Nanostructures intrinsically possessing two different structural or functional features, often called Janus nanoparticles, are emerging as a potential material for sensing, catalysis, and biomedical ...applications. Herein, we report the synthesis of plasmonic gold Janus nanostars (NSs) possessing a smooth concave pentagonal morphology with sharp tips and edges on one side and, contrastingly, a crumbled morphology on the other. The methodology reported herein for its synthesis using a single-step growth reaction is unique to any other Janus nanoparticle preparation involving either template-assisted growth or a masking technique. Interestingly, a coexistence of lower and higher index facets has been found in these Janus NSs. The general paradigm for synthesizing Au Janus NSs was investigated by understanding the mechanism of its kinetic control with the combinatorial effect of all the reagents responsible for its structure. Optical property of the Janus NSs has been realized by corelating their extinction spectrum with the simulated data. The size-dependent surface-enhanced Raman scattering (SERS) activity of these Janus NSs has been studied with 1,4-BDT as the model analyte. Finite-difference time-domain (FDTD) simulations for different sized particles revealed the distribution of electromagnetic hot-spots over the particles resulting in the enhancement of the SERS signal in a size dependent manner.
We investigate the effect of surfactant-mediated, asymmetric silver overgrowth of gold nanorods on their intrinsic optical properties. From concentration-dependent experiments, we established a close ...correlation of the extinction in the UV/vis/NIR frequency range and the morphological transition from gold nanorods to Au@Ag cuboids. Based on this correlation, a generic methodology for in situ monitoring of the evolution of the cuboid morphology was developed and applied in time-dependent experiments. We find that growth rates are sensitive to the substitution of the surfactant headgroup by comparison of benzylhexadecyldimethylammonium chloride (BDAC) with hexadecyltrimethylammonium chloride (CTAC). The time-dependent overgrowth in BDAC proceeds about 1 order of magnitude slower than in CTAC, which allows for higher control during silver overgrowth. Furthermore, silver overgrowth results in a qualitatively novel optical feature: Upon excitation inside the overlap region of the interband transition of gold and intraband of silver, the gold core acts as a retarding element. The much higher damping of the gold core compared to the silver shell in Au@Ag cuboids induces mirror charges at the core/shell interface as shown by electromagnetic simulations. Full control over the kinetic growth process consequently allows for precise tailoring of the resonance wavelengths of both modes. Tailored and asymmetric silver-overgrown gold nanorods are of particular interest for large-scale fabrication of nanoparticles with intrinsic metamaterial properties. These building blocks could furthermore find application in optical sensor technology, light harvesting, and information technology.
For large-scale fabrication of optical circuits, tailored subwavelength structures are required to modulate the refractive index. Here, we introduce a colloid-to-film-coupled nanocavity whose ...refractive index can be tailored by various materials, shapes, and cavity volumes. With this colloidal nanocavity setup, the refractive index can be adjusted over a wide visible wavelength range. For many nanophotonic applications, specific values for the extinction coefficient are crucial to achieve optical loss and gain. We employed bottom-up self-assembly techniques to sandwich optically active ternary metal-chalcogenides between a metallic mirror and plasmonic colloids. The spectral overlap between the cavity resonance and the broadband emitter makes it possible to study the tunable radiative properties statistically. For flat cavity geometries of silver nanocubes with sub-10 nm metallic gap, we found a fluorescence enhancement factor beyond 1000 for 100 cavities and a 112 meV Rabi splitting. In addition, we used gold spheres to extend the refractive index range. By this easily scalable colloidal nanocavity setup, gain and loss building blocks are now available, thereby leading to new generation of optical devices.
Plasmonic assemblies featuring high sensitivity that can be readily shifted by external fields are the key for sensitive and versatile sensing devices. In this paper, a novel fast-responsive ...plasmonic nanocomposite composed of a multilayer nanohole array and a responsive electrochromic polymer is proposed with the plasmonic mode appearance vigorously cycled upon orthogonal electrical stimuli. In this nanocomposite, the coaxially stacked plasmonic nanohole arrays can induce multiple intense Fano resonances, which result from the crosstalk between a broad surface plasmon resonance (SPR) and the designed discrete transmission peaks with ultrahigh sensitivity; the polymer wrapper could provide the sensitive nanohole array with real-time-varied surroundings of refractive indices upon electrical stimuli. Therefore, a pronounced pure electroplasmonic shift up to 72 nm is obtained, which is the largest pure electrotuning SPR range to our knowledge. The stacked nanohole arrays here are also directly used as a working electrode, and they ensure sufficient contact between the working electrode (plasmonic structure) and the electroactive polymer, thus providing considerably improved response speed (within 1 s) for real-time sensing and switching.
DNA nanostructures provide a powerful platform for the programmable assembly of nanomaterials. Here this approach is extended to synthesize rod‐like gold nanoparticles in a full DNA controlled ...manner. The approach is based on DNA molds containing elongated cavities. Gold is deposited inside the molds using a seeded‐growth procedure. By carefully exploring the growth parameters it is shown that gold nanostructures with aspect ratios of up to 7 can be grown from single seeds. The highly anisotropic growth is in this case controlled only by the rather soft and porous DNA walls. The optimized seeded growth procedure provides a robust and simple routine to achieve continuous gold nanostructures using DNA templating.
The fabrication of metal nanoparticles of a desired shape represents a considerable challenge. Here DNA molds are used to guide the growth of gold nanoparticles. By carefully tuning the reaction parameters, highly anisotropic particles with high aspect ratios are obtained whose shape is only controlled by the rather soft and porous DNA walls.
The transition dipole orientations of dye assemblies in heterostructures have a crucial impact on the efficiency of novel optoelectronic devices such as organic thin-film transistors and ...light-emitting diodes. These devices are frequently based on heterojunctions and tandem structures featuring multiple optical transitions. Precise knowledge of preferred orientations, spatial order, and spatial variations is highly relevant. We present a fast and universal large-area screening method to determine the transition dipole orientations in dye assemblies with diffraction-limited spatial resolution. Moreover, our hyperspectral imaging approach disentangles the orientations of different chromophores. As a demonstration, we apply our technique to dye monolayers with two optical transitions sandwiched between two ultrathin silicate nanosheets. A comprehensive model for dipole orientation distributions in monolayers reveals a long-range orientational order and a strong correlation between the two transitions.
In contrast to a macroscopic solid, the color and spectroscopic properties of gold nanoparticles change with size. Here, we propose a teaching platform for high school students (hereafter defined as ...students, attending secondary school in Europe, preparatory high schools in the US, or any educational institution of a comparable level) that explains the relationship between the perceived color of colloidal solutions and their physicochemical properties. For this aim, an open-source code, based on Mie theory and colorimetry, was developed to link the perceived colors to the plasmonic effects. Moreover, a set of such nanoparticles, varied in diameter, was created to support the theoretical description. The employed seed-mediated synthesis approach provides sufficient spherical roundness that is crucial for color matching. The proposed teaching platform is available online and can be linked to an outreach activity or practical laboratory course. Furthermore, we illustrate the linkage of nanoscience concepts to real-world examples (e.g., stained church glass), which can be used for promoting the potential of nanotechnology in other fields, including electronics, optics, and biomedicine adding to the interdisciplinarity of the teaching platform.
Quasi-two-dimensional conjugated polymers (q2DCPs) are polymers that consist of linear conjugated polymer chains assembled through non-covalent interactions to form a layered structure. In this work, ...w e report the synthesis of a novel crystalline q2D polypyrrole (q2DPPy) film at an air/H
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(95%) interface. The unique interfacial environment facilitated chain extension, prevented disorder, and resulted in a crystalline, layered assembly of protonated quinoidal chains with a fully extended conformation in its crystalline domains. This unique structure features highly delocalized π-electron systems within the extended chains, which is responsible for the low effective mass and narrow electronic bandgap. Thus w e investigated the temperature-dependent charge transport properties of q2DPPy using the van der Pauw (vdP) method and terahertz time-domain spectroscopy (THz-TDS). The vdP method revealed that the q2DPPy film exhibited a semiconducting behavior with a thermally activated hopping mechanism in long-range transport between the electrodes. Conversely, THz-TDS revealed a band-like transport, indicating intrinsic charge transport up to a record short-range high THz mobility of ∼107.1 cm
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Periodic arrays of plasmonic nanostructures can support surface lattice resonances emerging from coupling between localized and diffractive modes. This allows the confinement of light at the ...nanometer scale with significantly increased resonance lifetimes as compared to those of purely localized modes. Here, we demonstrate that self‐assembly of plasmonic hybrid nanoparticles allows the simple and fast fabrication of periodic plasmonic monolayers featuring macroscopic dimensions and easily controllable lattice spacings. Electromagnetic coupling between diffractive and localized modes is significantly enhanced when the arrays are embedded in a homogeneous refractive index environment. This is realized through spin‐coating of a polymer film on top of the colloidal monolayer. Narrow surface lattice resonances are detected by far‐field extinction spectroscopy while optical microscopy reveals a homogeneous coupling strength on cm‐sized substrates. The surface lattice resonance position is changed by manipulation of the refractive index of the polymer film through the immersion into different organic solvents. Capitalizing on the thermoresponsive behavior of the polymer film we modulate the surface lattice resonance by temperature in a fully reversible, dynamic manner. The findings demonstrate the potential of colloidal self‐assembly as a bottom‐up approach for the fabrication of future nanophotonic devices.
Dynamically tunable surface lattice resonances in self‐assembled particle arrays are demonstrated. Hexagonally ordered monolayers of wet‐chemically synthesized hybrid core–shell particles with silver nanoparticle cores enable coupling between plasmonic and diffractive modes. A swellable, temperature‐responsive polymer coating allows dynamic and fully reversible modulation of the surface lattice resonance position.
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