@phdthesis{Oberndorfer2022,
  author    = {Oberndorfer, Florian},
  title     = {Photoluminescence and Raman spectroscopy of doped nanomaterials},
  doi       = {10.25972/OPUS-27854},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-278540},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {This thesis includes measurements that were recorded by cooperation partners. The EPR spec- trosa mentioned in section 5.2 were recorded by Michael Auth from the Dyakonov Group (Ex- perimental Physics VI, Julius-Maximilians-Universit{\"a}t, W{\"u}rzburg). The TREFISH experiments and transient absorption in section 5.4 spectra were performed by Jašinskas et al. from the V. Gulbi- nas group (Center for Physical Sciences and Technology, Vilnius, Lithuania). This dissertation investigated the interactions of semiconducting single-walled carbon nanotubes (SWNTs) of (6,5) chirality with their environment. Shear-mixing provided high-quality SWNT sus- pensions, which was complemented by various film preparation techniques. These techniques were in turn used to prepare heterostructures with MoS2 and hBN, which were examined with a newly constructed photoluminescence microscope specifically for this purpose. Finally, the change of spectral properties of SWNTs upon doping was investigated in more detail, as well as the behaviour of charge carriers in the tubes themselves. To optimise the SWNT sample preparation techniques that supplied the other experiments, the sample quality of shear-mixed preparations was compared with that of sonicated samples. It was found that the quantum efficiency of sheared suspensions exceeds that of sonicated suspensions as soon as the sonication time exceeds 30 min. The higher PLQY is due to the lower defect concentration in shear-mixed samples. Via transient absorption, a mean lifetime of 17.3 ps and a mean distance between defects of 192.1 nm could be determined. Furthermore, it was found that the increased efficiency of horn sonication is probably not only due to higher shear forces acting on the SWNT bundles but also that the shortening of PFO-BPy strands plays a significant role. Sonication of very long polymer strands significantly increased their effectiveness in shear mixing. While previous approaches could only achieve very low concentrations of SWNTs in suspensions, pre-sonicated polymer yielded results which were comparable with much shorter PFO-BPy batches. Reference experiments also showed that different aggregation processes are relevant during production and further processing. Initial reprocessing of carbon nanotube raw material requires 7 h sonication time and over 24 h shear mixing before no increase in carbon nano concentration is detectable. However, only a few minutes of sonication or shear mixing are required when reprocessing the residue produced during the separation of the slurry. This discrepancy indicates that different aggregates are present, with markedly different aggregation properties. To study low-dimensional heterostructures, a PL microscope was set up with the ability to ob- serve single SWNTs as well as monolayers of other low-dimensional systems. Furthermore, sam- ples were prepared which bring single SWNTs into contact with 2D materials such as h-BN andMoS2 layers and the changes in the photoluminescence spectrum were documented. For h-BN, it was observed whether previous methods for depositing SWNTs could be transferred for photo- luminescence spectroscopy. SWNTs were successfully deposited on monolayers via a modified drip coating, with the limitation that SWNTs aggregate more at the edges of the monolayers. Upon contact of SWNTs with MoS2, significant changes in the emission properties of the mono- layers were observed. The fluorescence, which was mainly dominated by excitons, was shifted towards trion emission. Reference experiments excluded PFO-BPy and toluene as potential causes. Based on the change in the emission behaviour of MoS2, the most plausible explanation is a photoinduced charge transfer leading to delocalised charge carriers on MoS2. In contrast, on SWNTs, the introduction of additional charges would constitute a quenching centre, which would quench their PL emission, making them undetectable in the PL image. In the last chapter, the electronic properties of doped SWNTs and the behaviour of charge carri- ers inside the tubes should be investigated. First, the change in the conductivity of SWNT films with increasing doping levels was docu- mented. The resistance of the films drops drastically at minimum doping. After the initial in- troduction of charges, the resistance drops with increasing dopant concentration according to a double logarithmic curve. The initial drop could be due to a reduction of contact resistances within the SWNT network film, but this could not be further investigated within the scope of this PhD thesis. In cooperation with Andreas Sperlich and Michael Auth, the spin concentration of SWNTs at different doping levels was determined. The obtained concentrations were compared with the carrier concentrations determined from PL and absorption spectra. At low spin densities, good agreement with previous models was found. Furthermore, the presence of isolated spins strongly suggests a localised charge carrier distribution at temperatures around 10 K. When the charge density is increased, the spin density deviates significantly from the charge carrier con- centration. This discrepancy is attributed to the increasing delocalisation of charge carriers at high charge densities and the interactions of neighbouring spins. These results strongly indicate the existence of localised charge carriers in SWNTs at low temperatures. Next, the effect of doping on the Raman spectra of SWNT suspensions was investigated. In gen- eral, doping is expected to reduce the intensity of the Raman bands, i.e. a consequence of the reduced resonance gain due to bleaching of the S2 transition. However, similar to the resistivity measurements, the oscillator strength of the G+ band drops sharply in the first doping steps. It was also found that the G+ band decreases more than would be expected due to loss of reso- nance condition. Furthermore, the G- is bleached faster than the G+ band. All these anomalies suggest that resonance enhancement is not the only relevant effect. Another much faster deac- tivation path for the excitons may be introduced by doping. This would leave less time for the scattering process to occur and reduce the oscillator strength of the Raman bands. In cooperation with Vidmantas et al., the photoinduced charge carrier behaviour of SWNT/PCBM films was investigated. The required films were prepared by drop coating. The SWNT suspen- sions required for this were obtained from sheared SWNT preparations. Using transient absorp- tion and TREFISH, a number of charge transfer effects were identified and their dynamics in- vestigated: the recombination of neutral excitons (< 50 ps), the electron transfer from carbon nanotubes to PCBM molecules (< 1 ps), the decay of charge-transfer excitons (∼200 ps), the recombination of charge carriers between charge-transfer excitons (1 ns to 4 ns) and finally the propagation through the SWNT network (∼20 ns)},
  subject      = {Einwandige Kohlenstoff-Nanor{\"o}hre},
  language  = {en}
}
@phdthesis{Auth2020,
  author    = {Auth, Michael Tilman},
  title     = {Quantitative Electron Paramagnetic Resonance Studies of Charge Transfer in Organic Semiconductors},
  doi       = {10.25972/OPUS-18951},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-189513},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2020},
  abstract  = {In the present work we investigated various charge transfer processes, as they appear in the versatile world of organic semiconductors by probing the spin states of the corresponding charge carrier species via electron paramagnetic resonance (EPR) spectroscopy. All studied material systems are carbon-based compounds, either belonging to the group of polymers, fullerenes, or single-wall carbon nanotubes (SWNTs). In the first instance, we addressed the change of the open circuit voltage (Voc) with the fullerene blend stoichiometry in fullerene-based solar cells for organic photovoltaics (OPV). The voltage depends strongly on the energy separation between the lowest unoccupied molecular orbital (LUMO) of the donor and the highest occupied molecular orbital (HOMO) of the acceptor. By exploiting the Gaussian distribution of the charge carriers in a two-level system, and thus also their spins in the EPR experiment, it could be shown that the LUMOs get closer by a few to a few hundred meV when going from pure fullerene materials to a fullerene mixture. The reason for this strong energetic effect is likely the formation of a fullerene alloy. Further, we investigated the chemical doping mechanism of SWNTs with a (6,5)-chirality and their behaviour under optical excitation. In order to determine the unintentional (pre)-doping of SWNTs, EPR spectra of the raw material as well as after different purification steps were recorded. This facilitated the determination of nanotube defects and atmospheric p-doping as the causes of the measured EPR signals. In order to deliberately transfer additional charge carriers to the nanotubes, we added the redox-active substance AuCl3 where we determined an associated doping-yield of (1.5±0.2)\%. In addition, a statistical occupation model was developed which can be used to simulate the distribution of EPR active, i.e. unpaired and localised charge carriers on the nanotubes. Finally, we investigated the charge transfer behaviour of (6,5)-SWNTs together with the polymer P3HT and the fullerene PC60BM after optical excitation.},
  subject      = {Organische Halbleiter},
  language  = {en}
}
@phdthesis{Eckstein2019,
  author    = {Eckstein, Klaus},
  title     = {Linear and Nonlinear Spectroscopy of Doped Carbon Nanotubes},
  doi       = {10.25972/OPUS-18897},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-188975},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Doping plays a decisive role for the functionality of semiconductor-based (opto-)electronic devices. Hence, the technological utilization of semiconductors necessitates control and a fundamental understanding of the doping process. However, for low-dimensional systems like carbon nanotubes, neither concentration nor distribution of charge carriers is currently well known. The research presented in this thesis investigated the doping of semiconducting carbon nanotubes by spectroscopic methods. Samples of highly purified, intrinsic (6,5) single-wall carbon nanotubes were fabricated using polymer stabilization. Chapter 4 showed that both electro- and redox chemical \$p\$-doping lead to identical bleaching, blueshift, broadening and asymmetry of the S\$_1\$ exciton absorption band. The similar spectral changes induced by both doping schemes suggest that optical spectra can not be used to infer what process was used for doping. Perhaps more importantly, it also indicates that the distribution of charges and the character of the charge transfer states does not depend on the method by which doping was achieved. The detailed analysis of the doping-induced spectral changes in chapter 5 suggests that surplus charges are distributed inhomogeneously. The hypothesis of carrier localization is consistent with the high sensitivity of the S\$_1\$ exciton photoluminescence to additional charge carriers and with the stretched-exponential decay of the exciton population following ultrafast excitation. Both aspects are in good agreement with diffusion-limited contact quenching of excitons at localized charges. Moreover, localized charges act - similar to structural defects - as perturbations to the bandstructure as evidenced by a doping-induced increase of the D-band antiresonance in the mid-infrared spectrum. Quantum mechanical model calculations also suggest that counterions play a crucial role in carrier localization. Counterion adsorption at the nanotube surface is thus believed to induce charge traps of more than 100 meV depth with a carrier localization length on the order of 3 - 4 nm. The doping-induced bleach of interband absorption is accompanied by an absorption increase in the IR region below 600 meV. The observed shift of the IR peak position indicates a continuous transition from localized to rather delocalized charge carriers. This transition is caused by the increase of the overlap of charge carrier wavefunctions at higher charge densities and was modeled by classical Monte-Carlo simulations of intraband absorption. Chapter 6 discussed the spectroscopy of heavily (degenerately) doped nanotubes, which are characterized by a Drude-response of free-carrier intraband absorption in the optical conductivity spectrum. In the NIR spectral region, the S\$_1\$ exciton and X\$+^_1\$ trion absorption is replaced by a nearly 1 eV broad and constant absorption signal, the so-called H-band. The linear and transient absorption spectra of heavily doped nanotubes suggest that the H-band can be attributed to free-carrier interband transitions. Chapter 7 dealt with the quantification of charge carrier densities by linear absorption spectroscopy. A particularly good measure of the carrier density is the S\$_1\$ exciton bleach. For a bleach below about 50 \%, the carrier density is proportional to the bleach. At higher doping levels, deviations from the linear behavior were observed. For doping levels exceeding a fully bleached S\$_1\$ band, the determination of the normalized oscillator strength f\$\text{1st}\$ over the whole first subband region (trion, exciton, free e-h pairs) is recommended for quantification of carrier densities. Based on the nanotube density of states, the carrier density \$n\$ can be estimated using \$n = 0.74\,\text{nm}^{-1} \cdot (1 - f_\text{1st})\$. In the last part of this thesis (chapter 8), the time-resolved spectroelectrochemistry was extended to systems beyond photostable carbon nanotube films. The integration of a flowelectrolysis cell into the transient absorption spectrometer allows the investigation of in-situ electrochemically generated but photounstable molecules due to a continuous exchange of sample volume. First time-resolved experiments were successfully performed using the dye methylene blue and its electrochemically reduced form leucomethylene blue.},
  subject      = {Dotierung},
  language  = {en}
}
@phdthesis{Ruff2013,
  author    = {Ruff, Andreas},
  title     = {On the importance of electronic correlations in potassium-doped organic semiconductors},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83635},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2013},
  abstract  = {The present thesis is concerned with the impact of alkali metal-doping on the electronic structure of semiconducting organic thin films. The organic molecular systems which have been studied are the polycyclic aromatic hydrocarbons picene, pentacene, and coronene. Motivated by reports about exceptional behavior like superconductivity and electronic correlations of their alkali metal-doped compounds, high quality films fabricated from the above named molecules have been studied. The electronic structure of the pristine materials and their doped compounds has been investigated using photoelectron spectroscopy. Core level and valence band studies of undoped films yield excellent photoemission spectra agreeing with or even outperforming previously reported data from the literature. Alkali metal-doping manifests itself in a uniform manner in the electronic structure for all probed samples: Opposed to reports from the literature about metallicity and even superconductivity in alkali metal-doped picene, pentacene, and coronene, all films exhibit insulating nature with an energy gap of the order of one electron-volt. Remarkably, this is independent of the doping concentration and the type of dopant, i.e., potassium, cesium, or sodium. Based on the interplay between narrow bandwidths in organic semiconductors and sufficiently high on-molecule Coulomb repulsion, the non-metallicity is attributed to the strong influence of electronic correlations leading to the formation of a Mott insulator. In the case of picene, this is consolidated by calculations using a combination of density functional theory and dynamical mean-field theory. Beyond the extensive considerations regarding electronic correlations, further intriguing aspects have been observed. The deposition of thin picene films leads to the formation of a non-equilibrium situation between substrate and film surface. Here, the establishment of a homogeneous chemical potential is hampered due to the only weak van der Waals-interactions between the molecular layers in the films. Consequently, spectral weight is measurable above the reference chemical potential in photoemission. Furthermore, it has been found that the acceptance of additional electrons in pentacene is limited. While picene and coronene are able to host up to three extra electrons, in pentacene the limit is already reached for one electron. Finally, further extrinsic effects, coming along with alkali metal-doping, have been scrutinized. The oxidation of potassium atoms induced by the reaction with molecular oxygen in the residual gas of the ultra-high vacuum system turned out to significantly influence the electronic structure of alkali metal-doped picene and coronene. Moreover, also the applied X-ray and UV irradiation caused a certain impact on the photoemission spectra. Surprisingly, both effects did not play a role in the studies of potassium-doped pentacene.},
  subject      = {Organischer Halbleiter},
  language  = {en}
}