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Electronic Structure Carbon nanotubes possess large π -electronic systems similar to planar graphene 1 Reduced dimensionality around the circumference.

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Presentation on theme: "Electronic Structure Carbon nanotubes possess large π -electronic systems similar to planar graphene 1 Reduced dimensionality around the circumference."— Presentation transcript:

1 Electronic Structure Carbon nanotubes possess large π -electronic systems similar to planar graphene 1 Reduced dimensionality around the circumference of the SWNT causes a quantization of the allowed wavevectors One third of nanotubes have wavevectors passing through the degenerate K point resulting in metallic behaviour. The remaining two thirds are narrow gap semi-conductors Photoluminescence-Excitation Mapping of Single Walled Carbon Nanotubes Adrian Nish, Robin J Nicholas Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK References [1] Reich, Carbon Nanotubes (Wiley) 2004 [4] Campbell R., MPhys. Project Report [2] O’Connell et al., Science 297, 593 (2002) [5] Meyer et al., Ultramicroscopy 106, (2006) [3] Bachilo et al., Science 298, 2361 (2002) [6] Oyama et al., Carbon 44, (2006) Introduction Photoluminescence-Excitation mapping has become one of the most useful methods for characterising different samples of SWNTs in terms of their constituent semi-conducting species. Some of the background, experimental method and current work on analysing these spectra is presented here. Structure and diameter are given by integer values (n,m) from C = na 1 + ma 2 where a 1 & a 2 are lattice unit vectors and C is circumference Single-Walled Carbon Nanotubes Hexagonal network of carbon atoms rolled up to make a seamless cylinder Photoluminescence Intensity Distribution PLE Mapping Different ‘cuts’ have different helicities as defined by the ‘chiral angle‘ Sharp features in the density of electronic states lead to strong optical resonances Photoluminescence from SWNTs can be observed in the near infra red 2 Tube Axis kxkx kyky PL intensity of the different nanotube species (n,m) in a map is determined by their relative concentration and their fluorescence efficiency Using empirically determined positions, a simulated PLE map was produced to model dominant tube species found in SWNTs grown by the ‘HiPCO’ method Since no sample of known (n,m) concentration exists, we must guess the distribution of nanotube species Applying a Gaussian function to the diameter and chiral angle 5 distributions localises the strong emission to a few species, similar to the experimental spectra Simulation Experiment The diameter distribution is thought to be a function of the SWNT growth process However, the chiral angle dependency is possibly due to both the growth process and a strong dependency of the fluorescence yield on this angle 6 The intensity of PL is very sensitive to the wavelength of excitation Two systems are currently used for excitation: -75W Xenon lamp & 0.3m monochromator with a linewidth of <1nm 4 { nm } -Tunable Ti:Sapphire laser with a linewidth of 0.05nm { nm } A Si-CCD { nm} and InGaAs PDA { nm} are used for detection To a first approximation the energy gap in SWNTs is inversely proportional to diameter Trigonal warping of the graphene energy surfaces close to the K point results in a further chiral angle dependency Thus distinct energy gaps exist for all species (n,m) which may then be characterised using PL Photoluminescence excitation mapping yields distinct peaks which allow unambiguous identification of nanotube species present in a sample 3 PLE Map taken with the system in the above schematic


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