On-chip transporting arresting and characterizing individual nano-objects in biological ionic liquids.

Understanding and controlling the individual behavior of nanoscopic matter in liquids, the environment in which many such entities are functioning, is both inherently challenging and important to many natural and man-made applications. Here, we transport individual nano-objects, from an assembly in a biological ionic solution, through a nanochannel network and confine them in electrokinetic nanovalves, created by the collaborative effect of an applied ac electric field and a rationally engineered nanotopography, locally amplifying this field. The motion of so-confined fluorescent nano-objects is tracked, and its kinetics provides important information, enabling the determination of their particle diffusion coefficient, hydrodynamic radius, and electrical conductivity, which are elucidated for artificial polystyrene nanospheres and subsequently for sub-100-nm conjugated polymer nanoparticles and adenoviruses. The on-chip, individual nano-object resolution method presented here is a powerful approach to aid research and development in broad application areas such as medicine, chemistry, and biology.

A Plasmonic Painter's Method of Color Mixing for a Continuous Red-Green-Blue Palette.

The ability of mixing colors with remarkable results had long been exclusive to the talents of master painters. By finely combining colors in different amounts on the palette, intuitively, they obtain smooth gradients with any given color. Creating such smooth color variations through scattering by the structural patterning of a surface, as opposed to color pigments, has long remained a challenge. Here, we borrow from the painter's approach and demonstrate color mixing generated by an optical metasurface. We propose a single-layer plasmonic color pixel and a method for nanophotonic structural color mixing based on the additive red-green-blue (RGB) color model. The color pixels consist of plasmonic nanorod arrays that generate vivid primary colors and enable independent control of color brightness without affecting chromaticity by simply varying geometric in-plane parameters. By interleaving different nanorod arrays, we combine up to three primary colors on a single pixel. Based on this, two- and three-color mixing is demonstrated, enabling the continuous coverage of a plasmonic RGB color gamut and yielding a palette with a virtually unlimited number of colors. With this multiresonant color pixel, we show the photorealistic printing of color and monochrome images at the nanoscale, with ultrasmooth transitions in color and brightness. Our color-mixing approach can be applied to a broad range of scatterer designs and materials and has the potential to be used for multiwavelength color filters and dynamic photorealistic displays.

Wetting transitions in droplet drying on soft materials.

Droplet interactions with compliant materials are familiar, but surprisingly complex processes of importance to the manufacturing, chemical, and garment industries. Despite progress-previous research indicates that mesoscopic substrate deformations can enhance droplet drying or slow down spreading dynamics-our understanding of how the intertwined effects of transient wetting phenomena and substrate deformation affect drying remains incomplete. Here we show that above a critical receding contact line speed during drying, a previously not observed wetting transition occurs. We employ 4D confocal reference-free traction force microscopy (cTFM) to quantify the transient displacement and stress fields with the needed resolution, revealing high and asymmetric local substrate deformations leading to contact line pinning, illustrating a rate-dependent wettability on viscoelastic solids. Our study has significance for understanding the liquid removal mechanism on compliant substrates and for the associated surface design considerations. The developed methodology paves the way to study complex dynamic compliant substrate phenomena.

Self-Sustained Cascading Coalescence in Surface Condensation.

Sustained dropwise condensation of water requires rapid shedding of condensed droplets from the surface. Here, we elucidate a microfluidic mechanism that spontaneously sweeps condensed microscale droplets without the need for the traditional droplet removal pathways such as use of superhydrophobicity for droplet rolling and jumping and utilization of wettability gradients for directional droplet transport among others. The mechanism involves self-generated, directional, cascading coalescence sequences of condensed microscale droplets along standard hydrophobic microgrooves. Each sequence appears like a spontaneous zipping process, can sweep droplets along the microgroove at speeds of up to ∼1 m/s, and can extend for lengths more than 100 times the microgroove width. We investigate this phenomenon through high-speed in situ microscale condensation observations and demonstrate that it is enabled by rapid oscillations of a condensate meniscus formed locally in a filled microgroove and pinned on its edges. Such oscillations are in turn spontaneously initiated by coalescence of an individual droplet growing on the ridge with the microgroove meniscus. We quantify the coalescence cascades by characterizing the size distribution of the swept droplets and propose a simple analytical model to explain the results. We also demonstrate that, as condensation proceeds on the hydrophobic microgrooved surface, the coalescence cascades recur spontaneously through repetitive dewetting of the microgrooves. Lastly, we identify surface design rules for consistent realization of the cascades. The hydrophobic microgrooved textures required for the activation of this mechanism can be realized through conventional, scalable surface fabrication methods on a broad range of materials (we demonstrate with aluminum and silicon), thus promising direct application in a host of phase-change processes.

Nanoprinting organic molecules at the quantum level.

Organic compounds present a powerful platform for nanotechnological applications. In particular, molecules suitable for optical functionalities such as single photon generation and energy transfer have great promise for complex nanophotonic circuitry due to their large variety of spectral properties, efficient absorption and emission, and ease of synthesis. Optimal integration, however, calls for control over position and orientation of individual molecules. While various methods have been explored for reaching this regime in the past, none satisfies requirements necessary for practical applications. Here, we present direct non-contact electrohydrodynamic nanoprinting of a countable number of photostable and oriented molecules in a nanocrystal host with subwavelength positioning accuracy. We demonstrate the power of our approach by writing arbitrary patterns and controlled coupling of single molecules to the near field of optical nanostructures. Placement precision, high yield and fabrication facility of our method open many doors for the realization of novel nanophotonic devices.

Transparent Metasurfaces Counteracting Fogging by Harnessing Sunlight.

Surface fogging is a common phenomenon that can have significant and detrimental effects on surface transparency and visibility. It affects the performance in a wide range of applications including windows, windshields, electronic displays, cameras, mirrors, and eyewear. A host of ongoing research is aimed at combating this problem by understanding and developing stable and effective antifogging coatings that are capable of handling a wide range of environmental challenges "passively" without consumption of electrical energy. Here we introduce an alternative approach employing sunlight to go beyond state-of-the-art techniques, such as superhydrophilic and superhydrophobic coatings, by rationally engineering solar absorbing metasurfaces that maintain transparency, while upon illumination induce localized heating to significantly delay the onset of surface fogging or decrease defogging time. For the same environmental conditions, we demonstrate that our metasurfaces are able to reduce defogging time by up to 4-fold and under supersaturated conditions inhibit the nucleation of condensate outperforming conventional state-of-the-art approaches in terms of visibility retention. Our research illustrates a durable and environmentally sustainable approach to passive antifogging and defogging for transparent surfaces. This work opens up the opportunity for large-scale manufacturing that can be applied to a range of materials, including polymers and other flexible substrates.

Desublimation Frosting on Nanoengineered Surfaces.

Ice nucleation from vapor presents a variety of challenges across a wide range of industries and applications including refrigeration, transportation, and energy generation. However, a rational comprehensive approach to fabricating intrinsically icephobic surfaces for frost formation-both from water condensation (followed by freezing) and in particular from desublimation (direct growth of ice crystals from vapor)-remains elusive. Here, guided by nucleation physics, we investigate the effect of material composition and surface texturing (atomically smooth to nanorough) on the nucleation and growth mechanism of frost for a range of conditions within the sublimation domain (0 °C to -55 °C; partial water vapor pressures 6 to 0.02 mbar). Surprisingly, we observe that on silicon at very cold temperatures-below the homogeneous ice solidification nucleation limit (<-46 °C)-desublimation does not become the favorable pathway to frosting. Furthermore, we show that surface nanoroughness makes frost formation on silicon more probable. We experimentally demonstrate at temperatures between -48 °C and -55 °C that nanotexture with radii of curvature within 1 order of magnitude of the critical radius of nucleation favors frost growth, facilitated by capillary condensation, consistent with Kelvin's equation. Our findings show that such nanoscale surface morphology imposed by design to impart desired functionalities-such as superhydrophobicity-or from defects can be highly detrimental for frost icephobicity at low temperatures and water vapor partial pressures (<0.05 mbar). Our work contributes to the fundamental understanding of phase transitions well within the equilibrium sublimation domain and has implications for applications such as travel, power generation, and refrigeration.

Metasurfaces Leveraging Solar Energy for Icephobicity.

Inhibiting ice accumulation on surfaces is an energy-intensive task and is of significant importance in nature and technology where it has found applications in windshields, automobiles, aviation, renewable energy generation, and infrastructure. Existing methods rely on on-site electrical heat generation, chemicals, or mechanical removal, with drawbacks ranging from financial costs to disruptive technical interventions and environmental incompatibility. Here we focus on applications where surface transparency is desirable and propose metasurfaces with embedded plasmonically enhanced light absorption heating, using ultrathin hybrid metal-dielectric coatings, as a passive, viable approach for de-icing and anti-icing, in which the sole heat source is renewable solar energy. The balancing of transparency and absorption is achieved with rationally nanoengineered coatings consisting of gold nanoparticle inclusions in a dielectric (titanium dioxide), concentrating broadband absorbed solar energy into a small volume. This causes a > 10 °C temperature increase with respect to ambient at the air-solid interface, where ice is most likely to form, delaying freezing, reducing ice adhesion, when it occurs, to negligible levels (de-icing) and inhibiting frost formation (anti-icing). Our results illustrate an effective unexplored pathway toward environmentally compatible, solar-energy-driven icephobicity, enabled by respectively tailored plasmonic metasurfaces, with the ability to design the balance of transparency and light absorption.

Single entity resolution valving of nanoscopic species in liquids.

Investigating biological and synthetic nanoscopic species in liquids, at the ultimate resolution of single entity, is important in diverse fields1-5. Progress has been made6-10, but significant barriers need to be overcome such as the need for intense fields, the lack of versatility in operating conditions and the limited functionality in solutions of high ionic strength for biological applications. Here, we demonstrate switchable electrokinetic nanovalving able to confine and guide single nano-objects, including macromolecules, with sizes down to around 10 nanometres, in a lab-on-chip environment. The nanovalves are based on spatiotemporal tailoring of the potential energy landscape of nano-objects using an electric field, modulated collaboratively by wall nanotopography and by embedded electrodes in a nanochannel system. We combine nanovalves to isolate single entities from an ensemble, and demonstrate their guiding, confining, releasing and sorting. We show on-demand motion control of single immunoglobulin G molecules, quantum dots, adenoviruses, lipid vesicles, dielectric and metallic particles, suspended in electrolytes with a broad range of ionic strengths, up to biological levels. Such systems can enable nanofluidic, large-scale integration and individual handling of multiple entities in applications ranging from single species characterization and screening to in situ chemical or biochemical synthesis in continuous on-chip processes.

Spontaneous self-dislodging of freezing water droplets and the role of wettability.

Spontaneous removal of liquid, solidifying liquid and solid forms of matter from surfaces, is of significant importance in nature and technology, where it finds applications ranging from self-cleaning to icephobicity and to condensation systems. However, it is a great challenge to understand fundamentally the complex interaction of rapidly solidifying, typically supercooled, droplets with surfaces, and to harvest benefit from it for the design of intrinsically icephobic materials. Here we report and explain an ice removal mechanism that manifests itself simultaneously with freezing, driving gradual self-dislodging of droplets cooled via evaporation and sublimation (low environmental pressure) or convection (atmospheric pressure) from substrates. The key to successful self-dislodging is that the freezing at the droplet free surface and the droplet contact area with the substrate do not occur simultaneously: The frozen phase boundary moves inward from the droplet free surface toward the droplet-substrate interface, which remains liquid throughout most of the process and freezes last. We observe experimentally, and validate theoretically, that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, the droplet lifts from the substrate due to its incompressibility, density differences, and the asymmetric freezing dynamics with inward solidification causing not fully frozen mass to be displaced toward the unsolidified droplet-substrate interface. Depending on surface topography and wetting conditions, we find that this can lead to full dislodging of the ice droplet from a variety of engineered substrates, rendering the latter ice-free.

Ink-Free Reversible Optical Writing in Monolayers by Polymerization of a Trifunctional Monomer: Toward Rewritable "Molecular Paper".

A Langmuir-Blodgett film consisting of a dense array of trifunctional monomers bearing three 1,8-diazaanthracene units is polymerized at an air/water interface or after transfer on solid substrates. The transfer does not affect the excimer fluorescence of the film, indicating that the monomers' packing with their diazaanthracene units stacked face-to-face is retained-a prerequisite for successful polymerization. The monomer film can be polymerized in confined areas on solid substrates by UV irradiation with a confocal microscope laser. The underlying chemistry of the polymerization, a [4+4]-cycloaddition of the diazaanthracene units, leads to disappearance of the fluorescence in the irradiated regions which enables writing into the monolayer on a µm scale-thus the term "molecular paper." The reaction can be reversed by heating which leads to a recovery of the fluorescence and to erasing of the writing. Alternative pathways for this phenomenon are discussed and control experiments are conducted to rule them out.

Detergency and Its Implications for Oil Emulsion Sieving and Separation.

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil-water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water-soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro-/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh-and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of w ≈ d is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter-without any additional surface engineering or functionalization-to separate oil droplets (d < 100 µm) from a surfactant-stabilized emulsion with a flux of ∼11,000 L m-2 h-1 bar-1. We believe these findings can inform the design of future oil separation materials.

A Rapid Response Thin-Film Plasmonic-Thermoelectric Light Detector.

Light detection and quantification is fundamental to the functioning of a broad palette of technologies. While expensive avalanche photodiodes and superconducting bolometers are examples of detectors achieving single-photon sensitivity and time resolutions down to the picosecond range, thermoelectric-based photodetectors are much more affordable alternatives that can be used to measure substantially higher levels of light power (few kW/cm2). However, in thermoelectric detectors, achieving broadband or wavelength-selective performance with high sensitivity and good temporal resolution requires careful design of the absorbing element. Here, combining the high absorptivity and low heat capacity of a nanoengineered plasmonic thin-film absorber with the robustness and linear response of a thermoelectric sensor, we present a hybrid detector for visible and near-infrared light achieving response times of the order of 100 milliseconds, almost four times shorter than the same thermoelectric device covered with a conventional absorber. Furthermore, we show an almost two times higher light-to-electricity efficiency upon replacing the conventional absorber with a plasmonic absorber. With these improvements, which are direct results of the efficiency and ultra-small thickness of the plasmonic absorber, this hybrid detector constitutes an ideal component for various medium-intensity light sensing applications requiring spectrally tailored absorption coatings with either broadband or narrowband characteristics.

Printable Nanoscopic Metamaterial Absorbers and Images with Diffraction-Limited Resolution.

The fabrication of functional metamaterials with extreme feature resolution finds a host of applications such as the broad area of surface/light interaction. Nonplanar features of such structures can significantly enhance their performance and tunability, but their facile generation remains a challenge. Here, we show that carefully designed out-of-plane nanopillars made of metal-dielectric composites integrated in a metal-dielectric-nanocomposite configuration can absorb broadband light very effectively. We further demonstrate that electrohydrodynamic printing in a rapid nanodripping mode is able to generate precise out-of-plane forests of such composite nanopillars with deposition resolutions at the diffraction limit on flat and nonflat substrates. The nanocomposite nature of the printed material allows the fine-tuning of the overall visible light absorption from complete absorption to complete reflection by simply tuning the pillar height. Almost perfect absorption (∼95%) over the entire visible spectrum is achieved by a nanopillar forest covering only 6% of the printed area. Adjusting the height of individual pillar groups by design, we demonstrate on-demand control of the gray scale of a micrograph with a spatial resolution of 400 nm. These results constitute a significant step forward in ultrahigh resolution facile fabrication of out-of-plane nanostructures, important to a broad palette of light design applications.

Rapid-response low infrared emission broadband ultrathin plasmonic light absorber.

Plasmonic nanostructures can significantly advance broadband visible-light absorption, with absorber thicknesses in the sub-wavelength regime, much thinner than conventional broadband coatings. Such absorbers have inherently very small heat capacity, hence a very rapid response time, and high light power-to-temperature sensitivity. Additionally, their surface emissivity can be spectrally tuned to suppress infrared thermal radiation. These capabilities make plasmonic absorbers promising candidates for fast light-to-heat applications, such as radiation sensors. Here we investigate the light-to-heat conversion properties of a metal-insulator-metal broadband plasmonic absorber, fabricated as a free-standing membrane. Using a fast IR camera, we show that the transient response of the absorber has a characteristic time below 13 ms, nearly one order of magnitude lower than a similar membrane coated with a commercial black spray. Concurrently, despite the small thickness, due to the large absorption capability, the achieved absorbed light power-to-temperature sensitivity is maintained at the level of a standard black spray. Finally, we show that while black spray has emissivity similar to a black body, the plasmonic absorber features a very low infra-red emissivity of almost 0.16, demonstrating its capability as selective coating for applications with operating temperatures up to 400°C, above which the nano-structure starts to deform.

Proximal gap-plasmon nanoresonators in the limit of vanishing inter-cavity separation.

Nanostructured metal-insulator-metal (MIM) metasurfaces supporting gap-plasmons (GPs) show great promise due to their ability to manipulate or concentrate light at the nanoscale, which is of importance to a broad palette of technologies. The interaction between individual, proximal GP nanoresonators, reaching the point of first electrical connection, can have unexpected, important consequences and remains unexplored. Here we study the optical properties of a GP-metasurface in the limit of diminishing spacing between GP nanocavities and show that it maintains its nanoresonator array character, with negligible GP interaction, even at extremely close proximity between cavities. Then, at the point where inter-cavity electrical connection is first established, the surface abruptly transforms into a patterned metal-insulator-metal waveguide. We report detailed experimental evidence and explain the underlying physics through computational modeling and based on the properties and inherent symmetries of the electromagnetic field of the investigated metasurface. The novel phenomenon explored here can have a host of potential applications in the field of active plasmonic metamaterials, plasmonic photocatalysis and ultra-sensitive sensors.

Facile multifunctional plasmonic sunlight harvesting with tapered triangle nanopatterning of thin films.

Plasmonic absorbers have recently become important for a broad spectrum of sunlight-harvesting applications exploiting either heat generation, such as in thermal photovoltaics and solar thermoelectrics, or hot-electron generation, such as in photochemical and solid state devices. So far, despite impressive progress, combining the needed high performance with fabrication simplicity and scalability remains a serious challenge. Here, we report on a novel solar absorber concept, where we demonstrate and exploit simultaneously a host of absorption phenomena in tapered triangle arrays integrated in a metal-insulator-metal configuration to achieve ultrabroadband (88% average absorption in the range of 380-980 nm), wide-angle and polarization-insensitive absorption. Furthermore, this absorber is subwavelength in thickness (260 nm) and its fabrication is based on a facile, low-cost and potentially scalable method. In addition, the geometry of our design makes it compatible for both heat and hot electron generation.

A simple, versatile method for GFP-based super-resolution microscopy via nanobodies.

We developed a method to use any GFP-tagged construct in single-molecule super-resolution microscopy. By targeting GFP with small, high-affinity antibodies coupled to organic dyes, we achieved nanometer spatial resolution and minimal linkage error when analyzing microtubules, living neurons and yeast cells. We show that in combination with libraries encoding GFP-tagged proteins, virtually any known protein can immediately be used in super-resolution microscopy and that simplified labeling schemes allow high-throughput super-resolution imaging.

Resolution and enhancement in nanoantenna-based fluorescence microscopy.

Single gold nanoparticles can act as nanoantennas for enhancing the fluorescence of emitters in their near fields. Here we present experimental and theoretical studies of scanning antenna-based fluorescence microscopy as a function of the diameter of the gold nanoparticle. We examine the interplay between fluorescence enhancement and spatial resolution and discuss the requirements for deciphering single molecules in a dense sample. Resolutions better than 20 nm and fluorescence enhancement up to 30 times are demonstrated experimentally. By accounting for the tip shaft and the sample interface in finite-difference time-domain calculations, we explain why the measured fluorescence enhancements are higher in the presence of an interface than the values predicted for a homogeneous environment.