87 research outputs found
Replicating Nanostructures on Silicon by Low Energy Ion Beams
We report on a nanoscale patterning method on Si substrates using
self-assembled metal islands and low-energy ion-beam irradiation. The Si
nanostructures produced on the Si substrate have a one-to-one correspondence
with the self-assembled metal (Ag, Au, Pt) nanoislands initially grown on the
substrate. The surface morphology and the structure of the irradiated surface
were studied by high-resolution transmission electron microscopy (HRTEM). TEM
images of ion-beam irradiated samples show the formation of sawtooth-like
structures on Si. Removing metal islands and the ion-beam induced amorphous Si
by etching, we obtain a crystalline nanostructure of Si. The smallest
structures emit red light when exposed to a UV light. The size of the
nanostructures on Si is governed by the size of the self-assembled metal
nanoparticles grown on the substrate for this replica nanopatterning. The
method can easily be extended for tuning the size of the Si nanostructures by
the proper choice of the metal nanoparticles and the ion energy in
ion-irradiation. It is suggested that off-normal irradiation can also be used
for tuning the size of the nanostructures.Comment: 12 pages, 7 figures, regular paper submitted to Nanotechnolog
Growth modes of Fe(110) revisited: a contribution of self-assembly to magnetic materials
We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and
propose an overview or our contribution to the field. We show that the
Stranski-Krastanov growth mode, recognized for a long time in these systems, is
in fact characterized by a bimodal distribution of islands for growth
temperature in the range 250-700°C. We observe firstly compact islands
whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat
islands that display a preferred height, ie independant from nominal thickness
and deposition procedure (1.4nm for Mo, and 5.5nm for W on the average). We
used this effect to fabricate self-organized arrays of nanometers-thick stripes
by step decoration. Self-assembled nano-ties are also obtained for nucleation
of the flat islands on Mo at fairly high temperature, ie 800°C. Finally,
using interfacial layers and solid solutions we separate two effects on the
preferred height, first that of the interfacial energy, second that of the
continuously-varying lattice parameter of the growth surface.Comment: 49 pages. Invited topical review for J. Phys.: Condens. Matte
Slowing and cooling molecules and neutral atoms by time-varying electric field gradients
A method of slowing, accelerating, cooling, and bunching molecules and
neutral atoms using time-varying electric field gradients is demonstrated with
cesium atoms in a fountain. The effects are measured and found to be in
agreement with calculation. Time-varying electric field gradient slowing and
cooling is applicable to atoms that have large dipole polarizabilities,
including atoms that are not amenable to laser slowing and cooling, to Rydberg
atoms, and to molecules, especially polar molecules with large electric dipole
moments. The possible applications of this method include slowing and cooling
thermal beams of atoms and molecules, launching cold atoms from a trap into a
fountain, and measuring atomic dipole polarizabilities.Comment: 13 pages, 10 figures. Scheduled for publication in Nov. 1 Phys. Rev.
Raman analysis and mapping for the determination of COOH groups on oxidized single walled carbon nanotubes
Raman spectroscopy and mapping, coupled to molecular labelling, is used to analyse and
monitor the first stage of carbon nanotube functionalization, i.e. their oxidation, which
is usually performed to increase the number of surface carboxylic groups, allowing both
a better dispersion in solution and the further attachment of biomolecules. Since the abundance
of such surface groups is critical for the final application, it is important to develop a
reliable but simple and fast method to investigate their presence on the tube walls. The presented
data demonstrate the correlation between the presence and intensity of the Raman
peak ascribed to the labelling molecule and the production of COOH groups on the nanotube
walls. Between the analysed carbon nanotubes related spectral parameters, the G0
Raman peak position appears to be the most sensitive one to determine the degree of single
walled carbon nanotube labelling, which directly depends on the number of COOH groups
available for molecular attachment, i.e. on the efficiency of the oxidation treatment.
The presented results demonstrate that it
is possible to directly use the Raman signal of the THA labelling
molecule for the COOH groups determination. In particular,
the labelling is extremely useful to overcome the
limitations of Raman spectroscopy alone in determining the
degree of oxidation, so that such a method can be applied
to study the oxidation itself, with particular reference to its
efficiency and to the analysis of the presence of carbonaceous
impurities (fulvic acids residues) in acid treated SWCNT samples. We are presently working in this direction, also
applying the THA-method to monitoring different oxidation
techniques, such as microwave and plasma treatments
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