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

2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure.

Nanocrystals are able to self-assemble in hexagonal networks (2D) and in supracrystals (3D). Here it is shown that the interparticle distance is tuned by the presence of water molecules adsorbed at the nanocrystal interface and on the alkyl chains used as coating agents. By using an intrinsic property due to the nanocrystal ordering, a new, but destructive, method is proposed to detect defects on a large monolayer scale. The supracrystal growth mechanism changes with the nanocrystal size from a heterogeneous (layer-by-layer) to a homogeneous (growth in solution) process. Co supracrystals are highly stable after annealing at 350 !C with an improvement in the nanocrystal ordering, i.e., in the supracrystallinity. With Ag supracrystals it was possible, from the same batch of 5 nm Ag nanocrystals, to control the supracrystallinity with phase transitions of hcp to fcc and amorphous solids to hcp and bcc. Finally a tentative analogy between atoms and nanocrystals is proposed in the crystal growth process. These data open a new research area with a large potential for discovering new chemical and physical properties.

Source : 2D superlattices and 3D supracrystals of metal nanocrystals : a new scientific adventure.  M.P.Pileni J. Mater. Chem., 2011, 21, 16748 – 16758

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

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

Ordering at Various Scales: Magnetic Nanocrystals.

Here, it is shown that the internal crystallinity called nanocrystallinity of rather uniform Co nanoparticles can be improved by annealing. This induces marked changes in the magnetic properties such as an increase in the blocking temperature that can reach a value close to room temperature. It is shown that the acoustic breathing modes remain quite unchanged by changing the nanocrystallinity. Co nanocrystals with low size distribution are able to self-assemble either in fcc colloidal crystals called supracrystals or in #lms with voids. Collective intrinsic properties (chemical and physical) due to magnetic nanocrystal ordering in 2D and 3D superlattices are presented. Furthermore, when the nanocrystals are aligned, the magnetic properties of the assemblies are improved. By using magnetostatic bacteria, it is demonstrated that the magnetic anisotropy is mainly due to induced dipolar interactions with a low contribution of the in!uence of the orientation of the nanocrystal easy axes.

Source : Ordering at Various Scales: Magnetic Nanocrystals. I. Lisiecki and M. P. Pileni, J. Phys. Chem. C, 2012, 116, 3–14