Carbon nanotubes (CNTs) have extraordinary thermal and mechanical properties on an isolated nanotube level. Gaining these benefits on a macro-scale has been elusive due to the poor nanotube linkages (tube-to-tube as well as wall-to-wall for multiwall CNTs). Hence, in this study, ‘defect decoration’ at various nanotube locations using boron ion-implantation was followed by high-pressure treatments to 20-25 GPa in a diamond anvil cell at room temperature. This was done to promote intra nanotube wall-to-wall and inter tube-to-tube interlinking in double wall carbon nanotubes (DWCNTs). The present study has shown, for the first time, the close-interval monitoring of the D-band evolution with pressure up to 25 GPa for DWCNTs, in spite of intense obscuring signals from the diamond window.

The D to G⁺ (axial) band intensity ratio for implanted DWCNTs evolves with pressure following a trajectory consistent with an empirically determined trend for proliferation of sp³ bonds in carbon materials [A.C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000)]. A maximum of ~10% volume fraction of sp³ bonds was determined, with the sp³ interlinking being initiated at ~3 GPa and proliferates to the 10% volume fraction by ~10 GPa. The pressure dependences of the G⁺ band position for both implanted DWCNTs  and a control unimplanted sample delineated three pressure regimes: 0 ≤ P ≤ 3 GPa, 3 < P ≤ 11 GPa and P >11 GPa. The change in pressure dependence at ~3 GPa  is attributed to the onset in proliferation of sp³ interlinking in the implanted sample,  whereas in the  unimplanted sample such a change is due to disruption of the starting sp² bonded structure.   Upon further pressurization appreciable deviations from circular or polygonal cross-sectional shapes occur to the implanted  DWCNTs at 11-12 GPa compared with 18-19 GPa for unimplanted DWCNTs.   This occurrence of shape changes at earlier pressures in the implanted sample is attributed to coupling between tubes, from the sp³ interlinking.  The integrity of the interlinked tubular structures remains intact even up to 20-25 GPa, based on favourable signatures of DWCNT integrity by way of low wavenumber radial breathing modes still discerned in samples decompressed to ambient pressure. Thus, irreversible sp³ interlinking to the levels of 10% volume fraction have been obtained by combined ¹¹B⁺ implantation and pressurization of DWCNTs. The results of this study demonstrate the benefits of such coupled double- parameter manipulation of CNT bundles to produce novel interlinked CNT structures at low pressure thresholds and at room temperature.

Complementary UV-Raman spectroscopy studies show a definite, albeit small, intensity contribution above the reference baseline of the implanted and recovered material in the region of ~ 1100 cm^-1, where sp³ signatures are known to occur. This is compatible with a 10% volume fraction of sp³, upon comparison with reported spectra from systems in the literature with higher sp³ volume fractions.