Research Topics


Inorganic-Organic Semiconductor Nanoparticle Thin Films

Where thin films of semiconductor nanoparticles (S-NP) or organic semiconductors (OSC) individually have been applied with great success as core materials for optoelectronic devices like solar cells, blends of the materials are less efficient. Although these blends or hybrid materials bear the promise of bringing together the best of two worlds – low excitonic binding energy in the S-NP and defined surfaces in the OSC matrix – the complex interaction and lack of understanding of the interphase between the two materials has overshadowed the potential advantages so far.

Two core problems are the tendency to phase-segregate upon unspecific mixing of the two materials and the small grain sizes of nanoparticles with frequent hopping events which requires highly effective interparticle coupling to allow good carrier transport.

In this project, we seek to tackle both problems with the same approach: Covalent functionalization of the nanoparticle surface with organic semiconductor molecules. The OSC is selected such that it not only shows a large binding affinity to the S-NP but also provides suitable molecular orbitals for charge carriers in the S-NP to tunnel from one particle to the next.

Experimental methods include bottom-up colloidal chemistry synthesis of various semiconductor nanoparticles with size libraries of narrow dispersion, chemical synthesis of small organic semiconductor molecules, film deposition from solution by spin-coating, solvent annealing or dip-coating, spectroscopic characterization of the constituents and the hybrid film and full analysis of the electric transport properties in a field effect transistor set-up.


Structure-Transport Correlations in PbS Mesocrystals

This project aims at a detailed understanding of the optical and electrochemical interactions within a new organic-inorganic semiconductor hybrid nanostructure, namely PbS quantum dots (QDs) and selected organic semiconductor ligands (OSC). These ligands are chosen such that they contain functional groups to bind to the surface of PbS QDs and replace the native ligand sphere. This way, coupled organic-inorganic nanostructures are obtained which bear the potential of significant carrier delocalisation between the particle core and the ligand shell depending on the alignment of relevant energy levels between the two moieties. Such coupled nanostructures circumvent the current charge transport dilemma of solution-processed QDs by providing a channel for carrier transport through molecular orbitals of the organic semiconductor ligands.

These PbS QDs self-assemble into three-dimensional highly ordered superlattices, where the individual QDs are iso-oriented – so called mesocrystals. In close analogy to classical crystals in which atoms have been replaced by QDs, the coupling between QDs can be tuned by changing their orientation and structural order. Hence, structural properties are assumed to determine the electronic properties of this artificial QD solids.

To gain novel insight into the electronic and structural properties of these PbS QD films, our group focuses on understanding and controlling of the self-assembly of PbS QDs into conductive superlattices.

 

For this purpose we make use of a large portfolio of methods, such as:

  • PbS QD synthesis, self-assembly and ligand exchange with organic semiconductors
  • Device fabrication via microfabrication (optical lithography, soft-nanolithography, …)
  • Conductivity and field effect transistor measurements
  • Structural characterisation by means of SEM/TEM imaging and X-ray scattering (GISAXS, GIWAXS, nano-focused XRD)

With this project we seek to understanding the structure-property relationship in PbS QD superlattices, which facilitates the development of novel solution-processed nanomaterial-based devices for electronics, optics, thermoelectrics & spintronics.

This project is supported by several collaborations:

  • Prof. Monika Fleischer, University of Tübingen
  • Prof. Frank Schreiber, University of Tübingen
  • Prof. Ivan Vartaniants, Deutsches Elektronen Synchrotron, Hamburg

Key references

André, A.; Weber, M.; Wurst, K. M.; Maiti, S.; Schreiber, F.; Scheele, M. Electron-Conducting PbS Nanocrystal Superlattices with Long-Range Order Enabled by Terthiophene Molecular Linkers. ACS Applied Materials & Interfaces 2018, published online. https://pubs.acs.org/doi/10.1021/acsami.8b06044

Maiti, S. ; André, A.; Banerjee, R.; Hagenlocher, J.; Konovalov, O.; Schreiber, F.; Scheele, M. Monitoring Self-Assembly and Ligand Exchange of PbS Nanocrystal Superlattices at the Liquid/Air Interface in Real Time. J. Phys. Chem. Lett. 2018, 9, 739−744. http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.7b03278

Zaluzhnyy, I.; Kurta, R; André, A.; Gorobotsov, O.Y.; Rose, M.; Skopintsev, P.; Besedin, I.; Zozulya, A. V.; Sprung, M.; Schreiber, F.; Vartanyants, I. A.; and Scheele, M. Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking. Nano Lett 2017, 17, 3511–3517. http://pubs.acs.org/doi/full/10.1021/acs.nanolett.7b00584

André, A.; Theurer, C.; Lauth, J.; Maiti, S.; Hodas, M.; Samadi Khoshkhoo, M. ; Kinge, S.; Meixner, M.; Schreiber, F.; Siebbeles, L.; Braun, K.; Scheele, M. Structure, transport and photoconductance of PbS quantum dot monolayers functionalized with a Copper Phthalocyanine derivative. Chem. Comm. 2017, 53, 1700-1703. http://pubs.rsc.org/en/content/articlelanding/2017/cc/c6cc07878h


Electrochemical characterization of hybrid nanostructures

We are interested in investigating the electronic structure and their influence on the conductivity of quantum dots as well as hybrid nanostructured thin films, consisting of inorganic nanoparticles and organic semiconductors. To this end, we apply different electrochemical techniques like Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV), Electrochemical Gating (ECG) and Potential-modulated Absorption Spectroscopy (EMAS).

While standard techniques like CV and DPV always probe the complete density of states, spectroelectrochemical techniques are particulary advantageous in distinguishing between the electronic states of quantum dots and trap states. EMAS is introduced to study thin layer films and offers a particularly good signal-to-noise ratio that is not achieved with common spectroelectrochemical techniques. This lock-in technique is capable to isolate a signal which is several orders of mangnitude weaker than the background.

ECG is used to study the movement of charge carriers within a QD thin film. Through the in-situ measurement of the film conductance while oxidizing or reducing the film, information can be gained about the energy level over which the majority conductance occurs.


Instrumentation


Work in progress

Schlenk lines and Wet labs

Two well equipped wet labs, with standard synthesis equipment as well as high performance Schlenk lines, are used mainly for synthesis of colloidal systems and preparation of coupled organic-inorganic nanostructures.

UV-vis Spectrophotometer

Colloidal dispersions as well as bulk materials can be optically investigated via UV-vis-NIR spectroscopy with the help of a Cary5000 spectrophotometer from Agilent. This instrument enables measurements in the wavelength range of 175-3300nm and absorbance values of up to 8 a.u., which enables highly accurate measurements.

Glovebox Systems

Due to the vulnerability of many nanoparticle based mesocrystals to oxygen and water it is of uttermost importance to protect these sensitive systems from such influences. Thus, the preparations are commonly carried out under nitrogen atmosphere in the two glovebox systems GS Systems. Each glovebox comes along with specific preparation and measurement accessoires, e.g. custom electrochemical cells, field effect transistor setups, syringe pumps, spin-coater, etc.

Keithley Source Meters

For the electronic characterization of the mesocrystalline systems a very suitable method are field effect transistor measurements. As a voltage source and amperemeter two different Keithley Source Meters are available, one with Nanoampere (2634 B) and one with Picoampere measurement range (2636 B). Sample holders and faradaic cages for the measurement stations are custom made for this specific purpose.

Probe Station and Laser diodes

In the Lake Shore Cryotronics CRX-6.5K Probe Station, materials can be examined for their electronic properties in the temperature range of 8.5-350K. In addition to the usual simple characterization of a material with regard to its electronic properties at room temperature (concerning conductivity and charge carrier mobility), this allows statements to be made about the present transport mechanisms.

Additionally to those dark current investigations, it is also possible to examine the optoelectronic properties of the matierals inside the Probe Station (both under vacuum and nitrogen) and various laser solutions are available therefore. On the one hand, steady state photocurrents can be studied with the CW-mode of a laser (405nm, 636nm, 780nm); on the other hand, fast on/off time-resolved photocurrent investigations can be performed with both very fast on/off switchable laser diodes (with 635nm, 1310nm) and picosecond pulsed laser diodes (635nm, 780nm).