Ideal probe single-molecule experiments reveal the intrinsic dynamic heterogeneity of a supercooled liquid.

The concept of dynamic heterogeneity and the picture of the supercooled liquid as a mosaic of environments with distinct dynamics that interchange in time have been invoked to explain the nonexponential relaxations measured in these systems. The spatial extent and temporal persistence of these regions of distinct dynamics have remained challenging to identify. Here, single-molecule fluorescence measurements using a probe similar in size and mobility to the host o-terphenyl unambiguously reveal exponential relaxations distributed in time and space and directly demonstrate ergodicity of the system down to the glass transition temperature. In the temperature range probed, at least 200 times the structural relaxation time of the host is required to recover ensemble-averaged relaxation at every spatial region in the system.

Conformation-Dependent Photostability among and within Single Conjugated Polymers.

The relationship between photostability and conformation of 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) conjugated polymers was studied via excitation polarization modulation depth (M) measurements. Upon partial photobleaching, M distributions of collapsed, highly ordered MEH-PPV molecules shifted toward lower values. Conversely, M distributions of MEH-PPV molecules with random coil conformations moved toward higher values after partial photobleaching. Monte Carlo simulations of randomly distributed dipole moments along polymer chains subjected to partial photobleaching revealed that a statistical effect leads to an increase in peak M value. Decreases in M values seen experimentally in the population of MEH-PPV molecules with high M values, however, are due to conformation-dependent photostability within single MEH-PPV polymers. We show that, while folded MEH-PPV molecules are relatively more photostable than extended MEH-PPV molecules in an ensemble, extended portions of particular molecules are more photostable than folded domains within single MEH-PPV molecules.

Extraction of rotational correlation times from noisy single molecule fluorescence trajectories.

Monitoring single molecule probe rotations is an increasingly common approach to studying dynamics of complex systems, including supercooled liquids. Even with advances in fluorophore design and detector sensitivity, such measurements typically exhibit low signal to noise and signal to background ratios. Here, we simulated and analyzed orthogonally decomposed fluorescence signals of single molecules undergoing rotational diffusion in a manner that mimics experimentally collected data of probes in small molecule supercooled liquids. The effects of noise, background, and trajectory length were explicitly considered, as were the effects of data processing approaches that may limit the impact of noise and background on assessment of environmental dynamics. In many cases, data treatment that attempts to remove noise and background were found to be deleterious. However, for short trajectories below a critical signal to background threshold, a thresholding approach that successfully removed data points associated with noise and spared those associated with signal allowed for assessment of environmental dynamics that was as accurate and precise as would be achieved in the absence of noise.

Rh(1-x)Pd(x) nanoparticle composition dependence in CO oxidation by oxygen: catalytic activity enhancement in bimetallic systems.

Bimetallic 15 nm Rh(1-x)Pd(x) nanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O(2) in two pressure regimes: atmospheric pressure and 100-200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100-200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O(2) reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd-Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.

In situ multi-modal monitoring of solvent vapor swelling in polymer thin films.

Polymer processing techniques involving solvent vapor swelling are typically challenging to control and thus reproduce. Moreover, traditional descriptions of solvent swollen films lack microscopic detail. We describe the design and use of an apparatus that facilitates macroscopic and microscopic characterization of samples undergoing solvent vapor swelling in a controlled environment. The experimental design incorporates three critical characteristics: (1) a mass-flow controlled solvent vapor delivery system allows for precise control of the amount of solvent vapor delivered to the sample, (2) a sample prepared on a quartz crystal microbalance allows for real-time assessment of the extent of sample swelling, (3) a second sample prepared and assessed in parallel on a coverslip allows real-time fluorescence microscopy during swelling. We demonstrate that this apparatus allows for single-particle tracking, which in turn facilitates in situ monitoring of local environments within the solvent-swollen film.

Localizing exciton recombination sites in conformationally distinct single conjugated polymers by super-resolution fluorescence imaging.

To thoroughly elucidate how molecular conformation and photophysical properties of conjugated polymers (CPs) are related requires simultaneous probing of both. Previous efforts used fluorescence imaging with one nanometer accuracy (FIONA) to image CPs, which allowed simultaneous estimation of molecular conformation and probing of fluorescence intensity decay. We show that calculating the molecular radius of gyration for putative folded and unfolded poly(2-methoxy-5-(2'-ethylhexyloxy)1,4-phenylenevinylene) (MEH-PPV) molecules using FIONA underestimates molecular extension by averaging over emitters during localization. In contrast, employing algorithms based on single molecule high resolution imaging with photobleaching (SHRImP), including an approach we term all-frames SHRImP, allows localization of individual emitters. SHRImP processing corroborates that compact MEH-PPV molecules have distinct photophysical properties from extended ones. Estimated radii of gyration for isolated 168 kDa MEH-PPV molecules immobilized in polystyrene and exhibiting either stepwise or continuous intensity decays are found to be 12.6 and 25.3 nm, respectively, while the distance between exciton recombination sites is estimated to be ∼10 nm independent of molecular conformation.