Chirality-dependent mechanical response of empty and water-filled single-wall carbon nanotubes at high pressure

Abstract

The mechanical stability of single-wall carbon nanotubes (SWCNTs) at high pressure was studied by high-resolution resonant Raman and wavelength-dependent fluorescence-excitation (PLE) spectroscopy resolving the vibrational and electronic resonances of 18 individual chiralities and furthermore even resolving the different behaviour of empty (closed, pristine) and water-filled (opened) SWCNTs (diameter range = 0.6–1.42 nm). We find that water-filling exerts a stabilizing counter-pressure on the SWCNT walls, leading to an increasing difference between the radial breathing mode frequencies of water-filled and empty SWCNTs at elevated pressures. For small diameter SWCNTs (d < 1 nm) with a chiral angle of ∼12°, in particular for the (7,2) chirality, an anomalous behaviour is observed, revealing an increased mechanical instability for these SWCNTs. We furthermore ascribe the longstanding contradiction between experiments and theory on the collapse pressure of SWCNTs to the presence of filling in most experiments to date, while empty SWCNTs follow the theoretically predicted collapse behaviour.

Introduction

Single-wall carbon nanotubes (SWCNTs) are intensively studied for their peculiar electronic and optical properties that depend critically on their specific one-dimensional chiral structure, as described by the chiral indices (n,m) [1]. Being hollow cylindrical structures of sp²-hybridized carbon atoms, all located at their surface, they combine a high mechanical resilience with a low density and a high sensitivity to environmental changes. While the axial strength and stiffness of SWCNTs are in the order of 1 TPa and 50 GPa [2], respectively, the radial mechanical stability is lower and strongly dependent on the SWCNT diameter [3], with large diameter SWCNTs (d ≳ 5 nm) even collapsing at ambient pressure [4]. For the further development of SWCNT nanocomposites and nano electro-mechanical devices (NEMS) it is important to understand this radial mechanical stability of SWCNTs.

The radial mechanical stability of SWCNTs has previously been studied by probing their vibrational, electronic and structural properties at high pressure via resonant Raman scattering (RRS) [5], [6], [7], optical absorption spectroscopy [8], fluorescence-excitation (PLE) experiments [9], [10] and X-ray diffraction [11]. The various observations have been attributed to pressure-dependent cross-sectional deformations, from circular to oval and even peanut-shaped structures. However, in most of these low-resolution high pressure optical studies of SWCNTs, the intrinsic changes of the SWCNTs' vibrational and electronic properties are entangled with ensemble averaging over multiple chiral species that complicate the interpretation of the data. Furthermore, different pressure transmitting media [5] (PTM), interactions between SWCNTs (bundles versus individualized SWCNTs [10]) and filling of the SWCNTs (e.g. argon [5], fullerenes [6], [12] or even another nanotube in double-walled CNTs [7], [8]) can influence the results of the pressure behaviour. As such, a longstanding disagreement between various experiments and theoretical results exists: while models showed that for individual 1.4 nm diameter SWCNTs the collapse pressure should be of the order of 1–3 GPa [13], [14] most experiments found signatures of the collapse at much higher pressures (i.e. ∼10–20 GPa) [5], [6], [7], [8].

To enable high-resolution (chirality-resolved) optical spectroscopy of SWCNTs, the SWCNTs need to be solubilised with bile salt surfactants in aqueous solution [15], which, unlike other surfactants, provide an extremely homogeneous surrounding for the SWCNTs and thus also an increased resolution in optical spectroscopy. This has allowed us to spectrally resolve the features of empty (closed) and water-filled (opened) SWCNTs in bulk solutions, with water-filled SWCNTs showing a hardening (i.e. shift to higher frequency) and broadening of the radial breathing mode (RBM) vibrational frequency and a red-shift and broadening of the electronic transitions [16], [17], [18]. In earlier (high-pressure) studies of surfactant-solubilised SWCNTs [8], [10], harsh sonication was applied, thereby opening a significant fraction of the SWCNTs, creating a mixture of empty (closed) and water-filled (opened) SWCNTs [16], [18], [19], [20].

In this work, we compare high-resolution spectroscopy at high-pressure of bile-salt solubilised empty and water-filled SWCNTs to study the effect of the inner environment on the pressure response of 18 different individual SWCNT chiralities. The high-resolution spectroscopy even allows discriminating the RBMs of empty and water-filled SWCNTs within the same experiment in a mixed sample [17]. We further combine the high-pressure RRS-study with excitation wavelength-dependent high-pressure PLE experiments to correlate intensity changes in RRS with changes in the resonance conditions.