The Quantum Devices for Metrology Department carries out fundamental and applicative research in the field of superconducting and nano-devices. Research topics are focused on quantum effects observable in such materials and devices, with the aim of exploiting new devices and techniques for various aspects of metrology and measurement science, spanning from traditional electrical metrology to environmental monitoring, biological measurements and food metrology.
In view of optimizing the parameters of the Nb/Al-AlOx/Nb overdamped Josephson junctions already developed, measurement of the temperature dependence of the critical current has been carried out. The experimental data differ from the theoretical calculated curves of Superconductor-Insulator-Superconductor (SIS) and of metallic barrier junctions, and have a marked dependence on the proximity effect between the niobium electrode and the thick (100nm) Al film.
Temperature dependence of the critical current for Nb/Al-AlOx/Nb junctions with different aluminium thickness and barrier transparency. Experimental data are compared to theoretical curves for SIS and metallic barrier junctions.
SIS mixers with current density suitable for astronomical use at 220 GHz have been successfully produced and tested, and preliminary packaging of the devices in a waveguide mount has been implemented, using indium reflow soldering for resistance contacts.
SIS mixer receiver. Detail of the mounting package of the device.
I-V characteristic of the SIS at 4.2 K: the series of two junctions is seen.
The reflectance R of MgB2 thin films has been studied in view of application at photon detectors and bolometers. The experimental data are congruent with the reflectivity expected from MgB2 films with clean surface, confirming the good quality of the surface of the samples and the absence of surface oxide layer.
In the framework of the FIRB project "Silicon micromachined photodetectors based on MgB2 superconductor films", a bolometer of MgB2 on silicon nitride membrane has been realised. MgB2 film grown by co-deposition technique has been patterned by optical lithography and ion-milling technique. Thermal conductance evaluated by the electro-thermal method is lower than 5·10-6 W/K and the responsivity at zero frequency is about 1500 V/W.
Reflectance R of MgB2 thin films of different thickness.
MgB2 meander on 0.5 µm thick silicon membrane.
A method for the fabrication of magnesium diboride nanostructures has been developed. MgB2 films with smooth surface and thickness ranging between 50 and 150 nm have been structured by e-beam lithography and Ar ion-milling. Reproducible nanobridges (from 150 to 500 nm wide and 1 µm long) have been obtained by optimization of ion-milling parameters The e-beam nanopatterning technique is very flexible and makes it possible to obtain various types of nanostructures, allowing the fabrication of innovative superconducting nanodevices.
SEM micrographs of four MgB2 nanobridges.
Gas sensors based on nanostructured silicon have been fabricated and characterized. The IV curves of mesoporous silicon devices both in vacuum and in the presence of NO2 traces have been studied. By the use of different electrode configurations, the longitudinal (parallel to the sample surface) and transverse (perpendicular to the sample surface) components of the electrical conductance have been independently measured and compared, in order to optimize sensor responsivity. In this way, we succeeded in fabricating highly sensitive devices, and correlation between sensor responsivity and the morphology of nanostructured silicon has been observed.
SEM cross-section of nanostructured mesoporous Si;
Anisotropic response of nanostructured mesoporous Si to NO2.
Electron beam irradiation to define biomolecule nanopatterns on porous silicon has been employed. The e-beam is able to locally activate the material, so that proteins selectively bind to the irradiated regions. We have found that the process can be serially repeated, allowing the fabrication of biochips where various types of biomolecules are immobilized in different regions. Thus, the technique is suitable for the production of innovative biodevices.
Fluorescence microscopy image of a porous silicon biochip with different proteins (Glucose-Galactose Binding Protein (GGBP) and Glutamine Binding Protein (GlnBP)) immobilized on adjacent microspots.
The determination of the alcoholic degree in wines has been achieved by exploiting the optical properties of porous silicon and a new micro-sensor based on a Porous Silicon Oxide Microcavity (PSOM) has been developed. The sensitivity of this sensor has been compared with the official method of ethanol detections in wines imposed by Italian legislation and a good agreement has been observed.
The shift of the cavity mode, correlated to a change in the PSOM refractive index due to the physisorption of ethanol into the pores, has been measured in the presence of several wines and linear behaviour, increasing the alcoholic degree, has been observed. The selectivity of the PSOM to other volatile compounds of wines, such as acetic acid, has also been tested and excellent results have been obtained by monitoring wine evaporation.
Exploded view of the cell.
Linear behaviour of the sensor as a function of the alcoholic degree.