MagGen: A Graph-Aided Deep Generative Model for Inverse Design of Permanent Magnets.

A deep generative model based on a variational autoencoder (VAE), conditioned simultaneously by two target properties, is developed to inverse design stable magnetic materials. The structure of the physics-informed, property embedded latent space of the model is analyzed using graph theory. An impressive ∼96% of the generated materials are found to satisfy the target properties as per predictions from the target-learning branches. This is a huge improvement over approaches that do not condition the VAE latent space by target properties or that do not consider the connectivity of the parent materials from which the new materials are generated. This impressive feat is achieved by using a simple real-space-only representation that can be directly read from material cif files. Model predictions are finally validated by density functional theory calculations on a randomly chosen subset of materials. The performance of the present model is comparable or superior to that of models reported earlier. This model (MagGen) is applied to the problem of designing rare earth-free permanent magnets with promising results.

Computational screening of layered metal chalcogenide materials for HER electrocatalysts, and its synergy with experiments.

Layered materials have emerged as attractive candidates in our search for abundant, inexpensive and efficient hydrogen evolution reaction (HER) catalysts, due to larger specific area these offer. Among these, transition metal dichalcogenides have been studied extensively, while ternary transition metal tri-chalcogenides have emerged as promising candidates recently. Computational screening has emerged as a powerful tool to identify the promising materials out of an initial set for specific applications, and has been employed for identifying HER catalysts also. This article presents a comprehensive review of how computational screening studies based on density functional calculations have successfully identified the promising materials among the layered transition metal di- and tri-chalcogenides. Synergy of these computational studies with experiments is also reviewed. It is argued that experimental verification of the materials, predicted to be efficient catalysts but not yet tested, will enlarge the list of materials that hold promise to replace expensive platinum, and will help ushering in the much awaited hydrogen economy.

From the single-atom limit to the mixed-metal phase: finding the optimum condition for activating the basal plane of a FePSe3 monolayer towards HER.

Layered ternary transition metal tri-chalcogenides are some of the most promising candidates for hydrogen evolution reaction (HER) because of their ease of synthesis and affordability. However, majority of the materials in this category have HER active sites only at their edges, rendering a large portion of the catalyst useless. In this work, ways for activating the basal planes of one of these materials, namely, FePSe3, are explored. The effects of substitutional transition metal doping and external biaxial tensile strain on the HER activity of the basal plane of a FePSe3 monolayer are studied via first principles electronic structure calculations based on density functional theory. This study reveals that although the basal plane of the pristine material is inactive towards HER (value of H adsorption free energy, ΔGH* = 1.41 eV), 25% Zr, Mo, and Tc doping makes it more active (ΔGH* = 0.25, 0.22 and 0.13 eV, respectively). The effect of reducing the doping concentration, moving to the single-atom limit, on the catalytic activity is studied for Sc, Y, Zr, Mo, Tc and Rh dopants. For Tc, the mixed-metal phase FeTcP2Se6 is also studied. Among the unstrained materials, 25% Tc-doped FePSe3 gives the best result. Significant tunability of HER catalytic activity in the 6.25% Sc doped FePSe3 monolayer via strain engineering is also discovered. An external tensile strain of 5% reduces ΔGH* to ∼0 eV from 1.08 eV in the unstrained material, making this an attractive candidate for HER catalysis. The Volmer-Heyrovsky and Volmer-Tafel pathways are examined for some of the systems. A fascinating correlation between the electronic density of states and HER activity is also observed in most materials.

Polysaccharide Modified Cathodes for High-Capacity Rechargeable Nonaqueous Li-O2 Batteries.

Nonaqueous rechargeable Li-O2 batteries are recognized as possible alternatives to the currently established Li-ion battery technology for next-generation traction by virtue of their high specific energy. However, the technology is still far from commercial realization mainly due to the performance-limiting reactions at the cathode. The insulating discharge product, Li2O2, can passivate the cathode leading to issues such as low specific capacity and early cell death. Herein, the -OH functionalities at the cathode, incorporated by polysaccharide addition, are shown to enhance the discharge capacity and cyclability. The -OH functional group (high pKa) at the cathode helps to stabilize the intermediate, LiO2, via an energetically favorable pathway and delays the precipitation to Li2O2, without any parasitic reaction, unlike the other reported low pKa additives. The role of the functionalities is studied using various experimental techniques and first principles density functional theory based studies. This approach provides a rational design route for the cathodes that provide high capacities for the emergent Li-O2 batteries.

Role of water structure in alkaline water electrolysis.

Herein, with the help of experimental and first-principles density functional theory (DFT)-based studies, we have shown that structural changes in the water coordination in electrolytes having high alkalinity can be a possible reason for the reduced catalytic activity of platinum (Pt) in high pH. Studies with polycrystalline Pt electrodes indicate that electrocatalytic HER activity reduces in terms of high overpotential required, high Tafel slope, and high charge transfer resistances in concentrated aqueous alkaline electrolytes (say 6 M KOH) in comparison to that in low alkaline electrolytes (say 0.1 M KOH), irrespective of the counter cations (Na+, K+, or Rb+) present. The changes in the water structure of bulk electrolytes as well as that in electrode-electrolyte interface are studied. The results are compared with DFT-based analysis, and the study can pave new directions in studying the HER process in terms of the water structure near the electrode-electrolyte interface.

Ionization Energies and Ground-State Structures of Neutral Lan (n = 2-14) Clusters: A Combined Experimental and Theoretical Investigation.

Neutral lanthanum clusters are studied by photoionization time-of-flight mass spectroscopy, laser threshold photoionization spectroscopy, and density functional theory (DFT). Mass abundance spectra (MS) registered at multiple photoionization wavelengths in the range of 195-230 nm by single photon ionization reveal the production of all sizes, Lan (n ≥ 50), in good abundance, nullifying previously predicted low abundances for certain sizes in the 3-14 size range. Also, the MS do not reveal the extraordinary stability of any specific size, as one would expect, from previous theoretical predictions of 7- and 13-atom clusters as magic. Ionization energies (IEs) are measured for Lan (n = 2-14) clusters. DFT has been used to determine the stable geometric isomers for 2- to 10-atom clusters and to calculate their IEs. The theoretical IEs of 2-7 atom clusters are in decent agreement with their experimental values; however, the theoretical IEs are somewhat lower by ∼0.4 eV for n ≥ 8 than their experimental IEs.

Finding the catalytically active sites on the layered tri-chalcogenide compounds CoPS3 and NiPS3 for hydrogen evolution reaction.

Electronic structure calculations based on density functional theory are used to identify the catalytically active sites for the hydrogen evolution reaction on single layers of the two transition metal tri-chalcogenide compounds CoPS3 and NiPS3. Some of the under-coordinated P and S atoms at the edges are found to act as the active sites, the details of which depend on the coverage of H on the electrode. Overpotentials along the two possible pathways for HER are also estimated for the two materials. These findings not only resolve an apparent discrepancy between published experimental results and our earlier calculations, but also provide insights which can be used to enhance catalytic efficiency of these materials further.

Extending the Cyclability of Alkaline Zinc-Air Batteries: Synergistic Roles of Li+ and K+ Ions in Electrodics.

Tweaking the electrolyte of the anode compartment of zinc-air battery (ZAB) system is shown to be extending the charge-discharge cyclability of the cell. An alkaline zinc (Zn)-air cell working for ∼32 h (192 cycles) without failure is extended to >55 h (>330 cycles) by modifying the anode compartment with a mixture electrolyte of KOH and LiOH. The cell containing the mixture electrolyte has a low overpotential for charging along with high discharge capacity. The role of Li+ ions in tuning the electrode morphology and electrodics is studied both theoretically and experimentally. The synergistic effect of Li+ and K+ ions in the electrolyte on improved ZAB performance is proven. This study can pave new ways for the commercial implementation of ZAB, where it has already proven its potential in low-cost, high energy density, and mobility applications.

Exploring a low temperature glassy state, exchange bias effect, and high magnetic anisotropy in Co2C nanoparticles.

It is interesting to explore the connections between the exchange bias effect (EBE) and magnetic anisotropy (MA). It is often found that materials exhibiting a strong EBE also have enhanced MA. Here we explore 40 nm diameter Co2C nanoparticles (NPs) that exhibit ferromagnetism with a blocking temperature exceeding 300 K. We report the first observation of EBE in these Co2C NPs below 50 K. The effect arises from the exchange coupling of frozen ferromagnetic spins with a freely rotatable spin component. The dynamics of the freely rotatable component freezes in a temperature range between 5 K to 20 K resulting in low-temperature coexistence of a glassy behavior along with ferromagnetism. In fact, Co2C displays a unique separation of onset temperatures of spin freezing (∼20 K), vanishing of EBE (∼50 K), and magnetic blocking (⩾450 K). Our calculations show that Co2C NPs have a core-shell structure. Our study suggests that modifying chemical co-ordination in the shell is one of the effective routes to manipulating MA compared to manipulating EBE.

Catalytic properties of α-MnO2 for Li-air battery cathodes: a density functional investigation.

Details of the formation and dissociation of the first layer of Li2O2 on the α-MnO2(100) surface as the cathode in Li-air batteries have been studied using first principles density functional theory. The bias dependence of the electrochemical steps of charge (Li2O2 dissociation) and discharge (Li2O2 formation) via two different mechanisms has been studied. Discharge potential is found to be 2.94 V for the mechanism in which O2 adsorption is followed by lithiation. Charging potential for the reverse process is 3.37 V, giving an overpotential of 0.43 V, which is much lower than that on carbon electrodes. This is also in good agreement with experiments on α-MnO2 cathodes. In Li2O2 formation via the disproportionation of two LiO2 adsorbates, a maximum discharge potential of 2.61 V and a minimum charging potential of 3.48 V are obtained. The minimum energy pathway in this mechanism has a moderate kinetic barrier of 0.57 eV. Charging potentials of 3.37 V and 3.48 V imply that the typical charging potentials applied in the experiments (∼3.8 V) will dissociate the entire Li2O2 layer. These findings explain why α-MnO2 performs so well as a catalyst in Li-air battery cathodes, and suggest that a larger area of α-MnO2(100) can help reduce capacity loss.

Combinatorial Design and Computational Screening of Two-Dimensional Transition Metal Trichalcogenide Monolayers: Toward Efficient Catalysts for Hydrogen Evolution Reaction.

Recent experiments showed that some layered ternary transition metal trichalcogenide compounds are efficient catalysts for the hydrogen evolution reaction (HER). Motivated by these, we have combinatorially designed and computationally screened, through an efficient, automated approach based on density functional theory, single layers of such compounds, including those not reported in widely used crystal structure database like the International Crystal Structure Database (ICSD), for their efficiency as HER catalysts. On the basis of our theoretical prediction of overpotentials determined from the reaction coordinate mapping corresponding to the HER mechanism, 13 of these compounds are found to be promising catalysts, out of which three are suggested to be as efficient as platinum, the best known HER catalyst to date.

A method to detect immunoreactions on the basis of current vs. concentration slope - an electrochemical approach.

The emergence of novel infectious diseases is rising with time and is a major threat to the society. The recent outbreak of infectious coronavirus disease has made a huge impact in our lives. The massive outbreak of the disease revealed that there is room for development of new diagnostics tools and methods to screen huge numbers of samples in the shortest possible time. Our current work relates to an electronic diagnostic system and method that rapidly detects the presence of an antigen in solution. Our designed system is capable of separating the immunocomplex formation on the basis of the slope it produces in contrast to the controls, when oxidation peak current is plotted against the concentration of the reactant after electrochemistry measurement. In this system, antibody conjugated copper nanoparticles synthesized by the electroexplosion method has played the key role. The values of the slopes of copper nanoparticles (CuNPs) was found to be -3.7637, whereas those for CuNP conjugated Antibody and CuNPAntibodyAntigen were -2.3044 and -0.8332, respectively. Hence, the current method could become one of the easiest and fastest method for the electronic detection of an immune reaction and a good replacement for the time-consuming, label-based assays in multistep reaction.

Designing rare earth free permanent magnets: insights from small Co clusters.

With the ultimate goal of rational design of permanent magnets without rare earth elements, we study the magnetic properties of Co4 clusters doped with six different group 14 and group 15 elements, using first principles electronic structure methods, based on density functional theory. For the first time, low energy isomers and magnetic moments of pure Co4 and Co6 clusters, and the doped clusters are identified through a global search employing an evolutionary algorithm. The magnetic anisotropy energy (MAE) of these clusters is then calculated. No correlation is found between the type of dopant atom and MAE. Through further analyses we establish that MAE is largely determined by the sum of absolute values of the spin moments on the individual atoms, and the HOMO-LUMO gap of the system. The importance of these in designing new permanent magnetic materials is discussed.

Synthesis, docking, and preliminary in vitro/in vivo evaluation of MPP-dithiocarbamate-capped silver nanoparticle as dual-imaging agent for 5HT1A.

Methoxyphenyl piperazine is a versatile pharmacophore and has been exploited for targeting 5HT1A receptors. In the present study, silver nanoparticles were conjugated (capped) with methoxyphenyl piperazine-dithiocarbamate for application as targeted optical imaging agent at extremely low detection limits. Our results demonstrate an easy synthesis of the ligand methoxyphenyl piperazine-dithiocarbamate and silver nanoparticles and their conjugation was free from extraneous impurities.

Bimetallic Ag-Pt Sub-nanometer Supported Clusters as Highly Efficient and Robust Oxidation Catalysts.

A combined experimental and theoretical investigation of Ag-Pt sub-nanometer clusters as heterogeneous catalysts in the CO→CO2 reaction (COox) is presented. Ag9 Pt2 and Ag9 Pt3 clusters are size-selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first-principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano-aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O2 , and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.

A density functional study of silver clusters on a stepped graphite surface: formation of self-assembled nano-wires.

Adsorption and diffusion of silver adatoms and clusters containing up to eight atoms on an HOPG substrate with an armchair step are studied using density functional methods. Step edges act as attractive sinks for adatoms and clusters. The diffusion barrier of an Ag adatom along the step edge is much larger than that on a clean terrace. At zero temperature, Ag clusters either distort or dissociate by forming covalent bonds with the edge C atoms. At 600 K, Ag5 and Ag8 clusters diffuse to the step edges, and then break up so as to maximize Ag-C bonds. The Ag atoms try to form a nanowire structure along the step edge. At such high temperatures, diffusion of clusters along the step edge involves diffusion of individual Ag atoms not bonded to the edge C atoms. Assumption of complete immobility of clusters trapped at step edges in the Gates-Robins model is not valid at high temperatures in this particular system.

Do Ag(n) (up to n = 8) clusters retain their identity on graphite? Insights from first-principles calculations including dispersion interactions.

Adsorption of pre-formed Agn clusters for n = 1 - 8 on a graphite substrate is studied within the density functional theory employing the vdW-DF2 functional to treat dispersion interactions. Top sites above surface layer carbon atoms turn out to be most favorable for a Ag adatom, in agreement with experimental observations. The same feature is observed for clusters of almost all sizes which have the lowest energies when the Ag atoms are positioned over top sites. Most gas phase isomers retain their structures over the substrate, though a couple of them undergo significant distortions. Energetics of the adsorption can be understood in terms of a competition between energy cost of disturbing Ag-Ag bonds in the cluster and energy gain from Ag-C interactions at the surface. Ag3 turns out to be an exceptional candidate in this regard that undergoes significant structural distortion and has only two of the Ag atoms close to surface C atoms in its lowest energy structure.

Nature of valence transition and spin moment in Ag(n)V(+) clusters.

Evolution in the atomic structure, bonding characteristics, stability, and the spin magnetic moment of neutral and cationic AgnV clusters has been investigated using first-principles density functional approach with gradient corrected functional. It is shown that at small sizes, the V 4s states hybridize with Ag states to form 1S and 1P like superatomic orbitals, whereas the 3d states are localized on V giving the V atom an effective valence of 1 or 2. Starting from Ag8V(+), the V 3d states begin to participate in the bonding by hybridizing with the nearly free electron gas to form 1D superatomic orbitals increasing the V atom effective valence toward 5. For the cationic clusters, this changing valence results in three shell closures that lead to stable species. These occur for cationic clusters containing 5, 7, and 14 Ag atoms. The first two stable species correspond to filled 1S and 1P shells in two and three dimensions with a valence of 2 for V, whereas the closure at 14 Ag atoms correspond to filled 1S, 1P, and 1D shells with V site exhibiting a valence of 5. The transition from filled 1S and 1P shells to filled 1S, 1P, and 1D shells is confirmed by a quenching of the spin magnetic moment. The theoretical findings are consistent with the observed drops in intensity in the mass spectrum of AgnV(+) clusters after 5, 7, and 14 Ag atoms.

Structural, electronic and magnetic properties of binary transition metal aluminum clusters: absence of electronic shell structure.

Single Cr, Mn, Fe, Co and Ni doped Al clusters having up to 12 Al atoms are studied using density functional methods. The global minima of structure for all the clusters are identified, and their relative stability and electronic and magnetic properties are studied. FeAl4 and CoAl3 are found to have enhanced stability and aromatic behavior. In contrast to binary transition metal alkali and transition metal alkaline earth clusters, spherical shell models cannot describe the electronic structure of transition metal aluminum clusters.

Density functional study of structural defects in h-BNC2 sheets.

Structure, energetics, electronic and magnetic properties of single and double vacancies and Stone-Wales defects in h-BNC(2) sheets have been calculated using the planewave pseudopotential method within density functional theory. The formation energy of a defect strongly depends on its location within the sheet. In some cases, though not all, the energy ordering of various defects can be rationalized in terms of the strengths of various bonds that are broken or created during the defect formation. Single vacancy defects have rather low migration barriers, and the energy cost of double vacancies is smaller than that of two isolated single vacancies. Barriers of formation for Stone-Wales defects at the interfaces are large, but those for healing these defects are quite small. Therefore, they can heal easily even at moderate temperatures. Thus, double vacancies are the most likely defect structures in these sheets. Many of the defects possess finite magnetic moments. Unlike BN sheets and graphene, some of the double vacancies and Stone-Wales defects are also found to possess finite moment.