Structure and electronic properties of transition-metal/Mg bimetallic clusters at realistic temperatures and oxygen partial pressures

Composition, atomic structure, and electronic properties of TM$_x$Mg$_y$O$_z$ clusters (TM = Cr, Ni, Fe, Co, $x+y \leq 3$) at realistic temperature $T$ and partial oxygen pressure $p_{\textrm{O}_2}$ conditions are explored using the {\em ab initio} atomistic thermodynamics approach. The low-energy isomers of the different clusters are identified using a massively parallel cascade genetic algorithm at the hybrid density-functional level of theory. On analyzing a large set of data, we find that the fundamental gap E$_\textrm{g}$ of the thermodynamically stable clusters are strongly affected by the presence of Mg-coordinated O$_2$ moieties. In contrast, the nature of the transition metal does not play a significant role in determining E$_\textrm{g}$. Using E$_\textrm{g}$ of a cluster as a descriptor of its redox properties, our finding is against the conventional belief that the transition metal plays the key role in determining the electronic and therefore chemical properties of the clusters. High reactivity may be correlated more strongly with oxygen content in the cluster than with any specific TM type.Comment: 7 pages, 5 figure

Micro-structural origin of elongation in swift heavy ion irradiated Ni nanoparticles: A combined EXAFS and DFT study

Aiming for perpendicular magnetic storage usage, shape anisotropy is introduced in Ni nanoparticles (NPs) embedded inside a thin SiO$_2$ matrix using swift heavy ion irradiation (SHI). Systematic increase in NPs' aspect ratio along the direction of incident SHI beam is observed up to 5 × 10$^{13}$ ions/cm2 (5e13) fluence from grazing incidence small angle X-ray scattering measurements. Strikingly, at higher fluences the major dimension (along SHI beam) got reduced. This observation is totally intriguing as usually particle elongation increases with applied fluence. To understand this anomaly, as a first step, we have performed a combined near and far edge X-ray absorption spectroscopy (XANES and EXAFS) analysis. This shows irradiated Ni NPs sustain their metallic phase even with increased structural disorder and reduced atomic co-ordination. However an atypical reduction in local structural anisotropy beyond 5e13 fluence is observed from angle dependent EXAFS: following the same trend as NP elongation. To have a better insight, the role of the electronic spin of individual atoms in controlling particle shape is investigated. Using the lattice temperature profiles derived from thermal spike model, we have carried out ab initio molecular dynamics (MD) simulations to understand the structural distortion at higher temperatures. The experimentally observed structural and shape anisotropy in the irradiated NPs is only realized when spin polarization contributions were considered in our calculations. From the pair correlation function and atom-wise spin density plots, we conclude that presence of spin affects the second co-ordination shell and controls the atomic arrangements in such a manner that elongated structures are preferred in the irradiated system at intermediate fluence (i.e. 5e13). Further increase in MD temperature to 6000 K (corresponding to 1 × 1014 ions/cm2 fluence) results in disordered spin alignment and melting of moderately weak spin polarized Ni NPs: thus violating conventional trend of increasing particle elongation w.r.t SHI fluence

Tuning LSPR of Thermal Spike-Induced Shape-Engineered Au Nanoparticles Embedded in Si3_3N4_4 Thin-Film Matrix for SERS Applications

While gold nanoparticles (Au NPs) are widely used as surface-enhanced Raman spectroscopy (SERS) substrates, their agglomeration and dynamic movement under laser irradiation result in the major drawback in SERS applications, viz., the repeatability of SERS signals. We tune the optical and structural properties of size- and shape-modified Au NPs embedded in a thin silicon nitride (Si$_3$N$_4$) matrix by intense electronic excitation with swift heavy ion (SHI) irradiation with the aim of overcoming this classical SERS disadvantage. We demonstrate the shape evolution of a single layer of Au NPs inserted between amorphous Si$_3$N$_4$ thin films under fluences of 120 MeV Au$^{9+}$ ions ranging between 1 × 10$^{11}$ and 1 × 10$^{13}$ ions cm$^{-2}$. This shape modification results in the gradual blue shift of the localized surface plasmon resonance (LSPR) dip until 1 × 10$^{12}$ ions/cm$^2$ and then a sudden diminishment at 1 × 10$^{13}$ ions/cm$^2$. Finite domain time difference (FDTD) simulations further justify our experimental optical spectra. The dynamical NP aggregation and dissolution, in addition to NP elongation and deformation at different fluences, are noted from 2D grazing incidence small-angle X-ray scattering (GISAXS) profiles, as well as cross-sectional transmission electron microscopy (X-TEM). The systematic shape evolution of metal NPs embedded in the insulating matrix is shown to be due to thermal spike-induced localized melting and a localized pressure hike upon SHI irradiation. Utilizing this specific control over the characteristics of Au NPs, viz., shape, size, interparticle gap, and corresponding optical response via SHI irradiation, we demonstrate their applications as very stable SERS substrates, where the separation between NPs and analyte does not alter under laser illumination. Thus, these irradiated SERS active substrates with controlled NP size and gap provide the optimal conditions for creating localized electromagnetic hotspots that amplify the SERS signals, which do not alter with time or laser exposure. We found that the film irradiated with 1 × 10$^{11}$ exhibits the highest SERS intensity due to its optimal NP size distribution and shape. Thus, not only our study provides a SERS substrate for stable and repeatable signals but also the understanding depicted here opens new research avenues in designing SERS substrates, photovoltaics, optoelectronic devices, etc. with ion beam irradiation

Electronic excitation induced structural modification of FeCo nanoparticles embedded in silica matrix

Electronic excitation mediated energy loss by 120 MeV Au9+ swift heavy ions (SHIs) results in significant structural modifications of FeCo nanoparticles embedded in thin SiO2 matrix. The variations in local atomic structure and particle size/shape at different irradiation fluences are probed by extended x-ray absorption fine structure spectroscopy (EXAFS) and grazing incidence small angle x-ray scattering (GISAXS). The crystallinity and ordering of the films are found to first decrease and then increase with fluence. The observed alterations in co-ordination number of Fe/Co from EXAFS are correlated primarily with particle size modifications due to a transient thermal spike generated in the embedded nanoparticles by SHIs. The role of hydrogen desorption from nanoparticles is also highlighted

Revealing the Formation Mechanism and Optimizing the Synthesis Conditions of Layered Double Hydroxides for the Oxygen Evolution Reaction

Layered double hydroxides (LDHs), whose formation is strongly related to OH- concentration, have attracted significant interest in various fields. However, the effect of the real-time change of OH- concentration on LDHs’ formation has not been fully explored due to the unsuitability of the existing synthesis methods for in situ characterization. Here, the deliberately designed combination of NH3 gas diffusion and in situ pH measurement provides a solution to the above problem. The obtained results revealed the formation mechanism and also guided us to synthesize a library of LDHs with the desired attributes in water at room temperature without using any additives. After evaluating their oxygen evolution reaction performance, we found that FeNi-LDH with a Fe/Ni ratio of 25/75 exhibits one of the best performances so far reported.publishe

Enhancement in field emission current density of Ni nanoparticles embedded in thin silica matrix by swift heavy ion irradiation

The field emission (FE) properties of nickel nanoparticles embedded in thin silica matrix irradiated with 100 MeV Au+7 ions at various fluences are studied here. A large increase in FE current density is observed in the irradiated films as compared to their as deposited counterpart. The dependence of FE properties on irradiation fluence is correlated with surface roughness, density of states of valence band and size distribution of nanoparticles as examined with atomic force microscope, X-ray photoelectron spectroscopy, and grazing incidence small angle x-ray scattering. A current density as high as 0.48 mA/cm2 at an applied field 15 V/μm has been found for the first time for planar field emitters in the film irradiated with fluence of 5.0 × 1013 ions/cm2. This significant enhancement in the current density is attributed to an optimized size distribution along with highest surface roughness of the same. This new member of field emission family meets most of the requirements of cold cathodes for vacuum micro/nanoelectronic device