Jiuxiang Dai

Abstract

Strongly bound excitons in atomically thin transition metal dichalcogenides offer many opportunities to reveal the underlying physics of basic quasiparticles and many-body effects in the two-dimensional (2D) limit. Comprehensive reflection investigation on band-edge exciton transitions is essential to exploring wealthy light-matter interactions in the emerging 2D semiconductors, whereas angle-resolved reflection (ARR) characteristics of intralayer and interlayer excitons in 2D MoS₂ layers remain unclear. Herein, we report ARR spectroscopic features of A, B, and interlayer excitons in monolayer (ML) and bilayer (BL) MoS₂ on three kinds of photonic substrates, involving distinct exciton-photon interactions. In a BL MoS₂ on a protected silver mirror, the interlayer exciton with one-third amplitude of A exciton appears at 0.05 eV above the A exciton energy, exhibiting an angle-insensitive energy dispersion. When ML and BL MoS₂ lie on a SiO₂-covered silicon, the broad trapped-photon mode weakly couples with the reflection bands of A and B excitons by the Fano resonance effect, causing the asymmetric lineshapes and the redshifted energies. After transferring MoS₂ layers onto a one-dimensional photonic crystal, two high-lying branches of B-exciton polaritons are formed by the interactions between B excitons and Bragg photons, beyond the weak-coupling regime. This work provides ARR spectral benchmarks of A, B, and interlayer excitons in ML and BL MoS₂, gaining insights into the interpretation of light-matter interactions in 2D semiconductors and the design of their devices for practical photonic applications.

Nature Electronics, Published online: 22 December 2022; doi:10.1038/s41928-022-00879-8

Nature Electronics, Published online: 21 December 2022; doi:10.1038/s41928-022-00890-z

Nature Nanotechnology, Published online: 21 December 2022; doi:10.1038/s41565-022-01253-7

Abstract

The full E-field control of multiferroic interfacial magnetism is a long-standing challenge for micro-electromechanical systems (MEMS) and has the potential to transform electronics operation mechanisms. When scaling down conventional complementary metal-oxide semiconductor (CMOS) devices, increased heating dissipation becomes a top concern. Combining the highly correlated ferroic orders, notably the strongly coupled interfacial magnetoelectric (ME) interactions, may lead to devices beyond CMOS. These devices use the electric field to regulate magnetization, which opens up the prospect of downsizing, improved performance, and lower power consumption. To broadly survey this tremendous scope within the last five years, this review summarizes advances in voltage control of interfacial magnetism (VCIM) with various material system selection; controlling effects with different gating methods are also explored. Five classic mechanisms are demonstrated: strain, exchange bias, orbital reconstruction, and the electrostatic and electrochemical. The encouraging photovoltaic approach is also discussed. Each method’s capabilities and application scenarios are compared. Analyses of the comprehensive gating results of different magnetic coupling effects such as perpendicular magnetic anisotropy (PMA) and Ruderman—Kittel—Kasuya—Yosida (RKKY) are additionally made. At last, controlling of skyrmions and two-dimensional (2D) material magnetization is summarized, indicating that E-field gating offers a universal approach with few limitations for material selection. These results point to potential for E-field control interfacial magnetism and predict significant future advancements for spintronics.

Abstract

Single-layer MoS₂ produced by mechanical exfoliation is usually connected to thicker and multilayer regions. We show a facile laser trimming method to insulate single-layer MoS₂ regions from thicker ones. We demonstrate, through electrical characterization, that the laser trimming method can be used to pattern single-layer MoS₂ channels with regular geometry and electrically disconnected from the thicker areas. Scanning photocurrent microscope further confirms that in the as-deposited flake (connected to a multilayer area) most of the photocurrent is being generated in the thicker flake region. After laser trimming, scanning photocurrent microscopy shows how only the single-layer MoS₂ region contributes to the photocurrent generation. The presented method is a direct-write and lithography-free (no need of resist or wet chemicals) alternative to reactive ion etching process to pattern the flakes that can be easily adopted by many research groups fabricating devices with MoS₂ and similar two-dimensional materials.

It is known that the growth of large domains of single crystal 2D materials is important for device integration. Here, salt-assisted deposition approach is presented to grow atomically-thin, millimeter size 2D Bi₂O₂Se domains at a low-temperature. Because of the high quality of the salt-assisted-grown material, a few-layer Bi₂O₂Se-based FET represents an excellent high on/off ratio of ≈10⁷ and mobility of ≈287 cm² V⁻¹ s⁻¹. The results presented in this work, reveal the remarkable potential of 2D Bi₂O₂Se for future 2D electronics.

Abstract

Bi₂O₂Se is the most promising 2D material due to its semiconducting feature and high mobility, making it propitious channel material for high-performance electronics that demands highly crystalline Bi₂O₂Se at low-growth temperature. Here, a low-temperature salt-assisted chemical vapor deposition approach for growing single-domain Bi₂O₂Se on a millimeter scale with thicknesses of multilayer to monolayer is presented. Because of the advantage of thickness-dependent growth, systematical scrutiny of layer-dependent Raman spectroscopy of Bi₂O₂Se from monolayer to bulk is investigated, revealing a redshift of the A _1g mode at 162.4 cm⁻¹. Moreover, the long-term environmental stability of ≈2.4 nm thick Bi₂O₂Se is confirmed after exposing the sample for 1.5 years to air. The backgated field effect transistor (FET) based on a few-layered Bi₂O₂Se flake represents decent carrier mobility (≈287 cm² V⁻¹s⁻¹) and an ON/OFF ratio of up to 10⁷. This report indicates a technique to grow large-domain thickness controlled Bi₂O₂Se single crystals for electronics.

The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes’ mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. The most promising results in MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations are provided.

Abstract

The large and rapidly growing family of 2D early transition metal carbides, nitrides, and carbonitrides (MXenes) raises significant interest in the materials science and chemistry of materials communities. Discovered a little more than a decade ago, MXenes have already demonstrated outstanding potential in various applications ranging from energy storage to biology and medicine. The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes’ mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. Although research on the understanding of the friction and wear performance of MXenes under dry and lubricated conditions is still in its early stages, it has experienced rapid growth due to the excellent mechanical properties and chemical reactivities offered by MXenes that make them adaptable to being combined with other materials, thus boosting their tribological performance. In this perspective, the most promising results in the area of MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations that could be useful for experts as well as newcomers to MXenes research, in particular, to the emerging area of MXene tribology, are provided.

Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

Abstract

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

Atomically thin MoS₂ electrical switches are characterized for the first time in operando and in ambient conditions through a non-invasive spectroscopy-based method. The study sheds light on the – controversial and still debated – dynamics driving the switching mechanism. It reveals volatile metallic filaments percolating interatomic spacing in MoS₂ and stresses the impact of transfer residues on the electrical performance of switches.

Abstract

MoS₂ nanoswitches have shown superb ultralow switching energies without excessive leakage currents. However, the debate about the origin and volatility of electrical switching is unresolved due to the lack of adequate nanoimaging of devices in operando. Here, three optical techniques are combined to perform the first noninvasive in situ characterization of nanosized MoS₂ devices. This study reveals volatile threshold resistive switching due to the intercalation of metallic atoms from electrodes directly between Mo and S atoms, without the assistance of sulfur vacancies. A “semi-memristive” effect driven by an organic adlayer adjacent to MoS₂ is observed, which suggests that nonvolatility can be achieved by careful interface engineering. These findings provide a crucial understanding of nanoprocess in vertically biased MoS₂ nanosheets, which opens new routes to conscious engineering and optimization of 2D electronics.

Nature Electronics, Published online: 20 December 2022; doi:10.1038/s41928-022-00909-5

Nature Reviews Materials, Published online: 20 December 2022; doi:10.1038/s41578-022-00526-w

High performing non-centrosymmetric γ-NaAsSe₂ is stabilized by doping the As site with Sb, resulting in γ-NaAs_0.95Sb_0.05Se_2. Large single crystals are successfully obtained via zone refining and the Bridgman method. A giant second harmonic generation (SHG) coefficient of |d ₁₁| = 648 ± 74 pm V⁻¹ is observed at 2 µm. These properties make it a promising candidate for infrared laser applications.

Abstract

The dearth of suitable materials significantly restricts the practical development of infrared (IR) laser systems with highly efficient and broadband tuning. Recently, γ-NaAsSe₂ is reported, and it exhibits a large nonlinear second-harmonic generation (SHG) coefficient of 590 pm V⁻¹ at 2 µm. However, the crystal growth of γ-NaAsSe₂ is challenging because it undergoes a phase transition to centrosymmetric δ-NaAsSe₂. Herein, the stabilization of non-centrosymmetric γ-NaAsSe₂ by doping the As site with Sb, which results in γ-NaAs_0.95Sb_0.05Se₂ is reported. The congruent melting behavior is confirmed by differential thermal analysis with a melting temperature of 450 °C and crystallization temperature of 415 °C. Single crystals with dimensions of 3 mm × 2 mm are successfully obtained via zone refining and the Bridgman method. The purification of the material plays a significant role in crystal growth and results in a bandgap of 1.78 eV and thermal conductivity of 0.79 Wm⁻¹ K⁻¹. The single-crystal SHG coefficient of γ-NaAs_0.95Sb_0.05Se₂ exhibits an enormous value of |d ₁₁| = 648 ± 74 pm V⁻¹, which is comparable to that of γ-NaAsSe₂ and ≈20× larger than that of AgGaSe₂. The bandgap of γ-NaAs_0.95Sb_0.05Se₂ (1.78 eV) is similar to that of AgGaSe₂, thus rendering it highly attractive as a high-performing nonlinear optical material.

Nature Communications, Published online: 19 December 2022; doi:10.1038/s41467-022-35477-x