Highlight Publications 2018

Direct observation of polymer surface mobility via nanoparticle vibrations
Hojin Kim, Yu Cang, Eunsoo Kang, Bartlomiej Graczykowski, Maria Secchi, Maurizio Montagna, Rodney D. Priestley, Eric M. Furst, George Fytas
Direct observation of polymer surface mobility via nanoparticle vibrations
MPIP researchers in collaboration with Prof. Priestley (Princeton Univeristy), Hojin Kim, Prof. Furst (University of Delaware), Dr. Maria Secchi and Prof. Montagna (University of Trento) introduced a new technique (Brillouin Light Spectroscopy) to directly observe surface mobility and shear modulus of polymer colloids. When laser impinges an assembly of polymer nanoparticles (NP)s, the inelastically scattered light is frequency resolved by a high resolution Tandem Fabry-Perot Interferometer to record the particle vibration spectrum in the GHz frequency range. For individual non-interactive nanoparticles, the single lower frequency mode (1,2) of the spectrum (red in the scheme) splits into a blue shifted doublet and a new interaction induced (1,1) mode (blue in the scheme) emerges in the presence of attractive interparticle interactions. The proposed assignment and analysis of the particle vibration spectrum allows the measurement of the particle shear modulus and leads to the first direct observation of NP surface mobility. NP’s possess lower shear modulus than the contiguous films and this softening effect is not size but surface chemistry dependent. The results provide new insight in the glass transition phenomenon and NP elasticity. Understanding thermomechanical properties is crucial for applicatios such as pressure-sensitive adhesives, drug carriers, surface coating, and nanoparticle reinforced plastics.
About the Project
Dr B.Graczykowski is supported by the Alexander von Humboldt Foundation
This project is funded by ERC SmartPhon (No. 694977)
© Nature Communications (2018)
Schematic picture of low frequency vibration spectrum of individual (red line) and interacted (violet lines) nanoparticles in a colloidal cluster.
Synthesis of Triply Fused Porphyrin-Nanographene Conjugates
Qiang Chen, Luigi Brambilla, Lakshya Daukiya, Kunal S. Mali, Steven De Feyter, Matteo Tommasini, Klaus Müllen, Akimitsu Narita
Synthesis of Triply Fused Porphyrin-Nanographene Conjugates
Syntheses of Pi-extended porphyrins have attracted immense interests for their unique optical and electronic properties, which render them highly valuable for a variety of applications, e.g., as near-infrared (NIR) dyes, organic semiconductors, and nonlinear optical materials. Various aromatic hydrocarbons, including benzene, naphthalene, pyrene, azulene, corannulene, anthracene and coronene, have thus been fused to the meso- and β-positions of porphyrin. Large PAHs, which can be regarded as nanographenes, are known to exhibit attractive (opto)electronic properties and self-assembly behavior, which can be further fine- tuned by precise control over their size, shape, and edge structure. Fusion of porphyrin core to large PAHs has not been achieved because of lacking appropriate methods. In this paper, two unprecedented porphyrin fused nanographene molecules 1 and 2 have been synthesized by Scholl reaction of tailor-made precursors based on benzo[m]tetraphene-substituted porphyrins. The chemical structures were validated by a combination of high-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (HR MALDI- TOF MS), IR and Raman spectroscopy, and scanning tunnelling microscopy (STM). The UV- vis-near infrared absorption spectroscopy of 1 and 2 demonstrated broad and largely red- shifted absorption spectra extending up to 1000 and 1400 nm, respectively, marking the significant extension of the Pi-conjugated systems.
© John Wiley and Sons (2018)
Molecular structures and UV-vis-NIR absorption spectra of triply fused porphyrin nanographene conjugates 1 and 2, demonstrating the absorption extending to 1000 and 1400 nm, respectively (inset shows pictures of THF solutions of compound 1 and 2).
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
A. Tomadin, S.M. Hornett, H.I. Wang, E.M. Alexeev, A. Candini, C. Coletti, D. Turchinovich, M. Kläui, M. Bonn, F.H.L. Koppens, E. Hendry, M. Polini and K.J. Tielrooij
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
For many of the envisioned optoelectronic applications of graphene, it is crucial to understand the subpicosecond carrier dynamics immediately following photoexcitation and the effect of photoexcitation on the electrical conductivity—the photoconductivity. Whereas these topics have been studied using various ultrafast experiments and theoretical approaches, controversial and incomplete explanations concerning the sign of the photoconductivity, the occurrence and significance of the creation of additional electron-hole pairs, and, in particular, how the relevant processes depend on Fermi energy have been put forward. We present a unified and intuitive physical picture of the ultrafast carrier dynamics and the photoconductivity, combining optical pump–terahertz probe measurements on a gate-tunable graphene device, with numerical calculations using the Boltzmann equation. We distinguish two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy (EF ≲ 0.1 eV for our experiments), broadening of the carrier distribution involves interband transitions (interband heating). At higher Fermi energy (EF ≳ 0.15 eV), broadening of the carrier distribution involves intraband transitions (intraband heating). Under certain conditions, additional electron-hole pairs can be created [carrier multiplication (CM)] for low EF, and hot carriers (hot-CM) for higher EF. The resultant photoconductivity is positive (negative) for low (high) EF, which in our physical picture, is explained using solely electronic effects: It follows from the effect of the heated carrier distributions on the screening of impurities, consistent with the DC conductivity being mostly due to impurity scattering. The importance of these insights is highlighted by a discussion of the implications for graphene photodetector applications. The Mainz-based researchers Dr. H. Wang, Prof. D. Turchinovich, Prof. M. Kläui, and Prof. M. Bonn, in collaboration with scientists from various European labs, have now succeeded in understanding these processes. The project was led by Dr. K.-J. Tielrooij from ICFO in Spain, who was recently elected visiting professor at the MAINZ Graduate School.
© Fabien Vialla (2018)
Schematic representation of the ultrafast optical pump – terahertz probe experiment.
How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells
Stefan A.L. Weber, Ilka M. Hermes, Silver-Hamill Turren-Cruz, Christopher Gort, Victor W. Bergmann, Laurent Gilson, Anders Hagfeldt, Michael Graetzel, Wolfgang Tress, Rüdiger Berger
How the Formation of Interfacial Charge Causes Hysteresis in Perovskite Solar Cells
Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. Very efficient (>20 %) devices with perovskite layers of low defect density can be prepared by cheap and simple solution based processes at moderate temperatures (<150°C). For commercializing this technology, a stable and reliable operation is required. In perovskite solar cells, however, the output power strongly depends on the history of the device in terms of bias voltage (causing hysteresis) or illumination (known as light soaking effect). The underlying process is connected to the migration of ionic charges within the perovskite layer. In our study, we were able to map and follow the vertical charge distribution in the perovskite layer of an operating device. In particular, we found that thin layers of localized charge were forming at the electrode interfaces when we changed the external voltage or illuminated the device. Our results show that the formation and release of these ionic interface charges determine the time scales for current-voltage hysteresis in perovskite solar cells. Our study demonstrates that a precise control over the interfaces in perovskite solar cells is the key for controlling and suppressing hysteresis in perovskite solar cells.
© RCS (2018)
In this study, we use time-resolved Kelvin probe force micrscopy to investigate the mechanisms of current–voltage hysteresis in a hybrid lead-halide perovskite solar cell.
Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion
Yunbin Hu, Peng Xie, Marzio De Corato, Alice Ruini, Shen Zhao, Felix Meggendorfer, Lasse Arnt Straasø, Loic Rondin, Patrick Simon, Juan Li, Jonathan J Finley, Michael Ryan Hansen, Jean-Sébastien Lauret, Elisa Molinari, Xinliang Feng, Johannes V. Barth, Carlos-Andres Palma, Deborah Prezzi, Klaus Müllen, and Akimitsu Narita
Bandgap Engineering of Graphene Nanoribbons by Control over Structural Distortion
Amongst organic electronic materials, graphene nanoribbons (GNRs) offer extraordinary versatility as next-generation semiconducting materials for nanoelectronics and optoelectronics due to their tunable properties, including charge-carrier mobility, optical absorption and electronic bandgap, which are uniquely defined by their chemical structures. Although planar GNRs have been predominantly considered until now, non-planarity can be an additional parameter to modulate their property without changing the aromatic core. Herein, we report theoretical and experimental studies on two GNR structures with “cove”-type edges, having an identical aromatic core, but with alkyl side chains at different peripheral positions. The theoretical results indicate that installment of alkyl chains at the innermost positions of the “cove”-type edges can “bend” the peripheral rings of the GNR through steric repulsion between aromatic protons and the introduced alkyl chains. This structural distortion is theoretically predicted to reduce the bandgap by up to 0.27 eV, which is corroborated by experimental comparison of thus synthesized planar and non-planar GNRs through UV-Vis-near infrared absorption and photoluminescence excitation spectroscopy. Our results extend the possibility of engineering GNR properties, adding subtle structural distortion as a distinct and potentially highly versatile parameter.
© ACS (2018)
Molecular models and band structures of planar and non-planar graphene nanoribbons with the identical aromatic structure, demonstrating that the structural distortion induced by the proper positioning of alkyl chains can lower the bandgap.
Magnetic edge states and coherent manipulation of graphene nanoribbons
Michael Slota, Ashok Keerthi, William K. Myers, Evgeny Tretyakov, Martin Baumgarten, Arzhang Ardavan, Hatef Sadeghi, Colin J. Lambert, Akimitsu Narita, Klaus Müllen & Lapo Bogani
Magnetic edge states and coherent manipulation of graphene nanoribbons
Here we use molecular graphene nanoribbons functionalized with stable spin-bearing radical groups to demonstrate delocalized magnetic edge states and test theoretical models of the spin dynamics and spin–environment interactions. Comparison with a non-graphitized reference material enables us to clearly identify the characteristic behaviour of the radical-functionalized graphene nanoribbons. We quantify the parameters of spin–orbit coupling, define the interaction patterns and determine the spin decoherence channels. Even without any optimization, the spin coherence time is in the range of microseconds at room temperature, and we perform quantum inversion operations between edge and radical spins. Our approach provides a way of testing the theory of magnetism in graphene nanoribbons experimentally. The coherence times that we observe open up encouraging prospects for the use of magnetic nanoribbons in quantum spintronic devices.
© Lapo Bogani (2018)
Molecular graphene nanoribbons functionalized with stable spin-bearing nitronyl nitroxide radical groups, demonstrating delocalized magnetic edge state.
 
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