2D Layered Transition Metal Dichalcogenides Research Articles

Probing the Origin and Suppression of Vertically Oriented Nanostructures of 2D WS2 Layers.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) such as WS2 are promising materials for nanoelectronic applications. However, growth of the desired horizontal basal-plane oriented 2D TMD layers is often accompanied by the growth of vertical nanostructures that can hinder charge transport and, consequently, hamper device application. In this work, we discuss both the formation and suppression of vertical nanostructures during plasma-enhanced atomic layer deposition (PEALD) of WS2. Using scanning transmission electron microscopy studies, formation pathways of vertical nanostructures are established for a two-step (AB-type) PEALD process. Grain boundaries are identified as the principal formation centers of vertical nanostructures. Based on the obtained insights, we introduce an approach to suppress the growth of vertical nanostructures, wherein an additional step (C)—a chemically inert Ar plasma or a reactive H2 plasma—is added to the original two-step (AB-type) PEALD process. This approach reduces the vertical nanostructure density by 80%. It was confirmed that suppression of vertical nanostructures goes hand in hand with grain size enhancement. The vertical nanostructure density reduction consequently lowers film resistivity by an order of magnitude. Insights obtained in this work can contribute toward devising additional pathways, besides plasma treatments, for suppressing the growth of vertical nanostructures and improving the material properties of 2D TMDs that are relevant for nanoelectronic device applications.

(Invited) ALD and ALEt of Transition Metal Dichalcogenide Materials

Ultra-thin layer of two dimensional (2D) materials mainly transition metal dichalcogenide (TMD) semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in optoelectronic, energy, and sensor applications. At present, research on 2D-layered TMDs mostly rely on (i) flakes produced by exfoliation from bulk, (ii) synthesis by high temperature (800-1000oC) mostly by chemical vapor deposition or (iii) solution based methods. Although exciting results are being obtained, but all these methods have limitations such as poor quality of the material, lack of precise control of thickness, interface or ability to integrate them into complex 3D devices. Nevertheless, the biggest challenge in realizing their full potential has been the lack of doable material synthesis methods of such films with high uniformity, conformality and interfacing with other materials such as oxides and metals. Among the various thin film growth methods, atomic layer deposition (ALD) offers the best combination of a controlled layer-by-layer growth of the material with high conformality and suitable integration with other oxides such as dielectrics and metals. At the same time, precisely control thermal-based etching method for 2D-TMD materials are needed. Secondly at present, there are tremendous attention and efforts on development of the thermal vapor based well control etching methods especially on atomic layer etching (ALEt). The successful combination of both ALD and ALEt of 2D-TMD can pave the way to control the synthesis of 2D-TMD layers over large areas. This will be important for 2D-TMD material integration for future successful large-scale complex device structures fabrication. Here we will first discuss the ALD growth of 2D-TMDs (e.g. MoS2, HfS2 and MgS2) on metals (e.g. Mo, W and TiN), dielectric oxides (e.g. Al2O3, MgO, HfO2, and TiO2) and vice versa. Then precise control etching processes based on molybdenum hexafluoride (MoF6) and H2O vapor will be discussed. The MoF6-H2O precursors based etching process offer several advantages including low cost, low processing temperature, rapid and effective etching spanning the range from continuous etching to atomic layer etching (ALEt). We have used in-situ quartz crystal microbalance (QCM) and Fourier-transform infrared spectroscopy (FTIR) measurements to monitor the deposition and etching 2D-TMDs layers. Next, the deposited and etched 2D-TMDs thin films were analyzed by spectroscopic ellipsometry to determine the thickness and refractive index, and the composition was determined by X-ray photoelectron spectroscopy (XPS). These ex-situ measurements confirmed the 2D-TMDs deposition and etching behavior findings from our in-situ studies.

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures.

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD-grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

Controlled Vapor Growth and Nonlinear Optical Applications of Large‐Area 3R Phase WS2 and WSe2 Atomic Layers

Abstract2D layered 3‐rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large‐area WS2 and WSe2 atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large‐area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and time‐resolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization‐resolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS2, which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.

Band Alignment of MoTe2/MoS2 Nanocomposite Films for Enhanced Nonlinear Optical Performance

AbstractBand alignment is a key issue for the optoelectronics based on 2D layered transition metal dichalcogenides (TMDs) heterostructures. Herein, band alignment of MoTe2/MoS2 mixed heterostructure is measured with high‐resolution X‐ray photoelectron spectroscopy. The MoTe2/MoS2 heterostructure belongs to type‐II heterostructure with the conduction band offset of 0.46 eV and the valence band offset of 0.9 eV. The stronger saturable absorption is observed in MoTe2/MoS2 heterostructure film compared with that of pure MoTe2 and MoS2 nanofilms at the same condition. An energy‐level model combined with Runge–Kutta algorithm is used to understand the enhancement mechanism. It is found that the interlayer transition from MoTe2/MoS2 heterojunction plays an important role in the nonlinear optical enhancement. Meanwhile, band structure of MoTe2/MoS2 heterostructure is calculated by the first principles. The contributions of the MoTe2 and MoS2 to the heterojunction are almost equal and the valence band maximum and conduction band minimum exist in MoTe2 and MoS2 separately. This structure can form the interlayer carriers easily. The results suggest that the band alignment of TMDs paves the way for the type‐II heterostructure for enhanced nonlinear response in the development of optical modulator, ultrafast laser mode lockers, saturable absorbers, and optical switches.

Temperature dependant electronic charge transport characteristics at MX2 (M = Mo, W; X = S, Se)/Si heterojunction devices

Nanosheets of two dimensional (2D) transition metal dichalcogenides (WS2, MoS2 and WSe2) have been synthesized by two stage solvothermal mediated sonochemical exfoliation method. The as-synthesized high quality and stable dispersions in the form of few layer nanosheets were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy. The nanosheets were deposited on p-type silicon wafer to from WS2/p-Si, MoS2/p-Si, and WSe2/p-Si, heterojunction diodes. Temperature dependent transport properties and conduction behavior were analyzed using I–V characteristics. Thermionic emission transport model with T0 affected current transport mechanism across the junction was found as the most possible current transport model for all the three prepared diodes. The results provide an easy and large area preparation of 2D layered transition metal dichalcogenide semiconductors for unique electro-optical applications.

Electronic structure and exciton shifts in Sb-doped MoS2 monolayer

The effective manipulation of excitons is important for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this. While studies have shown that 2D semiconductors are ideal for excitonic devices, preparation of homogenous substitutional foreign-atom-doped 2D crystals is still difficult. Here we report the preparation of homogenous monolayer Sb-doped MoS2 single crystals via a facile chemical vapor deposition method. A and B excitons are observed in the Sb-doped MoS2 monolayer by reflection magnetic circular dichroism spectrum measurements. More important, compared with monolayer MoS2, the peak positions of two excitons show obvious shifts. Meanwhile, the degeneration of A exciton is also observed in the monolayer Sb-doped MoS2 crystal using photoluminescence spectroscopy, which is ascribed to the impurity energy levels within the band-gap, confirmed by density function theory. Our study opens a door to developing the doping of 2D layered transition metal dichalcogenides with group-V dopants, which is helpful for the fundamental study of the physical and chemical properties of transition metal dichalcogenides.

Effect of Ru Doping on the Properties of MoSe2 Nanoflowers

MoSe2 is a 2D layered transition metal dichalcogenide that has attracted much attention because its properties may be easily altered by both morphology control and doping by substitutional transition metals. Here, the study of Ru-doped MoSe2 nanoflowers is presented, and the effect of Ru doping on their optical, electronic, and catalytic properties is presented. A significant enhancement in their catalytic properties toward the hydrogen evolution reaction (HER) is evident, showing an overpotential as low as 143 mV (at 10 mA cm–2) for samples by substituting 11.4% of the Mo with Ru. In order to gain understanding of the dopants’ interaction with the host and the nature of the atomic-scale substrate for the catalytic reaction, density functional theory (DFT) calculations are employed to trace the modulation of the density of states (DOS) near the Fermi level and to model possible dopant sites. The Ru dopants have two additional d electrons and a high DOS near the Fermi level. The optical absorption spectra ...

Towards efficient photon management in nanostructured solar cells: Role of 2D layered transition metal dichalcogenide semiconductors

Photovoltaic devices are the source for future energy where the naturally abundant solar energy is converted into electricity using photoactive materials. An efficient photon management mechanism is essential for achieving high efficiency next generation solar cells. The use semiconducting two dimensional (2D) transition metal dichalcogenide (TMDCs) nanosheets light absorbing layer in modern solar cell is inevitable. Diverse large area synthesis tools to tailor nanosheets and its incorporation in solar cell cover the entire solar spectrum for enhanced photon management. In this review, the case to case use of TMDCs in various photovoltaic devices as an efficient absorber layer has been discussed. Experimental measures to overcome the limitations of pristine TMDC layers due to its atomic layer thickness were also reviewed. It can be seen that, the device performance of the TMDCs based solar cells is showing significant improvement in comparison with conversional silicon based photovoltaics.

Ion Beam Defect Engineering on ReS2/Si Photocathode with Significantly Enhanced Hydrogen Evolution Reaction

AbstractLoading 2D layered transition metal dichalcogenides (TMDs) on p‐type silicon photocathode is suitable for hydrogen production in solar‐driven photoelectrochemical (PEC) water splitting. Similarly, various nanostructured TMDs exposing more active sites are widely explored for improving the PEC performances of composite photoelectrodes. Here, defect engineering using a controllable argon ion beam bombardment is presented on ReS2/Si photocathode. The atomic vacancy defects are introduced on the 2D ReS2 to realize high‐density active sites, which significantly enhance the solar‐driven hydrogen evolution reaction performance of ReS2/Si photocathode. The highest photocurrent density of 18.5 mA cm−2 (at 0 V vs reversible hydrogen electrode) is achieved, under a simulated sun irradiation.

Resistive switching behaviors mediated by grain boundaries in one longitudinal Al/MoS2&PVP/ITO device

Nonvolatile memory devices based on 2D layered transition metal dichalcogenides have inspired tremendous amounts of research interests in recent years. In this work, resistive switching (RS) devices based on hybrid molybdenum disulphide and Polyvinylpyrrolidone (MoS2&PVP) is explored. One longitudinal Al/MoS2&PVP/ITO device exhibits three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and threshold switching (TS), which can be explained by the formation and rupture of conducting filaments (CFs) consisting of sulfur vacancy filaments in MoS2 nanoflakes and sulfur ions filaments in PVP. These CFs form from three different kinds of grain boundaries (GBs), intersecting-GB, bisecting-GB, and bridge-GB in which GBs attach one electrode, parallel to the two electrodes, and attach both electrodes, respectively, corresponding to the three modes of I-V characteristics. These three kinds of GBs coexist in one device and the CF forms randomly from one kind of them during sweeping loops. The mechanism was experimentally proved by our results of the increasing probability of typical bipolar RS in device with one layer of PVP thin film inserted into the interface of MoS2&PVP and ITO and other researchers’ results in all-surface MoS2 single-layer MoS2 devices.

Mobility Deception in Nanoscale Transistors: An Untold Contact Story

Mobility is a critical parameter that is routinely used for benchmarking the performance of field-effect transistors (FETs) based on novel nanomaterials. In fact, mobility values are often used to champion nanomaterials since high-performance devices necessitate high mobility values. The current belief is that the contacts can only limit the FET performance and hence the extracted mobility is an underestimation of the true channel mobility. However, here, such misconception is challenged through rigorous experimental effort, backed by numerical simulations, to demonstrate that overestimation of mobility occurs in commonly used geometries and in nanomaterials for which the contact interface, contact doping, and contact geometry play a pivotal role. In particular, dual-gated FETs based on multilayer MoS2 and WSe2 are used as case studies in order to elucidate and differentiate between intrinsic and extrinsic contact effects manifesting in the mobility extraction. The choice of 2D layered transition metal dichalcogenides (TMDCs) as the semiconducting channel is motivated by their potential to replace and/or coexist with Si-based aging FET technologies. However, the results are equally applicable to nanotube- and nanowire-based FETs, oxide semiconductors, and organic-material-based thin-film FETs.

Understanding the electrochemical reaction mechanism of VS2 nanosheets in lithium-ion cells by multiple in situ and ex situ x-ray spectroscopy

Recently, 2D layered transition metal dichalcogenides (TMDs) have attracted great scientific interest in electrochemical energy storage research. Vanadium disulfide (VS2) as an important family member of TMDs, is a promising electrode material for lithium-ion cells because of its remarkable electrical conductivity and high Li+ diffusion rate, but its electrochemical reaction mechanism is still poorly understood. Herein, we have prepared VS2 nanosheets as the electrode and systematically investigated its structural and chemical evolution during the electrochemical processes by employing both in situ and ex situ x-ray spectroscopy. The VS2 undergoes intercalation and conversion reactions in sequence during discharge and this process is found to be partially reversible during the subsequent charge. The decreased reversibility of the conversion reaction over extended cycles could be mainly responsible for the capacity fading of the VS2 electrode. In addition, the hybridization strength between S and V shows a strong dependence on the states of charge, as directly illustrated by the intensity change of the V–S hybridized states and pure V states. We have also found that the solid electrolyte interphase on the electrode surface is dynamically evolved during cycling, which may be a universal phenomenon for the conversion-based electrodes. This study is expected to be beneficial for the further development of high-performance VS2-based electrodes.

Ab Initio Calculations on the Electronic Structure and Photocatalytic Properties of Two‐Dimensional WS2 (0001) Nanolayers of Varying Thickness

2D layered transition metal dichalcogenides (TMDC) structurized in hexagonal 2H c phase possess space group P63/mmc and graphite‐type morphology (e.g., WS2 bulk), as well as strong chemical bonds and weak coupling between S‐W‐S three‐plane monolayers (MLs). Ab initio calculations on pristine WS2 (0001) nanolayers are performed using CRYSTAL14 code within the formalism of hybrid density functional theory and Hartree–Fock method (HSE06 Hamiltonian properly adapted to describe tungsten disulphide bulk). An ordered stack of graphene‐type WS2 (0001)‐oriented MLs (in absence of any point defects including dopants) with thikness varies from 1 to 40 MLs is found to be suitable for photocatalysis since irrespectively of this thickness the bandgap corresponds to the varied range of visible spectrum between its violet and red edges, respectively. The authors calculate the electronic structure of these nanolayers. The highest photocatalytic efficiency is achieved for WS2 2 ML with band gap △ϵgap ≈ 2.06 eV (yellow range of the visible spectrum). Additionally, all the edges of ordered pristine 2H c WS2 (0001) nanosheets (ϵVB and ϵCB levels corresponding to the top of the valence band and the bottom of the conduction band) are properly aligned relative to the oxidation and reduction potentials ( and ): ϵVB < < < ϵCB, which means that probability of photocatalytic electron‐hole recombination is rather minimal, being reduced with a decrease of stack thickness.

(Invited) Atomic Layer Deposition and Etching of 2D Transition Metal Dichalcogenide Materials

Ultra-thin layer of two dimensional (2D) materials mainly transition metal dichalcogenide (TMD) semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in optoelectronic, energy, and sensor applications. At present, research on 2D-layered TMDs mostly rely on (i) flakes produced by exfoliation from bulk, (ii) synthesis by high temperature (800-1000oC) mostly by chemical vapor deposition or (iii) solution based methods. Although exciting results are being obtained, but all these methods have limitations such as poor quality of the material, lack of precise control of thickness, interface or ability to integrate them into complex 3D devices. Nevertheless, the biggest challenge in realizing their full potential has been the lack of doable material synthesis methods of such films with high uniformity, conformality and interfacing with other materials such as oxides and metals. Among the various thin film growth methods, atomic layer deposition (ALD) offers the best combination of a controlled layer-by-layer growth of the material with high conformality and suitable integration with other oxides such as dielectrics and metals. At the same time, precisely control thermal-based etching method for 2D-TMD materials are needed. Secondly at present, there are tremendous attention and efforts on development of the thermal vapor based well control etching methods especially on atomic layer etching (ALEt). The successful combination of both ALD and ALEt of 2D-TMD can pave the way to control the synthesis of 2D-TMD layers over large areas. This will be important for 2D-TMD material integration for future successful large-scale complex device structures fabrication. Here we will first discuss the ALD growth of 2D-TMDs (e.g. MoS2, HfS2 and MgS2) on metals (e.g. Mo, W and TiN), dielectric oxides (e.g. Al2O3, MgO, HfO2, and TiO2) and vice versa. Then precise control etching processes based on molybdenum hexafluoride (MoF6) and H2O vapor will be discussed. The MoF6-H2O precursors based etching process offer several advantages including low cost, low processing temperature, rapid and effective etching spanning the range from continuous etching to atomic layer etching (ALEt). We have used in-situ quartz crystal microbalance (QCM) and Fourier-transform infrared spectroscopy (FTIR) measurements to monitor the deposition and etching 2D-TMDs layers. Next, the deposited and etched 2D-TMDs thin films were analyzed by spectroscopic ellipsometry to determine the thickness and refractive index, and the composition was determined by X-ray photoelectron spectroscopy (XPS). These ex-situ measurements confirmed the 2D-TMDs deposition and etching behavior findings from our in-situ studies.

(Invited) 2D MoS2 Memristors with Variable Switching Characteristics

Memristors has been extensively studied as an important candidate device structure for realizing non-Von Neumann computers, neuromorphic and analog computing systems. The conventional memristors are made from transition metal oxides such as TiOx, TaOx, WOx, and HfOx.1 The conductance states of such memristors can be adjusted through modulating the distribution of oxygen vacancies in the oxide channels. Recently, memristor-like transport behaviors have been also observed in the electronic devices made from emerging 2D-layered transition metal dichalcogenides (TMDCs, such as MoS2 and WSe2).2 Such emerging 2D-material-based memristors could be used for constructing novel neural networks with a significantly improved level of connectivity and enabling bio-realistic emulation of brain neuron functions. Therefore, it is highly desirable to further perform nanofabrication and nanoelectronics research to fully explore the applicability and advantage of 2D-TMDC-based memristors for practical neuromorphic applications. Here, we present a study on the nanofabrication and characterization of memristive devices based on mechanically printed few-layer MoS2 structures. This work shows that mechanically printed MoS2 memristors with critical thicknesses exhibit a very low threshold electric field for initiating memristive-switching processes, which is essential for the successful construction and operation of energy-efficient memristor-based neural networks. More importantly, such MoS2 memristors exhibit both analogue (i.e., gradient) and discrete switching characteristics, which could be further exploited for making reconfigurable memristors for both analog and digital computing applications. This work leveraged the unique transport properties of 2D layered MoS2 for memristor-related applications and advanced the scientific/technical knowledge for controlling memristive switching behaviors.

Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides.

Layered transition metal dichalcogenides (TMDs) offer monolayer 2D systems with diverse properties that extend beyond what graphene alone can achieve. The properties of TMDs are heavily influenced by the atomic structure and in particular imperfects in the crystallinity in the form of vacancy defects, grain boundaries, cracks, impurity dopants, ripples and edge terminations. This review will cover the current knowledge of the detailed structural forms of some of the most intensively studied 2D TMDs, such as MoS2, WSe2, MoTe2, WTe2, NbSe2, PtSe2, and also covers MXenes. The review will utilize results achieved using state-of-the-art aberration corrected transmission electron microscopy, including annular dark-field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS), showing how elemental discrimination can be achieved to understand structure at a deep level. The review will also cover the impact of single atom substitutional dopants, such as Cr, V and Mn, and electron energy loss spectroscopy used to understand the local bonding configuration. It is expected that this review will provide an atomic level understanding of 2D TMDs with a connection to imperfections that can arise from chemical vapour deposition synthesis, intentional doping, rips and tears, dislocations, strain, polycrystallinity and confinement to nanoribbons.

Asymmetrically Encapsulated Vertical ITO/MoS2 /Cu2 O Photodetector with Ultrahigh Sensitivity.

Strong light absorption, coupled with moderate carrier transport properties, makes 2D layered transition metal dichalcogenide semiconductors promising candidates for low intensity photodetection applications. However, the performance of these devices is severely bottlenecked by slow response with persistent photocurrent due to long lived charge trapping, and nonreliable characteristics due to undesirable ambience and substrate effects. Here ultrahigh specific detectivity (D*) of 3.2 × 1014 Jones and responsivity (R) of 5.77 × 104 A W-1 are demonstrated at an optical power density (Pop ) of 0.26 W m-2 and external bias (Vext ) of -0.5 V in an indium tin oxide/MoS2 /copper oxide/Au vertical multi-heterojunction photodetector exhibiting small carrier transit time. The active MoS2 layer being encapsulated by carrier collection layers allows us to achieve repeatable characteristics over large number of cycles with negligible trap assisted persistent photocurrent. A large D* > 1014 Jones at zero external bias is also achieved due to the built-in field of the asymmetric photodetector. Benchmarking the performance against existing reports in literature shows a viable pathway for achieving reliable and highly sensitive photodetectors for ultralow intensity photodetection applications.

General Strategy for Two-Dimensional Transition Metal Dichalcogenides by Ion Exchange

The ability to control and vary the atomic compositions of two-dimensional (2D) layered semiconductors is of considerable importance for tailoring their electronic and optoelectronic properties. The current methods to tailor chemical composition of 2D layered semiconductors is largely limited to vapor phase chemistry, and solution phase chemistry is insufficiently explored to date. Here, we report a general approach to prepare 2D layered transition metal dichalcogenides (TMDs) by ion exchange reactions in solution phase. By choosing four typical layered metal dichalcogenides including MoS2, MoSe2, WS2 and SnS2 as representative cases, the feasibility and versatility of both cation and anion exchange reactions in layered metal dichalcogenides are confirmed. Transient absorption results indicate that exciton lifetime of these samples as excitation energy is increased. The optoelectronic properties of these TMD nanosheets after the ion exchange exhibit potential for future devices. Our strategy can be employ...

Nature of carrier injection in metal/2D-semiconductor interface and its implications for the limits of contact resistance

Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. Till date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (Rc), particularly in the monolayer limit. In this work, we use a combination of theory and experiments in Au and Ni contacted monolayer MoS2 device to conclude the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers - the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multi-dimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.