Abstract
On a single-walled carbon nanotube (SWCNT) surface, water forms a peculiar adsorption layer comprising two monolayers, and the physical properties of this adsorption layer remain unclear. We studied the changes that occurred in this water adsorption layer from room temperature down to 140 K using photoluminescence (PL) spectroscopy of suspended SWCNTs. The PL emission energy exhibited complex changes with temperature depending on the chirality of the SWCNTs. These changes were described quantitatively on the basis of changes in the bandgap, the dielectric constant of the adsorption layer, and the strain imposed by the adsorption layer. The results suggested that the adsorption layer might be a two-dimensional disordered solid rather than a liquid.
Highlights
Measurements at low temperature are important in investigating the intrinsic physical properties of materials because the contribution of, or disturbance by, thermal energy is reduced at low temperature
In single-walled carbon nanotube (SWCNT) without a water adsorption layer, En,mðTÞ exhibited a simple temperature dependence that was unaffected by chirality
We assumed that the factors that simultaneously contributed to the temperature dependence of En,mðTÞ were the bandgap variations, environmental conditions, and axial strain due to the difference in thermal expansion between the SWCNTs and the adsorption layer
Summary
Measurements at low temperature are important in investigating the intrinsic physical properties of materials because the contribution of, or disturbance by, thermal energy is reduced at low temperature. Such measurements are effective in elucidating the optical properties of single-walled carbon nanotubes (SWCNTs). Spectroscopic measurements of SWCNTs at low temperatures have been reported, and photoluminescence (PL) spectra from SWCNTs with various morphologies at low temperatures have been investigated.. It is an important tool for the investigation of SWCNTs. At low temperatures, PL spectra exhibit a shift in the optical transition energy, sharpening of PL peaks, long decay lifetime of excitons, appearance of multiple subbands, depression of additional peaks assisted by phonons, and thermal strain effects.
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