Frank Cichos

Hot Brownian Motion

We derive the markovian description for the nonequilibrium brownian motion of a heated nanoparticle in a simple solvent with a temperature-dependent viscosity. Our analytical results for the generalized fluctuation-dissipation and Stokes-Einstein relations compare favorably with measurements of laser-heated gold nanoparticles and provide a practical rational basis for emerging photothermal tracer and nanoparticle trapping and tracking techniques.

Photothermal single-particle microscopy: detection of a nanolens.

Combining quantitative photothermal microscopy and light scattering microscopy as well as accurate MIE scattering calculations on single gold nanoparticles, we reveal that the mechanism of photothermal single-molecule/particle detection is quantitatively explained by a nanolensing effect. The lensing action is the result of the long-range character of the refractive index profile. It splits the focal detection volume into two regions. Our results lay the foundation for future developments and quantitative applications of single-molecule absorption microscopy.

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.

Blinking of single molecules in various environments

The fluorescence intermittency of various dye molecules in different environments was studied by wide-field fluorescence microscopy. The present work focuses on the analysis of long dark periods that are not due to triplet states. It is shown that the distributions of the length of dark periods follow power laws for all systems studied here. Furthermore, blinking kinetics is strongly influenced by the gas atmosphere to which the molecules are exposed. The presence of oxygen is of crucial importance.

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.

Nano-lens diffraction around a single heated nano particle

The action of a nanoscopic spherically symmetric refractive index profile on a focused Gaussian beam may easily be envisaged as the action of a phase-modifying element, i.e. a lens: Rays traversing the inhomogeneous refractive index field n(r) collect an additional phase along their trajectory which advances or retards their phase with respect to the unperturbed ray. This lens-like action has long been understood as being the mechanism behind the signal of thin sample photothermal absorption measurements [Appl. Opt. 34, 41-50 (1995)], [Jpn. J. Appl. Phys. 45, 7141-7151 (2006)], where a cylindrical symmetry and a different lengthscale is present. In photothermal single (nano-)particle microscopy, however, a complicated, though prediction-wise limited, electrodynamic scattering treatment was established [Phys. Rev. B 73, 045424 (2006)] during the emergence of this new technique. Our recent study [ACS Nano, DOI: 10.1021/nn300181h] extended this approach into a full ab-initio model and showed for the first time that the mechanism behind the signal, despite its nanoscopic origin, is also the lens-like action of the induced refractive index profile only hidden in the complicated guise of the theoretical generalized Mie-like framework. The diffraction model proposed here yields succinct analytical expressions for the axial photothermal signal shape and magnitude and its angular distribution, all showing the clear lens-signature. It is further demonstrated, that the Gouy-phase of a Gaussian beam does not contribute to the relative photothermal signal in forward direction, a fact which is not easily evident from the more rigorous EM treatment. The presented model may thus be used to estimate the signal shape and magnitude in photothermal single particle microscopy.

Coupled molecular dynamics/semiempirical simulation of organic solutes in polar liquids. II. Coumarin 153 in methanol and acetonitrile

In this paper we present a coupled molecular dynamics/semiempirical simulation of the solvation of the dye Coumarin 153 (C153) in two solvents, methanol and acetonitrile. In order to account for the solute electronic polarizability we use a semiempirical description to determine the charge distribution of the dye during the simulation. Solute–solvent and solvent–solvent interactions are described by empirical potentials. We examine the structure of the solvation shell, the purely electrostatic part of the solute–solvent interactions, shifts of the absorption and emissions spectra and the solvation dynamics of C153 in both solvents. In contrast with our first study of naphthalene in acetonitrile, the equilibrium simulations show structural changes in the solvation shell when electronic polarizability is included. The inclusion of electronic polarizability also enhances solute–solvent electrostatic interactions. Therefore, an increase of absorption and emission redshifts is observed compared to simulations ...

Optically controlled thermophoretic trapping of single nano-objects.

Brownian motion is driven by thermal fluctuations and becoming more efficient for decreasing size and elevated temperatures. Here, we show that despite the increased fluctuations local temperature fields can be used to localize and control single nano-objects in solution. By creating strong local temperature gradients in a liquid using optically heated gold nanostructures, we are able to trap single colloidal particles. The trapping is thermophoretic in nature, and thus no restoring body force is involved. The simplicity of the setup allows for an easy integration and scalability to large arrays of traps.

Stochastic Localization of Microswimmers by Photon Nudging

Force-free trapping and steering of single photophoretically self-propelled Janus-type particles using a feedback mechanism is experimentally demonstrated. Realtime information on particle position and orientation is used to switch the self-propulsion mechanism of the particle optically. The orientational Brownian motion of the particle thereby provides the reorientation mechanism for the microswimmer. The particle size dependence of the photophoretic propulsion velocity reveals that photon nudging provides an increased position accuracy for decreasing particle radius. The explored steering mechanism is suitable for navigation in complex biological environments and in-depth studies of collective swimming effects.