Raman study of individually dispersed single-walled carbon nanotubes under pressure

We report the resonant Raman spectra of individualized $(=debundled)$ single-walled carbon nanotubes (SWNTs) having diameters $d\sim 0.8–1.3\mathrm{nm}$ subject to compression up to $10.5\mathrm{GPa}$. Both SWNTs in water-surfactant dispersions as well as SWNTs deposited onto glass microfibers and compressed with methanol-ethanol were studied. In the low pressure regime $(<1\mathrm{GPa})$, linear and reversible up-shifts of the Raman bands associated with the radial breathing vibrational mode (RBM) and tangential $G$ mode were observed. The pressure derivative of the RBM frequency increases with increasing $d$ and, unexpectedly, is larger for metallic than for semiconducting tubes. Above $1–2\mathrm{GPa}$, RBM bands of additional SWNT species appear. This is due to pressure-induced shifts into resonance and broadening of the corresponding optical transitions. The latter could be quantified by photoluminescence (PL) measurements of the corresponding semiconducting tubes that are also reported here. Disappearance of the RBM and $G$ bands contributed by nanotubes with $d\sim 0.8–0.9$ and $\sim 1.2–1.3\mathrm{nm}$ at $\sim 10$ and $\sim 4\mathrm{GPa}$, respectively, is tentatively assigned to extensive radial deformation of the nanotubes, in accordance with theoretical predictions. After the application of several GPa pressure, a significant loss of Raman signals and a relative increase of the defect-induced $D$ band are found, in particular for metallic SWNTs. We attribute these and other irreversible effects in the Raman spectra to defects generated in nanotubes under compression (most likely via chemical processes). Comparable structural deterioration of semiconducting nanotubes is evidenced by strong irreversible changes in their PL spectra.

Figure 1

(Color online) Optical interband transition (resonant Raman excitation) energies, $E_{\mathrm{ii}}$, versus RBM frequencies, ${\omega }_{\mathrm{RBM}}$, for individualized $(n,m)$ nanotubes dispersed in water—SDS. The $E_{\mathrm{ii}}$ and ${\omega }_{\mathrm{RBM}}$ values as well as their $(n,m)$ assignment are compiled from the photoluminescence study of Bachilo et al. (Ref. 31) and the Raman data of Telg et al. (Ref. 35) and Fantini et al. (Ref. 36). The plotted energy range covers the first $\left({E_{11}}^M\right)$ and second $\left({E_{22}}^S\right)$ transitions of metallic (circles and italic $n,m$ indices) and semiconducting (squares) $\mathrm{HiPco}$ nanotubes, respectively. Triangles refer to the third interband transition energies $\left({E_{33}}^S\right)$ of semiconducting tubes, as calculated by Barone et al. (Ref. 37). The horizontal lines denote the laser excitation energies applied in this work.

Figure 2

The pressure dependence of Raman spectra in the RBM frequency range for $\mathrm{HiPco}$ nanotubes dispersed in water - $1\mathrm{wt}.\%$ sodium cholate and excited at 633, 785, and $514\mathrm{nm}$. The pressure was increased in steps of $\sim 0.2\mathrm{GPa}$ up to the largest indicated value and then decreased. Each RBM pattern is contributed by several $(n,m)$ nanotubes having optical transitions in resonance with the laser excitation line (Fig. 1). The asterisks indicate new resonant RBM features that appear due to pressure-induced shifts of optical transitions of corresponding nanotubes (see Sec. 3C for details).

Figure 3

(Color online) (a) The RBM Raman spectrum of water-$1\mathrm{wt}.\%$ sodium cholate dispersion of $\mathrm{HiPco}$ nanotubes at ambient pressure as excited at $633\mathrm{nm}$ (black). Also shown are Lorentzian fits of contributing bands as well as their assignment to $(n,m)$ nanotubes. Italic $n,m$ indices denote metallic nanotubes. (b) RBM frequency shifts under low pressure conditions for the assigned nanotubes (as a percentage of the RBM frequency at ambient pressure). Also shown are linear fits for (12,6) and (8,3) nanotubes.

Figure 4

Pressure derivatives of the RBM frequency of $\mathrm{HiPco}$ nanotubes dispersed in water-$1\mathrm{wt}.\%$ sodium cholate as determined in the low pressure regime $(⩽1\mathrm{GPa})$ as a function of the tube diameter (see also Table ). Larger error bars relate to relatively weak and poorly resolved RBM bands and corresponding uncertainties of the Lorentzian fits.

Figure 5

The pressure dependence of the Raman $G$ band of $\mathrm{HiPco}$ nanotubes dispersed in water - $1\mathrm{wt}.\%$ sodium cholate and excited at 633, 785, and $514\mathrm{nm}$. For each excitation wavelength and sample, the pressure was increased in steps to the largest indicated value and then decreased.

Figure 6

Pressure dependence of the frequency and bandwidth of the most intense $G$ band component $(G+)$ as excited at $633\mathrm{nm}$ for $\mathrm{HiPco}$ nanotubes (a) compressed directly in water-sodium cholate dispersion and (b) deposited onto glass microfibers and compressed using methanol-ethanol. The large datapoint scatter and wide bands above $\sim 3\mathrm{GPa}$ in (a) arise from nonhydrostatic conditions in solidified aqueous dispersions (see Sec. 3E for details).

Figure 7

Pressure dependence of the RBM and $G$ bands for $\mathrm{HiPco}$ nanotubes deposited from $1\mathrm{wt}.\%$ sodium cholate dispersions onto glass microfibers and compressed with methanol-ethanol. The excitation wavelength is $633\mathrm{nm}$. Each spectrum has been scaled to maximum intensity and is shifted vertically for clarity.

Figure 8

$D$ and $G$ bands at different laser excitation wavelengths for $\mathrm{HiPco}$ nanotubes deposited from a cholate dispersion onto glass microfibers and measured at ambient pressure. (1) initial sample and (2) the same sample recovered from the diamond anvil cell after compression with methanol-ethanol up to $8\mathrm{GPa}$ for $\sim 10\min$. Each spectrum has been scaled to the $G$ band intensity maximum and is vertically shifted for clarity. The asterisks indicate luminescence bands of a ruby microcrystal used for pressure calibration.

Figure 9

(Color online) Raman and photoluminescence (PL) emission spectra of nanotubes dispersed in ${\mathrm{D}}_2\mathrm{O}$ - $1\mathrm{wt}.\%$ sodium cholate (intensity versus wavelength of emitted/scattered light). Shown are spectra obtained for the same sample at ambient pressure, upon increasing compression and after pressure release. The laser excitation wavelength was $785\mathrm{nm}$. The spectra are each scaled to the $G$ band intensity maximum and are vertically shifted for clarity. Five PL bands are assigned to the labeled $(n,m)$ semiconducting tubes. The asterisk refers to the Raman line of the diamond anvils.

Figure 10

(Color online) Photoluminescence contour maps (intensity versus excitation and emission wavelengths) for $\mathrm{HiPco}$ nanotubes dispersed in ${\mathrm{D}}_2\mathrm{O}-1\mathrm{wt}.\%$ sodium cholate. The PL emission and excitation maxima correspond to ${E_{11}}^S$ and ${E_{22}}^S$ energies, respectively. (A) PL map at ambient pressure with $(n,m)$ indices of emitting semiconducting tubes; (B) under $4\mathrm{GPa}$; (C) after $4\mathrm{GPa}$ pressure release; (D) after compression of a fresh sample up to $6.5\mathrm{GPa}$ and pressure release. The PL intensity color schemes are linear, but different scaling factors were used for each of the four maps (maximum intensities scale as 25: 1: 6: 2 for A, B, C, and D, respectively).