Research

Synthesis of Si-based nanosheet bundles

A variety of Si-based nanosheet bundles have been synthesized from CaSi2 as a template using Metal-Chlorides, NH4Cl, IP6 and other chelate materials. A nanosheet bundle is also one of the important structures for technological applications. Not only the bundles in powder form but also the bundles rooted on Si substrates and were prepared. The morphological, structural and optical properties of the Si-based nanosheet bundles were clarified. The applications of the nanosheet bundles for thermoelectric generators and Li-ion batteries are now under investigation.

 

Syntheses of Si-based nanosheets
Low-dimensional materials have attracted much interest because of their enhanced or modified optical, electronic and mechanical properties compared to those of bulk materials. Hexagonal semiconducting MoSi2 nanosheets were synthesized for thermoelectric applications. In addition, Si-based nanosheets were also grown on Si substrates applications to electrochemical cells.

 

Growth of Ag Nanostructures as antiviral and antimicrobial agents

For many years, silver has been extensively investigated for its catalytic activities, anti-microbial activities, and used to avoid infections and prevent spoilage. Many researchers have focused on the anti-bacterial, ability to kill microorganisms and multi-functional properties of silver nano-particles. For practical uses, it is important to fabricate the nanostructures using a simple process with low costs. In this study, hexakisphosphate (IP6) was used as a silver carrier agent, and various shapes of Ag nanostructures were  grown.
 

Formation of Silicide nanowire bundles for thermoelectric applications

A variety of nanowire bundles and arrays based on semiconducting metal silicides have been synthesized using abundant and non-toxic materials. Two types of fabrication techniques of the nanostructure bundles or arrays, namely, direct growth and template synthesis using artificially fabricated nanostructured materials, are demonstrated. CrSi2 nanowire bundles were directly grown by the exposure of Si substrates to CrCl2 vapor at atmospheric pressure. Mg2Si/MgO composite nanowire, Mg2Si and MnSi1.7 nanowire bundles were synthesized using a SiOx nanofiber bundle, Si nanowire arrays as the templates. Easy fabrication of thermoelectric generators will be expected using the bundle as electrodes of the generators.
 

Shape and morphological modifications of Si nanowires/microrods

It is important issue to control shape and morphological properties of nanostructures, because the physical properties of the nanostructures are dependent on their dimensions and shape. The modification of the cross-sectional shape of the Si nanowires/microrods was successfully demonstrated, and Si nanowires with a round, hexagonal, square and triangular shapes have been synthesized. The triangular nanowire has a sawtooth faceting structures on the sidewall, which would be a further preferable structure to reduce the thermal conductivity for thermoelectric applications.Using the triangular thin nanowires, further improvement of thermoelectric generators will be expected. 
 

Explore new features of exotic silicides

Semiconducting silicides, which consist of non-toxic and abundant materials, have attracted much attention for their potential to create new classes of environmentally conscious electronics. Ca2Si, h-MoSi2,,,, were successfully synthesized, and the features of the silicides are theoretically explored.
 

Development of silicide nanocomposites, and their device applications for electric generators

Prof. Tatsuoka and two students visited three Universities in Taiwan as “Summer Visiting Program in Taiwan” provided by the Interchange Association Japan (IAJ), from September 23 to 29, 2011;

1) Prof. Li-Jen Chou, Department of Materials Science and Engineering, National Tsing Hua University

2) Prof. Yu-Ting Cheng, Department of Electronics Engineering, National Chiao Tung University

3) Prof. Jr Hau He, Department of Electrical Engineering, National Taiwan University

The joint research subject was entitled as “Development of silicide nanocomposites, and their device applications for electric generators”. During the 7-day visit in Taiwan, we successfully developed research exchanges with the three laboratories. This short visit brought new experimental achievements to us, and provided a chance to move to the next research stage. Based on the discussions we had, our future collaborations will be expected. In addition, all the students had effective exchange activities with each other through research and social activities, and enhanced their research motivation. The summer visiting program is reported through the IAJ website (Click here).

 

Silicide Synthesis for Thermoelectric Materials

II-group silicides have been recognized as a promising candidate for one of non-toxic environmentally friendly materials. Mg2Si and Mg2SiGe compounds are synthesized  for thermo-electric applications, and excellent n-type materials can be grown. However, it is difficult to obtain p-type semiconductors using the II-group silicides. Thus, Ca-silicides have been proposed as p-type semiconductors, and synthesized by heat treatment of Mg2Si powders under a Ca vapor. The Ca2Si tablets were prepared by sparking plasma sintering (SPS). Its structural, electrical and thermoelectric properties have been investigated. The silicides were also grown by mechanical alloying techniques, and the formation of new types of silicides can be expected.

 

MnSi1.7 for Thermophotovoltaic batteries

Higher manganese silicides (HMS) with a composition near MnSi1.7 are some of the semiconducting silicides with a direct band gap of about 0.7 eV, which are stable at high temperature, and are considered to be appropriate semiconducting materials for use in infrared photovoltaic devices, such as thermophotovoltaic batteries. MnSi1.7 has a tetragonal crystalline structure with a lattice constant of a = 0.553 nm and an unusually long c-axis up to 10 nm. The unusual crystalline structure of MnSi1.7 makes it difficult to grow high quality layers on highly symmetric Si substrates. A new and simple technique for growing semiconducting manganese silicide layers on Si substrates using MnCl2 has been developed, and the structural property of the resultant silicide layers is examined.
 

b-FeSi2 Semiconducting Silicides

Recently, semiconducting silicides, which consist of nontoxic and abundant materials, have attracted much attention for their potential to create new classes of environmentally freindly electronics. Much effort has been made to grow them, especially b-FeSi2 layers for the applications to  infrared photo-electronic devices.
 

Oxide Nano-structures & applications

Nano-sized materials have attracted much interest due to their optical, electronic and mechanical properties in comparison to those of bulk materials. Recently, b-Ga2O3 and other oxide, such as ZnO, a-Fe2O3 and CuO have attracted much attention as new functional materials. Nano-structures of the oxides with various morphologies have been synthesized by the thermal oxidation of specific seed materials with Ga.
 

SiOx nanofibers

One-dimensional materials have attracted much interest due to their optical, electronic and mechanical properties compared to those of bulk materials. Nanowires and nanotubes made of various kinds of materials have been intensively synthesized, such as SiOx, b-Ga2O3, CuO, ZnO, Fe2O3 for optical guides, nano-filter, O2 gas sensor, fuel cell, optical devices, piezo-electronic generator, H2S catalyst, photo-catalyst etc. For example,  Nanoscale connections in future integrated optical devices and low-loss optical wave guiding for developing future microphotonic devices with subwavelength-width structures, Nano-sized imaging bundle, Luminescence materials,  Scanning near-field optical microscopy disposal chip,  Nano-filters for bio-hazard protection and high-efficiency, long-life devices.
 

Safe and Abundant Materials

O, Si, Al, Fe, Ca, Na, K, Mg……… are abundant elements existing in the Earth crust. We are trying to synthesize new functional materials consisting these safe elements above. Silicides, Oxides and their compounds are categorized in the materials group, and new devices using the compounds will be under investigation for our future generations.
 

NASA Finds New Type of Comet Dust Mineral

According to NASA News Releases,
The mineral, a manganese silicide, was discovered within an interplanetary dust particle from comet 26P/Grigg-Skjellerup by Keiko Nakamura-Messenger research group. See the web site above. Our Mnsilicide crystals have contributed the analysis of the minerals.

 

Thermoelectrics Research in Caltech

I had stayed as a visiting faculty in The Caltech Thermoelectrics Team led by faculty associate Dr. G. Jeffrey Snyder , Materials Science, Division of Engineering & Applied Science, California Institute of Technology, from August 10, 2007- February 6, 2008. I grew silicide materials and characterized their thermoelectric properties. I have expected that newly silicides or Si-related compounds would be one of thermoelectrics materials with low price and simple synthesis process.
 

“New Interdisciplinary Education & Research Foundation Integrating the State-of-the-Art Plasma Science” was established under the competitive expense allocation determined at the dean’s discretion. Prof. Tatsuoka and Mr. Qing Yang presented their research achievements in nano-technological oxide fiber growth techniques at the project seminars.
 

Luminescence study in GIA & AGT Gem Laboratory

Since I visited GIA (Gemological Institute of America ) in 1997, Dr. Lu Taijin has characterized our nano-structured wide-gap oxides. SiOx nano fibers made from FeSi2 and Ga emit beautiful ocean blue light. The photo image is taken at AGT Gem laboratory in Japan using ultra-violet Photo-Luminescence (PL) Imaging system “DiamondView”.
 

Semiconductor Growth in Singapore

I had stayed as a visitor at Prof. C. K. Ong Lab,  Department of Physics, National University of Singapore, Singapore, Singapore, from September 7 to 16,1996.  We had a collaboration research using our Hot-Wall Epitaxy technique to fabricate magneto-semiconductor hybrid structures.
 

TEM analyses in Cambridge

I had stayed as a visitor, in Prof. Colin Humphreys research group, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK, from June 25 to August 23,1996, invited by Dr. P. D. Brown ( he is now Professor in University of Nottingham ). I analyzed the interface structures of magneto-semiconductor hybrid system, with Yan Xin and Rafal Dunin-Borkowski.
 

TEM Characterizations in Durham

I had stayed as a Visiting Research Fellow in Prof. Ken Durose Lab, when he was in Department of Physics, Durham University, Durham, UK, from October 12,1992 to October 1,1993. I characterized the dislocation arrangement in HgMnTe layers grown on CdTe and CdZnTe substrates.
 

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