Low-Temperature Vapor-Phase Growth of 2D Metal Chalcogenides

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

Interfacial Self-Assembly of Oriented Semiconductor Monolayer for Chemiresistive Sensing

Semiconductor nanofilm fabrication with advanced technology is of great importance for next-generation electronics/optoelectronics. Fabrication of high-quality and perfectly oriented semiconductor thin films and integration into high-performance electronic devices with low cost and high efficiency are huge challenges. Here we exquisitely utilized the Marangoni effect to perfectly guide tin disulfide (SnS2) nanocoins into an ordered assembly in milliseconds, resulting in an uniaxial-oriented monolayer semiconductor film. Further exploration revealed that the formed "crumple zone" at the interface caused by the Marangoni force endows the nanofilm with a rapid healable capability, which can be easily transferred to arbitrary substrates. As a proof of concept, the nanocoin-monolayer was transferred onto a micro-interdigitated electrode substrate to form a high-performance chemiresistive sensor that can effectively monitor the trace amounts of toxic gases. In addition, the assembled monolayer nanofilms can be conformally printed on freeform surfaces: both flat and nonflat substrates. This efficient and low-cost Marangoni force-assisted surface self-assembly (MFA-SSA) strategy is promising for advanced microelectronics and real industrial applications.

From Material to Cameras: Low-Dimensional Photodetector Arrays on CMOS

The last two decades have witnessed a dramatic increase in research on low-dimensional material with exceptional optoelectronic properties. While low-dimensional materials offer exciting new opportunities for imaging, their integration in practical applications has been slow. In fact, most existing reports are based on single-pixel devices that cannot rival the quantity and quality of information provided by massively parallelized mega-pixel imagers based on complementary metal-oxide semiconductor (CMOS) readout electronics. The first goal of this review is to present new opportunities in producing high-resolution cameras using these new materials. New photodetection methods and materials in the field are presented, and the challenges involved in their integration on CMOS chips for making high-resolution cameras are discussed. Practical approaches are then presented to address these challenges and methods to integrate low-dimensional material on CMOS. It is also shown that such integrations could be used for ultra-low noise and massively parallel testing of new material and devices. The second goal of this review is to present the colossal untapped potential of low-dimensional material in enabling the next-generation of low-cost and high-performance cameras. It is proposed that low-dimensional materials have the natural ability to create excellent bio-inspired artificial imaging systems with unique features such as in-pixel computing, multi-band imaging, and curved retinas.

Collagen membrane functionalized with magnesium oxide via room-temperature atomic layer deposition promotes osteopromotive and antimicrobial properties

Artificial bone grafting materials such as collagen are gaining interest due to the ease of production and implantation. However, collagen must be supplemented with additional coating materials for improved osteointegration. Here, we report room-temperature atomic layer deposition (ALD) of MgO, a novel method to coat collagen membranes with MgO. Characterization techniques such as X-ray photoelectron spectroscopy, Raman spectroscopy, and electron beam dispersion mapping confirm the chemical nature of the film. Scanning electron and atomic force microscopies show the surface topography and morphology of the collagen fibers were not altered during the ALD of MgO. Slow release of magnesium ions promotes bone growth, and we show the deposited MgO film leaches trace amounts of Mg when incubated in phosphate-buffered saline at 37 °C. The coated collagen membrane had a superhydrophilic surface immediately after the deposition of MgO. The film was not toxic to human cells and demonstrated antibacterial properties against bacterial biofilms. Furthermore, in vivo studies performed on calvaria rats showed MgO-coated membranes (200 and 500 ALD) elicit a higher inflammatory response, leading to an increase in angiogenesis and a greater bone formation, mainly for Col-MgO500, compared to uncoated collagen. Based on the characterization of the MgO film and in vitro and in vivo data, the MgO-coated collagen membranes are excellent candidates for guided bone regeneration.

Synthesis and Volatility Characterization of Mo(II) and W(II) Compounds for Thin Films

Mo(II) and W(II) compounds, Mo(η3-allyl)(CO)2(Tri-MEDA)Br (1), Mo(η3-allyl)(CO)2(TMEDA)Br (2), W(η3-allyl)(CO)2(Tri-MEDA)Br (3), and W(η3-allyl)(CO)2(TMEDA)Br (4) (Tri-MEDA = N,N,N'-trimethylethylenediamine), were synthesized and characterized. The molecular structures of 1 and 3 were nearly identical with a pseudo-octahedral geometry except for the different Mo and W metal centers. The thermogravimetric analysis of 1 and 3 showed approximately 53 and 64% residues at 550 °C, respectively, which were significantly higher than the values for the expected materials. However, 1 and 3 sublimed at 100 °C under 0.40 Torr and 120 °C under 0.50 Torr, respectively, confirming that they were volatile. For 1 and 3, the temperatures at a vapor pressure of 1 Torr and enthalpies of vaporization (ΔHvap) were 168.78 °C and 143.8 kJ mol-1, and 167.48 °C and 148.5 kJ mol-1, respectively. The tungsten compound (3) exhibited good durability for 5 weeks under a thermal stability test at a sublimation temperature of 120 °C.

Molybdenum(III) Amidinate: Synthesis, Characterization, and Vapor Phase Growth of Mo-Based Materials

The synthesis, characterization, and thermogravimetric analysis of tris(N,N'-di-isopropylacetamidinate)molybdenum(III), Mo(iPr-AMD)₃, are reported. Mo(iPr-AMD)₃ is a rare example of a homoleptic mononuclear complex of molybdenum(III) and fills a longstanding gap in the literature of transition metal(III) trisamidinate complexes. Thermogravimetric analysis (TGA) reveals excellent volatilization at elevated temperatures, pointing to potential applications as a vapor phase precursor for higher temperature atomic layer deposition (ALD), or chemical vapor deposition (CVD) growth of Mo-based materials. The measured TGA temperature window was 200-314 °C for samples in the 3-20 mg range. To validate the utility of Mo(iPr-AMD)₃, we demonstrate aerosol-assisted CVD growth of MoO₃ from benzonitrile solutions of Mo(iPr-AMD)₃ at 500 °C using compressed air as the carrier gas. The resulting films are characterized by X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. We further demonstrate the potential for ALD growth at 200 °C with a Mo(iPr-AMD)₃/Ar purge/300 W O₂ plasma/Ar purge sequence, yielding ultrathin films which retain a nitride/oxynitride component. Our results highlight the broad scope utility and potential of Mo(iPr-AMD)₃ as a stable, high-temperature precursor for both CVD and ALD processes.

The Road for 2D Semiconductors in the Silicon Age

Continued reduction in transistor size can improve the performance of silicon integrated circuits (ICs). However, as Moore's law approaches physical limits, high-performance growth in silicon ICs becomes unsustainable, due to challenges of scaling, energy efficiency, and memory limitations. The ultrathin layers, diverse band structures, unique electronic properties, and silicon-compatible processes of 2D materials create the potential to consistently drive advanced performance in ICs. Here, the potential of fusing 2D materials with silicon ICs to minimize the challenges in silicon ICs, and to create technologies beyond the von Neumann architecture, is presented, and the killer applications for 2D materials in logic and memory devices to ease scaling, energy efficiency bottlenecks, and memory dilemmas encountered in silicon ICs are discussed. The fusion of 2D materials allows the creation of all-in-one perception, memory, and computation technologies beyond the von Neumann architecture to enhance system efficiency and remove computing power bottlenecks. Progress on the 2D ICs demonstration is summarized, as well as the technical hurdles it faces in terms of wafer-scale heterostructure growth, transfer, and compatible integration with silicon ICs. Finally, the promising pathways and obstacles to the technological advances in ICs due to the integration of 2D materials with silicon are presented.

Two dimensional semiconducting materials for ultimately scaled transistors

Two dimensional (2D) semiconductors have been established as promising candidates to break through the short channel effect that existed in Si metal-oxide-semiconductor field-effect-transistor (MOSFET), owing to their unique atomically layered structure and dangling-bond-free surface. The last decade has witnessed the significant progress in the size scaling of 2D transistors by various approaches, in which the physical gate length of the transistors has shrank from micrometer to sub-one nanometer with superior performance, illustrating their potential as a replacement technology for Si MOSFETs. Here, we review state-of-the-art techniques to achieve ultra-scaled 2D transistors with novel configurations through the scaling of channel, gate, and contact length. We provide comprehensive views of the merits and drawbacks of the ultra-scaled 2D transistors by summarizing the relevant fabrication processes with the corresponding critical parameters achieved. Finally, we identify the key opportunities and challenges for integrating ultra-scaled 2D transistors in the next-generation heterogeneous circuitry.

Atomic Layer Deposition of Large-Area Polycrystalline Transition Metal Dichalcogenides from 100 °C through Control of Plasma Chemistry

Two-dimensional transition metal dichalcogenides, such as MoS₂, are intensely studied for applications in electronics. However, the difficulty of depositing large-area films of sufficient quality under application-relevant conditions remains a major challenge. Herein, we demonstrate deposition of polycrystalline, wafer-scale MoS₂, TiS₂, and WS₂ films of controlled thickness at record-low temperatures down to 100 °C using plasma-enhanced atomic layer deposition. We show that preventing excess sulfur incorporation from H₂S-based plasma is the key to deposition of crystalline films, which can be achieved by adding H₂ to the plasma feed gas. Film composition, crystallinity, growth, morphology, and electrical properties of MoS _x films prepared within a broad range of deposition conditions have been systematically characterized. Film characteristics are correlated with results of field-effect transistors based on MoS₂ films deposited at 100 °C. The capability to deposit MoS₂ on poly(ethylene terephthalate) substrates showcases the potential of our process for flexible devices. Furthermore, the composition control achieved by tailoring plasma chemistry is relevant for all low-temperature plasma-enhanced deposition processes of metal chalcogenides.

Atomic Layer Deposition of Metal Oxides and Chalcogenides for High Performance Transistors

Atomic layer deposition (ALD) is a deposition technique well-suited to produce high-quality thin film materials at the nanoscale for applications in transistors. This review comprehensively describes the latest developments in ALD of metal oxides (MOs) and chalcogenides with tunable bandgaps, compositions, and nanostructures for the fabrication of high-performance field-effect transistors. By ALD various n-type and p-type MOs, including binary and multinary semiconductors, can be deposited and applied as channel materials, transparent electrodes, or electrode interlayers for improving charge-transport and switching properties of transistors. On the other hand, MO insulators by ALD are applied as dielectrics or protecting/encapsulating layers for enhancing device performance and stability. Metal chalcogenide semiconductors and their heterostructures made by ALD have shown great promise as novel building blocks to fabricate single channel or heterojunction materials in transistors. By correlating the device performance to the structural and chemical properties of the ALD materials, clear structure-property relations can be proposed, which can help to design better-performing transistors. Finally, a brief concluding remark on these ALD materials and devices is presented, with insights into upcoming opportunities and challenges for future electronics and integrated applications.

Electrochemically deposited molybdenum disulfide surfaces enable polymer adsorption studies using quartz crystal microbalance with dissipation monitoring (QCM-D)

Polymer and small molecules are often used to modify the wettability of mineral surfaces which facilitates the separation of valuable minerals such as molybdenum disulfide (MoS₂) from gangue material through the process of froth flotation. By design, traditional methods used in the field for evaluating the separation efficacy of these additives fail to give proper access to adsorption kinetics and molecule conformation, crucial aspects of flotation where contact times may not allow for full thermodynamic equilibrium. Thus, there is a need for alternative methods for evaluating additives that accurately capture these features during the adsorption of additives at the solid/liquid interface. Here, we present a novel method for preparing MoS₂ films on quartz crystals used for Quartz Crystal Microbalance with Dissipation (QCM-D) measurements through an electrochemical deposition process. The resulting films exhibit well-controlled structure, composition, and thickness and therefore are ideal for quantifying polymer adsorption. After deposition, the sensors can be annealed without damaging the quartz crystal, resulting in a phase transition of the MoS₂ from the as-deposited, amorphous phase to the 2H semiconducting phase. Furthermore, we demonstrate the application of these sensors to study the interactions of additives at the solid/liquid interface by investigating the adsorption of a model polymer, dextran, onto both the amorphous and crystalline MoS₂ surfaces. We find that the adsorption rate of dextran onto the amorphous surface is approximately twice as fast as the adsorption onto the annealed surface. These studies demonstrate the ability to gain insight into the short-term kinetics of interaction between molecules and mineral surface, behavior that is key to designing additives with superior separation efficiency.

Endoepitaxial growth of monolayer mosaic heterostructures

The controllable growth of two-dimensional (2D) heterostructure arrays is critical for exploring exotic physics and developing novel devices, yet it remains a substantial synthetic challenge. Here we report a rational synthetic strategy to fabricate mosaic heterostructure arrays in monolayer 2D atomic crystals. By using a laser-patterning and an anisotropic thermal etching process, we create periodic triangular hole arrays in 2D crystals with precisely controlled size and atomically clean edges, which function as robust templates for endoepitaxial growth of another 2D crystal, to obtain monolayer mosaic heterostructures with atomically sharp heterojunction interfaces. Systematic microstructure and spectroscopic characterizations reveal periodic modulation of chemical compositions, lattice strains and electronic band gaps throughout the mosaic heterostructures. The robust growth of the monolayer mosaic heterostructures with a high level of synthetic control opens a pathway for band structure engineering and spatially modulating the potential landscapes in the atomically thin 2D crystals, establishing a designable material platform for fundamental studies and development of complex devices and integrated circuits from 2D heterostructures.

Effect of the Substrate on MoS2 Monolayer Morphology: An Integrated Computational and Experimental Study

Synthesis of two-dimensional materials, specifically transition metal dichalcogenides (TMDs), with controlled lattice orientations is a major barrier to their industrial applications. Controlling the orientation of as-grown TMDs is critical for preventing the formation of grain boundaries, thus reaching their maximum mechanical and optoelectronic performance. Here, we investigated the role of the substrate's crystallinity in the growth orientation of 2D materials using reactive molecular dynamics (MD) simulations and verified with experimental growth using the chemical vapor deposition (CVD) technique. We considered MoS₂ as our model material and investigated its growth on crystalline and amorphous silica and sapphire substrates. We revealed the role of the substrate's energy landscape on the orientation of as-grown TMDs, where the presence of monolayer-substrate energy barriers perpendicular to the streamlines hinder the detachment of precursor nuclei from the substrate. We show that MoS₂ monolayers with controlled orientations could not be grown on the SiO₂ substrate and revealed that amorphization of the substrate changes the intensity and equilibrium distance of monolayer-substrate interactions. Our simulations indicate that 0° rotated MoS₂ is the most favorable configuration on a sapphire substrate, consistent with our experimental results. The experimentally validated computational results and insight presented in this study pave the way for the high-quality synthesis of TMDs for high-performance electronic and optoelectronic devices.

Two-Dimensional Field-Effect Transistor Sensors: The Road toward Commercialization

The evolutionary success in information technology has been sustained by the rapid growth of sensor technology. Recently, advances in sensor technology have promoted the ambitious requirement to build intelligent systems that can be controlled by external stimuli along with independent operation, adaptivity, and low energy expenditure. Among various sensing techniques, field-effect transistors (FETs) with channels made of two-dimensional (2D) materials attract increasing attention for advantages such as label-free detection, fast response, easy operation, and capability of integration. With atomic thickness, 2D materials restrict the carrier flow within the material surface and expose it directly to the external environment, leading to efficient signal acquisition and conversion. This review summarizes the latest advances of 2D-materials-based FET (2D FET) sensors in a comprehensive manner that contains the material, operating principles, fabrication technologies, proof-of-concept applications, and prototypes. First, a brief description of the background and fundamentals is provided. The subsequent contents summarize physical, chemical, and biological 2D FET sensors and their applications. Then, we highlight the challenges of their commercialization and discuss corresponding solution techniques. The following section presents a systematic survey of recent progress in developing commercial prototypes. Lastly, we summarize the long-standing efforts and prospective future development of 2D FET-based sensing systems toward commercialization.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Atomic-Layer-Deposition-Based 2D Transition Metal Chalcogenides: Synthesis, Modulation, and Applications

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Growth Mechanisms and Morphology Engineering of Atomic Layer-Deposited WS2

Transition-metal dichalcogenides (TMDs) have attracted intense research interest for a broad range of device applications. Atomic layer deposition (ALD), a CMOS compatible technique, can enable the preparation of high-quality TMD films on 8 to 12 in. wafers for large-scale circuit integration. However, the ALD growth mechanisms are still not fully understood. In this work, we systematically investigated the growth mechanisms for WS₂ and found them to be strongly affected by nucleation density and film thickness. Transmission electron microscope imaging reveals the coexistence and competition of lateral and vertical growth mechanisms at different growth stages, and the critical thicknesses for each mechanism are obtained. The in-plane lateral growth mode dominates when the film thickness remains less than 5.6 nm (8 layers), while the vertical growth mode dominates when the thickness is greater than 20 nm. From the resulting understanding of these growth mechanisms, the conditions for film deposition were optimized and a maximum grain size of 108 nm was achieved. WS₂-based field-effect transistors were fabricated with electron mobility and on/off current ratio up to 3.21 cm² V^-1 s^-1 and 10⁵, respectively. Particularly, this work proves the capability of synthesis of TMD films in a wafer scale with excellent controllability of thickness and morphology, enabling many potential applications other than transistors, such as nanowire- or nanosheet-based supercapacitors, batteries, sensors, and catalysis.

Toward Low-Temperature Solid-Source Synthesis of Monolayer MoS2

Two-dimensional (2D) semiconductors have been proposed for heterogeneous integration with existing silicon technology; however, their chemical vapor deposition (CVD) growth temperatures are often too high. Here, we demonstrate direct CVD solid-source precursor synthesis of continuous monolayer (1L) MoS₂ films at 560 °C in 50 min, within the 450-to-600 °C, 2 h thermal budget window required for back-end-of-the-line compatibility with modern silicon technology. Transistor measurements reveal on-state current up to ∼140 μA/μm at 1 V drain-to-source voltage for 100 nm channel lengths, the highest reported to date for 1L MoS₂ grown below 600 °C using solid-source precursors. The effective mobility from transfer length method test structures is 29 ± 5 cm² V^-1 s^-1 at 6.1 × 10¹² cm^-2 electron density, which is comparable to mobilities reported from films grown at higher temperatures. The results of this work provide a path toward the realization of high-quality, thermal-budget-compatible 2D semiconductors for heterogeneous integration with silicon manufacturing.

Recent Progress in the Synthesis of MoS2 Thin Films for Sensing, Photovoltaic and Plasmonic Applications: A Review

In the surge of recent successes of 2D materials following the rise of graphene, molybdenum disulfide (2D-MoS2) has been attracting growing attention from both fundamental and applications viewpoints, owing to the combination of its unique nanoscale properties. For instance, the bandgap of 2D-MoS2, which changes from direct (in the bulk form) to indirect for ultrathin films (few layers), offers new prospects for various applications in optoelectronics. In this review, we present the latest scientific advances in the field of synthesis and characterization of 2D-MoS2 films while highlighting some of their applications in energy harvesting, gas sensing, and plasmonic devices. A survey of the physical and chemical processing routes of 2D-MoS2 is presented first, followed by a detailed description and listing of the most relevant characterization techniques used to study the MoS2 nanomaterial as well as theoretical simulations of its interesting optical properties. Finally, the challenges related to the synthesis of high quality and fairly controllable MoS2 thin films are discussed along with their integration into novel functional devices.

Stabilizing an ultrathin MoS2 layer during electrocatalytic hydrogen evolution with a crystalline SnO2 underlayer

Amorphous MoS₂ has been investigated abundantly as a catalyst for hydrogen evolution. Not only its performance but also its chemical stability in acidic conditions have been reported widely. However, its adhesion has not been studied systematically in the electrochemical context. The use of MoS₂ as a lubricant is not auspicious for this purpose. In this work, we start with a macroporous anodic alumina template as a model support, add an underlayer of SnO₂ to provide electrical conduction and adhesion, then provide the catalytically active, amorphous MoS₂ material by atomic layer deposition (ALD). The composition, morphology, and crystalline or amorphous character of all layers are confirmed by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, grazing incidence X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic water reduction performance of the macroporous AAO/SnO₂/MoS₂ electrodes, quantified by voltammetry, steady-state chronoamperometry and electrochemical impedance spectroscopy, is improved by annealing the SnO₂ layer prior to MoS₂ deposition. Varying the geometric parameters of the electrode composite yields an optimized performance of 10 mA cm⁻² at 0.22 V overpotential, with a catalyst loading of 0.16 mg cm⁻². The electrode's stability is contingent on SnO₂ crystallinity. Amorphous SnO₂ allows for a gradual dewetting of the originally continuous MoS₂ layer over wide areas. In stark contrast to this, crystalline SnO₂ maintains the continuity of MoS₂ until at least 0.3 V overpotential.

A molybdenum disulfide coating deposited on a macroporous substrate as an electrocatalyst is mobile on an underlying amorphous tin dioxide substrate, but remains continuous and impervious to acidic conditions on crystalline tin dioxide.

A wafer-scale synthesis of monolayer MoS2 and their field-effect transistors toward practical applications

Molybdenum disulfide (MoS2) has attracted considerable research interest as a promising candidate for downscaling integrated electronics due to the special two-dimensional structure and unique physicochemical properties. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Recently, a variety of investigations have focused on wafer-scale monolayer MoS2 synthesis with high-quality. The 2D MoS2 field-effect transistor (MoS2-FET) array with different configurations utilizes the high-quality MoS2 film as channels and exhibits favorable performance. In this review, we illustrated the latest research advances in wafer-scale monolayer MoS2 synthesis by different methods, including Au-assisted exfoliation, CVD, thin film sulfurization, MOCVD, ALD, VLS method, and the thermolysis of thiosalts. Then, an overview of MoS2-FET developments was provided based on large-area MoS2 film with different device configurations and performances. The different applications of MoS2-FET in logic circuits, basic memory devices, and integrated photodetectors were also summarized. Lastly, we considered the perspective and challenges based on wafer-scale monolayer MoS2 synthesis and MoS2-FET for developing practical applications in next-generation integrated electronics and flexible optoelectronics.

One-dimensional, space-confined, solid-phase growth of the Cu9S5@MoS2 core-shell heterostructure for electrocatalytic hydrogen evolution

Binary transition metal chalcogenide core-shell nanocrystals are considered the most promising nonprecious metal catalysts for large-scale industrial hydrogen production. Herein, we report a one-dimensional, space-confined, solid-phase strategy for the growth of a Cu₉S₅@MoS₂ core-shell heterostructure by combining electrospinning and chemical vapor deposition methods. The Cu₉S₅@MoS₂ core-shell nanocrystals were synthesized in situ on carbon nanofibers (Cu₉S₅@MoS₂/CNFs) by an S vapor graphitization process. Tuning of the MoS₂ shell numbers can be controlled by changing the mass ratio of the Cu and Mo precursors. We experimentally determined the effects of the thickness of the MoS₂ shell on the electrocatalytic activity for the hydrogen evolution reaction (HER) in acidic and alkaline solutions. When the mass ratio is 3:1, the Cu₉S₅@MoS₂/CNFs show the fewest MoS₂ shells with just 1-2 layers each and exhibit the best HER performance with small overpotentials of 116 mV and 114 mV in acidic and alkaline solutions, respectively, at a current density of 10 mA cm^-2. The core shell structures, with their unique Cu-S-Mo nanointerfaces, could enhance the electron transfer and surface area, thus increasing the performance of the HER. This work provides a facile method to design unique core shell assemblies in one-dimensional nanostructures.

Emerging Opportunities for 2D Semiconductor/Ferroelectric Transistor-Structure Devices

Semiconductor technology, which is rapidly evolving, is poised to enter a new era for which revolutionary innovations are needed to address fundamental limitations on material and working principle level. 2D semiconductors inherently holding novel properties at the atomic limit show great promise to tackle challenges imposed by traditional bulk semiconductor materials. Synergistic combination of 2D semiconductors with functional ferroelectrics further offers new working principles, and is expected to deliver massively enhanced device performance for existing complementary metal-oxide-semiconductor (CMOS) technologies and add unprecedented applications for next-generation electronics. Herein, recent demonstrations of novel device concepts based on 2D semiconductor/ferroelectric heterostructures are critically reviewed covering their working mechanisms, device construction, applications, and challenges. In particular, emerging opportunities of CMOS-process-compatible 2D semiconductor/ferroelectric transistor structure devices for the development of a rich variety of applications are discussed, including beyond-Boltzmann transistors, nonvolatile memories, neuromorphic devices, and reconfigurable nanodevices such as p-n homojunctions and self-powered photodetectors. It is concluded that 2D semiconductor/ferroelectric heterostructures, as an emergent heterogeneous platform, could drive many more exciting innovations for modern electronics, beyond the capability of ubiquitous silicon systems.

Modulation of the adsorption chemistry of a precursor in atomic layer deposition to enhance the growth per cycle of a TiO2 thin film

Atomic layer deposition (ALD) has scarcely been utilized in large-scale manufacturing and industrial processes due to its low productivity, even though it possesses several advantages for improving the device performance. The major cause of its low productivity is the slow growth rate, which is determined by the amount of chemisorbed precursor. The slow growth rate of ALD has become even more critical due to the introduction of heteroleptic-based precursors for achieving a higher thermal stability. In this study, we investigated the theoretical and experimental chemisorption characteristics of the Ti(CpMe5)(OMe)3 precursor during the ALD of TiO2. By density functional theory calculations, the relationship between the steric hindrance effect and the chemistry of a chemisorbed precursor was revealed. Based on the calculation result, a way for improving the growth per cycle by 50% was proposed and demonstrated, successfully.

Hybrid structure of PbS QDs and vertically-few-layer MoS2 nanosheets array for broadband photodetector

A novel three-dimensional (3D) vertically-few-layer MoS2 (V-MoS2) nanosheets- zero-dimensional PbS quantum dots (QDs) hybrid structure based broadband photodetector was fabricated, and its photoelectric performance was investigated in detail. We synthesized the V-MoS2 nanosheets by chemical vapor deposition, using the TiO2 layer as the induced layer, and proposed a possible growth mechanism. The use of the TiO2 induction layer successfully changed the growth direction of MoS2 from parallel to vertical. The prepared V-MoS2 nanosheets have a large specific surface area, abundantly exposed edges and excellent light absorption capacity. The V-MoS2 nanosheets detector was then fabricated and investigated, which exhibits a high sensitivity for 635 nm light, a fast response time and an excellent photoelectric response. The V-MoS2 nanosheets with a height of approximately 1 μm successfully broke the light absorption limit caused by the atomic thickness. Finally, we fabricated the PbS QDs/V-MoS2 nanosheets hybrid detector and demonstrated their potential for high-performance broadband photodetectors. The response wavelength of the hybrid detector extends from the visible band to the near-infrared band. The responsivity of the hybrid detector reaches 1.46 A W-1 under 1450 nm illumination. The combination of 3D MoS2 nanosheets and QDs further improves the performance of MoS2-based photodetector devices. We believe that the proposed zero-dimensional QDs and 3D vertical nanosheets hybrid structure broadband photodetector provides a promising way for the next-generation optoelectronic devices.

Morphology-controlled MoS2 by low-temperature atomic layer deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 are materials for multifarious applications such as sensing, catalysis, and energy storage. Due to their peculiar charge-transport properties, it is always desired to control their morphologies from vertical nanostructures to horizontal basal-plane oriented smooth layers. In this work, we established a low-temperature ALD process for MoS2 deposition using bis(t-butylimino)bis(dimethylamino)molybdenum(vi) and H2S precursors. The ALD reaction parameters, including reaction temperature and precursor pulse times, are systematically investigated and optimized. Polycrystalline MoS2 is conformally deposited on carbon nanotubes, Si-wafers, and glass substrates. Moreover, the morphologies of the deposited MoS2 films are tuned from smooth film to vertically grown flakes, and to nano-dots, by controlling the reaction parameters/conditions. It is noticed that our MoS2 nanostructures showed morphology-dependent optical and electrocatalytic properties, allowing us to choose the required morphology for a targeted application.

Electronic structures of WS2 armchair nanoribbons doped with transition metals

Armchair WS2 nanoribbons are semiconductors with band gaps close to 0.5 eV. If some of the W atoms in the ribbon are replaced by transition metals, the impurity states can tremendously affect the overall electronic structure of the doped ribbon. By using first-principles calculations based on density functional theory, we investigated substitutional doping of Ti, V, Cr, Mn, Fe, and Co at various positions on WS2 ribbons of different widths. We found that Fe-doped ribbons can have two-channel conduction in the middle segment of the ribbon and at the edges, carrying opposite spins separately. Many Co-doped ribbons are transformed into spin filters that exhibit 100% spin-polarized conduction. These results will be useful for spintronics and nanoelectronic circuit design.

Nanoscale Assembly of 2D Materials for Energy and Environmental Applications

Rational design of 2D materials is crucial for the realization of their profound implications in energy and environmental fields. The past decade has witnessed significant developments in 2D material research, yet a number of critical challenges remain for real-world applications. Nanoscale assembly, precise control over the orientational and positional ordering, and complex interfaces among 2D layers are essential for the continued progress of 2D materials, especially for energy storage and conversion and environmental remediation. Herein, recent progress, the status, future prospects, and challenges associated with nanoscopic assembly of 2D materials are highlighted, specifically targeting energy and environmental applications. Geometric dimensional diversity of 2D material assembly is focused on, based on novel assembly mechanisms, including 1D fibers from the colloidal liquid crystalline phase, 2D films by interfacial tension (Marangoni effect), and 3D nanoarchitecture assembly by electrochemical processes. Relevant critical advantages of 2D material assembly are highlighted for application fields, including secondary batteries, supercapacitors, catalysts, gas sensors, desalination, and water decontamination.

Area-Selective Atomic Layer Deposition of Two-Dimensional WS2 Nanolayers

With downscaling of device dimensions, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as WS₂ are being considered as promising materials for future applications in nanoelectronics. However, at these nanoscale regimes, incorporating TMD layers in the device architecture with precise control of critical features is challenging using current top-down processing techniques. In this contribution, we pioneer the combination of two key avenues in atomic-scale processing: area-selective atomic layer deposition (AS-ALD) and growth of 2D materials, and demonstrate bottom-up processing of 2D WS₂ nanolayers. Area-selective deposition of WS₂ nanolayers is enabled using an ABC-type plasma-enhanced ALD process involving acetylacetone (Hacac) as inhibitor (A), bis(tert-butylimido)-bis(dimethylamido)-tungsten as precursor (B), and H₂S plasma as the co-reactant (C) at a low deposition temperature of 250 °C. The developed AS-ALD process results in the immediate growth of WS₂ on SiO₂ while effectively blocking growth on Al₂O₃ as confirmed by in situ spectroscopic ellipsometry and ex situ X-ray photoelectron spectroscopy measurements. As a proof of concept, the AS-ALD process is demonstrated on patterned Al₂O₃/SiO₂ surfaces. The AS-ALD WS₂ films exhibited sharp Raman (E _2g ¹ and A _1g) peaks on SiO₂, a fingerprint of crystalline WS₂ layers, upon annealing at temperatures within the thermal budget of semiconductor back-end-of-line processing (≤450 °C). Our AS-ALD process also allows selective growth on various TMDs and transition metal oxides while blocking growth on HfO₂ and TiO₂. It is expected that this work will lay the foundation for area-selective ALD of other 2D materials.

Large area few-layer TMD film growths and their applications

Research on 2D materials is one of the core themes of modern condensed matter physics. Prompted by the experimental isolation of graphene, much attention has been given to the unique optical, electronic, and structural properties of these materials. In the past few years, semiconducting transition metal dichalcogenides (TMDs) have attracted increasing interest due to properties such as direct band gaps and intrinsically broken inversion symmetry. Practical utilization of these properties demands large-area synthesis. While films of graphene have been by now synthesized on the order of square meters, analogous achievements are difficult for TMDs given the complexity of their growth kinetics. This article provides an overview of methods used to synthesize films of mono- and few-layer TMDs, comparing spatial and time scales for the different growth strategies. A special emphasis is placed on the unique applications enabled by such large-scale realization, in fields such as electronics and optics.

Direct Growth of Continuous and Uniform MoS2 Film on SiO2/Si Substrate Catalyzed by Sodium Sulfate

Because of its unique electronic band structure, molybdenum disulfide (MoS₂) has been regarded as a star semiconducting material. However, direct growth of continuous and high-quality MoS₂ films on SiO₂/Si substrates is still very challenging. Here, we report a facile chemical vapor deposition (CVD) method based on synergistic modulation of precursor and Na₂SO₄ catalysis, realizing the centimeter scale growth of a continuous MoS₂ film on SiO₂/Si substrates. The as-grown MoS₂ film had an excellent spatial homogeneity and crystal quality, with an edge length of the composite domain as large as 632 μm. Both experimental and theoretical results proved that Na tended to bond with SiO₂ substrates rather than to interfere with as-grown MoS₂. Thus, they showed decent and uniform electrical performance, with electron mobilities as high as 5.9 cm² V^-1 s^-1_. We believe our method will pave a new way for MoS₂ toward real application in modern electronics.

Light and complex 3D MoS2/graphene heterostructures as efficient catalysts for the hydrogen evolution reaction

Multi-component 3D porous structures are highly promising hierarchical materials for numerous applications. Herein we show that atomic-layer deposition (ALD) of MoS₂ on graphene foams with variable pore size is a promising methodology to prepare complex 3D heterostructures to be used as electrocatalysts for the hydrogen evolution reaction (HER). The effect of MoS₂ crystallinity is studied and a trade-off between the high density of defects naturally presented in amorphous MoS₂ coatings and the highly crystalline phase obtained after annealing at 800 °C is established. Specifically, an optimal annealing at 500 °C is shown to yield improved catalytic performance with an overpotential of 180 mV, a low Tafel slope of 47 mV dec^-1, and a high exchange current of 17 μA cm^-2. The ALD deposition is highly conformal, and thus advantageous when coating 3D porous structures with small pore sizes, as required for real-world applications. This approach is enabled by conformal thin film deposition on porous structures with controlled crystallinity by tuning the annealing temperature. The results presented here therefore serve as an effective and general platform for the design of chemically and structurally tunable, binder-free, complex, lightweight, and highly efficient 3D porous heterostructures to be used for catalysis, energy storage, composite materials, sensors, water treatment, and more.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

A new metalorganic chemical vapor deposition process for MoS2 with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur

Crystalline MoS₂ thin films are deposited via MOCVD using a new molybdenum precursor, 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(N^tBu)₂(^tBu₂DAD)], and elemental sulfur.

Trickle Flow Aided Atomic Layer Deposition (ALD) Strategy for Ultrathin Molybdenum Disulfide (MoS2) Synthesis

The synthesis of large-area and high-quality two-dimensional (2D) MoS₂ is undoubtedly a significant challenge till now. In this work, an effective strategy for achieving 2D MoS₂ with enlarged grain size by Ni-foam-based trickle flow atomic layer deposition (ALD) was suggested, by which MoS₂ grain sizes up to 420 nm (monolayer sample) and 400 nm (five-layer sample) were obtained under the covering of 1 mm-thick Ni foam with a 2 mm gap from the substrate at 460 °C. In terms of specific ALD experiments, the Ni foam with a certain thickness placed on top of the substrate made the original precursor flow into a trickle-fluidization source flow, which decreased the nucleation density effectively. Thus, MoS₂ with enlarged grain sizes were obtained based on the typical ALD mechanism of large-scale vertical growth under the action of steric hindrance after a planar parallel growth around the crystal nucleus. In addition, the Ni foam also achieved a stable temperature field by enhancing the heat transfer around the substrate and thus improved its crystallinity.

Graphene and two-dimensional materials for silicon technology

The development of silicon semiconductor technology has produced breakthroughs in electronics-from the microprocessor in the late 1960s to early 1970s, to automation, computers and smartphones-by downscaling the physical size of devices and wires to the nanometre regime. Now, graphene and related two-dimensional (2D) materials offer prospects of unprecedented advances in device performance at the atomic limit, and a synergistic combination of 2D materials with silicon chips promises a heterogeneous platform to deliver massively enhanced potential based on silicon technology. Integration is achieved via three-dimensional monolithic construction of multifunctional high-rise 2D silicon chips, enabling enhanced performance by exploiting the vertical direction and the functional diversification of the silicon platform for applications in opto-electronics and sensing. Here we review the opportunities, progress and challenges of integrating atomically thin materials with silicon-based nanosystems, and also consider the prospects for computational and non-computational applications.

Molecular Routes to Two-Dimensional Metal Dichalcogenides MX2 (M = Mo, W; X = S, Se)

New synthetic access to two-dimensional transition metal dichalcogenides (TMDCs) is highly desired to exploit their extraordinary semiconducting and optoelectronic properties for practical applications. We introduce here an entirely novel class of molecular precursors, [M^IV(XEtN(Me)EtX)₂] (M^IV = Mo^IV, W^IV, X = S, Se), enabling chemical vapor deposition of TMDC thin films. Molybdenum and tungsten complexes of dianionic tridentate pincer-type ligands (HXEt)₂NR (R = methyl, tert-butyl, phenyl) produced air-stable monomeric dichalcogenide complexes, [W(SEtN(Me)EtS)₂] and [Mo(SEtN(Me)EtS)₂], displaying W and Mo centers in an octahedral environment of 4 S and 2 N donor atoms. Owing to their remarkable volatility and clean thermal decomposition, both Mo and W complexes, when used in the chemical vapor deposition (CVD) process, produced crystalline MoS₂ and WS₂ thin films. X-ray diffraction analysis and atomic-scale imaging confirmed the phase purity and 2D structural characteristics of MoS₂ and WS₂ films. The new set of ligands presented in this work open ups convenient access to a scalable and precursor-based synthesis of 2D transition metal dichalcogenides.

Electrocatalytic Performance of Titania Nanotube Arrays Coated with MoS2 by ALD toward the Hydrogen Evolution Reaction

The electrochemical splitting of water provides an elegant way to store renewable energy, but it is limited by the cost of the noble metals used as catalysts. Among the catalysts used for the reduction of water to hydrogen, MoS₂ has been identified as one of the most promising materials as it can be engineered to provide not only a large surface area but also an abundance of unsaturated and reactive coordination sites. Using Mo[NMe₂]₄ and H₂S as precursors, a desired thickness of amorphous MoS₂ can be deposited on TiO₂ nanotubes by atomic layer deposition. The identity and structure of the MoS₂ film are confirmed by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The electrocatalytic performance of MoS₂ is quantified as it depends on the tube length and the MoS₂ layer thickness through voltammetry, steady-state chronoamperometry, and electrochemical impedance spectroscopy. The best sample reaches 10 mA/cm² current density at 189 mV overpotential in 0.5 M H₂SO₄. All of the various geometries of our nanostructured electrodes reach an electrocatalytic proficiency comparable with the state-of-the-art MoS₂ electrodes, and the dependence of performance parameters on geometry suggests that the system can even be improved further.

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

Ångstrom-Scale, Atomically Thin 2D Materials for Corrosion Mitigation and Passivation

Metal deterioration via corrosion is a ubiquitous and persistent problem. Ångstrom-scale, atomically thin 2D materials are promising candidates for effective, robust, and economical corrosion passivation coatings due to their ultimate thinness and excellent mechanical and electrical properties. This review focuses on elucidating the mechanism of 2D materials in corrosion mitigation and passivation related to their physicochemical properties and variations, such as defects, out-of-plane deformations, interfacial states, temporal and thickness variations, etc. In addition, this review discusses recent progress and developments of 2D material coatings for corrosion mitigation and passivation as well as the significant challenges to overcome in the future.

New development of atomic layer deposition: processes, methods and applications

Atomic layer deposition (ALD) is an ultra-thin film deposition technique that has found many applications owing to its distinct abilities. They include uniform deposition of conformal films with controllable thickness, even on complex three-dimensional surfaces, and can improve the efficiency of electronic devices. This technology has attracted significant interest both for fundamental understanding how the new functional materials can be synthesized by ALD and for numerous practical applications, particularly in advanced nanopatterning for microelectronics, energy storage systems, desalinations, catalysis and medical fields. This review introduces the progress made in ALD, both for computational and experimental methodologies, and provides an outlook of this emerging technology in comparison with other film deposition methods. It discusses experimental approaches and factors that affect the deposition and presents simulation methods, such as molecular dynamics and computational fluid dynamics, which help determine and predict effective ways to optimize ALD processes, hence enabling the reduction in cost, energy waste and adverse environmental impacts. Specific examples are chosen to illustrate the progress in ALD processes and applications that showed a considerable impact on other technologies.

Synthesis of Thin-Film Metal Pyrites by an Atomic Layer Deposition Approach

Late 3d transition metal disulfides (MS₂ , M=Fe, Co, Ni, Cu, Zn) can crystallize in an interesting cubic-pyrite structure, in which all the metal cations are in a low-spin electronic configuration with progressive increase of the e_g electrons for M=Fe-Zn. These metal pyrite compounds exhibit very diverse and intriguing electrical and magnetic properties, which have stimulated considerable attention for various applications, especially in cutting-edge energy conversion and storage technologies. The synthesis of the metal pyrites is certainly very important, because highly controllable, reproducible, and reliable synthesis methods are virtually essential for both fundamental materials research and practical engineering. In this Concept, a new approach of (plasma-assisted) atomic layer deposition (ALD) to synthesize the thin-film metal pyrites (FeS₂ , CoS₂ , NiS₂ ) is introduced. The ALD synthesis approach allows for atomic-precision control over film composition and thickness, excellent film uniformity and conformality, and superior process reproducibility, and therefore it is of high promise for uniformly conformal metal pyrite thin-film coatings on complex 3D structures in general. Details and implications of this ALD approach are discussed in this Concept, mainly from a conceptual perspective, and it is envisioned that, with this new ALD synthesis approach, a significant amount of new studies will be enabled on both the fundamentals, and novel applications of the metal pyrite materials.

Armchair MoS2 nanoribbons turned into half metals through deposition of transition-metal and Si atomic chains

MoS₂ nanoribbons with armchair-terminated edges are semiconductors suitable for the tuning of electronic and magnetic properties. Our first-principles density function calculations reveal that a variety of transition-metal atomic chains deposited on some of the ribbons is able to transform the semiconductors into half metals, allowing transport of 100% spin-polarized currents. Furthermore, we found that a Si atomic chain is equally capable of achieving half metallicity when adsorbed on the same nanoribbon. These results should be useful for spintronic application.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.