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People currently working in our group:

Andris Gulans
Markus Scheidgen
Archana Manoharan
Christian Vorwerk
Claudia Draxl
Dmitrii Nabok
Helen Jurscha
Lorenzo Pardini
Maria Troppenz
Nora Illanes Salas
Olga Turkina
Pasquale Pavone
Santiago Rigamonti
Benedikt Hoock
Sven Lubeck
Eric Pierschel
Georg Huhs
Jungho Shin
Konstantin Lion
Martin Kuban
Sebastian Tillack
Axel Hübner
Ka Wai Lau
Saeideh Edalati-Boostan
Fabio Caruso
  • Andris Gulans
  • Markus Scheidgen
  • Archana Manoharan
  • Christian Vorwerk
  • Claudia Draxl
  • Dmitrii Nabok
  • Helen Jurscha
  • Lorenzo Pardini
  • Maria Troppenz
  • Nora Illanes Salas
  • Olga Turkina
  • Pasquale Pavone
  • Santiago Rigamonti
  • Benedikt Hoock
  • Sven Lubeck
  • Eric Pierschel
  • Georg Huhs
  • Jungho Shin
  • Konstantin Lion
  • Martin Kuban
  • Sebastian Tillack
  • Axel Hübner
  • Ka Wai Lau
  • Saeideh Edalati-Boostan
  • Fabio Caruso

Optoelectronic excitations at inorganic/organic interfaces:
Role of vibrations and time scales

This project is dedicated to the ab initio description of temperature-dependent exciton spectra of hybrid inorganic/organic semiconductor interfaces by a combination of density-functional theory (DFT) with many-body perturbation theory. Target systems include single molecules, mono- and multilayers of functionalized molecules like pyridine, oligo-acenes, -thiophenes and -phenylenes  adsorbed on ZnO, Si, and GaN  surfaces. 

  

Recent publications:

C. Draxl, D. Nabok, and K. Hannewald
Organic/Inorganic Hybrid Materials: Challenges for ab Initio Methodology
Acc. Chem. Res. 47, 3225-3232 (2014). 

We address the question to what extent we can quantitatively describe hybrid materials and where we even miss a qualitative description. We argue when and why, for example, standard DFT may fall short when it comes to the electronic structure of organic/metal interfaces or where the framework of MBPT can or must take over. Selected examples of organic/inorganic interfaces, structural properties, electronic bands, optical excitation spectra, and charge-transport properties as obtained from DFT and MBPT highlight which properties can be reliably computed for such materials. Read more...

 

L. Pithan, C. Cocchi, H. Zschiesche, C. Weber, A. Zykov, S. Bommel, S.J. Leake, P. Schäfer, C. Draxl, S.M. Kowarik
Light controls polymorphism in thin films of sexithiophene
Cryst. Growth Des. 15, 1319–1324 (2015)

We demonstrate that the bimodal growth behavior of sexithiophene can be influenced strongly by light. α-Sexithiophene films grown in the dark on KCL consist of high- and low-temperature polymorphs, whereas in films grown under laser illumination the high temperature phase is significantly suppressed. This shows that light can act as a new control parameter in growth of organic semiconductors. Read more...

S. Rigamonti, S. Botti, V. Veniard, C. Draxl, L. Reining, and F. Sottile
Estimating Excitonic Effects in the Absorption Spectra of Solids: Problems and Insight from a Guided Iteration Scheme
Phys. Rev. Lett. 114, 1463402 (2015).

A major obstacle for computing optical spectra of solids is the lack of reliable approximations for capturing excitonic effects within time-dependent density-functional theory. We show that the trustful prediction of strongly bound electron-hole pairs within this framework using simple approximations is still a challenge and that available promising results have to be revisited. Deriving a set of analytical formula we analyze and explain the difficulties. We deduce an alternative approximation from an iterative scheme guided by previously available knowledge, significantly improving the description of exciton binding energies. Finally, we show how one can "read" exciton binding energies from spectra determined in the random phase approximation, without any further calculation. Read more...

 

C. Cocchi and C. Draxl
Optical Spectra from Molecules to Crystals: Insight from Many-body Perturbation Theory
Phys. Rev. B 92, 205126 (2015).

Time-dependent density-functional theory (TDDFT) is successful in describing excitation energies of finite systems, already in its most simple form, the adiabatic local-density approximation (ALDA). By confronting TDDFT with many-body perturbation theory, we clarify when and why this method can be trusted for molecular materials, where many-body effects can be crucial. We show that ALDA provides accurate results for excitations with mainly single-particle character. Conversely, when electron-hole correlations as well as local-field effects become decisive, only a many-body approach can quantitatively predict absorption features. Read more...

Abstract Image

S.-A. Savu, G. Biddau, L. Pardini, R. Bula, H. F. Bettinger, C. Draxl, T. Chassé, and M. B. Casu
Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface
J. Phys. Chem. C 119, 12538 (2015)

Although physisorption is a widely occurring mechanism of bonding at the organic/metal interface, contradictory interpretations of this phenomenon are often reported. Photoemission and X-ray absorption spectroscopy investigations of nanorods of a substituted pentacene, 2,3,9,10-tetrafluoropentacene, deposited on gold single crystals reveal to be fundamental to identify the bonding mechanisms. We find fingerprints of a fractional charge transfer from the clean metal substrate to the physisorbed molecules. This phenomenon is unambiguously recognizable by a nonrigid shift of the core-level main lines while the occupied states at the interface stay mostly unperturbed, and the unoccupied states experience pronounced changes. The experimental results are corroborated by first-principles calculations. Read more...

 

People involved: 

Olga Turkina (PhD student)
Benjamin Höffling (postdoc)
Caterina Cocchi (postdoc)
Dmitrii Nabok (postdoc)
Andris Gulans (postdoc)
José Enrique Alvarez Roca (student assistant)

Giulio Biddau (2013-2014)