Self-assembly Of Metal Nanoparticles Research Articles

Plasmonic and diffractive nanostructures for light trapping—an experimental comparison

Metal nanoparticles and diffractive nanostructures are widely studied for enhancing light trapping efficiency in thin-film solar cells. Both have achieved high performance enhancements, but there are very few direct comparisons between the two. Also, it is difficult to accurately determine the parasitic absorption of metal nanoparticles. Here, we assess the light trapping efficiencies of both approaches in an identical absorber configuration. We use a 240 nm thick amorphous silicon slab as the absorber layer and either a quasi-random supercell diffractive nanostructure or a layer of self-assembled metal nanoparticles for light trapping. Both the plasmonic and diffractive structures strongly enhance the absorption in the red/near-infrared regime. At longer wavelengths, however, parasitic absorption becomes evident in the metal nanoparticles, which reduces the overall performance of the plasmonic approach. We have formulated a simple analytical model to assess the parasitic absorption and effective reflectivity of a plasmonic reflector and to demonstrate good agreement with the experimental data.

Self-assembled metallic nanoparticle superlattices on large-area graphene thin films: growth and evanescent waveguiding properties

Self-assembly of copper nanoparticle (Cu-np) superlattices on graphene thin films is demonstrated. These superlattices show visible light evanescent waveguiding properties.

Growth mechanisms of self-assembled gold nanoparticles in Deep Eutectic Solvent

Self-assembled metallic nanoparticles are attractive candidates for plasmonic heating, non-linear optical switching [1], bio-analytical, chemical [2], catalytic , and surface enhanced RAMAN scattering (SERS) [3]. These applications are strongly dependent on the shape, size, composition, size distribution and volume fraction of nanoparticles. Here, self-assembly of gold nanoparticles (AuNPs) was obtained by low energy sputter deposition on Deep Eutectic Solvent (DES ; choline chloride and urea) surfaces and elucidated by Small Angle X-ray Scattering (SAXS), Cryogenic Transmission Electron Microscopy (Cryo-TEM) and UV-Vis. Data analysis shows the formation of spherically shaped AuNPs of 5 nm in diameter with narrow size distributions. Moreover, analysis reveals that prolongation of gold-sputtering time has no effect on the size of the particles and only the concentration of AuNPs increases linearly. The growth of the maxima in evaluated structure factor S(q) and the distance distribution function G(r) at higher concentrations of AuNPs is caused by the interference effects. Moreover, it indicates that the particles are not arranged in random but have a self-assembly in short-range order. Prolonged gold-sputtering time leads to increase in the ordering of the AuNPs with strong interactions. It is proposed that the self-assembly of AuNPs is due to the ionic liquid template effects of DES and the balancing physical forces. Moreover, a disulfide based stabilizer bis ((2-Mercaptoethyl) trimethylammonium) disulfide dichloride was applied to supress the self-assembly. The stabilizer even reverses the self-assembled or agglomerated AuNPs back to stable 5 nm individual particles. The templating effect of DES is compared with the non-templating solvent Castor oil. Our results will also pave a way to understand and control self-assembly of metallic and bimetallic nanoparticles.

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

Self-assembly of metallic nanoparticles into one dimensional arrays

One-dimensional (1D) assemblies of nanoparticles are a burgeoning area of research due to their potential in electronics, photonics and sensing applications. They are also the focus of academic study, as they provide a link between the nanoscale world and meso- or macroscale objects. Despite their potential application in a variety of optoelectronic, electronic, photonic and magnetic applications, 1D assemblies of nanoparticles have not been as thoroughly studied or reviewed as their 2- and 3D counterparts. This review presents an overview of the methods used to prepare 1D arrays of metallic nanoparticles, covering both templated and template-free methods. The unique properties they possess are discussed, along with existing and predicted applications. The review finishes by addressing the current challenges facing the field of research and practical applications of 1D nanoparticle arrays.

Formation of oriented nanostructures in diamond using metallic nanoparticles

A simple, fast and cost-effective etching technique to create oriented nanostructures such as pyramidal and cylindrical shaped nanopores in diamond membranes by self-assembled metallic nanoparticles is proposed. In this process, a diamond film is annealed with thin metallic layers in a hydrogen atmosphere. Carbon from the diamond surface is dissolved into nanoparticles generated from the metal film, then evacuated in the form of hydrocarbons and, consequently, the nanoparticles enter the crystal volume. In order to understand and optimize the etching process, the role of different parameters such as type of catalyst (Ni, Co, Pt, and Au), hydrogen gas, temperature and time of annealing, and microstructure of diamond (polycrystalline and nanocrystalline) were investigated. With this technique, nanopores with lateral sizes in the range of 10–100 nm, and as deep as about 600 nm, in diamond membranes were produced without any need for a lithography process, which opens the opportunities for fabricating porous diamond membranes for chemical sensing applications.

Nanoparticle assembly for sensitive DNA detection using SERRS

SERRS (surface-enhanced resonance Raman scattering) is a vibrational technique, whereby a relatively weak Raman scattering effect is enhanced through the use of a visible chromophore and a roughened metal surface. The direct analysis of DNA by SERRS requires the modification of a nucleic acid sequence to incorporate a chromophore, and adsorption of the modified sequence on to a roughened metal surface. Aggregated metallic nanoparticles are commonly used in the analysis of dye-labelled DNA by SERRS, allowing for detection levels that rival those gained from standard fluorescence-based techniques. In the present paper, we report on how SERRS can be exploited for the analysis of clinically relevant DNA samples. We also report on the ability of nanoparticles to aggregate as the result of a biologically significant event, as opposed to the use of an external charge-modifying agent. The self-assembly of metallic nanoparticles is shown to be a promising new technique in the move towards extremely sensitive methods of DNA analysis by SERRS.

A bottom-up approach to fabricate optical metamaterials by self-assembled metallic nanoparticles

We introduce a novel bottom-up approach to fabricate by self assembly a metamaterial from metallic nanoparticles in a two-step process. In the first step, a metamaterial made of densely packed silver nanoparticles is required. The material dispersion with increasing nanoparticle densities, from dispersed to randomly packed nanoparticles, was measured by spectroscopic ellipsometry, demonstrating high permittivity values in the visible. In the second step, this material was used to prepare spherical clusters by a method based on oil-in-water emulsion. The optical properties of these clusters were equally investigated by spectroscopic means. Comparisons with rigorous numerical simulations clearly indicate that, depending on the cluster size, their spectral response can be unambiguously associated with the excitation of a magnetic dipole resonance. As a consequence, such spherical clusters are promising building blocks for future metamaterials possessing a magnetic response in the visible range.

Salt-mediated kinetics of the self-assembly of gold nanorods end-tethered with polymer ligands

Studies on the self-assembly of metal nanoparticles (NPs) in the presence of ions are motivated by the biosensing applications of NP clusters and the capability to control the morphology of clusters of oppositely charged NPs. The effect of ions has been explored for the self-assembly of metal NPs capped solely with ionic ligands, whereas, in general, the surface of NPs can be coated with a mixture of ligands interacting with each other by non-electrostatic forces. In the present work, we examined the kinetics of self-assembly of gold nanorods capped with a mixture of low-molecular weight ionic molecules and nonpolar polymer ligands. We show that in contrast with earlier reports on the effect of electrolytes on NP self-assembly, the driving force for the accelerated self-assembly of nanorods is the reduction in polymer solubility in the presence of ions, rather than the screening of the electric double layer of the charged ligands. The reported results are important for NP self-assembly occurring in mixed solvents, in which attraction forces between nonpolar ligands are governed by the balance between solvent-solvent and solvent-salt interactions. Furthermore, the addition of salts can be used to increase the rate of nanorod self-assembly, which, otherwise, is an intrinsically slow process.

Self-assembly of metallic nanoparticles into plasmonic rings

Metallic nanoparticles are self-assembled into plasmonic nanorings. The self-assembly is evaporation-induced and is driven using a template of dielectric microspheres. We obtain well-ordered arrays of metallic nanorings over large areas. The inner and outer diameters of the rings, as well as the pitch of the array, are fully controllable. The optical resonances supported by the plasmonic rings are then characterized using extinction spectroscopy. Our approach opens a simple and inexpensive road to create plasmonic structures that can find applications as metamaterials or substrates for enhanced spectroscopies.

Structure and optical properties of self-assembled multicomponent plasmonic nanogels

Multicomponent plasmonic nanogels (PNGs) capable of broadband absorption of light in the 400−700 nm wavelength range were synthesized by the self-assembly of metal nanoparticles with wormlike surfactant micelles. Small angle x-ray scattering and rheological experiments suggest that the nanoparticles bridge micelle fragments to aid the formation a stable gel phase with exceptional color uniformity. Their optical absorbance could be robustly tuned by changing the nanoparticle type (Au/Ag), size, shape, and/or concentration. The PNGs have relatively low viscosity and are thermoreversible. Potential applications to the manufacturing of coatings and interfaces for solar energy harvesting and reconfigurable optical devices can be envisioned.

Versatile fabrication of self-assembled metallic nanoparticle arrays

One of the challenging aspects of nanotechnology is the development of an effective and potentially universal method to place nanoparticles (NPs) into spatially well-defined, ordered, defect-free arrays. This can be achieved using “top-down” approaches, such as optical, electron beam, focused ion-beam and scanning probe lithography, or “bottom-up” approaches based on self-assembly. Here, we report the simple and rapid electrochemical generation of periodic surface defects, used to fabricate metallic NP arrays having good feature size and spacing control over a large area, without involving costly and time-consuming nanolithographic methods. Our high-throughput nanofabrication approach combines electrochemical anodization to quickly and reproducibly form a highly ordered Ta-based nanotemplate, in the form of inverted hemispherical caps (dimples), with the simplicity of thin metallic film dewetting techniques, forming a self-assembled metallic (individual metals or alloy) NP array. These can be used as nanoelectrode arrays that may have useful applications in analytical chemistry, biosensing, and electrocatalysis.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2–12 nm and interparticle distances D = 20–140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L10 phase, however, this is still less by a factor of 3–4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.

Self-assembly of silver nanoparticles into dendritic flowers from aqueous solution

Similar to folding of proteins into three-dimensional (3D) structure, self-assembly of metallic nanoparticles (NPs) into dendritic or other supramolecular structures is of greater interest, yet less understood. We observed spontaneous, template-free organisation of silver NPs into 3D, dendritic, elegant, flower-like structures from an aqueous solution containing AgNO3 and L-cysteine (Cys). The resultant NPs and dendritic structures were characterised using UV–Vis spectroscopy and scanning and transmission electron microscopy. The process initiated most likely by diffusion-limited aggregation in the liquid phase, which were further grown into elegant, dendritic flowers probably by evaporation of residual wetting layer of larger NPs. Electrostatic attraction between –COO− and –NH3 + groups of L-Cys on the surface of silver NPs may also have a role in their growth.

Nanoscale force induced size-selective separation and self-assembly of metal nanoparticles: sharp colloidal stability thresholds and hcp ordering

A simple and versatile nanoscale force induced precipitation approach for the separation of gold nanoparticles (AuNPs) was developed. The AuNPs show sharp size-dependent colloidal stability thresholds as a function of salt concentration. Upon separation, the AuNPs were electrostatically self-assembled onto silicon substrates by fine-tuning interparticle and particle-substrate forces, forming 2D AuNP networks with a high degree of hexagonal closest pack (hcp) superstructures.

Probing the average size of self-assembled metal nanoparticles using x-ray standing waves

We report application of x-ray standing-wave field for characterization of average vertical size of metal nanoparticles dispersed on a substrate surface with accuracies better than 1 nm. This method is applied to analyze the distribution of Fe, Co, and Au nanoparticles on Si substrate and W/C multilayer substrates. Results demonstrate that the method is a valuable tool to evaluate the surface morphology of nanoparticles over a large surface area. Unlike conventional probes such as atomic force microscopy or microinterferometry, the present method provides element-specific analysis. It also has the advantage of being able to examine nanoparticles in a liquid medium, or buried inside a coating layer. We anticipate that the proposed method has great potential to infer the internal structure of nanoparticles.

Charge Storage in Pentacene/Polymethylmethacrylate Memory Devices

The electrical behavior of organic metal-insulator-semiconductor (MIS) structures incorporating a layer of self-assembled metallic nanoparticles is described. These have been based on thermally evaporated pentacene (semiconductor) and spin-coated polymethylmethacrylate (insulator). The MIS devices containing the nanoparticles exhibited significant hysteresis in their capacitance-versus-voltage and conductance-versus-voltage characteristics, which was attributed to the charging and discharging of the nanoparticles. The memory structure reported here offers a useful advance in the development of flexible organic memory structures.

Processing and near-field optical properties of self-assembled plasmonic nanoparticle networks

We report on recent progress in the production and the deposition on surfaces of dielectric materials of self-assembled metallic nanoparticle systems able to organize into branched networks. We emphasize the interest of these new objects for tailoring novel near-field optical properties that could be the basis for optical energy transport in systems of extremely reduced sizes. The experimental optical spectra of such chain networks in solution can be very well reproduced by applying the coupled-dipole approximation scheme. In this paper, we apply this method to investigate the near-field optical properties of self-assembled plasmonic nanoparticle networks deposited on transparent surfaces.

One-step deposition of diamond-like carbon films containing self-assembled metallic nanoparticles, by femtosecond pulsed laser ablation

Femtosecond pulsed laser ablation has been used for the deposition of various thin films by co-ablating pure graphite and pure metallic targets (nickel or tantalum) under vacuum conditions with an amplified Ti:sapphire laser working at 800 nm, with about 1.5 mJ per pulse at a repetition rate of 1 kHz. The plasma plume created during the femtosecond pulsed laser deposition process has been investigated, by fast imaging and spectroscopic characterizations, depending on the ablated target and deposition conditions. Particular attention has been paid to the correlation between the film nanostructure and the plasma plume properties, in terms of ejected species and energies of impinging species on the substrate. The tantalum clusters in the tantalum-doped carbon films appear under two crystalline phases: a stable phase (α-Ta) corresponding to that of the target, and a metastable phase (β-Ta), already observed at high temperatures. The presence of this metastable phase (β-Ta) is correlated to the temperature of the clusters measured from the plasma plume investigation.

Self-Assembly of Metal Nanoparticles and Nanotubes on Bioengineered Flagella Scaffolds

The previously described FliTrx E. coli flagellin protein was genetically engineered to display rationally designed histidine, arginine-lysine, and aspartic acid-glutamic acid peptide loops on the solvent-accessible outer domain region. The resulting flagellin monomers were self-assembled to obtain the corresponding oligomeric flagella bionanotubes in which the peptide loops were 5 nm apart. These flagella nanotubes were equilibrated with solutions of various metal ions (Co(II), Cu(II), Cd(II), Ag(I), Au(I), and Pd(II)). Controlled reduction of these metal ions yielded ordered arrays of nanoparticles or nanotubes, and in some cases, extensive aggregation resulted in formation of metal nanotube bundles. Both metal nanoparticles and nanotubes were generated with Cu(II) and Au(I), depending on the initial concentration of Cu(II) ions, while Ag(I) consistently formed metal nanowires, even under relatively mild conditions of reduction. The covalent attachment of separately synthesized Au nanoparticles to the f...