Search Results (70)

Re₃Zr has recently been proposed in the cubic Pm-3n structure. Its alternative atomic arrangements and thermodynamic position within the Re–Zr system have not been systematically resolved. In this work, CALYPSO particle-swarm structure searches combined with density functional theory identify five Re₃Zr polymorphs with Cmcm, Fmmm, Immm, P4/mmm, and Fm-3m symmetries and compare them consistently with the reported Pm-3n phase. Formation enthalpies, a selected-phase partial convex hull, phonon dispersions, elastic properties, sound velocities, elastic Debye temperatures, electronic structures, and bonding characteristics are evaluated. All five CALYPSO structures lie below Pm-3n in fixed-composition energy, with Cmcm being the lowest. The Cmcm phase has a formation enthalpy of −0.257 eV atom⁻¹ and lies approximately 0.042 eV atom⁻¹ above the selected-phase partial hull, identifying it as a low-lying off-hull metastable candidate at 0 K. AIMD simulations at 300 and 1000 K show no obvious structural reconstruction of the Cmcm framework over the simulated time scales. All six structures exhibit no imaginary phonon frequencies and satisfy the corresponding mechanical-stability criteria. Their elastic responses are strongly structure dependent: P4/mmm has the largest Chen–Tian hardness estimates and elastic Debye temperature, whereas Fm-3m shows the weakest elastic anisotropy. All six polymorphs are metallic, with Fermi-level states dominated by Re-5d orbitals and predominantly delocalized bonding. These results provide a thermodynamically constrained 0 K reference for future synthesis, phase identification, and finite-temperature or kinetic investigations of hypothetical Re₃Zr polymorphs. Full article

In this study, we systematically decoupled reversible charge transitions via recombination and irreversible bulk trapping via ionization in solution-processed indium zinc oxide thin-film transistors (TFTs) under positive- and negative-bias-stress (PBS and NBS) conditions. We defined highly decoupled degradation behavior by completely evaluating time-dependent transfer characteristics and saturation leakage currents across a range of indium molarities (0.0125 M to 0.2 M). Results indicate that PBS-induced instability is likely governed by a reversible electrostatic neutralization process reducing total effective shallow and deep acceptor-like states, which are dynamically counteracted by interfacial recombination at the dielectric/semiconductor boundary. Conversely, severe degradation under NBS originated from irreversible bulk trapping triggered by the ionization of donor-like oxygen vacancies in a ZnO amorphous random network. Total effective trapped charges were calculated from threshold voltage shifts to clarify these defect kinetics quantitatively; these calculations demonstrated direct correlation with the integrated theoretical capacities of the deep and shallow acceptor-like gap-state distributions. Finally, we propose a comprehensive density of state–energy band alignment model incorporating thermal activation energies and flat-band voltages. This analytical framework proves that the composition-dependent Fermi level positioning rigorously limits and dictates complex bias-stress instabilities, offering profound insights for designing highly stable amorphous oxide semiconductor TFTs. Full article

A comprehensive first-principles investigation of bulk and surface Cu defects in Zn-based chalcogenides (ZnO, ZnS, and ZnSe) is presented, aimed at assessing the effect of Cu doping on the optoelectronic properties of these materials and at addressing the photocatalytic activity towards the hydrogen evolution reaction (HER). Defect formation energies, adiabatic and optical charge-transition levels of the bulk materials are determined, and their dependence on growth conditions and Fermi-level position is analysed. The results indicate that, whereas ZnO supports both donor- and acceptor-like Cu defects with pronounced Jahn-Teller distortions, ZnS and ZnSe predominantly stabilise substitutional Cu as a mid-gap acceptor with weaker electron-lattice coupling and similar absolute transition levels. Calculated vertical transition energies rationalise the characteristic emission of Cu-doped samples in terms of defect-mediated optical cycles. The focus is then placed on surface energetics, which differ markedly from bulk behaviour and critically influence photocatalytic performance. Explicit modelling of HER demonstrates that Cu substitution dramatically reduces the overpotential on ZnS and ZnSe by tuning hydrogen adsorption toward the Sabatier optimum, while in ZnO the beneficial effect of Cu doping is diminished by the excessive strengthening of the adsorbate-surface interactions. Finally, the measured HER activities are rationalised by proposing a defect-mediated mechanism involving electron trapping at the surface Cu site, cooperative proton adsorption, and hydride formation. These findings establish defect thermodynamics and surface charge localisation as key design parameters for optimising materials engineering strategies in photocatalytic applications. Full article

Magnesium antimonide (Mg₃Sb₂) has emerged as a promising high-performance thermoelectric material, yet its efficiency is fundamentally determined by intrinsic point defects. In this study, we present a comprehensive investigation of defects in the intermetallic compound Mg₃Sb₂ using first laws of thermodynamics and density functional theory (DFT) within the generalized gradient approximation (GGA). By calculating the energy of defect formation and the charge transition energy between energy levels, it was determined how the change in chemical potential associated with phase synthesis affects the phase stability and carrier concentrations. Calculations show that donor defects dominate in Mg-rich alloys, primarily antimony vacancies and magnesium atoms in interstitial positions. This means that in a phase with a slight magnesium excess, e.g., Mg_3.01Sb_1.99 at 1400 K, n-type conductivity dominates. In the opposite case, i.e., in an Sb-rich alloy, magnesium vacancies spontaneously form in the Wyckoff 1a position. These ionized acceptors induce strong self-compensation, blocking the Fermi level about 0.38 eV above the valence band maximum. As a result of this process, the Mg₃Sb₂ phase, at elevated temperatures, becomes the non-stoichiometric Mg_2.99Sb_2.01 phase, which causes the material to retain p-type conductivity and actively block doping-induced n-type conductivity. The conducted studies demonstrate that the homogeneity range of the Mg-Sb system, although traditionally considered narrow, has a significant impact on the semiconducting properties of the material. Furthermore, they also point to the need for continued research on high temperature in the area of synthetic defect engineering, interface engineering, and optimization of the thermoelectric properties of materials based on Mg-Sb alloys. Full article

This study introduces a switchable and tunable multimodal, multi-peak, perfect terahertz absorber, utilizing a composite structure of graphene and double concentric metal rings. From bottom to top, the absorber consists of a gold substrate, a SiO₂ dielectric layer, a patterned graphene layer, another SiO₂ dielectric layer, and double concentric metal rings on the top. The structure achieves three high-absorption resonance peaks in the far-infrared band: a relatively broad peak with 99.05% absorptance at 38.128 THz, and two extremely narrow peaks with 99.56% and 97.23% absorptance at 47.909 THz and 49.873 THz, respectively. Analysis of the absorption spectra and electric field distributions reveals that the generation mechanism of Peak I is Fabry–Pérot cavity resonance, while Peaks II and III result from the coupling between the high-order localized surface plasmons in the outer ring and the graphene surface plasmon polaritons. Benefiting from graphene’s excellent electrical tunability, the absorption peaks’ positions and intensities can be dynamically tuned by varying the Fermi level. The core innovation of this work lies in the high-level integration of multiple functionalities. By leveraging the sensitive response of Peak III to variations in the Fermi level, a four-input AND logic gate is embedded within the metamaterial absorber in this frequency band. The Fermi levels of four independent graphene regions serve as the binary inputs, while the absorption state of Peak III is defined as the logical output. Additionally, the two narrow peaks display high sensitivity to the surrounding refractive index, with sensitivities of 30.1 THz/RIU and 62.5 THz/RIU, demonstrating significant potential for sensing. This multifunctional integrated device combines tunable absorption, a logic gate, and sensing capabilities, making it promising for terahertz communication systems, intelligent sensing networks, and reconfigurable platforms. Full article

The study presents a comprehensive first-principles investigation of the structural, electronic, and magnetic properties of rocksalt scandium nitride (ScN) and its Cr- and Mn-doped derivatives using spin-polarized density-functional theory within the GGA + U (U_Cr = 3.5 eV, U_Mn = 2.7 eV) and HSE06 frameworks. Pristine ScN crystallizes in the cubic Fm$\bar{3}$m structure and exhibits narrow-gap semiconducting behavior, with an indirect band gap of 0.82 eV obtained from hybrid-functional calculations, in excellent agreement with reported theoretical values. Substitutional doping with Cr and Mn introduces localized 3d states near the Fermi level, driving a transition toward spin-polarized metallic or half-metallic behavior accompanied by robust ferromagnetism. Density-of-states and band-structure analyses reveal that magnetism and charge transport in the doped systems are dominated by exchange-split transition-metal 3d states hybridized with N-2p orbitals. Total energy calculations confirm ferromagnetic ground states for both Cr- and Mn-doped ScN, with Mn substitution yielding stronger exchange stabilization and higher magnetic moments. Magnetocrystalline anisotropy energies, evaluated using the force-theorem approach, are found to be negligibly small, indicating weak anisotropy consistent with the moderate spin–orbit coupling strength in ScN-based nitrides. Nevertheless, symmetry breaking around dopant sites gives rise to a finite Dzyaloshinskii–Moriya interaction, leading to weak spin canting and non-collinear magnetic tendencies. The interplay between magnetic exchange coupling, spin–orbit interaction, and local inversion symmetry breaking positions of Cr- and Mn-doped ScN as promising dilute magnetic semiconductors with tunable spin polarization and chiral magnetic interactions, offering a viable platform for nitride-based spintronic and magneto-electronic applications. Full article

Ohmic contact resistance is a persistent and increasingly dominant bottleneck limiting the practical performance of wide-bandgap (WBG) and ultrawide-bandgap (UWBG) power semiconductor devices. This review provides a comprehensive and comparative treatment of specific contact resistivity (${\rho }_c$) phenomena across five material systems—4H-SiC, GaN, β-Ga₂O₃, AlN/AlGaN, and diamond—spanning fundamental contact physics, characterization methodology, material-specific state of the art, device context, and advanced engineering strategies. A semi-empirical scaling analysis establishes that the minimum achievable ${\rho }_c$ increases by approximately one order of magnitude per 0.8–1.0 eV increase in bandgap, arising from the interplay of Fermi-level pinning, increasing carrier effective mass, and decreasing achievable near-surface doping concentration. The best demonstrated ${\rho }_c$ values range from ~3 × 10⁻⁸ Ω·cm² for GaN epitaxially regrown contacts to ~8 × 10⁻⁵ Ω·cm² for direct AlN metallization. The transition from alloyed to regrown contacts in GaN—delivering two orders of magnitude improvement—is identified as the paradigm model for UWBG contact development, with β-Ga₂O₃ most immediately positioned to follow this trajectory. Key challenges include the absence of p-type doping in β-Ga₂O₃, near-complete Fermi-level pinning in AlN, and the unsolved shallow-donor problem in diamond. Recommendations for standardized ${\rho }_c$ measurement protocols and priority research directions are presented. Full article

Deposition of amorphous boron (a-B) onto Si substrates via chemical decomposition of B₂H₆ molecules produces a-B/Si, heterojunctions which are the core parts of photodetectors used in vacuum ultraviolet (VUV) and potentially in extreme ultraviolet (EUV) lithography. However, fundamental questions regarding the limit on the thickness of the deposited a-B thin films and the intrinsic electronic nature of the B atoms adjacent to the Si substrate remain unanswered. Here we investigated the local structural and electronic properties of atomic-thin a-B layers at the Si{001} substrates using ab initio molecular dynamics (AIMD) techniques. The investigation revealed a rich variety of local chemical bonding and consequently interfacial electronic properties. For thin a-B layer(s)/Si systems, most of the a-B atoms at the interface formed (-B-Si-B-Si-) chains on the Si{001} surface. These B atoms were found to occupy the positions of the missing Si atoms and were bonded to the surficial Si atoms. The surficial Si atoms predominantly have two B neighbors. Localized defect states at the Fermi level for the interfacial Si and B atoms were found in the pseudo-gap. These states have a major influence on the electrical properties of the device. The predicted minimum thickness of the a-B films is about 1 to 2 nm, a useful metric for the manufacturing of a-B/Si devices. The information obtained here further helps us to understand the working mechanisms of a-B/Si interfaces for photon detection and constructing new core devices for potential applications in the field of metal/semiconductor heterojunctions for photon detection, photovoltaics, Schottky diodes and semiconductor devices. Full article

Solar-blind ultraviolet (UV) photodetectors based on wide-bandgap oxide semiconductors are highly desirable for environmental monitoring, flame sensing, and secure optical communication. Among them, Ga₂O₃ has attracted extensive attention due to its ultra-wide bandgap and intrinsic solar-blind response; however, its high dark current, weak built-in electric field, and defect-induced instability remain critical challenges, particularly for amorphous films prepared by scalable sputtering processes. Herein, a self-powered solar-blind UV photodetector based on a NiO/Ga₂O₃ heterojunction is demonstrated, in which the oxygen-vacancy concentration and band structure of sputtered Ga₂O₃ are systematically regulated by tailoring the Ar/O₂ sputtering atmosphere. Combined X-ray photoelectron spectroscopy, UV photoelectron spectroscopy, and optical measurements reveal that the variation in oxygen-vacancy concentration simultaneously modulates the Fermi-level position, band-edge alignment, and built-in potential at the NiO/Ga₂O₃ interface. As a result, the optimized heterojunction device exhibits a low dark current, pronounced rectifying behavior, and efficient carrier separation under zero bias, enabling self-powered operation. The photodetector delivers a responsivity of 47 mA W⁻¹, a detectivity of 7.52 × 10¹¹ Jones, and a high rejection ratio exceeding 10⁴ between 254 and 365 nm. Furthermore, stable and high-contrast UV imaging is successfully demonstrated, highlighting the practical applicability of the device. This work provides an effective methodology for modulating defects and band structure in high-performance solar-blind UV photodetectors based on sputtered wide-bandgap oxide heterojunctions. Full article

Hydrogen is frequently incorporated in alkaline-earth oxides during crystal growth or post-deposition annealing. For MgO, several studies in the past showed that interstitial monatomic hydrogen can also favourably bind with oxygen vacancies to form stable substitutional defect complexes (substitutional hydrogen or U-defect centers). The present study reports first-principles density-functional calculations of the formation energies of both interstitial and substitutional forms of the hydrogen impurity in MgO. Determination of the site-resolved densities of electronic states allowed for a detailed identification of the nature of the impurity-induced levels, both in the valence-energy region and inside the band gap of the host. The stability and diffusion mechanisms of both hydrogen defects was also studied with the aid of nudged elastic-band (NEB) calculations. Interstitial hydrogen was found to be an amphoteric defect with the lower formation energy for any realistic environment conditions (temperature and oxygen partial pressure). The NEB calculations showed that it is a fast-diffusing species when it is thermodynamically stable as a positively-charged state (bare proton). In contrast, the hydrogen-vacancy complex is a shallow donor, extremely stable against dissociation and virtually immobile as an isolated defect. Its formation is found to be favoured for a range of mid-gap Fermi-level positions where positively-charged interstitial hydrogen and neutral oxygen vacancies (F centers) are both thermodynamically stable low-energy defects. The present findings are consistent with the established consensus on the electrical activity of hydrogen in MgO as well as with experimental observations reporting the remarkable thermal stability of substitutional hydrogen defects and their ability to act as electron traps. Full article

Octachlorinated metal phthalocyanines (MPcCl₈, M = Co, Zn, VO) represent an underexplored class of functional materials with promising potential for chemiresistive sensing applications. This work is the first to determine the structure of single crystals of CoPcCl₈, revealing a triclinic (P-1) packing motif with cofacial molecular stacks and an interplanar distance of 3.381 Å. Powder XRD, vibrational spectroscopy, and elemental analysis confirm phase purity and isostructurality between CoPcCl₈ and ZnPcCl₈, while VOPcCl₈ adopts a tetragonal arrangement similar to its tetrachlorinated analogue. Thin films were fabricated via physical vapor deposition (PVD) and spin-coating (SC), with SC yielding highly crystalline films and PVD resulting in poorly crystalline or amorphous layers. Electrical measurements demonstrate that SC films exhibit n-type semiconducting behavior with conductivities 2–3 orders of magnitude higher than PVD films. Density functional theory (DFT) calculations corroborate the experimental findings, predicting band gaps of 1.19 eV (Co), 1.11 eV (Zn), and 0.78 eV (VO), with Fermi levels positioned near the conduction band, which is consistent with n-type character. Chemiresistive sensing tests reveal that SC-deposited MPcCl₈ films respond reversibly and selectively to ammonia (NH₃) and hydrogen sulfide (H₂S) at room temperature. ZnPcCl₈ shows the highest NH₃ response (45.3% to 10 ppm), while CoPcCl₈ exhibits superior sensitivity to H₂S (LOD = 0.3 ppm). These results suggest that the films of octachlorinated phthalocyanines produced by the SC method are highly sensitive materials for gas sensors designed to detect toxic and corrosive gases. Full article

We present a comprehensive first-principles study of nanoribbons made from the α-borophene sheet. This study looks at how edge shape, ribbon width, and magnetic ordering affect their structural, electronic, and transport properties. Ribbons cut along armchair (ac) and zigzag (zz) directions with various edge designs—armchair (a), single (s), and double (d) chains—are all stable. The double chain “dd” edges have the highest binding energies and the lowest edge energies, which aligns with near-bulk coordination. Our analysis of electronic structure and ballistic transport shows strong metallic characteristics in almost all configurations. Only the narrowest “3-ad” ribbon shows a small energy gap that disappears as the width increases. Zigzag ribbons (“zz”) display edge magnetism that depends on width, changing from non-magnetic to antiferromagnetic and finally to ferromagnetic states. Their spin-resolved transmission demonstrates clear spin filtering with polarization exceeding about 40%. Edge passivation affects these properties: hydrogen and fluorine reduce the “zz” edge magnetic moments and spin transport, while oxygen maintains finite magnetism. Near the Fermi level, many ribbons allow for multiple conducting channels. This feature supports low-resistance charge flow even for widths below 10 nm, while higher-energy transmission shows greater dependence on width. These findings position α-borophene nanoribbons as promising one-dimensional components for nanoelectronic connections and spintronic devices, combining high stability, adjustable edge magnetism, and strong metallic conduction. Full article

Motivated by the seminal discoveries in graphene, the exploration of novel physical phenomena in alternative two-dimensional (2D) materials has attracted tremendous attention. In this work, through theoretical investigation using first-principles calculations, we reveal that Mo-intercalated ${\mathrm{CrI}}_3$ bilayer exhibits ferromagnetic semiconductor behavior with a small easy-plane magnetocrystalline anisotropy energy (MAE) of 0.618 meV/Cr(Mo) between (100) and (001) magnetizations. The spin–orbit coupling (SOC) opens a narrow band gap at the Fermi level for both magnetization orientations with nonzero Chern number for realizing the quantum anomalous Hall effect (QAHE) in the former and with trivial topology in the latter. The small MAE implies the efficient experimental manipulation of magnetization between distinct topologies through an external magnetic field. Our findings provide compelling evidence that the QAHE in this system originates from the quantum spin Hall effect (QSHE), driven by intrinsic magnetism under broken time-reversal symmetry. These unique properties position Mo-intercalated ${\mathrm{CrI}}_3$ as a promising candidate for tunable spintronic applications. Full article

The electronic structures and optical properties of Mn-doped, Co-doped, and (Mn,Co)-co-doped Cu₂ZnSnS₄ were calculated and analyzed using the first-principles pseudopotential plane-wave approach. The results indicate that doping with Mn or Co increases the bond population and decreases the bond length of the S-Mn and S-Co bonds, respectively, enhancing their covalent character. The undoped Cu₂ZnSnS₄ exhibits a bandgap of 0.16 eV, whereas doping with Mn or Co introduces impurity levels near the Fermi level, resulting in bandgap narrowing. Within the visible light spectrum, the static dielectric constant ε₁(0) reaches its maximum value of 67.7 under co-doping conditions, and the absorption coefficient also attains its maximum value of 6.7 × 10⁴ cm⁻¹ under co-doping. Doping with Mn and Co induces a redshift (shift towards lower energy) in both the absorption peaks and dielectric function peaks, concomitantly increasing the probability of photon-induced electronic transitions. Conversely, doping shifts the reflectivity peaks towards higher energies (blue-shift), with the most pronounced blue-shift occurring under co-doping; the strongest reflectivity peaks remain below 43%. A prominent conductivity peak is observed at 1.7 eV. Doping shifts this peak position towards lower energies, with the maximum peak intensity reaching 1.6. These findings collectively suggest that Mn and Co doping effectively modulate key optical properties of Cu₂ZnSnS₄, such as its band gap and absorption coefficient, constituting an effective strategy for enhancing its optoelectronic transport characteristics. Full article

This study employs density functional theory (DFT) to investigate the electronic and optical properties of molybdenum (Mo) and chalcogen (S, Se, Te) co-doped anatase TiO₂. Two co-doping configurations were examined: Model 1, where the dopants are adjacent, and Model 2, where the dopants are farther apart. The incorporation of Mo into anatase TiO₂ resulted in a significant bandgap reduction, lowering it from 3.22 eV (pure TiO₂) to range of 2.52–0.68 eV, depending on the specific doping model. The introduction of Mo-4d states below the conduction band led to a shift in the Fermi level from the top of the valence band to the bottom of the conduction band, confirming the n-type doping characteristics of Mo in TiO₂. Chalcogen doping introduced isolated electronic states from Te-5p, S-3p, and Se-4p located above the valence band maximum, further reducing the bandgap. Among the examined configurations, Mo–S co-doping in Model 1 exhibited most optimal structural stability structure with the fewer impurity states, enhancing photocatalytic efficiency by reducing charge recombination. With the exception of Mo–Te co-doping, all co-doped systems demonstrated strong oxidation power under visible light, making Mo-S and Mo-Se co-doped TiO₂ promising candidates for oxidation-driven photocatalysis. However, their limited reduction ability suggests they may be less suitable for water-splitting applications. The study also revealed that dopant positioning significantly influences charge transfer and optoelectronic properties. Model 1 favored localized electron density and weaker magnetization, while Model 2 exhibited delocalized charge density and stronger magnetization. These findings underscore the critical role of dopant arrangement in optimizing TiO₂-based photocatalysts for solar energy applications. Full article