laboratoire pierre aigrain
électronique et photonique quantiques
laboratoire pierre aigrain

Seminar, 15th Decembre 2016 (14h00 L363-365)

Tobias Hertel (Wurzburg université)
Spectroscopy of doped carbon nanotubes

The control of charges in conventional doped semiconductors by chemical
or by physical means is key to the technologies driving the modern
information age. Not surprisingly, the investigation of chemical and
physical doping of semiconducting single-wall carbon nanotubes (s-SWNTs)
has received considerable attention for nearly two decades. However, the
assessment of doping levels and the prediction of associated changes in
the electronic structure of SWNTs, still presents challenges for theory
and experiment. Despite previous efforts, interest in mastering
SWNT-doping thus continues to be strong, being driven both, by questions
of fundamental as well as of applied significance.

Here we will look at some recent experimental work, investigating the
photophysical properties of chemically and of electrochemically
gate-doped s-SWNTs by stationary and by femtosecond time-resolved
spectroscopy, at the single-particle and at the ensemble levels. Through
these studies we aim at developing a better understanding of s-SWNT
doping and the associated changes of optical spectra for the purpose of
spectroscopic quantification of carrier concentrations and band gaps. At
the same time, we are interested in the role of electronic correlations
for the spectroscopy of s-SWNTs. We find that the intuitive and
frequently used independent particle model often does not facilitate a
satisfying description of phenomena observed by stationary or
time-resolved spectroscopy. Evidence for many particle effects induced
by strong electronic correlations is here provided by the experimental
search for band gap-renormalization in doped s-SWNTs and from
femtosecond time-resolved spectroscopy of intrinsic and doped s-SWNTs.
This suggests that electronic correlations play a very important role
for the electronic properties of SWNTs, much more so than typically
expected for bulk semiconductors, comparable perhaps to those of smaller
molecular systems.

We also discuss the use of different optical probes for assessing free
carrier band gaps of s-SWNTs and compare the effects of different doping
schemes for exciton- and trion-photophysics as well as for energy
transport and dissipation. The absorption and PL-spectra of highly
enriched (6,5) s-SWNTs, carefully doped by electrostatic gating or by
redoxactive species are full of surprises, revealing among others, the
presence of a second exciton sub-band trion for moderately doped s-SWNTs
as well as a previously unrecognized ultra-broad VIS-NIR absorption
feature resembling a classical 1D free-carrier absorption band for
highly doped s-SWNTs.