Adsorption effects on radial breathing mode of single-walled carbon nanotubes

For elucidation of the adsorption effects on the vibration properties of single-walled carbon nanotubes (SWNTs), photoluminescence and Raman scattering spectra from SWNTs at different vapor pressure of water were simultaneously measured and a molecular dynamics (MD) simulation was performed. The water vapor pressure dependence and its tube diameter $\left(d_{\mathrm{tube}}\right)$ dependence of the frequency of the radial breathing mode (RBM) peaks $\left({\omega }_{\mathrm{RBM}}\right)$ and the optical transition energy $\left(E_{ii}\right)$ indicate that the physical adsorption is quite important, and both ${\omega }_{\mathrm{RBM}}$ and $E_{ii}$ clearly depend on the number density of adsorption molecules on the SWNT surface. A simple adsorption model, where the vibrational coupling between the surrounding adsorption layer and SWNTs via van der Waals interaction is considered for RBM, reproduces the experimental and MD simulation results of ${\omega }_{\mathrm{RBM}}$ in a wide $d_{\mathrm{tube}}$ range for various SWNTs, such as isolated SWNTs in vacuum, SWNTs with adsorption water layer, and even bundled SWNTs. On the basis of the model, the variation of the relationship between ${\omega }_{\mathrm{RBM}}$ and $E_{ii}$ in a Kataura plot for various SWNT samples can also be understood generally as the “environmental effects.”

Figure 1

Snapshots of (a) water molecules around an SWNT and (b) bundled SWNTs in a MD simulation.

Figure 2

Water vapor pressure dependence of (a) RBM peak and (b) PL emission spectra simultaneously measured in different water vapor pressure. (c) Water vapor pressure dependence of ${\omega }_{\mathrm{RBM}}$ (open circle $\circ$) and PL emission peak wavelength (filled circle $•$).

Figure 3

Tube diameter dependence of ${\omega }_{\mathrm{RBM}}$ of SWNTs in vacuum, water-molecule adsorbed SWNTs, and bundled SWNTs. The experimental (open circle $\circ$ and open diamond $\diamond$) and MD simulation (filled circle $•$ and filled diamond $⧫$) results are shown. ${\omega }_{\mathrm{RBM}}$ of bundled SWNT ($▴$) calculated by a MD simulation are also shown. For reference, ${\omega }_{\mathrm{RBM}}$ of bundled SWNT ($▵$) are plotted [14]. The solid line (in vacuum), dashed curve (in water vapor), and dotted curve (bundled) are expressed by the adsorption model.

Figure 4

Relationship between ${\omega }_{\mathrm{RBM}}$ and the $E_{ii}$ (Kataura plot) obtained from suspended SWNTs. The PL and Raman scattering measurements were performed in vacuum (open diamond $\diamond$) and water vapor (open circle $\circ$). The plots of as-grown SWNTs ($\times$) [34] and surfactant wrapped SWNTs in solution ($+$) [33] are shown for reference. The open ($\square$) [34] and filled ($\blacksquare$) squares [35] present the Raman resonant energy and ${\omega }_{\mathrm{RBM}}$, which are calculated from the empirical equations for as-grown and supergrowth SWNT samples, respectively.