Séminaire Institut

Vendredi 21 Janvier 2011 à 14h00.

ELECTRONIC PROPERTIES OF C AND CNX – SWNT: A COMBINED TEM AND STM APPROACH

''The talk will review recent advances obtained on the relationships between structure and electronic properties of pure carbon and carbon doped single wall nanotubes (SWNT) by coupling two experimental tools: transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). After presenting the basic principles of these two techniques and their potentialities, we will first consider pure C-SWNT. The band gaps of semiconducting single wall carbon nanotubes are known to depend on their diameter and helicity. First experimental evidence was provided by pioneering STM experiments [1], where diameter dependence was analysed within the framework of a tight binding mode, which neglects many-body effects. However, in 2005, optical luminescence measurements have revealed the importance of these effects, by demonstrating the existence of huge excitonic effects, with an exciton binding energy equal to about 1/3 of the band gap [2]. In order to solve this controversy, we have coupled measurements using a low temperature STM operating under UHV conditions with TEM analyses of the structure and optical absorption measurements [3]. STS measurements have revealed a screening effect due to gold substrate which results in a gap reduction, depending on the tube-substrate distance. Thanks to a simple modeling of the screening effect, we have determined how to extract the intrinsic gap of the tube from STS measurements. Furthermore, comparing STS results with optical absorption measurements on the same sample provides a value of the exciton binding energy, fully compatible with luminescence experiments [3]. These results provide a deeper knowledge of many-body interactions in these 1D systems and solve the apparent controversy between the pioneering STM measurements, and optical experiments [1, 2]. Furthermore, STS allows us also to image directly the wave functions at the Van Hove singularities of the WNT density of states. Local conductance measurements show spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both semiconducting and metallic tubes, demonstrating the symmetry breaking of molecular orbitals in nanotubes. We show that the observed conductance images, including spectacular complementary effects when going from one singularity to the other, can be accounted for very accurately, using the simplest tight-binding model, confirmed by ab initio calculations [5]. Finally, we have studied the impact of nitrogen atoms on the electronic structure of C-SWNTs. Both TEM and STM techniques reveal the existence of different nitrogen configurations in the carbon network leading rise to coplex defects with specific spectroscopi signatures which will be discussed. [1] J.W. Wildoer et al, Nature 391, 59 (1998), Odom et al, Nature 391, 62 (1998) [2] G.Dukovic, et al., Nano Lett. 5, 2314_2318 (2005). [3] H. Lin et al, Nature Mat. (2010), on line 17-01-(2010) [5] H. Lin et al arXiv:0911.4437v1 [cond-mat.mtrl-sci]; Phys. Rev B (2010)''