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Mauser, Nina (2014): Antenna-enhanced optoelectronic probing of carbon nanotube devices. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



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.