Jiuxiang Dai

The wafer-scale synthesis and device processing of 2D materials (2DMs) for integrated circuit applications are reviewed, emphasizing the criticality of process integration towards practical manufacturing. The feasibility of transitioning from laboratory research to industrial-scale utilization is also discussed, while identifying potential integration issues that need to be addressed.

Abstract

The atomically thin nature and exceptional electrical properties of 2D materials (2DMs) have garnered significant interest in circuit applications. Researchers have developed circuits based on wafer-level 2DM fabrication and monolithic integration in the laboratory. Numerous studies have been conducted on discrete device processes; however, circuit manufacturing is a multifaceted and methodical engineering process that demands seamless integration of multiple procedures. Notably, the optimization of crucial processes holds paramount significance in achieving expected results. This review presents the existing research on process integration of 2DM devices and circuit applications. The selection of suitable 2DMs for circuit applications is outlined, considering their excellent theoretical properties and feasible high-quality growth processes. Drawing on highly mature semiconductor manufacturing process, while incorporating customized key processes, 2DM devices have the potential to strongly compete with, and even outperform, the conventional devices. Finally, the recent circuit applications of 2DMs are also discussed in detail. 2DM integrated circuits (2DM ICs) are now being practically applied in advanced manufacturing, transitioning from laboratory development to fabrication plant deployment. The implementation of underlying 2DM IC fabrication provides effective and unique solutions for More Moore, More than Moore, and Beyond CMOS technology routes.

Nature Reviews Materials, Published online: 18 August 2023; doi:10.1038/s41578-023-00583-9

UV photodetectors based on low-dimensional wide-bandgap semiconductors offer wearable, multidimensional and intelligent functions in the scenarios of imaging, communication, multispectral and/or weak light detection and flexible electronics. This Review focuses on the material design, dimensionality engineering and device engineering of wide-bandgap semiconductors in diversified UV applications.

This review highlightsrecent progress in the field of nanoelectronics utilizing metal-insulatortransition (MIT) behaviors in Mott insulators. It covers a wide range oftopics, from the microscopic interactions in condensed matter systems to themacroscopic device functionalities by various external stimuli. This review servesas an overview and a comprehensive understanding of the design of next-generation MIT-based nanoelectronics.

Abstract

Metal–insulator transition (MIT) coupled with an ultrafast, significant, and reversible resistive change in Mott insulators has attracted tremendous interest for investigation into next-generation electronic and optoelectronic devices, as well as a fundamental understanding of condensed matter systems. Although the mechanism of MIT in Mott insulators is still controversial, great efforts have been made to understand and modulate MIT behavior for various electronic and optoelectronic applications. In this review, recent progress in the field of nanoelectronics utilizing MIT is highlighted. A brief introduction to the physics of MIT and its underlying mechanisms is begun. After discussing the MIT behaviors of various Mott insulators, recent advances in the design and fabrication of nanoelectronics devices based on MIT, including memories, gas sensors, photodetectors, logic circuits, and artificial neural networks are described. Finally, an outlook on the development and future applications of nanoelectronics utilizing MIT is provided. This review can serve as an overview and a comprehensive understanding of the design of MIT-based nanoelectronics for future electronic and optoelectronic devices.

Nature Communications, Published online: 19 August 2023; doi:10.1038/s41467-023-40825-6

Molecular ferroelectrics contain stimuli-responsive structure and ionic building blocks, promising for ionically tailored multifunctionality. Here, the authors report molecular ionic ferroelectrics exhibiting the coexistence of room-temperature ionic conductivity and ferroelectricity.

A dual nanopatterning process of block copolymer self-assembly and transfer-printing can create periodic sub-20 nm structures on an eight-inch wafer scale by the printing of patterned ultrathin block copolymer film onto desired substrates. Various complex geometries of nano-in-micro structures, specifically angular/wave lines, rings, and dot-in-hole/dot-in-honeycomb patterns consisting of sub-20 nm SiO _x structures are successfully generated by the combined patterning method.

Abstract

Nanotransfer printing (nTP) is one of the most promising nanopatterning methods given that it can be used to produce nano-to-micro patterns effectively with functionalities for electronic device applications. However, the nTP process is hindered by several critical obstacles, such as sub-20 nm mold technology, reliable large-area replication, and uniform transfer-printing of functional materials. Here, for the first time, a dual nanopatterning process is demonstrated that creates periodic sub-20 nm structures on the eight-inch wafer by the transfer-printing of patterned ultra-thin (<50 nm) block copolymer (BCP) film onto desired substrates. This study shows how to transfer self-assembled BCP patterns from the Si mold onto rigid and/or flexible substrates through a nanopatterning method of thermally assisted nTP (T-nTP) and directed self-assembly (DSA) of Si-containing BCPs. In particular, the successful microscale patternization of well-ordered sub-20 nm SiO _x patterns is systematically presented by controlling the self-assembly conditions of BCP and printing temperature. In addition, various complex pattern geometries of nano-in-micro structures are displayed over a large patterning area by T-nTP, such as angular line, wave line, ring, dot-in-hole, and dot-in-honeycomb structures. This advanced BCP-replicated nanopatterning technology is expected to be widely applicable to nanofabrication of nano-to-micro electronic devices with complex circuits.

A novel in situ biasing approach has facilitated atomic-level observation of domain wall translational features in PZT/LSMO heterojunction via precise electric field control. The study uncovers a unique domain wall creeping behavior with morphology variation, driven by the competition between gradient, elastic, and electrostatic energies. These findings advance a fundamental understanding of domain wall properties and potential modulation in practical devices.

Abstract

As a promising candidate for next-generation nonvolatile memory devices, ferroelectric oxide films exhibit many emergent phenomena with functional applications, making understanding polarization switching and domain evolution behaviors of fundamental importance. However, tracking domain wall motion in ferroelectric oxide films with high spatial resolution remains challenging. Here, an in situ biasing approach for direct atomic-scale observations of domain nucleation and sideways motion is presented. By accurately controlling the applied electric field, the lateral translational speed of the domain wall can decrease to less than 2.2 Å s⁻¹, which is observable with atomic resolution STEM imaging. In situ observations on a capacitor structured PbZr_0.1Ti_0.9O₃/La_0.7Sr_0.3MnO₃ heterojunction demonstrate the unique creeping behavior of a domain wall under a critical electric field, with the atomic structure of the creeping domain wall revealed. Moreover, the evolution of the metastable domain wall forms an elongated morphology, which contains a large proportion of charged segments. Phase-field simulations unveil the competition between gradient, elastic, and electrostatic energies that decide this unique domain wall creeping and morphology variation. This work paves the way toward a complete fundamental understanding of domain wall physics and potential modulations of domain wall properties in real devices.

Nature, Published online: 23 August 2023; doi:10.1038/s41586-023-06247-6

A study using high-resolution scanning electrochemical cell microscopy attributes proton permeation through defect-free graphene and hexagonal boron nitride to transport across areas of the structure that are under strain.

Light: Science & Applications, Published online: 24 August 2023; doi:10.1038/s41377-023-01249-5

Enhancing light-matter interaction in 2D semiconductors coupled to Mie-resonant dielectric nanoantennas leads to the suppression of exciton-exciton annihilation, overcoming a fundamental limit for light emission in atomically thin materials.

This review summarizes the recent progress in the nucleation and growth of nanocrystals studied by in situ transmission electron microscopy (TEM), mainly focusing on the atomic migration dynamics, interface evolution, and structure transformation. Besides, challenges in the study of nanocrystal growth by TEM are discussed and perspectives on the future development of advanced in situ TEM techniques are provided.

Abstract

Nanocrystals play a key role in the modern energy, catalysis, semiconductor, and biology industries due to their unique structures and performances. However, controllable fabrication of ideal nanocrystals with the desired structures and properties is still challenging, which needs a deep understanding of their nucleation and growth process. Here, the research on nucleation and growth of nanocrystals studied by in situ transmission electron microscopy (TEM) is reviewed, mainly focusing on the atomic migration dynamics, interface evolution, and structure transformation. In addition, the challenges in the study of nanocrystal growth by TEM are discussed and the perspective on the future development of advanced in situ TEM techniques is provided. It is hoped that the review can give a deep insight into the nanocrystal nucleation and growth process, and further contribute to the rational design and precise fabrication of high-performance functional nanocrystals.

Upon pulsed photoexcitation, multiferroic bismuth ferrite thin film exhibits a sub-nanosecond melting of their periodic stripe domains. The disordered domains recover back to their original state over few nanosecond timescale. The ultrafast manipulation of domains can be used for fast speed operation in applications of ferroelectrics.

Abstract

Domain switching is crucial for achieving desired functions in ferroic materials that are used in various applications. Fast control of domains at sub-nanosecond timescales remains a challenge despite its potential for high-speed operation in random-access memories, photonic, and nanoelectronic devices. Here, ultrafast laser excitation is shown to transiently melt and reconfigure ferroelectric stripe domains in multiferroic bismuth ferrite on a timescale faster than 100 picoseconds. This dynamic behavior is visualized by picosecond- and nanometer-resolved X-ray diffraction and time-resolved X-ray diffuse scattering. The disordering of stripe domains is attributed to the screening of depolarization fields by photogenerated carriers resulting in the formation of charged domain walls, as supported by phase-field simulations. Furthermore, the recovery of disordered domains exhibits subdiffusive growth on nanosecond timescales, with a non-equilibrium domain velocity reaching up to 10 m s⁻¹. These findings present a new approach to image and manipulate ferroelectric domains on sub-nanosecond timescales, which can be further extended into other complex photoferroic systems to modulate their electronic, optical, and magnetic properties beyond gigahertz frequencies. This approach could pave the way for high-speed ferroelectric data storage and computing, and, more broadly, defines new approaches for visualizing the non-equilibrium dynamics of heterogeneous and disordered materials.

Biaxial heterostrain in bilayer graphene leads to a new atomic lattice relaxation to minimize the stacking energy. Elastic energy is stored in solitons hosting topological states. This demonstrates that heterostrain can be an additional tuning parameter in moiré materials opening new possibilities for moiré engineering.

Abstract

The study of moiré engineering started with the advent of van der Waals heterostructures, in which stacking 2D layers with different lattice constants leads to a moiré pattern controlling their electronic properties. The field entered a new era when it was found that adjusting the twist between two graphene layers led to strongly-correlated-electron physics and topological effects associated with atomic relaxation. A twist is now routinely used to adjust the properties of 2D materials. This study investigates a new type of moiré superlattice in bilayer graphene when one layer is biaxially strained with respect to the other—so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moiré engineering in van der Waals multilayers.

Nature, Published online: 16 August 2023; doi:10.1038/s41586-023-06145-x

The challenges and opportunities for the design of field-effect transistors are discussed and a vision of future transistors and potential innovation opportunities is provided.

Nature Nanotechnology, Published online: 17 August 2023; doi:10.1038/s41565-023-01492-2

A giant spin Hall effect with long spin diffusion length and coexisting with ferromagnetism is observed in AB-stacked MoTe2/WSe2 moiré hetero-bilayers.

Nature Physics, Published online: 17 August 2023; doi:10.1038/s41567-023-02174-5

Previous work has suggested that at very low temperatures TbInO3 hosts an unconventional quantum ground state. Terahertz time-domain spectroscopy measurements of its excitations show that related exotic effects can persist to room temperature.

npj 2D Materials and Applications, Published online: 17 August 2023; doi:10.1038/s41699-023-00416-x

Room temperature multiferroicity in a transition metal dichalcogenide

Nature Communications, Published online: 16 August 2023; doi:10.1038/s41467-023-40591-5

Transition metal monochalcogenides have been predicted to host interesting superconducting and topological properties, but their synthesis remains challenging. Here, the authors report a self-intercalation method driven by ionic liquid gating to obtain PdTe and NiTe single crystals from PdTe2 and NiTe2, respectively.

Orthorhombic κ-Ga₂O₃ is gaining attention for its spontaneous polarization and ferroelectric properties, suitable for fabrication of “polarization” doped field effect transistors . Crystalline quality is improved for thick layers on GaN/Sapphire or epitaxial lateral overgrowth templates. A doping concentration of 10¹² – 10¹⁴ cm⁻³ is achieved by Sn doping. This can be increased up to 10¹⁹ cm⁻³ by H-plasma, with persistent photoconductivity and photocapacitance pointing to DX-like nature..

Abstract

The structural and electrical properties of undoped and Sn doped κ-Ga₂O₃ layers grown by epitaxial lateral overgrowth on TiO₂/sapphire substrates using stripe and point masks show that the crystalline structure of the films can be greatly improved relative to conventional planar growth. The undoped films are semi-insulating, with the Fermi level pinned near E_C-0.7 eV, and deep electron traps at E_C-0.5 eV and E_C-0.3 eV are detectable in thermally stimulated current and photoinduced current transient spectra measurements. Low concentration Sn doping results in net donor concentrations of ≈ 10¹³ cm⁻³, and deep trap spectra determined by electron traps at E_C-0.5 eV, and deep acceptors with an optical ionization threshold near 2 and 3.1 eV. Treatment of the samples in hydrogen plasma at 330 °C increases the donor density near the surface to ≈ 10¹⁹ cm⁻³. Such samples show strong persistent photocapacitance and photoconductivity, indicating the possible DX-like character of the centers involved. For thin (5 µm) κ-Ga₂O₃ films grown on GaN/sapphire templates, p-type-like behavior is unexpectedly observed in electrical properties and  we  discuss the possible formation of a 2D hole gas at the κ-Ga₂O₃/GaN interface.

Nature Materials, Published online: 16 August 2023; doi:10.1038/s41563-023-01654-6

By means of a precise folding–tearing process, screw dislocations with helical cores — appearing in pairs and taking on a DNA-like double-helix structure — are engineered to control the growth of twisted bilayer graphene.

Nature Communications, Published online: 16 August 2023; doi:10.1038/s41467-023-40665-4

The authors present crackling noise microscopy, a method for measurement of the crackling of individual nanoscale features based on AFM nanoindentation. They use it to investigate crackling noise and avalanches in the domains and domain walls of ferroelectric materials.