MoS2 Flakes Research Articles

Understanding Substrate Effects on 2D MoS2 Growth: A Kinetic Monte Carlo Approach

AbstractControlling the morphology of 2D transition metal dichalcogenides (TMDs) plays a key role in their applications. Although chemical vapor deposition can achieve wafer‐scale growth of 2D TMDs, a comprehensive theoretical framework for effective growth optimization is lacking. Atomistic modeling methods offer a promising approach to delve into the intricate dynamics underlying the growth. In this study, kinetic Monte Carlo (kMC) simulations are employed to identify crucial parameters that govern the morphology of MoS2 flakes grown on diverse substrates. The simulations reveal that large adsorption rates significantly enhance growth speed, which however necessitates rapid edge migration to achieve compact triangles. Substrate etching can tune the adsorption–desorption process of adatoms and enable preferential growth within a specific substrate region, controlling the flake morphology. This study unravels the complex dynamics governing 2D TMD morphology, offering a theoretical framework for decision‐making in the design and optimization of TMD synthesis processes.

Optical Signal Investigation of Monolayer MoS2 Grown Via Glass-Assisted CVD On Patterned Surfaces

Enhancing photoluminescence (PL) in single-layer transition metal dichalcogenides has garnered significant interest, particularly for advancing high-performance 2D electronics and optoelectronics. The combination of surface engineering and contemporary growth methods has provided a platform for investigating optical signals. In this study, we present variations in PL and Raman signals of single-layer MoS2 flakes grown conformally using the glass-assisted CVD method on square-patterned surfaces with varying well depths. PL spectroscopy revealed a systematic and pronounced enhancement in intensities as the valley thickness decreased from 285 nm to 225 nm. Conversely, for the hill regions of the samples, the PL intensity initially increased with decreasing valley thickness and then decreased, despite the hill regions having a constant thickness of 300 nm. On the other hand, PL maps did not exhibit a systematic dependence of intensities on the hill-valley thickness distinction, contrary to expected results based on literature data for similar materials on flat surfaces. The origin of the intensity oscillations was attributed to possible mechanisms, including thickness-dependent interference and strain-related exciton funneling effects. Additionally, Raman measurements revealed irregular variations in intensity in hill regions, dependent on the thicknesses of the underlying SiO2 layers. Furthermore, we observed that the sizes of the flakes increased as the well depths of the underlying patterned surface decreased. This phenomenon might be attributed to alterations in the carrier gas flow pattern and varying temperature gradients between the hills and valleys. These results hold substantial potential to open new avenues for the integration of 2D transition metal dichalcogenides into on-chip electronic and optoelectronic devices.

Gold Nano-colloids impregnated in Langmuir-Blodgett Film of MoS2 flakes as SERS active platform: Fabrication and its application in Malathion detection

The current work focuses on fabrication of Langmuir-Blodgett (LB) films of MoS2 flakes with impregnated gold nano particles (AuNPs) as a noble Surface enhanced Raman Scattering (SERS) active hetero structure. The hot spots created over the heterostructures work as a potent agent for localization of electromagnetic field resulting in high enhancement of Raman Signals. The LB film of MoS2 flakes with and without the AuNPs were thoroughly characterized in our present work. The efficacy of Au–MoS2 substrate as a SERS sensing platform was investigated up to ultrasensitive concentrations using Raman probe molecules. Moreover, the reproducibility and homogeneity of the substrate was also tested with Raman mapping. The as prepared SERS sensing platform was engaged further for detection of Malathion at trace concentrations. The tests to check on service time was also performed. In the contemporary exploration the fabricated substrate reveals its accuracy and effectivity as a SERS sensor. Thus, in future this substrate can be a novel “Nano-Lab on Chip” device for ultrasensitive detections of chemical and bio-chemical composites.

Knot Architecture for Biocompatible and Semiconducting 2D Electronic Fiber Transistors.

Wearable devices have generally been rigid due to their reliance on silicon-based technologies, while future wearables will utilize flexible components for example transistors within microprocessors to manage data. Two-dimensional (2D) semiconducting flakes have yet to be investigated in fiber transistors but can offer a route toward high-mobility, biocompatible, and flexible fiber-based devices. Here, the electrochemical exfoliation of semiconducting 2D flakes of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) is shown to achieve homogeneous coatings onto the surface of polyester fibers. The high aspect ratio (>100) of the flake yields aligned and conformal flake-to-flake junctions on polyester fibers enabling transistors with mobilities μ ≈1 cm2 V-1 s-1 and a current on/off ratio, Ion/Ioff ≈102-104. Furthermore, the cytotoxic effects of the MoS2 and WSe2 flakes with human keratinocyte cells are investigated and found to be biocompatible. As an additional step, a unique transistor 'knot' architecture is created by leveraging the fiber diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (≈100 µm) and utilize it to make a MoS2 fiber transistor with a human hair that achieves mobilities as high as μ ≈15 cm2 V-1 s-1.

Silver induced chirality controlled spin filtration observed in ss-DNA functionalized with MoS2.

Chiral molecules can exhibit strong spin-orbit coupling, which can result in a large spin polarization. This is due to the fact that the energy levels of the electrons in a chiral molecule are strongly influenced by the chiral structure of the molecule, which can result in the separation of the energy levels for electrons with different spin orientations. We report a controlled spin-selective transmission of electrons through 20 base-paired poly-cytosine molecules functionalized with MoS2 flakes on ITO glass via the quantum mechanical tunneling effect. A reversion in spin polarization was observed after the silver ions interact with poly-cytosine due to the strong coordination of Ag(I) with cytosine-cytosine (C-C) mismatches, indicating the formation of duplex structural motifs, as confirmed by the circular dichroism spectroscopy at room temperature. Manipulating the spin of an electron through such a small molecule merely controlled by special cations could pave the way for major advances in spin-independent charge transport, advanced bioanalytical system design, and related applications.

Determining by Raman spectroscopy the average thickness and N-layer-specific surface coverages of MoS2 thin films with domains much smaller than the laser spot size.

Raman spectroscopy is a widely used technique to characterize nanomaterials because of its convenience, non-destructiveness, and sensitivity to materials change. The primary purpose of this work is to determine via Raman spectroscopy the average thickness of MoS2 thin films synthesized by direct liquid injection pulsed-pressure chemical vapor deposition (DLI-PP-CVD). Such samples are constituted of nanoflakes (with a lateral size of typically 50 nm, i.e., well below the laser spot size), with possibly a distribution of thicknesses and twist angles between stacked layers. As an essential preliminary, we first reassess the applicability of different Raman criteria to determine the thicknesses (or layer number, N) of MoS2 flakes from measurements performed on reference samples, namely well-characterized mechanically exfoliated or standard chemical vapor deposition MoS2 large flakes deposited on 90 ± 6 nm SiO2 on Si substrates. Then, we discuss the applicability of the same criteria for significantly different DLI-PP-CVD MoS2 samples with average thicknesses ranging from sub-monolayer up to three layers. Finally, an original procedure based on the measurement of the intensity of the layer breathing modes is proposed to evaluate the surface coverage for each N (i.e., the ratio between the surface covered by exactly N layers and the total surface) in DLI-PP-CVD MoS2 samples.

Identification and Structural Characterization of Twisted Atomically Thin Bilayer Materials by Deep Learning.

Two-dimensional materials are expected to play an important role in next-generation electronics and optoelectronic devices. Recently, twisted bilayer graphene and transition metal dichalcogenides have attracted significant attention due to their unique physical properties and potential applications. In this study, we describe the use of optical microscopy to collect the color space of chemical vapor deposition (CVD) of molybdenum disulfide (MoS2) and the application of a semantic segmentation convolutional neural network (CNN) to accurately and rapidly identify thicknesses of MoS2 flakes. A second CNN model is trained to provide precise predictions on the twist angle of CVD-grown bilayer flakes. This model harnessed a data set comprising over 10,000 synthetic images, encompassing geometries spanning from hexagonal to triangular shapes. Subsequent validation of the deep learning predictions on twist angles was executed through the second harmonic generation and Raman spectroscopy. Our results introduce a scalable methodology for automated inspection of twisted atomically thin CVD-grown bilayers.

PEG-PVP-Assisted Hydrothermal Synthesis and Electrochemical Performance of N-Doped MoS2/C Composites as Anode Material for Lithium-Ion Batteries.

Molybdenum disulfide shows promise as an anode material for lithium-ion batteries. However, its commercial potential has been constrained due to the poor conductivity and significant volume expansion during the charge/discharge cycles. To address these issues, in this study, N-doped MoS2/C composites (NMC) were prepared via an enhanced hydrothermal method, using ammonium molybdate and thiourea as molybdenum and sulfur sources, respectively. Polyethylene glycol 400 (PEG400) and polyvinylpyrrolidone (PVP) were added in the hydrothermal procedure as soft template surfactants and nitrogen/carbon sources. The crystal structure, morphology, elemental composition, and surface valence state of the N-doped MoS2/C composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS), respectively. The results indicate that the NMC prepared by this method are spherical particles with a nanoflower-like structure composed of MoS2 flakes, having an average particle size of about 500 nm. XPS analysis shows the existence of C and N elements in the samples as C-N, C-C, and pyrrolic N. As anodes for LIBs, the NMC without annealing deliver an initial discharge capacity of 548.2 mAh·g-1 at a current density of 500 mA·g-1. However, this capacity decays in the following cycles with a discharge capacity of 66.4 mAh·g-1 and a capacity retention rate of only 12% after 50 cycles. In contrast, the electrochemical properties of the counterparts are enhanced after annealing, which exhibits an initial discharge capacity of 575.9 mAh·g-1 and an ultimate discharge capacity of 669.2 mAh·g-1 after 70 cycles. The capacity retention rate decreases initially but later increases and elevated afterward to reach 116% at the 70th cycle, indicating an improvement in charge-discharge performance. The specimens after annealing have a smaller impedance, which indicates better charge transport and lithium-ion diffusion performance.

Novel Bi-Functional MoS2/α-Fe2O3 Nanocomposites for High Photocatalytic Performance

In this study, 3-dimensional molybdenum disulfide (MoS2) structures, integrated with hematite (α-Fe2O3) nanoparticles, were fabricated under a convenient two-step hydrothermal route. The fabricated photocatalytic nanocomposites consist of well-arranged MoS2 flakes, resembling spherical flower-like morphology, and the nanoparticulate α-Fe2O3 structures decorate the 3D network. By raising the α-Fe2O3 weight ratio, the composites’ specific surface area and morphology were not affected, regardless of the partial cover of the cavities for higher hematite content. Moreover, the crystallinity examination with XRD, Raman, and FTIR techniques revealed that the precursor reagents were fully transformed to well-crystalized MoS2 and Fe2O3 composites of high purity, as no organic or inorganic residues could be detected. The photocatalytic oxidation and reduction performance of these composites was evaluated against the tetracycline pharmaceutical and the industrial pollutant hexavalent chromium, respectively. The improvement in the removal efficiencies demonstrates that the superior photoactivity originates from the high crystallinity and homogeneity of the composite, in combination with the enhanced charge carriers’ separation in the semiconductors’ interface.

Photoluminescence enhancement of chemical vapor-deposited MoSe2 monolayers

The transition metal dichalcogenide (TMDC) materials have attracted a great interest owing to their superior features. The chemical vapor deposition (CVD) is very feasible to synthesis of TMDC materials. The thermal expansion coefficient difference between TMDC materials and substrate in the CVD synthesis can cause a strain, resulting in non-radiative recombination and PL decrement for these materials. Therefore, in this work, we studied acetone and isopropanol (IPA) treatment on the photoluminescence (PL) properties of CVD-grown MoSe2 monolayers. Firstly, monolayer MoSe2 flakes on SiO2/Si substrates were synthesized via the CVD method by optimizing synthesis parameters. The Raman and PL measurements were taken after acetone and IPA treatments were applied to grown samples. The results indicated an important PL enhancement was seen for acetone treatment. For IPA treatment, there was a PL peak position shifting with decreasing intensity due to its possible structural damage. The acetone, IPA, and transfer processes caused the releasing strain on MoSe2 by breaking the strong interaction between MoSe2 and substrate. For the whole treatment, PL peaks shift to blue with about 80 meV. As a result of the present study, acetone treatment was found as an easy and quick way to enhance radiative emission of CVD-grown MoSe2.

Monolayer-like Exciton Recombination Dynamics of Multilayer MoSe2 Observed by Pump-Probe Microscopy.

Transition metal dichalcogenides (TMDCs) have garnered considerable interest over the past decade as a class of semiconducting layered materials. Most studies on the carrier dynamics in these materials have focused on the monolayer due to its direct bandgap, strong photoluminescence, and strongly bound excitons. However, a comparative understanding of the carrier dynamics in multilayer (e.g., >10 layers) flakes is still absent. Recent computational studies have suggested that excitons in bulk TMDCs are confined to individual layers, leading to room-temperature stable exciton populations. Using this new context, we explore the carrier dynamics in MoSe2 flakes that are between ∼16 and ∼125 layers thick. We assign the kinetics to exciton-exciton annihilation (EEA) and Shockley-Read-Hall recombination of free carriers. Interestingly, the average observed EEA rate constant (0.003 cm2/s) is nearly independent of flake thickness and 2 orders of magnitude smaller than that of an unencapsulated monolayer (0.33 cm2/s) but very similar to values observed in encapsulated monolayers. Thus, we posit that strong intralayer interactions minimize the effect of layer thickness on recombination dynamics, causing the multilayer to behave like the monolayer and exhibit an apparent EEA rate intrinsic to MoSe2.

Vertical heterostructure of graphite-MoS2 for gas sensing.

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

Changes of phonon modes and electron transfer induced by interface interactions of Pd/MoS2 heterostructures.

As functional materials and nano-catalysts, Pd nanoparticles (NPs) are often used to modify two-dimensional (2D) materials. In the heterostructures of metal NPs and 2D transition metal dichalcogenides, the interface atomic configuration and interface effect greatly affect material properties and stability. Therefore, the rational design of interface structures and in-depth analysis of interface interactions are of vital importance for the preparation of specific functional devices. In this work, Pd NPs were deposited on mechanically exfoliated MoS2 flakes and the epitaxial relationship between Pd and MoS2 was observed, accompanied by distinct moiré patterns. Raman spectra of the Pd NPs/MoS2 heterostructure showed an E12g' vibration mode indicative of the local strain in MoS2. A new vibration mode A'1g appeared in the higher-frequency direction compared with the pristine A1g peak. Combined with X-ray photoelectron spectra and density functional theory calculations, the new vibration mode can be attributed to the bonding between Pd and MoS2. Besides, graphene was inserted between Pd NPs and MoS2, and the decoupling of the interfacial effect by graphene was investigated. This study will help deepen our understanding on the interaction mechanism between metals and MoS2, thereby enabling the modulation of optoelectronic properties and the performance of these hybrid materials.

In-situ methyl red doped MoS2 field effect transistor made by atomically thin MoS2 channel

Here we present the effects of Methyl Red (MR) doping on the electrical properties of the MoS2 field effect transistor (FET) in in-situ experimental system where the doping and electrical measurement are carried out in a single ultra-high vacuum chamber. A single-layered MoS2 flake was used as channel material for the FET preparation. N-type doping by MR was confirmed by the left shift of threshold voltage (Vth) and the surface vibrational change measured by Raman spectroscopy. The n-doping by MR is implied by the Fermi level shift towards greater binding energy with MR coverage, as proven by an X-ray photoelectron spectroscopy (XPS) measurement. It is estimated from XPS that the MoS2-FET device may detect the doping effect from the sub-monolayer MR. MR was detected on the MoS2 surface by the Time-of-Flight Secondary Ionization Mass Spectroscopy (ToF-SIMS). The findings may pave the way for innovations in electronic device design, with potential implications for fields ranging from nanoelectronics to sensor technology.

Physical vapor deposition-free scalable high-efficiency electrical contacts to MoS2

Fermi-level pinning caused by the kinetic damage during metallization has been recognized as one of the major reasons for the non-ideal behavior of electrical contacts, forbidding reaching the Schottky–Mott limit. In this manuscript, we present a scalable technique wherein Indium, a low-work-function metal, is diffused to contact a few-layered MoS2 flake. The technique exploits a smooth outflow of Indium over gold electrodes to make edge contacts to pre-transferred MoS2 flakes. We compare the performance of three pairs of contacts made onto the same MoS2 flake, the bottom-gold, top-gold, and Indium contacts, and find that the Indium contacts are superior to other contacts. The Indium contacts maintain linear I–V characteristics down to cryogenic temperatures with an extracted Schottky barrier height of ∼2.1 meV. First-principle calculations show the induced in-gap states close to the Fermi level, and the damage-free contact interface could be the reason for the nearly Ohmic behavior of the Indium/MoS2 interface.

Induced exfoliation by growing Cu1.96S nanoparticles on the edge defect sites of MoS2 and enhancing photothermal conversion

The composite structures may integrate advantages of each species and afford new functionalized materials with fascinating properties for specific applications. In this study, a novel approach is proposed to prepare MoS2/Cu1.96S composite structures through shear-assisted exfoliation and edge-defect-assisted exfoliation. A simple relationship between the edge defect of MoS2 flakes, the growth of Cu1.96S and facilitating the exfoliation of MoS2 is proposed and supported by the experimental results. The shear-assisted exfoliation process facilitates the exfoliation of MoS2, which enables the growth of Cu1.96S at the edge defect sites of MoS2 flakes. As Cu1.96S forms at these sites, further exfoliation of MoS2 occurs, obtaining MoS2/Cu1.96S composite structures. The adsorption behavior of Cu atoms at different edge defect sites of MoS2 flakes is studied using density functional theory, and a growth model is proposed to explain the exfoliation mechanism of MoS2 flakes. Moreover, the prepared MoS2/Cu1.96S composite structures show higher photothermal conversion efficiency than MoS2 flakes. This improvement is attributed to lattice vibration arising from phonon emission and a pronounced extinction band caused by the collective oscillation of free holes induced by copper vacancies. This finding opens a new pathway towards for advanced fabrications of complex or composite structures.

Exceptionally high-rate production of few-layer MoS2 by a scalable and cost-effective gas-driven shear exfoliation method and its application for supercapacitors

Two-dimensional (2D) few-layer molybdenum disulfide (MoS2) is a versatile future electronic material with fascinating unique properties, but the commercialization of its’ applications is still hampered by high-rate production of few-layer MoS2 flakes on low-cost. Herein, we present a simple, scalable and cost-effective gas-driven shear exfoliation method for producing high-aspect-ratio few-layer MoS2 flakes with exceptionally high production rate. The gas-driven exfoliated MoS2 flakes, with an average thickness of 3 nm, show a reasonable large lateral dimension, with more than 10% of the flakes >1.0 μm and ∼73 % of the flakes in the range of 0.3–1.0 μm. In a lab-scale setup, the production rate reaches an impressive high value of 1.23 g/h, while the exfoliation energy consumption is as low as 0.175 kW⋅h/g. At an industrial level, the production rate can exceed 3 kg/h, marking a significant advancement towards the mass production of few-layer MoS2 on low-cost. As a proof-of-concept, the as-exfoliated few-layer MoS2 was employed as an active material for supercapacitor, which exhibited a good electrochemical performance with the specific capacity of 101.8 mAh/g. After 5000 cycles, the capacitance retention sustained at a high level of 92.9%.

Heterostructured organic/MoS2 nanowall with synergistic SERS enhancement enabling direct and sensitive detection of contaminants

In recent years, the π-conjugated organic molecular system has attracted considerable attention for its surface-enhanced Raman scattering (SERS) properties. However, the practical application of π-conjugated organic materials as SERS substrates is limited due to the inefficient dissociation of excitons, weak enhancement, and poor reproducibility. Herein, we proposed a novel nanowall SERS heterocomposite based on 3D D(C7CO)-BTBT nanowalls and 2D few-layer MoS2 flakes. A significant SERS activity of the D(C7CO)-BTBT/MoS2 nanowall heterocomposite can be achieved by the charge transfer (CT) resonance enabled by the π-conjugated interactions between D(C7CO)-BTBT and MoS2 and the electromagnetic field spatial distribution induced by the D(C7CO)-BTBT nanowalls. The concentration limit of detection (LOD) of the methylene blue (MB) molecules absorbed on the heterostructured D(C7CO)-BTBT/MoS2 nanowall substrate is as low as 8.2 × 10−11 M, which is excellent among the reported organic-based substrates. A practical SERS detection for the 100 nm polystyrene (PS) nanoparticles on the D(C7CO)-BTBT/MoS2 substrates can be down to 4.5 × 105 ng/L, which is exceptional for the real-time monitoring of emerging contaminants based on SERS technology. This research not only provides a new SERS enhancement strategy for organic-based materials, but also may be helpful in exploring a potential candidate for SERS detection of emerging contaminants.

Toward an understanding of the mechanism of mixed-salt-mediated CVD growth of MoSe2

The use of liquid precursors in chemical vapor deposition (CVD) techniques is advantageous for growing large-area, uniform two-dimensional (2D) transition-metal dichalcogenides (TMDs) compared to conventional methods using solid precursors. While various liquid precursors have been explored, recent studies highlight the use of mixed-salt precursors for growing uniform and wafer-scale TMDs. In this study, we propose a growth mechanism and present our findings on the epitaxial growth of MoSe2 domains as a function of annealing/growth time and H2 flow rate using Na2MoO4 and Na2SeO3 mixed-salt precursors. We confirm that the increase in the annealing time enhances the distribution of spin-coated precursors, leading to a rise in flake number density. On the other hand, prolonged growth time results in better-aligned MoSe2 flakes along the c-sapphire substrate step-edges. A significant finding is the dynamic diffusion of dendritic structures within large domains over the growth period, owing to the constant dissolution and recrystallization in the presence of residual liquid alloys. An increase in the H2 flow during CVD growth yields small, triangular domains aligned with the step edges, a result of the efficient reduction of precursor alloys and subsequent selenization. Our results provide an insight on achieving uniform and aligned morphology in CVD growth of 2D TMDs using liquid-phase precursors, a crucial step toward large-area fabrication.

Multiplex Portable Biosensor for Bacteria Detection.

An advanced, cost-effective, and portable DNA biosensor capable of detecting multiple bacteria simultaneously has been developed. The biosensor comprises a fast and inexpensive potentiostat that controls the applied potential to a screen-printed electrochemical array platform functionalized with MoS2 flakes and bacterial DNA probes. The current response obtained by à la carte thionine functionalized carbon nanodots (Ty-CDs) is monitored as an electrochemical indicator of the hybridization event. The design of the potentiostat prioritizes achieving an optimal signal-to-noise ratio and incorporates a user-friendly interface compatible with various devices, including computers, mobile phones, and tablets. The device is compact, lightweight, and manufactured at a low cost. The key components of the potentiostat include a data acquisition board capable of analyzing multiple samples simultaneously and a controller board. The results of this study confirm the ability of the multiplex portable biosensor to successfully detect specific bacterial DNA sequences, demonstrating its reliability and superior performance compared with a traditional, more complex, and laboratory-oriented potentiostat.