|Titel:||Single charge carrier–defect complexes in CdSe and CdS nanowires observed by photoluminescence spectroscopy||Sonstige Titel:||Beobachtung einzelner Ladungsträger-Defekt-Komplexe in CdSe und CdS Nanodrähten mittels Photolumineszenzspektroskopie||Sprache:||Englisch||Autor*in:||Reich, Aina Johanna||GND-Schlagwörter:||Spektroskopie; Halbleiterphysik; Festkörperphysik; Physikalische Chemie; Nanowissenschaften; Nanotechnologie; Chemie||Erscheinungsdatum:||2019||Tag der mündlichen Prüfung:||2019-06-28||Zusammenfassung:||
This work investigates and compares the optical properties of CdSe and CdS quantum nanowires and bulk nanowires.
To properly understand the luminescence of the nanowires, an in-depth review of the luminescence of bulk CdS is done. The different types of luminescence bands are categorised and catalogued as a further reference point. This review is contrasted with the works that reviewed the CdSe bulk luminescence.
The measurements done in this work centre on nanowires produced via wet chemistry. Multiple samples from different synthesis batches were prepared for single wire analysis for both CdSe quantum nanowires and CdS bulk nanowires. In addition, single CdS nanowires were contacted via optical lithography and bulk CdSe nanowires were synthesised in an on-substrate synthesis. The samples were then analysed with a home-built confocal setup at sample temperatures between 5 K to 7 K.
The measurements on the CdSe quantum nanowires showed strong blinking of a multitude of individual luminescence bands for single quantum nanowires. The blinking was spectrally resolved at the highest time resolution possible while retaining acceptable signal–to–noise ratios in the spectra. At the same time a time-tagged time-resolved intensity trace was recorded for luminescence decay time analysis. These two simultaneous measurements were the main type of data collected for the analysis of the CdSe quantum nanowires. In the spectral traces, with the reference of the bulk spectroscopy data, three types of individually blinking groups of luminescence bands could be identified: a group of high energy bands, the near band edge emission and the trap emission. The high energy group is the least intense group, with its origin most likely due to free and weakly bound excitons. The near band edge emission is often the dominating group of alternating sharp emission lines that also show a simultaneous emission dynamic. They are situated just at lower energies of the high energy emission. Their origin is most likely strong exciton complexes or free–to–bound recombinations. The third signal group is the trap emission, which consists of broad zero-phonon lines and two or more phonon replicas. The origin of these lines are most likely donor–acceptor pair recombinations. All of the three types of lines can be observed within a broad energy range. Previously performed calculations confirm the assignments of the different charge carrier–defect complexes to these lines, as well as the strong influence of the defect environment. The behaviour of the individual luminescence bands is best described under the assumption that each line can be associated with a specific defect site in the wire volume. Also, the nanowire blinking is usually dominated either by near band–edge blinking or trap signal blinking. Both types follow power law behaviour and are likely due to trap ionisation and surface processes. In addition some processes that influence decay time but not intensity or intensity but not luminescence decay time can be observed, showing that the quantum nanowires are much more complex system than single nanocrystals.
For CdS bulk nanowires, as for CdSe quantum nanowires, observation of emission lines with strong resemblance to the bulk emission can be made, but with much stronger sensitivity to the excitation power and much less frequent. Hyperspectral imaging and fluorescence lifetime imaging were used to observe and characterise these defect-related lines. Signals in the energy regions of bound exciton lines, free–to–bound emission and of donor–acceptor pair emission could all be identified. Unexpected for a bulk system these lines showed strong variations in energy and also luminescence blinking.
The studies showed that CdSe quantum nanowires, but also CdS bulk wires are much more complex systems than previously thought. The luminescence spectra mirror this complexity and the electronic properties on the level of single defect processes.
|URL:||https://ediss.sub.uni-hamburg.de/handle/ediss/6003||URN:||urn:nbn:de:gbv:18-100037||Dokumenttyp:||Dissertation||Betreuer*in:||Mews, Alf (Prof. Dr.)|
|Enthalten in den Sammlungen:||Elektronische Dissertationen und Habilitationen|