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

Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals.

8 nm Co nanoparticles with various crystalline structures called polymorphs were produced using different synthetic procedures, such as using reverse micelles, the thermal decomposition of organometallics approach or the hot injection process. These 8 nm Co nanoparticles differing by their crystalline structures are exposed to oxygen at elevated temperature. The fcc Co polycrystalline nanoparticles produce either Co–CoO yolk–shell or CoO hollow structures whereas amorphous Co nanoparticles produce Co–CoO core–shell nanoparticles. Furthermore, single domains with either hcp or e crystalline structure behave differently upon oxygen diffusion. Co–CoO nanoparticles were produced from the hcp phase while CoO hollow nanoparticles were the product for e-phase Co nanocrystals.

Source : Crystal polymorphism: dependence of oxygen diffusion through 2D ordered Co nanocrystals. Z .Yang, J.Yang, J.Bergström, K.Khazen, and M. P. Pileni Phys. Chem. Chem. Phys., 2014,16, 9791-9796.

peer journals

Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size.

The size-dependent Kirkendall effect is studied by using Co nanoparticles.The sizes of Co nanoparticles differing by their crystal structures callednanocrystallinity, namely amorphous, polycrystalline fcc, single crystalline hcp, and single crystalline ε  phase, are modulated from 4 to 10 nm. The nanoparticles self-assembled in 2D superlattices and differing by their nanocrystallinities are subjected to oxygen at 200 ° C for 10 min. With single-domain nanocrystals differing by their crystalline structure (ε  and hcp  phases), marked changes in the fi nal structures are observed: upon increasing the nanocrystal size, the ε  phase favors formation of a hollow structure whereas a transition from single-domain hollow to multidomain core/shell structures takes place with the hcp  phase. With polycrystalline fcc  Co nanocrystals, a transition from a hollow to a yolk/shell structure is observed, whereas with amorphous cobalt, solid CoO nanoparticles are produced at the smaller size and are converted to the core/shell structure at the larger one. These differences in size effect are attributed to the change in the control of the inward fl ow of oxygen atoms and the outward fl ow of Co atoms with the crystalline structure of cobalt nanoparticles. Such a diffusion process described here on the Kirkendall effect can be studied for other metal nanocrystals.

Source : Control of the oxygen and cobalt atoms diffusion through Co nanoparticles differing by their crystalline structure and size. Z. Yang, N. Yang,  J. Yang,  J. Bergström  and M.P. Pileni Adv.Funct.Mater.,2015, 25, 891-897.

peer journals

Superior Oxygen Stability of N-Heterocyclic Carbene-coated Au Nanocrystals – Comparison with Dodecanethiol

The stability to oxygen-based treatments of Au nanocrystals (NCs) coated with different N -heterocyclic carbenes (NHCs) or dodecanethiol (DDT) was investigated. A dominant effect of the ligand type was observed with a significantly greater oxygen resistance of NHC-coated Au NCs compared to the thiol-based analogues. NHC-coated Au NCs are stable to 10 W oxygen plasma etching for up to 180 s whereas the integrity of DDT-coated Au NCs is strongly affected by the same treatment from 60–80 s. In the latter case, the average size of the NCs (from 2.6 to 6.3 nm) and the method of synthesis have no effect on the stability. NHC-coated Au NCs were found to generate of a lower amount of ligand-derived species under molecular oxygen treatment, which could account for the increased stability.

Source : Superior Oxygen Stability of N-Heterocyclic Carbene-coated Au Nanocrystals – Comparison with Dodecanethiol.
X.Ling, N. Schaeffer; S.Roland, M.P. Pileni Langmuir, 2015, 31, 12873-12882

peer journals

Surface Plasmon Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals

Silver nanocrystals differing by their coating agents and sizes are self-assembled in thin supracrystalline films. The surface plasmon resonance (SPR) properties of such assemblies are presented. Nanocrystal size, interparticle distance and coating agent play key roles in the plasmonic coupling of Ag nanocrystals within supracrystals. Here, we demonstrate experimentally the predictions for 2D self-assemblies remains valid for thin 3D superlattices. The absorption spectra in the visible range are markedly dependent on the incidence of the light source and confirm the apparition of a splitting of the dipolar surface band into two components upon increasing the incidence angle. The major parameter inducing the splitting of the SPR band is the relative ratio between the average distance of nanocrystals and their diameters. The nature of the coating agent is also of particular importance; it is hereby shown that theoretical predictions and experimental data are in agreement for alkylamine coating agents, whereas they differ for thiol-coated nanocrystals.

Source : Surface Plasmon Resonance Properties of Silver Nanocrystals Differing in Size and Coating Agent Ordered in 3D Supracrystals
J.Wei, N. Schaeffer, P.A. Albouy, and M.P.Pileni, Chem.Mat., 2015,27, 5614−5621.

peer journals

Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure.

The Kirkendall eff ect is a classical phenomenon in materials science, and it is referred to as a nonreciprocal interdiff usion process through an interface of two metals with strikingly diff erent atomic diff usivities, leading to a formation of vacancies called Kirkendall voids. The nanoscale Kirkendall eff ect has been vastly applied in the fabrication of hollow nanostructures after the fi rst report on the synthesis of Co-based hollow nanocrystals. In this Feature Article, we briefl y start with an introduction on the Kirkendall effect concept, followed by the general synthetic strategy toward the production of hollow Kirkendall voids. The overall synthetic strategies are based on the design of diff usion couples at the nanoscale, and then, we discuss the factors that govern the formation of Kirkendall voids at the nanoscale, from the viewpoint of the nanoparticle size, nanoparticle crystallinity, and nanoparticle environment. We conclude with a summary and perspectives on the design of hollow nanostructures governed by the Kirkendall effect.

Source : Nano Kirkendall Effect Related to Nanocrystallinity of Metal Nanocrystals: Influence of the Outward and Inward Atomic Diffusion on the Final Nanoparticle Structure
ZYang, N. Yang and M.P.Pileni J.Phys.Chem.C., 2015,119, 22249-22260