Metal-insulator-metal strip spectroscopic infrared photothermal absorber based on uniaxially oriented plasmonic lanthanum hexaboride films.

We report the fabrication of a mid-infrared device using LaB6 - Al2O3 - LaB6 trilayers, with an array of LaB6 strips as the top layer. Uniaxially oriented lanthanum hexaboride (LaB6) films self-organized in a (100) orientation were adopted together with a lithographic process using laser direct writing followed by reactive ion etching. The fabricated infrared absorbers based on our electromagnetic design exhibited excellent resonant absorption and flexible tunability by changing the periodicity and width of the top LaB6 strips. We examined the performance of epitaxial and sputtered LaB6 films by fabricating two different types of absorbers using sputtered LaB6(100) and epitaxial LaB6(100) films for the bottom mirror layers. Owing to a difference in crystallinity, the latter exhibited a lower background in the absorption spectra as well as in the thermal emission spectra, indicating its good spectral selectivity.

Influences of 1-Propanol and Methanol Additives on Crystallization of Thin Amorphous Solid Water Films.

In general, randomly oriented ice crystallites are formed by heating amorphous solid water (ASW) films at ∼160 K via homogeneous nucleation. Here, we demonstrate that monolayers of methanol and 1-propanol additives incorporated in the multilayer ASW film lead to heterogeneous nucleation at the substrate interface of Pt(111), as evidenced by the occurrence of epitaxial ice growth. The mobility of water in direct contact with the Pt(111) substrate is decreased relative to that in the bulk, but it can be increased via interactions with hydrophobic moieties of alcohols that are segregated to the interfacial region. As a result, heterogeneous nucleation occurs at ca. 160 K along with homogeneous nucleation in the film interior. However, the template effect is quenched when the alcohols are in direct contact with the substrate. The methanol adspecies deposited onto the ASW film surface induces heterogeneous nucleation at a temperature as low as 145 K, but the 1-propanol adspecies has no such an effect. Their different ability of heterogeneous nucleation at the free ASW film surface, as well as their uptake behaviors in the near surface region, is associated with the hydrophobic hydration of the alcohols resulting from different lengths of the aliphatic moiety.

A temperature programmed desorption study of interactions between water and hydrophobes at cryogenic temperatures.

It is considered that hydrophobic solutes dissolve in water via the formation of icelike cages in the first hydration shell. However, this conventional picture is currently under debate. We have investigated how hydrophobic species, such as D2, Ne, Ar, Xe, CH4, and C3H8, interact with water in composite films of amorphous solid water (ASW) based on temperature programmed desorption (TPD). The D2 and Ne species tend to be incorporated in ASW without being caged, whereas two distinct peaks assignable to the caged species are identifiable for the other solutes examined here. The low-temperature peak is observed preferentially for Ar and CH4 prior to crystallization. The hydrophobes are thought to be encapsulated in porous ASW films via reorganization of the hydrogen bond network up to 100 K; most of them are released in a liquidlike phase that occurs immediately before crystallization at ca. 160 K. The nature of hydrophobic hydration at cryogenic temperature appears to differ from that in normal water at room temperature because the former resembles crystalline ices in the local hydrogen-bond structure rather than the latter. No ordered structures assignable to clathrate hydrates were identified before and after crystallization.

Triggering Water and Methanol Activation for Solar-Driven H2 Production: Interplay of Dual Active Sites over Plasmonic ZnCu Alloy.

Methanol steam reforming (MSR) is a promising reaction that enables efficient production and safe transportation of hydrogen, but it requires a relatively high temperature to achieve high activity, leading to large energy consumption. Here, we report a plasmonic ZnCu alloy catalyst, consisting of plasmonic Cu nanoparticles with surface-deposited Zn atoms, for efficient solar-driven MSR without additional thermal energy input. Experimental results and theoretical calculations suggest that Zn atoms act not only as the catalytic sites for water reduction with lower activation energy but also as the charge transfer channel, pumping hot electrons into water molecules and subsequently resulting in the formation of electron-deficient Cu for methanol activation. These merits together with photothermal heating render the optimal ZnCu catalyst a high H2 production rate of 328 mmol gcatalyst-1 h-1 with a solar energy conversion efficiency of 1.2% under 7.9 Suns irradiation, far exceeding the reported conventional photocatalytic and thermocatalytic MSR. This work provides a potential strategy for efficient solar-driven H2 production and various other energy-demanding industrial reactions through designing alloy catalysts.

Simultaneous harvesting of radiative cooling and solar heating for transverse thermoelectric generation.

For any thermoelectric effects to be achieved, a thermoelectric material must have hot and cold sides. Typically, the hot side can be easily obtained by excess heat. However, the passive cooling method is often limited to convective heat transfer to the surroundings. Since thermoelectric voltage is proportional to the temperature difference between the hot and cold sides, efficient passive cooling to increase the temperature gradient is of critical importance. Here, we report simultaneous harvesting of radiative cooling at the top and solar heating at the bottom to enhance the temperature gradient for a transverse thermoelectric effect which generates thermoelectric voltage perpendicular to the temperature gradient. We demonstrate this concept by using the spin Seebeck effect and confirm that the spin Seebeck device shows the highest thermoelectric voltage when both radiative cooling and solar heating are utilized. Furthermore, the device generates thermoelectric voltage even at night through radiative cooling which enables continuous energy harvesting throughout a day. Planar geometry and scalable fabrication process are advantageous for energy harvesting applications.

Gires-Tournois resonators as ultra-narrowband perfect absorbers for infrared spectroscopic devices.

Ultra-narrowband perfect absorbers and emitters are proposed and realized by engineering multiple-beam interference in Gires-Tournois etalon with the presence of low metallic loss. The absorption mechanism and spectral characteristics of the Gires-Tournois resonators are numerically and experimentally investigated for three configurations: dielectric cavity on metal, metal-dielectric-metal resonator, and distributed Bragg reflector (DBR)-dielectric-metal resonator. Narrowband thermal emitters based on the metal-dielectric-metal cavity and (DBR)-dielectric-metal cavity are experimentally demonstrated with an emissivity of 0.8 and 0.82, and a quality factor of 21 and 85, respectively. A DBR-dielectric-metal resonator-based absorber is directly loaded onto a LiTaO 3 film for the first time to constitute an on-chip ultra-narrowband pyroelectric detector with an excellent quality factor of 151 at the absorption band of methane.

Strong coupling between phonon-polaritons and plasmonic nanorods.

We perform far-field spectroscopy of infrared metal antennas on silicon oxide layers of different thickness, where we find a splitting of the plasmonic resonance. This splitting can result in a transparency window, corresponding to suppression of antenna scattering, respectively "cloaking" of the antenna. Backed up by theory, we show that this effect is caused by strong coupling between the metal antenna plasmons and the surface phonon polaritons in the oxide layer. The effect is a kind of induced transparency in which the strength of the phonon-polariton field plays the crucial role. It represents a further tuning possibility for the optical performance of infrared devices.

Tamm plasmon selective thermal emitters.

Selective thermal emissions from the excitation of Tamm plasmon polaritons (TPPs) are demonstrated. A TPP structure is composed of a distributed Bragg reflector (DBR) and a thin metal film on top. The tunability of the thermal emission was experimentally achieved only by changing the DBR's photonic bandgap. Low cost and large area selective thermal emitters can be realized by TPP-based structures.

Terahertz-field-induced nonlinear electron delocalization in Au nanostructures.

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond time scale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field of THz pulses; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.

Insulator-to-Proton-Conductor Transition in a Dense Metal-Organic Framework.

Metal-organic frameworks (MOFs) are prone to exhibit phase transitions under stimuli such as changes in pressure, temperature, or gas sorption because of their flexible and responsive structures. Here we report that a dense MOF, ((CH3)2NH2)2[Li2Zr(C2O4)4], exhibits an abrupt increase in proton conductivity from <10(-9) to 3.9 × 10(-5) S/cm at 17 °C (activation energy, 0.64 eV) upon exposure to humidity. The conductivities were determined using single crystals, and the structures were analyzed by X-ray diffraction and X-ray pair distribution function analysis. The initial anhydrous structure transforms to another dense structure via topotactic hydration (H2O/Zr = 0.5), wherein one-fourth of the Li ions are irreversibly rearranged and coordinated by water molecules. This structure further transforms into a third crystalline structure by water uptake (H2O/Zr = 4.0). The abrupt increase in conductivity is reversible and is associated with the latter reversible structure transformation. The H2O molecules coordinated to Li ions, which are formed in the first step of the transformation, are considered to be the proton source, and the absorbed water molecules, which are formed in the second step, are considered to be proton carriers.

Transparent oxides forming conductor/insulator/conductor heterojunctions for photodetection.

Photoexcited hot electrons from conductors can be injected into the conduction bands of wide-bandgap materials, thus enabling the visible and near-infrared (NIR) photoactivities of light-harvesting devices. While metals have been dominantly used as conductors to excite hot electrons, we demonstrate that transparent conductive oxides (TCOs) can also be used for this purpose. Trilayer structures consisting of a thin dielectric layer sandwiched by TCOs show photoresponsiveness in UV, visible, as well as NIR wavelength range. As these trilayer structures are transparent, they can be used to monitor light without blocking it.

Plasmon-mediated photocatalytic activity of wet-chemically prepared ZnO nanowire arrays.

We report on measurements and simulations of the efficient sunlight-driven and visible-active photocatalysts composed of plasmonic metal nanoparticles and ZnO nanowire (NW) arrays fabricated via an all-wet-chemical route. Because of the coupling between the ZnO dielectric response and the excitation of the Ag or Au nanoparticles, efficient electronic excitation can be induced in the vicinity of the metal-ZnO interfaces because optically-excited plasmonic particles can not only concentrate the electromagnetic field at the ZnO/particle interface, but also act as efficient sources of plasmonic hot electrons to be injected into the conduction band of the ZnO catalyst. The catalytic activities of the fabricated ZnO NWs are examined by photodegradation of methylene blue and by photocurrent measurements in a photovoltaic configuration. Numerical electromagnetic simulations were used to understand the behavior of the light on the nanometer-scale to clarify the catalytic enhancement mechanisms in both the ultraviolet (UV) and visible (VIS) regions. In addition, simulation results indicated that a near-surface normal but slightly tilted ZnO NW array geometry would provide an increased optical path length and enhanced multiple scattering and absorption processes arising from the localized surface plasmon resonances of the nanoparticles. The results obtained here clarify the role of the plasmon resonance and provide us with useful knowledge for the development of metal-oxide nano-hybrid materials for solar energy conversion.

Conversion of Carbon Dioxide by Methane Reforming under Visible-Light Irradiation: Surface-Plasmon-Mediated Nonpolar Molecule Activation.

A novel CO2 photoreduction method, CO2 conversion through methane reforming into syngas (DRM) was adopted as an efficient approach to not only reduce the environmental concentration of the greenhouse gas CO2 but also realize the net energy storage from solar energy to chemical energy. For the first time it is reported that gold, which was generally regarded to be inactive in improving the performance of a catalyst in DRM under thermal conditions, enhanced the catalytic performance of Rh/SBA-15 in DRM under visible-light irradiation (1.7 times, CO2 conversion increased from 2100 to 3600 µmol g(-1) s(-1)). UV/Vis spectra and electromagnetic field simulation results revealed that the highly energetic electrons excited by local surface plasmon resonances of Au facilitated the polarization and activation of CO2 and CH4 with thermal assistance. This work provides a new route for CO2 photoreduction and offers a distinctive method to photocatalytically activate nonpolar molecules.

Rare-Earth-Metal-Free Solid-State Fluorescent Carbonized-Polymer Microspheres for Unclonable Anti-Counterfeit Whispering-Gallery Emissions from Red to Near-Infrared Wavelengths.

Colloidal carbon dots (CDs) have garnered much attention as metal-free photoluminescent nanomaterials, yet creation of solid-state fluorescent (SSF) materials emitting in the deep red (DR) to near-infrared (NIR) range poses a significant challenge with practical implications. To address this challenge and to engineer photonic functionalities, a micro-resonator architecture is developed using carbonized polymer microspheres (CPMs), evolved from conventional colloidal nanodots. Gram-scale production of CPMs utilizes controlled microscopic phase separation facilitated by natural peptide cross-linking during hydrothermal processing. The resulting microstructure effectively suppresses aggregation-induced quenching (AIQ), enabling strong solid-state light emission. Both experimental and theoretical analysis support a role for extended π-conjugated polycyclic aromatic hydrocarbons (PAHs) trapped within these microstructures, which exhibit a progressive red shift in light absorption/emission toward the NIR range. Moreover, the highly spherical shape of CPMs endows them with innate photonic functionalities in combination with their intrinsic CD-based attributes. Harnessing their excitation wavelength-dependent photoluminescent (PL) property, a single CPM exhibits whispering-gallery modes (WGMs) that are emission-tunable from the DR to the NIR. This type of newly developed microresonator can serve as, for example, unclonable anti-counterfeiting labels. This innovative cross-cutting approach, combining photonics and chemistry, offers robust, bottom-up, built-in photonic functionality with diverse NIR applications.

Bioinspired Carbonized Polymer Microspheres for Full-Color Whispering Gallery Mode Emission for White Light Emission, Unclonable Anticounterfeiting, and Chemical Sensing Applications.

Light-element-based fluorescent materials, colloidal graphene quantum dots, and carbon dots (CDs) have sparked an immense amount of scientific interest in the past decade. However, a significant challenge in practical applications has emerged concerning the development of solid-state fluorescence (SSF) materials. This study addresses this knowledge gap by exploring the unexplored photonic facets of C-based solid-state microphotonic emitters. The proposed synthesis approach focuses on carbonized polymer microspheres (CPMs) instead of conventional nanodots. These microspheres exhibit remarkable SSF spanning the entire visible spectrum from blue to red. The highly spherical shape of CPMs imparts built-in photonic properties in addition to its intrinsic CD-based attributes. Leveraging their excitation-dependent photoluminescence property, these microspheres exhibit amplified spontaneous emission, assisted by the whispering gallery mode resonance across the visible spectral region. Remarkably, unlike conventional semiconductor quantum dots or dye-doped microresonators, this single microstructure showcases adaptable resonant emission without structural/chemical modifications. This distinctive attribute enables a plethora of applications, including microcavity-assisted energy transfer for white light emission, highly sensitive chemical sensing, and secure encrypted anticounterfeiting measures. This interdisciplinary approach, integrating photonics and chemistry, provides a robust solution for light-element-based SSF with inherent photonic functionality and wide-ranging applications.

Porous gold nanodisks with multiple internal hot spots.

For increasing the number of internal hot spots in the individual plasmonic nanoparticles, porous Au nanostructures were synthesized by a hybrid approach combining a physical process, which defined the overall shapes and dimensions of the nanostructures, and a chemical process, which incorporated nanopores inside the patterned nanostructures. This approach allows us to synthesize lithographically designed Au nanodisks containing numerous internal Raman hot spots in the form of nanopores. The increased number of hot spots successfully improved SERS intensity, and this experimental result was further elucidated by numerical electromagnetic simulations. The highly improved and homogeneous SERS intensities illustrate the great potential of the porous plasmonic nanodisks as a sensitive molecular imaging agent.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2.

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Longitudinal and transverse coupling in infrared gold nanoantenna arrays: long range versus short range interaction regimes.

Interaction between micrometer-long nanoantennas within an array considerably modifies the plasmonic resonant behaviour; for fundamental resonances in the infrared already at micrometer distances. In order to get systematic knowledge on the relationship between infrared plasmonic resonances and separation distances dx and dy in longitudinal and transverse direction, respectively, we experimentally studied the optical extinction spectra for rectangularly ordered lithographic gold nanorod arrays on silicon wafers. For small dy, strong broadening of resonances and strongly decreased values of far-field extinction are detected which come along with a decreased near-field intensity, as indicated by near-field amplitude maps of the interacting nanoantennas. In contrast, near-field interaction over small dx does only marginally broaden the resonance. Our findings set a path for optimum design of rectangular nanorod lattices for surface enhanced infrared spectroscopy.

Surface-enhanced ATR-IR spectroscopy with interface-grown plasmonic gold-island films near the percolation threshold.

Flat nano-island films prepared by wet-chemical deposition were investigated with attenuated total reflection infrared (ATR-IR) spectroscopy and scanning electron microscopy (SEM) in order to analyze the correlation between film morphology and optical properties. Here we choose Au as representative coinage metal (Au, Ag, Cu) that shows strong structure-dependent surface-enhanced infrared absorption (SEIRA). Infrared spectra of octadecanethiol monolayers on films of different stages of morphologic development show effects that are characteristic for SEIRA, such as enhanced vibrational signals, Fano-type line shape, and adsorbate induced baseline shifts. Their extent was found to be strongly dependent on the structural details and the strongest enhancement occurs at the percolation threshold of the two-dimensional island system. Also films beyond percolation show significant enhancement due to residual nanoholes that are acting as hotspots.

Hydropower generation by transpiration from microporous alumina.

Traditional hydropower generation is one of the most sustainable energy sources; however, the local environmental impact of hydroelectric dams and reservoirs is serious, and hydroelectric power requires high-cost turbines and generators. Because these installations utilize gravitational potential energy of massive volumes of falling water, this sort of hydropower generation is unsuitable for ubiquitous, small-scale energy production. Here, we report that wetting and evaporation of pure water from a tiny block of porous alumina generates electrical current in the direction of water transpiration. The current induced in microporous alumina is associated with mass transport of water accompanying ions that accumulate near the negatively charged surface of alumina pores. Without any pre-treatment or additives, once evaporation commences, a 3 × 3 cm2 piece of alumina can generate an open-circuit voltage as large as 0.27 V. The power generation scheme we propose here is simple, clean, and versatile, and it can be employed anywhere, as it utilizes only spontaneous capillary action of water and Coulombic interaction at the alumina-water interface, without requiring any input of heat or light.