Shun Yu

Hydrodynamic alignment and assembly of nanofibrils resulting in strong cellulose filaments.

Cellulose nanofibrils can be obtained from trees and have considerable potential as a building block for biobased materials. In order to achieve good properties of these materials, the nanostructure must be controlled. Here we present a process combining hydrodynamic alignment with a dispersion–gel transition that produces homogeneous and smooth filaments from a low-concentration dispersion of cellulose nanofibrils in water. The preferential fibril orientation along the filament direction can be controlled by the process parameters. The specific ultimate strength is considerably higher than previously reported filaments made of cellulose nanofibrils. The strength is even in line with the strongest cellulose pulp fibres extracted from wood with the same degree of fibril alignment. Successful nanoscale alignment before gelation demands a proper separation of the timescales involved. Somewhat surprisingly, the device must not be too small if this is to be achieved.

Silver substrates for surface enhanced Raman scattering: Correlation between nanostructure and Raman scattering enhancement

The fabrication of substrates for Surface Enhanced Raman Scattering (SERS) applications matching the needs for high sensitive and reproducible sensors remains a major scientific and technological issue. We correlate the morphological parameters of silver (Ag) nanostructured thin films prepared by sputter deposition on flat silicon (Si) substrates with their SERS activity. A maximum enhancement of the SERS signal has been found at the Ag percolation threshold, leading to the detection of thiophenol, a non-resonant Raman probe, at concentrations as low as 10−10M, which corresponds to enhancement factors higher than 7 orders of magnitude. To gain full control over the developed nanostructure, we employed the combination of in-situ time-resolved microfocus Grazing Incidence Small Angle X-ray Scattering with sputter deposition. This enables to achieve a deepened understanding of the different growth regimes of Ag. Thereby an improved tailoring of the thin film nanostructure for SERS applications can be realized.

Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance

Optically transparent wood (TW) with transmittance as high as 85% and haze of 71% was obtained using a delignified nanoporous wood template. The template was prepared by removing the light-absorbing lignin component, creating nanoporosity in the wood cell wall. Transparent wood was prepared by successful impregnation of lumen and the nanoscale cellulose fiber network in the cell wall with refractive-index-matched prepolymerized methyl methacrylate (MMA). During the process, the hierarchical wood structure was preserved. Optical properties of TW are tunable by changing the cellulose volume fraction. The synergy between wood and PMMA was observed for mechanical properties. Lightweight and strong transparent wood is a potential candidate for lightweight low-cost, light-transmitting buildings and transparent solar cell windows.

Use of intermediate focus for grazing incidence small and wide angle x-ray scattering experiments at the beamline P03 of PETRA III, DESY.

We describe the new experimental possibilities of the micro- and nanofocus X-ray scattering beamline P03 of the synchrotron source PETRA III at DESY, Hamburg (Germany), which arise from experiments with smaller beam sizes in the micrometer range. This beamline has been upgraded recently to perform new kinds of experiments. The use of an intermediate focus allows for reducing the beam size of microfocused hard X-rays while preserving a large working distance between the focusing elements and the focus position. For the first time, this well-known methodology has been employed to grazing incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS). As examples, we highlight the applications to in situ studies using microfluidic devices in GISAXS geometry as well as the investigation of the crystallinity of thin films in GIWAXS geometry.

Patterned Diblock Co-Polymer Thin Films as Templates for Advanced Anisotropic Metal Nanostructures

We demonstrate glancing-angle deposition of gold on a nanostructured diblock copolymer, namely polystyrene-block-poly(methyl methacrylate) thin film. Exploiting the selective wetting of gold on the polystyrene block, we are able to fabricate directional hierarchical structures. We prove the asymmetric growth of the gold nanoparticles and are able to extract the different growth laws by in situ scattering methods. The optical anisotropy of these hierarchical hybrid materials is further probed by angular resolved spectroscopic methods. This approach enables us to tailor functional hierarchical layers in nanodevices, such as nanoantennae arrays, organic photovoltaics, and sensor electronics.

Monitoring structural dynamics of in situ spray-deposited zinc oxide films for application in dye-sensitized solar cells.

The spray-deposition technique is an effective and scalable method to deposit zinc oxide nanostructures, which are used as active layers for dye-sensitized solar cells (DSSCs) in the present study. The dynamics of structural evolution are studied with grazing incidence small-angle X-ray scattering during in situ spraying. Nanostructured films obtained through multiple spray shots provide suitable structural length scales, morphologies, and film thicknesses; this leads to reasonable performance in a DSSC with the highest short-circuit current density reported so far.

Adsorption geometry, molecular interaction, and charge transfer of triphenylamine-based dye on rutile TiO2(110)

The fast development of new organic sensitizers leads to the need for a better understanding of the complexity and significance of their adsorption processes on TiO(2) surfaces. We have investigated a prototype of the triphenylamine-cyanoacrylic acid (donor-acceptor) on rutile TiO(2) (110) surface with special attention on the monolayer region. This molecule belongs to the type of dye, some of which so far has delivered the record efficiency of 10%-10.3% for pure organic sensitizers [W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C. Pan, and P. Wang, Chem. Mater. 22, 1915 (2010)]. The molecular configuration of this dye on the TiO(2) surface was found to vary with coverage and adopt gradually an upright geometry, as determined from near edge x-ray absorption fine structure spectroscopy. Due to the molecular interaction within the increasingly dense packed layer, the molecular electronic structure changes systematically: all energy levels shift to higher binding energies, as shown by photoelectron spectroscopy. Furthermore, the investigation of charge delocalization within the molecule was carried out by means of resonant photoelectron spectroscopy. A fast delocalization (∼1.8 fs) occurs at the donor part while a competing process between delocalization and localization takes place at the acceptor part. This depicts the push-pull concept in donor-acceptor molecular system in time scale.

Probing evaporation induced assembly across a drying colloidal droplet using in situ small-angle X-ray scattering at the synchrotron source

Colloidal particles in a tiny drying droplet are forced to assemble due to attractive capillary forces. Jamming of the particles throughout the droplet remains either isotropic or anisotropic depending upon the drying kinetics and the physicochemical environment. In this work, we explore the dynamical evolution of such an assembly process across a single evaporative droplet by in situ scanning small-angle scattering using a micro-focused X-ray beam at the synchrotron source. A methodology has been elucidated to differentiate quantitatively between the isotropic and the anisotropic jamming process. Switching of jamming behaviour depending on the initial particle volume fraction in the droplet has been demonstrated. Three distinct stages of assembly, associated with droplet shrinkage, have been revealed even during isotropic jamming. This is in contrast to the drying of a pure liquid droplet under diffusion limited evaporation. It has been established that such in situ scattering measurements can also be used to estimate the temporal evolutions of the viscosity of a drying suspension as well as the diffusivity of nanoparticles in a droplet.

Molecular layers of ZnPc and FePc on Au(111) surface : Charge transfer and chemical interaction

We have studied zinc phthalocyanine (ZnPc) and iron phthalocyanine (FePc) thick films and monolayers on Au(111) using photoelectron spectroscopy and x-ray absorption spectroscopy. Both molecules are adsorbed flat on the surface at monolayer. ZnPc keeps this orientation in all investigated coverages, whereas FePc molecules stand up in the thick film. The stronger inter-molecular interaction of FePc molecules leads to change of orientation, as well as higher conductivity in FePc layer in comparison with ZnPc, which is reflected in thickness-dependent differences in core-level shifts. Work function changes indicate that both molecules donate charge to Au; through the π-system. However, the Fe3d derived lowest unoccupied molecular orbital receives charge from the substrate when forming an interface state at the Fermi level. Thus, the central atom plays an important role in mediating the charge, but the charge transfer as a whole is a balance between the two different charge transfer channels; π-system and the central atom.

Light-induced rearrangements of chemisorbed dyes on anatase(101)

Photoinduced molecular rearrangements are important in daily events essential for life such as visual perception and photo-protection of light harvesting complexes in plants. In this study we demonstrate that similar photoarrangements appear in an analogous technological application where the device performance is controlled by chromophores in sensitized anatase TiO(2), one of the main components for light-harvesting in dye-sensitized solar cells (DSC). STM reveals that illumination leads to distortions of organic dyes containing conjugated backbones and of cis-bis(isothiocyanate)-bis-(2,2-bipyridyl-4,4-dicarboxylate)ruthenium(II)-bis(tetrabutylammonium), known as N719. The dyes were adsorbed in a closed-packed mode on an anatase(101) single crystal surface and imaged in the dark and under white light illumination in an ultra-high vacuum (UHV). STM images of N719 clearly suggest rearrangements caused by rotation of the dye. Conversely, organic dyes rearrange by photoisomerization depending on the number of double bonds, their position in the molecular structure and on the ligand modifications.