Assembly Of Metal Nanoparticles Research Articles

A 3-dimensional Wentzel–Krammers–Brillouin calculation of the charging and retention times of metal nanoparticles in a dielectric matrix

When an assembly of metallic nanoparticles (mNPs) is found in a dielectric matrix over a Si substrate with an applied bias the mNPs are filled with electrons (or holes depending on the polarity of the bias.) It is customary to calculate the times for either charging (i.e. programming) or discharging when the bias is removed (i.e. retention) by either (i) simple capacitor models or (ii) solving the three dimensional (3D) Poisson equation and using a simplified 1D Wentzel–Krammers–Brillouin (WKB) model for the current despite the 3D nature of the potential profile. In this paper we use a fully 3D self-consistent method in which both potential and current are calculated by a 3D Poisson and a 3D WKB method respectively. The methodology has been applied in other cases and we now apply it to Non-Volatile-Memory structures with Pt NPs in a SiO2/HfO2 matrix fabricated by our group and reported in this paper.

Shape‐Controlled Metal Nanoparticles and Their Assemblies with Optical Functionalities

Metal nanoparticles (NPs) possess excellent optical, optoelectronic, and optochemical properties based on their surface plasmon resonance. However, for practical use, the morphology and assembly of metal NPs need to be controlled. Here, we review facile control methods including seed‐mediated growth accompanied with a comproportionation reaction of seeds to control their morphology and assembly. Several synthetic conditions have been modified to precisely control the morphology of metal NPs. Functionalized mesoporous oxides have also been used as hard templates to align metal nanorods and control their dimensions. The high performance of such metal nanorods in surface‐enhanced Raman scattering, polarization of light, and photocatalysis has been measured, and the reasons for their high performance are discussed.

Tuning and assembling metal nanostructures with DNA

Here, we highlight the strategies for the synthesis and tuning of a variety of metal nanostructures and nanoassemblies with oligonucleotides and their applications. We have discussed the importance of and need for various metal nanostructures and the role and suitability of DNA in building these nanostructures and nanoassemblies. A large part of this article is devoted to the discussion of DNA-mediated synthetic methods for metal nanostructures. The synthetic strategies are categorized into two groups - strategy 1 that uses DNA as the ligand for metal nanoparticles and subsequent assembly or modification and strategy 2 that uses DNA template-directed assemblies of metal nanoparticles.

Chiral Nematic Stained Glass: Controlling the Optical Properties of Nanocrystalline Cellulose-Templated Materials

Chiral nematic mesoporous materials decorated with metal nanoparticles have been prepared using the templated self-assembly of nanocrystalline cellulose (NCC). By adding small quantities of ionic compounds to aqueous dispersions of NCC and tetramethoxysilane (TMOS), the helical pitch of the chiral nematic structure could be manipulated in a manner complementary to the ratio of NCC/TMOS previously demonstrated by our group. We have studied the transformation of these ion-loaded composites into high surface area mesoporous silica and carbon films decorated with metal nanoparticles through calcination and carbonization, respectively. This general and straightforward approach to prepare chiral nematic metal nanoparticle assemblies may be useful in a variety of applications, particularly for their chiral optical properties.

Covalent assembly of metal nanoparticles on cellulose fabric and its antimicrobial activity

We develop an antimicrobial active robust metal-cellulose nanohybrid by covalent assembly of metal nanoparticles on cellulose fabric using a simple impregnation of thiol-modified cellulose fabric in colloidal silver (Ag) or palladium (Pd) nanoparticle solutions. The combined results of high resolution transmission electron microscopy (HR-TEM), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDXS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) reveal that the nanoparticles are highly loaded and dispersed in the thiol-modified cellulose fabric, and X-ray photoelectron spectroscopy (XPS) analysis reveals that the nanoparticles are immobilized in the fabric by a strong and stable covalent bond with thiol functional group. This robust covalent linkage between the nanoparticles and the fabric leads to a remarkable suppression of the release of metal nanoparticles from the fabric. In addition, the metal-cellulose nanohybrids show high antimicrobial activity in excess of 99.9 % growth inhibition of the microorganism. Thus, we anticipate that our metal-cellulose nanohybrid may not only protect cell damage caused by penetration and fixation of metal nanoparticles into the human body but also act as a sustainable biomedical textile.

Synthesis of an embedded metal nanoparticle planar assembly by low-energy ion irradiation of a thin discontinuous metal film sandwiched in silica

Synthesis of a planar assembly of metal nanoparticles embedded in silica by low-energy ion irradiation is presented here. Argon ions of 350 keV were used to irradiate SiO2/Au/SiO2 and SiO2/Ag/SiO2 tri-layered films, with oxide thicknesses of 40 nm and metal layer thickness of 2 nm, to synthesize well isolated nanoparticles without the need of annealing. The nanoparticles have an average diameter of about 6 nm as revealed by transmission electron microscopy. Simulations by three-dimensional kinetic lattice Monte Carlo were performed to understand the ion-induced nanoparticle array formation from the initially percolated as-deposited metal layer embedded in the silica matrix.

Controlled Assembly and Plasmonic Properties of Asymmetric Core–Satellite Nanoassemblies

The assembly of noble metal nanoparticles offers an appealing means to control and enhance the plasmonic properties of nanostructures. However, making nanoassemblies with easily modifiable gap distances with high efficiency has been challenging. Here, we report a novel strategy to assemble gold nanoparticles (AuNPs) into Janus-type asymmetric core-satellite nanostructures. Markedly different desorption efficiency between large and small AuNPs in ethanol allows us to prepare the asymmetric core-satellite nanoassemblies in a dispersed colloidal state with near 100% purity. The resulting nanoassemblies have well-defined structures in which a core AuNP (51 nm) is covered by an average of 13 ± 3 satellite AuNPs (13 nm) with part of the core surfaces left unoccupied. Strong surface plasmon coupling is observed from these nanoassemblies as a result of the close proximity between the core and the satellites, which appears significantly red-shifted from the surface plasmon resonance frequencies of the constituting nanoparticles. The dependence of the surface plasmon coupling on a gap distance of less than 3 nm is systematically investigated by varying the length of the alkanedithiol linkers. The asymmetric core-satellite nanoassemblies also serve as an excellent surface-enhanced Raman scattering substrate with an enhancement factor of ~10(6). Finally, we demonstrate that the presented assembly method is extendible to the preparation of compositionally heterogeneous core-satellite nanoassemblies.

Coupling of single nitrogen‐vacancy defect centers in diamond nanocrystals to optical antennas and photonic crystal cavities

AbstractWe demonstrate the ability to modify the emission properties and enhance the interaction strength of single‐photon emitters coupled to nanophotonic structures based on metals and dielectrics. Assembly of individual diamond nanocrystals, metal nanoparticles, and photonic crystal cavities to meta‐structures is introduced. Experiments concerning controlled coupling of single defect centers in nanodiamonds to optical nanoantennas made of gold bowtie structures are reviewed. By placing one and the same emitter at various locations with high precision, a map of decay rate enhancements was obtained. Furthermore, we demonstrate the formation of a hybrid cavity quantum electrodynamics system in which a single defect center is coupled to a single mode of a gallium phosphite photonic crystal cavity.

Molecular-mediated assembly of silver nanoparticles with controlled interparticle spacing and chain length

In the present work, we report on a one-pot method for the assembly of noble metal nanoparticles with tunable optical properties, assembly length and interparticle spacing. The synthetic colloidal route is based on the covalent binding among OH-terminated silver nanoparticles by means of dicarboxylic acids with a defined molecular length. As a result, the initially symmetric plasmon band of silver nanoparticles splits into two plasmonic modes when nanoparticles are assembled due to the strong near-field plasmon coupling. We noticed a very good correlation between the plasmon wavelength shift and the interparticle spacing that is represented by the universal scaling law of the surface plasmon resonance in metal nanoparticle dimers. A relationship between the plasmon coupling and the assembly size (represented by the number of nanoparticles) for two different interparticle distances has been experimentally found. Such a correlation has revealed the additional effect of the electronic polarizability of the linker on the propagation of the plasmon coupling between NPs.

Low absorption losses of strongly coupled surface plasmons in nanoparticle assemblies

Coupled surface plasmons in one-dimensional assemblies of metal nanoparticles have attracted significant attention because strong interparticle interactions lead to large electromagnetic field enhancements that can be exploited for localizing and amplifying electromagnetic radiation in nanoscale structures. Ohmic loss (i.e., absorption by the metal), however, limits the performance of any application due to nonradiative surface plasmon relaxation. While absorption losses have been studied theoretically, they have not been quantified experimentally for strongly coupled surface plasmons. Here, we report on the ohmic loss in one-dimensional assemblies of gold nanoparticles with small interparticle separations of only a few nanometers and hence strong plasmon coupling. Both the absorption and scattering cross-sections of coupled surface plasmons were determined and compared to electrodynamic simulations. A lower absorption and higher scattering cross-section for coupled surface plasmons compared to surface plasmons of isolated nanoparticles suggest that coupled surface plasmons suffer smaller ohmic losses and therefore act as better antennas. These experimental results provide important insight for the design of plasmonic devices.

Smectic‐Layer Alignments of Surface‐Modified Gold Nanoparticles in the Nanocomposite Induced by a Hydrogen‐Bonded Bent‐Core Liquid Crystalline Host under Electric Fields

There has been great interest recently in the fabrication of highly ordered metal nanoparticle (NP) assemblies. Large scale periodical arrangements of NPs are fascinating systems for the development of nanostructural materials exhibiting new electronic, magnetic, optical, and photonic properties. Many examples of the application of templates have appeared for the generation of ordered assemblies of NPs, by using, for example, surfacemodified polymers, self-assembled monolayers, lipid films, and DNA. In addition, the self-organization behavior of gold NPs surface-modified with rod-like, bent-core, and discotic liquid crystal (LC) surfactants has been reported. Moreover, several strategies have been proposed to template arrays of NPs by using block copolymer systems, in which the spatial arrangements are directed through self-assembly of the block copolymers. LC materials possessing a variety of mesomorphic structures can be aligned in a straightforward manner through the application of electric fields, magnetic fields, and polymer networks. The self-organization of LC molecules is determined by the characteristic arrangements of the mesophasic types, including the nematic, cholesteric, smectic, and columnar phases. Herein, we describe the synthesis of the novel hydrogenbonded bent-core LC host VPy-SiA (bearing a flexible carbosilane chain) and the gold nanocomposite (VPy-SiA/ AuNPs-S) by adding 5 wt% the surface-modified gold NPs (AuNPs-S) to the self-assembled LC host (VPy-SiA; Scheme 1). We developed a novel in situ technique for alignment of the nanocomposite VIPy-SiA/AuNPs-S under (DC/AC) electric fields, and examined the orientation behavior of the smectic layer structures using two-dimensional X-ray diffraction patterns and frozen transmission electron microscopy (FTEM). To the best of our knowledge, no previous reports have described the ordered layer arrangements of NPs in a nanocomposite—induced by compatibility of the NPs in the phase of the LC hosts—that could then be further aligned under electric fields. [a] W.-H. Chen, Y.-T. Chang, Prof. H.-C. Lin Department of Materials Science and Engineering National Chiao Tung University, Hsinchu 300 (Taiwan) Fax: (+8863)5724727 E-mail : linhc@cc.nctu.edu.tw [b] J.-J. Lee, W.-T. Chuang National Synchrotron Radiation Research Center Hsinchu, 30076 (Taiwan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102607. Scheme 1. Chemical structures of the hydrogen-bonded bent-core LC host VPy-SiA (containing the hydrogen bond acceptor VPy and the hydrogen bond donor SiA) and the surface-modified gold NPs AuNPs-S presenting the covalently bonded bent-core surfactant S.

Three-Dimensional Assembly of Gold Nanoparticles in Spherically Confined Microphase-Separation Structures of Block copolymers

Three-dimensional (3D) assemblies of metal nanoparticles are promising materials for plasmonic materials. Metal nanoparticles have unique surface plasmon resonances, ranging from ultraviolet to visible light wavelengths, depending on their metal species, size, and shape. The arrangements and spacing of the nanoparticles also strongly affect the plasmonic properties. The microphase separation of block copolymers can create a nanoscopic periodic structure, where metal nanoparticle assemblies are arranged along the microphase separation structure. In this paper, we construct 3D Au nanoparticle assemblies embedded in the microphase separation structures of confined block copolymers, in the pores of inverse opals. Polymer-stabilized Au nanoparticles were synthesized, and submicrometer poly(styrene) (PS) colloidal crystals were prepared using a simple coating method. After molding a PS colloidal crystal with poly(vinyl alcohol) (PVA), block copolymers and Au nanoparticles were introduced into the PVA inverse op...

Interfacing Colloidal Graphene Oxide Sheets with Gold Nanoparticles

Aqueous solutions of graphene oxide (GO) and citrate-stabilised gold nanoparticles (AuNPs) are two classic, negatively charged colloids. Using the surface plasmon resonance spectra of AuNPs as a probe, we illustrate how the two like-charged colloids interact with each other and in so doing, reveal the unique solution behaviour of GO. We demonstrate that the electrical double layer of the GO sheets in water plays a key role in controlling the interaction between GO and AuNPs, which displays a one-way gate effect. It is shown that GO can capture and disperse AuNPs in water in a controllable fashion, without the need for additional chemical linkers. This discovery allows the successful synthesis of uncapped, yet solution-dispersible metal-nanoparticle assemblies. Such metal nanostructures have long been pursued for nano-plasmonics and sensing applications, but have remained difficult to prepare using conventional polymer dispersants. This work also makes clear that the combination of the two-dimensional conformation of GO along with its large molecular size and self-contained functional groups allows it to act as a unique soluble nanocarrier/substrate (the thinnest, functionalised flat substrate possible in nature) for the synthesis of new, soluble functional materials.

Spatial distribution of enhanced optical fields in monolayered assemblies of metal nanoparticles: Effects of interparticle coupling

Near-field two-photon excitation images of assemblies of many gold nanospheres show characteristic feature that enhanced optical fields are confined at the rim parts of the assemblies. In the present report we analyzed the origin of this feature based on finite-difference time-domain (FDTD) approach as well as a simple point dipole model that incorporates the interparticle interaction with the dipole–dipole potential. It has been found that the simple point dipole model is useful for qualitative discussion on the optical field distribution in the metal nanoparticle assemblies. From the analysis, we have found that the interparticle interaction, which causes the propagation of the plasmon excitation in the assemblies, seems to be essential for the localization of the enhanced field at the rim. We propose that regular close-packed assemblies do not yield efficiently enhanced optical fields in visible to near-infrared region, and rather assemblies with large fluctuation are more advantageous to get highly enhanced fields.

Reversible assembly of metal nanoparticles induced by penicillamine. Dynamic formation of SERS hot spots

We report a systematic study of the surface modification of gold and silver nanoparticles with dl-penicillamine (PEN) and N-acetyl-dl-penicillamine (NAP), motivated by the possibility of inducing pH-controlled reversible nanoparticle assembly. The interaction of PEN and NAP with the metal nanoparticle surface was studied by isothermal titration calorimetry (ITC). The results indicate that equilibrium is reached with the formation of a submonolayer corresponding to ca. 40% and 64% of total surface coverage for PEN and NAP, respectively. Both PEN and NAP modified nanoparticles could be reversibly aggregated at acidic pH due to the protonation of the carboxylic groups, leading to a decrease in their stability by electrostatic interactions and the advent of hydrogen bonding interactions which promote interparticle linkage. The process was monitored by UV-Vis spectroscopy, transmission electron microscopy (TEM) and surface enhanced Raman scattering (SERS) spectroscopy. Interestingly, the SERS characterization demonstrated the pH-controlled formation of hot-spots.

Silver nanoparticles assemblies mediated by functionalized biomimetic oligomers

The interaction between biopolymers and metal nanoparticles (AgNPs) is a key element in the development of biomimetic nanomaterials with applications in catalysis, delivery, and recognition. Here we report a facile method for the functionalization of AgNPs by N-substituted glycine oligomers, "peptoids." Based on the established affinity between phenanthroline ligand and Ag(0), we synthesized a peptoid bearing 1,10-phenanthroline at the N-terminus (PHP). Treatment of AgNPs that were pre-stabilized by citrate ions, with PHP, leads to the formation of aggregates as suggested by UV-vis spectroscopy. Transmission electron microscopy (TEM) revealed that the replacement of citrate ions by PHP yields spherical assemblies of AgNPs. These peptoids/AgNPs hybrids, as well as the ability of functional biomimetic oligomers to mediate the assembly of metal nanoparticles, hold potential for applications in sensor materials, biology, and catalysis.

Acid-directed synthesis of SERS-active hierarchical assemblies of silver nanostructures

SERS-active silver hierarchical assemblies are synthesized in solution by the assistance of small acid molecules. We here demonstrate the acid-directed self-assembly of metal nanoparticles (MNPs) into large systems with complex structures, without the application of any polymer surfactant or capping agent. It is verified that small acid molecules (citric acid, mandelic acid, etc.) incorporated into conventional solution chemistry can direct the assembly of MNPs into well-defined hierarchical structures. The constructed assembled structures with highly roughened surfaces can be highly sensitive SERS platforms, and the fabricated core–shell Ag wires show especially high SERS sensitivity toward the analyte melamine. The prepared Ag particles with hierarchical structures show no evident polarization-dependent SERS behavior, and this isotropic feature may be an advantage for highly sensitive SERS tags, since no certain incident polarization is required for molecule detection. We believe the subsequent addition of acid to induce formation of self-assembled structures can be a general synthetic platform to fabricate metal structures with complex morphologies.

Ultrafine metallic Fe nanoparticles: synthesis, structure and magnetism.

The results of the investigation of the structural and magnetic (static and dynamic) properties of an assembly of metallic Fe nanoparticles synthesized by an organometallic chemical method are described. These nanoparticles are embedded in a polymer, monodisperse, with a diameter below 2 nm, which corresponds to a number of around 200 atoms. The X-ray absorption near-edge structure and Mössbauer spectrum are characteristic of metallic Fe. The structural studies by wide angle X-ray scattering indicate an original polytetrahedral atomic arrangement similar to that of β-Mn, characterized by a short-range order. The average magnetic moment per Fe atom is raised to 2.59 µB (for comparison, bulk value of metallic Fe: 2.2 µB). Even if the spontaneous magnetization decreases rapidly as compared to bulk materials, it remains enhanced even up to room temperature. The gyromagnetic ratio measured by ferromagnetic resonance is of the same order as that of bulk Fe, which allows us to conclude that the orbital and spin contributions increase at the same rate. A large magnetic anisotropy for metallic Fe has been measured up to (3.7 ± 1.0)·105 J/m3. Precise analysis of the low temperature Mössbauer spectra, show a broad distribution of large hyperfine fields. The largest hyperfine fields display the largest isomer shifts. This indicates a progressive increase of the magnetic moment inside the particle from the core to the outer shell. The components corresponding to the large hyperfine fields with large isomer shifts are indeed characteristic of surface atoms.

Correlation between structure and optoelectronic properties in a two‐dimensional nanoparticle assembly

AbstractThe optical properties of two‐dimensional assemblies of metal nanoparticles are strongly influenced by the morphological configuration of the metal particles in the layer. Therefore, we correlate the structural and optical properties of two‐dimensional, hexagonal gold nanoparticle arrays. We characterize the structure of the arrays using grazing‐incidence small angle X‐ray scattering (GISAXS). From the GISAXS pattern, we determine the size of the gold particles as well as the lattice spacing of the hexagonal assembly. Based upon these parameters we calculate the dielectric function of the gold particle array using the Maxwell–Garnett effective medium theory. We further deduce the absorption spectrum which closely follows the measured absorption and photoconductance spectrum. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Bioinspired Polymeric Template for 1D Assembly of Metallic Nanoparticles, Semiconductor Quantum Dots, and Magnetic Nanoparticles

A precise control of metallic-nanoparticle assembly is highly critical for the realization of tangible, high-performance devices or materials. Until recently, nanoparticle assembly using 1D templates had been limited to a narrow spectrum of nanoparticles as it was mostly dependent on the surface chemistry of the nanoparticles used. Inspired by the universal adhesive properties of mussels, we demonstrate a universal polymeric template for 1D assembly of various nanoparticles including, gold nanoparticles, iron oxide nanoparticles, and quantum dots. We find that the length of the 1D assembly is tunable using hyaluronic acid-graft-catechol templates with various contour lengths.