peer journals

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.

Source : Supra – and nanocrystallinities: a new scientific adventure M.P.Pileni. J.Phys. Cond.Mat., 2011, 23, 503102.

peer journals

Analogy Between Atoms in a Nanocrystal and Nanocrystals in a Supracrystal: Is It Real or Just a Highly Probable Speculation ?

Nanocrystals and supracrystals are arrangements of highly-ordered atoms and nanocrystals, respectively. At the nanometer scale, from face centered cubic (fcc) tetrahedral subunits, either single fcc nanocrystals such as cubooctahedra and octahedra or decahedral and icosahedral nanocrystals are produced. Such nanocrystals with different shapes are produced by soft chemistry. At the micrometer scale, very surprisingly, supracrystals having shapes similar to those obtained at the nanometer scale are produced. For example, large triangular nanocrystals as well as supracrystals are produced either by soft chemistry, from nanocrystals diffusion on a surface or by nanocrystals interactions in solution. The morphologies of nanocrystals, supracrystals and minerals, which are similar at various scales (nm and mm), are pointed out and an explanation of these similarities is undertaken.

Source : Analogy Between Atoms in a Nanocrystal and Nanocrystals in a Supracrystal: Is It Real or Just a Highly Probable Speculation ? N.Goubet and M. P. Pileni. J. Phys. Chem. Lett., 2011, 2, 1024–1031.

peer journals

Simultaneous Growths of Gold Colloidal Crystals.

Natural systems give the route to design periodic arrangements with mesoscopic architecture using individual nanocrystals as building blocks forming colloidal crystals or supracrystals . The collective properties of such supracrystals are one of the main driving forces in materials research for the 21st century with potential applications in electronics or biomedical environments. Here we describe two simultaneous supracrystal growth processes from gold nanocrystal suspension, taking place in solution and at the air− liquid interface. Furthermore, the growth processes involve the crystallinity selection of nanocrystals and induce marked changes in the supracrystal mechanical properties.

Source : Simultaneous Growths of Gold Colloidal Crystals. N. Goubet, H. Portalès, C. Yan, I.Arfaoui, P.A. Albouy, A. Mermet and M.P.Pileni J.Am..Chem.Soc, ., 2012, 134, 3714-3719.

 

peer journals

Unexpected electronic properties of micrometer-thick supracrystals of Au nanocrystals.

We investigated the electronic properties of highly ordered three-dimensional colloidal crystals of gold nanocrystals (7 ±  0.4 nm), called supracrystals. Two kinds of Au supracrystals with typical thicknesses of 300 nm and 5 ! m, respectively, are probed for the first time with scanning tunneling microscopy/spectroscopy at 5 K revealing similar power law behavior and showing homogeneous conductance with the fingerprint of isolated nanocrystal. Potential applications evading the size-related risks of nanocrystals could be then considered.

Source : Unexpected electronic properties of micrometer-thick supracrystals of Au nanocrystal. P.Yang, I. Arfaoui, T. Cren, N. Goubet and M.P. Pileni. Nano Lett., 2012, 12, 2051-2055.

peer journals

A phase-solution annealing strategy to control the Co nanocrystal anisotropy: Structural and magnetic investigations.

Here, we report a phase-solution annealinginduced structural transition of 7 nm-Co nanocrystals from the fcc polycrystalline phase to the hcp single-crystalline phase. For any annealing temperature, contrary to what was down in our previous paper (Langmuir 2011 , 27 , 5014), the same solvent (octyl ether) is used preventing any change in adsorbates related to various solvents on the nanocrystal surface. A careful transmission electron microscopy study, combined with the electron diff raction, confi rms the nanocrystal recrystallization mechanism. The annealing process results in neither coalescence nor oxidation. The converted nanocrystals can be easily manipulated and due to their low size dispersion self-organize on an amorphous-carbon-coated grid. Magnetic property investigations, keeping the same nanocrystal environment, show that the structural transition is accompanied by a signifi cant increase in both the blocking temperature (to a near room-temperature value) and the coercivity.

Source : A phase-solution annealing strategy to control the Co nanocrystal anisotropy: Structural and magnetic investigations. Z.Yang, M.Cavalier, M. Walls, P. Bonville, I. Lisiecki, M.P Pileni J.Phys.Chem.C. , 2012, 116, 15723-15730.

peer journals

Electronic Properties Probed by Scanning Tunneling Spectroscopy: From Isolated Gold Nanocrystal to Well-defined Supracrystals.

Scanning tunneling microscopy and spectroscopy at 5 K have been used to determine the electronic properties of 7-nm dodecanethiol-passivated Au nanocrystals in three different configurations: isolated nanocrystal, selforganized thin films (few nanocrystal layers), and large three-dimensional well-defined thick films (over 30 nanocrystal layers) called supracrystals. The electronic properties of both thin and thick well-ordered supracrystals are analyzed in scanning tunneling spectroscopy geometry through dI/dV curves and conductance mapping at different bias voltages. The single particles exhibit a typical dI/dV curve with a Coulomb gap of !360 meV and a Coulomb staircase. The dI/dV curve of the thin supracrystals presents a Coulomb blockade feature !100 meV narrower in width than that of the single nanocrystal but without well-defined staircase. On the contrary, the thick supracrystals exhibit a dI/dV curve showing a large Coulomb gap with a Coulomb-staircase-like structure. Generally, the conductance mapping is found to be very homogeneous for both supracrystals. Nevertheless, for some bias voltages, inhomogeneities across individual nanocrystals appear. Additionally, some of these inhomogeneities seem to be related to the supracrystal surface morphology. Finally, these slight variations in the conductance mapping across individual nanocrystals embedded in the supracrystal are discussed in terms of high degree of nanocrystal ordering, low nanocrystal size distribution, and nanocrystal crystallinity.

Source : Electronic Properties Probed by Scanning Tunneling Spectroscopy: From Isolated Gold Nanocrystal to Well-defined Supracrystals. P.Yang, I. Arfaoui, T. Cren, N. Goubet and M.P Pileni, Phys.Rev. B., 2012, 86, 075409.

peer journals

Crystallinity Segregation upon Selective Self- Assembling of Gold Colloidal Single Nanocrystals.

Spontaneous separation of single from polycrystalline5 nm gold nanocrystals (NCs) is observed in colloidal solution. This segregation takes place upon selfassembling of single crystalline NCs at the air − solvent interface and in precipitated superlattices. Polycrystalline NCs are observed to remain in the suspension. Transmission electron microscopy analysis of the size distribution of NCs issued from the diff erent populations indicates that the NC size does not change from each other, excluding therefore any size segregation in this process. Using both low-frequency Raman scattering and X-ray diff raction provides reliable characterization of nanocrystallinity for each population of NCs, thus confi rming the crystallinity segregation. The single crystalline NCs are found by electron diff raction to self-assemble into close-packed superlattices with long-range translational and orientational ordering, while polycrystalline NCs behave like spheres with no preferential orientation. The face-to-face orientational ordering, which is only observed for single crystalline NCs, supports the relevance of the specific crystallinity-related morphologies of these NCs in their better ability to self-assemble. Exploiting this spontaneous segregation would open up a simple alternative to other demanding routes for controlling crystallinity of nanocrystals and optimizing their properties for potential applications.

Source : Crystallinity Segregation upon Selective Self- Assembling of Gold Colloidal Single Nanocrystals. H.Portales, N. Goubet, S.Sirotki, E. Duval, A. Mermet, P. Albouy, and M.P. Pileni. Nano Lett., 2012, 12, 5292−5298.