Boosting in-plane anisotropy by periodic phase engineering in two-dimensional VO2 single crystals

Observation of optical anisotropy and a linear dichroism transition in layered silicon phosphide

The investigation of in-plane two-dimensional (2D) anisotropic materials has garnered significant attention due to their exceptional electronic, optical, and mechanical characteristics. The anisotropic optical properties and angle-dependent photodetectors based on 2D anisotropic materials have been extensively studied. However, novel in-plane anisotropic materials still need to be explored to satisfy for distinct environments and devices. Here, we report the remarkable anisotropic behavior of excitons and demonstrate a unique linear-dichroism transition of absorption between ultraviolet and visible light in layered silicon phosphide (SiP) through the analysis of polarization photoluminescence (PL) and absorbance spectra. Its high absorption linear dichroism ratio of 1.16 at 388 nm, 1.15 at 532 nm, and 1.19 at 733 nm is revealed, suggesting the brilliant non-isotropic responses. The robust periodic variation of the A₁ and A₂ Raman modes in 2D SiP materials allows for the determination of their crystal orientation. Furthermore, the presence of indirect excitons with phonon sidebands in the temperature-dependent PL spectra exhibits non-monotonic energy shifts with increasing temperature, which is attributed to an enhanced electron-phonon interaction and thermal expansion. Our findings provide valuable insights into the fundamental physical properties of layered SiP and offer guidelines for designing polarization-sensitive photodetectors and angle-dependent devices based on 2D anisotropic materials.

Emerging 2D Metal Oxides: From Synthesis to Device Integration

2D metal oxides have aroused increasing attention in the field of electronics and optoelectronics due to their intriguing physical properties. In this review, an overview of recent advances on synthesis of 2D metal oxides and their electronic applications is presented. First, the tunable physical properties of 2D metal oxides that relate to the structure (various oxidation-state forms, polymorphism, etc.), crystallinity and defects (anisotropy, point defects, and grain boundary), and thickness (quantum confinement effect, interfacial effect, etc.) are discussed. Then, advanced synthesis methods for 2D metal oxides besides mechanical exfoliation are introduced and classified into solution process, vapor-phase deposition, and native oxidation on a metal source. Later, the various roles of 2D metal oxides in widespread applications, i.e., transistors, inverters, photodetectors, piezotronics, memristors, and potential applications (solar cell, spintronics, and superconducting devices) are discussed. Finally, an outlook of existing challenges and future opportunities in 2D metal oxides is proposed.

Modulation of Switching Characteristics in a Single VO2 Nanobeam with Interfacial Strain via the Interconnection of Multiple Nanoscale Channels

We demonstrate the modulation of electrical switching properties through the interconnection of multiple nanoscale channels (∼600 nm) in a single VO₂ nanobeam with a coexisting metal-insulator (M-I) domain configuration during phase transition. The Raman scattering characteristics of the synthesized VO₂ nanobeams provide evidence that substrate-induced interfacial strain can be inhomogeneously distributed along the length of the nanobeam. Interestingly, the nanoscale VO₂ devices with the same channel length and width exhibit distinct differences in hysteric current-voltage characteristics, which are explained by theoretical calculations of resistance change combined with Joule heating simulations of the nanoscale VO₂ channels. The observed results can be attributed to the difference in the spatial distribution and fraction ratios of M-I domains due to interfacial strain in the nanoscale VO₂ channels during the metal-insulator transition process. Moreover, we demonstrate the electrically activated resistive switching characteristics based on the hysteresis behaviors of the interconnected nanoscale channels, implying the possibility of manipulating multiple resistive states. Our results may offer insights into the nanoscale engineering of correlated phases in VO₂ as the key materials of neuromorphic computing for which nonlinear conductance is essential.

Type-I PtS2/MoS2 van der Waals heterojunctions with tunable photovoltaic effects and high photosensitivity

Recent advances in two-dimensional (2D) materials play an essential role in boosting modern electronics and optoelectronics. Thus far, transition metal dichalcogenides (TMDs) as emerging members of 2D materials, and the van der Waals heterostructures (vdWHs) based on TMDs have been extensively investigated owing to their prominent capabilities and unique crystal structures. In this work, an original vdWH composed of molybdenum disulfide (MoS₂) and platinum disulfide (PtS₂) was comprehensively studied as a field-effect transistor (FET) and photodetector. A gate-tunable rectifying behavior was obtained, stemming from the band design of PtS₂/MoS₂ vdWH. Upon 685 nm laser illumination, it also exhibited a superior photodetection performance with a distinctly high photoresponsivity of 403 A W^-1, a comparable detectivity of 1.07 × 10¹¹ Jones, and an excellent external quantum efficiency of 7.32 × 10⁴%. More importantly, fast rise (24 ms) and decay (21 ms) times were obtained under 685 nm light illumination attributed to the unilateral depletion region structure. Further, the photovoltaic effect and photocurrent of the heterojunction could be modulated by a back gate voltage. All these results indicated that such 2D-TMD-based vdWHs provide a new idea for realizing high-performance electronic and optoelectronic devices.

First-principles investigation of in-plane anisotropies in XYTe4 monolayers with X = Hf, Zr, Ti and Y = Si, Ge

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe₄ (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe₄ reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe₄ monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe₄ differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe₄ monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.

Strain-Free Layered Semiconductors for 2D Transistors with On-State Current Density Exceeding 1.3 mA μm-1

High-mobility and air-stable two-dimensional (2D) Bi₂O₂Se semiconductor holds promise as an alternative fast channel material for next-generation transistors. However, one of the key challenges remaining in 2D Bi₂O₂Se is to prepare high-quality crystals to fabricate the high-performance transistors with a high on-state current density. Here, we present the free-standing growth of strain-free 2D Bi₂O₂Se crystals. An ultrahigh Hall mobility of 160 000 cm² V^-1 s^-1 is measured in strain-free Bi₂O₂Se crystals at 2 K, which enables the observation of Shubnikov-de Haas quantum oscillations and shows substantially higher (>4 times) mobility over previous in-plane 2D crystals. The fabricated 2D transistors feature an on-off current ratio of ∼10⁶ and a record-high on-state current density of ∼1.33 mA μm^-1, which is comparable to that of commercial Si and Ge n-type field-effect transistors (FETs) for similar channel length. Strain-free 2D Bi₂O₂Se provides a promising material platform for studying novel quantum phenomena and exploration of high-performance low-power electronics.

Atomistic Observation of the Local Phase Transition in MoTe2 for Application in Homojunction Photodetectors

Direct atomic-scale observation of the local phase transition in transition metal dichalcogenides (TMDCs) is critically required to carry out in-depth studies of their atomic structures and electronic features. However, the structural aspects including crystal symmetries tend to be unclear and unintuitive in real-time monitoring of the phase transition process. Herein, by using in situ transmission electron microscopy, information about the phase transition mechanism of MoTe₂ from hexagonal structure (2H phase) to monoclinic structure (1T' phase) driven by sublimation of Te atoms after a spike annealing is obtained directly. Furthermore, with the control of Te atom sublimation by modulating the hexagonal boron nitride (h-BN) coverage in the desired area, the lateral 1T'-enriched MoTe₂ /2H MoTe₂ homojunction can be one-step constructed via an annealing treatment. Owing to the gradient bandgap provided by 1T'-enriched MoTe₂ and 2H MoTe₂ , the photodetector composed of the 1T'-enriched MoTe₂ /2H MoTe₂ homojunction shows fast photoresponse and ten times larger photocurrents than that consisting of a pure 2H MoTe₂ channel. The study reveals a route to improve the performance of optoelectronic and electronic devices based on TMDCs with both semiconducting and semimetallic phases.