Mo 4d States Research Articles

Unraveling the strain tuning mechanism of interlayer excitons in WSe2/MoSe2 heterostructure

Atomically thin transition metal dichalcogenides (TMDs) exhibit rich excitonic physics, due to reduced dielectric screening and strong Coulomb interactions. Especially, some attractive topics in modern condensed matter physics, such as correlated insulator, superconductivity, topological excitons bands, are recently reported in stacking two monolayer (ML) TMDs. Here, we clearly reveal the tuning mechanism of tensile strain on interlayer excitons (IEXs) and intralayer excitons (IAXs) in WSe2/MoSe2 heterostructure (HS) at low temperature. We utilize the cryogenic tensile strain platform to stretch the HS, and measure by micro-photoluminescence (μ-PL). The PL peaks redshifts of IEXs and IAXs in WSe2/MoSe2 HS under tensile strain are well observed. The first-principles calculations by using density functional theory reveals the PL peaks redshifts of IEXs and IAXs origin from bandgap shrinkage. The calculation results also show the Mo-4d states dominating conduction band minimum shifts of the ML MoSe2 plays a dominant role in the redshifts of IEXs. This work provides new insights into understanding the tuning mechanism of tensile strain on IEXs and IAXs in two-dimensional (2D) HS, and paves a way to the development of flexible optoelectronic devices based on 2D materials.

DFT insight into magnetoelectronic and optical properties of Mo-modified BaTiO3

Herein, the magnetoelectronic and optical characteristics of Ba1-xMoxTiO3 (x = 25%, 50%, and 75%) compound are computed by employing density functional theory (DFT). The robust half metallic ferromagnetic (HMF) character has been observed for Mo-incorporated BaTiO3. This behavior of Mo-modified BaTiO3 is due to the highly asymmetric behavior in both spin channels, semiconducting in dn-spin and metallic nature in up-spin version. The computed total magnetic moment (MTot) is found to be 4.0 μB for all substitutional concentrations which is mainly due to the Mo-4d states. In addition, the optical parameters (extinction coefficient, reflectivity, optical conductivity, refractive index, dielectric function, and absorption coefficient) are also calculated. The computed results suggest the suitability of studied material for spintronic and optoelectronic devices.

Mo-modified (t-m)-ZrO2 with narrowed band gap and strong ability to activate reactants for photocatalytic CO2 reduction

ZrO2 is a promising candidate for photocatalytic CO2 reduction due to its high conduction potential, but it suffers from narrow sunlight absorption range and weak activation ability to reactants. Herein, we explore Mo-modified (t-m)-ZrO2 photocatalysts by a one-step hydrothermal method. As compared to bared ZrO2, their photoresponse extends to the visible light region, being attributed to the impurity band of Mo 4d states embedded in the ZrO2 band gap. Meanwhile, PL and electrochemical tests illustrate that the impurity band also leads to an increase in the carrier separation efficiency. MoO3 nanoparticles and doping Mo in Mo-modified ZrO2 photocatalysts promotes H2O and CO2 activation, respectively. Moreover, in situ FTIR and theoretical calculation revealed that Mo modification is also favorable for the generation of *COOH intermediate and the desorption of CO. As a result, the optimized Mo-modified ZrO2 exhibited a 27.3 μmol·g−1 of the CO yield with ∼100% selectivity, being 13 times higher activity than bared ZrO2. This work provides a guidance for the exploitation of wide bandgap semiconductors in the field of photocatalytic CO2 reduction.

Atomic coordination structural dynamic evolution of single-atom Mo catalyst for promoting H2 activation in slurry phase hydrocracking

Development of efficient catalysts with high atomic utilization and turnover frequency (TOF) for H2 activation in slurry phase hydrocracking (SPHC) is crucial for the conversion of vacuum residue (VR). Herein, for the first time, we reported a robust and stable single atoms (SAs) Mo catalyst through a polymerization-pyrolysis-in situ sulfurization strategy for activating H2 in SPHC of VR. An interesting atomic coordination structural dynamic evolution of Mo active sites was discovered. During hydrocracking of VR, the O atoms that coordinated with Mo were gradually replaced by S atoms, which led to the O/S exchange process. The coordination structure of the Mo SAs changed from pre-reaction Mo-O3S1 to post-reaction Mo-O1S3 coordination configurations, promoting the efficient homolytic cleavage activation of H2 into H radical species effectively. The evolved Mo SAs catalyst exhibited robust catalytic hydrogenation activity with the per pass conversion of VR of 65 wt%, product yield of liquid oils of 93 wt%, coke content of only 0.63 wt%, TOF calculated for total metals up to 0.35 s−1, and good cyclic stability. Theoretical calculation reveals that the significant variation of occupied Mo 4d states before and after H2 interaction has a direct bearing on the dynamic evolution of Mo SAs catalyst structure. The lower d-band center of Mo-O1S3 site indicates that atomic H diffusion is easy, which is conducive to catalytic hydrogenation. The finding of this study is of great significance to the development of high atom economy catalysts for the industrial application of heavy oil upgrading technology.

DFT study of TiO2 brookite (210) surface doped with silver and molybdenum

The most popular semiconductor in dye-sensitized solar cells (DSSCs) is titanium dioxide (TiO2) because of its low cost, non-toxicity, and good stability. However, the DSSCs still have low efficiency due to the low light absorption of TiO2 in the visible region. Understanding the properties of TiO2 can contribute to improving the efficiency of DSSCs. In this study, we use density functional theory to investigate the electronic and optical properties of TiO2 brookite (210) surface mono-doped and co-doped with 4d transition metals, silver, and molybdenum. Our results show that the band gap energy of brookite (210) surface is 3.514 eV, which reduces to 1.143 eV and 0.183 eV when doped with Ag and Mo, respectively. However, doping with both Ag and Mo yielded a band gap of 0.387 eV. The results suggest the presence of Ag and Mo 4d states below the conduction band minimum, which could be responsible for the narrowing of the band gap on the brookite (210) surface. Both mono-doped and co-doped brookite (210) surfaces have higher visible light absorbance compared to the undoped brookite (210) surface and extend to the near-infrared region.

An ab-initio study on two-dimensional semiconductor alloys: Monolayer Mo1-xCrxS2

The formation energy, structure, electronic and magnetic properties of the monolayer (ML) Mo1-xCrxS2 alloy are calculated based on density functional theory calculations within generalized gradient approximation method. It is found that Cr doping in the ML MoS2 is energetically unfavorable. Its lattice constant exhibits a negative bowing coefficient of −0.022 Å. The Mo–S bond length and the average bond length of the ML Mo1-xCrxS2 show a decline trend while the composition dependence of the Cr–S bond length is not monotonic. It is also found that the ML Mo1-xCrxS2 has a direct bandgap at K for any Cr concentration and its bandgap energy monotonically decreases with increasing Cr concentration. When the Cr concentration is no more than 0.6, the bandgap narrowing is due to the upward shift of K VBM and the downward shift of K CBM. If the Cr concentration goes on increasing, the bandgap narrowing is merely due to the upward movement of the K VBM. Besides, the location of the K CBM for Mo1-xCrxS2 is determined by the coupling interaction between the S-3p and Cr-3d states in the conduction band while the K VBM is dominated by two coupling interactions in the valence band. One is the coupling interaction between the S-3p and Cr-3d states. The other is the coupling interaction between the S-3p and Mo-4d states. In order to study the bandgap energy of the ML Mo1-xCrxS2 alloy quantitatively, a physical model is developed. It is observed that the model can give a good estimation by setting EI,Cr−3d,MoS2=1.68 eV, Cp−d=0.55 eV and b1=0.69 eV. The result of the magnetic properties shows that the magnetism of the ML Mo1-xCrxS2 system is very weak.

Half-metallic ferromagnetic properties of Mn-doped MoSe<sub>2</sub> and its heterostructure

<p indent="0mm">We use the first-principle method to investigate the crystal structure, electronic structure, and magnetic properties of a Mn-doped MoSe₂ monolayer (Mo_0.75Mn_0.25Se₂). The spin-up electrons exhibit metallic properties, whereas the spin-down electrons exhibit insulating properties, indicating the half-metallic properties of Mo_0.75Mn_0.25Se₂. The density of states shows a strong coupling between dopant Mn 3d orbitals via the Mo 4d and Se 4p states. We investigate the dispersion relation of the spin-spiral energy <italic>E</italic>(<bold>q</bold>) and the wave vector <bold>q</bold> for Mo_0.75Mn_0.25Se₂ under the generalized Bloch condition. The exchange coupling coefficient<italic> J</italic>_{<italic>i</italic>} (<italic>i</italic> = 1, 2, 3, and 4) is obtained using the Heisenberg interaction model. The next-nearest-neighbor interaction exchange coupling coefficient <italic>J</italic>₂ is non-negligible, although the first nearest neighbor, <italic>J</italic>₁, plays the most important role in the interaction between atoms. Because Mo_0.75Mn_0.25Se₂ is a two-dimensional (2D) metal material that can easily undergo oxidation, we use insulating MoSe₂ to fabricate a MoSe₂/Mo_0.75Mn_0.25Se₂/MoSe₂ heterostructure. First-principle calculations results show that the half-metallic ferromagnetic properties of Mo_0.75Mn_0.25Se₂ can be protected very well in the heterostructure. The findings of this study can promote the understanding of the mechanism of 2D half-metallic ferromagnetic properties and provide a general method for designing materials that show such properties with stability.

First-principles investigations of electronic, optical, and photocatalytic properties of Au-adsorbed MoSi2N4 monolayer

The electronic, optical, and photocatalytic properties of Au-adsorbed MoSi2N4 systems are predicted by first-principles calculations. Electronic structure analysis shows that the band gap is reduced from 1.89 eV (intrinsic MoSi2N4) to 0 eV (MoSi2N4–9Au). Calculation results of density of states indicate that the valence band edge is mainly contributed by the Mo 4d state, whereas the conduction band edge is contributed by both the Mo 4d and Si 3p states. As the number of adsorbed Au atoms is increased, the absorption coefficient increases from 3.0 × 104 cm−1 to 5.6 × 104 cm−1, meaning that more photons can be absorbed. Indeed, Au-adsorbed MoSi2N4 monolayer shows greater potential for applications such as water splitting and CO2 reduction based on redox potential analysis. The Au-adsorbed MoSi2N4 structures can effectively change the properties of materials, offering great potential in different fields.

Nonlocal Interactions in the Double Perovskite Sr2FeMoO6 from Core-Level X-ray Spectroscopy

The valence electronic structure of the half-metallic double perovskite Sr2FeMoO6 forms from a strongly hybridized band in the spin-down channel of Fe 3d and Mo 4d states that provides metallic conductivity and a gapped spin-up channel. The ground-state description has previously been explored in terms of many-body interactions where local and nonlocal interactions produce states with a combination of a charge-transfer configuration and intersite charge fluctuations. Here, we provide a qualitative understanding on nonlocal effects in Sr2FeMoO6 using a combination of core-level X-ray spectroscopies, specifically X-ray absorption, emission, and photoelectron spectroscopies. Our spectroscopic data indicate intersite Fe 4p–O 2p–Mo 4d interactions to be the origin of these nonlocalized transitions. Close to the Fermi level, this interaction is dominated by Mo 4d–O 2p character. When our data are compared against first-principles electronic structure calculations, we conclude that a full understanding of the nature of these states requires a spin-resolved description of the hybridization functions and that the nonlocal screening occurs predominantly through hybridization in the minority spin channel of the Mo 4d bands.

First-principles calculations to investigate the electronic and optical properties of (MoS2)4-n/(MoSSe)n lateral heterostructure

The electronic properties, work function, and optical properties of the lateral heterostructure formed after combining MoS2 and Janus MoSSe ((MoS2)4-n/(MoSSe)n, where n is the width of Janus MoSSe) have been investigated comparatively using first-principles calculations. The electronic structure analysis shows that the band gaps decrease from 1.69 eV for (MoS2)4/(MoSSe)0–1.41 eV for (MoS2)0/(MoSSe)4. Based on the calculated density of states calculations, the valence band edge (VBE) is mainly contributed by Mo 4d states, while the conduction band edge (CBE) is originated from the contribution of Mo 4d and Se 4p orbital hybridization. As the width n increases, the average positive charge on Mo atoms decreases, while the average charge on Se increases. As a consequence, there is a significant enhancement of the built-in electric field in the z-direction, inducing the work functions changed from 6.17 eV for (MoS2)4/(MoSSe)0–5.22 eV for (MoS2)0/(MoSSe)4. The results of this study imply that forming (MoS2)4-n/(MoSSe)n lateral heterostructure systems which can have adjustable work function and built-in electric field, bring new properties and great potential in different applications.

Structural and dielectric properties of Cu-doped α-ZnMoO4 ceramic system for enhanced green light emission and potential microwave applications

In this article, the synthesis of the Zn1-xCuxMoO4 ceramic system was carried out by a solid-state reaction route, and their structural, morphological, and optical properties were investigated. The samples were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, FT-IR spectroscopy, and UV–Vis spectrophotometry. Structural analysis confirms the formation of a triclinic structure with space group P $$\stackrel{-}{1}$$ and point group symmetry C1 without any secondary phase formation. The decrease in effective nuclear charge with the introduction of Cu2+ ion decreases the bandgap from 3.57 to 2.78 eV. The potential application in the visible (green) region of the electromagnetic spectrum is supported by the reduction of the bandgap. The increasing Cu2+ concentration is accompanied by Photoluminescence peaks being shifted toward larger wavelength side, covering a broad range of visible region from 300 to 600 nm. The broad range luminescence emission spectra that happened with ZnMoO4 occurred due to the electronic transition between (MoO4)2− complexes or between Mo4d and O2p states. Hakki–Coleman method was applied for the study of microwave dielectric parameters and shows increase of eobs from 8.35 to 12.52 with a simultaneous decrease in dielectric loss (tan δ) from 0.01 to 0.005, respectively. The corresponding quality factor (Q $$\times$$ f) was calculated. The observed high-quality factor could be used as high signal speed microwave materials.

First-principles calculations of the structural, electronic, mechanical and thermodynamic properties of MAX phase Mon+1GeCn (n = 1–3) compounds

A novel class of nitrides and carbides called MAX phase materials offers an exceptional blend of metal and ceramic properties together with new guidelines to structure modifications for a variety of technological and engineering applications. In this study, we predict the structural, electronic, elastic, mechanical, and thermodynamic properties of the Mon+1GeCn (n = 1, 2, and 3) MAX phase materials via first-principles (density functional theory (DFT)) calculations. The lattice parameters and formation energies are calculated for the simulated MAX phases under consideration. Five independent elastic constants (C11, C12, C13, C33, and C44 = C55) are determined and we have used them to calculate the bulk modulus (B), Young’s modulus (E), shear modulus (G), Poisson’s ratio (υ) and anisotropy index (A). From our calculations, the predicted electronic band structure and density of states plots show that for the investigated MAX phases, a major contribution in the electronic density of states around the Fermi energy originates from the Mo-4d states. The investigated compounds are metallic with electrical conductivity arising primarily due to the Mo-4d electrons. Thermal effects on some macroscopic properties (lattice constants, Debye temperature, heat capacity, and thermal expansion coefficient) of Mon+1GeCn (n = 1, 2, and 3) are investigated by employing the quasi-harmonic Debye model in a temperature range from 0 K to 1000 K and in the pressure range from 0 GPa to 40 GPa as well.

Bioinspired Mo tape-porphyrin as an efficient and selective electrocatalyst for ammonia synthesis

The efficient N2 fixation to NH3 in the biological system benefits from the electron transfer and catalysis of nitrogenase under ambient conditions, where N2 reduction occurs at Mo and Fe sites. Herein, the electrochemical N2 reduction reaction (NRR) on bioinspired Mo tape-porphyrin (denoted as MoP) has been systematically investigated on the basis of first-principles calculations. The anchoring of Mo atom to the biomolecular porphyrin changes the charge distribution and activates more delocalized π electrons in the fully conjugated π-system. The results reveal that N2 favourably coordinated with Mo atom in end-on configuration and the electronic and magnetic properties of MoP are changed. The most promising reaction pathway for NRR on MoP is via the distal route with overpotential of 0.22 V. The excellent conductivity, highly localized and spin-polarized Mo-4d states of MoP endow it preeminent NRR activity at room temperature. Furthermore, the competitive 2-electron hydrogen evolution reaction (HER) is sufficiently mitigated and circumvented during the NRR progress. The high symmetry of MoP enables a simple synthetic pathway. Our findings provide a ingenious strategy for catalytic N2 reduction and a practical manufacturing method for designing efficient and selective electrocatalyst.

Template-confined growth of X-Bi2MoO6 (X: F, Cl, Br, I) nanoplates with open surfaces for photocatalytic oxidation; experimental and DFT insights of the halogen doping

The present work describes a series of two-dimensional architecture of CTAB-assisted X-Bi2MoO6 (X: F, Cl, Br, I) with open surfaces for adsorption and photodegradation of RhB. The effects of Halogen-doping and surfactant on the physicochemical properties of Bi2MoO6 are investigated by different analysis. Based on the XRD patterns, the crystal planes of Bi2MoO6 were affected by the substitution of X− anions for the host O2− (mainly by F−). FESEM images confirmed the confined growth of nanoplates under the influence of CTAB template, which leads to surface doping of more halogen ions within oriented nucleation process. The results indicated all of the X-doped catalysts, especially F-doped sample, show enhanced photocatalytic activity with different levels, due to lower band gap, improved charge separation, and surface properties. The results from density functional theory (DFT) calculations revealed in all the X-Bi2MoO6 cases, the valance band maximum and conduction band minimum is mostly composed of O 2p and Mo 4d states, and the gaps decrease with the insertion of halogen atoms, which are in excellent agreement with experimental data. The study of the template-confined Halogen-doped Bi2MoO6 may furnish new perspective into the fabrication of a series of photocatalysts with better light-harvesting capacity in a sustainable manner.

CeMo2B5: A New Type of Arrangement of Puckered Boron Hexagonal Rings

REMo2B5 (RE = Ce, Pr, Nd) and CeW2B5 have been synthesized by arc melting with further annealing at 1300 °C. CeMo2B5 crystallizes in a new structure type with space group Pnma, a = 11.0298(2) Å, b = 3.12080(5) Å, c = 9.9594(2) Å, Z = 4. Its structural arrangement reveals puckered [B6]‐hexagonal rings, trigonal‐prismatic‐ and empty pyramidal‐tetrahedral‐slabs. The structure can be related to the derivatives of AlB2 and CrB prototypes. The boron atoms form a two‐dimensional network of corrugated ribbons, each composed of four edge‐linked [B6] rings infinitely extending along the b‐axis. CeMo2B5 is a Pauli paramagnet. Electrical transport measurements together with the calculated density of states at the Fermi level of 3.81 states eV–1 f.u.–1 indicate CeMo2B5 to be a typical metallic system. Electronic structure calculations reveal significant hybridization of Ce 4f5d states with Mo 4d states. The chemical bonding scenario is dominated by chains of [Ce2Mo2] clusters with multi‐center bonds. These clusters are connected via metallic Mo–Mo bonds. This metal‐framework, in turn, is stabilized by bonding interactions with the boron ribbons to which it donates electrons.

Adsorption and migration behavior of molybdenum atom on graphite (0001) surface

Molybdenum atom adsorption and migration behavior on clean graphite (0001) surface have been investigated systematically by using first-principles calculations based on the density functional theory. The Mo atom was not adsorpted at the bridge site of graphite (0001) surface because Mo atom moves towards to the hollow site after geometry optimization. Mo atom has been predicted to adsorb preferentially at the hollow site of graphite (0001) surface because of the larger adsorption energy and the smaller work function. It is found from electronicproperties that Mo adsorption on graphite (0001) surface belongs to chemisorptions, which is attributed to the electrons of Mo 4d states transfer to C atoms of graphite (0001) surface. The calculated adsorptive rate suggests that the Mo atom adsorption at the hollow site of graphite (0001) surface is more stable with the increasing of temperature. The optimum energy migration paths for Mo atom diffusion from the hollow site to the nearby hollow site is hollow 1 site → bridge site → hollow 2 site by using LST/QST transition state seeking method.

Effect of electron doping on the magnetic properties and electronic structures of Ba2MnMoO6

The electron-carrier-doping effects in an antiferromagnetic insulator Ba2MnMoO6 were investigated by La doping using the first-principles calculation. We examined the magnetic properties and the electronic structures for different doping ratios of La. The results indicate that a transition from an antiferromagnetic (AFM) insulating Ba2MnMoO6 to a AFM half-metallic Ba2-xLaxMnMoO6 (x ≤ 1.0) and then a AFM metallic phase (x = 1.5, 2.0) occurs upon La doping. The injected electron accommodated into the Mo 4d states accounts for the metallic properties. Moreover, the electron correlation plays a crucial role in the determination of the electronic structure. The evaluated magnetic ordering temperatures for the doping systems are all higher than room temperature revealing the potential applications.

Promising electronic structure of double perovskite Sr2TiMoO6: Spin-polarized DFT+U approach

The electronic structure of double perovskite Sr2TiMoO6 have been systematically investigated using first principle. The spin-polarized electronic band structure and the density of states reveal that the spin-up channel has metal behavior while the spin-down channel expresses semiconductor behavior with direct band gap of 2.92 eV. For valence band, the O-2p states is the mainly contributor for both spin-up and spin-down channels above -5.3eV. Between -7.7eV to -5.3eV, valence band is formed by O-2p states with admixture of Mo-4d states. For conduction band, from range 4 eV to 7.4 eV is mostly formed by Sr-3d. On the other hand, O-2p, Mo-4d as well as Ti-3d states mainly consists of the conduction band near Fermi energy, so those states need to be more concerned. The spin splitting may lead to unusual thermoelectric transport properties.

Mapping Catalytically Relevant Edge Electronic States of MoS2.

Molybdenum disulfide (MoS2) is a semiconducting transition metal dichalcogenide that is known to be a catalyst for both the hydrogen evolution reaction (HER) as well as for hydro-desulfurization (HDS) of sulfur-rich hydrocarbon fuels. Specifically, the edges of MoS2 nanostructures are known to be far more catalytically active as compared to unmodified basal planes. However, in the absence of the precise details of the geometric and electronic structure of the active catalytic sites, a rational means of modulating edge reactivity remain to be developed. Here we demonstrate using first-principles calculations, X-ray absorption spectroscopy, as well as scanning transmission X-ray microscopy (STXM) imaging that edge corrugations yield distinctive spectroscopic signatures corresponding to increased localization of hybrid Mo 4d states. Independent spectroscopic signatures of such edge states are identified at both the S L2,3 and S K-edges with distinctive spatial localization of such states observed in S L2,3-edge STXM imaging. The presence of such low-energy hybrid states at the edge of the conduction band is seen to correlate with substantially enhanced electrocatalytic activity in terms of a lower Tafel slope and higher exchange current density. These results elucidate the nature of the edge electronic structure and provide a clear framework for its rational manipulation to enhance catalytic activity.

Effects of (Mo/W, N) codoping on electronic structures of NaNbO3 based on hybrid density functional calculations

The influences of a series of (N, Mo, W) monodoping and N–Mo(W) codoping on the electronic structures of sodium niobate have been systematically investigated by density functional theory (DFT) calculations with the hybrid B3LYP functional. The results indicated that some localized N 2p states were formed above the top of valence band (VB) after N-doping. In the case of Mo(W)-doping, the position of VB shifted up, meanwhile, the hybridization of Nb 4d and Mo 4d(W 5d) states moved the conduction band (CB) downward, leading to narrowing the band gap. As for N–Mo codoped NaNbO3, the localized Mo 4d states were formed near the bottom of CB while the localized N 2p states appeared above the top of VB. When N–W codoping, the W 5d p states mixed with Nb 4d states and then moved the top of CB downward as well as some localized N 2p states were formed above the valence band (O 2p), leading to the obvious reduction of the photon transition energy compared with that of NaNbO3.