192 research outputs found
Lateral resolution of electrostatic force microscopy for mapping of dielectric interfaces in ambient conditions
The attainable lateral resolution of electrostatic force microscopy (EFM) in an ambient air environment on dielectric materials was characterized on a reference sample comprised of two distinct, immiscible glassy polymers cut in a cross-section by ultramicrotomy. Such a sample can be modeled as two semi-infinite dielectrics with a sharp interface, presenting a quasi-ideal, sharp dielectric contrast. Electric polarizability line profiles across the interface were obtained, in both lift-mode and feedback-regulated dynamic mode EFM, as a function of probe/surface separation, for different cases of oscillation amplitudes. We find that the results do not match predictions for dielectric samples, but comply well or are even better than predicted for conductive interfaces. A resolution down to 3 nm can be obtained by operating in feedback-regulated EFM realized by adopting constant-excitation frequency-modulation mode. This suggests resolution is ruled by the closest approach distance rather than by average separation, even with probe oscillation amplitudes as high as 10 nm. For better comparison with theoretical predictions, effective probe radii and cone aperture angles were derived from approach curves, by also taking into account the finite oscillation amplitude of the probe, by exploiting a data reduction procedure previously devised for the derivation of interatomic potentials
Polarization-modulation near-field optical microscope for quantitative local dichroism mapping
A couple of experimental techniques have been implemented to an aperture near-field scanning optical microscopy (NSOM) to obtain reliable measurement of sample dichroism on the local scale. First, a method to test NSOM tapered fiber probes toward polarization conservation into the near optical field is reported. The probes are characterized in terms of the in-plane polarization of the near field emerging from their aperture, by using a thin dichroic layer of chromophore molecules, structured along stretched polymeric chains, to probe such polarization when approached in the near-field region of the probe. Second, to assure that the light intensity coupled in the fiber is polarization independent, an active system operating in real time has been realized. Such combination of techniques allowed quantitative imaging of local dichroism degree and average orientation by means of dual-phase lock-in demodulation of the optical signal. Translation of the coupled light polarization state in the near field has been observed for one-half of the tested probes. For the others, the tip acts as a polarizer, and therefore showed it was not suitable for polarization modulation NSOM measurements
Extended model for the interaction of dielectric thin films with an electrostatic force microscope probe
To improve measurements of the dielectric permittivity of nanometric portions by means of Local Dielectric Spectroscopy (LDS), we introduce an extension to current analytical models for the interpretation of the interaction between the probe tip of an electrostatic force microscope (EFM) and a thin dielectric film covering a conducting substrate. Using the proposed models, we show how more accurate values for the dielectric constant can be obtained from single-frequency measurements at various probe/substrate distances, not limited to a few tip radii
Electrostatic force microscopy and potentiometry of realistic nanostructured systems
We investigate the dependency of electrostatic interaction forces on applied
potentials in Electrostatic Force Microscopy (EFM) as well as in related local
potentiometry techniques like Kelvin Probe Microscopy (KPM). The approximated
expression of electrostatic interaction between two conductors, usually
employed in EFM and KPM, may loose its validity when probe-sample distance is
not very small, as often realized when realistic nanostructured systems with
complex topography are investigated. In such conditions, electrostatic
interaction does not depend solely on the potential difference between probe
and sample, but instead it may depend on the bias applied to each conductor.
For instance, electrostatic force can change from repulsive to attractive for
certain ranges of applied potentials and probe-sample distances, and this fact
cannot be accounted for by approximated models. We propose a general
capacitance model, even applicable to more than two conductors, considering
values of potentials applied to each of the conductors to determine the
resulting forces and force gradients, being able to account for the above
phenomenon as well as to describe interactions at larger distances. Results
from numerical simulations and experiments on metal stripe electrodes and
semiconductor nanowires supporting such scenario in typical regimes of EFM
investigations are presented, evidencing the importance of a more rigorous
modelling for EFM data interpretation. Furthermore, physical meaning of Kelvin
potential as used in KPM applications can also be clarified by means of the
reported formalism.Comment: 20 pages, 7 figures, 1 tabl
Quantitative Nanofriction Characterization of Corrugated Surfaces by Atomic Force Microscopy
Atomic Force Microscopy (AFM) is a suitable tool to perform tribological
characterization of materials down to the nanometer scale. An important aspect
in nanofriction measurements of corrugated samples is the local tilt of the
surface, which affects the lateral force maps acquired with the AFM. This is
one of the most important problems of state-of-the-art nanotribology, making
difficult a reliable and quantitative characterization of real corrugated
surfaces. A correction of topographic spurious contributions to lateral force
maps is thus needed for corrugated samples. In this paper we present a general
approach to the topographic correction of AFM lateral force maps and we apply
it in the case of multi-asperity adhesive contact. We describe a complete
protocol for the quantitative characterization of the frictional properties of
corrugated systems in the presence of surface adhesion using the AFM.Comment: 33 pages, 9 figures, RevTex 4, submitted to Journal of Applied
Physic
Dominance of charged excitons in single quantum dot photoluminescence spectra
Single InxGa1-xAs/GaAs quantum dot photoluminescence spectra, obtained by low-temperature near-field scanning optical microscopy, are compared with theoretically derived optical spectra. The spectra show shell filling as well as few-particle fine structure associated with neutral and charged multiexcitons, in good agreement with the many-body calculations. There appears to be a greater tendency to charged-exciton formation, which is discussed in terms of the high diffusivity of photogenerated electrons
Nerve growth factor is an autocrine survival factor for memory B lymphocytes
AbstractProduction of nerve growth factor (NGF) was assessed in cultures of human T and B lymphocytes and macrophages. NGF was constitutively produced by B cells only, which also expressed surface p140trk-A and p75NGFR molecules and hence efficiently bound and internalized the cytokine. Neutralization of endogenous NGF caused disappearance of Bcl-2 protein and apoptotic death of resting lymphocytes bearing surface IgG or IgA, a population comprising memory cells, while surface IgM/IgD âvirginâ B lymphocytes were not affected. In vivo administration of neutralizing anti-NGF antibodies caused strong reduction in the titer of specific IgG in mice immunized with tetanus toxoid, nitrophenol, or arsonate and reduced numbers of surface IgG or IgA B lymphocytes. Thus, NGF is an autocrine survival factor for memory B lymphocytes
Nanomaterials-based strategies for piezoelectric cochlear implants
Nanomaterials-based strategies for piezoelectric cochlear implant
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