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Many-body effects in electronic bandgaps of carbon nanotubes measured by scanning tunnelling spectroscopy
(Result of the month 05/2010) |
Single-walled carbon nanotubes provide an ideal system for studying the properties of one-dimensional (1D) materials, where strong electron–electron interactions are expected. Optical measurements have recently reported the existence of excitons in semiconducting nanotubes, revealing the importance of many-body effects.
Surprisingly, pioneering electronic structure calculations and scanning tunnelling spectroscopy (STS) experiments report the same gap values as optical experiments. Here, an experimental STS study of the bandgap of single-walled semiconducting nanotubes, demonstrates a continuous transition from the gap reduced by the screening resulting from the metal substrate to the intrinsic gap dominated by many-body interactions. These results provide a deeper knowledge of many-body interactions in these 1D systems and a better understanding of their electronic properties, which is a prerequisite for any application of nanotubes in the ultimate device miniaturization for molecular electronic or spintronics.
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3D view of the topographic image (40 x 40nm2) with a colour scale corresponding to the differential conductance image at 12mV (black for
low value, yellow for high value). The tube marked by a triangle is in contact with the substrate, whereas the tube on top of the bundle, marked by a circle is at a certain distance from the substrate. |
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Figure 1 shows a STM image an ensemble of nanotubes adsorbed onto a Au(111) surface. The dI/dV signal close to the Fermi level used for the color mapping allows to distinguish easily the metallic and semiconducting tubes that have respectively a finite or zero local density of states at this energy. Such a bundle of tubes provides the possibility to measure the electronic structure nanotubes at different distances from the substrate. |
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dI/dV spectra taken at the positions indicated in image 1. |
The dI/dV spectra of image 2 measured on the tubes of the bundle show series of peaks corresponding to the Van Hove singularities that are characteristics of one dimensional systems. Around the Fermi level one can see the larger value for the density of states of the metallic tube (blue) as compared to the semiconducting tubes (red and green). The spectrum measured on the gold substrate displays a constant density of states (black) as expected for a metal. The semiconducting tube 2, that is separated from the substrate by the bundle clearly displays a larger bandgap than tube 1, which is directly in contact with the metallic substrate. We showed that the interaction between the tube and the image charge that develops in the metallic substrate reduces significantly the bandgap of the tube. From STS spectra, we can therefore estimate the intrinsic bandgap of a semiconducting tube and compare it with optical absorption experiments. The difference between these values gave us an estimate of the exciton binding energy. These findings allow to solve the apparent controversy between STM measurements and optical measurements. |
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Authors:
H. Lin1,2, J. Lagoute1, V. Repain1, C. Chacon1, Y. Girard1, J.-S. Lauret3, F. Ducastelle2, A. Loiseau2 and S. Rousset1
(1) Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot, CNRS, UMR7162, 75205 Paris Cedex 13, France,
(2) Laboratoire d’Etude des Microstructures, ONERA-CNRS, BP72, 92322 Châtillon, France,
(3) Laboratoire de Photonique Quantique et Moléculaire, Institut d’Alembert, Ecole Normale Supérieure de Cachan, 94235 Cachan Cedex, France.
Publications:
Many-body effects in electronic bandgaps of carbon nanotubes measured by scanning tunnelling spectroscopy
H. Lin, J. Lagoute, V. Repain, C. Chacon, Y. Girard, J.-S. Lauret, F. Ducastelle, A. Loiseau and S. Rousset
Nature Materials, 9, 235, 2010 | DOI: 10.1038/NMAT2624
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This result has been obtained with :
LT STM
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