Order and interactions in colloidal nanoparticle superlattices

• Applicant: Professor Dr. Holger Lange
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2011 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 189145350

• Final Report Year: 2019

Final Report Abstract

The project aimed for the use of Raman spectroscopy to investigate nanoparticle superstructures. Such superstructures are ordered arrays of nanoparticles and their unique optical and electronic properties offer a huge potential for applications in the field of optoelectronics. The self-assembly into the superstructures is governed by the balance of attractive and repulsive forces. While the synthesis of superstructures was much more difficult than anticipated, inelastic scattering of visible light, Raman spectroscopy, proved to be a very useful tool for the investigation of the semiconductor nanoparticles themselves. Raman spectroscopy allows to quantify the lattice strain in nanoparticles. This is especially important for core/shell nanoparticles. The lattice mismatch between the core and the shell materials results in surface reconstructions at the core/shell interface and strain in the core and the shell. Concomitantly formed surface traps result in a reduction of the optical emission properties. We performed a systematic investigation of core/shell nanoparticles with successive ion layer adsorption and reaction grown shells. We observed an increase of strain with increasing shell size and were able to estimate a “saturation regime” at which size increments no longer result in additional strain. This finding contributed to the advancement of core/shell nanoparticles for the use in light emitters such as white light diodes or lasers. With a thorough investigation of the Raman features and concomitant ab-initio calculation, we were able to reason an alloyed interface between the nanoparticle core and the shell. This finding is of significant importance as the interface between the core and the shell has a huge influence on the optical and structural properties of the nanoparticles. Several theoretical studies have highlighted the importance of a better understanding of the interface and we now provide a tool for a more detailed experimental investigation. In the further course of the project, we confirmed a one-mode behavior of alloyed nanoparticles. This knowledge allows to differentiate between core/shell and alloyed structures and to rule out minority domains in alloys. The Raman investigations were concluded with a demonstration of strain tuning by using alloyed shells. We showed the general route to obtain strain-free core/shell nanoparticle configurations. Each of the mentioned topics resulted in peer-reviewed publications and helps advertising and advancing the use of Raman spectroscopy for the investigation of semiconductor nanoparticles. Regarding the second topic, self-organization, we developed a protocol to functionalize gold nanoparticles with polystyrene ligands with a direct phase transfer. Larger polymer ligands allow the stabilization of gold nanoparticles with diameters above 20 nm and enable the synthesis of well-ordered nanoparticle monolayers. The protocol is fast and easy to reproduce by others. We now have access to nanoparticle films on scales of several hundreds of micrometers. First studies addressed the mechanic forces between the nanoparticles by using pressure-dependent X-ray experiments. The films perform well as surface-enhanced Raman scattering substrates and tip-enhanced Raman experiments in the concluding phase of the project point towards exciting new results concerning the origin of the surface enhancement. More sophisticated studies on the collective properties of self-organized nanoparticles can be expected from the recently started Cluster of Excellence Advanced Imaging of Matter. The concluded project resulted in several ideas for time-resolved X-ray experiments.

Publications

• Interfacial alloying in CdSe/CdS heteronanocrystals, a Raman spectroscopy analysis. Chemistry of Materials 2012, 24, 311
  N. Tschirner, H. Lange, A. Schliwa, A. Biermann, C. Thomsen, K. Lambert, R. Gomes, and Z. Hens
  
• Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles. Langmuir, 2012, 24, 8862
  H. Lange, B.H. Juárez, A. Carl, M. Richter, N.G. Bastus, H. Weller, C. Thomsen, R. von Klitzing, and A. Knorr,
  
• Homogeneously Alloyed CdSe 1−x Sx Quantum Dots: An Efficient Synthesis for Full Optical Tunability. Chemistry of Materials, 2013, 25, 2388
  T. Aubert, M. Cirillo, S. Flamee, R. van Deun, H. Lange, C. Thomsen, and Z. Hens
  
• Radical Initiated Reactions on Biocompatible CdSe-Based Quantum Dots: Ligand Cross-Linking, Crystal Annealing, and Fluorescence Enhancement. J. Phys. Chem. C, 2013, 117, 8570
  C. Schmidtke, H. Lange, H. Tran, J. Ostermann†, H. Kloust, N.G. Bastus, J.P. Merkl, C. Thomsen, and H. Weller
  
• ’Flash’ Synthesis of CdSe/CdS Core-Shell Quantum Dots. Chemistry of Materials, 2014, 26, 1154
  M. Cirillo, T. Aubert, R. Gomes, R. van Deun, P. Emplit, A. Biermann, H. Lange, C. Thomsen, E. Brainis, and Z. Hens
  
• Size-Dependent Phase Transfer Functionalization of Gold Nanoparticles To Promote Well-Ordered Self-Assembly. Langmuir, 2017, 33, 14437
  F. Schulz, S. Tober, and H. Lange
  
• Heterogeneous local order in self-assembled nanoparticle films revealed by X-ray cross-correlations. IUCrJ, 2018, 5, 354
  F. Lehmkühler, F. Schulz, M.A. Schroer, L. Frenzel, H. Lange, and G. Grübel
  
• Strain Engineering in InP/(Zn,Cd)Se Core/Shell Quantum Dots. Chemistry of Materials, 2018, 30, 4393
  M. Rafipoor, D. Dupont, H. Tornatzky, M.D. Tessier, J. Maultzsch, Z. Hens, and H. Lange