Close-packed Assemblies Research Articles

Breakage Characteristics and Crushing Mechanism of Cement Materials Using Point-Loading Tests and the Discrete-Element Method

A point-loading device was used to research the breakage characteristics of single particles and to study their crushing mechanism. A series of point-loading tests were completed on the basis of cement ellipsoid particles modeled in various sizes and strengths with cement pastes. A close-packing method of equal spheres with several close-packed assemblies was generated, and the corresponding numerical models for cement ellipsoid particles were established by the discrete-element method. Experimental and numerical tests show that the high-strength materials are brittle, whereas the low-strength materials are ductile, and vice versa. A comparison of the experimental and numerical results show reasonable, rational results, and indicate that the force-displacement relationship is approximately linear for this type of granular material. A measurement-sphere method was used to trace the energy and stress distribution in the particle samples. The accumulation of dilatant deformation causes the collapse of the local skeleton structures in the cement ellipsoid particles.

Surface Plasmon Tunability and Emission Sensitivity of Ultrasmall Fluorescent Copper Nanoclusters

Ultrasmall copper nanoparticles have been synthesized using copper(II) salt as precursor by hydrazine reduction in the presence of citric acid and cetyltrimethylammonium bromide facilitating the growth of stable copper nanoparticles with an average diameter of <2 nm. The corresponding surface plasmon resonances were monitored under variable microenvironments, and it is seen that these tiny copper nanoparticles form aggregates under stipulated reaction conditions. It is noted that ultrasmall copper nanoparticles do not exhibit any characteristic surface plasmon band in the visible region; rather, a continuous absorption is seen over the entire UV–vis region. However, a well-defined plasmon absorption band makes its appearance while the particles are aggregated in close-packed assembly. These results demonstrate that the maximum of surface plasmon resonance is red-shifted from that of isolated particles because of electromagnetic interaction between the particles. The aggregation process is manifested upon changes of pH, anionic surfactant, etc. and is not reversible, i.e., the aggregates could not be re-dispersed into ultrasmall particles. The effect of addition of electrolyte has been monitored to study the surface plasmon damping of the copper nanoparticles. The plasmonic sensitivity of the copper nanoparticle aggregates has been elicited by the determination of amino acid chain length with exquisite sensitivity because of enormous electromagnetic field at the junction of the particles in the aggregates. Interestingly, the as-synthesized ultrasmall copper nanoclusters exhibit excellent fluorescence properties with a narrow emission profile. The emission properties of these copper nanoclusters have been utilized as an indicator for selective and ultrasensitive detection of highly toxic HgII ions in water in the nanomolar detection limit.

Temperature-dependent magnetic and structural ordering of self-assembled magnetic array of FePt nanoparticles

Hexagonal close-packed 2D assembly of monodispersed FePt nanoparticles by the co-reduction method have been synthesised with particles size ~3 nm. The X-ray diffraction (XRD) studies reveal a structural transformation from a chemically disordered faced-centred cubic (FCC) phase to a chemically ordered face-centred tetragonal (FCT) phase with heat treatment. The percentage of phase transformation from FCC to FCT has also been investigated by the XRD profile fitting. Annealing at 750 °C gives 95 ± 1 % FCT phase. The structural transformation was further confirmed by the presence of two superlattice rings corresponding to (001) and (110) by the selected area electron diffraction. Transmission electron microscopy (TEM) shows the grain growth with annealing temperature. High resolution TEM shows the presence of the carbonaceous layer over FePt after annealing. This carbonaceous layer has been investigated by the Raman spectroscopy. Magnetic measurement shows the presence of magnetic hard phase with coercivity of 1 Tesla. The soft/hard compositions have also been investigated by the hysteresis measurements.

Mechanical Control of Molecular Aggregation and Fluorescence Switching/Enhancement in an Ultrathin Film

Optical responses of molecular aggregates and assemblies are often different from that of the individual molecules. Self-assembly approaches provide little physical control on the extent of aggregation. Mechanical compression of amphiphilic molecules (with chromophore/fluorophore head groups) at the air-water interface, followed by transfer as Langmuir-Blodgett (LB) films, should prove to be an elegant route to molecular assemblies with systematically tunable aggregation and optical responses. This concept is demonstrated using monolayer LB films of a diaminodicyanoquinodimethane (DADQ)-based amphiphile fabricated at different surface pressures. Films deposited above a threshold pressure exhibit a strong blue-shift in the absorption and fluorescence relative to those deposited below; computational investigations suggest that this is due to the formation of 2-dimensional close-packed assemblies. Significantly, the blue emission of the films deposited above the threshold pressure increases with compaction, demonstrating aggregation-induced fluorescence enhancement in ultrathin films, a phenomenon well-established in crystals and nanocrystals of selected classes of molecules including the DADQs. The sharp contrast with aggregation-induced fluorescence quenching observed with most dye molecules is illustrated by a parallel investigation of LB films of a hemicyanine-based amphiphile. The present study illustrates the efficacy of simple mechanical compression and the LB technique in fabricating ultrathin films with tailored supramolecular assembly and optical responses.

Controlling the Dimensionality and Structure of Supramolecular Porphyrin Assemblies by their Functional Substituents: Dimers, Chains, and Close‐Packed 2D Assemblies

Repulsive interactions: a staging of supramolecular aggregation from (0D) clusters to (1D) chains and (2D) assemblies as a function of molecular coverage of dipolar porphyrins adsorbed on the Ag(111) surface is described. It displays a complex interplay of both attractive and repulsive molecule-molecule interactions, the emergence of chirality, and the registry of the substrate.

Graphene modulated 2D assembly of plasmonic gold nanostructure on diamond-like carbon substrate for surface-enhanced Raman scattering

A close-packed 2D assembly of plasmonic gold nanostructure was fabricated on a diamond-like carbon film by a two-step electrodeposition via reduced graphene oxide (rGO) modulation. The oxygen functionalities at the rGO surface which were controlled by changing electrochemical reduction time of GO provided reactive sites for nucleation and growth of gold nanoparticles. The Raman intensity of rhodamine B obtained from the gold assembly showed an 860-fold increase compared with that from Si reference, indicating a coupling of localized electromagnetic field enhancement and chemical enhancement. Our research offered a novel way for metallic plasmonic fabrication to realize their potential applications in biochemical analysis, environmental monitoring, disease detection and food safety.

Hierarchical porous gold electrodes: Preparation, characterization, and electrochemical behavior

Hierarchical porous gold films with a well-defined bimodal architecture have been made by electrodepositing gold at a constant current around a close-packed assembly of raspberry-like latex spheres (1200/60nm) followed by template removal. Electrodeposition was stopped when the gold was either ½ layer or 1½ layer thick as evident from oscillations in the potential vs time traces. Scanning electron microscopy (SEM) images show the hierarchical pore structure with an ensemble of small ∼20nm openings located in a large ∼1200nm diameter macropore. Prior to electrochemical characterization, the electrodes were cleaned either chemically and/or via UV radiation and X-ray photoelectron spectroscopy (XPS) was used to evaluate the presence of residual polystyrene. Of the three cleaning methods investigated, sonication in chloroform–acetone followed by UV radiation proved best. The surface area of the hierarchical porous gold electrodes, determined by integrating the area under the gold oxide peak, was 4× larger than a bare gold electrode and 2× larger than a macroporous gold electrode prepared using unimodal, 1200nm diameter latex spheres as the template. The electrochemical performance of the electrodes relative to the macroporous gold and flat gold was undertaken using cyclic voltammetry. The results show that the non-Faradaic current scales linearly with electrode area while the Faradaic current of a diffusing electrochemically reversible redox probe (ferrocene methanol) does not. For an adsorbed redox couple (ferrocene hexanethiol), the voltammetric wave shapes and surface coverage were different for the different electrodes.

Close-packed SnO2 nanocrystals anchored on amorphous silica as a stable anode material for lithium-ion battery

A sol–gel route has been used to synthesize close-packed SnO2 nanocrystals anchored on amorphous silica as a potential anode material for lithium-ion battery. The materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR, transmission electron microscopy (TEM) and electrochemical techniques. The electrochemical performance of the SnO2/silica composites shows higher capacity and good cycle stability compared with that of the bare SnO2 electrode. It is believed that the good performance as a stable anode material originates from the unique structure of the close-packed nanocrystalline assemblies and the amorphous porous silica as inactive material to mediate the massive volume expansion and contraction of SnO2 during lithiation and delithiation processes. It has been proved for the first time that the close-packed architecture of SnO2 nanocrystals ensure adequate amount of active component for the lithium storage, resulting in a reasonable lithium storage capability for the present system. On the other hand, the crystalline/amorphous interactions should be one of the most fundamental factors to improve the electrochemical stability of the SnO2/silica hybrid electrode.

Transparent, Superhydrophobic Surfaces from One-Step Spin Coating of Hydrophobic Nanoparticles

We study the nonwettability and transparency from the assembly of fluorosilane modified silica nanoparticles (F-SiO(2) NPs) via one-step spin-coating and dip-coating without any surface postpassivation steps. When spin-coating the hydrophobic NPs (100 nm in diameter) at a concentration ≥ 0.8 wt % in a fluorinated solvent, the surface exhibited superhydrophobicity with an advancing water contact angle greater than 150° and a water droplet (5 μL) roll-off angle less than 5°. In comparison, superhydrophobicity was not achieved by dip-coating the same hydrophobic NPs. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed that NPs formed a nearly close-packed assembly in the superhydrophobic films, which effectively minimized the exposure of the underlying substrate while offering sufficiently trapped air pockets. In the dip-coated films, however, the surface coverage was rather random and incomplete. Therefore, the underlying substrate was exposed and water was able to impregnate between the NPs, leading to smaller water contact angle and larger water contact angle hysteresis. The spin-coated superhydrophobic film was also highly transparent with greater than 95% transmittance in the visible region. Further, we demonstrated that the one-step coating strategy could be extended to different polymeric substrates, including poly(methyl methacrylate) and polyester fabrics, to achieve superhydrophobicity.

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.

Effect of Magnetic Dipole Interaction on Magnetic Properties of Fe Nanoparticle Assemblies

Fe/Au nanoparticle (NPs) assemblies with various Fe NPs densities were prepared by simultaneous agglomeration of Fe NPs and Au NPs in solution. The density of the Fe NPs was widely controlled below 43% by modifying the mixture ratio of both NP solutions. Only dipole interaction effectively induced magnetic interaction between Fe NPs in this system. With the increase in the volume fractions of Fe NPs from 0.5% to 43%, the collective behavior of magnetic moments of Fe NPs was induced by strong magnetic dipole interactions. As a result, the tendency of soft magnetic properties was found. The diameter analysis revealed that the magnetic dipole interaction energy depending on close-packed assembly consisting of only Fe NPs was 13 times that of the magnetic dipole interaction energy between two Fe NPs. The maximum magnetic dipole field was estimated to be approximately 3-5 × 105 A/m for around 8 nm of the assembly of Fe NPs. This value is much larger than that of the anisotropic field of Fe NPs. Therefore, the magnetic properties in this system were dominated by magnetic dipole interactions.

New topotactic synthetic route to mesoporous silicon carbide

Mesoporous silicon carbide (SiC) was synthesized by a one-pot thermal reduction of SiO2/C composites by metallic Mg at the remarkably low temperature of 800 °C. Two distinct mesostructured silica were used as hard templates for composite preparation: a hexagonal 3D close-packed assembly of Stober silica spheres and an ordered mesoporous SBA15 silica. In the latter case, SiC has crystallized in its 2H–SiC hexagonal phase, which is rather unique at such a low temperature. Composites were obtained by impregnation/polymerisation/carbonisation of a molecular carbon precursor within the porous structure of the silica template. After thermal treatment at a moderate temperature in the presence of Mg and subsequent by-products rinsing off, both prepared SiC showed distinct mesoporous structures related to the initial SiO2 architectures. By comparison of the mesoporous characteristics, resulting SiC was found to retain the carbon structures of the pristine composites. The description of the synthetic mechanism of this topotactic reaction contrasts with the usual assumption stating the templating role of silica.

Tunable Pyramidal Assemblies of Nanoparticles by Convective/Capillary Deposition on Hydrophilic Patterns Made by AFM Oxidation Lithography

Close-packed pyramidal assemblies of 100 nm latex nanoparticles were made by convective/capillary deposition on hydrophilic patterns created by oxidation lithography using atomic force microscopy (AFM). We demonstrated that the substrate temperature during convective/capillary assembly is a key experimental parameter in finely tuning the geometry of these pyramids and thus the total number of nanoparticles forming each 3D assembly. The volume and shape of these nanoparticle assemblies are discussed and compared to simulations.

Aggregation and Contingent Metal/Surface Reactivity of 1,3,8,10‐Tetraazaperopyrene (TAPP) on Cu(111)

The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPP-Cu) chains.

Formation of magnetic nanotubes by the cooperative self-assembly of chiral amphiphilic molecules and Fe3O4 nanoparticles

Single- and double-walled magnetic nanotubes are obtained in a one-step liquid phase reaction by the cooperative self-assembly of chiral amphiphiles and nanoparticles on cooling of heated mixtures of N-dodecanoyl-L-serine and Fe(3)O(4) nanoparticles in toluene. The nanotubes are composed of well-ordered, close-packed nanoparticle assemblies, and can be transformed into chiral magnetic nanostructures, such as helical coils, by subsequent calcination. The nanoparticle assemblies and their variations on calcination are attributed to the collective organization of the surfactant molecules adsorbed on the nanoparticles and the freely dispersed chiral molecules, and the dewetting effects guided by the primitive constitution of the chiral amphiphilic molecular assemblies.

Surface plasmon resonance of gold nanoparticles assemblies at liquid | liquid interfaces

Surface plasmon resonance (SPR) was observed when a planar close-packed assembly of gold nanoparticles (Au NPs) is adsorbed at the water|1,2-dichloroethane interface. Aqueous gold nanoparticles, 13 or 16 nm in diameter, are deposited at the interface by adding methanol to form a close-packed film with a visible gold mirror reflectance. By total internal reflection of a light beam on the interface, the angular dependence of the interfacial reflectivity was measured in a pseudo-Kretschmann configuration and compared to Fresnel simulations for a homogeneous gold film. The experimental angles for minimum reflectivity were found to match the simulated values. Then, the fluorescence of dye molecules co-adsorbed within 13 and 16 nm gold nanoparticles assemblies at the liquid|liquid interface was measured. The fluorescence intensity under SPR is revealed to be much greater than under total internal reflection conditions, yielding an enhancement factor of approximately 30 and 50 for 13 and 16 nm Au NPs assemblies, respectively. Also, the fluorescence lifetime was found to decrease under SPR conditions.

The role of symmetry in building up zeolite frameworks from layered zeolite precursors having ferrierite and CAS layers

Layered zeolite precursors represent an emerging class of materials that expand the zeolite field in a new direction and have already produced new intriguing discoveries. One is a new fundamental insight that a zeolite framework can assemble by two pathways, directly in 3-D and via a layered precursor. Also new types of materials such as pillared and delaminated zeolites have been synthesized. Starting with the discovery of MCM-22 precursor in 1990s, other framework structures have now been obtained by topotactic condensation, including classical zeolites such as sodalite and ferrierite. The recent structural diversity observed with FER and CAS layers, which were found to produce two frameworks each, i.e. FER/CDO and CAS/NSI, respectively, are herein rationalized as resulting from the absence of in-plane mirror symmetry. A systematic treatment is envisioned, such as consideration of translational and pseudo-translational interactions in condensation of elementary shapes into close-packed assemblies, providing a blueprint for a general approach to analyze new precursors in the future. Other already documented instances of structural diversity include the presence of both types of packing in one preparation (CAS and NSI) and incomplete pairing of silanols connecting FER layers in ESR-12.

Close‐Packed Gold‐Nanocrystal Assemblies Deposited with Complete Selectivity into Lithographic Trenches

Close-packed gold-nanocrystal assemblies are selectively deposited into trenches (300 nm × 300 nm) fabricated by electron-beam lithography. The base of the trenches consists of highly doped silicon with 30 nm silica walls. The assemblies are formed using electric-field mediated deposition of8 nm gold spheres from organic solvents.

Combining Convective/Capillary Deposition and AFM Oxidation Lithography for Close-Packed Directed Assembly of Colloids

We combine convective/capillary deposition and oxidation lithography by atomic force microscopy to direct the close-packed assembly of colloids on SiOx patterns fabricated on silicon substrates previously functionalized with a hydrophobic monolayer of octadecyltrimethoxysilane. The efficiency of this original generic method, which is well adapted to integrate colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. A three-step mechanism involving convective flow and capillary forces appears to be responsible for these close-packed assemblies of colloids onto SiOx patterns.

Biomolecule induced nanoparticle aggregation: Effect of particle size on interparticle coupling

Gold nanoparticles of variable sizes have been prepared by reducing HAuCl 4 with trisodium citrate by Frens' method. It has been found that the gold particles under consideration produce well-ordered aggregates upon interaction with a biomolecule, glutathione in variable acidic pH condition and exhibit pronounced changes in their optical properties arising due to electromagnetic interaction in the close-packed assembly. The effect of nanoparticle size on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects amongst the gold particles have been investigated. The optical properties of the gold aggregates have been accounted in the light of Maxwell-Garnett effective medium theory considering the changes in the filling factor in different aggregates produced by variable sizes of gold colloids. The aggregates have been characterized by UV–vis spectroscopy, FTIR, Raman, XRD and TEM studies. It has been observed that a new peak appearing at a longer wavelength intensifies and shifts further to the red from the original peak position depends on the particle size, concentration of glutathione and pH of the solution. On the basis of the first appearance of a clearly defined new peak at longer wavelength, a higher sensitivity of glutathione detection has been achieved with gold nanoparticles of larger dimension.