A colloidal solution of Si quantum dots (QDs) is a versatile material for electronics, optoelectronics and biophotonics. Recently, we have developed a new type of all-inorganic Si QDs that can be dispersed in alcohol and water almost perfectly without organic ligands, and exhibit size-dependent luminescence in the near-infrared range in water. The unique property of the QDs arises from the unique structure. The QD has a core-shell structure; the core is heavily B and P codoped crystalline Si and the shell is an amorphous layer composed of B, Si and P. The shell induces negative potential on the surface and prevents agglomeration of Si QDs in polar solvents. Furthermore, isolation of an exciton in a crystalline core from environment by the shell makes the luminescence insensitive to the change of the environment and robust for chemical treatments. To the ligand-free surface of codoped Si QDs, molecules in a solution can access easily, and thus a variety of charge-transfer-induced processes such as a photocatalytic effect and chemical doping are expected.
In this presentation, we first discuss the structure of codoped Si QDs based on the data obtained by atom probe tomography and transmission electron microscopy. We then discuss the size dependence of the energy state structure, especially that of the donor and acceptor levels, based on the data obtained by scanning tunneling spectroscopy, photoemission yield spectroscopy and photoluminescence spectroscopy. We then discuss charge transfer interaction between Si QDs and adsorbed molecules, including the photocatalytic activity and chemical doping. We show the formation of a variety of nanocomposites composed of codoped Si QDs and noble metal nanostructures, and discuss the enhanced optical responses due to the coupling with the localized surface plasmon resonances of metal nanostructures. Finally, we discuss the electrical transport property of a Si QDs film produced from the colloidal solution.
Prof. Fujii leads an active research group in the field of mesoscopic materials with research interests including the synthesis and optical properties of group IV semiconductor nanostructures, impurity doping in semiconductor nanostructures, plasmonic nanostructures, and nanocomposite materials. He graduated from Kobe University in March 1992, and has held the following positions:
1992 – 1994 Panasonic Corporation, Semiconductor Research Center
1995 – 1997Graduate School of Sci. and Tech., Kobe University (Research Associate).
1997 – Faculty of Engineering, Kobe University (Research Associate).
(2001 – 2002) Physics Department, Technical University of Munich, Germany (Supported by Alexander von Humboldt foundation)
2003 – 2008 Faculty of Engineering, Kobe University (Associate Professor).
2009 – Graduate School of Engineering, Kobe University (Professor).