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Research Projects

The Research Unit is composed of four experienced researchers, three researchers with recent first full-time appointments, as well as one junior reseacher and one career researcher. Each of the quoted project areas, A, B, and C, is addressed by a team of scientists who work together on complementary fields in the specific areas as outlined in the following.

Project A - Synthesis and Chemical Analysis: Prof. Dr. Stefanie Dehnen and Prof. Dr. Peter R. Schreiner

Stefanie Dehnen (spokesperson) is an expert in the synthesis and structural and chemical characterization of metal chalcogenide clusters, among them the prototype compounds for the proposed work. Peter R. Schreiner (deputy spokesperson) is an expert in organic synthesis of diamondoid molecules and other cage structures with a diversity of core moieties and substituents. The two groups have already collaborated and published together in the recent past. Within the Research Unit, they interact by complementary syntheses of structurally related inorganic, organic, and hybrid clusters. Both scientists additionally have a quantum chemical background, which makes them ideal project partners of the physical and theoretical expert groups. Stefanie Dehnen has many further collaborations with other researchers of the team. Peter R. Schreiner is a member of the Center for Materials Research (LaMa) at JLU, with access to a large array of analytical instruments.

Project A1 - Synthesis and Characterization of Amorphous Inorganic Cluster Compounds (Dehnen)

We have recently discovered that adamantane-type clusters of the general formula [(RT)4S6] (R = organic ligand; T = Ge, Sn) exhibit extreme non-linear optical properties and show white-light generation (WLG) upon irradiation with a commercially available continuous-wave (CW) GaAs laser diode, if the compounds are amorphous and comprise ligands R with a small-to-medium-size aromatic system. For compounds that show long-range order and/or comprise other organic ligands, strong second-harmonic generation (SHG) is observed instead. In the meantime, we have investigated a few selected compounds in order to get a basic understanding of the underlying mechanism of the WLG in contrast to SHG. We know that the anharmonicity of the electronic ground-state potential of the clusters as well as the local, mesoscopic order define the optical nonlinearity, but we do not know so far, whether the anharmonicity is caused by a molecular lack of symmetry or a lack of long range order in the bulk alone, or by both of the said aspects. Hence, in this project A1 we will introduce a greater variety of clusters with different core composition and different substituents, which are then subject to further investigation within the other projects. Not only the preparation of homogeneous [(RT)4E6] clusters (one R, one T, one E) will be addressed, but also (mainly) the synthesis of clusters with heterogeneous composition (R/R’, T/T’, E/E’), as well as defined cluster mixtures, like blends of inorganic-organic and purely organic clusters. Especially with regard to the second funding period, the library of compounds will be extended to clusters with other inversion-free architectures adopting an overall (near) tetrahedral or threefold symmetry. The general objectives of this project can be specified as follows: 1) Preparation of new clusters with adamantane-based topology. 2) Influence and control of amorphousness and particle size. 3) Pre-screening of structural, vibrational and electronic properties by DFT studies. 4) Expansion of the compound library to other architectures with (near) tetrahedral or threefold symmetry. The number of potential compounds that accord with the general formula [(RT)4E6] is virtually countless for heterogeneous clusters. For this, a careful pre-selection of R/T/E combinations is necessary, which will be achieved by own pre-screening of the compounds by means of molecular quantum chemical studies, and in close collaboration with the theorists and physicists in FOR 2824.

Project A2 - Synthesis and Characterization of Functionalized Diamondoids and Heterodiamondoids (Schreiner)

This proposal is part of the application for funding of a Research Unit (FOR 2824) on “Amorphous Molecular Materials with Extreme Non-Linear Optical Properties“. The current project focuses on the preparation of organic white-light emitting materials based on diamondoid cores that are equipped with functionality or substitution (or both) to alter the molecular properties that, in turn, determine the emission characteristics. This also includes initial particle size determinations to allow pre-screening of suitable candidates, whose cluster morphologies as well as physical properties will then be measured in the physical groups of FOR 2824. We will also undertake preliminary computational screens utilizing density functional theory to evaluate the effects of heteroatom and functional group substitution on HOMO-LUMO energy separations, polar effects, and electron spin distributions. Four general fundamental questions will be addressed: 1) What is the connection between molecular topology / symmetry and solid-state morphology? This will be addressed through a substitution pattern that generates a particular molecular symmetry. 2) What is the effect of the HOMO-LUMO gap on the emission properties? This will be tackled with systematic albeit simple series of molecular structures that allow fine-tuning of the HOMO-LUMO gap through electron donor and electron acceptor substitution. Additionally, “internal doping” with heteroatoms will affect the electronic states dramatically as the heteroatom lone pairs are “inserted” between the HOMO and the LUMO. 3) Is there a cross-connection between particle or crystal size and the ability of the material to emit white light? Here, it will be essential to determine particle sizes of the solid materials, and to reproducibly generate defined grain size distributions. 4) Outlook (second funding period): How can the organic material be designed to exhibit long-term stability when used in integrated devices? The long-term overall goal is to reproducibly prepare novel organic white-light emitting materials that display long-term stability and ease of usability in a variety of devices.

Project B - Physical Characterization: Prof. Dr. Sangam Chatterjee, Prof. Dr. Kerstin Volz, Prof. Dr. Wolf-Christian Pilgrim, Dr. Nils W. Rosemann and Prof. Dr. Marina Gerhard

Samgam Chatterjee has been appointed Heisenberg Professor for Spectroscopy and Optics at JLU. He is an expert for diffraction-limited time-resolved spectroscopy and nonlinear optics and was the first to measure the extreme non-linear optical response of the compounds addressed in this research unit. Kerstin Volz is an expert in semiconductor growth and structural characterization. Both have published many papers together and/or together with Stefanie Dehnen. In particular, the electron microscopy facility of the Structure and Technology Research Laboratory (STRL) run by Kerstin Volz makes her an outstanding collaborator for many groups worldwide. She contributed to the initial findings of the subject matter in that she realized the deposition of the [(RSn)₄S₆] clusters on GaAs and Si (001) surfaces. Structural correlations in the disordered materials on micro- and mesoscopic length scales are supported by Wolf-Christian Pilgrim who is an expert in the analysis of structure and dynamics in liquids and glasses using different synchrotron- and neutron techniques at national and international large scale facilities. A first collaboration of Wolf-Christian Pilgrim with Stefanie Dehnen on the potential packing of [(RSn)₄S₆] clusters is currently underway using a combined approach of Anomalous X-Ray Scattering (AXS) and Reverse Monte Carlo simulations (RMC).

For the second funding period, FOR 2824 welcomes two new principal investigators: Nils W. Rosemann is junior researcher at the Karlsruhe Institute of Technology (KIT). He is an expert in time-resolved and nonlinear optics with focus on combining those techniques with integrated photonics. In his doctoral thesis he was the first to explore the extreme non-linear optical response of the inorganic cluster compounds that are investigated in this research unit. Marina Gerhard joins the research unit with her expertise in temperature dependent, time and spatially resolved photoluminescence spectroscopy, with a particular background in organic polymers and molecular materials. She leads the semiconductor spectroscopy junior research group at the Faculty of Physics, University of Marburg.

Project B1 - Optical Spectroscopy of Condensed-Phase (Hetero-) Diamondoid Systems (Chatterjee)

This project aims at investigating the nonlinear optical properties of condensed-phase (hetero-) diamondoiden solids. In particular, the role of the microscopic, i.e., molecular structure and symmetries as well as of the mesoscopic order in the condensed phase, i.e., degree of crystallinity or amorphousness will be studied regarding their influence on the nonlinear conversion mechanism and their efficiencies as well as the resulting coherence properties.Therefore, exemplary prescreened samples from the materials library synthesized by projects A1, A2 of this research unit will be studied. Additionally, structurally more simple compounds are investigated as benchmark systems for comparison with the modelling in the theory projects C1, C2. The latter render numerically feasible test systems for the advanced descriptions developed within the framework of this research unit. Typical experiments include the linear and nonlinear absorption spectra, the latter via z-scan, (time-resolved) photoluminescence characterization, nonlinear emission spectra and conversion efficiencies which yield the nonlinear optical coefficients. Additionally, the temporal and spatial coherence properties will be explored using interferometric techniques, both in Michelson and Hanbury Brown-Twiss geometries. This way, the coherence functions of first and second order will be determined for selected samples. Finally, electron paramagnetic resonance will be applied to suited derivatives to identify the local chemical environment and identify the potential influence of triplet states. All results will be correlated with the insights gained in projects B2, B3 of the research unit devoted to in-depth structural characterization on comparable of even identical samples.

Project B2 - Electron microscopy and electron diffraction (Volz)

This project addresses the structure of various molecular assemblies investigated in the FOR 2824 by different electron microscopic techniques. The questions, which will be answered, are the exact arrangement of the atoms in crystalline solids and the structure of defects in crystalline material. Moreover, the mesoscopic disorder in amorphous / crystalline mixtures will be examined as well as the nearest neighbor arrangement in amorphous solids. Together with the partner projects, the information on the structure will be correlated to the synthesis of the materials as well as to their optical properties in experiment and in theory.To answer these questions, the sample preparation will be optimized for the materials under investigation and the imaging conditions will be adopted to the mostly radiation sensitive specimen. High angle annular dark field imaging as well as annular bright field imaging in scanning transmission electron microscopy (STEM) will be used – in combination with image simulation – to tackle the exact atomic arrangement of crystalline structures. For amorphous / crystalline mixtures Fluctuation Electron Microscopy (FEM) in diffraction as well as in STEM mode will be applied to derive statistical information on grain properties. For entirely amorphous specimens, the Radial Distribution Function (RDF) will be determined for the ensemble as well as for the constituents individually by using (energy filtered) electron diffraction. All the methods described above will also be developed further with respect to the materials studied in the project.

Project B3 - Determination of Structural Correlations in Amorphous Materials by Means of Synchrotron- and Neutron Scattering and Reverse Monte-Carlo-Simulations (Pilgrim)

Microscopic structural and dynamical correlations of the amorphous and partly amorphous materials, subject matter of the research unit FOR 2824, will be determined in this subproject by means of different modern x-ray-, synchrotron- and neutron-scattering methods. The structural correlations between molecular cluster-cores and/or the organic side groups leading to the specific and yet unresolved optical properties of these materials are to be determined (i.e. White-Light-Emission (WLE) vs. Second-Harmonics-Generation (SHG)). So far, it is only known that a specific size of the π-electron system in the side groups as well as a minimum solid state amorphization is needed, to sustain the outstanding WLE-effect. Other substituents as well as a too large portion of crystallinity lead to materials with optical SHG-behavior. In the requested research sub project we aim to find out, if there is a relationship between structural correlations and optical properties and which molecular structural properties control the solid-formation crystalline vs. amorphous. The use of different scattering probes (x-ray and neutrons) and the subsequent analysis of the measured scattering laws by means of Reverse-Monte-Carlo (RMC) simulation allows to draw attention selectively either to the cluster cores or to the organic side groups, respectively, which allows reliable structure refinement with respect to these cluster components.Furthermore, since the WLE-materials convert infrared radiation into white light, and also as preliminary for a possible second funding period, first inelastic neutron- and x-ray scattering experiments shall be carried out. From the results we will get first insight into the relation between electronic and vibrational properties of these solids.

Project B4 - On-chip supercontinuum generation and advanced spectroscopy on adamantane-type cluster molecules (Rosemann)

The main goal of this project is to exploit the extreme nonlinear optical properties of the research unit's amorphous molecular materials to generate white light on a chip. We will build on our previous work in which we converted the continuous wave output of a commercial laser diode into visible light by placing the amorphous molecular material directly on the emitting surface of the laser diode. We will improve on this initial approach by specifically designing waveguide structures that create a defined interaction between the infrared light and the molecular materials to enhance the generation of white light. These functionalized waveguide structures then provide a white light laser source driven by a standard laser diode with a total area comparable to that of a laser diode. Such a small design simplifies its use in lab-on-a-chip systems or even at the tip of an endoscope.

At the same time and in close collaboration with the partner projects, we will perform advanced spectroscopy on the amorphous molecular materials. We will investigate how the generation of white light is affected when excited by far-infrared light with a wavelength in the µm range. By tuning the far-infrared excitation, we can control whether the energy is mainly transferred into the electronic system of the molecules or whether it introduces vibrations of the backbone. In this way, we can refine our understanding of the white light generation mechanism and the role of the electronic system of the molecules. We will also conduct experiments to influence and control the generation of white light. By placing the molecular materials in an electric field, we can manipulate the electronic system of the molecule to study its contribution to white light generation. Finally, we will establish the generation of white light by molecular materials in an optical resonator. This will introduce optical feedback into the nonlinear process and increase its efficiency. In addition, the generation of white light will be fully controlled. By tuning the resonant frequency of the resonator, certain parts of the generated white light spectrum can be enhanced. This concept enables us to convert the broadband white light emission into a narrow emission that can be tuned over an extremely wide spectral range.

Overall, these experiments will foster the understanding of the observed non-linear white-light generation and the correlation to characteristic attributes of the amorphous molecular materials.

Project B5 - Investigation of the static and dynamical structure-property-relationships of diamondoid cluster compounds through optical spectroscopy (Gerhard)

In order to tailor novel diamondoid cluster compounds for efficient white light generation (WLG), it is essential to comprehend the interrelation between the WLG mechanism and the underlying micro- and nanostructure. The goal of project B5 is to complement methods of structural characterization by optical techniques such as in particular absorption and (time resolved) photoluminescence spectroscopy. Beside establishing correlations between optical spectra and the nanomorphology, we will also explore the optical properties through spatial mapping on length scales > 1 µm by photoluminescence microscopy. Moreover, we will explore the role of dynamic disorder such as lattice vibrations, rotation of the ligands or conformational changes in the excited state. In this context, low temperature and temperature dependent optical experiments are of particular relevance and the temperature-induced broadening of emission lines as well as the low temperature spectra will yield important quantities to estimate the electron-lattice interactions, which will be further compared to theoretical work.  Also, the presence of phonon side bands in emission spectra can be informative about the energies of the dominant vibrational modes. A particular challenge in this context will be inhomogeneous broadening, which potentially masks the relevant signatures. To see beyond such ensemble averaging effects, line narrowing techniques will be employed. Our experiments are on the one hand based on the intrinsic optical properties of the cluster compounds, but we will also study compounds with attached small luminescent molecules with absorption edges energetically well below the absorption onset of the cluster materials. These molecules and in particular their temperature dependent optical properties will ideally serve as probes for the dynamical properties of the local environment. Overall, we intend to establish optical spectroscopy as a non-destructive and efficient probe for the morphology of the growing library of cluster compounds, which will ideally pave the way for a more targeted research of compounds with improved WLG properties.

Project C - Theory: Prof. Dr. Doreen Mollenhauer and Prof. Dr. Simone Sanna

Doreen Mollenhauer has been an early career researcher at JLU since 2014, addressing several topics including ligand-stabilized clusters and weak intermolecular interactions. She has recently made some very interesting contributions in these fields and is experienced in collaborating with synthetic chemists and physicists. Simone Sanna is a theoretical physicist and has accepted the offer for a professorship for Theoretical Physics (W2), with emphasis on solid state spectroscopy, at JLU in 2017. He is experienced in computational studies on defect structures and optical excitations, hence his expertise combines two key areas of the planned studies within the Research Unit.

Project C1 - Modelling Cluster Structures and Properties at Different Length Scales (Mollenhauer)

A new spectrally broad band white light emitter based on adamantane-like cluster material has been discovered. This material in combination with cheap, low-power continuous-wave infrared laser diodes has the potential to be used in high-brilliance applications. The first investigated cluster material [(StySn)4S6] consists of a tin-sulfide-based adamantane-like core saturated with organic styrene ligands. Investigations of clusters with modified adamantane core composition and different organic ligands suggest that a disorder in the macroscopic structure and delocalized π electrons at ligands are required to generate white light. Furthermore, another nonlinear optical effect, namely the second harmonic generation of the fundamental laser light, was found for clusters that show no white light generation. The recent studies demonstrate the need to understand the structural and electronic features of the cluster material as well as the underlying nonlinear optical effects in order to design and fine-tuning cluster materials to the desired properties. This theoretical research project will focus on the calculation and modelling of the geometric structures of single clusters up to solid state material and related electronic and vibrational properties. Therefore, numerous variations of the cluster core and the organic ligands will be considered. The project will closely collaborate with the theoretical project of C2 (Sanna) which focuses on the modelling of the nonlinear optical effects and all experimental groups of the research group. The systematic study of single clusters will allow to identify trends in structure and properties of white light or second harmonic generating clusters. Therefore, the interplay between the aromatic ligands and the adamantane core will be analyzed in detail. The investigation of cluster dimers to tetramers will allow first insights into crystal growth leading to crystalline or amorphous material. The study will answer the question how the cluster interaction can be influenced by cluster composition. Based on the understanding of the fundamental structural features of the cluster interaction, approaches to model amorphous-like cluster solid state structures will be developed. The modelled amorphous-like solid state cluster structures will contribute to the understanding of short- and long-range order in the cluster material and manipulation of this. The calculation of the cluster material at different length scales connected to each other will allow structural and property-related insights beneficial to an optimization for desired nonlinear optical effects.

Project C2 - Electronic and Structural Excitations in Molecular Clusters and Molecular Cluster Aggregates (Sanna)

In the present project, we aim at gaining a thorough understanding of the interplay between atomic structure, electronic configuration and chemical variations of molecular clusters/cluster aggregates and their optical response. The focus is set on the parameter-free determination of the spectroscopic properties of the investigated molecular compounds. Linear and nonlinear optical spectra, neutron scattering experiments, Raman spectra and photoluminescence signatures shall be modeled for isolated molecules and periodic molecular crystals. The effect of structural disorder on the optical properties will be modeled with the help of the ground state geometries calculated from first principles within the project C1. We perform calculations at different levels of accuracy, ranging from the independent particle approximation to more refined calculation schemes including quasiparticle effects and the electron-hole interaction. An accurate estimation (e.g., within state-of-the-art many-body perturbation theory) of the spectroscopic properties of the extended systems of interest in FOR 2824 is beyond the capability of modern supercomputers, especially concerning larger cluster aggregates. We will therefore extend our calculation schemes in order to implement and apply real-time TD-DFT based approaches, which allow for an efficient calculation of optical nonlinearities with adequate accuracy even for extended systems, including molecular crystals. On the one hand, we deal with well-known model systems such as the prototypical [(PhSi)4S6] adamantane shaped compound, for which structural studies already exist. On the other hand, we address the chemical variations as proposed in project area A. Fundamental questions regarding the prerequisites for the optical nonlinearities such as order/disorder, symmetry and chemical composition are explored. Starting with structural data determined in project C1, our project provides the optical response of the investigated systems, thus helping the interpretation of the measurements in A1, A2, B1, B2 and B3 and inspiring the synthetization of new compounds with tailored optical properties. Thereby our atomistic models offer two crucial advantages. The first advantage is that the optical response is calculated as a function of the incident wavelength. The knowledge of the full optical spectrum will help to establish the correlation between excitation wavelength and optical nonlinearities, and ascertain whether other light sources than IR radiation can be employed to drive the material. A second great advantage of our models is the possibility to calculate the optical response of metastable structures that are not experimentally accessible, such as metastable clusters or ordered structures of amorphous compounds. This allows to disentangle the different effects that simultaneously act to determine the optical response, and address the physical questions concerning the single aspects.