Density Functional Theoretical Analysis Research Articles

Microwave Quasi‐Solid State to Construct Strong Metal‐Support Interactions with Interfacial Electron‐Enriched Ru for Anion Exchange Membrane Electrolysis

AbstractRegulating the metal‐support interaction of the anchored metal nanoclusters is recognized as valid approach to optimize the electrocatalytic performance through tuning the interfacial electronic structure. However, developing novel support and understanding the interfacial electron accumulation on modulating the reaction kinetics are still elusive. Herein, highly‐dispersed Ruthenium (Ru) nanoclusters anchored onto phosphorous doped molybdenum boride (Ru/P‐MoB) is developed through ultrafast microwave‐plasma (60 s) approach. The synthesized Ru/P‐MoB impressively promote the hydrogen evolution with low overpotentials of 34, 45, and 40 mV to drive 10 mA cm−2 in alkaline freshwater, alkaline seawater and acid media. Specially, it presents superior turnover frequency and mass/specific activity relative to Pt/C, Ru/C, and Ru/MoB. Moreover, the anion exchange membrane (AEM) electrolyzer cell based on Ru/P‐MoB can achieve 500 and 1000 mA cm−2 with small voltages of 1.71 and 1.78 V with good durability. Experimental and density functional theoretical (DFT) analysis reveal that the strong metal‐support interactions (Ru─Mo and Ru─P bonds) with generated interfacial electron‐enriched Ru, and then favoring the water‐molecule adsorption/dissociation and optimal H intermediate adsorption free energy. This work provides novel designing avenue to exploit electrocatalysts with outstanding catalytic performance under high current density at practical high‐temperature.

Electrochemical detection of fluoride ions using 4-aminophenyl boronic acid dimer modified electrode

Fluoride is a widely used component in dental products, offering benefits such as cavity prevention through bacterial growth inhibition. However, excessive fluoride consumption can lead to fluorosis, which causes tooth discoloration and pitting, and in severe cases, skeletal fluorosis with joint pain and stiffness. Therefore, monitoring fluoride levels in dental products and drinking water is essential. In this study, we developed a rapid electrochemical detection method for fluoride ions using a preanodized screen-printed carbon electrode modified with 4-aminophenylboronic acid dimer (SPCE*/APBAD). The preanodization process involved applying a constant potential of +2 V for 5 min to capture APBAD. The resulting APBAD facilitated a chemical reaction between fluoride ions and boronic acid, forming a fluorophenylboronate complex. This complexation reaction enabled the detection of fluoride ions in a wide linear range (5 µM to 30 mM) with a detection limit of 0.3 µM. The detection behaviour of fluoride ion using SPCE*/APBAD has been studied using cyclic voltammetry, potentiometry, Density Functional Theoretical calculations and flow injection analysis. Furthermore, the practical utility of the sensor was validated by successfully detecting fluoride ions in toothpaste and water samples. The developed method offers a rapid and reliable means of monitoring fluoride ion levels for ensuring safe and beneficial use in dental products and drinking water.

Deciphering the sensing mechanism in a sarin gas surrogate, diethylchlorophosphate specific probe: Colorimetric observation and theoretical analysis

The extreme toxicity of organophosphates (OPs) nerve agents poses a significant public health risk and national safety. Increasing interest in developing reliable and accurate methods for simple and fast detection of these toxic, i.e., poisonous chemicals, is necessary and urgent. In this contribution, [(E)-3-(4-(dimethylamino)phenyl)allylidene)amino)phenol] (DPAP), a cinnamalimine-based chromogenic probe with high selectivity and sensitivity towards the detection of sarin mimic diethylchlorophosphate (DCP), has been introduced. The detection limit of DPAP towards the detection of DCP is in the nM region. The developed probe DPAP shows reddish pink color upon treatment with DCP and reverses to its original one by adding triethylamine (TEA); it is observed eight times reversibility by alternative addition of DCP and TEA. The detailed mechanism has been elucidated through the density functional theoretical analysis depicting the energy profile diagram and Frontier molecular orbital analysis. To establish the practical application, we have demonstrated a paper strips-based test kit to establish the capacity of DPAP for on-spot detection and quantification of DCP. A dip-stick experiment has also been performed to identify DCP in the gaseous phase within the various OPs, inorganic phosphates, and other toxic analytes to establish the probe, DPAP, as a proficient chemosensor. To detect the response capacity of probe DPAP towards the identification and quantification of DCP, a time-dependent UV–visible spectrophotometric experiment, and dip-stick method have been performed for 0–6 min. A smartphone-based readout technique has been executed for on-spot detection and quantification of DCP in remote areas. The present contribution introduced an excellent probe and technique for identifying and quantifying sarin gas in actual threatening situations. It would be helpful to monitor the environmental toxicology from exposure to these toxic nerve agents.

Anisotropic mechanical response of a 2D covalently bound fullerene lattice

Contrary to the monotype intercell connection in a hexagonal structure material, newly synthesized stable monolayer C60 fullerene (mC60) lattice possesses two kinds of intercell connection, and anisotropic properties are thus expected. Herein, we have investigated the anisotropy mechanical properties of the mC60 using first-principles calculations within the framework of density functional theory (DFT) and theoretical analysis. Uniaxial tensile properties of mC60 are orientation-dependent, and the ultimate tensile strength and the work-to-fracture of mC60 reach their maxima at 15° and minima at 75°, respectively. A theoretical expression, based on strain-governed bond failure criterion, has been developed for the strength-loading direction relationship of mC60. The theory captures well with the orientation-dependent strength from DFT calculations. We further illustrate that mC60's band gap may be largely tuned through strain engineering. Our atomistic insights and the theoretical on the structure – mechanical property relationship might be helpful in the exploring of the functioning and application of mC60, which may be further generalized to the mechanical analysis of other monolayer lattices.

DFT analysis on the adsorption of melamine in Ga12-N12/P12 nanocages: solvent effects, SERS analysis, reactivity properties

Due to its negative effects on people, melamine contamination in food products are detected and filtered. Amongst several sensory schemes for the screening of melamine poisoning, one of the most promising techniques is the use of nanomaterial based sensing for real time applicability in industries. In the current work, we have looked into the way melamine binds to Ga12-N12/P12 nanocages. Surface-enhanced Raman scattering (SERS), a successful spectroscopic technique is used to monitor melamine. Density functional theoretical (DFT) computations were used to study the sensing properties of melamine (Me) with Ga12-N12/P12 nanocages. Reactivity and Mulliken charge analyses show charge transfer from melamine to nanocage. Me-Ga12-N12 and Me-Ga12P12 clusters have adsorption energies of −47.54 and −33.12 kcal/mol, respectively. All nanocage-Me systems have a significant increase in polarizability. The electron densities revealed non-covalent interactions in the adsorbed systems. All adsorption energies in aqueous media are negative, indicating an attractive and exothermic reaction, with maximum value in water for Me-Ga12N12 and in acetone for Me-Ga12P12. Evidence of SERS is observed due to the enhancement of different vibrational modes. Communicated by Ramaswamy H. Sarma

Gap switching in metal-organic coordination chains

We predict two switchable one-dimensional (1D) spin-polarized semiconductors based on metal-organic coordination chains constructed out of Fe, V, and zwitterionic quinone (ZQ) molecules using first-principle density functional theoretical analysis. The Fe-ZQ coordination chain can be converted from a semiconductor to a half-metal when oxidized by chlorine (Cl). Upon chlorination, the magnetic moment of the Fe-ZQ is increased from 4 μB to 5 μB, per iron atom. In addition, the bimetallic (Fe-ZQ-V-ZQ) ferromagnetic semiconducting coordination chain with a very small energy gap of only 90 meV can be converted to an antiferromagnetic semiconductor with a large gap of more than 1 eV when oxidized by chlorine. Its magnetic moment is found to be 8 μB per heterobimetallic unit (Fe and V) after chlorination, and 7 μB without chlorine. These unique properties, namely a switchable or reversible electronic and magnetic characteristics with a transition between different semiconducting states, make these coordination chains to be highly promising candidates for specific applications as multi-functional switch in nanoelectronics and spintronics.

Density functional theoretical analysis of micro-adsorption of isotopes of hydrogen molecule and atom by uranium

Tritium, the heaviest among hydrogen isotopes can be safely stored as metal hydride (tritide). The (depleted) uranium metal is being considered in international thermonuclear experimental reactor (ITER) because of its compatible physicochemical properties. The fundamental understanding of U–H isotopes interaction will help in the efficient storage of H isotopes in depleted uranium metal. Hence, density functional theoretical (DFT) analysis has been performed to investigate the micro-adsorption of hydrogen and its isotopic molecules on uranium atom. The geometrical configurations of UXn, U (X2)n and UX4-(X2)n (X = H, D, T; n = 1–9) cluster were analyzed using different DFT functional (PBE, PBEO, M06 and M06-2X). In the case of U-(H2)4 and U-(H2)5, one H2 molecule was seen to be dissociated to give UH2(H2)3 and UH2(H2)4 cluster as observed earlier in the experiment. The formation of U (X2)n and UX4-(X2)n microclusters was explained by calculating binding enthalpy, natural population analysis and electron density at the bond critical points (BCP). Further, more insights were derived by computing the type of interactions between U and H2 isotopic molecules. The Kubas interaction of a U atom with a H2 molecule was identified by an elongation of the H–H bond without breakage and a reduction in its stretching frequency due to binding. The interaction of U and H2 isotopic molecules was confirmed to be Kubas type of interaction whose strength lies in between the covalent bond of metal hydrides and the van der Waals bond of materials. Further, σ-donation from H2 to d and f orbitals of U atom and π-back-donation from U to the anti-bonding orbital of H2, and atoms-in-molecules analysis indicates that the electron density at the bond critical points of the bound H2 is similar to that of Kubas systems. The Kubas type of interaction suggests that the reversible adsorption of hydrogen molecules might be favored with U metal. From structural and binding enthalpy (BE) analysis, UH4-(H2)8 polyhydride was predicted to be the largest super polyhydride and found to be stable by 8.2 kcal/mol over UH4-(H2)6 polyhydride and thus confirmed its plausibility. To the best of our knowledge UH4-(H2)8 is the largest metal polyhydride ever been reported with twenty hydrogen atoms displaying high gravimetric density of 7.80, 14.47 and 20.21 wt% for H2, D2 and T2 respectively. The present DFT results thus draw further attention for more computational and experimental studies in this important uranium-hydrogen system for efficient reuse of depleted uranium metal as tritium storage material.

Raman and first-principles study of the pressure-induced Mott-insulator to metal transition in bulk FePS3

Members of a recently discovered class of two-dimensional materials based on transition metal phosphorous trichalcogenides exhibit an antiferromagnetic ground state and they have potential applications in spintronics. In particular, FePS3 is a Mott insulator with a band gap of ∼ 1.5 eV. In this study, we used Raman spectroscopy and first-principles density functional theoretical analysis to examine the stability of the structure and electronic properties of FePS3 under pressure. Raman spectroscopy detected two phase transitions at 4.6 GPa and 12 GPa, which were characterized by changes in the pressure coefficients of the mode frequencies and the number of symmetry allowed modes. FePS3 transformed from the ambient monoclinic C2/m phase with a band gap of 1.54 eV to another monoclinic C2/m (band gap of 0.1 eV) phase at 4.6 GPa, which was followed by another transition to the metallic trigonal P-31m phase at 12 GPa. Our findings complement those obtained recently in high pressure X-ray diffraction studies. The calculated elastic properties indicated increases in the bulk, shear, and Young's moduli, as well as a significant reduction in the universal elastic anisotropy index as the crystal changed from the ambient monoclinic C2/m phase to the high pressure trigonal P-31m phase.

Redox Activity of Noninnocent 2,2'-Bipyridine in Zinc Complexes: An Experimental and Theoretical Study.

We report on a systematical reactivity study of β-diketiminate zinc complexes with redox-active 2,2′-bipyridine (bpy). The reaction of LZnI (L = HC[C(Me)N(2,6-iPr2C6H3)]2) with NaB(C6F5)4 in the presence of bpy yielded [LZn(bpy)][B(C6F5)4] (1), with bpy serving as a neutral ligand, whereas reduction reactions of LZnI with 1 or 2 equiv of KC8 in the presence of bpy gave the radical complex LZn(bpy) (2) and [2.2.2-Cryptand-K][LZn(bpy)] (3), in which bpy either acts as a π-radical anion or a diamagnetic dianion, respectively. The paramagnetic nature of 2 was confirmed via solution magnetic susceptibility measurements, and UV–vis spectroscopy shows that 2 exhibits absorption bands typical for bpy radical species. The EPR spectra of 2 and its deuterated analog 2-d8 demonstrate that the spin density is localized to the bpy ligand. Density functional theoretical calculations and natural bond orbital analysis were employed to elucidate the electronic structure of complexes 1–3 and accurately reproduced the structural experimental data. It is shown that reduction of the bpy moiety results in a decrease in the β-diketiminate co-ligand bite angle and elongation of the Zn–N(β-diketiminate) bonds, which act cooperatively and in synergy with the bpy ligand by decreasing Zn–N(bpy) bond lengths to stabilize the energy of the LUMO.

Magneto-Optical Stark Effect in Fe-Doped CdS Nanocrystals

Fe2+ doping in II-VI semiconductors, due to the absence of energetically accessible multiple spin state configurations, has not given rise to interesting spintronic applications. In this work, we demonstrate for the first time that the interaction of homogeneously doped Fe2+ ions with the host CdS nanocrystal with no clustering is different for the two spin states and produces two magnetically inequivalent excitonic states upon optical perturbation. We combine ultrafast transient absorption spectroscopy and density functional theoretical analysis within the ground and excited states to demonstrate the presence of the magneto-optical Stark effect (MOSE). The energy gap between the spin states arising due to MOSE does not decay within the time frame of observation, unlike optical and electrical Stark shifts. This demonstration provides a stepping-stone for spin-dependent applications.

Mesomorphism in H-bonded binary mixtures of guanazole with nonyloxy benzoic acid

The ability of liquid crystalline materials to feature self-assembly behavior has garnered attention in designing new aggregates for various functional applications. The structural influence of hydrogen-donating and accepting moieties on the liquid crystalline behavior of hydrogen-bonded assemblies of guanazole (G) with liquid crystalline nonyloxy benzoic acid (9OBA) is systematically investigated. Significant variations in the mesogenic behavior of the structurally diverse assemblies, ranging from smectic C to nematic phases were observed and evidenced by the X-ray diffraction as well as thermal studies. These binary mixtures exhibited low working temperatures as well as low crystallization temperatures. A comprehensive density functional theoretical analysis provided insight into the structure-property relationships of the various guanazole:9OBA supramolecular architecture.

New route to estimate the Mott-Hubbard and charge transfer parameters: An optical and x-ray absorption studies

X-ray absorption spectroscopy (XAS) and optical absorption spectroscopy (OAS) measurements have been performed on polycrystalline samples of PrFeO3+δ sintered at different temperatures. XAS measurements carried out at Fe K-edge confirms the increase in the concentration of Fe4+ ions in PrFeO3+δ with sintering temperature (TS). The optical band gap (Eg) of these samples appears to be correlated with that of the bandwidth (dBW-Fe-3d) of the “Fe-3d” orbital. The values of dBW-Fe-3d have been estimated from XAS data collected at Fe L3-edge. The increase in the dBW-Fe-3d is attributed to be due to the splitting of Fe-3d (t2g)III and Fe-3d (eg)II states due to the presence of Fe4+ Jahn-Teller ions. The OAS spectral analysis suggests that the band gap decreases systematically with TS. The relative intensity of d-d transition has been found to scale with the concentration of Fe4+ ions and the width of the said d-d transition seems to be correlated with dBW-Fe-3d. Using the estimated values of ‘Eg’ and d-bandwidth, the value of charge transfer parameter ‘Δ’ has been estimated. The obtained value of Δ is used to simulate the band gap of PrFeO3 using density functional theoretical analysis. Thus, using the combination of x-ray absorption and optical spectroscopies we have demonstrated the new way to investigate and analyse the electronic structure of strongly correlated electron systems.

Molecular structure, Hirshfeld surface and density functional theoretical analysis of a NLO active chalcone derivative single crystal—A quantum chemical approach

Nonlinear optically active thienyl chalcone, with molecular formula C16H15ClO4S has been synthesized by adopting claisen-schimdt condensation reaction method. FT-IR and NMR spectral study was used to identify the functional groups present in the newly synthesized compound and confirm the molecular structure. Based on the solubility study DMF is chosen as solvent to grow single crystals of the compound using slow evaporation (solution growth) technique. The single crystal XRD study reveals that the grown crystal belongs to the orthorhombic crystal system with Pbca space group and the associated cell parameters are, a = 16.6575 (6) Å, b = 8.1147 (3) Å, c = 23.6618 (9) Å, α = β = γ = 90°. The linear optical properties were studied using UV-–visible, Photoluminescence method. The energy band gap of the material was calculated using tauc's plots. The refractive index (1.47) of the crystal was determined using Brewster's angle method. Second harmonic generation (SHG) test was carried out using Kurtz and Perry powder technique and SHG efficiency of 1.82 times that of urea was estimated. The thermal stability of the compound was tested by thermo gravimetric and differential thermal analysis. Various inter contacts present within the crystal were visualized through 3-D molecular hirshfeld surface analysis and their percentage contribution towards the total hirshfeld surface area has been quantized using 2D finger print plot method. The theoretical investigation was carried out using DFT method and HOMO-LUMO energy band gap was determined. The different global chemical reactivity descriptors (GCRDs) parameters were calculated. The molecular electrostatic potential map of the molecule was analyzed and the sites of electrophilic and nucleophilic attacks were predicted. The molecular NLO properties such as static dipole moment, mean polarizability, anisotropy of polarizability and first order hyper polarizability values were theoretically determined.

Pressure-induced isostructural electronic topological transitions in 2H-MoTe2: x-ray diffraction and first-principles study

Synchrotron x-ray diffraction measurements on powder 2H-MoTe2 (P63/mmc) up to ∼46 GPa have been performed along with first-principles based density functional theoretical analysis to probe the isostructural transition in low pressure regime and two electronic topological transitions (ETT) of Lifshitz-type in high pressure regime. The low pressure isostructural transition at ∼7 GPa is associated with the lattice parameter ratio c/a anomaly and the change in the compressibility of individual layers. The pressure dependence of the volume by linearizing the Birch–Murnaghan equation of state as a function of Eulerian strain shows a clear change of the bulk modulus at the ETT pressure of ∼20 GPa. The minimum of c/a ratio around 32 GPa is associated with the change in topology of electron pockets marked as second ETT of Lifshitz-type. We do not observe any structural transition up to the maximum applied pressure of ∼46 GPa under quasi-hydrostatic condition.

Development of a Schottky barrier diode using molecular network of 1-alkyl-2-(arylazo) imidazole prepared by Langmuir–Blodgett technique

In this paper we report the photophysical and electrical characteristics of our synthesized imidazole derivative namely l-alkyl-2-(arylazo) imidazole (AAI) organized in Langmuir-Blodgett (LB) films towards the development of organic Schottky barrier diode (SBD). Presence of long alkyl chain in the molecule make it suitable for LB technology and may favour aggregations of the molecules in LB films under barrier compression. The interfacial properties of the formation of Langmuir monolayer of AAI at air-water interface have been studied by surface pressure versus area per molecule (π-A) isotherm and compressibility modulus. Hysteresis study of the isotherm confirmed the formation of stable monolayer of AAI molecular assemblies at air-water interface. The aggregation of AAI molecules in LB films is confirmed by Ultraviolet–visible (UV–vis) absorption, fluorescence emission spectroscopy and Atomic force microscopic (AFM) methods. AAI molecules also formed network type morphology in LB films due to their closure association and aggregations or cluster of molecules which were evidenced by AFM technique. The compression induced aggregations of AAI is reflected as broadened nature of monomeric absorption band along with the emergence of a new absorption band at longer wavelength side for LB film deposited at surface pressure of 25 mN/m. The optical absorption bands of AAI and the corresponding transitions between different electronic states have been simulated by density functional theoretical (DFT) analysis and is compared with the experimental results. Nonlinear behaviour of the current-voltage characteristics and various relevant electrical parameters of the LB film device at the interface of metal electrode (gold) satisfy the necessary conditions showing the properties of a Schottky barrier diode.

Inhomogeneous superconductivity in high-density nonmagnetic cobalt in a polycrystalline Co film

We report the observation of inhomogeneous superconductivity (ISC) in the recently discovered high-density nonmagnetic (NM) phase of Co in thin films below an onset temperature () of 5.4 K in the absence of external magnetic field, via four-probe measurements of resistivity. Further, the point-contact spectroscopy studies also confirm superconductivity in this system. We attribute the observed ISC to the presence of nanoscale grains of high-density non-magnetic Co (FCC structure) in a thin film of conductive, normal Co (HCP structure) which is magnetic. Incomplete superconducting transition found in the bulk measurements suggests that the observed phenomenon is due to ISC of nanoscale grains of NM phase of Co. In addition, using first-principles density functional and BCS theoretical analysis of Co under hydrostatic and volume-preserving-biaxial strains, we demonstrate that superconducting of its NM phase increases anomalously with strain near its transformation to ferromagnetic phase, as a result of softening of N phonon due to strong electron-phonon coupling that is further enhanced with the biaxial strain.

Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral p-Type GeSe Crystal.

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: Pnma), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: R3m), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe2. We show ultralow lattice thermal conductivity (κL) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the p-type rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)0.9(AgBiSe2)0.1 are directly evident in the low temperature heat capacity (Cp) and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κL and enhances the thermoelectric performance in rhombohedral (GeSe)0.9(AgBiSe2)0.1 crystals.

Ultralow Thermal Conductivity in Chain-like TlSe Due to Inherent Tl+ Rattling.

Understanding the mechanism that correlates phonon transport with chemical bonding and solid-state structure is the key to envisage and develop materials with ultralow thermal conductivity, which are essential for efficient thermoelectrics and thermal barrier coatings. We synthesized thallium selenide (TlSe), which is comprised of intertwined stiff and weakly bonded substructures and exhibits intrinsically ultralow lattice thermal conductivity (κL) of 0.62-0.4 W/mK in the range 295-525 K. Ultralow κL of TlSe is a result of its low energy optical phonon modes which strongly interact with the heat carrying acoustic phonons. Low energy optical phonons of TlSe are associated with the intrinsic rattler-like vibration of Tl+ cations in the cage constructed by the chains of (TlSe2)nn-, as evident in low temperature heat capacity, terahertz time-domain spectroscopy, and temperature dependent Raman spectroscopy. Density functional theoretical analysis reveals the bonding hierarchy in TlSe which involves ionic interaction in Tl+-Se while Tl3+-Se bonds are covalent, which causes significant lattice anharmonicity and intrinsic rattler-like low energy vibrations of Tl+, resulting in ultralow κL.

Lattice deformation on flat-band modulation in 3D Hopf-linked carbon allotrope: Hopfene

Flat bands form in a 3D Hopf-linked graphene crystal or a 3D carbon allotrope named Hopfene, which qualitatively differ from bands of only graphenes. This paper discusses carbon-hexagon deformation on the level shift of a flat band via density-functional-theoretical (DFT) analysis to set the flatband level to the Fermi level, viz., to utilize its large density of states for magnetic- and electronic-property research. Tight-binding (TB) analysis is also performed for a comparison with the DFT analysis; here, a qualitative agreement between TB and DFT bands is obtained. The DFT analysis shows an almost linear flatband level shift to the lattice-deformation rate, where electron-interaction effects are included within the Kohn-Sham method. To tune the flatband level so that it fits the Fermi level, a double-heterolike structure is also proposed as a way of hexagon-deformation control.

Prediction of Coupled Electronic and Phononic Ferroelectricity in Strained 2D h-NbN: First-Principles Theoretical Analysis

Using first-principles density functional theoretical analysis, we predict coexisting ferroelectric and semimetallic states in a two-dimensional monolayer of h-NbN subjected to an electric field and in-plane strain (ε). At strains close to ε=4.85%, where its out-of-plane spontaneous polarization changes sign without inverting the structure, we demonstrate a hysteretic response of its structure and polarization to an electric field, and uncover a three-state (P=±P_{o}, 0) switching during which h-NbN passes through Dirac semimetallic states. With first-principles evidence for a combination of electronic and phononic ferroelectricity, we present a simple model that captures the energetics of coupled electronic and structural polarization, and show that electronic ferroelectricity arises in a material which is highly polarizable (small band gap) and exhibits a large electron-phonon coupling leading to anomalous dynamical charges. These insights will guide the search for electronic ferroelectrics, and our results on 2D h-NbN will stimulate development of piezofield effect transistors and devices based on the multilevel logic.