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.

This project aims at a detailed understanding of the electronic interactions within a new class of materials, namely self-assembled superlattices of inorganic semiconductor nanocrystals (or quantum dots) and atomically precise nanoclusters, which are promising candidates for solution-processed next-generation devices, such as transistors, photodetectors, solar cells and LEDs.

The nanocrystals and nanoclusters (NCs) self-assemble into three-dimensional highly ordered superlattices. In close analogy to classical crystals in which atoms have been replaced by NCs, the coupling between NCs can be tuned by changing their size, distance, and ligand coverage, as well as their orientation and structural order. Hence, structural properties dictate the electronic properties of this artificial NC-based solids.

To gain novel insight into the electronic and structural properties of these superlattices, our group focuses on understanding and controlling of the self-assembly of semiconducting nanocrystals and atomically precise nanoclusters into conductive superlattices.

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

• NC synthesis, self-assembly and ligand exchange
• Device fabrication via microfabrication (optical and electron beam lithography, soft-nanolithography, HIM and FIB milling, …)
• Conductivity and field effect transistor measurements
• Structural characterisation by means of SEM/TEM imaging and X-ray scattering (GISAXS, GIWAXS, nano-focused XRD)

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
• Prof. Andreas Schnepf, University of Tübingen

Maier, A.; Lapkin, D.; Mukharamova, N.; Frech, P.; Assalauova, D.; Ignatenko, A.; Khubbutdinov, R.; Lazarev, S.; Sprung, M.; Laible, F.; Loeffler, R.; Previdi, N.; Bräuer, A.; Guenkel, T.; Fleischer, M.; Schreiber, F.; Vartanyants, I.A.; Scheele, M. Structure-transport correlation reveals anisotropic charge transport in coupled PbS nanocrystal superlattices. Adv. Mater. 2020, 32, 2002254. https://doi.org/10.1002/adma.202002254

Fetzer, F.*; Maier, A.*; Hodas, M.; Geladari, O.; Braun, K.; Meixner, A. J.; Schreiber, F.; Schnepf, A.; Scheele, M. Structural order enhances charge carrier transport in self-assembled Au-nanoclusters. Nat. Commun. 2020, 11, 6188. https://doi.org/10.1038/s41467-020-19461-x

Maier, A.; Löffler, R.; Scheele, M. Fabrication of nanocrystal superlattice microchannels by soft-lithography for electronic measurements of single‑crystalline domains. Nanotechnology 2020, 31, 405302. https://doi.org/10.1088/1361-6528/ab9c52

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, 10, 24708-24714. https://pubs.acs.org/doi/10.1021/acsami.8b06044

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

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.