Xingxing Zhang

Dissimilar transition metal dichalcogenides (TMDs) are grown concurrently and location‐selectively by a new method. Precise control over the transition‐metal‐precursor vapor pressure allows successful lateral and vertical heterojunction growth, as well as growth of p‐ and n‐type TMDs at desired locations. A new synthetic strategy for future (opto)electronic applications is thus provided.

Abstract

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large‐area electronics and circuits strongly relies on wafer‐scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal‐guided selective growth (MGSG), is reported. The success of control over the transition‐metal‐precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p‐ and n‐type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom‐up complementary metal‐oxide‐semiconductor inverter based on p‐type WSe₂ and n‐type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

The metallic 1T′‐MoS₂ monolayer is an ideal platform for novel topological electronic states to emerge, and it also exhibits excellent energy conversion and storage properties. Solution processing is introduced to realize phase‐pure 1T′‐MoS₂ monolayers with large lateral size. The high‐quality and large‐sized 1T′‐MoS₂ nanosheets exhibit thickness‐dependent superconductivity, which opens up insights into the novel topological states of metastable 2D transition metal dichalcogenide materials.

Abstract

The development of transition metal dichalcogenides has greatly accelerated research in the 2D realm, especially for layered MoS₂. Crucially, the metallic MoS₂ monolayer is an ideal platform in which novel topological electronic states can emerge and also exhibits excellent energy conversion and storage properties. However, as its intrinsic metallic phase, little is known about the nature of 2D 1T′‐MoS₂, probably because of limited phase uniformity (<80%) and lateral size (usually <1 µm) in produced materials. Herein, solution processing to realize high phase‐purity 1T′‐MoS₂ monolayers with large lateral size is demonstrated. Direct chemical exfoliation of millimeter‐sized 1T′ crystal is introduced to successfully produce a high‐yield of 1T′‐MoS₂ monolayers with over 97% phase purity and unprecedentedly large size up to tens of micrometers. Furthermore, the large‐sized and high‐quality 1T′‐MoS₂ nanosheets exhibit clear intrinsic superconductivity among all thicknesses down to monolayer, accompanied by a slow drop of transition temperature from 6.1 to 3.0 K. Prominently, unconventional superconducting behavior with upper critical field far beyond the Pauli limit is observed in the centrosymmetric 1T′‐MoS₂ structure. The results open up an ideal approach to explore the properties of 2D metastable polymorphic materials.

A highly sensitive infrared light photo‐detector is fabricated by transferring multilayer PdSe₂ on a germanium nanocones array with a strong light‐trapping effect. The as‐assembled PdSe₂/GeNCs hybrid heterojunction devices can also record simple near‐infrared images.

Abstract

In this study, a sensitive infrared photodetector (IRPD) composed of a germanium nanocones (GeNCs) array and PdSe₂ multilayer is presented, which is obtained by a straightforward selenization approach. The as‐assembled PdSe₂/GeNCs hybrid heterojunction exhibits obvious photovoltaic behavior to 1550 nm illumination, which renders the IRPD a self‐driven device without external power supply. Further device analysis reveals that the PdSe₂/GeNCs hybrid based IRPD exhibits high sensitivity to 1350, 1550, and 1650 nm illumination with excellent stability and reproducibility. The responsivity and external quantum efficiency is as high as 530.2 mA W⁻¹ and 42.4%, respectively. Such a relatively good device performance is related to the strong light trapping effect of GeNCs array, according to the theoretical simulation based on finite‐difference time‐domain. It is also found that the IRPD shows an abnormal sensitivity to IR illumination with a wavelength of 2200 nm. Finally, the present individual IRPD can also record the simple “F” image produced by 1550 nm, suggesting the promising application of the PdSe₂/GeNCs hybrid device in future infrared optoelectronic systems.

Probing van der Waals interactions at two-dimensional heterointerfaces

Probing van der Waals interactions at two-dimensional heterointerfaces, Published online: 25 March 2019; doi:10.1038/s41565-019-0405-2

MoS2 is shown to exhibit a stronger vdW interaction with graphite than with hexagon boron nitride, which is well described by Lifshitz theory and utilized to construct 2D heterostructures in a sophisticated way

In optically excited 2D phototransistors, charge transport is often affected by photodoping effects. Recently, it was shown that such effects are especially strong and persistent for graphene/h-BN heterostructures, and that they can be used to controllably tune the charge neutrality point of graphene. In this work we investigate how this technique can be extended to h-BN encapsulated monolayer MoSe 2 phototransistors at room temperature. By exposing the sample to 785 nm laser excitation we can controllably increase the charge carrier density of the MoSe 2 channel by Δ n   ≈  4.45  ×  10 12 cm −2 , equivalent to applying a back gate voltage of ~60 V. We also evaluate the efficiency of photodoping at different illumination wavelengths, finding that it is strongly correlated with the light absorption by the MoSe 2 layer, and maximizes for excitation on-resonance with the A exciton absorption. This indicates that the photodoping ...

In article number 1805582, Bin Wu, Yunqi Liu, and co‐workers demonstrate a library of h‐BN epitaxy on graphene–h‐BN templates of various dimensionalities, including a two‐dimensional homo‐/heteromaterial surface and one‐dimensional interfaces of homo‐/heteromaterials. They provide a framework that allows the description of various types of kinetic growth by combined geometric and structural modelling. This work provides a general viewpoint for understanding epitaxial growth in complex systems.

Large‐domain‐size ultrathin MTe₂ (M = V, Nb, Ta) nanoplates with the thickness down to the monolayer regime are prepared using 2D WSe₂ or WS₂ as the growth substrate. The atomically flat dangling‐bond‐free surface of WSe₂ (WS₂) ensures a minimized diffusion barrier for the successful realization of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) nanosheets.

Abstract

2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe₂ or WS₂ as the growth substrate, the synthesis of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe₂/WSe₂ (WS₂) vertical heterojunctions is reported. Comparison with the growth on the SiO₂/Si substrate under the same conditions reveals that the utilization of the dangling‐bond‐free WSe₂ or WS₂ as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe₂ (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.

Spin-polarized electrons in monolayer MoS₂

Spin-polarized electrons in monolayer MoS₂, Published online: 11 March 2019; doi:10.1038/s41565-019-0397-y

Under the application of a magnetic field, the 2D electron gas in a gated MoS2 monolayer becomes spin-polarized, the Coulomb interaction probably being key to the symmetry breaking.

Quantum interference (QI) in a multilayer WSe₂ is clearly observed at room temperature via graphene contacts. A vertical WSe₂ homojunction exhibits a clean and sharp interface, becoming a good platform to simultaneously investigate QI and Coulomb drag effects. This study provides a novel approach for the realization of advanced quantum electronics operating at high temperatures.

Abstract

Mesoscopic fluctuations, manifesting the quantum interference (QI) of electrons, have been theoretically proposed in bilayer Coulomb drag systems. Unfortunately, these phenomena are usually observed at cryogenic temperatures, which severely limits their novel physics for pragmatic applications. In this paper, observation of room‐temperature QI and Coulomb drag in a multilayer WSe₂ transistor is reported via graphene contacts separately at its top and bottom layers. The central layers of WSe₂ act as an insulating region with a width of few nanometers, which spatially separates the top and bottom conducting channels and provides a strong Coulomb interaction between them, leading to large conductance oscillations at room temperature. The gradual suppression of the oscillations with the increase in the applied magnetic field and/or injected current further confirms the QI phenomenon. With the decrease in temperature, the Coulomb drag effect is exhibited in the system owing to the increased thickness of the insulating region. This study reveals a novel approach for realization of advanced quantum electronics operating at high temperatures.

In article number 1805582, Bin Wu, Yunqi Liu, and co‐workers demonstrate a library of h‐BN epitaxy on graphene–h‐BN templates of various dimensionalities, including a two‐dimensional homo‐/heteromaterial surface and one‐dimensional interfaces of homo‐/heteromaterials. They provide a framework that allows the description of various types of kinetic growth by combined geometric and structural modelling. This work provides a general viewpoint for understanding epitaxial growth in complex systems.

Controllable synthesis of 2D layered 3‐rhombohedral phase WS₂ and WSe₂ atomic layers with full‐covered top layers is realized by physical vapor deposition. Compared to the 2‐hexagonal phase, 3‐rhombohedral phase layers show a unique photoluminescence and Raman spectra, and more importantly quadratically increasing second harmonic generation intensity with respect to layer numbers, which is promising for nonlinear optics.

Abstract

2D layered 3‐rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large‐area WS₂ and WSe₂ atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large‐area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and time‐resolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization‐resolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS₂, which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.

2D metal chalcogenides, including two‐dimensional metal mono‐, di‐, and tri‐chalcogenides, are a large family of van der Waals layered materials. They possess different crystal structures contributing to distinct physical characteristics. Numerous studies have focused on their electronic and optoelectronic applications. Two‐dimensional metal chalcogenides have been considered promising candidates for future practical applications.

Abstract

Emerging 2D metal chalcogenides present excellent performance for electronic and optoelectronic applications. In contrast to graphene and other 2D materials, 2D metal chalcogenides possess intrinsic bandgaps, versatile band structures, and superior atmospheric stability. The many categories of 2D metal chalcogenides ensure that they can be applied to various practical scenarios. 2D metal monochalcogenides, dichalcogenides, and trichalcogenides are the three main categories of these materials. They have distinct crystal structures resulting in different characteristics. Some basic device characteristics, such as the charge carrier characteristics, scattering mechanisms, interfacial contacts, and band alignments of heterojunctions, are vital factors for practical device applications that ensure that the desired properties can be achieved. Various electronic, optoelectronic, and photonic applications based on 2D metal chalcogenides have been extensively investigated. 2D metal chalcogenides are considered as competitive candidates for future electronic and optoelectronic applications.

Large‐domain‐size ultrathin MTe₂ (M = V, Nb, Ta) nanoplates with the thickness down to the monolayer regime are prepared using 2D WSe₂ or WS₂ as the growth substrate. The atomically flat dangling‐bond‐free surface of WSe₂ (WS₂) ensures a minimized diffusion barrier for the successful realization of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) nanosheets.

Abstract

2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe₂ or WS₂ as the growth substrate, the synthesis of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe₂/WSe₂ (WS₂) vertical heterojunctions is reported. Comparison with the growth on the SiO₂/Si substrate under the same conditions reveals that the utilization of the dangling‐bond‐free WSe₂ or WS₂ as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe₂ (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.