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Concept: Waves in plasmas


In this work, we focused on the label-free detection of simple protein binding using near-infrared light-responsive plasmonic nanoshell arrays with a controlled interparticle distance. The nanoshell arrays were fabricated by a combination of colloidal self-assembly and subsequent isotropic helium plasma etching under atmospheric pressure. The diameter, interparticle distance, and shape of nanoshells can be tuned with nanometric accuracy by changing the experimental conditions. The Au, Ag, and Cu nanoshell arrays, having a 240-nm diameter (inner, 200-nm polystyrene (PS) core; outer, 20-nm metal shell) and an 80-nm gap distance, exhibited a well-defined localized surface plasmon resonance (LSPR) peak at the near-infrared region. PS@Au nanoshell arrays showed a 55-nm red shift of the maximum LSPR wavelength of 885 nm after being exposed to a solution of bovine serum albumin (BSA) proteins for 18 h. On the other hand, in the case of Cu nanoshell arrays before/after incubation to the BSA solution, we found a 30-nm peak shifting. We could evaluate the difference in LSPR sensing performance by changing the metal materials.

Concepts: Nanoparticle, Serum albumin, Surface plasmon resonance, Plasmon, Extraordinary optical transmission, Surface plasmon, Plasma oscillation, Waves in plasmas


We illustrate the important trade-off between far-field scattering effects, which have the potential to provide increased optical path length over broad bands, and parasitic absorption due to the excitation of localized surface plasmon resonances in metal nanoparticle arrays. Via detailed comparison of photocurrent enhancements given by Au, Ag and Al nanostructures on thin-film GaAs devices we reveal that parasitic losses can be mitigated through a careful choice of scattering medium. Absorption at the plasmon resonance in Au and Ag structures occurs in the visible spectrum, impairing device performance. In contrast, exploiting Al nanoparticle arrays results in a blue shift of the resonance, enabling the first demonstration of truly broadband plasmon enhanced photocurrent and a 22% integrated efficiency enhancement.

Concepts: Nanoparticle, Nanotechnology, Gold, Solar cell, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors.

Concepts: Fundamental physics concepts, Laser, Resonator, Wave, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


The miniaturization of integrated optical circuits below the diffraction limit for high-speed manipulation of information is one of the cornerstones in plasmonics research. By coupling to surface plasmons supported on nanostructured metallic surfaces, light can be confined to the nanoscale, enabling the potential interface to electronic circuits. In particular, gap surface plasmons propagating in an air gap sandwiched between metal layers have shown extraordinary mode confinement with significant propagation length. In this work, we unveil the optical properties of gap surface plasmons in silver nanoslot structures with widths of only 25 nm. We fabricate linear, branched and cross-shaped nanoslot waveguide components, which all support resonances due to interference of counter-propagating gap plasmons. By exploiting the superior spatial resolution of a scanning transmission electron microscope combined with electron energy-loss spectroscopy, we experimentally show the propagation, bending and splitting of slot gap plasmons.

Concepts: Diffraction, Optics, Condensed matter physics, Gold, Wave, Wavelength, Surface plasmon resonance, Waves in plasmas


Plasmonic nanostructures have played a significant role in the field of nanotechnology due to their unprecedented ability to concentrate light at the nanometre scale, which renders them precious for various sensing applications. The adsorption of plasmonic nanoparticles and nanostructures onto solid substrates in a controlled manner is a crucial process for the fabrication of nanoplasmonic devices, in which the nanoparticles amplify the electromagnetic fields for enhanced device performance. In this perspective article we summarize recent developments in the fabrication of flexible nanoplasmonic devices for sensing applications based on surface enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR) shifts. We introduce different types of flexible substrates such as filter paper, free-standing nanofibres, elastomers, plastics, carbon nanotubes and graphene, for the fabrication of low-cost flexible nanoplasmonic devices. Various techniques are described that allow impregnation of such flexible substrates with plasmonic nanoparticles, including solution processes, physical vapour deposition and lithographic techniques. From the discussion in this Perspective, it is clear that highly sensitive and reproducible flexible plasmonic devices can currently be fabricated on a large scale at relatively low-cost, toward real-world applications in diagnostics and detection.

Concepts: Electromagnetism, Nanoparticle, Nanotechnology, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


The optical properties of metal nanoparticles have attracted wide interest. Recent progress in controlling nanoparticles with atomic precision (often called nanoclusters) provide new opportunities for investigating many fundamental questions, such as the transition from excitonic to plasmonic state–which is a central question in metal nanoparticle research because it provides insights into the origin of surface plasmon resonance (SPR) as well as the formation of metallic bond. However, this question still remains elusive because of the extreme difficulty in preparing atomically precise nanoparticles larger than 2 nm. Here we report the synthesis and optical properties of an atomically precise Au279(SR)84 nanocluster. Femtosecond transient absorption spec-troscopic analysis reveals that the Au279 nanocluster shows a laser power dependence in its excited state lifetime, indicating metallic state of the particle, in contrast with the nonmetallic electronic structure of the Au246(SR)80 nanocluster. Steady-state absorption spectra reveal that the nascent plasmon band of Au279 at 506 nm shows no peak shift even down to 60 K, consistent with plasmon behavior. The sharp transition from nonmetallic Au246 to metallic Au279 is surprising and will stimulate future theoretical work on the transition and many other relevant issues.

Concepts: Nanoparticle, Atom, Nanotechnology, Gold, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


Plasmonic biosensing has been demonstrated to be a powerful technique for quantitative determination of molecular analytes and kinetic analysis of biochemical reactions. However, interfaces of most plasmonic biosensors are made of noble metals, such as gold and silver, which are not compatible with industrial production technologies. This greatly limits biosensing applications beyond biochemical and pharmaceutical research. Here, we propose and investigate copper-based biosensor chips fully fabricated with a standard complementary metal-oxide-semiconductor (CMOS) process. The protection of thin copper films from oxidation is achieved with SiO2 and Al2O3 dielectric films deposited onto the metal surface. In addition, the deposition of dielectric films with thicknesses of only several tens of nanometers significantly improves the biosensing sensitivity, owing to better localization of electromagnetic field above the biosensing surface. According to surface plasmon resonance (SPR) measurements, the copper biosensor chips coated with thin films of SiO2 (25 nm) and Al2O3 (15 nm) show 55% and 75% higher sensitivity to refractive index changes, respectively, in comparison to pure gold sensor chips. To test biomolecule immobilization, the copper-dielectric biosensor chips are coated with graphene oxide linking layers and used for the selective analysis of oligonucleotide hybridization. The proposed plasmonic biosensors make SPR technology more affordable for various applications and provide the basis for compact biosensors integrated with modern electronic devices.

Concepts: Electromagnetism, Aluminium, Gold, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas, Biosensor


We present the synthesis, separation, and characterization of covalently-bound multimers of para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters. The multimers were synthesized by performing a ligand-exchange reaction of a pre-characterized Au102(p-MBA)44 nanocluster with biphenyl-4,4'-dithiol (BPDT). The reaction products were separated using gel electrophoresis yielding several distinct bands. The bands were analyzed by transmission electron microscopy (TEM) revealing monomer, dimer, and trimer fractions of the nanocluster. TEM analysis of dimers in combination with molecular dynamics simulations suggest that the nanoclusters are covalently bound via a disulfide bridge between BPDT molecules. The linking chemistry is not specific to Au102(p-MBA)44. The same approach yields multimers also for a larger monodisperse p-MBA-protected cluster of approximately 250 gold atoms, Au∼250(p-MBA)n. While the Au102(p-MBA)44 is not plasmonic, the Au∼250(p-MBA)n nanocluster supports localized surface plasmon resonance (LSPR) at 530 nm. Multimers of the Au∼250(p-MBA)n exhibit additional transitions in their UV-vis spectrum at 630 nm and 810 nm, indicating the presence of hybridized LSPR modes. Well-defined structures and relatively small sizes make these systems excellent candidates for connecting ab initio theoretical studies and experimental quantum plasmonics. Moreover, our work opens new possibilities in the controlled synthesis of advanced monodisperse nanocluster superstructures.

Concepts: Electron, Nanoparticle, Nanotechnology, Gold, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


The aggregation of gold nanoparticles (AuNPs) is known to induce an enhancement of localized surface plasmon resonance due to the coupling of plasmonic fields of adjacent nanoparticles. Here we show that AuNPs aggregation also causes a significant enhancement of chemiluminescence in the presence of luminophores. The phenomenon is used to introduce a rapid and sensitive DNA detection method that does not require amplification. DNA probes conjugated to AuNPs were used to detect a DNA target sequence specific to the fungus Ceratocystis fagacearum, causal agent of oak wilt. The hybridization of the DNA target with the DNA probes results in instantaneous aggregation of AuNPs into nanoballs, leading to a significant enhancement of luminol chemiluminescence. The enhancement reveals a linear correlation (R(2) = 0.98) to the target DNA concentration, with a limit of detection down to 260 fM (260 × 10(-15) M), two orders of magnitude higher than the performance obtained with plasmonic colorimetry and absorption spectrometry of single gold nanoparticles. Furthermore, the detection can be performed within 22 min using only a portable luminometer.

Concepts: Spectroscopy, Nanoparticle, Nanotechnology, Gold, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas


Doped metal oxide nanocrystals represent an exciting frontier for colloidal synthesis of plasmonic materials, displaying unique optoelectronic properties and showing promise for a variety of applications. However, fundamental questions about the nature of doping in these materials remain. In this article, the strong influence of radial dopant distribution on the optoelectronic properties of colloidal indium tin oxide nanocrystals is reported. Comparing elemental depth-profiling by X-ray photoelectron spectroscopy (XPS) with detailed modeling and simulation of the optical extinction of these nanocrystals using the Drude model for free electrons, a correlation between surface segregation of tin ions and the average activation of dopants is observed. A strong influence of surface segregation of tin on the line shape of the localized surface plasmon resonance (LSPR) is also reported. Samples with tin segregated near the surface show a symmetric line shape that suggests weak or no damping of the plasmon by ionized impurities. It is suggested that segregation of tin near the surface facilitates compensation of the dopant ions by electronic defects and oxygen interstitials, thus reducing activation. A core-shell model is proposed to explain the observed differences in line shape. These results demonstrate the nuanced role of dopant distribution in determining the optoelectronic properties of semiconductor nanocrystals and suggest that more detailed study of the distribution and structure of defects in plasmonic colloidal nanocrystals is warranted.

Concepts: Spectroscopy, Nanoparticle, Condensed matter physics, Surface plasmon resonance, Plasmon, Plasma oscillation, Waves in plasmas