Espectroscopia Raman ressonante em cadeias de carbono e de enxofre

Abstract

In addition to the more well-known allotropic forms of carbon, such as graphite, diamond, graphene and carbon nanotubes, there are also linear carbon chains, based on the hybridization sp; these latter structures are purely one-dimensional and are also known as carbynes (Cn). The carbon sp comprises the lower limit of a solid. Its properties are exceptional and the theoretical forecasts put it in a prominent place in front of other materials. These chains when isolated are extremely unstable under ambient conditions and in this context the internal volume of carbon nanotubes (CNTs) is a suitable place to stabilize C n , even in extreme conditions of pressure and temperature. This behavior is also verified for the sulfur chains (S). The Cn (S) chains, when encapsulated by carbon nanotubes, form the hybrid system Cn@CNTs (S@CNTs). In this work we study the behavior of these two structures when submitted to high pressure conditions using as main tool the resonant Raman spectroscopy technique. The results of the measurements in the linear carbon chains encapsulated by double-walled cabon nanotubes (Cn@DWCNT) showed that the transfer of charge (TC) from the tube into the chain is responsible for the exponential reduction of the band frequency of the Cn (ωCn) chains as the pressure on the system increases. strand is responsible for the exponential reduction of the Cn band frequency (ωCn) as the pressure on the system increases. this is because pressure induced TC increases the bond length C≡C. In contrast to this behavior, the frequencies associated with the modes of the sulfur chains (ωS) showed a linear increase with pressure. The spectral modifications showed a change in the hybridization from sp to sp2 to the linear carbon chain, due to the formation of a reversible bond between the strand and the inner tube (∼13 GPa), acquiring a form of zig-zag. We also verify that in the pressure cycle 0−39 GPa this formed bond is irreversible and produces deformations only to the walls of the internal CNT. All of these observations were substantiated and supported by the calculations of density functional theory, presenting an excellent agreement with the experiments. In addition, we studied the behavior of Cn @MWCNTs at higher pressures (0−28 GPa) compared to that recently described in the literature (0−10 GPa). For the sample under study, we observed that it is possible to probe the C n -C n bond between long chains and between short chains, simply by changing the laser energy, since the energy gap of the chains is inversely proportional to its length. However, to trigger the Cn -Cn bond between long chains, a pressure greater than 5 GPa (pressure that triggers the bonding between short chains) is required, since they are more stable. This interpretation is based on calculations of molecular dynamics from an earlier work by N.F. Andrade et al. Finally, the results of the Raman measurements in the S@SWCNTs samples showed that S chains (square-projection 3D helices) are little susceptible to pressure, being reversible in all investigated cycles (maximum pressure of 40 GPa). This system reversibility suggests that there is no bond formation between the S chains and the CNT wall, in contrast to the Cn chains. An interesting behavior of the S chains is that the manifestation of their resonance depends on the fact that the encapsulating CNT is metallic and in addition, it is necessary that the distance between the S atoms of the chain and carbon of the wall of the nanotubo is short. This latter factor can be identified by pressing the system so that it is possible to observe a change in the intensity profile in the band of the chains of S. The experimental and theoretical results described in this thesis provide new information on the behavior of hybrid systems Cn@DWCNTs and Cn@MWCNTs when subjected to high pressure conditions. This work also suggests a convincing response to the G-band signal of the S-chains encapsulated by small diameter SWCNTs.