Christian von Borczyskowski

Electron-phonon coupling and localization of excitons in single silicon nanocrystals.

We report a detailed photoluminescence (PL) study on single silicon nanocrystals produced by laser pyrolysis. The PL spectra reveal nearly homogeneously broadened zero-phonon lines coupled to Si-O-Si phonon transitions in the SiO2 shell. A systematic investigation of electron-phonon coupling is reported on the basis of single nanocrystals. The stepwise localization of electron and hole at the Si-SiO2 interface for nanocrystals smaller than d approximately 2.7 nm is driven by electron-phonon coupling. From the localization energies the effective Bohr radii of the (localized) electron and hole are estimated to be in the range of 1-2 bond lengths of Si-O and Si-Si.

Influence of self-trapped states on the fluorescence intermittency of single molecules

We present data on photoinduced fluorescence intermittency of single terrylene molecules embedded in polymer films. Intermittency statistics follow power laws on time scales from tens of milliseconds to tens of seconds. Power law exponents vary with the polarity of the medium while the probability of long dark periods is drastically increased in the more polar matrix. Our experiments support a picture, which assumes a molecule charged by photoexcitation and coupled to a broad manifold of (charged) self-trapped states stabilized by the dielectric response of the surrounding matrix. This model is able to explain long living dark states both for semiconductor nanoparticles and fluorescent dye molecules making use of a unique microscopic description. It also takes into account a competitive photoinduced irreversible bleaching of the molecular state.

Formation and optical properties of self-organized pentameric porphyrin arrays

Abstract Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II 1,4-bis[5-(10,15,20-tri-p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP)2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions in the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP)2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-singlet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

Formation Principles and Ligand Dynamics of Nanoassemblies of CdSe Quantum Dots and Functionalised Dye Molecules

Functional dye molecules, such as porphyrins, attached to CdSe quantum dots (QDs) through anchoring meso-pyridyl substituents, form quasi-stable nanoassemblies. This fact results in photoluminescence (PL) quenching of the QDs both due to Förster resonance energy transfer (FRET) and the formation of non-radiative surface states under conditions of quantum confinement (non-FRET). The formation process is in competition with the ligand dynamics. At least two timescales are found for the formation of the assemblies: 1) one faster than 60 s attributed to saturation of empty attachment sites and 2) one slower than 600 s, which is attributed to a reorganisation of the tri-n-octylphosphine oxide (TOPO) ligand shell. Finally, this process results in almost complete exchange of the TOPO shell by porphyrin dye molecules. Following a Stern-Volmer analysis, we established a microscopic description of PL quenching and assembly formation. Based on this formalism, we determined the equilibrium constant for assembly formation between QDs and the pyridyl-functionalised dye molecules to be K ≈ 10(5) - 10(7)  M(-1), which is several orders of magnitude larger than that of the TOPO ligands. Our results give additional insights into the non-FRET PL quenching processes involved and show that the QD surface is inhomogeneous with respect to the involved attachment and detachment processes. In comparison with other methods, such as NMR spectroscopy, the advantage of our approach is that ligand dynamics can be investigated at extremely low ratios of dye molecules to QDs.

Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate

The tunneling methyl groups in dimethyl‐s‐tetrazine (DMST) doped single crystals of durene were investigated by high resolution optical spectroscopy using spectral hole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius‐type behavior above 3.5 K. The activation energy of 20 cm−1 also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.

Interaction of multiporphyrin systems with molecular oxygen in liquid solutions: extra-ligation and screening effects

Abstract Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer→(ZnOEP) 2 Ph→(ZnOEP) 2 →(ZnHTPP) 2 →(ZnOEChl) 2 with relative lowering of excited S 1 - and T 1 -states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T 1 -states non-radiative decay (the most pronounced for dimers with higher lying T 1 -levels). For pyridinated dimers at 293 K T 1 -states quenching by molecular oxygen depends on the spacer flexibility and donor–acceptor interactions with PYR. In self-assembled triads and pentads energy and electron transfer (within a few ps) takes place from Zn-dimers to pyridyl substituted porphyrin extra-ligand, H 2 P, followed by the effective population of H 2 P T 1 -state. For these systems, bimolecular constants of H 2 P T 1 -states quenching by O 2 decrease by 1.4–1.8 times with respect to those found for individual monomeric porphyrins. This effect is explained by the screening action of a strongly quenched Zn–porphyrin dimer subunit limiting the access of oxygen molecule to the excited extra-ligand.

Inkjet printing as a tool for the patterned deposition of octadecylsiloxane monolayers on silicon oxide surfaces

We present a case study about inkjet printing as a tool for molecular patterning of silicon oxide surfaces with hydrophobic functionality, mediated by n-octadecyltrichlorosilane (OTS) molecules. In contrast to state-of-the-art techniques such as micro contact printing or chemical immersion with subsequent lithography processes, piezo drop-on-demand inkjet printing does not depend on physical masters, which allows an effective direct-write patterning of rigid or flexible substrates and enables short run-lengths of the individual pattern. In this paper, we used mesithylene-based OTS inks, jetted them in droplets of 10 pL on a silicon oxide surface, evaluated the water contact angle of the patterned areas and fitted the results with Cassies law. For inks of 2.0 mM OTS concentration, we found that effective area coverages of 38% can be obtained. Our results hence show that contact times of the order of hundred milliseconds are sufficient to form a pattern of regions with OTS molecules adsorbed to the surface, representing at least a fragmented, inhomogeneous self-assembled OTS monolayer (OTS-SAM).

Influence of the dielectric environment on the photoluminescence intermittency of CdSe quantum dots.

We show experimentally that the photoluminescence intermittency (blinking) of single CdSe quantum dots (QDs) is influenced by the dielectric properties of the embedding environment (matrix), the type of ligands and the capping shell. For the on-times, we observe (and tentatively explain) a strong deviation from the commonly reported inverse power law behaviour, which can be taken into account by an exponential cut-off at long times. We assign this component to the photoejection of the electron, while the power law behaviour is a combination of hole- and electron-trapping processes. The cut-off times and their distributions depend strongly on the polarity of the environment. Also, the off-times show, though on a much longer timescale, deviations from the inverse power laws. We suggest a model including surface states and self-trapped states, which quantitatively explains the experimental observations.

Photoinduced hole trapping in single semiconductor quantum dots at specific sites at silicon oxide interfaces

Blinking dynamics of CdSe/ZnS semiconductor quantum dots (QD) are characterized by (truncated) power law distributions exhibiting a wide dynamic range in probability densities and time scales both for off- and on-times. QDs were immobilized on silicon oxide surfaces with varying grades of hydroxylation and silanol group densities, respectively. While the off-time distributions remain unaffected by changing the surface properties of the silicon oxide, a deviation from the power law dependence is observed in the case of on-times. This deviation can be described by a superimposed single exponential function and depends critically on the local silanol group density. Furthermore, QDs in close proximity to silanol groups exhibit both high average photoluminescence intensities and large on-time fractions. The effect is attributed to an interaction between the QDs and the silanol groups which creates new or deepens already existing hole trap states within the ZnS shell. This interpretation is consistent with the trapping model introduced by Verberk et al. (R. Verberk, A. M. van Oijen and M. Orrit, Phys. Rev. B, 2002, 66, 233202).

Identification of Different Donor-Acceptor Structures via Förster Resonance Energy Transfer (FRET) in Quantum-Dot-Perylene Bisimide Assemblies

Nanoassemblies are formed via self-assembly of ZnS capped CdSe quantum dots (QD) and perylene bisimide (PBI) dyes. Upon assembly formation the QD photoluminescence is quenched, as can be detected both via single particle detection and ensemble experiments in solution. Quenching has been assigned to FRET and NON-FRET processes. Analysis of FRET allows for a distinction between different geometries of the QD dye assemblies. Time-resolved single molecule spectroscopy reveals intrinsic fluctuations of the PBI fluorescence lifetime and spectrum, caused by rearrangement of the phenoxy side groups. The distribution of such molecular conformations and their changed dynamics upon assembly formation are discussed in the scope of FRET efficiency and surface ligand density.