1,008 research outputs found
Scanning probe microscopy with SQUID-on-tip sensor
In this thesis, we present the work developed in the past four years, on the fabrication, improvements and applications of a promising scanning probe technique, based on the SQUID technology. The nanoSQUID sensors improved drastically, but most of them lie on the plane of a large substrate and are therefore complicated to apply as scanning sensors. Although there have been demonstrations of magnetic imaging by scanning samples in proximity to such SQUIDs, the geometry is not amenable to a generally applicable microscopy. The nanoSQUIDs, that we used for our investigations, unlike conventional planar SQUIDs, can have diameters down to 50nm and are positioned on the apex of a sharp tip, hence their name a SQUID-on-tip (SOT)
IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models
Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of
the interstellar medium. In dense molecular clouds, it is also present in the
solid-phase as a constituent of the mixed water-dominated ices covering dust
grains. Its presence in the solid-phase is inferred from its infrared (IR)
signals. In experimental observations of solid CO/water mixed samples, its IR
frequency splits into two components, giving rise to a blue- and a redshifted
band. However, in astronomical observations, the former has never been
observed. Several attempts have been carried out to explain this peculiar
behaviour, but the question still remains open. In this work, we resorted to
pure quantum mechanical simulations in order to shed some light on this
problem. We adopted different periodic models simulating the CO/HO ice
system, such as single and multiple CO adsorption on water ice surfaces, CO
entrapped into water cages and proper CO:HO mixed ices. We also simulated
pure solid CO. The detailed analysis of our data revealed how the quadrupolar
character of CO and the dispersive forces with water ice determine the
energetic of the CO/HO ice interaction, as well as the CO spectroscopic
behaviour. Our data suggest that the blueshifted peak can be assigned to CO
interacting {\it via} the C atom with dangling H atoms of the water ice, while
the redshifted one can actually be the result of CO involved in different
reciprocal interactions with the water matrix. We also provide a possible
explanation for the lack of the blueshifted peak in astronomical spectra. Our
aim is not to provide a full account of the various interstellar ices, but
rather to elucidate the sensitivity of the CO spectral features to different
water ice environments.Comment: MNRAS, accepte
Stochastic model of solvent exchange in the first coordination shell of aqua Ions
Ion microsolvation is a basic, yet fundamental, process of ionic solutions underlying many relevant phenomena in either biological or nanotechnological applications, such as solvent reorganization energy, ion transport, catalytic activity, and so on. As a consequence, it is a topic of extensive investigations by various experimental techniques, ranging from X-ray diffraction to NMR relaxation and from calorimetry to vibrational spectroscopy, and theoretical approaches, especially those based on molecular dynamics (MD) simulations. The conventional microscopic view of ion solvation is usually provided by a "static" cluster model representing the first ion-solvent coordination shell. Despite the merits of such a simple model, however, ion coordination in solution should be better regarded as a complex population of dynamically interchanging molecular configurations. Such a more comprehensive view is more subtle to characterize and often elusive to standard approaches. In this work, we report on an effective computational strategy aiming at providing a detailed picture of solvent coordination and exchange around aqua ions, thus including the main structural, thermodynamic, and dynamic properties of ion microsolvation, such as the most probable first-shell complex structures, the corresponding free energies, the interchanging energy barriers, and the solvent-exchange rates. Assuming the solvent coordination number as an effective reaction coordinate and combining MD simulations with enhanced sampling and master-equation approaches, we propose a stochastic model suitable for properly describing, at the same time, the thermodynamics and kinetics of ion-water coordination. The model is successfully tested toward various divalent ions (Ca2+, Zn2+, Hg2+, and Cd2+) in aqueous solution, considering also the case of a high ionic concentration. Results show a very good agreement with those issuing from brute-force MD simulations, when available, and support the reliable prediction of rare ion-water complexes and slow water exchange rates not easily accessible to usual computational methods
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