Cecilia Noguez

On the origin of the optical activity displayed by chiral-ligand-protected metallic nanoclusters.

Ligand-protected metallic clusters exhibit optical activity when chiral molecules are used as protecting units. Various mechanisms, such as the inherently chiral metallic cluster core, the dissymmetric field effect, and the chiral footprint model, have been proposed as possible explanations of the nonzero circular dichroism (CD) spectra found for these nanoscale materials. This communication presents a first-principles theoretical study of the CD spectrum of the [Au(25)(SR)(18)](-) cluster that was undertaken to gain insight into the physicochemical origin of the optical activity measured for the glutathione-protected [Au(25)(SG)(18)](-) cluster. The calculated CD spectrum of the cysteine-protected cluster, with R(cys) = C(beta)H(2)-C(alpha)H(NH(2))-COOH, shows good agreement with the experimental data obtained for the glutathione-protected cluster. Analysis of the calculated CD spectra of the peculiar two-shell metallic core and the two distinct thiolate-Au binding modes existing in the [Au(25)(SR(cys))(18)](-) cluster showed that the weak CD signal due to the slight distortion of cluster core is enhanced by the dissymmetric location of the ligands forming the Au-S binding modes. This result shows that the mechanisms proposed to explain the optical activity of chiral-ligand-protected metallic clusters cannot be differentiated but are acting concurrently. It is also predicted that the CD line shape should be highly sensitive to the orientation of the thiolate ligands forming the cluster protecting layer and to the stability of the thiolate-Au binding modes.

Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation

The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance unambiguously show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

Optical circular dichroism of single-wall carbon nanotubes

The circular dichroism (CD) spectra of single-wall carbon nanotubes are calculated using a dipole approximation. The calculated CD spectra show features that allow us to distinguish between nanotubes with different angles of chirality, and diameters. These results provide theoretical support for the quantification of chirality and its measurement, using the CD line shapes of chiral nanotubes. It is expected that this information would be useful to motivate further experimental studies.

Large optical birefringence by anisotropic silver nanocomposites

A large optical birefringence of oriented Ag nanoellipsoids embedded in silica was measured using an ellipsometric technique. The two main surface plasmon resonances associated with the axes of the ellipsoid were tuned, allowing us to quantify the light transmission through the samples when placed and rotated between crossed and parallel polarizers. This birefringence can be physically associated with the selective optical absorption of one component of the linear polarization of the incident light with respect to the anisotropic axis of the sample, depending on the wavelength used to perform the measurement.

Chiral atomically thin films

Chiral materials possess left- and right-handed counterparts linked by mirror symmetry. These materials are useful for advanced applications in polarization optics, stereochemistry and spintronics. In particular, the realization of spatially uniform chiral films with atomic-scale control of their handedness could provide a powerful means for developing nanodevices with novel chiral properties. However, previous approaches based on natural or grown films, or arrays of fabricated building blocks, could not offer a direct means to program intrinsic chiral properties of the film on the atomic scale. Here, we report a chiral stacking approach, where two-dimensional materials are positioned layer-by-layer with precise control of the interlayer rotation (θ) and polarity, resulting in tunable chiral properties of the final stack. Using this method, we produce left- and right-handed bilayer graphene, that is, a two-atom-thick chiral film. The film displays one of the highest intrinsic ellipticity values (6.5 deg μm(-1)) ever reported, and a remarkably strong circular dichroism (CD) with the peak energy and sign tuned by θ and polarity. We show that these chiral properties originate from the large in-plane magnetic moment associated with the interlayer optical transition. Furthermore, we show that we can program the chiral properties of atomically thin films layer-by-layer by producing three-layer graphene films with structurally controlled CD spectra.

A new diagrammatic summation for the effective dielectric response of composites

We extended a previously developed diagrammatic formulation for the calculation of the effective dielectric response of composites prepared as a random, homogeneous, and isotropic distribution of small spherical inclusions in an otherwise homogeneous matrix. This is done within the long‐wavelength, dipolar approximation in the low‐density regime of inclusions. We propose a new diagrammatic summation and we compare our results with two recently reported computer simulations.

Designing the plasmonic response of shell nanoparticles: Spectral representation

A spectral representation formalism in the quasistatic limit is developed to study the optical response of nanoparticles, such as nanospheres, nanospheroids, and concentric nanoshells. A transfer matrix theory is formulated for systems with an arbitrary number of shells. The spectral representation formalism allows us to analyze the optical response in terms of the interacting surface plasmons excited at the interfaces by separating the contributions of the geometry from those of the dielectric properties of each shell and surroundings. Neither numerical nor analytical methods can do this separation. These insights into the physical origin of the optical response of multishelled nanoparticles are very useful for engineering systems with desired properties for applications in different fields ranging from materials science and electronics to medicine and biochemistry.

Ab Initio Electronic Circular Dichroism of Fullerenes, Single‐Walled Carbon Nanotubes, and Ligand‐Protected Metal Nanoparticles

The versatility and applicability of a time-perturbed density functional method implemented within the SIESTA program package to calculate electronic circular dichroism of diverse nanoparticles is discussed. Results for nanostructures, such as fullerenes, single-wall carbon nanotubes, as well as metallic nanoparticles composed of up to hundreds of atoms were examined by comparison with previously reported experimental and theoretical results. In all cases, the calculated electronic circular dichroism shows very good consistency with other calculations, and a remarkable agreement with experiments. It is concluded that such a high-level method provides theoretical support for the quantification, understanding, and prediction of chirality and its measurement in nanostructures. It is expected that this information would be useful to motivate further experimental studies and interpretation of optical activity in terms of electronic circular dichroism in novel nanostructures.

Theoretical Study of Surface Plasmon Resonances in Hollow Gold-Silver Double-Shell Nanostructures †

A theoretical model has been developed to study the optical properties of metallic multishell structures on the nanometer scale. The Mie theory was generalized for multiconcentric spherical shell nanostructures and employed to determine the effects and importance of the different parameters of the system such as thickness, size, and other material properties, for instance, the medium index of refraction. A unique hollow gold-silver double-shell structure is used as an example to test the model developed with recent experiments. The surface plasmon resonance (SPR) absorption spectrum of this structure has been calculated as a function of various parameters, including shell thickness and diameter. Using parameters similar to those previously reported experimentally, very good agreement has been found between calculated and experimentally measured SPR spectra, which validates the model. The results provide new insights into the fundamental properties of complex metal nanostructures that give us the ability to control the optical response, which has important implications in the synthesis of new metal nanostructures as well as their application in emerging technologies.

Atomic structure of small and intermediate-size silver nanoclusters.

Molecular dynamic simulations were performed to study the morphology and binding energy of the most stable isomers of silver clusters with diameters of less than 2 nm. A 5-fold symmetry was found in most cases, and a novel morphology for the clusters of 39 and 116 silver atoms was identified. This morphology can be understood in terms of decahedral and icosahedral geometries, which are intercalated, as we explain in detail. These kind of structures have been observed for gold and now are predicted for small and intermediate silver nanoparticles.