Showing papers by "Etienne Snoeck published in 2005"

Multimillimetre-large superlattices of air-stable iron–cobalt nanoparticles

Abstract: Self-organization of nanoparticles into two- and three-dimensional superlattices on a large scale is required for their implementation into nano- or microelectronic devices1,2. This is achieved, generally after a size-selection process3,4, through spontaneous self-organization on a surface5,6,7,8,9,10,11, layer-by-layer deposition12 or the three-layer technique of oversaturation3,14, but these techniques consider superlattices of limited size. An alternative method developed in our group involves the direct formation in solution of crystalline superlattices, for example of tin nanospheres, iron nanocubes or cobalt nanorods, but these are also of limited size15,16,17. Here, we report the first direct preparation in solution of multimillimetre-sized three-dimensional compact superlattices of nanoparticles. The 15-nm monodisperse FeCo particles adopt an unusual short-range atomic order that transforms into body-centred-cubic on annealing at 500 ∘C. The latter process produces an air-stable material with magnetic properties suitable for radiofrequency applications.

Selective growth of PbSe on one or both tips of colloidal semiconductor nanorods.

Abstract: PbSe nanocrystals with rock-salt structure are grown on the tips of colloidal CdS and CdSe nanorods. The facets of wurtzite rods provide a substrate with various degrees of reactivity for the growth of PbSe. The presence of dangling Cd bonds may explain subtle differences between nonequivalent facets resulting in the selective nucleation of PbSe only on one of the two tips of each CdS rod. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties.

Epitaxial growth and magnetic exchange anisotropy in Fe3O4/NiO bilayers grown on MgO(001) and Al2O3(0001)

Abstract: Epitaxial Fe3O4/NiO bilayers were epitaxially grown on MgO(001) and Al2O3(0001) substrates to investigate the influence of the fully spin compensated (001) and the non-compensated (111) NiO interface planes between the ferromagnetic (F) and antiferromagnetic (AF) layers on the AF/F exchange coupling. Bilayers of different magnetite thicknesses and constant NiO thickness were investigated. The structural characterizations indicate a perfect epitaxy of the two layers for the both growth directions in the two Fe3O4/NiO/MgO(001) and NiO/Fe3O4/Al2O3(0001) systems. An epitaxial ferrimagnetic (Ni,Fe)Fe2O4 phase is observed at the AF/F interface when the NiO oxide is grown on the top of the Fe3O4 layer while a perfectly flat AF/F interface is observed in the Fe3O4/NiO/MgO(001) system exhibiting only a very slight interdiffusion. Magnetic measurements indicate a relative strong bias at 300 K for the bilayers grown on Al2O3(0001), which decreases with the inverse of the ferrimagnetic layer thickness as theoretically expected. On the contrary, a zero exchange biasing is observed at 300 K for the bilayers grown on MgO(001).

Quantitative strain and stress measurements in Ge/Si dual channels grown on a Si0.5Ge0.5 virtual substrate

Abstract: The incorporation of compressive strained Ge/tensile strained Si bi-layers in the active regions of MOSFETs is a promising route for creating ultimate Si-based devices due to the considerable increase of the mobility of spatially confined holes/electrons. The main challenge in device application is to be able to control and manipulate strain within such thin layers. This paper reports on quantitative measurements of strain in a structure consisting of a 8 nm Ge/5 nm Si heterostructure grown by chemical vapour deposition on top of a relaxed Si0.5Ge0.5 buffer layer. Geometric phase analysis of high resolution TEM images is used to measure the strain within Ge and Si layers. The in-plane stress within each layer is deduced. Experimental results are compared with the predictions of elasticity theory and discussed in terms of effect of defect formation.

Complex angular dependence of exchange bias on (001) epitaxial NiO-Co bilayers

Abstract: We have sputter-deposited NiO-Co bilayers on MgO(001) substrates. NiO and Co grow epitaxially on MgO and reproduce its fcc structure. The high quality of our samples, in terms of flatness and crystallographic coherence of the interface, allows the observation of an additional fourfold magnetic anisotropy term by standard magnetometry. This term is induced by interfacial interaction assigned to the same origin as exchange bias. Additional measurements of exchange bias azimuthal dependence versus the crystallographic axes of the film plane reveal unusual behaviors with several sign changes related to this fourfold anisotropy.

II-VI and II1-xMnxVI semiconductor nanocrystals formed by the pressure cycle method

Abstract: II–VI and II1−x Mn x VI nanocrystals were prepared by the pressure cycle method using the Paris–Edinburgh cell. The recovered samples are nanocrystals in the cubic phase zinc-blend (ZB) structure and were characterized using transmission electron microscopy, electron diffraction, X-ray diffraction and Raman scattering. Transmission electron micrographs show that these nanocrystals are nearly spherical with diameters ranging from 20 to 50 nm depending on the sample under investigation. The Raman scattering measurements confirm the existence of II–VI nanocrystals in the cubic phase (ZB). The magnetic properties of Cd0.5Mn0.5Te nanoparticles were found to vary with the particle size and were different from those observed for the Cd0.5Mn0.5Te bulk initial samples. The χ vs. T data show temperature hysteresis due to spin-glass form, which occurs at T g=21 K, for both the bulk as well as for the recovered nanoparticle samples. The zero-field cooled and field-cooled χ vs. T curves for the nanoparticles showed a ...