Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on-off current ratios >10(6), as well as carrier mobilities up to 230 cm(2)/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-k (deionized water) top gate. SnS2 transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states.

Individual multichromophoric dendrimer molecules, bearing eight perylenecarboximide chromophores at the rim, immobilized in a thin polyvinylbutyral (PVB) film were studied by far-field fluorescence microscopy. Fluorescence intensity trajectories as a function of time (transients), spectra, and decay traces were recorded separately or simultaneously. For comparison, similar measurements have been performed on a model compound containing one perylenecarboximide chromophore. Collective on/off jumps of the fluorescence intensity were observed for single dendrimer molecules, resembling previously reported collective jumps for the emission of single light-harvesting antenna systems. Spectra and decays of both non-interacting and dimer-like interacting chromophoric sites could be distinguished within an individual dendrimer. Transitions between the different spectral forms and decay times, observed for a single molecule, underline the dynamic character of the interactions among the chromophores. Evidence for a stepwise bleaching process of the multichromophoric system was found. Furthermore, the single-molecule data incontestably prove the assumptions stated in the ensemble model.

Single-molecule techniques offer a unique tool for studying the dynamical behavior of individual molecules and provide the possibility to construct distributions from individual events rather than from a signal stemming from an ensemble of molecules. In biological systems, known for their complexity, these techniques make it possible to gain insights into the detailed spectrum of molecular conformational changes and activities. Here, we report on the direct observation of a single lipase-catalyzed reaction for extended periods of time (hours), by using confocal fluorescence microscopy. When adding a profluorescent substrate, the monitored enzymatic activity appears as a trajectory of ''on-state'' and ''off-state'' events. The waiting time probability density function of the off state and the state-correlation function fit stretched exponentials, independent of the substrate concentration in a certain range. The data analysis unravels oscillations in the logarithmic derivative of the off-state waiting time probability density function and correlations between off-state events. These findings imply that the fluctuating enzyme model, which involves a spectrum of enzymatic conformations that interconvert on the time scale of the catalytic activity, best describes the observed enzymatic activity. Based on this model, values for the coupling and reaction rates are extracted.single enzyme activity ͉ two-state trajectories D ynamics of chemical reactions are conventionally investigated by ensemble measurements. Recent advances in single-molecule spectroscopy have enabled the real-time study of biophysical processes (1-10) and conformational changes (11, 12) of single biomolecules. These studies have demonstrated that new information about such processes can be extracted from single-molecule measurements. In particular, deviations from the standard Michaelis-Menten behavior (13,14), which is expected for bulk enzymatic activity, have been observed (6 -8, 12).Motivated by these findings, we examined the enzymatic activity of individual molecules of the 33-kDa lipase B from Candida antarctica molecules (15, 16) by using confocal fluorescence microscopy. This lipase catalyzes the hydrolysis of esters in aqueous solution following the same reaction mechanism as that of a serine protease (17). To study the catalysis by single lipase, we used a fluorogenic substrate, namely the nonf luorescent ester 2Ј,7Ј-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester, which upon hydrolysis forms a highly fluorescent carboxylic acid product (18,19). This method enabled us to probe the enzymatic activity by monitoring the fluorescence emission from single enzymes. The fluorescence emission displayed blinking of ''on'' and ''off'' events depending on the presence (or absence) of the fluorescent product in the confocal focus (20). By using this approach, we have been able to obtain long trajectories (for time periods of hours) suitable for reliable statistical analysis while varying the concentration of the substrate, thus ...

T he current renaissance of the use of the fluorescence resonance energy-transfer (FRET) (1) process between two weakly coupled dipoles, one acting as donor and the other acting as acceptor, results from various elegant and successful studies of distance changes in individual biomolecules by using singlemolecule spectroscopy (SMS). This type of study is often referred to as single-pair Förster resonance ET (2-4). Recent advances in fluctuation microscopy allow the observation of changes in the ET efficiency in single-pair Förster resonance ET systems at the millisecond-to-nanosecond time scale (5, 6). It has been suggested that, in the latter time scale, thermally excited conformational dynamics of (bio)macromolecules will become accessible (5). Thus far, FRET measurements at these time scales have overlooked competitive Förster-type ET pathways, which might complicate data analysis and interpretation of the FRET data. The FRET process involves nonradiative transfer from a donor to an acceptor. For molecules within the weak coupling limit, Förster derived an expression for the rate constant of dipole-dipole-induced ET (7). This relation shows that the ET efficiency scales with the sixth power of the distance between the chromophores:Here, R 0 is the distance at which the efficiency equals 50%, i.e., the distance at which an equal probability exists for the excited chromophore to relax to the ground state via emission of a photon or to undergo ET. It includes the spectral properties and the relative orientation (in terms of the orientation factor 2 ) of donor and acceptor transition dipoles. If the distances and orientations of the chromophores are kept constant, the ET efficiency is determined by the spectral overlap of the corresponding transitions, i.e., the overlap of the emission spectrum of the donor and the absorption spectrum of the acceptor. Therefore, FRET can also occur between identical molecules if the Stokes shift is small enough to allow for a sufficient overlap of the emission and absorption spectrum of the chromophores. This so-called homo-transfer or energy hopping represents a key mechanism for energy transport in some light-harvesting complexes (8).However, Förster-type resonance ET is not restricted to the nonradiative transfer of energy from a donor in the excited state to a ground-state acceptor. Transfer processes that are allowed within the Förster formalism are those for which there are no changes in electron spin in the acceptor transition. Therefore, even ET processes between donor and acceptor molecules with different spin multiplicity likewise are possible. Hence, the transfer of excitation energy from a chromophore residing in the first excited singlet (S) state to another chromophore residing either in the triplet (T) or in an excited singlet state are possible and competitive ET pathways.In the present contribution, we demonstrate that, because of the specific excitation conditions that are applied in SMS, indeed several Förster-type ET pathways are prevalent in single bichromopho...