Optical Properties of Organic Semiconductors: from Submonolayers to Crystalline Films
| Autor: | Nitsche, Robert |
|---|---|
| Přispěvatelé: | Leo, Karl, Laubschat, Clemens, Sokolowski, Moritz |
| Jazyk: | angličtina |
| Rok vydání: | 2005 |
| Předmět: |
Absorption
Au(111) Differentielle Reflexionsspektroskopie Dipolmodell Exzitonen Glas Glimmer Graphit HBC HOPG Kramers-Kronig Oszillatorstärke PTCDA dünne Filme optische Eigenschaften optische Konstanten Au(111) Differential Reflectance Spectroscopy Glass HBC HOPG Kramers-Kronig PTCDA absorption dielectric constants dipole model excitons graphite mica optical constants optical properties oscillator strength thin films Absorption Dipol Dünne Schicht Exziton Glimmer Gold Graphit Optische Eigenschaft Optische Konstante Oszillatorstärke Perylendianhydrid Reflexionsspektroskopie ddc:530 |
| Popis: | We have measured the optical properties of films of the organic semiconductors PTCDA (3,4:9,10-perylene-tetracarboxylic dianhydride) and HBC (peri-hexabenzocoronene), prepared by Organic Molecular Beam Expitaxy (OMBE), on different substrates by means of Differential Reflectance Spectroscopy (DRS). The optical setup enables us to directly follow the thickness dependent optical properties of the organic films, starting from submonolayer coverage up to thicker films on the order of 20 monolayers (ML) film thickness. Due to the different optical nature of the different substrates used, i.e., mica, glass, Au(111), and HOPG, the direct interpretation of the DRS signal is not feasible. Therefore, we have proposed a method by which the calculation of the optical constants n (index of refraction) and k (absorption index) of thin films on arbitrary substrates from just one spectral measurement (in our case the DRS) becomes possible. The results fulfill a priori a Kramers-Kronig consistency and no specific model is needed to express the spectral behavior of the optical constants. Based on our method, we have successfully calculated the optical constants, and therefore the absorption behavior, of films of different thickness of PTCDA on mica, glass, Au(111), and HOPG, as well as of HBC on mica, glass, and HOPG. Extrinsic effects due to island growth or the presence of a polarizable substrate (screening) have been accounted for. We have introduced a finite dipole model which considers the extended geometry and anisotropy of the organic molecules. The calculated absorption behavior is discussed in great detail in terms of spectral changes with varying film thickness, different growth modes, degree of ordering of the films, interactions with the substrates and oscillator strength. A direct observation of a monomer-dimer transition in solid films could be observed for the first time. Our results indicate an exciton delocalization over about 4 molecules for both molecules. |
| Databáze: | OpenAIRE |
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