Oxide-mediated recovery of field-effect mobility in plasma-treated MoS2

Monolithic three-dimensional tier-by-tier integration via van der Waals lamination

Two-dimensional (2D) semiconductors have shown great potential for monolithic three-dimensional (M3D) integration due to their dangling-bonds-free surface and the ability to integrate to various substrates without the conventional constraint of lattice matching1-10. However, with atomically thin body thickness, 2D semiconductors are not compatible with various high-energy processes in microelectronics11-13, where the M3D integration of multiple 2D circuit tiers is challenging. Here we report an alternative low-temperature M3D integration approach by van der Waals (vdW) lamination of entire prefabricated circuit tiers, where the processing temperature is controlled to 120 °C. By further repeating the vdW lamination process tier by tier, an M3D integrated system is achieved with 10 circuit tiers in the vertical direction, overcoming previous thermal budget limitations. Detailed electrical characterization demonstrates the bottom 2D transistor is not impacted after repetitively laminating vdW circuit tiers on top. Furthermore, by vertically connecting devices within different tiers through vdW inter-tier vias, various logic and heterogeneous structures are realized with desired system functions. Our demonstration provides a low-temperature route towards fabricating M3D circuits with increased numbers of tiers.

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.

Nitrogen Pretreatment of Growth Substrates for Vacancy-Saturated MoS2

Two-dimensional transition-metal dichalcogenides (2D TMDCs) are considered promising materials for optoelectronics due to their unique optical and electric properties. However, their potential has been limited by the occurrence of atomic vacancies during synthesis. While post-treatment processes have demonstrated the passivation of such vacancies, they increase process complexity and affect the TMDC's quality. We here introduce the concept of pretreatment as a facile and powerful route to solve the problem of vacancies in MoS2. Low-temperature nitridation of the sapphire substrate prior to growth provides a nondestructive method to MoS2 modification without introducing new processing steps or increasing the thermal budget. Spectroscopic characterization and atomic-resolution microscopy reveal the incorporation of nitrogen from the sapphire surface layer into chalcogen vacancies. The resulting MoS2 with nitrogen-saturated defects shows a decrease in midgap states and more intrinsic doping as confirmed by ab initio calculations and optoelectronic measurements. The demonstrated pretreatment method opens up new routes toward future, high-performance 2D electronics, as evidenced by a 3-fold reduction in contact resistance and a 10-fold improved performance of 2D photodetectors.

On the switching mechanism and optimisation of ion irradiation enabled 2D MoS2 memristors

Memristors are prominent passive circuit elements with promising futures for energy-efficient in-memory processing and revolutionary neuromorphic computation. State-of-the-art memristors based on two-dimensional (2D) materials exhibit enhanced tunability, scalability and electrical reliability. However, the fundamental of the switching is yet to be clarified before they can meet industrial standards in terms of endurance, variability, resistance ratio, and scalability. This new physical simulator based on the kinetic Monte Carlo (kMC) algorithm reproduces the defect migration process in 2D materials and sheds light on the operation of 2D memristors. The present work employs the simulator to study a two-dimensional 2H-MoS₂ planar resistive switching (RS) device with an asymmetric defect concentration introduced by ion irradiation. The simulations unveil the non-filamentary RS process and propose routes to optimize the device's performance. For instance, the resistance ratio can be increased by 53% by controlling the concentration and distribution of defects, while the variability can be reduced by 55% by increasing 5-fold the device size from 10 to 50 nm. Our simulator also explains the trade-offs between the resistance ratio and variability, resistance ratio and scalability, and variability and scalability. Overall, the simulator may enable an understanding and optimization of devices to expedite cutting-edge applications.

Biaxially Strained MoS2 Nanoshells with Controllable Layers Boost Alkaline Hydrogen Evolution

Strain in layered transition-metal dichalcogenides (TMDs) is a type of effective approach to enhance the catalytic performance by activating their inert basal plane. However, compared with traditional uniaxial strain, the influence of biaxial strain and the TMD layer number on the local electronic configuration remains unexplored. Herein, via a new in situ self-vulcanization strategy, biaxially strained MoS₂ nanoshells in the form of a single-crystalline Ni₃ S₂ @MoS₂ core-shell heterostructure are realized, where the MoS₂ layer is precisely controlled between the 1 and 5 layers. In particular, an electrode with the bilayer MoS₂ nanoshells shows a remarkable hydrogen evolution reaction activity with a small overpotential of 78.1 mV at 10 mA cm^-2 , and negligible activity degradation after durability testing. Density functional theory calculations reveal the contribution of the optimized biaxial strain together with the induced sulfur vacancies and identify the origin of superior catalytic sites in these biaxially strained MoS₂ nanoshells. This work highlights the importance of the atomic-scale layer number and multiaxial strain in unlocking the potential of 2D TMD electrocatalysts.

Few-Layered MnAl2S4 Dielectrics for High-Performance van der Waals Stacked Transistors

The gate dielectric layer is an important component in building a field-effect transistor. Here, we report the synthesis of a layered rhombohedral-structured MnAl₂S₄ crystal, which can be mechanically exfoliated down to the monolayer limit. The dielectric properties of few-layered MnAl₂S₄ flakes are systematically investigated, whereby they exhibit a relative dielectric constant of over 6 and an electric breakdown field of around 3.9 MV/cm. The atomically smooth thin MnAl₂S₄ flakes are then applied as a dielectric top gate layer to realize a two-dimensional van der Waals stacked field-effect transistor, which uses MoS₂ as a channel material. The fabricated transistor can be operated at a small drain-source voltage of 0.1 V and gate voltages within ranges of ±2 V, which exhibit a large on-off ratio over 10⁷ at 0.5 V and a low subthreshold swing value of 80 mV/dec. Our work demonstrates that the few-layered MnAl₂S₄ can work as a dielectric layer to realize high-performance two-dimensional transistors, and thus broadens the research on high-κ 2D materials and may provide new opportunities in developing low-dimensional electronic devices with a low power consumption in the future.

Photothermally Crumpled MoS2 Film as an Omnidirectionally Stretchable Platform

Molybdenum disulfide (MoS₂ ) is considered a fascinating material for next-generation semiconducting applications due to its outstanding mechanical stability and direct transition characteristics comparable to silicon. However, its application to stretchable platforms still is a challenging issue in wearable logic devices and sensors with noble form-factors required for future industry. Here, an omnidirectionally stretchable MoS₂ platform with laser-induced strained structures is demonstrated. The laser patterning induces the pyrolysis of MoS₂ precursors as well as the weak adhesion between Si and SiO₂ layers. The photothermal expansion of the Si layer results in the crumpling of SiO₂ and MoS₂ layers and the field-effect transistors with the crumpled MoS₂ are found to be suitable for strain sensor applications. The electrical performance of the crumpled MoS₂ depends on the degree of stretching, showing the stable omnidirectional stretchability up to 8% with approximately four times higher saturation current than its initial state. This platform is expected to be applied to future electronic devices, sensors, and so on.

Improved Temporal Response of MoS2 Photodetectors by Mild Oxygen Plasma Treatment

Temporal response is an important factor limiting the performance of two-dimensional (2D) material photodetectors. The deep trap states caused by intrinsic defects are the main factor to prolong the response time. In this work, it is demonstrated that the trap states in 2D molybdenum disulfide (MoS₂) can be efficiently modulated by defect engineering through mild oxygen plasma treatment. The response time of the few-layer MoS₂ photodetector is accelerated by 2–3 orders of magnitude, which is mainly attributed to the deep trap states that can be easily filled when O₂ or oxygen ions are chemically bonded with MoS₂ at sulfur vacancies (SV) sites. We characterized the defect engineering of plasma-exposed MoS₂ by Raman, PL and electric properties. Under the optimal processing conditions of 30 W, 50 Pa and 30 s, we found 30-fold enhancements in photoluminescence (PL) intensity and a nearly 2-fold enhancement in carrier field-effect mobility, while the rise and fall response times reached 110 ms and 55 ms, respectively, at the illumination wavelength of 532 nm. This work would, therefore, offer a practical route to improve the performance of 2D dichalcogenide-based devices for future consideration in optoelectronics research.

Bridging the gap between atomically thin semiconductors and metal leads

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS₂, towards the quantum limit), the high field-effect mobility (up to 358,000 cm²V^-1s^-1 in WSe₂), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design.

Ultrasensitive and Selective Field-Effect Transistor-Based Biosensor Created by Rings of MoS2 Nanopores

The ubiquitous field-effect transistor (FET) is widely used in modern digital integrated circuits, computers, communications, sensors, and other applications. However, reliable biological FET (bio-FET) is not available in real life due to the rigorous requirement for highly sensitive and selective bio-FET fabrication, which remains a challenging task. Here, we report an ultrasensitive and selective bio-FET created by the nanorings of molybdenum disulfide (MoS₂) nanopores inspired by nuclear pore complexes. We characterize the nanoring of MoS₂ nanopores by scanning transmission electron microscopy, Raman, and X-ray photoelectron spectroscopy spectra. After fabricating MoS₂ nanopore rings-based bio-FET, we confirm edge-selective functionalization by the gold nanoparticle tethering test and the change of electrical signal of the bio-FET. Ultrahigh sensitivity of the MoS₂ nanopore edge rings-based bio-FET (limit of detection of 1 ag/mL) and high selectivity are accomplished by effective coupling of the aptamers on the nanorings of the MoS₂ nanopore edge for cortisol detection. We believe that MoS₂ nanopore edge rings-based bio-FET would provide platforms for everyday biosensors with ultrahigh sensitivity and selectivity.

Synthesis and Characterization of Metallic Janus MoSH Monolayer

Janus transition-metal dichalcogenides (TMDCs) are emerging as special 2D materials with different chalcogen atoms covalently bonded on each side of the unit cell, resulting in interesting properties. To date, several synthetic strategies have been developed to realize Janus TMDCs, which first involves stripping the top-layer S of MoS₂ with H atoms. However, there has been little discussion on the intermediate Janus MoSH. It is critical to find the appropriate plasma treatment time to avoid sample damage. A thorough understanding of the formation and properties of MoSH is highly desirable. In this work, a controlled H₂-plasma treatment has been developed to gradually synthesize a Janus MoSH monolayer, which was confirmed by the TOF-SIMS analysis as well as the subsequent fabrication of MoSSe. The electronic properties of MoSH, including the high intrinsic carrier concentration (∼2 × 10¹³ cm^-2) and the Fermi level (∼ - 4.11 eV), have been systematically investigated by the combination of FET device study, KPFM, and DFT calculations. The results demonstrate a method for the creation of Janus MoSH and present the essential electronic parameters which have great significance for device applications. Furthermore, owing to the metallicity, 2D Janus MoSH might be a potential platform to observe the SPR behavior in the mid-infrared region.

Understanding the Synergistic Oxidation in Dichalcogenides through Electrochemiluminescence Blinking at Millisecond Resolution

The oxidation of transition metal dichalcogenides (TMDCs) has been extensively studied and applied in electronics, optics, and energy sources because of its tunable structure and performance. However, due to the lack of appropriate technology, dynamically observe the oxidation process remains an arduous task. Herein, the synergistic oxidation between edge and basal plane in molybdenum disulfide (MoS₂ ) is observed through electrogenerated chemiluminescence (ECL) blinking with a millisecond resolution. In addition, the ECL method provides a simple, convenient, and quick way to judge structural changes. The transient elevation of the ECL intensity proved the intermittent doping of oxygen at MoS₂ , which generates O-atom active sites. High ECL intensity enhanced from the produced hydroperoxide intermediates eases the monitoring of MoS₂ particles. Further study shows that the formation of sulfur vacancies at MoS₂ , by the edge activation of hydrogen peroxide and the migration of oxygen to the basal plane, is more conducive to oxygen doping that favors the formation of MoOMo as new active sites to induce bursts. The revealing of sulfur vacancy-governed blinking from MoS₂ indicates a complex interaction between oxygen and MoS₂ . The same phenomenon is observed on tungsten disulfide (WS₂ ), which provides new information about the oxidation feature of 2D dichalcogenides.

Bidirectional doping of two-dimensional thin-layer transition metal dichalcogenides using soft ammonia plasma

Because of suitable band gap and high mobility, two-dimensional transition metal dichalcogenide (TMD) materials are promising in future microelectronic devices. However, controllable p-type and n-type doping of TMDs is still a challenge. Herein, we develop a soft plasma doping concept and demonstrate both n-type and p-type doping for TMDs including MoS₂ and WS₂ through adjusting the plasma working parameters. In particular, p-type doping of MoS₂ can be realized when the radio frequency (RF) power is relatively small and the processing time is short: the off-state current increases from ∼10^-10 A to ∼10^-8 A, the threshold voltage is positively shifted from -26.2 V to 8.3 V, and the mobility increases from 7.05 cm² V^-1 s^-1 to 16.52 cm² V^-1 s^-1. Under a relatively large RF power and long processing time, n-type doping was realized for MoS₂: the threshold voltage was negatively shifted from 6.8 V to -13.3 V and the mobility is reduced from 10.32 cm² V^-1 s^-1 to 3.2 cm² V^-1 s^-1. For the former, suitable plasma treatment can promote the substitution of N elements for S vacancies and lead to p-type doping, thus reducing the defect density and increasing the mobility value. For the latter, due to excessive plasma treatment, more S vacancies will be produced, leading to heavier n-type doping as well as a decrease in mobility. We confirm the results by systematically analyzing the optical, compositional, thickness and structural characteristics of the samples before and after such soft plasma treatments via Raman, photoluminescence (PL), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Due to its nondestructive and expandable nature and compatibility with the current microelectronics industry, this potentially generic method may be used as a reliable technology for the development of diverse and functional TMD-based devices.

Rapid Degradation of the Electrical Properties of 2D MoS2 Thin Films under Long-Term Ambient Exposure

The MoS₂ thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS₂ thin films under ambient exposure of up to a year. The MoS₂ thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current-voltage (I-V) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear I-V curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS₂ thin film gradually oxidizes and 13.1% of MoO₃ and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS₂, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS₂ thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E_2g ¹ and A_1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS₂. It is also found that coating the MoS₂ thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure.

Wafer-Scale Oxygen-Doped MoS2 Monolayer

Monolayer MoS₂ is an emergent 2D semiconductor for next-generation miniaturized and flexible electronics. Although the high-quality monolayer MoS₂ is already available at wafer scale, doping of it uniformly remains an unsolved problem. Such doping is of great importance in view of not only tailoring its properties but also facilitating many potential large-scale applications. In this work, the uniform oxygen doping of 2 in wafer-scale monolayer MoS₂ (MoS_{2- x} O_x ) with tunable doping levels is realized through an in situ chemical vapor deposition process. Interestingly, ultrafast infrared spectroscopy measurements and first-principles calculations reveal a reduction of bandgaps of monolayer MoS_{2- x} O_x with increased oxygen-doping levels. Field-effect transistors and logic devices are also fabricated based on these wafer-scale MoS_{2- x} O_x monolayers, and excellent electronic performances are achieved, exhibiting promise of such doped MoS₂ monolayers.

Hidden Vacancy Benefit in Monolayer 2D Semiconductors

Monolayer 2D semiconductors (e.g., MoS₂ ) are of considerable interest for atomically thin transistors but generally limited by insufficient carrier mobility or driving current. Minimizing the lattice defects in 2D semiconductors represents a common strategy to improve their electronic properties, but has met with limited success to date. Herein, a hidden benefit of the atomic vacancies in monolayer 2D semiconductors to push their performance limit is reported. By purposely tailoring the sulfur vacancies (SVs) to an optimum density of 4.7% in monolayer MoS₂ , an unusual mobility enhancement is obtained and a record-high carrier mobility (>115 cm² V^-1 s^-1 ) is achieved, realizing monolayer MoS₂ transistors with an exceptional current density (>0.60 mA µm^-1 ) and a record-high on/off ratio >10¹⁰ , and enabling a logic inverter with an ultrahigh voltage gain >100. The systematic transport studies reveal that the counterintuitive vacancy-enhanced transport originates from a nearest-neighbor hopping conduction model, in which an optimum SV density is essential for maximizing the charge hopping probability. Lastly, the vacancy benefit into other monolayer 2D semiconductors is further generalized; thus, a general strategy for tailoring the charge transport properties of monolayer materials is defined.

Scalable Integration of Coplanar Heterojunction Monolithic Devices on Two-Dimensional In2Se3

The formation of lateral heterojunction arrays within two-dimensional (2D) crystals is an essential step to realize high-density, ultrathin electro-optical integrated circuits, although the assembling of such structures remains elusive. Here we demonstrated a rapid, scalable, and site-specific integration of lateral 2D heterojunction arrays using few-layer indium selenide (In₂Se₃). We use a scanning laser probe to locally convert In₂Se₃ into In₂O₃, which shows a significant increase in carrier mobility and transforms the metal-semiconductor junctions from Schottky to ohmic type. In addition, a lateral p-n heterojunction diode within a single nanosheet is demonstrated and utilized for photosensing applications. The presented method enables high-yield, site-specific formation of lateral 2D In₂Se₃-In₂O₃-based hybrid heterojunctions for realizing nanoscale devices with multiple advanced functionalities.

Gas flow through atomic-scale apertures

Gas flows are often analyzed with the theoretical descriptions formulated over a century ago and constantly challenged by the emerging architectures of narrow channels, slits, and apertures. Here, we report atomic-scale defects in two-dimensional (2D) materials as apertures for gas flows at the ultimate quasi-0D atomic limit. We establish that pristine monolayer tungsten disulfide (WS₂) membranes act as atomically thin barriers to gas transport. Atomic vacancies from missing tungsten (W) sites are made in freestanding (WS₂) monolayers by focused ion beam irradiation and characterized using aberration-corrected transmission electron microscopy. WS₂ monolayers with atomic apertures are mechanically sturdy and showed fast helium flow. We propose a simple yet robust method for confirming the formation of atomic apertures over large areas using gas flows, an essential step for pursuing their prospective applications in various domains including molecular separation, single quantum emitters, sensing and monitoring of gases at ultralow concentrations.

Thickness Trends of Electron and Hole Conduction and Contact Carrier Injection in Surface Charge Transfer Doped 2D Field Effect Transistors

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O₂ plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS₂, MoSe₂, MoTe₂, WS₂, and WSe₂ flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the doping-induced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

In Situ Oxygen Doping of Monolayer MoS2 for Novel Electronics

In 2D semiconductors, doping offers an effective approach to modulate their optical and electronic properties. Here, an in situ doping of oxygen atoms in monolayer molybdenum disulfide (MoS₂ ) is reported during the chemical vapor deposition process. Oxygen concentrations up to 20-25% can be reliable achieved in these doped monolayers, MoS_{2- x} O_x . These oxygen dopants are in a form of substitution of sulfur atoms in the MoS₂ lattice and can reduce the bandgap of intrinsic MoS₂ without introducing in-gap states as confirmed by photoluminescence spectroscopy and scanning tunneling spectroscopy. Field effect transistors made of monolayer MoS_{2- x} O_x show enhanced electrical performances, such as high field-effect mobility (≈100 cm² V^-1 s^-1 ) and inverter gain, ultrahigh devices' on/off ratio (>10⁹ ) and small subthreshold swing value (≈80 mV dec^-1 ). This in situ oxygen doping technique holds great promise on developing advanced electronics based on 2D semiconductors.

Effect of localized helium ion irradiation on the performance of synthetic monolayer MoS2 field-effect transistors

Helium ion irradiation is a known method of tuning the electrical conductivity and charge carrier mobility of novel two-dimensional semiconductors. Here, we report a systematic study of the electrical performance of chemically synthesized monolayer molybdenum disulfide (MoS₂) field-effect transistors irradiated with a focused helium ion beam as a function of increasing areal irradiation coverage. We determine an optimal coverage range of approx. 10%, which allows for the improvement of both the carrier mobility in the transistor channel and the electrical conductance of the MoS₂, due to doping with ion beam-created sulfur vacancies. Larger areal irradiations introduce a higher concentration of scattering centers, hampering the electrical performance of the device. In addition, we find that irradiating the electrode-channel interface has a deleterious impact on charge transport when contrasted with irradiations confined only to the transistor channel.

Inkjet-defined site-selective (IDSS) growth for controllable production of in-plane and out-of-plane MoS2 device arrays

Along with the increasing interest in MoS2 as a promising electronic material, there is also an increasing demand for nanofabrication technologies that are compatible with this material and other relevant layered materials. In addition, the development of scalable nanofabrication approaches capable of directly producing MoS2 device arrays is an imperative task to speed up the design and commercialize various functional MoS2-based devices. The desired fabrication methods need to meet two critical requirements. First, they should minimize the involvement of resist-based lithography and plasma etching processes, which introduce unremovable contaminations to MoS2 structures. Second, they should be able to produce MoS2 structures with in-plane or out-of-plane edges in a controlled way, which is key to increase the usability of MoS2 for various device applications. Here, we introduce an inkjet-defined site-selective (IDSS) method that meets these requirements. IDSS includes two main steps: (i) inkjet printing of microscale liquid droplets that define the designated sites for MoS2 growth, and (ii) site-selective growth of MoS2 at droplet-defined sites. Moreover, IDSS is capable of generating MoS2 with different structures. Specifically, an IDSS process using deionized (DI) water droplets mainly produces in-plane MoS2 features, whereas the processes using graphene ink droplets mainly produce out-of-plane MoS2 features rich in exposed edges. Using out-of-plane MoS2 structures, we have demonstrated the fabrication of miniaturized on-chip lithium ion batteries, which exhibit reversible lithiation/delithiation capacity. This IDSS method could be further expanded as a scalable and reliable nanomanufacturing method for generating miniaturized on-chip energy storage devices.

Hydrogen Plasma Exposure of Monolayer MoS2 Field-Effect Transistors and Prevention of Desulfurization by Monolayer Graphene

Atomic vacancies related to structural disorder and doping variation influence carrier transport in monolayer transition-metal dichalcogenide devices. Here, we investigate the effect of hydrogen plasma exposure (HPE) on monolayer MoS₂ field-effect transistors (FETs). We observe that a 1% increase in sulfur vacancy after HPE results in incremental 0.06 eV of the Schottky barrier. Short-range scattering from the sulfur vacancies reduces the carrier mobility of monolayer MoS₂ by 2 orders of magnitude. Despite the defects and grain boundaries formed during the chemical vapor deposition and transferring process, the surface desulfurization induced by the proton exposure and thermally accelerated oxidation can be blocked by monolayer graphene cladding with a van der Waals contact distance of 2.5 Å. The material-level study indicates a promising route for a low-cost and robust fabrication of smart sensor circuits on a monolithic MoS₂ wafer, where the bare MoS₂ FETs can serve as proton sensors, with their electronic readout processed by a logic circuit of graphene-protected pristine FETs with a high on/off ratio.

Layer-Selective Synthesis of MoS2 and WS2 Structures under Ambient Conditions for Customized Electronics

Transition metal dichalcogenides (TMDs) have attracted significant interest as one of the key materials in future electronics such as logic devices, optoelectrical devices, and wearable electronics. However, a complicated synthesis method and multistep processes for device fabrication pose major hurdles for their practical applications. Here, we introduce a direct and rapid method for layer-selective synthesis of MoS₂ and WS₂ structures in wafer-scale using a pulsed laser annealing system (λ = 1.06 μm, pulse duration ∼100 ps) in ambient conditions. The precursor layer of each TMD, which has at least 3 orders of magnitude higher absorption coefficient than those of neighboring layers, rigorously absorbed the incoming energy of the laser pulse and rapidly pyrolyzed in a few nanoseconds, enabling the generation of a MoS₂ or WS₂ layer without damaging the adjacent layers of SiO₂ or polymer substrate. Through experimental and theoretical studies, we establish the underlying principles of selective synthesis and optimize the laser annealing conditions, such as laser wavelength, output power, and scribing speed, under ambient condition. As a result, individual homostructures of patterned MoS₂ and WS₂ layers were directly synthesized on a 4 in. wafer. Moreover, a consecutive synthesis of the second layer on top of the first synthesized layer realized a vertically stacked WS₂/MoS₂ heterojunction structure, which can be treated as a cornerstone of electronic devices. As a proof of concept, we demonstrated the behavior of a MoS₂-based field-effect transistor, a skin-attachable motion sensor, and a MoS₂/WS₂-based heterojunction diode in this study. The ultrafast and selective synthesis of the TMDs suggests an approach to the large-area/mass production of functional heterostructure-based electronics.

Conversion of Intercalated MoO3 to Multi-Heteroatoms-Doped MoS2 with High Hydrogen Evolution Activity

Lack of effective strategies to regulate the internal activity of MoS₂ limits its practical application for hydrogen evolution reactions (HERs). Doping of heteroatoms without forming aggregation or an edge enrichment is still challenging, and its effect on the HER needs to be further explored. Herein, a two-step method is developed to obtain multi-metal-doped H-MoS₂ , which includes intercalation of the layered MoO₃ precursor with a following sulfurization. Benefiting from the capability of the intercalation method to uniformly and simultaneously introduce different elements into the van der Waals gap, this method is universal to obtain multi-heteroatoms co-doped MoS₂ without forming clusters, phase separation, and an edge enrichment. It is demonstrated that the doping of adjacent cobalt and palladium monomers on MoS₂ greatly enhances the HER catalytic activity. The overpotential at 10 mA cm^-2 and Tafel slope of Co and Pd co-doped MoS₂ is found to be 49.3 mV and 43.2 mV dec^-1 , respectively, representing a superior acidic HER catalytic activity. This intercalation-assisted method also provides a new and general strategy to synthesize uniformly doped transition metal dichalcogenides for various applications.

Effects of Ion Energy and Density on the Plasma Etching-Induced Surface Area, Edge Electrical Field, and Multivacancies in MoSe2 Nanosheets for Enhancement of the Hydrogen Evolution Reaction

Plasma functionalization can increase the efficiency of MoSe₂ in the hydrogen evolution reaction (HER) by providing multiple species but the interactions between the plasma and catalyst are not well understood. In this work, the effects of the ion energy and plasma density on the catalytic properties of MoSe₂ nanosheets are studied. The through-holes resulting from plasma etching and multi-vacancies induced by plasma-induced damage enhance the HER efficiency as exemplified by a small overpotential of 148 mV at 10 mA cm^-2 and Tafel slope of 51.6 mV dec^-1 after the plasma treatment using a power of 20 W. The interactions between the plasma and catalyst during etching and vacancies generation are evaluated by plasma simulation. Finite element and first-principles density functional theory calculations are also conducted and the results are consistent with the experimental results, indicating that the improved HER catalytic activity stems from the enhanced electric field and more active sites on the catalyst, and reduced bandgap and adsorption energy arising from the etched through-holes and vacancies, respectively. The results convey new fundamental knowledge about the plasma effects and means to enhance the efficiency of catalysts in water splitting as well insights into the design of high-performance HER catalysts.

Embedded Metal Oxide Plasmonics Using Local Plasma Oxidation of AZO for Planar Metasurfaces

New methods for achieving high-quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon-near-zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al-doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi-layer devices. To demonstrate the performance of the carrier-modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible-range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross-talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide-based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology.

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

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS₂) using transmission electron microscopy analysis. We find that MoS₂ starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO₃) nanosheets. The α-MoO₃ nanosheets so formed are stacked layer-by-layer on the underlying MoS₂ via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS₂ is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.

Interface engineering of two-dimensional transition metal dichalcogenides towards next-generation electronic devices: recent advances and challenges

Over the past decade, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interest for future electronics owing to their atomically thin thickness, compelling properties and various potential applications. However, interface engineering including contact optimization and channel modulations for 2D TMDCs represents fundamental challenges in ultimate performance of ultrathin electronics. This article provides a comprehensive overview of the basic understanding of contacts and channel engineering of 2D TMDCs and emerging electronics benefiting from these varying approaches. In particular, we elucidate multifarious contact engineering approaches such as edge contact, phase engineering and metal transfer to suppress the Fermi level pinning effect at the metal/TMDC interface, various channel treatment avenues such as van der Waals heterostructures, surface charge transfer doping to modulate the device properties, and as well the novel electronics constructed by interface engineering such as diodes, circuits and memories. Finally, we conclude this review by addressing the current challenges facing 2D TMDCs towards next-generation electronics and offering our insights into future directions of this field.

Surface Plasmon Resonance-Enhanced Near-Infrared Absorption in Single-Layer MoS2 with Vertically Aligned Nanoflakes

The development of MoS₂ with two- or three-dimensional heterostructures can provide a significant breakthrough for the enhancement of photodetection abilities such as increase in light absorption and expanding the detection ranges. Till date, although the synthesis of a MoS₂ layer with three-dimensional nanostructures using a chemical vapor deposition (CVD) process has been successfully demonstrated, most studies have concentrated on electrochemical applications that utilize structural strengths, for example, a large specific surface area and electrochemically active sites. Here, for the first time, we report spectral light absorption induced by plasmon resonances in single-layer MoS₂ (SL-MoS₂) with vertically aligned nanoflakes grown by a CVD process. Treatment with oxygen plasma results in the formation of a substoichiometric phase of MoO_x in the vertical nanoflakes, which exhibit a high electron density of 4.5 × 10¹³ cm^-2. The substoichiometric MoO_x with a high electron-doping level that is locally present on the SL-MoS₂ surface induces an absorption band in the near-infrared (NIR) wavelength range of 1000-1750 nm because of the plasmon resonances. Finally, we demonstrate the enhancement of photodetection ability by broadening the detection range from the visible region to the NIR region in oxygen-treated SL-MoS₂ with vertically aligned nanoflakes.

Defect-Engineered Atomically Thin MoS2 Homogeneous Electronics for Logic Inverters

Ultrathin molybdenum disulfide (MoS₂ ) presents ideal properties for building next-generation atomically thin circuitry. However, it is difficult to construct logic units of MoS₂ monolayer using traditional silicon-based doping schemes, such as atomic substitution and ion implantation, as they cause lattice disruption and doping instability. An accurate and feasible electronic structure modulation strategy from defect engineering is proposed to construct homogeneous electronics for MoS₂ monolayer logic inverters. By utilizing the energy-matched electron induction of the solution process, numerous pure and lattice-stable monosulfur vacancies (V_monos ) are introduced to modulate the electronic structure of monolayer MoS₂ via a shallow trapping effect. The resulting modulation effectively reduces the electronic concentration of MoS₂ and improves the work function by 100 meV. Under modulation of V_monos , an atomically thin homogenous monolayer MoS₂ logic inverter with a voltage gain of 4 is successfully constructed. A brand-new and practical design route of defect modulation for 2D-based circuit development is provided.

Recent advances in plasma modification of 2D transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have recently attracted great interest because of their tantalising prospects for a broad range of applications including electronics, optoelectronics, and energy storage. Unlike bulk materials, the device performance of atomically thin 2D materials is determined by the interface, thickness and defects. Plasma processing is very effective for diverse modifications of nanoscale 2D TMDC materials, owing to its uniquely controllable, effective processes and energy efficiency. Herein, we critically discuss selected recent advances in plasma modification of 2D TMDC materials and their optical and electronic (including optoelectronic) properties of relevance to applications in hydrogen production, gas sensing and energy storage devices. Challenges and future research opportunities in the relevant research field are presented. This review contributes to directing future advances of plasma processing of TMDC materials for targeted applications.

Direct Synthesis of a Self-Assembled WSe2 /MoS2 Heterostructure Array and its Optoelectrical Properties

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next-generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self-assembled WSe₂ /MoS₂ heterostructures through facile solution-based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel- and cross-aligned heterostructures. The realized WSe₂ /MoS₂ -based p-n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W^-1 and response speed of 16 µs. As a feasible application, a WSe₂ /MoS₂ -based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution-based growth of hierarchical structures with various alignments could offer a method for the further development of large-area electronic and optoelectronic applications.

NiPS3 nanoflakes: a nonlinear optical material for ultrafast photonics

Ultrafast photonics based on two-dimensional (2D) materials has been used to investigate light-matter interactions and laser generation, as well as light propagation, modulation, and detection. Here, 2D metal-phosphorus trichalcogenides, which are known for applications in catalysis and electrochemical storage, also exhibit advantageous photonic properties as nanoflakes that are only a few layers thick. By using an open-aperture Z-scan system, few-layer NiPS3 nanoflakes exhibited a large modulation depth of 56% and a low saturable intensity of 16 GW cm-2 at 800 nm. When NiPS3 nanoflakes were used as a saturable absorber at 1066 nm, highly stable mode-locked pulses were generated. Thus, these results revealed the nonlinear optical properties of NiPS3 nanoflakes which have potential photonics applications, such as modulators, switches, and thresholding devices.

High-Yield Electrochemical Production of Large-Sized and Thinly Layered NiPS3 Flakes for Overall Water Splitting

Achieving large-sized and thinly layered 2D metal phosphorus trichalcogenides with high quality and yield has been an urgent quest due to extraordinary physical/chemical characteristics for multiple applications. Nevertheless, current preparation methodologies suffer from uncontrolled thicknesses, uneven morphologies and area distributions, long processing times, and inferior quality. Here, a sonication-free and fast (in minutes) electrochemical cathodic exfoliation approach is reported that can prepare large-sized (typically ≈150 µm² ) and thinly layered (≈70% monolayer) NiPS₃ flakes with high crystallinity and pure phase structure with a yield ≈80%. During the electrochemical exfoliation process, the tetra-n-butylammonium salt with a large ionic diameter is decomposed into gaseous species after the intercalation and efficiently expands the tightly stratified bulk NiPS₃ crystals, as revealed by in situ and ex situ characterizations. Atomically thin NiPS₃ flakes can be obtained by slight manual shaking rather than sonication, which largely preserves in-plane structural integrity with large size and minimum damage. The obtained high quality NiPS₃ offers a new and ideal model for overall water splitting due to its inherent fully exposed S and P atoms that are often the active sites for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Consequently, the bifunctional NiPS₃ exhibits outstanding performance for overall water splitting.

Paving the way towards green catalytic materials for green fuels: impact of chemical species on Mo-based catalysts for hydrodeoxygenation

A series of Mo-based catalysts were synthesized by tuning the sulfidation temperature to produce mixtures of MoO3 and MoS2 as active phases for the hydrodeoxygenation (HDO) of palmitic acid. Differences in the oxidation states of Mo, and the chemical species present in the catalytic materials were determined by spectroscopic techniques. Palmitic acid was used as a fatty-acid model compound to test the performance of these catalysts. The catalytic performance was related to different chemical species formed within the materials. Sulfidation of these otherwise inactive catalysts significantly increased their performance. The catalytic activity remains optimal between the sulfidation temperatures of 100 °C and 200 °C, whereas the most active catalyst was obtained at 200 °C. The catalytic performance decreased significantly at 400 °C due to a higher proportion of sulfides formed in the materials. Furthermore, the relative proportion of MoO3 to MoS2 is essential to form highly active materials to produce O-free hydrocarbons from biomass feedstock. The transition from MoS2 to MoO3 reveals the importance of Mo-S and Mo-O catalytically active species needed for the HDO process and hence for biomass transformation. We conclude that transitioning from MoS2 to MoO3 catalysts is a step in the right direction to produce green fuels.

Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers

Atom-thick two-dimensional materials usually possess unique properties compared to their bulk counterparts. Their properties are significantly affected by defects, which could be uncontrollably introduced by irradiation. The effects of electromagnetic irradiation and particle irradiation on 2H MoS 2 two-dimensional nanolayers are reviewed in this paper, covering heavy ions, protons, electrons, gamma rays, X-rays, ultraviolet light, terahertz, and infrared irradiation. Various defects in MoS 2 layers were created by the defect engineering. Here we focus on their influence on the structural, electronic, catalytic, and magnetic performance of the 2D materials. Additionally, irradiation-induced doping is discussed and involved.

Doping engineering and functionalization of two-dimensional metal chalcogenides

Two-dimensional (2D) layered metal chalcogenides (MXs) have significant potential for use in flexible transistors, optoelectronics, sensing and memory devices beyond the state-of-the-art technology. To pursue ultimate performance, precisely controlled doping engineering of 2D MXs is desired for tailoring their physical and chemical properties in functional devices. In this review, we highlight the recent progress in the doping engineering of 2D MXs, covering that enabled by substitution, exterior charge transfer, intercalation and the electrostatic doping mechanism. A variety of novel doping engineering examples leading to Janus structures, defect curing effects, zero-valent intercalation and deliberately devised floating gate modulation will be discussed together with their intriguing application prospects. The choice of doping strategies and sources for functionalizing MXs will be provided to facilitate ongoing research in this field toward multifunctional applications.

Rubbing-Induced Site-Selective Growth of MoS2 Device Patterns

The superior electronic and mechanical properties of two-dimensional layered transition-metal dichalcogenides could be exploited to make a broad range of devices with attractive functionalities. However, the nanofabrication of such layered material-based devices still needs resist-based lithography and plasma etching processes for patterning layered materials into functional device features. Such patterning processes lead to unavoidable contaminations, to which the transport characteristics of atomically thin-layered materials are very sensitive. More seriously, such lithography-introduced contaminants cannot be safely eliminated by conventional semiconductor cleaning approaches. This challenge seriously retards the manufacturing of large arrays of layered material-based devices with consistent characteristics. Toward addressing this challenge, we introduce a rubbing-induced site-selective growth method capable of directly generating few-layer MoS₂ device patterns without the need of any additional patterning processes. This method consists of two critical steps: (i) a damage-free mechanical rubbing process for generating microscale triboelectric charge patterns on a dielectric surface and (ii) site-selective deposition of MoS₂ within rubbing-induced charge patterns. Our microscopy characterizations in combination with finite element analysis indicate that the field magnitude distribution within triboelectric charge patterns determines the morphologies of grown MoS₂ patterns. In addition, the MoS₂ line patterns produced by the presented method have been implemented for making arrays of working transistors and memristors. These devices exhibit a high yield and good uniformity in their electronic properties over large areas. The presented method could be further developed into a cost-efficient nanomanufacturing approach for producing functional device patterns based on various layered materials.

Oxygen plasmas: a sharp chisel and handy trowel for nanofabrication

Although extremely chemically reactive, oxygen plasmas feature certain properties that make them attractive not only for material removal via etching and sputtering, but also for driving and sustaining nucleation and growth of various nanostructures in plasma bulk and on plasma-exposed surfaces. In this minireview, a number of representative examples is used to demonstrate key mechanisms and unique capabilities of oxygen plasmas and how these can be used in present-day nano-fabrication. In addition to modification and functionalisation processes typical for oxygen plasmas, their ability to catalyse the growth of complex nanoarchitectures is emphasized. Two types of technologies based on oxygen plasmas, namely surface treatment without a change in the size and shape of surface features, as well as direct growth of oxide structures, are used to better illustrate the capabilities of oxygen plasmas as a powerful process environment. Future applications and possible challenges for the use of oxygen plasmas in nanofabrication are discussed.

First principles mechanistic study of self-limiting oxidative adsorption of remote oxygen plasma during the atomic layer deposition of alumina

Plasma-enhanced atomic layer deposition (ALD) of metal oxides is rapidly gaining interest, especially in the electronics industry, because of its numerous advantages over the thermal process. However, the underlying reaction mechanism is not sufficiently understood, particularly regarding saturation of the reaction and densification of the film. In this work, we employ first principles density functional theory (DFT) to determine the predominant reaction pathways, surface intermediates and by-products formed when constituents of O2-plasma or O3 adsorb onto a methylated surface typical of TMA-based alumina ALD. The main outcomes are that a wide variety of barrierless and highly exothermic reactions can take place. This leads to the spontaneous production of various by-products with low desorption energies and also of surface intermediates from the incomplete combustion of -CH3 ligands. Surface hydroxyl groups are the most frequently observed intermediates and are formed as a consequence of the conservation of atoms and charge when methyl ligands are initially oxidized (rather than from subsequent re-adsorption of molecular water). Anionic intermediates such as formates are also commonly observed at the surface in the simulations. Formaldehyde, CH2O, is the most frequently observed gaseous by-product. Desorption of this by-product leads to saturation of the redox reaction at the level of two singlet oxygen atoms per CH3 group, where the oxidation state of C is zero, rather than further reaction with oxygen to higher oxidation states. We conclude that the self-limiting chemistry that defines ALD comes about in this case through the desorption by-products with partially-oxidised carbon. The simulations also show that densification occurs when ligands are removed or oxidised to intermediates, indicating that there may be an inverse relationship between Al/O coordination numbers in the final film and the concentration of chemically-bound ligands or intermediate fragments covering the surface during each ALD pulse. Therefore reactions that generate a bare surface Al will produce denser films in metal oxide ALD.