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Comment on “(Au–Ag)144(SR)60alloy nanomolecules” by C. Kumara and A. Dass, Nanoscale, 2011,3, 3064
No full textA recent paper in this journal reported the synthesis and characterization via electrospray ionization mass
spectroscopy and UV-vis spectroscopy of (Au–Ag)144(SR)60 alloy nanomolecules with different compositions,
ranging from 1 : 0 to 1 : 0.75 Au : Ag ratios. The UV-vis spectra of such systems were found to
exhibit absorption peaks at 310 nm, 425 nm and 560 nm, interpreted as reminiscent of the silver surface
plasmon resonance band due to simple atomic replacement of Au by Ag atoms in a fixed structural
framework. On the basis of a comparison of experimentally observed and theoretically simulated optical
absorption spectra, we conclude that the experimental situation must be more complicated, and that
further work is needed to achieve atomistic insight into these fascinating systems
Optical Activity of Metal Nanoclusters Deposited on Regular and Doped Oxide Supports from First-Principles Simulations
Full text linkWe report a computational study and analysis of the optical absorption processes of Ag20 and Au20 clusters deposited on the magnesium oxide (100) facet, both regular and including point defects. Ag20 and Au20 are taken as models of metal nanoparticles and their plasmonic response, MgO as a model of a simple oxide support. We consider oxide defects both on the oxygen anion framework (i.e., a neutral oxygen vacancy) and in the magnesium cation framework (i.e., replacing Mg++ with a transition metal: Cu++ or Co++). We relax the clusters’ geometries via Density-Functional Theory (DFT) and calculate the photo-absorption spectra via Time-Dependent DFT (TDDFT) simulations on the relaxed geometries. We find that the substrate/cluster interaction induces a broadening and a red-shift of the excited states of the clusters, phenomena that are enhanced by the presence of an oxygen vacancy and its localized excitations. The presence of a transition-metal dopant does not qualitatively affect the spectral profile. However, when it lies next to an oxygen vacancy for Ag20, it can strongly enhance the component of the cluster excitations perpendicular to the surface, thus favoring charge injection
Optical properties of Pt and Ag-Pt nanoclusters from TDDFT calculations: Plasmon suppression by pt poisoning
No full textThe optical properties of alloyed Ag−Pt nanoclusters are theoretically
investigated as a function of composition and chemical ordering via a time-dependent
density-functional-theory (TDDFT) approach. Clusters with icosahedral structure ranging in
size between 55 and 146 atoms are considered, large enough to start observing strong
adsorption peaks related to surface plasmon resonances (SPR) in pure Ag systems. Strikingly
it is found that even the modest Pt content here considered, ranging between 14% and 24%,
is sufficient to substantially damp the optical response of these clusters. The effect is most
disruptive when Pt atoms are scattered at the cluster surface, where the Ag SPR is mostly
located, especially at the cluster apex, while the most intense residual peaks occur as Pt 5d →
Ag 5p transitions at a Pt(core)/Ag(shell) interface and are strongly blue-shifted by 0.7−1.0
eV with respect to the analogous Ag peaks. Smaller Pt13 and Pt38 clusters are also studied for
comparison, finding a nonplasmonic behavior but a strong involvement of Pt 5d orbitals in
the optical response
Optical properties of nanoalloys
No full textThe optical properties of multi-component metal nanostructures (or nanoalloys) are the subject of an
intense and rapidly growing experimental and theoretical activity. In this perspective article, we first
provide a survey of the most recent developments in the field, concerning both theoretical methods,
especially at the first-principles level, and novel results, distinguishing for the convenience of
presentation the sub-field of monolayer-protected multi-component metal clusters from the other alloy
nanosystems. We then discuss a few general concepts which can be drawn from this survey, and offer a
few suggestions on the most promising directions for future research. We hope that making the point in
this fast developing field will provide a framework and a perspective useful to trigger future studies and
advancements
Ab initio modelling of oxygen vacancy arrangement in highly defective HfO2 resistive layers
No full textWe report ab initio results for sub-stoichiometric HfOx with different oxygen vacancy densities, useful in
exploring microscopic mechanisms that govern the operation of RRAM devices. We demonstrate that
oxygen vacancy filaments are energetically more stable than randomly distributed defects. Furthermore,
the stability of the filaments increases with the number of confined oxygen vacancies. Energetic and
structural analyses show that bonds between neighboring coordinative unsaturated Hf atoms promote
filament stability, and electron trapping, due to electron injection, increases the cohesive energy until
the injection is moderate. The highly oxygen deficient configuration of the filaments leads to a
substantial lowering of the HfOx band gap, which locally increases the conductivity of the system.
Charge injection and electric fields modify the mobility of oxygen ions in the proximity of the filament.
The simulations suggest that oxygen ion diffusion can lead to an asymmetric reduction of filament
thickness and thus to its progressive disruption where the vacancy cohesion energy is lower.We report ab initio results for sub-stoichiometric HfOxwith different oxygen vacancy densities, useful in exploring microscopic mechanisms that govern the operation of RRAM devices. We demonstrate that oxygen vacancy filaments are energetically more stable than randomly distributed defects. Furthermore, the stability of the filaments increases with the number of confined oxygen vacancies. Energetic and structural analyses show that bonds between neighboring coordinative unsaturated Hf atoms promote filament stability, and electron trapping, due to electron injection, increases the cohesive energy until the injection is moderate. The highly oxygen deficient configuration of the filaments leads to a substantial lowering of the HfOx band gap, which locally increases the conductivity of the system. Charge injection and electric fields modify the mobility of oxygen ions in the proximity of the filament. The simulations suggest that oxygen ion diffusion can lead to an asymmetric reduction of filament thickness and thus to its progressive disruption where the vacancy cohesion energy is lower
Coupling between Plasmonic and Molecular Excitations: TDDFT Investigation of an Ag-Nanorod/BODIPY-Dye Interaction
Full text linkA time-dependent density functional theory (TDDFT) computational approach is employed to study the optical coupling between a plasmonic system (a Ag-50 nanorod) and a fluorescent dye (BODIPY). It is found that the BODIPY dye can interact with a plasmonic system in a rather different and selective way according to the mutual orientation of the fragments. Indeed, (i) the plasmon excitation turns out to be sensitive to the presence of the BODIPY transition and (ii) this can lead to amplify or suppress the resonance accordingly to the relative orientation of the corresponding transition dipoles. To understand the coupling mechanism, we analyze the shape of the induced density in real space and the Individual Component Map of the Oscillator Strength (ICM-OS) plots and achieve a simple rationalization and insight on the origin and features of the coupling. The resulting possibility of understanding plasmon/fluorophore interactions by simple qualitative arguments opens the way to a rational design of hybrid (plasmon + dye) systems with the desired optical behavior
Bottom-up approach to innovative memory devices: I. Intrinsic and environmental effects on the molecular component
No full textThe 50 nm-thick polystyrene (PS) film, involved in some innovative memory devices, contains 8-hydroxyquinoline
(8HQ) molecules and gold nanoparticles. A model where molecular localized properties directly
reflect on macroscopic behavior of a complex system has been tested in the present work, which is focused
on the structural and electronic properties of the 8HQ-PS mixture modeled in a continuum scheme: one 8HQ
molecule with a polarizable continuum model (PCM) whose reliability has been checked by comparison with
periodic DFT calculations of 8HQ-PS crystalline structures. A comprehensive study of the keto-enolic
tautomerization of 8HQ has been performed, at the DFT level using B3LYP, LC-PBE, and M052X functionals
and a polarized double- basis set. The energetics of the obtained structures (minima and transition states)
have been refined by single point calculations at the CCSD(T) level with the aug-cc-pVDZ basis set. Our
calculations predict the enolic tautomer to be the most stable for the isolated and PS-solvated 8HQ in its
neutral form, with a tautomerization barrier much larger than the thermal energy at the working conditions.
The opposite trend has been found for the charged (both positive and negative) 8HQ, with ketonic tautomers
being the most stable. In a first approximation of weak interaction with the aluminum electrodes, the electricfield
effects have also been taken into account for the calculations of electron affinities and ionization potentials
of 8HQ molecules. The electron and hole injection barriers issuing from these results are in good agreement
with the experimental observations
Combined experimental and computational study of the photoabsorption of the monodoped and nondoped nanoclusters Au24Pt(SR)18, Ag24Pt(SR)18, and Ag25(SR)18
Full text linkAssessing the accuracy of first-principles computational approaches is instrumental to predict electronic excitations in metal nanoclusters with quantitative confidence. Here we describe a validation study on the optical response of a set of monolayer-protected clusters (MPC). The photoabsorption spectra of Ag-25(DMBT)(18)(-), Ag24Pt(DMBT)(18)(2-) and Au24Pt(SC4H9)(18), where DMBT is 2,4-dimethylbenzenethiolate and SC4H9 is n-butylthiolate, have been obtained at low temperature and compared with accurate TDDFT calculations. An excellent match between theory and experiment, with typical deviations of less than 0.1 eV, was obtained, thereby validating the accuracy and reliability of the proposed computational framework. Moreover, an analysis of the TDDFT simulations allowed us to ascribe all relevant spectral features to specific transitions between occupied/virtual orbital pairs. The doping effect of Pt on the optical response of these ultrasmall MPC systems was identified and discussed
Dichroism of plasmonic chiral nanoalloys by rational design
Full text linkTime-dependent density functional theory (TDDFT) simulations are conducted on a series of chiral gold/silver alloy nanowires to explore whether silver doping can produce an enhancement of circular dichroism at the plasmon resonance in these systems, and to identify the quantum-mechanical origin of the observed effects. We find a strong plasmonic dichroism when one or two helixes of gold atoms are substituted by silver in a linear chiral nanotube, whose pure gold counterpart does not display any plasmonic dichroism, and we rationalize this finding in terms of “decoupling” the destructive interference of excitations in the pure gold nanotube via alloying. However, further attempts to increase the plasmonic dichroism by considering multi-shell gold nanowires in which one entire shell is doped with silver did not produce the desired effect, but rather a decrease in circular dichroism. We show that this latter result is due to a more severe destructive interference in the dipole excitation contributions, and suggest that further amplification should be possible in principle by properly tuning simultaneously the nanowire structure and chemical ordering
Circularly Polarized Plasmons in Chiral Gold Nanowires via Quantum-Mechanical Design
No full textTime-dependent density functional theory (TDDFT) simulations are
conducted on a series of chiral gold nanowires to explore whether an enhancement of
circular dichroism at the plasmon resonance is possible and identify its quantummechanical
origin. We find that in linear two-dimensional chiral nanowires the
dichroic response is suppressed by destructive interference of nearly degenerate
components with opposite signs, pointing to this phenomenon as a common and
likely origin of the difficulty encountered so far in achieving a plasmonic CD response
in experiment and suggesting nevertheless that these opposite components could be
“decoupled” by using multiwall arrangements. In contrast, we predict a giant dichroic
response for nanowires with three-dimensional helical coiling. We rationalize this
finding via an electronic structure analysis of longitudinal and transversal plasmonic
excitations and their coupling into chiral components, and we propose a simple
formula for the chiral response as a function of structural parameters (nanowire
length and coiling number)
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