2018-08-09, Werschler, Florian, Lindner, Benjamin, Hinz, Christopher, Conradt, Frieder, Gumbsheimer, Pascal, Negele, Carla, de Roo, Tjaard, Mecking, Stefan, Leitenstorfer, Alfred, Seletskiy, Denis V.

A strong increase of spontaneous radiative emission from colloidally synthesized CdSe/CdS/PMMA hybrid particles is achieved when manipulated into plasmonic bullseye resonators with the tip of an atomic force microscope (AFM). This type of antenna provides a broadband resonance, which may be precisely matched to the exciton ground state energy in the inorganic cores. Statistically analyzing the spectral photoluminescence (PL) of a large number of individual coupled and uncoupled CdSe/CdS/PMMA quantum dots, we find an order of magnitude of intensity enhancement due to the Purcell effect. Time-resolved PL shows a commensurate increase of the spontaneous emission rate with radiative lifetimes below 230 ps for the bright exciton transition. The combination of AFM and PL imaging allows for sub-200 nm localization of the particle position inside the plasmonic antenna. This capability unveils a different coupling behavior of dark excitonic states: even stronger PL enhancement occurs at positions with maximum spatial gradient of the nearfield, effectively adding a dipolar component to original quadrupole transitions. The broadband maximization of light-matter interaction provided by our nanoengineered compound systems enables an attractive class of future experiments in ultrafast quantum optics.

2017, Werschler, Florian, Lindner, Benjamin, Hinz, Christopher, de Roo, Tjaard, Mecking, Stefan, Seletskiy, Denis V., Leitenstorfer, Alfred

An order-of-magnitude enhancement in the radiative recombination rate is observed in single CdSe/CdS/PMMA colloidal quantum dots when coupled to gold plasmonic resonators, consistent with the measured increase in the excitonic photoluminescence signal.

2016-06-14, de Roo, Tjaard, Huber, Steffen, Mecking, Stefan

Hybrid particles consisting of II–VI semiconductor quantum dots and conjugated polymers are increasingly relevant, but access is limited by the usual step-growth nature of polymer formation. We report on a grafting-from approach by controlled Pd(II)-mediated polymerization to yield CdSe/CdS nanocrystals with a defined number of polyfluorene chains grown from their surface, as concluded from MALDI-TOF analysis and quantitative end-capping. Further studies underline the importance of matching the monomers’ and the surface-bound initiators’ reactivity.

2014, Jung, Christoph, de Roo, Tjaard, Mecking, Stefan

Composite nanoparticles from poly[(9,9-di-n-octylfluoren-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) and poly(9,9-di-n-hexylfluoren-2,7-diyl) (PF) with embedded inorganic nanoparticles (TiO₂, CdSe, and CdSe/CdS) are prepared through kinetic trapping by rapid turbulent mixing in a multi-inlet vortex mixer without the need for polymer functionalization. High contents of inorganic materials up to 50–60 wt% are realized for all composites. The influence of flow ratios, sodium dodecyl sulfate (SDS) concentration, and absolute flow rates on the particle size and morphology is studied. High water-to-THF ratios and high total flow rates around 2 m s⁻¹ yield particle sizes below 50 nm. By adjusting these parameters, controlled particle sizes between 30 to several hundred nanometers are obtained. Composite particles from CdSe/CdS and F8BT or PF show a strong quenching of the polymer emission and near exclusive emission from the inorganic nanocrystal, which indicates an efficient energy transfer with fluorescence quantum yields of 23% for the F8BT/CdSe/CdS composites and 21% for the PF/CdSe/CdS composites. The dispersions are colloidally stable for several months.

2017, Gothe, Bastian, de Roo, Tjaard, Will, Johannes, Unruh, Tobias, Mecking, Stefan, Halik, Marcus

The use of functional oligomers of π-conjugated oligofluorenes led to a region-selective assembly of amorphous monolayers which exhibit robust lateral charge transport pathways in selfassembled monolayer field-effect transistors over long distances and even in mixed monolayers of semiconducting and insulating molecules. This oligomer concept might stimulate a new molecular design of self-assembling semiconducting materials.

2017, de Roo, Tjaard

2016, Werschler, Florian, Hinz, Christopher, de Roo, Tjaard, Mecking, Stefan, Leitenstorfer, Alfred, Seletskiy, Denis V.

The role of discrete acoustic phonons in radiative emission from single CdSe/CdS/PMMA nanoparticles is analyzed via photoluminescence and lifetime measurements. Controlled coupling between uncharged exciton fine-structure states dramatically enhances the brightness of the lowest-energy transition.

2017, Werschler, Florian, Lindner, Benjamin, Hinz, Christopher, de Roo, Tjaard, Mecking, Stefan, Seletskiy, Denis V., Leitenstorfer, Alfred

Polymer-capped colloidal semiconductor quantum dots [1] offer a robust material system for time-resolved analysis and individual control of ultrafast charge carrier dynamics [2]. This goal necessitates a strong enhancement of light-matter interaction. In this work we demonstrate an efficient coupling of individual CdSe/CdS/PMMA quantum dots (QDs) to plasmonic resonators formed out of multiple concentric rings patterned in a 280-nm thick gold layer. Such a “bullseye” design allows for efficient manipulation of single QDs into the center of the antenna while imposing no polarization selectivity [3]. As a result, we achieve a dramatic increase of the photoluminescence (PL) emission intensity and the radiative decay rates due to the Purcell effect.

2016-09-14, Werschler, Florian, Hinz, Christopher, Froning, Florian, Gumbsheimer, Pascal, Haase, Johannes, Negele, Carla, de Roo, Tjaard, Mecking, Stefan, Leitenstorfer, Alfred, Seletskiy, Denis V.

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

2015, Knittel, Vanessa, Fischer, Marco P., de Roo, Tjaard, Mecking, Stefan, Leitenstorfer, Alfred, Brida, Daniele

We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence.