We present fluorescence decay measurements of single ZnS covered CdSe nanocrystals. It is shown that the fluorescence decay time is fluctuating during the investigation leading to a multiexponential decay even for a single nanocrystal. In combination with measurements of the fluorescence blinking behavior we find that a high fluorescence intensity is correlated with a long fluorescence decay time. This is consistent with a model of fluctuating nonradiative decay channels leading to variable dynamic quenching processes of the excited state.

The electrochemical behavior of arrays of Au nanoparticles assembled on Au electrodes modified by 11-mercaptoundecanoic acid (MUA) and poly-L-lysine (PLYS) was investigated as a function of the particle number density. The self-assembled MUA and PLYS layers formed compact ultrathin films with a low density of defects as examined by scanning tunneling microscopy. The electrostatic adsorption of Au particles of 19 +/- 3 nm on the PLYS layer resulted in randomly distributed arrays in which the particle number density is controlled by the adsorption time. In the absence of the nanoparticles, the dynamics of electron transfer involving the hexacynoferrate redox couple is strongly hindered by the self-assembled film. This effect is primarily associated with a decrease in the electron tunneling probability as the redox couple cannot permeate through the MUA monolayer at the electrode surface. Adsorption of the Au nanoparticles dramatically affects the electron-transfer dynamics even at low particle number density. Cyclic voltammetry and impedance spectroscopy were interpreted in terms of classical models developed for partially blocked surfaces. The analysis shows that the electron transfer across a single particle exhibits the same phenomenological rate constant of electron transfer as for a clean Au surface. The apparent unhindered electron exchange between the nanoparticles and the electrode surface is discussed in terms of established models for electron tunneling across metal-insulator-metal junctions.

In this letter, we describe the preparation of a versatile polymer ligand, which can be attached to CdSe/ZnS semiconductor nanocrystals via a phase transfer reaction. The ligand is based on a chain of reactive esters, which can, in principle, be substituted by any compound containing amino-functionalities. The polymer/nanocrystal complexes are characterized in terms of structure and photostability.

We investigate the polarization state of the fluorescence emission of individual CdSe/ZnS nanocrystals and try to establish a correlation with their crystallographic properties. The crystallinity and orientation of the particles are examined by transmission electron microscopy (TEM). These micrographs are compared with simulated TEM images. The polarization of the fluorescence light is investigated with a method in which the orientation of the transition dipole can be determined within seconds even though the fluorescence shows strong intensity fluctuations. The results show that even spherical particles emit elliptically polarized light, which can in principle be explained by a 2D transition dipole (“bright plane”). For a few particles, we could directly compare the polarization state with the crystallographic structure of the same nanocrystals. The findings reveal that a well-defined polarization state can even be observed for particles with stacking faults and polycrystalline nanocrystals.

Colloidal semiconductor nanocrystals (NCs) are color tunable inorganic fluorophores, which consist of an inorganic core and an outer shell of molecular ligands.[1] While the optical properties can be adjusted by the composition [2] and the size of the core, [3] the ease with which the NCs can be chemically processed is mainly governed by the molecular structure of the ligands. In this context we define a ligand as consisting of three parts: a functional anchor group that attaches to the inorganic NC surface, an outer functional group, and a spacer in between. The architecture of the ligand is of major importance for assembling the NCs into larger supramolecular structures [4] or conjugating them to biological samples where they serve as fluorescence labels.[5] For these purposes it is essential that both the anchor and outer functionality can form strong chemical bonds. We will address this problem by introducing a novel type of polymer ligand, where the bond strength is increased by the presence of multiple anchor groups. Since our model ligands are decorated with dye molecules, we can employ fluorescence resonance energy transfer (FRET) to study the formation and stability of the NC-dye couples, where the NCs are the donors and the dyes the acceptors [6,7] . Based on the ensemble absorption and fluorescence spectra, the amount of dye attached to a given NC [6] and the average distance between the NC and the dye molecules can be estimated.[8] However, since the relative orientation and structure of the FRET couples can differ significantly from one complex to the next, we extend our measurements to single NC-dye complexes. Our experiments clearly show that static and dynamic heterogeneities occur in the ensemble of NC-dye couples, a result which is of importance for a FRET-based analysis of intermolecular distances.The CdSe/ZnS core-shell NCs are synthesized by an established method [3,9] in the presence of phosphonic acids [10] and trioctylphosphine oxide (TOPO) as ligands. The surface molecules can be subsequently cross linked to form multianchor ligands to stabilize the ligand shell.

No abstract

SummaryLocal fluorescence probes based on CdSe semiconductor nanocrystals were prepared and tested by recording scanning near-field optical microscopy (SNOM) images of calibration samples and fluorescence resonance energy transfer SNOM (FRET SNOM) images of acceptor dye molecules inhomogeneously deposited onto a glass substrate. Thousands of nanocrystals contribute to the signal when this probe is used as a local fluorescence source while only tens of those (the most apical) are involved in imaging for the FRET SNOM operation mode. The dip-coating method used to make the probe enables diminishing the number of active fluorescent nanocrystals easily. Prospects to realize FRET SNOM based on a single fluorescence centre using such an approach are briefly described.

We present a method to establish a correlation between the crystalline structure and the fluorescence properties of isolated semiconductor nanocrystals. By using ultrathin silicon nitride substrates with markers, we have localized and investigated the same particles in both a high-resolution transmission electron microscope (TEM) and a confocal optical microscope. We have found that the observation of strong fluorescence emission does not require single domain particles. Additionally, we have correlated the size and shape of a particle as determined by TEM and its spectral properties like emission wavelength and spectral diffusion.