CVD-grown Monolayer MoS2 Research Articles

Influence of seeding promoters on the properties of CVD grown monolayer molybdenum disulfide

Chemical vapor deposition (CVD) is the most efficient method to grow large-area two dimensional (2D) transition metal dichiacogenides (TMDCs) in high quality. Monolayer molybdenum disulfide (MoS2) and seed-assistant are the mostly selected 2D TMDC and growth strategy for such CVD processes, respectively. Though the advantages of seed catalysts in facilitating the nucleation, achieving higher yield and better repeatability, as well as their effects on the morphologies of as-grown MoS2 have been studied, the influence of seeding promoters on both optical and electrical properties of as-grown monolayer MoS2 is not known comprehensively, which is indeed critical for understanding fundamental physics and developing practical application of such emerging 2D semiconductors. In this report, we systematically investigated the effect of different seeding promoters on the properties of CVD-grown monolayer MoS2. It is found that different seed molecules lead to different impacts on the optical and electrical properties of as-grown monolayer MoS2. Among three different seed catalysts (perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), copper phthalocyanine (CuPc), and crystal violet (CV)), PTAS performs better in obtaining large area monolayer MoS2 with good optical quality and high electrical mobility than the other two. Our work gives a guide for modifying the properties of as-grown monolayer MoS2 and other 2D transition metal dichalcogenides in seeding promoters-assisted synthesis process.

Anomalous lattice vibrations of CVD-grown monolayer MoS2 probed using linear polarized excitation light.

A novel physical phenomenon and advanced application have been explored in 2D low-dimensional van der Waals layered materials due to their reduced in-plane symmetry. The light-matter interaction is observed upon rapid characterization of the 2D material's crystal orientation. Here, the effects of the sample's rotation angle and the incident light's linear polarization angle on the Raman scattering of chemical vapor deposition (CVD)-grown monolayer MoS2 were investigated. The results show that the crystal orientation of monolayer MoS2 can be distinguished by analyzing the intensity ratio and frequency difference of its two dominant Raman vibration modes. In addition, an increase in the incident light's power intensity causes the Raman peaks to red shift due to the photothermal effect. Strikingly, it was found that, with an increase in the incident linear polarization angle, the out-of-plane A1g phonon mode red shifts, while the in-plane E2g1 phonon mode blue shifts. The frequency difference consequently decreases from 19.5 cm-1 to 17.4 cm-1. The anomalous lattice vibrations of monolayer MoS2 originate from the built-in strain introduced by the SiO2/Si substrate. This work paves the way for the investigation and characterization of 2D MoS2, providing further understanding of the light-matter interaction in 2D materials, which is beneficial for advanced studies on anisotropic MoS2 based electronic and photoelectric information technologies and sensing applications.

Effects of contact material on complex excitonic behaviour of monolayer MoS2

We investigated the optical behaviour of excitons in monolayer MoS2 prepared on a 60 nm-thick metal (Au) ring structure patterned on SiO2. CVD-grown monolayer MoS2 was transferred to the metal structure and placed in contact with three different environments simultaneously: SiO2, Au metal and air. The optical properties of the transferred MoS2 on the substrates were characterized through micro-photoluminescence (μ-PL) measurements. PL peaks were observed near ∼1.82 eV for defect-related emission, ∼1.92 eV for an A exciton, which includes a neutral exciton and a trion, and ∼2.1 eV for a B exciton. The optical band gap and exciton-phonon interaction of the samples on the three substrates were investigated by measuring the temperature-dependent photoluminescence.

CVD-grown monolayer MoS2 in bioabsorbable electronics and biosensors

Transient electronics represents an emerging technology whose defining feature is an ability to dissolve, disintegrate or otherwise physically disappear in a controlled manner. Envisioned applications include resorbable/degradable biomedical implants, hardware-secure memory devices, and zero-impact environmental sensors. 2D materials may have essential roles in these systems due to their unique mechanical, thermal, electrical, and optical properties. Here, we study the bioabsorption of CVD-grown monolayer MoS2, including long-term cytotoxicity and immunological biocompatibility evaluations in biofluids and tissues of live animal models. The results show that MoS2 undergoes hydrolysis slowly in aqueous solutions without adverse biological effects. We also present a class of MoS2-based bioabsorbable and multi-functional sensor for intracranial monitoring of pressure, temperature, strain, and motion in animal models. Such technology offers specific, clinically relevant roles in diagnostic/therapeutic functions during recovery from traumatic brain injury. Our findings support the broader use of 2D materials in transient electronics and qualitatively expand the design options in other areas.

Gas adsorbates are Coulomb scatterers, rather than neutral ones, in a monolayer MoS2 field effect transistor

Direct current (DC) and low-frequency (LF) noise analyses of a chemical vapor deposition (CVD)-grown monolayer MoS2 field effect transistor (FET) indicate that time-varying carrier perturbations originate from gas adsorbates. The LF noise analysis supports that the natural desorption of physisorbed gas molecules, water and oxygen, largely reduces the interface trap density (NST) under vacuum conditions (∼10-8 Torr) for 2 weeks. After a longer period of 8 months under vacuum, the carrier scattering mechanism alters, in particular for the low carrier density (Nacc) region. A decrease of both NST and the scattering parameter αSC with desorption of surface adsorbates from MoS2, explains the enhanced carrier mobility and the early turn-on of the device. The stabilized carrier behavior is verified with γ = 0.5 in the formula αSC ∝ Nacc-γ, as in Si-MOSFETs. Our results support that the gas adsorbates work as charged impurities, rather than neutral ones.

Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors

Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS2 induce significant effects on the electrical and low frequency noise characteristics of the MoS2. Here, we investigated the electrical properties and noise characteristics of MoS2 field effect transistors (FETs) made with CVD-grown monolayer MoS2. The electrical and noise characteristics of MoS2 FETs were analysed and compared for the MoS2 channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS2 FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

Substrate-induced strain and charge doping in CVD-grown monolayer MoS2

Due to its electronic-grade quality and potential for scalability, two-dimensional (2D) MoS2 synthesized by chemical vapor deposition (CVD) has been widely explored for electronic/optoelectronic applications. As 2D MoS2 can be considered a 100% surface, its unique intrinsic properties are inevitably altered by the substrate upon which it is grown. However, systematic studies of substrate-layer interactions in CVD-grown MoS2 are lacking. In this study, we have analyzed built-in strain and charge doping using Raman and photoluminescence spectroscopy in 2D MoS2 grown by CVD on four unique substrates: SiO2/Si, sapphire, Muscovite mica, and hexagonal boron nitride. We observed decreasing strain and charge doping in grown MoS2 as the substrates become less rough and more chemically inert. The possible origin of strain was investigated through atomic force microscopy roughness measurements of the as-grown layer and substrate. Our results provide direction for device optimization through careful selection of the growth substrate and pave the way for further investigations to unravel the complex nature of the 2D monolayer-substrate interface.

Ambient effects on electrical characteristics of CVD-grown monolayer MoS2 field-effect transistors

Monolayer materials are sensitive to their environment because all of the atoms are at their surface. We investigate how exposure to the environment affects the electrical properties of CVD-grown monolayer MoS2 by monitoring electrical parameters of MoS2 field-effect transistors as their environment is changed from atmosphere to high vacuum. The mobility increases and contact resistance decreases simultaneously as either the pressure is reduced or the sample is annealed in vacuum. We see a previously unobserved, non-monotonic change in threshold voltage with decreasing pressure. This result could be explained by charge transfer on the MoS2 channel and Schottky contact formation due to adsorbates at the interface between the gold contacts and MoS2. Additionally, from our electrical measurements it is plausible to infer that at room temperature and pressure water and oxygen molecules adsorbed on the surface act as interface traps and scattering centers with a density of several 1012 cm−2 eV−1, degrading the electrical properties of monolayer MoS2.

Orientation-specific transgranular fracture behavior of CVD-grown monolayer MoS2 single crystal

In recent times, there has been a rapidly growing interest in fracture behavior of two-dimensional materials since it is crucial for device performances. Here, we report an orientation-specific transgranular fracture behavior of a CVD-grown monolayer MoS2 single crystal investigated by various means. The underlying mechanism proposed that micro-cracks nucleated at sulfur vacancies propagate along the energy-favored zigzag directions upon fast quenching induced thermal strain, which results in an orientation-specific fracture behavior. The corresponding photoluminescence characteristic peaks undergo a blue-shift by ∼165 meV, suggesting compressive strain resided, in sharp contrast to tensile strain in a normal CVD-grown MoS2 single crystal. In addition, the combined photoemission electron microscopy and kelvin force microscopy results show the obvious surface potential variation between fractured MoS2 microflakes in some regions, ascribed to inhomogeneous interactions between MoS2 and the underlying substrate. The results reported here deepen the understanding of the fracture behavior of monolayer single crystalline MoS2, which is also adoptable in other transition metal dichalcogenide materials.

In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2

We perform in-situ two-cycle thermal cycling and annealing studies for a transferred CVD-grown monolayer MoS2 on a SiO2/Si substrate, using spatially resolved micro-Raman and PL spectroscopy. After the thermal cycling and being annealed at 305 deg C twice, the film morphology and film-substrate bonding are significantly modified, which together with the removal of polymer residues cause major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the PL peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the film-substrate bonding, and thus the strain and doping, is unstable under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.

Analysis of the interface characteristics of CVD-grown monolayer MoS2 by noise measurements

We investigated the current–voltage and noise characteristics of two-dimensional (2D) monolayer molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD). A large number of trap states were produced during the CVD process of synthesizing MoS2, resulting in a disordered monolayer MoS2 system. The interface trap density between CVD-grown MoS2 and silicon dioxide was extracted from the McWhorter surface noise model. Notably, generation–recombination noise which is attributed to charge trap states was observed at the low carrier density regime. The relation between the temperature and resistance following the power law of a 2D inverted-random void model supports the idea that disordered CVD-grown monolayer MoS2 can be analyzed using a percolation theory. This study can offer a viewpoint to interpret synthesized low-dimensional materials as highly disordered systems.

Effective shape-controlled growth of monolayer MoS2 flakes by powder-based chemical vapor deposition

In the recent years, transition-metal dichalcogenides such as MoS2 have attracted considerable attention owing to their unique structure and electronic properties. Chemical vapor deposition (CVD) is a popular method for producing MoS2 flakes with different shapes. Here, we report an effective method for achieving a broad range of shape evolution in CVD-grown monolayer MoS2 flakes. By controlling the S and MoO3 temperatures, the shape of the monolayer MoS2 flakes was varied from hexagonal to triangular via intermediate shapes such as truncated and multi-apex triangles. The shape evolution of the MoS2 flakes can be explained by introducing the term “nominal Mo:S ratio”, which refers to the amount of loaded MoO3 and evaporated S powders. By using the nominal Mo:S ratio, we predicted the potential reaction atmosphere and effectively controlled the actual proportion of MoO3−x with respect to S in the growth region, along with the growth temperature. From the systematic investigation of the behavior of the shape evolution, we developed a shape-evolution diagram, which can be used as a practical guide for producing CVD-grown MoS2 flakes with desired shapes.

Enhancement of carrier mobility in MoS2 field effect transistors by a SiO2 protective layer

Molybdenum disulfide is a promising channel material for field effect transistors (FETs). In this paper, monolayer MoS2 grown by chemical vapor deposition (CVD) was used to fabricate top-gate FETs through standard optical lithography. During the fabrication process, charged impurities and interface states are introduced, and the photoresist is not removed cleanly, which both limit the carrier mobility and the source-drain current. We apply a SiO2 protective layer, which is deposited on the surface of MoS2, in order to avoid the MoS2 directly contacting with the photoresist and the ambient environment. Therefore, the contact property between the MoS2 and the electrodes is improved, and the Coulomb scattering caused by the charged impurities and the interface states is reduced. Comparing MoS2 FETs with and without a SiO2 protective layer, the SiO2 protective layer is found to enhance the characteristics of the MoS2 FETs, including transfer and output characteristics. A high mobility of ∼42.3 cm2/V s is achieved, which is very large among the top-gate CVD-grown monolayer MoS2 FETs.

(Invited) Mechanical Energy Harvesting Using Triboelectric/Piezoelectric Properties from Two-Dimensional Materials

Graphene has not been used as an active material in energy harvesting devices and systems. Recently, a new type of power-generating device that converts mechanical energy into electricity using triboelectricity was intensively studied. The triboelectric effect depends on various factors, such as the electron affinity, work function, friction, chemical structure, pressure, surface roughness, and humidity. A number of theoretical studies on the electrostatic behavior of graphene have been reported, and it has been concluded that graphene can store an electric charge for a period of time, which adds to its suitability for triboelectric nanogenerators. We designed and fabricated flexible transparent triboelectric nanogenerators by using monolayer, bilayer, trilayer, and quadlayer graphene. The dependence of the power output performance of the graphene triboelectric nanogenerators on the number of graphene layers is also discussed in detail in terms of the work function and friction, which arises due to different electronic relations between randomly and regularly stacked graphene layers. Furthermore, this talk will introduce a direct observation of the directional dependent piezoelectric effect in CVD grown monolayer MoS2 using lateral piezoresponse force microscopy measurements and the piezoelectric power output from piezoelectric nanogenerators fabricated with monolayer MoS2.

Electrical Properties of Synthesized Large-Area MoS₂ Field-Effect Transistors Fabricated with Inkjet-Printed Contacts.

We report the electrical properties of synthesized large-area monolayer molybdenum disulfide (MoS2) field-effect transistors (FETs) with low-cost inkjet-printed Ag electrodes. The monolayer MoS2 film was grown by a chemical vapor deposition (CVD) method, and the top-contact Ag source/drain electrodes (S/D) were deposited onto the films using a low-cost drop-on-demand inkjet-printing process without any masks and surface treatments. The electrical characteristics of FETs were comparable to those fabricated by conventional deposition methods such as photo- or electron beam lithography. The contact properties between the S/D and the semiconductor layer were also evaluated using the Y-function method and an analysis of the output characteristic at the low drain voltage regimes. Furthermore, the electrical instability under positive gate-bias stress was studied to investigate the charge-trapping mechanism of the FETs. CVD-grown large-area monolayer MoS2 FETs with inkjet-printed contacts may represent an attractive approach for realizing large-area and low-cost thin-film electronics.

Integrated Flexible and High‐Quality Thin Film Transistors Based on Monolayer MoS2

Integrated thin film transistors based on CVD-grown high-quality monolayer MoS2 are reported. Every device has the stable and uniformity mobility ˜13.9 ± 2 cm2 v-1 s-1 and an on/off ratio higher than 105. These MoS2 field effect transistors exhibit remarkably high mechanical flexibility with no obvious change of the electrical characteristics upon strain ˜1%.

Size-tunable synthesis of monolayer MoS2 nanoparticles and their applications in non-volatile memory devices.

We report the CVD synthesis of a monolayer of MoS2 nanoparticles such that the nanoparticle size was controlled over the range 5-100 nm and the chemical potential of sulfur was modified, both by controlling the hydrogen flow rate during the CVD process. As the hydrogen flow rate was increased, the reaction process of sulfur changed from a "sulfiding" process to a "sulfo-reductive" process, resulting in the growth of smaller MoS2 nanoparticles on the substrates. The size control, crystalline quality, chemical configuration, and distribution uniformity of the CVD-grown monolayer MoS2 nanoparticles were confirmed. The growth of the MoS2 nanoparticles at different edge states was studied using density functional theory calculations to clarify the size-tunable mechanism. A non-volatile memory device fabricated using the CVD-grown size-controlled 5 nm monolayer MoS2 nanoparticles as a floating gate showed a good memory window of 5-8 V and an excellent retention period of a decade.

Spectroscopic Visualization of Grain Boundaries of Monolayer Molybdenum Disulfide by Stacking Bilayers.

Polycrystalline growth of molybdenum disulfide (MoS2) using chemical vapor deposition (CVD) methods is subject to the formation of grain boundaries (GBs), which have a large effect on the electrical and optical properties of MoS2-based optoelectronic devices. The identification of grains and GBs of CVD-grown monolayer MoS2 has traditionally required atomic resolution microscopy or nonlinear optical imaging techniques. Here, we present a simple spectroscopic method for visualizing GBs of polycrystalline monolayer MoS2 using stacked bilayers and mapping their indirect photoluminescence (PL) peak positions and Raman peak intensities. We were able to distinguish a GB between two MoS2 grains with tilt angles as small as 6° in their grain orientations and, based on the inspection of several GBs, found a simple empirical rule to predict the location of the GBs. In addition, the large number of twist angle domains traced through our facile spectroscopic mapping technique allowed us to identify a continuous evolution of the coupled structural and optical properties of bilayer MoS2 in the vicinity of the 0° and 60° commensuration angles which were explained by elastic deformation model of the MoS2 membranes.

Superlinear composition-dependent photocurrent in CVD-grown monolayer MoS2(1-x)Se2x alloy devices.

Transition metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional materials that are promising for electronics and photonics. To date, optoelectronic measurements in these materials have shown the conventional behavior expected from photoconductors such as a linear or sublinear dependence of the photocurrent on light intensity. Here, we report the observation of a new regime of operation where the photocurrent depends superlinearly on light intensity. We use spatially resolved photocurrent measurements on devices consisting of CVD-grown monolayers of TMD alloys spanning MoS2 to MoSe2 to show the photoconductive nature of the photoresponse, with the photocurrent dominated by recombination and field-induced carrier separation in the channel. Time-dependent photoconductivity measurements show the presence of persistent photoconductivity for the S-rich alloys, while photocurrent measurements at fixed wavelength for devices of different alloy compositions show a systematic decrease of the responsivity with increasing Se content associated with increased linearity of the current-voltage characteristics. A model based on the presence of different types of recombination centers is presented to explain the origin of the superlinear dependence on light intensity, which emerges when the nonequilibrium occupancy of initially empty fast recombination centers becomes comparable to that of slow recombination centers.

Controlling Grain Size and Continuous Layer Growth in Two-Dimensional MoS<sub>2</sub> Films for Nanoelectronic Device Application

We report that control over the grain size and lateral growth of monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film, yielding a uniform large-area monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film, can be achieved by submitting the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> surfaces of the substrates to oxygen plasma treatment and modulating substrate temperature in chemical vapor deposition (CVD) process. Scanning electron microscopy and atomic force microscopy images and Raman spectra revealed that the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> lateral growth could be controlled by the surface treatment conditions and process temperatures. Moreover, the obtained monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films showed excellent scalable uniformity covering a centimeter-scale SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates, which was confirmed with Raman and photoluminescence mapping studies. Transmission electron microscopy measurements revealed that the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film of the monolayer was largely single crystalline in nature. Back-gate field effect transistors based on a CVD-grown uniform monolayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film showed a good current on/off ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> and a field effect mobility of 7.23 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s. Our new approach to growing MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films is anticipated to advance studies of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or other transition metal dichalcogenide material growth mechanisms and to facilitate the mass production of uniform high-quality MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films for the commercialization of a variety of applications.