Weng Yu-Xiang

Explanation of Effect of Added Water on Dye-Sensitized Nanocrystalline TiO2 Solar Cell: Correlation between Performance and Carrier Relaxation Kinetics

Time-resolved mid-IR transient absorption spectroscopy is employed to explore the mechanism of improving the performance of dye-sensitized TiO2 solar cell (DSSC) when a certain amount of H2O is added into the electrolyte. The relaxation kinetics of dye-sensitized TiO2 nanocrystalline film and the corresponding DSSC performance are investigated under different conditions. It is found that the interfacial charge recombination is retarded and electron injection efficiency is increased in the water vapour and in the electrolyte when D2O is added. The values of open-circuit photovoltage Voc and the short-circuit photocurrent Jsc of the cells are linearly correlated to the product of the two decay time constants. We also observed that Voc well correlates with electron injection efficiency. It provides a preliminary microscopic account for the function of the added water in improving the performance of DSSCs.

Investigation of mechanisms of enhanced open-circuit photovoltage of dye-sensitized solar cells based the electrolyte containing 1-hexyl-3-methylimidazolium iodide

The open-circuit photovoltage is improved by adding 1-hexyl-3-methylimidazolium iodide (HMImI) into the electrolyte. To investigate the mechanisms of the increase of the open-circuit photovoltage, we take the Mott–Schottky analysis and time-resolved mid-infrared absorption spectroscopy to study the band edge movement of TiO2 and the rate of back electron transfer, respectively. The results indicate that the negative shift of the conduction band of TiO2 is a predominant factor to increase the open-circuit photovoltage for the electrolyte containing HMImI.

Nanosecond-time-resolved infrared spectroscopic study of fast relaxation kinetics of protein folding by means of laser-induced temperature-jump

Elucidating the initial kinetics of folding pathways is critical to the understanding of the protein folding mechanism. Transient infrared spectroscopy has proved a powerful tool to probe the folding kinetics. Herein we report the construction of a nanosecond laser-induced temperature-jump (T-jump) technique coupled to a nanosecond time-resolved transient mid-infrared (mid-IR) spectrometer system capable of investigating the protein folding kinetics with a temporal resolution of 50 ns after deconvolution of the instrumental response function. The mid-IR source is a liquid N2 cooled CO laser covering a spectral range of 5.0?m (2000 cm?1)~ 6.5?m (1540 cm?1). The heating pulse was generated by a high pressure H2 Raman shifter at wavelength of 1.9?m. The maximum temperature-jump could reach as high as 26?1?C. The fast folding/unfolding dynamics of cytochrome C was investigated by the constructed system, providing an example.

Experimental study on the chirped structure of the white-light continuum generation by femtosecond laser spectroscopy

The chirped structure of the white-light continuum generation (WLCG) pulse produced by focusing 800nm laser pulse with a pulse duration of 150fs (FWHM: full-width-at-half-maximum) onto a 2.4 mm thick sapphire plate was investigated by the optical Kerr gate technique with normal hexane as the optical Kerr gate medium. The observed WLCG was positively chirped, the measured anti-Stokes spectrum of WLCG ranges from 449 to 580nm with a temporal span of 2.56ps. When using metal reflecting mirrors to eliminate the group velocity dispersion (GVD) effect, we found that a span of 1.3ps still remained, indicating that the chirped pulse cannot be accounted for simply by GVD of the pulse propagation in the dispersive media. Our results suggest that the light-induced refractive index change due to the third-order nonlinear optical effect leads to an additional positive group velocity dispersion, which contributes to an important portion of the observed temporal broadening of the chirped WLCG. In addition to using reflective optical elements instead of dispersive optical elements, an effective way of reducing the chirp is to minimize the optical path length of the WLCG medium.

Synchrotron Small-Angle X-Ray Scattering Investigation on Integral Membrane Protein Light-Harvesting Complex LH2 from Photosynthetic Bacterium Rhodopseudomonas Acidophila

Structures of membrane protein in solution are different from that in crystal phase. We present the primary results of small angle x-ray scattering (SAXS) resolved topological structures of a light harvesting antenna membrane protein complex LH2 from photosynthetic bacteria Rhodopseudomonas acidophila in detergent solution for the first time. Our results show that the elliptical shape of the LH2 complex in solution clearly deviates from its circular structure in crystal phase determined by x-ray diffraction. This result provides an insight into the structure and function interplay in LH2.

Synchrotron small angle x-ray scattering study of dye-sensitized/-unsensitized TiO2 nanoparticle colloidal solution

The wide-gap semiconductor TiO2 nanoparticles with and without dye sensitization have been studied by small angle x-ray scattering using synchrotron radiation. Surface properties of the colloidal TiO2 nanoparticles have been analysed in terms of the surface fractal dimensions (Ds), showing that Ds changes from 3.25 to 2.34 when TiO2 nanoparticles are sensitized by ATRA (all-trans-retinoic acid), which reveals that the surface of the particles become relatively smooth after dye sensitization. The size distribution of gyration radius of TiO2 nanoparticles in the colloids M(Rg) has been successfully determined by the Shull–Roess method. The main peak of M(Rg) for the unsensitized TiO2 colloid is located at 2.1nm, corresponding to a spherical diameter of 5.4nm and this value for the ATRA sensitized TiO2 increases to 2.4nm, indicating a spherical diameter of 6.4nm. Such a size enlargement of TiO2 nanoparticles suggests that there is a coating of ATRA on the TiO2 surface, supporting the view that a monolayer of the dye has been attached to the surface of the TiO2 nanoparticle.

Growth and form in biology: Generation of the plant morphology by spontaneous symmetry breaking based on a pressure field

Considering the role of mechanical forces playing in the morphogenetic pattern formation, we propose a second-order differential equation for the growth and form of plants based on the turgor pressure field at the organ and cellular level. The solutions can well describe various kinds of morphological features of flowers under certain hypotheses. The plant morphology is considered as the spontaneous symmetry breaking of a circular growing boundary, while the employed hypotheses are subjected to further experimental confirmation.

Synchronous Measurement of Ultrafast Anisotropy Decay of the B850 in Bacterial LH2 Complex

Ultrafast anisotropic decay is a prominent parameter revealing ultrafast energy and electron transfer; however, it is difficult to be determined reliably owing to the requirement of a simultaneous availability of the parallel and perpendicular polarized decay kinetics. Nowadays, any measurement of anisotropic decay is a kind of approach to the exact simultaneity. Here we report a novel method for a synchronous ultrafast anisotropy decay measurement, which can well determine the anisotropy, even at a very early time, as the rising phase of the excitation laser pulse. The anisotropic decay of the B850 in bacterial light harvesting antenna complex LH2 of Rhodobacter sphaeroides in solution at room temperature with coherent excitation is detected by this method, which shows a polarization response time of 30 fs, and the energy transfer from the initial excitation to the bacteriochlorophylls in B850 ring takes about 70 fs. The anisotropic decay that is probed at the red side of the absorption spectrum, such as 880 nm, has an initial value of 0.4, corresponding to simulated emission, while the blue side with an anisotropy of 0.1 contributes to the ground-state bleaching. Our results show that the coherent excitation covering the whole ring might not be realized owing to the symmetry breaking of LH2: from C9 symmetry in membrane to C2 symmetry in solution.

New method for fast morphological characterization of organic polycrystalline films by polarized optical microscopy

A new method to visualize the large-scale crystal grain morphology of organic polycrystalline films is proposed. First, optical anisotropic transmittance images of polycrystalline zinc phthalocyanine (ZnPc) films vacuum deposited by weak epitaxial growth (WEG) method were acquired with polarized optical microscopy (POM). Then morphology properties including crystal grain size, distribution, relative orientation, and crystallinity were derived from these images by fitting with a transition dipole model. At last, atomic force microscopy (AFM) imaging was carried out to confirm the fitting and serve as absolute references. This method can be readily generalized to other organic polycrystalline films, thus providing an efficient way to access the large-scale morphologic properties of organic polycrystalline films, which may prove to be useful in industry as a film quality monitoring method.

Coherent excitons at different orientation arrangements of local transition dipole moments in circular light-harvesting complexes

The coherent exciton plays an important role in the photosynthetic primary process, and its functions are deeply dependent on the orientation arrangements of local transition dipole moments (TDMs). We theoretically and systematically study the physical property of the coherent exciton at different orientation arrangements of the local TDMs in circular light-harvesting (LH) complexes. Especially, if the orientation arrangements are different, the delocalized TDMs of the coherent excitons and the energy locations of the optically active coherent excitons (OACEs) can be obviously different, and then there are more manners to capture, store and transfer light energy in and between LH complexes. Similarly, if the orientation arrangements are altered, light absorption and radiative intensities can be converted fully between the OACEs in the upper and lower coherent exciton bands, and then the blue and red shifts of the absorption and radiative bands of the pigment molecules can occur simultaneously at some orientation arrangements. If the systems are in the vicinities of the critical orientation arrangements, the weak static disorder or small thermal excitation can destroy the coherent electronic excitations, and then the coherent exciton cannot exist any more.