Oriented Attachment of ZnO Nanocrystals

Self-organization of nanoparticles is a major issue to synthesize mesoscopic structures. Among the possible mechanisms leading to self-organization, the oriented attachment is efficient yet not completely understood. We investigate here the oriented attachment process of ZnO nanocrystals preformed in the gas phase. During the deposition in high vacuum, about 60% of the particles, which are uncapped, form larger crystals through oriented attachment. In the present conditions of deposition, no selective direction for the oriented attachment is noticed. To probe the driving force of the oriented attachment, and more specifically the possible influence of the dipolar interaction between particles, we have deposited the same nanocrystals in the presence of a constant electric field. The expected effect was to enhance the fraction of domains resulting from the oriented attachment due to the increased interaction of the particle dipoles with the electric field. The multiscale analytical and statistical analysis (TEM coupled to XRD) shows no significant influence of the electric field on the organization of the particles. We therefore conclude that the dipolar interaction between nanocrystals is not the prominent driving force in the process. Consequently, we argue, in accordance with recent theoretical and experimental investigations, that the surface reduction, possibly driven by Coulombic interaction, may be the major mechanism for the oriented attachment process

Ga and Al degeneratly doped ZnO Nanocrystals for Mid IR plasmonics

International audienc

Is Graphitic Silicon Carbide (Silagraphene) Stable?

Graphene is considered to be the most likely candidate for the postsilicon era; however, the problem with its zero band gap is challenging to overcome. A close relative of silicon, silicon carbide is expected to have a stable 2D polymorph which happens to be a wide-gap semiconductor. Unfortunately, the so-called silagraphene has proven to be elusive. To date, neither theoretical nor experimental studies have been conclusive. Here, we employ computational methods to determine the stable arrangements of silagraphene and establish their accurate band structure. We also experimentally validate our models by preparing and characterizing a number of graphitic features. Silagraphene exhibits a wide spectrum of optoelectronic properties (360-690 nm) as well as an unusual band structure with highly anisotropic transport properties, which originates from its nondispersive band near its K-point. This feature makes direct-indirect gap crossover extremely sensitive to ambient conditions, making silagraphene suitable for a range of sensors

Metallic ZnO nanocrystals for Mid IR plasmonics

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Metallic ZnO nanocrystals for Mid IR plasmonics: [Invited]

International audiencePlasmonics has achieved tremendous improvements over the recent decade in sensing (e.g. SERS). However, plasmonics based on noble metal electron gas allows enhancing electromagnetic fields only in the visible and near-IR range. Unfortunately, many molecules exhibit vibrational modes in the mid-IR. In order to enhance the coupling of Mid-IR light with molecular vibrations and thus to improve the sensitivity of IR absorption spectroscopy (the so-called SEIRA: surface enhanced IR absorption), it is necessary to design materials with tunable electron gas density. This is possible in degenerate semiconductors [1, 2]. Among these, oxides and most specifically ZnO are second to none. In the present paper, we address the issue of the design of Ga or Al doped ZnO nanocrystals for Mid-IR plasmonics. We first demonstrate the tunability of the local surface plasmon resonance (LSPR) of the nanocrystals upon Ga (or Al) concentration [3]. The nanocrystals are synthesized by a physical route, relying on the adiabatic supersonic expansion of a plasma. The resulting nanocrystal films are studied structurally by TEM, XDR, chemically by RBS and XPS and optically by FTIR and PhotoAcoustic FTIR. The effect of high doping level in ZnO nanocrystals is investigated and the solubility of Ga and Al in our process is determined. The LSPR of oxide nanocrystals is characterized by a large FWHM as can be seen on figure 1. This characteristic is related to several parameters. Among them, the role of the dopant location and activation is crucial. Consequently we examine both experimentally [4] and numerically [5] their influence on our nanocrystals. We show that the dopant activation can be greatly enhanced if the nanocrystals are synthesized in an O-poor environment. However, no further post-deposition treatment, such as annealing, as proven to be efficient to further improve the dopant activation. This questions the dopant stability in degenerate nanocrystals.Embedding the nanocrystals in a dielectric matrix (Al2O3) does not improve the activation of the dopants. However, it forbids the self-organization of the crystals (a process known as Oriented Attachment) and consequently leads to a reduced FWHM of the plasmon resonance. Eventually, the conditions to observe SEIRA effect will be discussed

Metallic ZnO nanocrystals for Mid IR plasmonics

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Interplay between local structure and magnetic properties of graded exchange-coupled Co@FePt nanocomposite films

International audienceMagnetically-hard nanocomposites are attractive materials for integration in various microsys-tems and for building next generation permanent magnets. However, exploiting their full potential requires to control their microstructure at the nanometre scale. Studying these materials in model systems synthesised by nanofabrication routes gives interesting insights about the interplay between the microstructure and the magnetic performances. Here, by using a combination of mass-selected low energy cluster beam deposition and electron-beam evaporation, we have prepared nanocomposite films where Co nanoinclusions are integrated in a hard magnetic FePt matrix. Local atomic structures and element selective magnetic properties of such nanocomposites have been thoroughly investigated using polarisation dependent hard X-ray absorption spectroscopies. These results demonstrate that magnetically soft inclusions are stabilised at room temperature, emphasising the role of the interdiffusion in the preparation of nanocomposites