03/2011 - Postdoc at nanotech@surfaces, EMPA Zurich (Switzerland) - 2dimensional metal-organic frameworks - Organic molecules on hexagonal boron nitride (nanomesh)
10/2009 - 12/2010 Postdoc at Institute of Inorganic Chemistry, University of Zurich (Switzerland) - Luminescent solar concentrators based on artificial photosynthesis
01/2008 - 09/2009 Postdoc at Institute for Integrative Nanosciences, Leibniz Institute for Solid State and Materials Research (IFW) Dresden (Germany) - Thermoelectric characterization of nanomembranes - Fabrication and optical properties of hybrid materials
10/2002 - 12/2007 Diploma and Ph.D. studies at Institute for Applied Photophysics (IAPP), University of Technology Dresden (Germany) - Optical properties of ultra-thin layers of organic molecules - In situ optical spectroscopy + organic molecular beam epitaxy
Awards - Solar Energy Journal Best Paper Awards 2011 - Harry Dember Award 2003 (http://iapp.de/zap/?Dember-Preis)
Abstract: Doping-induced absorption changes of organic molecules on an insulating solid are reported. The charge transfer between alkali metal atoms and individual molecules on a surface leads to new electronic transitions identified with optical absorption spectroscopy. Progressive doping allows the discrimination of neutral, monoanionic and dianionic molecules in the solid state through examination of the spectra and rate equation modeling.
Abstract: In a recent paper, we discussed the optical properties of a heterostructure consisting of a highly ordered monolayer of quaterrylene (QT), electronically decoupled from the gold substrate by a predeposited epitaxial monolayer of hexa-peri-hexabenzocoronene (HBC). Here we now present the detailed structural investigation of this organic double-layer system. We show that the structure of the heterosystem can be identified as line-on-line coincidence (lol), a new type of epitaxy discovered by us previously for the system 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on HBC on highly oriented pyrolytic graphite (HOPG). Additionally, we provide evidence on the basis of advanced potential energy calculations that indeed energetic gain drives this lol growth mode.
Abstract: We report on the optical absorbance behavior of ultrathin 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films epitaxially grown on Au(1 1 1) and Au(1 0 0). Although PTCDA on Au(1 1 1) has been classified before as weakly interacting without noticeable metal-organic charge transfer (CT), we present clear optical evidence for ionized PTCDA at the interface. Unexpectedly, the charging of PTCDA is not restricted to the layer of contact, but extends into the second monolayer at least. A new feature in the optical spectra is identified as PTCDA cation, and we reason that the second monolayer is (partly) positively charged. We propose the CT to be related to the interface dipole formation between the first and second PTCDA monolayers on Au. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: In this article, Au/titanylphthalocyanine/n-GaAs diodes incorporating ultrathin films of the archetypal organic semiconductor titanylphthalocyanine were investigated by ballistic electron emission microscopy/spectroscopy (BEEM/S). BEEM/S measurements were used to determine the transmission of ballistic electrons through titanylphthalocyanine as a function of energy and temperature.
Abstract: Sharpening of optical spectra caused by commensurate growth of an organic adlayer on salt single crystals is reported. The structure is elucidated by atomic force microscopy and advanced potential energy calculations. Continued deposition or annealing induces a rearrangement of the molecular monolayer into 3D crystallites, demonstrating the crucial role of the Coulomb interaction with the substrate to form the unexpected commensurate structure.
Abstract: In this work we present an experimental investigation of hot electron transport in thin titanylphthalocyanine films grown on GaAs. Ballistic electron emission microscopy/spectroscopy is used as the experimental method. The transmission through a Au/titanylphthalocyanine/GaAs heterostructure is evaluated. In addition, an approach to calculate the attenuation length in titanylphthalocyanine is presented.
Abstract: Au/hexa-peri-hexabenzocoronene [C42H18/(HBC)]/GaAs heterostructures were investigated by ballistic electron emission microscopy. At room temperature, the Schottky barrier height at the Au/HBC interface was measured to be 1.3 eV, while the Fermi level at the HBC-GaAs interface is pinned 1.2 eV below the GaAs conduction band. Decreasing the temperature down to T=10 K, the Au/HBC Schottky barrier height increases to 1.55 eV and the Fermi level pinning at the HBC-GaAs interface reaches a value of 1.4 eV, close to the valence band of GaAs. These high values make HBC a promising interfacial layer in order to increase, for example, the open circuit voltage of GaAs Schottky barrier solar cells. (c) 2008 American Institute of Physics.
Abstract: We present a detailed study of the thickness dependence of the photoluminescence of ultrathin organic films on Au(111). For the evaluation of the observed intensities a new differential analysis method is introduced. The obtained quantum yield indicates a complete quenching in the thickness range below 4 monolayers (ML) for the two crystal polymorphs and saturates above 12 ML. Several models of energy transfer were applied to our data. We conclude that only consideration of both exciton diffusion and dissociation at the organic-metal interface in combination with classical energy transfer results in a complete and satisfactory description.
Abstract: Electronic coupling effects of organic adsorbates to metallic surfaces are examined optically. Covering a Au(111) substrate with an ultrathin (similar to 0.3 nm) nanographene-like layer of flat lying molecules leads to decoupling of further molecular layers to a large extent. Our work suggests that not only inorganic salts or oxides can be used as insulating spacers but also particular organic monolayers.
Abstract: In this letter, Au/tanylphthalocyanine/GaAs diodes incorporating ultrathin films of the archetypal organic semiconductor titanylphthalocyanine were investigated by ballistic electron emission microscopy (BEEM)/spectroscopy. The presence of the organic semiconductor increases the BEEM threshold voltage compared to reference Au/GaAs diodes. The Schottky barrier heights were measured for various temperatures and show an approximately linear increase from 1.2 eV at room temperature to 1.5 eV at T=10 K. (c) 2007 American Institute of Physics.
Abstract: We investigate the thickness-dependent dielectric function of ultrathin titanyl phthalocyanine (TiOPc) layers on Au(111) by in situ spectroscopy during film growth. The absorption features reveal a complex structural development of the film, described by the observation of different crystal phases. The experiment allows addressing the thickness dependence of the oscillator strength in ultrathin films for the first time. Surprisingly, the oscillator strength increases with thickness by more than a factor of 3 and does not saturate until a thickness of 20 monolayers is reached.
Abstract: The authors present an investigation of highly ordered epitaxial thin films of large organic molecules, namely quaterrylene derivatives. Even though the raw material quaterrylene tetracarboxylic diimide (QTCDI) starts to decompose below its sublimation temperature, epitaxial monolayers could be grown. By modifying the preparation procedure, highly ordered domains of quaterrylene dicarboxylic monoimide, being the main decomposition product of QTCDI, could also be produced. Although both molecules bear strong similarities, they grow in a dissimilar fashion influenced by their substituents and molecular dipole and quadrupole moments. The structures are characterized by scanning tunneling microscopy and, in case of QTCDI, low energy electron diffraction.
Abstract: We present a combined low energy electron diffraction and scanning tunneling microscopy study on the growth of the first and second monolayers (MLs) of quaterrylene on single crystalline Au(111). For the first ML, we observe a point-on-line coincident growth of flat lying molecules which agrees well with our potential energy calculations. In contrast, we observe alternating upstanding and flat lying molecules in the second layer. This behavior nicely demonstrates that the substrate has a very strong influence on the structure of the first layer, whereas the second layer is much weakly affected and exhibits properties comparable to the bulk. (c) 2006 American Institute of Physics.
Abstract: We describe the influence of the substrate on the optical properties of ultrathin hexa-peri-hexabenzocoronene (HBC) layers deposited by organic molecular beam epitaxy. For that purpose, in situ differential reflectance spectroscopy (DRS) was employed, providing unsurpassed sensitivity for the thickness-dependent optical analysis. From the DR spectra, the optical functions were extracted using a numerical algorithm. The obtained spectra vary significantly for different substrates. We interpret these variances as being due to the different growth modes of HBC films altered by the influence of the respective substrate. HBC shows polycrystalline island structures on fused quartz in atomic force microscopy. On the other hand, layer-by-layer growth along with the formation of quasi-one-dimensional stacks on highly oriented pyrolytic graphite is known from previously published structural examinations. In this contribution, the optically observed monomer -> dimer -> oligomer transition, indicated by the occurrence of isosbestic points, is related to the findings for perylenetetracarboxylic dianhydride layers on mica.
Abstract: We report an investigation of the excitonic properties of thin crystalline films of the archetypal organic semiconductor PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) grown on poly- and single crystalline surfaces. A sensitive setup capable of measuring the optical properties of ultrathin organic molecular crystals via differential reflectance spectroscopy (DRS) is presented. This tool allows to carry out measurements in situ, i.e., during the actual film growth, and over a wide spectral range, even on single crystalline surfaces with high symmetry or metallic surfaces, where widely used techniques like reflection anisotropy spectroscopy (RAS) or fluorescence excitation spectroscopy fail. The spectra obtained by DRS resemble mainly the absorption of the films if transparent substrates are used, which simplifies the analysis. In the case of mono- to multilayer films of PTCDA on single crystalline muscovite mica(0001) and Au(111) substrates, the formation of the solid state absorption from monomer to dimer and further to crystal-like absorption spectra can be monitored.
Abstract: We investigated photoluminescence of the phosphorescent molecule platinum(II)-2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin. In neat films prepared by organic molecular beam deposition or precipitation from solution, we observe a near infrared emission feature around 1.65 eV, caused by aggregation effects. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 325th Wilhelm and Else Heraeus Seminar on Organic Molecular Solids, Bad Honnef, GERMANY, MAY 17-19, 2004
Abstract: We present luminescence quenching experiments and determine the exciton diffusion length in polycrystalline thin films of PTCDA (perylene-3,4,9,10-tetracarboxylic-dianhydride). From an analysis of time-resolved experiments, we can distinguish between exciton transport during an ultra-fast initial relaxation phase and transport in the long-living emitting states. The temperature dependence of the exciton diffusion constant in the emitting states indicates thermally activated hopping. (C) 2004 Elsevier B.V. All rights reserved.
Notes: 325th Wilhelm and Else Heraeus Seminar on Organic Molecular Solids, Bad Honnef, GERMANY, MAY 17-19, 2004
Abstract: We directly follow the evolution of the absorption spectrum from a single molecule to a dimer and further to a one-dimensional molecular stack: We determine the optical absorption properties of ordered monolayer to multilayer films of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) on muscovite mica(0001) surfaces by in situ differential reflectance spectroscopy. The data clearly show the transition from the single molecule to a dimer spectrum, followed by the exciton delocalization to a molecular crystal exciton. The accompanying spectral shifts compare favorably with recent model concepts.
Abstract: The aim of this work is the in situ monitoring of the growth of molecular thin films on either insulating (potassium chloride, -bromide and mica) or gold substrates by optical spectroscopy. The influence of the molecular arrangement and an optionally added doping on the properties is studied on perylene-tetracarboxy-dianhydride (PTCDA) and titanyl phthalocyanine (TiOPc), deposited by molecular beam epitaxy.
The impact of perfect matching between the substrate’s lattice constants and the dimensions of the molecules, appears in narrow absorption and emission bands in case of commensurate growth of PTCDA on potassium chloride. This arrangement and its metastability are proven by accompanying atomic force microscopy and advanced potential energy calculations. Once potassium can be added stepwise to monolayers of neutral PTCDA molecules, the spectral development towards PTCDA anions can be followed and assigned to the reached levels of charging.
The crystal growth of TiOPc molecules in phase II takes place on a wetting layer with phase I arrangement, proving earlier results by scanning tunneling microscopy. Measuring the thickness-dependent optical properties with (sub-)monolayer resolution allows a deeper understanding of the dependences of both, the oscillator strength and the efficiency of luminescence quenching, on the molecules’ orientations with respect to each other and on their distance to the substrate.
