@phdthesis{Babocsi2005,
  author    = {Babocsi, Krisztina},
  title     = {Characterization of II-VI semiconductor nanostructures by low wavenumber raman- and four-wave-mixing spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12551},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2005},
  abstract  = {Es ist bekannt, dass r{\"a}umlich eingeschr{\"a}nkte Ladungstr{\"a}ger in niederdimensionalen Halbleitern zur Verst{\"a}rkung optischer und elektronischer Eigenschaften solcher Nanostrukturen beitragen. Die Physik des "Quantum Confinements" ist trotz umfangreicher Nachforschungen noch immer nicht v{\"o}llig verstanden. Die vorliegende Arbeit beinhaltet eine qualitative Studie quasi-nulldimensionaler II-VI Halbleiter Nanostrukturen. Es wurden handels{\"u}bliche und w{\"a}rmebehandelte CdSxSe1-x Quantenpunkte (QDs) mittels linearer und nicht-linearer Spektroskopie untersucht. Im Rahmen nicht-resonanter Raman Spektroskopie wurden Schl{\"u}sseleigenschaften der QDs, wie z.B. der Durchmesser und die Gr{\"o}ßenverteilung, bestimmt. Die Anordnung der Energieniveaus in einer atom-{\"a}hnlichen Struktur hat die Verst{\"a}rkung der Intensit{\"a}t akustischer Phononen zur Folge, welche im Bulk nicht nachgewiesen werden k{\"o}nnen. In Nanokristallen sind nur zwei Sorten akustischer Vibrationen Raman-aktiv: Die kugelsymmetrischen (l = 0) und die quadrupolaren (l=2) Vibrationen, die durch linear polarisierte Laserpulse selektiv angeregt werden k{\"o}nnen. Die Gr{\"o}ße der QDs wurde durch Ber{\"u}cksichtigen der Abh{\"a}ngigkeit der Vibrationsfrequenz akustischer Phononen von dem Durchmesser des Nanokristalls berechnet. Die Gr{\"o}ßenverteilung der QDs ist aus dem normalisierten FWHM ("full width at half maximum") der symmetrischen Vibration bestimmt worden. Die Relaxationsprozesse in Quantenpunkten finden auf einer Pikosekundenskala statt, zu deren Untersuchung ultraschnelle Spektroskopiemethoden mit Laserpulsen im Femtosekundenbereich notwendig sind. Es wurden in einer Glasmatrix eingebettete CdS0.6Se0.4 QDs von 9.1 nm Durchmesser mittels Fs-VWM- und Fs-PPT-Spektroskopie untersucht. In beiden F{\"a}llen wurden zirkular polarisierte Fs-Laserpulse eingesetzt. Es ist gezeigt worden, dass die Auswahlregeln f{\"u}r die Polarisation sehr stark von der Symmetrie der Nanokristalle abh{\"a}ngig sind. Es ist gezeigt worden, dass die angeregten Nanokristalle der Symmetriegruppe C2v oder niedriger angeh{\"o}ren und der Nachweis einer hexagonalen Struktur der Nanokristalle wurde erbracht. Die G{\"u}ltigkeit des Vier-Niveau-Modells wurde ebenfalls nachgewiesen. Dieses Modell enth{\"a}lt einen Grundzustand, zwei Exzitonzust{\"a}nde und einen Biexzitonenzustand. Das Entstehen der VWM- und PPT-Signale in verbotenen Polarisationsgeometrien wurde durch das Auftreten starker Coulomb-Wechselwirkung zwischen Exzitonen, die sich in demselben QD befinden, und durch die niedrige Symmetrie der QDs erkl{\"a}rt. Aufgrund der quadratischen Abh{\"a}ngigkeit der Intensit{\"a}ten der VWM-Signale von der Intensit{\"a}t der PPT-Signale, konnten die Ergebnisse der VWM-Messungen durch PPT-Untersuchungen gepr{\"u}ft werden. Die Effizienz der Methode der zirkular polarisierten Fs-VWM-Spektroskopie wurde bei der Untersuchung von in einer Glasmatrix eingebetteten w{\"a}rmebehandelten CdSe Quantenpunkten noch einmal best{\"a}tigt. Die Aufmerksamkeit auf Nicht-Phonon-Relaxationsmechanismen des Grund- und angeregten Zustands des Exzitons gerichtet. Außerdem konnte die Abh{\"a}ngigkeit der Kristallasymmetrie von der Nanopartikelgr{\"o}ße und von den Wachstumsbedingungen abgesch{\"a}tzt werden. Es zeigte sich, dass qualitativ hochwertige Quantenpunkte am effizientesten durch lange Wachstumszeiten bei niedrigen Temperaturen hergestellt werden k{\"o}nnen. Dabei haben die Nanokristalle gen{\"u}gend Zeit f{\"u}r „Nukleation" und nehmen eine symmetrischere Form an. Außerdem ist es nachgewiesen worden, dass die Exzitonrelaxation sehr stark von den Coulomb-Wechselwirkungen zwischen den Ladungstr{\"a}gern abh{\"a}ngt. Die Relaxationsprozesse der Exzitonen werden sowohl durch die Auger Selbstionisation, als auch durch den anschließenden Einfang der Ladungstr{\"a}ger in tiefen Fallen (an der Quantenpunktoberfl{\"a}che und/oder in der dielektrischen Matrix) deutlich verlangsamt. Dadurch wird die Lebensdauer der Exzitonen deutlich verk{\"u}rzt und liegt im Pikosekundenbereich. Die Relaxation der Exzitonen von h{\"o}heren Energieniveaus in den Grundzustand erfolgt auch auf zwei Wegen: Am Anfang des Relaxationsprozesses (t31 ~ 200 fs) ist Auger-Thermalisierung der Ladungstr{\"a}ger f{\"u}r die Relaxation des Elektrons von seinem angeregten 1pe Zustand auf sein niedrigeres 1se Energieniveau verantwortlich. W{\"a}renddessen erfolgt die Relaxation des Lochs sehr schnell {\"u}ber sein dichtes Spektrum von Valenzbandzust{\"a}nden. Diesem Prozess folgt unmittelbar der Einfang der Ladungstr{\"a}ger in tiefen Fallen, die sich an der Nanokristall-Glasmatrix-Grenzfl{\"a}che befinden. Diese Fallen sind eine direkte Konsequenz der Asymmetrie des Nanokristalls: je zahlreicher und je tiefer die Fallen, desto h{\"o}her ist die Asymmetrie des Kristalls. Im Rahmen dieser Arbeit ist eine komplette Charakterisierung der in einer Glas- matrix eingebetteten CdSSe-Quantenpunkte gelungen. Die wichtigsten Eigenschaften, wie z.B. die Gr{\"o}ße und die Gr{\"o}ßenverteilung der Quantenpunkte, sind durch polarisierte Raman-Messungen bestimmt worden. Um ein komplettes Bild {\"u}ber die Nanokristalle zu bekommen, sind weitere nicht-lineare Spektroskopiemethoden eingesetzt worden. Polarisierte VWM Spektroskopie wurde zur Untersuchung verschiedener Quantenpunktensembles erfolgreich eingesetzt und daraus sind wertvolle Informationen {\"u}ber die Symmetrie der Nanokristalle gewonnen worden. Weiterhin sind die Exzitonrelaxationsmechanismen beschrieben worden, die die Verst{\"a}rkung der optischen nicht-linearen Eigenschaften und starke Coulomb-Wechselwirkungen zwischen Exzitonen erkl{\"a}ren. Durch die Untersuchung der Auswirkung verschiedener Wachstumsbedingungen auf die Symmetrie der QDs stellt diese Arbeit einen erg{\"a}nzenden Beitrag zu Herstellungsverfahren qualitativ hochwertiger Quantenpunkte dar.},
  subject      = {Zwei-Sechs-Halbleiter},
  language  = {en}
}
@phdthesis{Dem2003,
  author    = {Dem, Claudiu Dorin},
  title     = {Design and construction of a device for light scattering studies on airborne particles},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9605},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2003},
  abstract  = {This thesis is concerned with the development of an on-line in-situ device for a chemical characterisation of flowing aerosols. The thesis describes the principles and most important features of such a system, allowing also on-line measurements using Raman spectroscopy as a diagnostic technique An analysis of the effect of forced oscillations on the motion of the particle dispersed in a gas flow is given in Chapter 2. Also the most important particle parameters are introduced. A review of the particle/fluid interaction in laminar air flows and the response of the particle is presented. In Chapter 3 the behaviour of the particle under different external conditions (ion bombardment and electric fields) is extended. A brief review of the most important particle charging theories (diffusion, field, and alternating potential charging) shows, that the effect of the electrical properties (represented by the dielectric constant) of the particles affects the charging process. A non-contact method for particle charge measurement was also presented. In the second part of the chapter, the interaction between the electric field and the charged particle for the purpose of particle trapping is illustrated. The most common systems like the two or four ring electrodynamic balance and the quadrupole trap are pointed out. In Chapter 4 a short review of the possibility of using scattered light to study aerosol particles is presented. First, the conditions and the facilities of using the Mie theory for particle size and refractive index determination are mentioned, then some features concerning the classical treatment of the Raman effect are presented Supported by the theoretical considerations exposed in Chapter 2, 3, and 4 the construction and the tests of different devices are presented in Chapter 5. Following the goal of the thesis, first an overview of the used materials and methods for particle generation is presented. Then, the constructed charging devices are described (from the mechanical and electrical point of view) and compared by measuring the acquired charge on the particle. Charged particles can be trapped in different containers. Two types of axially symmetric electrodynamic balances (two ring or an extended four ring configuration) were presented. For a deeper understanding these systems were studied using analytic and numerical methods. Considering the presented purpose of the work another type of trapping system has been developed, namely the quadrupole trap. A similar theoretical characterisation (in term's of Mathieu equation) as for the electrodynamic balance was presented pointing out some specific features of this system. The incoming particle stream will be focused to the centre of the system simultaneously also the applied DC and AC potential onto the tube electrodes, yields a stable trapping of one or more particles. Chapter 6 consists of two parts: the system for single particle and for many particles investigation. The individual devices presented in Chapter 5 are now put together. The first part presents the method and the experimental realisation of a set-up for solid particle injection. In order to suppress the phase injection disadvantage found for the electrodynamic balance a developed program processes the information obtained from a particle cloud through an adequate electronic detection system, and reduces the number of particles until just one single particle is trapped. The method for one particle investigation can be extended for many particles. Using the presented set-up the particles are moved from one quadrupole to another and transformed from a particle cloud to a particle stream. A linearity between an external vertical mounted detector and the formed image of the particle stream on the CCD camera has been observed and used for simultaneous detection of many particles by Raman spectroscopy. For both methods Raman results are presented. One limitation of Raman Spectroscopy is the relatively long integration time needed for adequate signal-to-noise ratio. There are two factors which influence the integration time: first the incident radiation and the detector sensitivity, and second the intensity of the Raman bands. Using a CCD detector, the desired detector sensitivity should be achieved. So, the improvement of the signal-to-noise ratio should be the next goal in the system development. In order to reduce the integration time an optical system including optic fibres and the integration of an FT-Raman module operating in the visible region is planed. The goal of this work was to develop and construct an instrument for on-line in-situ single particle investigation by Raman spectroscopy. With the presented experimental set-up and the developed program the purpose of the work, the on-line in-situ near atmospheric pressure aerosol investigation was achieved. The Raman spectroscopy has been used successfully for a chemical characterisation of the aerosol particles.},
  subject      = {Aerosol},
  language  = {en}
}
@phdthesis{Tarcea2004,
  author    = {Tarcea, Nicolae},
  title     = {Light as a universal tool : Microcapsule sizing by elastic light scattering and mineral investigation by in situ Raman spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9383},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2004},
  abstract  = {The present work consist of two major parts. The first part, extending over chapters 1, 2, 3 and 4, addresses the design and construction of a device capable of determining the shell thickness and the core size for monolayer spherical particles in a flow. The second part containing chapters 5, 6, 7, 8, 9 and 10, concentrate on the use of Raman spectroscopy as a space application, namely for use as a tool for in situ planetary investigations. This part directly addresses the MIRAS project, a study run under the auspices of Federal Ministry of Education and Research, BMBF and German Aerospace Center, DLR under national registration number 50OW0103. MIRAS stands for "Mineral Investigation by in situ Raman Spectroscopy". Microcapsule Sizing by Elastic Light Scattering The industrial development of processes based on microcapsules depends on the possibility to provide clear and complete information about the properties of these microcapsules. However, the tools for an easy and efficient determination of the microcapsule properties are lacking, several methods being often required to describe adequately the microcapsule behavior. Methods for evaluating the individual size and size distribution of both the core and the shell are required together with methods for measuring the mechanical strength, stability in appli-cation media, permeability of the shell, etc. Elastic light scattering measurements provide a possible way of determining properties such as core size, shell size and refractive index. The design and con-struction of a device capable of measuring the above mentioned parameters for a core-shell particle is the subject of the first part of this thesis. The basic principle of measurement for the device proposed here consists of an-alyzing one particle at a time by recording the elastic light scattering pattern at angles between approx. 60 and 120 grad. By comparing the experimentally recorded phase functions with the previously calculated phase functions stored in a database, the geometry of the scattering object can be identified. In our case the geometry is characterized by two parameters: the shell thickness and the core radius. In chapter 2 a short overview on the methods used for sizing microparticles is given. Different sizing methods are compared, and the advantages and disadvan-tages for the general problem of sizing are shortly discussed. It is observed that all sizing methods that are based on elastic light scattering theories are ensemble methods. Chapter 3 focusses on the theories used for calculating the theoretical scattering patterns with emphasize on the Mie theory. The generalization of Mie theory for layered particles is shortly presented and the far field intensity approximations are discussed. The last chapter (4) of this first part describes the experimental approach for building an automatic microcapsule sizer. The approach started by O. Sbanski [76] with the development of a software packet for calculating and storing theoret-ical phase functions for core-shell particles was continued with the designing and construction of a measuring device. The hardware construction and the software with all implemented corrections imposed by the individual setup components are described in detail. For the laser, the monochromaticity, the intensity profile of the beam as well as the planarity of the equi-phase fronts are taken into consid-eration. The flow cell with three different designs is described, and the influences of the employed design on the light scattering patterns are discussed together with the optical system used for recording the experimental phase functions. The detection system formed by two identical linear CCD arrays is presented together with the software approach used for data acquisition. Ways of improving the quality and the speed of the analyzing process are discussed. The final section presents measurements run on samples made of homogeneous spheres and also on samples containing industrial microcapsules. Mineral Investigation by in situ Raman Spectroscopy The envisaged future planetary missions require space-born instruments, which are highly miniaturized with respect to volume and mass and which have low needs of power. A micro Raman spectrometer as a stand alone device on a planetary surface (e.g. Mars) offers a wide spectrum of possibilities. It can assess the chemical analysis via determination of the mineral composition, detect organic molecules in the soil, identify the principal mineral phases, etc. The technical developments in the last years have introduced a new generation of small Raman systems suitable for robotic mineral characterization on planetary surfaces [20, 95]. Two different types of spectrometer were considered for the MIRAS study. As supporting laboratory experiments for the MIRAS study, the measure-ments on standard minerals and on SNC Mars meteorites are discussed in chapter 6. The following SNC meteorites have been investigated: Sayh al Uhaymir 060, Dar al Gani 735, Dar al Gani 476, Northwest Africa 856, Los Angeles, Northwest Africa 1068 and Zagami. Pyrite as a hitherto undescribed phase in the picritic (olivin-phyric) shergottite NWA 1068 as well as reduced carbon (e.g. graphite) and anatase in the shergottite Say al Uhaymir 060 are new findings for this class of meteorites. A detailed description of the proposed designs for MIRAS, with the compo-nents used for building the test version on a breadboard is covered in chapter 7. The scientific as well as the mission requirements imposed on the instrument are discussed. The basic design is presented and the main components that are brought together to build the device being the laser unit, the Raman head, the Rayleigh filtering box, and the spectral sensor (spectrometer with a matching de-tector) are described. The two proposed designs, one based on an acousto-optic tunable filter (AOTF) and the other based on a dispersive hadamard transform spectrometer are compared to each other. The actual breadboard setup with the detailed description of the components follows in Section 7.3. Further de-velopment of a Raman spectrometer for planetary investigations is proposed in combination with a microscope as part of the Extended-MIRAS project. The software developed for controlling the breadboard version of MIRAS is described in chapter 8 together with a short description of the structure of a relational database used for in house spectra management. The measuring pro-cedures and the data processing steps are presented. Spectra acquired with the MIRAS breadboard version based on the AOTF are shown in chapter 9. The final chapter addresses a rather different possibility of using Raman spectroscopy for planetary investigations. The chapter summarizes the content of four tech-nical notes that were established within the study contracted by the European Space Agency with firma Kayser-Threde in Munich concerning the possibility of applying Raman spectroscopy in the field of remote imaging.},
  language  = {en}
}