Controlled Growth of Gold Nanoparticles Preorganized in Langmuir–Blodgett Monolayers

A method is described for the in situ growth of substrate-supported organized gold nanoparticles. Upon exposure to an aqueous solution of a gold salt and a mild reducing agent, the particle size can be significantly increased without any loss of superstructure organization. Furthermore, no secondary nucleation is observed. The surface-supported regrowth procedure can be combined with the Langmuir–Blodgett technique to produce a rich library of plasmonic nanoparticle assemblies. Controlled particle regrowth plays a crucial role in this assembly method because only relatively small metallic nanoparticles can be directly dispersed in polymeric Langmuir–Blodgett films. The versatility of the method is demonstrated through the fabrication of several specific nanoparticle structures, including contiguous plasmonic rings, core–satellite structures, and necklace assemblies. Plasmon extinction spectra are presented for the various nanoparticle superstructures and illustrate the importance of controlling both particle size and assembly architecture in achieving targeted optical properties. The reported approach constitutes a viable bottom-up assembly route for the fabrication of surface-supported nanoparticle superstructures for plasmonic applications

Controlled 2D Organization of Gold Nanoparticles in Block Copolymer Monolayers

The organization of organic-capped gold nanoparticles in PS-<i>b</i>-PMMA monolayers is investigated. The preferred location of the particles within the block copolymer template is found to depend on both nanoparticle size and the length of the aliphatic capping agent. In the case of relatively short ligands, the particles behave as hard spheres and their incorporation in the polymer matrix can be qualitatively rationalized by entropic considerations. Three distinct arrangements are observed. Particles that are small, relative to the radius of gyration of the host polymer, evenly disperse within the PS domains, whereas the largest particles are considered form ordered, island-like aggregates. Particles of intermediate size exhibit the most striking arrangement, being relegated to the PS-PMMA interface to form organized ring structures. The tendency of these particles to assemble at the interface is sufficiently strong to force a modification of the polymer morphology to accommodate the particles at higher loadings. As the number of particles is increased, the circular PS-<i>b</i>-PMMA surface micelles elongate to form nanostrands

Size-Dependent Extinction Coefficients and Transition Energies of Near-Infrared β‑Ag₂Se Colloidal Quantum Dots

Our investigations of silver selenide colloidal quantum dots, emitting in the biologically important near-infrared region, demonstrate the size-dependence of their optical properties. Ag₂Se nanocrystals were prepared in the orthorhombic phase with their average radius varying from 0.95 to 4.7 nm as observed by transmission electron microscopy. The high purity of the samples, established by energy-dispersive X-ray spectroscopy and X-ray diffraction, allowed for the accurate determination of the Ag₂Se content of colloidal suspensions by a thermogravimetric method. The energy of the first observed transition is found to decrease asymptotically with colloidal quantum dot size, tending toward a value of 1.1 eV, a value significantly above the β-Ag₂Se bulk bandgap. Furthermore, the molar extinction coefficient of this absorption is proportional to <i>r</i>₀^2.7±0.2, where <i>r</i>₀ is the cQD radius. At higher energies, the extinction coefficient eventually follows the classically predicted cubic power law with <i>r</i>₀

Size-Modulation of Plasmonic Nanorings Obtained by the Self-Assembly of Gold Nanoparticles and Block Copolymers

Metal nanoparticles exhibit interesting optical properties due to the collective excitation of conduction electrons called the plasmon. Within appropriate metal nanostructures, cooperative plasmon modes appear and the resonance plasmon frequency is modified. This article reports a simple method for the formation of such structures, in the form of self-assembled nanorings. Rings of alkanethiol-capped gold nanoparticles are obtained by the Langmuir–Blodgett technique and a block copolymer (PS-<i>b</i>-P2VP) template. With this approach, organized nanoparticle arrangements covering a large surface area are obtained. Furthermore, geometric parameters such as ring diameter, ring-to-ring separation, and ring width can be systematically varied by the addition of homopolymer or in situ nanoparticle regrowth. Optical extinction spectra recorded for the nanoparticle rings depend both on ring diameter and particle size. In particular, after in situ particle regrowth, the plasmon extinction spectrum exhibits a red-shift that increases with ring diameter. Theoretical spectra generated with the discrete dipole approximation indicate that this spectral shift can be attributed to plasmon coupling that extends over an increasing number of particles as the ring is enlarged

Plasmonic Properties of Self-Assembled Gold Nanocrescents: Implications for Chemical Sensing

A bottom-up approach, the Langmuir–Blodgett technique, is used for the preparation of composite thin films of gold nanoparticles and polymers: poly(styrene-b-2-vinylpyridine), poly-2-vinylpyridine, and polystyrene. The self-assembly of poly(styrene-b-2-vinylpyridine) at the air–water interface leads to the formation of surface micelles, which serve as a template for the organization of gold nanoparticles into ring assemblies. By using poly-2-vinylpyridine in conjunction with low surface pressure, the distance between nanostructures can be increased, allowing for optical characterization of single nanostructures. Once deposited on a solid substrate, the preorganized gold nanoparticles are subjected to further growth by the reduction of additional gold, leading to a variety of nanostructures which can be divided into two categories: nanocrescents and circular arrays of nanoparticles. The optical properties of individual structures are investigated by optical dark-field spectroscopy and numerical calculations. The plasmonic behavior of the nanostructures is elucidated through the correlation of optical properties with structural features and the identification of dominant plasmon modes. Being based on a self-assembly approach, the reported method allows for the formation of interesting plasmonic materials under ambient conditions, at a relatively large scale, and at low cost. These attributes, in addition to the resonances located in the near-infrared region of the spectrum, make nanocrescents candidates for biological and chemical sensing

Comprehensive Solid-State Characterization of Rare Earth Fluoride Nanoparticles

The combination of multinuclear solid-state NMR spectroscopy and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles revealed their general formula to be (H₃O)­Y₃F₁₀·<i>x</i>H₂O. ¹H magic-angle spinning (MAS) NMR experiments provided information on sample purity and served as an excellent probe of the zeolithic incorporation of atmospheric water. ¹⁹F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yielded spectra featuring three unique ¹⁹F resonances arising from three different fluorine sites within the (H₃O)­Y₃F₁₀·<i>x</i>H₂O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles led to changes in the integrated peak intensities in the ¹⁹F MAS NMR spectra; the origin of this behavior is discussed in terms of ¹⁹F longitudinal relaxation. ¹⁹F–⁸⁹Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under MAS conditions indicated that two distinct yttrium environments are present, and on the basis of the relative peak intensities, the population of one of the two sites is closely linked to the nanoparticle size. Both ¹⁹F MAS and ¹⁹F–⁸⁹Y VACP/MAS experiments indicated small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF₃ nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application