High-pressure behavior of polyiodides confined into single-walled carbon nanotubes: A Raman study

Abstract

The high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range ͑0-17 GPa͒. Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species.

Role of the pressure transmitting medium on the pressure effects in DWCNTs

“…It has been demonstrated that the homogeneous filling of the SWCNT cavity with an inner wall, forming what is called a double-walled carbon nanotube (DWCNT), stabilizes the outer tube [8][9][10][11][12][13]. In contrast, filling SWCNTs with C 70 or iodine molecules, which is considered as a case of inhomogeneous filling, leads to the destabilization of the nanotubes [8,10,11,14,15]. This was attributed to the inhomogeneous interaction, i.e., van der Waals forces, between the nanotube wall and the inner filler.…”

Metallization of single-wall carbon nanotube thin films induced by gas phase iodination

“…In our case, the up-shift is not so big (2 cm -1 ). According to our estimations, the gaseous filling results in the lower doping degree comparing with that in case of filling from the iodine melt [16]. Meanwhile, doping with molten iodine is much more destructive for SWCNTs than the gaseous iodination.…”

“…The reason is its high intercalation ability, high level of detection inside nanotubes and a high efficiency of SWCNT modification [13]. Usually it is injected inside SWCNTs as elemental iodine dissociated into the building blocks of I 3 -and I -in the molten or gaseous phases [16]. As a result, the polyiodide structures are formed inside nanotubes or between nanotubes in the bundles [17].…”

Analytical Methods for Iodine and Iodides

“…Although solid iodine has a raman-active mode at 180 cm −1 , no spectral features are seen in the infrared (Ir) spectra as a result of the symmetry selection rules [24][25][26]. The ν 1 symmetric mode for molten and gaseous iodine are at 194 and 213 cm −1 , respectively [27,28].…”

“…Iodine molecule (I 2 ) and the lewis base donors I − and I 3 − can be regarded as the main building blocks of polyiodides [24, 30b, 32]. Polyiodides do not exist as discrete entities but are always a combination of I 2 molecules and I − and/or I 3 − anion chains [24,25]. They can be described by the following formula: mI 2 + nI − → I (2m+n) n− , leading to several possible structures.…”