathin non-layered FeS crystal on SiO₂/Si substrate remains a challenge. Herein, the influence of temperature and molecular sieves is reported on the synthesis of ultrathin FeS nanosheets with a thickness as low as 2.3 nm by molecular sieves-assisted chemical vapor deposition (CVD). The grown FeS nanosheets exhibit a non-layered hexagonal NiAs structure and belong to the P6₃/mmc space group. The inverted symmetry broken structure is confirmed by the angle-resolved second harmonic generation (SHG) test. In particular, the 2D FeS nanosheets exhibit exceptional metallic behavior, with conductivity up to 1.63 × 10⁶ S m⁻¹ at 300 K for an 8 nm thick sample, which is higher than that of reported 2D metallic materials. This work provides a significant contribution to the synthesis and characterization of 2D non-layered Fe-based materials.

Nature Nanotechnology, Published online: 20 August 2024; doi:10.1038/s41565-024-01734-x

Nature Communications, Published online: 20 August 2024; doi:10.1038/s41467-024-50226-y