J. Feldmann

Technische Universität München, München, Bavaria, Germany

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Publications (368)1802.1 Total impact

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    ABSTRACT: Copper (I) based catalysts, such as Cu2S, are considered to be very promising materials for photocatalytic CO2 reduction. A common synthesis route for Cu2S via cation exchange from CdS nanocrystals requires Cu (I) precursors, organic solvents and neutral atmosphere, but these conditions are not compatible with in situ applications in photocatalysis. Here we propose a novel cation exchange reaction which takes advantage of the reducing potential of photoexcited electrons in the conduction band of CdS and proceeds with Cu (II) precursors in an aqueous environment and under aerobic conditions. We show that the synthesized Cu2S photocatalyst can be efficiently used for the reduction of CO2 to carbon monoxide and methane, achieving formation rates of 3.02 μmol h-1g-1 and 0.13 μmol h-1g-1, respectively, and suppressing competing water reduction. The process opens new pathways for the preparation of new efficient photocatalysts from readily available nanostructured templates.
    No preview · Article · Oct 2015 · Journal of the American Chemical Society
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    ABSTRACT: Organometal halide perovskites have recently emerged displaying a huge potential for not only photovoltaic, but also light emitting applications. Exploiting the optical properties of specifically tailored perovskite nanocrystals could greatly enhance the efficiency and functionality of applications based on this material. In this study, we investigate the quantum size effect in colloidal organometal halide perovskite nanoplatelets. By tuning the ratio of the organic cations used, we can control the thickness and consequently the photoluminescence emission of the platelets. Quantum mechanical calculations match well with the experimental values. We find that not only do the properties of the perovskite, but also those of the organic ligands play an important role. Stacking of nanoplatelets leads to the formation of minibands, further shifting the bandgap energies. In addition, we find a large exciton binding energy of up to several hundreds of meV for nanoplatelets thinner than three unit cells, partially counteracting the blueshift induced by quantum confinement. Understanding of the quantum size effects in perovskite nanoplatelets and the ability to tune them provide an additional method with which to manipulate the optical properties of organometal halide perovskites.
    No preview · Article · Sep 2015 · Nano Letters
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    ABSTRACT: Carbon dots (CDs) have attracted rapidly growing interest in recent years due to their unique and tunable optical properties, the low cost of fabrication and their wide-spread uses. However, due to the complex structure of CDs, both the molecular ingredients and the intrinsic mechanisms governing photoluminescence of CDs are poorly understood. Among other features, a large Stokes shift of over 100 nm and a photoluminescence spectrally dependent on the excitation wavelength, have so far not been adequately explained. In this paper we investigate the properties of CDs and develop a model system to mimic the optical properties of the CDs. This system was comprised of three types of polycyclic aromatic hydrocarbon (PAH) molecules with fine-tuned concentrations embedded in a polymer matrix. We show the Stokes shift to be due to the self-trapping of an exciton in the PAH network. The width and the excitation dependence of the emission comes from a selective excitation of PAHs with slightly different energy gaps and from energy transfer between them. These insights will help to tailor the optical properties of CDs and help their implementation into applications, e.g. light-emitting devices and biomarkers. This could also lead to "artificial" tunable carbon dots by locally modifying the composition and consequently the optical properties of composite PAH films.
    Full-text · Article · Aug 2015 · Nano Letters
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    ABSTRACT: In transient absorption (TA) spectra, the bleach features originating from state filling are overlapped by their energy-shifted derivatives, arising from excited state energy level shifts. This makes the direct extraction of carrier dynamics from a single-wavelength time-trace misleading. Fitting TA spectra in time, as Gaussian functions and their derivative-like shifted Gaussians, allows to individually extract the real dynamics of both photobleached transitions, and their energy shifts. In CdTe nanorods (NRs) we found a delayed heating of holes due to the release of the large excess energy in the electron relaxation process. The slow hole-trapping process is consistent with a high number of surface trap states in these model NRs. Our results show that only a correct disentanglement of bleaching and energy shift contributions provides a reliable framework to extract the underlying carrier relaxation dynamics, including trapping, non-radiative recombination, and eventually carrier multiplication.
    No preview · Article · Aug 2015
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    ABSTRACT: In this article, we report how Janus particles, composed of a silica sphere with a gold half-shell, can be not only stably trapped by optical tweezers but also displaced controllably along the axis of the laser beam through a complex interplay between optical and thermal forces. Scattering forces orient the asymmetric particle, while strong absorption on the metal side induces a thermal gradient, resulting in particle motion. An increase in the laser power leads to an upward motion of the particle, while a decrease leads to a downward motion. We study this reversible axial displacement, including a hysteretic jump in the particle position that is a result of the complex pattern of a tightly focused laser beam structure above the focal plane. As a first application we simultaneously trap a spherical gold nanoparticle and show that we can control the distance between the two particles inside the trap. This photonic micron-scale " elevator " is a promising tool for thermal force studies, remote sensing, and optical and thermal micromanipulation experiments. S tructures that are capable of using energy from their environment to exhibit self-propulsion at the nano-and microscale are of great interest for controlling processes in microfluidic chips as well as aiding in therapeutics, diagnostics, and performing in vivo tasks. 1−3 Motion at this scale presents challenges because viscous forces dominate inertial forces. In order to swim in the low Reynolds number regime, some biological organisms exhibit nonreciprocal motion, which has been the inspiration of many artificial microswimmers. 4−7
    Full-text · Article · Feb 2015
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    ABSTRACT: CdSe/CdS core-shell nanocrystals with controlled CdS shell thickness and CdSe core size were synthesized for several different values of these two parameters. The particles in aqueous dispersion were in situ decorated with Ni nanoparticles and evaluated for photocatalytic hydrogen generation capacity. The highest H-2 production quantum yield at 457 nm illumination was 4.7%, while at 530 nm it rose to 9.1%. We have found that ensuring equal absorption at illumination wavelength the H-2 formation rate increases with the shell thickness, but decreases with the increasing core size. Interestingly, the same trend was observed for the photoluminesce quantum yield and lifetime for samples without Ni decoration, suggesting that they reflect the balance between charge transfer and avoidance of recombination sites which is crucial for photocatalysis with semiconductor nanocrystals. Moreover, the core-shell nanocrystals constitute a very convenient model system in which the charge carrier dynamics in photocatalytic applications can be studied. In comparison with well-known CdSe/CdS dot-in-rod structures they are less efficient at blue illumination, but they allow for significant extension of the usable absorption range.
    No preview · Article · Jan 2015 · Zeitschrift für Physikalische Chemie
  • Miao Li · Theobald Lohmüller · Jochen Feldmann
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    ABSTRACT: The controlled injection of nanoscopic objects into living cells with light offers promising prospects for the development of novel molecular delivery strategies or intracellular biosensor applications. Here, we show that single gold nanoparticles from solution can be patterned on the surface of living cells with a continuous wave laser beam. In a second step, we demonstrate how the same particles can then be injected into the cells through a combination of plasmonic heating and optical force. We find that short exposure times are sufficient to perforate the cell membrane and inject the particles into cells with a survival rate of >70%.
    No preview · Article · Dec 2014 · Nano Letters
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    ABSTRACT: We demonstrate strong coupling of nanocavities in metal films, sparked by propagating surface plasmons. Unlike the coupling of metallic nanoparticles which decays over distances of tens of nanometers, the metallic nanocavities display long range coupling at distances of hundreds of nanometers for the properly selected metal/wavelength combinations. Such strong coupling drastically changes the symmetry of the charge distribution around the nanocavities as is evidenced by the nonlinear optical response of the medium. We show that when strongly coupled, equilateral triangular nanocavities lose their individual symmetry to adopt the lower symmetry of the coupled system and respond like a single dipolar entity. A quantitative model is suggested for the transition from individual to strongly coupled nanocavities.
    No preview · Article · Nov 2014 · Journal of optics
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    ABSTRACT: We present an optofluidic method that allows the two-dimensional vectorial near-field mapping of oscillatory flows with micron-scale resolution. An oscillatory flow created by a microsource (an optically trapped silica particle set to oscillate in a dipole-type mode) is detected by another twin silica particle independently trapped and located in the vicinity of the source. Fourier analysis of the motion of the detecting particle at different points in space and time renders the vectorial velocity map around the oscillating microsphere. The method introduced here paves the way for in-situ characterization of fast mixing microscale devices and for new detection methods able to provide location and recognition (due to the field pattern) of moving sources that may be applied to both artificial and living microobjects, including macromolecules, cells, and microorganisms.
    Full-text · Article · Oct 2014
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    ABSTRACT: Photocatalytic conversion of solar energy to fuels, such as hydrogen, is attracting enormous interest, driven by the promise of addressing both energy supply and storage. Colloidal semiconductor nanocrystals have been at the forefront of these efforts owing to their favourable and tunable optical and electronic properties as well as advances in their synthesis. The efficiency of the photocatalysts is often limited by the slow transfer and subsequent reactions of the photoexcited holes and the ensuing high charge recombination rates. Here we propose that employing a hydroxyl anion/radical redox couple to efficiently relay the hole from the semiconductor to the scavenger leads to a marked increase in the H2 generation rate without using expensive noble metal co-catalysts. The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g(-1) h(-1), respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full water splitting.
    No preview · Article · Aug 2014 · Nature Materials
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    ABSTRACT: We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle's position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.
    No preview · Article · Jul 2014 · Journal of Visualized Experiments
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    ABSTRACT: Inspired by biological microswimmer strategies, we consider the design of helical silica particles. We characterize optically induced rotation of these “microscrews”, and discuss their applications as highly controlled microswimmers in the low Reynold’s number regime.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: We report on a combined study of Rayleigh and Raman scattering spectroscopy, 3D electron tomography, and discrete dipole approximation (DDA) calculations of a single complex shaped gold nanoparticle (NP). Using the exact reconstructed 3D morphology of the NP as input for the DDA calculations, the experimental results can be reproduced with unprecedented precision and detail. We find that not only the exact NP morphology but also the surroundings including the points of contact with the substrate are of crucial importance for a correct prediction of the NP optical properties. The achieved accuracy of the calculations allows determining how many of the adsorbed molecules have a major contribution to the Raman signal, a fact that has important implications for analyzing experiments and designing sensing applications.
    No preview · Article · Apr 2014 · ACS Nano
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    ABSTRACT: We report that plasmonic nanoantennas made by DNA origami can be used as reliable and efficient probes for surface enhanced Raman spectroscopy (SERS). The nanoantenna is built up by two gold nanoparticles that are linked together by a three-layered DNA origami block at a separation distance of 6 nm in order to achieve plasmonic coupling and the formation of a plasmonic 'hot spot'. The plasmonic properties of the hybrid structure are optically characterized by dark-field imaging and polarization-dependent spectroscopy. SERS measurements on molecules that are embedded in the DNA origami which bridges the nanoantenna gap were performed in order to demonstrate the excellent performance of these structures for enhancing spectroscopic signals. A strong enhancement of the Raman signal was recorded from measurements on single hot spots compared to measurements in bulk. Finally, we show that the laser polarization with respect to the dimer orientation has a strong impact on the SERS performance.
    No preview · Article · Apr 2014 · Nano Letters
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    ABSTRACT: Selected Lectures Presented at Symposium on Ultrafast Dynamics of the 7th International Conference on Materials for Advanced TechnologiesSingapore, 30 June – 5 July 2013Edited by: G G Gurzadyan (NTU, Singapore), G Lanzani (Istituto Italiano di Tecnologia, Italy), C Soci (NTU, Singapore), T C Sum (NTU, Singapore)
    No preview · Conference Paper · Apr 2014
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    ABSTRACT: Rod-shaped CdTe–Cu2−xTe nano-heterostructures with tunable dimensions of both sub-units and a type II band alignment were prepared by Cd2+/Cu+ cation exchange. The light absorption properties of the heterostructures are dominated by the excitonic and plasmonic contributions arising, respectively, from the CdTe and the Cu2−xTe sub-units. These results were confirmed over a wide range of sub-unit length fractions through optical modelling based on the discrete dipole approximation (DDA). Although assuming electronically independent sub-units, our modelling results indicate a negligible ground state interaction between the CdTe exciton and the Cu2−xTe plasmon. This lack of interaction may be due to the low spectral overlap between exciton and plasmon, but also to localization effects in the vacancy-doped sub-unit. The electronic interaction between both sub-units was evaluated with pump-probe spectroscopy by assessing the relaxation dynamics of the excitonic transition. In particular, the CdTe exciton decays faster in the presence of the Cu2−xTe sub-unit, and the decay gets faster with increasing its length. This points towards an increased probability of Auger mediated recombination due to the high carrier density in the Cu2−xTe sub-unit. This indication is supported through length-fraction dependent band structure calculations, which indicate a significant leakage of the CdTe electron wavefunction into the Cu2−xTe sub-unit that increases along with the shortening of the CdTe sub-unit, thus enhancing the probability of Auger recombination. Therefore, for the application of type II chalcogenide–chalcogenide heterostructures based on Cu and Cd for photoenergy conversion, a shorter Cu-based sub-unit may be advantageous, and the suppression of high carrier density within this sub-unit is of high importance.
    Full-text · Article · Apr 2014
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    ABSTRACT: This feature article discusses the optical trapping and manipulation of plasmonic nanoparticles, an area of current interest with potential applications in nanofabrication, sensing, analytics, biology and medicine. We give an overview over the basic theoretical concepts relating to optical forces, plasmon resonances and plasmonic heating. We discuss fundamental studies of plasmonic particles in optical traps and the temperature profiles around them. We place a particular emphasis on our own work employing optically trapped plasmonic nanoparticles towards nanofabrication, manipulation of biomimetic objects and sensing.
    No preview · Article · Mar 2014 · Nanoscale
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    ABSTRACT: The sensitization of dispersed P25 TiO2 nanoparticles (NPs) and macroporous TiO2 films with water-soluble and air-stable PbS quantum dots (QDs) capped with l-glutathione (GSH) ligands was investigated. Optimum sensitization was achieved by careful adjustment of the surface charges of TiO2 and PbS QDs by controlling the pH of the QD solution. Efficient electron transfer from photoexcited PbS QDs via the GSH ligands into the conduction band of TiO2 was demonstrated by photoluminescence (PL) spectroscopy of PbS-sensitized P25 nanoparticles. The PbS QD-sensitized porous TiO2 electrodes were used to prepare quantum-dot-sensitized solar cells (QDSSCs) utilizing a CuxSy counter electrode and aqueous polysulfide electrolyte. Cells with up to 64% injection efficiency, 1.1% AM 1.5 conversion efficiency, and short circuit current density of 7.4 mA cm–2 were obtained. The physical parameters of the cells were investigated using impedance spectroscopy.
    No preview · Article · Mar 2014 · The Journal of Physical Chemistry C
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    ABSTRACT: We report on a highly sensitive approach to measure and quantify the time dependent changes of the flow generated by the flagella bundle rotation of single bacterial cells. This is achieved by observing the interactions between a silica particle and a bacterium, which are both trapped next to each other in a dual beam optical tweezer. In this configuration, the particle serves as a sensitive detector where the fast-Fourier analysis of the particle trajectory renders, it possible to access information about changes of bacterial activity.
    Full-text · Article · Mar 2014 · Applied Physics Letters
  • Wei Li · Thomas Dittrich · Frank Jäckel · Jochen Feldmann
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    ABSTRACT: Pyrite nanocrystals are currently considered as a promising material for large scale photovoltaic applications due to their non-toxicity and large abundance. While scalable synthetic routes for phase-pure and shape controlled colloidal pyrite nanocrystals have been reported, their use in solar cells has been hampered by the detrimental effects of their surface defects. Here, we report a systematic study of optical and electronic properties of pyrite nanocrystal thin films employing a series of different ligands varying both the anchor and bridging group. The effect of the ligands on the optical and electronic properties is investigated by UV-vis/NIR absorption spectroscopy, current voltage characteristic measurements and surface photovoltage spectroscopy. We find that the optical absorption is mainly determined by the anchor group. The absorption onset in the thin films shifts up to ∼100 meV to the red. This is attributed to changes in the dielectric environment induced by different anchors. The conductivity and photoconductivity, on the other hand, are determined by combined effects of anchor and bridging group, which modify the effective hopping barrier. Employing different ligands, the differential conductance varies over four orders of magnitude. The largest redshift and differential conductance are observed for ammonium sulfides and thiolated aromatic linkers. Pyridine and long chain amines, on the other hand, lead to smaller modifications. Our findings highlight the importance of surface functionalization and interparticle electronic coupling in the use of pyrite nanocrystals for photovoltaic devices.
    No preview · Article · Mar 2014 · Small

Publication Stats

16k Citations
1,802.10 Total Impact Points


  • 2000-2015
    • Technische Universität München
      • Walter Schottky Institut (WSI)
      München, Bavaria, Germany
  • 1997-2015
    • Ludwig-Maximilians-University of Munich
      • • Photonics and Optoelectronics Group
      • • Center for Nanoscience (CeNS)
      • • Department of Physics
      München, Bavaria, Germany
  • 2007
    • Trinity College Dublin
      • School of Physics
      Dublin, Leinster, Ireland
  • 2001
    • University of Colorado at Boulder
      Boulder, Colorado, United States
    • University of Colorado
      Denver, Colorado, United States
    • Belarussian State Institute of Metrology
      Myenyesk, Minsk, Belarus
  • 1965-2000
    • Philipps University of Marburg
      • Faculty of Physics
      Marburg, Hesse, Germany
  • 1999
    • University of Münster
      Muenster, North Rhine-Westphalia, Germany
  • 1989
    • Phillips University
      Missouri, United States