Computational simulations were used to investigate the dynamics and resulting structures of several para-phenylenevinylene (PPV) based polymers and oligomers (PPV, 2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene --> MEH-PPV and 2,5,2',5'-tetrahexyloxy-7,8'-dicyano-p-phenylenevinylene --> CN-PPV). The results show how the morphology and structure are controlled to a large extent by the nature of the solute-solvent interactions in the initial solution-phase preparation. Secondary structural organization is induced by using the solution-phase structures to generate solvent-free single molecule nanoparticles. Isolation of these single molecule nanostructures from microdroplets of dilute solution results in the formation of electrostatically oriented nanostructures at a glass surface. Our structural modeling suggests that these oriented nanostructures consist of folded PPV conjugated segments with folds occurring at tetrahedral defects (sp3 C-C bonds) within the polymer chain. This picture is supported by detailed experimental fluorescence and scanning probe microscopy studies. We also present results from a fully quantum theoretical treatment of these systems which support the general conclusion of structure-mediated photophysical properties.

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We report the observation of uniformly oriented transition moments perpendicular to the support substrate in single molecules of a conjugated polymer (MEH-PPV) isolated by ink-jet printing techniques. Fluorescence imaging combined with atomic force microscopy and polarization modulation studies, supported by molecular mechanics simulation, provides compelling evidence of polymer nanoparticle (single-molecule) structures with an extraordinary degree of intramolecular order. This is a general technique for preparing oriented nanostructures from structurally similar polymers.

High-resolution fluorescence imaging of isolated nanoparticles of a common semiconducting polymer (poly-[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene, MEH-PPV), produced by ink-jet printing techniques, has revealed highly uniform transition moment orientation perpendicular to the glass substrate. In contrast with the broad emission spectra associated with bulk or single molecules of these species in thin films, we observe narrow photoluminescence emission spectra (10-15 nm fwhm) from individual oriented polymer nanostructures with no evidence of spectral diffusion on time scales of several hundred seconds. The distribution of center frequencies (from several hundred individual nanoparticle measurements) shows clearly defined peaks that can be correlated with excitonic traps of integer multiples of monomer conjugation lengths (8, 9, 10, and 11). The observation of discrete emission characteristics in this important class of materials suggests exciting possibilities in photonics and molecular optoelectronics.

Microcantilevers, such as those used in atomic force microscopy, undergo Brownian motion due to mechanical thermal noise. The root mean square amplitude of the Brownian motion of a cantilever typically ranges from 0.01–0.1 nm, which limits its use in practical applications. Here we describe a technique by which the Brownian amplitude and the Q factor in air and water can be amplified by three and two orders of magnitude, respectively. This technique is similar to a positive feedback oscillator, wherein the Brownian motion of the vibrating cantilever controls the frequency output of the oscillator. This technique can be exploited to improve sensitivity of microcantilever-based chemical and biological sensors, especially for sensors in liquid environments.

We examine the role of solution-phase polymer geometries on the favorability of forming oriented nanostructures from single conjugated polymer molecules formed from microdroplets of ultradilute solution. To our knowledge, this is the first time single molecule fluorescence correlation spectroscopy has been used to extract structural information from solution-phase fluorescence measurements. We find that production of oriented species is strongly favored in “poorer” solvents, where the polymer chains have more compact solution-phase structures. While charging during droplet generation is essential for orientation, we conclude that most of the internal organization associated with these interesting structures takes place in solution, independent of the droplets.

We show that continuously illuminated single europium ions incorporated in yttrium oxide (Eu 3+ :Y 2 O 3 ) nanocrystals (5-15 nm diameter) undergo on-off blinking on a variable time scale ranging from hundreds of milliseconds to several seconds. We observe both a pump-intensity dependent "duty factor" (on-time as a percentage of total measurement time), and quantum jumps between at least three well-defined luminescence intensity levels (bright states) from individual nanoparticles. Interesting switching or oscillation between different bright levels was also observed with a modulation rate that is dependent on pump-laser intensity. These features of single-ion luminescence are not observed for larger particles with multiple chromophores. We propose that these effects derive from pump laser-induced fluctuations between different quasi-stable Eu 3+ symmetry sites that effectively modulate the electric dipole transition moment.