Research

Color Centers in Semiconductors: Spin and Photophysics

Color centers in semiconductors or insulators show a rich photophysics. Possessing a total electron spin they may be utilized to store quantum information and thus pave the way for the development of solid state quantum computing. The nitrogen-vacancy center (NV) in diamond as well as the divacancy  and the silicon vacancy in silicon carbide (SiC) have emerged as promising candidates for implementing solid state quantum bits. Furthermore such defects have important applications as sensors of magnetic and electric fields or temperatur.

Optical excitation of the high-spin ground state and subsequent spin-selective recombination via unknown intermediate low-spin states enables spin-initialization mediated by intersystem crossings. Together with spin-dependent luminescence this provides all-optical control of the defect spins. The host material and the defect parameters like the fluorecensce wavelength, lifetimes of the optical transitions and non-radiative recombination, as well as spin relaxation times make the defects suitable for different applications.

 

Although the photo physics of a vast variety of systems, including defects, was successfully addressed in the framework of many bodyperturbation theory (GW and BSE) and time dependent density functional theory, these approaches do not provide direct access to the important low-spin excited states. As an approach towards the spin physics of defects the solid state group has developed an ab initio CI-hamiltonian employing a restricted basis and an effective screened coulomb interaction.  On this basis the solid state group explores the photo and spin physics of candidate defect centers in diamond and silicon carbide or other materials.

  • M. Bockstedte, F. Schutz, Th. Garratt, V. Ivady, A. Gali, Ab initio description of highly correlated states in defects for realizing quantum bits, Nature Partner Journal Quantum Materials, 3, 31 (2018).
  • K. Szasz, V. Ivady, I. I. Abrikosov, E. Janzén, M. Bockstedte and A. Gali, Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit, Phys. Rev. B 91, 121201(R) (2015).
  • M. Widmann, M. Niethammer, D. Yu. Fedyanin, I. A. Khramtsov, T. Rendler, I. D. Booker, J. Ul Hassan, N. Morioka, Y.-C. Chen, I. G. Ivanov, N. T. Son, T. Ohshima, M. Bockstedte, A. Gali, C. Bonato, Sang-Yun Lee, J, Wrachtrup, Electrical Charge State Manipulation of Single Silicon Vacancies in a Silicon Carbide Quantum Optoelectronic Device, Nano Lett. 19, 7173 (2019).

Adsorbates at Surfaces

Adsorption of functional molecules on nanostructured metal oxide surfaces, thin films grown on a metal substrate, or nanocrystalline assemblies are pivotal for accessing novel functional materials for a wide range of applications, including photovoltaic, sensing, or catalysis.

Porphyrins as a flexible class of molecules enable vast possibilities of tailoring their functional properties towards specific applications. The solid state physics group collaborates with the materials science group and partners at the Friedrich-Alexander-Universität Erlangen-Nürnberg in the funCOS research unit.

“Molecular landscaping”, the vision of funCOS, includes site-selective adsorption and formation of molecular assemblies via linker groups, tuning of the adsorbate-adsorbate interaction and alteration of the electronic and chemical properties via functional/linker groups or the metal center. These topics directly depend on the metal oxide substrate as key elements of structure formation via the adsorbate-oxide interaction, in particular, at low-coordinated sites or nanostructures. Challenges for a theoretical modeling lie in the development of atomic-scale models in collaboration with experiment and their rationalization in terms of a structure-property relationship. The solid state physics group focuses on  metalation of free-base porphyrins at metal oxide surfaces and nano particles, porperties of porphyrin films, their electronic structure, and photophysics

  • A. Classen, R. Pöschel, G. Di Filippo, T. Fauster, O. B. Malcioğlu and M. Bockstedte, Electronic Structure of Tetraphenylporphyrin layers on Ag(100), Phys. Rev. B 95, 115414 (2017).
  • D. Thomele, G. R. Bourret, J. Bernardi, M. Bockstedte and O. Diwald, Hydroxylation Induced Alignment of Metal Oxide Nanocubes, Angew. Chem. Int. Ed. 56, 1407 (2017).
  • M. D. Weber, J. E. Wittmann, A. Burger, O. B. Malcioğglu, J. Segarra-Marti, A. Hirsch, P. B. Coto, M. Bockstedte and R. D. Costa, From white to Red: Electric-Field Dependent Chromaticity of Light-Emitting Electrochemical Cells based on Archetypal Porphyrins, Adv. Funct. Mater. 26, 6737 (2016).

Reactions at Surfaces

The surface of condensed matter is capable to alter the properties of adsorbates such that the mutualreaction among the adsorbed species occurs at rates that are unavailable in other environments likethe gas-phase. Yet, as a catalyst, the surface is not only the host - for supplying favorable conditionsfor reactions - it also plays an active role by e.g. providing electrons. Indeed, surface structures on the nanoscale scale, such as metal clusters or oxygen vacancies on metal oxides, are sources of reactive electrons. Also photo-excited electrons can efficiently attach to adsorbates on metal or insulator surfaces and induce their dissociation.

Polar solvents represent a unique class of catalysts because of their ability to solvate charged carriers and excess electrons. This ability allows for the localization and subsequent solvation of electrons also at the surface of solvent fi lms or ices. The electron solvation occurs as a dynamical response of the solvent to the charge. As compared to catalysts like metal oxides, where the reactive electron often stems from pre-defined sites, the evolution of the solvated state adds another degree of complexity to electron-induced processes. The interplay between solvated electrons and adsorbates contains fascinating physical mechanisms, yet an atomic scale understanding is missing.

 

In a highly complementary approach employing theoretical modeling , high-resolution scaning tunneling spectroscopy (STM), and two photon-photoemission spectroscopy (2PPE), the solid state physics group joined forces with the groups of Prof. Dr. Karina Morgenstern (Ruhr-Universität Bochum) and Prof. Dr. Uwe Bovensiepen (Universität Duisburg-Essen) to investigate the mechanisms at work in the dissociative electron attachment of halogenated bencenes  with varying electron affinity C6H5X, X=F,Cl,Br. 

  • Ph. Auburger, I. Kemeny, C. Bertram, M. Ligges, M. Bockstedte, U. Bovensiepen, K. Morgenstern, Microscopic insight into electron-induced dissociation of aromatic molecules on ice, Phys. Rev. Lett., 121, 206001 (2018).
  • M. Bockstedte, A. Michl, M. Kolband, M. Mehlhorn and K. Morgenstern, Incomplete Bilayer Termination of the Ice (0001) Surface, J. Phys. C 120, 1097 (2016).