Nano Scale Materials

In nano-sized materials, their surface and interface significantly influence on their electronic characteristics. Recently, atomically thin nano-materials such as single-wall carbon nanotubes, graphene, two-dimensional chalcogenides, are highly attracted due to their extraordinary physical properties and their potentials for device applications, but it is noteworthy that their physical properties are strongly related to their surface structures, such as chirality, edge-structures, defects, broken inversion-symmetry, and so on. Precise control of their surface and interfaces is crucial for clarification of their novel properties, and is fundamental for their electric, opto-electric, thermo-electric, magnetic applications. In this group, we focus on understanding and controlling of physical properties on the surface of nano-materials.

Control of optical properties of SWCNTs

Single-Wall Carbon Nanotubes (SWCNTs) are cylindrical graphitic tubes with diameter of 1 nm, and their electronic properties strongly depend on how their graphene sheet is rolled (called chirality). They show various colors depending on their chiralities, and we have revealed that metallic SWCNTs with diameter of 1.4, 1.0 and 0.8 nm show cyan, magenta, and yellow colors. The colors are caused by the optical transitions between van-Hove singularities in density of states, reflecting one-dimensional nature of SWCNTs. We tuned their colors by electron or hole injections through electric double layers formation using ionic liquids.
App. Phys. Express 1, 034003 (2008)
Adv. Mater. 23, 2811 (2011)

Control of conducting and thermoelectric properties of SWCNTs

Thermoelectics is a very important technology to convert waste heat to electric power. Seebeck coefficient is a parameter for characterizing the generation of electric voltage from the difference of temperature. It is of great importance to prepare negative and positive Seebeck coefficient materials for high-performance thermoelectric devices. We achieved to prepare across-bandgap p-type and n-type control over the Seebck coefficients of semiconducting SWCNTs through an electric double layer transistor setup using ionic liquids.

Control of physical properties of molecules encapsulated inside SWCNTs

We have found that molecules can change their structures inside the nanotubes, see this paper (Nature Nanotechnology 2, 422 - 425 (2007) ). We are investigating various physical properties of molecules encapsulated inside SWCNTs.

According to Gauss’s law of electromagnetism, “surface” charges on conductive cylindrical tubes cannot influence on the materials located inside the cavity. However, in the nanoscale, the meaning of “surface” should be more strictly defined. What is the depth of "surface"? If the "surface" is limited to approximately several Á in depth, the surface charge influeces on the molecules encapsulated inside SWCNTs. We found that the charges in molecules encapsulated inside SWCNTs can be manipulated through carrier injections using electric double layers.
Physical review letters 110 (8), 086801 (2013)

Control of physical properties of other materials such as nanowire, transition metal dichalcogenizes, and e.t.c.

Under investigation.