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Raman spectroscopy is a powerful technique to characterize materials. It probes non-destructively chemical composition, crystallinity, defects, strain and coupling phenomena. However, the Raman response of surfaces or thin films is often weak and obscured by dominant bulk signals. Here we overcome this limitation by placing a transferable porous gold membrane (PAuM) on top of the surface of interest. Slot-like nanopores in the membrane act as plasmonic slot antennas and enhance the Raman response of the surface or thin film underneath. Simultaneously, the PAuM suppresses the penetration of the excitation laser into the bulk, efficiently blocking the bulk Raman signal. Using graphene as a model surface, we show that these two simultaneous effects lead to an increase in the surface-to-bulk Raman signal ratio by three orders of magnitude. We find that 90% of the Raman enhancement occurs within the top 2.5 of the material, demonstrating truly surface-sensitive Raman scattering. To validate our approach, we analyze the surface of a LaNiO3 thin film. We observe a Raman mode splitting for the LaNiO3 surface-layer, which is spectroscopic evidence that the surface structure differs from the bulk. This result underpins that PAuM give direct access to Raman signatures of surfaces and their structural properties.

Recovery of biofilms over time and validation of the precision of our rheometer measuring setup

We present a comprehensive model to account for the behavior of suspended nanoparticles under magnetohydrodynamic conditions. The Lorentz force not only drags nanoparticle flocs toward the walls reducing the distance between flocs resulting a more negative total pair interaction potential energy, but also produces extra magnetic-induced stresses inside a floc leading to a change of pair interaction distance thus giving rise to a less negative total potential energy. The model explains quite well the recent experimental results showing that magnetic field assists aggregation in laminar or weak turbulent flows, but favors floc disruption in turbulent regime., Comment: 10 pages, 2 figures

The behavior of a colloidal suspension of rod-like {\it fd} viruses in the nematic phase, subjected to steady state and transient shear flows is studied. The monodisperse nature of these rods combined with relatively small textural contribution to the overall stress make this a suitable model system to investigate the effects of flow on the non-equilibrium phase diagram. Transient rheological experiments are used to determine the critical shear rates at which director tumbling, wagging and flow-aligning occurs. The present model system enables us to study the effect of rod concentration on these transitions. The results are in quantitatively agreement with the Doi-Edwards-Hess model. Moreover, we observe that there is a strong connection between the dynamic transitions and structure formation, which is not incorporated in theory., Comment: 13 pages, 12 figures, to appear in Langmuir

Active systems such as microorganisms and self-propelled particles show a plethora of collective phenomena, including swarming, clustering, and phase separation. Control over the propulsion direction and switchability of the interactions between the individual self-propelled units may open new avenues in designing of materials from within. Here, we present a self-propelled particle system, consisting of half-gold-coated titania (TiO2) particles, in which we can quickly and on-demand reverse the propulsion direction, by exploiting the different photocatalytic activities on both sides. We demonstrate that the reversal in propulsion direction changes the nature of the hydrodynamic interaction from attractive to repulsive and can drive the particle assemblies to undergo both fusion and fission transitions. Moreover, we show these active colloids can act as nucleation sites, and switch rapidly the interactions between active and passive particles, leading to reconfigurable assembly and disassembly. Our experiments are qualitatively described by a minimal hydrodynamic model., Nature Communications, 11 (1), ISSN:2041-1723

Wet processing of graphene sheets is a potentially interesting route for economically viable creation of graphene based composites. In the present work flow dichroism and small angle light scattering are used to investigate the dispersion of functionalized graphene sheets in suspension and their response to shear flow. In line with expectations from scaling theory at rest, the functionalized graphene sheets are present as Brownian, flat sheets, and there is no evidence of significant crumpling. More surprisingly, we find that the rate dependent orientation of these molecularly thin sheets can be described by numerical predictions for hard spheroidal sheets making quantitative predictions of the flow induced orientation possible. Further comparing the flow induced orientation of thick gold decahedra with the thin graphene sheets shows that, except for effects of polydispersity, the flow induced orientation is predicted well quantitatively. Adequate prediction of the effects of flow on orientation of graphene sheets makes it possible to design wet processed graphene based composite materials. The authors thank Luiz Lis-Marzan, Jorge Perez-Juste, and Isabel Pastoriza-Santos for providing oblate gold spheroids. J.V. acknowledges the Swiss National Science Foundation (project number 200021-157147), and G.N. acknowledges funding from the NSERC (grant number RGPIN-2017-03783). The authors thank the reviewers for insightful comments on the article, as these comments led us to improve it.