Effects of light-ion low-fluence implantation on the pressure response of double-walled carbon nanotubes

Low-fluence light-ion $\left({}^{11}\mathrm{B}^{+}\right)$ medium-energy (150 keV/ion) implantation preprocessing of double-walled carbon nanotubes (DWCNTs) has been effected to “decorate” them with defects. These are intended to serve as nucleation sites for potential $s{p}^3$ interlinking between tube walls in close proximity, following on strong deformation of the tube cross sections under cold compression to 20–25 GPa in diamond anvil cells. The pressure response of such implanted DWCNTs has been monitored in situ using Raman spectroscopy, and compared with those of unimplanted reference DWCNTs. Pressure dependences of the $G^{+}$ mode frequency and D to $G^{+}$ band intensity ratio, and radial breathing modes, have been monitored. These Raman signatures show that some degree of mechanical softening occurs in the implanted tube bundles, without major disruption to tube integrity in both samples. Consequently, the collapse pressure to racetrack or peanut shaped cross-sectional profiles is lowered substantially, from $P_{\mathrm{c}}\sim 18\mathrm{GPa}$ in the reference tube bundles to $P_{\mathrm{c}}\sim 11\mathrm{GPa}$ in the implanted case. Defect structures also proliferate more readily in the implanted sample under pressure. Therefore, the light-ion low-fluence implantation lowers the threshold pressure for deformation of tube cross sections to high-curvature profiles decorated with defects. Considerations of whether irreversible $s{p}^3$ interlinking at low volume fractions is discerned in the Raman data from implanted tube bundles under compression, and the stability of such bonding is discussed.

Figure 1

Raman spectrum (average of 64 spectra) of the unimplanted sample at ambient pressure, with intensities normalized to the G band. Inset shows an example deconvolution of the RBMs using a minimum number of Lorentzians.

Figure 2

(a) Compression sequence of Raman spectra of starting unimplanted DWCNTs in the range of characteristic D and G band signatures. The intense Raman signature appearing in the range $1332-1420{\mathrm{cm}}^{-1}$, for spectra collected at pressure other than ambient, is the first order peak of the diamond anvil. Spectra were collected at room temperature and normalized to the G band intensity. Faint bars to guide the eye, track intensity maxima from the spectral analysis, e.g., in (b), lower panels. (b) Left panel shows deconvolution of the spectrum acquired at ambient pressure, into respective components representing DWCNT Raman signatures. Two Lorentzians represent $G^{+}$ ($1597{\mathrm{cm}}^{-1}$) and $G^{-}$ ($1570{\mathrm{cm}}^{-1}$) of the overall G band. Right panel has exemplary deconvolution of the spectrum acquired at 12.6 GPa. Assignment of the low intensity peaks is described in the text (Sec. 3a).

Figure 3

(a) Compression sequence of the Raman spectra of implanted DWCNTs in the range of the characteristic D and G band signatures. Strong Raman signature appearing in the range $1332-1420{\mathrm{cm}}^{-1}$, for spectra at high pressure, is the first order peak of the diamond anvil. All spectra were collected at room temperature and normalized to the G band. Faint bars to guide the eye, track intensity maxima from the spectral analysis, e.g., in (b) lower panels. (b) Left panel shows deconvolution of spectrum at ambient pressure, into the respective components representing DWCNT Raman signatures. Two Lorentzians represent the $G^{+}$ ($1588{\mathrm{cm}}^{-1}$) and $G^{-}$ ($1560{\mathrm{cm}}^{-1}$) of the overall G band. A $D^{\prime }$ band ($\sim 1625\mathrm{c}{\mathrm{m}}^{\text{–}1}$) arises at ambient pressure (see Supplemental Material [15], Fig. S3). Right panel has exemplary deconvolution of the spectrum acquired at 16.5 GPa. Neglecting the low intensity peaks at 1555 and $1592{\mathrm{cm}}^{-1}$ leads to discrepancies in the cumulative fit compared with the data. These have the same origin as those of Fig. 2, described in the text.

Figure 4

(a) Evolution of D band position with pressure, for starting unimplanted DWCNTs. (b) Evolution of the $G^{+}$ band position with pressure, for the starting unimplanted DWCNTs. Grey solid line shows the expected maximum trajectory of the G band due to phonon hardening only, employing a model from graphite, after Hanfland et al. [30]. Insets show cross sections for pristine DWCNTs, obtained at 15 and 20 GPa from MD simulations, discussed in the text Sec. 3b [15]. (c) Change in D band position with pressure for the implanted sample. (d) Change in $G^{+}$ band position with pressure for the implanted sample. Grey line shows expected trend due to normal mode hardening, based on extrapolated behavior of the low-pressure regime. In all panels dashed lines through the data guide the eye and arrows delineate where changes in pressure dependences (preceding average slope) occur for the well resolved $G^{+}$ band. Estimates of error bars are discussed at midway and start of Secs. 2 and 3, respectively.

Figure 5

Two-stage change in D to $G^{+}$ band intensity ratio with pressure. The faint bars are to guide the eye and delineate regimes of different pressure dependences discussed in the text (Sec. 3c). Supplemental Material [15], Fig. S4, has the corresponding area ratios.

Figure 6

(a) Raman spectra of reference and processed DWCNTs: unimplanted starting material (blue), implanted material before pressurization (grey), unimplanted and recovered from 20.4 GPa (green), and implanted and recovered from 20.4 GPa (orange). All spectra are normalized to the G band peak intensity. (b) Corresponding Raman spectra of (a) in the RBM region. All spectra are normalized to the G band. Spectra are displaced from one another by an equidistant gap for clarity.

Figure 7

(a) UV-Raman spectra of starting and processed DWCNTs measured using 244-nm laser excitation: starting DWCNT material (blue), unimplanted and recovered from 20.4 GPa (green), implanted and recovered (orange). The inset is a zoom-in to the region of interest. (b) The region of interest, after subtracting out the reference spectrum for starting DWCNTs from the spectra in (a).

Figure 8

One of the possible interlinks from MD simulations representing a DWCNT with a single vacancy on the outer tube yielding three $s{p}^3$ interlinks to the inner tube.