- Research Philosophy
- Against the backdrop of global environmental and energy issues, in recent
years there has been a focus on research into energy conservation in human
society and the creation and use of renewable energy. Since the beginning
of this century, there has been a rapid and visible spread of lightweight,
flexible light-emitting and power-generating elements based on functional
organic semiconductors that make up smartphones and TV screens. More recently,
nanosheets made of one or a few atomic layers, and nanometer-scale structures
and particles made of several to several thousand atoms have been attracting
attention as next-generation nanomaterials. Furthermore, it is hoped
that the aggregation and combination of these nanoscale functional material
units will lead to the creation of advanced nanodevices that will support
advanced human society in the mid-21st century and beyond. The unique functions
found in nanomaterials and their composites are governed by the behavior
of the carriers (electrons and holes) that move around within them. For
example, when a solar cell is constructed using nanomaterials, light carriers
that carry electric current (electric power) are generated by light (sunlight),
and they move between functional units at an ultrafast time scale of femtoseconds
(1000 trillionths of a second). In order to understand the origin of the
functions of nanomaterials and create more highly controlled devices, it
is essential to clarify the behavior of carriers on such ultrafast time
scales using precise observation methods. We are promoting research to
further refine the functions of current nanodevices, etc., by establishing
technologies to precisely create nanomaterials and their composites controlled
on the atomic and molecular scales, and clarifying the origin of their
functions from the perspective of ultrafast carrier dynamics.
- Research Subject
- Endohedral metallofullerenes
Hyper-aromatic organic semiconductors
Semiconductor quantum dots
Metallic nanostructures
Nanoclusters etc.
- Research content 1
- The fabrication of semiconductor quantum dot superlattices and the elucidation
of ultrafast photoexcitation dynamics: When semiconductor and metal particles
are reduced to the nanoscale (nano means one millionth of a millimeter),
material functions that cannot be seen at normal sizes appear. In particular,
semiconductor nanoparticles (quantum dots: QDs) are widely researched as
nanomaterials with excellent luminescent properties, and are already widely
used in the market as light-emitting elements that convert electrical energy
into light, such as QD displays. In addition, when semiconductor QDs are
arranged in a regular pattern, they are expected to be applied as elements
that convert light energy into electricity (solar cells), and research
is being carried out in this area. In order to determine the principles
of “photovoltaic conversion”, which converts electrical energy into light
or light energy into electricity, and to propose more advanced element
designs, it is necessary to observe the state of quantum dots that express
their function through light irradiation in detail. Therefore, this research
group is promoting research to observe the ultra-fast photoexcitation dynamics
that are deeply involved in the expression of function, as well as the
fabrication of a regular array structure of semiconductor QDs. Starting
with the development of time-resolved spectroscopy equipment based on femtosecond
pulsed lasers, we aim to clarify and utilize the “aggregation effect” that
appears for the first time when semiconductor QDs are arranged in a regular
pattern through characteristic high-precision spectroscopic measurements
that are unprecedented even on a global scale. Research content 1 Endohedral
fullerenes Superatoms Aromatic organic semiconductors Semiconductor quantum
dots Metal nanostructures Nanoclusters, etc.
- Emission of semiconductor quantum dots
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- Research content 2
- Elucidation of photoexcitation dynamics in organic semiconductor thin films
It is no exaggeration to say that device technology (organic electronics)
based on functional organic semiconductors such as organic EL and organic
solar cells is one of the most advanced technologies to have developed
in this century. Organic EL, which was considered revolutionary 20 years
ago as a small monochrome clock display on mobile phones, is now dominating
the market as ultra-thin large-screen TVs and flexible displays. However,
it has become clear that the operating principles of these organic devices
differ slightly from those of conventional inorganic materials (such as
silicon), and in order to further develop organic electronics towards the
middle of this century, it is necessary to investigate the behavior (dynamics)
of the charges on the surface and interface of organic semiconductor thin
films in detail, and to establish design guidelines for optimizing their
functions and advancing them to a higher level. In this research theme,
we are developing a study that uses functional organic molecules used as
the components of organic devices to create ultra-thin films of polycyclic
aromatic organic molecules, which form the basic framework, on a substrate,
and then observes the functional expression that occurs as a result of
photoexcitation in the film or at the interface between the film and the
substrate using photoelectron spectroscopy with a femtosecond pulsed laser
as the light source.
Main research results
M. Shibuta, K. Yamamoto, T. Ohta, T. Inoue, K. Mizoguchi, M. Nakaya, T.
Eguchi, A. Nakajima, “Confined Hot Electron Relaxation at the Molecular
Heterointerface of the Size-Selected Plasmonic Noble Metal Nanocluster
and Layered C60”ACS Nano 15, pp. 1199–1209 (2021).
K. Stallberg, M. Shibuta, U. Höfer, “Temperature effects on the formation
and the relaxation dynamics of metal-organic interface states”Physical
Review B 102, pp. 121401(R)-1−5 (2020).
M. Shibuta, N. Hirata, T. Eguchi, A. Nakajima, “Photoexcited State Confinement
in Two-Dimensional Crystalline Anthracene Monolayer at Room Temperature”
ACS Nano 11, pp. 4307–4314 (2017).
M. Shibuta, N. Hirata, T. Eguchi, A. Nakajima, “Probing of an Adsorbate-Specific
Excited State on an Organic Insulating Surface by Two-Photon Photoemission
Spectroscopy”Journal of the American Chemical Society 136, pp. 1825–1831
(2014).
- Research content 3
- Imaging of propagating surface plasmon polaritons
At the interface between a metal and a dielectric (including a vacuum),
there is a collective oscillation mode of free electrons called a surface
plasmon. The term 'surface plasmon' may not be very familiar, but this
principle has been put to use in the coloring of glass ornaments in the
past, and in the modern era, for example, in the development of high-efficiency
solar cells. Among these, surface plasmon polaritons (SPPs), which are
propagating surface plasmons that couple with light at flat interfaces
such as gold and silver, are attracting attention as a technology element
(plasmonics) that can be used to efficiently store and convert the energy
of visible and near-infrared light, which makes up a large proportion of
the sunlight that reaches the earth's surface, into devices and other applications.
In addition, since the propagation speed of SPP is close to the speed of
light (more than 90% of the speed of light in the near-infrared region),
it is expected to be useful for the integration of optical communication
circuits.
The aim of this research theme is to establish imaging technology for
propagating surface plasmon polaritons (SPPs), which are essential for
the development of plasmonics. It is difficult to capture the propagation
of SPPs, which do not generally involve radiation, using a normal optical
microscope, and it is only possible to do so by imaging the spatial distribution
of another phenomenon emitted from the photoexcited state, namely, photoelectron
emission. Until now, we have succeeded in clearly visualizing the propagation
mode of SPP using a photoelectron microscope with a femtosecond laser,
and have also shown that it is possible to precisely evaluate the relationship
(dispersion) between the propagation speed of SPP and the speed of light. Recently,
we have been working on developing a new, highly versatile SPP imaging
technique that converts propagating SPP into light using highly efficient
luminescent materials, and also on overcoming the limitations of the observation
principle of observing photoelectrons, and making it possible to observe
SPP at “buried interfaces” that are difficult to detect using photoelectron
emission with appropriate sensitizers.
Main research results
M. Shibuta, A. Nakajima, “Spectroscopic imaging of photoexcited states
at a polycrystalline copper metal surface via two-photon photoelectron
emission microscopy"Chemical Physics Letters, Accepted (2022)
K. Yamagiwa, M. Shibuta, A. Nakajima, “Visualization of Surface Plasmon
Polaritons Propagating at the Buried Organic-Metal Interface with Silver
Nanocluster Sensitizers”ACS Nano 14, pp. 2044–2052 (2020).
K. Yamagiwa, M. Shibuta, A. Nakajima, “Two-photon photoelectron emission
microscopy for surface plasmon polaritons at the Au(111) surface decorated
with alkanethiolate self-assembled monolayers”Physical Chemistry Chemical
Physics 19, pp. 13455–13461 (2017).
Overview of SPP imaging
- Collaborative research partners
- Keio University Nakajima Laboratory
Osaka University Akai Laboratory
Hokkaido University Taketsgu Laboratory
Institute for Molecular Science Kera Group
Philipps Universitaet Marburg Prof. U. Höfer Laboratory
Research Institute for Material and Chemical Measurement (AIST)
Kyoto University Teranishi Laboratory
Osaka Metropolitan University Kanasaki Laboratory
Friedrich-Schiller-Universität Jena (Germany）) Prof. Torsten Fritz Laboratory
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