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

Supracrystals of Au-nanocrystals differing by their diameters: Influence of the Thickness and Nanocrystallinity on the Electronic Properties. 

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.

Source : Supracrystals of Au-nanocrystals differing by their diameters: Influence of the Thickness and Nanocrystallinity on the Electronic Properties.  P.Yang, I.Arfaoui, T. Cren, N. Goubet, M.P. Pileni J.Phys.Cond.Mat, 2013, 25 335300-5.

peer journals

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 singledomainor a polycrystalline phase maintaining the same chemical environment. We obtain quasi-sphericalnanoparticles 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.

Source : Size and nanocrystallinity controlled gold nanocrystals: synthesis, electronic and mechanical properties. N. Goubet,  I. Tempra,  J. Yang,  G. Soavi,  D. Polli,  G. Cerullo  and M. P. Pileni Nanoscale, 2015,7, 3237–3246.