Mo-S Bonds Research Articles

Computational Synthesis of MoS2 Layers by Reactive Molecular Dynamics Simulations: Initial Sulfidation of MoO3 Surfaces.

Transition metal dichalcogenides (TMDC) like MoS2 are promising candidates for next-generation electric and optoelectronic devices. These TMDC monolayers are typically synthesized by chemical vapor deposition (CVD). However, despite significant amount of empirical work on this CVD growth of monolayered crystals, neither experiment nor theory has been able to decipher mechanisms of selection rules for different growth scenarios, or make predictions of optimized environmental parameters and growth factors. Here, we present an atomic-scale mechanistic analysis of the initial sulfidation process on MoO3 surfaces using first-principles-informed ReaxFF reactive molecular dynamics (RMD) simulations. We identify a three-step reaction process associated with synthesis of the MoS2 samples from MoO3 and S2 precursors: O2 evolution and self-reduction of the MoO3 surface; SO/SO2 formation and S2-assisted reduction; and sulfidation of the reduced surface and Mo-S bond formation. These atomic processes occurring during early stage MoS2 synthesis, which are consistent with experimental observations and existing theoretical literature, provide valuable input for guided rational synthesis of MoS2 and other TMDC crystals by the CVD process.

Synthesis, Structure, and Electrical Properties of the Mo12 Cluster Sulfide Hg∼2.8KMo12S14 Containing Mercury Chains.

The new compound Hg∼2.8KMo12S14 was synthesized by diffusing mercury into the metastable KMo12S14 compound at 350 °C. Its crystallographic structure, solved from single-crystal X-ray diffraction, shows that the Mo-S framework is maintained during the synthesis. It is based on Mo12S14S6 units interlinked via Mo-S bonds as in the parent compound. The mercury forms linear chains with Hg-Hg distances of about 2.72 Å in the tunnels delimited by the Mo12S14S6 units. Superconductivity was observed below 2.5 K by electrical resistivity and magnetic susceptibility measurements.

Thermodynamic calculations and dynamics simulation on thermal-decomposition reaction of MoS2 and Mo2S3 under vacuum

In this paper, thermal decomposition process of molybdenum concentrate in vacuum was thermodynamically analyzed, and decomposition mechanism of MoS system in molybdenum was mainly studied. To further investigate and analyze the electronic properties in different temperature, the simulation calculations of MoS2(0 0 1) surface and Mo2S3(0 0 1) surface density of states, electron density difference, electronic orbital and Mulliken overlap population were carried out by density functional theory(DFT) formalism. The dynamics simulations results of MoS2(0 0 1) surface and Mo2S3(0 0 1) surface were obtained the temperature range of phase transforming are 1573 K–1673 K and from 1473 K to 1573 K at 20Pa, respectively. The interaction of MoS2 and Mo2S3 shows that thermal-decomposition of MoS2 and Mo2S3 does react, when new SS bonds and MoMo bond are formed instead of MoS bond fracture. The experimental results of the thermal decomposition of MoS2 and Mo2S3 were in accordance with the thermodynamics theoretical calculation results and ab-initio molecular dynamics simulation results.

Investigation on MoS2(1-x)Te2x Mixture Alloy Fabricated by Co-sputtering Deposition

ABSTRACTWe report the synthesis of MoS2(1-x)Te2x by co-sputtering deposition and effect of mixture on its bandgap. The deposition was carried out at room temperature, and the sputtering power on individual MoS2 and MoTe2 targets were varied to obtain films with different compositions. Investigation with X-ray photoelectron spectroscopy confirmed the formation of Mo-Te and Mo-S bonds after post-deposition annealing (PDA), and one of the samples exhibited composition ratio of Mo:S:Te = 1:1.2:0.8 and 1:1.9:0.1 achieving 1:2 ratio of metal to chalcogen. Bandgap of MoS1.2Te0.8 and MoS1.9Te0.1 was evaluated with Tauc plot analysis from the extinction coefficient obtained by spectroscopic ellipsometry measurements. The obtained bandgaps were 1.0 eV and 1.3 eV. The resulting bandgap was lower than that of bulk MoS2 and higher than that of bulk MoTe2 suggesting mixture of both materials was achieved by co-sputtering.

Effects of atomic vacancies and temperature on the tensile properties of single-walled MoS2 nanotubes.

Using molecular dynamics simulations, we study the effects of Mo and S atomic vacancies and different temperatures on the tensile properties of single-walled MoS2 nanotubes through a series of tensile tests. Both armchair and zigzag MoS2 nanotubes under uniaxial tensions show phase transitions. Two types of Mo-S bonds play different roles in this phase transition of MoS2 nanotubes. Moreover, the influences of Mo and S atomic vacancies and temperature on the Young's modulus, ultimate strength and fracture strain of single-walled MoS2 nanotubes are investigated systematically. The results show that Mo and S atomic vacancies have no influence on the Young's modulus of MoS2 nanotubes. However, Mo atomic vacancies result in a significant decrease of ultimate strength and fracture strain of MoS2 nanotubes, while S atomic vacancies have a relatively small influence on the fracture properties of MoS2 nanotubes. With an increase in temperature, the Young's modulus and ultimate strength decrease. When the temperature is higher than 300 K, the fracture is changed from brittle to ductile together with an enhanced fracture strain.

Catalytic characteristics of active corner sites in Co[formula omitted]Mo[formula omitted]S nanostructure hydrodesulfurization – A mechanism study based on DFT calculations

A non-periodic computational CoMoS model consisting of un-promoted Mo-edge (Mo-E), Co-mixed Mo-edge (Mo-X), Co-edge (Co-E) and corner (Corner) sites was used to examine the structural effect of CoMoS nanocluster on its hydrodesulfurization (HDS) reaction activity. A density functional theory (DFT) with numerical atomic functions was used to predict the fine structures of Mo-E, Mo-X, Co-E and Corner under common HDS condition. The elementary thiophene HDS reactions on Mo-X, Co-E and Corner sites were compared. The results show that the Mo-E has short and long MoS bonds; the former is more stable. The hydrogen dissociation on short MoS bond is endothermic and with high activation energy, whereas the hydrogen dissociation on long MoS bond is just the opposite. The Mo-X has only short MoS bonds. The Co atoms on the Mo-X and Corner exhibit a square planar coordinated structure with four sulfur atoms with a CoMo atom pair. The CoMo atom pair can easily form coordinatively unsaturated active site (CUS) by H2S desorption. The CUSs on the Mo-X and Corner sites preferably bond with radical containing sulfides to spin-restricted sulfides, leading to high hydrogenolysis activity of the CS bonds. The di-sulfur atom pairs on the Co-E and Corner sites are effective to dissociate the hydrogen molecule and reduce steric hindrance from adsorbing sulfur intermediates during HDS, leading to high hydrogenation activity at the Co-E and Corner sites. Thiophene HDS reactions can process completely on the Corner site which contains CoMo and di-sulfur atom pairs. Hence, increasing Corner sites in CoMoS is effective and beneficial to HDS catalysis.

DFT Study on the Interaction of Tris(benzene-1,2-dithiolato)molybdenum Complex with Water. A Hydrolysis Mechanism Involving a Feasible Seven-Coordinate Aquomolybdenum Intermediate.

In the present work, the reactivity of the tris(benzene-1,2-dithiolato)molybdenum complex ([Mo(bdt)3]) toward water is studied by means of the density functional theory (DFT). DFT calculations were performed using the M06, B3P86, and B3PW91 hybrid functionals for comparison purposes. The M06 method was employed to elucidate the reaction pathway, relative stability of the intermediate products, nature of the Mo-S bond cleavage, and electronic structure of the involved molybdenum species. This functional was also used to study the transference of electrons from the molybdenum center toward the ligands. The reaction pathway confirms that [Mo(bdt)3] undergoes hydrolysis, yielding dihydroxo-bis(benzene-1,2-dithiolato)molybdenum complex ([Mo(OH)2(bdt)2]) and benzenedithiol. The reaction takes place through seven transition structures, one of them involving an aquo seven-coordinate molybdenum intermediate stabilized by a lone pair (LP) LPO→LPMo hyperconjugative interaction. This heptacoordinate species allows understanding of the observed oxygen atom exchange between water and tertiary phosphines mediated by these complexes. Calculations also show that [Mo(C2H4S2)3] and [Mo(OH)2(C2H4S2)2] have d2 and d0 electronic configuration, and hence an electron pair must be transferred during the course of the hydrolysis. The frontier molecular orbital (FMO) analysis concludes that the electron pair is transferred in the rupture of the second Mo-S bond, from the occupied donating Mo dx2-y2 orbital to the unoccupied C2H4(SH)2 S-C σ* ligand orbital. This result is supported by the bond dissociation energy calculations, which demonstrate that the neutral dissociation of the second Mo-S bond is energetically the more favorable.

The Role of Interfacial Electronic Properties on Phonon Transport in Two-Dimensional MoS2 on Metal Substrates.

We investigate the role of interfacial electronic properties on the phonon transport in two-dimensional MoS2 adsorbed on metal substrates (Au and Sc) using first-principles density functional theory and the atomistic Green's function method. Our study reveals that the different degree of orbital hybridization and electronic charge distribution between MoS2 and metal substrates play a significant role in determining the overall phonon-phonon coupling and phonon transmission. The charge transfer caused by the adsorption of MoS2 on Sc substrate can significantly weaken the Mo-S bond strength and change the phonon properties of MoS2, which result in a significant change in thermal boundary conductance (TBC) from one lattice-stacking configuration to another for same metallic substrate. In a lattice-stacking configuration of MoS2/Sc, weakening of the Mo-S bond strength due to charge redistribution results in decrease in the force constant between Mo and S atoms and substantial redistribution of phonon density of states to low-frequency region which affects overall phonon transmission leading to 60% decrease in TBC compared to another configuration of MoS2/Sc. Strong chemical coupling between MoS2 and the Sc substrate leads to a significantly (∼19 times) higher TBC than that of the weakly bound MoS2/Au system. Our findings demonstrate the inherent connection among the interfacial electronic structure, the phonon distribution, and TBC, which helps us understand the mechanism of phonon transport at the MoS2/metal interfaces. The results provide insights for the future design of MoS2-based electronics and a way of enhancing heat dissipation at the interfaces of MoS2-based nanoelectronic devices.

A Chemical-Adsorption Strategy to Enhance the Reaction Kinetics of Lithium-Rich Layered Cathodes via Double-Shell Surface Modification.

Sluggish surface reaction kinetics hinders the power density of Li-ion battery. Thus, various surface modification techniques have been applied to enhance the electronic/ionic transfer kinetics. However, it is challenging to obtain a continuous and uniform surface modification layer on the prime particles with structure integration at the interface. Instead of classic physical-adsorption/deposition techniques, we propose a novel chemical-adsorption strategy to synthesize double-shell modified lithium-rich layered cathodes with enhanced mass transfer kinetics. On the basis of experimental measurement and first-principles calculation, MoO2S2 ions are proved to joint the layered phase via chemical bonding. Specifically, the Mo-O or Mo-S bonds can flexibly rotate to bond with the cations in the layered phase, leading to the good compatibility between the thiomolybdate adsorption layer and layered cathode. Followed by annealing treatment, the lithium-excess-spinel inner shell forms under the thiomolybdate adsorption layer and functions as favorable pathways for lithium and electron. Meanwhile, the nanothick MoO3-x(SO4)x outer shell protects the transition metal from dissolution and restrains electrolyte decomposition. The double-shell modified sample delivers an enhanced discharge capacity almost twice as much as that of the unmodified one at 1 A g(-1) after 100 cycles, demonstrating the superiority of the surface modification based on chemical adsorption.

Fabrication of MoS2 Thin Film at Low Temperature By Atmospheric-Pressure Solution Based Mist CVD

Recently, layered transitional metal dichalcogenide (TMD) materials which are bound by vdW forces between the layers have received much attention owing to their excellent properties. In this study, we focused on layered molybdenum disulfide (MoS2) thin film fabrication by mist CVD. MoS2 is a semiconductor with a non-zero energy band gap of 1.8 eV which is close to the typical band gap energy of a-Si:H (1.7 eV), contrary to graphene with a zero energy band gap [1]. The fabrication of MoS2 layered thin film by mechanical exfoliation (ME), chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) have been previously studied and reported in literatures. However, there are some problems to be solved for scaling up those processes to industrial level. In ME, size of layered MoS2 thin films are smaller than 1×1 mm2. In CVD and MBE, large amount of energy is consumed by the heater set at 700-1000°C and the vacuum pump keeping the low pressure of 10-5 Pa. [2-5]. Therefore, the development of a new method which is suitable for industrial expansion is very important. Thus, in this study, we demonstrate the fabrication of MoS2 by mist CVD. mist CVD is one of functional thin-film fabrication methods carried out under open-air atmospheric-pressure, which is developed for advanced control of precursor streams in order to achieve the fabrication of uniform thin films.In the experiment, MoS2 was fabricated by a homemade fine-channel type mist CVD system, which was designed to achieve the uniform flow and homogeneous temperature in the reactor, details of which were reported elsewhere [6]. The quartz substrates were used. Prior to the thin-film growth, substrates were cleaned by acetone, isopropanol, and deionized water each for 2 min. Thin films were fabricated by using a precursor solution containing a mixture of (NH4)6Mo7O24・4H2O and thiourea dissolved in methanol. The growth temperature was varied from 200 to 450°C at 25°C intervals. Raman spectra of samples were evaluated using a micro-Raman system with incident laser wavelengths of 532 nm (2.33 eV). The binding states of the samples were investigated by X-ray Photoelectron spectroscopy (XPS) operated at 15 kV and 26.7 mA with the unmonochromated Al Kα source. The crystal structure of the samples was identified by grazing incidence X-ray diffraction (GIXD). In this paper, results of Raman spectroscopy and XPS are shown and discussed.In the results, the slight change of the color on the substrate surfaces of some samples was seen after fabrication of the MoS2 thin film and was seemed that uniform thin films were obtained on 30×30 mm2 square substrate. Raman peaks around 380 cm-1 (E1 2g) and 410 cm-1 (A1g) were observed from the samples prepared at 300 - 450°C, except the samples prepared at temperature lower than 300°C. Raman peaks around 383.0 cm-1 (E1 2g) and 407.8 cm-1 (A1g) in bulk MoS2 are reported in the references [7]. XPS analysis of the samples prepared at 350°C shows binding energies corresponding to Mo3d (229.25, 232.58, and 235.48 eV) and S2p (162.22 and 162.96 eV) levels in Fig.2. According to the literature, binding energy peaks corresponding to Mo-S bonds; Mo3d5/2 (229.25 eV), Mo3d3/2 (232.2 eV), S2p3/2 (162.1 eV), and S2p1/2 (163.1 eV) as well as corresponding to Mo-O bonds; Mo3d5/2 (232.8 eV) and Mo3d3/2 (235.5 eV) has been observed in the case of XPS analysis of bulk MoS2 powders [8]. A small amount of MoO3 is included in MoS2 powder, as evident from low intensity of those binding energy peaks corresponding Mo-O bonds compared with that of Mo-S bonds in this literature. Similar results were obtained in XPS analysis of MoS2 thin film prepared by mist CVD at 350°C. From these results, it is suggested that layered MoS2 thin films with high quality were successfully fabricated by mist CVD. Remarkably, the growth temperature of MoS2thin-films by mist CVD is comparatively lower temperature than that of the traditional CVD. In summary, we have demonstrated the successful fabrication of MoS2 thin-film at atmospheric pressure and relatively low growth temperature by mist CVD. Reference [1] K. Ueno and K. Tsukagoshi, Jpn. J. Appl. Phys., 83 (2014) 274 [2] H. Li, et. al., Acc. Chem. Res., 47 (2014) 1067[3] Yi-H. Lee, et. al., Adv. Mater., 24 (2012) 2320[4] M.R. Laskar, et. al., Appl. Phys. Lett., 102 (2013) 252108 [5] H.F. Liu, et. al., Chem. Vap. Deposition, 21 (2015) 241[6] T. Kawaharamura, Jpn. J. Appl. Phys., 53 (2014) 05FF08[7] H. Li, et. al., Adv. Funct. Mater, 22 (2012) 1385[8] M.A. Baker, et. al., Appl. Surface Sci., 150 (1999) 255

Structural characteristic correlated to the electronic band gap inMoS2

The structural evolution with pressure in bulk $\mathrm{Mo}{\mathrm{S}}_{2}$ has been investigated by high-pressure x-ray diffraction using synchrotron radiation. We found that the out-of-plane S-Mo-S bond angle \ensuremath{\theta} increases and that in in-plane angle $\ensuremath{\phi}$ decreases linearly with increasing pressure across the known semiconducting-to-metal phase transition, whereas the Mo-S bond length and the S-Mo-S trilayer thickness display only little change. Extrapolating the experimental result along the in-plane lattice parameter with pressure, both S-Mo-S bond angles trend to those found in monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$, which manifests as a structural characteristic closely correlating the electronic band gap of $\mathrm{Mo}{\mathrm{S}}_{2}$ to its physical forms and phases, e.g., monolayer as direct band gap semiconductor, multilayer or bulk as indirect band gap semiconductor, and high-pressure $(g19\phantom{\rule{0.16em}{0ex}}\mathrm{GPa})$ bulk form as metal. Combined with the effects of bond strength and van der Waals interlayer interactions, the structural correlations between the characteristic bond angle and electronic band gaps are readily extendible to other transition metal dichalcogenide systems ($M{X}_{2}$, where $M=\mathrm{Mo}$, W and $X=\mathrm{S}$, Se, Te).

Fluorescent MoS2 Quantum Dots: Ultrasonic Preparation, Up-Conversion and Down-Conversion Bioimaging, and Photodynamic Therapy.

Small size molybdenum disulfide (MoS2) quantum dots (QDs) with desired optical properties were controllably synthesized by using tetrabutylammonium-assisted ultrasonication of multilayered MoS2 powder via OH-mediated chain-like Mo-S bond cleavage mode. The tunable up-bottom approach of precise fabrication of MoS2 QDs finally enables detailed experimental investigations of their optical properties. The synthesized MoS2 QDs present good down-conversion photoluminescence behaviors and exhibit remarkable up-conversion photoluminescence for bioimaging. The mechanism of the emerging photoluminescence was investigated. Furthermore, superior (1)O2 production ability of MoS2 QDs to commercial photosensitizer PpIX was demonstrated, which has great potential application for photodynamic therapy. These early affording results of tunable synthesis of MoS2 QDs with desired photo properties can lead to application in fields of biomedical and optoelectronics.

EXAFS reveals two Mo environments in the nitrogenase iron-molybdenum cofactor biosynthetic protein NifQ.

Mo and Fe K-edge EXAFS analysis of NifQ shows the presence of a [MoFe3S4] cluster and a second independent Mo environment that includes Mo-O bonds and Mo-S bonds. Both environments are relevant to FeMo-co biosynthesis and may represent different stages of Mo biochemical transformations catalyzed by NifQ.

Preparation of un-promoted molybdenum HDS catalysts supported on titania by equilibrium deposition filtration: Optimization of the preparative parameters and investigation of the promoting action of titania

The present work deals with un-promoted molybdenum hydrodesulphurization (HDS) catalysts supported on titania (anatase) which have been prepared by “Equilibrium Deposition Filtration” (EDF). The catalysts were characterized using various techniques (XRD, N2 adsoption–desorption isotherms, TPR, DRS and XPS) and evaluated at various temperatures and atmospheric pressure in a fixed bed differential reactor, using the HDS of thiophene as a probe reaction.The calcination of the carrier before its usage has been optimized. The adjustment of the impregnation parameters allowed regulating the Mo loading in the monolayer region and thus optimizing the catalytic performance. A linear decrease of the intrinsic activity with the Mo surface density and a concomitant volcano trend for the specific activity have been found.Monomer, monopodal Mo species are deposited on the anatase surface in the pH range 7.0–4.5 and polymer Mo species at pH 2. The former are transformed into tripodal ones after calcination. Thus, the monolayer capacity of the anatase surface is decreased leading to a partial association of these species into oligomer ones where the mean number of TiOMo bridges per Mo atom is lower than three. The extent of this transformation increased with the Mo surface density. The DRS, XPS and TPR results suggested that electron transfer takes place through the TiOMo bridges from the anatase surface oxygen to the Mo4d orbitals.The monomer tripodal Mo species are transformed into relatively small Mo sulphided edge clusters in tilted or perpendicular configuration bonded on the anatase surface by TiOMo and/or TiSMo bridges. The aforementioned transfer in the electronic density from the surface oxygens, and likewise from the surface sulfur, to Mo4d orbitals weakens the MoS bonds of the clusters and stabilizes S-deficient structures creating coordinative unsaturated active sites. This explains the very high intrinsic activity of these small Mo sulphided clusters.The oligomer Mo species, on the other hand, are transformed into relatively larger Mo S-deficient sulphided clusters, also with tilted or perpendicular configuration with respect to the anatase surface. The number the MoOTi and MoSTi bridges per Mo atom is smaller than that in the small Mo sulphided clusters, resulting to less coordinative unsaturated active sites per Mo atom. The above explained the aforementioned trends for the intrinsic and specific activities.

Electronic Structure and Multisite Basicity of the Pyramidal Phosphinidene-Bridged Dimolybdenum Complex [Mo2(η(5)-C5H5)(μ-κ(1):κ(1),η(5)-PC5H4)(η(6)-C6H3(t)Bu3)(CO)2(PMe3)

The title phosphinidene complex could be sequentially protonated with HBF4·OEt2 or [H(OEt2)2](BAr'4) to give the phosphido-bridged derivatives [Mo2Cp(μ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X and then the hydrides [Mo2Cp(H)(μ-κ(1):κ(1),η(5)-HPC5H4)(η(6)-HMes*)(CO)2(PMe3)]X2 (X = BF4, BAr'4; Ar' = 3,5-C6H3(CF3)2; Mes* = 2,4,6-C6H2(t)Bu3). Density functional theory (DFT) calculations revealed that the most favored site for initial electrophilic attack is the metallocene Mo atom, but attachment of the electrophile to the phosphinidene P atom gives more stable products. This was in agreement with all other reactions investigated, which invariably involved the attachment of the added electrophile at the P site. Thus, the title compound reacted with S8 at 223 K to give the thiophosphinidene-bridged complex [Mo2Cp{μ-κ(1):κ(1),η(5)-P(S)C5H4}(η(6)-HMes*)(CO)2(PMe3)], a poorly stable molecule which reacted with MeI at room temperature to give the corresponding thiolatophosphido derivative, isolated as [Mo2Cp{μ-κ(1):κ(1),η(5)-P(SMe)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4) (P-S = 2.128(4) Å) after anion exchange with Na(BAr'4). Reaction of the title compound with MeI proceeded smoothly to give the corresponding methylphosphido derivative, isolated analogously as [Mo2Cp{μ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe3)](BAr'4). The related complex [Mo2Cp{μ-κ(1):κ(1),η(5)-P(Me)C5H4}(η(6)-HMes*)(CO)2(PMe2Ph)](BAr'4) (P-C(Me) = 1.841(5) Å) could be prepared analogously from the neutral precursor [Mo2Cp{μ-κ(1):κ(1),η(5)-PC5H4}(η(6)-HMes*)(CO)2(PMe2Ph)]. In contrast, reaction of the title complex with ethylene sulfide involved opening of the C2S ring and formation of new P-C and Mo-S bonds (1.886(7) and 2.493(2) Å, respectively), with displacement of the PMe3 ligand, to give the phosphido-thiolato complex [Mo2Cp{μ-κ(2)(P,S):κ(1)P,η(5)-P(C2H4S)C5H4}(η(6)-HMes*)(CO)2]. All these derivatives of the title complex displayed an unusual trigonal pyramidal-like environment around the bridging P atom, with the added electrophile placed in the Mo2P plane as a result of the directionality of the relevant frontier orbital of the phosphinidene complex, according to DFT calculations.

Bandgap engineering of monolayer MoS2 under strain: A DFT study

In this paper, density functional theory calculations are used to investigate the monolayer MoS2 in terms of the strain by analyzing the structure parameters: the bandgap, the density of states (DOS) and the Milliken charges. The calculations indicate that an increasing external stain tends to depress the ripple structure with a shorter S-S interlayer spacing and to enlarge the length of the Mo-S bond. Tensile strain dramatically alteres the bandgap; however, compressive strain almost does not. The change in the bandgap is explained by an analysis of the DOS, the partial density of states (PDOS), the structure parameters and the Mulliken charge distribution. The effects of strain on the Mulliken charge and the length of the Mo-S band cause bandgap differences under tensile and compressive strain.

Influence of decomposition time and H2 pressure on properties of unsupported ammonium tetrathiomolybdate-derived MoS2 catalysts

Abstract Background Molybdenum sulfide (MoS2) catalysts to be used for hydrodesulfurization (HDS) processes were prepared via the reductive thermal decomposition of ammonium tetrathiomolybdate at fixed temperature (653 K) by varying decomposition times and H2 pressures. Both parameters were found to strongly influence textural and catalytic properties of the resulting MoS2 catalysts. Methods Nitrogen sorption, FT-IR, and XRD analyses revealed the effect of varying decomposition times (3 to 7 h) and H2 pressure (20 to 1,000 psig) on the morphology and structure of the catalysts. Dibenzothiophene (DBT) was used to assess catalytic efficiency for HDS reactions. Results The influence of time on specific surface was minimal at low pressures but increased at higher decomposition pressures. Vibrational energies of Mo-S bonds in FT-IR indicate that MoS2 catalysts prepared at higher pressures exhibit weaker Mo-S bonds. Analysis of XRD patterns point towards an increase in stacking and crystallite size with increasing pressure; interlayer rotation about both the a- and c-axes of the stacks was also observed. Catalytic testing results show that conversion increases at higher values of decomposition time and pressure. Partially hydrogenated products were also observed at higher pressures, and the ratio of partially to fully hydrogenated DBT was calculated as an additional measure of catalytic efficiency. Conclusions Decomposition time and H2 pressure during ammonium tetrathiomolybdate (ATM) thermal decomposition have a significant impact on the morphological and catalytic properties of the derived MoS2 catalysts. Samples prepared for 5 h at 1,000 psig exhibited the highest conversion of DBT and the lowest ratio of partially to fully hydrogenated products.

Quantum mechanical rippling of a MoS2 monolayer controlled by interlayer bilayer coupling.

Nanoscale corrugations are of great importance in determining the physical properties of two-dimensional crystals. However, the mechanical behavior of atomically thin films under strain is not fully understood. In this Letter, we show a layer-dependent mechanical response of molybdenum disulfide (MoS(2)) subject to atomistic-precision strain induced by 2H-bilayer island epitaxy. Dimensional crossover in the mechanical properties is evidenced by the formation of star-shaped nanoripple arrays in the first monolayer, while rippling instability is completely suppressed in the bilayer. Microscopic-level quantum mechanical simulations reveal that the nanoscale rippling is realized by the twisting of neighboring Mo-S bonds without modifying the chemical bond length, and thus invalidates the classical continuum mechanics. The formation of nanoripple arrays significantly changes the electronic and nanotribological properties of monolayer MoS(2). Our results suggest that quantum mechanical behavior is not unique for sp(2) bonding but general for atomic membranes under strain.

Controllable Synthesis of Band-Gap-Tunable and Monolayer Transition-Metal Dichalcogenide Alloys

The electronic and optical properties of transition metal dichalcogenide (TMD) materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x) monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

Strain-tunable electronic and transport properties of MoS2 nanotubes

Using density functional theory calculations, we have investigated the mechanical properties and strain effects on the electronic structure and transport properties of molybdenum disulfide (MoS2) nanotubes. At a similar diameter, an armchair nanotube has a higher Young’s modulus and Poisson ratio than its zigzag counterpart due to the different orientations of Mo-S bond topologies. An increase in axial tensile strain leads to a progressive decrease in the band gap for both armchair and zigzag nanotubes. For armchair nanotube, however, there is a semiconductor-to-metal transition at the tensile strain of about 8%. For both armchair and zigzag nanotubes, the effective mass of a hole is uniformly larger than its electron counterpart, and is more sensitive to strain. Based on deformation potential theory, we have calculated the carrier mobilities of MoS2 nanotubes. It is found that the hole mobility is higher than its electron counterpart for armchair (6, 6) nanotube while the electron mobility is higher than its hole counterpart for zigzag (10, 0) nanotube. Our results highlight the tunable electronic properties of MoS2 nanotubes, promising for interesting applications in nanodevices, such as opto-electronics, photoluminescence, electronic switch and nanoscale strain sensor.