Raman spectroscopy: from ferroelastic domain identification to strain tuning

This cumulative Habilitation thesis summarizes several examples related to the application of Raman spectroscopy for the investigation of coupling phenomena induced by epitaxially, mechanically or piezoeletrically applied strain. Methods for quantitative determination of strain by Raman spectroscopy are proposed for some materials such as BiFeO3 or strained Si. Raman spectroscopy was also used for understanding temperature induced phase transitions or orbital ordering, which are intimately related to specific phonon modes, as in the case of BiCrO3 or LaVO3, respectively. A method based on the Raman tensor formalism, which allows an assignment of the BiFeO3 Raman modes of pure as well as mixed character/symmetries is proposed. Relying on this assignment it is shown that Raman spectroscopy is a powerful tool for the investigation of ferroelastic domain formation in multiferoic materials, being able to probe the tilt of the domain walls

Small molecule tandem organic photovoltaic cells incorporating an α-NPD optical spacer layer

We report an improvement in power conversion efficiency in a small molecule tandem organic photovoltaic (OPV) device by the optimisation of current balancing of the sub-cells using an optical spacer layer. A co-deposited layer of N,N’-bis(1-naphthyl)-N,N′-diphenyl-1,1’-biphenyl-4,4’-diamine (α-NPD) and molybdenum oxide was used as the optical spacer layer and provided a highly transparent and conductive layer. Optical simulations showed the addition of the optical spacer in a boron subphthalocyanine (SubPc)/C60 based tandem OPV device increased the SubPc absorption in the front sub-cell and resulted in current balancing through the device. Fabricated tandem OPV devices showed similar trends, with the power conversion efficiency increasing from 2.3% to 4.2% with the addition of an optimised optical spacer thickness. External quantum efficiency and total absorption efficiency measurements back up the optical model data which attribute the increased performance to improved SubPc absorption in the front sub-cell, balancing the photocurrents of the two sub-cells

Discovery, Crystal Growth, and Characterization of Garnet Eu2PbSb2Zn3O12

Single crystal specimens of previously unknown garnet Eu2PbSb2Zn3O12 were grown in a reactive PbO:PbF2 flux medium. The crystals were characterized by a combination of X-ray crystallography, magnetization measurements, and the optical techniques of Raman, photoluminescence, and UV/Vis spectroscopy. The material exhibits Van Vleck paramagnetism associated with the J = 0 state of Eu3+, which was possible to accurately fit to a theoretical model. Band structure calculations were performed and compared to the experimental band gap of 1.98 eV. The crystals demonstrate photoluminescence associated with the 4f 6 configuration of the Eu3+ ions sitting at the distorted 8-coordinate garnet A site. The title compound represents a unique quinary contribution to a relatively unexplored area of rare earth bearing garnet crystal chemistry. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Probing orbital ordering in LaVO3_{3} epitaxial films by Raman scattering

Single crystals of Mott-Hubbard insulator LaVO3 exhibit spin and orbital ordering along with a structural change below ≈140 K. The occurrence of orbital ordering in epitaxial LaVO3films has, however, been little investigated. By temperature-dependent Raman scatteringspectroscopy, we probed and evidenced the transition to orbital ordering in epitaxial LaVO3film samples fabricated by pulsed-laser deposition. This opens up the possibility to explore the influence of different epitaxial strain (compressive vs. tensile) and of epitaxy-induced distortions of oxygen octahedra on the orbital ordering, in epitaxial perovskite vanadate films

Resonant Raman Scattering of ZnS, ZnO, and ZnS/ZnO Core/shell quantum dots

Resonant Raman scattering by optical phonon modes as well as their overtones was investigated in ZnS and ZnO quantum dots grown by the Langmuir–Blodgett technique. The in situ formation of ZnS/ZnO core/shell quantum dots was monitored by Raman spectroscopy during laser illumination

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs

Raman spectroscopy: from ferroelastic domain identification to strain tuning

This cumulative Habilitation thesis summarizes several examples related to the application of Raman spectroscopy for the investigation of coupling phenomena induced by epitaxially, mechanically or piezoeletrically applied strain. Methods for quantitative determination of strain by Raman spectroscopy are proposed for some materials such as BiFeO3 or strained Si. Raman spectroscopy was also used for understanding temperature induced phase transitions or orbital ordering, which are intimately related to specific phonon modes, as in the case of BiCrO3 or LaVO3, respectively. A method based on the Raman tensor formalism, which allows an assignment of the BiFeO3 Raman modes of pure as well as mixed character/symmetries is proposed. Relying on this assignment it is shown that Raman spectroscopy is a powerful tool for the investigation of ferroelastic domain formation in multiferoic materials, being able to probe the tilt of the domain walls

Characterisation of Si-Si bonded wafers and low-k silica xerogel films by means of optical spectroscopies

In dieser Arbeit werden als Untersuchungsverfahren für die Charakterisierung von gebondeten Siliziumwafern und Siliziumoxid-Xerogel-Schichten spektroskopische Ellipsometrie mit variablem Einfallswinkel (VASE) und Fouriertransformations-infrarotspektroskopie (FTIR) eingesetzt. Aus dem Verhalten der LO- und TO-Moden in den Infrarotspektren werden Veränderungen der Dicke und Struktur der vergrabenen Grenzfläche zweier Wafer bei einer Wärmebehandlung abgeleitet. Es werden Mechanismen für das Tieftemperaturbonden von Wafern, die auf der Entwicklung der chemischen Spezies an der vergrabenen Grenzfläche basieren, vorgeschlagen. Die chemischen Spezies wurden durch interne Vielfachtransmissionsinfrarotspektroskopie ermittelt. Aus ellipsometrischen Messungen wurden Dicke, optische Konstanten, Porosität und Porenabmessungen von Siliziumoxid-Xerogel bestimmt. Mittels VASE und FTIR wurde der Einfluss unterschiedlicher Hydrophobisierungsprozesse auf die Eigenschaften von Xerogel-Schichten untersucht. Weiterhin wurden die elektronischen und ionischen Beiträge zur statischen Dielektrizitätskonstanten bestimmt

Raman spectroscopy: from ferroelastic domain identification to strain tuning

This cumulative Habilitation thesis summarizes several examples related to the application of Raman spectroscopy for the investigation of coupling phenomena induced by epitaxially, mechanically or piezoeletrically applied strain. Methods for quantitative determination of strain by Raman spectroscopy are proposed for some materials such as BiFeO3 or strained Si. Raman spectroscopy was also used for understanding temperature induced phase transitions or orbital ordering, which are intimately related to specific phonon modes, as in the case of BiCrO3 or LaVO3, respectively. A method based on the Raman tensor formalism, which allows an assignment of the BiFeO3 Raman modes of pure as well as mixed character/symmetries is proposed. Relying on this assignment it is shown that Raman spectroscopy is a powerful tool for the investigation of ferroelastic domain formation in multiferoic materials, being able to probe the tilt of the domain walls