Tuning the threshold voltage of MoS2 field-effect transistors via surface treatment

Implementation of excellent spin-filtering effect in half-metallic electrode-based single-molecule optoelectronic devices

The application of half-metallic materials in single-molecule optoelectronic devices opens a promising way in advancing device performance and functionality, thus addressing a research question of significance. Here we propose a series of single-molecule devices with half-metallic FeN4-doped armchair graphene nanoribbon as electrodes and metalloporphyrin (MPr) molecules as photoresponsive materials for photon harvesting, which are driven by photogalvanic effects (PGEs). Through the quantum transport simulations, we systematically investigated the spin-polarized photocurrents under the linearly polarized light illumination in these devices. Since the exclusive opening only exists in the spin-up channel of the half-metallic nanoribbons, these devices can generate a large photocurrent in the spin-up direction whereas suppressing the spin-down photocurrent. Consequently, they exhibit an effective spin-filtering effect at numerous photon energies. Our study unveils the excellent spin-filtering effect achieved in single-molecule optoelectronic devices with half-metallic electrodes, showing instructive significance for the future design of new optoelectronic devices.

Precursor-Confined Chemical Vapor Deposition of 2D Single-Crystalline SexTe1-x Nanosheets for p-Type Transistors and Inverters

Two-dimensional (2D) tellurium (Te) is emerging as a promising p-type candidate for constructing complementary metal-oxide-semiconductor (CMOS) architectures. However, its small bandgap leads to a high leakage current and a low on/off current ratio. Although alloying Te with selenium (Se) can tune its bandgap, thermally evaporated SexTe1-x thin films often suffer from grain boundaries and high-density defects. Herein, we introduce a precursor-confined chemical vapor deposition (CVD) method for synthesizing single-crystalline SexTe1-x alloy nanosheets. These nanosheets, with tunable compositions, are ideal for high-performance field-effect transistors (FETs) and 2D inverters. The preformation of Se-Te frameworks in our developed CVD method plays a critical role in the growth of SexTe1-x nanosheets with high crystallinity. Optimizing the Se composition resulted in a Se0.30Te0.70 nanosheet-based p-type FET with a large on/off current ratio of 4 × 105 and a room-temperature hole mobility of 120 cm2·V-1·s-1, being eight times higher than thermally evaporated SexTe1-x with similar composition and thickness. Moreover, we successfully fabricated an inverter based on p-type Se0.30Te0.70 and n-type MoS2 nanosheets, demonstrating a typical voltage transfer curve with a gain of 30 at an operation voltage of Vdd = 3 V.

Environmentally sustainable implementations of two-dimensional nanomaterials

Rapid advancement in nanotechnology has led to the development of a myriad of useful nanomaterials that have novel characteristics resulting from their small size and engineered properties. In particular, two-dimensional (2D) materials have become a major focus in material science and chemistry research worldwide with substantial efforts centered on their synthesis, property characterization, and technological, and environmental applications. Environmental applications of these nanomaterials include but are not limited to adsorbents for wastewater and drinking water treatment, membranes for desalination, and coating materials for filtration. However, it is also important to address the environmental interactions and implications of these nanomaterials in order to develop strategies that minimize their environmental and public health risks. Towards this end, this review covers the most recent literature on the environmental implementations of emerging 2D nanomaterials, thereby providing insights into the future of this fast-evolving field including strategies for ensuring sustainable development of 2D nanomaterials.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS₂ and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS₂. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS₂.

Improved detectivity and response speed of MoS2 phototransistors based on the negative-capacitance effect and defect engineering

Due to the unique crystal structure, outstanding optoelectronic properties and a tunable band gap from 1.2-1.8 eV, two-dimensional molybdenum disulfide (MoS₂) has attracted extensive attention as a promising candidate for future photodetectors. In this work, a negative-capacitance (NC) MoS₂ phototransistor is fabricated by using H f _0.5 Z r _0.5 O ₂ (HZO) as ferroelectric layer and Al₂O₃ as matching layer, and a low subthreshold swing (SS) of 39 mV/dec and an ultrahigh detectivity of 3.736×10¹⁴ cmHz^1/2W^-1 are achieved at room temperature due to the NC effect of the ferroelectric HZO. Moreover, after sulfur (S) treatment on MoS₂, the transistor obtained a lower SS of 33 mV/dec, a detectivity of 1.329×10¹⁴ cmHz^1/2W^-1 and specially a faster response time of 3-4 ms at room temperature, attributed to the modulation of photogating effect induced by S-vacancy passivation in MoS₂ by the S treatment. Therefore, the combination of the defect engineering on MoS₂ and the NC effect from ferroelectric thin film could provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.

Tuning Threshold Voltage of Electrolyte-Gated Transistors by Binary Ion Doping

Electrolyte-gated transistors (EGTs) operating at low voltages have attracted significant attention in widespread applications, including neuromorphic devices, nonvolatile memories, chemical/biosensors, and printed electronics. To increase the practicality of the EGTs in electronic circuits, systematic control of threshold voltage (Vth), which determines the power consumption and noise margin of the circuits, is essential. In this study, we present a simple strategy for systematically tuning Vth to almost half of the operating potential range of the EGT by controlling the electrochemical doping of electrolyte ions into organic p-type semiconductors. The type of anion in the ionogel determines Vth as well as other transistor characteristics, such as the subthreshold swing and mobility, because the positive hole carriers are the majority carriers. More importantly, Vth can be finely controlled by binary anion doping using ionogels with two anions with varying molar fractions at a fixed cation. In addition, the binary anion doping successfully controls the inversion characteristics of ion-gated inverters. As unlimited combinations of ion pairs are possible for ionogels, this study opens a route for controlling the device characteristics to expand the practicality and applicability of ionogel-based EGTs for next-generation ionic/electronic devices.

Vacancy Defects in 2D Transition Metal Dichalcogenide Electrocatalysts: From Aggregated to Atomic Configuration

Vacancy defect engineering has been well leveraged to flexibly shape comprehensive physicochemical properties of diverse catalysts. In particular, growing research effort has been devoted to engineering chalcogen anionic vacancies (S/Se/Te) of 2D transition metal dichalcogenides (2D TMDs) toward the ultimate performance limit of electrocatalytic hydrogen evolution reaction (HER). In spite of remarkable progress achieved in the past decade, systematic and in-depth insights into the state-of-the-art vacancy engineering for 2D-TMDs-based electrocatalysis are still lacking. Herein, this review delivers a full picture of vacancy engineering evolving from aggregated to atomic configurations covering their development background, controllable manufacturing, thorough characterization, and representative HER application. Of particular interest, the deep-seated correlations between specific vacancy regulation routes and resulting catalytic performance improvement are logically clarified in terms of atomic rearrangement, charge redistribution, energy band variation, intermediate adsorption-desorption optimization, and charge/mass transfer facilitation. Beyond that, a broader vision is cast into the cutting-edge research fields of vacancy-engineering-based single-atom catalysis and dynamic structure-performance correlations across catalyst service lifetime. Together with critical discussion on residual challenges and future prospects, this review sheds new light on the rational design of advanced defect catalysts and navigates their broader application in high-efficiency energy conversion and storage fields.

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

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

On the Contact Optimization of ALD-Based MoS2 FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance

Despite the extensive ongoing research on MoS₂ field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS₂ interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS₂ contacts are based on exfoliated MoS₂, whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS₂ films and Ti/Au contacts are demonstrated, using current-voltage (I-V) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O₂/Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS₂ surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O₂/Ar plasma changes the MoS₂ chemical state and enhances the MoS₂ electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiS _x layer at the Ti-to-MoS₂ interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS₂ films.

Electrically Tunable Room Temperature Hysteresis Crossover in Underlap MoS2 Field-Effect Transistors

Clockwise to anticlockwise hysteresis crossover in current-voltage transfer characteristics of field-effect transistors (FETs) with graphene and MoS₂ channels holds significant promise for nonvolatile memory applications. However, such crossovers have been shown to manifest only at high temperature. In this work, for the first time, we demonstrate room temperature hysteresis crossover in few-layer MoS₂ FETs using a gate-drain underlap design to induce a differential response from traps near the MoS₂-HfO₂ channel-gate dielectric interface, also referred to as border traps, to applied gate bias. The appearance of trap-driven anticlockwise hysteresis at high gate voltages in underlap FETs can be unambiguously attributed to the presence of an underlap since transistors with and without the underlap region were fabricated on the same MoS₂ channel flake. The underlap design also enables room temperature tuning of the anticlockwise hysteresis window (by 140×) as well as the crossover gate voltage (by 2.6×) with applied drain bias and underlap length. Comprehensive measurements of the transfer curves in ambient and vacuum conditions at varying sweep rates and temperatures (RT, 45 °C, and 65 °C) help segregate the quantitative contributions of adsorbates, interface traps, and bulk HfO₂ traps to the clockwise and anticlockwise hysteresis.

Theoretical analysis of thermal boundary conductance of MoS2-SiO2 and WS2-SiO2 interface

Understanding the physical processes involved in interfacial heat transfer is critical for the interpretation of thermometric measurements and the optimization of heat dissipation in nanoelectronic devices that are based on transition metal dichalcogenide (TMD) semiconductors. We model the phononic and electronic contributions to the thermal boundary conductance (TBC) variability for the MoS2-SiO2 and WS2-SiO2 interface. A phenomenological theory to model diffuse phonon transport at disordered interfaces is introduced and yields G = 13.5 and 12.4 MW K-1 m-2 at 300 K for the MoS2-SiO2 and WS2-SiO2 interface, respectively. We compare its predictions to those of the coherent phonon model and find that the former fits the MoS2-SiO2 data from experiments and simulations significantly better. Our analysis suggests that heat dissipation at the TMD-SiO2 interface is dominated by phonons scattered diffusely by the rough interface although the electronic TBC contribution can be significant even at low electron densities (n ≤ 1012 cm-2) and may explain some of the variation in the experimental TBC data from the literature. The physical insights from our study can be useful for the development of thermally aware designs in TMD-based nanoelectronics.

Hysteresis-reversible MoS2 transistor

Sulfur vacancy dominant hysteresis in MoS₂ transistors is observed. By decorating with Pt, the hysteresis behavior could switch from sulfur vacancy dominant to interfacial dominant, thereby realizing a hysteresis-reversible MoS₂ transistor.

Impact of thermal annealing in forming gas on the optical and electrical properties of MoS2monolayer

Technological applications involving 2D MoS2require transfer of chemical vapor deposition (CVD) grown material from its original substrate and subsequent lithographic processes. Inevitably, those steps contaminate the surface of the 2D material with polymeric residues affecting the electronic and optical properties of the MoS2. Annealing in forming gas is considered an efficient treatment to partially remove such residues. However, hydrogen also interacts with MoS2creating or saturating sulfur vacancies. Sulfur vacancies are known to be at the origin of n-doping evident in the majority of as-grown MoS2samples. In this context, investigating the impact of thermal annealing in forming gas on the electronic and optical properties of MoS2monolayer is technologically important. In order to address this topic, we have systematically studied the evolution of CVD grown MoS2monolayer using Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy and transport measurements through a series of thermal annealing in forming gas at temperatures up to 500 °C. Efficient removal of the polymeric residues is demonstrated at temperatures as low as 200 °C. Above this value, carrier density modulation is identified by photoluminescence, x-ray photoelectron spectroscopy and electrical characterization and is correlated to the creation of sulfur vacancies. Finally, the degradation of the MoS2single layer is verified with annealing at or above 350 °C through Raman and photocurrent measurements.

Atomic Layer MoS2xTe2(1-x) Ternary Alloys: Two-Dimensional van der Waals Growth, Band gap Engineering, and Electrical Transport

Ternary alloys in two-dimensional (2D) transition-metal dichalcogenides allow band gap tuning and phase engineering and change the electrical transport type. A process of 2D van der Waals epitaxial growth of molybdenum sulfide telluride alloys (MoS_2xTe_2(1-x), 0 ≤ x ≤ 1) is presented for synthesizing few-atomic-layer films on SiO₂ substrates using metal-organic chemical vapor deposition. Raman spectra, X-ray photoelectron spectra, photoluminescence (PL), and electrical transport properties of few-atomic-layer MoS_2xTe_2(1-x) (0 ≤ x ≤ 1) films are systematically investigated. The strong PL peaks at 80 K from MoS_2xTe_2(1-x) (0.45 ≤ x ≤ 0.93) reveal a composition-controllable optical band gap (E_g = 1.55-1.91 eV at 80 K). Electrical transport properties of MoS_2xTe_2(1-x) alloys, where 0 ≤ x ≤ 0.8 and 0.93 ≤ x ≤ 1, exhibit p-type and n-type semiconducting behaviors, respectively. Remarkably, an increase in the Te composition of a few-atomic-layer MoS_2xTe_2(1-x) (0 ≤ x ≤ 1) film left-shifts the threshold voltage of a MoS_2xTe_2(1-x) (0 ≤ x ≤ 1) field-effect transistor. The narrower band gap energies of MoS_2xTe_2(1-x) films with higher Te content cause a decrease in the on/off current ratios.

Artificial Intelligence Algorithm Enabled Industrial-Scale Graphene Characterization

No characterization method is available to quickly perform quality inspection of 2D materials produced on an industrial scale. This hinders the adoption of 2D materials for product manufacturing in many industries. Here, we report an artificial-intelligence-assisted Raman analysis to quickly probe the quality of centimeter-large graphene samples in a non-destructive manner. Chemical vapor deposition of graphene is devised in this work such that two types of samples were obtained: layer-plus-islands and layer-by-layer graphene films, at centimeter scales. Using these samples, we implemented and integrated an unsupervised learning algorithm with an automated Raman spectroscopy to precisely cluster 20,250 and 18,000 Raman spectra collected from layer-plus-islands and layer-by-layer graphene films, respectively, into five and two clusters. Each cluster represents graphene patches with different layer numbers and stacking orders. For instance, the two clusters detected in layer-by-layer graphene films represent monolayer and bilayer graphene based on their Raman fingerprints. Our intelligent Raman analysis is fully automated, with no human operation involved, is highly reliable (99.95% accuracy), and can be generalized to other 2D materials, paving the way towards industrialization of 2D materials for various applications in the future.

Optimizing Al-doped ZrO2 as the gate dielectric for MoS2 field-effect transistors

In this work, we investigate the effects on the electrical properties of few-layered MoS₂ field-effect transistors (FETs) following Al incorporation into ZrO₂ as the gate dielectrics of the devices. A large improvement in device performance is achieved with the Al-doped ZrO₂ gate dielectric when Zr:Al = 1:1. The relevant MoS₂ transistor exhibits the best electrical characteristics: high carrier mobility of 40.6 cm² V^-1 s^-1 (41% higher than that of the control sample, and an intrinsic mobility of 68.0 cm² V^-1 s^-1), a small subthreshold swing of 143 mV dec^-1, high on/off current ratio of 6 × 10⁶ and small threshold voltage of 0.71 V. These are attributed to the facts that (i) Al incorporation into ZrO₂ can decrease its oxygen vacancies; densify the dielectric film; and smooth the gate dielectric surface, thus reducing the traps at/near the Zr_0.5Al_0.5O _y /MoS₂ interface and the gate leakage current; (ii) adjusting the dielectric constant of the gate dielectric to an appropriate value, which achieves a reasonable trade-off between the gate screening effect on the Coulomb-impurity scattering and the surface optical phonon scattering. These results demonstrate that optimized Zr_0.5Al_0.5O_y is a potential gate dielectric material for MoS₂ FET applications.

A Two-Dimensional MoS2 Catalysis Transistor by Solid-State Ion Gating Manipulation and Adjustment (SIGMA)

A variety of methods including tuning chemical compositions, structures, crystallinity, defects and strain, and electrochemical intercalation have been demonstrated to enhance the catalytic activity. However, none of these tuning methods provide direct dynamical control during catalytic reactions. Here we propose a new method to tune the activity of catalysts through solid-state ion gating manipulation and adjustment (SIGMA) using a catalysis transistor. SIGMA can electrostatically dope the surface of catalysts with a high electron concentration over 5 × 10¹³ cm^-2 and thus modulate both the chemical potential of the reaction intermediates and their electrical conductivity. The hydrogen evolution reaction (HER) on both pristine and defective MoS₂ were investigated as model reactions. Our theoretical and experimental results show that the overpotential at 10 mA/cm² and Tafel slope can be in situ, continuously, dynamically, and reversibly tuned over 100 mV and around 100 mV/dec, respectively.

Selective Engineering of Chalcogen Defects in MoS2 by Low-Energy Helium Plasma

Structural defects in two-dimensional transition-metal dichalcogenides can significantly modify the material properties. Previous studies have shown that chalcogen defects can be created by physical sputtering, but the energetic ions can potentially displace transition-metal atoms at the same time, leading to ambiguous results and in some cases, degradation of material quality. In this work, selective sputtering of S atoms in monolayer MoS₂ without damaging the Mo sublattice is demonstrated with low-energy helium plasma treatment. Based on X-ray photoelectron spectroscopy analysis, wide-range tuning of S defect concentration is achieved by controlling the ion energy and sputtering time. Furthermore, characterization with scanning transmission electron microscopy confirms that by keeping the ion energy low, the Mo sublattice remains intact. The properties of MoS₂ at different defect concentrations are also characterized. In situ device measurement shows that the flake can be tuned from a semiconducting to metallic-like behavior by introducing S defects due to the creation of mid-gap states. When the defective MoS₂ is exposed to air, the S defects are soon passivated, with oxygen atoms filling the defect sites.

Functionalized MoS2-erlotinib produces hyperthermia under NIR

BACKGROUND

Molybdenum disulfide (MoS2) has been widely explored for biomedical applications due to its brilliant photothermal conversion ability. In this paper, we report a novel multifunctional MoS2-based drug delivery system (MoS2-SS-HA). By decorating MoS2 nanosheets with hyaluronic acid (HA), these functionalized MoS2 nanosheets have been developed as a tumor-targeting chemotherapeutic nanocarrier for near-infrared (NIR) photothermal-triggered drug delivery, facilitating the combination of chemotherapy and photothermal therapy into one system for cancer therapy.

RESULTS

The nanocomposites (MoS2-SS-HA) generated a uniform diameter (ca. 125 nm), exhibited great biocompatibility as well as high stability in physiological solutions, and could be loaded with the insoluble anti-cancer drug erlotinib (Er). The release of Er was greatly accelerated under near infrared laser (NIR) irradiation, showing that the composites can be used as responsive systems, with Er release controllable through NIR irradiation. MTT assays and confocal imaging results showed that the MoS2-based nanoplatform could selectively target and kill CD44-positive lung cancer cells, especially drug resistant cells (A549 and H1975). In vivo tumor ablation studies prove a better synergistic therapeutic effect of the joint treatment, compared with either chemotherapy or photothermal therapy alone.

CONCLUSION

The functionalized MoS2 nanoplatform developed in this work could be a potent system for targeted drug delivery and synergistic chemo-photothermal cancer therapy.

Threshold Voltage Control of Multilayered MoS2 Field-Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement-Mode Logic Gates

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂ ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^-2 at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.

The organic-2D transition metal dichalcogenide heterointerface

Since the first isolation of graphene, new classes of two-dimensional (2D) materials have offered fascinating platforms for fundamental science and technology explorations at the nanometer scale. In particular, 2D transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been intensely investigated due to their unique electronic and optical properties, including tunable optical bandgaps, direct-indirect bandgap crossover, strong spin-orbit coupling, etc., for next-generation flexible nanoelectronics and nanophotonics applications. On the other hand, organics have always been excellent materials for flexible electronics. A plethora of organic molecules, including donors, acceptors, and photosensitive molecules, can be synthesized using low cost and scalable procedures. Marrying the fields of organics and 2D TMDs will bring benefits that are not present in either material alone, enabling even better, multifunctional flexible devices. Central to the realization of such devices is a fundamental understanding of the organic-2D TMD interface. Here, we review the organic-2D TMD interface from both chemical and physical perspectives. We discuss the current understanding of the interfacial interactions between the organic layers and the TMDs, as well as the energy level alignment at the interface, focusing in particular on surface charge transfer and electronic screening effects. Applications from the literature are discussed, especially in optoelectronics and p-n hetero- and homo-junctions. We conclude with an outlook on future scientific and device developments based on organic-2D TMD heterointerfaces.

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

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS₂) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS₂ can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO _x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO _x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

Split-gated point-contact for electrostatic confinement of transport in MoS2/h-BN hybrid structures

Electrostatically defined nanoscale devices on two-dimensional semiconductor heterostructures are the building blocks of various quantum electrical circuits. Owing to its atomically flat interfaces and the inherent two-dimensional nature, van der Waals heterostructures hold the advantage of large-scale uniformity, flexibility and portability over the conventional bulk semiconductor heterostructures. In this letter we show the operation of a split-gate defined point contact device on a MoS₂/h-BN heterostructure, the first step towards realizing electrostatically gated quantum circuits on van der Waals semiconductors. By controlling the voltage on the split-gate we are able to control and confine the electron flow in the device leading to the formation of the point contact. The formation of the point contact in our device is elucidated by the three characteristic regimes observed in the pinch-off curve; transport similar to the conventional FET, electrostatically confined transport and the tunneling dominated transport. We explore the role of the carrier concentration and the drain-source voltages on the pinch-off characteristics. We are able to tune the pinch-off characteristics by varying the back-gate voltage at temperatures ranging from 4 K to 300 K.

Spin-dependent thermoelectric effects in Fe-C6 doped monolayer MoS2

By using the non-equilibrium Green’s function with density functional theory, we have studied the thermal spin transport properties of Fe-C₆ cluster doped monolayer MoS₂. The results show that the device has a perfect Seebeck effect under temperature difference without gate voltage or bias voltage. Moreover, we also find the thermal colossal magnetoresistance effect, which is as high as 10⁷%. The competition between spin up electrons and spin down holes of the parallel spin configuration leads to peculiar behavior of colossal magnetoresistance and thermo-current, which is essential for the design of thermal transistors. These results are useful in future MoS₂-based multifunctional spin caloritronic devices.

Topography preserved microwave plasma etching for top-down layer engineering in MoS2 and other van der Waals materials

A generic and universal layer engineering strategy for van der Waals (vW) materials, scalable and compatible with the current semiconductor technology, is of paramount importance in realizing all-two-dimensional logic circuits and to move beyond the silicon scaling limit. In this letter, we demonstrate a scalable and highly controllable microwave plasma based layer engineering strategy for MoS₂ and other vW materials. Using this technique we etch MoS₂ flakes layer-by-layer starting from an arbitrary thickness and area down to the mono- or the few-layer limit. From Raman spectroscopy, atomic force microscopy, photoluminescence spectroscopy, scanning electron microscopy and transmission electron microscopy, we confirm that the structural and morphological properties of the material have not been compromised. The process preserves the pre-etch layer topography and yields a smooth and pristine-like surface. We explore the electrical properties utilising a field effect transistor geometry and find that the mobility values of our samples are comparable to those of the pristine ones. The layer removal does not involve any reactive gasses or chemical reactions and relies on breaking the weak inter-layer vW interaction making it a generic technique for a wide spectrum of layered materials and heterostructures. We demonstrate the wide applicability of the technique by extending it to other systems such as graphene, h-BN and WSe₂. In addition, using microwave plasma in combination with standard lithography, we illustrate a lateral patterning scheme making this process a potential candidate for large scale device fabrication in addition to layer engineering.

p-Type transition-metal doping of large-area MoS2 thin films grown by chemical vapor deposition

Two-dimensional transition metal dichalcogenides (e.g. MoS₂) have recently emerged as a promising material system for electronic and optoelectronic applications. A major challenge for these materials, however, is to realize bipolar electrical transport properties (i.e. both p-type and n-type conduction), which is critical for enhancing device performance and functionalities. Here, we demonstrate the transition metal zinc as a p-type dopant in the otherwise n-type MoS₂, through systematic characterizations of large area Zn-doped MoS₂ thin films grown by a one-step chemical vapor deposition (CVD) approach. Raman characterization and X-ray photoelectron spectroscopy studies identified millimeter-scale, monolayer films with 1-2% Zn as dopants. Zinc doping suppresses n-type conductivity in MoS₂ and shifts its Fermi level downwards. The stability and p-type nature of Zn dopants were further confirmed by density-functional-theory calculations of formation energies and electronic band structures. The electrical transport properties of Zn-MoS₂ films can be influenced by stoichiometry, and p-type gate transfer characteristics were realized by thermal treatment under a sulfur atmosphere. Our work highlights transition-metal doping followed by sulfur vacancy elimination in CVD grown films as a promising route for achieving large area p-type transition metal dichalcogenide films that are essential for practical applications in electronics and optoelectronics.

Impact and Origin of Interface States in MOS Capacitor with Monolayer MoS2 and HfO2 High-k Dielectric

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS₂) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS₂ channel and gate dielectric is fundamentally important for enhancing the carrier transport properties. Here, we investigate the frequency dispersion mechanism in a metal-oxide-semiconductor capacitor (MOSCAP) with a monolayer MoS₂ and an ultra-thin HfO₂ high-k gate dielectric. We show that the existence of sulfur vacancies at the MoS₂-HfO₂ interface is responsible for the generation of interface states with a density (D_it) reaching ~7.03 × 10¹¹ cm⁻² eV⁻¹. This is evidenced by a deficit S:Mo ratio of ~1.96 using X-ray photoelectron spectroscopy (XPS) analysis, which deviates from its ideal stoichiometric value. First-principles calculations within the density-functional theory framework further confirms the presence of trap states due to sulfur deficiency, which exist within the MoS₂ bandgap. This corroborates to a voltage-dependent frequency dispersion of ~11.5% at weak accumulation which decreases monotonically to ~9.0% at strong accumulation as the Fermi level moves away from the mid-gap trap states. Further reduction in D_it could be achieved by thermally diffusing S atoms to the MoS₂-HfO₂ interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS₂ devices with carrier transport enhancement.

Green synthesis of luminescent and defect-free bio-nanosheets of MoS2: interfacing two-dimensional crystals with hydrophobins

High quality luminescent nanosheets of MoS₂ interfaced with the amphiphilic protein Vmh2.

Triethanolamine doped multilayer MoS2 field effect transistors

As a result of the TEOA doping process, the electrical performances of multilayer MoS₂ FETs were enhanced at room temperature.

Thickness-Dependent Binding Energy Shift in Few-Layer MoS2 Grown by Chemical Vapor Deposition

The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS2 shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.

Temperature-controlled spin filter and spin valve based on Fe-doped monolayer MoS2

The thermal transport properties of an iron-doped molybdenum disulfide system were explored theoretically using the density functional theory calculations combined with the Keldysh non-equilibrium Green's function approach. The results indicate that a perfect spin filtering effect and spin Seebeck effect are induced thermally. Excellently, there exists thermal colossal magnetoresistances, which exhibit a transition between positive and negative that can be tuned using temperature. These features were elucidated using the band structures of the electrodes and the transmission function together with current spectra. Our findings may be helpful in the design of highly efficient spin caloritronic devices.

Nonequilibrium spin injection in monolayer black phosphorus

Nonequilibrium spin injection and spin-polarized quantum transport in monolayer black phosphorus are studied using the first principles method.

One-pot synthesis of multidimensional conducting polymer nanotubes for superior performance field-effect transistor-type carcinoembryonic antigen biosensors

Aptamer FET sensors based on carboxylated polypyrrole multidimensional nanotubes show ultrahigh sensitivity and selectivity toward CEA, and superior lifetimes.