Wafer-Scalable Single-Layer Amorphous Molybdenum Trioxide

Analysis of p-Type Doping in Graphene Induced by Monolayer-Oxidized TMDs

Doping is one of the most difficult technological challenges for realizing reliable two-dimensional (2D) material-based semiconductor devices, arising from their ultrathinness. Here, we systematically investigate the impact of different types of nonstoichiometric solid MOx (M are W or Mo) dopants obtained by oxidizing transition metal dichalcogenides (TMDs: WSe2 or MoS2) formed on graphene FETs, which results in p-type doping along with disorders. From the results obtained in this study, we were able to suggest an analytical technique to optimize the optimal UV-ozone (UVO) treatment to achieve high p-type doping concentration in graphene FETs (∼2.5 × 1013 cm-2 in this study) without generating defects, mainly by analyzing the time dependency of D and D' peaks measured by Raman spectroscopy. Furthermore, an analysis of the structure of graphene sheets using TEM indicates that WOx plays a better protective role in graphene, compared to MoOx, suggesting that WOx is more effective for preventing the degradation of graphene during UVO treatment. To enhance the practical application aspect of our work, we have fabricated a graphene photodetector by selectively doping the graphene through oxidized TMDs, creating a p-n junction, which resulted in improved photoresponsivity compared to the intrinsic graphene device. Our results offer a practical guideline for the utilization of surface charge transfer doping of graphene toward CMOS applications.

Spike rate dependent synaptic characteristics in lamellar, multilayered alpha-MoO3 based two-terminal devices - efficient way to control the synaptic amplification

Brain-inspired computing systems require a rich variety of neuromorphic devices using multi-functional materials operating at room temperature. Artificial synapses which can be operated using optical and electrical stimuli are in high demand. In this regard, layered materials have attracted a lot of attention due to their tunable energy gap and exotic properties. In the current study, we report the growth of layered MoO3 using the chemical vapor deposition (CVD) technique. MoO3 has an energy gap of 3.22 eV and grows with a large aspect ratio, as seen through optical and scanning electron microscopy. We used transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy for complete characterisation. The two-terminal devices using platinum (Pt/MoO3/Pt) exhibit superior memory with the high-resistance state (HRS) and low-resistance state (LRS) differing by a large resistance (∼MΩ). The devices also show excellent synaptic characteristics. Both optical and electrical pulses can be utilised to stimulate the synapse. Consistent learning (potentiation) and forgetting (depression) curves are measured. Transition from long term depression to long term potentiation can be achieved using the spike frequency dependent pulsing scheme. We have found that the amplification of postsynaptic current can be tuned using such frequency dependent spikes. This will help us to design neuromorphic devices with the required synaptic amplification.

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.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Comparative Study between Sulfurized MoS2 from Molybdenum and Molybdenum Trioxide Precursors for Thin-Film Device Applications

Two-dimensional (2D) materials have been studied as an emerging class of nanomaterials owing to their attractive properties in nearly every field of science and technology. Molybdenum disulfide (MoS₂) is one of the more promising candidates of these atomically thin 2D materials for its technological potential. The facile synthesis of MoS₂ remains a matter of broad interest. In this study, MoS₂ was synthesized by chemical vapor deposition sulfurization at various temperatures (550 °C, 650 °C, and 750 °C) of either precursor molybdenum metal (Mo) or molybdenum trioxide (MoO₃) deposited on silicon/silicon dioxide (Si/SiO₂) via e-beam evaporation. Monolayer, bilayer, and few layers sulfurized samples have been grown and verified by Raman, photoluminescence spectroscopy, XRD, XPS, and AFM. MoO₃ sulfurization provided monolayer growth in comparison to Mo sulfurization under the same conditions and precursor thicknesses. Optical microscopy showed the homogeneous nature of grown samples. A main finding of this work is that MoO₃ sulfurization produced higher quality MoS₂ as compared to those grown by an Mo precursor. Device characteristics based on monolayer MoO₃ sulfurized MoS_2-x include nonvolatile resistive switching with I_on/I_off ≈ 10⁴ at a relatively low operating bias of ±1 V. In addition, field-effect transistor characteristics revealed p-type material growth with a carrier mobility ∼ 41 cm² V^-1 s^-1, which is in contrast to typically observed n-type characteristics.

Graphene and Beyond: Recent Advances in Two-Dimensional Materials Synthesis, Properties, and Devices

Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field.

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.