peer journals

How to control the crystalline structure of supracrystals of 5-nm Ag nanocrystals ?

Supracrystals of 5-nm silver nanocrystals are characterized by various structures, ranging from face-centered-cubic (fcc), to hexagonal-close-packed (hcp), to body-centered-cubic (bcc) structures. Here, it is shown that the transition from fcc to hcp is solvent-dependent and attributed to specifi c stacking processes, depending on the evaporation kinetics. Hence, at a fi xed substrate temperature, the most volatile solvents (such as hexane and toluene) favor the growth of fcc superlattices, whereas with solvents that have a higher boiling point (such as octane, decane, and dodecane), hcp supracrystals are produced. In contrast, the formation of bcc structures is shown to be solvent-independent and is attributed to van der Waals attractions. The chain length of the coating agent and the deposition temperature govern the transition from compact (fcc/hcp) to bcc supracrystals. The experimentally phase transitions are interpreted by theoretical approaches.

Source : How to control the crystalline structure of supracrystals of 5-nm Ag nanocrystals ?A. Courty, J. Richardi, P.A. Albouy and M.P. Pileni, Chem Mat., 2011, 23, 4186-4192.

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

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

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

Supra and Nano crystallinity: 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.

Source : Supra and Nano crystallinity : Specific properties related to crystal growth mechanisms and nanocrystallinity. M.P.Pileni, Account Chem Res., 2012,45, 1965-1972.

 

peer journals

Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations.

Under solvent saturation, a precipitation of fullgrownsupracrystals on the one hand and the formation of well defined supracrystalline fi lms at the air− liquid interface on the other hand were previously observed for the fi rst time (J. Am. Chem. Soc. 2012 , 134 , 3714− 3719). Here, these two simultaneous growth processes are studied by additional experiments and by Brownian dynamics simulations. The thickness of the supracrystalline films and the concentration of free nanocrystals within the solution are measured as a function of the nanocrystal size. The simulations show that the fi rst process of supracrystal growth is due to a homogeneous nucleation favored by solvent-mediated ligand interactions, while the second one is explained in terms of a diff usion process caused by a decrease in the surface energy when the particles penetrate the air− liquid interface. It is also verifi ed that the presence of thiol molecules at the air− solution interface does not hinder the formation of supracrystalline films.

Source : Simultaneous Interfacial and Precipitated Supracrystals of Au Nanocrystals: Experiments and Simulations. N Goubet, J. Richardi, P.A. Albouy and M. P. Pileni. J.Phys.Chem.B., 2013, 117, 4510!4516 .