MoS2 Single Crystals Research Articles

Electronic structure and exciton shifts in Sb-doped MoS2 monolayer

The effective manipulation of excitons is important for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this. While studies have shown that 2D semiconductors are ideal for excitonic devices, preparation of homogenous substitutional foreign-atom-doped 2D crystals is still difficult. Here we report the preparation of homogenous monolayer Sb-doped MoS2 single crystals via a facile chemical vapor deposition method. A and B excitons are observed in the Sb-doped MoS2 monolayer by reflection magnetic circular dichroism spectrum measurements. More important, compared with monolayer MoS2, the peak positions of two excitons show obvious shifts. Meanwhile, the degeneration of A exciton is also observed in the monolayer Sb-doped MoS2 crystal using photoluminescence spectroscopy, which is ascribed to the impurity energy levels within the band-gap, confirmed by density function theory. Our study opens a door to developing the doping of 2D layered transition metal dichalcogenides with group-V dopants, which is helpful for the fundamental study of the physical and chemical properties of transition metal dichalcogenides.

Structural Determination and Nonlinear Optical Properties of New 1T‴-Type MoS2 Compound.

Noncentrosymmetric MoS2 semiconductors (1H, 3R) possess not only novel electronic structures of spin-orbit coupling (SOC) and valley polarization but also remarkable nonlinear optical effects. A more interesting noncentrosymmetric structure, the so-called 1T‴-MoS2 layers, was predicted to be built up from [MoS6] octahedral motifs by theoreticians, but the bulk 1T‴ MoS2 or its single crystal structure has not been reported yet. Here, we have successfully harvested 1T‴ MoS2 single crystals by a topochemical method. The new layered structure is determined from single-crystal X-ray diffraction. The crystal crystallizes in space group P31m with a cell of a = b = 5.580(2) Å and c = 5.957(2) Å, which is a √3 a × √3 a superstructure of 1T MoS2 with corner-sharing Mo3 triangular trimers observed by the STEM. 1T‴ MoS2 is verified to be semiconducting and possesses a band gap of 0.65 eV, different from metallic nature of 1T or 1T' MoS2. More surprisingly, the 1T‴ MoS2 does show strong optical second-harmonic generation signals. This work provides the first layered noncentrosymmetric semiconductor of edge-sharing MoS6 octahedra for the research of nonlinear optics.

An approach to high-throughput growth of submillimeter transition metal dichalcogenide single crystals.

High-throughput growth of large size transition metal dichalcogenide (TMD) single crystals is an important challenge for their applications in the next generation electronic and optoelectronic integration devices. Here we report the high-throughput growth of submillimeter monolayer TMD single crystals by two-stage space confined chemical vapor deposition, where the nucleation density of TMD crystals is significantly decreased for the growth of large size monolayer crystals by the space confinement effect. Moreover, high-throughput growth of submillimeter TMD crystals is also achieved by stacking the substrates along the perpendicular direction to the flow of the reaction gases. The mobilities of the TMD materials produced in this way are up to 1.2, 17.0 and 25.0 cm2 (V s)-1 for monolayer WS2, WSe2 and MoS2 single crystals, respectively. The results demonstrate that two-stage space confined growth is a highly promising method for high-throughput fabrication of high-quality submillimeter monolayer TMD single crystals, which will pave a new pathway to large-scale production of TMD-based electronic and optoelectronic devices.

Na-assisted fast growth of large single-crystal MoS2 on sapphire

Monolayer molybdenum sulfide (MoS2), a typical semiconducting transition metal dichalcogenide, has emerged as a perfect platform for next-generation electronics and optoelectronics due to its sizeable band gap and strong light–matter interactions. Nevertheless, the controlled growth of a monolayer MoS2 single-crystal with a large-domain size and high crystal quality still faces great challenges. Herein, we demonstrate the fast growth of a large-domain monolayer MoS2 on the c-plane sapphire substrate with the assistance of sodium chloride (NaCl) crystals as the intermediate promoter. Particularly, the volatilization temperature of the NaCl crystal and the growth temperature of MoS2 are established to be the key parameters that influence the growth efficiency of MoS2 at an optimized growth condition. Monolayer triangular MoS2 domain with an edge length ∼300 μm is obtained within 1 min, featured with a growth rate ∼5 μm s–1. The Na element from the NaCl crystal is found to be able to facilitate the two dimensional growth of monolayer MoS2. This work thus offers novel insights into the high-efficiency production of large-domain monolayer MoS2 on insulating growth substrates.

High-performance transistors based on monolayer CVD MoS2 grown on molten glass

Transition metal dichalcogenides (TMDCs) are emerging two-dimensional materials for their potential in next-generation electronics. One of the big challenges is to realize a large single-crystal TMDCs film with high mobility, which is critical for channel materials. Herein, we report an optimized atmospheric pressure chemical vapor deposition method for growing large single-crystal monolayer MoS2 on molten glass substrate with domain size up to 563 μm. Better interface quality can be achieved using high-κ dielectrics with respect to the conventional thermal SiO2. Mobility up to 24 cm2 V−1 s−1 at room temperature and 84 cm2 V−1 s−1 at 20 K can be obtained. This low-cost growth of high-quality, large single-crystal size of two dimensional materials provides a pathway for high-performance two dimensional electronic devices.

Room temperature magnetron sputtering and laser annealing of ultrathin MoS2 for flexible transistors

Pulsed magnetron sputtering and subsequent laser annealing represents a scalable route for producing two-dimensional (2D) semiconducting grade molybdenum disulfide (MoS2) directly on the surface of flexible polymer substrates. In this study the room temperature magnetron sputtering was used to deposit 10 nm thick, amorphous MoS2 films on flexible PDMS and rigid SiO2/Si substrates. The films were then crystallized using an ultrafast 248 nm pulsed laser to produce polycrystalline 2HMoS2 over large areas. Raman and XPS analysis confirmed that pulsed laser annealing with 1.04 mJ/mm2 energy density induced hexagonal 2H crystal structure, while preserving MoS2 chemical composition and avoiding formation of oxide phases or damage to the temperature-sensitive polymer surface. Top-gated field effect transistor (FET) devices with laser annealed sputter grown MoS2 were directly fabricated on PDMS surfaces. Oxygen substitution of sulfur in sputter grown MoS2 and polycrystallinity of laser annealed 2HMoS2 films resulted in reduced mobility values, when compared to mechanically exfoliated and chemical vapor deposition grown single crystal 2D MoS2. However, the described approach is intrinsically scalable and provides a direct growth route for the fabrication of 2D transition metal dichalcogenide semiconducting devices on the surface of flexible and stretchable polymers.

Synthesis mechanism of MoS2 layered crystals by chemical vapor deposition using MoO3 and sulfur powders

Monolayer MoS2 layered crystals have attracted significant attention owing to their potential applicability in emerging devices, and chemical vapor deposition (CVD) is the best method so far to obtain monolayer MoS2 single crystals. Although many studies have been published on MoS2 monolayer crystals grown by CVD, there is a lack of understanding of its synthesis pathway. Therefore, in this paper, we studied the mechanism of the synthesis pathway when monolayer MoS2 crystals are synthesized by conventional CVD using MoO3 and sulfur powders. It was found that solid MoO2 produced by the reduction of MoO3 by sulfur plays a very important role in the synthesis of MoS2 layered crystals as an intermediate phase.

Reduction of Aqueous CO2 to 1-Propanol at MoS2 Electrodes

Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS2 or thin-film MoS2 electrodes yields 1-propanol as the major CO2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of −0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO2 to 1-propanol were ∼3.5% for MoS2 single crystals and ∼1% for thin films with low edge-site densities. Reduction of CO2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e– and 18 H+ in a process that involves the formation of 2 C–C bonds. NMR analyses using 13CO2 showed the production of 13C-labeled 1-propanol. In all cases, the vast majority of the Faradaic current resulted in hydrogen evolution via water reduction. H2S was detected qualitatively when single-crystal MoS2 electrodes were used, indicating that some desulfidization of si...

Observation of interlayer excitons in MoSe2 single crystals

Interlayer excitons are observed coexisting with intralayer excitons in bi-layer, few-layer, and bulk MoSe2 single crystals by confocal reflection contrast spectroscopy. Quantitative analysis using the Dirac-Bloch-Equations provides unambiguous state assignment of all the measured resonances. The interlayer excitons in bilayer MoSe2 have a large binding energy of 153 meV, narrow linewidth of 20 meV, and their spectral weight is comparable to the commonly studied higher-order intralayer excitons. At the same time, the interlayer excitons are characterized by distinct transition energies and permanent dipole moments providing a promising high temperature and optically accessible platform for dipolar exciton physics.

Conversion of Single Crystal (NH4)2Mo3S13·H2O to Isomorphic Pseudocrystals of MoS2 Nanoparticles

We have prepared nanocrystals of MoS2 across a range of length scales by heating single crystals of the molecular precursor (NH4)2Mo3S13·H2O. Rod-shaped crystals of the polysulfide precursor retain their original morphology after heating at temperatures up to 1000 °C and undergo complete conversion to MoS2 while acting as a template for the confined formation of MoS2 nanocrystals. This solid state transformation proceeds with the release of gaseous species without blowing the crystals apart and leads to formation of pores embedded into a nanocrystalline assembly of the templated nano-MoS2. The obtained assemblies of MoS2 nanocrystals have the exact same shape of the original rod-shaped (NH4)2Mo3S13·H2O crystals indicative of a pseudomorphic shape-retentive process. Such crystal-shaped nanocrystal assemblies show electrical conductivity values similar to a bulk MoS2 single crystal with electron carrier concentration of 1.5 × 1014 cm–3 and mobility of 7 cm2/(V s). The nanocrystals of MoS2 were grown at temp...

Experimental determination of thermal expansion of natural MoS2

We report helium diffraction from natural MoS2 single crystal. The high quality of the samples studied lead to the appearance of sharp and intense in-plane and out-of-plane diffraction peaks, with unusually low background. The pronounced out-of-plane features observed confirm the high corrugation of the 2D surface unit cell. The observation of diffraction features along the two main high symmetry directions allows determining the in-plane surface lattice constant with high accuracy. The measured lattice constant along and is Å. Within experimental error, the MoS2 lattice parameter was found to remain constant in the temperature range between 90 and 522 K, in good agreement with previously reported DFT based calculations.

Role of the Growth Temperature in MoS₂ Growth by Chemical Vapor Deposition.

In this paper, we discuss the effect of synthesis temperature on the lateral growth of MoS2 thin films in chemical vapor deposition. With increasing temperature, surface coverage with MoS2 triangular islands is significantly improved due to an increase in the density of nuclei and fully continuous MoS2 thin film is grown when the growth temperature reached 800 °C. The MoS2 triangular islands grown at the temperature from 650 to 750 °C are monolayer and highly crystalline, whereas the large-area continuous film grown at the temperature of 800 °C is composed of double-layer or overlapping MoS2 nanosheets. Our research provides that synthesis temperature is the key to growth large area and high quality single crystal MoS2 films.

A Facile Space-Confined Solid-Phase Sulfurization Strategy for Growth of High-Quality Ultrathin Molybdenum Disulfide Single Crystals.

Single-crystal transition metal dichalcogenides (TMDs) and TMD-based heterojunctions have recently attracted significant research and industrial interest owing to their intriguing optical and electrical properties. However, the lack of a simple, low-cost, environmentally friendly, synthetic method and a poor understanding of the growth mechanism post a huge challenge to implementing TMDs in practical applications. In this work, we developed a novel approach for direct formation of high-quality, monolayer and few-layer MoS2 single crystal domains via a single-step rapid thermal processing of a sandwiched reactor with sulfur and molybdenum (Mo) film in a confined reaction space. An all-solid-phase growth mechanism was proposed and experimentally/theoretically evidenced by analyzing the surface potential and morphology mapping. Compared with the conventional chemical vapor deposition approaches, our method involves no complicated gas-phase reactant transfer or reactions and requires very small amount of solid precursors [e.g., Mo (∼3 μg)], no carrier gas, no pretreatment of the precursor, no complex equipment design, thereby facilitating a simple, low-cost, and environmentally friendly growth. Moreover, we examined the symmetry, defects, and stacking phase in as-grown MoS2 samples using simultaneous second-harmonic-/sum-frequency-generation (SHG/SFG) imaging. For the first time, we observed that the SFG (peak intensity/position) polarization can be used as a sensitive probe to identify the orientation of TMDs' crystallographic axes. Furthermore, we fabricated ferroelectric programmable Schottky junction devices via local domain patterning using the as-grown, single-crystal monolayer MoS2, revealing their great potential in logic and optoelectronic applications. Our strategy thus provides a simple, low-cost, and scalable path toward a wide variety of TMD single crystal growth and novel functional device design.

Structure Re‐determination and Superconductivity Observation of Bulk 1T MoS2

Abstract2H MoS2 has been intensively studied because of its layer‐dependent electronic structures and novel physical properties. Though the metastable 1T MoS2 with a [MoS6] octahedron was observed over the microscopic area, the true crystal structure of 1T phase has not been strictly determined. Moreover, the true physical properties have not been demonstrated from experiments owing to the challenge for the preparation of pure 1T MoS2 crystals. 1T MoS2 single crystals were successfully synthesized and the crystal structure of 1T MoS2 re‐determined from single‐crystal X‐ray diffraction. 1T MoS2 crystallizes in the space group P m1 with a cell of a=b=3.190(3) Å and c=5.945(6) Å. The individual MoS2 layer consists of MoS6 octahedra sharing edges with each other. More surprisingly, the bulk 1T MoS2 crystals undergo a superconducting transition of Tc=4 K, which is the first observation of superconductivity in pure 1T MoS2 phase.

Structure Re-determination and Superconductivity Observation of Bulk 1T MoS2.

2H MoS2 has been intensively studied because of its layer-dependent electronic structures and novel physical properties. Though the metastable 1T MoS2 with a [MoS6 ] octahedron was observed over the microscopic area, the true crystal structure of 1T phase has not been strictly determined. Moreover, the true physical properties have not been demonstrated from experiments owing to the challenge for the preparation of pure 1T MoS2 crystals. 1T MoS2 single crystals were successfully synthesized and the crystal structure of 1T MoS2 re-determined from single-crystal X-ray diffraction. 1T MoS2 crystallizes in the space group P3‾ m1 with a cell of a=b=3.190(3) Å and c=5.945(6) Å. The individual MoS2 layer consists of MoS6 octahedra sharing edges with each other. More surprisingly, the bulk 1T MoS2 crystals undergo a superconducting transition of Tc =4 K, which is the first observation of superconductivity in pure 1T MoS2 phase.

Crystal‐Field Tuning of Photoluminescence in Two‐Dimensional Materials with Embedded Lanthanide Ions

AbstractLanthanide (Ln) group elements have been attracting considerable attention owing to the distinct optical properties. The crystal‐field surroundings of Ln ions in the host materials can determine their energy level splitting, which is of vital importance to tailor their optical properties. 2D MoS2 single crystals were utilized as the host material to embed Eu3+ and energy‐level splitting was achieved for tuning its photoluminescence (PL). The high anisotropy of the 2D host materials makes them distort the degenerate orbitals of the Ln ions more efficiently than the symmetrical bulk host materials. A significant red‐shift of the PL peak for Eu3+ was observed. The strategy for tailoring the energy level splitting of Ln ions by the highly designable 2D material crystal field provides a new method to extend their optical properties.

Crystal-Field Tuning of Photoluminescence in Two-Dimensional Materials with Embedded Lanthanide Ions.

Lanthanide (Ln) group elements have been attracting considerable attention owing to the distinct optical properties. The crystal-field surroundings of Ln ions in the host materials can determine their energy level splitting, which is of vital importance to tailor their optical properties. 2D MoS2 single crystals were utilized as the host material to embed Eu3+ and energy-level splitting was achieved for tuning its photoluminescence (PL). The high anisotropy of the 2D host materials makes them distort the degenerate orbitals of the Ln ions more efficiently than the symmetrical bulk host materials. A significant red-shift of the PL peak for Eu3+ was observed. The strategy for tailoring the energy level splitting of Ln ions by the highly designable 2D material crystal field provides a new method to extend their optical properties.

Single crystal monolayer MoS2 triangles with wafer-scale spatial uniformity by MoO3 pre-deposited chemical vapor deposition

Two-dimensional transition metal dichalcogenides (TMDs) show a potential application in photoelectric device due to their excellent electrical and optical properties. Here, we report that the MoO3 pre-deposited chemical vapor deposition (CVD) is used to synthesize single crystal monolayer MoS2 triangles on 4in. wafer. We found that the wafer-scale uniformity of MoS2 can be greatly improved by regularly depositing MoO3 particles on substrate before CVD growth. Therefore, a piece of cleanroom wiper was used as a template for implementing precise control of MoO3 pre-deposition. We found that the optimal deposition size of MoO3 particles and the distance between MoO3 particles are about 15μm and 0.9mm, respectively. Both microscopic and spectroscopic characterization results demonstrate that the as-grown MoS2 is highly uniform in space distribution and crystal structure. The electronic performance of MoS2 synthesized by our method is comparable to or even slightly better than those in common CVD samples. The role of MoO3 pre-deposition is not only to effectively control the MoS2 nucleation density but also to overcome poor diffusion of MoO3 source. Our method is expected to accelerate the industrial synthesis of the atomically thin TMD materials.

Piezoreflectance study of Nb-doped MoS2 single crystals

We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transition of the Nb-doped MoS2 single crystals from 25 to 300 K using piezoreflectance (PzR). The energies and broadening parameters of the A and B excitons of the Nb-doped MoS2 samples have been determined accurately via a detailed line shape fit of the PzR spectra. we can infer that the Nb ions are most likely intercalated between the van der Waals gap and stabilize the rhombohedral 3R phase of the MoS2 crystal.

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

Toward the large-area deposition of MoS2 layers, we employ metal-organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS2 grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm2 V-1 s-1 at cryogenic temperatures.