Mie Magnetic Structural Colors from Short Chains of TiO2 Nanospheres.

We demonstrate distinctive structural colors within a small footprint by using a short chain of nanospheres. Rather than using high-index materials like Si (n ∼ 4), which ensure strong modal confinement, TiO2 is employed. TiO2 has an intermediate index (n ∼ 2), promoting stronger modal coupling between the magnetic dipoles of each particle. This approach enables selective engineering of the magnetic response and yields larger spectral changes compared to that of Si. Despite the lower refractive index, the absence of absorption in TiO2 also produces higher scattering intensities than Si. We develop a quasistatic analytical model that describes the dipolar modal coupling in a trimer and use it to reveal distinct magnetic field strengths in the outer or central particle depending on the polarization of incident light. These results suggest pathways to manipulate the magnetic field in chains of particles and create vibrant structural colors with simple configurations.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review.

Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

Nanoscale Single-Element Color Filters.

Visible-light filters constructed from nanostructured materials typically consist of a metallic grating and rely on the excitation of surface plasmon polaritons (SPPs). In order to operate at full efficiency, the number of grating elements needs to be maximized such that light can couple more efficiently to the SPPs through improved diffraction. Such conditions impose a limitation on the compactness of the filter since a larger number of grating elements represents a larger effective size. For emerging applications involving nanoscale transmitters or receivers, a device that can filter localized excitations is highly anticipated but is challenging to realize through grating-type filters. In this work, we present the design of an optical filter operating with a single element, marking a departure from diffractive plasmonic coupling. Our device consists of a ZnO nanorod enclosed by two layers of Ag film. For diffraction-limited light focused on the nanorod, narrow passbands can be realized and tuned via variation of the nanorod diameter across the visible spectrum. The spectral and spatial filtering originates from scattering cancellation localized at the nanorod due to the cavity and nanorod exhibiting opposite effective dipole moments. This ability to realize high-performance optical filtering at the ultimate size introduces intriguing possibilities for nanoscale near-field communication or ultrahigh resolution imaging pixels.

Thickness measurements using photonic modes in monochromated electron energy-loss spectroscopy.

Characteristic energies of photonic modes are a sensitive function of a nanostructures' geometrical parameters. In the case of translationally invariant planar waveguides, the eigen-energies reside in the infrared to ultraviolet parts of the optical spectrum and they sensitively depend on the thickness of the waveguide. Using swift electrons and the inherent Cherenkov radiation in dielectrics, the energies of such photonic states can be effectively probed via monochromated electron energy-loss spectroscopy (EELS). Here, by exploiting the strong photonic signals in EELS with 200 keV electrons, we correlate the energies of waveguide peaks in the 0.5-3.5 eV range with planar thicknesses of the samples. This procedure enables us to measure the thicknesses of cross-sectional transmission electron microscopy samples over a 1-500 nm range and with best-case accuracies below ± 2%. The measurements are absolute with the only requirement being the optical dielectric function of the material. Furthermore, we provide empirical formulation for rapid and direct thickness estimations for a 50-500 nm range. We demonstrate the methodology for two semiconducting materials, silicon and gallium arsenide, and discuss how it can be applied to other dielectrics that produce strong optical fingerprints in EELS. The asymptotic form of the loss function for two-dimensional materials is also discussed.

Barrier height measurement of metal contacts to Si nanowires using internal photoemission of hot carriers.

Barrier heights between metal contacts and silicon nanowires were measured using spectrally resolved scanning photocurrent microscopy (SPCM). Illumination of the metal-semiconductor junction with sub-bandgap photons generates a photocurrent dominated by internal photoemission of hot electrons. Analysis of the dependence of photocurrent yield on photon energy enables quantitative extraction of the barrier height. Enhanced doping near the nanowire surface, mapped quantitatively with atom probe tomography, results in a lowering of the effective barrier height. Occupied interface states produce an additional lowering that depends strongly on diameter. The doping and diameter dependencies are explained quantitatively with finite element modeling. The combined tomography, electrical characterization, and numerical modeling approach represents a significant advance in the quantitative analysis of transport mechanisms at nanoscale interfaces that can be extended to other nanoscale devices and heterostructures.

Soft elastomeric nanopillar stamps for enhancing absorption in organic thin-film solar cells.

An elastomeric poly(dimethylsiloxane) (PDMS) block engraved with periodically arrayed nanopillars serves as a transferable light-trapping stamp for encapsulated organic thin-film solar cells. Diffracted light rays from the stamp interfere with one another and self-focus onto the active layer of the solar cell, generating enhanced absorption, as indicated in the current density-voltage measurements.

Diameter and polarization-dependent Raman scattering intensities of semiconductor nanowires.

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.

Directional Self-Assembly of Nanoparticles Coated with Thermoresponsive Block Copolymers and Charged Small Molecules.

Herein, we report the directional stimuli-responsive self-assembly of gold nanoparticles (AuNPs) coated with a thermoresponsive block copolymer (BCP), poly(ethylene glycol)-b-poly(N-isopropylacrylamide) (PEG-b-PNIPAM) and charged small molecules. AuNPs modified with PEG-b-PNIPAM possessing a AuNP/PNIPAM/PEG core/active/shell structure undergo temperature-induced self-assembly into one-dimensional (1D) or two-dimensional (2D) structures in salt solutions, with the morphology varying with the ionic strength of the medium. Salt-free self-assembly is also realized by modulating the surface charge by the codeposition of positively charged small molecules; 1D or 2D assemblies are formed depending on the ratio between the small molecule and PEG-b-PNIPAM, consistent with the trend observed with the bulk salt concentration. A series of charge-controlled self-assembly at various conditions revealed that the temperature-induced BCP-mediated self-assembly reported here provides an effective means for on-demand directional self-assembly of nanoparticles (NPs) with controlled morphology, interparticle distance, and optical properties, and the fixation of high-temperature structures.

Raman concentrators in Ge nanowires with dielectric coatings.

Raman spectroscopy is a powerful tool for investigating many fundamental properties of nanostructures, but extrinsic effects including background scattering and laser-induced heating can limit the analysis of intrinsic properties. A thin SiO2 dielectric coating is found to enhance the Raman signal from a single Ge nanowire by a factor of two as a result of wave interference. Consequently, the coated nanowire experiences less heating than a bare nanowire at equivalent signal intensities. The results demonstrate a simple and effective method to extend the limits of Raman analysis on single nanostructures and facilitate their characterization.

Spatially resolved plasmonically enhanced photocurrent from Au nanoparticles on a Si nanowire.

Semiconducting nanowires have been demonstrated as promising light-harvesting units with enhanced absorption compared to bulk films of equivalent volume. However, for small diameter nanowires, the ultrahigh aspect ratio constrains the absorption to be polarization selective by responding primarily to the transverse magnetic (TM) light. While this effect is useful for polarization-sensitive optoelectronic devices, practical light-harvesting applications demand efficient light absorption in both TM and transverse electric (TE) light. In this study, we engineer the polarization sensitivity and the charge carrier generation in a 50 nm Si nanowire by decorating the surface with plasmonic Au nanoparticles. Using scanning photocurrent microscopy (SPCM) with a tunable wavelength laser, we spatially and spectrally resolve the local enhancement in the TE photocurrent resulting from the plasmonic near-field response of individual nanoparticles and the broad-band enhancement due to surface-enhanced absorption. These results provide guidance to the development and the optimization of nanowire-nanoparticle light-harvesting systems.

Active modulation of reflective structural colors.

Dynamically controllable reflective structural colors have been garnering growing attention due to increasing commercial interests in smart displays (e.g., colorimetric labels), colored e-readers, etc. To comply with the requirements of future displays, several strategies have been proposed by various research groups to achieve wide color tuning ranges in the visible regime, high on/off contrast ratios, and fast response times. In this review, we first introduce ways to create optical resonances including plasmonic, photonic, and plasmonic-photonic hybrid structures that form the basis of color generation. We then outline strategies that control the refractive index contrast between the system and the surrounding as a means to actively change the reflective structural colors, backed with representative examples from the literature. Finally, we provide a comparison of dynamic structural colors based on various switching mechanisms summarizing performance metrics that are important for future displays, and conclude with an outlook on current challenges.

Direct detection of hole gas in Ge-Si core-shell nanowires by enhanced Raman scattering.

We report the direct detection of hole accumulation in the core of Ge-Si core-shell nanowire heterostructures by a Fano resonance between free holes and the F2g mode in Raman spectra. Raman enhancements of 10-10 000 with respect to bulk were observed and explained using finite difference time domain simulations of the electric fields concentrated in the nanowire. Numerical modeling of the radial carrier concentration revealed that the asymmetric line-shape is strongly influenced by inhomogeneous broadening.

Broadband plasmonic microlenses based on patches of nanoholes.

This paper reports a new type of diffractive microlens based on finite-areas of two-dimensional arrays of circular nanoholes (patches). The plasmonic microlenses can focus single wavelengths of light across the entire visible spectrum as well as broadband white light with little divergence. The focal length is determined primarily by the overall size of the patch and is tolerant to significant changes in patch substructure, including lattice geometry and local order of the circular nanoholes. The optical throughput, however, depends sensitively on the patch substructure and is determined by the wavelengths of surface plasmon resonances. This simple diffractive lens design enables millions of broadband plasmonic microlenses to be fabricated in parallel using soft nanolithographic techniques.

Mesoporous Solid and Yolk-Shell Titania Microspheres as Touchless Colorimetric Sensors with High Responsivity and Ultrashort Response Times.

Touchless user interfaces offer an attractive pathway toward hygienic, remote, and interactive control over devices. Exploiting the humidity generated from fingers or human speech is a viable avenue for realizing such technology. Herein, titania microspheres including solid and yolk-shell structures with varying microstructural characteristics were demonstrated as high-performance, ultrafast, and stable optical humidity sensors aimed for touchless control. When water molecules enter the microporous network of the microspheres, the effective refractive index of the microsphere increases, causing a detectable change in the light scattering behavior. The microstructural properties of the microspheres, namely, the pore characteristics, crystallinity, and particle size, were examined in relation to the humidity-sensing performance, establishing optimum structural conditions for realizing humidity-responsive wavelength shifts above 100 nm, near full-scale relative humidity (RH) responsivity, ultrashort response times below 30 ms, and prolonged lifetimes. These optimized microspheres were used to demonstrate a colorimetric touchless sensor that responds to humidity from a finger and a microcontroller-based detector that translates the moisture pattern from human speech to electrical signals in real time. These results provide practical strategies for enabling humidity-based touchless user interfaces.

Concurrent Imaging of Surface-Enhanced Raman and Mie Scattering from Built-in Nanogap Plasmonic Particles.

We report a bimodal imaging method that can spatially resolve and concurrently correlate SERS and background-free Mie scattering signals. By examining two types of nanoparticle assemblies with different types of plasmonic junctions, namely raspberry-like metamolecules (raspberry-MMs) containing intraparticle nanogaps and groups of Au nanocubes forming interparticle gaps, we were able to rapidly screen SERS-active particles among the entire population of nanoparticles. Ratiometric analysis of SERS/Mie scattering revealed distinct behaviors for these intra- and interparticle nanogaps. In particular, raspberry-MMs showed a high fraction of SERS-active particles with the SERS intensity essentially insensitive to the nanoparticle aggregation state and a predictable environmental dependence. In comparison, nanocube clusters exhibited highly heterogeneous SERS/Mie scattering ratios and unpredictable intensity fluctuations but higher maximum SERS intensity. This dual-imaging approach constitutes an in situ visualization tool that enables simultaneous and stoichiometric analysis of dual signals consisting of elastic and inelastic scattering, which can significantly improve the reliability of SERS measurements.

Using electrons as a high-resolution probe of optical modes in individual nanowires.

While nanowires show increasing promise for optoelectronic applications, probing the subwavelength details of their optical modes has been a challenge with light-based techniques. Here we report the excitation of dielectric optical waveguide modes in a single GaN nanowire using transition radiation generated by a 1 nm diameter electron beam. This spatially resolved study opens important gateways to probing the optical modes of more complex nanostructures, fundamental for optimization of optoelectronic device performance.

Mie Resonant Structural Colors.

Structural colors refer to colors produced by the interference of light scattered by judiciously arranged nano- or microscopic structures. In this Forum Article, we discuss the use of Mie resonant scattering in structural colors with dielectric and metal-dielectric hybrid structures to achieve notable figures of merit in pixel size and gamut range. Compared with plasmonic structures, resonant dielectric and hybrid structures are subjected to less loss while providing strong field confinement and large scattering cross sections, making them appealing for realizing vibrant colors at ultrahigh resolutions. We outline the basic principles behind Mie resonances in analytically solvable structures and highlight the relation between these resonances and color with demonstrations in dielectric metasurfaces. Mie resonant colors occurring in nonplanar designs including disordered systems are also explored. We review recent advances in dynamic and reversibly tunable Mie resonant colors and conclude by providing an outlook for future research directions.

Realizing Vibrant and High-Contrast Reflective Structural Colors from Lossy Metals Supporting Dielectric Gratings.

Despite their distinctive chemical properties, lossy metals are generally avoided in the design of structural colors because the optical losses can degrade the color vibrancy. Herein, we demonstrate a strategy that allows lossy metals supporting near-wavelength dielectric gratings to achieve high color vibrancy by benefiting from the optical loss rather than suffering from it. By exciting the grating rotated 45° relative to the incident field, s-polarized (s-pol) and p-polarized (p-pol) light each excites a spectrally distinct resonance, described by a treatment of coupled waveguide-array modes, that retards the phase over the wavelength. Owing to the birefringence, a cross-polarized reflection spectrum displays two sharp peaks from each component that decreases the monochromaticity. We show that lossy metals can minimize the p-pol contribution, leaving the sharp s-pol response to determine the spectrum and generate high color vibrancy. Through this scheme, we demonstrate that lossy metal substrates including Pt, a catalytically active metal, and W, a CMOS-compatible metal, can achieve larger sRGB gamut coverage ratios of 90% and 69%, respectively, than that of 55% from Ag, while maintaining similar pixel contrast ratios to that of Ag.

Active-Material-Independent Color-Tunable Semitransparent Organic Solar Cells.

Semitransparent colorful organic solar cells (OSC) provide exciting opportunities for harnessing sunlight as colored windows. Previously, color filter (CF) electrodes on (OSC) were demonstrated via vacuum-deposition techniques, resulting in deposition-induced damage. Thus, we present CF integrated organic photovoltaics (CF-OPVs) using solution-processed TiO2-AcAc as the dielectric component. The noninvasive processing substantially expands the range of usable active materials, allowing the device to display pure and vibrant colors that are independent of the inherent color of the active material and show superior optical and photovoltaic characteristics. These results provide practical pathways to realizing colored semitransparent solar cells.

Light-Induced Surface Potential Modification in MoS2 Monolayers on Au Nanostripe Arrays.

In this work, the surface potential (VS) of exfoliated MoS2 monolayers on Au nanostripe arrays with period of 500 nm was investigated using Kelvin probe force microscopy. The surface morphology showed that the suspended MoS2 region between neighboring Au stripes underwent tensile-strain. In the dark, the VS of the MoS2 region on the Au stripe (VS-Au) was larger than that of the suspended MoS2 region (VS-S). However, under green light illumination, VS-Au became smaller than VS-S. To explain the VS modification, band diagrams have been constructed taking into consideration not only the local strain but also the electronic interaction at the MoS2/Au interface. The results of this work provide a basis for understanding the electrical properties of MoS2-metal contacts and improving the performance of MoS2-based optoelectronic devices.