Continuous-Gradient Plasmonic Nanostructures Fabricated by Evaporation on a Partially Exposed Rotating Substrate

A Microshutter for the Nanofabrication of Plasmonic Metal Alloys with Single Nanoparticle Composition Control

Alloying offers an increasingly important handle in nanomaterials design in addition to the already widely explored size and geometry of nanostructures of interest. As the key trait, the mixing of elements at the atomic level enables nanomaterials with physical or chemical properties that cannot be obtained by a single element alone, and subtle compositional variations can significantly impact these properties. Alongside the great potential of alloying, the experimental scrutiny of its impact on nanomaterial function is a challenge because the parameter space that encompasses nanostructure size, geometry, chemical composition, and structural atomic-level differences among individuals is vast and requires unrealistically large sample sets if statistically relevant and systematic data are to be obtained. To address this challenge, we have developed a microshutter device for spatially highly resolved physical vapor deposition in the lithography-based fabrication of nanostructured surfaces. As we demonstrate, it enables establishing compositional gradients across a surface with single nanostructure resolution in terms of alloy composition, which subsequently can be probed in a single experiment. As a showcase, we have nanofabricated arrays of AuAg, AuPd, and AgPd alloy nanoparticles with compositions systematically controlled at the level of single particle rows, as verified by energy dispersive X-ray and single particle plasmonic nanospectroscopy measurements, which we also compared to finite-difference time-domain simulations. Finally, motivated by their application in state-of-the-art plasmonic hydrogen sensors, we investigated PdAu alloy gradient arrays for their hydrogen sorption properties. We found distinctly composition-dependent kinetics and hysteresis and revealed a composition-dependent contribution of a single nanoparticle response to the ensemble average, which highlights the importance of alloy composition screening in single experiments with single nanoparticle resolution, as offered by the microshutter nanofabrication approach.

Plasmonic Nanostructure Engineering with Shadow Growth

Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.

Optimization of Colloidal Gold Nanoparticles on Porous Anodic Aluminum Oxide Substrates for Refractometric Sensing

A new composite metal-insulator-metal (MIM) system consisting of exceptionally dense non-close-packed (NCP) arrays of gold or silver nanoparticles, porous anodic aluminum oxide (PAAO), and bulk aluminum substrate interacts strongly with visible light and may become a very useful component for optical applications. The proposed MIM structure can be synthesized using accessible lithography-free chemical and physical processes (anodization and capillary force assisted colloidal particle deposition) that are suitable for the low-cost production of specialized devices. Here, we present a systematic study to determine the essential MIM structure parameters (nanoparticle size and PAAO layer thickness) for localized surface plasmon resonance (LSPR) refractometric sensing. A performance comparison was done by recording the spectra of scattered light upon angled illumination in media with different refractive indices. A clear advantage for maximizing the signal to background ratio was observed in the case of 60 and 80 nm Au nanoparticles with a PAAO thickness in a narrow range between 300 and 375 nm. Sensitivity exceeding a 200 nm peak wavelength shift per refractive index unit was found for 60 nm Au nanoparticles on approximately 500-nm-thick PAAO. The experimental observations were supported by finite-difference time-domain (FDTD) simulations.

Etching-free high-throughput intersectional nanofabrication of diverse optical nanoantennas for nanoscale light manipulation

The capabilities to manipulate light-matter interaction at the nanoscale lie at the core of many promising photonic applications. Optical nanoantennas, made of metallic or dielectric materials, have seen a rapid development for their remarkable optical properties facilitating the coupling of electromagnetic waves with subwavelength entities. However, high-throughput and cost-effective fabrication of these nanoantennas is still a daunting challenge. In this work, we provide a versatile nanofabrication method capable of producing large scale optical nanoantennas with different shapes. It is developed from colloidal lithography with no dry etching required. Furthermore, both metallic and all-dielectric nanoantennas can be readily fabrication in a high-throughput fashion. Au and Si nanodisks were fabricated and employed to assemble heterostructures with monolayer tungsten disulfide. Strong coupling is observed in both systems between plasmon modes (Au nanodisks) or anapole modes (Si nanodisks) with excitons. We believe that this nanofabrication method could find a wide range of applications with the diverse optical nanoantennas it can engineer.

Unveiling radial breathing mode in a particle-on-mirror plasmonic nanocavity

Plasmonic radial breathing mode (RBM), featured with radially oscillating charge density, arises from the surface plasmon waves confined in the flat nanoparticles. The zero net dipole moment endows the RBM with an extremely low radiation yet a remarkable intense local field. On the other hand, owing to the dark mode nature, the RBMs routinely escape from the optical measurements, severely preventing their applications in optoelectronics and nanophotonics. Here, we experimentally demonstrate the existence of RBM in a hexagonal Au nanoplate-on-mirror nanocavity using a far-field linear-polarized light source. The polarization-resolved scattering measurements cooperated with the full-wave simulations elucidate that the RBM originates from the standing plasmon waves residing in the Au nanoplate. Further numerical analysis shows the RBM possesses the remarkable capability of local field enhancement over the other dark modes in the same nanocavity. Moreover, the RBM is sensitive to the gap and nanoplate size of the nanocavity, providing a straightforward way to tailor the wavelength of RBM from the visible to near-infrared region. Our approach provides a facile optical path to access to the plasmonic RBMs and may open up a new route to explore the intriguing applications of RBM, including surface-enhanced Raman scattering, enhanced nonlinear effects, nanolasers, biological and chemical sensing.

Laser-Scanning-Guided Assembly of Quasi-3D Patterned Arrays of Plasmonic Dimers for Information Encryption

The application of plasmonic dimeric nanostructures in color displays, data storage, and especially metamaterials necessitates the patterning of dimers into ordered arrays, but controllable assembly of plasmonic nanoparticles into patterned dimer arrays on substrates still remains a challenge. Here, a facile laser-scanning-based strategy to fabricate quasi-3D patterned arrays of plasmonic nanoparticle dimers with controlled orientation for plasmonic information encryption is reported. Laser scanning of polymer-covered plasmonic nanoparticle (e.g., gold) arrays selectively exposes the surface of irradiated nanoparticle via localized photothermal heating, guiding the assembly of another type of nanoparticles onto the exposure nanoparticle surface to form dimers on substrates. This combined top-down/bottom-up approach is highly flexible in forming high-resolution patterns of plasmonic dimers from nanoparticles of different sizes and shapes. The z-axis orientation, interparticle spacing, and nanoparticle size and shape of plasmonic dimers can be precisely tuned, enabling the modulation of the coupled resonances of the dimer arrays. Moreover, it is demonstrated that the patterned dimer arrays can be used in information encryption where their plasmonic color can be repeatedly displayed and erased. This work provides an important addition to tools for the fabrication of patterned complex plasmonic nanostructures from as-synthesized nanoparticles with broad applications.

Room-temperature valleytronic transistor

Valleytronics, based on the valley degree of freedom rather than charge, is a promising candidate for next-generation information devices beyond complementary metal-oxide-semiconductor (CMOS) technology^1-4. Although many intriguing valleytronic properties have been explored based on excitonic injection or the non-local response of transverse current schemes at low temperature^4-7, demonstrations of valleytronic building blocks similar to transistors in electronics, especially at room temperature, remain elusive. Here, we report a solid-state device that enables a full sequence of generating, propagating, detecting and manipulating valley information at room temperature. Chiral nanocrescent plasmonic antennae⁸ are used to selectively generate valley-polarized carriers in MoS₂ through hot-electron injection under linearly polarized infrared excitation. These long-lived valley-polarized free carriers can be detected in a valley Hall configuration^9-11 even without charge current, and can propagate over 18 μm by means of drift. In addition, electrostatic gating allows us to modulate the magnitude of the valley Hall voltage. The electrical valley Hall output could drive the valley manipulation of a cascaded stage, rendering the device able to serve as a transistor free of charge current with pure valleytronic input/output. Our results demonstrate the possibility of encoding and processing information by valley degree of freedom, and provide a universal strategy to study the Berry curvature dipole in quantum materials.

Arrays of Plasmonic Nanoparticle Dimers with Defined Nanogap Spacers

Plasmonic molecules are building blocks of metallic nanostructures that give rise to intriguing optical phenomena with similarities to those seen in molecular systems. The ability to design plasmonic hybrid structures and molecules with nanometric resolution would enable applications in optical metamaterials and sensing that presently cannot be demonstrated, because of a lack of suitable fabrication methods allowing the structural control of the plasmonic atoms on a large scale. Here we demonstrate a wafer-scale "lithography-free" parallel fabrication scheme to realize nanogap plasmonic meta-molecules with precise control over their size, shape, material, and orientation. We demonstrate how we can tune the corresponding coupled resonances through the entire visible spectrum. Our fabrication method, based on glancing angle physical vapor deposition with gradient shadowing, permits critical parameters to be varied across the wafer and thus is ideally suited to screen potential structures. We obtain billions of aligned dimer structures with controlled variation of the spectral properties across the wafer. We spectroscopically map the plasmonic resonances of gold dimer structures and show that they not only are in good agreement with numerically modeled spectra, but also remain functional, at least for a year, in ambient conditions.

Patterned plasmonic gradient for high-precision biosensing using a smartphone reader

Smartphone-compatible biosensors are believed to be one of the key techniques for improving the quality of diagnosis in remote areas. However, to date, few smartphone-compatible biosensors can reach the specifications of their conventional counterparts due to the limitations of consumer-grade detectors carried by phones. To circumvent this issue, we reported a metasurface-inspired bio-sensor, patterned plasmonic gradient (PPG), which transduces local index information into 2D patterns. By harnessing the powerful imaging and computational capability of modern smartphones, the PPG is sensitive enough to detect tiny refractive index changes induced by a submonolayer of molecules with high precision (Δn < 0.001) in a large dynamic range. It allows us to monitor the conjugation process between biotin and a trace amount of streptavidin (15 nM, 20 μL) in real-time. With high sensitivity and accuracy, the PPG provides a high performance bio-sensing solution for the places where professional equipment is inaccessible.

Plasmon Excited Ultrahot Carriers and Negative Differential Photoresponse in a Vertical Graphene van der Waals Heterostructure

Photogenerated nonequilibrium hot carriers play a key role in graphene's intriguing optoelectronic properties. Compared to conventional photoexcitation, plasmon excitation can be engineered to enhance and control the generation and dynamics of hot carriers. Here, we report an unusual negative differential photoresponse of plasmon-induced "ultrahot" electrons in a graphene-boron nitride-graphene tunneling junction. We demonstrate nanocrescent gold plasmonic nanostructures that substantially enhance the absorption of long-wavelength photons whose energy is greatly below the tunneling barrier and significantly boost the electron thermalization in graphene. We further analyze the generation and transfer of ultrahot electrons under different bias and power conditions. We find that the competition among thermionic emission, the carrier-cooling effect, and the field effect results in a hitherto unusual negative differential photoresponse in the photocurrent-bias plot. Our results not only exemplify a promising platform for detecting low-energy photons, enhancing the photoresponse, and reducing the dark current but also reveal the critically coupled pathways for harvesting ultrahot carriers.

Advances in nanoplasmonic biosensors for clinical applications

Plasmonic biosensors can be conveniently used as portable diagnostic devices for attaining timely and cost-effective clinical outcomes. Nanoplasmonics technology opens the way for sensor miniaturization, multiplexing and point of care testing.

Variable Thickness Porous Anodic Alumina/Metal Film Bilayers for Optimization of Plasmonic Scattering by Nanoholes on Mirror

Continuously variable thickness porous anodic aluminum oxide (PAAO) films were obtained using electrochemical oxidation of bulk aluminum sheet while both electrodes were simultaneously withdrawn from the electrolyte solution. The thickness gradient was controlled by the withdrawal rate (1-10 mm/min range) and thickness variation demonstrated from below 50 nm to above 1 micrometer. The thickness increased linearly with the sample lateral coordinate, whereas the nanopore structure (diameter and interpore distance) remained unchanged. Effects of the initial pore growth and capillary forces are discussed. The presented method can be used for tuning optimal PAAO thickness for optical and other applications as exemplified by finding maximum plasmonic scattering in structured Al-PAAO-Au multilayers. Enhanced scattering from porous gold film separated by a specific-thickness PAAO layer from aluminum mirror surface is demonstrated.

SERS polarization-dependent effects for an ordered 3D plasmonic tilted silver nanorod array

Hexagonal close-packed tilted Ag nanorod arrays that exhibit excellent uniformity and reproducibility were prepared. The tilt angle was easily controlled by regulating the sputtering angle, accompanied by a reduction and constancy in the gap size of adjacent nanorods, which is 30° and 90° relative to the sputtering direction. The surface enhanced Raman spectroscopy (SERS) technique was used to characterize the interaction of tilted Ag nanorod arrays with polarized laser excitation. Interestingly, the SERS polarization-dependence increased with increasing tilt angle of the Ag nanorods. To elucidate the essential factors responsible for this SERS result, three-dimensional (3D) electromagnetic enhancement distribution for the proposed system was numerically simulated based on p- and s-polarization excitation. Most importantly, the fundamental reasons for the polarization dependence of SERS were obtained by a quantitative 3D numerical simulation of hotspot distribution for adjacent nanorods.

Advanced Plasmonic Materials for Dynamic Color Display

Plasmonic structures exhibit promising applications in high-resolution and durable color generation. Research on advanced hybrid plasmonic materials that allow dynamically reconfigurable color control has developed rapidly in recent years. Some of these results may give rise to practically applicable reflective displays in living colors with high performance and low power consumption. They will attract broad interest from display markets, compared with static plasmonic color printing, for example, in applications such as digital signage, full-color electronic paper, and electronic device screens. In this progress report, the most promising recent examples of utilizing advanced plasmonic materials for the realization of dynamic color display are highlighted and put into perspective. The performances, advantages, and disadvantages of different technologies are discussed, with emphasis placed on both the potential and possible limitations of various hybrid materials for dynamic plasmonic color display.

Switchable Plasmonic Metasurfaces with High Chromaticity Containing Only Abundant Metals

Plasmonic color generation offers several advantages but is also limited by the cost and availability of noble metals like gold. In this work, we present color-tunable metasurfaces with high chromaticity and reflectivity consisting of an aluminum mirror, a dielectric spacer, and a plasmonic nanohole array in copper. Copper is shown to be an excellent alternative to gold when properly protected from oxidation and makes it possible to generate a wide RGB gamut covering 27% of the standard RGB. By patterning the metasurfaces into microscale pixel triplets, color photos can be well reproduced with high resolution over wafer-sized areas. Further, we demonstrate active modulation of the reflected intensity using an electrochromic conductive polymer deposited on top of the nanostructures by screen printing. This technology opens up for ultrathin and flexible reflective displays in full color, that is, plasmonic electronic paper, compatible with large-scale sustainable production.

Controlled Patterning of Plasmonic Dimers by Using an Ultrathin Nanoporous Alumina Membrane as a Shadow Mask

We report on design and fabrication of patterned plasmonic dimer arrays by using an ultrathin anodic aluminum oxide (AAO) membrane as a shadow mask. This strategy allows for controllable fabrication of plasmonic dimers where the location, size, and orientation of each particle in the dimer pairs can be independently tuned. Particularly, plasmonic dimers with ultrasmall nanogaps down to the sub-10 nm scale as well as a large dimer density up to 1.0 × 10¹⁰ cm^-2 are fabricated over a centimeter-sized area. The plasmonic dimers exhibit significant surface-enhanced Raman scattering (SERS) enhancement with a polarization-dependent behavior, which is well interpreted by finite-difference time-domain (FDTD) simulations. Our results reveal a facile approach for controllable fabrication of large-area dimer arrays, which is of fundamental interest for plasmon-based applications in surface-enhanced spectroscopy, biochemical sensing, and optoelectronics.

Facile design of ultra-thin anodic aluminum oxide membranes for the fabrication of plasmonic nanoarrays

Ultra-thin anodic aluminum oxide (AAO) membranes are efficient templates for the fabrication of patterned nanostructures. Herein, a three-step etching method to control the morphology of AAO is described. The morphological evolution of the AAO during phosphoric acid etching is systematically investigated and a nonlinear growth mechanism during unsteady-state anodization is revealed. The thickness of the AAO can be quantitatively controlled from ∼100 nm to several micrometers while maintaining the tunablity of the pore diameter. The AAO membranes are robust and readily transferable to different types of substrates to prepare patterned plasmonic nanoarrays such as nanoislands, nanoclusters, ultra-small nanodots, and core-satellite superstructures. The localized surface plasmon resonance from these nanostructures can be easily tuned by adjusting the morphology of the AAO template. The custom AAO template provides a platform for the fabrication of low-cost and large-scale functional nanoarrays suitable for fundamental studies as well as applications including biochemical sensing, imaging, photocatalysis, and photovoltaics.

Plasmonic Metasurfaces with Conjugated Polymers for Flexible Electronic Paper in Color

A flexible electronic paper in full color is realized by plasmonic metasurfaces with conjugated polymers. An ultrathin large-area electrochromic material is presented which provides high polarization-independent reflection, strong contrast, fast response time, and long-term stability. This technology opens up for new electronic readers and posters with ultralow power consumption.

Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators

Three-dimensional chiral plasmonic nanostructures have been shown to be able to dramatically boost photon-spin selective light-matter interactions, potentially leading to novel photonics, molecular spectroscopy, and light-harvesting applications based on circularly polarized light. Here, we show that chiral split-ring gold nanoresonators interfaced to a wide band gap semiconductor exhibit a contrast in hot-electron transfer rate between left-handed and right-handed visible light that essentially mimics the far-field circular dichroism of the structures. We trace down the origin of this effect to the differential excitation of the thinnest part of the split-ring structures using dichroic-sensitive cathodoluminescence imaging with nanometer spatial resolution. The results highlight the intricate interplay between the near-field and far-field chiral response of a nanostructure and establishes a clear link to the emerging field of hot carrier plasmonics with numerous potential applications in photocatalysis and solar light harvesting.