Effects of plasmon coupling on circular dichroism of chiral nanoparticle arrays.

Arrays and ensembles of chiral nanostructures have potential applications in the field of enantioselective sensors, metamaterials, and metasurfaces. In particular, chiral nanostructures fabricated through chemical and bottom-up approaches have attracted much attention from the viewpoint of cost and scalability, but the heterogeneity of the unit nanostructure constituting the array or ensemble often deteriorates its chiroptical responses. Here, we report that their deteriorated responses can be recovered or even enhanced further by interparticle plasmon coupling. We employed chiral silver (Ag) hexamers as models for electromagnetic simulations and investigated the effect of their parameters, such as interparticle spacing, chiral purity, and enantiomeric excess, on their g-factor, which is an index for chiroptical responses. The maximum value of g-factor (gmax) of the Ag hexamer surpasses that of the chiral monomer and augments with decreasing interparticle spacing. This enhancement in g-factor is observed even when chiral purity and enantiomeric excess are less than 100%, showing the potent role of plasmon coupling in amplifying chiroptical responses. Furthermore, our research highlights the amplification of the effect of plasmon coupling on the gmax value of infinite periodic chiral nanostructures. These results corroborate the potential of plasmon coupling to improve chiroptical responses by precisely controlling the interparticle spacing of chiral plasmonic nanostructures, thus mitigating the loss of g-factor caused by low purity and enantiomeric excess of the nanostructures fabricated by chemical and bottom-up approaches.

Well-dispersed Au co-catalyst deposited on a rutile TiO2 photocatalyst via electron traps.

We deposited Au nanoparticles as a co-catalyst onto a TiO2 photocatalyst by reducing [AuCl4]- using electrons trapped in the oxygen vacancies of TiO2. The dispersibility and hydrogen production ability of the Au co-catalyst are higher than those prepared using the conventional photodeposition method.

Inactivation and spike protein denaturation of novel coronavirus variants by CuxO/TiO2 nano-photocatalysts.

In order to reduce infection risk of novel coronavirus (SARS-CoV-2), we developed nano-photocatalysts with nanoscale rutile TiO2 (4-8 nm) and CuxO (1-2 nm or less). Their extraordinarily small size leads to high dispersity and good optical transparency, besides large active surface area. Those photocatalysts can be applied to white and translucent latex paints. Although Cu2O clusters involved in the paint coating undergo gradual aerobic oxidation in the dark, the oxidized clusters are re-reduced under > 380 nm light. The paint coating inactivated the original and alpha variant of novel coronavirus under irradiation with fluorescent light for 3 h. The photocatalysts greatly suppressed binding ability of the receptor binding domain (RBD) of coronavirus (the original, alpha and delta variants) spike protein to the receptor of human cells. The coating also exhibited antivirus effects on influenza A virus, feline calicivirus, bacteriophage Qß and bacteriophage M13. The photocatalysts would be applied to practical coatings and lower the risk of coronavirus infection via solid surfaces.

Site-selective introduction of MnO2 co-catalyst onto gold nanocubes via plasmon-induced charge separation and galvanic replacement for enhanced photocatalysis.

For energy harvesting with plasmonic photocatalysis, it is important to optimize geometrical arrangements of plasmonic nanomaterials, electron (or hole) acceptors, and co-catalysts so as to improve the charge separation efficiency and suppress charge recombination. Here, we employ a photocatalytic system with Au nanocubes on TiO2 and introduce MnO2 as an oxidation co-catalyst onto the nanocubes via site-selective oxidation based on plasmon-induced charge separation (PICS). However, it has been known that PbO2 is the only material that can be deposited onto Au nanomaterials through PICS with sufficient site-selectivity. Here we addressed this issue by introducing an indirect approach for MnO2 deposition via site-selective PbO2 deposition and subsequent galvanic replacement of PbO2 with MnO2. The indirect approach gave nanostructures with MnO2 introduced at around the top part, bottom part, or entire surface of the Au nanocubes on a TiO2 electrode. The activity of those plasmonic photocatalysts was strongly dependent on the location of MnO2. The key to improving the activity is to separate MnO2 from TiO2 to prevent recombination of the positive charges in MnO2 with the negative ones in TiO2.

Alginate-enabled green synthesis of S/Ag1.93S nanoparticles, their photothermal property and in-vitro assessment of their anti-skin-cancer effects augmented by a NIR laser.

We report herein the design and synthesis of colloidally-stable S/Ag1.93S nanoparticles, their photothermal conversion properties and in vitro cytotoxicity toward A431 skin cancer cells under the excitation of a minimally-invasive 980 nm near-infrared (NIR) laser. Micron-sized S particles were first synthesized via acidifying Na2S2O3 using biocompatible sodium alginate as a surfactant. In the presence of AgNO3 and under rapid microwave-induced heating, alginate reduced AgNO3 to nascent Ag which reacted with molten S in situ forming S/Ag1.93S nanoparticles. The nanoparticles were characterized using a combination of X-ray diffraction, electron microscopies, elemental analysis, zeta-potential analysis and UV-VIS-NIR spectroscopy. The average particles size was controlled between 40 and 60 nm by fixing the mole ratio of Ag+:S2O32-. When excited by a 980 nm laser, S/Ag1.93S nanoparticles (~40 nm) produced with the least amount of AgNO3 exhibited a respectable photothermal conversion efficiency of circa 62% with the test aqueous solution heated to a hyperthermia-inducing 52 °C in 15 min. At 0.7 W/cm2, the viability of A431 skin cancer cells incubated with 7.0 ± 0.2 µg/mL of S/Ag1.93S nanoparticles reduced to 14 ± 0.6%, while an A431 cell control maintained an 80% cell viability. These results suggested that S/Ag1.93S nanoparticles may have good potential in reducing metastatic skin carcinoma.

Visualization of nano-localized and delocalized oxidation sites for plasmon-induced charge separation.

Oxidation reaction sites for plasmon-induced charge separation at Au nanocubes on TiO2 were visualized on the basis of deposition and dissolution reactions. For Pb2+ oxidation, PbO2 was deposited selectively at resonance sites of the nanocube, while oxidation polymerization of pyrrole to polypyrrole and oxidative dissolution of Au took place over the entire nanocube surface. The localized and delocalized reaction sites are explained in terms of a relationship between oxidation potentials of the electron donors and potentials of the entire nanocube and localized holes.

Plasmonic hole ejection involved in plasmon-induced charge separation.

Since the finding of plasmon-induced charge separation (PICS) at the interface between a plasmonic metal nanoparticle and a semiconductor, which has been applied to photovoltaics including photodetectors, photocatalysis including water splitting, sensors and data storage in the visible/near-infrared ranges, injection of hot electrons (energetic electrons) into semiconductors has attracted attention almost exclusively. However, it has recently been found that behaviours of holes are also important. In this review, studies on the hot hole ejection from plasmonic nanoparticles are described comprehensively. Hole ejection from plasmonic nanoparticles on electron transport materials including n-type semiconductors allows oxidation reactions to take place at more positive potentials than those involved in a charge accumulation mechanism. Site-selective oxidation is also one of the characteristics of the hole ejection and is applied to photoinduced nanofabrication beyond the diffraction limit. Hole injection into hole transport materials including p-type semiconductors (HTMs) in solid-state cells, hole ejection through a HTM for stabilization of holes, hole ejection to a HTM for efficient hot electron ejection, voltage up-conversion by the use of hot carriers and electrochemically assisted hole ejection are also described.

Photoinduced Chirality Switching of Metal-Inorganic Plasmonic Nanostructures.

Chiral plasmonic nanodevices whose handedness can be switched reversibly between right and left by external stimulation have attracted much attention. However, they require delicate DNA nanostructures and/or continuous external stimulation. In this study, those issues are addressed by using metal-inorganic nanostructures and photoinduced reversible redox reactions at the nanostructures, namely, site-selective oxidation due to plasmon-induced charge separation under circularly polarized visible light (CPL) and reduction by UV-induced TiO2 photocatalysis. We irradiate gold nanorods (AuNRs) supported on TiO2 with right- or left-CPL to generate electric fields with chiral distribution around each AuNR and to deposit PbO2 at the sites where the electric fields are localized, for fixing the chirality to the AuNR. The nanostructures thus prepared exhibit circular dichroism (CD) based on longitudinal and transverse plasmon modes of the AuNRs. Their chirality given by right-CPL (or left-CPL) is locked until PbO2 is rereduced under UV light. After unlocking by UV, the chirality can be switched by left-CPL (or right-CPL) irradiation, resulting in reversed CD signals and locking the switch again. The handedness of the chiral plasmonic nanodevice can be switched reversibly and repeatedly.

Controlling the oxidation state of molybdenum oxide nanoparticles prepared by ionic liquid/metal sputtering to enhance plasmon-induced charge separation.

Nanoparticles composed of molybdenum oxide, MoO x , were successfully prepared by room-temperature ionic liquid (RTIL)/metal sputtering followed by heat treatment. Hydroxyl groups in RTIL molecules retarded the coalescence between MoO x NPs during heat treatment at 473 K in air, while the oxidation state of Mo species in MoO x nanoparticles (NPs) could be modified by changing the heat treatment time. An LSPR peak was observed at 840 nm in the near-IR region for MoO x NPs of 55 nm or larger in size that were annealed in a hydroxyl-functionalized RTIL. Photoexcitation of the LSPR peak of MoO x NPs induced electron transfer from NPs to ITO electrodes.

Accelerated site-selective photooxidation on Au nanoparticles via electrochemically-assisted plasmonic hole ejection.

In order to induce electrochemical reactions by localized surface plasmon resonance (LSPR), semiconductors have been employed as electron or hole acceptors for plasmon-induced charge separation (PICS) in most cases. Here we replaced a semiconductor with a potential-controlled transparent electrode, and achieved accelerated photooxidation reactions at selected local sites on plasmonic metal nanoparticles. We demonstrate site-selective PbO2 deposition at the tips and sides of Au nanorods and PbO2 deposition and Au dissolution at the top and bottom of Au nanocubes, through the selective excitation of different LSPR modes. Energetic electron-hole pairs are generated at a plasmonic resonance site, and oxidation reactions are driven by hole ejection at the site. The complementary electrons are removed via the positively biased electrode, and consumed at a counter electrode by reduction reactions. In the case of the PbO2 deposition, formation of PbO2 nuclei is triggered by the hole ejection, and PbO2 is grown further in an electrochemical manner at an improved rate.

Electrochemical modulation of plasmon-induced charge separation behaviour at Au-TiO2 photocathodes.

Plasmon-induced charge separation (PICS) at the interface between a plasmonic nanoparticle and a semiconductor becomes less efficient as the plasmon resonance wavelength increases, because the energy of a photon may not be sufficiently higher than the interfacial Schottky barrier height. In this study, we developed PICS photocathodes by coating Au nanoparticles of different sizes on an ITO electrode with a thin TiO2 layer, and applied negative potentials to those photocathodes so as to suppress back electron transfer and improve the PICS photocurrent responses. The photocurrent enhancement factor was increased as the particle size was decreased, and enhancement of about two orders of magnitude was observed for small Au nanoparticles when bias voltage of 0.5 V was applied. In some cases the photocurrent enhancement was accompanied by a slight redshift of the photocurrent peak, which was caused by a lowered barrier. This technique would be useful for tuning the photocurrents when it is applied to devices such as electrochemical LSPR sensors and photodetectors.

Plasmon-induced charge separation at the interface between ITO nanoparticles and TiO2 under near-infrared irradiation.

Plasmon-induced charge separation (PICS) by continuous electron injection from plasmonic compound nanoparticles to a semiconductor was achieved by using solid-state cells based on tin-doped indium oxide (ITO) nanoparticles with a short capping agent and a TiO2 film. The cells extended the PICS range to longer wavelengths and exhibited photoresponses to 1500-2200 nm near-infrared light.

Local trapping of energetic holes at gold nanoparticles on TiO2.

Oxidation sites for plasmon-induced charge separation at gold nanocubes and nanorods on TiO2 were visualized by PbO2 deposition, and the sites were localized at plasmonic resonance sites. This indicates that energetic holes generated at those sites dominate the reactions, which can be applied to photo-nanofabrication beyond the diffraction limit.

Chiral Plasmonic Nanostructures Fabricated by Circularly Polarized Light.

The chirality of materials results in a wide variety of advanced technologies including image display, data storage, light management including negative refraction, and enantioselective catalysis and sensing. Here, we introduce chirality to plasmonic nanostructures by using circularly polarized light as the sole chiral source for the first time. Gold nanocuboids as precursors on a semiconductor were irradiated with circularly polarized light to localize electric fields at specific corners of the cuboids depending on the handedness of light and deposited dielectric moieties as electron oscillation boosters by the localized electric field. Thus, plasmonic nanostructures with high chirality were developed. The present bottom-up method would allow the large-scale and cost-effective fabrication of chiral materials and further applications to functional materials and devices.

Effects of particle size and annealing on plasmon-induced charge separation at self-assembled gold nanoparticle arrays.

Two-dimensional periodic Au nanoparticle arrays were constructed on TiO2 thin films by a micelle lithography method and seed-mediated photoelectrochemical growth. Their adjustable interparticle distance allows investigation of a particle size effect on plasmon-induced charge separation (PICS) efficiencies without interference from particle aggregation or plasmon coupling. External or internal PICS efficiencies were found to increase and decrease, respectively, with an increase in particle diameter from 25 to 38 nm. Improvement of the contact between Au nanoparticles and TiO2 by annealing enhanced the intensity of a plasmonic interface mode and both external and internal PICS efficiencies.

Plasmonic behaviour and plasmon-induced charge separation of nanostructured MoO3-x under near infrared irradiation.

Plasmon-induced charge separation (PICS) allows direct conversion of localized surface plasmon resonance (LSPR) to electron flows and photoelectrochemical reactions. However, PICS has only been achieved using plasmonic noble metal nanoparticles, not with compound nanoparticles. In order to achieve compound PICS, MoO3-x nanostructures were prepared that exhibit LSPR in the near infrared region by using metal oxides or metal nanoparticles as templates. Solid-state cells based on the MoO3-x nanostructure were developed. Their photoresponse to 700-1400 nm infrared light was investigated and analyzed on the basis of their PICS mechanisms.

Hydrogen evolution from water based on plasmon-induced charge separation at a TiO2/Au/NiO/Pt system.

Metal-semiconductor plasmonic nanostructures are capable of converting light energy through plasmon-induced charge separation (PICS), providing fruitful new strategies to utilize solar energy in various fields, including photocatalysis. Here, we enhance the PICS efficiencies for hydrogen evolution from water at a Pt cathode coupled with a TiO2/Au photoanode by coating the TiO2/Au with a p-type NiO layer on which a Pt co-catalyst is deposited. PICS occurs at the Au-TiO2 interface under visible light. The electrons injected from the Au nanoparticles into TiO2 are transported to the Pt cathode and cause hydrogen evolution from water, the action spectrum of which matches the plasmonic extinction spectrum of the Au nanoparticles. The NiO layer extracts the separated positive charges from the Au nanoparticles, accumulates the charges and drives methanol oxidation at the Pt co-catalyst on NiO with the positive charges. As a result of the introduction of the Pt-modified NiO layer, the rates of methanol oxidation and accompanying hydrogen evolution at zero bias voltage were improved by ∼3.5 times. The NiO layer may also protect the Au nanoparticles from self-oxidation.

Photoassisted bottom-up construction of plasmonic nanocity.

Herein, photoassisted self-construction of nanocity as a novel plasmonic metasurface was achieved. It is an ensemble of nanobuildings, and the height of each nanobuilding is greater than its depth. Plasmonic nanocity exhibits a vertical resonance mode in addition to distal longitudinal and proximal longitudinal resonance modes, such that it can be applied to chromatic angular polarizers, sophisticated image recording, and high density data storage. Further growth of nanobuildings to bulky and tall nanocuboids leads to asymmetric and dichroic scattering, which can be applied in security printing.

Tunable plasmon resonance of molybdenum oxide nanoparticles synthesized in non-aqueous media.

Plasmonic compound nanoparticles (NPs) have attracted great interest because they are prepared at lower cost and show unique optical properties. However, full replacement of the plasmonic noble metal NPs with the compound NPs has been difficult because most of the compound NPs exhibit plasmon resonance in the infrared range owing to low free carrier density and mobility. In order to overcome this limitation, we developed a new synthetic method for plasmonic MoO2 and MoO3-x NPs. Those NPs exhibit plasmon resonance at ∼500 nm and 600-1000 nm, respectively, likely because of high carrier densities. The plasmonic properties of the NPs are tunable by changing the synthetic conditions or oxidizing and reducing the NPs. Their refractive index sensitivities are 115-260 nm RIU-1. Those molybdenum oxide NPs are expected to substitute for plasmonic noble metal NPs in optical, electronic, sensing and light harvesting devices and materials.

Semi-transparent Perovskite Solar Cells Developed by Considering Human Luminosity Function.

Semi-transparent solar cells draw a great deal of attention because their applications include, for instance, photovoltaic windows. General approach to semi-transparent cells is using thin active layers or island-type structures. Here we take human luminosity function into account, and develop solar cells that harvest photons in the wavelength regions in which human eyes are less sensitive to light. We used an organic-inorganic hybrid perovskite, which is sensitive to light particularly in the blue and deep-blue regions, and plasmonic silver nanocubes that enhance light harvesting in the red and deep-red ranges. In order to tune the plasmonic wavelength to that range, we took advantage of electrode-coupled plasmons (ECPs). We prepared non-plasmonic semi-transparent solar cells, and reduced the active layer thickness and introduced ECPs, so that the visual transparency index and power conversion efficiency of the cell were improved by 28% and 6%, respectively, of the initial values.