CVD-grown Monolayer MoS2 Research Articles

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum.

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Enhancing hydrogen evolution reaction activity through defects and strain engineering in monolayer MoS2.

Molybdenum disulfide (MoS2) has recently emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER). However, the poor in-plane electrical conductivity and inert basal plane activity pose major challenges in realizing its practical application. Herein, we demonstrate a new approach to induce biaxial strain into CVD-grown MoS2 monolayers by draping it over an array of patterned gold nanopillar arrays (AuNAs) as an efficient strategy to enhance its HER activity. We vary the magnitude of applied strain by changing the inter-pillar spacing, and its effect on the HER activity is investigated. To capitalize on the synergistic effect of improved ΔG H via strain engineering and leverage basal plane activation by introduction of sulphur vacancies, we further exposed the strained MoS2 monolayers to oxygen plasma treatment to create S-vacancies. The strained MoS2 on AuNAs with optimal inter-pillar spacing is exposed to oxygen plasma treatment for different durations, and we study its electrocatalytic activity towards the HER using on-chip microcell devices. The strained and vacancy-rich monolayer MoS2 draped on AuNAs with a 0.5 μm inter-pillar spacing and exposed to plasma for 50 s (S0.5μmV50-MoS2) is shown to exhibit remarkable improvement in HER activity, with an overpotential of 53 mV in 0.5 M H2SO4. Thus, the synergistic creation of additional vacancy defects, along with strain-induced active sites, results in enhancement in HER performance of CVD-grown monolayer MoS2. The present study provides a highly promising route for engineering 2D electrocatalysts towards efficient hydrogen evolution.

Oxidation-induced modulation of photoresponsivity in monolayer MoS2 with sulfur vacancies.

Two-dimensional transition metal dichalcogenides (TMDs), such as MoS2, hold great promise for next-generation electronics and optoelectronics due to their unique properties. However, the ultrathin nature of these materials renders them vulnerable to structural defects and environmental factors, which significantly impact their performance. Sulfur vacancies (VS) are the most common intrinsic defects in MoS2, and their impact on device performance in oxidising environments remains understudied. This study investigates the impact of VS defects on the photoresponsivity of CVD-grown monolayer MoS2 devices, when exposed to oxidising environments at high temperatures. Our findings reveal a dynamic process of defect generation and healing through oxygen passivation, leading to a significant difference in photocurrent between environments. Temperature-dependent analysis shows defect healing and a notable reduction in defect density upon cooling. This study provides crucial insights into the stability and performance of 2D materials-based devices under varying environmental conditions, essential for designing and controlling the performance of TMD-based devices. Our results pave the way for the development of robust and reliable 2D materials-based electronics and optoelectronics.

Mobility Enhancement in CVD-Grown Monolayer MoS2 Via Patterned Substrate-Induced Nonuniform Straining.

The extraordinary mechanical properties of two-dimensional transition-metal dichalcogenides make them ideal candidates for investigating strain-induced control of various physical properties. Here we explore the role of nonuniform strain in modulating optical, electronic, and transport properties of semiconducting, chemical vapor deposited monolayer MoS2, on periodically nanostructured substrates. A combination of spatially resolved spectroscopic and electronic properties explore and quantify the differential strain distribution and carrier density on a monolayer, as it conformally drapes over the periodic nanostructures. The observed accumulation in electron density at the strained regions is supported by theoretical calculations which form the likely basis for the ensuing ×60 increase in field effect mobility in strained samples. Though spatially nonuniform, the pattern-induced strain is shown to be readily controlled by changing the periodicity of the nanostructures thus providing a robust yet useful macroscopic control on strain and mobility in these systems.

Probing Sulfur Vacancies in CVD-Grown Monolayer MoS2 on SiO2/Si in the Temperature Range 750–900°C

This work reports morphologically alike, high-quality monolayer MoS2 flakes with a similar strain at various growth temperatures (750–900°C) achieved by adjusting sulfur temperature. The growth dynamics of MoS2 are correlated with changes in the photoluminescence (PL) and Raman peak positions. Monolayer MoS2 crystals are synthesized at different growth temperatures from 750°C to 900°C using chemical vapor deposition (CVD). We examined the structural quality and aimed to extract the recombination mechanisms in MoS2 using low-temperature, variable, and low-laser-intensity PL measurements. Our studies of the defect-associated bound exciton emission are well correlated with the blueshift in the A1g mode of Raman spectra, blueshift in PL spectra, and x-ray photoelectron spectroscopy results for crystal grown at 900°C. Our research findings not only shed light on a thorough, non-intrusive method for modifying growth parameters to enhance optical performance, but they also suggest a way to modify the optical characteristics of MoS2 while maintaining the morphology.

Size and shape control of CVD-grown monolayer MoS2

Molybdenum disulfide (MoS2) has attracted great attention because of its unique physical properties and wide potential applications in electronic, optoelectronic devices. Chemical vapor deposition (CVD) turned out to be a powerful tool for producing MoS2 thin layers. Nonetheless, how the concentration of vapor-phase precursor influences the size and shape change of MoS2 is not clearly understood yet. In this work, we designed a gradient of sulfur (S) concentration during CVD growth of MoS2 to investigate how S concentration affects its size and shape change. The shape of MoS2 film changes from triangle to hexagon, then back to triangle as S concentration decreases. We also analyzed the uniformity and crystal quality of the CVD-grown MoS2 by using optical microscopy, Raman, photoluminescence (PL), circularly polarized PL, and atomic force microscopy.

Strain-Induced Performance Enhancement of a Monolayer Photodetector via Patterned Substrate Engineering.

Two-dimensional (2D) materials exhibit tremendous potential for applications in next-generation photodetectors. Currently, approaches aiming at enhancing the device's performance are limited, mainly relying on complex hybrid systems such as heterostructures and sensitization. Here, we propose a new strategy by constructing patterned nanostructures compatible with the conventional silicon substrate. Using CVD-grown monolayer MoS2 on the periodical nanocone arrays, we demonstrate a high-performance MoS2 photodetector via manipulating strain distribution engineered by the substrate at the nanoscale. Compared to the pristine MoS2 counterpart, the strained MoS2 photodetector exhibits a much enhanced performance, including a high signal-to-noise ratio over 105 and large responsivity of 3.2 × 104 A W-1. The physical mechanism responsible for the enhancement is discussed by combining Kelvin probe force microscopy with theoretical simulation. The enhanced performances can be attributed to the improved light absorption, the fast separation of photo-excited carriers, and the suppression of dark currents induced by the designed periodical nanocone arrays. This work depicts an alternative method to achieve high-performance optoelectronic devices based on 2D materials integrated with semiconductor circuits.

Highly Sensitive Photodetectors Based on Monolayer MoS2 Field-Effect Transistors.

Molybdenum disulfide (MoS2) is a promising candidate for the development of high-performance photodetectors, due to its excellent electric and optoelectronic properties. However, most of the reported MoS2 phototransistors have adopted a back-gate field-effect transistor (FET) structure, requiring applied gate bias voltages as high as 70 V, which made it impossible to modulate each detecting device in the fabricated array. In this paper, buried-gate FETs based on CVD-grown monolayer MoS2 were fabricated and their electric and photoelectric properties were also systematically investigated. A photoresponsivity of around 6.86 A/W was obtained at 395 nm, under the conditions of zero gate bias voltage and a light power intensity of 2.57 mW/cm2. By application of a buried-gate voltage of 8 V, the photoresponsivity increased by nearly 10 times. Furthermore, the response speed of the buried-gate MoS2 FET phototransistors is measured to be around 350 ms. These results pave the way for MoS2 photodetectors in practical applications.

Analysis of Schottky barrier heights and reduced Fermi-level pinning in monolayer CVD-grown MoS2 field-effect-transistors

Chemical vapor deposition (CVD)-grown monolayer (ML) molybdenum disulfide (MoS2) is a promising material for next-generation integrated electronic systems due to its capability of high-throughput synthesis and compatibility with wafer-scale fabrication. Several studies have described the importance of Schottky barriers in analyzing the transport properties and electrical characteristics of MoS2 field-effect-transistors (FETs) with metal contacts. However, the analysis is typically limited to single devices constructed from exfoliated flakes and should be verified for large-area fabrication methods. In this paper, CVD-grown ML MoS2 was utilized to fabricate large-area (1 cm × 1 cm) FET arrays. Two different types of metal contacts (i.e. Cr/Au and Ti/Au) were used to analyze the temperature-dependent electrical characteristics of ML MoS2 FETs and their corresponding Schottky barrier characteristics. Statistical analysis provides new insight about the properties of metal contacts on CVD-grown MoS2 compared to exfoliated samples. Reduced Schottky barrier heights (SBH) are obtained compared to exfoliated flakes, attributed to a defect-induced enhancement in metallization of CVD-grown samples. Moreover, the dependence of SBH on metal work function indicates a reduction in Fermi level pinning compared to exfoliated flakes, moving towards the Schottky–Mott limit. Optical characterization reveals higher defect concentrations in CVD-grown samples supporting a defect-induced metallization enhancement effect consistent with the electrical SBH experiments.

Highly Photoluminescent Monolayer MoS2 and WS2 Achieved via Superacid Assisted Vacancy Reparation and Doping Strategy (Laser Photonics Rev. 15(12)/2021)

Monolayer Photoluminescence In article number 2100104 by Weizhen Liu, Haiyang Xu, and co-workers, a novel organic superacid trifluoromethanesulfonic (TFMS) is explored to significantly improve the photoluminescence quantum yield of CVD-grown monolayer MoS2 and WS2 to 14.1% and 56.7%, respectively. Through in-depth study and analysis with scanning transmission electron microscopy and density functional theory calculation, luminescence enhancement mechanism is attributed to a synergistic effect of TFMS-assisted sulfur vacancy repairation and p-type doping.

(Invited) Optoelectronic Synapses with Two Dimensional Materials for Neuromorphic Computing

Optoelectronic synapses are devices that behave like photodetectors with analog memory. In neuromorphic computing, these devices have the potential to perform imaging and object identification using the same device. We first demonstrate that the strong photogeneration efficiency of methylammonium lead bromide perovskite quantum dots (PQDs) can be exploited by growing PQDs from the lattice of a single layer graphene by a defect-mediated process. The rationale for designing this hybrid superstructure stems from the ability of PQDs to absorb light and generate charge carriers. The charges generated are transferred to graphene, which transports the carriers across the active layer of the device. Through the implementation of this thin superstructure in a phototransistor geometry, we produce a photoresponsivity of 1.4×108 AW-1 at 430 nm and a specific detectivity (D*) of 4.72×1015 Jones, which is by far the best responsivity and detectivity across similar devices reported to date. This is promising for the development of highly efficient optoelectronic materials for high-speed communications, sensing, ultra-sensitive cameras and high-resolution imaging and displays. The graphene-PQD superstructure behaves as an optoelectronic synapse with low energy consumption of 36.75 pJ/ spike that mimics crucial characteristics of its biological equivalent, with unique optical potentiation and electrical habituation function. We also utilized the persistent photoconductivity of 2D MoS2 layers to demonstrate an optoelectronic synapse. A CVD-grown monolayer MoS2 FET exhibits substantial enhancement in drain current when illuminated with a laser of 450 nm wavelength as well as a negative shift in its threshold voltage due to the generation of photocarriers. The device undergoes optical potentiation and electrical depression. The device is “optically” potentiated by applying light pulses with ON/OFF interval of 5 s for each at 450 nm wavelength – measured at a constant drain bias of 1.0 V with VG = -2 V. Also, it is “electrically” depressed by applying 50 electrical pulses with ON/OFF interval of 1 s and 9 s, respectively – measured at a drain bias of -100 mV with VG = -2 V. These results confirm the active and simultaneous tunability of conductance essential for the electrical/optical training of optoelectronic synapse in neural network applications. The negative voltage pulses applied at the drain de-traps the holes from the SiO2/MoS2 interface, thus gradually depress the device to its initial low conductance state.

Isoemissive Photoluminescence from a Quaternary System of Valley-Polarized, Defect-Bound Excitons and Trions in Two-Dimensional Transition Metal Dichalcogenides

Interest in the use of local band extrema, known as valleys, as a possible information carrier has in recent years been advanced by the isolation of 2D transition metal dichalcogenides (TMDCs). In some monolayer TMDCs, the structural inversion asymmetry leads to spin-valley locked states that present potential advantages for information storage and manipulation beyond existing charge- and spin-based semiconductor technologies. However, understanding of the role of defects on exciton recombination and valley scattering remains lacking and limits progress toward applications. Here, we report gate voltage-dependent polarized photoluminescence measurements on CVD-grown monolayer WS2 and MoS2 field-effect transistors and observe a quaternary system of valley polarized excitons and trions. Intriguingly, we find the spectra to exhibit isoemissive points, which were previously seen only as evidence of a binary system. We propose a rate model to explain the existence of isoemission in a quaternary system and find that the presence of isoemissive points imply a kinetic relationship between recombination and valley scattering. This relationship sheds light on the inverse relation seen between the polarization degree and photoluminescence intensity. Further consideration of the phenomenon also led to the identification of a possible new mechanism for charge carrier detrapping using the energy released from trion recombination.

Effect of precursor ratio on the morphological and optical properties of CVD-grown monolayer MoS2 nanosheets

Atmosphere pressure chemical vapor deposition (CVD) is one of the most powerful methods of synthesizing high quality and large area MoS2 films with a reasonable cost. In our work, the large-scale and high crystalline quality monolayer MoS2 nanosheets were synthesized on Silicon substrate with a 300 nm oxide layer using MoO3 and S powders as precursors by an atmosphere pressure CVD. The results suggest that the surface morphology, crystalline quality and luminescence of CVD-grown MoS2 nanosheets can be tunable by controlling the precursor ratio (the effective Mo: S ratio). Excessive S-rich atmosphere is favor to synthesize large-size and high crystalline quality monolayer MoS2 nanosheets with sharp corners and straight edges. This study may provide insight into the synthesis of large-scale and high crystalline quality MoS2 films.

Electrothermal transport induced material reconfiguration and performance degradation of CVD-grown monolayer MoS2 transistors

Device and material reliability of 2-dimensional materials, especially CVD-grown MoS2, has remained un-addressed since 2011 when the first TMDC transistor was reported. For its potential application in next generation electronics, it is imperative to update our understanding of mechanisms through which MoS2 transistors’ performance degrades under long-term electrical stress. We report, for CVD-grown monolayer MoS2, results on temporal degradation of material and device performance under electrical stress. Both low and high field regimes of operation are explored at different temperatures, gate bias and stress cycles. During low field operation, current is found to saturate after hundreds of seconds of operation with the current decay time constant being a function of temperature and stress cycle. High field operation, especially at low temperature, leads to impact ionization assisted material and device degradation. It is found that high field operation at low temperature results in amorphization of the channel and is verified by device and kelvin probe force microscopy (KPFM) analyses. In general, a prolonged room temperature operation of CVD-grown MoS2 transistors lead to degraded gate control, higher OFF state current and negative shift in threshold voltage (VT). This is further verified, through micro-Raman and photoluminescence spectroscopy, which suggest that a steady state DC electrical stress leads to the formation of localized low resistance regions in the channel and a subsequent loss of transistor characteristics. Our findings unveil unique mechanism by which CVD MoS2 undergoes material degradation under electrical stress and subsequent breakdown of transistor behavior. Such an understanding of material and device reliability helps in determining the safe operating regime from device as well as circuit perspective.

Spin-Valley Locking Effect in Defect States of Monolayer MoS2.

Valley pseudospin in two-dimensional (2D) transition-metal dichalcogenides (TMDs) allows optical control of spin-valley polarization and intervalley quantum coherence. Defect states in TMDs give rise to new exciton features and theoretically exhibit spin-valley polarization; however, experimental achievement of this phenomenon remains challenges. Here, we report unambiguous valley pseudospin of defect-bound localized excitons in CVD-grown monolayer MoS2; enhanced valley Zeeman splitting with an effective g-factor of -6.2 is observed. Our results reveal that all five d-orbitals and the increased effective electron mass contribute to the band shift of defect states, demonstrating a new physics of the magnetic responses of defect-bound localized excitons, strikingly different from that of A excitons. Our work paves the way for the manipulation of the spin-valley degrees of freedom through defects toward valleytronic devices.

Dynamics of cleaning, passivating and doping monolayer MoS2 by controlled laser irradiation

Lattice defects (mainly chalcogen vacancies) drastically affect the optoelectronic properties of monolayer transition metal dichalcogenides (TMDs) grown by chemical vapor deposition (CVD). They can be passivated through charge-transfer doping by laser irradiation in air. Here we perform a systematic in situ study to elucidate the passivation mechanism upon laser irradiation and show a way to controllably n-dope CVD-grown monolayer MoS2 on SiO2 substrates. By combining resonance Raman and photoluminescence (PL) spectroscopy we show that an increase in defect density correlates with a redshifted PL emission and hence an increase in electron density. Density functional theory (DFT) calculations identify chalcogen vacancies to be facilitators (not the source) of n-doping, and population of mid-gap levels upon doping lowers the activation barrier for O2 adsorption from 0.3 to 0.03 eV. Laser irradiation aids in the oxygen-passivation of chalcogen vacancies, manifested by an increase in PL intensity and blueshifted emission, and this blueshift is determined by the laser power density. The passivation occurs on two timescales, with the removal of surface adsorbates first, followed by oxygen adsorption at the sulfur vacancy sites.

Thermally stimulated current spectroscopy of traps in CVD grown monolayer MoS2

We have investigated defects in large area monolayer MoS2 films using thermally stimulated current (TSC) spectroscopy. Films are grown on c-sapphire substrates using a microcavity based chemical vapor deposition technique. A theoretical framework to analyze TSC data for a two dimensional semiconductor has been developed. The study reveals the existence of two traps with average activation energies of 670 and 850 meV. The density of these traps shows an increase followed by saturation as the sample goes through repeated thermal cycles in vacuum. Interestingly, the density returns to its initial level when the sample is exposed to the ambient condition for a sufficiently long time, suggesting that these defects are passivated by certain adsorbate groups/molecules in the ambient condition. It has been found that annealing in a sulfur environment substantially reduces the concentration of these traps. This indicates that the traps must be related to sulfur deficiency related defects.

PbI2-MoS2 Heterojunction: van der Waals Epitaxial Growth and Energy Band Alignment.

van der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations. Herein, PbI2 was thermally deposited at high-vacuum conditions onto CVD-grown monolayer MoS2 flakes in a vdW epitaxial manner to form 3D-2D heterojunctions, which are promising for vdW epitaxial growth of perovskite films. X-ray diffraction, X-ray photoemission spectroscopy, Raman, and atomic force microscopy measurements reveal the structural properties of the high-quality heterojunctions. Photoluminescence (PL) measurements reveal that the PL emissions from the bottom MoS2 flakes are greatly quenched compared to their as-grown counterparts, which can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. Strong interlayer excitons can be detected at the PbI2/MoS2 interface, indicating an effective type II band alignment, which can be further confirmed by ultraviolet photoemission spectroscopy measurements. The results provide a new material platform for the application of the vdW heterojunctions in electronic and optoelectronic devices.

Detection of both optical polarization and coherence transfers to excitonic valley states in CVD-grown monolayer MoS2

We experimentally demonstrate the generation and detection of both excitonic valley polarization and coherence in CVD-grown monolayer MoS2 under nearly resonant excitation in polarized photoluminescence. By circular and linear photoexcitation close to the A-exciton energy, where the excitation energy is 37 meV above the A-exciton energy, 54% and 32% of valley polarization and coherence, respectively, are observed at T = 20 K. By tuning the optical polarization and the polarization plane of photoexcitation, valley polarization and its coherence are systematically controlled. This indicates the coherent control of an excitonic valley two-level system, an important step for valley quantum information.

Thermal-Assisted Vertical Electron Injections in Few-Layer Pyramidal-Structured MoS2 Crystals.

The interlayer screening effects and charge conduction mechanisms in atomically thin two-dimensional (2D) materials are crucial for electronics and optoelectronics applications. However, such effects remain largely unexplored in chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS2) crystals. Here, we report a controllable CVD-grown monolayer MoS2 and layer-by-layer pyramidal-structured MoS2 crystals with an oxidized Mo foil precursor. The interlayer screening effects and charge conduction mechanisms in the pyramidal-structured MoS2 crystals are studied. Although the Fowler-Nordheim (FN) tunneling model is widely adopted to describe the vertical charge transport mechanism at the 2D semiconductor/bulk metal interface, we found that such a mechanism cannot satisfactorily explain the electrical measurement obtained from our CVD-grown MoS2 samples. Instead, our analysis reveals that Richardson-Schottky (RS) emission is the dominant transport mechanism when Vbias < 1 V. Our findings provide a fundamental understanding of the charge conduction mechanism in CVD-grown MoS2 crystals, which is crucial for development of MoS2 electronics and optoelectronics devices.