Rapid Flame Synthesis of Atomically Thin MoO3 down to Monolayer Thickness for Effective Hole Doping of WSe2

Innovations of metallic contacts on semiconducting 2D transition metal dichalcogenides toward advanced 3D-structured field-effect transistors

2D semiconductors, represented by transition metal dichalcogenides (TMDs), have the potential to be alternative channel materials for advanced 3D field-effect transistors, such as gate-all-around field-effect-transistors (GAAFETs) and complementary field-effect-transistors (C-FETs), due to their inherent atomic thinness, moderate mobility, and short scaling lengths. However, 2D semiconductors encounter several technological challenges, especially the high contact resistance issue between 2D semiconductors and metals. This review provides a comprehensive overview of the high contact resistance issue in 2D semiconductors, including its physical background and the efforts to address it, with respect to their applicability to GAAFET structures.

A dual-electrode label-free immunosensor based on in situ prepared Au-MoO3-Chi/porous graphene nanoparticles for point-of-care detection of cholangiocarcinoma

A novel label-free electrochemical immunosensor was prepared for the detection of carbohydrate antigen 19-9 (CA19-9) and carcinoembryonic antigen (CEA) as biomarkers of cholangiocarcinoma (CCA). A nanocomposite of gold nanoparticles, molybdenum trioxide, and chitosan (Au-MoO3-Chi) was layer-by-layer assembled on the porous graphene (PG) modified a dual screen-printed electrode using a self-assembling technique, which increased surface area and conductivity and enhanced the adsorption of immobilized antibodies. The stepwise self-assembling procedure of the modified electrode was further characterized morphologically and functionally. The electroanalytical detection of biomarkers was based on the interaction between the antibody and antigen of each marker via linear sweep voltammetry using ferrocyanide/ferricyanide as an electrochemical redox indicator. Under optimized conditions, the fabricated immunosensor showed linear relationships between current change (ΔI) and antigen concentrations in two ranges: 0.0025-0.1 U mL-1 and 0.1-1.0 U mL-1 for CA19-9, and 0.001-0.01 ng mL-1 and 0.01-1.0 ng mL-1 for CEA. The limits of detection (LOD) were 1.0 mU mL-1 for CA19-9 and 0.5 pg mL-1 for CEA. Limits of quantitation (LOQ) were 3.3 mU mL-1 for CA19-9 and 1.6 pg mL-1 for CEA. The selectivity of the developed immunosensor was tested on mixtures of antigens and was then successfully applied to determine CA19-9 and CEA in human serum samples, producing satisfactory results consistent with the clinical method.

High-Performance Contact-Doped WSe2 Transistors Using TaSe2 Electrodes

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have garnered significant attention due to their potential for next-generation electronics, which require device scaling. However, the performance of TMD-based field-effect transistors (FETs) is greatly limited by the contact resistance. This study develops an effective strategy to optimize the contact resistance of WSe2 FETs by combining contact doping and 2D metallic electrode materials. The contact regions were doped using a laser, and the metallic TaSe2 flakes were stacked on doped WSe2 as electrodes. Doping the contact areas decreases the depletion width, while introducing the TaSe2 contact results in a lower Schottky barrier. This method significantly improves the electrical performance of the WSe2 FETs. The doped WSe2/TaSe2 contact exhibits an ultralow Schottky barrier height of 65 meV and a contact resistance of 11 kΩ·μm, which is a 50-fold reduction compared to the conventional Cr/Au contact. Our method offers a way on fabricating high-performance 2D FETs.

Selective Tumor Hypoxia Targeting Using M75 Antibody Conjugated Photothermally Active MoOx Nanoparticles

Photothermal therapy (PTT) mediated at the nanoscale has a unique advantage over currently used cancer treatments, by being spatially highly specific and minimally invasive. Although PTT combats traditional tumor treatment approaches, its clinical implementation has not yet been successful. The reasons for its disadvantage include an insufficient treatment efficiency or low tumor accumulation. Here, we present a promising new PTT platform combining a recently emerged two-dimensional (2D) inorganic nanomaterial, MoOx, and a tumor hypoxia targeting element, the monoclonal antibody M75. M75 specifically binds to carbonic anhydrase IX (CAIX), a hypoxia marker associated with many solid tumors with a poor prognosis. The as-prepared nanoconjugates showed highly specific binding to cancer cells expressing CAIX while being able to produce significant photothermal yield after irradiation with near-IR wavelengths. Small aminophosphonic acid linkers were recognized to be more effective over the combination of poly(ethylene glycol) chain and biotin-avidin-biotin bridge in constructing a PTT platform with high tumor-binding efficacy. The in vitro cellular uptake of nanoconjugates was visualized by high-resolution fluorescence microscopy and label-free live cell confocal Raman microscopy. The key to effective cancer treatment may be the synergistic employment of active targeting and noninvasive, tumor-selective therapeutic approaches, such as nanoscale-mediated PTT. The use of active targeting can streamline nanoparticle delivery increasing photothermal yield and therapeutic success.

High-Performance WSe2 Top-Gate Devices with Strong Spacer Doping

Because of the lack of contact and spacer doping techniques for two-dimensional (2D) transistors, most high-performance 2D devices have been produced with nontypical structures that contain electrical gating in the contact regions. In the present study, we used chloroauric acid (HAuCl4) as a strong p-dopant for WSe2 monolayers used in transistors. The HAuCl4-doped devices exhibited a record-low contact resistance of 0.7 kΩ·μm under a doping concentration of 1.76 × 1013 cm-2. In addition, an extrinsic carrier diffusion phenomenon was discovered in the HAuCl4-WSe2 system. With a suitably designed spacer length for doping, a normally off, high-performance underlap top-gate device can be produced without the application of additional gating in the contact and spacer regions.

Self-Aligned Top-Gate Structure in High-Performance 2D p-FETs via van der Waals Integration and Contact Spacer Doping

The potential of 2D materials in future CMOS technology is hindered by the lack of high-performance p-type field effect transistors (p-FETs). While utilization of the top-gate (TG) structure with a p-doped spacer area offers a solution to this challenge, the design and device processing to form gate stacks pose serious challenges in realization of ideal p-FETs and PMOS inverters. This study presents a novel approach to address these challenges by fabricating lateral p+-p-p+ junction WSe2 FETs with self-aligned TG stacks in which desired junction is formed by van der Waals (vdW) integration and selective oxygen plasma-doping into spacer regions. The exceptional electrostatic controllability with a high on/off current ratio and small subthreshold swing (SS) of plasma doped p-FETs is achieved with the self-aligned metal/hBN gate stacks. To demonstrate the effectiveness of our approach, we construct a PMOS inverter using this device architecture, which exhibits a remarkably low power consumption of approximately 4.5 nW.

Large-Scale Complementary Logic Circuit Enabled by Al2O3 Passivation-Induced Carrier Polarity Modulation in Tungsten Diselenide

Achieving effective polarity control of n- and p-type transistors based on two-dimensional (2D) materials is a critical challenge in the process of integrating transition metal dichalcogenides (TMDC) into complementary metal-oxide semiconductor (CMOS) logic circuits. Herein, we utilized a proficient and nondestructive method of electron-charge transfer to achieve a complete carrier polarity conversion from p-to n-type by depositing a thin layer of aluminum oxide (Al2O3) onto tungsten diselenide (WSe2). By utilizing the Al2O3 passivation layer, we observed precisely tuned n-type behavior in contrast to transistors fabricated on the as-grown WSe2 film without any passivation layer, which display prominent p-type behavior. The polarity-transformed n-type WSe2 transistor from the pristine p-type shows the maximum ON current of ∼0.1 μA accompanied by a high electron mobility of 7 cm2 V-1 s-1 at a drain voltage (VDS) of 1 V. We successfully showcased a homogeneous CMOS inverter utilizing 2D-TMDC which exhibits an impressive voltage gain of 7 at VDD = 5 V. Moreover, this effective polarity control approach was further expanded upon to successfully demonstrate a range of logic circuits such as AND, OR, NAND, NOR logic gates, and SRAM. The proposed methodology possesses significant promise for facilitating the advancement of high-density circuitry components utilizing 2D-TMDC.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

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

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

Modulation Doping of Single-Layer Semiconductors for Improved Contact at Metal Interfaces

Single layers of two-dimensional (2D) materials hold the promise for further miniaturization of semiconductor electronic devices. However, the metal-semiconductor contact resistance limits device performance. To mitigate this problem, we propose modulation doping, specifically a doping layer placed on the opposite side of a metal-semiconductor interface. Using first-principles calculations to obtain the band alignment, we show that the Schottky barrier height and, consequently, the contact resistance at the metal-semiconductor interface can be reduced by modulation doping. We demonstrate the feasibility of this approach for a single-layer tungsten diselenide (WSe₂) channel and 2D MXene modulation doping layers, interfaced with several different metal contacts. Our results indicate that the Fermi level of the metal can be shifted across the entire band gap. This approach can be straight-forwardly generalized for other 2D semiconductors and a wide variety of doping layers.

The Trend of 2D Transistors toward Integrated Circuits: Scaling Down and New Mechanisms

2D transition metal chalcogenide (TMDC) materials, such as MoS₂ , have recently attracted considerable research interest in the context of their use in ultrascaled devices owing to their excellent electronic properties. Microprocessors and neural network circuits based on MoS₂ have been developed at a large scale but still do not have an advantage over silicon in terms of their integrated density. In this study, the current structures, contact engineering, and doping methods for 2D TMDC materials for the scaling-down process and performance optimization are reviewed. Devices are introduced according to a new mechanism to provide the comprehensive prospects for the use of MoS₂ beyond the traditional complementary-metal-oxide semiconductor in order to summarize obstacles to the goal of developing high-density and low-power integrated circuits (ICs). Finally, prospects for the use of MoS₂ in large-scale ICs from the perspectives of the material, system performance, and application to nonlogic functionalities such as sensor circuits and analogous circuits, are briefly analyzed. The latter issue is along the direction of "more than Moore" research.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Two-Dimensional Crystals as a Buffer Layer for High Work Function Applications: The Case of Monolayer MoO3

We propose that the crystallinity of two-dimensional (2D) materials is a crucial factor for achieving highly effective work function (WF) modification. A crystalline 2D MoO₃ monolayer enhances substrate WF up to 6.4 eV for thicknesses as low as 0.7 nm. Such a high WF makes 2D MoO₃ a great candidate for tuning properties of anode materials and for the future design of organic electronic devices, where accurate evaluation of the WF is crucial. We provide a detailed investigation of WF of 2D α-MoO₃ directly grown on highly ordered pyrolytic graphite, by means of Kelvin probe force microscopy (KPFM) and ultraviolet photoemission spectroscopy (UPS). This study underlines the importance of a controlled environment and the resulting crystallinity to achieve high WF in MoO₃. UPS is proved to be suitable for determining higher WF attributed to 2D islands on a substrate with lower WF, yet only in particular cases of sufficient coverage. KPFM remains a method of choice for nanoscale investigations, especially when conducted under ultrahigh vacuum conditions. Our experimental results are supported by density functional theory calculations of electrostatic potential, which indicate that oxygen vacancies result in anisotropy of WF at the sides of the MoO₃ monolayer. These novel insights into the electronic properties of 2D-MoO₃ are promising for the design of electronic devices with high WF monolayer films, preserving the transparency and flexibility of the systems.

Selective Electron Beam Patterning of Oxygen-Doped WSe2 for Seamless Lateral Junction Transistors

Surface charge transfer doping (SCTD) using oxygen plasma to form a p-type dopant oxide layer on transition metal dichalcogenide (TMDs) is a promising doping technique for 2D TMDs field-effect transistors (FETs). However, patternability of SCTD is a key challenge to effectively switch FETs. Herein, a simple method to selectively pattern degenerately p-type (p+ )-doped WSe2 FETs via electron beam (e-beam) irradiation is reported. The effect of the selective e-beam irradiation is confirmed by the gate-tunable optical responses of seamless lateral p+ -p diodes. The OFF state of the devices by inducing trapped charges via selective e-beam irradiation onto a desired channel area in p+ -doped WSe2 , which is in sharp contrast to globally p+ -doped WSe2 FETs, is realized. Selective e-beam irradiation of the PMMA-passivated p+ -WSe2 enables accurate control of the threshold voltage (Vth ) of WSe2 devices by varying the pattern size and e-beam dose, while preserving the low contact resistance. By utilizing hBN as the gate dielectric, high-performance WSe2 p-FETs with a saturation current of -280 µA µm-1 and on/off ratio of 109 are achieved. This study's technique demonstrates a facile approach to obtain high-performance TMD p-FETs by e-beam irradiation, enabling efficient switching and patternability toward various junction devices.

Evidence of phase stability, topological phonon and temperature-induced topological phase transition in rocksalt SnS and SnSe

Both SnS and SnSe have been experimentally and theoretically confirmed as topological crystalline insulators in native rocksalt structure. Here, phononic structure, thermodynamic properties and temperature dependent electron-phonon interaction (EPI) have investigated for both the materials in rocksalt phase. Previously performed theoretical studies have predicted the phase instability of SnS in this crystal structure at ambient condition. But, after a detailed study performing on the phonon calculation of SnS, we have predicted the phase stability of SnS with considering the Sn 4porbitals as valence states inab-initiocalculation. The importance of long range Coulomb forces along with the themodynamical properties are also described in detailed for both materials. The computed value of Debye temperature (ΘD) for SnS (SnSe) is ∼315.0 K (∼201.7 K). The preliminary evidence of topological phonon is found alongX-Wdirection, where the linear band touching is observed as compared to type II Weyl phononic material ZnSe (Liuet al2021Phys. Rev.B103094306). The topological phase transition is seen for these materials due to EPI, where non-linear temperature dependent bandgap is estimated. The predicted value of transition temperature for SnS (SnSe) is found to be ∼700 K, where after this temperature the non-trivial to trivial topological phase is seen. The strength of EPI shows more stronger impact on the electronic structure of SnS than SnSe material. The reason of non-linear behaviour of bandgap with rise in temperature is discussed with the help of temperature dependent linewidths and lineshifts of conduction band and valence band due to EPI. The present study reveals the phase stability of SnS along with the comparative study of thermal effect on EPI of SnS and SnSe. Further, the possibility of temperature induced topological phase transition provides one of important behaviour to apply these two materials for device making application.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Wafer-Scalable Single-Layer Amorphous Molybdenum Trioxide

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

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.

Ultrahigh-Quality Infrared Polaritonic Resonators Based on Bottom-Up-Synthesized van der Waals Nanoribbons

van der Waals nanomaterials supporting phonon polariton quasiparticles possess extraordinary light confinement capabilities, making them ideal systems for molecular sensing, thermal emission, and subwavelength imaging applications, but they require defect-free crystallinity and nanostructured form factors to fully showcase these capabilities. We introduce bottom-up-synthesized α-MoO₃ structures as nanoscale phonon polaritonic systems that feature tailorable morphologies and crystal qualities consistent with bulk single crystals. α-MoO₃ nanoribbons serve as low-loss hyperbolic Fabry-Pérot nanoresonators, and we experimentally map hyperbolic resonances over four Reststrahlen bands spanning the far- and mid-infrared spectral range, including resonance modes beyond the 10th order. The measured quality factors are the highest from phonon polaritonic van der Waals structures to date. We anticipate that bottom-up-synthesized polaritonic van der Waals nanostructures will serve as an enabling high-performance and low-loss platform for infrared optical and optoelectronic applications.

High-specific-power flexible transition metal dichalcogenide solar cells

Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact–TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoO_x capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4 W g⁻¹ for flexible TMD (WSe₂) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46 W g⁻¹, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.

Ultrathin transition metal dichalcogenides (TMDs) hold promise for next-generation lightweight photovoltaics. Here, the authors demonstrate the first flexible high power-per-weight TMD solar cells with notably improved power conversion efficiency.

Electronic and Vibrational Decoupling in Chemically Exfoliated Bilayer Thin Two-Dimensional V2O5

The synthesis of high-quality two-dimensional (2D) transition metal oxides is challenging compared to 2D transition metal dichalcogenides as a result of the exotic surface changes that can appear during formation. Herein, we report the synthesis of bilayer 2D V₂O₅ nanosheets with a thickness of ∼1 nm using the chemical exfoliation method and a comprehensive study on the vibrational and optical properties of bilayer 2D V₂O₅. We report, for the first time, a thickness-dependent blue shift of 1.33 eV in the optical bandgap, which signifies the emergence of electronic decoupling in bilayer 2D V₂O₅. In addition, a thickness-dependent vibrational decoupling of phonon modes observed via Raman spectroscopy fingerprinting was verified by computing the lattice vibrational modes using the density functional perturbation theory. We demonstrate that the manifestation of the electronic and vibrational decoupling can be used as a benchmark to confirm the successful formation of bilayer 2D V₂O₅ from its bulk counterpart.

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

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

High-Performance p-n Junction Transition Metal Dichalcogenide Photovoltaic Cells Enabled by MoOx Doping and Passivation

Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage (V_OC) and therefore limited power conversion efficiency (PCE) in TMD photovoltaics. Here, we simultaneously address these fundamental issues with a simple MoO_x (x ≈ 3) surface charge-transfer doping and passivation method, applying it to multilayer tungsten disulfide (WS₂) Schottky-junction solar cells with initially near-zero V_OC. Doping and passivation turn these into lateral p-n junction photovoltaic cells with a record V_OC of 681 mV under AM 1.5G illumination, the highest among all p-n junction TMD solar cells with a practical design. The enhanced V_OC also leads to record PCE in ultrathin (<90 nm) WS₂ photovoltaics. This easily scalable doping and passivation scheme is expected to enable further advances in TMD electronics and optoelectronics.

Identification of Point Defects in Atomically Thin Transition-Metal Dichalcogenide Semiconductors as Active Dopants

Selective doping in semiconductors is essential not only for monolithic integrated circuity fabrications but also for tailoring their properties including electronic, optical, and catalytic activities. Such active dopants are essentially point defects in the host lattice. In atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs), the roles of such point defects are particularly critical in addition to their large surface-to-volume ratio, because their bond dissociation energy is relatively weaker, compared to elemental semiconductors. In this Mini Review, we review recent advances in the identifications of diverse point defects in 2D TMDC semiconductors, as active dopants, toward the tunable doping processes, along with the doping methods and mechanisms in literature. In particular, we discuss key issues in identifying such dopants both at the atomic scales and the device scales with selective examples. Fundamental understanding of these point defects can hold promise for tunability doping of atomically thin 2D semiconductor platforms.

High Current Density in Monolayer MoS2 Doped by AlOx

Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low-temperature (<200 °C) substoichiometric AlO_x provides a stable n-doping layer for monolayer MoS₂, compatible with circuit integration. This approach achieves carrier densities >2 × 10¹³ cm^-2, sheet resistance as low as ∼7 kΩ/□, and good contact resistance ∼480 Ω·μm in transistors from monolayer MoS₂ grown by chemical vapor deposition. We also reach record current density of nearly 700 μA/μm (>110 MA/cm²) along this three-atom-thick semiconductor while preserving transistor on/off current ratio >10⁶. The maximum current is ultimately limited by self-heating (SH) and could exceed 1 mA/μm with better device heat sinking. With their 0.1 nA/μm off-current, such doped MoS₂ devices approach several low-power transistor metrics required by the international technology roadmap.

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.

Electrostatically Enhanced Electron-Phonon Interaction in Monolayer 2H-MoSe2 Grown by Molecular Beam Epitaxy

The enhancement of electron-phonon interaction provides a reasonable explanation for gate-tunable phonon properties in some semiconductors where multiple inequivalent valleys are simultaneously occupied upon charge doping, especially in few-layer transition metal dichalcogenides (TMDs). In this work, we report var der Waals epitaxy of 2H-MoSe₂ by molecular beam epitaxy (MBE) and gate-tunable phonon properties in monolayer and bilayer MoSe₂. In monolayer MoSe₂, we find that out-of-plane phonon mode A_1g exhibits a strong softening and shifting toward lower wavenumbers at a high electron doping level, while in-plane phonon mode E_2g¹ remains unchanged. The softening and shifting of the out-of-plane phonon mode could be attributed to the increase of electron-phonon interaction and the simultaneous occupation of electrons in multiple inequivalent valleys. In bilayer MoSe₂, no corresponding changes of phonon modes are detected at the same doping level, which could originate from the occupation of electrons only in single valleys upon high electron doping. This study demonstrates electrostatically enhanced electron-phonon interaction in monolayer MoSe₂ and clarifies the relevance between occupation of multiple valleys and phonon properties by comparing Raman spectra of monolayer and bilayer MoSe₂ at different doping levels.

Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2

Forming pits on molybdenum disulfide (MoS₂ ) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS₂ , and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS₂ monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS₂ monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS₂ , rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS₂ interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS₂ .

Optical-Based Thickness Measurement of MoO3 Nanosheets

Considering that two-dimensional (2D) molybdenum trioxide has acquired more attention in the last few years, it is relevant to speed up thickness identification of this material. We provide two fast and non-destructive methods to evaluate the thickness of MoO₃ flakes on SiO₂/Si substrates. First, by means of quantitative analysis of the apparent color of the flakes in optical microscopy images, one can make a first approximation of the thickness with an uncertainty of ±3 nm. The second method is based on the fit of optical contrast spectra, acquired with micro-reflectance measurements, to a Fresnel law-based model that provides an accurate measurement of the flake thickness with ±2 nm of uncertainty.

Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study

Due to their atomic thicknesses and semiconducting properties, two-dimensional transition metal dichalcogenides (TMDCs) are gaining increasing research interest. Among them, Hf- and Zr-based TMDCs demonstrate the unique advantage that their oxides (HfO₂ and ZrO₂) are excellent dielectric materials. One possible method to precisely tune the material properties of two-dimensional atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. In the present work, the molecular adsorption of NO₂ and NH₃ on the two-dimensional trigonal prismatic (1H) and octahedral (1T) phases of Hf and Zr dichalcogenides (S, Se, Te) is studied using dispersion-corrected periodic density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are investigated. In addition, the effects of the molecular dopants (NH₃ and NO₂) on the electronic structure of the materials are studied. It was observed that the adsorbed NH₃ donates electrons to the conduction band of the Hf (Zr) dichalcogenides, while NO₂ receives electrons from the valance band. Furthermore, the NO₂ dopant affects than NH₃ significantly. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This study explores, not only the properties of the two-dimensional 1H and 1T phases of Hf and Zr dichalcogenides (S, Se, Te), but also tunes their electronic properties by adsorbing non-bonded dopants.

Rapid synthesis of vertically aligned α-MoO3 nanostructures on substrates

We report a new procedure for large scale, reproducible and fast synthesis of polycrystalline, dense, vertically aligned α-MoO3 nanostructures on conducting (FTO) and non-conducting substrates (Si/SiO2) by using a simple, low-cost hydrothermal technique. The synthesis method consists of two steps, firstly formation of a thermally evaporated Cr/MoO3 seed layer, and secondly growth of the nanostructures in a highly acidic precursor solution. In this report, we document a growth process of vertically aligned α-MoO3 nanostructures with varying growth parameters, such as pH and precursor concentration influencing the resulting structure. Vertically aligned MoO3 nanostructures are valuable for different applications such as electrode material for organic and dye-sensitized solar cells, as a photocatalyst, and in Li-ion batteries, display devices and memory devices due to their high surface area.

Synthesis of large-area uniform Si2Te3 thin films for p-type electronic devices

Two-dimensional (2D) p-n junctions are basic components of various functional devices. However, the shortage of natural p-type 2D semiconductors makes it difficult to achieve both p-type and n-type transport in high-performance multifunctional devices. Here, continuous and uniform p-type Si2Te3 thin films are grown on SiO2/Si substrates, which are simultaneously used as an in situ Si source. Large-size 2D films with dimensions of ∼8 × 2 cm2 are prepared for the first time using a reliable and simple chemical vapor deposition (CVD) technique. Film growth occurs via the vapor-liquid-solid mechanism, allowing the film thickness to be controlled by the substrate temperature. As the Si2Te3 film thickness increases from 3 to 8 nm, the bandgap decreases from 2.07 to 1.65 eV. Moreover, the directly grown thin films possess high crystallinity, showing electronic properties that are comparable to those of MoTe2 crystals and MoS2 films. Therefore, this large-area growth of p-type Si2Te3 enriches the 2D semiconductor library and opens up a new platform for the study of p-type Si2Te3, which has potential for application in p-n junctions.

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.

Enhanced photoresponse of epitaxially grown ZnO by MoO3 surface functionalization

Enhanced photoresponse of epitaxially grown ZnO has been observed with MoO₃ surface functionalization, which is attributed to the larger upward band bending at the interface.

Thickness-Insensitive Properties of α-MoO3 Nanosheets by Weak Interlayer Coupling

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO₃ nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO₃ is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO₃ in MoO₃ showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO₃ of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO₃ nanosheets for nanodevice applications.

Contact Engineering High-Performance n-Type MoTe2 Transistors

Semiconducting MoTe₂ is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe₂ transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO_x encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe₂ and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.

2DMatPedia, an open computational database of two-dimensional materials from top-down and bottom-up approaches

Two-dimensional (2D) materials have been a hot research topic in the last decade, due to novel fundamental physics in the reduced dimension and appealing applications. Systematic discovery of functional 2D materials has been the focus of many studies. Here, we present a large dataset of 2D materials, with more than 6,000 monolayer structures, obtained from both top-down and bottom-up discovery procedures. First, we screened all bulk materials in the database of Materials Project for layered structures by a topology-based algorithm and theoretically exfoliated them into monolayers. Then, we generated new 2D materials by chemical substitution of elements in known 2D materials by others from the same group in the periodic table. The structural, electronic and energetic properties of these 2D materials are consistently calculated, to provide a starting point for further material screening, data mining, data analysis and artificial intelligence applications. We present the details of computational methodology, data record and technical validation of our publicly available data ( http://www.2dmatpedia.org/ ).

Degenerately Doped Transition Metal Dichalcogenides as Ohmic Homojunction Contacts to Transition Metal Dichalcogenide Semiconductors

In search of an improved strategy to form low-resistance contacts to MoS₂ and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO₃/MoS₂ and MoO₂/MoS₂ bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS₂ very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS₂, resulting in a highly transparent contact region.

Enhancing Electrocatalytic Water Splitting by Strain Engineering

Electrochemical water splitting driven by sustainable energy such as solar, wind, and tide is attracting ever-increasing attention for sustainable production of clean hydrogen fuel from water. Leveraging these advances requires efficient and earth-abundant electrocatalysts to accelerate the kinetically sluggish hydrogen and oxygen evolution reactions (HER and OER). A large number of advanced water-splitting electrocatalysts have been developed through recent understanding of the electrochemical nature and engineering approaches. Specifically, strain engineering offers a novel route to promote the electrocatalytic HER/OER performances for efficient water splitting. Herein, the recent theoretical and experimental progress on applying strain to enhance heterogeneous electrocatalysts for both HER and OER are reviewed and future opportunities are discussed. A brief introduction of the fundamentals of water-splitting reactions, and the rationalization for utilizing mechanical strain to tune an electrocatalyst is given, followed by a discussion of the recent advances on strain-promoted HER and OER, with special emphasis given to combined theoretical and experimental approaches for determining the optimal straining effect for water electrolysis, along with experimental approaches for creating and characterizing strain in nanocatalysts, particularly emerging 2D nanomaterials. Finally, a vision for a future sustainable hydrogen fuel community based on strain-promoted water electrolysis is proposed.

Polymorphic expressions of ultrathin oxidic layers of Mo on Au(111)

Ultrathin oxidic layers of Mo (i.e. O/Mo) on the Au(111) substrate are investigated using first-principles density-functional theory calculations. Various polymorphic structural models of these O/Mo layers are proposed and compared with previous experimental results - covering both spectroscopic and microscopic approaches of characterization. We find that, through the control of metal-oxygen coordination in these ultrathin oxidic O/Mo films on Au(111), the oxidation state of Mo atoms in the O/Mo layers can be modulated and reduced without intentional creation of oxygen vacancies. This is also assisted by a charge transfer mechanism from the Au substrate to these oxidic films, providing a direct means to tune the surface electronic properties of ultrathinoxide films on metal substrates.

Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe2 Transistors

We investigate the valley Hall effect (VHE) in monolayer WSe₂ field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n -doped MoS₂, we observe the VHE in WSe₂ in both the n - and p -doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p -doped regime. Assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.

Electrophoresis Assembly of Novel Superhydrophobic Molybdenum Trioxide (MoO₃) Films with Great Stability

This work presents a hydrothermal synthesis approach to produce novel schistose molybdenum trioxide (MoO₃) powders with wide application, and introduces a facile electrophoresis assembly technique to construct the superhydrophobic MoO₃ films (SMFs) with contact angle up to 169 ± 1° at normal pressure and temperature. The microstructures and chemical compositions of product were analyzed by field emission scanning electron microcopy (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD). The wettability and stability studies indicate that the SMFs all show great resistance in various environments with adjusting factors, including droplets with different surface tension, pH, relative humidity, etc., and the stability can be maintained at least for five months. Notably, this paper will provides a valuable reference for designing novel oxide powders and their high-efficient hydrophobic film formation with self-cleaning or water proof properties.

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.

Monitoring the electronic, thermal and optical properties of two-dimensional MoO2 under strain via vibrational spectroscopies: a first-principles investigation

This study presents the electronic, mechanical, thermal, vibrational and optical properties of the MoO₂ monolayer under the effect of biaxial and uniaxial compressive/tensile strain, using first-principles calculations based on density functional theory.

HfSe2 and ZrSe2: Two-dimensional semiconductors with native high-κ oxides

The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO₂ as a high-quality "native" insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe₂ and ZrSe₂ have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable "high-κ" native dielectrics HfO₂ and ZrO₂, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 10⁶; on current, ~30 μA/μm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature.