van der Waals Epitaxial Formation of Atomic Layered α-MoO3 on MoS2 by Oxidation

Spectro-Microscopic Perceptions into Oxidation Behavior of Large-Scale Molybdenum Disulfide and its Photoelectrical Correlation

Despite the encouraging properties and research of 2D MoS2 , an ongoing issue associated with the oxidative instability remains elusive for practical optoelectronic applications. Thus, in-depth understanding of the oxidation behavior of large-scale and homogeneous 2D MoS2 is imperative. Here the structural and chemical transformations of large-area MoS2 multilayers by air-annealing with altered temperature and time via combinatorial spectro-microscopic analyses (Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy) are surveyed. The results gave indications pertaining to temperature- and time-dependent oxidation effects: i) heat-driven elimination of redundant residues, ii) internal strain stimulated by the formation of MoO bonds, iii) deterioration of the MoS2 crystallinity, iv) layer thinning, and v) morphological transformation from 2D MoS2 layers to particles. Photoelectrical characterization of the air-annealed MoS2 is implemented to capture the link between the oxidation behavior of MoS2 multilayers and their photoelectrical properties. The photocurrent based on MoS2 air-annealed at 200 °C is assessed to be 4.92 µA, which is 1.73 times higher than that of pristine MoS2 (2.84 µA). The diminution in the photocurrent of the photodetector based on MoS2 air-annealed above 300 °C in terms of the structural, chemical, and electrical conversions induced by the oxidation process is further discussed.

Physical adsorption and oxidation of ultra-thin MoS2crystals: insights into surface engineering for 2D electronics and beyond

The oxidation mechanism of atomically thin molybdenum disulfide (MoS2) plays a critical role in its nanoelectronics, optoelectronics, and catalytic applications, where devices often operate in an elevated thermal environment. In this study, we systematically investigate the oxidation of mono- and few-layer MoS2flakes in the air at temperatures ranging from 23 °C to 525 °C and relative humidities of 10%-60% by using atomic force microscopy (AFM), Raman spectroscopy and x-ray photoelectron spectroscopy. Our study reveals the formation of a uniform nanometer-thick physical adsorption layer on the surface of MoS2, which is attributed to the adsorption of ambient moisture. This physical adsorption layer acts as a thermal shield of the underlying MoS2lattice to enhance its thermal stability and can be effectively removed by an AFM tip scanning in contact mode or annealing at 400 °C. Our study shows that high-temperature thermal annealing and AFM tip-based cleaning result in chemical adsorption on sulfur vacancies in MoS2, leading to p-type doping. Our study highlights the importance of humidity control in ensuring reliable and optimal performance for MoS2-based electronic and electrochemical devices and provides crucial insights into the surface engineering of MoS2, which are relevant to the study of other two-dimensional transition metal dichalcogenide materials and their applications.

Atomic-Scale Mechanisms of MoS2 Oxidation for Kinetic Control of MoS2/MoO3 Interfaces

Oxidation of transition metal dichalcogenides (TMDs) occurs readily under a variety of conditions. Therefore, understanding the oxidation processes is necessary for successful TMD handling and device fabrication. Here, we investigate atomic-scale oxidation mechanisms of the most widely studied TMD, MoS₂. We find that thermal oxidation results in α-phase crystalline MoO₃ with sharp interfaces, voids, and crystallographic alignment with the underlying MoS₂. Experiments with remote substrates prove that thermal oxidation proceeds via vapor-phase mass transport and redeposition, a challenge to forming thin, conformal films. Oxygen plasma accelerates the kinetics of oxidation relative to the kinetics of mass transport, forming smooth and conformal oxides. The resulting amorphous MoO₃ can be grown with subnanometer to several-nanometer thickness, and we calibrate the oxidation rate for different instruments and process parameters. Our results provide quantitative guidance for managing both the atomic scale structure and thin-film morphology of oxides in the design and processing of TMD devices.

Molten Salt-Assisted Catalytic Preparation of MoS2/α-MoO3/Graphene as High-Performance Anode of Li-Ion Battery

We report on the facile and scalable catalytic conversion of natural graphite and MoS2 minerals into α-MoO3 nanoribbons incorporated into hexagonal MoS2 and graphene nanosheets, and evaluate the structural, morphological and electrochemical performances of the hybrid nanostructured material obtained. Mechanochemical treatment of raw materials, followed by catalytic molten salt treatment leads to the formation of nanostructures with promising electrochemical performances. We examined the effect of processing temperature on the electrochemical performance of the products. At 1100 °C, an excellent Li-ion storage capacity of 773.5 mAh g−1 is obtained after 180 cycles, considerably greater than that of MoS2 (176.8 mAh g−1). The enhanced capacity and the rate performance of this electrode are attributed to the well-integrated components, characterized by the formation of interfacial molybdenum oxycarbide layer during the synthesis process, contributing to the reduced electrical/electrochemical resistance of the sample. This unique morphology promotes the charge and ions transfer through the reduction of the Li-ion diffusion coefficient (1.2 × 10−18 cm2 s−1), enhancing the pseudocapacitive performance of the electrode; 59.3% at the scan rate of 0.5 mV s−1. This article provides a green and low-cost route to convert highly available natural graphite and MoS2 minerals into nanostructured hybrid materials with promising Li-ion storage performance.

Morphotaxy of Layered van der Waals Materials

Layered van der Waals (vdW) materials have attracted significant attention due to their materials properties that can enhance diverse applications including next-generation computing, biomedical devices, and energy conversion and storage technologies. This class of materials is typically studied in the two-dimensional (2D) limit by growing them directly on bulk substrates or exfoliating them from parent layered crystals to obtain single or few layers that preserve the original bonding. However, these vdW materials can also function as a platform for obtaining additional phases of matter at the nanoscale. Here, we introduce and review a synthesis paradigm, morphotaxy, where low-dimensional materials are realized by using the shape of an initial nanoscale precursor to template growth or chemical conversion. Using morphotaxy, diverse non-vdW materials such as HfO₂ or InF₃ can be synthesized in ultrathin form by changing the composition but preserving the shape of the original 2D layered material. Morphotaxy can also enable diverse atomically precise heterojunctions and other exotic structures such as Janus materials. Using this morphotaxial approach, the family of low-dimensional materials can be substantially expanded, thus creating vast possibilities for future fundamental studies and applied technologies.

Highly Sensitive NO2 Gas Sensors Based on MoS2@MoO3 Magnetic Heterostructure

Recently, two-dimensional (2D) materials and their heterostructures have attracted considerable attention in gas sensing applications. In this work, we synthesized 2D MoS₂@MoO₃ heterostructures through post-sulfurization of α-MoO₃ nanoribbons grown via vapor phase transport (VPT) and demonstrated highly sensitive NO₂ gas sensors based on the hybrid heterostructures. The morphological, structural, and compositional properties of the MoS₂@MoO₃ hybrids were studied by a combination of advanced characterization techniques revealing a core-shell structure with the coexistence of 2H-MoS₂ multilayers and intermediate molybdenum oxysulfides on the surface of α-MoO₃. The MoS₂@MoO₃ hybrids also exhibit room-temperature ferromagnetism, revealed by vibrating sample magnetometry (VSM), as a result of the sulfurization process. The MoS₂@MoO₃ gas sensors display a p-type-like response towards NO₂ with a detection limit of 0.15 ppm at a working temperature of 125 °C, as well as superb selectivity and reversibility. This p-type-like sensing behavior is attributed to the heterointerface of MoS₂-MoO₃ where interfacial charge transfer leads to a p-type inversion layer in MoS₂, and is enhanced by magnetic dipole interactions between the paramagnetic NO₂ and the ferromagnetic sensing layer. Our study demonstrates the promising application of 2D molybdenum hybrid compounds in gas sensing applications with a unique combination of electronic and magnetic properties.

Elucidation of Novel Potassium-Mediated Oxidation and Etching of Two-Dimensional Transition Metal Dichalcogenides

Preparation of edge-rich two-dimensional (2D) transition metal dichalocogenides (TMDs) has been actively investigated with the aim to improve their electrical and catalytic properties. Here, we elucidate the role of potassium ions in oxidation of TMDs and suggest a consequent novel anisotropic etching mechanism driven by self-running oxide droplets. We discover that potassium-mediated oxidation of MoS₂ leads to the formation of K-intercalated hexagonal-phase molybdenum oxides (h-K_xMoO₃), whereas orthorhombic-phase oxides are formed in the absence of potassium ions. Metastable h-K_xMoO₃ appears to have decomposed into oxide droplets at higher temperature. Self-running of the oxide droplets leads to layer-by-layer anisotropic etching of MoS₂ along the armchair direction. The motion of the droplets appears to be triggered by the surface energy instability between the oxide droplets and the underlying MoS₂ layer. This study opens new possibilities to design and manufacture novel edge-rich 2D TMDs that do not follow the equilibrium Wulff shape by modulating their oxidation with the assistance of alkali metals and also offers fundamental insights into the interactions between nanodroplets and 2D materials toward edge engineering.

Bandgap prediction of two-dimensional materials using machine learning

The bandgap of two-dimensional (2D) materials plays an important role in their applications to various devices. For instance, the gapless nature of graphene limits the use of this material to semiconductor device applications, whereas the indirect bandgap of molybdenum disulfide is suitable for electrical and photo-device applications. Therefore, predicting the bandgap rapidly and accurately for a given 2D material structure has great scientific significance in the manufacturing of semiconductor devices. Compared to the extremely high computation cost of conventional first-principles calculations, machine learning (ML) based on statistics may be a promising alternative to predicting bandgaps. Although ML algorithms have been used to predict the properties of materials, they have rarely been used to predict the properties of 2D materials. In this study, we apply four ML algorithms to predict the bandgaps of 2D materials based on the computational 2D materials database (C2DB). Gradient boosted decision trees and random forests are more effective in predicting bandgaps of 2D materials with an R2 >90% and root-mean-square error (RMSE) of ~0.24 eV and 0.27 eV, respectively. By contrast, support vector regression and multi-layer perceptron show that R2 is >70% with RMSE of ~0.41 eV and 0.43 eV, respectively. Finally, when the bandgap calculated without spin-orbit coupling (SOC) is used as a feature, the RMSEs of the four ML models decrease greatly to 0.09 eV, 0.10 eV, 0.17 eV, and 0.12 eV, respectively. The R2 of all the models is >94%. These results show that the properties of 2D materials can be rapidly obtained by ML prediction with high precision.

2D MoO3-xSx/MoS2 van der Waals Assembly: A Tunable Heterojunction with Attractive Properties for Photocatalysis

Two-dimensional (2D) van der Waals (vdW) heterostructures currently have attracted much attention in widespread research fields where semiconductor materials are key. With the aim of gaining insights into photocatalytic materials, we use density functional theory (DFT) calculations within the HSE06 functional to analyze the evolution of optoelectronic properties and high-frequency dielectric constant profiles of various 2D MoO_3-xS_x/MoS₂ heterostructures modified by chemical and physical approaches. Although the MoO₃/MoS₂ heterostructure is a type III heterojunction associated with a metallic character, we found that exchanging the terminal oxo atoms of the MoO_3-xS_x single layer (SL) with sulfur enables shifting its CB position above the VB position of the MoS₂ SL. This trend gives rise to a type II heterojunction where the band gap and charge transfer within the two layers are driven continuously by the S concentration in the MoO_3-xS_x SL. This fine-tuning leads to a versatile type II heterostructure proposed to provide a direct Z-scheme system valuable for photocatalytic water splitting.

Fabrication of a novel ternary heterojunction composite Ag2MoO4/Ag2S/MoS2 with significantly enhanced photocatalytic performance

A novel ternary heterojunction Ag₂MoO₄/Ag₂S/MoS₂ was successfully fabricated via a facile two-step method. The prepared ternary heterojunction showed much enhanced catalytic activity compared with monomers and binary heterojunctions.

Ab-initio investigation of preferential triangular self-formation of oxide heterostructures of monolayer [Formula: see text]

Triangular growth patterns of pristine two-dimensional (2D) transition metal dichalcogenides (TMDs) are ubiquitous in experiments. Here, we use first-principles calculations to investigate the growth of triangular shaped oxide islands upon layer-by-layer controlled oxidation in monolayer and few-layer [Formula: see text] systems. Pristine 2D TMDs with a trigonal prismatic geometry prefer the triangular growth morphology due to structural stability arising from the edge chalcogen atoms along its three sides. Our ab-initio energetics and thermodynamic study show that, since the Se atoms are more susceptible to oxygen replacement, the preferential oxidation happens along the Se zigzag lines, producing triangular islands of transition metal oxides. The thermodynamic stability arising from the preferential triangular self-formation of TMD based oxide heterostructures and their electronic properties opens a new avenue for their exploration in advanced electronic and optoelectronic devices.