A high performance photothermal film with spherical shell-type metallic nanocomposites for solar thermoelectric conversion

Plasmonic Nanoparticles Boost Solar-to-Electricity Generation at Ambient Conditions

Sunlight-to-electricity conversion using solar thermoelectric generators (STEGs) is a proven technology to meet our ever-growing energy demand. However, STEGs are often operated under a vacuum with customized thermoelectric materials to achieve high performance. In this work, the incorporation of plasmonic gold nanoparticle (AuNP) based solar absorbers enabled the efficient operation of STEGs under ambient conditions with commercially available thermoelectric devices. AuNPs enhanced the performance of STEG by ∼9 times, yielding an overall solar-to-electricity conversion efficiency of ∼9.6% under 7.5 W cm-2 solar irradiance at ambient conditions. Plasmonic heat dissipated by AuNPs upon solar irradiation was used as the thermal energy source for STEGs. High light absorptivity, photothermal conversion efficiency (∼95%), and thermal conductivity of AuNPs enabled the efficient generation and transfer of heat to STEGs, with minimal radiative and convective heat losses. The power generated from plasmon-powered STEGs is used to run electrical devices as well as produce green hydrogen via the electrolysis of water.

Ultralarge Pixel Array Photothermal Film based on 3D Self-Suspended Microbridge Structure for Infrared Scene Projection

Infrared emitter is highly desirable for applications in infrared imaging and infrared stealth technology. It is also a core device in infrared scene generation. Light-driven photothermal film has attracted considerable interest due to its outstanding photothermal properties and easy fabrication. However, the existing photothermal films suffer from low photothermal conversion efficiency (PCE) as well as small sizes. The improvement of the PCE is usually achieved at the expense of dynamic frame rate. Here, this work designs and fabricates a photothermal film based on 3D self-suspended microbridge structure. Silicon (Si) microbridges are introduced into each microstructure to manipulate the thermal conductivity of the films. By optimizing the parameters of the Si microbridges, the high PCE and fast frame rate are both achieved. Moreover, the 3D structure microbridge film is 4-inch in diameter, forming an ultralarge array with over 2200 × 2200 pixels. Finally, a high PCE infrared scene projector is realized based on this photothermal film. A visible image is projected on the film, the 3D-microstructured photothermal film absorbs the visible light and emits an infrared image same as the visible one with high resolution and fast frame rate due to the excellent photothermal properties.

Plasmonic photothermal properties of silver nanoparticle grating films

The plasmon-induced photothermal effect offers effective light-to-heat conversion systems. In this study, we fabricate plasmonic photothermal silver nanoparticle (AgNP) grating films to produce highly effective plasmon-induced heat generation films. AgNP films provide effective heat generation by localized surface plasmon excitation in the void of the AgNP films. The heat generated at a AgNP film by irradiation of solar light is 3.4 times higher than that generated at the reference flat evaporated-Ag film. Furthermore, simultaneous excitation of localized surface plasmons and propagating surface plasmons is confirmed to be obtained on AgNP grating films by finite-difference time-domain simulation and reflectivity measurements. The AgNP grating film is created by the nanoimprinting technique. The grating structure on AgNPs further enhances electric field intensity in the large area of the film, which results in higher heat generation. Thus, 5.4 times higher heat generation is achieved compared with that of the reference flat evaporated-Ag film.

Precipitation of Pt Nanoparticles inside Ion-Track-Etched Capillaries

Ion-track-etched capillaries containing nanoparticles of precious metals (e.g., Pt, Au, and Ag) can be applied to plasmonic absorber materials. The precipitation of homogeneous and highly dispersed precious metal nanoparticles inside capillaries represents a key process. Ion-track-etched capillaries (diameter: ~500 nm, length: ~25 μm) were created in polyimide film by 350 MeV Xe irradiation (3 × 107 ions/cm2) and chemical etching (using a sodium hypochlorite solution). The films with capillaries were immersed in an aqueous solution containing 0.1–10 mmol/L H2PtCl6 and 0.5 vol% C2H5OH, and then irradiated with a 2 MeV electron beam up to a fluence of 1.4 × 1016 e/cm2. The Pt particles inside the capillaries were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation of Pt nanoparticles and isolated aggregates inside the capillaries was confirmed by TEM. The Pt nanoparticles tended to aggregate under increasing concentrations of H2PtCl6 in the aqueous solution; meanwhile, no changes in nanoparticle size were noted under increasing electron beam fluence. The results suggest that the proposed method can be used to form metal nanoparticles in nanosized capillaries with a high aspect ratio.

High-temperature stable plasmonic and cavity resonances in metal nanoparticle-decorated silicon nanopillars for strong broadband absorption in photothermal applications

Plasmonic metal nanoparticles in conjunction with the cavity mode resonance in crystalline silicon (c-Si) nanopillars (NPs) can help achieve strongly enhanced broadband light absorption far beyond the limit of bulk c-Si. However, a major concern arises from the stability of metal nanoparticles, particularly at a high temperature, as the diffusion and conglomeration of the nanoparticles will undermine the very basis for the advantageous plasmonic effect. We here carried out a systematic investigation of the thermal stability of different metal nanoparticles coated on 3D Si-based NPs and found that simple Al2O3 encapsulation could help stabilize the gold (Au) particles coated on Si NPs even when subjected to annealing at >1073 K while accomplishing excellent broadband optical absorption (∼95%) from 200 nm to 2500 nm. This could be assigned mainly to the excellent dispersion retention capability of the Al2O3-encapsulated Au nanoparticles and the beneficial plasmon resonance absorption among the Au nanoparticles and Si NPs, as also revealed from the FDTD simulation analysis. Finally, a rapid vapor generation application was demonstrated based on the optimized Au/Si NPs, where salt water drops could be directly injected onto the high-temperature photo-heated Au/Si NPs and could vaporize/bounce off quickly without leaving any salt precipitation on the surface. This new strategy can also pave the way for high-performance Si-based photothermal applications.

Effects of geometry and composition of soft polymer films embedded with nanoparticles on rates for optothermal heat dissipation

Embedding soft matter with nanoparticles (NPs) can provide electromagnetic tunability at sub-micron scales for a growing number of applications in healthcare, sustainable energy, and chemical processing. However, the use of NP-embedded soft material in temperature-sensitive applications has been constrained by difficulties in validating the prediction of rates for energy dissipation from thermally insulating to conducting behavior. This work improved the embedment of monodisperse NPs to stably decrease the inter-NP spacings in polydimethylsiloxane (PDMS) to nano-scale distances. Lumped-parameter and finite element analyses were refined to apportion the effects of the structure and composition of the NP-embedded soft polymer on the rates for conductive, convective, and radiative heat dissipation. These advances allowed for the rational selection of PDMS size and NP composition to optimize measured rates of internal (conductive) and external (convective and radiative) heat dissipation. Stably reducing the distance between monodisperse NPs to nano-scale intervals increased the overall heat dissipation rate by up to 29%. Refined fabrication of NP-embedded polymer enabled the tunability of the dynamic thermal response (the ratio of internal to external dissipation rate) by a factor of 3.1 to achieve a value of 0.091, the largest reported to date. Heat dissipation rates simulated a priori were consistent with 130 μm resolution thermal images across 2- to 15-fold changes in the geometry and composition of NP-PDMS. The Nusselt number was observed to increase with the fourth root of the Rayleigh number across thermally insulative and conductive regimes, further validating the approach. These developments support the model-informed design of soft media embedded with nano-scale-spaced NPs to optimize the heat dissipation rates for evolving temperature-sensitive diagnostic and therapeutic modalities, as well as emerging uses in flexible bioelectronics, cell and tissue culture, and solar-thermal heating.

Solution-Processed Plasmonic-Dielectric Sunlight-Collecting Nanofilms for Solar Thermoelectric Application

It is important but remains a challenge to develop solution-processed plasmonic solar thermoelectricity films on various substrates, without strictly considering hierarchical plasmonic-dielectric-metal structures, to harvest a wide range of visible to near-infrared sunlight. We simply fabricate plasmonic silica metastructure sunlight-collecting nanofilms on highly reflective Cu and Si surfaces by introducing spin coating (with an Ag and silica colloidal mixture, a spin coater, and a heating plate) and low-temperature annealing (in an oven at 200 °C for 1 h) processes. The approximately 250 nm thick metastructure consists of a top 60 nm thick silica layer as an antireflective film and a bottom 190 nm thick Ag nanoparticle-silica hybrid film as a sunlight harvester. The metastructure film reduces the reflectivity of Cu (>90%) and Si (25-35%) to less than 5% at visible to near-infrared frequencies. The metastructure film on the Cu sheet has an absorptance of 0.95 and a thermal emittance of 0.06, ideal for high-performance sunlight absorbers. The solar thermoelectric powers of the film-coated Cu and Si are 15.4 and 4.7 times those of the uncoated Cu and Si substrates, respectively. The metastructure film on Cu exhibited a similar or slightly higher performance than that of a top-class vapor-deposited commercialized absorber film on Cu, demonstrating the robustness of the present method.

Near-field surface plasmon field enhancement induced by rippled surfaces

The occurrence of plasmon resonances on metallic nanometer-scale structures is an intrinsically nanoscale phenomenon, given that the two resonance conditions (i.e., negative dielectric permittivity and large free-space wavelength in comparison with system dimensions) are realized at the same time on the nanoscale. Resonances on surface metallic nanostructures are often experimentally found by probing the structures under investigation with radiation of various frequencies following a trial-and-error method. A general technique for the tuning of these resonances is highly desirable. In this paper we address the issue of the role of local surface patterns in the tuning of these resonances as a function of wavelength and electric field polarization. The effect of nanoscale roughness on the surface plasmon polaritons of randomly patterned gold films is numerically investigated. The field enhancement and relation to specific roughness patterns is analyzed, producing many different realizations of rippled surfaces. We demonstrate that irregular patterns act as metal-dielectric-metal local nanogaps (cavities) for the resonant plasmonic field. In turn, the numerical results are compared to experimental data obtained via aperture scanning near-field optical microscopy.

Assembly-Induced Thermogenesis of Gold Nanoparticles in the Presence of Alternating Magnetic Field for Controllable Drug Release of Hydrogel

Films of gold nanoparticles are easily fabricated by layer-by-layer assembly. With increasing number of layers a transition of the electric property from insulating to conducting can be achieved. This conductivity leads to controllable thermogenesis of the film, which can be employed for drug release of loaded hydrogels.

A Rapid Response Thin-Film Plasmonic-Thermoelectric Light Detector

Light detection and quantification is fundamental to the functioning of a broad palette of technologies. While expensive avalanche photodiodes and superconducting bolometers are examples of detectors achieving single-photon sensitivity and time resolutions down to the picosecond range, thermoelectric-based photodetectors are much more affordable alternatives that can be used to measure substantially higher levels of light power (few kW/cm²). However, in thermoelectric detectors, achieving broadband or wavelength-selective performance with high sensitivity and good temporal resolution requires careful design of the absorbing element. Here, combining the high absorptivity and low heat capacity of a nanoengineered plasmonic thin-film absorber with the robustness and linear response of a thermoelectric sensor, we present a hybrid detector for visible and near-infrared light achieving response times of the order of 100 milliseconds, almost four times shorter than the same thermoelectric device covered with a conventional absorber. Furthermore, we show an almost two times higher light-to-electricity efficiency upon replacing the conventional absorber with a plasmonic absorber. With these improvements, which are direct results of the efficiency and ultra-small thickness of the plasmonic absorber, this hybrid detector constitutes an ideal component for various medium-intensity light sensing applications requiring spectrally tailored absorption coatings with either broadband or narrowband characteristics.

Filter-free, junctionless structures for color sensing

A simple structure, efficient color splitting, sufficient output of electrical signals, and low power consumption are the important characteristics of contemporary devices for color sensing. In this study, we developed filter-free, junctionless structures that exhibited a superior photo-thermo-electrical response under a low bias voltage and a short response time in milliseconds. Although our compact sensor had a simple single-layer trench-like aluminum (Al) structure, it could perform multiple functions, including light harvesting, color-selective absorption, photo-thermo-electrical transformation, and the ability to collect photoinduced differences in electrical signals. This device exploited near-field surface plasmon resonance and cavity effects to enhance the intensity of the electric field and the color-selective absorption, ultimately resulting in significant current signals in its structured Al film. This strategy significantly simplifies not only the components of the color sensor but also its fabrication; for example, red, green, and blue color detection devices could be prepared simultaneously through a single lithography, etching, and deposition step. With its ability to provide functional filter-free, junctionless structures, this strategy has great potential for the production of devices that operate on different kinds of substrates, thereby bridging various applications of color sensing technologies.

On-chip integration of novel Au electrode with a higher order three-dimensional layer stack nanostructure for surface-enhanced Raman spectroscopy

We develop a novel in situ surface-enhanced Raman spectroscopy (SERS) platform with three-dimensional nanostructure gold electrodes using the competitive self-assembly between dielectrophoresis and convective aggregation.