Resonant Raman Scattering Studies of SWCNT Templated Extreme Nanowires of Hgte and Pbi

By filling the central pore of a single walled carbon nanotube (SWCNT) it is possible to form nanowires of a wide range of inorganic materials that are one unit cell in diameter. These are extreme nanowires, i.e. at the limit of miniaturisation of nanowires. Nanowires of these diameters show extreme quantum confinement effects and in some cases have crystal structures which cannot be stabilised as 3D crystals. Wide ranges of scientifically and technologically interesting materials have already been used to produce such nanowires. These include ferromagnetic materials, e.g. Fe and Co, phase change materials, e.g. GeTe, ferroelectric materials, e.g. SbSeI, and semiconductors, e.g. HgTe. It is likely that extreme nanowires will have entirely new properties, not just modified versions of the properties of the materials from which they are formed. Such properties would be interesting both fundamentally and technologically.  However, as yet there has only been very limited characterisation of the properties of nanowires formed inside single walled carbon nanotubes and the fundamental physics is not yet really understood.

We will present a comprehensive study of the Raman spectra of the vibrational modes of HgTe extreme nanowires inside single walled carbon nanotubes alongside atomic resolution TEM of the samples. Both TEM and the vibrational spectra clearly indicate that HgTe forms nanowires with an entirely new crystal structure when infiltrated into 1.3-1.4nm diameter SWCNTs. The strength of the Raman scattering is strongly dependent on incoming photon energy, with a strong resonance centred at 1.77eV which tails off more slowly to low energy than to high. Whilst all of the fundamental and combination modes are resonant at the same basic energy, the shape of the resonance varies for different features. It is clear that the form of the resonance has a lot to tell us about the electron-phonon coupling in these 1-Dimensional structures.  We will present the dependence of the Raman scattering on excitation and scattered polarisation, which is entirely in line with what would be expected for nanowires. However the temperature dependence of the Raman scattering is anomalous, showing a decrease in scattering intensity by a factor of ten with increasing temperature from 4K to 300K. We will also present results of electrochemically gated Raman scattering from HgTe nanowires which allow us to determine the band alignment of the template tubes and extreme nanowire. In addition to a comprehensive discussion of Raman scattering from HgTe nanowires, we will also present Raman scattering from PbI2 and CoI2 nanowires formed in SWCNTs and prove that the physics demonstrated by the HgTe nanowires is more generally applicable.