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

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

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

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

peer journals

Self-organization of inorganic nanocrystals: Unexpected chemical and physical properties.

Here we point out that the nanocrystals well ordered in compact hexagonal networks are highly stable compared to the same nanocrystals either isolated on a substrate or ordered in a less compact manner. The emergence of unexpected collective physical intrinsic properties results in the nanocrystals being ordered over a long distance in colloidal crystals called supracrystals. Some morphologies of nanocrystals ordered, at the micrometer scale, in 3D superlattices called supracrystals are similar to those obtained with atoms in nanocrystals. From a comparison between vibrational and magnetic properties of supracrystals and aggregates composed of the same nanocrystals, it is proposed that nanocrystals in a supracrystal could behave as atoms in a nanocrystal. From these data a possible analogy between nanocrystals in a supracrystal and atoms in nanocrystals is proposed.

Source : Self-organization of inorganic nanocrystals: Unexpected chemical and physical properties. M.P.Pileni Journal of Colloid and Interface Science, 2012,  388 , 1–8.

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

Crack patterns in superlattices made of maghemite nanocrystals

Here, 11 nm g-Fe2O3 nanocrystals characterized by a low size distribution of 5% and dispersed in chloroform are deposited onto a substrate as well-defined films. The film thickness is controlled by the initial concentration of the colloidal solution. After drying, the film shows a network of well-defined cracks. It is demonstrated using scanning electron microscopy and small-angle X-ray diffraction that the nanocrystals within the film are self-assembled in ordered lattices. It is shown that the nanocrystals selfassemble in superlattices before the cracks appear. In contrast to what was previously observed with a disordered film of maghemite nanocrystals with large size distribution, the top surface of the films is covered with patterns formed before crack formation. The cracks preferentially follow the direction of the hexagonal array observed on the top surface of the film by using a field emission gun scanning electron microscope. The average crack distance as a function of the film height follows the same linear scaling law as in amorphous nanocrystal assemblies. The size of superlattices of nanocrystals evolves with film height, but is usually significantly smaller than the average crack distance except on the border of the films.

Source : Crack patterns in superlattices made of maghemite nanocrystals A.T.Ngo, J.Richardi and M.P.Pileni,  Phys. Chem. Chem. Phys., 2013, 15, 10666-10672.