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Ab-Initio Prediction of Intrinsic Half-Metallicity in Binary Alkali–Metal Chalcogenides: KX (X=S, Se and Te)
摘要： First-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate the structural, electronic and magnetic properties of KX (X ? S, Se and Te) binary alkali–metal chalcogenides compounds. These compounds in different crystalline phases, NaCl (B1), CsCl (B2), ZB (B3), NiAs (B81T, WZ (B4) and Pnma, were calculated within the generalized gradient approximation (GGA-PBE) and the modified Becke–Johnson approach (mBJ-GGA-PBE) for the exchange–correlation energy and potential. We found that the most stable phase for the KX binary compounds is the nonmagnetic Pnma phase. The calculated lattice parameters, bulk moduli, their first-pressure derivatives and internal parameters are in good agreement with the other theoretical data. The electronic band structure and density of states show that half-metallic and magnetic character arises, which can be attributed to the presence of spin-polarized p orbitals in the group VI elements. KX (X ? S, Se and Te) compounds, except for KSe and KTe in the CsCl and NiAs phases, show HM character in all phases, with an integer magnetic moment of 1 (cid:1)B per formula unit and HM gaps.
关键词： alkali metals,Density functional theory,half-metals,chalcogenides,ferromagnetism
更新于2025-09-23 15:22:29
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Phase Transition in Two-dimensional Tellurene under Mechanical Strain Modulation
摘要： We carry out computational simulations based on density functional theory (DFT) to investigate different phases of two-dimensional (2-D) tellurene. These phases are classified by their characteristic space groups and symmetry elements. Correlations of these phases to the bulk crystalline tellurium structure are also illustrated. Our specific interests include mechanical property calculations for different phases and the possible phase transitions between them. Simulation results show that these 2-D Te phases have very different elastic moduli due to their different atomic bonding and relaxed structures. Moreover, compression along the in-plane directions facilitates the α → β phase transition, while in-plane tensile strains always make the α-phase more stable than the β-phase. However, the energy difference between the two phases is comparable to or even much smaller than the thermal energy kT, depending on the in-plane strain direction. We find that further increase of the tensile strain along the chain direction beyond a critical value, ca. 12%, may lead to a possible α → γ phase transition. As the tensile strain is above 15%, the γ-phase will be more stable than the α-phase, accompanied by a further reduced transition energy barrier.
关键词： phase transition,elastic modulus,Tellurene,strain engineering,density functional theory (DFT),2-D materials
更新于2025-09-23 15:22:29
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Mixing Thermodynamics and Photocatalytic Properties of GaP-ZnS solid solutions
摘要： Preparation of solid solutions represents an effective means to improve the photocatalytic properties of semiconductor-based materials. Nevertheless, the effects of site-occupancy disorder on the functional properties of materials are difficult to predict and consequently many experimental trials may be required before achieving enhanced photocatalytic activity. Here, first-principles methods are employed to estimate the mixing free energy and the structural and electronic properties of (GaP)x(ZnS)1?x solid solutions. The method relies on a multi-configurational supercell approach that takes into account the configurational and vibrational contributions to the free energy. Phase competition among the zinc-blende and wurtzite polymorphs is also considered. Overall excellent agreement with the available experimental data is demonstrated, namely: 1) zinc-blende is energetically most favorable, 2) the solid solution energy band gap lies within the 2–3 eV range, and 3) the energy band gap of the solid solution is direct for compositions x ≤ 75%. It is found that at ambient conditions, (GaP)x(ZnS)1?x solid solutions with x ≈ 25%, 50% and 75% render promising hydrogen evolution photocatalysts for water splitting under visible light, owing to their favorable energy band gaps and band levels relative to vacuum.
关键词： multiconfigurational supercell approach,semiconductor solid solutions,density functional theory calculations,photocatalytic materials
更新于2025-09-23 15:22:29
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The Niecke Biradicals and Their Congeners - The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties
摘要： Biradicals within organic chemistry are known to be of fleeting existence. They can be traced only by femtosecond spectroscopy and/or corresponding trapping experiments. The conceptual understanding of a biradical originates mainly from gas phase experiments. These shaped the biradicals as highly unstable species, their lifetimes determined by reactions, which are merely entropy controlled. On the other hand, biradicals in π-systems of non-Kekule type, lead to species, which either have distinct singlet or triplet ground states. Yet they are still species confined to spectroscopic investigations. The matter changed by the introduction of sterically protecting groups into chemistry, in order to stabilize hitherto highly reactive compounds. With the new growing field of phosphorus organic chemistry, both fields merged and the concept of a stable biradical came to light. The concept matured in subsequent years and it is now a flowering field with promising aspects in materials science. The Niecke-type biradicaloid, which is a basic unit in this area, owes its stability by protection of the phosphorus position with sterically demanding substituents, in particular with the “super-mesityl” (Mes*) group. The electron withdrawing TMS groups at the carbon positions enhance the singlet stability. On this basis it was possible to isolate and structurally characterize these non-Kekule type structures. The description of these systems as biradicals or biradicaloids is determined by their high reactivity and they offer versatility for a variety of chemical reactions. From the point of theory this refers to small energy gaps between adiabatic singlet and triplet states. However, the understanding as biradicals is different to the Doering concept of a biradical vs. a biradicaloid, as anticipated in the transition state of the Cope automerization. In the Niecke-type biradicaloid a corresponding biradical structure is promoted with enhanced pyramidalization at the phosphorus atoms or in more general by a decrease of the Lewis basicity of the lone pairs. The Niecke-type biradical can be converted into a Bertrand-type biradical with trans-annular bonding. This requires changing the donor-ability at the phosphorus centers. This prediction could be proven by either addition with Lewis acids or complexation with transition metal fragments. In the last part of this article, it is briefly shown how biradicals can be utilized as units for building extended π-systems. They feature an interesting target for the design of interesting materials for nonlinear optics.
关键词： Phosphorus,Rings,Computational chemistry,Bond theory
更新于2025-09-23 15:22:29
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Prediction of Second-Order Nonlinear Optical Properties of D–π–A Compounds Containing Novel Fluorene Derivatives: A Promising Route to Giant Hyperpolarizabilities
摘要： Herein, ?rst attempt has been made to utilize ?uorene-based dye-sensitized solar cell (DSSCs) dye JK-201 as potential nonlinear optical (NLO) material and for the theoretical designing of novel NLO chromophores JK-D1–JK-D12. DFT/TDDFT calculations were performed to compute the effect of p-linkers and acceptors-steered modulation on electronic, photophysical and NLO properties of JK-201 and JK-D1–JK-D12. Results illustrate that computed kmax (484.74 nm) and experimentally calculated kmax (481 nm) of JK-201 was found in good agreement. Maximum red shifted absorption spectrum was observed in JK-D12 with 599.38 nm. JK-D1–JK-D12 showed narrow energy gap and broader absorption spectrum as compared to JK-201. NBO analysis con?rmed the formation of charge separation state due to robust range of electrons/charge transfer from donor to acceptor via p-bridge. Giant NLO response was observed in all compounds. Particularly, JK-D12 displayed surprisingly large hai and btot computed 1376.74 (a.u.) and 405,731.84 (a.u.) respectively. Although literature is ?ooded with D–p–A compounds investigated for their DSSCs properties, but research reports on their NLO properties and utilization as NLO materials are completely deserted. Our research will open new horizons to explore DSSCs materials for NLO applications. This theoretical framework also exposed that ?uorene-substituted chromophores are excellent NLO candidates for modern hi-tech applications.
关键词： D–p–A organic chromophores,Acceptor units,Molecular modeling,Density functional theory,NLO response properties,p-Conjugated linkers
更新于2025-09-23 15:22:29
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Decoherence of the Radiation from an Accelerated Quantum Source
摘要： Decoherence is the process via which quantum superposition states are reduced to classical mixtures. Decoherence has been predicted for relativistically accelerated quantum systems; however, examples to date have involved restricting the detected field modes to particular regions of space-time. If the global state over all space-time is measured, then unitarity returns and the decoherence is removed. Here, we study a decoherence effect associated with accelerated systems that cannot be explained in this way. In particular, we study a uniformly accelerated source of a quantum field state—a single-mode squeezer. Even though the initial state of the field is vacuum (a pure state) and the interaction with the quantum source in the accelerated frame is unitary, we find that the final state detected by inertial observers appears to be decohered, i.e., in a mixed state. This unexpected result may indicate new directions in resolving inconsistencies between relativity and quantum theory. We extend this result to a two-mode state and find that entanglement is also decohered.
关键词： Unruh Effect,Quantum Field Theory,Accelerated Systems,Decoherence,Quantum Source
更新于2025-09-23 15:22:29
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Low-cost, Inkjet-printed UHF RFID Tag based System for Internet of Things Applications using Characteristic Modes
摘要： The radio frequency identification (RFID) has emerged Internet of things (IoT) into the identification of things. This paper presents, a low-cost smart refrigerator system for future internet of things applications. The proposed smart refrigerator is used for automatic billing and restoring of beverage metallic cans. The metallic cans can be restored by generating a product shortage alert message to a nearby retailer. To design a low-cost and low-profile tag antenna for metallic items is very challenging, especially when mass production is required for item-level tagging. Therefore, a novel ultra-high frequency (UHF) radio frequency identification (RFID) tag antenna is designed for metallic cans by exploiting the metallic structure as the main radiator. Applying Characteristics mode analysis (CMA), we observed that some characteristic modes associated with the metallic structure could be exploited to radiate more effectively by placing a suitable inductive load. Moreover, a low cost, printed (using conductive ink) small loop integrated with meandered dipole used as an inductive load, which was also connected with RFID chip. The 3-dB bandwidth of the proposed tag covers the whole UHF band ranging from 860 - 960 MHz when embedded with metal cans. The measured read range of the RFID tag is more than 2.5 meters in all directions to check the robustness of the proposed solution. To prove the concept, a case study was performed by placing the tagged metallic cans inside a refrigerator for automatic billing, 97.5 % tags are read and billed successfully. This study paves the way for tagging metallic bodies for tracking applications in domains ranging from consumer devices to infotainment solutions, which enlightens a vital aspect for the Internet of Things (IoT).
关键词： Characteristic mode theory (CMT),smart refrigerator,radio frequency identification (RFID) tag antenna,Internet of Things (IoT)
更新于2025-09-23 15:22:29
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Bond theory, Terahertz spectra and dielectric studies in donor-acceptor (Nb-Al) substituted ZnTiNb <sub/>2</sub> O <sub/>8</sub> system
摘要： As a donor and acceptor separately, Nb5+ and Al3+ are used to substitute Ti4+ in the ixiolite ZnTiNb2O8 system for the first time. The dielectric responses in the terahertz range of this system are initially studied based on the data from terahertz time-domain transmitted spectroscopy. Combined with ligand field theory, the formation of a secondary phase of ZnAl2O4 is reasonably explicated. Then, the origins of the lower dielectric loss and the terahertz wave absorption coefficient are determined to be the high chemical bond covalency and effective phase control. For the composition of ZnTi0.85(Al0.5Nb0.5)0.15Nb2O8 sintered at 1180 °C, a low dielectric loss (< 5×10-3 @ 0.5 THz) and absorption coefficient (< 10 cm-1 @ 0.5 THz) are obtained, which make this kind of material propitious for developing terahertz dielectric devices.
关键词： complex chemical bond theory,covalency,microwave and terahertz,dielectric properties,donor-acceptor substituted
更新于2025-09-23 15:22:29
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[IEEE 2018 31st International Vacuum Nanoelectronics Conference (IVNC) - Kyoto, Japan (2018.7.9-2018.7.13)] 2018 31st International Vacuum Nanoelectronics Conference (IVNC) - Comparison of the Lepetit Field emission currentdensity calculations with the Modinos-Forbes uncertainty limits
摘要： This poster makes some preliminary comparisons between (a) the results of atomic-level theories of metal field electron emission (FE) recently reported by Lepetit, and (b) the Modinos-Forbes uncertainty limits relating to the accuracy of zero-temperature Murphy-Good theory (which is based, like most existing FE theory, on an unrealistic smooth-surface conceptual model). It is found that there is broad consistency between the new results and the old uncertainty limits. The poster briefly indicates some further steps that would now be useful, on the long and winding path towards developing accurate and reliable field emission theory.
关键词： field electron emission theory,Fowler-Nordheim theory
更新于2025-09-23 15:21:21
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Enhanced Charge Transport and Increased Active Sites on α-Fe <sub/>2</sub> O <sub/>3</sub> (110) Nanorod Surface Containing Oxygen Vacancies for Improved Solar Water Oxidation Performance
摘要： The e?ect of oxygen vacancies (VO) on α-Fe2O3 (110) facet on the performance of photoelectrochemical (PEC) water splitting is researched by both experiments and density functional theory (DFT) calculations. The experimental results manifest that the enhancement in photocurrent density by the presence of VO is related with increased charge separation and charge-transfer e?ciencies. The electrochemical analysis reveals that the sample with VO demonstrates an enhanced carrier density and reduced charge-transfer resistance. The results of DFT calculation indicate that the better charge separation is also contributed by the decrease of potential on the VO surface, which improves the hole transport from the bulk to the surface. The reduced charge-transfer resistance is owing to the greatly increased number of active sites. The current study provides important insight into the roles of VO on α-Fe2O3 photoanode, especially on its surface catalysis. The generated lesson is also helpful for the improvement of other PEC photoanode materials.
关键词： oxygen vacancies,α-Fe2O3,photoelectrochemical water splitting,charge-transfer efficiencies,density functional theory,charge separation
更新于2025-09-23 15:21:21