MoS2 Film Thickness Research Articles

Influence of MoS2 film thickness for nitric oxide gas sensing applications

Influence of MoS2 film thickness for nitric oxide gas sensing applications

One step sputtered MoS2 field-effect transistor

In this study, variable thickness MoS2 which is one of the most important 2D materials, was grown in a single step radio frequency magnetron sputtering (RFMS) method from a MoS2 target. Successful large area, continuous and homogeneous formation of MoS2 thin film have been shown by Raman mapping measurements. Structural, morphological, elemental composition of MoS2 films grown at different thicknesses was performed using SEM and XPS. Dozens of MoS2-based FET was fabricated with a single lithography step on the same substrate.MoS2 based FETs showed a mobility value of 0.07- 0.16 cm2/Vs increasing with MoS2 film thickness and a current on/off ratio of about 103-104. It has been demonstrated that the FETs show ambipolar characteristics.

Optical and electrical tunability in vertically aligned MoS2 thin films prepared by DC sputtering: Role of film thickness

Insights into the fundamentals of structure-property relations are one of the most important key parameters, which can be utilized for tailoring material properties such as electrical, and optical for the photonics, optoelectronics, energy storage, and sensing devices. Herein, we report a controlled single-step, large area growth of highly crystalline vertically grown edge exposed MoS2 thin films using DC magnetron sputtering. To understand a correlation between microstructural and material properties, we have prepared MoS2 films of different thicknesses (∼1 nm–440 nm) by varying sputtering deposition times (DT = 1 s–10 min). We have attempted to augment our prevailing understanding of structure-property relations of MoS2 in order to provide large tunability in the optical and electrical properties. Spectroscopic ellipsometry results suggest that the complex refractive index (n and k), dielectric function (Ɛ1 and Ɛ2), and optical bandgap (Eg = 1.35–1.88 eV) are highly dependent on MoS2 film thickness. The MoS2 electrical resistance was observed in the range of ∼15 kΩ–98 MΩ, displaying an inverse relationship with the film thickness.

Influence of two-dimensional molybdenum disulfide on the surface enhanced Raman scattering effect in a silver nanocavity

Surface-enhanced Raman scattering (SERS) is commonly associated with metal nanostructures. However, recently large Raman enhancement has also been reported for two-dimensional (2D) semiconductors. In the present work, we exam systematically the influence of thin 2D molybdenum disulfide (MoS2) films on the SERS properties of a typical silver (Ag) nanocavity formed by a single Ag nanoparticle (NP) and a flat Ag surface. The dependence of the localized electric field, Raman enhancement and spatial resolution on the NP-MoS2 film distance, the thickness of MoS2 film, as well as the NP radius were quantitatively calculated and analyzed. The results show that the presence of MoS2 film could efficiently enhance the plasmonic coupling between the NP and the substrate, leading to more significant SERS enhancement. Thinner MoS2 film and shorter NP-film distance will further improve the localized plasmonic enhancement. The calculated Raman enhancement factor reaches 9.1 ×1010 with monolayer MoS2 film under the 660-nm excitation. We also show that the spatial resolution for Raman spectroscopy could be significantly improved by reducing the NP radius, and the maximum resolutions is 4.5 nm at the radius size of 20 nm. Our results offer a theoretical guidance for better understanding the SERS properties of the novel 2D material and its further applications.

Heterogeneous integration of AuNRs monolayer with MoS2 film assembled for highly efficient surface-enhanced Raman scattering and significant in improvement electrical conductivity

Integration techniques that combine substrates, metals, and semiconductor components were developed to achieve excellent mechanical compliance and electrical performance simultaneously for wearable electronics. Herein, heterogeneous integration of AuNRs monolayer with MoS2 film on glass slide (or polyethylene terephthalate, PET) substrate had been fabricated into Janus gold nanorods/molybdenum disulfide (AuNRs-MoS2) film with large-area metamaterials by a modified self-assembly methods. Janus AuNRs-MoS2 film showed the high efficient surface-enhanced Raman scattering and significantly enhanced electrical conductivity. The SERS intensity of AuNRs-MoS2 film was obviously higher than that of AuNRs film and MoS2 film due to the densely packed nanoparticles. In addition, the conductivity of Janus AuNRs/MoS2 film increased rapidly as the thickness of MoS2 increased. The conductivity of Janus AuNRs/MoS2 film on PET was obviously higher than that of film on glass slide with similar thickness of MoS2 film. This work provides a platform to inspire novel bilayer approaches with surface enhanced properties.

Effect of MoS2 film thickness on electroluminescence performance of diamond/boron/MoS2/diamond composite films

Diamond/boron/MoS2/diamond composite film electroluminescence (EL) devices with different MoS2 thickness were fabricated on p-type heavy doped silicon substrates. Its composition, microstructure and EL characteristics were tested and discussed. The experimental results show that the EL spectrum of the device has two luminous peaks in the visible region, which is located in the green region and the red region respectively. As the thickness of MoS2 increased from 10 nm to 80 nm, the peak of red luminescence shifted from 676 nm to 704 nm, and the breakdown voltage of an EL device gradually decreased from 420 V to 140 V, meanwhile the EL intensity first increases and then decreases. When thickness of MoS2 is 30 nm, the brightness of the EL device reaches the maximum.

Absorption coefficient estimation of thin MoS2 film using attenuation of silicon substrate Raman signal

Abstract A simple, non-destructive, and convenient method using Raman spectroscopy and Atomic Force Microscopy for extracting the absorption coefficient of MoS2 is presented. The attenuation of the substrate Raman signal intensity due to the MoS2 overlayer is found to be dependent on the MoS2 film thickness estimated from the AFM measurements. Using the light attenuation model from the measurements, the experimentally extracted absorption coefficient of the thin MoS2 flakes is determined to be 2.8 × 106 cm−1. This simple technique is capable of estimating the absorption coefficient of other two-dimensional layered materials.

Ultra-thin MoS2 shell deposited on Ag nanowires for tuning surface-enhanced Raman spectroscopy

In this article, we report an ultra-thin shell of MoS2 (3–7 nm) deposited on Ag nanowires (AgNWs) to form a core-shell hybrid system by the sol-gol method. Since AgNWs is easy to be oxidized, a protection shell is necessary. Compared with commonly used hydrothermal method for MoS2, it could produce a more thinner and homogeneous shell with the least 3 nm thickness of MoS2 shell, which is a breaking-through of making MoS2 shell based on noble nanoparticles. Results show that the obtained hybrid system has great value on surface-enhanced Raman spectroscopy (SERS) due to surface plasmon resonance (SPR) effect of Ag nanowires. Ultra-thin MoS2 shell was suitable for protecting AgNWs and simultaneously had excellent SERS signature, and this effect was rare for MoS2 shells. By decreasing the thickness of MoS2 film from 7 to 3 nm, the intensity of SERS signal could be enhanced. It was observed that the microstructure of this AgNWs@MoS2 system consist of crystal AgNWs surrounded by crystal MoS2. AgNWs@MoS2 showed excellent SERS effect with Rhodamine 6G (R6G) and N719 dyes, the detection limit could reach to 10−5 M. Finally for comparison, local electromagnetic field enhancement of AgNWs@SiO2 was also simulated to indicate AgNWs@MoS2 better.

Relation between film thickness and surface doping of MoS2 based field effect transistors

Ultra-thin MoS2 film doping through surface functionalization with physically adsorbed species is of great interest due to its ability to dope the film without reduction in the carrier mobility. However, there is a need for understanding how the thickness of the MoS2 film is related to the induced surface doping for improved electrical performance. In this work, we report on the relation of MoS2 film thickness with the doping effect induced by the n-dopant adsorbate poly(vinyl-alcohol). Field effect transistors built using MoS2 films of different thicknesses were electrically characterized, and it was observed that the ION/OFF ratio after doping in thin films is more than four orders of magnitudes greater when compared with thick films. Additionally, a semi-classical model tuned with the experimental devices was used to understand the spatial distribution of charge in the channel and explain the observed behavior. From the simulation results, it was revealed that the two-dimensional carrier density induced by the adsorbate is distributed rather uniformly along the complete channel for thin films (<5.2 nm) contrary to what happens for thicker films.

Interface Engineering with MoS2 -Pd Nanoparticles Hybrid Structure for a Low Voltage Resistive Switching Memory.

Metal oxide-based resistive random access memory (RRAM) has attracted a lot of attention for its scalability, temperature robustness, and potential to achieve machine learning. However, a thick oxide layer results in relatively high program voltage while a thin one causes large leakage current and a small window. Owing to these fundamental limitations, by optimizing the oxide layer itself a novel interface engineering idea is proposed to reduce the programming voltage, increase the uniformity and on/off ratio. According to this idea, a molybdenum disulfide (MoS2 )-palladium nanoparticles hybrid structure is used to engineer the oxide/electrode interface of hafnium oxide (HfOx )-based RRAM. Through its interface engineering, the set voltage can be greatly lowered (from -3.5 to -0.8 V) with better uniformity under a relatively thick HfOx layer (≈15 nm), and a 30 times improvement of the memory window can be obtained. Moreover, due to the atomic thickness of MoS2 film and high transmittance of ITO, the proposed RRAM exhibits high transparency in visible light. As the proposed interface-engineering RRAM exhibits good transparency, low SET voltage, and a large resistive switching window, it has huge potential in data storage in transparent circuits and wearable electronics with relatively low supply voltage.

Band alignment at interfaces of synthetic few-monolayer MoS2 with SiO2 from internal photoemission

Electron band alignment at interfaces of SiO2 with directly synthesized few-monolayer (ML) thin semiconducting MoS2 films is characterized by using field-dependent internal photoemission of electrons from the valence band of MoS2 into the oxide conduction band. We found that reducing the grown MoS2 film thickness from 3 ML to 1 ML leads to ≈400 meV downshift of the valence band top edge as referenced to the common energy level of the SiO2 conduction band bottom. Furthermore, comparison of the MoS2 layers grown by a H-free process (sputtering of Mo in sulfur vapor) to films synthesized by sulfurization of metallic Mo in H2S indicates a significant (≈500 meV) electron barrier increase in the last case. This effect is tentatively ascribed to the formation of an interface dipole due to the interaction of hydrogen with the oxide surface.

Thickness-dependent phase transition and optical behavior of MoS2 films under high pressure

We report the Raman and photoluminescence spectroscopic analysis of layered MoS2 under hydrostatic pressure up to ∼ 30 GPa. Unlike previous studies, throughout this work, a special treatment is applied to submerge monolayer, bilayer, multilayer (∼ 200 layers), and bulk MoS2 samples directly into silicone oil without a supporting substrate in a diamond anvil cell, thereby eliminating possible interference from substrate–film contact. A thickness-dependent trend is observed for the 2Hc-to-2Ha phase transition: The transition pressure increases from 19.0 to 25.6 GPa as the system thickness is reduced from bulk to multilayer MoS2; a further decrease in thickness to the bilayer structure increases the transition pressure to ∼ 36 GPa, as predicted theoretically. This exceeds our measured pressure range, indicating the weakening of interlayer repulsive interactions as the MoS2 film thickness is reduced. Our experiment also reveals a monotonic trend of Raman peak shifting vs. film thickness under applied pressure, suggesting that the Raman vibration modes are more responsive to external pressure in thinner films. The photoluminescence emission peak of the monolayer MoS2 exhibits a blue shift under applied pressure at the rate of 23.8 meV·GPa−1. These results show that the structural and optical properties of MoS2 can be effectively modified by applying hydrostatic pressure.

Electrical and photovoltaic properties of residue-free MoS2 thin films by liquid exfoliation method

Molybdenum disulfide (MoS2) film fabricated by a liquid exfoliation method has significant potential for various applications, because of its advantages of mass production and low-temperature processes. In this study, residue-free MoS2 thin films were formed during the liquid exfoliation process and their electrical properties were characterized with an interdigitated electrode. Then, the MoS2 film thickness could be controlled by centrifuge condition in the range of 20 ∼ 40 nm, and its carrier concentration and mobility were measured at about 7.36 × 1016 cm−3 and 4.67 cm2 V−1 s−1, respectively. Detailed analysis on the films was done by atomic force microscopy, Raman spectroscopy, and high-resolution transmission electron microscopy measurements for verifying the film quality. For application of the photovoltaic device, a Au/MoS2/silicon/In junction structure was fabricated, which then showed power conversion efficiency of 1.01% under illumination of 100 mW cm−2.

Performance Investigation of Multilayer MoS2 Thin-Film Transistors Fabricated via Mask-free Optically Induced Electrodeposition.

Transition metal dichalcogenides, particularly MoS2, have recently received enormous interest in explorations of the physics and technology of nanodevice applications because of their excellent optical and electronic properties. Although monolayer MoS2 has been extensively investigated for various possible applications, its difficulty of fabrication renders it less appealing than multilayer MoS2. Moreover, multilayer MoS2, with its inherent high electronic/photonic state densities, has higher output driving capabilities and can better satisfy the ever-increasing demand for versatile devices. Here, we present multilayer MoS2 back-gate thin-film transistors (TFTs) that can achieve a relatively low subthreshold swing of 0.75 V/decade and a high mobility of 41 cm2·V-1·s-1, which exceeds the typical mobility value of state-of-the-art amorphous silicon-based TFTs by a factor of 80. Ag and Au electrode-based MoS2 TFTs were fabricated by a convenient and rapid process. Then we performed a detailed analysis of the impacts of metal contacts and MoS2 film thickness on electronic performance. Our findings show that smoother metal contacts exhibit better electronic characteristics and that MoS2 film thickness should be controlled within a reasonable range of 30-40 nm to obtain the best mobility values, thereby providing valuable insights regarding performance enhancement for MoS2 TFTs. Additionally, to overcome the limitations of the conventional fabrication method, we employed a novel approach known as optically induced electrodeposition (OIE), which allows the flexible and precise patterning of metal films and enables rapid and mask-free device fabrication, for TFT fabrication.

Thickness-Dependent Binding Energy Shift in Few-Layer MoS2 Grown by Chemical Vapor Deposition

The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS2 shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.

The role of MoS2 as an interfacial layer in graphene/silicon solar cells.

The role of MoS2 as an effective interfacial layer in graphene/silicon solar cells is systematically investigated by varying MoS2 film annealing temperature and thickness. It is found that the power conversion efficiency (PCE) is increased by ∼100% from ∼2.3% to ∼4.4% with 80 °C annealed MoS2 film whereas it drops significantly to ∼0.6% with 200 °C annealed MoS2 film. The results are well explained based on the device energy band diagram. That is, the incorporation of MoS2(80) films leads to the formation of type II structure, facilitating hole transport; while valence band mismatch is formed with MoS2(200) films due to the increase in the work function of MoS2. Besides, the PCE increases gradually with decreasing MoS2 film thickness, and "saturates" at about 2 nm. The PCE can be further enhanced to ∼6.6% with the aid of silicon surface passivation. Our work demonstrates that MoS2 is an excellent interfacial layer to improve the PCE with low-temperature annealing (80 °C in air), which may be helpful in developing efficient and low-cost G/Si solar cells.

Investigation of the optical properties of MoS2 thin films using spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) characterization of layered transition metal dichalcogenide (TMD) thin films grown by vapor phase sulfurization is reported. By developing an optical dispersion model, the extinction coefficient and refractive index, as well as the thickness of molybdenum disulfide (MoS2) films, were extracted. In addition, the optical band gap was obtained from SE and showed a clear dependence on the MoS2 film thickness, with thinner films having a larger band gap energy. These results are consistent with theory and observations made on MoS2 flakes prepared by exfoliation, showing the viability of vapor phase derived TMDs for optical applications.

Study of Surface Films of the ZDDP and the MoDTC with Crystalline and Amorphous Overbased Calcium Sulfonates by XPS

The role of amorphous or crystalline overbased detergent and sulfur containing AW/EP additives on the surface films created in a tribometer was investigated. In the crystalline overbased sulfonate containing samples the weld point was higher than for the equivalent blend using an amorphous overbased sulfonate. The calcium carbonate core of the overbased sulfonates was deduced to be present in the surface film when combined with either ZDDP or MoDTC. For samples containing ZDDP the crystalline overbased sulfonate, reduced the phosphate film thickness, while for the MoDTC it reduced the MoS2 film thickness. In both cases significantly more carbon, likely as calcium carbonate (which acts to improve the EP/AW properties of the lubricant), was present when using the crystalline overbased calcium sulfonate compared to the amorphous overbased calcium sulfonate.