Optical Properties of Nanostructures
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The fabrication of nanostructures requires a deeper understanding of the physical phenomena involved at this scale. Low-dimensional quantum structures have shown to have unique optical and electronic properties, which have been employed in the fabrication of new opto-electronic devices. In particular, the shape and size of low-dimensional structures are crucial parameters to determine their physical properties. It is well known that the distribution of electronic states on a nanostructure depends on its size, and the confinement potential acting on the electrons is associated to its shape. Therefore, the exact knowledge of the size and shape of a nanostructure is of decisive importance in the development of the science and technology at the nanometer scale. Furthermore, the characterization of these parameters are important issues either in fundamental research or in technological applications, covering from growth and characterization to device processing. The chiral nature of the clusters, which means they exist in distinct right-handed and left-handed variations, dramatically affects the way in which they absorb polarized light. This optical effect had been predicted theoretically to occur in metal nanostructures, and it has been observed experimentally in a special class of clusters.
Electromagnetic Field Enhancement at the Edge of Metal NanostructuresThe role of localized surface plasmon resonances near the surface of a silver or gold wedge is discussed based on quasistatic theory. Strongly enhanced electromagnetic field intensities at moderate distances of 1~nm from the wedge are found by manipulating morphology and dielectric environment. The theory also shows that wedge structures can have high enhancements over a broad range of wavelengths, which is of relevance to surface-enhanced Raman scattering and Tip Enhanced Raman Scattering measurements and for improving plasmon enhanced photovoltaic devices. More ... |
The Role of Morphology in the Enhanced Optical Activity of Ligand-Protected Metal NanoparticlesIn recent years, many research has been dedicated to understand and predict the interaction of light with nanostructures, which exhibit a wide variety of interesting physical properties that can be tailored by altering their size, morphology, composition, and environment. One emerging area in nanoscience is the phenomenon where the optical activity can be influenced and enhanced due to the presence of metal nanoparticles, with possible technological implications in sensing and labeling chiral molecules, asymmetric catalysis, pharmacology, toxicology, among other applications. In this Perspective article, we talk about the theoretical aspects of this phenomenon and the relationship between morphology and the optical activity enhancement in ligand-protected metal nanoparticles. More ... |
Designing the plasmonic response of shell nanoparticles: spectral representationA spectral representation formalism in the quasi-static 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 multi-shelled 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. More ... |
Optically active nanoparticles: fullerenes, carbon nanotubes, and metal nanoparticles A brief topical review of the optical activity of fullerenes, carbon nanotubes, and metallic clusters, is given. Using a recently developed first-principles formalism, the particular case of the optical activity of the chiral C$_{84}$ fullerene with D$_2$ symmetry is studied in detail. In particular, the optical activity of the four possible isomers with this symmetry is studied by calculating the electronic circular dichroism (CD) and compared with experimental data. Although a similar line shape is found for the lowest-energy isomer, the width and intensity of the main experimental peaks are not well reproduced. On the other hand, it is found that the average CD of the mixture of the three lower-energy isomers is in better agreement with experiments. It is expected that this review would be useful as an introduction to optical activity at the nanometer scale, and motivate further experimental studies and the interpretation of natural optical activity in nanoparticles. More ...
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| 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 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 [Au25(SR)18]− cluster that was undertaken to gain insight into the physicochemical origin of the optical activity measured for the glutathione-protected [Au25(SG)18]− cluster. Analysis of the calculated CD spectra 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. 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. More ... |
Understanding Optical Activity in Single-Walled Carbon Nanotubes from First-Principles Studies. The origin of optical activity in single-walled carbon nanotubes (SWNTs) is investigated by performing first-principles calculations of the circular dichroism (CD) spectrum. The calculated CD is in excellent agreement with experiments, which is understood in terms of the density of states and optical absorption, providing the nanotubes’ absolute configuration. These results determine which nanotubes are present or not in CD measurements and their chirality, providing a framework to understand the enantioselectivity process in recent experiments. Additionally, these results offer theoretical support to understand chirality at the nanoscale and convey selectivity in synthesis, separation, and analysis using carbon nanotubes, which are important issues in molecular recognition, nanocatalysts, DNA assembly, as well as in biofunctionalization based on SWNT technology. More ...
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