Properties Of MoS2 Thin Films Research Articles

Intercalation of quaternary ammonium cations as a key factor of electron storage in MoS2 thin films

Electrochemical intercalation of cations within two-dimensional transition metal dichalcogenides presents a promising route for tailoring their optoelectronic properties. We have now succeeded in modulating the optical properties of MoS2 thin films through electrochemical intercalation of quaternary ammonium cations. The spectroelectrochemical experiments conducted with varying sizes of the intercalant revealed the size-dependent stability of the intercalated MoS2 nanosheets. The observed absorption change of the exciton bands is reversible and arises from the storage of electrons in MoS2 nanosheets and the subsequent weakening of interlayer van der Waals interactions following cation intercalation. This structural change is evidenced by the emergence of A*1g out-of-plane Raman mode. Additionally, the photoelectron spectroscopy reveals the emergence of a lower binding energy component of Mo 3d and the shift in Fermi level to higher energies, confirming the presence of stored electrons in cation intercalated-MoS2. The underlying mechanism of intercalation-induced property modifications in MoS2 discussed in the present study is useful in developing strategies for energy conversion devices.

Post-annealing in ultra-high vacuum or nitrogen plasma for MoS2 thin films deposited by magnetron sputtering

The thin films of amorphous molybdenum disulfide were deposited at room temperature by magnetron sputtering technique. Post-annealing process in ultra-high vacuum (∼10−8 Pa) or nitrogen plasma environments at the temperatures of 300, 400, 500, and 700 °C have been first proposed to enhance the microstructure and optical properties of MoS2 thin films. The phase transformation of MoS2 thin films from amorphous to polycrystalline was characterized by in situ reflection high-energy electron diffraction during the post-annealing process. The microstructure of MoS2 thin films was also analyzed by Raman spectrum and X-ray diffractometer after the post-annealing process. In addition, the thermal analysis of the differential scanning calorimeter and optical measurement of photoreflectance confirmed the phase transformation of MoS2 thin films. The analysis of photoreflectance also estimated the exciton transition at the bandgap energy of 2.038 eV at 0 K, attributed by the crystalline MoS2 film annealed at 700 °C in ultra-high vacuum. The surface chemical composition of MoS2 thin films has been identified by X-ray photoelectron spectroscopy, but the desulfurization of MoS2 was observed after post-annealing in ultra-high vacuum. Moreover, the preferred orientation of (004) plane in the MoS2 films was performed as the increase in post-annealing temperature.

Nanostructured MoS2 thin films: Effect of substrate temperature on microstructure, optical, and electrical properties

Molybdenum disulfide (MoS2), a two-dimensional transition-metal dichalcogenide, has the potential for applications in next-generation optoelectronic devices. In this work, MoS2 thin films were deposited by using radio frequency magnetron sputtering on glass and silicon substrates at different substrate temperatures. The effect of growth temperature on crystalline structure, morphology, compositional, optical, and electrical properties of MoS2 thin films was systematically evaluated. It is observed that surface morphology depended on the substrate temperature. Nanowormlike structures formed at the surface of films deposited at 100 and 200 °C. Raman analysis indicated that the mode separation distance for films deposited at room temperature was close to 25 cm−1 of the bulk value. However, mode separation was higher than 27 cm−1 when the substrate temperature was high. This can be attributed to sulfur vacancy in the MoS2 lattice and to strain formation. The bandgap of thin films was estimated to be in the range of 2.3–2.8 eV. X-ray photoelectron spectroscopy was used to investigate chemical composition as well as the effect of the substrate temperature on sulfur vacancies in films. Mo(IV)/S ratios were found to be 1.29, 1.94, and 1.87 for substrate temperatures of RT, 100 °C, and 200 °C, respectively. The conductivity of MoS2 thin films varied considerably with the substrate temperature during deposition. The highest conductivity, 10−13 S/cm, was observed at 300 K measurement temperature in films deposited at room temperature.

Defects engineering and enhancement in optical and structural properties of 2D-MoS2 thin films by high energy ion beam irradiation

Transition metal dichalcogenides (TMDs) like molybdenum disulfide (MoS2) have extra-ordinary physical and chemical peculiarities which can be additionally modified by ion beam irradiation. In the present work, we illustrate the tailoring of MoS2 thin films by employing ion beam irradiation. Silicon ion beam with 100 MeV energy was used to have an apparent impact on structural, morphological and optical properties of MoS2 thin films. Sputtered MoS2 films were irradiated with 100 MeV Si8+ ion beam with a fluence of 1E13 ions/cm2. The X-ray diffraction (XRD) technique validates the structural analysis of MoS2 films with hexagonal phase and reduction in the crystallinity is observed after irradiation owing to the generation of defects by high energy ion beam irradiation. MoS2 thin films display Raman modes at ⁓378 cm−1 and ⁓411 cm−1 corresponding to the one molybdenum atom and two sulfur atoms of the same layer vibrates in the converse in-plane and out-of-plane direction respectively. Intensity of Raman peaks is reduced after the employment of ion beam irradiation. Slight reduction in transmittance is observed after irradiation. The intensity of broad PL emission spectra observed at 360 nm showed the enhancement in PL intensity after ion beam irradiation. Rutherford backscattering spectroscopy (RBS) substantiated the presence of molybdenum (Mo) and sulfur (S) elements and 440 nm thickness of the samples. The investigation of the thickness and atomic fraction of the specimen was also governed by simulating the RBS spectrum using SIMNRA software. To demonstrate the changes occur in grain size and surface roughness parameters in ion irradiated and pristine MoS2 thin films were analyzed by AFM (atomic force microscopy). The alterations of binding energy of Mo 3d and S 2p region of MoS2 pristine and ion irradiated thin films were analyzed by X-ray photoelectron spectroscopy (XPS).

Residual Oxygen Effects on the Properties of MoS2 Thin Films Deposited at Different Temperatures by Magnetron Sputtering

Molybdenum disulfide (MoS2) thin films were deposited at different temperatures (150 °C, 225 °C, 300 °C, 375 °C, and 450 °C) on quartz glass substrates and silicon substrates using the RF magnetron sputtering method. The influence of deposition temperature on the structural, optical, electrical properties and deposition rate of the obtained thin films was investigated by X-ray diffraction (XRD), Energy Dispersive Spectrometer (EDS), Raman, absorption and transmission spectroscopies, a resistivity-measuring instrument with the four-probe method, and a step profiler. It was found that the MoS2 thin films deposited at the temperatures of 150 °C, 225 °C, and 300 °C were of polycrystalline with a (101) preferred orientation. With increasing deposition temperatures from 150 °C to 300 °C, the crystallization quality of the MoS2 thin films was improved, the Raman vibrational modes were strengthened, the deposition rate decreased, and the optical transmission and bandgap increased. When the deposition temperature increased to above 375 °C, the molecular atoms were partially combined with oxygen atoms to form MoO3 thin film, which caused significant changes in the structural, optical, and electrical properties of the obtained thin films. Therefore, it was necessary to control the deposition temperature and reduce the contamination of oxygen atoms throughout the magnetron sputtering process.

Modulation of electronic properties of MoS2 thin films by benzyl viologen treatment for IR detection

In this research, benzyl viologen (BV) is incorporated to enhance the n-type carriers in rapid thermally grown MoS2 thin films. A (002) oriented XRD peak, around 2θ = 14.1°, has been observed in BV treated MoS2 thin films. Raman spectra depicted the presence of two characteristics peaks around 389 cm−1 and 408 cm−1 corresponding to E12g and A1g modes of MoS2, respectively. The ideality factor and built-in voltage for the MoS2/Si heterojunction, treated with 25 mM and 6 coats of BV solution, were found to be 1.27 and 0.30 V, respectively for annealing temperature of 300 °C. The carrier concentration, calculated from Mott-Schottky analysis, is found to rise with the increase in BV concentration and annealing temperature up to 300 °C. The BV treated MoS2/Si heterojunction revealed the current On/Off ratio, rise and fall time of 520, 1.9 s and 1.7 s, respectively for the exposure of 940 nm IR light. With 850 nm IR illumination, the current On/Off ratio, rise and fall time decreased to 322, 1.4 s and 1.3 s, respectively. The results obtained provide the preliminary information for the fabrication of efficient next generation IR detectors.

Study of Optical, Electrochemical, and Morphological Properties of MoS2 Thin Films Prepared by Thermal Evaporation

Study of Optical, Electrochemical, and Morphological Properties of MoS2 Thin Films Prepared by Thermal Evaporation

Tribological properties of MoS2 thin films deposited by RF magnetron sputtering technique

This study aims to reduce the friction and wear losses of AISI440C steel, which is preferred in some machine elements like bearing balls, valves, oil pumps, cutters, springs, cams and scissors. The AISI440C substrates were coated with thin film MoS2 which provides low friction and wear. During the coating, steel disc groups were obtained by changing the parameters of deposition including the radio frequency sputtering power and the deposition temperature. The thickness value of the lubricating MoS2 films were determined with a scanning electron microscope. The hardness and elasticity modules of the MoS2 coatings were determined. The data showing friction coefficient-distance relation of wear tests was obtained and the average friction coefficients, wear amounts and wear trace widths were reached. In this way, how changes in substrate deposition temperature and RF sputtering power affect tribological properties were interpreted. As a result, tribological optimisation of the most suitable coating pair parameter was provided.

Investigations on electronic and optical properties of Ag:MoS2 co-sputtered thin films

MoS2 semiconductor is a promising candidate for photovoltaic applications. The effect of Ag incorporation on optical properties of MoS2 thin films was observed in this study. First principle calculations were performed before experimental investigations using well known Wien2k code for estimation of variation in parameters with incorporation of 12.5% Ag content in MoS2. Pure and Ag incorporated MoS2 thin films were fabricated using magnetron sputter deposition technique. Experimentally found optical parameters approximately follow the similar trend observed in simulation. A sharp increase of absorption in Ag incorporated MoS2 thin film makes it more suitable for photovoltaic applications.

Modulation of microstructural and electrical properties of rapid thermally synthesized MoS2 thin films by the flow of H2 gas

Two dimensional transition metal dichalcogenides (TMDC) have gained potential attention of the researchers, owing to their fascinating electronic and optoelectronic properties. Among the TMDC, the widely studied molybdenum disulphide (MoS2) stands out as a prototypical material. In this work, rapid thermal process (RTP) is adopted for the deposition of MoS2 thin film, using sputtered molybdenum (Mo) thin film and thermally evaporated sulphur (S) film. The microstructural and electrical characteristics of the MoS2 thin films were studied by varying the H2 flow rate in the rapid thermal processing furnace. Field Emission Scanning Electron Microscope images showed the improvement in the uniformity of the MoS2 thin films with the increase in the flow rate of H2 gas. The X-ray diffraction peak has been observed at 14.1°, corresponding to the (002) plane of the hexagonal MoS2 (2H). Raman spectra depict two peaks around 400 cm−1 for A1g and E2g1active modes, which a fingerprint of 2H MoS2. The FTIR studies also depicted the enhancement of MoS2 characteristics peaks with the increase in H2 flow rate. In order to study the effect of substrate, MoS2 films were grown on both Si and SiO2/Si substrates. Among the various H2 flow rates used during the RTP, MoS2 thin films grown with 20 sccm H2 gas at 800 °C has shown better microstructural properties. In order to study the semiconductor properties of MoS2 thin films, the carrier concentration has been calculated from the heterojunction capacitance and is found to be ~1016 cm−3. The comprehensive study on the effect of H2 flow rate on the properties of MoS2 thin films imparts the underlying role of hydrogen atoms in the deposition process, which paves a way for the controlled synthesis of low dimensional materials.

Electrical properties tunability of large area MoS2 thin films by oxygen plasma treatment

MoS2 thin films prepared via sulfurization of molybdenum films have attracted great attention due to their advantage for scalable synthesis with a large area coverage. However, the MoS2 thin films are typically more resistive than their exfoliated and co-evaporation chemical vapor deposition based counterparts. The ability to modulate the electrical property of MoS2 thin films will have a significant impact on scalable device applications in electronics, sensors, and catalysis. Here, we report the tuning of electrical transport properties of large area MoS2 thin films with different oxygen plasma exposure times. The electrical transport measurements of the pristine and plasma treated samples reveal that with increasing oxygen plasma treatment, the resistance of the MoS2 thin films first decreases by almost an order of magnitude and then increases again. The x-ray photoelectron spectroscopy measurements show that the S:Mo ratio continuously decreases with increasing plasma exposure time. For a short plasma exposure time, the resistance decrease can be explained due to the creation of sulfur vacancies leaving unsaturated electrons with molybdenum (Mo) atoms which act as electron donors. With increasing plasma exposure, more sulfur vacancies and hence more Mo atoms are created, many of which get converted to insulating MoO3 resulting in an increase in the resistance of the MoS2 thin film. The results presented here are a major step forward in realizing the overreaching goals of MoS2 thin films for practical device applications.

Temperature-tuned band gap properties of MoS2 thin films

MoS2 is one of the fascinating members of transition metal dichalcogenides and has attracted great attention due to its various optoelectronic device applications and its characteristic as two-dimensional material. The present paper reports the structural and temperature tuned optical properties of MoS2 thin films grown by RF magnetron sputtering technique. It was observed that the atomic composition ratio of Mo:S was nearly equal to 1:2 and the deposited thin films have hexagonal crystalline structure exhibiting Raman peaks around 376 and 410 cm−1. The band gap energies were determined as 1.66 and 1.71 eV at 300 and 10 K, respectively and temperature dependency of band gap energy was analyzed by means of Varshni and O’Donnell-Chen models.

Effect of Growth Temperature on Physical Properties of MoS2 Thin Films Synthesized by CVD

Due to the application of two-dimensional crystals in different fields, high-quality growth of these materials has attracted more attention from researchers. High-quality monolayer MoS2 with single crystals up to 20 microns in size has been formed on Si substrate by the chemical vapor deposition method. A comprehensive study was carried out on the prepared MoS2 thin films using optical microscopy, atomic force microscopy, x-ray diffraction (XRD) analysis, and Raman spectroscopy. It was concluded that the growth temperature affected the morphology and structure of the synthesized MoS2 sheets. The XRD spectra confirmed that the peak intensity and resolution were highly dependent on the growth temperature. Raman spectroscopy showed that monolayer MoS2 was grown on the silicon substrate at higher temperature, as proved by the Raman frequency difference (∼ 19 cm−1) between two characteristic modes ($$ {\hbox{E}}^{1}_{{2{\rm{g}}}} $$ and A1g). Atomic force micrographs of the films showed the evolution of the surface morphology as a function of the growth temperature.

Thermal annealing effects on the electrophysical characteristics of sputtered MoS2 thin films by Hall effect measurements

Transition-metal dichalcogenide (MoS2) is gaining increasing attention as a promising high-performance material in practical applications because of its wide-range electrical properties. However, the effect of thermal annealing on the electrophysical characteristics of MoS2 remains unclear. In this study, the physical and electrophysical properties of MoS2 thin films were characterized by Hall effect measurements. Large-area and high-quality MoS2 thin films were obtained by RF sputtering and then subjected to ex situ thermal annealing. As the thermal annealing temperature was increased, the crystallinity of the MoS2 thin films improved and the corresponding Fermi level shifted toward the valence band. At thermal annealing temperatures above 900 °C, the conductivity type of the MoS2 thin films changed from the intrinsic n-type to p-type due to the emergence of MoO3. In addition, the orientation of the Hall coefficient was indefinite under thermal annealing, implying the electrical transport anisotropy of the MoS2 thin films, which can be attributed to the spin–orbit coupling of the exposed Mo atoms in the films with sulfur vacancies. Furthermore, the Hall mobility and carrier concentration of the annealed MoS2 thin films were up to 4.40 cm2 V−1 s−1 and 12.5 × 1016 cm−3, respectively, at a Hall testing temperature of 305 K and an external magnetic field of 0.5 T.

Pd- and Au-Decorated MoS2 Gas Sensors for Enhanced Selectivity

To date, chemoresistive gas sensors based on metal oxide semiconductors (MOS) have been the most attractive sensor types for the practical application. However, with the emerging concept of Internet of Everything, high operating temperatures over 300 °C of the gas sensors based on MOS must be reduced to achieve low power consumption and overcome a limited battery capacity of mobile devices. The 2-dimensional materials like MoS2 have been, therefore, one of the most recently studied materials for gas sensors with capability of operation at significantly lower temperatures. However, lacking selectivity toward various target gas species limited their application to gas sensors. Herein, we investigated the effects of noble metals (Pd and Au) decoration on the gas sensing properties of MoS2 thin films. Due to the electronic sensitization of the noble metal catalysts and the formation of Pd hydride, overall gas responses and selectivity were significantly improved toward tests gas species. These results provide clear understandings on the effects of the surface noble metal catalysts on gas sensing properties of MoS2.

Chemical Vapor Deposition Growth of Large-Area Monolayer MoS2 and Fabrication of Relevant Back-Gated Transistor**Supported by the National Natural Science Foundation of China under Grant No 61774064.

A closed two-temperature-zone chemical vapor deposition (CVD) furnace was used to grow monolayer molybdenum disulfide (MoS2) by optimizing the temperature and thus the evaporation volume of the Mo precursor. The experimental results show that the Mo precursor temperature has a large effect on the size and shape transformation of the monolayer MoS2, and at a lower temperature of <760°C, the size of the triangular MoS2 increases with the elevating temperature, while at a higher temperature of >760°C, the shape starts to change from a triangle to a truncated triangle. A large-area triangular monolayer MoS2 with a side length of 145 μm is achieved at 760°C. Further, the as-grown monolayer MoS2 is used to fabricate back-gated transistors by means of electron beam lithography to evaluate the electrical properties of MoS2 thin films. The MoS2 transistors with monolayer MoS2 grown at 760°C exhibit a high on/off current ratio of 106, a mobility of 1.92 cm2/Vs and a subthreshold swing of 194.6 mV/dec, demonstrating the feasible approach of CVD deposition of monolayer MoS2 and the fabrication of transistors on it.

Effect of substrate on the growth and properties of MoS2 thin films grown by plasma-enhanced atomic layer deposition

Plasma-enhanced atomic layer deposition was used to grow molybdenum disulfide films using (tBuN)2(NMe2)2Mo and a remote H2S-Ar plasma as coreactants on three different substrates: thermal oxide on silicon, c-plane sapphire, and epitaxial c-plane GaN on sapphire. Depositions were carried out at 250 °C. The substrates’ effect on the growth of MoS2 was investigated through resonance Raman spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. In addition, transmission electron microscopy was performed on films deposited on electron-transparent silicon nitride membranes. Films of 2H-MoS2 were deposited with atomic-level control of thickness under the deposition conditions studied. By analyzing the resonance Raman spectrum, it was found that higher degrees of crystallinity could be achieved on GaN or Al2O3 substrates compared to thermally oxidized silicon.

Carrier transport properties of MoS2 field-effect transistors produced by multi-step chemical vapor deposition method

We report the transistor properties of MoS2 thin films formed with a multi-step chemical vapor deposition (CVD) method. The established multi-step CVD technique has four steps: MoO3 thermal evaporation, annealing for MoO3 crystallization, sulfurization, and post-annealing. We found that the MoS2 transistor properties were greatly affected by the post-annealing temperature (TPA). The films worked as ambipolar transistors below TPA = 1000 °C. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a TPA of 1000 °C. X-ray photoelectron spectroscopy measurements revealed that the films annealed below 1000 °C had sulfur-rich compositions (S/Mo &amp;gt; 2). The excess S atoms were reduced by elevating the annealing temperature to produce an almost stoichiometric composition (S/Mo = 2) at 1000 °C. These results indicate that excess sulfurs are responsible for the ambipolar operation by acting as acceptors that generate holes. Moreover, the high-temperature annealing at 1000 °C had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 0.20 ± 0 .12 cm2/V s.

Investigation of the dynamic bending properties of MoS2 thin films by interference colours.

A non-contact method for the observation of the elastic deformation of 2D molybdenum disulfide (MoS2) thin films using an ordinary optical microscope is reported. A pulsed laser is used to rapidly increase the bending deformation of the MoS2 thin films via heating. The bending angle of the MoS2 thin films shows high stability, changing only 5% in forty days without external forces. However, the bending angle of the MoS2 thin films substantially decreases after being wetted with the volatile polar solvent tetrahydrofuran (THF), because of its low surface tension. By removing the nano-Newton scale forces on the MoS2 thin films, the bending angle increases significantly within 4 minutes, and this feature of the thin films shows great potential for use in the fabrication of micro-force sensors. This is the first attempt to study the mechanical properties of 2D materials by optical methods. Further utilization of industrially manufactured MoS2 thin films for detecting micro-force qualitatively on the basis of their excellent bending properties would significantly reduce the production costs of micro-force sensors.

Gate-induced electron-state tuning of MoS2: first-principles calculations

The electronic structure of electrostatically doped MoS2 thin films is investigated on the basis of first-principles total-energy calculations. We find that electron injection leads to a rapid downward shift in the energy of the unoccupied nearly free electron (NFE) state relative to other conduction bands. The NFE state finally crosses the Fermi level at an electron density of 0.81 × 1014 cm−2 that is attributable to the strong local electric field induced by charge accumulation near the surface. Electrons accommodated in the NFE state play an important role in determining the conducting properties of MoS2 thin films.