We recently found that polyvinylpyrrolidone (PVP)-protected metal nanoparticles dispersed in water/butanol mixture spontaneously float to the air/water interface and form two-dimensional assemblies ...due to classical surface excess theory and Rayleigh-Bénard-Marangoni convection induced by butanol evaporation. In this study, we found that by leveraging this principle, a unique structure is formed where hetero gold nanospheres (AuNPs)/gold nanostars (AuNSs) complexes are dispersed within AuNP two-dimensional assemblies, obtained from a mixture of polyvinylpyrrolidone-protected AuNPs and AuNSs that interact electrostatically with the AuNPs. These structures were believed to form as a result of AuNPs/AuNSs complexes formed in the water/butanol mixture floating to the air/water interface and being incorporated into the growth of AuNP two-dimensional assemblies. These structures were obtained by optimizing the amount of mixed AuNSs, with excessive addition resulting in the formation of random three-dimensional network structures. The AuNP assemblies dispersed with AuNPs/AuNSs complexes exhibited significantly higher Raman (surface-enhanced resonance Raman scattering: SERRS) activity compared to simple AuNP assemblies, while the three-dimensional network structure did not show significant SERRS activity enhancement. These results demonstrate the excellent SERRS activity of AuNP two-dimensional assemblies dispersed with hetero AuNPs/AuNSs complexes.
In this study, we investigated the viscoelastic properties of metal nanoparticle monolayers at the air/water interface by dilational rheology under periodic oscillation of surface area. Au ...nanoparticles capped with oleylamine form a stable, dense monolayer on a Langmuir film balance. The stress response function of a nanoparticle monolayer was first analyzed using the classical Kelvin–Voigt model, yielding the spring constant and viscosity. The obtained results suggest that the monolayer of nanoparticles is predominantly elastic, forming a two-dimensional physical gel. As the global shape of the signal exhibited a clear nonlinearity, we further analyzed the data with the higher modes in the Fourier series expansion. The imaginary part of the higher mode signal was stronger than the real part, suggesting that the dissipative term mainly causes the nonlinearity. Intriguingly, the response function measured at larger strain amplitude became asymmetric, accompanied by the emergence of even modes. The significance of interactions between nanoparticles was quantitatively assessed by calculating the potential of mean force, indicating that the lateral correlation could reach up to the distance much larger than the particle diameter. The influence of surface chemical functions and core metal has also been examined by using Au nanoparticles capped with partially fluorinated alkanethiolate and Ag nanoparticles capped with myristic acid. The combination of dilational rheology and correlation analyses can help us precisely control two-dimensional colloidal assembly of metal nanoparticles with fine-adjustable localized surface plasmon resonance.
We demonstrate a flexible transmissive color filter based on a hybrid metallic–optical Tamm state device composed of a thin metallic film on top of dual dielectric distributed Bragg reflectors (DBRs) ...to simultaneously excite the Tamm plasmon (TP), the optical Tamm state (OTS), and the Fabry–Pérot (FP) resonant eigenmodes for spectrally achieving triple transmittance peaks in the visible light range. We show that the resonant eigenmodes confined inside the device can be tuned at will by simply adjusting the designed Bragg wavelengths of the dual DBRs and retain an ultranarrow bandwidth regardless of resonant wavelength, creating the desired chromaticity points and constructing a large color gamut space in the CIE coordinate. We further show that, due to the fabrication simplicity of our color filter involving only thin-film deposition, the proposed structure can be easily integrated onto flexible substrates, leading to tunable transmittance spectrum as well as the color appearance by simply changing its bending curvature. The tunable color filter reported herein can be employed for various applications such as aesthetical color decorations in architectures, low-cost and portable spectral analyzers, and optical strain/deformation sensors.
Photocatalysts are practically used for decomposition of harmful and fouling organic compounds. Among the photocatalytic reactions, remote oxidation via airborne species is a relatively slow process, ...so that a sensitive technique for its detection has been awaiting. Here, we investigated an airborne remote photocatalytic reaction of a TiO2 photocatalyst modified with Pt nanoparticles as co-catalysts via the color change caused by a decomposition of a multilayered silver nanoparticle sheet. The silver nanoparticle sheet fabricated by the Langmuir–Schaefer method on a gold substrate exhibits a unique multicolor depending upon the number of layers. The color originates from multiple light trapping in the stratified sheets that has a metamaterial characteristic along with an intra- and interlayer coupling of localized surface plasmon resonance (LSPR). The stepwise decomposition of the sheets was confirmed by the colorimetric data, which exhibited not only a monotonic decrease but also a maximized absorption of light when the film thickness reached the optimal thickness for light trapping or when the oxidation of the Ag core started. Scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface plasmon resonance (SPR) spectroscopy data provided a complete view of the decomposition process of this inorganic–organic nanocomposite film, and simulation by the transfer-matrix method explained a simultaneous plasmonic response rationally. The influence of the humidity and gas flow rate on the airborne remote photocatalytic reaction kinetics was examined by this colorimetric detection method, and it suggests that H2O in air plays an essential role in the reaction.
Micro-photoluminescence (PL) mapping was investigated for Al-coated InGaN/GaN quantum wells (QWs), which showed huge PL enhancement by the surface plasmon (SP) resonance. The obtained images show ...inhomogeneity at the micro-meter scale; in addition, the region with lower PL intensities tend to have a longer PL wavelength for bare QWs. This correlation changed with an Al coating, positive correlations were observed in an area with a relatively short peak wavelength with blue-shift. Conversely, negative correlations were observed at longer peak wavelengths. These results suggest that the quantum-confined Stark effect (QCSE) was screened by the enhanced electrical-field of the SP resonance.
We report the fabrication technique and optical properties of multilayered two-dimensional (2D) gold nanoparticle sheets (“Au nanosheet”). The 2D crystalline monolayer sheet composed of Au ...nanoparticles shows an absorption peak originating from a localized surface plasmon resonance (LSPR). It was found that the absorption spectra dramatically change when the monolayers are assembled into the multilayers on different substrates (quartz or Au). In the case of the multilayers on Au thin film (d = 200 nm), the LSPR peak is shifted to longer wavelength at the near-IR region by increasing the number of layers. The absorbance also depends on the layer number and shows the nonlinear behavior. On the other hand, the multilayers on quartz substrate show neither such LSPR peak shift nor nonlinear response of absorbance. The layer number dependence on metal surfaces can be interpreted as the combined effects between the near-field coupling of the LSPR and the far-field optics of the stratified metamaterial films, as proposed in our previous study. We also report the spectroscopic properties of hybrid multilayers composed of two kinds of monolayers, i.e., Au nanosheet and Ag nanosheet. The combination of the different metal nanoparticle sheets realizes more flexible plasmonic color tuning.
We have developed a new methodology to fabricate high-resolution DNA microarrays (2500 dots/cm2) in combination with microcontact printing (μCP) and surface plasmon resonance (SPR) imaging. A novel ...COOH-terminated PEG-disulfide with para-carborane (p-carborane) was synthesized as an initiator chemically bound to a gold substrate for surface coupling reactions with NH2-terminated DNA. The hybridization between target DNA immobilized on gold nanoparticles and probe DNA arrayed on flat gold substrates was successfully demonstrated by SPR imaging, where nonspecific adsorption was not observed at the array background. The gold nanoparticles on the array give quite high contrast even at low surface coverage with gold nanoparticles (∼10%) by the enhancement effect of optical signals based on nanoscale phenomena. This enhancement effect can be well described by the simulation based on the Maxwell-Garnett theory for the effective dielectric constants and Fresnel's equation.
Constructing polaritonic devices in monolithic, ultra‐compact photonic architectures with monolayer‐featured exciton‐emitters is decisive to exploit the coherent superposition between entangled ...photonic and excitonic eigenstates for potential realizations of optical nonlinearities, macroscopic condensations, and superfluidity. Here, a feasible strategy for exciton‐polariton formations is demonstrated by implementing a Tamm‐plasmon (TP) polaritonic device with the active material composed of single‐monolayered perovskite (CsPbBr3) quantum dots (QDs). The metallic character of the TP configuration is able to concentrate its resonance mode into a confined region beyond the diffraction limit, which highly overlaps, both spatially and spectrally, with the single‐monolayered CsPbBr3 QDs embedded inside. The mode volume of the device is hence reduced dramatically, leading to an enhanced light–matter coupling strength for the polaritonic emission at room temperature. In particular, it is found that the dispersion relation of the TP polaritonic device is tunable by detuning the excitonic and photonic eigenmodes and that the polariton–polariton interaction energy is strongly dependent on the polariton's spin state. The presented strategy is a determinant step toward the realization of strong light–matter coupling and polariton spintronics in the CsPbBr3 QDs with a single‐monolayered feature.
This work demonstrates a feasible strategy for room temperature polaritonic emissions by implementing a Tamm‐plasmon polaritonic device embedded with the CsPbBr3 quantum dots with a single‐monolayered feature.
Homogeneously self-assembled colloidal semiconductor quantum dot monolayers (QD-SAMs) over large areas are promising materials for thin film optoelectronic device applications, especially for ...display. Although tuning of emission colors from QDs is generally achieved during wet chemical synthesis and before monolayer formation, we propose in this study a simple and effective method to adjust emission colors after the formation of QD-SAMs by a simple one-step heat treatment. CdSe-based core/shell or core/double shell structured QDs (CdSe/ZnS, CdSe/CdZnS, and CdSe/CdS/ZnS) covered with an optimal set of hydrophobic ligands can form homogeneous and stable QD-SAMs at the air–water interface. The QD-SAMs are subsequently transferred onto hydrophobized glass substrates by the Langmuir–Schaefer (LS) method and thermally treated in air. We found a blueshift of more than 35 nm for the emission wavelength (red to green) by a thermal treatment at 280 °C for 150 min with CdSe/ZnS QD-SAMs. The color can be adjusted by changing the heating temperature and the treatment time. The wavelength shift is in the order of CdSe/ZnS(4L) > CdSe/ZnS(6L) = (CdSe/CdZnS) > (CdSe/CdS/ZnS). The energy dispersive X-ray (EDX) microanalysis of a single QD reveals that the blueshift is mainly caused by atomic diffusion-induced alloying of core/shell type QDs. The main problem of this method is the decreasing emission intensity caused by oxidation during the heat treatment; however, this problem can be solved with the use of a SiO2 protective coating on the QD-SAMs. We believe that this simple technique is useful for manufacturing RGB-colored ultrathin QD-SAM films for QD displays such as QD film display, QD color-filter display, and QD light emitting diode.
Gold nanoparticles capped by an unsymmetrical azobenzene disulfide, 4-hexyl-4‘-(12-(dodecyldithio)dodecyloxy)azobenzene (C6AzSSC12), were synthesized in order to investigate the efficiency of ...azobenzene photoisomerization on colloidal gold surfaces. The nanoparticles were synthesized by a two-step method to avoid the direct contact of azobenzene units with a reducing agent. The average size of the particles was determined to be ∼5.2 ± 1.3 nm from transmission electron microscope (TEM) images. The CH2 antisymmetric (∼2919 cm-1) and symmetric (∼2850 cm-1) stretching bands in the FTIR spectra of the nanocomposite confirmed the all-trans conformation of alkyl chains in the C6AzSSC12 on the colloidal gold. The photoisomerization reaction of the C6AzSSC12-capped gold nanoparticles was studied by UV−vis absorption spectroscopy in toluene. The reaction kinetics was identical to that of the free C6AzSSC12 molecules dissolved in toluene, with no deviations from a first-order plot for both trans-to-cis and cis-to-trans photoisomerization, suggesting no steric hindrance throughout the whole reaction process. The free volume guaranteed by the 50% dilution of the dye functions due to the unsymmetrical disulfide structures, as well as their noncompact molecular tails owing to the assembly on the curved colloidal gold, must be responsible for such a highly efficient photoreaction. Sedimentation of the nanoparticles arose in toluene subsequent to the photoisomerization of the capping azobenzene molecules from trans to cis isomers. This phenomenon can be interpreted as resulting from differences in the degree of solvation between the azobenzene isomers.
