Defects In MoS2 Research Articles

On‐Chip Integrated High Gain Complementary MoS2 Inverter Circuit with Exceptional High Hole Current P‐Channel Field‐Effect Transistors

AbstractIntegration of complementary logic circuits using 2D layered MoS2 can overcome the fundamental device limitations of silicon‐based Complementary metal–oxide–semiconductor (CMOS) technology. For the high functionality of complementary MoS2 logic circuits, realizing high‐current p‐channel MoS2 field‐effect transistors (p‐FETs) is essential yet quite challenging. Herein, passivation of surface defects of MoS2 by atomic oxygen is proposed theoretically by ab initio simulations in favor of mitigating the Fermi level pinning. Experimentally, an atomic layer passivation (ALP) technique combined with a slow metal‐deposition strategy is developed to reduce contact resistance to p‐channel MoS2 and report a more than tenfold increase in the contact hole conductance. The fabricated p‐channel MoS2 FETs achieve a record high saturation current of 45 µA µm−1 with a 0.7 µm channel length under a VDS of −1 V. With these techniques, an electron–hole pairing in MoS2 similar to the Si counterpart is achieved for complementary circuit applications. An on‐chip integrated MoS2 inverter with Pt electrodes is demonstrated with a high gain of 20 at 3 V supply.

Threshold voltage modulation in monolayer MoS2 field-effect transistors via selective gallium ion beam irradiation

Electronic regulation of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is a crucial step towards next-generation optoelectronics and electronics. Here, we demonstrate controllable and selective-area defect engineering in 2D molybdenum disulfide (MoS2) using a focused ion beam with a low-energy gallium ion (Ga+) source. We find that the surface defects of MoS2 can be tuned by the precise control of ion energy and dose. Furthermore, the field-effect transistors based on the monolayer MoS2 show a significant threshold voltage modulation over 70 V after Ga+ irradiation. First-principles calculations reveal that the Ga impurities in the monolayer MoS2 introduce a defect state near the Fermi level, leading to a shallow acceptor level of 0.25 eV above the valence band maximum. This defect engineering strategy enables direct writing of complex pattern at the atomic length scale in a controlled and facile manner, tailoring the electronic properties of 2D TMDCs for novel devices.

Electrode-Induced Self-Healed Monolayer MoS2 for High Performance Transistors and Phototransistors.

Contact engineering for monolayered transition metal dichalcogenides (TMDCs) is considered to be of fundamental challenge for realizing high-performance TMDCs-based (opto) electronic devices. Here, an innovative concept is established for a device configuration with metallic copper monosulfide (CuS) electrodes that induces sulfur vacancy healing in the monolayer molybdenum disulfide (MoS2 ) channel. Excess sulfur adatoms from the metallic CuS electrodes are donated to heal sulfur vacancy defects in MoS2 that surprisingly improve the overall performance of its devices. The electrode-induced self-healing mechanism is demonstrated and analyzed systematically using various spectroscopic analyses, density functional theory (DFT) calculations, and electrical measurements. Without any passivation layers, the self-healed MoS2 (photo)transistor with the CuS contact electrodes show outstanding room temperature field effect mobility of 97.6 cm2 (Vs)-1 , On/Off ratio > 108 ,low subthreshold swing of 120mV per decade, high photoresponsivity of 1 × 104 A W-1 , and detectivity of 1013 jones, which are the best among back-gated transistors that employ 1L MoS2 . Using ultrathin and flexible 2D CuS and MoS2 , mechanically flexible photosensor is also demonstrated, which shows excellent durability under mechanical strain. These findings demonstrate a promising strategy in TMDCs or other 2D material for the development of high performance and functional devices including self-healable sulfide electrodes.

The mechanistic difference of 1T-2H MoS2 homojunctions in persulfates activation: Structure-dependent oxidation pathways

Two-dimensional layered MoS2 recently demonstrates great potentials in water purification via adsorption and catalysis. The structure-catalysis relationship of MoS2 has not been well addressed. Herein, we investigated the mechanistic difference of 1 T-2H MoS2 homojunctions in catalytic activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) for degrading butyl paraben (BPB). PMS/1 T-2H MoS2 attains higher performance in BPB removal compared to PDS/1 T-2H MoS2. 1 T phase in MoS2 primarily contributed to PMS activation to produce SO4−, OH and 1O2, while the defects in MoS2 coordinated the PDS activation to generate OH, 1O2 and O2−. Nevertheless, OH-induced BPB removal was the top priority for PMS/1 T-2H MoS2 and PDS/1 T-2H MoS2. The different activation pathways and reactive species resulted in the varying BPB removal, by-product distributions and toxicity. This work provides new insights into the different functions of MoS2 in persulfates activation and the guidance in rational design of oxidant-oriented MoS2 composites for sewage purification.

Steric Hindrance- and Work Function-Promoted High Performance for Electrochemical CO Methanation on Antisite Defects of MoS2 and WS2.

CO methanation from electrochemical CO reduction reaction (CORR) is significant for sustainable environment and energy, but electrocatalysts with excellent selectivity and activity are still lacking. Selectivity is sensitive to the structure of active sites, and activity can be tailored by work function. Moreover, intrinsic active sites usually possess relatively high concentration compared to artificial ones. Here, antisite defects MoS2 and WS2 , intrinsic atomic defects of MoS2 and WS2 with a transition metal atom substituting a S2 column, were investigated for CORR by density functional theory calculations. The steric hindrance from the special bowl structure of MoS2 and WS2 ensured good selectivity towards CO methanation. Coordination environment variation of the active sites, the under-coordinated Mo or W atoms, effectively lowered the work function, making MoS2 and WS2 highly active for CO methanation with the required potential of -0.47 and -0.49 V vs. reversible hydrogen electrode, respectively. Moreover, high concentration of active sites and minimal structural deformation during the catalytic process of MoS2 and WS2 enhanced their attraction for future commercial application.

One-step construction of sulfide heterostructures with P doping for efficient hydrogen evolution

Molybdenum disulfide (MoS2) exhibits the modest properties toward hydrogen evolution due to its poor electrical conductivity and a limited number of active sites. To make up for the defects in MoS2, we built hierarchical P-doped MoS2/NiS2@carbon hollow nanospheres (P–MoS/NiS@CHNS) electrocatalyst via one-step hydrothermal method owning to the priority of the reaction between Ni2+ and H2S. Motivated by high-efficient electron transfer at the interface of heterostructure and the activation of MoS2 by P dopant, P–MoS/NiS@CHNS exhibits optimal Gibbs free energies (ΔGH∗) and superior HER performance with low overpotential (ƞ10) of 110 ​mV ​at 10 ​mA ​cm−2 and small Tafel slope of 61 ​mV dec−1. The focus of this work is to optimize the HER performance of MoS2 through P-doping and heterostructure construction, which can further provide a reference for the design of MoS2 based catalysts.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects.

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg2+) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS2-based field-effect transistors (FETs) and exploited them as platforms for Hg2+ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS2 lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS2 when exposed to Hg2+ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg2+ exposure. Transfer characteristics in MoS2 FETs provided further evidence that Hg2+ acts as a p-dopant of MoS2. Interestingly, we observed a strict correlation of doping with the concentration of Hg2+, following a semi-log trend. Hg2+ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg2+ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Influence of Lithium Intercalation on Magnetic Properties of MoS2 Nanosheets

MoS2 is a member of a large class of 2-dimensional transition metal dichalcogenides (TMDs) with general formula MX2 (M-transition metal, X-chalcogen atom). In these 2D materials X-M-X monolayer comprises of M-layer sandwiched between two X-layers. Monolayers are held by weak van der Waals interaction in bulk compounds. Among various TMDs reported MoS2 has received great attention due to its exclusive properties like large spin-orbit coupling, layer dependent bandgap (1.2 eV to 1.8 eV) and high on/off current ratio making it a promising candidate for spintronics devices.1 Bulk MoS2 is diamagnetic in nature, hence it is necessary to tune its ferromagnetic properties to make it a potential candidate for spintronics applications. Several experiments like transition metal doping, inducing defects in MoS2 and structural phase transition have been shown to influence the ferromagnetic properties.MoS2 exists in different polymorphs, namely 1T, 2H and 3R. Mo in 2H and 3R phase resides in trigonal prismatic geometry. In this coordination electrons in d-orbital pair up oppositely resulting in a net zero magnetic moment. Whereas, in 1T phase Mo residing in octahedral geometry stabilizes in spin-polarized state giving net magnetic moment of 2μB per MoS2. This provides a pathway for enabling MoS2 ferromagnetic through structural phase transition (i.e. 3R/2H to 1T). Intercalating ions within the interlayer sites between adjacent MoS2 layers which takes place with an associated charge transfer to molybdenum enable such structural phase transition. This process leads to the trigonal prismatic (2H/3R) coordination of molybdenum unstable. Hence the system stabilizes in octahedral geometry (1T) via intralayer atomic plane gliding through transverse S-layer displacement. In this work, we utilize MoS2 nanosheets synthesized from wet chemical method to investigate the magnetic properties of Li+ intercalated MoS2 nanosheets. MoS2 nanosheets with different degrees of disorder can be obtained by controlling the annealing temperature.2 Planar and edge defect-rich MoS2 (MS-DR) are obtained upon annealing at 500 °C and defect-suppressed ordered layers of MoS2 nanosheets (MS-DS) are obtained upon annealing at 900 °C for 1 h. The former shows larger magnetization (0.10 emu/g) compared to the latter (0.03 emu/g). After lithium intercalation these MoS2 nanosheets show enhanced magnetization, 0.37 emu/g (for MS-DR) and 0.22 emu/g (MS-DS). Since the deintercalation process retains Li within the nanosheets based on the extent of defects present in MoS2, these nanosheets retain ferromagnetism even after deintercalation process. This presentation will discuss in detail the ferromagnetic properties of MoS2 nanosheets upon Li intercalation/deintercalation process.

Influence of chemical treatment on strain and charge doping in vertically stacked monolayer–bilayer MoS2

We report simultaneous Raman scattering and photoluminescence (PL) mapping results to study the strain and doping effects of chemical treatment with bis(trifluoromethane) sulfonimide (TFSI) on the optical phonon, exciton, and trion characteristics of a vertically stacked monolayer–bilayer (1L–2L) MoS2 structure. Correlation analysis between the E′ and A1′ phonon energies revealed that tensile strain developed in the TFSI-treated MoS2 mainly by the filling of sulfur vacancies: 0.13% and 0.10% for 1L and 2L MoS2, respectively. In addition, TFSI-induced changes in the electron densities evaluated from the Raman correlation analysis were estimated to be −0.38×1013 cm−2 and −1.21×1013 cm−2 for 1L and 2L MoS2, respectively. The larger p-doping effect in 2L than in 1L MoS2 was attributed to a relatively higher defect density in the 2L region of the pristine MoS2, followed by a subsequent healing of the defects via chemical doping. The TFSI-induced change in electron density estimated from the PL result was in excellent agreement with the Raman correlation analysis. Furthermore, the Raman mapping and PL histogram analyses showed that structural defects in MoS2 could be effectively healed by chemical treatment.

Hydrothermally synthesized MoS2-multi-walled carbon nanotube composite as a novel room-temperature ammonia sensing platform

MoS2 and its composite with multi-walled carbon nanotube (MWCNTs) were synthesized using facile hydrothermal approach. Structural and vibrational analysis revealed the presence of various defects in MoS2 and its composite with MWCNT. Two-terminal devices were made on a quartz substrate with pre-deposited silver contacts. The resulting Ag/MoS2/Ag and Ag/MoS2-MWCNT/Ag devices exhibit n-type semiconducting behavior and have shown room-temperature ammonia detection down to 12 ppm level. In case of former, the corresponding response time (tresponse = 400 s) and recovery time (trecovery = 280 s) are very large with limit of detection down to 1.2 ppm. The latter, i.e., Ag/MoS2-MWCNT/Ag device on the other hand exhibits faster response-recovery (65 and 70 s, respectively) features along with enhanced relative response for various ammonia concentrations ranging from 12 to 325 ppm. Further, the composite device also displays superior selectivity to its counterpart. It is argued that these differences might arise from different adsorption energy for different gas molecules on the surface of MoS2 or MoS2-MWCNT semiconducting channel. Present results suggest that composite materials such as MoS2-MWCNT may serve as a novel gas sensing platform with superior sensing characteristics and selectivity features.

Band Bending and Valence Band Quantization at Line Defects in MoS2

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS2, grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upward bending of valence and conduction bands toward the line defects is found for the 4|4E mirror twin boundary and island edges but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upward band bending takes place. Focusing on the common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS2 mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS2. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary.

Effects of the Intercalant and the Temperature in Hybrid-MoS2 Nanodots Fabrication and Their Photoluminescence Enhancement

In this study, we have proposed a simple method to produce temperature-dependent variablesize of MoS2 nanodots (NDTs) by using MoS2 powder as a pre-cursor and KNa-tartrate as the intercalant. Due to defects in MoS2, the optical properties are strongly modified and show blue luminescence under UV irradiation of MoS2 NDTs. The temperature variable is used to gradually introduce defects in 2D materials to obtain nanodots with different particle sizes. When the synthesis temperature is increased from 120 °C to 200 °C, the particle size is reduced from ~ 120 nm to ~ 2.5 nm. Next, an enhancement of photoluminescent magnitude and a red shift were observed in photoluminescence spectra from MoS2 NDTs solutions. These results offer a route toward tailoring the optical properties of 2D nanomaterials by controlling the size/temperature/synthesis method.

Defects suppression in MoS2 caused by W doped for enhanced response/recovery behaviors against NO2

MoS2 is a promising material for room temperature NO2 gas detection, but there are still some great challenges to MoS2-based gas sensors, such as slow response/recovery speed. Here, MoS2 with different W doped ratios are synthesized by the hydrothermal method, and W doping is utilized to suppress defects in MoS2. The results show that the speed of MoS2 is greatly improved through W doped, and its response/recovery times can be reduced by almost an order of magnitude, from hundreds to tens of seconds. Moreover, P-N transition to sensing behavior against NO2 is observed with an increase of the W doped ratio. This work provides a useful guideline on the wide application of MoS2.

Atomistic Positioning of Defects in Helium Ion Treated Single-Layer MoS2.

Structuring materials with atomic precision is the ultimate goal of nanotechnology and is becoming increasingly relevant as an enabling technology for quantum electronics/spintronics and quantum photonics. Here, we create atomic defects in monolayer MoS2 by helium ion (He-ion) beam lithography with a spatial fidelity approaching the single-atom limit in all three dimensions. Using low-temperature scanning tunneling microscopy (STM), we confirm the formation of individual point defects in MoS2 upon He-ion bombardment and show that defects are generated within 9 nm of the incident helium ions. Atom-specific sputtering yields are determined by analyzing the type and occurrence of defects observed in high-resolution STM images and compared with Monte Carlo simulations. Both theory and experiment indicate that the He-ion bombardment predominantly generates sulfur vacancies.

Interaction of Pb2+ ions in water with two-dimensional molybdenum disulfide

The removal of heavy metal contaminants from water is important for public health, and recently many two-dimensional (2D) materials with high specific surface areas are being studied as promising new active components in water purification. In particular, 2D MoS2 nanosheets have been used for the removal of various heavy metals, but usually in either in complex geometries and composites, or in the chemically exfoliated metallic 1T-MoS2 phase. However, the interaction of heavy metals dissolved in water with unmodified semiconducting 2H-MoS2 is not well studied. In this paper, we report a detailed fundamental investigation of how Pb2+ ions interact with 2H-MoS2. We observe small solid clusters that form on the MoS2 surfaces after exposing them to Pb(NO3)2 aqueous solutions as shown by atomic force microscopy and transmission electron microscopy, and for liquid phase exfoliated MoS2 we observe the nanosheets precipitating out of dispersion along with insoluble solid granules. We use a combination of x-ray photoelectron spectroscopy and x-ray diffraction to identify these solid clusters and granules as primarily PbSO4 with some PbMoO4. We put forth an interaction mechanism that involves MoS2 defects acting as initiation sites for the partial dissolution in aqueous oxygenated conditions which produces MoO42− and SO42− ions to form the solids with Pb2+. These results are an important contribution to our fundamental understanding of how MoS2 interacts with metal ions and will influence further efforts to exploit MoS2 for water remediation applications.

Mechanical Properties of Monolayer MoS2 with Randomly Distributed Defects.

The variation of elastic constants stiffness coefficients with respect to different percentage ratios of defects in monolayer molybdenum disulfide (MLMoS2) is reported for a particular set of atomistic nanostructural characteristics. The common method suggested is to use conventional defects such as single vacancy or di vacancy, and the recent studies use stone-walled multiple defects for highlighting the differences in the mechanical and electronic properties of 2D materials. Modeling the size influence of monolayer MoS2 by generating defects which are randomly distributed for a different percentage from 0% to 25% is considered in the paper. In this work, the geometry of the monolayer MoS2 defects modeled as randomized over the domain are taken into account. For simulation, the molecular static method is adopted and study the effect of elastic stiffness parameters of the 2D MoS2 material. Our findings reveals that the expansion of defects concentration leads to a decrease in the elastic properties, the sheer decrease in the elastic properties is found at 25%. We also study the diffusion of Molybdenum (Mo) in Sulphur (S) layers of atoms within MoS2 with Mo antisite defects. The elastic constants dwindle in the case of antisite defects too, but when compared to pure defects, the reduction was to a smaller extent in monolayer MoS2. Nevertheless, the Mo diffusion in sulfur gets to be more and more isotropic with the increase in the defect concentrations and elastic stiffness decreases with antisite defects concentration up to 25%. The distribution of antisite defects plays a vital role in modulating Mo diffusion in sulfur. These results will be helpful and give insights in the design of 2D materials.

Towards the evaluation of defects in MoS2 using cryogenic photoluminescence spectroscopy.

Characterization of the type and density of defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) is important as the nature of these defects strongly influences the electronic and optical properties of the material, especially its photoluminescence (PL). Defect characterization is not as straightforward as it is for graphene films, where the D and D' Raman scattering modes easily indicate the density and type of defects in the graphene layer. Thus, in addition to the Raman scattering analysis, other spectroscopic techniques are necessary to perform detailed characterization of atomically thin TMD layers. We demonstrate that PL spectroscopy performed at liquid helium temperatures reveals the key fingerprints of defects in TMDs and hence provides valuable information about their origin and concentration. In our study, we address defects in chemical vapor deposition (CVD)-grown MoS2 monolayers. A significant difference is observed between the as-grown monolayers compared with the CVD-grown monolayers transferred onto a Si/SiO2 substrate, which contain extra defects due to the transfer process. We demonstrate that the temperature-dependent Raman and PL micro-spectroscopy techniques enable disentangling the contributions and locations of various defect types in TMD systems.

Defect-Assisted Contact Property Enhancement in a Molybdenum Disulfide Monolayer.

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS2) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS2 in contact properties. Remarkably, MoS2 devices with Ag contacts show a field-effect mobility of 12.2 cm2 V-1 s-1, an on/off current ratio of 7 × 107, and a photoresponsivity of 1020 A W-1, which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag-MoS2 orbital hybridization at the interface, which facilitates efficient charge transfer between MoS2 and Ag. Interestingly, X-ray photoelectron spectroscopic analysis reveals that Ag2S was formed in our defect-containing CVD-grown MoS2 monolayer, but such orbital hybridization is not observed in a nearly defect-free exfoliated MoS2. This distinction shows that defects existing in MoS2 enable Ag to effectively couple to MoS2 and correspondingly enhance multiple electrical and photoelectrical properties.

Enhanced Photo‐Response of Mos2 Photodetectors by a Laterally Aligned SiO2 Nanoribbon Array Substrate

AbstractAchieving both high photocurrent and small dark current is required for the enhanced performance of molybdenum disulfide (MoS2) photodetector (PD). In the two‐dimensional transition metal dichalcogenide PD, inevitable recombinations occur highly at intrinsic defects of MoS2 and impede photo‐generated carrier releasement into electrodes, resulting in a poor PD performance. To address this issue without introducing a superiorly high‐crystalline MoS2 monolayer and/or complex PD architecture, we for the first time report a facile method of simply transferring the MoS2 onto a periodically aligned silicon dioxide nanoribbons (SNR) array substrate fabricated by 325 nm laser interference lithography. Interestingly, two different n‐doping states are arranged alternately on the MoS2 layer, depending on the underlying region of contact substrate (pristine SiO2 and SNR). The different n‐doping levels induce internal electric fields by which photo‐generated carriers are separated, reducing the recombination chance. The MoS2 PD on the SNR array substrate shows an improved photocurrent to dark current ratio of ∼360 (∼7 times larger than that of the reference PD on the pristine SiO2 substrate), while producing a small dark current of ∼10−12 A at VG=0 V. Our method paves the way for enhancing the performance of other 2D materials‐based optoelectronic devices.

First-principles study of coupled effect of ripplocations and S-vacancies in MoS2

Recent experiments have revealed ripplocations, atomic-scale ripplelike defects on samples of MoS2 flakes. We use quantum mechanical calculations based on density functional theory to study the effect of ripplocations on the structural and electronic properties of single-layer MoS2, and, in particular, the coupling between these extended defects and the most common defects in this material, S-vacancies. We find that the formation of neutral S-vacancies is energetically more favorable in the ripplocation. In addition, we demonstrate that ripplocations alone do not introduce electronic states into the intrinsic bandgap, in contrast to S-vacancies. We study the dependence of the induced gap states on the position of the defects in the ripplocation, which has implications for the experimental characterization of MoS2 flakes and the engineering of quantum emitters in this material. Our specific findings collectively aim to provide insights into the electronic structure of experimentally relevant defects in MoS2 and to establish structure-property relationships for the design of MoS2-based quantum devices.