Shunsheng Ye

Optical properties of SiO2@M (M = Au, Pd, Pt) core–shell nanoparticles: material dependence and damping mechanisms

Optical properties of SiO2@Pd and SiO2@Pt core–shell nanoparticles (NPs) are investigated experimentally and theoretically, combined with a systematic comparison with those of traditionally investigated SiO2@Au NPs. Theoretical calculations show that both the plasmon hybridization effect and the retardation effect influence the spectral peak position of all three kinds of core–shell NPs. These two effects compete with each other. Plasmon hybridization is a dominant influence in the case of a thinner shell, whereas the retardation effect becomes more important when the shell becomes thicker. As a result, the dipolar plasmon band reveals an initial blue shift, and then red shift, with the increase of shell thickness. Furthermore, the influences of core size and shell material on the competition are discussed. Finally, the relative strengths of absorption and scattering associated with the LSPR of the three kinds of core–shell NPs are investigated. For SiO2@Pd and SiO2@Pt NPs, extinction is found to be dominated by absorption when shell thickness is less than ∼20 nm, and a crossover from absorption dominance to scattering dominance takes place with the further increase of shell thickness. In contrast, scattering is always the main decay process for SiO2@Au NPs, which contribute more than 90% intensity of the extinction despite the shell thickness.

Panchromatic plasmonic color patterns: from embedded Ag nanohole arrays to elevated Ag nanohole arrays

Plasmonic structural color is one of the most fascinating applications of recently fast developing plasmonics, which is a promising candidate technology for information processing, color displays and optical measurement devices. However, the implementation of plasmonic structural color in modern optical and spectral imaging systems demands strongly a simple fabrication process, low power consumption, and complex color pattern integration. Thus far, tuning of the plasmonic color has been generally achieved by morphology alteration or adjusting the lattice constants of plasmonic nanostructures. Nevertheless, this strategy suffers greatly from high cost and low throughput when designing complex color patterns. Herein, by precisely controlling the refractive indices on two sides of Ag nanohole arrays (NAs) that are embedded between a silica coating and a glass substrate, we are capable of filtering white light into individual colors across the entire visible band. Moreover, the straightforward strategy we propose is compatible with the traditional photolithography process, with which complex color patterns can be easily achieved. We further experimentally demonstrate that these engineered colored samples can be used as chromatically switchable anti-counterfeit tags. We anticipate that the method we demonstrate can provide a new approach for the fabrication of compact color filtering devices.

Morphology-Patterned Anisotropic Wetting Surface for Fluid Control and Gas–Liquid Separation in Microfluidics

This article shows morphology-patterned stripes as a new platform for directing flow guidance of the fluid in microfluidic devices. Anisotropic (even unidirectional) spreading behavior due to anisotropic wetting of the underlying surface is observed after integrating morphology-patterned stripes with a Y-shaped microchannel. The anisotropic wetting flow of the fluid is influenced by the applied pressure, dimensions of the patterns, including the period and depth of the structure, and size of the channels. Fluids with different surface tensions show different flowing anisotropy in our microdevice. Moreover, the morphology-patterned surfaces could be used as a microvalve, and gas-water separation in the microchannel was realized using the unidirectional flow of water. Therefore, benefiting from their good performance and simple fabrication process, morphology-patterned surfaces are good candidates to be applied in controlling the fluid behavior in microfluidics.

Nanotransfer printing of gold disk, ring and crescent arrays and their IR range optical properties

We demonstrate a facile method to fabricate gold plasmonic microstructures based on the combination of colloidal lithography and a nanotransfer printing method. Poly(dimethylsiloxane) PDMS hemisphere arrays were fabricated through colloidal lithography and used as a “stamp” for the nanotransfer printing. Three kinds of plasmonic microstructures, gold disk, ring and crescent arrays, were fabricated by transferring gold “ink” onto the PDMS stamp, then to the substrate based on covalent “glue”. By adjusting the pressure applied during the printing process, the diameter of the as-prepared gold disks and gold rings can be precisely controlled, and these plasmonic arrays all exhibited significant diameter dependent LSPR properties in the NIR or Mid-IR range. In addition, by obliquely depositing gold ink onto the PDMS stamp, a gold crescent array with asymmetrical geometry was also prepared on the substrate. Owing to the asymmetric structure of the gold crescents, the gold crescent array showed significant polarization dependent LSPR properties in the Mid-IR range. We believe that these as-prepared gold plasmonic microstructures could show promising potential for application as real-time, label-free plasmonic sensing platforms in the IR range.

The fabrication of long-range ordered nanocrescent structures based on colloidal lithography and parallel imprinting

A method for fabricating nanocrescent structures is presented based on a combination of colloidal lithography and parallel imprinting. In this process, non-close-packed colloidal spheres were prepared by a simple lift-up soft lithography technique, and subsequently the individual particles were used as shadow masks to angle deposit a layer of silver on the silicon substrates. Then, the silver-coated samples were etched to get silicon crescent nanohole arrays, which served as templates to mold patterned photocurable resin membranes. The patterned photocurable resin membranes were used to print gold nanocrescent nanostructures onto glass substrates. The size of the opening and the width of the gold nanostructures could be freely adjusted by changing the azimuth angle and tilt angle. Very importantly, the central angle of the nanocrescents could be adjusted in the range of 0°-360°. This method provides a low-cost and highly reproducible way to prepare complex nanostructure arrays for applications related to near field enhancement materials, optical sensors and surface-enhanced Raman spectroscopy, etc.

Multifunctional Reversible Fluorescent Controller Based on a One-Dimensional Photonic Crystal

With the aim to build a multifunctional solid fluorescent controller, a one-dimensional photonic crystal and CdSe fluorescent single layer were separated on the opposite sides of quartz substrates. The separation structure remarkably facilitates materials selection for the fluorescent controller, which allows one to freely choose the fluorescent substance and constituents of 1DPC from a wide range of available materials with the best desirable properties and without caring about the interactions between them. Fluorescent enhancement and weakened effect were successfully achieved when the excitation light was irradiated from different sides of the fluorescent device. In addition, the fluorescent intensity can be altered reversibly along with environmental pH values according to the change of a pH-responsive one-dimensional photonic crystal layer, which is quite different from a previously reported quenching mode. Meanwhile, the original position of the photonic stop band is essential for deciding what pH value would produce the best effect of fluorescent control. It provides a way to adjust the effect of fluorescent controller according to certain applied situations. The mechanism of fluorescent variation was confirmed by the assistance of a finite-difference time-domain simulation. Furthermore, this device is also able to modulate fluorescent wavelength and full width at half-maximum by overlapping the photonic stop band and the emission of CdSe. Therefore, this method offers a universal strategy for the fabrication of fluorescent controllers.

Unidirectional Wetting of Liquids on “Janus” Nanostructure Arrays under Various Media

We report the unidirectional wetting behavior of liquids (water and oil) on Janus silicon cylinder arrays (Si-CAs) under various media (air, water, and oil). The Janus cylinders were prepared by chemical modification of nanocylinders with different molecules on two sides. Through adjusting surface energies of the modified molecules, the as-prepared surfaces could control the wetting behavior of different types of liquids under various media. We discuss the regularity systematically and propose a strategy for preparing anisotropic wetting surfaces under arbitrary media. That is, to find two surface modification molecules with different surface energies, one of the molecules is easy to be wetted by the liquid under the corresponding media, while the other one is difficult. Additionally, by introducing thermal-responsive polymer brushes onto one part of Janus Si-CAs, the surfaces show thermal-responsive anisotropic wetting property under various media. We believe that due to the excellent unidirectional wettability under various media, the Janus surfaces could be applied in water/oil transportation, oil-repellent and self-cleaning coatings, water/oil separation, microfluidics, and so on.

Facile fabrication of homogeneous and gradient plasmonic arrays with tunable optical properties via thermally regulated surface charge density

A facile strategy is reported for the electrostatic self-assembly of homogeneous and gradient plasmonic nanoparticle arrays with tunable interparticle distances and optical properties. The interparticle distance is dominated by the surface charge density of the substrate, which is tuned via thermal annealing according to the temperature-dependent molecular mobility of the polymer. Oxygen plasma is employed to endow neutral polystyrene (PS) films with sufficient charges, enabling subsequent electrostatic adsorption. The density of surface charges can be readily tuned via a thermal annealing step after plasma treatment, which is confirmed by quantitative analyses of oxygen and nitrogen using X-ray photoelectron spectroscopy. Afterwards, PS films with regulated charge densities reshape the double layers around nanoparticles to various degrees during the assembly, leading to tunable interparticle separations. UV-Vis spectroscopy reveals tunable plasmonic properties owing to the critical role of interparticle separation in plasmon coupling. Here such structures are demonstrated to act as wavelength-selective substrates for multiplexed acquisition of surface enhanced Raman scattering. Alternatively, by applying a temperature gradient in the annealing step, we create a macroscopic surface with a continuous gradient in plasmonic properties. Such a “plasmonic library” can be a promising material for fast screening of interparticle distance or extinction spectrum in specific applications on one single substrate.

From 1D to 3D: a new route to fabricate tridimensional structures via photo-generation of silver networks

A rapid and cost effective method has been developed to fabricate 3 dimensional (3D) ordered structures by photo-generating silver networks inside a 1D layered heterogeneous laminate composed of poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA). By designing the photo-mask meticulously, the silver nanoparticles (NPs) produced by UV light aggregate to form frameworks in different forms, which perform as the anisotropic component, i.e. the building blocks, thus converting the 1D structure into 3D. Formation of silver NPs increases the refractive index (RI) of the PVA layers, thus bringing optical change to the 1D laminar structure, which allows us to trace the silver formation process by measuring the change of RI value and reflectance spectra. The 1D layered structure is a good building matrix for 3D construction because the total number of layers and the layer thickness can be finely tuned flexibly which allow us to further study the various properties caused by the structural modulation. By utilizing photo-reactive silver ions, an innovative strategy of fabricating 3D structures through in situ photo-generating silver building blocks inside the laminar matrix is established, which makes it possible to fabricate 3D micro/nano composite materials with potential applications in fields such as sensors and photonic meta-materials.

Highly sensitive deep-silver-nanowell arrays (d-AgNWAs) for refractometric sensing

Large-area deep-silver-nanowell arrays (d-AgNWAs) for plasmonic sensing were manufactured by combining colloidal lithography with metal deposition. In contrast to most previous studies, we shed light on the outstanding sensitivity afforded by deep metallic nanowells (up to 400 nm in depth). Using gold nanohole arrays as a mask, a silicon substrate was etched into deep silicon nanowells, which acted as a template for subsequent Ag deposition, resulting in the formation of d-AgNWAs. Various geometric parameters were separately tailored to study the changes in the optical performance and further optimize the sensing ability of the structure. After several rounds of selection, the best sensing d-AgNWA, which had a Ag thickness of 400 nm, template depth of 400 nm, hole diameter of 504 nm, and period of 1 μm, was selected. It had a sensitivity of 933 nm·RIU–1, which is substantially higher than those of most common thin metallic nanohole arrays. As a proof of concept, the as-prepared structure was employed as a substrate for an antigen-antibody recognition immunoassay, which indicates its great potential for label-free real-time biosensing.