Partial Charge Density Research Articles

Structure–compressibility correlation among MoBx

Molybdenum forms a large number of borides with various stoichiometries. In this paper the trends in the compressibility behaviour among MoBx (x: 0.5, 1, 2 and 3) were investigated. High-pressure structural stability studies on MoB3 were carried out up to a pressure of 40 GPa using a Mao–Bell-type diamond anvil cell. The hexagonal structure remained stable up to 40 GPa. High-pressure X-ray diffraction data yielded a bulk modulus of 254 GPa and were in good agreement with computed bulk moduli of 258–295 GPa with various potentials. First-principles calculations were carried out to investigate the structural, elastic and mechanical properties of molybdenum borides based on generalized gradient approximation proposed by Perdew–Burke–Ernzerhof. The compressibility behaviour of MoBx (x = 0.5, 1, 2 and 3) showed parabolic trend and was explained in terms of electronic structure and mainly arose due to the partial charge density of boron.

Prediction of atomically thin two-dimensional single monolayer SnGe with high carrier mobility: a DFT study

(a) Electron localization function, (b) projected band structure and (c and d) partial charge densities of CBM and VBM of SnGe.

Modulation of Double Zwitterionic Block Copolymer Aggregates by Zwitterion-Specific Interactions.

Transformable double hydrophilic block copolymer assemblies are valid as a biocompatible smart macromolecular system. The molecular mechanisms in the spontaneous assembly of double zwitterionic diblock copolymers composed of a poly(carboxybetaine methacrylate) (PCB2) and a poly(sulfobetaine methacrylate) (PSB4) chains (PCB2-b-PSB4) were investigated by the modulation of the aggregates in response to nondetergent zwitterions. The PCB2-b-PSB4 diblock copolymers with a high degree of polymerization PSB4 block produced aggregates in salt-free water through "zwitterion-specific" interactions. The PCB2-b-PSB4 aggregates were dissociated by the addition of nondetergent sulfobetaine (SB4) and carboxybetaine (CB2) molecules, while the aggregates showed different aggregation modulation processes for SB4 and CB2. Zwitterions with different charged groups from SB4 and CB2, glycine and taurine, hardly disrupted the PCB2-b-PSB4 aggregates. The PCB2-b-PSB4 aggregate modulation efficiency of SBs associated with the intercharge hydrocarbon spacer length (CSL) rather than the symmetry with the SB in the PSB chain. These zwitterion-specific modulation behaviors were rationalized based on the nature of zwitterions including partial charge density, dipole moment, and hydrophobic interactions depending on the charged groups and CSL.

Hydrostatic pressure-tuning of thermoelectric properties of CsSnI3 perovskite by first-principles calculations

Pressure engineering is a conventional approach to modify the material’s interatomic bond length, density, forces as well as other intrinsic properties. The effects of hydrostatic pressure on the electronic and thermoelectric properties of n-type halide perovskite CsSnI3 have been investigated using first-principles approach with spin–orbit coupling included. Electronic structure analysis illustrates that the material is a direct gap semiconductor. The partial charge density calculation suggested that electrons from Cs atoms do not contribute to the valence bonding instead assist in balancing the overall charge distribution. The material’s bandgap decreases with applied pressure up to 1.7 Gpa, where band inversion was observed, after which further compression widened the energy gap. Accompanying with Boltzmann's theory, it was shown that CsSnI3 is a potential candidate for thermoelectric energy harvesting. Hence, materials modification through pressure application is an effective approach to tune the electronic structure and thermoelectric properties of CsSnI3. We achieved the optimum ZT of 1.26 at 1 Gpa pressure and 600 K temperature, close the band inversion region.

Polyvinylpyrrolidone assisted transformation of Cu-MOF into N/P-co-doped Octahedron carbon encapsulated Cu3P nanoparticles as high performance anode for lithium ion batteries

The large volume expansion and poor electrical conductivity of copper phosphide (Cu3P) during the cycle limit their further application as anode of lithium-ion batteries. Therefore, polyvinylpyrrolidone (PVP) modified Cu3(BTC)2-derived (BTC = 1, 3, 5-Benzentricarboxylic acid) in-situ N/P-co-doped Octahedron carbon encapsulated Cu3P nanoparticles (Cu3P@NPC) are successfully prepared through a two-step process of carbonization and phosphating. The N/P-co-doped Octahedron carbon matrix improves the conductivity of Cu3P and moderates the volume expansion during the lithiation/delithiation process. Meanwhile, the interaction between the Cu3P and the doped carbon matrix is methodically explored by using density functional theory (DFT). Through the analysis of the partial charge density, the density of states and the Bader charge, and the calculation results verify the correctness of the experimental observation results, that is, Cu3P@NPC has good electrochemical performance. The results show that Cu3P@NPC, as the anode of Lithium-ion batteries, has excellent electrochemical performance: it exhibits satisfactory rate performance (251.9 mAh g−1 at 5.0 A g−1) and excellent cycle performance (336.4 mAh g−1 at 1 A g−1 over 1000 cycles). This article provides an effective strategy for the encapsulation of metal phosphide nanoparticles in a doped carbon matrix.

First-principles magnetic treatment of the uranium nitride (100) surface and effect on corrosion initiation

The magnetic properties of uranium nitride (UN) surfaces are not well understood experimentally or computationally but they have a significant effect on UN performance as a nuclear fuel. We investigated ferromagnetic (FM), antiferromagnetic (AFM), nonmagnetic (NM), and three hybrid magnetic structures of the most stable UN surface (100). To account for electron correlation and metastability, a U-ramp was performed to an effective Hubbard U-term of 2.0 eV. FM was found to be the most energetically favorable magnetic structure. Type 1 AFM slab was optimized to a new magnetic structure consisting of (100) planes with either all spin-up electrons, all spin-down electrons, or half spin-up and half spin-down electrons on uranium atoms. After OH adsorption to simulate corrosion initiation, the AFM, FM, and NM structures yield relatively similar bond lengths but varying bond angles, adsorption energies, and electronic profiles. Partial charge density maps show varying degradation mechanisms across magnetic structures. Electron localization function reveals more charge localized to AFM uranium atoms with spin-down electrons than uranium atoms with spin-up electrons. This leads to different properties depending on if an adsorbate interacts with a spin-up or spin-down terminated AFM surface. This work supports the physical accuracy of future computational studies toward corroborating with experiments and addressing UN fuel corrosion.

Strain Modulation of Black Phosphorene for the Hydrogen Evolution Reaction Activity

The first‐principles calculation within the density functional theory (DFT) is used to study the hydrogen evolution reaction (HER) activity and related electronic properties of monolayer black phosphorus under the uniaxial strains. Herein, it is found that different compressive and tensile strains would modulate the Gibbs free energy (ΔGH*), partial charge density of the lowest unoccupied state (εLUS), and band structure of black phosphorene. Specifically, as the ΔGH* decreases, the HER activity of phosphorene system with uniaxial strain is improved except for the +3% strain case. Importantly, the HER catalytic performance of phosphorene can be reduced by 0.4 eV and the bandgap decreased to 0.23 eV by applying the −10% strain. Comparing with strained‐free case, the larger partial charge density of εLUS on the surface of black phosphorene shows more H adsorption sites for electrocatalysis. Moreover, the HER activity is related to the partial charge density of εLUS rather than the value of εLUS site. These results reveal that strain could effectively modulate the HER performance for electrocatalysis. And it is expected to provide further understanding in practical applications.

Developing orbital-free quantum crystallography: the local potentials and associated partial charge densities

This work extends the orbital-free density functional theory to the field of quantum crystallography. The total electronic energy is decomposed into electrostatic, exchange, Weizsacker and Pauli components on the basis of physically grounded arguments. Then, the one-electron Euler equation is re-written through corresponding potentials, which have clear physical and chemical meaning. Partial electron densities related with these potentials by the Poisson equation are also defined. All these functions were analyzed from viewpoint of their physical content and limits of applicability. Then, they were expressed in terms of experimental electron density and its derivatives using the orbital-free density functional theory approximations, and applied to the study of chemical bonding in a heteromolecular crystal of ammonium hydrooxalate oxalic acid dihydrate. It is demonstrated that this approach allows the electron density to be decomposed into physically meaningful components associated with electrostatics, exchange, and spin-independent wave properties of electrons or with their combinations in a crystal. Therefore, the bonding information about a crystal that was previously unavailable for X-ray diffraction analysis can be now obtained.

Semiconductor-metal transition induced by combined electric field and external strain in bilayer phosphorene

The gap modulation by effectively external control is an intriguing feature of the black phosphorene which may enable a flexible design and optimization of electronics. In this paper, combined electric field and external strain effects are systematically investigated to tailor the electrical properties of bilayer black phosphorene (BP) in four stacking order by using density functional theory (DFT). The electronic structures and density of states of bilayer black phosphorene can be modulated by the two external controls, the direct semiconductor-indirect semiconductor-metal transitions emerge under uniaxial in-plain strains and vertical electric field. The band gap is more likely to be modulated by the electric field under compressive strain along armchair direction for the AB stacking structures. The greater the compressive strain, the gap closed at a lower electric field. The partial density of states (PDOS) shows the energy band was mainly contributed by p orbital, and the partial charge density of the VBM and CBM further indicates the shifting trend of electrons and holes between the double layers. Our results provide potential reference information for nano-electronic and optoelectronic applications.

Pressure-induced electrides and metallic phases in the Y–Cl system

Pressure can profoundly change the electronic structure, leading to the formation of new phases and materials with exotic properties. Herein, using evolutionary algorithms and density functional theory, we systematically investigate the behaviour of materials in the yttrium–chlorine binary system under pressure. Electrons are found to be spatially confined at low pressures in yttrium chlorides and tend to form new electrides. In particular, a novel yttrium chloride, Y3Cl2, is predicted to be thermodynamically and lattice dynamically stable at approximately 10 GPa. Further analyses of the electron localization function and partial charge density identify trigonal Y3Cl2 as a new 2D high-pressure electride with partially localized electrons contributing to the conduction. By further increasing the pressure, electrons in the yttrium–chlorine binary system tend to delocalize with the electrides decomposing into two new compounds (Y2Cl and YCl2) and a new YCl phase (space group P63/mmc) above 20 GPa. These newly discovered phases are all metallic in their stable pressure range according to band structure simulations without interstitial electron localization. The discovery of these unconventional yttrium chlorides may inspire strategies to search for low-pressure electrides in other rare-earth halogenide systems.

Synthesis of Mg–Al layered double hydroxides-functionalized hydrochar composite via an in situ one-pot hydrothermal method for arsenate and phosphate removal: Structural characterization and adsorption performance

• Mg–Al LDHs-FHC composite was synthesized via an in situ one-pot hydrothermal route. • The Mg:Al molar ratio and hydrothermal temperature were systematically optimized. • Synthetic conditions play significant roles in arsenate and phosphate removal. • Mechanism involves intercalation, inner-sphere, and outer-sphere surface complexes. • Competitive adsorption behavior mainly occurred at the binding sites on Mg(OH) 2 . In recent years, hydrochar has been recognized as an ideal and eco-friendly adsorbent for the removal of various pollutants, while it has a relatively low adsorption affinity toward negatively charged contaminants, such as arsenate and phosphate. Thus, to address serious worldwide water pollution caused by arsenate and phosphate, the Mg–Al layered double hydroxides-functionalized hydrochar (Mg–Al LDHs-FHC) composite was synthesized via an in situ one-pot hydrothermal method. The Mg:Al molar ratios (2:1–4:1) and hydrothermal temperatures (150–210 °C) played significant roles in influencing the structural properties of the Mg–Al LDHs-FHC composites. Specifically, the Mg–Al LDHs-FHC composite prepared at Mg:Al molar ratio of 2:1 and hydrothermal temperature of 150 °C showed the best arsenate (100%) and phosphate (99.7%) removal due to the highest charge density (4.132 e/nm 2 ) and the largest interlayer d -spacing (0.8371 nm), facilitating strong electrostatic adsorbent–adsorbate interaction and easy intercalation into the interlayer. Single-component isotherm data fit Langmuir and Freundlich models, while binary-component data used extended and ideal adsorbed solution theory combining these models. Overall, the adsorption affinity of the composite followed the order arsenate > phosphate (single-component system) and arsenate < phosphate (binary-component system). Spectroscopic analyses demonstrated that the different physicochemical characteristics of arsenate and phosphate, such as partial charge density, ionic radius, and the coordinate bonding characteristics were responsible for the competitive adsorption behavior. Stable reusability at least five times indicates that the Mg–Al LDHs-FHC composite could be a promising adsorbent for simultaneous arsenate and phosphate removal from water.

Efficient elimination of the pollutants in eutrophicated water with carbon strengthened expanded graphite based photocatalysts: Unveiling the synergistic role of metal sites

Metal sites (Ni, Bi or Ag) were introduced into carbon strengthened expanded graphite (CEG) based photocatalysts, and performed as a novel strategy to enhance the elimination of Microcystis aeruginosa and microcystin-LR from water. Results show that metal doping can efficiently improve the adsorption of harmful algae and enhance the photocatalytic activities in inactivation of harmful algae and degradation of MC-LR. Among the CEG catalysts, Ni-CEG can achieve the highest removal rate up to 90.6% for algal cells with 5 h visible light irradiation, while Bi-CEG catalyst provides the best performance for MC-LR degradation with the removal rate of 80.9% in 6 h visible light irradiation. In general, considering the coexistence of algal cells and microcystin-LR, Bi-CEG is proved to be an excellent candidate for the remediation of eutrophicated waters since it can achieve the efficient removal of both harmful algae and MC-LR. DFT calculations indicate that metal doping can transform the photocatalysts into n-type semiconductor, and provide the mid-gap state. In addition, the partial charge density distribution near Fermi level was mainly composed by the metal dopants, which can enhance the interaction with harmful algae and MC-LR.

First-principles studies of MoF6 absorption on hydroxylated and non-hydroxylated metal oxide surfaces and implications for atomic layer deposition of MoS2

Significant interest in two-dimensional transition metal dichalcogenides has led to numerous experimental studies of their synthesis using scalable vapor phase methods, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD). ALD typically allows lower deposition temperatures, and nucleation of chemical precursors requires reactions with surface functional groups. A common first-principles method used to study ALD modeling is the calculation of activation energy for a proposed reaction pathway. In this work we calculated the partial charge densities, local density of states (LDoS), Bader charge analysis, adsorption energies, and charge density difference using density functional theory (DFT) to investigate the nucleation of MoF6 on three oxide surfaces, including Al2O3, HfO2, and MgO. Our findings indicate that hydroxyl groups (OH) help lower the reaction barrier during the first half-cycle of MoF6 and promote the chemisorption of a precursor on the oxide substrates. This discovery is supported by the formation of highly ionic MFx (M = metal, x = 1, 2, 3) bonds at the oxide surfaces. By comparing surfaces with and without hydroxyl groups, we highlight the importance of surface chemistry.

First‐principle calculations of CrN(200)/Ni(111) interface: Atomic structure, stability, and electronic properties

Atomic structure, adhesion energy, and electronic properties of the bonding nature at CrN(200)/Ni(111) interfacial region are studied by first‐principle calculations based on density functional theory (DFT). A new method for building the heterostructure of CrN(200)/Ni(111) is proposed to solve the large lattice mismatching between CrN(200) and Ni(111) slab. Three models (OT, ST, and TL) with different interface distance are built up, and the total energy for each of them is calculated. Comparing with other stacking sequence, TL sites with 1.9‐Å spacing at the interfacial region shows the biggest value of adhesion energy after geometry relaxation. Furthermore, the contour of charge density, partial charge density of states, and Bader charge values of the TL sites with 1.9‐Å spacing are plotted out and calculated. Results indicate that covalency bonding between Ni and N ions and metallic bonding between Ni and Cr atoms are formed at the interfacial region.

Two-Dimensional Black Arsenic Phosphorus for Ultrafast Photonics in Near- and Mid-Infrared Regimes.

Black arsenic phosphorus (b-AsP), as one kind of novel two-dimensional (2D) materials, bridges the band gap between black phosphorus and graphene. Thanks to its great advantages, including high carrier mobility, excellent in-plane anisotropy, and broad tunability band gap, b-AsP has aroused great interest in fields of photonics and photoelectronics. In this paper, ultrathin 2D b-AsP nanomaterials were fabricated by the liquid-phase exfoliation method, and their strong broadband linear and nonlinear absorptions were characterized by ultraviolet-visible-infrared and Z-scan technology. The experimental determination of the nonlinear absorption coefficient and low saturation intensity of b-AsP were -0.23 cm/GW and 3.336 GW/cm2, respectively. Based on density functional theory, the partial charge density and band structure at the conduction band minimum and valence band maximum were calculated, which further proves the excellent optical properties of 2D b-AsP. By first using 2D b-AsP as a novel saturable absorber in both erbium-doped and thulium-doped fiber lasers, mode-locked soliton pulses can stably operate at 1.5 and 2 μm. The laser pulses generated by 2D b-AsP possess higher stability to resist self-splitting than those generated by other 2D material-based mode-lockers. These experimental results highlight that 2D b-AsP has great application potential as a novel optical material in ultrafast photonics from near- to mid-infrared regimes.

Strain-Tuned Nodal Ring in Two-Dimensional Zn3C6S6 Monolayers

The nodal ring material has recently attracted wide attention due to its singular properties and potential applications in spintronics. Here, two-dimensional Zn3C6S6 is calculated and discussed by using first-principle calculations. We found that two-dimensional Zn3C6S6 can generate a nodal ring at 10% compressive strain, and the existence of the ring is proved by a partial charge density map. And as the compressive strain increases, the nodal ring does not disappear. At the same time, the stability of the electron-orbit coupling to the nodal ring is applied. Our findings indicate that the two-dimensional Zn3C6S6 is promising in new electronic and spintronic applications.

Size-dependent electronic, optical and photocatalytic properties of Ti3C2O2 quantum dots studied by first-principles calculations

Abstract Due to quantum confinement effect, quantum dots (QDs) would show totally different electronic and optical properties from two-dimensional (2D) nanosheets. For the first time, we have systemically investigated the optical, electronic and photocatalytic properties of Ti3C2O2 QDs based on the first-principles calculations. Our results show that the most stable structures of Ti3C2O2 QDs have features of hexagonal unit centered by O atom. The energy gaps of QDs with H passivation vary from 2.76 to 1.14 eV with the increase of lateral size. The lowest unoccupied molecular orbital (LUMO) is mainly contributed by Ti:3d, while the highest occupied molecular orbital (HOMO) is mainly contributed by C:2p and Ti:3d. The corresponding partial charge density at HOMO or LUMO is distributed around Ti and C atoms in the outside surface of QDs. The QDs show large absorption coefficient with the order of 105 cm−1 at visible light range, while the first absorption peaks vary from 2.8 to 1.4 eV (443–886 nm). Most importantly, the Ti3C2O2 QDs present size-dependent photocatalytic selectivity for CO2 reduction and photocatalytic activity for water splitting, caused by the shift of energy level positions of HOMO and LUMO. Our study reveal the potential of Ti3C2O2 QD in electronic and photocatalytic applications and provide new ideals into the design of photocatalysis based on QD materials.

True bulk As-antisite defect in GaAs(1 1 0) identified by DFT calculations and probed by STM/STS measurements

We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk characteristics using first-principles methods with experimental verifications. We found that the AsGa in the third-layer mimics the geometry, partial charge density and more importantly, the density of states of AsGa in bulk GaAs. Notably, the mid-gap state induced by AsGa in bulk GaAs is well-reproduced by the AsGa in the third layer of GaAs(1 1 0). Simulated and experimental STM images show an “asymmetric two-lobe” feature in the region around the defect. Using local density of states (LDOS) and STS spectra, we propose three prominent peaks with characteristic energy levels corresponding to the third layer AsGa. The above results present the first report of surface electronic signatures of true bulk defect near the surface of GaAs(1 1 0).

Theoretical Study on the Electronic Structure of Heavy Alkali-Metal Suboxides.

On the metal-rich side of the phase diagrams of the Rb-O, Cs-O, and Rb-Cs-O systems, one can find a variety of stoichiometries: for example, Rb9O2, Rb6O, Cs4O, Cs7O, Cs11O3, RbCs11O3, and Rb7Cs11O3. They may be termed heavy alkali-metal suboxides. The application of the standard electron-counting scheme to these compounds suggests the presence of surplus electrons. This motivated us to carry out a theoretical study using the first-principles density functional theory (DFT) method. The structures of these compounds are based on either a formally cationic Rb9O2 or Cs11O3 cluster. The analyses of the partial charge density just below the Fermi level and the electron localization function (ELF) have revealed that there exist surplus electrons in interstitial regions of all the investigated suboxides so that the excess positive charge of the cluster can be compensated. Density of states (DOS) calculations suggest that all of the compounds are metallic. Therefore, the suboxides listed above may be regarded as a new family of metallic electrides, where coreless electrons reside in interstitial spaces and provide a conduction channel. Except for the phases of Rb9O2 and Cs11O3, the suboxide structures include both the cationic clusters and alkali-metal matrix. Several charge analyses indicate that the interstitial surplus-electron density can be assigned to the alkali-metal atoms in the metal matrix, leading to the possibility of the presence of negatively charged alkali-metal atoms, namely Rb- (rubidide) and Cs- (caeside) ions, a.k.a. alkalides. In Rb6O, Rb-, Rb0, and Rb+ are found to coexist in the same crystal structure. Similarly, in Cs7O, one can find the three types of Cs atoms. However, in Cs4O, no Cs0 state is identified. In the Rb-Cs-O ternary suboxides, Rb takes a negatively charged anion state or neutral state, while all of the Cs atoms are found to be cationic because they get involved in the Cs11O3 cluster and all the Rb atoms exist in interstitial sites. Orbital interactions between the clusters are analyzed to understand how the condensation of the clusters into the solid happens and how the electride nature ensues. These clusters are found to have some superatomic character.

Electronic and magnetic properties of the Janus MoSSe/WSSe superlattice nanoribbon: a first-principles study.

The electronic structure properties of Janus MoSSe/WSSe superlattice nanoribbons (SLNRs) are investigated by first-principles calculations. The band structure, density of states and partial charge density are calculated. The results show that the ribbon width, combination ratio and period length have a great effect on the properties of SLNRs. Adjusting the distribution of outer S and Se atoms can restrict the spontaneous bending of the Janus nanoribbons while the properties are well preserved. The zigzag MoSSe/WSSe SLNRs (Z-SLNRs) show semiconductor and half-metal behavior depending on the ribbon width, which is quite different from the metallic properties of pristine MoSSe or WSSe nanoribbons. By modulating the ratio and period length of the two construction units, the bandgap and magnetic properties of Z-SLNRs can be tuned. The armchair MoSSe/WSSe SLNRs (A-SLNRs) are semiconductors, and the bandgap is similar to that of the corresponding armchair MoSSe or WSSe nanoribbons. As the width increases, the bandgaps of A-SLNRs show a variant behavior. It is found that A-SLNRs are all semiconductors regardless of the composition ratio, and the bandgaps also vary with the composition ratio of the constituent materials.