MoS2 Devices Research Articles

UV-assisted fluctuation-enhanced gas sensing by ink-printed MoS2 devices

In this work, MoS2 flakes were printed on ceramic substrates and investigated toward 1–10 ppm of nitrogen dioxide (NO2), 2–12 ppm of ammonia (NH3), and 2–12 ppm acetone (C3H6O) under UV light (275 nm). The structure of overlapping MoS2 flakes and UV light assistance affected high responsivity to NO2 when DC resistance was monitored, and superior sensitivity to NH3 was obtained from the low-frequency noise spectra. MoS2 exhibited response and recovery times in hundreds of seconds and stability throughout the experiments conducted within a few months. MoS2 sensor exhibited a resistance drift during the detection of a specific relaxation time. Subtracting the baseline burden with exponential drift exposed the direction of changes induced by oxidizing and reducing gases and reduced DL to 80 ppb, 130 ppb, and 360 ppb for NO2, NH3, and C3H6O, respectively. The fluctuation-enhanced sensing (FES) revealed that the adsorption of NO2 on MoS2 decreases the noise intensity, whereas adsorbed NH3 increases the fluctuations of current flowing through the sensor, and these changes are proportional to the concentration of gases. The noise responses for NO2 and NH3 were opposite and higher than DC resistance responses with subtracted baseline (an increase of 50% for 10 ppm of NO2 and an increase of more than 600% for 12 ppm of NH3), showing that FES is a highly sensitive tool to detect and distinguish between these two gases. This way, we introduce a simple and low-cost method of gas sensor fabrication using ink-printed MoS2 and the possibility of enhancing its sensitivity through data processing and the FES method.

Flame aerosol deposition Y3Al5O12:Yb 3+ , Er 3+ nanoparticles on monolayer MoS2 for broadening near-infrared photoelectroresponse

Monolayer molybdenum disulfide (MoS2) is a two-dimensional material with a direct bandgap widely used in photoelectric detection. It exhibits excellent characteristics such as an ultra- high specific surface area, strong light interaction, and good carrier mobility. However, due to the bandgap limitation, monolayer MoS2 can only efficiently detect visible wavelengths less than 680 Combining lanthanide-doped upconversion nanoparticles with monolayer MoS2 to realize near- infrared (NIR) photodetection is a feasible strategy. Thus, Y3Al5O12:Yb 3+ , Er3+ up-conversion nanoparticles are deposited on the surface of a monolayer MoS2 by flame aerosol deposition (FAD), significantly extending their photoelectric response range to the near-infrared 980 nm. The Y3Al5O12:Yb 3+ , Er3+ nanoparticles synthesized by flame can achieve efficient up-conversion from near-infrared (~980 nm) to visible light (~450, ~550, ~650 nm). Therefore, the visible light can be subsequently absorbed by monolayer MoS2. The carefully designed Y3Al5O12:Yb 3+ , Er3+ /MoS2 photodetector shows strong responsivity and highly specific detectivity in the near-infrared spectrum. Under the bias voltage of 3 V and laser irradiation of 980 nm at 2.8 μW, the newly designed MoS2 device achieves a responsivity of 16.82 A/W and a specific detectivity of 1.47×10 12 Jones, as the traditional monolayer MoS2 usually responds to zero. This work can further expand the application of two-dimensional materials and their composites in near-infrared photoelectric detection

Ultra‐low power consumption flexible sensing electronics by dendritic bilayer MoS2

AbstractTwo‐dimensional transition metal dichalcogenides (2D TMDs) are promising as sensing materials for flexible electronics and wearable systems in artificial intelligence, tele‐medicine, and internet of things (IoT). Currently, the study of 2D TMDs‐based flexible strain sensors mainly focuses on improving the performance of sensitivity, response, detection resolution, cyclic stability, and so on. There are few reports on power consumption despite that it is of significant importance for wearable electronic systems. It is still challenging to effectively reduce the power consumption for prolonging the endurance of electronic systems. Herein, we propose a novel approach to realize ultra‐low power consumption strain sensors by reducing the contact resistance between metal electrodes and 2D MoS2. A dendritic bilayer MoS2 has been designed and synthesized by a modified CVD method. Large‐area edge contact has been introduced in the dendritic MoS2, resulting in decreased the contact resistance significantly. The contact resistance can be down to 5.4 kΩ μm, which is two orders of magnitude lower than the conventional MoS2 devices. We fabricate a flexible strain sensor, exhibiting superior sensitivity in detecting strains with high resolution (0.04%) and an ultra‐low power consumption (33.0 pW). This study paves the way for future wearable and flexible sensing electronics with high sensitivity and ultra‐low power consumption.image

The Impact of Microwave Annealing on MoS2 Devices Assisted by Neural Network-Based Big Data Analysis.

Microwave annealing, an emerging annealing method known for its efficiency and low thermal budget, has established a foundational research base in the annealing of molybdenum disulfide (MoS2) devices. Typically, to obtain high-quality MoS2 devices, mechanical exfoliation is commonly employed. This method's challenge lies in achieving uniform film thickness, which limits the use of extensive data for studying the effects of microwave annealing on the MoS2 devices. In this experiment, we utilized a neural network approach based on the HSV (hue, saturation, value) color space to assist in distinguishing film thickness for the fabrication of numerous MoS2 devices with enhanced uniformity and consistency. This method allowed us to precisely assess the impact of microwave annealing on device performance. We discovered a relationship between the device's electrical performance and the annealing power. By analyzing the statistical data of these electrical parameters, we identified the optimal annealing power for MoS2 devices as 700 W, providing insights and guidance for the microwave annealing process of two-dimensional materials.

Memristive feature and mechanism induced by laser-doping in defect-free 2D semiconductor materials

Memristors as non-volatile memory devices have gained numerous attentions owing to their advantages in storage, in-memory computing, synaptic applications, etc. In recent years, two-dimensional (2D) materials with moderate defects have been discovered to exist memristive feature. However, it is very difficult to obtain moderate defect degree in 2D materials, and studied on modulation means and mechanism becomes urgent and essential. In this work, we realized memristive feature with a bipolar switching and a configurable on/off ratio in a two-terminal MoS2 device (on/off ratio ~100), for the first time, from absent to present using laser-modulation to few-layer defect-free MoS2 (about 10 layers), and its retention time in both high resistance state and low resistance state can reach 2 × 104 s. The mechanism of the laser-induced memristive feature has been cleared by dynamic Monte Carlo simulations and first-principles calculations. Furthermore, we verified the universality of the laser-modulation by investigating other 2D materials of TMDs. Our work will open a route to modulate and optimize the performance of 2D semiconductor memristive devices.

Curcumin-Assisted Synthesis of MoS2 Nanoparticles as an Electron Transport Material in Perovskite Solar Cells.

Recently, two-dimensional (2D) transition metal dichalcogenides (2D TMDs), such as molybdenum sulfide (MoS2) and molybdenum selenide (MoSe2), have been presented as effective materials for extracting the generated holes from perovskite layers. Thus, the work function of MoS2 can be tuned in a wide range from 3.5 to 4.8 eV by adjusting the number of layers, chemical composition, elemental doping, surface functionalization, and surface states, depending on the synthetic approach. In this proposed work, we attempt to synthesize MoS2 nanoparticles (NPs) from bulk MoS2 using two steps: (1) initial exfoliation of bulk MoS2 into few-layer MoS2 by using curcumin-cholesteryl-derived organogels (BCC-ED) and curcumin solution in ethylene diamine (C-ED) under sonication; (2) ultrasonication of the subsequently obtained few-layer MoS2 at 60-80 °C, followed by washing of the above chemicals. The initial treatment with the BCC-ED/C-ED undergoes exfoliation of bulk MoS2 resulted in few-layer MoS2, as evidenced by the morphological analysis using SEM. Further thinning or reduction of the size of the few-layer MoS2 by prolonged ultrasonication at 60-80 °C, followed by repeated washing with DMF, resulted in uniform nanoparticles (MoS2 NPs) with a size of ~10 nm, as evidenced by morphological analysis. Since BCC-ED and C-ED produced similar results, C-ED was utilized for further production of NPs over BCC-ED owing to the ease of removal of curcumin from the MoS2 NPs. Utilization of the above synthesized MoS2 NPs as an ETL layer in the cell structure FTO/ETL/perovskite absorber/spiro-OMeTAD/Ag enhanced the efficiency significantly. The results showed that MoS2 NPs as an ETL exhibited a power conversion efficiency (PEC) of 11.46%, a short-circuit current density of 18.65 mA/cm2, an open-circuit voltage of 1.05 V, and a fill factor of 58.66%, at the relative humidity of 70 ± 10% (open-air conditions) than that of the ED-treated MoS2 devices without curcumin. These results suggest that the synergistic effect of both curcumin and ED plays a critical role in obtaining high-quality MoS2 NPs, beneficial for efficient charge transport, lowering the crystal defect density/trap sites and reducing the charge recombination rate, thus, significantly enhancing the efficiency.

High performance Pt-anchored MoS2 based chemiresistive ascorbic acid sensor

Ascorbic acid (AA), known as vitamin C, is a vital bioactive compound that plays a crucial role in several metabolic processes, including the synthesis of collagen and neurotransmitters, the removal of harmful free radicals, and the uptake of iron by cells in the human intestines. As a result, there is an absolute need for a highly selective, sensitive, and economically viable sensing platform for AA detection. Herein, we demonstrate a Pt-decorated MoS2 for efficient detection of an AA biosensor. MoS2 hollow rectangular structures were synthesized using an easy and inexpensive chemical vapor deposition approach to meet the increasing need for a reliable detection platform. The synthesized MoS2 hollow rectangular structures are characterized through field effect scanning electron microscopy (FESEM), energy-dispersive spectroscopy elemental mapping, Raman spectroscopy, and x-ray photoelectron spectroscopy. We fabricate a chemiresistive biosensor based on Pt-decorated MoS2 that measures AA with great precision and high sensitivity. The experiments were designed to evaluate the response of the Pt-decorated MoS2 biosensor in the presence and absence of AA, and selectivity was evaluated for a variety of biomolecules, and it was observed to be very selective towards AA. The Pt-MoS2 device had a higher response of 125% against 1 mM concentration of AA biomolecules, when compared to that of all other devices and 2.2 times higher than that of the pristine MoS2 device. The outcomes of this study demonstrate the efficacy of Pt-decorated MoS2 as a promising material for AA detection. This research contributes to the ongoing efforts to enhance our capabilities in monitoring and detecting AA, fostering advancements in environmental, biomedical, and industrial applications.

Reverse distribution of self-driven photocurrent response hotspots in layer-dependent MoS2 devices

In van der Waals materials, the electronic property, including the bandgap as well as the charge carrier mobility and lifetime, significantly changes as its thickness shrinks to the monolayer limit. While such characteristic brings additional freedom and convenience in regard to device design and fabrication, it also raises uncertainty in terms of the semiconductor device functionality due to the competing roles of those factors in tuning the charge carrier transport. In this work, we perform scanning photocurrent microscopy experiments on different sets of MoS2 homojunctions under zero bias voltage and show the reverse photocurrent distribution among them. Specifically, in the combination of 1L–3L MoS2, the band offset is large and thus dominates the photocarrier separation. In the combination of 3L–7L MoS2, by contrast, the effect of band offset is negligible, and the mismatch in minority carrier diffusion length takes charge of the photoresponse. The authors hope that the findings presented here offer a perspective on the current transport and thus functionality realization in layered materials.

Strain and defect engineering of high-performance monolayer two-dimensional material/Ag nanoparticles optoelectronic FET sensor through multiple laser shock

The development of sensor devices that detect low-concentrations hydrogen at room temperature is becoming increasingly important for the safe use of hydrogen. In this study, the introduction of strain and defects in two-dimensional materials has been realised by laser shock and its application in gas sensing. The strain and defect density of monolayer MoS2 increase after multiple laser shock, providing more active sites for gas adsorption and improving the ability to adsorb hydrogen atoms. Under the synergistic effect of strain and defects, the response of non-noble metal nanoparticles/MoS2 photoelectric hydrogen sensors is accelerated, and their sensing performance (83% for 200 ppm) is 9.5 times higher than that of pure MoS2 devices (8.7% for 200 ppm) at room temperature (25 ℃). As a proof of concept, laser shock induced 2D material strain and defect engineering has opened up new ideas for improving the performance of photoelectric gas sensors at room temperature.

Thin-film photodiode based on novel SiC/MoS2 composite by RF-sputtering for fast response photodetection

A Schottky -photodiode and pn-photodiode were fabricated based on a SiC/MoS2 composite semiconductor material using a physical vapour deposition (PVD) method. The surface topology and structural features of the nanostructured SiC/MoS2 thin films were investigated using scanning electron microscope (SEM), Fourier Transform Infrared spectroscopy (FTIR), and grazing incidence X-Ray diffraction (GIXRD) analysis. The two-dimensional nature of these materials enables easiness integration for vertical device design with novel properties. The performance parameters of the fabricated devices were characterized. The electrical performance of the fabricated SiC/MoS2 Schottky and pn photodiodes were investigated according to their current-voltage (I–V) characteristics within a bias voltage range from −4 to 4 V. Here, we report highly efficient photocurrent generation from vertical SiC/MoS2 composite device fabricated using asymmetric metal contacts, exhibiting an external quantum efficiency (EQE) of up to 14.68% for pn -photodiode and 6.68% for Schottky -photodiode. Evaluated to in-plane MoS2 devices, the vertical design of these devices has a much larger junction area which is essential for achieving highly efficient photovoltaic devices. As well as spectral responsivity of 28.25 mA/W and 63.22 mA/W for Schottky and pn-photodiodes respectively also, response time of 1 ms for Schottky-photodiode and 977.5 μs for pn-photodiode were measured and discussed.

Defects Contributing to Hysteresis in Few-Layer and Thin-Film MoS2 Memristive Devices.

Molybdenum disulfide, a two-dimensional material extensively explored for potential applications in non-von Neumann computing technologies, has garnered significant attention owing to the observed hysteresis phenomena in MoS2 FETs. The dominant sources of hysteresis reported include charge trapping at the channel-dielectric interface and the adsorption/desorption of molecules. However, in MoS2 FETs with different channel thicknesses, the specific nature and density of defects contributing to hysteresis remain an intriguing aspect requiring further investigation. This study delves into memristive devices with back-gate modulated channel layers based on CVD-deposited flake-based and thin-film-based MoS2 FETs, with a few-layer (FL) and thin-film (TF) channel thickness. Analysis of current-voltage (I-V) and conductance-frequency (Gp/ω-f) measurements led to the conclusion that the elevated hysteresis observed in TF MoS2 devices, as opposed to FL devices, stems from a substantial contribution from intrinsic defects within the channel volume, surpassing that of interface defects. This study underscores the significance of considering both intrinsic defects within the bulk and the interface defects of the channel when analyzing hysteresis in MoS2 FETs, particularly in TF FETs. The selection between FL and TF MoS2 devices depends on the requirements for memristive applications, considering factors such as hysteresis tolerance and scaling capabilities.

Insulating 6,6-Phenyl-C61-butyric Acid Methyl Ester on Transition-Metal Dichalcogenides: Impact of the Hybrid Materials on the Optical and Electrical Properties.

In this study we develop a strategy to insulate 6,6 -Phenyl C61 butyric acid methyl ester (PCBM) on the basal plane of transition metal dichalcogenides (TMDs). Concretely single layers of MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 and ultrathin MoO2 and WO2 were grown via chemical vapor deposition (CVD). Then, the thiol group of a PCBM modified with cysteine reacts with the chalcogen vacancies on the basal plane of TMDs, yielding PCBM-MoS2, PCBM-MoSe2, PCBM-WS2, PCBM-WSe2, PCBM-WTe2, PCBM-MoO2 and PCBM-WO2. Afterwards, all the hybrid materials were characterized using several techniques, including XPS, Raman spectroscopy, TEM, AFM, and cyclic voltammetry. Furthermore, PCBM causes a unique optical and electrical impact in every TMDs. For MoS2 devices, the conductivity and photoluminescence (PL) emission achieve a remarkable enhancement of 1700 % and 200 % in PCBM-MoS2 hybrids. Similarly, PCBM-MoTe2 hybrids exhibit a 2-fold enhancement in PL emission at 1.1 eV. On the other hand, PCBM-MoSe2, PCBM-WSe2 and PCBM-WS2 hybrids exhibited a new interlayer exciton at 1.29-1.44, 1.7 and 1.37-154 eV along with an enhancement of the photo-response by 2400, 3200 and 600 %, respectively. Additionally, PCBM-WTe2 and PCBM-WO2 showed a modest photo-response, in sharp contrast with pristine WTe2 or WO2 which archive pure metallic character.

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing.

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS2.

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS2) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p-n junction, achieves multifunctionality. The ambipolar MoS2 device operates as an ambipolar transistor (on/off ratios exceeding 107 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 106), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 109 with a maximum current as high as 10-4 A), and a photodetector. Finally, we demonstrate a complementary inverter (gain of ∼24 at Vdd = 1.5 V), which is highly facile to fabricate without the need for complex heterostructures and doping processes. Our study provides strategies to achieve high-performance ambipolar MoS2 devices and to effectively utilize drain bias for electrical switching.

Oxidation-induced modulation of photoresponsivity in monolayer MoS2 with sulfur vacancies.

Two-dimensional transition metal dichalcogenides (TMDs), such as MoS2, hold great promise for next-generation electronics and optoelectronics due to their unique properties. However, the ultrathin nature of these materials renders them vulnerable to structural defects and environmental factors, which significantly impact their performance. Sulfur vacancies (VS) are the most common intrinsic defects in MoS2, and their impact on device performance in oxidising environments remains understudied. This study investigates the impact of VS defects on the photoresponsivity of CVD-grown monolayer MoS2 devices, when exposed to oxidising environments at high temperatures. Our findings reveal a dynamic process of defect generation and healing through oxygen passivation, leading to a significant difference in photocurrent between environments. Temperature-dependent analysis shows defect healing and a notable reduction in defect density upon cooling. This study provides crucial insights into the stability and performance of 2D materials-based devices under varying environmental conditions, essential for designing and controlling the performance of TMD-based devices. Our results pave the way for the development of robust and reliable 2D materials-based electronics and optoelectronics.

Flexible SnO2–MoS2 based memristive device exhibiting stable and enhanced memory phenomenon

Flexible non-volatile memory devices have been gaining interest in expanding the digital data storage world. Due to the burgeoning advancement in the healthcare industry, the Internet of Things, and wearable electronics, the demand for ultra-thin, low-power, and flexible memory is increasing. Further, the advancement of synthesis procedures for two-dimensional nanomaterials having better optical, electrical, and mechanical strength with flexibility has fuelled the flexible memory device area, as commonly used flash memory is approaching its physical limit. In this context, the present work reports the flexible resistive switching memory device based on pure molybdenum disulfide (MoS2) and tin oxide (SnO2) based nanocomposite powder synthesized using the simple hydrothermal process. The nanocomposite formation was characterized using x-ray diffraction and Raman spectroscopic techniques. The memory device was fabricated by spin-coating the pure MoS2, pure SnO2, and MoS2–SnO2 nanocomposite film over the ITO-PET flexible substrate. The device was completed by thermally evaporating the thin Al layer through a shadow mask. It was found that MoS2–SnO2 based memory devices exhibited improved switching performance having a higher I ON/I OFF ratio, and lower switching voltage in comparison to pure MoS2 and pure SnO2-based devices (I ON/I OFF ratio ∼100, V = 0.5 V). Furthermore, to check the stability and cyclic performance of the fabricated device, the retention and endurance test was also performed, and the MoS2–SnO2 device retained the HRS and LRS states up to 2 × 103s and showed stable performance up to 100 switching cycles without much degradation, respectively. It should be mentioned that the presently proposed ReRAM device based on SnO2 and MoS2 with flexible and low-power features had excellent potential for use in the wearable device industry.

Design and Simulation of a High‐Efficiency CuInS2 Thin‐Film Solar Cell with a Thin CIS Current Augmenter

Ternary chalcopyrite compound cupper indium disulfide (CuInS2)‐based photovoltaic cells are designed and numerically computed. In this design, n‐CdS and p++‐MoS2 have been used as window and back surface field (BSF) layers, respectively. The modeled n‐CdS/p‐CuInS2/p++‐MoS2 device provides power conversion efficiency (PCE) of 24.6% with JSC of 26.98 mA cm−2, VOC of 1.05 V, and fill factor (FF) of 86.93%. This efficiency is ≈60% higher the than n‐CdS/p‐CuInS2 single heterostructure. Furthermore, the use of a second CuInSe2 (CIS) absorber layer between CuInS2 absorber and MoS2 BSF layer significantly enhances the short‐circuit current. As a result, the current expressively improves to 38.48 mA cm−2, yielding an efficiency of 32.4%. The greater built‐in potential in absorber/BSF interface is attributed to the high JSC and VOC, causing the increment of efficiency. Besides, the photon management by Bragg reflector with 90% back and front reflectance recycling effect further increases the PCE to 36.1%. This work indicates the prospects of CuInS2 solar cells with a CuInSe2 current augmenter and MoS2 BSF layers for the fabrication of efficient CuInS2 solar cells.

Semi-classical Monte Carlo study of the impact of tensile strain on the performance limits of monolayer MoS2 n-channel MOSFETs

The effects of tensile strain and contact transmissivity on the performance limits of monolayer molybdenum disulfide (MoS2) nanoscale n-channel MOSFETs are studied using a semi-classical Monte Carlo method. Density functional theory calculations were performed to parametrize the electronic band structure of MoS2 subject to tensile and shear strain. Tensile strain decreases the bandgap, increases the inter-valley band-edge energy separation between the light-mass K-valleys and heavier-mass Q-valleys, and decreases the K-valley effective mass in a way that depends on the direction and the amount of the applied strain. Biaxial tensile strain and uniaxial tensile strain along the x- or y-directions are found to have the largest effect. In bulk materials, low-field phonon-limited electron mobility is enhanced, peak and saturation drift velocities are increased, and high-field negative differential resistance becomes more pronounced. Both 200 and 15 nm gate length MoS2 MOSFETs with end-contacts with ideal (unity) and more realistic (significantly sub-unity) contact interface transmissivity were simulated. These MoS2 devices exhibited substantial sensitivity to strain with ideal contact transmissivity, and more so for the 15 nm quasi-ballistic device scale than 200 nm long-channel devices. However, the results showed much less strain sensitivity for devices with more realistic contact transmissivities, which may be good or bad depending on whether strain-insensitive or strain-sensitive performance is desired for a particular application and may be possible to modify with improved contact geometries.

Electron and Hole-Doping for Influencing the Electronic Transport Properties of MoS₂ Devices at Room Temperature

Electron and Hole-Doping for Influencing the Electronic Transport Properties of MoS₂ Devices at Room Temperature

Unveiling the origin of n-type doping of natural MoS2: carbon

MoS2 has attracted intense interest in many applications. Natural MoS2 and field-effect transistors made of it generally exhibit n-type characteristics, but its origin is unknown. Herein, we show that C is the origin of the universal n-type doping of natural MoS2. Photoemission spectroscopies reveal that while many MoS2 samples with C detected are n-type, some without C exhibit p-type characteristics. The C-free, p-type MoS2 changes to n-type over time with the concurrent appearance of C that is out-diffused from bulk, indicating that C induces the n-type doping. The C-origin is verified by C-deposition and supported by theoretical calculations. This carbon appears as nanometer-scale defects frequently observed in scanning tunneling microscopy. In addition, we propose, based on the calculations, that S vacancies are responsible for the p-type characteristics, which contrasts with the widespread belief. This work provides new perspectives on MoS2 doping and presents a new direction for fabricating reliable MoS2 devices.