Large-area MoS2 Research Articles

Controlling Gold-Assisted Exfoliation of Large-Area MoS2 Monolayers with External Pressure.

Gold-assisted exfoliation can fabricate centimeter- or larger-sized monolayers of van der Waals (vdW) semiconductors, which is desirable for their applications in electronic and optoelectronic devices. However, there is still a lack of control over the exfoliation processes and a limited understanding of the atomic-scale mechanisms. Here, we tune the MoS2-Au interface using controlled external pressure and reveal two atomic-scale prerequisites for successfully producing large-area monolayers of MoS2. The first is the formation of strong MoS2-Au interactions to anchor the top MoS2 monolayer to the Au surface. The second is the integrity of the covalent network of the monolayer, as the majority of the monolayer is non-anchored and relies on the covalent network to be exfoliated from the bulk MoS2. Applying pressure or using smoother Au films increases the MoS2-Au interaction, but may cause the covalent network of the MoS2 monolayer to break due to excessive lateral strain, resulting in nearly zero exfoliation yield. Scanning tunneling microscopy measurements of the MoS2 monolayer-covered Au show that even the smallest atomic-scale imperfections can disrupt the MoS2-Au interaction. These findings can be used to develop new strategies for fabricating vdW monolayers through metal-assisted exfoliation, such as in cases involving patterned or non-uniform surfaces.

Influence of High-κ Dielectrics Integration on ALD-Based MoS2 Field-Effect Transistor Performance.

The integration of high-κ dielectrics on MoS2 field-effect transistors (FETs) is essential for the realization of MoS2 in ultrascaled nanoelectronic devices and circuits. Most studies covering this topic are based on exfoliated MoS2 flakes or chemical vapor deposition (CVD) grown MoS2 films, whereas other techniques, such as atomic layer deposition (ALD), are also gaining attention for the growth of MoS2 in recent years. In this work, we grow large-area MoS2 by means of plasma-enhanced (PE-)ALD and evaluate the influence of high-κ dielectrics on the properties of ALD-based MoS2 FETs through electrical characterization combined with surface-chemical and high-resolution scanning transmission electron microscopy (HR-STEM) analyses. We grow HfO x , AlO x , or both by means of PE-ALD or thermal ALD on our fabricated devices and show that, in addition to the dielectric constant, three other major parameters related to the processing of the dielectrics can simultaneously affect the MoS2 FET electrical characteristics and govern its doping. These parameters are the stoichiometry of the dielectric, its carbon impurity content, and the degree to which the MoS2 surface oxidizes upon the dielectric growth. When grown at 100 °C, our HfO x films are oxygen-vacant whereas our AlO x films are oxygen-rich. In addition, carbon impurities are incorporated into the dielectrics at low deposition temperatures, being one of the likely causes of the MoS2 FET overall n-type performance in all of the studied cases. Our investigations also reveal that PE-ALD of HfO x or AlO x oxidizes the MoS2 surface, whereas thermal ALD AlO x leaves MoS2 almost intact. In this respect, if thermal ALD AlO x of proper thickness is grown between MoS2 and HfO x , it can reduce the degree to which the MoS2 surface oxidizes by HfO x and meanwhile improve the total dielectric constant, altogether leading to the most optimal electrical performance in ALD-based MoS2 FETs.

(Invited) 2D Semiconductors from Spin-on Molecular Chemistries for Direct Large-Scale Integration

In this talk, I will discuss our recent developments centered on a process to produce fully soluble solutions from a unique molecular chemistry. This enables the creation of wafer-scale monolayers of MoS2, a prominent 2D semiconductor for logic and memory devices. This molecule can be dissolved in common solvents and spin-coated onto various substrates, including crystalline substrates, printed electrodes, and polymers. It can also be integrated into gate-all-around (GAA) 3D transistor structures for logic devices. Our method of rapidly synthesizing large-area MoS2 films, particularly in monolayer form via spin-coating, represents a novelty, as it hasn't been demonstrated previously with a single-source chemistry. This advancement is a significant step toward scalable synthesis of wafer-scale 2D semiconductor thin films, eliminating the need for post-growth wafer transfer techniques, a requirement with films grown by chemical vapor deposition (CVD) methods. Our fabrication process highlights the potential of using a single-source chemical precursor to develop monolayer 2D semiconductors that exhibit commendable transport characteristics and optical properties, which enhance with rising crystallization temperatures. Consequently, it holds promise as a semiconductor for microelectronic transistor channels in both back-end-of-line (BEOL) and front-end-of-line (FEOL) architectures.

Controlled epitaxial growth of strain-induced large-area bilayer MoS2 by chemical vapor deposition based on two-stage strategy

Two-dimensional (2D) bilayer transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their promising applications in the fields of electronics, optoelectronics, valleytronics and nonlinear optics. However, the precise synthesis of large-area, high-yield and uniform bilayer MoS2 semiconductors remains a significant challenge. Herein, we have developed one two-stage chemical vapor deposition strategy based on strain engineering, enabling the controlled preparation of large-area (4 × 6 cm2) bilayer MoS2 nanostructures. Systematic characterizations indicate that compressive strain was introduced during the growth of first layer MoS2, which effectively induces the synthesis of the second layer MoS2. First-principles calculations based on density functional theory further reveal the mechanism of strain induced controllable growth of bilayer MoS2. Field-effect transistors based on AA and AB stacking bilayer MoS2 have been fabricated exhibiting excellent electronic properties. Our work provides a new pathway for the precise preparation of bilayer TMDCs nanostructures, offering experimental support for their application in the field of electronic and optoelectronic devices.

Molybdenum Disulfide-Assisted Spontaneous Formation of Multistacked Gold Nanoparticles for Deep Learning-Integrated Surface-Enhanced Raman Scattering.

Several fabrication methods have been developed for label-free detection in various fields. However, fabricating high-density and highly ordered nanoscale architectures by using soluble processes remains a challenge. Herein, we report a biosensing platform that integrates deep learning with surface-enhanced Raman scattering (SERS), featuring large-area, close-packed three-dimensional (3D) architectures of molybdenum disulfide (MoS2)-assisted gold nanoparticles (AuNPs) for the on-site screening of coronavirus disease (COVID-19) using human tears. Some AuNPs are spontaneously synthesized without a reducing agent because the electrons induced on the semiconductor surface reduce gold ions when the Fermi level of MoS2 and the gold electrolyte reach equilibrium. With the addition of polyvinylpyrrolidone, a two-dimensional large-area MoS2 layer assisted in the formation of close-packed 3D multistacked AuNP structures, resembling electroless plating. This platform, with a convolutional neural network-based deep learning model, achieved outstanding SERS performance at subterascale levels despite the microlevel irradiation power and millisecond-level acquisition time and accurately assessed susceptibility to COVID-19. These results suggest that our platform has the potential for rapid, low-damage, and high-throughput label-free detection of exceedingly low analyte concentrations.

Two-step process for the fabrication of direct FLG\\MoS2 heterostructures

MoS2 has proven its efficacy in flexible electronics, transistor devices, and various biological and chemical applications. However, it is still challenging to achieve large-area MoS2 monolayers with desired material quality and electrical properties to fulfill the requirement for practical applications. Moreover, the main strategy for the preparation of a 2D heterostructure it is based on the sequential stacking of the layered materials using wet or dry transfer methods which introduces many defects. This paper presents an economically viable and straightforward two-step methodology to obtain MoS2 thin films, encompassing magnetron sputtering deposition of Mo and subsequent annealing in a sulfur-rich environment. This approach successfully yielded MoS2 thin films on Si\\SiO2 substrates. Additionally, heterostructures consisting of few layer graphene (FLG) and MoS2 were directly obtained using the same method. The utilization of grazing incidence X-ray diffraction verified the formation of the hexagonal MoS2 phase, a finding further confirmed by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) investigations revealed the successful sulfurization process, with surface-bound oxides forming only subsequent to air exposure. Comprehensive assessment involving X-ray reflectivity, atomic force microscopy and XPS collectively inferred the fabrication of thin films comprised of a small number of MoS2 layers covering the entire substrate. Electrical assessments exhibited an electrical hysteresis, demonstrating its potential for memristor applications. Overall, this study outlines a cost-effective fabrication method for producing nanoscale MoS2 thin films with excellent properties, avoiding the use of toxic gases such as H2S. These findings contribute to the potential development of cutting-edge applications.

CVD growth of large-area, continuous, and defect-free MoS2 multilayer films from solution-cast seed nanoflakes

Defects in transition metal dichalcogenides are among the primary source of their chemical reactivity, including their response to water. The CVD growth of defect-free continuous molybdenum disulfide (MoS2) films has been particularly challenging. In this work, we demonstrate the growth of large-area continuous films of MoS2 using solution-cast MoS2 seed nanoflakes, both with and without the assistance of alkali halide. The MoS2 seed nanoflakes serve as nucleation sites for the in-plane growth of few-layer MoS2, which is accompanied by localized vertical growth of MoS2 particles. For films grown with the combination of seed and NaCl, very low oxygen content was observed together with the stoichiometric ratio of Mo: S being close to the ideal value of 1: 2. These films show humidity-insensitive electrical response over a wide interval (5%–95% RH). This is attributed to very low concentrations of edge and basal-plane defects, leading to low water dissociation and restricted proton transport.

Recombination dynamics and manybody effect of excitons in large-area monolayer MoS2 capped with (111) NiO epitaxial layer

CVD grown monolayer MoS2 films on c-sapphire substrates are vacuum annealed and capped with (111) NiO epitaxial films using pulsed laser deposition technique. Time, energy and polarization resolved optical techniques are used to understand the effect of capping on the excitonic properties of the monolayer MoS2. It has been observed that trion contribution in the photoluminescence (PL) spectra increases after the capping, suggesting an enhancement of electron concentration in the conduction band. This has been attributed to the capping driven reduction of physisorbed air molecules from the sulphur vacancy ( VS ) sites. Note that the air molecules act as passivating agents for the VS -donors. Low temperature polarization resolved PL spectroscopy and ultrafast pump and probe transient absorption spectroscopy (TAS) show an increase of the biexcitonic population in the system after the encapsulation. The TAS study further reveals longer lifetime for both A and B excitons in capped samples implying a reduction of non-radiative recombination rate of the excitons after the capping. It has also been observed that in the capped samples, K/K′ valleys are populated with trions under sufficiently high pump powers. This has been attributed to the lower non-radiative recombination rates of the high energy photoexcited carriers and the faster transfer of either electrons or holes from the high energy pockets to the K/K′ valleys. The study further reveals different many-body effects in excitons and trions.

Large area MoSe2 and MoSe2/Bi2Se3 films on sapphire (0001) for near-infrared photodetection

The fabrication of heterojunction-based photodetectors (PDs) is well known for the enhancement of PDs performances, tunable nature of photoconductivity, and broadband application. Herein, the PDs based on MoSe2 and MoSe2/Bi2Se3 heterojunction on sapphire (0001) substrates were deposited using a r.f. magnetron sputtering system. The high-resolution x-ray diffraction and Raman spectroscopy characterizations disclosed the growth of the 2-H phase of MoSe2 and the rhombohedral phase of Bi2Se3 thin films on sapphire (0001). The chemical and electronic states of deposited films were studied using x-ray photoelectron spectroscopy and revealed the stoichiometry growth of MoSe2. We have fabricated metal-semiconductor–metal type PD devices on MoSe2 and MoSe2/Bi2Se3 heterojunction and the photo-response measurements were performed at external voltages of 0.1–5 V under near-infrared (1064 nm) light illumination. The bare MoSe2 PD device shows positive photoconductivity behavior whereas MoSe2/Bi2Se3 heterojunction PD exhibits negative photoconductivity. It was found that the responsivity of MoSe2 and MoSe2/Bi2Se3 heterojunction PDs is ~ 1.39 A W−1 and ~ 5.7 A W−1, respectively. The enhancement of photoresponse of MoSe2/Bi2Se3 PD nearly four-fold compared to bare MoSe2 PD shows the importance of heterojunction structures for futuristics optoelectronic applications.

Nonconventional Strain Engineering for Uniform Biaxial Tensile Strain in MoS2 Thin Film Transistors.

Strain engineering has been employed as a crucial technique to enhance the electrical properties of semiconductors, especially in Si transistor technologies. Recent theoretical investigations have suggested that strain engineering can also markedly enhance the carrier mobility of two-dimensional (2D) transition-metal dichalcogenides (TMDs). The conventional methods used in strain engineering for Si and other bulk semiconductors are difficult to adapt to ultrathin 2D TMDs. Here, we report a strain engineering approach to apply the biaxial tensile strain to MoS2. Metal-organic chemical vapour deposition (MOCVD)-grown large-area MoS2 films were transferred onto SiO2/Si substrate, followed by the selective removal of the underneath Si. The release of compressive residual stress in the oxide layer induces strain in MoS2 on top of the SiO2 layer. The amount of strain can be precisely controlled by the thickness of oxide stressors. After the transistors were fabricated with strained MoS2 films, the array of strained transistors was transferred onto plastic substrates. This process ensured that the MoS2 channels maintained a consistent tensile strain value across a large area.

Excitonic Rydberg States in a Trilayer to Monolayer H2-Aided CVD-Grown Large-Area MoS2 Film with Excellent UV to Visible Broad Band Photodetection Applications.

The diverse nature of optoelectronic properties of few-layer or monolayer MoS2 is generally dominated by A and B excitons. Occasionally, strong Coulombic interactions within the 2D monolayer led to the creation of hydrogen-like Rydberg states of excitons in MoS2 similar to other 2D monolayers. In this paper, a simple process is used to convert trilayer MoS2 films to a monolayer by introducing H2 gas during chemical vapor deposition. Remarkably, alongside the usual A, B excitons, and A- trion, the appearance of the Rydberg states is evidenced by photoluminescence spectra even at room temperature; also, there is an increase in their areal percentage with an increase in H2 content. The s-type excited Rydberg states up to the fourth order (n = 5) and third order (n = 4) of A and B excitons, respectively, have been probed from the photoluminescence spectra at 93 K. Unprecedentedly, the first-order derivative of room-temperature photocurrent spectrum reveals the Rydberg states concurrently and elaboratively. Furthermore, the large-area MoS2 films exhibit photoresponse in a broad UV to visible region with excellent photosensitivity (∼102) toward both UV and visible lights. Not only does this provide a profound understanding of the excitonic Rydberg states but also highlights the considerable potential of large-area monolayer MoS2 overcoming the difficulty of tiny flake-related 2D device endeavors.

Direct liquid injection pulsed-pressure MOCVD of large area MoS2 on Si/SiO2.

Large-scale, high-quality growth of transition metal dichalcogenides (TMD) of controlled thickness is paramount for many applications in opto- and microelectronics. This paper describes the direct growth of well-controlled large area molybdenum disulfide (MoS2) on Si/SiO2 substrates by direct liquid injection pulsed-pressure metal-organic chemical vapor deposition (DLI-PP-MOCVD) using low-toxicity precursors. It is shown that control of the deposited thickness can be achieved by carefully tuning the amount of molybdenum precursor evaporated and that continuous layers are routinely obtained. Homogeneity and reproducibility have also been examined, as well as the average size of the grains. When targeting monolayer thickness, the MoS2 showed near stoichiometry (S/Mo = 1.93-1.95), low roughness and high photoluminescence (PL) quantum yield, equivalent to exfoliated monolayers and CVD MoS2 grown on the same substrates.

Large area van der Waals MoS2–WS2 heterostructures for visible-light energy conversion

Large area MoS2–WS2 van der Waals heterostructures have been fabricated using a physical deposition process. The type-II band alignment promotes electro-optical photoconversion and enhances organic pollutant photodissociation.

Exploring kinetic control for the growth of layered MoS2(1-x)Se2x alloy and its electrocatalytic activity in hydrogen evolution reaction

Although controlled growth of monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) alloys has recently been achieved, a comprehensive understanding of their growth mechanism remains a major challenge due to the complex nucleation kinetics. In this study, the synthesis of large-area ternary MoS2(1-x)Se2x alloys by atmospheric pressure chemical vapor deposition (CVD) is effectively and efficiently controlled. Both experimental and theoretical investigations are conducted to explore the influence of precursors on growth kinetics. The results suggest that the introduction of Se not only reduces nucleation sites but also lowers diffusion energy, thereby promoting lateral growth of the alloy, which is supported by density functional theory (DFT) calculations. Furthermore, the electrocatalytic performance of MoS2 film with discretely distributed triangle sheets is further enhanced by the incorporation of Se atoms during the hydrogen evolution reaction (HER). The Gibbs free energy of adsorbed hydrogen atom (ΔGH*) has been calculated by DFT, showing that the replacement of S with Se at the edge site compared with the in-plane site would significantly reduce the Gibbs free energy. This study is crucial for improving the comprehension of the growth dynamics of TMD alloys and promoting their utilization in electrocatalytic hydrogen generation.

Negative capacitance field-effect transistors based on ferroelectric AlScN and 2D MoS2

Al0.68Sc0.32N (AlScN) has gained attention for its outstanding ferroelectric properties, including a high coercive field and high remnant polarization. Although AlScN-based ferroelectric field-effect transistors (FETs) for memory applications have been demonstrated, a device for logic applications with minimal hysteresis has not been reported. This study reports on the transport characteristics of a MoS2 negative capacitance FET (NCFET) based on an AlScN ferroelectric material. We experimentally demonstrate the effect of a dielectric layer in the gate stack on the memory window and subthreshold swing (SS) of the NCFET. We show that the hysteresis behavior of transfer characteristics in the NCFET can be minimized with the inclusion of a non-ferroelectric dielectric layer, which fulfills the capacitance-matching condition. Remarkably, we also observe the NC effect in MoS2/AlScN NCFETs arrays based on large-area monolayer MoS2 synthesized by chemical vapor deposition, showing the SS values smaller than its thermionic limit (∼36 to 60 mV/dec) and minimal variation in threshold voltages (<20 mV).

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition.

In this paper, we present the preparation of few-layer MoS2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (ns from ~1 × 1013 to ~3.4 × 1013 cm-2 from T = 300 K to 500 K), with a donor ionization energy of Ei = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS2/GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of ϕB ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS2 on GaN in electronics/optoelectronics.

Toolbox of Advanced Atomic Layer Deposition Processes for Tailoring Large-Area MoS2 Thin Films at 150 °C.

Two-dimensional MoS2 is a promising material for applications, including electronics and electrocatalysis. However, scalable methods capable of depositing MoS2 at low temperatures are scarce. Herein, we present a toolbox of advanced plasma-enhanced atomic layer deposition (ALD) processes, producing wafer-scale polycrystalline MoS2 films of accurately controlled thickness. Our ALD processes are based on two individually controlled plasma exposures, one optimized for deposition and the other for modification. In this way, film properties can be tailored toward different applications at a very low deposition temperature of 150 °C. For the modification step, either H2 or Ar plasma can be used to combat excess sulfur incorporation and crystallize the films. Using H2 plasma, a higher degree of crystallinity compared with other reported low-temperature processes is achieved. Applying H2 plasma steps periodically instead of every ALD cycle allows for control of the morphology and enables deposition of smooth, polycrystalline MoS2 films. Using an Ar plasma instead, more disordered MoS2 films are deposited, which show promise for the electrochemical hydrogen evolution reaction. For electronics, our processes enable control of the carrier density from 6 × 1016 to 2 × 1021 cm-3 with Hall mobilities up to 0.3 cm2 V-1 s-1. The process toolbox forms a basis for rational design of low-temperature transition metal dichalcogenide deposition processes compatible with a range of substrates and applications.

Vertical growth of compact and large-area MoS2 nanosheet arrays on Ti3C2Tx for efficient bifunctional electrochemical energy storage and hydrogen evolution

The developments in hydrogen evolution reaction (HER) and supercapacitor technologies for electrochemical energy storage and conversion have received considerable attention. Although MoS2 is electrochemically active for both HER and supercapacitors, limited active sites, slow ionic transport, and poor conductivity lead to its poor capacitance and electrocatalytic activity. Herein, hierarchical Ti3C2Tx/MoS2/Ti3C2Tx@CC (TMT@CC) composites were well-designed as electrodes for both HER and supercapacitors. Flexible TMT@CC electrodes with an area as large as ∼ 80 cm2 and optimal mass-loading of 17.9 mg cm−2 were achieved. The inner layer Ti3C2Tx in the composites provides ideal nucleation sites for the growth of MoS2 arrays, and the outermost Ti3C2Tx effectively anchors the vertically arrayed MoS2. The hierarchically vertical structure provides strong interfacial coupling and shortens ion diffusion paths, leading to high stability and fast ion/electron transport kinetics. Due to such a synergistic effect, the flexible binder-free TMT@CC electrodes exhibited high areal capacitance (5.06 F cm−2 at 5 mA cm−2) for supercapacitors and low overpotential (119 mV versus RHE at 10 mA cm−2) for HER catalyst. Furthermore, a high energy density of 0.125 mWh cm−2 at a power density of 1.5 mW cm−2 has been achieved from the TMT@CC-based symmetric supercapacitor. Our strategy can be expanded to other vertically arrayed hierarchical structures as electrode materials of efficient HER and supercapacitors.

CVD Synthesis of MoS2 Using a Direct MoO2 Precursor: A Study on the Effects of Growth Temperature on Precursor Diffusion and Morphology Evolutions.

In this study, the influence of growth temperature variation on the synthesis of MoS2 using a direct MoO2 precursor was investigated. The research showed that the growth temperature had a strong impact on the resulting morphologies. Below 650 °C, no nucleation or growth of MoS2 occurred. The optimal growth temperature for producing continuous MoS2 films without intermediate-state formation was approximately 760 °C. However, when the growth temperatures exceeded 800 °C, a transition from pure MoS2 to predominantly intermediate states was observed. This was attributed to enhanced diffusion of the precursor at higher temperatures, which reduced the local S:Mo ratio. The diffusion equation was analyzed, showing how the diffusion coefficient, diffusion length, and concentration gradients varied with temperature, consistent with the experimental observations. This study also investigated the impact of increasing the MoO2 precursor amount, resulting in the formation of multilayer MoS2 domains at the outermost growth zones. These findings provide valuable insights into the growth criteria for the effective synthesis of clean and large-area MoS2, thereby facilitating its application in semiconductors and related industries.

CVD synthesis of monolayer MoS2 using Na compounds as additives to enhance two-dimensional growth

Abstract Molybdenum disulfide (MoS2) is two-dimensional transition metal dichalcogenides that has garnered significant interest as a potential alternative to graphene. The synthesis of large-area monolayer MoS2 via CVD is highly anticipated. To investigate the growth-promoting effect of sodium, NaCl and Na2SO4 were used as additives for CVD synthesis, as they have a similar mp to the commonly used MoO3 precursor. The increase in the domain size of monolayer MoS2 with using Na compounds was confirmed through X-ray photoelectron spectroscopy analysis. The results suggested that the growth of MoS2 may have been promoted by the reaction of Na with S. Moreover, it was found that monolayer MoS2 with improved crystallinity and larger domain size could be synthesized using Na2SO4 as an additive compared to NaCl.