Enhanced Photoluminescence of Multiple Two-Dimensional van der Waals Heterostructures Fabricated by Layer-by-Layer Oxidation of MoS2

Potassium hydroxide treatment of layered WSe2 with enhanced electronic performances

2D WSe2-based electronic devices have received much research interest. However, it is still a challenge to achieve high electronic performance in WSe2-based devices. In this work, we report greatly enhanced performances of different thickness WSe2 ambipolar transistors and demonstrate homogeneous WSe2 inverter devices, which are obtained by using a semiconductor processing-compatible layer removal technique via chemical removal of the surface top WOx layer formed by O2 plasma treatment. Importantly, monolayer WSe2 was realised after several consecutive removal processes, demonstrating that the single layer removal is accurate and reliable. After subsequent removal of the top layer WOx by KOH, the fabricated WSe2 field-effect transistors exhibit greatly enhanced electronic performance along with the high electron and hole mobilities of 40 and 85 cm2 V-1 s-1, respectively. Our work demonstrates that the layer removal technique is an efficient route to fabricate high performance 2D material-based electronic devices.

Optical grade transformation of monolayer transition metal dichalcogenides via encapsulation annealing

Monolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for optoelectronic applications due to their direct band gap and strong light-matter interactions. However, exfoliated TMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significant defects and associated doping in the synthesized TMD crystals. Here, we report the improvement of optical properties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. Monolayer WSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime after encapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitons through dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport characteristics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, which also support dedoping of annealed TMDs. This work provides an innovative approach to elevate the optical grade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.

Deep-Learning-Assisted Discriminative Detection of Vitamin B12 and Vitamin B9 by Fluorescent MoSe2 Quantum Dots

A facile and environmentally mindful approach for the synthesis of MoSe2 QDs was developed via the hydrothermal method from bulk MoSe2. In this, the exfoliation of MoSe2 was enhanced with the aid of an intercalation agent (KOH), which could reduce the exfoliation time and increase the exfoliation efficiency to form MoSe2 QDs. We found that MoSe2 QDs display blue emission that is suitable for different applications. This fluorescence property of MoSe2 QDs was harnessed to fabricate a dual-modal sensor for the detection of both vitamin B12 (VB12) and vitamin B9 (VB9), employing fluorescence quenching. We performed a detailed study on the fluorescence quenching mechanism of both analytes. The predominant quenching mechanism for VB12 is via Förster resonance energy transfer. In contrast, the recognition of VB9 primarily relies on the inner filter effect. We applied an emerging and captivating approach to pattern recognition, the deep-learning method, which enables machines to "learn" patterns through training, eliminating the need for explicit programming of recognition methods. This attribute endows deep-learning with immense potential in the realm of sensing data analysis. Here, analyzing the array-based sensing data, the deep-learning technique, "convolution neural networks", has achieved 93% accuracy in determining the contribution of VB12 and VB9.

Recent progress in plasma modification of 2D metal chalcogenides for electronic devices and optoelectronic devices

Two-dimensional metal chalcogenides (2D MCs) present a great opportunity for overcoming the size limitation of traditional silicon-based complementary metal-oxide-semiconductor (CMOS) devices. Controllable modulation compatible with CMOS processes is essential for the improvement of performance and the large-scale applications of 2D MCs. In this review, we summarize the recent progress in plasma modification of 2D MCs, including substitutional doping, defect engineering, surface charge transfer, interlayer coupling modulation, thickness control, and nano-array pattern etching in the fields of electronic devices and optoelectronic devices. Finally, challenges and outlooks for plasma modulation of 2D MCs are presented to offer valuable references for future studies.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Reduced Defect Density in MOCVD-Grown MoS2 by Manipulating the Precursor Phase

Advancements in the synthesis of large-area, high-quality two-dimensional transition metal dichalcogenides such as MoS2 play a crucial role in the development of future electronic and optoelectronic devices. The presence of defects formed by sulfur vacancies in MoS2 results in low photoluminescence emission and imparts high n-type doping behavior, thus substantially affecting material quality. Herein, we report a new method in which single-phase (liquid) precursors are used for the metal-organic chemical vapor deposition (MOCVD) growth of a MoS2 film. Furthermore, we fabricated a high-performance photodetector (PD) and achieved improved photoresponsivity and faster photoresponse in the spectral range 405-637 nm compared to those of PDs fabricated by the conventional MOCVD method. In addition, the fabricated MoS2 thin film showed a threshold voltage shift in the positive gate bias direction owing to the reduced number of S vacancy defects in the MoS2 lattice. Thus, our method significantly improved the synthesis of monolayer MoS2 and can expand the application scope of high-quality, atomically thin materials in large-scale electronic and optoelectronic devices.

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.

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.

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.

A simple KPFM-based approach for electrostatic- free topographic measurements: the case of MoS2on SiO2

A simple implementation of Kelvin probe force microscopy (KPFM) is reported that enables recording topographic images in the absence of any component of the electrostatic force (including the static term). Our approach is based on a close loop z-spectroscopy operated in data cube mode. Curves of the tip-sample distance as a function of time are recorded onto a 2D grid. A dedicated circuit holds the KPFM compensation bias and subsequently cut off the modulation voltage during well-defined time-windows within the spectroscopic acquisition. Topographic images are recalculated from the matrix of spectroscopic curves. This approach is applied to the case of transition metal dichalcogenides (TMD) monolayers grown by chemical vapour deposition on silicon oxide substrates. In addition, we check to what extent a proper stacking height estimation can also be performed by recording series of images for decreasing values of the bias modulation amplitude. The outputs of both approaches are shown to be fully consistent. The results exemplify how in the operating conditions of non-contact AFM under ultra-high vacuum (nc-AFM), the stacking height values can dramatically be overestimated due to variations in the tip-surface capacitive gradient, even though the KPFM controller nullifies the potential difference. We show that the number of atomic layers of a TMD can be safely assessed, only if the KPFM measurement is performed with a modulated bias amplitude reduced at its strict minimum or, even better, without any modulated bias. Last, the spectroscopic data reveal that certain kind of defects can have a counterintuitive impact on the electrostatic landscape, resulting in an apparent decrease of the measured stacking height by conventional nc-AFM/KPFM compared to other sample areas. Hence, electrostatic free z-imaging proves to be a promising tool to assess the existence of defects in atomically thin TMD layers grown on oxides.

Exciton-dominant photoluminescence of MoS2 by a functionalized substrate

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS₂ are strongly influenced by the functionalized substrate. The PL of 1L MoS₂ placed on the oxygen plasma-treated SiO₂ substrate was highly p-doped owing to the functional groups of -OH on SiO₂, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS₂ on plasma-treated SiO₂ substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS₂ on the plasma-treated substrate. The MoS₂ field-effect transistors fabricated on plasma-treated SiO₂ also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS₂, which provides clues as to why MoS₂ exfoliated on various substrates always exhibited different properties in previous studies.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Highly Responsive Pd-Decorated MoO3 Nanowall H2 Gas Sensors Obtained from In-Situ-Controlled Thermal Oxidation of Sputtered MoS2 Films

Among transition metal oxides, MoO₃ is a promising material due to its layered structure and different oxidation states, making it suitable for different device applications. One of the methods used to grow MoO₃ is radio frequency magnetron sputtering (RFMS), which is the most compatible method in industry. However, obtaining nanostructures by RFMS for metal oxides is challenging because of compact morphology film formation. In this study, α-MoO₃ with vertical nanowalls is obtained by a two-step process; deposition of magnetron-sputtered MoS₂ vertical nanowalls and postoxidation of these structures without changing the morphology. In situ transmittance and electrical measurements are performed to control the oxidation process, which shed light on understanding the oxidation of MoS₂ nanowalls. The transition from MoS₂ to α-MoO₃ is investigated with partially oxidized MoS₂/MoO₃ samples with different thicknesses. It is also concluded that oxidation starts from nanowalls perpendicular to the substrate and lasts with oxidation of basal planes. Four different thicknesses of α-MoO₃ nanowall samples are fabricated for H₂ gas sensors. Also, the effect of Pd deposition on the H₂-sensing properties of sensors is deeply investigated. An outstanding response of 3.3 × 10⁵ as well as the response and recovery times of 379 and 304 s, respectively, are achieved from the thinnest Pd-loaded sample. Also, the gas-sensing mechanism is explored by gasochromic measurements to investigate the sensor behaviors under the conditions of dry air and N₂ gas as the carrier gas.

Coupling of N-Doped Mesoporous Carbon and N-Ti3 C2 in 2D Sandwiched Heterostructure for Enhanced Oxygen Electroreduction

2D heterostructures provide a competitive platform to tailor electrical property through control of layer structure and constituents. However, despite the diverse integration of 2D materials and their application flexibility, tailoring synergistic interlayer interactions between 2D materials that form electronically coupled heterostructures remains a grand challenge. Here, the rational design and optimized synthesis of electronically coupled N-doped mesoporous defective carbon and nitrogen modified titanium carbide (Ti₃ C₂ ) in a 2D sandwiched heterostructure, is reported. First, a F127-polydopamine single-micelle-directed interfacial assembly strategy guarantees the construction of two surrounding mesoporous N-doped carbon monolayers assembled on both sides of Ti₃ C₂ nanosheets. Second, the followed ammonia post-treatment successfully introduces N elements into Ti₃ C₂ structure and more defective sites in N-doped mesoporous carbon. Finally, the oxygen reduction reaction (ORR) and theoretical calculation prove the synergistic coupled electronic effect between N-Ti₃ C₂ and defective N-doped carbon active sites in the 2D sandwiched heterostructure. Compared with the control 2D samples (0.87-0.88 V, 4.90-5.15 mA cm^-2 ), the coupled 2D heterostructure possesses the best onset potential of 0.90 V and limited density current of 5.50 mA cm^-2 . Meanwhile, this catalyst exhibits superior methanol tolerance and cyclic durability. This design philosophy opens up a new thought for tailoring synergistic interlayer interactions between 2D materials.

Wafer-Scalable Single-Layer Amorphous Molybdenum Trioxide

Molybdenum trioxide (MoO₃), an important transition metal oxide (TMO), has been extensively investigated over the past few decades due to its potential in existing and emerging technologies, including catalysis, energy and data storage, electrochromic devices, and sensors. Recently, the growing interest in two-dimensional (2D) materials, often rich in interesting properties and functionalities compared to their bulk counterparts, has led to the investigation of 2D MoO₃. However, the realization of large-area true 2D (single to few atom layers thick) MoO₃ is yet to be achieved. Here, we demonstrate a facile route to obtain wafer-scale monolayer amorphous MoO₃ using 2D MoS₂ as a starting material, followed by UV-ozone oxidation at a substrate temperature as low as 120 °C. This simple yet effective process yields smooth, continuous, uniform, and stable monolayer oxide with wafer-scale homogeneity, as confirmed by several characterization techniques, including atomic force microscopy, numerous spectroscopy methods, and scanning transmission electron microscopy. Furthermore, using the subnanometer MoO₃ as the active layer sandwiched between two metal electrodes, we demonstrate the thinnest oxide-based nonvolatile resistive switching memory with a low voltage operation and a high ON/OFF ratio. These results (potentially extendable to other TMOs) will enable further exploration of subnanometer stoichiometric MoO₃, extending the frontiers of ultrathin flexible oxide materials and devices.

Recent trends in covalent functionalization of 2D materials

Covalent functionalization of the surface is more crucial in 2D materials than in conventional bulk materials because of their atomic thinness, large surface-to-volume ratio, and uniform surface chemical potential. Because...