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Register a new userSize dependent rearrangements in monometallic clusters
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Morphology and its stability are essential features to address physicochemical properties of metallic nanoparticles. By means of Molecular Dynamics based simulations we show a complex dependence on the size and material of common structural mechanisms taking place in mono-metallic nanoparticles at icosahedral magic sizes. We show that the well known Lipscomb s Diamond Square Diamond mechanisms, single step screw dislocation motions of the whole cluster, take place only below a given size which is material dependent. Above that size, layer by layer dislocations and/or surface peeling are likely to happen, leading to low symmetry defected motifs. The material dependence of this critical size is similar to the crossover sizes among structural motifs, based on the ration between the bulk modulus and atomic cohesive energy.
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The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics (SD) simulations coupled to molecular dynamics (MD-SD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering (SRMO). Analytical approximations to the magnetization as a function of temperature and size are proposed.
The vibrational properties of alkaline-earth metal fluoride clusters (BaF2)n (n=1-6) are investigated in the framework of density functional theory. The calculated Raman and Infrared (IR) spectra reveals shift in Raman and IR peak position towards lower frequency region with the increase in the cluster size. Further the calculated spectra have been compared with the experimental vibrational spectra of bulk BaF2 crystal. Even though the smaller size cluster lacks translational symmetry, the structural and vibrational characteristic of (BaF2)5-6 are nearer to bulk counterpart.
We present a systematic study of the photo-absorption spectra of various Si$_{n}$H$_{m}$ clusters (n=1-10, m=1-14) using the time-dependent density functional theory (TDDFT). The method uses a real-time, real-space implementation of TDDFT involving full propagation of the time dependent Kohn-Sham equations. Our results for SiH$_{4}$ and Si$_{2}$H$_{6}$ show good agreement with the earlier calculations and experimental data. We find that for small clusters (n<7) the photo-absorption spectrum is atomic-like while for the larger clusters it shows bulk-like behaviour. We study the photo-absorption spectra of silicon clusters as a function of hydrogenation. For single hydrogenation, we find that in general, the absorption optical gap decreases and as the number of silicon atoms increase the effect of a single hydrogen atom on the optical gap diminishes. For further hydrogenation the optical gap increases and for the fully hydrogenated clusters the optical gap is larger compared to corresponding pure silicon clusters.
The plasmonic properties of sphere-like bcc Na nanoclusters ranging from Na$_{15}$ to Na$_{331}$ have been studied by real-time time-dependent local density approximation calculations. The optical absorption spectrum, density response function and static polarizability are evaluated. It is shown that the effect of the ionic background (ionic species and lattice) of the clusters accounts for the remaining discrepancy in the principal (surface plasmon) absorption peak energy between the experiments and previous calculations based on a jellium background model. The ionic background effect also pushes the critical cluster size where the maximum width of the principal peak occurs from Na$_{40}$ predicted by the previous jellium model calculations to Na$_{65}$. In the volume mode clusters (Na$_{27}$, Na$_{51}$, Na$_{65}$, Na$_{89}$ and Na$_{113}$) in which the density response function is dominated by an intense volume mode, a multiple absorption peak structure also appears next to the principal peak. In contrast, the surface mode clusters of greater size (Na$_{169}$, Na$_{229}$, Na$_{283}$ and Na$_{331}$) exhibit a smoother and narrower principal absorption peak because their surface plasmon energy is located well within that of the unperturbed electron-hole transitions, and their density responses already bear resemblance to that of classical Mie theory. Moreover, it is found that the volume plasmon that exist only in finite size particles, gives rise to the long absorption tail in the UV region. This volume plasmon manifests itself in the absorption spectrum even for clusters as large as Na$_{331}$ with an effective diameter of $sim$3.0 nm.
In this work we develop a simple theory for the green photoluminescence of ZnO quantum dots (QDs) that allows us to understand and rationalize several experimental findings on fundamental grounds. We calculate the spectrum of light emitted in the radiative recombination of a conduction band electron with a deeply trapped hole and find that the experimental behavior of this emission band with particle size can be understood in terms of quantum size effects of the electronic states and their overlap with the deep hole.We focus the comparison of our results on detailed experiments performed for colloidal ZnO nanoparticles in ethanol and find that the experimental evolution of the luminescent signal with particle sizeat room temperature can be better reproduced by assuming the deep hole to be localized at the surface of the nanoparticles. However, the experimental behavior of the intensity and decay time of the signal with temperature can be rationalized in terms of holes predominantly trapped near the center of the nanoparticles at low temperatures being transferred to surface defects at room temperature. Furthermore, the calculated values of the radiative lifetimes are comparable to the experimental values of the decay time of the visible emission signal.We also study the visible emission band as a function of the number of electrons in the conduction band of the nanoparticle, finding a pronounced dependence of the radiative lifetime but a weak dependence of energetic position of the maximum intensity.
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