Using direct nanoimprinting of ferroelectric films to prepare devices exhibiting bi-directionally tunable surface plasmon resonances

Computational Sensing Using Low-Cost and Mobile Plasmonic Readers Designed by Machine Learning

Plasmonic sensors have been used for a wide range of biological and chemical sensing applications. Emerging nanofabrication techniques have enabled these sensors to be cost-effectively mass manufactured onto various types of substrates. To accompany these advances, major improvements in sensor read-out devices must also be achieved to fully realize the broad impact of plasmonic nanosensors. Here, we propose a machine learning framework which can be used to design low-cost and mobile multispectral plasmonic readers that do not use traditionally employed bulky and expensive stabilized light sources or high-resolution spectrometers. By training a feature selection model over a large set of fabricated plasmonic nanosensors, we select the optimal set of illumination light-emitting diodes needed to create a minimum-error refractive index prediction model, which statistically takes into account the varied spectral responses and fabrication-induced variability of a given sensor design. This computational sensing approach was experimentally validated using a modular mobile plasmonic reader. We tested different plasmonic sensors with hexagonal and square periodicity nanohole arrays and revealed that the optimal illumination bands differ from those that are "intuitively" selected based on the spectral features of the sensor, e.g., transmission peaks or valleys. This framework provides a universal tool for the plasmonics community to design low-cost and mobile multispectral readers, helping the translation of nanosensing technologies to various emerging applications such as wearable sensing, personalized medicine, and point-of-care diagnostics. Beyond plasmonics, other types of sensors that operate based on spectral changes can broadly benefit from this approach, including e.g., aptamer-enabled nanoparticle assays and graphene-based sensors, among others.

Flexible Plasmonic Sensors

Mechanical flexibility and the advent of scalable, low-cost, and high-throughput fabrication techniques have enabled numerous potential applications for plasmonic sensors. Sensitive and sophisticated biochemical measurements can now be performed through the use of flexible plasmonic sensors integrated into existing medical and industrial devices or sample collection units. More robust sensing schemes and practical techniques must be further investigated to fully realize the potentials of flexible plasmonics as a framework for designing low-cost, embedded and integrated sensors for medical, environmental, and industrial applications.

Electrostatically tunable plasmonic devices fabricated on multi-photon polymerized three-dimensional microsprings

Electrostatically tunable plasmonic devices on three-dimensional (3D) microsprings were fabricated using multi-photon polymerization followed by metal deposition. These plasmonic devices comprised a nanostructured Au microplate and two 3D microsprings. The maximum plasmon excitation efficiency was 35%, a value achieved with incident light of wavelength 632.8 nm. The efficiency could be continuously changed from almost zero to maximum by inclining the microplates with the application of DC voltage up to 50 V. Such dynamic functionality is useful for the realization of highly integrated optoelectronic devices and tunable metamaterials.

Ultrasensitive SERS detection of trinitrotoluene through capillarity-constructed reversible hot spots based on ZnO-Ag nanorod hybrids

A simple and efficient self-approach strategy was used to apply ultrasensitivity and self-revive ZnO-Ag hybrid surface-enhanced Raman scattering (SERS) sensors for the highly sensitive and selective detection of explosive TNT in both solution and vapour conditions. The good ultrasensitive sensing performance is a result of the abundant Raman hot spots, which were spontaneously formed in a reversible way by the self-approaching of flexible ZnO-Ag hybrid nanorods driven by the capillary force of solvent evaporation. Moreover, the enhancement effect was repeatedly renewed by the reconstruction of molecular bridges, which could selectively detect TNT with a lower limit of 4 × 10(-14) M. In addition, TNT vapor was also tested under this sensor, whereby once the ZnO-Ag NRs hybrid substrate was dipped in TNT, this substrate could detect the existence of TNT even in 5 detection cycles via a capillarity-constructed reversible hot spots approach. Compared with other pure Ag-based SERS sensors, this ZnO-Ag hybrid SERS sensor could rapidly self-revive SERS-activity by simple UV light irradiation and could retain stable SERS sensitivity for one month when used for TNT detection. This stable and ultrasensitive SERS substrate demonstrates a new route to eliminate the oxidized inactive problem of traditional Ag-based SERS substrates and suggests promising use in the applications of such hybrids as real-time online sensors for explosives detection.

A hanging plasmonic droplet: three-dimensional SERS hotspots for a highly sensitive multiplex detection of amino acids

3D hotspots in a hanging plasmonic droplet result in an ultrahigh Raman Scattering for the ultratrace and multiplex identification of amino acids.

Photovoltaic enhancement due to surface-plasmon assisted visible-light absorption at the inartificial surface of lead zirconate-titanate film

PZT film of 300 nm thickness was deposited on tin indium oxide (ITO) coated quartz by a sol-gel method. Four metal electrodes, such as Pt, Au, Cu and Ag, were used as top electrodes deposited on the same PZT film by sputtering at room temperature. In ITO-PZT-Ag and ITO-PZT-Au structures, the visible light (400-700 nm) can be absorbed partially by a PZT film, and the maximum efficiency of photoelectric conversion of the ITO-PZT-Ag structure was enhanced to 0.42% (100 mW cm(-2), AM 1.5G), which is about 15 times higher than that of the ITO-PZT-Pt structure. Numerical simulations show that the natural random roughness of polycrystalline-PZT-metal interface can offer a possibility of coupling between the incident photons and SPs at the metal surface. The coincidence between the calculated SP properties and the measured EQE spectra reveals the SP origin of the photovoltaic enhancement in these ITO-PZT-metal structures, and the improved photocurrent output is caused by the enhanced optical absorption in the PZT region near the metal surface, rather than by the direct charge-transfer process between two materials.

Enhancement of photocurrent in ferroelectric films via the incorporation of narrow bandgap nanoparticles

A novel nanostructured ferroelectric photovoltaic material, consisting of the ferroelectric lead zirconate titanate (PZT) film and Ag(2) O semiconductor nanoparticles of comparatively narrow bandgap, has demonstrated a remarkable enhancement in the photovoltaic effects and the highest light-electricity conversion efficiency among those PZT-based photodiodes previously reported. This work sheds light on the design and enhanced performance of new optoelectronic and solar energy devices.

A stretch-tunable plasmonic structure with a polarization-dependent response

We experimentally demonstrate a stretchable plasmonic structure composed of a monolayer array of gold semishells with dielectric cores on an elastic PDMS substrate. The composite structure is fabricated using simple and inexpensive self-assembly and transfer-printing techniques, and it supports Bragg-type surface plasmon resonances whose frequencies are sensitive to the arrangement of the metallic semishells. Under uniaxial stretching, the lattice symmetry of this plasmonic structure can be reconfigured from hexagonal to monoclinic, leading to resonance frequency shifts from 200 THz to 191 THz for the TM polarization and from 200 THz to 198 THz for the TE polarization with a strain up to 20%, respectively. Compared with previously reported tunable plasmonic structures, the reconfiguration of lattice symmetry offers a promising approach to tune the surface plasmon resonance with a polarization-dependent response at the standard telecommunication band, and such tunable plasmonic structure might be exploited in realizing photonic devices such as sensors, switches and filters.

Strain effects on the SERS enhancements for spherical silver nanoparticles

We demonstrate in the present work through the utilization of classical Mie scattering theory in conjunction with a radiation damping and dynamic depolarization-corrected electrostatic approximation the significant effect that mechanical strain has on the optical properties of spherical silver nanoparticles. Through appropriate modifications of the bulk dielectric functions, we find that the application of tensile strain generates significant enhancements in the local electric field for the silver nanoparticles, leading to large SERS enhancements of more than 300% compared to bulk, unstrained nanoparticles when a 5% tensile strain is applied. While the strain-induced SERS enhancements are found to be strongest for nanoparticle diameters where radiation damping effects are minimized, we find that the surface plasmon resonance wavelengths are relatively unchanged by mechanical strain, and that the various measures of the far field optical efficiencies (absorption, scattering, extinction) can be enhanced by up to 150% through the application of tensile strain. The present findings indicate the opportunity to actively engineer and enhance the optical properties of silver nanoparticles through the application of mechanical deformation.

Actively tuned plasmons on elastomerically driven Au nanoparticle dimers

We demonstrate a novel way to actively tune surface plasmons by fabricating plasmonic nanostructures on stretchable elastomeric films. This allows reversible modification of the metal geometry on the nanometer scale. Using 100 nm scale Au nanoparticle dimers whose spacing is stretch-tuned reveals radically different spectral tuning than previously reported for sub-10-nm nanoparticles, but which can be explained by a revised interpretation of existing models. Tuning plasmons in this way offers a much more robust way than lithography to interrogate the physics of localized plasmons and has applications in optimized surface-enhanced luminescence and Raman scattering.

Rapid nanoimprinting and excellent piezoresponse of polymeric ferroelectric nanostructures

Nanostructured ferroelectric patterns are promising for a wide range of applications, including sensing and actuation, data storage, photonics, spintronics, and energy conversion and storage. In this work, a rapid nanoimprinting technique is developed to pattern ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers in just 3 min, which exhibit excellent ferroelectricity and piezoresponse without any post-imprinting annealing. The effects of imprinting conditions have been thoroughly investigated, and the optimal imprinting parameters for excellent pattern transfer have been identified. The application of the imprinted polymeric patterns as a ferroelectric nonvolatile memory for data storage has also been demonstrated and discussed.

The morphology of Au@MgO nanopeapods

The structure of metal nanoparticles embedded inside dielectric nanowires/nanotubes, namely nanopeapods, has been of increasing interest due to their unusual photoresponse and optical adsorption properties. This paper presents a type of new inorganic nanopeapod: faceted Au nanoparticles inside MgO nanowires. The Au self-assembles into a nanoparticle chain during the vapor-liquid-solid growth of the MgO nanowires for which gold also serves as the catalyst. Surprisingly such a chain can follow the whole axis of the MgO nanowires even if the latter zigzag, provided that the amount of gold is sufficient. It is shown that such Au@MgO nanopeapods form not only under metalorganic chemical vapor deposition conditions (Lai et al 2009 Appl. Phys. Lett. 94 022904), but also under our conventional vapor transport deposition condition. This new nanopeapod material might be a candidate for the study of electronic and/or plasmonic wave transport along nanowires.