Wunshain Fann

Mass production and dynamic imaging of fluorescent nanodiamonds.

Fluorescent nanodiamond is a new nanomaterial that possesses several useful properties, including good biocompatibility, excellent photostability and facile surface functionalizability. Moreover, when excited by a laser, defect centres within the nanodiamond emit photons that are capable of penetrating tissue, making them well suited for biological imaging applications. Here, we show that bright fluorescent nanodiamonds can be produced in large quantities by irradiating synthetic diamond nanocrystallites with helium ions. The fluorescence is sufficiently bright and stable to allow three-dimensional tracking of a single particle within the cell by means of either one- or two-photon-excited fluorescence microscopy. The excellent photophysical characteristics are maintained for particles as small as 25 nm, suggesting that fluorescent nanodiamond is an ideal probe for long-term tracking and imaging in vivo, with good temporal and spatial resolution.

Electrospinning of continuous aligned polymer fibers

Electrospinning is a technique employed for preparing polymer fibers having diameters in the range of 10 μm–10 nm using high electrostatic field. In this letter, we report the formation of aligned polymer fibers, several centimeters in length, with separation between the fibers in the range of 5–100 μm. Achieving alignment is an important step toward the exploitation of these fibers in applications. We have employed about 4500 V and a separation distance of about 1–3 cm between the electrodes. Smaller distance between electrodes, we believe, provides better control on the formation of the fibers.

Experimental and theoretical investigations of absorption and emission spectra of the light-emitting polymer MEH-PPV in solution

Abstract We report the absorption and photoluminescence spectra of the MEH-PPV polymer in a chloroform solvent. In the analysis, we assume that the polymer consists of short conjugated segments which are called oligomers with different lengths. The appearance of each chain length is controlled by a distribution function. Based on the conformational disorder caused by torsional motion, we found that the distribution function should take a Gaussian form with its center and width being adjustable parameters. While the energy level of the emissive S1 state of the oligomer and the corresponding oscillator strength are calculated from exciton theory. In addition, the associated vibrational motions are considered in determining the spectral shape in which both displacement and distortion of the potential surfaces are taken into account. With this information, the calculation result can fit the experiment reasonably well.

White-light electroluminescence from soluble oxadiazole-containing phenylene vinylene ether-linkage copolymer

We report a promising oxadiazole-containing phenylene vinylene ether-linkage copolymer, which can emit nearly white light from a single-layer light-emitting diode. The emission spectrum is composed of a red component originating from the new excited dimer in addition to the blue-green component from an individual lumophore and excimer. This excited dimer is formed under a strong electric field inside the diode and cannot be produced by photoexcitation, which is different from the excimer or exciplex that is often found both in photoluminescence and electroluminescence, and it is termed the “electromer.”

Temperature effect on the electronic spectra of poly(p-phenylenevinylene)

Abstract The experimental temperature-dependent absorption, phololuminescence and photoluminescence excitation spectra of poly ( p -phenylenevinylene) (PPV) are presented and a molecular theory is used to analyze the results. In order to model the complicated system such as PPV, the structural inhomogeneity has to be included. The structural inhomogeneity is represented by the different Huang-Rhys factors associated with different chain lengths. The theoretical spectra agree qualitatively with the experimental results.

Aggregated states of luminescent conjugated polymers in solutions

Abstract In order to study the electronic properties of the interchain state in light-emitting polymers, we have performed spectroscopic experiments for 2,5-dioctyloxy PPV (DOO-PPV) in both good (chloroform) and poor (2-methyl-tetrahydrofuran) solvents. The concentration-dependent absorption, photoluminescence (PL) spectra and dynamics of time-resolved PL can be explained by the coexistence of an intrachain and one kind of aggregated states in poor solution. The estimated lifetime and transition moment of the aggregated state lead to significant quantum efficiency of PL which is about 0.75 times that of the intrachain state. The PL spectrum for the aggregated state resembles that of a thin film, implying that the emissive species in thin film is more likely to be an aggregated state.

DIRECT MEASUREMENTS OF THE TRUE VIBRATIONAL AMPLITUDES IN SHEAR FORCE MICROSCOPY

A new method to measure the tip’s true vibrational amplitude in the shear force microscopy is demonstrated. The measurements are based on modifications of the beam diffraction method. In addition to the dither vibration, the equilibrium position of the tip is set to move along the direction of the vibration. The ratio of ac amplitude to the derivative of dc amplitude gives the tip’s vibration. The sensitivity of this method is determined by the size of the laser focal spot times the ratio of the ac to dc signals of the diffracted light.

Structure and charge transport properties in MEH-PPV

The charge carrier transport in MEH-PPV is investigated with respect to different molecular weight distributions and with different tetrahedral defect densities. The defect density influences the charge transport behaviors significantly. With smaller defect density, MEH-PPV exhibits better charge transport which further depends upon the morphology. Position disorder parameter which is due to the morphology difference dominates the charge transport properties of low defect samples.

Vibration dynamics of tapered optical fiber probes

The motion of tapered fiber probes was studied both theoretically and experimentally. A continuum-mechanical model, including both the intrinsic and the external loss, is proposed to account for the vibration dynamics of the probe. The intrinsic loss was found to be the dominant damping factor experimentally. Analytical solutions based on a realistic probe geometry were obtained for the model in the presence of intrinsic loss. The results are compared with the measured overall motions of the tapered probe. The calculations agree well with the experimental results.

Molecular orbital calculations of electronic excited states in poly(p-phenylene vinylene)

Abstract The electronic structure of model oligomers (monomers, dimers, …, hexamers) of poly( p -phenylene vinylene) were studied using the intermediate neglect of differential overlap method with spectroscopic parametrization (INDO/S). The calculated first singlet-to-singlet transitions compare favorably with the experimental spectra. The results from the exciton theory are also presented. Our results show that the conjugation length is finite and the interactions between polymer chains play a very important role in optical properties. The theoretical results for the triplet-to-triplet transitions are in accord with the experimental values, supporting the experimentally ascribed triplet-to-triplet photoinduced absorption feature in oligomers.