Jiuxiang Dai

A new topology diagram was developed for the efficient and designed synthesis of 2D polymers and open frameworks, which led to the creation of novel 2D sp²-carbon materials via a C=C bond-formation reaction. The sp²-carbon frameworks are highly emissive and exhibit two-photon up-conversion luminescence, create novel 2D semiconductors with low band gaps yet tunable band structures, and achieve ultrahigh charge mobilities close to theoretical maxima.

Abstract

Despite rapid progress over the past decade, most polycondensation systems even upon a small structural variation of the building units eventually result in amorphous polymers other than the desired crystalline covalent organic frameworks. This synthetic dilemma is a central and challenging issue of the field. Here we report a novel approach based on module-patterned polymerization to enable efficient and designed synthesis of crystalline porous polymeric frameworks. This strategy features a wide applicability to allow the use of various knots of different structures, enables polycondensation with diverse linkers, and develops a diversity of novel crystalline 2D polymers and frameworks, as demonstrated by using the C=C bond-formation polycondensation reaction. The new sp²-carbon frameworks are highly emissive and enable up-conversion luminescence, offer low band gap semiconductors with tunable band structures, and achieve ultrahigh charge mobilities close to theoretically predicted maxima.

npj 2D Materials and Applications, Published online: 21 December 2021; doi:10.1038/s41699-021-00276-3

Iron-rich talc as air-stable platform for magnetic two-dimensional materials

Nature Electronics, Published online: 21 December 2021; doi:10.1038/s41928-021-00683-w

Inorganic molecular crystal films of antimony trioxide can be fabricated using thermal evaporation deposition and used as a van der Waals dielectric in molybdenum disulfide field-effect transistors.

Nature Electronics, Published online: 21 December 2021; doi:10.1038/s41928-021-00688-5

A vertical transistor and resistive memory can be integrated on a single vertical III–V semiconductor nanowire on silicon, creating a compact cell capable of Boolean logic operations.

Crystalline MoS₂ layers are successfully formed at wafer-scale sizes by a pulsed metal–organic chemical vapor deposition process at a low temperature of 350 °C. MoS₂ growth mode is controlled by adjusting the growth pressure and Ar/H₂S flow rate ratio. The photodetection and gas sensing characteristics are investigated using MoS₂ layers with different morphologies, and potential uses in various devices are suggested.

Abstract

2D semiconductor materials with layered crystal structures have attracted great interest as promising candidates for electronic, optoelectronic, and sensor applications due to their unique and superior characteristics. However, a large-area synthesis process for various applications and practical mass production is still lacking. In particular, there is a limitation in that a high process temperature and a very long process time are required to deposit a crystallized 2D material on a large area. Herein, pulsed metal–organic chemical vapor deposition (p-MOCVD) is proposed for the growth of wafer-scale crystalline MoS₂ thin films to overcome the existing limitations. In the p-MOCVD process, precursors are repeatedly injected at regular intervals to enhance the migration of precursors on the surface. As a result, crystalline MoS₂ is successfully synthesized at the lowest temperature (350 °C) reported so far in a very short process time of 550 s. In addition, it is found that the horizontal and vertical growth modes of MoS₂ can be effectively controlled by adjusting key process parameters. Finally, various applications are presented by demonstrating the photodetector (detectivity = 18.1 × 10⁶ at light power of 1 mW) and chemical sensor (response = 38% at 100 ppm of NO₂ gas) devices.

The transition metal dichalcogenide MoTe₂ thin flakes thinner than ≈ 19.5 nm do not experience a monoclinic-to-orthorhombic phase transition and still have a monoclinic 1T′ structure at temperatures down to 80 K, which provides a material platform for realizing 3D second-order topological insulator states in van der Waals materials at low temperatures.

Abstract

A van der Waals material, MoTe₂ with a monoclinic 1T′ crystal structure is a candidate for 3D second-order topological insulators (SOTIs) hosting gapless hinge states and insulating surface states. However, due to the temperature-induced structural phase transition, the monoclinic 1T′ structure of MoTe₂ is transformed into the orthorhombic T _d structure as the temperature is lowered, which hinders the experimental verification and electronic applications of the predicted SOTI state at low temperatures. Here, systematic Raman spectroscopy studies of the exfoliated MoTe₂ thin flakes with variable thicknesses at different temperatures, are presented. As a spectroscopic signature of the orthorhombic T _d structure of MoTe₂, the out-of-plane vibration mode D at ≈ 125 cm^–1 is always visible below a certain temperature in the multilayer flakes thicker than ≈ 27.7 nm, but vanishes in the temperature range from 80 to 320 K when the flake thickness becomes lower than ≈ 19.5 nm. The absence of the out-of-plane vibration mode D in the Raman spectra here demonstrates not only the disappearance of the monoclinic-to-orthorhombic phase transition but also the persistence of the monoclinic 1T′ structure in the MoTe₂ thin flakes thinner than ≈ 19.5 nm at low temperatures down to 80 K, which may be caused by the high enough density of the holes introduced during the gold-enhanced exfoliation process and exposure to air. The MoTe₂ thin flakes with the low-temperature monoclinic 1T′ structure provide a material platform for realizing SOTI states in van der Waals materials at low temperatures, which paves the way for developing a new generation of electronic devices based on SOTIs.

In this work, ultrabroadband photoelectric detectors covering visible, infrared, terahertz, and millimeter wave simultaneously are demonstrated through two kinds photoelectric effect synergy of photoexcited electron–hole pairs and electromagnetic induced well effect based on metal–Te–metal structure. The strategy of utilizing full photoelectric effect provides an effective route to achieve ultrabroadband photodetection with high sensitivity and fast response.

Abstract

Ultrabroadband photodetection is of great significance in numerous cutting-edge technologies including imaging, communications, and medicine. However, since photon detectors are selective in wavelength and thermal detectors are slow in response, developing high performance and ultrabroadband photodetectors is extremely difficult. Herein, one demonstrates an ultrabroadband photoelectric detector covering visible, infrared, terahertz, and millimeter wave simultaneously based on single metal–Te–metal structure. Through the two kinds of photoelectric effect synergy of photoexcited electron–hole pairs and electromagnetic induced well effect, the detector achieves the responsivities of 0.793 A W⁻¹ at 635 nm, 9.38 A W⁻¹ at 1550 nm, 9.83 A W⁻¹ at 0.305 THz, 24.8 A W⁻¹ at 0.250 THz, 87.8 A W⁻¹ at 0.172 THz, and 986 A W⁻¹ at 0.022 THz, respectively. It also exhibits excellent polarization detection with a dichroic ratio of 468. The excellent performance of the detector is further verified by high-resolution imaging experiments. Finally, the high stability of the detector is tested by long-term deposition in air and high-temperature aging. The strategy provides a recipe to achieve ultrabroadband photodetection with high sensitivity and fast response utilizing full photoelectric effect.

The unusual phases, characteristics, and applications of metal chalcogenides are introduced. First, the unusual phases of metal chalcogenides from different classes and their excellent properties are discussed. Then the methods for producing the unusual phases are introduced. Followed by an outlook and discussions on how to prepare the unusual phase metal dichalcogenides in terms of synthetic methodology and potential applications.

Abstract

Layered metal chalcogenides, as a “rich” family of 2D materials, have attracted increasing research interest due to the abundant choices of materials with diverse structures and rich electronic characteristics. Although the common metal chalcogenide phases such as 2H and 1T have been intensively studied, many other unusual phases are rarely explored, and some of these show fascinating behaviors including superconductivity, ferroelectrics, ferromagnetism, etc. From this perspective, the unusual phases of metal chalcogenides and their characteristics, as well as potential applications are introduced. First, the unusual phases of metal chalcogenides from different classes, including transition metal dichalcogenides, magnetic element-based chalcogenides, and metal phosphorus chalcogenides, are discussed, respectively. Meanwhile, their excellent properties of different unusual phases are introduced. Then, the methods for producing the unusual phases are discussed, specifically, the stabilization strategies during the chemical vapor deposition process for the unusual phase growth are discussed, followed by an outlook and discussions on how to prepare the unusual phase metal dichalcogenides in terms of synthetic methodology and potential applications.

Lanthanide ion-doped inorganic halide perovskite nanocrystals are presented as non-destructive dispersants capable of not only dispersing transition metal dichalcogenide nanosheets in the liquid phase but also enhancing the photodetection properties of the nanosheets, leading to a high performance near-infrared image sensor.

Abstract

Transition metal dichalcogenide (TMD) nanosheets exfoliated in the liquid phase are of significant interest owing to their potential for scalable and flexible photoelectronic applications. Although various dispersants such as surfactants, oligomers, and polymers are used to obtain highly exfoliated TMD nanosheets, most of them are electrically insulating and need to be removed; otherwise, the photoelectric properties of the TMD nanosheets degrade. Here, inorganic halide perovskite nanocrystals (NCs) of CsPbX₃ (X = Cl, Br, or I) are presented as non-destructive dispersants capable of dispersing TMD nanosheets in the liquid phase and enhancing the photodetection properties of the nanosheets, thus eliminating the need to remove the dispersant. MoSe₂ nanosheets dispersed in the liquid phase are adsorbed with CsPbCl₃ NCs. The CsPbCl₃ nanocrystals on MoSe₂ efficiently withdraw electrons from the nanosheets, and suppress the dark current of the MoSe₂ nanosheets, leading to flexible near-infrared MoSe₂ photodetectors with a high ON/OFF photocurrent ratio and detectivity. Moreover, lanthanide ion-doped CsPbCl₃ NCs enhance the ON/OFF current ratio to >10⁶. Meanwhile, the dispersion stability of the MoSe₂ nanosheets exfoliated with the perovskite NCs is sufficiently high.

The excellent optoelectrical properties are revealed for ultrathin metallic ruthenium (Ru) film, having a surface oxide layer, prepared by layer-by-layer technique. The figure-of-merit (FOM) achieved for Ru film is the best among the thin films fabricated using a wet-chemistry process with 2D nanosheets. The observed FOM is even comparable to that of conventional indium–tin–oxide electrodes.

Abstract

The growing industrial demand for flexible optoelectric devices has led to intensive researches on highly flexible transparent electrode materials such as graphene, reduced graphene-oxide (r-GO), Ag-nanowire, and 2D metal oxides. However, except Ag-nanowire, transparent electrode materials having optoelectric properties comparable to that of indium–tin–oxide (ITO) have not yet been developed. In this study, an ultrathin ruthenium film with a ruthenium oxide (RuO₂) subsurface layer has been introduced as a flexible transparent electrode. The metallic Ru thin film is fabricated from a RuO₂ nanosheet using a layer-by-layer coating technique, followed by thermal reduction. The thin film (≈6 nm) reveals comparable sheet resistance and transmittance as that of conventional ITO electrodes. The high transmittance (≈79%) of the metal thin film in the visible range is attributed to the presence of an oxide subsurface layer which acts as antireflection. The Ru film (with oxide subsurface layer) with figure-of-merit ≈3.4 × 10⁻⁴ Ω⁻¹ shows the best performance among the thin films fabricated using a wet-chemistry process with 2D nanosheets including graphene, r-GO, and other metal oxides. In addition, the high mechanical flexibility of Ru thin film makes it next-generation flexible transparent conducting electrodes, beyond graphene, r-GO, and 2D metal oxides.

A wafer-scale InN/In₂S₃ nanorod array with good homogeneity is grown on Si substrates for ultrafast photodetection. Following analysis of the synthesis mechanism of the In₂S₃/InN/Si heterojunction, the photodetector device exhibits excellent self-powered properties and an ultrafast photoresponse with a rise/fall time of 22/32 µs. The core–shell nanostructure hybrid-heterojunction introduces a novel idea for wafer-scale nano-photodetectors.

Abstract

Self-powered photodetectors have paved the way for electronic applications in fields such as civilian communication, infrared mapping, and industrial automatic control. However, most self-powered photodetectors have faced photoresponse-speed and device-scale bottlenecks. Herein, a novel, self-powered detector with an ultrafast response speed based on a core–shell InN/In₂S₃ nanorod array is proposed. A wafer-scale InN/In₂S₃ nanorod array with good homogeneity is synthesized on Si substrates via a simple two-step method involving molecular beam epitaxy and chemical vapor deposition. The photodetector device exhibits excellent self-powered properties and a high current on/off ratio of 5 × 10³. Further analyses determined that the device have an excellent photovoltaic responsivity and detectivity of 140 mA·W⁻¹ and 4.0 × 10¹⁰ Jones, respectively (0 V). Impressively, the device exhibits an ultrafast photoresponse with a rise/fall time of 22/32 µs. The self-powered InN/In₂S₃ photodetector with an ultrafast response speed shows superior potential for electronic applications. The core–shell nanostructure hybrid heterojunction introduces a novel idea for wafer-scale nano-photodetectors.

Optical harmonic generation (OHG) is reviewed in 2D materials. First, 2D materials are introduced with respect to their optical harmonic properties. Second, various types of characterization modes and tuning techniques in 2D materials are reviewed using their OHG properties. Finally, a review of 2D materials based device applications is presented based on their OHG properties.

Abstract

2D materials are emerging as ideal candidates for fundamental investigations and new technologies due to their unique optoelectronic properties. Giant nonlinear susceptibility and perfect phase matching in 2D materials lead to extraordinary nonlinear light matter interactions, thus enabling several potential applications and fundamental scientific discoveries in nonlinear optics. For instance, second harmonic generation in 2D materials play an important role in optical devices such as, lasers, tunable waveguides, electro-optic modulators, and switches. This review will discuss optical harmonic generation (OHG) processes, various characterization modes, and tuning techniques in 2D materials. The future prospectives for OHG in 2D materials is discussed. The extremely promising attributes of combining nonlinear optics and 2D materials is becoming a highly important multidisciplinary field.

A 2D ultrathin Z-scheme ZnIn₂S₄/g-C₃N₄ heterojunction is precisely constructed by using a ligand-assisted hydrothermal method. The optimized sample exhibits excellent photocatalytic H₂ evolution performance (14.799 mmol g⁻¹ h⁻¹) without Pt as a cocatalyst. The experimental characterizations and DFT calculations comprehensively demonstrate the roles of morphology control, interfacial regulation, and especially the Z-scheme mechanism in improving the photocatalytic performance.

Abstract

2D layered nanomaterials as photocatalysts have attracted much attention in the field of solar hydrogen production due to their unique electronic structure and abundant active sites. Nevertheless, the rational design and interfacial regulation of 2D Z-scheme heterojunction are still challenging. Herein, an ultrathin 2D ZnIn₂S₄/g-C₃N₄ Z-scheme heterojunction is precisely constructed via in-situ growth of ZnIn₂S₄ on the g-C₃N₄. By carefully regulating the interface structure in heterojunction, the hydrogen evolution performance can be greatly improved. The optimized photocatalyst exhibits a remarkable photocatalytic activity without Pt as cocatalyst, which is primarily ascribed to the synergistic effect of abundant active sites, enhanced photoresponse, and valid interfacial charge transfer channels. Meanwhile, the spectroscopic analyses and density functional theory (DFT) calculation results comprehensively prove that the promoted interfacial charge separation in 2D Z-scheme heterojunction is another key factor for the enhanced photocatalytic performance. This work offers a new avenue for the rational design of ultrathin Z-scheme heterojunction photocatalysts with improved photocatalytic performance through interfacial engineering.

Light: Science & Applications, Published online: 20 December 2021; doi:10.1038/s41377-021-00685-5

Two-dimensional terahertz strong-field spectroscopy reveals wave-mixing processes up to eighth order in a free-running quantum cascade laser, unraveling its sub-cycle gain dynamics and nonlinearities in a regime of negative absorption.

We address the impact of crystal phase disorder on the generation of helicity-dependent photocurrents in layered MoTe 2 , which is one of the van der Waals materials to realize the topological type-II Weyl semimetal phase. Using scanning photocurrent microscopy, we spatially probe the phase transition and its hysteresis between the centrosymmetric, monoclinic 1Tʹ phase to the symmetry-broken, orthorhombic T ##IMG## [http://ej.iop.org/images/2053-1583/9/1/011002/tdmac3e03ieqn1.gif] {$_\textrm{d}$} phase as a function of temperature. We find a highly disordered photocurrent response in the intermediate temperature regime. Moreover, we demonstrate that helicity-dependent and ultrafast photocurrents in MoTe 2 arise most likely from a local breaking of the electronic symmetries. Our results highlight the prospects of local domain morphologies and ultrafast relaxation dynamics on the optoelectronic properties of low-dimensional van der Waals cir...

Author(s): Giovanni Marini and Matteo Calandra

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with ...

[Phys. Rev. Lett. 127, 257401] Published Fri Dec 17, 2021