Quasiphase Transition in a Single File of Water Molecules Encapsulated in (6,5) Carbon Nanotubes Observed by Temperature-Dependent Photoluminescence Spectroscopy

Molecules confined inside single-walled carbon nanotubes (SWCNTs) behave quite differently from their bulk analogues. In this Letter we present temperature-dependent (4.2 K up to room temperature) photoluminescence (PL) spectra of water-filled and empty single-chirality (6,5) SWCNTs. Superimposed on a linear temperature-dependent PL spectral shift of the empty SWCNTs, an additional stepwise PL spectral shift of the water-filled SWCNTs is observed at $\sim 150\mathrm{K}$. With the empty SWCNTs serving as an ideal reference system, we assign this shift to temperature-induced changes occurring in the single-file chain of water molecules encapsulated in the tubes. Our molecular dynamics simulations further support the occurrence of a quasiphase transition of the orientational order of the water dipoles in the single-file chain.

Figure 1

(a) A representative wide-field PL image of DOC-wrapped empty (6,5) SWCNTs obtained at 4.2 K (localized exciton emission). The white circles mark individual SWCNTs. (b) Schematic molecular models of water-filled and empty (6,5) SWCNTs, demonstrating that the tube is just wide enough to fit a single linear chain of water molecules.

Figure 2

Representative PL spectra of (a) empty and (b) water-filled SWCNTs at different temperatures. PL spectra are normalized and shifted vertically by the temperature they were measured at (indicated by the horizontal gray lines). Gaussian fits to the experimental data are presented in red (empty) and blue (filled). Peak positions of $E_{11}$, as obtained from the Gaussian fits, are marked as red and blue diamonds and connected with the peak of the curve by vertical dashed lines. Magenta and cyan lines connect the $E_{11}$ points at different temperature.

Figure 3

(a) Mean PL peak positions ($E_{11}$) and standard errors of the mean for the empty (red, open circles) and water-filled (blue, filled circles) SWCNTs measured at different temperatures. The mean values and standard errors of the mean were determined from wide-field PL spectra measured for three different samples, with at least ten different positions for each sample at each temperature. The values of the empty SWCNTs are fitted linearly (magenta). (b) $\mathrm{\Delta }{E}_{11}=E_{11}(T)-E_{11}(4.2\mathrm{K})$ for empty and water-filled SWCNTs. The values of the empty SWCNTs are fitted linearly (magenta). (c) Energy difference for water-filled and empty SWCNTs $(|E_{11}^F-E_{11}^E|)$ as a function of temperature.

Figure 4

Orientational probability distribution $P(\theta )$ per solid angle ($d\mathrm{\Omega }$) of the water dipoles relative to the nanotube axis for different temperatures, using the contributions from the central seven (of the total 11) water molecules in the MD simulation (see also the Supplemental Material [46]).