"Engineering of the electronic and opto-electronic properties of carbon nanotubes via artificially induced defects and electrostatic doping "
Who: Gilles Buchs (Swiss Center for Electronics and Microtechnology)
Place: Donostia International Physics Center (DIPC). Paseo Manuel de Lardizabal, 4, Donostia
Date: Tuesday, 2 June 2015, 12:00
Engineering of the electronic and opto-electronic properties of carbon nanotubes via artificially induced defects and electrostatic doping
Gilles Buchs, Swiss Center for Electronics and Microtechnology (CSEM), Switzerland
The remarkable electronic and optical properties of single-walled carbon nanotubes (SWNTs) make them one of the most promising candidates for novel nanoelectronic and optoelectronic devices. Due to their quasi-one-dimensional geometry, the electronic bands of SWNTs can be engineered by means of artificially induced defects or/and by electrostatic doping.
Firstly, results of low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS)
studies of SWNTs exposed to different ion species with different kinetic energies will be presented. In particular, intratube quantum dots showing particle-in-a-box-like states with level spacing up to 200 meV in metallic SWNTs by means of low dose medium energy Ar+ irradiation will be displayed. By means of Fourier-transform scanning tunneling spectroscopy (FTSTS) combined with the results of a Fabry-Pérot electron resonator model, clear signatures for inter- and intra-valley scattering of electrons confined between consecutive defects are shown [1-3]. Similar results for semiconducting SWNTs  will be presented and their possible implications for SWNT-based quantum optics will be discussed.
Secondly, scanning photocurrent microscopy (SPCM) investigations of individual suspended SWNTs
devices where the potential profile can be engineered by means of local gates will be introduced, in
particular the generation of in-situ p-n junctions showing ideal diode behaviour . More specifically, a study addressing the fundamental understanding of the generation of photocurrent in semiconducting SWNTs will be presented . By placing the laser spot at the centre of a suspended nanotube and recording the photocurrent for the entire gate voltage space, a two-dimensional photocurrent map is obtained, from which it can be deduced that for specific gate voltages, the electrons are travelling ?uphill?, i.e. against the electric field. This remarkable result cannot be explained by the well-known photovoltaic effect, but it can be understood through a subtle thermoelectric effect. It is also shown that both photovoltaic and photothermal currents are always present in the nanotube device and that their dominant or non-dominant character strongly depends on the properties of the metal contacts. Such important findings bring a better understanding of the photocurrent generation mechanisms in nano-carbon based optoelectronics, allowing for more control in the design of future devices such as solar cells.
Finally, some possible applications of the above in the field of femtosecond laser pulses generation will be discussed.
 G. Buchs, D. Bercioux et al., Phys. Rev. Lett. 102, 245505 (2009)
 D. Bercioux, G. Buchs et al., Phys. Rev. B 83, 165439 (2011)
 L. Mayrhofer and D. Bercioux, Phys. Rev. B 84, 115126 (2011)
 In preparation
 G. Buchs et al., J. Appl. Phys. 110, 074308 (2011)
 G. Buchs, S. Bagiante and G. A. Steele, Nature Communications 5, 4987 (2014)