The interaction of molecules and material surfaces is one of the fundamental topics in surface science. Also, understanding the behavior of molecules in nano-space is increasingly important in nano- and biotechnologies. A single-walled carbon nanotube (SWNT) is a monolayered material composed of all surface atoms, which provide a simple van der Waals potential originated from a single layer both for the outer surface (quasi 2D) and inner nano-space (quasi 1D). Furthermore, the quasi 1D electronic structure of SWNT allows resonant optical transitions, which are sensitively affected by the surrounding environment of SWNT. SWNTs are thus an ideal material for examining the behaviors of molecules on the surface or in the nano-space by means of optical spectroscopy.

We use a singly suspended SWNT between mesa structures to minimize the interactions with the substrate and other nanotubes, and expose it directly to the ambient gas. Semiconducting SWNTs suspended in space exhibit resonant photoluminescence (PL) and Raman scattering, and their optical responses depend on the state of molecules adsorbed on the SWNT surface or encapsulated in the tube [1-3]. Therefore, gas molecule adsorption/desorption on the SWNT surface or inside of the tube can be probed by PL and Raman spectroscopy.

We have found that water molecules form an adsorption layer of two-monolayer thickness on the hydrophobic SWNT surface as a result of hydrogen-bonding network of water molecules [2]. The water adsorption layer has a sizable effect on the radial breathing mode frequency in Raman scattering [4]. The phase of water molecules can also affect those properties of SWNTs. The phase diagrams of the water both outside and inside of SWNT will be constructed based on the results of spectroscopy. Furthermore, PL peak energies from hybrids of single-stranded DNA and SWNT were found to depend on the DNA base type, namely thymine, adenine, cytosine, and guanine [5]. The base type dependence was attributed to the adsorption energy of the DNA bases on the SWNT surface.

[1] S. Chiashi, et al. Nano Lett. 8, 3097 (2008).

[2] Y. Homma, et al. Phys. Rev. Lett. 110, 157402 (2013).

[3] S. Chiashi, et al. J. Phys. Chem. Lett. 5, 408 (2014).

[4] S. Chiashi, et al. Phys. Rev. B 91, 155415 (2015).

[5] M. Ito, et al. J. Phys. Chem. C 119, 21141 (2015).