arXiv subjects

Maryam Mohammadi

Publications and source records attributed to Maryam Mohammadi.

16 recordsLinked to original sources

arXiv2026

Improved Heat Dissipation in CsPbBr${_3}$-hBN Heterostructures

Metal halide perovskite semiconductors are promising materials for optoelectronic and photonic devices, including solar cells and next-generation coherent light sources. However, their low thermal conductivity limits the practical operation of devices under high excitation levels. Integrating thermally conductive, large band-gap two-dimensional (2D) materials into perovskite devices could suppress heat accumulation, while preserving their optical properties. Here, we show that planar hot-pressed (PHP) cesium lead bromide (CsPbBr${_3}$) thin films capped with few-layer 2D hexagonal boron nitride (hBN) are less affected by laser-induced heating under high-power continuous-wave excitation than uncapped perovskite samples. A large-scale semidry transfer method was developed to integrate 2D hBN onto PHP CsPbBr${_3}$ thin films. The process is chemically and thermally compatible with perovskites. Microscopic and spectroscopic analyses show that the hBN capping layer does not alter the morphology and optical properties of the perovskite thin film. The PHP CsPbBr${_3}$-hBN heterostructure exhibits a thermal conductivity of 3 W/(m * K), approximately seven times higher than that of the bare perovskite films of 0.45 W/(m * K). Heat diffusion simulations confirm enhanced heat dissipation in the heterostructure relative to bare perovskite films. Our experiments demonstrate an effective approach to enhancing heat dissipation in perovskite devices using transparent, thermally conductive 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

An accurate flatness measure to estimate the generalization performance of CNN models

Flatness measures based on the spectrum or the trace of the Hessian of the loss are widely used as proxies for the generalization ability of deep networks. However, most existing definitions are either tailored to fully connected architectures, relying on stochastic estimators of the Hessian trace, or ignore the specific geometric structure of modern Convolutional Neural Networks (CNNs). In this work, we develop a flatness measure that is both exact and architecturally faithful for a broad and practically relevant class of CNNs. We first derive a closed-form expression for the trace of the Hessian of the cross-entropy loss with respect to convolutional kernels in networks that use global average pooling followed by a linear classifier. Building on this result, we then specialize the notion of relative flatness to convolutional layers and obtain a parameterization-aware flatness measure that properly accounts for the scaling symmetries and filter interactions induced by convolution and pooling. Finally, we empirically investigate the proposed measure on families of CNNs trained on standard image-classification benchmarks. The results obtained suggest that the proposed measure can serve as a robust tool to assess and compare the generalization performance of CNN models, and to guide the design of architecture and training choices in practice.

cs.LG↗

arXiv2025

Facile Synthesis and On-Chip Color Tuning of CsPbBr$_{3}$@CsPbBr$_{3-x}$TFA$_{x}$ Nanoplatelets via Ion Engineering

Metal halide perovskites (MHPs) have emerged as attractive optoelectronic materials because of high fluorescence quantum yield, broad color tunability, and excellent color purity. However, the ionic nature of MHPs makes them susceptible to polar solvents, leading to defect-induced nonradiative recombination and photoluminescence (PL) quenching. Here, we present a combined in-synthesis ($\textit{in situ}$) and post-synthesis ion engineering to suppress nonradiative recombination and integrate multicolor MHP arrays on-chip through a perovskite-compatible photolithography process and $\textit{in situ}$ vapor-phase anion exchange. CsPbBr$_{3}$@CsPbBr$_{3-x}$TFA$_{x}$ nanoplatelets were grown on-chip via a single-step solution process incorporating trifluoroacetate (TFA$^{-}$) pseudohalides. X-ray photoelectron spectroscopy revealed that TFA$^{-}$ passivate uncoordinated Pb$^{2+}$ ions on nanoplatelet surface and suppresses the formation of metallic lead (Pb$^{0}$). This decreases the non-radiative recombination centers and yields a PL peak at 520 nm with a linewidth of 14.56$\% \pm$ 0.5 nm. The nanoplatelets were patterned via a top-down photolithography process and selectively masked with a PMMA/Al$_{2}$O$_{3}$ stack to enable vapor-phase anion exchange. The PL peak shifted in the unmasked regions from 520 nm to 413 nm, resulting in distinct green and blue emission arrays. Our method enables the scalable fabrication of highly luminescent, two-color MHP arrays with tailored optical properties, advancing their integration into next-generation optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Versatile Top-Down Patterning Technique for Perovskite On-Chip Integration

Metal-halide perovskites (MHPs) have exciting optoelectronic properties and are under investigation for various applications, such as photovoltaics, light-emitting diodes, and lasers. An essential step towards exploiting the full potential of this class of materials is their large-scale, on-chip integration with high-resolution, top-down patterning. The development of such patterning methods for perovskite films is challenging because of their intrinsic ionic nature and adverse reactions with the solvents used in standard lithography processes. Here, we introduce a versatile and precise method comprising photolithography and reactive ion etching (RIE) processes that can be tuned to accommodate different perovskite compositions and morphologies. Our method utilizes conventional photoresists at reduced temperatures to create micron-sized features down to 1 $μ$m, providing high reproducibility from chip to chip. The patterning technique is validated through atomic force microscopy (AFM), X-ray diffraction (XRD), optical spectroscopy, and scanning electron microscopy (SEM). It enables the scalable and high-throughput on-chip monolithic integration of MHPs.

cond-mat.mtrl-sci↗

arXiv2025

Wafer-scale Synthesis of Mithrene and its Application in 2D Heterostructure UV Photodetectors

Silver phenylselenide (AgSePh), known as mithrene, is a two-dimensional (2D) organic-inorganic chalcogenide (MOC) semiconductor with a wide direct band gap, narrow blue emission and in-plane anisotropy. However, its application in next-generation optoelectronics is limited by crystal size and orientation, as well as challenges in large-area growth. Here, we introduce a controlled tarnishing step on the silver surface prior to the solid-vapor-phase chemical transformation into AgSePh thin films. Mithrene thin films were prepared through thermally assisted conversion (TAC) at 100°C, incorporating a pre-tarnishing water (H${_2}$O) vapor pulse and propylamine (PrNH${_2}$) as a coordinating ligand to modulate Ag${^+}$ ion reactivity and facilitate the conversion of Ph${_2}$Se${_2}$ into an active intermediate. The AgSePh thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and grazing incidence wide-angle X-ray scattering (GIWAXS). The pre-tarnishing process, combined with organic ligands, resulted in large crystals exceeding 1 $μ$m and improved homogeneous in-plane orientation, while also enabling the selective, wafer-scale synthesis of mithrene on 100 mm wafers. Furthermore, the films were integrated on planar graphene field-effect phototransistors (GFETs) and demonstrated photoresponsivity beyond 100 A/W at 450 nm, highlighting mithrene's potential for blue light-detection applications.

cond-mat.mtrl-sci↗

arXiv2025

Unisolvence of randomized MultiQuadric Kansa collocation for convection-diffusion with mixed boundary conditions

We make a further step in the open problem of unisolvence for unsymmetric Kansa collocation, proving that the MultiQuadric Kansa method with fixed collocation points and random fictitious centers is almost surely unisolvent, for stationary convection-diffusion equations with mixed boundary conditions on general domains. For the purpose of illustration, the method is applied in 2D with fictitious centers that are local random perturbations of predetermined collocation points.

math.NA↗

arXiv2025

Advanced 3D-Printed Multiphasic Scaffold with Optimal PRP Dosage for Chondrogenesis of BM-MSCs in Osteochondral Tissue Engineering

In osteochondral tissue engineering (OCTE), simultaneously regenerating subchondral bone and cartilage tissue presents a significant challenge. Multiphasic scaffolds were created and manufactured using 3D printing to address this issue. Excellent interfacial mechanical properties and biocompatibility enhance the growth and chondrogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs). The subchondral bone bottom layer is mimicked by incorporating varying concentrations of graphene oxide (GO) (0%, 1%, and 2% w/v) into a bioink composed of alginate (Alg) and gelatin (Gel). Based on evaluations of mechanical and biocompatibility properties, 1% GO is selected for further studies. Subsequently, the GO concentration is kept constant while varying the platelet-rich plasma (PRP) dosage in the multiphasic scaffolds. Different PRP dosages (0%, 1%, 2%, and 3% w/v) are integrated into the Alg-Gel bioink to simulate cartilage tissues. Results indicate that 3D-printed scaffolds containing 1% or 2% PRP exhibit favorable biomechanical properties, with no significant differences observed. However, BM-MSCs exposed to 2% PRP demonstrate enhanced adhesion, growth, and viability. Additionally, real-time PCR and Alcian blue staining confirm increased chondrogenic expression and glycosaminoglycans (GAGs) synthesis. This work highlights the promising potential of 3D-printed multiphasic frameworks in the development of OCTE.

q-bio.TO↗

arXiv2024

Room-temperature Distributed Feedback CsPbBr$_3$ Perovskite Laser Integrated on a Silicon Nitride Waveguide Platform

Silicon photonic integrated circuits (PICs) require cost-effective laser sources that can be monolithically integrated. The low cost and low-temperature solution processability of metal halide perovskites (MHPs) make them attractive alternatives to established III-V compound semiconductors for on-chip laser sources in PICs. Cesium lead bromide (CsPbBr$_3$) perovskites are emerging materials for green light-emitting diodes and lasers. To date, amplified spontaneous emission (ASE) at room temperature has been frequently achieved in CsPbBr$_3$ thin films, while reports on lasing are more limited. Here, we demonstrate a first-order grating distributed feedback (DFB) CsPbBr$_3$ thin-film laser operating at room temperature. Planar hot-pressed (PHP)-CsPbBr$_}$, with a low ASE threshold of 14.5 $μ$Jcm$^{-2}$ under 0.3 nanosecond (ns) pump pulses, was monolithically integrated into a silicon nitride (Si$_3$N$_4$) waveguide platform via a compatible top-down patterning process. The first-order grating DFB PHP-CsPbBr$_3$ thin-film laser operated at 540 nm in the green spectral region, where III-V lasers have limitations, and exhibited a lasing threshold of 0.755 mJcm$^{-2}$ at room temperature. This work marks a significant step toward utilizing MHPs for on-chip green lasers in PICs for commercial applications.

physics.app-ph↗

arXiv2024

Spectroscopic ellipsometry of CsPbCl${_3}$ perovskite thin films

Designing optoelectronic devices based on cesium lead chloride (CsPbCl${_3}$) perovskites requires accurate values of their optical constants. Unfortunately, experimental data for this material is very limited thus far. Therefore, here, we applied spectroscopic ellipsometry (SE) to measure the complex optical constants of thermally evaporated CsPbCl${_3}$ thin films with different thicknesses on Si/SiO${_2}$ substrates. The data were corroborated with scanning electron microscopy (SEM) images and absorption spectroscopy. An optical dispersion model was developed to derive the complex optical constants and film thicknesses. The Tauc-Lorentz model, in conjunction with two harmonic oscillators, was used to extract the required parameters. The extinction coefficient spectrum exhibited a sharp absorption edge at 411 nm, consistent with the absorption spectrum. In addition, the optical bandgap of the film was calculated from the absorption spectra and SE data. The experimental values agree well with the simulation results, with values of $\sim$ 2.99 eV for different film thicknesses. This work provides fundamental information for designing and modeling CsPbCl3-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Unisolvence of Kansa collocation for elliptic equations by polyharmonic splines with random fictitious centers

We make a further step in the unisolvence open problem for unsymmetric Kansa collocation, proving nonsingularity of Kansa matrices with polyharmonic splines and random fictitious centers, for second-order elliptic equations with mixed boundary conditions. We also show some numerical tests, where the fictitious centers are local random perturbations of predetermined collocation points.

math.NA↗

arXiv2024

New boundes for Sombor index of Graphs

In this paper, we find some bounds for the Sombor index of the graph G by triangle inequality, arithmetic index, geometric index, forgotten index (F(G)), arithmetic-geometric (AG) index, geometric-arithmetic (GA) index, symmetric division deg index (SDD(G)) and some central and dispersion indices. The bounds could state estimated values and error intervals of the Sombor index to show limits of accuracy. The error intervals are written as inequalities.

math.CO↗

arXiv2021

A shape preserving quasi-interpolation operator based on a new transcendental RBF

It is well-known that the univariate Multiquadric quasi-interpolation operator is constructed based on the piecewise linear interpolation by |x|. In this paper, we first introduce a new transcendental RBF based on the hyperbolic tangent function as a smooth approximant to f(r)=r with higher accuracy and better convergence properties than the multiquadric. Then Wu-Schaback's quasi-interpolation formula is rewritten using the proposed RBF. It preserves convexity and monotonicity. We prove that the proposed scheme converges with a rate of O(h^2). So it has a higher degree of smoothness. Some numerical experiments are given in order to demonstrate the efficiency and accuracy of the method.

math.NA↗

arXiv2017

Glass transition temperature of PMMA/modified alumina nanocomposite: Molecular dynamic study

In this study, the effect of alumina and modified alumina nanoparticles in a PMMA/alumina nanocomposite was investigated. To attain this goal, the glass transition behavior of poly methyl methacrylate (PMMA), PMMA/alumina and PMMA/hydroxylated alumina nanocomposites were investigated by molecular dynamic simulations (MD). All the MD simulations were performed using the Materials Studio 6.0 software package of Accelrys. To obtain the glass transition temperature, the variation of density vs. temperature was obtained. The temperature at which the slope of the density-temperature curve observably changes is defined as the glass transition temperature (Tg). The effect of alumina nanoparticles on the Tg was related to the free volume and the mobility of chain segments and the interaction between the alumina nanoparticles and the polymer. The mobility of the chain segments was investigated based on the mean square displacement and radius gyration. The results show that the increasement the Tg of the PMMA/hydroxylated alumina nanocomposite is more than that of the PMMA/alumina nanocomposite due to the modification of the alumina nanoparticles.

cond-mat.mtrl-sci↗

arXiv2017

Thermal diffusivity of PMMA/Alumina Nano Composites Using Molecular Dynamic Simulation

Fire protection mainly takes place through physical or chemical pathways or both of them. The thermal diffusivity is one of the basic thermophysical properties which can connect the chemical structure. It is well-known in the literature that there exist relationships between the thermal diffusivity and the thermal stability. Also, the thermal diffusivity can be related to some fire retardant properties such as total-heat-release (THR), time-to-ignition (TTI) and peak-heat-rare-release (PHRR) that are of the most important parameters in the evaluation of potential fire hazard of a given material. Metal oxides, as one of the most promising flame retardant additives, improve the fire-retardant and the thermal stability properties of polymers. In the present study, molecular dynamic (MD) simulations based on the united atom model are utilized to study the effect of alumina nanoparticles on the thermal diffusivity of isotactic Polymethyl methacrylate (is-PMMA) polymer.

physics.comp-ph↗

arXiv2016

On the Fractional Derivatives of Radial Basis Functions

The paper provides the fractional integrals and derivatives of the Rie\-mann-Liouville and Caputo type for the five kinds of radial basis functions (RBFs), including the powers, Gaussian, multiquadric, Matern and thin-plate splines, in one dimension. It allows to use high order numerical methods for solving fractional differential equations. The results are tested by solving two fractional differential equations. The first one is a fractional ODE which is solved by the RBF collocation method and the second one is a fractional PDE which is solved by the method of lines based on the spatial trial spaces spanned by the Lagrange basis associated to the RBFs.

math.NA↗

arXiv2012

A Mixed Integer Programming Model Formulation for Solving the Lot-Sizing Problem

This paper addresses a mixed integer programming (MIP) formulation for the multi-item uncapacitated lot-sizing problem that is inspired from the trailer manufacturer. The proposed MIP model has been utilized to find out the optimum order quantity, optimum order time, and the minimum total cost of purchasing, ordering, and holding over the predefined planning horizon. This problem is known as NP-hard problem. The model was presented in an optimal software form using LINGO 13.0.

math.OC↗