Strong photoluminescence enhancement of MoS(2) through defect engineering and oxygen bonding

Controlled Gas Molecules Doping of Monolayer MoS2 via Atomic-Layer-Deposited Al2O3 Films

MoS₂ as atomically thin semiconductor is highly sensitive to ambient atmosphere (e.g., oxygen, moisture, etc.) in optical and electrical properties. Here we report a controlled gas molecules doping of monolayer MoS₂ via atomic-layer-deposited Al₂O₃ films. The deposited Al₂O₃ films, in the shape of nanospheres, can effectively control the contact areas between ambient atmosphere and MoS₂ that allows precise modulation of gas molecules doping. By analyzing photoluminescence (PL) emission spectra of MoS₂ with different thickness of Al₂O₃, the doped carrier concentration is estimated at ∼2.7 × 10¹³ cm^-2 based on the mass action model. Moreover, time-dependent PL measurements indicate an incremental stability of single layer MoS₂ as the thicknesses of Al₂O₃ capping layer increase. Effective control of gas molecules doping in monolayer MoS₂ provides us a valuable insight into the applications of MoS₂ based optical and electrical devices.

Janus Monolayer Transition-Metal Dichalcogenides

The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe₂, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Oxygen Passivation Mediated Tunability of Trion and Excitons in MoS_{2}

Using wide spectral range in situ spectroscopic ellipsometry with systematic ultrahigh vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS_{2} isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the band structure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This Letter unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS_{2}.

Two-photon absorption and subband photodetection in monolayer MoS2

We develop a theoretical model to quantify the two-photon absorption (2 PA) coefficients of monolayer MoS₂. Based on two-dimensional excitons, our model reveals the 2 PA coefficient spectrum on the order of 0.01-0.1 cm/MW in the near-infrared for monolayer MoS₂. As compared to the band theory for bulk semiconductors, these coefficients are enhanced by at least one order of magnitude. Our model is in agreement with light-intensity-dependent photocurrent measurements on a monolayer MoS₂, subband photodetector with femtosecond laser pulses.

Interplay between many body effects and Coulomb screening in the optical bandgap of atomically thin MoS2

Due to its unique layer-number dependent electronic band structure and strong excitonic features, atomically thin MoS₂ is an ideal 2D system where intriguing photoexcited-carrier-induced phenomena can be detected in excitonic luminescence. We perform micro-photoluminescence (PL) measurements and observe that the PL peak redshifts nonlinearly in mono- and bi-layer MoS₂ as the excitation power is increased. The excited carrier-induced optical bandgap shrinkage is found to be proportional to n^4/3, where n is the optically-induced free carrier density. The large exponent value of 4/3 is explicitly distinguished from a typical value of 1/3 in various semiconductor quantum well systems. The peculiar n^4/3 dependent optical bandgap redshift may be due to the interplay between bandgap renormalization and reduced exciton binding energy.

Ambipolar MoS2 Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization

One of the main challenges to exploit molybdenum disulfide (MoS₂) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS₂ FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS₂ surface can be tailored at nanoscale using soft O₂ plasma treatments. The morphological, chemical, and electrical modifications of MoS₂ surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS₂ to a broader distribution (from 0.2 to 0.8 eV) after 600 s O₂ plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS₂ multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS₂ FETs, fabricated by self-aligned deposition of source/drain contacts in the O₂ plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio I_on/I_off ≈ 10³ and field-effect mobilities of 11.5 and 7.2 cm² V^-1 s^-1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O₂ plasma functionalized MoS₂ surface.

Atomic process of oxidative etching in monolayer molybdenum disulfide

The microscopic process of oxidative etching of two-dimensional molybdenum disulfide (2D MoS₂) at an atomic scale is investigated using a correlative transmission electron microscope (TEM)-etching study. MoS₂ flakes on graphene TEM grids are precisely tracked and characterized by TEM before and after the oxidative etching. This allows us to determine the structural change with an atomic resolution on the edges of the domains, of well-oriented triangular pits and along the grain boundaries. We observe that the etching mostly starts from the open edges, grain boundaries and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant termination of the triangular pits, and profound terraces and grooves are observed on the etched edges. Based on the statistical TEM analysis, we reveal possible routes for the kinetics of the oxidative etching in 2D MoS₂, which should also be applicable for other 2D transition metal dichalcogenide materials like MoSe₂ and WS₂.

Using defects to store energy in materials - a computational study

Energy storage occurs in a variety of physical and chemical processes. In particular, defects in materials can be regarded as energy storage units since they are long-lived and require energy to be formed. Here, we investigate energy storage in non-equilibrium populations of materials defects, such as those generated by bombardment or irradiation. We first estimate upper limits and trends for energy storage using defects. First-principles calculations are then employed to compute the stored energy in the most promising elemental materials, including tungsten, silicon, graphite, diamond and graphene, for point defects such as vacancies, interstitials and Frenkel pairs. We find that defect concentrations achievable experimentally (~0.1–1 at.%) can store large energies per volume and weight, up to ~5 MJ/L and 1.5 MJ/kg for covalent materials. Engineering challenges and proof-of-concept devices for storing and releasing energy with defects are discussed. Our work demonstrates the potential of storing energy using defects in materials.

Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS₂ samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

Atomically thin transition metal dichalcogenides can be grown on large scale using chemical vapour deposition which, however, determines presence of grain boundaries. Here, the authors report that third-harmonic generation imaging provides excellent sensitivity and fast speed for grain boundary visualization in MoS₂.

Nonlinear photoluminescence in monolayer WS2: parabolic emission and excitation fluence-dependent recombination dynamics

Recombination dynamics during photoluminescence (PL) in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are complicated and can be easily affected by the surroundings because of their atomically thin structures. Herein, we studied the excitation power and temperature dependence of the recombination dynamics on the chemical vapor deposition-grown monolayer WS₂via a combination of Raman, PL, and time-resolved PL spectroscopies. We found a red shift and parabolic intensity increase in the PL emission of the monolayer WS₂ with the increasing excitation power and the decay time constants corresponding to the recombination of trions and excitons from transient PL dynamics. We attributed the abovementioned nonlinear changes in the PL peak positions and intensities to the combination of increasing carrier interaction and band structure renormalization rather than to the thermal effect from a laser. Furthermore, the excitation power-dependent Raman measurements support our conclusion. These findings and understanding will provide important information for the development of TMD-based optoelectronics and photonics.

Realizing Stable p-Type Transporting in Two-Dimensional WS2 Films

Two-dimensional (2D) semiconductors have become promising candidates for nanoelectronics applications due to their unique layered structure and rich physical properties. However, the significant lack of reproducible p-type doping methods that can avoid the instability induced by the widely used charge transfer doping method greatly limits the applications of these semiconductors in complementary metal-oxide-semiconductor (CMOS) integrated digital circuits. This work presents a new scheme to realize stable p-type doping for WS₂ with excellent layer controllability, wafer-level uniformity, and high reproducibility at the same time. The p-type WS₂ was produced by introducing substitutional doping of sulfur with nitrogen atoms during the sulfurization of WO_xN_y film. Nitrogen atoms acted as acceptors moving the Fermi level of WS₂ toward the valance band. Both experimental and theoretical investigations were designed to study the physical properties of the films fabricated. The WS₂ based field-effect transistors exhibited a well-defined p-type behavior with a large on/off current ratio of ∼10⁵ and a high hole mobility of ∼18.8 cm² V^-1 s^-1. This opens up a promising method to realize stable p-type doping of 2D materials, which is very attractive for future large-scale 2D CMOS device applications.

Electrostatic Screening of Charged Defects in Monolayer MoS2

Defects in monolayer transition-metal dichalcogenides (TMDCs) may lead to unintentional doping, charge-carrier trapping, and nonradiative recombination. These effects impair electronic and optoelectronic technologies. Here we show that charged defects in MoS₂ monolayers can be effectively screened when they are in contact with an ionic liquid (IL), leading to an increase in photoluminescence (PL) yield by up to two orders of magnitude. The extent of this PL enhancement by the IL correlates with the brightness of each pretreated sample. We propose the existence of two classes of nonradiative recombination centers in monolayer MoS₂: (i) charged defects that relate to unintentional doping and may be electrostatically screened by ILs and (ii) neutral defects that remain unaffected by the presence of ILs.

Adsorption energy of oxygen molecules on graphene and two-dimensional tungsten disulfide

Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.

Optically active charge transfer in hybrids of Alq3 nanoparticles and MoS2 monolayer

Organic/inorganic hybrid structures have been widely studied because of their enhanced physical and chemical properties. Monolayers of transition metal dichalcogenides (1L-TMDs) and organic nanoparticles can provide a hybridization configuration between zero- and two-dimensional systems with the advantages of convenient preparation and strong interface interaction. Here, we present such a hybrid system made by dispersing π-conjugated organic (tris (8-hydroxyquinoline) aluminum(III)) (Alq₃) nanoparticles (NPs) on 1L-MoS₂. Hybrids of Alq₃ NP/1L-MoS₂ exhibited a two-fold increase in the photoluminescence of Alq₃ NPs on 1L-MoS₂ and the n-doping effect of 1L-MoS₂, and these spectral and electronic modifications were attributed to the charge transfer between Alq₃ NPs and 1L-MoS₂. Our results suggested that a hybrid of organic NPs/1L-TMD can offer a convenient platform to study the interface interactions between organic and inorganic nano objects and to engineer optoelectronic devices with enhanced performance.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition

We show that hexagonal domains of monolayer tungsten disulfide (WS₂) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS₂ hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS₂ domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.

Enhancement of Exciton Emission from Multilayer MoS2 at High Temperatures: Intervalley Transfer versus Interlayer Decoupling

It is very important to obtain a deeper understand of the carrier dynamics for indirect-bandgap multilayer MoS₂ and to make further improvements to the luminescence efficiency. Herein, an anomalous luminescence behavior of multilayer MoS₂ is reported, and its exciton emission is significantly enhanced at high temperatures. Temperature-dependent Raman studies and electronic structure calculations reveal that this experimental observation cannot be fully explained by a common mechanism of thermal-expansion-induced interlayer decoupling. Instead, a new model involving the intervalley transfer of thermally activated carriers from Λ/Γ point to K point is proposed to understand the high-temperature luminescence enhancement of multilayer MoS₂ . Steady-state and transient-state fluorescence measurements show that both the lifetime and intensity of the exciton emission increase relatively to increasing temperature. These two experimental evidences, as well as a calculation of carrier population, provide strong support for the proposed model.

Ultrabroadband MoS2 Photodetector with Spectral Response from 445 to 2717 nm

Photodetectors with excellent detecting properties over a broad spectral range have advantages for the application in many optoelectronic devices. Introducing imperfections to the atomic lattices in semiconductors is a significant way for tuning the bandgap and achieving broadband response, but the imperfection may renovate their intrinsic properties far from the desire. Here, by controlling the deviation from the perfection of the atomic lattice, ultrabroadband multilayer MoS₂ photodetectors are originally designed and realized with the detection range over 2000 nm from 445 nm (blue) to 2717 nm (mid-infrared). Associated with the narrow but nonzero bandgap and large photoresponsivity, the optimized deviation from the perfection of MoS₂ samples is theoretically found and experimentally achieved aiming at the ultrabroadband photoresponse. By the photodetection characterization, the responsivity and detectivity of the present photodetectors are investigated in the wavelength range from 445 to 2717 nm with the maximum values of 50.7 mA W^-1 and 1.55 × 10⁹ Jones, respectively, which represent the most broadband MoS₂ photodetectors. Based on the easy manipulation, low cost, large scale, and broadband photoresponse, this present detector has significant potential for the applications in optoelectronics and electronics in the future.

Impact of Interfacial Defects on the Properties of Monolayer Transition Metal Dichalcogenide Lateral Heterojunctions

We explored the impact of interfacial defects on the stability and optoelectronic properties of monolayer transition metal dichalcogenide lateral heterojunctions using a density functional theory approach. As a prototype, we focused on the MoS₂-WSe₂ system and found that even a random alloy-like interface with a width of less than 1 nm has only a minimal impact on the band gap and alignment compared to the defect-less interface. The largest impact is on the evolution of the electrostatic potential across the monolayer. Similar to defect-less interfaces, a small number of defects results in an electrostatic potential profile with a sharp change at the interface, which facilitates exciton dissociation. Differently, a large number of defects results in an electrostatic potential profile switching smoothly across the interface, which is expected to reduce the capability of the heterojunction to promote exciton dissociation. These results are generalizable to other transition metal dichalcogenide lateral heterojunctions.

Manipulation of local optical properties and structures in molybdenum-disulfide monolayers using electric field-assisted near-field techniques

Remarkable optical properties, such as quantum light emission and large optical nonlinearity, have been observed in peculiar local sites of transition metal dichalcogenide monolayers, and the ability to tune such properties is of great importance for their optoelectronic applications. For that purpose, it is crucial to elucidate and tune their local optical properties simultaneously. Here, we develop an electric field-assisted near-field technique. Using this technique we can clarify and tune the local optical properties simultaneously with a spatial resolution of approximately 100 nm due to the electric field from the cantilever. The photoluminescence at local sites in molybdenum-disulfide (MoS₂) monolayers is reversibly modulated, and the inhomogeneity of the charge neutral points and quantum yields is suggested. We successfully etch MoS₂ crystals and fabricate nanoribbons using near-field techniques in combination with an electric field. This study creates a way to tune the local optical properties and to freely design the structural shapes of atomic monolayers using near-field optics.

Heterogeneous Defect Domains in Single-Crystalline Hexagonal WS2

Single-crystalline monolayer hexagonal WS₂ is segmented into alternating triangular domains: sulfur-vacancy (SV)-rich and tungsten-vacancy (WV)-rich domains. The WV-rich domain with deep-trap states reveals an electron-dedoping effect, and the electron mobility and photoluminescence are lower than those of the SV-rich domain with shallow-donor states by one order of magnitude. The vacancy-induced strain and doping effects are investigated via Raman and scanning photoelectron microscopy.

Atomic Defects in Two-Dimensional Materials: From Single-Atom Spectroscopy to Functionalities in Opto-/Electronics, Nanomagnetism, and Catalysis

Two-dimensional layered graphene-like crystals including transition-metal dichalcogenides (TMDs) have received extensive research interest due to their diverse electronic, valleytronic, and chemical properties, with the corresponding optoelectronics and catalysis application being actively explored. However, the recent surge in two-dimensional materials science is accompanied by equally great challenges, such as defect engineering in large-scale sample synthesis. It is necessary to elucidate the effect of structural defects on the electronic properties in order to develop an application-specific strategy for defect engineering. Here, two aspects of the existing knowledge of native defects in two-dimensional crystals are reviewed. One is the point defects emerging in graphene and hexagonal boron nitride, as probed by atomically resolved electron microscopy, and their local electronic properties, as measured by single-atom electron energy-loss spectroscopy. The other will focus on the point defects in TMDs and their influence on the electronic structure, photoluminescence, and electric transport properties. This review of atomic defects in two-dimensional materials will offer a clear picture of the defect physics involved to demonstrate the local modulation of the electronic properties and possible benefits in potential applications in magnetism and catalysis.

Tunable Doping in Hydrogenated Single Layered Molybdenum Disulfide

Structural defects in the molybdenum disulfide (MoS₂) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS₂ electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in monolayered MoS₂ to tune its structural defects. We demonstrate that the electronic properties of single layer MoS₂ can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS₂ flakes. The successful incorporation of hydrogen in MoS₂ leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS₂ confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS₂ hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements.

Enhanced Photoluminescence of Solution-Exfoliated Transition Metal Dichalcogenides by Laser Etching

Using a conventional Raman experimental apparatus, we demonstrate that the photoluminescent (PL) yield from ultrasonication-exfoliated transition metal dichalcogenides (TMDs) (MoS₂ and WS₂) can be increased by up to 8-fold by means of a laser etching procedure. This laser etching process allows us to controllably pattern and reduce the number of layers of the solution-exfoliated material, overcoming the key drawback to solvent-based exfoliation of two-dimensional (2D) semiconducting materials for applications in optoelectronics. The successful laser thinning of the exfoliated 2D crystals was investigated systematically by changes in both Raman and PL spectra. A simple proof-of-principle of the scalability of this laser etching technique for solution-exfoliated TMD crystals was also demonstrated. As well as being applicable for individual materials, it is also possible to use this simple laser etching technique to investigate the structure of solution-generated van der Waals heterostructures, consisting of layers of both MoS₂ and WS₂.

Controllable growth of monolayer MoS2 by chemical vapor deposition via close MoO2 precursor for electrical and optical applications

MoO₂ is used as a new source material for the growth of large area and high optical quality monolayer MoS₂. However, a systematic study of the growth parameters is still missing and large-area growth of discreet single crystals is still challenging. Hereby, we report the shape evolution of monolayer growth of MoS₂ and develop a methodology to achieve centimeter-scaled discrete MoS₂ by adopting MoO₂ as Mo source material in an atmospheric-pressure chemical vapor deposition process. Our results indicate the growth of monolayer MoS₂ could benefit from the precise control of the introduction time of sulfur and the S/MoO₂ ratio in experiments. Micro-Raman and photoluminescence spectra confirm the properties of the material. E-beam lithography was utilized to make contact with the as-grown MoS₂ located at the selective area. The electrical properties of MoS₂ with different morphologies were compared. In the end, the persistent photoconductivity properties of monolayer MoS₂ were emphasized and the underlying mechanism was proposed. These studies demonstrate a better understanding of the growth and application of MoS₂-based 2D materials.

Near-field spectral mapping of individual exciton complexes of monolayer WS2 correlated with local defects and charge population

Exciton transitions are mostly responsible for the optical properties of transition metal dichalcogenide monolayers (1L-TMDs). Extensive studies of optical and structural characterization indicated that the presence of local structural defects and charge population critically influence the exciton emissions of 1L-TMDs. However, due to large variations of sample and experimental conditions, the exact mechanism of the exciton emission influenced by local structural defects and charge population is not clearly understood. In this work by using near-field scanning optical imaging and spectroscopy, we directly visualized spatially- and spectrally-resolved emission profiles of excitons, trions and defect bound excitons in CVD-grown monolayer tungsten disulfide (1L-WS₂) with ∼70 nm spatial resolution. We found that exciton emission is spatially uniform while emission of trions and defect bound excitons was strongly modulated by the presence of structural features such as defects and wrinkles. We also visually observe a strong correlation between local charge accumulation and the trion formation upon increased photo-excitation.

Unimer-Assisted Exfoliation for Highly Concentrated Aqueous Dispersion Solutions of Single- and Few-Layered van der Waals Materials

We suggest a unimer-assisted exfoliation method for the exfoliation of van der Waals two-dimensional (2D) materials such as graphene, MoS₂, and h-BN and show that the micellar size is a critical parameter for enhancing the exfoliation efficiency. To explain the effectiveness of the unimers in the exfoliation, the influence of the micellar size of a biocompatible block copolymer, Pluronic F-68, is evaluated in view of the yield and thickness of exfoliated 2D flakes. By the addition of water-soluble alcohols, the surfactants exist in the form of a unimer, which facilitates the intercalation into the layered materials and their exfoliation. The results showed that the high exfoliation efficiency could be achieved by controlling the micellar size mostly to be unimers; the average yield rate of MoS₂ exfoliation was 4.51% per hour, and the very high concentration of 1.45 mg/mL was obtained by sonication for 3 h. We also suggested the dielectrophoresis technique as a method for forming a film composed of 2D flakes for diverse applications requiring electrical signals. The unimer-assisted exfoliation method will be substantially utilized to achieve highly concentrated aqueous dispersion solutions of 2D materials.

Two-dimensional WS2 nanoribbon deposition by conversion of pre-patterned amorphous silicon

We present a method for area selective deposition of 2D WS₂ nanoribbons with tunable thickness on a dielectric substrate. The process is based on a complete conversion of a pre-patterned, H-terminated Si layer to metallic W by WF₆, followed by in situ sulfidation by H₂S. The reaction process, performed at 450 °C, yields nanoribbons with lateral dimension down to 20 nm and with random basal plane orientation. The thickness of the nanoribbons is accurately controlled by the thickness of the pre-deposited Si layer. Upon rapid thermal annealing at 900 °C under inert gas, the WS₂ basal planes align parallel to the substrate.

Tunable Volatile-Organic-Compound Sensor by Using Au Nanoparticle Incorporation on MoS2

Controlling the charge concentrations of two-dimensional (2D) materials is a critical requirement for realizing versatility and potential application of these materials in high-performance electronics and sensors. In order to exploit the novel chemical-sensing characteristics of 2D materials for sensitive and selective sensors, various functionalization methods are needed to ensure efficient doping of channels based on 2D materials. In the present study, the gas-sensing performance of MoS₂ has been significantly enhanced by controlled Au nanoparticle functionalization. By using the difference in reduction potential between the Au precursor and MoS₂ work functions, MoS₂ prepared by chemical exfoliation process was decorated with nanoparticles with sizes of tens of nanometers. The n-doping effect of Au nanoparticles was observed, that is, these particles were found to have facilitated in electron charge transfer from Au to MoS₂. The controlled n-doping effect enables the tuning of the sensing of hydrocarbon-based volatile organic compounds (VOCs) and oxygen-functionalized compounds by MoS₂. A significant step has therefore been made with this study toward solving the limitations imposed by previous MoS₂-based sensors, which mostly produce a single response to various VOC analytes. This controllable chemical doping process for tuning the VOC-sensing performance of MoS₂ can eventually be used in early detection using multichannel sensing systems that have different responses and recognize patterns for target analytes.

Shape-Dependent Defect Structures of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition

Monolayer MoS₂ crystals with tailored morphologies have been shown to exhibit shape-dependent properties and thus have potential applications in building nanodevices. However, a deep understanding of the relationship between the shape and defect structures in monolayer MoS₂ is yet elusive. Monolayer MoS₂ crystals in polygonal shapes, including triangle, tetragon, pentagon, and hexagon, are grown using the chemical vapor deposition technique. Compared with other shapes, the hexagon MoS₂ crystal contains more electron-donor defects that are mainly due to sulfur vacancies. In the triangular shapes, the defects are mainly distributed at the vertices of the shapes while they are located at the center of hexagonal shapes. On the basis of the Coulomb interaction of exciton and trion, quantitative calculations demonstrate a high electron density (∼10¹²/cm²) and high Fermi level (E_C - E_F = 15 meV) for hexagonal shape at room temperature, compared to triangular shapes (∼10¹¹/cm², E_C - E_F ≈ 30 meV). These findings verify that a much higher number of donor-like sulfur vacancies are formed in hexagonal MoS₂ shapes. This property allows more electrons or trions to localize in such sites through the physical/chemical adsorption of O₂/H₂O, which results in a strong enhancement of the light emission efficiency in the hexagonal crystal. The findings provide a better understanding of the formation of shape-dependent defect structures of monolayer MoS₂ crystals and are inspiring for applications in fabricating nanoelectronic and optoelectronic devices through defect engineering.

Large area growth of vertically aligned luminescent MoS2 nanosheets

Vertically aligned MoS₂ nanosheets (NSs) with exposed edges were successfully synthesized over a large area (∼2 cm²). The NSs were grown using an ambient pressure chemical vapor deposition technique via rapid sulfurization of sputter deposited thick molybdenum films. Extensive characterization of the grown MoS₂ NSs has been carried out using high resolution scanning and transmission electron microscopy (SEM & TEM). A special care was given to the TEM lamella preparation process by means of a focused ion beam to preserve the NS growth direction. The cross-section TEM measurements revealed the growth of densely packed, vertically aligned and straight MoS₂ NSs. Additional characterization techniques such as atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence (PL) were used to evaluate the MoS₂ NSs. These studies revealed the high crystallinity and quality of the synthesized NSs. The MoS₂ NSs show visible light emission similar to mechanically exfoliated monolayer MoS₂ NSs. The striking PL signal comes from the exposed edges as shown by experimental and theoretical calculations. The vertical MoS₂ NSs also exhibit a hydrophobic character with a contact angle of 114°. The as-grown MoS₂ NSs would be highly useful in the development of catalysis, nano-optoelectronics, gas-sensing and bio-sensing device applications.

Enhanced photoresponse of ZnO quantum dot-decorated MoS2 thin films

This paper reports on high photo responsivity (R_λ ∼ 1913 AW⁻¹) of MoS₂ photodetector by decorating a thin layer of ZnO quantum dots on MoS₂.

Enhanced light–matter interaction of a MoS2 monolayer with a gold mirror layer

The light–matter interaction of the MoS₂ monolayer can be enhanced on a substrate with a gold mirror layer.

Influence of O2, H2O and airborne hydrocarbons on the properties of selected 2D materials

Adsorption of molecules from the ambient environment significantly changes the optical, electrical, electrochemical, and tribological properties of 2D materials.

Layer-number dependent and structural defect related optical properties of InSe

We present systematic investigations on the layer-dependent optical properties of InSe and modify its excitonic states by electron beam irradiation.

Auger Recombination in Chemical Vapor Deposition-Grown Monolayer WS2

Reduced dimensionality and strong Coulombic interactions in monolayer semiconductors lead to enhanced many-body interactions. Here, we report Auger recombination, i.e., exciton-exciton annihilation, in large-area chemical vapor deposition-grown monolayer WS₂. Using ultrafast spectroscopy, we experimentally determine the Auger rate to be 0.089 ± 0.001 cm²/s at room temperature, which is an order of magnitude greater than the bulk value. This nonradiative recombination pathway dominates, regardless of excitation energy, for exciton densities greater than 8.0 ± 0.6 × 10¹⁰ cm^-2 and below the Mott density. Higher-energy excitation above the A exciton resonance may initially produce a hot electron-hole gas that precedes exciton formation. Therefore, we use resonant excitation of the A exciton to ensure accuracy and avoid artifacts associated with other photogenerated species.

Oxygen-Incorporated MoS2 Nanosheets with Expanded Interlayers for Hydrogen Evolution Reaction and Pseudocapacitor Applications

Two-dimensional transition-metal dichalcogenides (TMDs) nanosheets have attracted tremendous research interest. Engineering the structure of MoS₂ may result in desirable performance for energy applications. In this work, oxygen-incorporated MoS₂ nanosheets with expanded interlayers have been synthesized by a solvothermal reaction. The oxygen-incorporated MoS₂ nanosheets with rich defects demonstrate excellent hydrogen evolution reaction activity with a small Tafel slope of 42 mV decade^-1 as well as excellent long-term stability. Interestingly, a large expanded ∼8.40 Å interlayer of (002) faces can be achieved by controlling the reaction time. This material also shows excellent long-term cycling stability (up to 20 000 cycles) as well as high specific capacitance for pseudocapacitors. We believe that the structural modification strategy can be applied for other TMDs to further optimize the performance for various applications.

The Effect of VMoS3 Point Defect on the Elastic Properties of Monolayer MoS2 with REBO Potentials

Structural defects in monolayer molybdenum disulfide (MoS2) have significant influence on the electric, optical, thermal, chemical, and mechanical properties of the material. Among all the types of structural defects of the chemical vapor phase-grown monolayer MoS2, the VMoS3 point defect (a vacancy complex of Mo and three nearby S atoms) is another type of defect preferentially generated by the extended electron irradiation. Here, using the classical molecular dynamics simulation with reactive empirical bond-order (REBO) potential, we first investigate the effect of VMoS3 point defects on the elastic properties of monolayer MoS2 sheets. Under the constrained uniaxial tensile test, the elastic properties of monolayer MoS2 sheets containing VMoS3 vacancies with defect fraction varying from 0.01 to 0.1 are obtained based on the plane anisotropic constitutive relations of the material. It is found that the increase of VMoS3 vacancy concentration leads to the noticeable decrease in the elastic modulus but has a slight effect on Poisson's ratio. The maximum decrease of the elastic modulus is up to 25 %. Increasing the ambient temperature from 10 K to 500 K has trivial influences on the elastic modulus and Poisson's ratio for the monolayer MoS2 without defect and with 5 % VMoS3 vacancies. However, an anomalous parabolic relationship between the elastic modulus and the temperature is found in the monolayer MoS2 containing 0.1 % VMoS3 vacancy, bringing a crucial and fundamental issue to the application of monolayer MoS2 with defects.

Improving the luminescence enhancement of hybrid Au nanoparticle-monolayer MoS2 by focusing radially-polarized beams

Monolayer transition-metal dichalcogenides (TMDs) have grown as fantastic building blocks for optoelectronic applications, owing to their direct band gap, transparency, and mechanical flexibility. Since the luminescence of monolayer TMDs suffers from low light absorption and emission, surface plasmons, which confine light at subwavelength and enhance the local electric field, are utilized to boost both excitation and emission fields of TMDs, enabling strong light-matter interaction at the nano-scale. Meanwhile, radially-polarized beams (RPBs) as new and attractive excitation source have found many applications in surface plasmon polaritons, optical tweezer and so on. Here, by using RPBs, we demonstrate the photoluminescence (PL) enhancement of monolayer molybdenum disulfide (MoS₂) hybridized with 210 nm-diameter gold nanoparticle (AuNP) is improved by about 1.37-fold compared with linearly-polarized beams (LPBs). Besides, the PL enhancement with RPBs depends on the size of AuNP as well. With 210nm-diameter AuNP, the PL enhancement is more than 1.5-fold higher than that with 60nm-diameter AuNP. This study highlights that RPBs are superior to LPBs for tuning the near-field system response and shows that RPBs drive a valuable avenue to further study the emerging two-dimentional materials.

Modulating Electronic Properties of Monolayer MoS2 via Electron-Withdrawing Functional Groups of Graphene Oxide

Modulation of the carrier concentration and electronic type of monolayer (1L) MoS₂ is highly important for applications in logic circuits, solar cells, and light-emitting diodes. Here, we demonstrate the tuning of the electronic properties of large-area 1L-MoS₂ using graphene oxide (GO). GO sheets are well-known as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy. The optical and electronic properties of GO-treated 1L-MoS₂ are dramatically changed. The photoluminescence intensity of GO-treated 1L-MoS₂ is increases by more than 470% compared to the pristine sample because of the increase in neutral exciton contribution. In addition, the A_1g peak in Raman spectra shifts considerably, revealing that GO treatment led to the formation of p-type doped 1L-MoS₂. Moreover, the current vs voltage (I-V) curves of GO-coated 1L-MoS₂ field effect transistors show that the electron concentration of 1L-MoS₂ is significantly lower in comparison with pristine 1L-MoS₂. Current rectification is also observed from the I-V curve of the lateral diode structure with 1L-MoS₂ and 1L-MoS₂/GO, indicating that the electronic structure of MoS₂ is significantly modulated by the electron-withdrawing functional group of GO.

Sequential Solvent Exchange Method for Controlled Exfoliation of MoS2 Suitable for Phototransistor Fabrication

In this study, flakes of molybdenum disulfide (MoS₂) with controlled size and thickness are prepared through sequential solvent exchange method by sonication in dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) solvents. While NMP acts more effectively in reducing the thickness of flakes, DMF shows better potential in conserving the lateral size of nanosheets. The distribution of size and thickness of nanoflakes as a function of sonication time verifies that extended sonication results in dramatic drop of the dimension of the exfoliated flakes. This technique leads to the formation of few-layered MoS₂ flakes without further drop of their lateral dimensions. It has been observed that by exposing the bulk MoS₂ powders to oxygen plasma, the exfoliation process is accelerated without converting to 2H-MoS₂ structures. Finally, a phototransistor has been fabricated based on few-layered MoS₂ layers with a field effect mobility of ∼2.1 cm² V^-1 s^-1 showing a high response to laser excitation of 532 nm wavelength.

Temperature-dependent photoluminescence emission and Raman scattering from Mo1-x W x S2 monolayers

2D transition metal dichalcogenide (TMD) alloys with tunable band gaps have recently gained wide interest due to their potential applications in future nanoelectronics and optoelectronics. Here, we report the temperature-dependent photoluminescence (PL) and Raman spectra of Mo1-x W x S2 monolayers with W composition x = 0, 0.29, 0.53, 0.66 and 1 in the temperature range 93-493 K. We observed a linear temperature dependence of PL emission energy and Raman frequency. The PL intensity is enhanced at high temperature (>393 K). The temperature coefficients are negative for both PL and Raman bands, which may result from anharmonicity, thermal expansion and composition disorder.

Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects

Owing to their excellent physical properties, atomically thin layers of molybdenum disulfide (MoS₂ ) have recently attracted much attention due to their nonzero-gap property, exceptionally high electrical conductivity, good thermal stability, and excellent mechanical strength, etc. MoS₂ -based devices exhibit great potential for applications in optoelectronics and energy harvesting. Here, a comprehensive review of various doping strategies is presented, including wet doping and dry doping of atomically crystalline MoS₂ thin layers, and the progress made so far for their doping-based prospective applications is also discussed. Finally, several significant research issues for the prospects of doped-MoS₂ in industry, as a guide for 2D material community, are also provided.

Effect of Al2O3 Deposition on Performance of Top-Gated Monolayer MoS2-Based Field Effect Transistor

Deposition of high-k dielectrics on two-dimensional MoS₂ is an important process for successful application of the transition-metal dichalcogenides in electronic devices. Here, we show the effect of H₂O reactant exposure on monolayer (1L) MoS₂ during atomic layer deposition (ALD) of Al₂O₃. The results showed that the ALD-Al₂O₃ caused degradation of the performance of 1L MoS₂ field effect transistors (FETs) owing to the formation of Mo-O bonding and trapping of H₂O molecules at the Al₂O₃/MoS₂ interface. Furthermore, we demonstrated that reduced duration of exposure to H₂O reactant and postdeposition annealing were essential to the enhancement of the performance of top-gated 1L MoS₂ FETs. The mobility and on/off current ratios were increased by factors of approximately 40 and 10³, respectively, with reduced duration of exposure to H₂O reactant and with postdeposition annealing.

The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2

We report on preparation dependent properties observed in monolayer WS₂ samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO₂, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS₂) exhibit distinctly different optical properties than transferred WS₂ (x-WS₂). In the case of CVD growth on Si/SiO₂, following transfer to fresh Si/SiO₂ there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS₂ is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS₂ for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS₂. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties.

Optical Gain in MoS2 via Coupling with Nanostructured Substrate: Fabry-Perot Interference and Plasmonic Excitation

Despite the direct band gap of monolayer transition metal dichalcogenides (TMDs), their optical gain remains limited because of the poor light absorption in atomically thin, layered materials. Most approaches to improve the optical gain of TMDs mainly involve modulation of the active materials or multilayer stacking. Here, we report a method to enhance the optical absorption and emission in MoS2 simply through the design of a nanostructured substrate. The substrate consisted of a dielectric nanofilm spacer (TiO2) and metal film. The overall photoluminescence intensity from monolayer MoS2 on the nanostructured substrate was engineered based on the TiO2 thickness and amplified by Fabry-Perot interference. In addition, the neutral exciton emission was selectively amplified by plasmonic excitations from the local field originating from the surface roughness of the metal film with spacer thicknesses of less than 10 nm. We further demonstrate that the quality factor of the device can also be engineered by selecting a spacer material with a different refractive index.

Two-dimensional lateral heterojunction through bandgap engineering of MoS2 via oxygen plasma

The present study explores the structural, optical (photoluminescence (PL)), and electrical properties of lateral heterojunctions fabricated by selective exposure of mechanically exfoliated few layer two-dimensional (2D) molybdenum disulfide (MoS2) flakes under oxygen (O2)-plasma. Raman spectra of the plasma exposed MoS2 flakes show a significant loss in the structural quality due to lattice distortion and creation of oxygen-containing domains in comparison to the pristine part of the same flake. The PL mapping evidences the complete quenching of peak A and B consistent with a change in the exciton states of MoS2 after the plasma treatment, indicating a significant change in its band gap properties. The electrical transport measurements performed across the pristine and the plasma-exposed MoS2 flake exhibit a gate tunable current rectification behavior with a rectification ratio up to 1.3  ×  10(3) due to the band-offset at the pristine and plasma-exposed MoS2 interface. Our Raman, PL, and electrical transport data confirm the formation of an excellent lateral heterojunction in 2D MoS2 through its bandgap modulation via oxygen plasma.

Nanoforging Single Layer MoSe2 Through Defect Engineering with Focused Helium Ion Beams

Development of devices and structures based on the layered 2D materials critically hinges on the capability to induce, control, and tailor the electronic, transport, and optoelectronic properties via defect engineering, much like doping strategies have enabled semiconductor electronics and forging enabled introduction the of iron age. Here, we demonstrate the use of a scanning helium ion microscope (HIM) for tailoring the functionality of single layer MoSe2 locally, and decipher associated mechanisms at the atomic level. We demonstrate He(+) beam bombardment that locally creates vacancies, shifts the Fermi energy landscape and increases the Young's modulus of elasticity. Furthermore, we observe for the first time, an increase in the B-exciton photoluminescence signal from the nanoforged regions at the room temperature. The approach for precise defect engineering demonstrated here opens opportunities for creating functional 2D optoelectronic devices with a wide range of customizable properties that include operating in the visible region.