Omnidirectional 3D nanoplasmonic optical antenna array via soft-matter transformation

Progress of shrink polymer micro- and nanomanufacturing

Traditional lithography plays a significant role in the fabrication of micro- and nanostructures. Nevertheless, the fabrication process still suffers from the limitations of manufacturing devices with a high aspect ratio or three-dimensional structure. Recent findings have revealed that shrink polymers attain a certain potential in micro- and nanostructure manufacturing. This technique, denoted as heat-induced shrink lithography, exhibits inherent merits, including an improved fabrication resolution by shrinking, controllable shrinkage behavior, and surface wrinkles, and an efficient fabrication process. These merits unfold new avenues, compensating for the shortcomings of traditional technologies. Manufacturing using shrink polymers is investigated in regard to its mechanism and applications. This review classifies typical applications of shrink polymers in micro- and nanostructures into the size-contraction feature and surface wrinkles. Additionally, corresponding shrinkage mechanisms and models for shrinkage, and wrinkle parameter control are examined. Regarding the size-contraction feature, this paper summarizes the progress on high-aspect-ratio devices, microchannels, self-folding structures, optical antenna arrays, and nanowires. Regarding surface wrinkles, this paper evaluates the development of wearable sensors, electrochemical sensors, energy-conversion technology, cell-alignment structures, and antibacterial surfaces. Finally, the limitations and prospects of shrink lithography are analyzed.

Laser additive nano-manufacturing under ambient conditions

Additive manufacturing at the macroscale has become a hot topic of research in recent years. It has been used by engineers for rapid prototyping and low-volume production. The development of such technologies at the nanoscale, or additive nanomanufacturing, will provide a future path for new nanotechnology applications. In this review article, we introduce several available toolboxes that can be potentially used for additive nanomanufacturing. We especially focus on laser-based additive nanomanufacturing under ambient conditions.

Tunable optical response of bowtie nanoantenna arrays on thermoplastic substrates

Thermally responsive polymers present an interesting avenue for tuning the optical properties of nanomaterials on their surfaces by varying their periodicity and shape using facile processing methods. Gold bowtie nanoantenna arrays are fabricated using nanosphere lithography on prestressed polyolefin (PO), a thermoplastic polymer, and optical properties are investigated via a combination of spectroscopy and electromagnetic simulations to correlate shape evolution with optical response. Geometric features of bowtie nanoantennas evolve by annealing at temperatures between 105 °C and 135 °C by releasing the degree of prestress in PO. Due to the higher modulus of Au versus PO, compressive stress occurs on Au bowtie regions on PO, which leads to surface buckling at the two highest annealing temperatures; regions with a 5 nm gap between bowtie nanoantennas are observed and the average reduction is 75%. Reflectance spectroscopy and full-wave electromagnetic simulations both demonstrate the ability to tune the plasmon resonance wavelength with a window of approximately 90 nm in the range of annealing temperatures investigated. Surface-enhanced Raman scattering measurements demonstrate that maximum enhancement is observed as the excitation wavelength approaches the plasmon resonance of Au bowtie nanoantennas. Both the size and morphology tunability offered by PO allows for customizing optical response.

Mechanically Self-Assembled, Three-Dimensional Graphene-Gold Hybrid Nanostructures for Advanced Nanoplasmonic Sensors

Hybrid structures of graphene and metal nanoparticles (NPs) have been actively investigated as higher quality surface enhanced Raman spectroscopy (SERS) substrates. Compared with SERS substrates, which only contain metal NPs, the additional graphene layer provides structural, chemical, and optical advantages. However, the two-dimensional (2D) nature of graphene limits the fabrication of the hybrid structure of graphene and NPs to 2D. Introducing three-dimensionality to the hybrid structure would allow higher detection sensitivity of target analytes by utilizing the three-dimensional (3D) focal volume. Here, we report a mechanical self-assembly strategy to enable a new class of 3D crumpled graphene-gold (Au) NPs hybrid nanoplasmonic structures for SERS applications. We achieve a 3D crumpled graphene-Au NPs hybrid structure by the delamination and buckling of graphene on a thermally activated, shrinking polymer substrate. We also show the precise control and optimization of the size and spacing of integrated Au NPs on crumpled graphene and demonstrate the optimized NPs' size and spacing for higher SERS enhancement. The 3D crumpled graphene-Au NPs exhibits at least 1 order of magnitude higher SERS detection sensitivity than that of conventional, flat graphene-Au NPs. The hybrid structure is further adapted to arbitrary curvilinear structures for advanced, in situ, nonconventional, nanoplasmonic sensing applications. We believe that our approach shows a promising material platform for universally adaptable SERS substrate with high sensitivity.

Magnetic field induced quantum dot brightening in liquid crystal synergized magnetic and semiconducting nanoparticle composite assemblies

The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce 'meta-materials' has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting quantum dots (QDs) are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (<100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold peak intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.

Electrochemically Created Highly Surface Roughened Ag Nanoplate Arrays for SERS Biosensing Applications

Highly surface-roughened Ag nanoplate arrays are fabricated using a simple electrodeposition and in situ electrocorrosion method with inorganic borate ions as capping agent. The electrocorrosion process is induced by a change in the local pH value during the electrochemical growth, which is used to intentionally carve the electrodeposited structures. The three dimensionally arranged Ag nanoplates are integrated with substantial surface-enhanced Raman scattering (SERS) hot spots and are free of organic contaminations widely used as shaping agents in previous works, making them excellent candidate substrates for SERS biosensing applications. The SERS enhancement factor of the rough Ag nanoplates is estimated to be > 109. These Ag nanoplate arrays are used for SERS-based analysis of DNA hybridization monitoring, protein detection, and virus differentiation without any additional surface modifications or labelling. They all exhibit an extremely high detection sensitivity, reliability, and reproducibility.

Thermally-induced miniaturization for micro- and nanofabrication: progress and updates

The field of micro- and nanofabrication has developed extensively in the past several decades with rising interest in alternative fabrication techniques. Growth of these areas has been driven by needs that remain unaddressed by traditional lithographical methods: inexpensive, upscalable, biocompatible, and easily integrated into complete lab-on-a-chip (LOC) systems. Shape memory polymers (SMPs) have been explored as an alternative substrate. This review first focuses on structure fabrication at the micron and nanoscale using specifically heat-shrinkable SMPs and highlights the innovative improvements to this technology in the past several years. The second part of the review illustrates demonstrated applications of these micro- and nanostructures fabricated from heat-shrinkable SMP films. The review concludes with a discussion about future prospects of heat-shrinkable SMP structures for integration into LOC systems.

Plasmonic nanocomposites: polymer-guided strategies for assembling metal nanoparticles

Noble metal nanoparticles that support localized surface plasmon resonances (LSPRs) have the unique ability to manipulate and confine light at subwavelength dimensions. Utilizing these capabilities in devices and coatings requires the controlled organization of metal nanoparticles into ordered or hierarchical structures. Polymer grafts can be used as assembly-regulating molecules that bind to the nanoparticle surface and guide nanoparticle organization in solution, at interfaces, and within condensed phases. Here, we present an overview of polymer-directed assembly of plasmonic nanoparticles. We discuss how polymer grafts can be used to control short-range nanoparticle interactions that dictate interparticle gap distance and orientation. We also discuss how condensed polymer grafts can be used to control long-range order within condensed nanoparticle-polymer blends. The assembly of shaped plasmonic nanoparticles that have potential applications in enhanced spectroscopy and optical metamaterials is highlighted. We end with a summary of promising new directions toward the fabrication of plasmonic nanocomposites that are responsive and possess three-dimensional order.

An optimal substrate design for SERS: dual-scale diamond-shaped gold nano-structures fabricated via interference lithography

Dual-scale diamond-shaped gold nanostructures (d-DGNs) with larger scale diamond-shaped gold nanoposts (DGNs) coupled to smaller scale gold nanoparticles have been fabricated via interference lithography as a highly reliable and efficient substrate for surface enhanced Raman scattering (SERS). The inter- and intra-particle plasmonic fields of d-DGNs are varied by changing the periodicity of the DGNs and the density of gold nanoparticles. Because of the two different length scales in the nanostructures, d-DGNs show multipole plasmonic peaks as well as dipolar plasmonic peaks, leading to a SERS enhancement factor of greater than 10(9). Simulations are carried out by finite-difference time-domain (FDTD) methods to evaluate the dependence of the inter- and intra-particle plasmonic field and the results are in good agreement with the experimentally obtained data. Our studies reveal that the combination of two different length scales is a straightforward approach for achieving reproducible and great SERS enhancement by light trapping in the diamond-shaped larger scale structures as well as efficient collective plasmon oscillation in the small size particles.

Angle-insensitive structural colours based on metallic nanocavities and coloured pixels beyond the diffraction limit

To move beyond colorant-based pigmentation display technologies, a variety of photonic and plasmonic crystal based structures have been designed and applied as colour filters. Nanostructure based colour filtering offers increased efficiencies, low power consumption, slim dimensions, and enhanced resolution. However, incident angle tolerance still needs to be improved. In this work, we propose a new scheme through localized resonance in metallic nanoslits by light funneling. Angle insensitive colour filters up to ±80 degrees have been achieved, capable of wide colour tunability across the entire visible band with pixel size beyond the diffraction limit (~λ/2). This work opens the door to angle insensitive manipulation of light with structural filtering.

Energetic-assisted scanning thermal lithography for patterning silver nanoparticles in polymer films

Energetic-assisted scanning thermal lithography (SThL) was demonstrated with the addition of benzoyl peroxide (BPO) for patterning silver nanoparticles. SThL samples were prepared by spin-coating poly(methyl methacrylate) (PMMA) thin films preloaded with BPO and silver nitrate precursors. Localized thermal analysis via probe heating demonstrated that the BPO decomposition in the polymer film took place at the temperature of 80 °C. Above this temperature, the thermal probe initiated the decomposition of the peroxide, which resulted in the in situ discharge of exothermal energy to compensate the joule shortage and the rapid cooling in the SThL thin film samples. The additional joule energy thermally enhanced the synthesis of silver nanoparticles, which were patterned and embedded in the PMMA thin film. Surface plasmon resonance scattering of these silver nanoparticles was observed by dark-field optical microscopy, whereas the nanoparticle distribution was examined by transmission electron microscopy. Variations in the scanning probe temperatures and peroxide concentrations were carefully investigated to optimize the thermal lithography efficiency upon the addition of energetics.

Laser patterning of conductive gold micronanostructures from nanodots

Gold nanodots were used as the precursory material to form micronanopatterns under pinpoint scanning by a tightly focused femtosecond laser beam. Different from the widely reported metal ions photoreduction mechanism, here gradient force in an optical trap generated around the laser focus is considered as the major mechanism for particle accumulation (focusing). It has been proven to be an effective method for gold micronanostructure fabrication, and the electronic resistivity of the resulting metals reached as high as 5.5 × 10(-8) Ω m, only twice that of the bulk material (2.4 × 10(-8) Ω m). This merit makes it a novel free interconnection technology for micronanodevice fabrication.