154 research outputs found
Pressure stability and low compressibility of intercalated cagelike materials: the case of silicon clathrates
We study the behavior under pressure (up to 35 GPa) of intercalated silicon clathrates, combining x-ray diffraction experiments and ab initio calculations. We show that endohedral doping does not introduce a strong modification of the compressibility of the empty clathrate network and that in particular cases can raise it to values equivalent to the one of the silicon diamond phase. Intercalation can also prevent the collapse of the cage structure up to pressures at least 3 times higher than in the empty clathrate. Further we find that the stability of all studied silicon clathrate networks as well as stressed silicon diamond is limited to average Si-Si interatomic distances higher than 2.30 Angstrom
Guest displacement in silicon clathrates
We study both theoretically and experimentally the structure of the doped silicon clathrate II NaxSi34. We find that contrary to published works, the sodium atoms do not retain the T-d symmetry inside the Si-28 cages and move about 1 A away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent Si-28 cage, leads to a "dimerization" of sodium atoms. As a consequence, Rietveld refinements of x-ray diffraction spectra and transport, vibrational, and electronic properties must be revisited
A new class of low compressibility materials: Clathrates of silicon and related materials
We discuss the high pressure properties of different silicon clathrate structures that we have investigated by means of X-ray diffraction and ab initio calculations. Compressibility transition pressures or phase transformations are interpreted as a function of the nature of the guest atom intercalation, The compressibility of the clathrate structure is in all cases close to that of silicon diamond whereas transition pressures or the high pressure phases are extremely depending on the nature of the guest atom. We address the implications for obtaining a metallic material as hard as diamond
Micro-SQUID technique for studying the temperature dependence of switching fields of single nanoparticles
An improved micro-SQUID technique is presented allowing us to measure the
temperature dependence of the magnetisation switching fields of single
nanoparticles well above the critical superconducting temperature of the SQUID.
Our first measurements on 3 nm cobalt nanoparticle embedded in a niobium matrix
are compared to the Neel Brown model describing the magnetisation reversal by
thermal activation over a single anisotropy barrier.Comment: 3 pages, 4 figures; conference proceeding: 1st Joint European
Magnetic Symposia (JEMS'01), Grenoble (France), 28th August - 1st September,
200
Superconductivity in doped sp3 semiconductors: The case of the clathrates
We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba-8@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I-8@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C-60
Structural and magnetic properties of CoPt mixed clusters
In this present work, we report a structural and magnetic study of mixed
Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters,
avoiding any magnetic interactions between particles. Transmission Electron
Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters
are about 2nm in diameter and crystallized in the A1 fcc chemically disordered
phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing
Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters
conserve these properties, interaction with matrix atoms being limited to their
first atomic layers. Considering that 60% of particle atoms are located at
surface, this interactions leads to a drastic change in magnetic properties
which were investigated with conventional magnetometry and X-Ray Magnetic
Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are
weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon
as silicides are formed. Magnetic volume of clusters embedded in MgO is close
to the crystallized volume determined by GIWAXS experiments. Cluster can be
seen as a pure ferromagnetic CoPt crystallized core surrounded by a
cluster-matrix mixed shell. The outer shell plays a predominant role in
magnetic properties, especially for clusters embedded in niobium which have a
blocking temperature 3 times smaller than clusters embedded in MgO
Interface magnetic anisotropy in cobalt clusters embedded in a platinum or niobium matrix
A low concentration of cobalt clusters with a fcc structure and containing
almost one thousand atoms are embedded in two different metallic matrices:
platinum and niobium. Samples have been prepared using a co-deposition
technique. Cobalt clusters preformed in the gas phase and matrix atoms are
simultaneously deposited on a silicon substrate under Ultra High Vacuum
conditions. This original technique allows to prepare nanostructured systems
from miscible elements such as Co/Pt and Co/Nb in which clusters keep a pure
cobalt core surrounded with an alloyed interface. Magnetic measurements
performed using a Vibrating Sample Magnetometer (VSM) reveal large differences
in the magnetic properties of cobalt clusters in Pt and Nb pointing out the key
role of cluster/matrix interfaces.Comment: 7 pages (LaTeX), 12 PostScript figures, 1 PostScript tabl
YAG nano-light sources with high Ce concentration
We investigate the luminescence properties of 10 nm YAG nanoparticles doped
with Ce ions at 0.2%, 4% and 13% that are designed as active probes for
Scanning Near field Optical Microscopy. They are produced by a physical method
without any subsequent treatment, which is imposed by the desired application.
The structural analysis reveals the amorphous nature of the particles, which we
relate to some compositional defect as indicated by the elemental analysis. The
optimum emission is obtained with a doping level of 4%. The emission of the YAG
nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder
whereas the 13% doped particles hardly exhibit any luminescence. In the latter
case, the presence of Ce4+ ions is confirmed, indicating that the Ce
concentration is too high to be incorporated efficiently in YAG nanoparticles
in the trivalent state. By a unique procedure combining cathodoluminescence and
Rutherford backscattering spectrometry, we demonstrate that the enhancement of
the particles luminescence yield is not proportional to the doping
concentration, the emission enhancement being larger than the Ce concentration
increase. Time-resolved photoluminescence reveals the presence of quenching
centres likely related to the crystalline disorder as well as the presence of
two distinct Ce ions populations. Eventually, nano-cathodoluminescence
indicates that the emission and therefore the distribution of the doping Ce
ions and of the defects are homogeneous
Correlation between the Extraordinary Hall Effect and Resistivity
We study the contribution of different types of scattering sources to the
extraordinary Hall effect. Scattering by magnetic nano-particles embedded in
normal-metal matrix, insulating impurities in magnetic matrix, surface
scattering and temperature dependent scattering are experimentally tested. Our
new data, as well as previously published results on a variety of materials,
are fairly interpreted by a simple modification of the skew scattering model
Fluorescent oxide nanoparticles adapted to active tips for near-field optics
We present a new kind of fluorescent oxide nanoparticles with properties well
suited to active-tip based near-field optics. These particles with an average
diameter in the range 5-10 nm are produced by Low Energy Cluster Beam
Deposition (LECBD) from a YAG:Ce3+ target. They are studied by transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), classical
photoluminescence, cathodoluminescence and near-field scanning optical
microscopy (NSOM). Particles of extreme photo-stability as small as 10 nm in
size are observed. These emitters are validated as building blocks of active
NSOM tips by coating a standard optical tip with a 10 nm thick layer of
YAG:Ce3+ particles directly in the LECBD reactor and by subsequently performing
NSOM imaging of test surfaces.Comment: Changes made following Referee's comments; added references; one
added figure. See story on this article at:
http://nanotechweb.org/cws/article/tech/3606
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