Jiuxiang Dai

Nature Nanotechnology, Published online: 23 May 2022; doi:10.1038/s41565-022-01126-z

Nature Electronics, Published online: 23 May 2022; doi:10.1038/s41928-022-00770-6

Nature Electronics, Published online: 23 May 2022; doi:10.1038/s41928-022-00764-4

Directly grown graphene/Mo₂C vertical heterostructures provide an ideal platform for exploring the interplay of massless Dirac fermion and superconductivity. This low-temperature transport reveals the geometric resonance by the interaction of the carriers with the boundaries of the potential well formed at the graphene/Mo₂C interface, and the potential well can be effectively modified through superconducting proximity effect in heterostructures.

Abstract

The realization of high-quality heterostructures or hybrids of graphene and superconductor is crucial for exploring various novel quantum phenomena and devices engineering. Here, the electronic transport on directly grown high-quality graphene/Mo₂C vertical heterostructures with clean and sharp interface is comprehensively investigated. Owing to the strong interface coupling, the graphene layer feels an effective confinement potential well imposed by two-dimensional (2D) Mo₂C crystal. Employing cross junction device geometry, a series of resonance-like magnetoresistance peaks are observed at low temperatures. The temperature and gate voltage dependences of the observed resonance peaks give evidence for geometric resonance of electron cyclotron orbits with the formed potential well. Moreover, it is found that both the amplitude of resonance peaks and conductance fluctuation exhibit different temperature-dependent behaviors below the superconducting transition temperature of 2D Mo₂C, indicating the correlation of quantum fluctuations and superconductivity. This study offers a promising route toward integrating graphene with 2D superconducting materials, and establishes a new way to investigate the interplay of massless Dirac fermion and superconductivity based on graphene/2D superconductor vertical heterostructures.

Abstract

Because of profound applications of two-dimensional molybdenum disulfide (MoS₂) and its heterostructures in electronics, its thermal stability has been spurred substantial interest. We employ a precision muffle furnace at a series of increasing temperatures up to 340 °C to study the oxidation behavior of continuous MoS₂ films by either directly growing mono- and few-layer MoS₂ on SiO₂/Si substrate, or by mechanically transferring monolayer MoS₂ or hexagonal boron nitride (h-BN) onto monolayer MoS₂ substrate. Results show that monolayer MoS₂ can withstand high temperature at 340 °C with less oxidation while the few-layer MoS₂ films are completely oxidized just at 280 °C, resulting from the growth-induced tensile strain in few-layer MoS₂. When the tensile strain of films is released by transfer method, the stacked few-layer MoS₂ films exhibit superior thermal stability and typical layer-by-layer oxidation behavior at similarly high temperature. Counterintuitively, for the MoS₂/h-BN heterostructure, the h-BN film itself stacked on top is not damaged and forms many bubbles at 340 °C, whereas the underlying monolayer MoS₂ film is oxidized completely. By comprehensively using various experimental characterization and molecular dynamics calculations, such anomalous oxidation behavior of MoS₂/h-BN heterostructure is mainly due to the increased tensile strain in MoS₂ film at elevated temperature.

Nature Communications, Published online: 24 May 2022; doi:10.1038/s41467-022-30498-y

npj 2D Materials and Applications, Published online: 24 May 2022; doi:10.1038/s41699-022-00308-6

npj 2D Materials and Applications, Published online: 24 May 2022; doi:10.1038/s41699-022-00310-y

npj 2D Materials and Applications, Published online: 24 May 2022; doi:10.1038/s41699-022-00312-w

npj 2D Materials and Applications, Published online: 23 May 2022; doi:10.1038/s41699-022-00311-x

Light: Science & Applications, Published online: 20 May 2022; doi:10.1038/s41377-022-00813-9

The construction of a superhydrophilic bionic interface layer (Bio-IL) significantly suppresses the coffee-ring effect during printing of large-area perovskite films via regulating the nucleation rate (υ_N) and radial transport rate (υ_RT) of the perovskite precursor, which induces a homogeneous large-area flexible perovskite film and high-performance inverted flexible perovskite solar cells with an efficiency of 21.08%.

Abstract

The inhomogeneity, poor interfacial contact, and pinholes caused by the coffee-ring effect severely affect the printing reliability of flexible perovskite solar cells (PSCs). Herein, inspired by the bio-glue of barnacles, a bionic interface layer (Bio-IL) of NiO _x /levodopa is introduced to suppress the coffee-ring effect during printing perovskite modules. The coordination effect of the sticky functional groups in Bio-IL can pin the three-phase contact line and restrain the transport of perovskite colloidal particles during the printing and evaporation process. Moreover, the sedimentation rate of perovskite precursor is accelerated due to the electrostatic attraction and rapid volatilization from an extraordinary wettability. The superhydrophilic Bio-IL affords an even spread over a large-area substrate, which boosts a complete and uniform liquid film for heterogeneous nucleation as well as crystallization. Perovskite films on different large-area substrates with negligible coffee-ring effect are printed. Consequently, inverted flexible PSCs and perovskite solar modules achieve a high efficiency of 21.08% and 16.87%, respectively. This strategy ensures a highly reliable reproducibility of printing PSCs with a near 90% yield rate.

Publication date: May 2022

Source: Materials Today, Volume 55

Author(s): Laurie Donaldson

Passive 2D–3D Interfaces

In article number 2111343, Alex Henning, Ian D. Sharp, and co-workers develop a novel transparent and conductive support structure for the scalable integration and direct growth of 2D materials. The structure comprises a nanocrystalline carbon layer coated with nanometer-thin alumina and provides an electronically passive 2D–3D interface, which enables facile interfacial charge transport and significantly enhances the photoluminescence of monolayer MoS₂.

Abstract

Due to their superior hydrophilicity and conductivity, ultra-high volumetric capacitance, and rich surface-chemistry properties, MXenes exhibit unique and excellent performance in catalysis, energy storage, electromagnetic shielding, and life sciences. Since they are derived from ceramics (MAX phase) through etching, one of the challenges in MXenes preparation is the inevitable exposure of metal atoms on their surface and embedding of anions and cations. Because the as-obtained MXenes are always in a thermodynamically metastable state, they tend to react with trace oxygen or oxygen-containing groups to form metal oxides or degrade, leading to sharply declined activity and impaired performance. Therefore, improving the stability of MXenes-based materials is of practical significance in relevant applications. Unfortunately, there lacks a comprehensive review in the literature on relevant topics. To help promote the wide applications of MXenes, we review from the following aspects: (i) insights into the factors affecting the stability of MXenes-based materials, including oxidation of MXenes flakes, stability of MXenes colloidal solutions, and swelling and degradation of MXenes thin-film, (ii) strategies for enhancing the stability of MXenes-based materials by optimizing MAX phase synthesis and modifying the MXenes preparation, and (iii) techniques for further increasing the stability of freshly prepared MXenes-based materials via controlling the storage conditions, and forming shielding on the surface and/or edge of MXenes flakes. Finally, some outlooks are proposed on the future developments and challenges of highly active and stable MXenes. We aim to provide guidance for the design, preparation, and applications of MXenes-based materials with excellent stability and activity.

The 2D transition metal dichalcogenide alloy, Mo_{1− x} W _x S₂, is synthesized to investigate the influence of configurational entropy on the piezoelectrical property. Mo_0.46W_0.54S₂, in which two cations have similar concentrations and the maximum configurational entropy is attained, exhibits the best piezoelectric properties. Combined with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

Abstract

Piezoelectricity in 2D transition metal dichalcogenides (TMDs) has attracted considerable interest because of their excellent flexibility and high piezoelectric coefficient compared to conventional piezoelectric bulk materials. However, the ability to regulate the piezoelectric properties is limited because the entropy is constant for certain binary TMDs other than multielement ones. Herein, in order to increase the entropy, a ternary TMDs alloy, Mo_{1− x} W _x S₂, with different W concentrations, is synthesized. The W concentration in the Mo_{1− x} W _x S₂ alloy can be controlled precisely in the low-supersaturation synthesis and the entropy can be tuned accordingly. The Mo_0.46W_0.54S₂ alloy (x = 0.54) has the highest configurational entropy and best piezoelectric properties, such as a piezoelectric coefficient of 4.22 pm V⁻¹ and a piezoelectric output current of 150 pA at 0.24% strain. More importantly, it can be combined into a larger package to increase the output current to 600 pA to cater to self-powered applications. Combining with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

Superlattice Heterojunctions

In article number 2110099, Cesar Moreno, Diego Peña, Miguel Pruneda, Aitor Mugarza, and co-workers report the synthesis of an atomically precise nitrogen-doped nanoporous graphene that electronically behaves as a nanometer-scale 2D lateral superlattice heterostructure with unprecedented band discontinuities down to the single-bond limit. The atomically sharp nanoporous heterojunctions endow this nanomaterial with a multifunctionality that can be very relevant for photodetection, excitonic solar cells, water splitting, or selective nanosieving.

The progress made toward the development of a modular compact modeling technology for graphene field-effect transistors under DC, transient, AC, and noise analysis, is reported. This includes non-idealities such as extrinsic-, short-channel-, traps-, self-heating-, and non-quasi-static-effects. An overview of the challenges ahead in graphene transistor modeling to enable computer-aided circuit design is also provided.

Abstract

The progress made toward the definition of a modular compact modeling technology for graphene field-effect transistors (GFETs) that enables the electrical analysis of arbitrary GFET-based integrated circuits is reported. A set of primary models embracing the main physical principles defines the ideal GFET response under DC, transient (time domain), AC (frequency domain), and noise (frequency domain) analysis. Another set of secondary models accounts for the GFET non-idealities, such as extrinsic-, short-channel-, trapping/detrapping-, self-heating-, and non-quasi static-effects, which can have a significant impact under static and/or dynamic operation. At both device and circuit levels, significant consistency is demonstrated between the simulation output and experimental data for relevant operating conditions. Additionally, a perspective of the challenges during the scale up of the GFET modeling technology toward higher technology readiness levels while drawing a collaborative scenario among fabrication technology groups, modeling groups, and circuit designers, is provided.

Heating up the silver-mediated exfoliation of MoS₂ enables exfoliation yield on par with the top performer gold for the first time. A temperature dependence of the exfoliation yield is observed, suggesting a thermally activated process limited by oxidation for higher temperatures. Thereby, the critical role of temperature as a key variable in metal-mediated exfoliations is highlighted.

Abstract

The need for high-quality large-scale monolayers of layered materials pushes the development of scalable gold-mediated exfoliations. Gold proves to be a suitable adhesive for exfoliation of several 2D materials. However, the extension to other noble metals remains underwhelming as gold outperforms all previously studied metals by a large margin. This is attributed to compromised stability against oxidation and surface contamination of less noble metals, leading to nonideal interfaces for exfoliation. The closest competitor to gold is silver, where gold still leads by a factor 100 regarding exfoliated layer size. In this work, a silver-mediated exfoliation process performing on par with gold is presented. The combination of freshly cleaved silver surfaces with a low-temperature annealing is found to be crucial. The exfoliation yield shows a dependence with annealing temperature, leading to loss in exfoliation performance for higher temperature. Raman studies indicate inhomogeneous strain for the MoS₂/Ag interface at these temperatures, which hints at the competing factors of thermal activation versus oxidation of silver. Finally, a transfer process is implemented to promote silver to a fully functional exfoliation substrate. Ultimately, heating up exfoliations tips the strict balance between interfacial interactions and surface contaminations toward robust high monolayer yield exfoliation as demonstrated for silver.