Paper & Patents

 Chemisorption on ferromagnetic and non-magnetic surfaces is discussed within the Newns–Anderson–Grimley model along with the Stoner model of ferromagnetism. In the case of ferromagnetic surfaces, the adsorption energy is formulated in terms of the change in surface magnetic moments. Using such a formulation, we address the issue of how an adsorbate's binding strength depends on the magnetic moments of the surface and how the adsorption process reduces/enhances the magnetic moments of the surface. Our results indicate a possible scaling relationship of adsorption energy in terms of surface magnetic moments. In the case of non-magnetic surfaces, we formulate a modified Stoner criterion and discuss the condition for the appearance of magnetism due to chemisorption on an otherwise non-magnetic surface.

Structural studies of FeP powder is done using X-ray diffraction data. In addition to the pure hexagonal phase, we noticed the existence of an additional phase in pristine Fe,P powder. The additional phase is found to be Fe, P-type tetragonal phase as confirmed by two phases Rietveld analysis of XRD data of ball milled powder. The heat treatment method used to get homogeneous FeP phase which is hexagonal. The current findings are important as they can affect the physical properties such as magnetoelastic coupling and magnetocaloric effect.

We present a module to calculate the mobility and conductivity of semiconducting materials using Rode's algorithm. This module uses a variety of electronic structure inputs derived from the Density Functional Theory (DFT). We have demonstrated good agreement with experimental results for the case of Cadmium Sulfide (Cds). We also provide a comparison with the widely used method, the so-called relaxation time approximation (RTA) and demonstrated a favorable improvement of the results compared to RTA. The present version of the module is interfaced with the Vienna ab initio simulation package (VASP).

The scaling relationships between the adsorption energies of different reaction intermediates have a tremendous effect in the field of surface science, particularly in predicting new catalytic materials. In the last few decades, these scaling laws have been extensively studied and interpreted by a number of research groups which makes them almost universally accepted. In this work, we report the breakdown of the standard scaling law in magnetic bimetallic transition metal (TM) surfaces for hydrogenated species of oxygen (O), carbon (C), and nitrogen (N), where the adsorption energies are estimated using density functional theory (DFT). We propose that the scaling relationships do not necessarily rely solely on the adsorbates, they can also be strongly dependent on the surface properties. For magnetic bimetallic TM surfaces, the magnetic moment plays a vital role in the estimation of adsorption energy, and therefore towards the linear scaling relation.

Using machine learning (ML) approach, we unearthed a new III-V semiconducting material having an optimal bandgap for high-efficient photovoltaics with the chemical composition of Gallium-Boron-Phosphide (GaBP2, space group: Pna2). ML predictions are further validated by state-of-the-art ab initio density functional theory simulations. The stoichiometric Heyd-Scuseria-Ernzerhof bandgap of GaBP, is noted to be 1.65 eV, a close ideal value (1.4-1.5 eV) to reach the theoretical Queisser-Shockley limit. The calculated electron mobility is similar to that of silicon. Unlike perovskites, the newly discovered material is thermally, dynamically and mechanically stable. Above all the chemical composition of GaBP2 is nontoxic and relatively earth abundant, making it a new generation of PV material. Using ML, we showed that with a minimal set of features, the bandgap of III-III-V and II-IV-V semiconductor can be predicted up to an RMSE of less than 0.4 eV. We have presented a set of scaling laws, which can be used to estimate the bandgap of new III-III-V and II-IV-V semiconductor, with three different crystal phases, within an RMSE of 0.4 eV.

We propose transition metal substituted Fe2P as a new promising material for spin-transfer torque magnetic random-access memory (STT-MRAM) application. Using first-principles calculations based on density functional theory and Monte Carlo simulations, we demonstrate that this material can be used as a ferromagnetic electrode in the magnetic tunnel junction (MT) of STT-MRAM due to its moderate perpendicular magnetic anisotropy, high ferromagnetic transition temperature, and large tunnel magnetoresistance. This work is expected to provide a basis for the development of a new class of Fe2P-based electrode materials for STT-MRAM devices.

First-principles theoretical analysis of the catalytic activity of van der Waals hetero-structures of 1HMoS2 and graphene substituted with three chemical types of nitrogen species (1) Graphitic (G). (1) Pyridinic (Pn) and (iii) Pyrrolic (Pr), for application in catalysis of hydrogen evolution reaction (HER) has been presented. Graphitic and pyrrolic N substituents result in n-type electronic structure, whereas substitution of pyridinic Nimparts p-type electronic character to the hetero-structure. Work functions (v ) of the hetero-structures suggest that graphitic N-graphene:MoS2 hetero-structure (p = 3,8 eV) is expected to be effective in catalysing the reduction of Ht to evolve H2. 1H-MoS2 monolayer in the hetero-structure contributes by enabling increased H20 adsorption and offsetting the band edge energies optimal for the catalytic activity. Near optimum Gibbs free energy of H-adsorption (AG) were obtained for graphitic (AGH 0.29 eV) and pyrrolic (AGH -0.2 eV) N-graphene:MoS2 hetero structures. Our work showcases how catalytic and electronic properties of the N-doped graphene:MoS2 hetero-structure depends on the chemical identity of N-sites and uncovers a route to 2D heterostructures with high catalytic activity.

Iron sulfides are fascinating catalytic phases because these include S-modified Fe δ+ (δ ≤ 2) species functioning as H 2 O 2 activators to form [rad] OH used for oxidatively degrading aqueous contaminants (e.g., phenol). As an initial step for locating S-modified metal species (M δ+ ) that outperform Fe δ+ in catalytic H 2 O 2 cleavage, hexagonal metal sulfides (MS) were synthesized using Mn, Fe, Co, Ni, and Cu to understand electric potential-assisted H 2 O 2 scission kinetics on M δ+ species. Ni δ+ species were found to show the greatest [rad] OH productivity among all M δ+ species studied, mainly resulting from the Lewis acidic nature of Ni δ+ species adequate to expedite the liberation of [rad] OH species. This was partially evidenced by H 2 O 2 activation/phenol degradation runs on M δ+ species, wherein initial H 2 O 2 activation rate (-r H2O2,0 ) or initial phenol degradation rate (-r PHENOL,0 ) of Ni δ+ species was 3–9 times those of the other M δ+ species. Ni δ+ species, therefore, were located in the middle of the volcano-shaped curve plotting -r H2O2,0 (or -r PHENOL,0 ) versus the type of M δ+ . Kinetic assessment of M δ+ species under fine-tuned reaction environments also showed that regardless of varying H 2 O 2 concentrations, M δ+ species were found to retain their -r H2O2,0 values in the absence of electric potentials. Conversely, M δ+ species could enhance -r PHENOL,0 values at larger electric potentials, where greater energies were likely exerted on M δ+ species. This indeed corroborated that [rad] OH desorption from M δ+ species was the rate-determining step to direct catalytic H 2 O 2 scission. In addition to heterogeneous catalytic nature of Ni δ+ species in fragmenting H 2 O 2 , outstanding H 2 O 2 scission ability provided by Ni δ+ species could also compensate for their moderate catalytic stability at pH-neutral condition.

Magnetic exchange interactions in pure and vanadium (V)-doped Fe16N2 are studied within the framework of density functional theory (DFT). The Curie temperatures were obtained via both mean field approximation (MFA) and Monte Carlo (MC) calculations based on interactions that were obtained through DFT. The Curie temperature (TC) for pure Fe16N2 that was obtained under MFA is substantially larger than the experimental value, suggesting the importance of thermal fluctuations. At zero field, the calculated magnetic susceptibility shows a sharp peak at T = TC that corresponds to the presence of localized d-states. From the nature of the exchange interactions, we have determined the reason for the occurrence of the giant magnetic moment in this material, which remained a mystery for decades. Finally, we posit that Fe16N2 can also act as a satisfactory spin injector for III–V semiconductors, in addition to its application as a permanent magnet, since it has very high spin polarization (compared to elemental ferromagnets) and smaller lattice mismatch (compared to half-metallic Heusler alloys) with conventional III–V semiconductors such as GaAs and InGaAs. We demonstrate this application in the case of Fe16N2(001)/InGaAs(001) hetero-structures, which exhibit substantial spin polarization in the semiconductor (InGaAs) region. PACS number: 82.65.My, 82.20.Pm, 82.30.Lp, 82.65.Jv.

We present a detailed ab-initio study of semi-classical transport in n-ZnSe using Rode's iterative method. Inclusion of ionized impurity, piezoelectric, acoustic deformation and polar optical phonon scattering and their relative importance at low and room temperature for various n-ZnSe samples are discussed in depth. We have clearly noted that inelastic polar optical phonon scattering is the most dominant scattering mechanism over most of the temperature region. Our results are in good agreement with the experimental data for the mobility and conductivity obtained at different doping concentrations over a wider range of temperatures. Also we compare these results with the ones obtained with relaxation time approximation (RTA) which clearly demonstrate the superiority of the iterative method over RTA.