Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties

岩佐, 豪; イワサ, タケシ; IWASA, Takeshi

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Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties

https://ir.soken.ac.jp/records/1463

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Item type

学位論文 / Thesis or Dissertation(1)

公開日

2010-03-25

タイトル

Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties

タイトル

Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties

言語

eng

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資源タイプ識別子

http://purl.org/coar/resource_type/c_46ec

資源タイプ

thesis

著者名

岩佐, 豪

フリガナ

イワサ, タケシ

著者

IWASA, Takeshi

学位授与機関

学位授与機関名

総合研究大学院大学

学位名

博士（理学）

学位記番号

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総研大甲第1219号

研究科

値

物理科学研究科

専攻

値

07 構造分子科学専攻

学位授与年月日

2009-03-24

学位授与年度

値

2008

要旨

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The aim of this thesis is to theoretically study geometric, electronic, and optical prop- erties of one-nanometer sized cluster compounds. The thesis is composed of two parts. In the first part, the geometric and electronic properties of gold-thiolate cluster com- pounds, which have recently been studied experimentally, are revealed. I will discuss how the local geometric structures are related to the electronic properties of the com- pounds. In the second part, optical response theory that is applicable to the nan- ocluster compounds is developed. Special emphasis is placed on nonuniform electronic excitations induced by near-fields. 　　Let me briefly review history of metal nanoclusters. Research in nanocluster com- pounds has its root on the study of bare metal clusters in gaseous phase, where size- dependent physicochemical properties are the main concern. However, most of these bare clusters are energetically and chemically unstable. In the past few decades, metal clusters protected by organic molecules have been synthesized in solution, and some of these cluster compounds were found to be stable even in the air.　Although these nanocluster compounds were expected to be promising candidates for functional nanomaterials in a wide range of nanotechnologies, it is not trivial to characterize their detailed structures. Reducing the size of clusters to the 1 nm scale, their geometries and other properties become much more sensitive to the change in size and chemical compositions. In such circumstances, sub-nanometer sized gold-cluster compounds have intensively been synthesized with the definitive determination on the chemical compositions. Despite the brilliant results, even their geometrical structures have not sufficiently been characterized. Furthermore, the studies on their optical properties are still in the juvenile stage. For these reasons, I theoretically study the geometric, electronic, and optical properties of some representative cluster compounds at the 1 nm scale. 　　The geometric and electronic structures of a gold-methanethiolate [Au25(SCH3)18]+ are investigated by carrying out the density functional theory (DFT) calculations. The obtained optimized structure consists of a planar Au7 core cluster and Au-S com- plexes, where the Au7 plane is enclosed by a Au12(SCH3)12 ring and sandwiched by two Au3(SCH3)3 ring clusters. This geometry differs in shape and bonding from a gener- ally accepted geometrical motif of gold-thiolate clusters that a spherical gold cluster is superficially ligated by thiolate molecules. This newly optimized gold-methanthiolate cluster shows a large HOMO-LUMO gap, and calculated X-ray diffraction and absorp- tion spectra successfully reproduce the experimental results. On another gold cluster compound [Au25(PH3(SCH3)Cl2]2+, which consists of two icosahedral Au13 clus- ters bridged by methanethiolates sharing a vertex gold atom and terminated by chlo- rine atoms, the DFT calculation provides very close structure to the experimentally obtained gold cluster [Au25(PPh3)10(SC2H5)5CL2]2+. I further demonstrate that a vertex-sharing triicosahedral gold cluster [Au37(PH3)10(SCH3)10Cl2]+ is also achieved by bridging the core Au13 units with the methanethiolates. A comparison between the absorption spectra of the bi- and triicosahedral clusters shows that the new elec- tronic levels due to each oligomeric structure appear sequentially, whereas other elec- tronic properties remain almost unchanged compared to the individual icosahedral Au13 cluster. These theoretical studies have elucidated the fundamental properties of the promising building blocks such as geometric structures and stability of real cluster compounds in terms of the detailed electronic structures. As a next step, I have to gain a further insight into the dynamical optical properties of cluster compounds. In partic- ular for discussing photoinduced dynamics in nanoclusters or nanocluster assemblies, inter-cluster near-field interactions should be understood properly. The conventional light-matter interaction based on available lasers is quite different from the near-field interaction. The electric fields of available lasers usually have the wavelength much longer than the size of the local structure of the cluster compounds. In other words, the 1-nm-sized cluster compounds feel the almost uniform electromagnetic field and thus the local structures of the compounds cannot be resolved. In contrast, a near-field interaction occurs at the same scale of the cluster compounds and is thus expected to be used to observe the local structure of the 1 nm sized materials. The difficulty in their theoretical description arises from the fact that the near-field has a non-uniform local structure. For these reasons, I will develop an optical response theory that is applicable to 1-nm-sized clusters interacting with the near-field. 　　The optical response theory is developed in a general form on the basis of the multipolar Hamiltonian derived from the minimal coupling Hamiltonian by a canon- ical transformation. The light-matter interaction in the multipolar Hamiltonian is described in terms of the space integral of inner product of polarization and electric field. whereas the minimal coupling Hamiltonian uses momentum and vector poten- tial, which are rather inconvenient for practical computations. Noteworthy is the fact that the polarization in the integral can be treated entirely without any approxima- tions. This means an infinite order of multipole moments is taken into account. Thus the present approach is a generalization of the optical response formulation beyond the dipole approximation. I have incorporated the optical response theory with the nonuniform light-matter interaction into an electron-dynamics simulation approach based on the time-dependent density functional theory (TDDFT) in real space. To elucidate the electron dynamics of 1 nm-sized molecules induced by the nonuniform light-matter interaction, the integrated TDDFT approach has been applied to and computationally solved for a test molecular system, NC6N, in the dipole radiation field. Several unprecedented electronic excitation modes were induced owing to the nonuniform light-matter interaction using the near-field in contrast to the uniform light-matter interaction that corresponds to the conventional dipole approximation. For example, high harmonics were generated more easily. It has also been found that the near-field with different phase and spatial structure promotes or suppresses high harmonics. 　　In conclusion, I have revealed the geometric and electronic properties of gold- thiolate nanocluster compounds and developed optical response theory in an effort to understand nonuniform light-matter interaction between near-filed and 1 nm-sized cluster compounds.

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Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties

× 岩佐, 豪

× イワサ, タケシ

× IWASA, Takeshi

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