@misc{oai:ir.soken.ac.jp:00001463, author = {岩佐, 豪 and イワサ, タケシ and IWASA, Takeshi}, month = {2016-02-17, 2016-02-17}, note = {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., application/pdf, 総研大甲第1219号}, title = {Theoretical Investigations of Cluster Compounds on the 1 nm Scale: Geometric, Electronic, and Optical Properties}, year = {} }