Hybrid One-Dimensional Plasmonic-Photonic Crystals for Optical Detection of Bacterial Contaminants.

Photonic crystal-based biosensors hold great promise as low-cost devices for real-time monitoring of a variety of biotargets, for example, bacterial contaminants in food. Here, we report the proof-of-concept for a new colorimetric sensor of bacterial contamination, which is based on a novel hybrid plasmonic-photonic device. Our system consists of a layer of silver, a plasmonic metal exhibiting a well-known bioactivity, on top of a one-dimensional photonic crystal. We attribute the bioresponsivity to the formation of polarization charges at the Ag/bacterium interface within a sort of "bio-doping" mechanism. Interestingly, this triggers a blue shift in the photonic response. As an example, we assessed the validity of our approach by detecting one of the most hazardous contaminants, Escherichia coli. This work demonstrates that our device can be a low-cost and portable platform for the detection of common bacterial contaminants.

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy.

We introduce laser-assisted Time-Resolved SEM (TR-SEM), joining Scanning Electron Microscopy and laser light excitation, to probe the long-term temporal evolution of optically excited charge distributions at the surface of Metal Ammonium Lead Triiodide (MAPbI3) hybrid perovskite thin films. Laser-assisted TR-SEM relies on the optically induced local modification of Secondary Electron (SE) detection yield to provide mapping of photoexcited potentials and charge dynamics at surfaces, and qualifies as a complementary approach to near-field probe microscopies and nonlinear photoemission spectroscopies for photovoltage measurements. Real-time imaging of evolving field patterns are provided on timescales compatible with SEM scanning rates, so that temporal resolution in the millisecond range can be ultimately envisaged. MAPbI3 is an outstanding light-sensitive material candidate for applications in solar light harvesting and photovoltaics, also appealing as an active system for light generation. In this work, the real time temporal evolution of optically induced SE contrast patterns in MAPbI3 is experimentally recorded, both under illumination by a 405 nm blue laser and after light removal, showing the occurrence of modifications related to photoinduced positive charge fields at surface. The long term evolution of these surface fields are tentatively attributed to ion migration within the film, under the action of the illumination gradient and the hole collecting substrate. This optical excitation is fully reversible in MAPbI3 over timescales of hours and a complete recovery of the system occurs within days. Permanent irradiation damage of the material is avoided by operating the SEM at 5 keV of energy and 1-10 pA of primary current. Optical excitation is provided by intense above-bandgap illumination (up to 50 W/cm2). TR-SEM patterns show a strong dependence on the geometry of SE collection. Measurements are taken at different axial orientations of the sample with respect to the entrance of the in-column detection system of the SEM and compared with numerical modeling of the SE detection process. This enables to single out the information regarding the local potential distribution. Results are interpreted by combining data about the spectral distribution of emitted SEs with the configuration of the electric and magnetic fields in the specimen chamber. The present modeling sets a robust basis for the understanding of photoinduced SE electron contrast.

Graphene as an Ideal Buffer Layer for the Growth of High-Quality Ultrathin Cr2O3 Layers on Ni(111).

Metal-oxide nanostructures play a fundamental role in a large number of technological applications, ranging from chemical sensors to data storage devices. As the size of the devices shrinks down to the nanoscale, it is mandatory to obtain sharp and good quality interfaces. Here, it is shown that a two-dimensional material, namely, graphene, can be exploited as an ideal buffer layer to tailor the properties of the interface between a metallic substrate and an ultrathin oxide. This is proven at the interface between an ultrathin film of the magnetoelectric antiferromagnetic oxide Cr2O3 and a Ni(111) single crystal substrate. The chemical composition of the samples has been studied by means of X-ray photoemission spectroscopy, showing that the insertion of graphene, which remains buried at the interface, is able to prevent the oxidation of the substrate. This protective action leads to an ordered and layer-by-layer growth, as revealed by scanning tunneling microscopy data. The structural analysis performed by low-energy electron diffraction indicates that the oxide layer grown on graphene experiences a significant compressive strain, which strongly influences the surface electronic structure observed by scanning tunneling spectroscopy.

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy.

The excitation dynamics of defects in insulators plays a central role in a variety of fields from Electronics and Photonics to Quantum computing. We report here a time-resolved measurement of electron dynamics in 100 nm film of aluminum oxide on silicon by Ultrafast Scanning Electron Microscopy (USEM). In our pump-probe setup, an UV femtosecond laser excitation pulse and a delayed picosecond electron probe pulse are spatially overlapped on the sample, triggering Secondary Electrons (SE) emission to the detector. The zero of the pump-probe delay and the time resolution were determined by measuring the dynamics of laser-induced SE contrast on silicon. We observed fast dynamics with components ranging from tens of picoseconds to few nanoseconds, that fits within the timescales typical of the UV color center evolution. The surface sensitivity of SE detection gives to the USEM the potential of applying pump-probe investigations to charge dynamics at surfaces and interfaces of current nano-devices. The present work demonstrates this approach on large gap insulator surfaces.

Is a Knowledge of Surface Topology and Contact Angles Enough to Define the Drop Impact Outcome?

It is well known that a superhydrophobic surface may not be able to repel impacting droplets because of the so-called Cassie-to-Wenzel transition. It has been proven that a critical value of the receding contact angle (θR) exists for the complete rebound of water, recently experimentally measured to be 100° for a large range of impact velocities. On the contrary, in the present work, no rebound was observed when low-surface-tension liquids such as hexadecane (σ = 27.5 mN/m at 25 °C) are concerned, even for very low impact velocities and very high values of θR and low contact angle hysteresis. Therefore, the critical threshold of θR ≈ 100° does not sound acceptable for all liquids and for all hydrophobic surfaces. For the same Weber numbers, a Cassie-to-Wenzel state transition occurs after the impact as a result of the easier penetration of low-surface-tension fluids in the surface structure. Hence, a criterion for the drop rebound of low-surface-tension liquids must consider not only the contact angle values with surfaces but also their surface tension and viscosity. This suggests that, even if it is possible to produce surfaces with enhanced static repellence against oils and organics, generally the realization of synthetic materials with self-cleaning and antisticking abilities in dynamic phenomena, such as spray impact, remains an unsolved task. Moreover, it is demonstrated that the chemistry of the surface, the physicochemical interactions with the liquid drops, and the possible wettability gradient of the surface asperity also play important roles in determining the critical Weber number above which impalement occurs. Therefore, the classical numerical simulations of drop impact on dry surfaces are definitively not able to capture the final outcomes of the impact for all possible fluids if the surface topology and chemistry and/or the wettability gradient in the surface structure are not properly reflected.

Size Evolution of Ordered SiGe Islands Grown by Surface Thermal Diffusion on Pit-Patterned Si(100) Surface.

The ordered growth of self-assembled SiGe islands by surface thermal diffusion in ultra high vacuum from a lithographically etched Ge stripe on pit-patterned Si(100) surface has been experimentally investigated. The total surface coverage of Ge strongly depends on the distance from the source stripe, as quantitatively verified by Scanning Auger Microscopy. The size distribution of the islands as a function of the Ge coverage has been studied by coupling atomic force microscopy scans with Auger spectro-microscopy data. Our observations are consistent with a physical scenario where island positioning is essentially driven by energetic factors, which predominate with respect to the local kinetics of diffusion, and the growth evolution mainly depends on the local density of Ge atoms.