Doping of Fullerene‐Like MoS2 Nanoparticles with Minute Amounts of Niobium

Inorganic fullerene‐like closed‐cage nanoparticles of MoS₂ and WS₂ (IF‐MoS₂; IF‐WS₂), are synthesized in substantial amounts and their properties are widely studied. Their superior tribological properties led to large scale commercial applications as solid lubricants in numerous products and technologies. Doping of these nanoparticles can be used to tune their physical properties. In the current work, niobium (Nb) doping of the nanoparticles is accomplished to an unprecedented low level (≤0.1 at%), which allows controlling the work function and the band gap. The Nb contributes a positive charge, which partially compensates the negative surface charge induced by the intrinsic defects (sulfur vacancies). The energy diagram and position of the Fermi level on the nanoparticles surface is determined by Kelvin probe microscopy and optical measurements. Some potential applications of these nanoparticles are briefly discussed.     

The effect of carrier gas flow rate on the growth of MoS<Subscript>2</Subscript> nanoflakes prepared by thermal chemical vapor deposition

Molybdenum disulfide nanoflakes (MoS₂) are superior material for their semiconducting properties. For bulk and monolayer MoS₂ the band gap changes from indirect-to-direct, respectively. So, it exhibits promising prospects in the applications of optoelectronics and valleytronics, such as solar cells, transistors, photodetectors, etc. In this research, the influence of different Ar flow rates as the carrier gas, is investigated for growing MoS₂ nanoflakes on silicon substrates using one-step thermal chemical vapor deposition by simultaneously evaporating of solid sources like sulfur and molybdenum trioxide powders. The structural and optical properties of the obtained nanoflakes are assessed by using X-ray diffraction pattern, scanning electron microscopy, UV–visible absorption, photoluminescence and Raman spectroscopy. It is shown that, Ar gas flow rate is strongly affects on the final products as few-layer MoS₂ structures. Moreover, the abundance of MoS₂ in comparison to MoO₂ and MoO₃ structures, in the obtained nanoflakes, is influenced by the Ar flow rate.     

The rise of two-dimensional MoS<Subscript>2</Subscript> for catalysis

Two-dimensional (2D) MoS₂ is used as a catalyst or support and has received increased research interest because of its superior structural and electronic properties compared with those of bulk structures. In this article, we illustrate the active sites of 2D MoS₂ and various strategies for enhancing its intrinsic catalytic activity. The recent advances in the use of 2D MoS₂-based materials for applications such as thermocatalysis, electrocatalysis, and photocatalysis are discussed. We also discuss the future opportunities and challenges for 2D MoS₂-based materials, in both fundamental research and industrial applications.     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

Combined spectroscopy and microscopy of supported MoS2 nanoparticles

Supported molybdenum-sulfide nanoparticles are known catalysts for petroleum hydrodesulfurization as well as for electrochemical hydrogen evolution. In this study, we investigate molybdenum-sulfide nanoparticles supported on Au(1 1 1) using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), aiming to correlate spectroscopically determined chemical states with atomically resolved nanostructure. The results of this study allow us to conclude the following: (1) the XPS results from our model system are in good agreement with previously published results on supported MoS₂ for industrial applications, validating in part the fidelity of the model system; (2) STM reveals that catalytically active, crystalline MoS₂ nanoparticles exhibiting the well-known metallic edge state are only present after a post-deposition annealing step in the synthesis procedure, without which the particles exhibit amorphous shapes and incomplete sulfidation; and (3) the sulfided nanoparticles are found to be stable in air at room temperature.     

Controllable growth and characterizations of hybrid spiral-like atomically thin molybdenum disulfide

Monolayer MoS₂ is an emerging two-dimensional semiconductor with wide-ranging potential applications in novel electronic and optoelectronic devices. Here, we reported controlled vapor phase growth of hybrid spiral-like MoS₂ crystals investigated by multiple means of X-Ray photoemission spectroscopy, scanning electron microscopy, atomic force microscopy, kelvin probe force microscopy, Raman and Photoluminescence techniques. Morphological characterizations reveal an intriguing hybrid spiral-like MoS₂ feature whose lower planes are AB Bernal stacking and upper structure is spiral. We ascribe the hybrid spiral-like structure to a screw dislocation drive growth mechanism owing to lower supersaturation and layer-by-layer growth mode. In addition, the electrostatic properties of MoS₂ microflakes with hybrid spiral structures are obvious inhomogeneous and dependent on morphology manifested by kelvin probe force microscopy. Our work deepens the understanding of growth mechanisms of CVD-grown MoS₂, which is also adoptable to other TMDC materials.     

Formation of self-assembled quantum dots of iron oxide thin films by spray pyrolysis from non-aqueous medium

Quantum dots (QDs) of iron oxide have been deposited onto ITO coated glass substrates by spray pyrolysis technique, using ferric chloride (FeCl₃·7H₂O) in non-aqueous medium as a starting material. The non-aqueous solvents namely methanol, ethanol, propanol, butanol and pentanol were used as solvents. The effect of solvents on the film structure and morphology was studied. The structural, morphological, compositional and optical properties were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX), and optical absorption measurement techniques.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

水热法合成纳米花状二硫化钼及其微观结构表征

本文以钼酸钠、硫代乙酰胺为前驱体, 硅钨酸为添加剂, 成功用水热法合成高纯度纳米花状二硫化钼. 产物特性用X射线衍射(XRD)、能量色散谱(EDS)、扫描电子显微镜(SEM)进行表征. XRD和EDS图显示实验产物为二硫化钼, 且其结晶度和层状堆垛良好. SEM图谱则表明二硫化钼为纳米花状结构, 颗粒直径300 nm左右, 由几十上百片花瓣组成, 每片花瓣厚度十个纳米左右. 通过以硅钨酸为变量的梯度实验, 研究发现, 硅钨酸对于纳米花状MoS₂的形成具有重要作用, 不添加硅钨酸, 无法形成纳米花状MoS₂, 此外, 硅钨酸的剂量会影响合成MoS₂的大小和形貌. 本文还对纳米花状二硫化钼的形成机理做了初步的讨论.     

Effects of annealing on structural and morphological properties of e-beam evaporated AgGaSe2 thin films

Polycrystalline AgGaSe₂ thin films were deposited by using single crystalline powder of AgGaSe₂ grown by vertical Bridgman-Stockbarger technique. Post-annealing effect on the structural and morphological properties of the deposited films were studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA) measurements. XRD analysis showed that as-grown films were in amorphous structure, whereas annealing between 300 and 600 °C resulted in polycrystalline structure. At low annealing temperature, they were composed of Ag, Ga₂Se₃, GaSe, and AgGaSe₂ phases but with increasing annealing temperature AgGaSe₂ was becoming the dominant phase. In the as-grown form, the film surface had large agglomerations of Ag as determined by EDXA analysis and they disappeared because of the triggered segregation of constituent elements with increasing annealing temperature. Detail analyses of chemical composition and bonding nature of the films were carried out by XPS survey. The phases of AgO, Ag, Ag₂Se, AgGaSe₂, Ga, Ga₂O₃, Ga₂Se₃, Se and SeO₂ were identified at the surface (or near the surface) of AgGaSe₂ thin films depending on the annealing temperature, and considerable changes in the phases were observed.     

Atomic-registry-dependent electronic structures of sulfur vacancies in ReS2 studied by scanning tunneling microscopy/spectroscopy

Rhenium disulfide (ReS₂) is regarded as a promising candidate for optoelectronic applications (e.g., infrared photodetector), as it maintains a direct bandgap regardless of the number of layers unlike other typical transition metal dichalcogenides. Therefore, it is very important to understand and control the defects of ReS₂ for enhancing the performance of photodevices. In this work, we studied the electronic structures of ReS₂ affected by sulfur vacancies of different atomic registries at the atomic scale. The atomic and electronic structures of the mechanically exfoliated ReS₂ flakes were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), and were confirmed using density functional theory (DFT) calculations. The atomic structural models indicate four distinguishable atomic registries of sulfur vacancies on one face of ReS₂. Energetically, these atomic vacancies prefer to locate on the bottom side of the top monolayer of ReS₂ flakes. Only two among four possible kinds of vacancies could be observed using STM and STS, and they were identified using additional DFT calculations. We believe that our results regarding the identification of the defects and understanding the corresponding effects for electronic structures will provide important insights to enhance the performances of ReS₂-based optoelectronic devices in the future.     

Electrical behavior of the Au/MoS2 interface studied by light emission induced by scanning tunneling microscopy

Light emitted in the tunneling junction of a scanning tunneling microscope has been used to establish the electrical characteristics of nanojunctions made of Au islands deposited on flat MoS₂ surfaces. It is shown that these characteristics are those of rectifying contacts when the gold islands are isolated and that they evolve toward those of ohmic contacts when the island density increases. It is observed that the rectifying behavior also evolves over time as on infinite metal/semiconductor contacts. Using the STM tip, single gold islands can be manipulated on the MoS₂ surface so that their electrical behavior can be changed depending on their position with regard to the other islands.     

Structural and mechanical properties of MoS2–Ix nanotubes and Mo6SxIy nanowires

Using a scanning probe microscope, we investigate the structure, electronic and mechanical properties of MoS₂–I_x nanotubes and Mo₆S_xI_y nanowires. The electronic properties are interestingly very sensitive to the stoichiometry of the nanowires, which can be controlled by adjusting the synthesis conditions. In addition to that, we find also remarkable mechanical properties where molecules can be cut and recombined or deformed without any loss of structural integrity. We demonstrate this by deforming Mo₆S_xI_y nanowires to highly strained configurations without causing irreversible changes to their structures. The rupturing and/or welding process of these nanowires, using AFM manipulation, shows that the molecules stretch to more than 30% of its relaxed configuration before plastic deformation occurs.     

STM/STS characterization of platinum silicide nanostructures grown on a Pt(1 1 1) surface

Formation of the platinum silicides nanostructures and their electronic properties have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy. The investigated structures have been grown by solid state epitaxy upon deposition of the Si atoms (coverage about 0.2 ML) and sequential annealing at temperature range 600-1170 K. The formation of the Pt₂Si and PtSi islands was investigated until the Si atoms embedded into the Pt substrate at the 1170 K. The images of the silicides structures and Pt substrates with atomic resolution have been recorded. The evolution of the spectroscopic curves both for substrates and nanostructures, corresponding to the structural and sizes changes, have been shown.     

Theoretically exploring the possible configurations,the electronic and transport properties of MoS2-OH bilayer

Inspired by MoS₂-OH bilayer framework (Zhu et al. 2019 [19]), first principles calculations are applied to explore its possible configurations as well as their electronic and transport properties. The calculated results indicate O-MoS₂ and OH…O-MoS₂ are two primary configuration in MoS₂-OH bilayer. It shows negligible difference in electronic structure between O-MoS₂ and pure MoS₂, but a flat band arise at the Fermi level in OH…O-MoS₂. Their contact characteristics show larger binding energy with selected metals and smaller contact barrier with Pt electrode. Besides, the currents of both O-MoS₂ and OH…O-MoS₂ are enlarged compared with that of pure MoS₂ in finite bias, indicating MoS₂-OH bilayer may be potential candidate for future electron device applications.     

Robust Fabrication of Quantum Dots on Few‐Layer MoS2 by Soft Hydrogen Plasma and Post‐Annealing

Apart from unique properties of layered transition‐metal dichalcogenide nanosheets like MoS₂, quantum dots (QDs) from these layered materials promise novel science and applications due to their quantum confinement effect. However, the reported fabrication techniques for such QDs all involve the use of liquid organic solvents and the final material extraction from such liquid dispersions. Here a novel and convenient dry method for the synthesis of MoS₂ quantum dots interspersed on few‐layer MoS₂ using soft hydrogen plasma treatment followed by post‐annealing is demonstrated. The size of MoS₂ nanodots can be well controlled by adjusting the working pressure of hydrogen plasma and post‐thermal annealing. This method relies on the cumulative hydrogen ion bombardment effect which can destroy the hexagonal structure of the top MoS₂ layer and disintegrate the top layer into MoS₂ nanodots and even QDs. Post‐thermal annealing can further reduce the size. Such MoS₂ quantum dots interspersed on few‐layer MoS₂ exhibit two new photoluminescence peaks at around 575 nm because of the quantum confinement effect. This dry method is versatile, scalable, and compatible with the semiconductor manufacturing processes, and can be extended to other layered materials for applications in hydrogen evolution reaction, catalysis, and energy devices.     

Deposition and characterization of layer-by-layer sputtered AgGaSe2 thin films

Sputtering technique has been used for the deposition of AgGaSe₂ thin films onto soda-lime glass substrates using sequential layer-by-layer deposition of GaSe and Ag thin films. The analysis of energy dispersive analysis of X-ray (EDXA) indicated a Ga-rich composition for as-grown samples and there was a pronounce effect of post-annealing on chemical composition of AgGaSe₂ thin film. X-ray diffraction (XRD) measurements revealed that Ag metallic phase exists in the amorphous AgGaSe₂ structure up to annealing temperature 450 °C and then the structure turned to the single phase AgGaSe₂ with the preferred orientation along (1 1 2) direction with the annealing temperature at 600 °C. The surface morphology of the samples was analyzed by scanning electron microscopy (SEM) measurements. The structural parameters related to chalcopyrite compounds have been calculated. Optical properties of AgGaSe₂ thin films were studied by carrying out transmittance and reflectance measurements in the wavelength range of 325-1100 nm at room temperature. The absorption coefficient and the band gap values for as-grown and annealed samples were evaluated as 1.55 and 1.77 eV, respectively. The crystal-field and spin-orbit splitting levels were resolved. These levels (2.03 and 2.30 eV) were also detected from the photoresponse measurements almost at the same energy values. As a result of the temperature dependent resistivity and mobility measurements in the temperature range of 100-430 K, it was found that the decrease in mobility and the increase in carrier concentration following to the increasing annealing temperature attributed to the structural defects (tetragonal distortion, vacancies and interstitials).     

Substrate dependent properties of electrodeposited EuTe thin films

In electrodeposition, substrate besides providing mechanical support to the electrodeposit, affects significantly the structural and morphological properties of a film. Electrodeposition and characterization of EuTe thin films onto different substrates such as stainless steel (SS), titanium (Ti), copper (Cu), fluorine-doped tin oxide (F:SnO₂) covered glasses have been described. The deposition potentials have been estimated from the polarization curves. The reaction mechanism is proposed for the formation of EuTe electrodeposits. Preparative parameters such as deposition potential, current density, and deposition time are studied. The films have been characterized by X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive analysis by X-rays (EDAX) techniques. The electrodeposited EuTe films are polycrystalline on all the substrates with same cubic crystal structure. The SEM studies reveal that the surface morphology is different for the substrates studied. However, no cracks have been observed in the SEM micrographs. The AFM images show large spherical grains supporting the polycrystalline nature of the samples. The EDAX analysis shows that the EuTe films are nearly stoichiometric, slightly rich in tellurium.     

Effect of nanostructured MoS2 morphology on the glucose sensing of electrochemical biosensors

In this study, the effects of the morphological characteristics of MoS₂ nanomaterials on the glucose sensing of electrochemical biosensors were explored. Nanostructured MoS₂ materials, including nanoparticles (NPs), nanoflowers (NFs), and nanoplatelets (NPLs), were prepared via a simple hydrothermal method. The structure and morphological characteristics of MoS₂ nanomaterials were examined through X-ray diffraction, field emission scanning electron microscopy, and Raman spectroscopy. Electrochemical properties were analyzed through cyclic voltammetry. Results showed that the obtained sensitivity was 64, 68.7, and 77.6 μAmM⁻¹ cm⁻² for MoS₂ NP-, MoS₂ NF-, and MoS₂ NPL-based biosensors, respectively. The limit of detection (LOD) of all MoS₂-based glucose biosensors was 0.081 mM. In addition, the pH, temperature, glucose oxidase (GOx) concentration, reproducibility, specificity, and stability of glucose biosensors with different MoS₂ morphologies were also investigated and indicated the oxidation current response of the MoS₂ NPL-based glucose biosensor was higher than that of MoS₂ NF- and NP-based biosensors.     

Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI₂/MoS₂ into the CH₃NH₃PbI₃/MoS₂ heterostructures via CH₃NH₃I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH₃NH₃PbI₃/MoS₂. Both MoS₂ and CH₃NH₃PbI₃ related photoluminescence (PL) intensity quenching in CH₃NH₃PbI₃/MoS₂ implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH₃NH₃PbI₃ is opposite to that of MoS₂ (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH₃NH₃PbI₃/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.