Electronic Structure Analysis Research Articles

Simulation of interlayer coupling for electroactive covalent organic framework design.

Porous, stacked two-dimensional covalent organic frameworks (2D COFs) bearing semiconducting linkers can support directional charge transfer across adjacent layers of the COF. To better inform the current and possible future design rules for enhancing electron and hole transport in such materials, an understanding of how linker selection and functionalization affects interlayer electronic couplings is essential. We report electronic structure simulation and analysis of electronic couplings across adjacent linker units and to encapsulated species in functionalized electroactive 2D COFs. The detailed dependence of these electronic couplings on interlayer interactions is examined through scans along key interlayer degrees of freedom and through configurational sampling from equilibrium molecular dynamics on semiempirical potential energy surfaces. Beyond affirming the sensitivity of the electronic coupling to interlayer distance and orientation, these studies offer guidance toward linker functionalization strategies for enhancing charge carrier transport in electroactive 2D COFs.

The tendency of V segregation in Pd/V(110) and Pd/V(100) surfaces induced by H adsorption

This study employs density functional theory-based calculations to investigate the tendency of V atom segregation in Pd/V(100) and Pd/V(110) surfaces upon adsorption of H at varying coverage (0.25, 0.50, 0.75, and 1.0 ML). Geometric, energetic, and electronic structure analyses were performed to elucidate the stability of H on the surfaces, the V atoms segregation tendency, and the interactions of atoms in the systems. By calculating the relative energies, we found that Pd atoms will favor residing in the topmost layer of pristine Pd/V. Segregation of V atom in pristine Pd/V(100) is more endothermic than in Pd/V(110). For H-Pd/V systems, a tendency of V segregation was observed for 0.50–1.0 ML H coverage on Pd/V(110). V atom segregation was not predicted in H-Pd/V(100). Due to the more endothermic V atom segregation process in pristine (100) than in (110) facet, a larger energy is necessary to induce V segregation by H adsorption in (100). The adsorption energies, charge density difference distributions, and density of states revealed the stronger H-V interaction compared to H-Pd interaction. Hence, H adsorption stabilizes the V atoms in the topmost layers and could induce V segregation in the surface.

Electronic structure and optical spectral analysis of the MnO42− anion with consideration of site and Jahn–Teller distortion

The ground state of 3d1 MnO42− was studied by density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods in terms of a variety of molecular point group structures to ascertain the site and Jahn–Teller (JT) distortion effect. Modeling results from UB3LYP/6-31+G(d) calculations with natural bond orbital analysis show the four Mn–O bonds are coordinate covalent. The one-electron matrix elements from CASSCF(AILFT) (ab initio ligand field theory) with a second order perturbation treatment were used to calculate the parameters of the angular overlap model. These allowed the calculation of JT stabilization energies and first and second order JT coupling constants for MnO42− with C2v and D2d symmetry. Absorption spectra and excited state transition energies were calculated assuming the lattice distorted geometry was C2v with time-dependent DFT (TDDFT) using the unrestricted coulomb-attenuating UCAM-B3LYP density functional and with the spin-adapted spin-flip DFT using the UBH&HLYP density functional, both with the AUG-CC-PVTZ basis set. The mean absolute deviation of the calculations from experimental excited states for the ligand field and ligand-to-metal charge transfer (LMCT) bands was better than 0.1 eV. Several new assignments for LMCT excited states were made on the basis of the TDDFT excitation results.

Investigation of the Influence of Alloy Atomic Doping on the Properties of Cu-Sn Alloys Based on First Principles

In order to design Cu-Sn alloys with excellent overall performance, the structural stability, mechanical properties, and electronic structure of X-doped Cu-Sn alloys were systematically calculated using first-principles calculations. The calculation results of the cohesive energy indicate that the Cu-Sn-X structures formed by X atoms (X = Ag, Ca, Cd, Mg, Ni, Zr) doping into Cu-Sn can stably exist. The Cu-Sn-Ni structure is the most stable, with a cohesive energy value of −3.84 eV. Doping of X atoms leads to a decrease in the bulk modulus, Possion’s ratio and B/G ratio. However, doping Ag and Ni atoms can improve the shear modulus, Young’s modulus, and strain energy of the dislocation. The doping of Ni has the highest enhancement on shear modulus, Young’s modulus, and strain energy of the dislocation, with respective values as follows: 63.085 GPa, 163.593 GPa, and 1.689 W/J·m−1. The analysis of electronic structure results shows that the covalent bond between Cu and X is the reason for the performance differences in Cu-Sn-X structures.

A combined density functional theory and microkinetics simulations study of electrochemical CO2 reduction on Cu8/SnO2(110): The crucial role of hydrogen coverage

The electrochemical reduction of CO2 (eCO2R) is a promising approach for converting CO2 into valuable chemicals and fuels using renewable energy sources. We investigated the mechanism of eCO2R for a small Cu8 cluster placed on SnO2 containing O vacancies using density functional theory and predicted current density and selectivity by microkinetics simulations within the computational hydrogen electrode model. Low and high H coverages were modeled by Cu8/SnO2-x and Cu8H6/SnO2-x models, using statistical methods to identify their most stable structures. Different CO2 adsorption modes on Cu8/SnO2-x and Cu8H6/SnO2-x surface models, all containing an O vacancy, resulted in distinct reaction pathways, leading to either HCOOH or CO. The preferred formation of HCOOH occurred upon CO2 adsorption on an O vacancy on the Cu8H6/SnO2-x surface, followed by sequential hydrogenation to HCOO and HCOOH. Adsorption of CO2 on Cu8/SnO2-x opened a facile pathway to CO. Electronic structure analysis revealed that differences in charge donation of Cu to the antibonding orbitals of CO2 can explain the predicted selectivity differences. The preferred adsorption mode of CO2 is bidentate at the Cu-SnO2-x interface. Our findings emphasize the role of H coverage on Cu on the selectivity of eCO2R for Cu/SnOx catalysts.

A Bifunctional Electrocatalyst of Flower-Like Cr-Doped CoP/Fe2P Microsphere for Efficient Overall Water Splitting

Abstract Developing efficient, low-price non-noble metal-based electrocatalysts for overall water splitting in alkaline medium remains a formidable challenge. In our work, Cr-doped CoP/Fe2P (Cr-CoP/Fe2P) flower-like microsphere was synthesized through a simple hydrothermal and phosphating process. The resulting Cr-CoP/Fe2P electrocatalyst shows significantly enhanced oxygen evolution reaction performance (262 mV @ 10 mA cm-2) and has a satisfactory hydrogen evolution reaction performance (114 mV @ 10 mA cm-2), coupled with favorable stability in an alkaline medium. Furthermore, when assembling Cr-CoP/Fe2P into an electrolytic cell, the two-electrode system can provide a current density of 10 mA cm-2 at a voltage of 1.61 V. At high current density, the performance of the electrolytic cell composed of Cr-CoP/Fe2P is superior to that of noble metal catalyst electrode pair. Electronic structure analysis and various characterizations confirm that Cr doping and the formation of CoP/Fe2P heterogeneous interfaces redistribute the electron densities of the active sites, enlarge the specific surface area, and enhance the aerophobicity of the catalysts, thereby improving the electrocatalytic property. This work provides a referable method for engineering highly efficient and stable non-noble polymetallic phosphides, which serve as bifunctional electrocatalyst for overall water splitting.

High capacity hydrogen storage structure based on a two-dimensional material (Ga2C6) with strong adsorption properties

The use of solid-state devices is a promising approach to address the current technological requirements for sustainable clean energy applications. Within this context, it has become apparent that density functional theory (DFT) has emerged as a powerful tool to study intrinsic electronic characteristics and shows great potential to lead the way to novel materials exploration. Here, we have investigated H2 adsorption on Ga2C6 monolayer, a prospective model for the storage of H2 molecule because of its structural stability. The hydrogen molecules favor being adsorbed at the carbon hexagon center with a binding energy of 0.39 eV. The thermodynamic stability of the Ga2C6 monolayer upon adsorption of 22H2 molecules was evaluated at room temperature using ab-initio molecular dynamics (AIMD) simulations. It is found from the electronic structure analysis that the semiconducting Ga2C6 sheet maintain its behavior after 22H2 molecules were adsorbed. Owing to the small activation energy of about 0.0198 eV, the process of H2 molecule migration at the surface easy proceeds. This investigation demonstrates a strong (GC) gravimetric capacity around 9.65 wt%, and a desorption temperature (TD) around 213.68 K. The above findings imply that the Ga2C6 monolayer may serve as a suitable medium for hydrogen adsorption.

Adsorption behaviors of ethyltriethoxysilane on the calcium silicate hydrate surface: Effect of Ca/Si ratio and adsorption sites

Silane coupling agents (SCAs) are utilized to enhance the impermeability of concrete surfaces, whose adhesion force is affected by the interfacial interactions in between that remain unclear. In this study, we probe the adsorption tendencies of ethyltriethoxysilane (ETES) onto C-S-H surfaces by employing density functional theory (DFT) calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations. Results suggest that significant chemisorption occurs at the CaOH(1.5), SiOH(1.0), Ca(1.0) sites C-S-H surfaces. Electronic structures analysis proved that the chemisorption of hydroxyl sites arises from the overlap of H and O atomic orbitals, resulting in the formation of hydrogen bonds. The chemisorption of the Ca(1.0) site is caused by electrostatic adsorption between calcium and oxygen. Adsorption energy results show: ECaOH(1.5) > ECa(1.0) > ESiOH(1.0). Moreover, ETES remains transverse adsorption on low Ca/Si C-S-H surfaces, while displaying longitudinal adsorption on high. Interaction analysis illustrates the configuration discrepancy arising from the absence of bridge silicon-oxygen tetrahedrons which possess vdW interactions with ethyl. Notably, transverse adsorption covers a wider area on the C-S-H surface thereby enhancing the hydrophobic effect. In summary, the utilization of ETES presents notable advantages within the context of low Ca/Si ratio cement. This study contributes valuable insights into the design and application of silane coatings.

Synthesis, Crystal Structure, Hirshfeld Surface, Energy Frame Work and DFT Analysis of Ethyl-4-aminobenzoate Oxalate Monohydrate Salt

Using the single crystal X-ray diffraction method, the title molecule ethyl-4-aminobenzoate oxalate monohydrate has been synthesized, crystallized and characterized. The Hirshfeld surfaces computational approach was used to analyze the intermolecular interactions in the crystal structure. The aforementioned compound crystallizes in a monoclinic crystal system of space group P21/c with the following cell parameters: a = 15.475(3), b = 5.7117(9), c = 14.029(3), = 101.764(6)º, V = 1214.0(4) 3 and Z = 4. The molecules in the crystal structure are connected by the N1-H1C···O7, O5-H2···O3, O7-H1···O4 and C8-H8A···Cg type intermolecular interaction, where Cg is the centroid of the ring. For the title molecule, the inter contact O···H (48.4%) contributes more to the Hirshfeld surfaces. The proton transfer from oxalic acid’s phenolic group to ethyl-4-aminobenzoate’s amine group, which results in the creation of salt, is revealed by the crystal structure. The ground state of ethyl-4-aminobenzoate oxalate monohydrate was also studied theoretically using the density functional method (DFT/B3LYP) with a basis set of 6–311++G(d,p). Charge transfer happens within the molecule, as shown by the computed HOMO and LUMO energies, DOS and MESP, as well as by the optimized molecular structure, electronic structural analysis and UV spectrum analysis.

Paramagnetic Effects in NMR Spectroscopy of Transition-Metal Complexes: Principles and Chemical Concepts.

ConspectusMagnetic resonance techniques represent a fundamental class of spectroscopic methods used in physics, chemistry, biology, and medicine. Electron paramagnetic resonance (EPR) is an extremely powerful technique for characterizing systems with an open-shell electronic nature, whereas nuclear magnetic resonance (NMR) has traditionally been used to investigate diamagnetic (closed-shell) systems. However, these two techniques are tightly connected by the electron-nucleus hyperfine interaction operating in paramagnetic (open-shell) systems. Hyperfine interaction of the nuclear spin with unpaired electron(s) induces large temperature-dependent shifts of nuclear resonance frequencies that are designated as hyperfine NMR shifts (δHF).Three fundamental physical mechanisms shape the total hyperfine interaction: Fermi-contact, paramagnetic spin-orbit, and spin-dipolar. The corresponding hyperfine NMR contributions can be interpreted in terms of through-bond and through-space effects. In this Account, we provide an elemental theory behind the hyperfine interaction and NMR shifts and describe recent progress in understanding the structural and electronic principles underlying individual hyperfine terms.The Fermi-contact (FC) mechanism reflects the propagation of electron-spin density throughout the molecule and is proportional to the spin density at the nuclear position. As the imbalance in spin density can be thought of as originating at the paramagnetic metal center and being propagated to the observed nucleus via chemical bonds, FC is an excellent indicator of the bond character. The paramagnetic spin-orbit (PSO) mechanism originates in the orbital current density generated by the spin-orbit coupling interaction at the metal center. The PSO mechanism of the ligand NMR shift then reflects the transmission of the spin polarization through bonds, similar to the FC mechanism, but it also makes a substantial through-space contribution in long-range situations. In contrast, the spin-dipolar (SD) mechanism is relatively unimportant at short-range with significant spin polarization on the spectator atom. The PSO and SD mechanisms combine at long-range to form the so-called pseudocontact shift, traditionally used as a structural and dynamics probe in paramagnetic NMR (pNMR). Note that the PSO and SD terms both contribute to the isotropic NMR shift only at the relativistic spin-orbit level of theory.We demonstrate the advantages of calculating and analyzing the NMR shifts at relativistic two- and four-component levels of theory and present analytical tools and approaches based on perturbation theory. We show that paramagnetic NMR effects can be interpreted by spin-delocalization and spin-polarization mechanisms related to chemical bond concepts of electron conjugation in π-space and hyperconjugation in σ-space in the framework of the molecular orbital (MO) theory. Further, we discuss the effects of environment (supramolecular interactions, solvent, and crystal packing) and demonstrate applications of hyperfine shifts in determining the structure of paramagnetic Ru(III) compounds and their supramolecular host-guest complexes with macrocycles.In conclusion, we provide a short overview of possible pNMR applications in the analysis of spectra and electronic structure and perspectives in this field for a general chemical audience.

Theoretical study of electrochemical hydrogen evolution reaction of Pt–Co diatomic sites catalyst

Hydrolytic Hydrogen is a clean hydrogen production method. To mitigate the substantial costs associated with water electrolysis, electrocatalysts are crucial in decreasing the cathodic reaction involved in hydrogen hydrolysis, specifically, the overpotential for hydrogen evolution reaction (HER). Compared to traditional catalysts and single-atom catalysts, Pt–Co diatomic sites catalyst exhibit excellent hydrogen evolution reaction activity and stability. We have established eight different structures of dual-center metal catalysts based on graphene planes. A systematic study was conducted on the catalytic performance and mechanism of these configurations, among which the structure with the best hydrogen evolution reaction activity was CoN4/PtC2N2, with a Gibbs free energy (ΔG) of 0.004 eV. Electronic structure analysis confirms that H follows the Sabatier mechanism on diatomic catalysts, and the catalyst exhibits excellent HER performance for moderate adsorption strength of H atoms. The d-band centers of Co and Pt atoms in the electrocatalyst CoN4–PtC2N2 are lower than those of a single system, indicating that the electronic effect between the CoN4 and PtC2N2 sites can enhance the inherent HER activity of the catalyst. This study expands the application scope of dual-site catalysts and provides new insights for the design and synthesis of dual-site hydrogen evolution catalysts.

Structural, Optical, and Electronic Properties of Epitaxial β‐(AlxGa1‐x)2O3 Films for Optoelectronic Devices

AbstractBandgap engineering in monoclinic gallium oxide (β‐Ga2O3) is a powerful strategy for designing semiconductor optoelectronic devices with specific functionalities. In this work, aluminum doping is utilized to modulate the bandgap of Ga2O3. By growing epitaxial β‐(AlxGa1‐x)2O3 (0≤ x≤ 0.84) films on c‐plane sapphire substrates using RF magnetron sputtering, it allowed to tune the energy bandgap, achieving values as high as 6.10 eV. The increased luminescence intensity is attributed to the recombination between donor and acceptor transitions induced by Al doping, resulting in more defects. Additionally, the luminescent band experienced blueshifts due to the enhanced bandgaps. Moreover, density of functional theory (DFT) simulations confirmed the sensitivity of the bandgap to Al content, distinguishing between Ga‐dominated (x <  0.5) and Al‐dominated (x >  0.5) β‐(AlxGa1‐x)2O3. Notably, the bandgap increased more rapidly in Ga‐dominated structures compared to Al‐dominated ones. The electronic structure analysis revealed a redistribution of Ga d states from valence to conduction bands, attributed to the introduction of numerous Al p states. These combined experimental and detailed electronic structure investigations proved crucial insights for designing the structure and exploring potential applications of β‐(AlxGa1‐x)2O3 in photonic devices.

Theoretical Study of p-Block Metal Single-Atom-Loaded Carbon Nitride Catalyst for Photocatalytic Water Splitting.

Graphitic carbon nitride (g-C3N4), recognized for its considerable potential as a heterogeneous photocatalyst in water splitting, has attracted extensive research interest. By using density functional theory (DFT) calculations, the regulatory role of p-block metal (PM) single atoms on the photocatalytic activity of g-C3N4 in overall water splitting was systematically explored. The incorporation of PM atoms (Ge, Sn and Pb) led to a reduction in the overpotentials required for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Combined with the electronic structures analysis via hybrid functional, it was found that the introduction of Ge, Sn or Pb optimizes the positions of the valence band maximum (VBM) and the conduction band minimum (CBM), providing a robust driving force for HER and ensuring substantial driving force for OER. Meanwhile, the presence of these three PMs induces the spatial separation of VBM and CBM, inhibiting the recombination of carriers. These findings have significant implications for the design and preparation of efficient photocatalysts.

Bifunctional mechanism of oxygen evolution reaction and oxygen reduction reaction induced by surface reconstruction in Perovskite electrocatalysts

Elucidation of the relationship among crystal structure, material compositions, and electrocatalytic performance is beneficial to the design of electrocatalysts with high activity. Specially, for bifunctional electrocatalysts, there is a lack of the unified understanding of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) mechanisms. Based on the first-principles calculations, LaCoO3 surface reconstructions with different OH coverage under OER/ORR condition are determined. Thereby, the different reaction pathways for OER and ORR with the cooperation of Co and O sites are revealed, under which Co and lattice O sites are activated dynamically, and an overpotential of 0.35 V for the OER and 0.44 V for the ORR are obtained. Furthermore, the octahedra lattice distortion-related bifunctional mechanism (DRBM) is proposed from electron structure analysis, and the bifunctional activity of different B-site doped LaCoO3 (LaCoMO3) are studied. Based on this mechanism, we find that OER, ORR, and their bifunctional activity of LaCoMO3 all exhibit a volcano trend as a function of the O-B-O bond angle formed by Co and lattice oxygen. Consequently, we predict that Mn, Cr, Fe, and Ti doped LaCoO3 electrocatalysts exhibit remarkable bifunctional activity, which is corroborated by the reported experimental results.

Effects of Connectivity Isomerization on Electron Transport Through Thiophene Heterocyclic Molecular Junction.

Connectivity isomerization of the same aromatic molecular core with different substitution positions profoundly affects electron transport pathways and single-molecule conductance. Herein, we designed and synthesized all connectivity isomers of a thiophene (TP) aromatic ring substituted by two dihydrobenzo[b]thiophene (BT) groups with ethynyl spacers (m,n-TP-BT, (m,n = 2,3; 2,4; 2,5; 3,4)), to systematically probe how connectivity contributes to single-molecule conductance. Single-molecule conductance measurements using a scanning tunneling microscopy break junction (STM-BJ) technique show ∼12-fold change in conductance values, which follow an order of 10-4.83 G0 (2,4-TP-BT) < 10-4.78 G0 (3,4-TP-BT) < 10-4.06 G0 (2,3-TP-BT) < 10-3.75 G0 (2,5-TP-BT). Electronic structure analysis and theoretical simulations show that the connectivity isomerization significantly changes electron delocalization and HOMO-LUMO energy gaps. Moreover, the connectivity-dependent molecular structures lead to different quantum interference (QI) effects in electron transport, e.g., a strong destructive QI near E = EF leads the smallest conductance value for 2,4-TP-BT. This work proves a clear relationship between the connectivity isomerization and single-molecule conductance of thiophene heterocyclic molecular junctions for the future design of molecular devices.

Detecting Hachimoji DNA: An Eight-Building-Block Genetic System with MoS2 and Janus MoSSe Monolayers.

In the pursuit of personalized medicine, the development of efficient, cost-effective, and reliable DNA sequencing technology is crucial. Nanotechnology, particularly the exploration of two-dimensional materials, has opened different avenues for DNA nucleobase detection, owing to their impressive surface-to-volume ratio. This study employs density functional theory with van der Waals corrections to methodically scrutinize the adsorption behavior and electronic band structure properties of a DNA system composed of eight hachimoji nucleotide letters adsorbed on both MoS2 and MoSSe monolayers. Through a comprehensive conformational search, we pinpoint the most favorable adsorption sites, quantifying their adsorption energies and charge transfer properties. The analysis of electronic band structure unveils the emergence of flat bands in close proximity to the Fermi level post-adsorption, a departure from the pristine MoS2 and MoSSe monolayers. Furthermore, leveraging the nonequilibrium Green's function approach, we compute the current-voltage characteristics, providing valuable insights into the electronic transport properties of the system. All hachimoji bases exhibit physisorption with a horizontal orientation on both monolayers. Notably, base G demonstrates high sensitivity on both substrates. The obtained current-voltage (I-V) characteristics, both without and with base adsorption on MoS2 and the Se side of MoSSe, affirm excellent sensing performance. This research significantly advances our understanding of potential DNA sensing platforms and their electronic characteristics, thereby propelling the endeavor for personalized medicine through enhanced DNA sequencing technologies.

Strain tunable optical and transport properties of intrinsic ferromagnetic semiconductor CrOCl monolayer

Based on density functional theory, the electronic structure, transport, and optical properties of monolayer CrOCl are studied theoretically. The electronic structure analysis shows that monolayer CrOCl is a ferromagnetic semiconductor with an indirect band gap of 2.45 eV, and the electronic properties of monolayer CrOCl can be changed significantly by applying external strain. When 12 % uniaxial tensile strain is applied along the a-axis, the band gap of monolayer CrOCl decreases to 1.94 eV, the hole mobility can reach 1520.11 cm2 V−1·s−1, and the mobility difference between holes and electrons increase to 1471.53 cm2 V−1·s−1. Furthermore, the applied strain, regardless of tensile or compressive, can cause the redshift of the optical absorption peak of monolayer CrOCl, thereby improving the absorption performance in the visible light region. In addition, it was found that the in-plane absorption of bilayer CrOCl increased to nearly twice as compared with the monolayer one. These results indicate that single and few layers of CrOCl have the potential application prospect in the field of optoelectronic devices.

Development of dual-beamline photoelectron momentum microscopy for valence orbital analysis.

The soft X-ray photoelectron momentum microscopy (PMM) experimental station at the UVSOR Synchrotron Facility has been recently upgraded by additionally guiding vacuum ultraviolet (VUV) light in a normal-incidence configuration. PMM offers a very powerful tool for comprehensive electronic structure analyses in real and momentum spaces. In this work, a VUV beam with variable polarization in the normal-incidence geometry was obtained at the same sample position as the soft X-ray beam from BL6U by branching the VUV beamline BL7U. The valence electronic structure of the Au(111) surface was measured using horizontal and vertical linearly polarized (s-polarized) light excitations from BL7U in addition to horizontal linearly polarized (p-polarized) light excitations from BL6U. Such highly symmetric photoemission geometry with normal incidence offers direct access to atomic orbital information via photon polarization-dependent transition-matrix-element analysis.

Van der Waals Epitaxy of Weyl-Semimetal Td-WTe2.

Epitaxial growth of WTe2 offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe2 grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of Td -WTe2 ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of ≃110 nm, which is, on overage, more than three times larger than previous results. WTe2 films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of Td-WTe2 and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe2 nucleation becomes negligible. The quality of monolayer WTe2 films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe2 and previous reports. We found electron pockets at the Fermi level, indicating a n-type doping of WTe2 with an electron density of n = 2.0 ± 0.5 × 1012 cm-2 for each electron pocket.

On the Diffusion of Ionic Liquids in ILs@ZIF-8 Composite Materials: A Density Functional Theory Study.

Recently, composite materials consisting of ionic liquids (ILs) and metal-organic frameworks (MOFs) have attracted a great deal of attention due to their fantastic properties. Many theoretical studies have been performed on their special structures and gas separation applications. Yet, the mechanism for the diffusion of ILs inside MOF channels still remains unclear. Here, the DFT calculations (e.g., rigid and relaxed potential energy surface, PES, scan) together with frontier orbital analysis, natural charge analysis, and energy decomposition analysis were performed to investigate the diffusion behavior of a typical IL, [C4mim][PF6], into the ZIF-8 SOD cage. The PES profiles indicate that it is quite difficult for the cation [C4min]+ to diffuse into the cage of ZIF-8 through the pristine pores because of the large imidazole steric hindrance, which results in a large energy barrier of ca. 40 kcal·mol-1 at the least. Interestingly, the PES reveals that a successful diffusion could be obtained by thermal contributions, which enlarge the pore size through swing effects at higher temperatures. For example, both [C4mim]+ and [PF6]- could easily diffuse through the channel of the ZIF-8 SOD cage when the pore size was increased to 6.9 Å. Subsequently, electronic structure analyses reveal that the main interactions between [PF6]- or [C4mim]+ and ZIF-8 are the steric repulsion interactions. Finally, the effects of the amounts of [C4mim][PF6] on the ZIF-8 structures were investigated, and the results show that two pairs of [C4mim][PF6] per SOD cage are the most stable in terms of the interaction between energies and structural changes. With these findings, we propose that the high-temperature technique could be employed during the synthesis of IL@MOF membranes, to enrich their family members and their industrial applications.