Optische Spektroskopie an Chalkogenen und Porphyrinen

In the present work, various chalcogens and a porphyrin compound were investigated by optical spectroscopy. The photoconductivity of these compounds was considered by photocurrent spectroscopy. In addition, WSe2 monolayers were investigated on different substrates by photoluminescence. The first part of this work deals with photocurrent spectroscopy of various chalcogen compounds, namely various selenidostannates in ionic liquids, K2Hg2Se3 and K2Hg2Te3, and a porphyrin [H6TPyP][BiCl6]2. For this purpose, a newly established photocurrent setup was presented and characterized. With this setup it is possible to perform spectrally resolved measurements. Currents of up to a few 100 fA can be measured with the photocurrent setup. In addition, current-voltage curves can also be recorded by changing the detection technology. The measurement of the different selenidostannates in ionic liquids of different dimensions showed that all the samples tested show a photocurrent depending on the applied voltage and the wavelength of the incident light. The spectrally resolved measurements show good agreement with the absorption measurements shown in [112]. The 1D samples show a distinct photocurrent below their main absorption edge, which gives an indication of optically active conduction states below the band edge. These states are not present in the 2D and 3D samples. All samples show in their current-voltage curves a clear Schottky behavior with breakdown voltages of less than 10 V. K2Hg2Se3 combines the properties of a salt with those of a semiconductor. The band edge of the material determined by the photocurrent measurements could be confirmed by absorption measurements. It shows a pronounced photocurrent of about 3 nA at 10 V applied voltage and a dark current of about 0.1 nA at 10 V applied voltage. The photocurrent and absorption measurements show good agreement with the 1.36 eV bandgap calculated by DFT methods. Replacing selenium with tellurium in K2Hg2Se3 yielded the third sample tested. This sample was prepared with the aim of improving the electrical conductivity in contrast to K2Hg2Se3. In contrast to K2Hg2Se3, K2Hg2Te3 shows a higher dark current in the range of 100 nA as well as a higher photocurrent in the range of 200 nA at 10 V applied voltage. The goal of higher electrical conductivity could thus be confirmed. K2Hg2Te3 shows a pronounced photocurrent, but the band gap of K2Hg2Te3, which was also confirmed by absorption measurements, is shifted to lower energies in contrast to K2Hg2Se3. The final sample was a porphyrin named [H6TPyP][BiCl6]2. The absorption as well as the photocurrent of this sample were compared with those of the already known porphyrin [H6TPyP]Cl6. It was shown that the optical properties of [H6TPyP][BiCl6]2 can be attributed mainly to the porphyrin derivative. The [BiCl6]3- anions provide only more pathways for non-radiative recombination compared to the simple hydrochloride salt [H6TPyP]Cl6. [H6TPyP][BiCl6]2 shows a relatively high dark current of about 20 nA at 15 V applied voltage. Compared to the dark current, the photocurrent proved to be very small. It only assumed values in the range of 600 fA at 10 V applied voltage and illumination with the complete white light spectrum. In the second part of this thesis, the photoluminescence and the time-resolved photoluminescence of WSe2 monolayers deposited on different substrates were investigated. The investigations were carried out both at room temperature and at 10K. The spectral components of excitons, trions, biexcitons, and bound states, which contribute to the photoluminescence spectrum, were identified. The energetic position and intensity of these spectral contributions were compared dependent on to the excitation density for the different substrates. At room temperature, a small change in the energetic position of the exciton emission could be detected as a function of the refractive index of the substrate. At low temperatures, the monolayers on the different substrates showed a relatively similar energetic position of the individual states. Interestingly, CVD grown WSe2 on sapphire shows a very similar emission behavior at low temperatures as exfoliated WSe2 on sapphire. At room temperature, on the other hand, CVD grown WSe2 on sapphire behaves differently in its emission properties than the exfoliated samples. The emission of the CVD grown monolayer on sapphire shows a significant redshift in the energetic position which can be attributed to tensions caused by the growing conditions. Monolayers, where a low intensity ratio of excitons to trions and a high degree of polarization has been found at room temperature, show a biexitonic state and most different species at low temperatures. The reproducibility of the emission of monolayers on different substrates is influenced by the roughness of the substrate and the size of the monolayer. For samples with uneven substrate, the reproducibility of the photoluminescence is limited. The same applies to too small monolayer flakes. Another influence on the reproducibility of the μPL spectra seems to be that of heating the sample after production. Samples that have not been baked out will typically have a lower PL intensity compared to baked samples on the same substrate material due to adhesive residue or other contamination of the sample. The use of hBN as a buffer material between substrate and monolayer leads to a very similar appearance of all μPL spectra. By using hBN, the type of substrate underlying the hBN does not seem to play too much a role. The decay times of the photoluminescence of the monolayers on the different substrates show a dependence on the pumping density. With increasing excitation density, the photoluminescence drops faster. This behavior is due to exciton-exciton annihilation. At 10 K, on the other hand, the occurrence of excitonic states determines the cooldowns of photoluminescence. The decay times of the WSe2 monolayers on different substrates are very reproducible. Only small variations can be detected. The samples in which hBN was used as a buffer layer have a faster decay time than their counterpart without hBN.