peer journals

Dependence of the localized surface plasmon resonance of noble metal quasispherical nanoparticles on their crystallinity-related morphologies.

The absorption spectra of 5 nm noble metal nanoparticles (Ag, Au, and Cu) with typical morphologies of multiply twinned particles (MTPs) and single crystals are calculated by using the discrete dipole approximation method. Among the considered morphologies, it is found that icosahedral, cuboctahedral and truncated octahedral particles behave like quasispherical particles whereas the optical response of the decahedral particles significantly differs from the others. This result, which originates from the shape anisotropy of the decahedron, points out the capacity to discriminate decahedral MTPs from a population of particles with mixed crystallinities and related quasispherical shapes.

Source : Dependence of the localized surface plasmon resonance of noble metal quasispherical nanoparticles on their crystallinity-related morphologies. P.Yang, H.Portales and M.P.Pileni. J.Chem.Phys., 2011, 134, 024507-1/6.

peer journals

Low Sensitivity of Acoustic Breathing Mode Frequency in Co Nanocrystals upon Change in Nanocrystallinity.

Cobalt nanocrystals (NCs) with narrow size distribution and polycrystalline structure in their native form are synthesized in reverse micelles. After annealing at 350 !C, these NCs are transformed into single crystalline phase with hexagonal close-packed structure. The vibrational dynamics of NCs di!ering by their nanocrystallinity is studied by femtosecond pump!probe spectroscopy. By recording the di!erential re »ectivity signal in the native and annealed Co NCs, the frequency of their fundamental breathing acoustic mode can be measured in the time domain. A small decrease of the breathing mode frequency is observed in single crystalline Co NCs compared to that measured in polycrystals, indicating low sensitivity of their fundamental radial mode upon change in crystallinity. This result is in agreement with predictions from calculations using the resonant ultrasound approach.

Source : Low Sensitivity of Acoustic Breathing Mode Frequency in Co Nanocrystals upon Change in Nanocrystallinity. D.Polli I. Lisiecki, H. Portalès, G. Cerullo and M.P. Pileni. ACS Nano., 2011, 5, 5785-5791.

peer journals

How Can the nanocrystallinity of 7 nm Spherical Co nanoparticles dispersed in solution be improved?

We report a solution-phase annealing of spherical Co nanocrystals synthesized in reverse micelles and coated with dodecanoic acid. The deposition of a drop of solution on a transmission electron microscope grid shows that a progressive increase in the temperature to 316 ! C results in the progressive crystallographic transition from a polycrystalline and probably face-centered cubic Co phase to the single-crystalline hexagonal close-packed (hcp) Co phase. These nanocrystals are highly stable against oxidation and coalescence. We stress that, to our knowledge, this constitutes the firrst example in the literature of pure hcp-Co spherical single crystals dispersed in solution. These nanocrystals can be freely manipulated and, due to their, low size dispersion, can self-organize on various substrates.

Source : How Can the nanocrystallinity of 7 nm Spherical Co nanoparticles dispersed in solution be improved? M. Cavalier, M. Wall, I. Lisiecki, and M.P. Pileni. Langmuir, 2011, 27, 5014–5020.

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

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

Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals. 

Here it is demonstrated that the di!usion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen di!usion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward di!usion of Co ions is favored over the inward di!usion of oxygen, producing hollow CoO single-domain nanocrystals.

Source : Nanocrystallinity and the Ordering of Nanoparticles in 2D Superlattices: Controlled Formation of either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals. 
Z.Yang, I.Lisiecki, M. Walls, M.P. Pileni ACS Nano. , 2013,  7, 1342–1350.

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.