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

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

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 .

peer journals

Modulating the Physical Properties of Isolated and Self-Assembled Nanocrystals by Change in Their Nanocrystallinity

For self-assembled nanocrystals in three-dimensional (3D) superlattices, called supracrystals, the crystalline structure of themetal nanocrystals (either single domain or polycrystalline) or nanocrystallinity is likely to induce signi! cant changes in the physical properties. Previous studies demonstrated that spontaneous nanocrystallinity segregation takes place in colloidal solution upon selfassembling of 5 nm dodecanethiol-passivated Au nanocrystals. This segregation allows the exclusive selection of single domain and polycrystalline nanoparticles and consequently producing supracrystals with these building blocks. Here, we investigate the in » uence of nanocrystallinity on di# erent properties of nanocrystals with either single domain or polycrystalline structure. In particular, the in » uence of nanocrystallinity on the localized surface plasmon resonance of individual nanocrystals dispersed in the same dielectric media is reported. Moreover, the frequencies of the radial breathing mode of single domain and polycrystalline nanoparticles are measured. Finally, the orientational ordering of single domain nanocrystals markedly changes the supracrystal elastic moduli compared to supracrystals of polycrystalline nanocrystals.

Source : Modulating the Physical Properties of Isolated and Self-Assembled Nanocrystals by Change in Their Nanocrystallinity. N. Goubet, C.Yan, D. Polli, H.Portalès, I.Arfaoui, G. Cerullo and M. P. Pileni Nano Lett., 2013,13, 504−508.

peer journals

Hierarchy in Au nanocrystal ordering in Supracrystals: I.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 (C16 H33− SH). The infl uences 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 fi lm sitting on a disordered nanocrystal fi lm, supracrystal films grown layer-by-layer, and fi nally supracrystals grown in solution with various well-defi ned shapes.

Source : Hierarchy in Au nanocrystal ordering in Supracrystals: A potential approach to detect new physical properties. Y.F. Wan, N. Goubet, P. A. Albouy and M. P. Pileni. Langmuir, 2013,, 29, 7456−7463.

peer journals

Soft Supracrystals of Au Nanocrystals with Tunable Mechanical Properties

The elastic properties of highly ordered three-dimensional colloidal crystals of gold nanocrystals (called supracrystals) are reported. This study is based on the simultaneous growth of two kinds of gold nanocrystal supracrystals that range in size from 5 nm to 8 nm: interfacial supracrystals and precipitated supracrystals. The elastic properties are deduced from nanoindentationmeasurements performed with an atomic force microscope. The Young’s modulus of the interfacial supracrystals, which grow layer-by-layer and formwell-defined films, is compared to that of precipitated supracrystals, which are produced by homogeneous growth in solution. For the precipitated supracrystals, characterized by a thickness larger than 1 μ  m, the Oliver and Pharr model is used to determine the elastic moduli, which are in the gigapascal range and decrease with increasing nanocrystal size. For the interfacial supracrystals, with 300 nm average thickness, a second model (plate model) is applied in addition to the Oliver and Pharr model. These two models confirm independently that the interfacial fi lms are very soft with Young’s modulus in the range of 80–240 MPa. This result reveals a totally new feature of nanocrystal solids, never emphasized before. It is shown that these changes in the Young’s modulus are related to the supracrystal growth mechanism.

Source : Soft Supracrystals of Au Nanocrystals with Tunable Mechanical Properties C.Yan, I.Arfaoui, N.Goubet and M.P.Pileni Adv.Funct.Mat., 2013, 23, 2315-2321.