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Namitha Anna Koshi

Publications and source records attributed to Namitha Anna Koshi.

11 recordsLinked to original sources

Integrating Density Functional Theory with Deep Neural Networks for Accurate Voltage Prediction in Alkali-Metal-Ion Battery Materials

Accurate prediction of the voltage of battery materials plays a pivotal role in the advancement of energy storage technologies and the rational design of high-performance cathode materials. In this work, we present a deep neural network (DNN) model, built using PyTorch, to estimate the average voltage of cathode materials across Li-ion, Na-ion, and other alkali-metal-ion batteries. The model is trained on an extensive dataset from the Materials Project, incorporating a wide range of specific structural, physical, chemical, electronic, thermodynamic, and battery descriptors, ensuring a comprehensive representation of material properties. Our model exhibits strong predictive performance, as corroborated by first-principles density functional theory (DFT) calculations. The close alignment between the DNN predictions and the DFT outcomes highlights the robustness and accuracy of our machine learning framework to effectively select and identify viable battery materials. Using this validated model, we successfully proposed novel Na-ion battery compositions, with their predicted behavior confirmed by rigorous computational assessment. By seamlessly integrating data-driven prediction with first-principles validation, this study presents an effective framework that significantly accelerates the discovery and optimization of advanced battery materials, contributing to the development of more reliable and efficient energy storage technologies.

cond-mat.mtrl-sci

Multi-site mixing and entropy stabilization of CsPbI$_{3}$ with potential application in photovoltaics

Metal halide perovskite solar cells have achieved dramatic improvements in their power conversion efficiency in the recent past. Since compositional engineering plays an important role in optimizing material properties, we investigate the effect of alloying at Cs and Pb sites on the energetics and electronic structure of CsPbI$_{3}$ using cluster expansion method in combination with first-principles calculations. For Ge-mixing at Pb-site, the $α$ and $β$-phases are considered with emphasis on the electronic structure, transition probability, absorption coefficient, efficiency, and carrier mobility of higher-symmetry configurations. CsPb$_{0.50}$Ge$_{0.50}$I$_{3}$ (Cs$_{2}$PbGeI$_{6}$) which takes up a double perovskite (elpasolite) structure has a direct band gap with no parity-forbidden transitions. Further, we utilize the alloy entropic effect to improve the material stability and optoelectronic properties of CsPbI$_{3}$ by multi-element mixing. For the proposed mixed compositions, the Fr{ö}hlich electron-phonon coupling constant is determined. Scattering rates and electron mobility are obtained from first-principles inputs. These lower Pb-content inorganic perovskites offer great promise as efficient solar cell materials for photovoltaic applications.

cond-mat.mtrl-sci

Customizing PBE Exchange-Correlation functionals: A comprehensive approach for band gap prediction in diverse semiconductors

Accurate band gap prediction in semiconductors is crucial for materials science and semiconductor technology advancements. This paper extends the Perdew-Burke-Ernzerhof (PBE) functional for a wide range of semiconductors, tackling the exchange and correlation enhancement factor complexities within Density Functional Theory (DFT). Our customized functionals offer a clearer and more realistic alternative to DFT+U methods, which demand large negative U values for elements like Sulfur (S), Selenium (Se), and Phosphorus (P). Moreover, these functionals are more cost-effective than GW or Heyd-Scuseria-Ernzerhof (HSE) hybrid functional methods, therefore, significantly facilitating the way for unified workflows in analyzing electronic structure, dielectric constants, effective masses, and further transport and elastic properties, allowing for seamless calculations across various properties. We point out that such development could be helpful in the creation of comprehensive databases of band gap and dielectric properties of the materials without expensive calculations. Furthermore, for the semiconductors studied, we show that these customized functionals and the Strongly Constrained and Appropriately Normed semilocal density functional (SCAN) perform similarly in terms of the band gap.

cond-mat.mtrl-sci

Tuning the Electronic and Magnetic Properties of Double Transition Metal MCrCT$_2$ (M = Ti, Mo) Janus MXenes for Enhanced Spintronics and Nanoelectronics

Janus MXenes, a new category of two-dimensional (2D) materials, shows promising potential for advances in optoelectronics, spintronics and nanoelectronics. Our theoretical investigations not only provide interesting insights but also highlight the promise of Janus MCrCT$_2$ (M = Ti, Mo; T = O, F, OH) MXenes for future spintronic applications and highlight the need for their synthesis. Electronic structure analysis shows different metallic and semi-metallic properties: MoCrCF$_2$ exhibits metallic properties, TiCrC(OH)$_2$ and MoCrCO$_2$ exhibit near semi-metallicity with spin polarization values of 61\% and 86\%, respectively, while TiCrCO$_2$ and TiCrCF$_2$ are completely half-metallic with 100\% spin polarization at the Fermi level. All studied Janus MXenes exhibit intrinsic ferromagnetism, which is mainly attributed to the chromium (Cr) atoms, as shown by the spin density difference plots. Among them, the TiCrCO$_2$ monolayer stands out with the highest exchange constant and ferromagnetic transition temperature (T$_c$). Notably, the O-terminated Janus MXenes exhibit weak perpendicular magnetic anisotropy, in contrast to the in-plane anisotropy observed for F and OH-terminated MXenes, making them particularly interesting for future spintronic applications which we further demonstrate with micromagnetic simulation which reveal distinct current-induced switching behaviors in these Janus MXenes with different surface terminations.

cond-mat.mtrl-sci

Straintronics using the monolayer-Xene platform -- a comparative study

Monolayer silicene is a front runner in the 2D-Xene family, which also comprises germanene, stanene, and phosphorene, to name a few, due to its compatibility with current silicon fabrication technology. Here, we investigate the utility of 2D-Xenes for straintronics using the ab-initio density functional theory coupled with quantum transport based on the Landauer formalism. With a rigorous band structure analysis, we show the effect of strain on the K-point, and calculate the directional piezoresistances for the buckled Xenes as per their critical strain limit. Further, we compare the relevant gauge factors, and their sinusoidal dependences on the transport angle akin to silicene and graphene. The strain-insensitive transport angles corresponding to the zero gauge factors are 81 degree and 34 degree for armchair and zigzag strains, respectively, for silicene and germanene. For stanene as the strain limit is extended to 10% and notable changes in the fundamental parameters, the critical angle for stanene along armchair and zigzag directions are 69 degree and 34 degree respectively. The small values of gauge factors are attributed to their stable Dirac cones and strain-independent valley degeneracies. We also explore conductance modulation, which is quantized in nature and exhibits a similar pattern with other transport parameters against a change in strain. Based on the obtained results, we propose the buckled Xenes as an interconnect in flexible electronics and are promising candidates for various applications in straintronics.

cond-mat.mes-hall

Iridium-doping as a strategy to realize visible light absorption and p-type behavior in BaTiO3

BaTiO3 is typically a strong n-type material with tuneable optoelectronic properties via doping and controlling the synthesis conditions. It has a wide band gap that can only harness the ultraviolet region of the solar spectrum. Despite significant progress, achieving visible-light absorbing BTO with tuneable carrier concentration has been challenging, a crucial requirement for many applications. In this work, a p-type BTO with visible-light absorption is realized via iridium doping. Detailed analysis using advanced spectroscopy tools and computational electronic structure analysis is used to rationalize the n- to p-type transition after Ir doping. Results offered mechanistic insight into the interplay between the dopant site occupancy, the dopant position within the band gap, and the defect chemistry affecting the carrier concentration. A decrease in the Ti3+ donor levels concentration and the mutually correlated oxygen vacancies upon Ir doping is attributed to the p-type behavior. Due to the formation of Ir3+ or Ir4+ in-gap energy levels within the forbidden region, the optical transition can be elicited from or to such levels resulting in visible-light absorption. This newly developed Ir-doped BTO can be a promising p-type perovskite-oxide with imminent applications in solar fuel generation, spintronics and optoelectronics.

cond-mat.mtrl-sci

High-frequency complex impedance analysis of the two-dimensional semiconducting MXene-$Ti_2CO_2$

The two-dimensional compound group of MXenes, which exhibit unique optical, electrical, chemical, and mechanical properties, are an exceptional class of transition metal carbides and nitrides. In addition to traditional applications in Li-S, Li-ion batteries, conductive electrodes, hydrogen storage, and fuel cells, the low lattice thermal conductivity coupled with high electron mobility in the semiconducting oxygen-functionalized MXene-$Ti_2CO_2$ has led to the recent interests in high-performance thermoelectric and nanoelectronic devices. Apart from the above dc-transport applications, it is crucial to also understand ac-transport across them, given the growing interest in applications surrounding wireless communications and transparent conductors. In this work, we investigate using our recently developed $ab~initio$ transport model, the real and imaginary components of electron mobility and conductivity to conclusively depict carrier transport beyond the room temperature for frequency ranges upto the terahertz range. We also contrast the carrier mobility and conductivity with respect to the Drude's model to depict its inaccuracies for a meaningful comparison with experiments. Our calculations show the effect of acoustic deformation potential scattering, piezoelectric scattering, and polar optical phonon scattering mechanisms. Without relying on experimental data, our model requires inputs calculated from first principles using density functional theory. Our results set the stage for providing ab-initio based ac-transport calculations given the current research on MXenes for high frequency applications.

cond-mat.mtrl-sci

Can magneto-transport properties provide insight into the functional groups in semiconducting MXenes?

The Hall scattering factor of Sc2CF2, Sc2CO2 and Sc2C(OH)2 is calculated using Rode's iterative approach by solving the Boltzmann transport equation. This is carried out in conjunction with calculations based on density functional theory. The electrical transport in Sc2CF2, Sc2CO2, and Sc2C(OH)2 is modelled by accounting for both elastic (acoustic and piezoelectric) and inelastic (polar optical phonon) scattering. Polar optical phonon (POP) scattering is the most significant mechanism in these MXenes. We observe that there is a window of carrier concentration where Hall factor acts dramatically; Sc2CF2 obtains an incredible high value of 2.49 while Sc2CO2 achieves a very small value of approximately 0.5, and Sc2C(OH)2 achieves the so called ideal value of 1. We propose in this paper that such Hall factor behaviour has significant promise in the field of surface group identification in MXenes, an issue that has long baffled researchers.

cond-mat.mtrl-sci

Electrical and magneto transport in 2D semiconducting MXene Ti2CO2

The Hall scattering factor is formulated using Rode's iterative approach to solving the Boltzmann transport equation in such a way that it may be easily computed within the scope of ab-inito calculations. Using this method in conjunction with density functional theory based calculations, we demonstrate that the Hall scattering factor in electron-doped Ti2CO2 varies greatly with temperature and concentration, ranging from 0.2 to around 1.3 for weak magnetic fields. The electrical transport was modelled primarily using three scattering mechanisms: piezoelectric scattering, acoustic scattering, and polar optical phonons. Even though the mobility in this material is primarily limited by acoustic phonons, piezoelectric scattering also plays an important role which was not highlighted earlier.

cond-mat.mtrl-sci

Silicene for flexible electronics

The outstanding properties of graphene have laid the foundation for exploring graphene-like two-dimensional systems, commonly referred to as 2D-Xenes. Amongst them, silicene is a front-runner owing to its compatibility with current silicon fabrication technologies. Recent works on silicene have unveiled its useful electronic and mechanical properties. The rapid miniaturization of silicon devices and the useful electro-mechanical properties of silicene necessitates the exploration for potential applications of silicene flexible electronics in the nano electro-mechanical systems. Using a theoretical model derived from the integration of \textit{ab-initio} density-functional theory and quantum transport theory, we investigate the piezoresistance effect of silicene in the nanoscale regime. Like graphene, we obtain a small value of piezoresistance gauge factor of silicene, which is sinusoidally dependent on the transport angle. The small gauge factor of silicene is attributed to its robust Dirac cone and strain-independent valley degeneracy. Based on the obtained results, we propose to use silicene as an interconnect in flexible electronic devices and a reference piezoresistor in strain sensors. This work will hence pave the way for exploring flexible electronics applications in other 2D-Xene materials.

cond-mat.mes-hall

Probing Photo-excited Charge Carrier Trapping and Defect Formation in Synergistic Doping of SrTiO3

Strontium titanate (SrTiO3) is widely used as a promising photocatalyst due to its unique band edge alignment with respect to the oxidation and reduction potential corresponding to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, further enhancement of the photocatalytic activity in this material could be envisaged through the effective control of oxygen vacancy states. This could substantially tune the photoexcited charge carrier trapping under the influence of elemental functionalization in SrTiO3, corresponding to the defect formation energy. The charge trapping states in SrTiO3 decrease through the substitutional doping in Ti sites with p-block elements like Aluminium (Al) with respect to the relative oxygen vacancies. With the help of electronic structure calculations based on density functional theory (DFT) formalism, we have explored the synergistic effect of doping with both Al and Iridium (Ir) in SrTiO3 from the perspective of defect formation energy, band edge alignment and the corresponding charge carrier recombination probability to probe the photoexcited charge carrier trapping that primarily governs the photocatalytic water splitting process. We have also systematically investigated the ratio-effect of Ir:Al functionalization on the position of acceptor levels lying between Fermi and conduction band in oxygen deficient SrTiO3, which governs the charge carrier recombination and therefore the corresponding photocatalytic efficiency.

cond-mat.mtrl-sci