Exciton Dynamics in Perfluoropentacene Single Crystals

The realization of the first bipolar junction transistor in the year 1948 by Bardeen, Brattain and Shockley [1] sparked off the semiconductor industry, which gradually revolutionized the way we live. Nowadays, semiconductors are the fundamental building blocks of every high-tech electronic device, most notably the computer which has become an inescapable part of our daily lives. Besides voltage and current control capabilities, semiconductors exhibit intriguing opto-electronic properties; the best known and commercially most successful applications are light emitting diodes (LEDs), laser diodes, charged coupled devices (CCD) and solar cells.[2, 3, 4] Due to the broad variety of material systems, they cover virtually the complete optical spectrum while simultaneously being cost-efficient and easy to miniaturize. Until the late 90ies, commercially available devices were exclusively based on inorganic semiconductors, primarily on Silicon. However, over the last decade, the class of organic semiconductors has gained an increasing amount of interest, e.g., now one of the most popular smartphone’s display1 is based on OLED2-technology. Flexibility upon stress and deeper color contrasts are typically named as their main advantages over conventional liquid crystal displays (LCD). While organic semiconductor devices are already well established as light emitters, they are still in research state as light harvesters. In general, organic solar cells offer high photon cross sections in combination with similar flexibility as OLED displays. Additionally, they exhibit the potential for low-cost mass-production, including innovative and versatile procedures such as ink-jet printing.[6, 7] However, two major challenges still exist which need to be addressed before organic solar cells become compatible: the long-term stability and the quantum efficiency.[8] The fast degradation of organic solar cells is caused by oxidation, reduction and thermal instabilities. Research in this field focuses on the synthesis of new organic molecules, thus, it can be assigned to the organic chemistry sector. Quantum efficiencies are determined by the microscopic photon to carrier conversion, i.e., the photovoltaic effect, therefore, it is predominantly a research topic of solid state physics. This thesis focuses mainly on aspects of the quantum efficiency in the polyacene Perfluoropentacene (PFP) and its underlying decay processes, namely the electronic relaxation dynamics after optical excitation. In particular, the process of singlet exciton fission is analyzed which promises to double the quantum efficiencies, as it converts one singlet exciton into two triplet excitons.[9] Excitons are correlated electron and hole pairs: neutral excitations of the crystal after absorption of a photon. Singlet exciton fission was first proposed in 1968 in order to explain the drastic photoluminescence quench of Tetracene crystals compared to Anthracene crystals.[10] It has gained renewed attention lately, due to its potential application in the growing field of organic solar cells. However, the microscopic understanding is still in its infancy which hampers essential progress in this field; for instance, the influence of the geometrical order of the molecules within the crystal on singlet exciton fission has only been analyzed theoretically. The reason is the lack of single crystal samples allowing for the correlation of molecular packing and electronic dynamics.[9] This issue is resolved in Chapter 5 for the model system of PFP single crystals, where for the first time the singlet exciton fission dynamics are observed along the three crystal axes by polarization-resolved pump-probe spectroscopy. Moreover, the efficient coupling direction is identified as well as the preceding electronic species of the two triplet excitons. Although spectroscopic analysis on polyacenes date back to the 40ies [11], lack of computational power and interest lead to the sad state that even interpretations of the linear absorption are still debated today. However, basic knowledge of the linear absorption is essential in order to interpret the non-linear dynamics. Therefore, Chapter 4 serves as a precursor, where the linear absorption of the PFP samples is interpreted using phenomenological models. Here, first indications are given for a dominant coupling direction within the PFP crystal which are then confirmed in Chapter 5. Furthermore, the amount of exciton splitting in PFP is determined, also known as the Davydov-splitting.[12] It is induced by dipole coupling between the two basis molecules of the crystal lattice during excitation. In Chapter 6 the focus is shifted to inorganic semiconductors. The chapter introduces a fast and convenient method to determine dephasing times of induced coherent exciton polarizations with more precision than a common lineshape analysis of the absorption spectrum. In pump-probe spectroscopy, the transients of the coherent oscillations are exploited to serve as phase indicators for the several excitonic transitions. These transients are observed during the coherent regime before pump and probe pulses perfectly overlap in time.[13] As a proof of principle, the methodology is applied to a set of Germanium quantum well samples and evaluated in respect to their optical quality. In addition, the main dephasing mechanism in Germanium quantum wells is identified. These three chapters capture the results of the thesis and are preceded by introductory chapters covering basic light-matter interactions and experimental details; they are succeeded by a conclusion chapter summarizing the essential findings

An early inhabitant of Illinois - the trilobite

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Fluid and Thermal Analysis of Pre-Columbian Tiwanaku (500–1100 CE) Raised-Field Agricultural Systems of Bolivia

Raised-field agricultural systems have received attention from scholars involved in the analysis of prehistoric agricultural intensification in the New World. This paper discusses the function of raised fields associated with the Tiwanaku society (500–1100 CE) located on the southern rim of Lake Titicaca in Bolivia. The overnight internal heat storage capacity of Tiwanaku raised-field berms located at the high-altitude (~3810 masl) Bolivian altiplano is analyzed through ANSYS (version 4.2B) finite difference methods to provide an understanding of ancient agricultural engineers’ knowledge regarding how to protect crops from nightly subzero freezing temperatures and water saturation. The present analysis concludes that enhanced berm heat storage capacity derived from solar radiation into multi-layered moist berm agricultural soils, together with radiative heating of berm-surrounding swale water (swale water depth determined from excavation into the groundwater aquifer), was an essential Tiwanaku design element of raised-field agriculture to protect crops from freezing damage during both wet and dry seasons. This paper reports the ANSYS temperature distribution results derived from a raised-field berm swale computer model of ancient excavated raised fields in the form of a 24 h heat input and cooling cycle, which indicates the presence of an internal berm heat storage effect designed to protect crops from freezing damage. The calculations performed use specific hydrological and climatological conditions characteristic of the littoral and near-shore environment of Lake Titicaca. The use of the ANSYS finite element code to investigate the source of internal berm heat storage protecting crops from freezing temperatures, compared to the field test results from experimental use of reconstructed ancient, raised fields, provides an understanding of the technologies developed by Tiwanaku agricultural engineers to increase raised-field agricultural production

Low-energy radioactive beam experiments using the UM-UND solenoid RNB apparatus at the UND tandem: Past, present and future

Approximately ten years ago (1987) one of the first operational low-energy radioactive nuclear beam (RNB) facilities was put into place at the University of Notre Dame (UND) as a joint project between the University of Michigan (UM) and UND. The key elements to the success of the project were the installation of a large-bore 3.5 Tesla superconducting solenoid (supplied by UM) to collect and focus secondary radioactive beams, combined with an upgrade of the UND FN Tandem, the latter including the addition of a high-intensity sputter ion source. The resulting secondary beams (8Li,8Li, 6He,6He, 7Be,7Be, 8B,8B, 18Fm,18Fm, …) are generally produced by selective, high-cross-section direct reactions. These beams are sufficiently intense (viz. 104/s104/s to 108/s108/s) to permit measurement of many low-energy reaction cross sections of interest to nuclear astrophysics, nuclear reaction theory, and high-isospin nuclear physics. A review of past and recent RNB data obtained with this apparatus will be presented together with plans for a major upgrade using a pair of 6T solenoids (M. Y. Lee et al.−this conference). © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87532/2/369_1.pd

Reaction mechanisms for weakly-bound, stable nuclei and unstable, halo nuclei on medium-mass targets

An experimental overview of reactions induced by the stable, but weakly-bound nuclei 6Li, 7Li and 9Be, and by the exotic, halo nuclei 6He, 8B, 11Be and 17F on medium-mass targets, such as 58Ni, 59Co or 64Zn, is presented. Existing data on elastic scattering, total reaction cross sections, fusion processes, breakup and transfer channels are discussed in the framework of a CDCC approach taking into account the breakup degree of freedom.Comment: 7 pages, 6 figures, Invited Talk given by C. Beck to the 10th International Conference on Nucleus-Nucleus Collisions, August 16-21, 2009 Beijing, China; Paper submitted to the NN2009 Proceedings, Nuclear Physics A (to be published

Spektroskopie an Ge-basierten Halbleiterstrukturen auf Silizium

Anrege-Abfrage-Spektroskopie an Germanium Halbleiternanostrukturen auf Silizium. Es konnte ein gigantischer AC Stark Effekt in Germanium beobachtet werden, der nach 150fs abgeklungen ist. Außerdem wurde die Abkühlung des Lochsystems auf Gittertemperatur analysiert. Die Abkühlung erfolgt im Vergleich zu direkten Halbleitern um eine Größenordnung langsamer. Abschließend konnte optische Verstärkung in dotiertem Germanium nachgewiesen werden