Nanoparticle Dimers Research Articles

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing.

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

Surface and Tip-Enhanced Raman Spectroscopy at the Plasmonic Hot Spot of a Coordination Complex-Conjugated Gold Nanoparticle Dimer

We report a synthetic method to generate surface enhanced Raman scattering (SERS) active gold nanodimer, by employing rationally designed co-ordination complex of europium (Eu) metal with 4-mercaptobenzoic acid (4-MBA) and heterocyclic phenanthroline ligands. The reaction between the Raman active 4-MBA ligand of the Eu-complex and citrate-stabilized Au nanoparticles (cit-Au NPs) led to selective dimerization in considerable yield and precise control over hot-spot engineering to maximize SERS. Additionally, scanning probe microscopy (SPM)-correlated tip-induced Raman enhancement using He–Ne laser focusing on the plasmonically coupled Au nanodimers helped obtain intense Raman signal at the spatial resolution of single dimer. The average SERS amplification was measured to be on the order of 109, whereas that in the presence of tip-enhancement was found to be 104. This extraordinary Raman activity of an inorganic complex-coupled gold nanodimer therefore augurs well for applying chemical synthetic strategies i...

Effect of the size of silver nanoparticles on SERS signal enhancement

The localized surface plasmon resonance arising from plasmonic materials is beneficial in solution-based and thin-film sensing applications, which increase the sensitivity of the analyte being tested. Silver nanoparticles from 35 to 65 nm in diameter were synthesized using a low-temperature method and deposited in a monolayer on a (3-aminopropyl)triethoxysilane (APTES)-functionalized glass slide. The effect of particle size on monolayer structure, optical behavior, and surface-enhanced Raman scattering (SERS) is studied. While increasing particle size decreases particle coverage, it also changes the localized surface plasmon resonance and thus the SERS activity of individual nanoparticles. Using a laser excitation wavelength of 633 nm, the stronger localized surface plasmon resonance coupling to this excitation wavelength at larger particle sizes trumps the loss in surface coverage, and greater SERS signals are observed. The SERS signal enhancement accounts for the higher SERS signal, which was verified using a finite element model of a silver nanoparticle dimer with various nanoparticle sizes and separation distances.

The Sedimentation of Colloidal Nanoparticles in Solution and Its Study Using Quantitative Digital Photography

AbstractSedimentation and diffusion are important aspects of the behavior of colloidal nanoparticles in solution, and merit attention during the synthesis, characterization, and application of nanoparticles. Here, the sedimentation of nanoparticles is studied quantitatively using digital photography and a simple model based on the Mason–Weaver equation. Good agreement between experimental time‐lapse photography and numerical solutions of the model is found for a series of gold nanoparticles. The new method is extended to study for the first time the gravitational sedimentation of DNA‐linked gold nanoparticle dimers as a model system of a higher complexity structure. Additionally, simple formulas are derived for estimating suitable parameters for the preparative centrifugation of nanoparticle solutions.

Exploring the scattering directionality and light interaction in nanoparticle dimers of different semiconductors

Assuming as a starting point our recent work on a dimer of silicon nanoparticles with light scattering directionality, we have explored the light interaction between the incoming and scattered electric fields in dimers made of other different semiconductors. The scattering directionality is achieved by accomplishing Kerker's conditions. By directing the scattered light towards the gap of the dimer, interferential effects can be used to achieve high or low light intensities as a basis of all-optical nanoswitches. A comparison between dimers of different materials is shown. Full Text: PDF ReferencesR. Gómez-Medina, B. García-Cámara, I. Súarez-Lacalle, F. González, F. Moreno, M. Nieto-Vesperias and J.J. Sáenz, "Electric and magnetic dipolar response of germanium nanospheres: interference effects, scattering anisotropy, and optical forces", J. Nanophoton. 5 053512 (2011). CrossRef B. Rolly, B. Stout and N. Bonod, "Boosting the directivity of optical antennas with magnetic and electric dipolar resonant particles", Opt. Express 20 20376 (2012). CrossRef B. García-Cámara, R. Gómez-Medina, J.J. Sáenz, and B. Sepúlveda, "Sensing with magnetic dipolar resonances in semiconductor nanospheres", Opt. Express 21 23007-23020 (2013). CrossRef B. García-Cámara et al., "All-Optical Nanometric Switch Based on the Directional Scattering of Semiconductor Nanoparticles", J. Phys. Chem. C. 119, 19558?19564 (2015). CrossRef A.I. Barreda, H. Saleh, A. Litman, F. González, J-M. Geffrin, and F. Moreno, "Electromagnetic polarization-controlled perfect switching effect with high-refractive-index dimers and the beam-splitter configuration", Nat. Commun. 8, 13910 (2017). CrossRef R. Vergaz et al., "Control of the Light Interaction in a Semiconductor Nanoparticle Dimer Through Scattering Directionality", IEE Phot. Jour., 8(3), 4501410 (2016) CrossRef B. García-Cámara et al., "Size Dependence of the Directional Scattering Conditions on Semiconductor Nanoparticles", IEEE Photon. Technol. Lett. 27(19), 2059?2062 (2015). CrossRef

Single-molecule surface-enhanced Raman spectroscopy of 4,4′-bipyridine on a prefabricated substrate with directionally arrayed gold nanoparticle dimers

In this study, single-molecule detection on a prefabricated substrate through surface-enhanced Raman spectroscopy (SERS) with 4,4′-bipyridine molecules was achieved. The use of a substrate with directionally arrayed gold nanoparticle dimers was proposed for the single-molecule detection and identification of a wide range of bio/chemical molecules. Around 50 Raman measurements and statistical analyses were performed to demonstrate a single-molecule SERS. At 10−11 M, the distribution was fitted by three Gaussian curves, whereas the distribution of Raman intensities was fitted by one Gaussian curve at 10−5 M. The probability of molecule detection is consistent with the Poisson distribution. This result indicates the possibility of detecting 0, 1, and 2 molecules. Thus, we confirmed that the developed substrates achieved single-molecule SERS detection and identification.

Dimers and Trimers of Hollow Silicon Nanoparticles: Manipulating the Magnetic Hotspots

We study magnetic and electric hotspots in the gaps of nanoparticle dimers and trimers composed of hollow silicon nanoparticles. We find that the hollow size markedly influences the overall shape of the field enhancement, in particular, the peak magnitude and the peak position. The peak shifts about 100–200 nm as the hollow size varies. Tuned magnetic hotspots allow one to better enhance the Raman optical activity and circular dichroism of molecules, the interaction of light and nanomagnets, and so on.

Aluminum Nanoparticles with Hot Spots for Plasmon‐Induced Circular Dichroism of Chiral Molecules in the UV Spectral Interval

Plasmonic nanocrystals with hot spots are able to localize optical energy in small spaces. In such physical systems, near‐field interactions between molecules and plasmons can become especially strong. This paper considers the case of a nanoparticle dimer and a chiral biomolecule. In this model, a chiral molecule is placed in the gap between two plasmonic nanoparticles, where the electromagnetic hot spot occurs. Since many important biomolecules have optical transitions in the UV spectral region, the case of aluminum nanoparticles is considered, as they offer strong electromagnetic enhancements in the blue and UV spectral intervals. The calculations in this study show that the complex composed of a chiral molecule and an Al dimer exhibits strong circular dichroism (CD) signals in the plasmonic spectral region. In contrast to the standard Au and Ag nanocrystals, the Al system may have a much better spectral overlap between the typical biomolecule's optical transitions and the nanocrystals' plasmonic band. Overall, it is found that Al nanocrystals used as CD antennas exhibit unique properties as compared to other commonly studied plasmonic and dielectric materials. The plasmonic systems investigated in this study can be potentially used for sensing chirality of biomolecules, which is of interest in applications such as drug development.

The Computational Studies of Plasmon Interaction

In this paper, an interaction of metal nanoparticles that appears in the extinction spectra was investigated. The mutual coupling between the nanoparticles, the effect of size difference, and the interparticle separation in silver nanoparticle dimers are studied by computer discrete dipole approximation methods. The obtained results show that nanoparticle interaction results in the distinct collective modes, known as the low-energy bonding modes and the higher-energy antibounding modes. The spectral position of the modes is analyzed as a function of the ratio of interparticle distance to particle size that reduces the dependency on the particle size itself. The optical spectra of nanoparticles that form the fractal cluster were investigated. It was found that the number of spectral bands increase with the growth of the number of nanoparticles in the fractal cluster, which are described within the plasmon hybridization model.

One-dimensional plasmonic hotspots located between silver nanowire dimers evaluated by surface-enhanced resonance Raman scattering

Hotspots of surface-enhanced resonance Raman scattering (SERRS) are localized within 1 nm at gaps or crevices of plasmonic nanoparticle (NP) dimers. We demonstrate SERRS hotspots with volumes that are extended in one dimension tens of thousand times compared to standard zero-dimensional hotspots using gaps or crevices of plasmonic nanowire (NW) dimers.

Metal-Substrate-Mediated Plasmon Hybridization in a Nanoparticle Dimer for Photoluminescence Line-Width Shrinking and Intensity Enhancement.

Metal-film-coupled nanoparticles with subnanometer particle-film gaps possess an ultrasmall mode volume, responsible for a variety of intriguing phenomena in plasmonic nanophotonics. Due to the large radiative loss associated with dipolar coupling, however, the plasmonic-film-coupled nanocavities usually feature a low-quality factor, setting an ultimate limit of the increased light-matter interaction strength. Here, we demonstrate a plasmonic nanocavity composed of a metal-film-coupled nanoparticle dimer, exhibiting a significantly improved quality factor. Compared to a silica-supported dimer, the spectral line width of the nanocavity plasmon resonance is reduced by a factor of ∼4.6 and is even smaller than its monomer counterpart (∼30% reduction). Comprehensive theoretical analyses reveal that this pronounced resonance narrowing effect can be attributed to intense film-mediated plasmon hybridization between the bonding dipolar and quadrupolar gap modes in the dimer. More importantly, the invoking of the dark quadrupole resonance leads to a giant photoluminescence intensity enhancement (∼200 times) and dramatic emission line-width narrowing (∼4.6 times), compared to the silica-supported dimer. The similar spectral characteristics of the measured plasmonic scattering and photoluminescence emission indicate that the radiative decay of the coupled plasmons in the nanocavity is the origin of the observed photoluminescence, consistent with a proposed phenomenological model. Numerical calculations show that the intensity enhancement is mainly contributed by the dimer-film gap rather than the interparticle gap. These findings not only shed more light on the hybridized interaction between plasmon modes but also deepen the understanding of photoluminescence emission in coupled plasmonic nanostructures.

Building Electromagnetic Hot Spots in Living Cells via Target-Triggered Nanoparticle Dimerization.

Electromagnetic hot spots of surface-enhanced Raman scattering have been extensively employed for bioanalysis in solution or on a substrate, but building hot spots in living systems for probing targets of interest has not been achieved yet because of the complex and dynamic physiological environment. Herein, we show that a target-programmed nanoparticle dimerization can be combined with the background-free Raman reporters (alkyne, C≡C; nitrile, C≡N) for multiplexed imaging of microRNAs (miRNAs) in living cells. The in situ formation of plasmonic dimers results in an intense hot spot, thus dramatically enhancing the Raman signals of the reporters residing in the hot spot. More significantly, the reporters exhibit single nonoverlapping peaks in the cellular Raman-silent region (1800-2800 cm-1), thus eliminating spectral unmixing and background interference. A 3D Raman mapping technique was harnessed to monitor the spatial distribution of the dimers and thus the multiple miRNAs in cells. This approach could be extended to probe other biomarkers of interest for monitoring specific pathophysiological events at the live-cell level.

Radiative heat transfer in many-body systems: Coupled electric and magnetic dipole approach

The many-body radiative heat transfer theory [P. Ben-Abdallah, S.-A. Biehs, and K. Joulain, Phys. Rev. Lett. 107, 114301 (2011)] only considered the contribution from the electric dipole moment. For metal particles, however, the magnetic dipole moment due to eddy current plays an important role, which can further couple with the electric dipole moment to introduce crossed terms. In this work, we develop coupled electric and magnetic dipole (CEMD) approach for the radiative heat transfer in a collection of objects in mutual interaction. Due to the coupled electric and magnetic interactions, four terms, namely the electric-electric, the electric-magnetic, the magnetic-electric and the magnetic-magnetic terms, contribute to the radiative heat flux and the local energy density. The CEMD is applied to study the radiative heat transfer between various dimers of nanoparticles. It is found that each of the four terms can dominate the radiative heat transfer depending on the position and composition of particles. Moreover, near-field many-body interactions are studied by CEMD considering both dielectric and metallic nanoparticles. The near-field radiative heat flux and local energy density can be greatly increased when the particles are in coupled resonances. Surface plasmon polariton and surface phonon polariton can be coupled to enhance the radiative heat flux.

Direct observation of localized surface plasmon field enhancement by Kelvin probe force microscopy

A surface plasmon (SP) is a fundamental excitation state that exists in metal nanostructures. Over the past several years, the performance of optoelectronic devices has been improved greatly via the SP enhancement effect. In our previous work, the responsivity of GaN ultraviolet detectors was increased by over 30 times when using Ag nanoparticles. However, the physics of the SP enhancement effect has not been established definitely because of the lack of experimental evidence. To reveal the physical origin of this enhancement, Kelvin probe force microscopy (KPFM) was used to observe the SP-induced surface potential reduction in the vicinity of Ag nanoparticles on a GaN epilayer. Under ultraviolet illumination, the localized field enhancement induced by the SP forces the photogenerated electrons to drift close to the Ag nanoparticles, leading to a reduction of the surface potential around the Ag nanoparticles on the GaN epilayer. For an isolated Ag nanoparticle with a diameter of ~200 nm, the distribution of the SP localized field is located within 60 nm of the boundary of the Ag nanoparticle. For a dimer of Ag nanoparticles, the localized field enhancement between the nanoparticles was the strongest. The results presented here provide direct experimental proof of the localized field enhancement. These results not only explain the high performance of GaN detectors observed with the use of Ag nanoparticles but also reveal the physical mechanism of SP enhancement in optoelectronic devices, which will help us further understand and improve the performance of SP-based optoelectronic devices in the future.

Live-Cell Pyrophosphate Imaging by in Situ Hot-Spot Generation.

Controlling the electromagnetic hot-spot generation is essential for surface-enhanced Raman scattering (SERS) assays. Current hot-spot-based SERS assays have been extensively studied in solutions or on substrates. However, probing biospecies by controlling the hot-spot assembly in living systems has not been demonstrated thus far. Herein, we report a background-free SERS probe for imaging pyrophosphate (PPi), a biochemically significant anion, in living cells. Intracellular PPi is able to induce the nanoparticle dimerization, thus creating an intense electromagnetic hot spot and dramatically enhancing the signal of the Raman reporters residing in the hot spot. More impressively, the reporter we used in this study provides a strong and sharp single peak in the cellular Raman-silent region (1800-2800 cm-1), thus eliminating the possible background interference. This strategy could be readily extended to detect other biomarkers by only replacing the recognition ligands.

Interstitial-Dependent Enhanced Photoluminescence: A Near-Field Microscopy on Single Spheroid to Dimer, Tetramer, and Few Particles Gold Nanoassembly

Interstitials to be active or inactive depend on the characteristics of nanoaggregate. In this perspective, near-field scanning optical microscopy has been utilized to probe well-defined monomer, dimer, tetramer, and small assembly of gold nanoparticles of 60 nm diameter with spatial resolution of ∼35 nm. Confinement of localized surface plasmon resonances (LSPRs)-mediated near electromagnetic (EM) field distribution was observed through two-photon induced photoluminescence (TPI-PL) since TPI-PL emission is proportional to the fourth power of induced EM-field. It was revealed that the intensity of TPI-PL observed in closely connected nanoparticles was very strong with reference to those obtained in the presence of isolated nanoparticles of the same size. Among the closely connected nanoparticles, interparticle gap and interparticle axes played a vital role in TPI-PL emission enhancement, as was evident and observed directly through TPI-PL intensity distribution. Correlated local geometry and emission dist...

High-yield colloidal synthesis of monometallic Au nanorod–Au nanoparticle dimers and their application in SERS

Dimeric nanostructures have attracted much attention owing to their unique structure and excellent physiochemical properties.

Silica-encapsulated gold nanoparticle dimers for organelle-targeted cellular delivery.

Silica-encapsulated gold nanoparticle dimers were self-assembled through a single-insertion process using capillary force and can be utilized as an advanced drug-delivery and sensing platform for organelle-targeting in cancer cells.

In situ construction of polymer-encapsulated Au nanoparticle dimers based on a C–C coupling reaction

A polymer-encapsulated Au nanoparticle dimer was fabricated via C–C coupling reaction. The strong effect of LSPR, SERS and SPR catalysis were observed in the gap. It is expected to provide rich information for understanding SERS mechanism.

Anticorrelation of Photoluminescence from Gold Nanoparticle Dimers with Hot-Spot Intensity.

Bulk gold shows photoluminescence (PL) with a negligible quantum yield of ∼10–10, which can be increased by orders of magnitude in the case of gold nanoparticles. This bears huge potential to use noble metal nanoparticles as fluorescent and unbleachable stains in bioimaging or for optical data storage. Commonly, the enhancement of the PL yield is attributed to nanoparticle plasmons, specifically to the enhancements of scattering or absorption cross sections. Tuning the shape or geometry of gold nanostructures (e.g., via reducing the distance between two nanoparticles) allows for redshifting both the scattering and the PL spectra. However, while the scattering cross section increases with a plasmonic redshift, the PL yield decreases, indicating that the common simple picture of a plasmonically boosted gold luminescence needs more detailed consideration. In particular, precise experiments as well as numerical simulations are required. Hence, we systematically varied the distance between the tips of two gold bipyramids on the nanometer scale using AFM manipulation and recorded the PL and the scattering spectra for each separation. We find that the PL intensity decreases as the interparticle coupling increases. This anticorrelation is explained by a theoretical model where both the gold-intrinsic d-band hole recombination probabilities as well as the field strength inside the nanostructure are considered. The scattering cross section or the field strength in the hot-spot between the tips of the bipyramids are not relevant for the PL intensity. Besides, we not only observe PL supported by dipolar plasmon resonances, but also measure and simulate PL supported by higher order plasmonic modes.