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内容記述 |
The aim of this thesis is to theoretically study geometric, electronic, and optical prop-<br />erties of one-nanometer sized cluster compounds. The thesis is composed of two parts.<br /> In the first part, the geometric and electronic properties of gold-thiolate cluster com-<br />pounds, which have recently been studied experimentally, are revealed. I will discuss<br />how the local geometric structures are related to the electronic properties of the com-<br />pounds. In the second part, optical response theory that is applicable to the nan-<br />ocluster compounds is developed. Special emphasis is placed on nonuniform electronic<br />excitations induced by near-fields.<br /> Let me briefly review history of metal nanoclusters. Research in nanocluster com-<br />pounds has its root on the study of bare metal clusters in gaseous phase, where size-<br />dependent physicochemical properties are the main concern. However, most of these<br > bare clusters are energetically and chemically unstable. In the past few decades,<br />metal clusters protected by organic molecules have been synthesized in solution, and<br />some of these cluster compounds were found to be stable even in the air. Although<br />these nanocluster compounds were expected to be promising candidates for functional<br />nanomaterials in a wide range of nanotechnologies, it is not trivial to characterize their<br />detailed structures. Reducing the size of clusters to the 1 nm scale, their geometries<br />and other properties become much more sensitive to the change in size and chemical<br />compositions. In such circumstances, sub-nanometer sized gold-cluster compounds<br />have intensively been synthesized with the definitive determination on the chemical<br />compositions. Despite the brilliant results, even their geometrical structures have not<br />sufficiently been characterized. Furthermore, the studies on their optical properties<br />are still in the juvenile stage. For these reasons, I theoretically study the geometric,<br />electronic, and optical properties of some representative cluster compounds at the 1 <br />nm scale.<br /> The geometric and electronic structures of a gold-methanethiolate [Au<small>25</small>(SCH<small>3</small>)<small>18</small>]<sup>+</sup><br />are investigated by carrying out the density functional theory (DFT) calculations.<br />The obtained optimized structure consists of a planar Au<small>7</small> core cluster and Au-S com-<br />plexes, where the Au<small>7</small> plane is enclosed by a Au<small>12</small>(SCH<small>3</small>)<small>12</small> ring and sandwiched by two<br />Au<small>3</small>(SCH<small>3</small>)<small>3</small> ring clusters. This geometry differs in shape and bonding from a gener-<br />ally accepted geometrical motif of gold-thiolate clusters that a spherical gold cluster is<br />superficially ligated by thiolate molecules. This newly optimized gold-methanthiolate<br />cluster shows a large HOMO-LUMO gap, and calculated X-ray diffraction and absorp-<br />tion spectra successfully reproduce the experimental results. On another gold cluster<br />compound [Au<small>25</small>(PH<small>3</small>(SCH<small>3</small>)Cl<small>2</small>]<sup>2+</sup>, which consists of two icosahedral Au<small>13</small> clus-<br />ters bridged by methanethiolates sharing a vertex gold atom and terminated by chlo-<br />rine atoms, the DFT calculation provides very close structure to the experimentally<br />obtained gold cluster [Au<small>25</small>(PPh<small>3</small>)<small>10</small>(SC<small>2</small>H<small>5</small>)<small>5</small>CL<small>2</small>]<sup>2+</sup>. I further demonstrate that a<br />vertex-sharing triicosahedral gold cluster [Au<small>37</small>(PH<small>3</small>)<small>10</small>(SCH<small>3</small>)<small>10</small>Cl<small>2</small>]<sup>+</sup> is also achieved<br />by bridging the core Au<small>13</small> units with the methanethiolates. A comparison between<br />the absorption spectra of the bi- and triicosahedral clusters shows that the new elec-<br />tronic levels due to each oligomeric structure appear sequentially, whereas other elec-<br />tronic properties remain almost unchanged compared to the individual icosahedral<br />Au<small>13</small> cluster. These theoretical studies have elucidated the fundamental properties of<br />the promising building blocks such as geometric structures and stability of real cluster<br />compounds in terms of the detailed electronic structures. As a next step, I have to gain<br />a further insight into the dynamical optical properties of cluster compounds. In partic-<br />ular for discussing photoinduced dynamics in nanoclusters or nanocluster assemblies,<br />inter-cluster near-field interactions should be understood properly. The conventional<br />light-matter interaction based on available lasers is quite different from the near-field<br />interaction. The electric fields of available lasers usually have the wavelength much<br />longer than the size of the local structure of the cluster compounds. In other words,<br />the 1-nm-sized cluster compounds feel the almost uniform electromagnetic field and<br />thus the local structures of the compounds cannot be resolved. In contrast, a near-field<br />interaction occurs at the same scale of the cluster compounds and is thus expected to<br />be used to observe the local structure of the 1 nm sized materials. The difficulty in<br />their theoretical description arises from the fact that the near-field has a non-uniform<br />local structure. For these reasons, I will develop an optical response theory that is<br />applicable to 1-nm-sized clusters interacting with the near-field.<br /> The optical response theory is developed in a general form on the basis of the<br />multipolar Hamiltonian derived from the minimal coupling Hamiltonian by a canon-<br />ical transformation. The light-matter interaction in the multipolar Hamiltonian is<br />described in terms of the space integral of inner product of polarization and electric<br />field. whereas the minimal coupling Hamiltonian uses momentum and vector poten-<br />tial, which are rather inconvenient for practical computations. Noteworthy is the fact<br />that the polarization in the integral can be treated entirely without any approxima-<br />tions. This means an infinite order of multipole moments is taken into account. Thus<br />the present approach is a generalization of the optical response formulation beyond<br />the dipole approximation. I have incorporated the optical response theory with the<br />nonuniform light-matter interaction into an electron-dynamics simulation approach<br />based on the time-dependent density functional theory (TDDFT) in real space. To <br />elucidate the electron dynamics of 1 nm-sized molecules induced by the nonuniform<br />light-matter interaction, the integrated TDDFT approach has been applied to and<br />computationally solved for a test molecular system, NC<small>6</small>N, in the dipole radiation<br />field. Several unprecedented electronic excitation modes were induced owing to the<br />nonuniform light-matter interaction using the near-field in contrast to the uniform<br />light-matter interaction that corresponds to the conventional dipole approximation.<br />For example, high harmonics were generated more easily. It has also been found that<br />the near-field with different phase and spatial structure promotes or suppresses high<br />harmonics.<br /> In conclusion, I have revealed the geometric and electronic properties of gold-<br />thiolate nanocluster compounds and developed optical response theory in an effort<br />to understand nonuniform light-matter interaction between near-filed and 1 nm-sized<br />cluster compounds. |
要旨・審査要旨 (396.0 kB)
