Evolution of the Magnetic and Optical Properties in CocoreAushell and (CoRh)(core) Au-shell Core-Shell Nanoparticles

The magnetic and optical properties of CocoreAushell and (CoRh)(core)AU(shell) core-shell nanoparticles (14 shells, face centered cubic structure, 321 atoms) are investigated using spin-polarized density functional theory in the generalized gradient approximation. For CocoreAushell the properties show a clear dependence on the size of the Co core. For (CoRh)(core)Au-shell we vary the Co:Rh composition. We show that the distribution of the magnetic moments within the nanoparticle depends on strongly/weakly on the chemical environment for the Co/Rh atoms. The optical absorption shows sensitivity to both composition and chemical arrangement. In particular, (CoRh)(core)Au-shell nanoparticles show a blue shift for increasing Co concentration, whereas (RhCo)(core)Au-shell nanoparticles show a red shift for increasing Co concentration. The investigated nanoparticles uniquely couple the magnetism of the core to the optical properties of the Au shell and thus pave the way to optical tracking

Localized surface plasmon resonances in a hybrid structure consisting of a mono-layered Al sheet and Ti3C2F MXene

MXenes are a novel class of two-dimensional materials that exhibit unique light-matter interactions. In this work, using quantum-mechanical simulations based on the time dependent density functional theory, we investigate the electronic and optical properties of a hybrid structure consisting of a mono-layered aluminum (Al) sheet and Ti3C2F MXene. As a key result of this work, we reveal that the coupling of a mono-layered Al sheet on top of Ti3C2F MXene causes interlayer charge transfer accompanied by strong signatures of localized surface plasmon resonances (LSPRs) in the visible region of the electromagnetic spectrum. Our theoretical findings demonstrate a promising strategy to generate LSPRs in MXene-based heterostructures

Impact of Adsorption of Straight Chain Alcohol Molecules on the Optical Properties of Calcite (10.4) Surface

Calcium carbonate plays a central role in controlling the chemistry of the oceans, biomineralization and oil production, to name a few. In this work, using density functional theory with semiempirical dispersion corrections and simplified TD-DFT using Tamm–Dancoff approximation, we investigated the impact of the adsorption of straight chain alcohol (ethanol and pentanol) molecules on the optical properties of a calcite (10.4) surface. Our results show that ethanol and/or pentanol molecules form a well-ordered monolayer (through their hydroxyl group with carbon chains sticking away in a standing-up position) on the calcite (10.4) surface. Additionally, we found intriguing modulations in the photoabsorption spectra and circular dichroism spectra. In particular, the latter was a unique optical fingerprint for a molecule-adsorbed calcite (10.4) surface. Our findings provide useful insights into the structural and optical features of calcite-based systems at the atomic level

Theoretical study of magnetism, structure and chemical order in transition-metal alloy clusters

University of KasselThe principal motivation of this thesis was to enrich the fundamental understanding of the structural and magnetic properties exhibited by the TM nanoalloy clusters in view of applications in cluster-based magnetic nanometer devices. The results presented in this thesis open the way to several near future investigations and developments in this field. Specific conclusions concerning each chapter have already been mentioned at the end of the corresponding chapters. For this reason the present final chapter is quite compact. In the following, I briefly discuss how the thesis has been progressed. Chapter 1 through 3 are devoted to present the essential background material for the calculations performed in this thesis, while chapters 4 through 7 demonstrate the results. In the first part, i.e., in chapters 4 and 5, we combined the state-of-the-art Hohenberg-Kohn Sham's DFT with a global optimization technique based on a graph theory method. This method has been used to perform a thorough and systematic study on the interplay between cluster structure, magnetism and the chemical order in the Fe$_m$Rh$_n$ and Co$_m$Pd$_n$ nanoclusters having $N = m+n \leq 8$ atoms. For $N = m+n \leq 6$ a thorough sampling of all cluster topologies has been performed. We would like to emphasis that this kind of study is rather unique. For $N = 7$ and $8$ only a few representative topologies were considered including both open and compact structures. Choosing a small set of representative topologies for $N = 7$ and $8$ is justified by the fact that the number of isomers (number of local minima on the PES) increases exponentially with the cluster size $N$. Indeed, the computational complexity increases even more rapidly in the case of binary clusters, since one has to take into account all possible homotops. For all the clusters the entire concentration range is systematically investigated, and the different initial magnetic configurations such as ferro- and anti-ferromagnetic coupling are considered. The results in the case of FeRh clusters are the following: an increase of the average magnetic moment ($\overline\mu_N$) and magnetic stabilization energy ($\Delta E_m$) with increasing Fe concentration, the presence of small differences in the average magnetic moment ($\overline\mu_N$) between low-lying isomers, the dominant role of the $d$-electron spin polarization within the PAW spheres, the enhancement of the Fe moments upon Rh doping, and a general tendency to maximize the number of mixed bonds. We have adopted a similar approach in the case of Co$_m$Pd$_n$ clusters having $N = m+n \leq 8$ atoms. The main results in this case are the following: The optimized cluster structures have a tendency to maximize the number of nearest-neighbor CoCo pairs. An increase of $\overline\mu_N$ and $\Delta E_m$ is observed with increasing Co concentration. The magnetic order is ferromagnetic-like (FM) for all ground-state structures. However, an antiferromagnetic-like (AF) order has been obtained in some of the first exited isomers. The maximal local spin polarization for Co and Pd atoms are found in the equiatomic compositions (Co$_2$Pd$_2$, Co$_3$Pd$_3$ and Co$_4$Pd$_4$). We found that taking into account spin-orbit (SO) interactions in FeRh and CoPd clusters does not alter the ground-state structures found by using the scalar relativistic (SR) calculations. FeRh and CoPd clusters are expected to develop a variety of further interesting behaviors, which still remain to be explored. For instance, larger FeRh cluster should show a more complex dependence of the magnetic order as a function of concentration. In particular for large Rh content one should observe a transition from FM-like to AF-like order with increasing cluster size, in agreement with the AF phase found in solids for more than 50\% Rh concentration. Moreover, the metamagnetic transition observed in bulk FeRh alloys also puts forward the possibility of similar interesting phenomena in nanoalloys as a function of temperature. In chapter 6 we have developed a DFT based spin-polarized basing-hopping algorithm. This methodological approach has been applied to study the structural and magnetic properties of pure and alloy TM nanoclusters. This method is found to be very impressive. For instance, in the case of pure clusters, we obtained several Jahn-Teller distorted magnetic isomers of the same basic structural motif which, being similar, would have been most likely missed by using the graph or topographical scheme employed in the first part of this thesis. A similar situation has also been encountered in the case of mixed clusters, where we identified several Jahn-Teller distorted magnetic isomers having the same composition and a similar distributions of the two kinds of atoms. We have discussed extensively the technicalities used for choosing the ideal move parameters for the optimizations. Moreover, we have implemented a \textit{window acceptance criterion}. Moreover, in the case of mixed clusters we swap or exchange the positions of the dissimilar atoms on the fly. We noticed that this method greatly enhances the performance of our calculations by significantly reducing the CPU time. For the small clusters (e.g. Fe$_6$, Rh$_6$ and Fe$_3$Rh$_3$) the main result is the presence of dominant (or frequently visited) isomers. We found that this is an intrinsic feature of the basin-hopping method. This is interpreted as a consequence of the reduced system size and the resulting small number of low-lying isomers. The dominant isomers govern the overall computational demand of the sampling and are therefore the relevant isomers for the performance analysis. In the case of larger clusters (e.g. Fe$_{13}$, Fe$_6$Rh$_7$ and Rh$_{13}$) we applied a similar computational procedure as the one used in the case of small clusters. In fact, both pure and mixed clusters display remarkable structural and magnetic diversity. We found that isomers having similar shape but with a small distortion among each other can exhibit often quite different magnetic moments. This has been interpreted as a probable artifact of the spin-rotational symmetry breaking introduced by the spin-polarized LDA or GGA. In the case of Fe$_6$Rh$_7$ cluster, an implementation consisting of small move distances of about 0.15$a$ (where $a$ is the length of the shortest bond) in combination with swapping of Fe and Rh atoms, could identify all the relevant isomers including the ground-state. The ground-state structure of Fe$_{13}$ cluster is found to be an icosahedral structure, whereas Rh$_{13}$ and Fe$_6$Rh$_7$ isomers relax into cage-like and layered-like structures, respectively. The possibility of combining the spin-polarized density-functional theory with some other global optimization techniques such as minima-hopping method could be the next step in this direction. This combination is expected to be an ideal sampling approach by having the advantage of efficiently avoiding search over irrelevant regions of the potential energy surface. In chapter 7 we investigated the composition dependence of orbital magnetism and magnetic anisotropy energy in FeRh nanoclusters. A remarkable non-monotonous dependence of the MAE is observed as a function of Fe content, i.e., upon going from pure Fe to pure Rh. This leads to an important increase of the MAE, which reaches about 3 times the value for pure clusters at the optimal Fe concentration. This offers multiple possibilities of tailoring the magneto-anisotropic behavior in nanoalloys. In conclusion, it is our hope to see the experimental verification of the reported results in this thesis

Band Edge Optical Excitation of Pyridine-Adsorbed CuAg Nanoparticles

Understanding the structure–property relationship of multielement nanoparticles is vital for developing novel nanodevices. In the present paper, via a combination of a basin hopping global sampling method, a symmetry-orbit shell optimization technique, and density functional theory reoptimizations, we determine the energetically most stable CuAg face-centered cubic nanoparticles. The calculated structures show a clear tendency toward Cu_coreAg_shell chemical ordering by populating the more cohesive Cu in the core region and of Ag in the shell region. Further, using time-dependent density functional theory (TDDFT) calculations, we analyze the band edge optical excitations of the nanoparticles with pyridine molecule on top. With the help of charge difference density plots, we found dramatic modifications in the electron density distribution of the nanoparticles. We believe that the present theoretical findings will be useful for the development of novel nanosensors

Temperature-Dependent Electronic Structure of Bixbyite α-Mn2O3 and the Importance of a Subtle Structural Change on Oxygen Electrocatalysis

α-Mn2O3 is an inexpensive Earth-abundant mineral that is used as an electrode material in various kinds of electrochemical devices. The complex bixbyite structure of α-Mn2O3, and its subtle orthorhombic → cubic phase transformation near room temperature has made it challenging to accurately determine its electronic proper- ties. We used high-resolution X-ray diffraction to study the temperature-dependent structures of phase-pure α-Mn2O3 prisms. Our measurements show a clear change in the crystal phase from orthorhombic → cubic between 293K and 300K. We input the Rietveld refined high-resolution crystal structures collected at various temperatures (273, 293, 300, 330K) directly into density functional theory (DFT) calculations to model their electronic properties. These calculations indicate that the orthorhombic phase α-Mn2O3 is a narrow bandgap semiconductor as expected. However, temper- atures higher than 300K transform the α-Mn2O3 into the cubic phase, causing the molecular orbitals of the Mn 3d and O 2p bands to overlap and mix covalently, mak- ing the material behave as a semimetal. This subtle change in crystal structure will affect the bulk conductivity of the material as well as Mn-O-Mn bond distances that influence the quality of its catalytic active sites for oxygen electrochemistry. Elec- trochemical oxygen evolution (OER) and oxygen reduction reaction (ORR) experi- ments performed at various temperatures (∼ 288K to 323K) using the same prepared electrode show a marked enhancement in both OER and ORR performance that is attributed to the higher activity of the cubic phase.</div

Interplay between Chemical and Magnetic Order in FeRh Clusters

The structure, chemical order, and magnetic behavior in small FeRh clusters having <i>N</i> ≤ 19 atoms have been investigated theoretically. For <i>N</i> ≤ 6 atoms, a thorough global geometry optimization is performed by considering all possible cluster topologies, while for 7 ≤ <i>N</i> ≤ 19 only a few representative structures are considered. In all cases, the starting structures are fully relaxed in the entire range of concentrations and spin polarizations. The calculations are based on a generalized-gradient approximation to density-functional theory. The results are analyzed systematically as a function of size and composition. The optimized cluster structures are compact with a clear tendency to maximize the number of nearest-neighbor FeRh pairs. For very small sizes, the low-lying isomers present usually a topology different from that of the optimal structure, while for larger clusters the lowest-energy isomerizations imply mainly changes in the chemical order. The most stable structures are in general ferromagnetic. Antiparallel spin arrangements are found in some low-lying isomers. An important enhancement of the local Fe moments is observed as result of Rh doping. This is shown to be a consequence of an increase in the number of Fe d holes due to Fe–Rh charge transfer. The local moments at the Rh atoms, which are significant already in small pure Rh clusters, are not strongly enhanced by Fe doping. Nevertheless, the overall stability of magnetism, as measured by the energy gained upon spin polarization, increases with Fe content. The influence of spin–orbit interactions on the cluster stability and spin order is discussed

Free Energy Surfaces and Barriers for Vacancy Diffusion on Al(100), Al(110), Al(111) Reconstructed Surfaces

Metadynamics is a popular enhanced sampling method based on the recurrent application of a history-dependent adaptive bias potential that is a function of a selected number of appropriately chosen collective variables. In this work, using metadynamics simulations, we performed a computational study for the diffusion of vacancies on three different Al surfaces [reconstructed Al(100), Al(110), and Al(111) surfaces]. We explored the free energy landscape of diffusion and estimated the barriers associated with this process on each surface. It is found that the surfaces are unique regarding vacancy diffusion. More specically, the reconstructed Al(110) surface presents four metastable states on the free energy surface having sizable and connected passage-ways with an energy barrier of height 0.55 eV. On the other hand, the reconstructed Al(100)/Al(111) surfaces exhibit two/three metastable states, respectively, with an energy barrier of height 0.33 eV. The findings in this study can help to understand surface vacancy diffusion in technologically relevant Al surfaces