Highlight Publications 2018

Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature
Hai I. Wang, Ivan Infante, Stephanie ten Brinck, Enrique Cánovas, and Mischa Bonn
Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature
The efficiency of a single bandgap solar cell, such as silicon, is limited to ~31%. This limit is primarily the result of very rapid relaxation of electrons, once they are generated by a photon. For photons that have energies exceeding the bandgap energy, this excess energy is lost in this relaxation process. In order to circumvent such losses, and increase solar cell efficiency, the extraction of hot carriers towards selective contacts is required to be faster than thermalization in the absorber. Quantum dot (QD)-sensitized oxides have long been proposed as an appealing system to harvest hot carriers for solar energy conversion. Previous work has demonstrated the possibility of hot electron extraction in quantum dot (QD)-sensitized systems, but only at low temperatures (e.g. 77 Kelvin). Here, we demonstrate a room-temperature hot electron transfer with unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous oxide. Such achievement is realized by enhancing the electronic coupling between QDs and oxides, which ensures an ultrafast hot electron transfer process (sub-100 fs) that can effectively compete with the hot carrier thermalization processes. These results provide new insights into circumventing thermal losses in sensitized systems, with potential relevance for low-cost solar energy conversion schemes.
© ACS (2018)
Room-temperature hot electron transfer with unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous oxide
Detaching microparticles from a liquid surface
Frank Schellenberger, Periklis Papadopoulos, Michael Kappl, Stefan A.L. Weber, Doris Vollmer, Hans-Jürgen Butt
Detaching microparticles from a liquid surface
The work required to detach microparticles from fluid interfaces depends on the shape of the liquid meniscus. However, measuring the capillary force on a single microparticle and simultaneously imaging the shape of the liquid meniscus has not yet been accomplished. To correlate force and shape, we combined a laser scanning confocal microscope with a colloidal probe setup. While moving a hydrophobic microsphere (radius 5-10 µm) in and out of a 2-5 µm thick glycerol film, we simultaneously measured the force and imaged the shape of the liquid meniscus. In this way we verified the fundamental equations (D.F. James, J. Fluid Mech. 63, 657 (1974); A.D. Scheludko, A.D. Nikolov, Colloid Polymer Sci. 253, 396 (1975)) which describe the adhesion of particles in flotation, deinking of paper, the stability of Pickering emulsions and particle-stabilized-foams. Comparing experimental results with theory showed, however, that the receding contact angle has to be applied, which can be much lower than the static contact angle obtained right after jump-in of the particle.
© MPI-P (2018)
Macroscopic and microscopic particle withdrawn from a liquid. The microsphere was imaged with a confocal microscope while simultaneously the force was measured.
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Wei Huang, Jeehye Byun, Irina Rörich, Charusheela Ramanan, Paul W. M. Blom, Hao Lu, Di Wang, Lucas Caire da Silva, Run Li, Lei Wang, Katharina Landfester, Kai A. I. Zhang
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Complex multiple‐component semiconductor photocatalysts can be constructed that display enhanced catalytic efficiency via multiple charge and energy transfer, mimicking photosystems in nature. In contrast, the efficiency of single‐component semiconductor photocatalysts is usually limited due to the fast recombination of the photogenerated excitons. Here, we report the design of an asymmetric covalent triazine framework as an efficient organic single‐component semiconductor photocatalyst. Four different molecular donor–acceptor domains are obtained within the network, leading to enhanced photogenerated charge separation via an intramolecular energy transfer cascade. The photocatalytic efficiency of the asymmetric covalent triazine framework is superior to that of its symmetric counterparts; this was demonstrated by the visible‐light‐driven formation of benzophosphole oxides from diphenylphosphine oxide and diphenylacetylene.
© Wiley VCH (2018)
Asymmetric Covalent Triazine Framework for Enhanced Visible Light Photoredox Catalysis via Energy Transfer Cascade
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
Wang, Xiao-Ye; Urgel, José I.; Barin, Gabriela Borin; Eimre, Kristjan; Di Giovannantonio, Marco; Milani, Alberto; Tommasini, Matteo; Pignedoli, Carlo A.; Ruffieux, Pascal; Feng, Xinliang; Fasel, Roman; Müllen, Klaus; Narita, Akimitsu
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
Bottom-up synthesis of graphene nanoribbons (GNRs) has significantly advanced during the past decade, providing various GNR structures with tunable properties. The synthesis of chiral GNRs, however, has been underexplored and only limited to (3,1)-GNRs. We report herein the surface-assisted synthesis of the first heteroatom-doped chiral (4,1)-GNRs from the rationally designed precursor 6,16-dibromo-9,10,19,20-tetraoxa-9a,19a-diboratetrabenzo[a,f,j,o]perylene. The structure of the chiral GNRs has been verified by scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy in combination with theoretical modeling. Due to the presence of oxygen–boron–oxygen (OBO) segments on the edges, lateral self-assembly of the GNRs has been observed, realizing well-aligned GNR arrays with different modes of homochiral and heterochiral inter-ribbon assemblies.
© JACS (2018)
Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons
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.
 
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