arXiv subjects

Ahmed Zubair

Publications and source records attributed to Ahmed Zubair.

At least 19 recordsLinked to original sources

arXiv2026

First-Principles Investigation of Multimodal Toxic Gas Sensing in Carbon-Tuned hBN-Graphene Alloys: Chemiresistive, Work-Function, and Optical Responses

Compact, reliable, and cost-effective gas sensors have become a highly demanding subject for safety management in the medical sector, chemical manufacturing, food quality monitoring, agriculture, and industrial safety. Hazardous gas emissions need to be controlled and monitored with fast-responsive and highly sensitive sensing devices. A first-principles study employing density functional theory (DFT) was used to investigate the adsorption behavior of Cl2, CO, CO2, NO, NO2, and HCN gas molecules with our proposed alloys, which consisted of hexagonal boron nitride (hBN) and graphene (Gr). The alloy consisting of 22% carbon (BNGr-2) was found to be most competent for sensing Cl2, CO, CO2, and HCN with sufficient adsorption energy, charge transfer, and bandgap alteration. However, NO and NO2 gas molecules showed more engagement with 33% carbon-proportioned alloy (BNGr-3) in terms of adequate gas sensing properties. NOx gases exhibited the most chemiresistive sensitivity towards the adsorbents. Other gases also showed significant chemiresistive sensitivity and distinct selectivity ratios, which would facilitate these alloys as chemiresistive sensors. Besides, noticeable work function variation (~20%) of these systems manifested potential as work function based sensors. Cl2 and NO2 showed strong physical adsorption, while the rest of the gases were weakly to moderately physisorbed, resulting in very short recovery times (10-1 ~ 10-6 seconds). Additionally, the distinctive absorption spectra observed for the gas analyte systems highlighted the potential of the proposed alloys as optical gas sensors. Temperature variation revealed that all gas molecules can be freed from the adsorbent BNGr-2 at 425 K. These findings imply hBN-Gr alloys as promising gas sensors for pollution auditing.

cond-mat.mtrl-sci↗

arXiv2026

Surrogate-Assisted Inverse Design and Temperature-Dependent Electrothermal Analysis of an All-Oxide Narrowband Thermophotovoltaic Emitter

A narrowband emitter aligning with the bandgap of the underlying solar cell is essential for improving the spectral efficiency and thermal stability of thermophotovoltaic (TPV) systems. Emitters based on oxide materials present a promising solution to the optical and mechanical performance degradation of traditional emitters, which employ metal-dielectric structures that experience metal oxidation and structural deterioration at high temperatures. Here, we presented a surrogate-assisted inverse-design framework for a narrowband 1D grating emitter comprising ITO and Al2O3 layers on a sapphire substrate. We performed Bayesian optimization over the trained ExtraTrees surrogates on a penalty-augmented objective containing a peak-emission constraint (E_peak > 0.90) while minimizing the full width at half maximum (FWHM) and maximizing the fraction of emission concentrated within the selected peak-centered spectral band to acquire a high, narrowband peak emission. The surrogate model was trained using a dataset of emission spectra obtained from the finite-difference time-domain (FDTD) by systematically varying the layers' thicknesses and the structure's period as the input features, and the narrowband emission's figure of merit (FOM) ((E_peak), wavelength of peak emission (_peak), FWHM, in-band fraction (f_in) for determining the amount of emission outside the peak band, and concentration of peak emission near the peak) was used as the prediction target. The resulting set of predictions for the optimized structure was further validated using the FDTD method.

physics.optics↗

arXiv2026

Hybrid Nanocone-Nanohole Light Trapping for High-Efficiency Thin-Film Silicon Solar Cells

Thin-film silicon solar cells exhibit weak absorption at longer wavelengths of visible light due to their limited optical thickness. To overcome this inherent limitation, a hybrid nanocone-nanohole light-management architecture is proposed by integrating front-surface silicon nanocones with embedded air-filled nanoholes in the absorber layer. The nanocones provide a gradual refractive-index transition that suppresses front-surface reflection, while the nanoholes enhance optical confinement through multiple scattering and internal reflection. The structural parameters are systematically optimized using finite-difference time-domain (FDTD) simulations coupled with electrical device modeling. Under optimized doping conditions and isothermal steady-state operation, the optimized design yields a short-circuit current density of 34.64 mA cm-2, an open-circuit voltage of 0.82 V, a fill factor of 86.27%, and a power conversion efficiency of 24.42%, representing a 17.74% improvement over a comparable nanohole-based design. Furthermore, coupled opto-electro-thermal simulations show that the proposed hybrid nanocone-nanohole structure retains 95.21% of its efficiency under non-isothermal operation up to45 degree celsius, demonstrating excellent thermal stability. These results highlight the potential of the proposed hybrid nanocone-nanohole design as an effective light-management strategy for high-efficiency and thermally robust thin-film silicon solar cells.

physics.optics↗

arXiv2026

Functionalization-Driven Charge Redistribution Enabling Ultra-High-Capacity V2B MBene Anode for Li/Na ion batteries: A First-Principles study

Amid increasing global demand for clean, sustainable energy, the search for novel electrode materials has emerged as a crucial link to advancing future energy storage technologies. Here, we explored the potential of N-functionalized 2D MBene V2BN2 as anode materials for Li- and Na-ion batteries using first-principles calculations. Phonon dispersion and ab initio molecular dynamics calculations were employed to assess the dynamic and thermal stability of the material. The intrinsic metallic properties of V2BN2 were revealed through electronic band structures and density of states analyses. Importantly, Bader charge analysis demonstrates substantial charge redistribution upon Li/Na adsorption, leading to stronger ion-substrate interactions compared to the pristine counterpart. This redistribution plays a decisive role in enhancing Li/Na ion adsorption and stabilizing ion accommodation. Furthermore, owing to favorable multilayer adsorption of Li and Na ions, V2BN2 exhibited high theoretical specific capacities of 1524 and 762 mAh/g, as well as low open circuit voltages of 0.73 and 0.23 V for Li and Na, respectively. In addition, the energy barriers were calculated to be 0.49 and 0.29 eV for Li- and Na-ion transport, respectively, indicating rapid ion transport and excellent rate capability. These results indicate that V2BN2 holds significant potential as an anode material for next-generation rechargeable ion batteries.

cond-mat.mtrl-sci↗

arXiv2026

Advancing Lead-Free FASnI3 Perovskite Solar Cells via Cu-Doped NiOx HTL Engineering: An Optoelectronic Analysis

Efficient lead-free perovskite solar cells remain limited by inefficient charge transport and significant recombination losses. In this work, copper (Cu) doped nickel oxide (NiOx) is employed as the hole transport layer (HTL) in a formamidinium tin iodide (FASnI3) perovskite solar cell to improve hole extraction and suppress carrier losses. The optoelectronic response of the proposed device architecture is evaluated through a coupled numerical framework combining finite-difference time-domain (FDTD) optical simulations with finite element method (FEM)-based electrical modeling. Electrical analysis solves the Poisson equation together with the drift-diffusion and carrier continuity equations, while accounting for radiative, non-adiative, and interfacial recombination mechanisms. Systematic optimization of the thicknesses and doping concentrations of the constituent layers was performed to establish a high-performance device configuration. Following optimization of layer dimensions and carrier densities, the device achieved a power conversion efficiency (PCE) of 24.71%. Integrating an anti-reflection coating (ARC) effectively suppressed reflection losses, yielding a 5.47% relative enhancement in PCE. Consequently, the optimized device achieved a peak PCE of 26.06%, with a short-circuit current density (Jsc) of 28.98 mA/cm2, an open-circuit voltage (Voc) of 1.051 V, and a fill factor (FF) of 85.64%. These findings demonstrate that Cu-doped NiOx functions as a highly promising HTL to boost charge collection and maximize overall efficiency in lead-free FASnI3 solar cells.

physics.app-ph↗

arXiv2026

Machine learning based prediction of optical properties in two-dimensional Mo-W-S-Se-Te transition-metal dichalcogenide alloys through physics-informed sampling

Two-dimensional transition-metal dichalcogenide (TMD) alloys provide a compositionally tunable platform for controlling the optical and electronic properties. However, systematic prediction of their dielectric response across multicomponent alloy spaces remains challenging owing to the combinatorial cost of first-principles calculations. In this work, we combined ab initio optical-property calculations with a tabular foundation-model regression to predict the real and imaginary components of the frequency-dependent dielectric function for Mo-W-S-Se-Te TMD alloys. A dataset of 99 alloy structures spanning binary, ternary, quaternary, and quinary compositions was generated using density functional theory (DFT). The resulting polarization-dependent dielectric spectra were used to train a tabular prior-fitted network (TabPFN) and evaluated against the conventional Extra Trees and XGBoost models. To accommodate the in-context capacity limit of TabPFN, we introduced a non-uniform, physics-informed energy subsampling strategy that concentrates sampling in the optically active region above the band gap, where interband absorption is strongest. Trained solely on quaternary alloys, our TabPFN reconstructed the dielectric spectra of held-out quaternary compositions with an R2 > 0.98 and a mean absolute error below 0.10 for all four dielectric components, outperforming both baselines while requiring no gradient-based training or hyperparameter tuning. Our model further predicted derived optical quantities, including refractive index, extinction coefficient, and absorption coefficient. Additionally, our model generalized in a zero-shot manner to binary, ternary, and quinary alloys absent from the training set, with quinary predictions achieving an R2 > 0.97.

cond-mat.mtrl-sci↗

arXiv2026

Strain- and Electric-Field-Tunable Valley Polarization in Mo0.75V0.25Te2(Mo3VTe8) for Valleytronic Application

Valley polarization in 2D TMDs is promising for low-power valleytronic and spin-valley information processing, but time-reversal symmetry in pristine nonmagnetic TMDs keeps the K+ and K- valleys degenerate, limiting device applications. In this work, we investigated the structural stability, electronic properties, and tunable valley polarization of V-alloyed MoTe2 monolayer, Mo0.75V0.25Te2, using first-principles density functional theory (DFT) calculations. Substitutional alloying of MoTe2 with V introduced magnetic exchange interaction, which, together with spin-orbit coupling (SOC), lifted the valley degeneracy at the unequal valleys. The alloyed structure was found to be energetically and dynamically stable due to the absence of imaginary phonon modes. In pristine MoTe2, SOC produced spin splittings of 34.0 meV and 218.9 meV in the conduction bands and valence bands, respectively, but no valley polarization was observed. In contrast, Mo0.75V0.25Te2 exhibited spontaneous valley polarization of 37.3 meV in the conduction band and 78.2 meV in the valence band. The valley polarization was further enhanced by external electric fields and biaxial strain. A transverse electric field along the crystal c axis produced the maximum valley splitting of 132.8 meV in the valence band, whereas biaxial tensile strain increased the valence band valley splitting up to 160.8 meV. The maximum conduction band valley splitting reached 54.4 meV under 2% biaxial compressive strain. These results demonstrated that V alloying, combined with electric-field and strain engineering, provides an effective strategy for achieving large and tunable valley polarization in MoTe2. Thus, Mo0.75V0.25Te2 can be considered a promising 2D platform for tunable valleytronic device applications, such as transistors and sensors.

cond-mat.mtrl-sci↗

arXiv2026

Robust Spin Splitting and Strain-Controlled Optical Response in Monolayer CrC2N4 for Valleytronic and Optoelectronic Applications

Monolayer CrC2N4 recently emerged as a promising two-dimensional semiconductor, yet its spin-orbit-coupled (SOC) physics and strain-tunable optical response remained largely unexplored. Here, we investigated the electronic, valley, charge-transfer, and optical properties of pristine and biaxially strained monolayer CrC2N4 using first-principles calculations. The monolayer exhibited a direct band gap at the K/K' valleys. SOC produced valley contrasting out-of-plane spin polarization, yielding a moderate valence band spin splitting of 51.9 meV and a small conduction band spin splitting of 1.7 meV. Orbital-resolved analysis showed that the edge states were mainly governed by Cr-d and N-p hybridization, while Bader analysis indicated polar-covalent bonding through charge transfer toward N atoms. Biaxial strain in the range of -4% to +4% tuned the band gap from 1.987 to 1.421 eV and drove an indirect-to-direct gap transition near -1% strain. Tensile strain enhanced the Berry curvature and red-shifted the optical response toward the visible-near-infrared region. These results suggested monolayer CrC2N4 as a promising platform for strain-engineered valleytronic and optoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2026

Nature-Inspired Hyperuniform Nanohole Patterning for Robust Broadband Absorption Enhancement in Perovskite Solar Cells

Nature-inspired hyperuniform disorder offers a promising route to broadband light trapping in ultrathin perovskite solar cells by avoiding narrowband, illumination-sensitive responses commonly associated with periodic nanophotonic textures. Here, we introduce a nature-inspired ingenious hyperuniform nanohole architecture integrated into the front glass of a planar MAPbI$_3$ perovskite solar cell, serving as a junction-preserving strategy to enhance optical absorption and photovoltaic performance. In comparison with planar and periodic textures, the hyperuniform architecture redistributed incident light across a broader spectrum of in-plane momentum states, strengthened near-interface electromagnetic fields, and improved long-wavelength coupling into the absorber, thereby increasing the effective optical path length without altering the electronically active interfaces. To quantify these effects, we employed a coupled three-dimensional multiphysics framework that integrates finite-difference time-domain (FDTD) optical simulations with drift-diffusion electrical modeling. The optimized design exhibited broadband absorption enhancement, weak polarization dependence, and strong angular tolerance, while suppressing interference-driven spectral oscillations and reducing sensitivity to patterned-layer thickness. Relative to the planar structure, the hyperuniform architecture increased the short-circuit current density from 21.57 to 23.92 mAcm$^{-2}$ and improved the power conversion efficiency from 21.03% to 23.62%, while maintaining $\mathrm{V_{oc}}$ at 1.13 V and preserving a high fill factor of 87.66%. In addition to statistical pattern-invariant performance, stochastic radius-variation analysis indicated a positive enhancement in photocurrent and under fabrication-relevant dimensional disorder.

physics.app-ph↗

arXiv2026

High-Efficiency Hexagonal Nanowire MAPbI3 Perovskite Solar Cell with Broadband Light Trapping

Perovskite solar cells (PSCs) have emerged as strong contenders for the next generation of photovoltaic (PV) technologies due to their exceptional light absorption properties, tunability, and affordability in manufacturing. Here, we presented an ingenious hexagonal nanowire (HNW)-based PSC that achieves broadband absorption, minimizes reflectance, and offers robust polarization insensitivity by improving light-matter interaction and increasing charge-collection efficiency. The rotational symmetry of the HNW configuration yielded polarization-independent absorbance under both TE and TM illumination across the visible and near-infrared spectra. The optimization of the geometrical parameters of CH3NH3PbI3-based HNW structure, including diameter, period, and fill ratio, offered a wide rangeof variations that influenced both optical properties and device performance. To further intensify photon confinement, a dielectric SiO2 sphere is partially embedded in the ITO layer, improving long-wavelength absorbance and increasing electron-hole pair generation near the active region. We analyzed the finite-difference time-domain (FDTD) method to examine the optical properties of our proposed structure. This study demonstrates that our proposed structure has achieved a higher generation rate, enhanced absorbance, and a higher optical short-circuit current density (Jsc) of 29.53 mA/cm2. Electrical performance is assessed by solving the coupled drift-diffusion and Poisson equations for the dynamics of carrier transport. The optimized HNW structure achieved a notable power conversion efficiency of 24.2%, highlighting a strong connection between optical confinement and effective carrier transport. These attributes render the proposed HNW PSC a viable option for high-performance PV systems and scalable thin-film solar technologies.

physics.optics↗

arXiv2025

Unveiling architectural and optoelectronic synergies in lead-free perovskite/perovskite/kesterite triple-junction monolithic tandem solar cells

The widespread use of lead-based materials in tandem solar cells raises critical environmental and health concerns due to their inherent toxicity and risk of contamination. To address this challenge, we focused on lead-free tandem architectures based on non-toxic, environmentally benign materials such as tin-based perovskites and kesterites, which are essential for advancing sustainable photovoltaic technologies. In this study, we present the proposition, design, and optimization of two distinct lead-free monolithic tandem solar cell architectures - an all-perovskite dual-junction device employing potassium tin iodide (KSnI3) and formamidinium tin triiodide (FASnI3) as absorbers for the top and bottom subcells, respectively, and a triple-junction monolithic tandem structure incorporating KSnI3, FASnI3, and Ag-doped copper zinc tin selenide (ACZTSe) as absorbers for the top, middle, and bottom subcells, respectively. We simulated the optical and electrical characteristics of these devices using the finite-difference time-domain and finite element methods, explicitly considering radiative, non-radiative, and surface recombination mechanisms. The optimized all-perovskite dual-junction solar cell achieved a power conversion efficiency (PCE) of 27.3%, with short-circuit current density (Jsc) of 14.74 mA/cm2, open-circuit voltage (Voc) of 2.227 V, and fill factor (FF) of 83.14%. Conversely, the optimized triple-junction hybrid perovskite-kesterite architecture secured an elevated PCE of 30.69%, along with Jsc of 13.184 mA/cm2, Voc of 2.766 V, and FF of 84.18%. These findings reveal the strong potential of lead-free perovskite and kesterite material based absorbers in promoting high-performance hybrid tandem solar cells, highlighting their importance in advancing sustainable and efficient photovoltaic technologies.

physics.optics↗

arXiv2025

Strain-Tunable Spin Filtering and Valley Splitting Coexisting with Anomalous Hall Effect in 2D Half-Metallic VSe2/VN Heterostructure: Toward a Unified Spintronic-Valleytronic Platform

Rapid progress in valleytronics and spintronics is limited by the scarcity of two-dimensional materials that simultaneously provide robust valley splitting and strong spin selectivity. Here we showed that a van der Waals heterostructure (VSe2/VN) built from hexagonal VSe2 and hexagonal VN addressed this gap. Using first-principles density functional theory, phonon, ab initio molecular dynamics stability tests, Bader charge analysis, and Wannier-based Berry-curvature calculations, we demonstrated an energetically and dynamically stable heterostructure that exhibited interlayer charge transfer and a work function intermediate between the constituent monolayers. The electronic structure showed small indirect PBE gap (108.9 meV), with HSE06 indicating a half-metallic tendency; a sizable conduction-band valley splitting (ΔCKK' = 22.9 meV for spin-up and ΔCKK' = 61.3 meV for spin-down); and pronounced spin asymmetry, where the spin-down channel showed a wide semiconducting gap (0.64 eV) while the spin-up channel was nearly gapless. These features yielded a high zero-strain spin-filter efficiency P = 75.4%, tunable to 82.5% under +4% biaxial tensile strain. The heterostructure also supported non-zero, valley-contrasting Berry curvature, and a large anomalous Hall conductivity (peak sigmaxy = 568.33 S/cm). Importantly, mean-field estimation placed the ferromagnetic Curie temperature near room temperature at zero strain (Tc = 284.04 K), while Tc decreased to 183.9 K at +4% strain, the magnetic order remained robust to cryogenic temperatures, providing a beneficial tuning knob to balance spin-filter performance with thermal stability in device-relevant regimes. These results identified VSe2/VN as a practical, strain-tunable platform for integrated valleytronic, spintronic devices, and for exploring anomalous Hall and valley-dependent transport phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Multipolar Origin and Active Control of High-Q Quasi-BIC Fano Resonances in Dielectric Metasurfaces for Sensing Applications

We designed an ingenious all-dielectric metasurface, employing cuboid structures patterned with bow-tie-shaped nanoholes, exhibiting multiple Fano resonances induced by quasi-bound states in the continuum (quasi-BICs) through structural asymmetry. Among them, several resonant modes demonstrated high quality factors in the range of 10^3-10^4, along with near-unity modulation depth and strong spectral contrast. The optical responses were analyzed utilizing the finite-difference time-domain (FDTD) method, with Fano profiles fitted to theoretical models and the BIC governed modes validated via the squared inverse ratio law. Furthermore, multipolar decomposition and electromagnetic spatial field profile revealed the origins of the resonance, while LC circuit modeling provided additional physical insight into the Fano profiles. The proposed metasurface also exhibited strong polarization dependence, indicating its potential for active optical switching. Finally, refractive index sensing performance, including the detection of Vibrio cholerae, reached a sensitivity of 342 nm/RIU and a figure of merit of 217.14 RIU^-1. Advancing the control of high-Q quasi-BIC Fano resonances, this study highlights Fano resonators' potential for refractive index sensing and active switching.

physics.optics↗

arXiv2025

Broadband High-Temperature Multilayer Pyramid-Shaped Metamaterial Thermal Absorber for Thermophotovoltaic applications

A broadband, thermally stable absorber is essential for thermophotovoltaic (TPV) systems to simultaneously convert solar and industrial waste heat into usable energy to meet growing power demands. Here, we proposed an ingenious polarization-independent truncated pyramid-shaped symmetric multilayer metamaterial absorber in a metal-insulator-metal-insulator (MIMI) architecture with almost complete absorption over a broad wavelength range. A total of six structures (W/AlN, Mo/AlN, Ta/AlN, Rh/MgO, Rh/SiO2, Re/BN) were designed, and the materials were selected based on their lattice matching to prevent delamination at interfaces between layers. The absorption mechanism was studied at room temperature using the finite difference time domain (FDTD) method, and the structure was optimized through a brute force design approach, which illustrates a best average absorption of 98.2% till 4000 nm and 97.73% till 5072 nm wavelength for the W/AlN structure with metal and dielectric thicknesses of 60 nm and 17.5 nm, respectively. Moreover, W/AlN structure exhibits over 96% average absorption up to 50 degree incident angles irrespective of polarizations. The thermal stability was evaluated using the finite element method (FEM) by determining von Mises stress at elevated temperatures. Thermal analysis revealed that only W/AlN can withstand around 1700 K temperature and 1500 times the incident power before permanent deformation. A temperature-dependent Drude-Lorentz model was used to further analyze the effect of absorption on the optical performance of the highly absorptive and thermally stable W/AlN structure. Additionally, we determined the effect of the concentration factor, and the operating temperature on the system efficiency by considering the emission loss of the heated absorber.

physics.optics↗

arXiv2025

Pristine and transition metal doped 2D AlSb as high performance electrocatalyst for selective CO2 reduction: A first-principles study

Electrochemical CO2 reduction reaction (CO2RR) using 2D nanomaterials has emerged as a sophisticated approach to mitigate industrial CO2 emissions. In this work, the potential application of pristine as well as strategically Fe, Co, Ni-doped 2D AlSb was examined as a CO2RR electrocatalyst. The recation pathways of CO2RR intermediate complexes, overpotential, stability, efficiency, and selectivity were studied using density functional theory (DFT). Outstanding overpotentials were achieved with pristine and doped 2D AlSb: Ni-doped 2D AlSb was selective for HCOOH (0.12eV) and CH4 (0.28eV), and Fe-doped 2D AlSb for HCHO (0.31eV) and CH3OH (0.31eV). The opposing effects of hydrogen evolution reaction (HER) was mitigated with the application of electric potential and solution pH. The main reasons for the enhancement of catalytic effect due to doping with Fe, Co, and Ni are bandgap reduction and creation of states at the edge of the valence band due to the 3d orbitals of these dopants. Interestingly, the Fe-doped 2D AlSb catalyst exhibited the highest catalytic activity. Excellent electrocatalytic properties of pristine and doped 2D AlSb make them suitable as CO2RR catalysts contributing towards a green and sustainable energy ecosystem.

cond-mat.mtrl-sci↗

arXiv2025

Hierarchically structured, high-efficiency and thermally robust perovskite solar cells with band-engineered double-hole layers

Despite the promising optoelectronic properties of methylammonium lead iodide (MAPbI3)-based perovskite solar cells (PSCs), their commercial viability is hindered by interfacial energy misalignment, suboptimal light absorption, and thermal instability. Here, we present a comprehensive theoretical framework to enhance the power conversion efficiency (PCE) of MAPbI3 based PSC through integrated electronic and morphological engineering. Firstly, to address interfacial recombination and inefficient hole extraction at the perovskite/HTL junction, we introduced a double hole transport layer (HTL) stack comprising CuO and I2O5-doped Spiro-OMeTAD, which significantly improved energy level alignment and carrier selectivity. Comprehensive multiphysics simulations, combining finite-difference time-domain (FDTD) optical analysis with finite element method (FEM) based electrical and thermal modeling, demonstrated that optimized doping concentrations and thickness parameters within the CuO/Spiro-OMeTAD hole transport layers can enhance the PCE to 22.78%. However, planar architectures, while offering ease of fabrication and scalability, exhibit weak near-UV and near-infrared absorption, whereas nanostructures attain superior light trapping but incur significant fabrication complexity, underscoring the need for balanced design strategies. To address these inherent limitations, we propose a hierarchical ellipsoidal patterned solar cell (HEPSC), wherein a top-layer ellipsoidal nanostructure is introduced across the full device stack. This design enhances broadband light trapping and optical confinement throughout the active layers while maintaining fabrication feasibility through geometrically realistic structuring.

physics.optics↗

arXiv2025

Tunable Valley Polarization and Anomalous Hall Effect in Ferrovalley NbX2 and TaX2 (X = S, Se, Te): A First-Principles Study

Two-dimensional transition metal dichalcogenides lack inversion symmetry and have broken time-reversal symmetry due to the honeycomb structure and intrinsic ferromagnetism, which leads to their valley polarization. Here, we explored the electronic and magnetic properties of the novel ferrovalley materials 1H-NbS2, 1H-NbSe2, 1H-NbTe2, 1H-TaS2, 1H-TaSe2, and 1H-TaTe2 using first-principles calculations based on density functional theory. The materials are dynamically stable bipolar magnetic semiconductors. Among the magnetic semiconductors, NbSe2 showed the maximum Curie temperature of 176.25 K. For these materials, the ferromagnetic state was more favorable than the antiferromagnetic state, indicating robust ferrovalley characteristics. These ferrovalley materials showed a giant tunable valley polarization at K and K' points in the Brillouin zone without applying any external factors due to intrinsic exchange interactions of transition metal d-orbital electrons and spin-orbit coupling. TaTe2 exhibited an outstanding valley splitting of 541 meV. Reversing Bloch electrons' magnetic moment caused an alteration of valley polarization. Additionally, the application of uniaxial and biaxial strain led to the manipulation and variation of the bandgap and valley polarization. Berry curvature exhibited opposite signs and unequal magnitudes at K and K' points, which led to the anomalous valley Hall effect in these materials. NbS2, NbSe2, and NbTe2 exhibited Berry curvature at unstrained crystals, whereas Berry curvature appeared only in TaSe2 and TaTe2 with the application of strain. These ferrovalley materials exhibited distinct band gaps for spin-up and spin-down electrons, enabling the selective transport of spin-polarized electrons.

cond-mat.mtrl-sci↗

arXiv2025

Tunable Coloration in Core-Shell Plasmonic Nanopixels Based on Organic Conductive Polymers: A First-Principles and FDTD Study

From raindrops to planets, the scattering of electromagnetic fields introduces exciting phenomena that can be utilized for display devices. Here, we designed an electrochromic nanoparticle on mirror (eNPoM) structure with core-shell geometries for low-power nanoscale pixels with rapid coloration abilities based on four electrochromic organic conducting polymers utilizing the first-principles calculations based on density functional theory (DFT) and the finite-difference time-domain (FDTD) simulations. Au nanoparticles are coated with electrochromic conductive polymers (such as PANI, PEDOT, PPy, and PTh) and positioned on the metal mirror. The electric field enhancement and the impact of shell thickness are analyzed. Dielectric properties of all polymers resulting from atomistic calculation were utilized for FDTD simulation, which helps to correlate the direct relationship between polymer structure and optical properties. Notably, the study reveals significant wavelength tunability of 100nm, 40nm, 70nm, and over 40nm using PANI, PEDOT, PPy, and PTh shells, respectively. Additionally, the potential for RGB color production using a TiN layer on the mirror is explored. For the first time, complex structures such as bow tie and gear were utilized to model the nanopixels studied and a significant absorption peak shift was observed. Chromaticity coordinates in the CIE 1931 color space and CIELAB2000 color difference quantify color change capabilities during the redox cycle, and a comparative analysis of organic and inorganic materials highlights the prospects of the proposed plasmonic nanopixels.

physics.optics↗