A variety of electronic and optoelectronic devices based on carbon nanotubes (CNTs) has
been implemented during the last two decades. For their optoelectronic characterization,
diffraction-limited techniques such as photocurrent (PC) and electroluminescence (EL)
microscopy were employed. However, for the full characterization of these nano-devices,
novel techniques providing nanoscale spatial resolution are desired. This work presents
antenna-enhanced optoelectronic probing as a new scanning probe technique for the investigation
of nanoelectronic devices. Based on tip-enhanced near-field optical microscopy,
sub-diffraction spatial resolution is achieved by employing an optical antenna for the focusing
of light. It is applied to study PC and EL signals with a spatial resolution better than
40 nm for the first time. Complemented with antenna-enhanced Raman and topography
images, new insights into the optoelectronic properties of CNT based devices are gained.
In the first part of this thesis, an antenna-enhanced photocurrent microscopy study is
demonstrated. The signal enhancement mechanism of PC signals is investigated and
compared with expectations based on theory. While in spectroscopic applications both
the excitation AND the emission rate is enhanced, in optoelectronic applications either
the excitation OR the emission rate is affected by the antenna. Theory predicts therefore
a weaker total signal enhancement and a lower spatial resolution of optoelectronic signals
compared to Raman scattering by a factor of √2, which is experimentally confirmed.
Then, two applications are presented. First, CNT-metal interfaces are studied and an
exponential decay of the band bending at the contacts with a decay length of about
500 nm is revealed. Second, sub-diffraction potential modulations along the CNT channel
of another device are probed that remain undetected using confocal microscopy. Combined
with high-resolution spectroscopic images of the Raman signal, defects can be excluded as
the cause for these modulations. Correlating the PC with the topographic profile reveals
charges associated with a particle on the sample substrate as the possible origin.
In the second part, antenna-enhanced electroluminescence microscopy is introduced. The
EL emitted by a heterogeneous CNT network is studied with a resolution better than
40 nm. For the first time, pinning of the EL emission to a point-like region of smaller than
20 nm is observed. This strong localization occurs at a junction of at least one metallic
and one semiconducting CNT. By probing the PC signal at this junction, the presence
of a strong local electric field is revealed, probably caused by a Schottky contact. This
allows to identify impact excitation as the most likely origin of the EL emission. A second
device, based on a single CNT, was investigated and, in contrast to the network device,
the size of the EL source is extended over a length of more than 100 nm.
| Identifer | oai:union.ndltd.org:MUENCHEN/oai:edoc.ub.uni-muenchen.de:17623 |
| Date | 30 October 2014 |
| Creators | Mauser, Nina |
| Publisher | Ludwig-Maximilians-Universität München |
| Source Sets | Digitale Hochschulschriften der LMU |
| Detected Language | English |
| Type | Dissertation, NonPeerReviewed |
| Format | application/pdf |
| Relation | http://edoc.ub.uni-muenchen.de/17623/ |
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