Superstructures of water-dispersive hydrophobic nanocrystals: specific properties.

Here, we describe water-soluble superstructures of hydrophobic nanocrystals that have been developed in recent years. We will also report on some of their properties which are still in their infancy. One of these structures, called "cluster structures", consists of hydrophobic 3D superlattices of Co or Au nanocrystals, covered with organic molecules acting like parachutes. The magnetic properties of Co "cluster structures" a retained when the superstructures is dispersed in aqueous solution. With Au "cluster structures", the longer wavelength optical scattered spectra are very broad and red-shifted, while at shorter wavelengths the localized surface plasmonic resonance of the scattered nanocrystals is retained. Moreover, the maximum of the long-wavelength signal spectra is linearly dependent on the increase in assembly size. The second superstructure was based on liquid-liquid instabilities favoring the formation of Fe3O4 nanocrystal shells (colloidosomes) filled or unfilled with Au 3D superlattices and also spherical solid crystal structures are called supraballs. Colloidosomes and supraballs in contact with cancer cells increase the density of nanocrystals in lysosomes and near the lysosomal membrane. Importantly, the structure of their organization is maintained in lysosomes for up to 8 days after internalization, while the initially dispersed hydrophilic nanocrystals are randomly aggregated. These two structures act as nanoheaters. Indeed, due to the dilution of the metallic phase, the penetration depth of visible light is much greater than that of homogeneous metallic nanoparticles of similar size. This allows for a high average heat load overall. Thus, the organic matrix acts as an internal reservoir for efficient energy accumulation within a few hundred picoseconds. A similar behavior was observed with colloidosomes, supraballs and "egg" structures, making these superstructures universal nanoheaters, and the same behavior is not observed when they are not dispersed in water (dried and deposited on a substrate). Note that colloidosomes and supraballs trigger local photothermal damage inaccessible to isolated nanocrystals and not predicted by global temperature measurements.

Supraballs as spherical solid 3D superlattices of hydrophobic nanocrystals dispersed in water: nanoarchitectonics and properties.

Herein, we use a water-dispersive 3D suprastructure of ferrite (Fe3O4) nanocrystals called supraballs. They are solid spherical assemblies of hydrophobic nanocrystals with a rather low Young's modulus compared to similar 3D superlattices deposited on a substrate. Using atomic force microscopy methods, their nanomechanical properties are measured, which show small flexibility and deformation. This suprastructure behaves as a nanoheater and remains self-assembled after internalization in cancer cells. Furthermore, when subjected to light, the percentage of dead cells compared to the nanocrystals used as building blocks and dispersed in the solution increases.

Size and nanocrystallinity controlled gold nanocrystals: synthesis, electronic and mechanical properties.

The influence of nanocrystallinity on the electronic and mechanical properties of metal nanoparticles is still poorly understood, due to the difficulty in synthesizing nanoparticles with a controlled internal structure. Here, we report on a new method for the selective synthesis of Au nanoparticles in either a single-domain or a polycrystalline phase maintaining the same chemical environment. We obtain quasi-spherical nanoparticles whose diameter is tunable from 6 to 13 nm with a resolution down to ≈0.5 nm and narrow size distribution (4-5%). The availability of such high-quality samples allows the study of the impact of the particle size and nanocrystallinity on a number of parameters, such as plasmon dephasing time, electron-phonon coupling, period and damping time of the radial breathing modes.

Electronic properties of supracrystals of Au nanocrystals: influence of thickness and nanocrystallinity.

Well-defined superlattices of colloidal nanocrystals, called supracrystals, are expected to have interesting physical properties. While the electronic properties of thin supracrystals have been extensively studied in the planar configuration, little is known about electron transport through micrometer-thick supracrystals. Here, we investigate the electronic properties of supracrystals made of Au nanocrystals with diameters of 5, 6, 7 and 8 nm using scanning tunneling microscopy/spectroscopy at low temperatures. The current-voltage characteristics show power-law dependences with exponents varying strongly with supracrystal thicknesses from 30 nm to a few microns. The crystallinity of these nanocrystals, called nanocrystallinity, is exclusively single domain for 5 nm nanocrystals and a mixture of single and polycrystalline phase for 6, 7 and 8 nm nanocrystals. We observed that supracrystals made of 5 nm nanocrystals have a different behavior than supracrystals made of 6, 7 and 8 nm nanocrystals and this might be related to the nanocrystallinity. These results help us to better understand the electron transport mechanism in such miniscule structures built from a bottom-up approach.

Hierarchy in Au nanocrystal ordering in supracrystals: a potential approach to detect new physical properties.

Here we describe the morphologies of Au nanocrystals self-assembled in fcc 3D superlattices called supracrystals. The average size of the nanocrystals is either 5 or 7 nm with a very small size distribution (<7%). The coating agents used to stabilize the nanocrystals are dodecanethiol (C12H25-SH), tetradecanethiol (C14H29-SH), and hexadecanethiol (C16H33-SH). The influences of the evaporation time, the volume of the chamber used to evaporate the toluene solvent, and the substrate temperature are studied. For nanocrystals characterized by the same size and coating agent, the supracrystal morphologies markedly change on increasing the evaporation time from 8 to 9 to 25 h whereas a slight change takes place on increasing the chamber volume. The nanocrystals' ability to self-order in supracrystals decreases upon increasing the chain length of the coating agent from dodecanethiol (C12) to tetradecanethiol (C14) to hexadecanethiol (C16). Decreasing the evaporation rate (25 h) and/or increasing the substrate temperature (50 °C) improves the nanocrystal ordering in fcc supracrystals. A hierarchy in nanocrystal ordering has the following sequence disordered assemblies, supracrystal film sitting on a disordered nanocrystal film, supracrystal films grown layer-by-layer, and finally supracrystals grown in solution with various well-defined shapes.

Self organization of inorganic nanocrystals: unexpected chemical and physical properties.

Here we point out that the nanocrystals well ordered in compact hexagonal networks are highly stable compared to the same nanocrystals either isolated on a substrate or ordered in a less compact manner. The emergence of unexpected collective physical intrinsic properties results in the nanocrystals being ordered over a long distance in colloidal crystals called supracrystals. Some morphologies of nanocrystals ordered, at the micrometer scale, in 3D superlattices called supracrystals are similar to those obtained with atoms in nanocrystals. From a comparison between vibrational and magnetic properties of supracrystals and aggregates composed of the same nanocrystals, it is proposed that nanocrystals in a supracrystal could behave as atoms in a nanocrystal. From these data a possible analogy between nanocrystals in a supracrystal and atoms in nanocrystals is proposed.

Supra- and nanocrystallinity: specific properties related to crystal growth mechanisms and nanocrystallinity.

The natural arrangement of atoms or nanocrystals either in well-defined assemblies or in a disordered fashion induces changes in their physical properties. For example, diamond and graphite show marked differences in their physical properties though both are composed of carbon atoms. Natural colloidal crystals have existed on earth for billions of years. Very interestingly, these colloidal crystals are made of a fixed number of polyhedral magnetite particles uniform in size. Hence, opals formed of assemblies of silicate particles in the micrometer size range exhibit interesting intrinsic optical properties. A colorless opal is composed of disordered particles, but changes in size segregation within the self-ordered silica particles can lead to distinct color changes and patterning. In this Account, we rationalize two simultaneous supracrystal growth processes that occur under saturated conditions, which form both well-defined 3D superlattices at the air-liquid interface and precipitated 3D assemblies with well-defined shapes. The growth processes of these colloidal crystals, called super- or supracrystals, markedly change the mechanical properties of these assemblies and induce the crystallinity segregation of nanocrystals. Therefore, single domain nanocrystals are the primary basis in the formation of these supracrystals, while multiply twinned particles (MTPs) and polycrystals remain dispersed within the colloidal suspension. Nanoindentation measurements show a drop in the Young's moduli for interfacial supracrystals in comparison with the precipitated supracrystals. In addition, the value of the Young's modulus changes markedly with the supracrystal growth mechanism. Using scanning tunneling microscopy/spectroscopy, we successfully imaged very thick supracrystals (from 200 nm up to a few micrometers) with remarkable conductance homogeneity and showed electronic fingerprints of isolated nanocrystals. This discovery of nanocrystal fingerprints within supracrystals could lead to promising applications in nanotechnology.

Supra- and nanocrystallinities: a new scientific adventure.

Nanomaterials exist in the interstellar medium, in biology, in art and also metallurgy. Assemblies of nanomaterials were observed in the early solar system as well as silicate particle opals. The latter exhibits unusual optical properties directly dependent on particle ordering in 3D superlattices.The optical properties of noble metal nanoparticles (Ag, Au and Cu) change with the ordering of atoms in the nanocrystals, called nanocrystallinity. The vibrational properties related to nanocrystallinity markedly differ with the vibrational modes studied. Hence, a drastic effect on nanocrystallinity is observed on the confined acoustic vibrational property of the fundamental quadrupolar modes whereas the breathing acoustic modes remain quasi-unchanged. The mechanical properties characterized by the Young's modulus of multiply twinned particle (MTP) films are markedly lower than those of single nanocrystals.Two fcc supracrystal growth mechanisms, supported by simulation, of Au nanocrystals are proposed: heterogeneous and homogeneous growth processes. The final morphology of nanocrystal assemblies, with either films by layer-by-layer growth characterized by their plastic deformation or well-defined shapes grown in solution, depends on the solvent used to disperse the nanocrystals before the evaporation process.At thermodynamic equilibrium, two simultaneous supracrystal growth processes of Au nanocrystals take place in solution and at the air-liquid interface. These growth processes are rationalized by simulation. They involve, on the one hand, van der Waals interactions and, on the other hand, the attractive interaction between nanocrystals and the interface.Ag nanocrystals (5 nm) self-order in colloidal crystals with various arrangements called supracrystallinities. As in bulk materials, phase diagrams of supracrystals with structural transitions from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) and body-centered-cubic (bcc) structures are observed. They depend on the chain length of the coating agent and on the solvent used to disperse the nanocrystals before evaporation. The transition from fcc to hcp is attributed to specific stacking processes depending on evaporation kinetics whereas the formation of bcc supracrystals is attributed to van der Waals attractions.These results open up a new research area, which currently suffers from an extensive lack of knowledge.

Inorganic nanocrystals self ordered in 2D superlattices: how versatile are the physical and chemical properties?

Here we point out some parameters, such as substrate roughness, deposition modes, alkyl chain length of the coating agents and solvent used to disperse the nanocrystals, that play a role in the control of nanocrystal ordering in 2D superlattices. The interparticle distances do not follow the rules which imply that they are controlled by the length of the alkyl chain used as coating agent. The solvent in which the nanocrystals are dispersed is a key parameter. From DDA simulations it is observed that collective optical properties of 5 nm noble nanoparticles self-organized in a 2D hexagonal planar array markedly differ with the nanomaterial used (Au and Ag). There is no splitting of the surface plasmon resonance observed for Au nanoparticles, whereas for Ag nanoparticles, with a similar geometry, a splitting is observed when decreasing the interparticle spacing d/(2R) to around one nanoparticle radius due to the strengthening of the dipolar interactions between the nanoparticles. Here it is shown by using AMB-1 magnetostatic bacteria that the change in the hysteresis loop when the nanoparticles are aligned is preferentially due to induced dipolar interactions rather than to the orientation of the easy axes. Intrinsic collective chemical properties due to the nanocrystals ordering are presented.

Assemblies of aligned magnetotactic bacteria and extracted magnetosomes: what is the main factor responsible for the magnetic anisotropy?

The origin of the magnetic anisotropy is explained in an assembly of aligned magnetic nanoparticles. For that, nanoparticles synthesized biologically by Magnetospirillum magneticum AMB-1 magnetotactic bacteria are used. For the first time, it is possible to differentiate between the two contributions arising from the alignment of the magnetosome easy axes and the strength of the magnetosome dipolar interactions. The magnetic anisotropy is shown to arise mainly from the dipolar interactions between the magnetosomes.

Immunity of coated self-ordered silver nanocrystals: a new intrinsic property due to the nanocrystal ordering.

New materials made of two- and three-dimensional superlattices of nanoparticles exhibit unique collective properties arising from the ordering of the nanoparticles. Here, dodecanethiol-coated silver nanocrystals self-assembled in 2D were subjected to oxygen plasma using the reactive ion etching process. The careful investigation by transmission electronic microscopy (TEM) of the same areas before and after exposure allows one to distinguish two behaviors under plasma treatment, which are determined by the level of ordering of the 2D organizations. Higher ordered self-assemblies remain unchanged, while less ordered organizations coalesce into larger nanocrystals with spheroidal shapes that could be single crystals. Electron energy-loss spectroscopy (EELS) measurements show that there is no oxidation of the silver either on the large coalesced nanocrystals or on the stable self-assemblies. A new intrinsic property, immunity of highly self-ordered nanocrystals provided by the ordering, is reported, and a mechanism of the coalescence is proposed.

Mesoscopic void structures in cobalt nanocrystal films formed from drying concentrated colloidal solutions.

Here we report the formation of void (hole) structures when concentrated colloidal solutions of magnetic nanocrystals are subjected to a magnetic field during slow evaporation. This presents a new type of solid mesostructure obtained by self-assembly of nanocrystals. The voids are characterized by a cylindrical shape with either circular or elliptical base. We show that the morphology of these patterns is essentially controlled by the fraction of the volume occupied by the magnetic phase to the total volume of the film. Monte Carlo simulations carried out using a Stockmayer fluid model agree remarkably well with the experiments for the formation of void structures in the range of considered volume fractions.

Self-organization of confined dipolar particles in a parallel field.

Monte Carlo simulations of a Stockmayer fluid confined between two parallel walls are performed to investigate self-organization of magnetic nanocrystals in a field parallel to the walls as a function of density, field strength, and wall separation. In order to study the formation of mesoscopic structures, a large number of up to 12,000 particles have to be used. The particles organize into periodically spaced cylindrical-like columns whose width typically varies between 5 and 9 particle diameters at low density. At small heights the columns are quenched due to the parallel walls, while larger wall separations can accommodate several layers of columns in good agreement with experiments. An increase in density entails a clear increase in column thickness, whereas an increase in field strength seems to have the opposite effect.

Supracrystals of inorganic nanocrystals: an open challenge for new physical properties.

When naturally occurring spherical objects self-organize, the physical properties of the material change. For example, a colorless opal is the result of a disordered aggregate of silica particles. When the silica particles are ordered, however, the opal takes on color, which is determined by the size of the self-assembled particles. In this Account, we describe how these 3D arrangements of nanomaterials can self-organize in 3D arrays called supracrystals; the 3D arrays can fall into the familiar categories of face-centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body-centered (bcc) crystals. The collective properties of these 2D and 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk. Comparison between the approach to saturation of the magnetic curve for supracrystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles, either highly crystallized or amorphous. We also demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self-ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supracrystal, analogous to a breathing mode vibration of atoms in a nanocrystal. Furthermore, we used 10 nm gamma-Fe(2)O(3) nanocrystals to gain new insight into the scaling law of crack patterns. We found that isotropic and directional crack patterns follow the same universal scaling law over a film height varying by 3 orders of magnitude. These data have led us to propose general analogies between supracrystals of nanocrystals, individual nanocrystals, and the molecules in the bulk phase for certain physical properties based on the ordering of the material. As we continue to study the physical properties of the ordered and disordered arrangements of nanomaterials, we will be able to go further in these analogies. And this exploration leads to new questions: first and foremost, is this behavior general?

Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations.

Control of nanocrystal (NC) crystallinity currently raises great interest because of its potential benefits in both physics modeling and technological applications. Advances in methods for synthesizing perfect single-crystalline NCs are recent, so that the effect of crystallinity on NC properties has received only limited study and still needs to be properly investigated. Here, we report that crystallinity of gold NCs dramatically modifies their vibrations. Using low-frequency Raman scattering, we clearly demonstrate that single-domain NCs vibrate differently than their multiply twinned counterparts, through the splitting of the quadrupolar vibrations, which is only observed for the former. Using the resonant ultrasound approach, we calculate the vibrational frequencies of a gold sphere and show that elastic anisotropy induces a lift of degeneracy of the quadrupolar mode in good agreement with our experimental measurements. These findings open up challenging perspectives on using Raman spectroscopy to characterize nanocrystallinity.

Self-organization of magnetic nanoparticles: a Monte Carlo study.

To understand the self-organization of magnetic nanocrystals in an applied field, we perform Monte Carlo simulations of Stockmayer fluids confined between two parallel walls. The system is examined in the gas-liquid coexistence region of its phase diagram and the field is applied perpendicular to the walls. Gibbs ensemble simulations are carried out to determine the phase coexistence curves of the confined Stockmayer fluid. In canonical simulations, different types of organizations appear dependent on particle density: columns, walls, and elongated and spherical holes. The morphology and size of structures are in good agreement with results obtained by free energy minimization and experiments. The influence of a distribution of particle sizes on the particle organization is investigated.

Tuning of solid phase in supracrystals made of silver nanocrystals.

Decanethiol-passivated silver nanocrystals are shown, by small-angle X-ray diffraction, to organize into hexagonal close packed or face centered cubic (fcc) structures depending on the substrate temperature. When the nanocrystals are passivated by dodecanethiols, fcc and body centered cubic lattices as well as disordered arrangements are observed. The different crystalline phases correspond to thermodynamic equilibrium states. The passivant chain length is shown to control the interactions between the nanocrystals and consequently the superlattice structure.

2D self-organization of core/shell Co(hcp)/Co nanocrystals.

In this paper we report the preparation of ordered hexagonal 2D arrays of core/shell Cohcp/CoO nanocrystals. A full structural investigation has been carried out using high-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy.

Use of reverse micelles to make either spherical or worm-like palladium nanocrystals: influence of stabilizing agent on nanocrystal shape.

Depending on the water content, reverse micelles induce the formation of fcc metallic palladium worm-like nanocrystals made of spheres. After extraction from the nanoreactor, either spheres or worm-like nanocrystals are obtained, and it was found that the binding energy between the coating agent and the Pd surface is a key parameter in shape control (i.e., in the surface reconstruction).

Large triangular single crystals formed by mild annealing of self-organized silver nanocrystals.

Nanocrystal organizations represent a new generation of materials with specific properties compared with those of isolated nanocrystals and of the bulk material. Here, we present the first intrinsic crystalline growth properties of highly ordered mono- and multilayers of 5 nm silver nanocrystals. Triangular single crystals with face-centred-cubic structures are obtained by annealing the ordered nanocrystals under atmospheric pressure at 50 degrees C. The triangles are mixed with well-crystallized coalesced particles of various shapes. Their size depends on the initial nanocrystal ordering range on the substrate, which is local on amorphous carbon and highly extended on highly oriented pyrolitic graphite (HOPG). Hence, the single-crystal size is larger on HOPG than on amorphous carbon. These observations show that the crystalline growth properties of silver nanocrystals can be tailored by controlling their organization. Furthermore, on HOPG an epitaxial orientation of the triangles is observed.