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|Title: ||Photoabsorption of gold nanoparticles: a TDDFT analysis by cluster model molecules|
|Authors: ||Stener, Mauro|
|Keywords: ||gold nanoparticles|
|Issue Date: ||18-Mar-2009|
|Publisher: ||Università degli studi di Trieste|
|Abstract: ||The optical properties of gold nanoparticles are the object of study of the present Ph. D. thesis. Different clusters in various conditions have been considered in order to simulate the absorption spectra at theoretical level and to correlate to the electronic structure the information obtained from the interpretation of the spectra. The study of electronic excitations has been carried out within the TDDFT theory implemented in the ADF code.
The gold nanoparticles have different and interesting physical and chemical properties and in the scientific community they have attracted great attention at both experimental and theoretical level. The properties for which they have been widely used, are mainly catalysis, absorption and selective oxidation, while those which are now being studied with increasing interest are optical, electronic and magnetic properties. Biology, biophysics and medicine are the applicative fields in which recently the gold nanoparticles have greater impact.
In order to contribute to the study of the optical properties of the gold nanoparticles, we have tried to propose theoretical models with the goal to rationalize the experimental findings and to realize a computational model for new nanostructured materials with specific optical properties. We have therefore addressed this issue by handling several aspects.
Below it will be described the path followed during the Ph. D. research indicating the chapters of the second part of the thesis in which the various arguments are treated in detail.
Starting with both experimental and theoretical data found in literature, we have focused at first on naked gold clusters, to simulate the note SPR absorption band ascribed to the electron collective oscillations. We have tried to push the size of the cluster to the maximum computationally allowed to reproduce the red-shift/size increasing trend, observed in the UV-visible spectra, and to draw as closely as possible to the minimum size of the systems synthesized in the laboratory. Within the limitations due to high density of states in the case of larger clusters, we have tried to rationalize the results in terms of conventional quantum chemistry arguments. (Chapter 4)
We have concentrated then on the refining of the computational technique, introducing the spin-orbit coupling in the calculation of valence excitation spectra of small regular icosahedrical bimetallic systems with closed shell. In this context we have discussed the differences between a SR (Scalar relativistic) and a SO (spin-orbit) TDDFT calculation scheme and also those due to different heteroatoms present in the centre of the gold cage. Thus, we have identified a diagonal trend between two elements of Group V B (V and Nb) and two from Group VI B (Mo and W) of the Periodic Table. (Chapters 5 and 6)
Finally we have moved to the study of the electronic excitation phenomena from the valence to the core, simulating the XANES spectra of systems functionalized with metilthiolate. The experimental molecular models that we have compared, were similar in size but different in composition (dodecanethiolate) and in structure. Nevertheless, grounded on calculations, we have given a more detailed interpretation of the intense experimental band: it is the result of two unresolved bands which involve both S-Au and S-C antibonding states. Moreover, depending on the structure of the cluster, and then the types of interactions present, we have observed changes in the spectral patterns. (Chapter 7)
In the first part of the thesis have been developed some general issues and some theoretical aspects with which deal the above chapters. In particular, in Chapter 1 is considered the wide nanoparticles main theme, in Chapter 2, the electronic spectroscopy and Chapter 3 the theoretical DFT and TDDFT methods with particular emphasis on aspects related to the relativistic formalism.|
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