Department of General Education

Optical and Electrical Properties of Oxide Semiconductor Nanoparticles

Optical and Electrical Properties of Oxide Semiconductor Nanoparticles

We are interested in oxide semiconductor nanoparticles (NPs) such as indium tin oxide (ITO) and zinc oxide (ZnO), which have potential device applications as a transparent conductor. Unlike physical manufacturing processes requiring high vacuum and expensive experimental setups, new inexpensive chemical thermolysis to produce ITO NPs has been developed. The picture shows the chemically synthesized colorful ITO NPs in toluene with Sn different concentration (from the left, 0%, 1%, 3%, and 5.5%). We are also attempting to dope the ITO NPs with transition metal and rare-earth element to explore magnetic semiconductor. In order to understand the condensed matter physics of these materials, various experiments are performed. The optical properties are investigated by examining the photoluminescence using our He-Cd laser. Furthermore, quantum transport phenomena are revealed by measuring magnetoresistance using our cryogen-free superconducting magnet.

Graphene Transistor

Graphene is a promising candidate for future electronic devices owing to the high mobility of its carriers and its perfect two-dimensionality. However, single-layer graphene transistors have a low on-off ratio due to the zero bandgap. Therefore, new transistors using double-layer graphene have been proposed, among which are the bilayer pseudospin field-effect transistor and the symmetric graphene tunneling field-effect transistor. Chemical-vapor-deposited (CVD) graphene is better for fabricating devices on a large scale. One of the challenges with CVD graphene is that the transfer process typically results in contamination, which is very difficult to remove. A good method to overcome this issue is Ti cleaning. We found that the charge-neutral point shifted to a lower back-gate voltage and the mobility increased owing to Ti cleaning. We are investigating Ti-cleaned Hall bars under high magnetic fields in order to examine quantum magneto-transport properties. The picture shows one of the single-layer graphene Hall bars measured in our study. Furthermore, we are also measuring Raman spectra in order to examine which a stain or a charge doping is dominant for Ti-cleaned graphene. These our results will stimulate further investigations on the physics of graphene and applications such as two-dimensional materials.

Structure of Atomic Nuclei from Nuclear Force

The atomic nuclei are the many-body system consisting of protons and neutrons. We investigate the variety of the structures of nuclei by solving the motion of protons and neutrons from the view point of the nuclear force. The nuclear force acts attractively between proton and neutron and makes a nucleus bound. This force is explained by Dr. Hideki Yukawa, the first Japanese who received the Nobel Prize. Yukawa’s nuclear force is essential to build the nuclei and all the elements in the universe. However, the theoretical treatment of this force in nuclei is still difficult and challenging. The aim of our research is to understand the nuclei from Yukawa's nuclear force using the new many-body theory. The key of our theory is that we adopt the favorable components (configurations) of nuclei which describe the effect of nuclear force between the constituents at the maximum.

Knot Theory

In mathematics, there is a fascinating field of study called "knot theory", which studies knotted circles in various spaces. We consider knots made of a soft material that you can imagine as being made of knotted rubber bands. We regard two knots as the same if we can deform one into the other without cutting or pasting. For instance, the two knots illustrated in the figure are the same, although they look different. We mostly perform calculations using figures. You may think that knot theory is like a game, but it is actually a profound study that has broad applications not only in mathematics, but also in other fields of science, such as quantum physics and biochemistry.

Litho-stratigraphic and petrographic study on the tuff layers in the Paleogene Kobe Group, southwest Japan

A photograph showing an epiclastic tuff in the study area.
A photograph showing an epiclastic tuff in the study area.

The Paleogene Kobe Group distributed in the Sanda Basin in southwest Japan is composed mainly of non-marine sedimentary rocks, such as sandstone, mudstone and conglomerate. Although more than nine tuff layers are also interlayered in the Kobe Group, the correlation of the tuff layers is still indistinct. In this study, detailed a lithostratigraphic survey is carried out to clarify the precise correlation of the tuff layers. Analyses of the mineral compositions and chemical characteristics of plagioclase, biotite, and amphibole are used to discuss the effectiveness of petrographic analysis for discrimination of the tuff layers in the Kobe Group. The results of this study will contribute to further studies seeking to determine the source vent of the Kobe Group tuff layers.

Evolution of a shrimp group, Palaemonoidea

Evolution of a shrimp group, Palaemonoidea

The biodiversity of the various animals and plants living in coral reef habitats is the highest on Earth. One shrimp group living in coral reefs, subfamily Pontoniinae (family Palaemonidae), has over 500 species. Many species of these shrimp are associated with various invertebrate animals such as Porifera, Coelenterata, Mollusca, Echinodermata, and Chordata. The shrimp, especially those with commensal habits, are morphologically diverse and may have adapted to associate with various host animals. Recently, using DNA sequence data and morphology, our phylogenetic research revealed that Pontoniinae species evolved from several different lineages. We are trying to understand the evolution of these shrimp, including the deep-sea species and many other species of the superfamily Palaemonoidea, and are analyzing their phylogeny.