Pz Orbitals Research Articles

Layer Sliding And Twisting Induced Electronic Transitions In Correlated Magnetic 1t‐Nbse2 Bilayers

AbstractCorrelated 2D layers, like 1T‐phases of TaS2, TaSe2, and NbSe2, exhibit rich tunability through varying interlayer couplings, which promotes the understanding of electron correlation in the 2D limit. However, the coupling mechanism is, so far, poorly understood and is tentatively ascribed to interactions among the orbitals of Ta or Nb atoms. Here, it is theoretically shown that the interlayer hybridization and localization strength of interfacial Se pz orbitals, rather than Nb orbitals, govern the variation of electron‐correlated properties upon interlayer sliding or twisting in correlated magnetic 1T‐NbSe2 bilayers. Each of the layers is in a star‐of‐David (SOD) charge‐density‐wave phase. Geometric and electronic structures and magnetic properties of 28 different stacking configurations are examined and analyzed using density‐functional‐theory calculations. It is found that the SOD contains a localized region, in which interlayer Se pz hybridization plays a paramount role in varying the energy levels of the two Hubbard bands. These variations lead to three electronic transitions among four insulating states, which demonstrate the effectiveness of interlayer interactions to modulate correlated magnetic properties in a prototypical correlated magnetic insulator.

Local electrons perturbation of V2CTx via defect and strain for efficient hydrogen production

The electron density of electrocatalysts plays a vital role in governing the activity, but modifying the surface electronic structure to improve catalytic activity remains largely an empirical process, and rational designs are still extremely rare. Herein, we report a theory-led discovery that the introduction of oxygen (O)-defects and strain to tailor the local electrons on the V2CTx surface for hydrogen evolution reaction (HER). Our theoretical analysis shows that O-defects and strain can perturb the local electrons on the V2CTx surface and precisely modulate the pz orbitals of O to be half-occupied, achieving an optimal hydrogen adsorption-free energy (ΔGH*). To validate our theoretical predictions, defective and strained exfoliated V2CTx sheets are constructed by in situ heating and capillary forces, which exhibit an overpotential of 134 mV at 10 mA cm−2 and Tafel slope of 48.4 mV dec−1, exceeding other reported MXene phase catalysts without any performance-enhancing additives. Both theoretical and experimental data exhibit the same volcanic trend of overpotentials and unveil that the proper combinations of O-defects and strain can achieve highly efficient HER performance. This reliable strategy to tailor the local electrons of V2CTx will open a window for improving the catalytic performance of its structural analog.

Novel octa-graphene-like structures based on GaP and GaAs.

The discovery of graphene gave way to the search for new two-dimensional structures. In this regard, octa-graphene is a carbon allotrope consisting of 4- and 8-membered rings in a single planar sheet, drawing the research community's attention to study their inorganic analogs. Considering the promising properties of octa-graphene-like structures and the role of GaAs and GaP in semiconductor physics, this study aims to propose, for the first time, two novel inorganics buckled nanosheets based on the octa-graphene structure, the octa-GaAs and octa-GaP. This work investigated the structural, electronic, and vibrational properties of these novel octa-graphene-based materials. The octa-GaP and octa-GaAs have an indirect band gap transition with a valence band maximum between M and Г points and a conduction band minimum at Г point with energy of 3.05 eV and 2.56 eV, respectively. The QTAIMC analysis indicates that both structures have incipient covalent in their bonds. The vibrational analysis demonstrates the occurrence of ΓRaman = 6Ag + 6Bg and ΓRaman = 12A' + 12B″ for octa-GaP and octa-GaAs, respectively. The symmetry reduction of octa-GaAs leads to activating inactive modes observed in the octa-GaP structure. The frontier crystalline orbitals are composed by Ga(px) and P(py and pz) orbitals for octa-GaP and Ga(px and py) and As(s, py, and pz) for octa-GaAs in the valence bands while in the conduction bands by Ga(py, pz, and s) for both compounds and P(px and pz) and As(py). The phonon bands demonstrate the absence of the negative frequency modes and the structural stability of these new nanosheets. This report aims to reveal the fundamental properties of both newfound materials for stimulating experimental research groups in the search for synthesis routes to obtain this structure. This work used the DFT/B3LYP approach implemented in the CRYSTAL17 computational package. Ga, As, and P atomic centers were described by triple-zeta valence with polarization (TZVP) basis set. The vibrational analysis was carried out via coupled-perturbed Hartree-Fock/Kohn Sham (CPHF/KS) method, and the chemical bonds were evaluated via the quantum theory of atoms in molecules and crystals (QTAIMC).

Coexistence of Electronic and Phononic Dirac Points in TiB2 Monolayer with Inverse Sandwich Motif

Although many Dirac semimetals (DSMs) have been theoretically proposed, the ideal Dirac fermions coexisting with Dirac phonons in a DSM are rare. In this work, we proposed a general strategy to assemble two-dimensional (2D) ideal DSMs using the cluster of transition metal boride with an inverse sandwich configuration. We illustrated this strategy using first-principles calculations, resulting in the creation of a 2D titanium diboride named 2c-TiB2 with a space group of P21/c. This monolayer was found to be thermodynamically, dynamically, and thermally stable. Additionally, it was revealed to be a DSM with an ideal Dirac cone at the Fermi level, and its topology arises from the band inversion between the pz orbitals of B atoms and dx2–y2 orbitals of Ti atoms. Furthermore, the monolayer exhibits intersecting Dirac phonons along high-symmetry path Γ–X. The quantized Berry phase, inversed eigenvalues, and apparent electronic and phononic topological edge states all served as further evidence of the topological protection originating from C2x symmetry. The theoretical investigation into inverse sandwich structured boride monolayer not only predicts the coexistence of Dirac fermions and Dirac phonons in 2D TiB2, but also provides a novel approach to design topological states in 2D nanomaterials.

Tailing the Magnetoelectric Properties of Cr2Ge2Te6 by Engineering Covalently Bonded Cr Self-Intercalation: Ferromagnetic Half-Metal

Two-dimensional intrinsic ferromagnetic half-metals (HMs) are important for spintronics. Based on a systematic research of CrGeTe3 (CGT) bilayers and multilayers with the self-intercalated (SI) Cr (CrSI) atom, we find that self-intercalation can enhance interlayer magnetic coupling, which results in a super-exchange interaction between interlayers. The CGT bilayer keeps half-metallicity with the interlayer ferromagnetic (FM) order after Cr’s self-intercalation, independent of CrSI atoms’ concentration and stacking orders. Moreover, SI-CGT multilayers with interlayer FM order transform from HMs into normal spin-polarized metals, as the states at the Fermi level increase. Moreover, the magnetic anisotropy energies (MAEs) of SI-CGT-AA and SI-CGT-AB are −0.160 and −0.42 meV/f.u., respectively, which are modulated by the CrSI atoms. The MAEs of SI-CGT-AA and SI-CGT-AB are different, as the hybridization interactions between Cr’s d orbitals are different from each other. SI-CGT bilayers’ magnetic easy axis (EA) switches from the original [001] of CGT to the [100] direction, independent of the stacking orders. It is inferred that the MAE is mainly contributed by the hybridization between Te’s px and py, py and pz orbitals, and that this hybridization interaction is obviously weakened as CrSI atom is inserted. The SI-CGT bilayers and multilayers show good dynamic and thermal stability at 300 and 500 K. These findings offer a promising way to manipulate the interlayer exchange interaction and magnetoelectric properties of CGT multilayers and other vdW magnets.

Orbital distortion and electric field control of sliding ferroelectricity in a boron nitride bilayer

Recent experiments confirm that two-dimensional boron nitride (BN) films possess room-temperature out-of-plane ferroelectricity when each BN layer is sliding with respect to each other. This ferroelectricity is attributed to the interlayered orbital hybridization or interlayer charge transfer in previous work. In this work, we attempt to understand the sliding ferroelectricity from the perspective of orbital distortion of long-pair electrons. Using the maximally localized Wannier function method and first-principles calculations, the out-of-plane pz orbitals of BN are investigated. Our results indicate that the interlayer van der Waals interaction causes the distortion of the N pz orbitals. Based on the picture of out-of-plane orbital distortion, we propose a possible mechanism to tune the ferroelectric polarization by external fields, including electric field and stress field. It is found that both the polarization intensity and direction can be modulated under the electric field. The polarization intensity of the system can also be controlled by stress field perpendicular to the plane. This study will provide theoretical help in the device design based on sliding ferroelectrics.

Graphynes and Graphdiynes for Energy Storage and Catalytic Utilization: Theoretical Insights into Recent Advances

Carbon allotropes have contributed to all aspects of people's lives throughout human history. As emerging carbon-based low-dimensional materials, graphyne family members (GYF), represented by graphdiyne, have a wide range potential applications due to their superior physical and chemical properties. In particular, graphdiyne (GDY), as the leader of the graphyne family, has been practically applied to various research fields since it was first successfully synthesized. GYF have a large surface area, both sp and sp2 hybridization, and a certain band gap, which was considered to originate from the overlap of carbon 2pz orbitals and the inhomogeneous π-bonds of carbon atoms in different hybridization forms. These properties mean GYF-based materials still have many potential applications to be developed, especially in energy storage and catalytic utilization. Since most of the GYF have yet to be synthesized and applications of successfully synthesized GYF have not been developed for a long time, theoretical results in various application fields should be shared to experimentalists to attract more intentions. In this Review, we summarized and discussed the synthesis, structural properties, and applications of GYF-based materials from the theoretical insights, hoping to provide different viewpoints and comments.

Activating Bi p‐orbitals in Dispersed Clusters of Amorphous BiOx for Electrocatalytic Nitrogen Reduction

AbstractCatalytic strategies based on main group metals are significantly less advanced than those of transition metal catalysis, leaving untapped areas of potentially fruitful research. We here demonstrate an effective approach for the modulation of Bi 6p energy levels during the construction of atomically dispersed clusters of amorphous BiOx. Bi oxidation state is proposed to strongly affects the nitrogen fixation activity, with the half‐occupied pz orbitals of the Bi2+ ions being highly efficient toward electron injection into the inert N2 molecule. With sufficient catalytic sites to adsorb and activate N2, the bonding between N2 and catalyst is able to be in situ identified. The catalyst shows an outstanding Faraday efficiency (≈30 %) and high yield (≈113 μg h−1 mg−1cat) in NH3 production, outperforming most of the existing catalysts in aqueous solution. These results lay the basis for developing the potential of p‐block elements for catalysis of multi‐electron reactions.

Activating Bi p-orbitals in Dispersed Clusters of Amorphous BiOx for Electrocatalytic Nitrogen Reduction.

Catalytic strategies based on main group metals are significantly less advanced than those of transition metal catalysis, leaving untapped areas of potentially fruitful research. We here demonstrate an effective approach for the modulation of Bi 6p energy levels during the construction of atomically dispersed clusters of amorphous BiOx . Bi oxidation state is proposed to strongly affects the nitrogen fixation activity, with the half-occupied pz orbitals of the Bi2+ ions being highly efficient toward electron injection into the inert N2 molecule. With sufficient catalytic sites to adsorb and activate N2 , the bonding between N2 and catalyst is able to be in situ identified. The catalyst shows an outstanding Faraday efficiency (≈30 %) and high yield (≈113 μg h-1 mg-1 cat ) in NH3 production, outperforming most of the existing catalysts in aqueous solution. These results lay the basis for developing the potential of p-block elements for catalysis of multi-electron reactions.

Electronic structure of covalent networks of triangular graphene flakes embedded in hBN

Covalent networks of triangular graphene flakes ([n]triangulenes) embedded in hexagonal boron nitride (hBN) were theoretically investigated using density functional theory. Our calculations reveal that the electronic structure of these in-plane heterostructures comprising B, C, and N atoms strongly depends on the arrangements of the constituent triangular graphene flakes and border atom species. Heterostructures comprising a copolymer of [n]triangulene and [m]triangulene embedded in hBN are tiny gap semiconductors or metals for which flat dispersion bands emerge near and at the Fermi level. A heterostructure comprising [3]triangulene is a semiconductor with a moderate direct gap of 0.7 eV, in which the band edges exhibit a flat band nature throughout the Brillouin zone. These flat band states are attributed to the hybridization between the non-bonding states of the triangulenes and the pz orbitals of the B and N atoms at the borders.

Dominant changes in centre Fe atom of decamethyl-ferrocene from ferrocene in methylation

Staggered decamethyl-ferrocene (*Fc) becomes the lower energy conformer at low temperature, whereas the eclipsed conformer of ferrocene (Fc) is more stable. The powerful infrared (IR) spectroscopy which has remarkably provided signatures of ferrocene (Fc) in eclipsed and staggered conformers recently is employed to investigate methylation of Fc. The most significant consequences of the full methylation of Fc in the IR spectra are the blue shift of the band at ~ 800 cm−1 in Fc to ~ 1500 cm−1 in *Fc, and the enhancement of the C–H stretch band at ~ 3200 cm−1 region in *Fc. Further analysis reveals large impact of Fc methylation on core electron energies of the centre Fe atom (1s22s22p63s23p6). The Fe core electron energy changes can be as large as ~ 10 kcal mol−1 and are directional—the Fe 2pz and 3pz orbitals along the *Cp–Fe–*Cp axis (Cp centroids, vertical) change more strongly than other Fe core electrons in px and py orbitals. The directional inner shell energy changes are evidenced by larger inner shell reorganization energy. Energy decomposition analysis (EDA) indicates that methyl groups in *Fc apparently change the physical energy components with respect to Fc. The large steric energy of *Fc evidences that the closest hydrogens on adjacent methyl groups of the same *Cp ring in crystal structure are 0.2–0.4 Å closer than the hydrogens on nearest-neighbour methyl groups on opposing rings in *Fc. A significant increase in Pauli repulsive energy contributes to the large repulsive steric energy in *Fc.

Giant band gap bowing in two dimensional Pb[formula omitted]Sn[formula omitted]O alloys and Janus PbSnO monolayers

Two-dimensional monolayer alloys can be experimentally synthesized and the monolayer form of α-PbO and ultrathin SnO were recently synthesized. The band gap values of the materials can be tuned by varying the host atom composition by foreign atoms. Thus, desired band gap value can be obtained for various applications. In this study, we investigated electronic, mechanical and vibrational properties of energetically the most stable Pb1−xSnxO monolayer alloys and dynamically stable Janus PbSnO (J-PbSnO) monolayer using first-principles density functional theory calculations. We found that seven Pb1−xSnxO monolayer alloys can be synthesized by the exothermic reactions. However, to synthesize J-PbSnO monolayer, experimentally suitable environment conditions requires such as which is done to synthesize J-MoSSe monolayer. We found both with PBE and HSE06 functionals that PbO and Pb0.75Sn0.25 compounds have direct band gap values while the remaining compounds have indirect band gap values. In addition, the band gap values of Pb1−xSnxO monolayer alloys do not scale linearly with the changing of the composition and exhibit giant band gap bowing. This non-linearity is attributed to electronegativity and atomic radius differences between Pb and Sn atoms. In addition, we observed that in Pb1−xSnxO monolayer alloys localization of valence band maximum states are mostly distributed among Sn pz orbitals whereas the conduction band minimum states localized around Pb px and py orbitals by the increasing of Sn concentration. We believe that our theoretical predictions will guide the experimental realization of these Pb1−xSnxO monolayer alloys for various applications.

Topological line defects in hexagonal SiC monolayer.

Defect engineering of two-dimensional (2D) materials offers an unprecedented route to increase their functionality and broaden their applicability. In light of the recent synthesis of the 2D Silicon Carbide (SiC), a deep understanding of the effect of defects on the physical and chemical properties of this new SiC allotrope becomes highly desirable. This study investigates 585 extended line defects (ELDs) in hexagonal SiC considering three types of interstitial atom pairs (SiSi-, SiC-, and CC-ELD) and using computational methods like Density Functional Theory, Born-Oppenheimer Molecular Dynamics, and Kinetic Monte-Carlo (KMC). Results show that the formation of all ELD systems is endothermic, with the CC-ELD structure showing the highest stability at 300 K. To further characterize the ELDs, simulated scanning tunneling microscopy (STM) is employed, and successfully allow identify and distinguish the three types of ELDs. Although pristine SiC has a direct band gap of 2.48 eV, the presence of ELDs introduces mid-gap states derived from the pz orbitals at the defect sites. Furthermore, our findings reveal that the ELD region displays enhanced reactivity towards hydrogen adsorption, which was confirmed by KMC simulations. Overall, this research provides valuable insights into the structural, electronic, and reactivity properties of ELDs in hexagonal SiC monolayers and paves the way for potential applications in areas such as catalysis, optoelectronics, and surface science.

Robust Room Temperature Ferromagnetism In Cobalt Doped Graphene by Precision Control of Metal Ion Hybridization

AbstractGraphene‐based magnetic materials exhibit novel properties and promising applications in the development of next‐generation spintronic devices. Modern synthesis techniques have paved the way to design precisely the local environments of metal atoms anchored onto a nitrogen‐doped graphene matrix. Herein, it is demonstrated that grafting cobalt (Co) into the graphene lattice induces robust and stable room‐temperature ferromagnetism. These comprehensive experiments and first‐principles calculations unambiguously identify that the mechanism for this unusual ferromagnetism is π‐d orbital hybridization between Co dxz and graphene pz orbitals. Here, it is found that the magnetic interactions of Co–carbon ions are mediated by the spin‐polarized graphene pz orbitals, and room temperature ferromagnetism can be stabilized by electron doping. It is also found that the electronic structure near the Fermi level, which sets the nature of spin polarization of graphene pz bands, strongly depends on the local environment of the Co moiety. This is the crucial, previously missing, ingredient that enables control of the magnetism. Overall, these observations unambiguously reveal that engineering the atomic structure of metal‐embedded graphene lattices through careful d to p orbital interactions opens a new window of opportunities for developing graphene‐based spintronics devices.

Deciphering the electronic-level mechanism of Na+ transport in a graphdiyne desalination membrane with periodic nanopores

With the advancement of industry and population growth, the crisis of freshwater shortage has turned into a formidable dilemma, with nearly one billion people worldwide experiencing an urgent requirement for healthy freshwater. Recently, two-dimensional porous carbon materials with pore sizes of 0.45–0.55 nm have been confirmed to demonstrate a rejection rate of 100 %. However, there are still challenges in fabricating periodic defect-free nanopores in monolayers, so the development of these desalination membranes has somewhat stagnated at the investigation stage. Here, we have conducted a thorough study of desalination performance using molecular dynamics (MD) and density functional theory (DFT) with a view to delineating the electronic-level mechanism of Na+ transport in periodic nanopores with a self-cleaning effect. The results further suggested that synthesizable graphdiyne was one of the most promising potential desalination membranes by virtue of its periodic pore structure, electronic properties, and structural stability. Graphdiyne membranes have a salt rejection rate close to 100 % and require less pressure compared to other 2D materials. We then found that the transition-state energy of Na+ in a graphdiyne membrane was satisfactory (0.03 eV) due to significant charge-transfer between the carbon atoms in the periodic pores and Na+ in the pz orbitals. This blocked the filtration and agglomeration of Na+, while imparting the graphdiyne membrane with metallicity. Therefore, in striking contrast to conventional polymeric thin films, the periodic conductive network is expected to enable the reuse of reverse osmosis membranes through the application of an electric field, thereby increasing their service life. We present herein a new approach for the design and screening of next-generation desalination membranes, which is only one step away from solving the problem of introducing periodic nanopores in two-dimensional carbon materials.

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Engineering electronic structures of Janus monolayer group-III monochalcogenides via biaxial strain

We investigate the effects of biaxial strain and spin-orbit coupling on the electronic structures of Janus monolayer group-III monochalcogenides based on first-principle calculations. Due to the spin-orbit coupling and mirror symmetry breaking, Rashba band splitting appears at Γ point of the bottom conduction band and the band gap decreases significantly. The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain, which originate from the change of interaction between pz orbitals. As the lattice constant decreases, the interaction between pz orbitals is enhanced, leading to the decrease (increase) of the energy of corresponding bonding (anti-bonding) state, which is the origin of the indirect-direct-indirect transition of the band gap. In addition, when tensile strain is applied, the pz orbital component of Te atoms increases, leading to the enhancement of the Rashba effect. These results are of great significance for the band engineering and spin-orbitronics of two-dimensional materials.

Carrier Doping Modulates 2D Intrinsic Ferromagnetic Mn2Ge2Te6 Monolayer, High Curie Temperature, Large Magnetic Crystal Anisotropy

The Mn2Ge2Te6 shows intrinsic ferromagnetic (FM) order, with Curie temperature (Tc) of 316 K. The FM order origins from superexchange interaction between Mn and Te atoms. Mn2Ge2Te6 is half-metal (HM), and spin-\b{eta} electron is a semiconductor with gap of 1.462 eV. Mn2Ge2Te6 tends in-plane anisotropy (IPA), with magnetic anisotropy energy (MAE) of -13.2 meV/f.u.. The Mn2Ge2Te6 shows good dynamical and thermal stability. Moreover, Mn2Ge2Te6 presents good ferromagnetic and half-metallic stability under charge doping. The carriers doping could effectively tune magnetic and electronic properties. Specifically, the magnetic moment, exchange parameter, and MAE could be efficiently tuned. The total magnetic moment changes linearly with charges doping. The exchange parameters could be controlled by the doping carriers. The carriers doping could modulate MAE to -18.4 (+0.4 e), -0.85 (-1.6 e), 1.31 (-2.4 e) meV/f.u., by changing hybridization between Te atoms' py and pz orbitals. Mn2Ge2Te6 with intrinsic ferromagnetism, high tunable MAE, good stability of ferromagnetism and half-metallicity could help researchers to investigate its wide application in the electronics and spintronics.

Regulation of Electronic Structures to Boost Efficient Nitrogen Fixation: Synergistic Effects between Transition Metals and Boron Nanotubes.

Borophene possesses outstanding physical and chemical properties and thus demonstrates great application potential in catalysis. However, the lack of a controllable strategy for regulating the electronic structures of borophene for efficient catalysis limits the exploration of this material for a "black-box" model. Herein, taking advantage of the synergistic effects between metals and boron nanotubes (BNT), we report a core-shell structure that encapsulates early transition-metal nanowires into BNT (TMs@BNT) to improve the inherent electronic structures of primitive borophene for an efficient electrochemical nitrogen reduction reaction (eNRR). These filled BNT with disconnected π conjugation and vacant boron (B) pz orbitals enable the regulation of electronic states of B atoms in spatial extent and occupancy that has a great effect on the adsorption strength of intermediates. Using first-principles calculations, we demonstrate that the *N2H adsorption energy (ΔE*N2H) is strongly correlated with the intrinsic activity trends and that the variation of ΔE*N2H is attributed to the distribution of 2p states and charge of B atoms. Finally, we utilize the coupling of the d 2p states between B atoms and metals to obtain a quantitative explanation for synergistic effects and conclude that metals with a lower d-band center (εTMd) raise the average 2p state energy (ε̅2p) of B through two-level quantum coupling, which is the physical origin of this interaction. Therefore, two candidates (Mo@BNT and W@BNT) with lower εTMd are screened, benefiting from their high eNRR activity (limiting potentials of -0.75 and -0.77 V, respectively) and high selectivity. This work explores the activity origin, constructs a bridge between electronic structures and activity trends, and paves the way for future eNRR studies.

Quantum anomalous Hall and valley quantum anomalous Hall effects in two-dimensional d0 orbital XY monolayers

We propose a new family of the d0 orbital XY (X = K, Rb, Cs; Y = N, P, As, Sb, Bi) monolayers with abundant and novel topology and valley properties. The KN, RbN, RbP, RbAs, CsP, CsAs, and CsSb monolayers possess remarkable quantum anomalous Hall effect (QAHE). CsSb monolayer also exhibits extraordinary valley QAHE with giant splitting. Moreover, the topological properties of XY monolayers can be effciently tuned by the in-plane strain, owing to the strain-induced band inversion between the px;y and pz orbitals. Our findings suggest that the d0 orbital XY monolayers can be good candidates for promising applications in the spintronics and multifunctional topological-based devices.