International and National Journals
C. Baratto, G. Faglia, M. Pardo, M. Vezzoli, L. Boarino, M. Maffei, S. Bossi, G. Sberveglieri
Monitoring plants health in greenhouse for space missions
Sensors and Actuators B: Chemical, Vol. 108, nos. 1-2, pp. 278-284, 2005.
We developed a sensor array made up of metal oxide thin films for the detection of stress molecules produced by plants in greenhouses for manned long term space missions. This sensor array was coupled with other sensors for illumination, CO2 and humidity to realize an advanced hybrid electronic nose that was successfully employed to monitor plant growth inside a greenhouse. The electronic nose was able to detect the onset of abiotic and biotic plant stresses. Volatile organic compounds (VOCs) emission by plants was also evaluated by head space analysis performed by gas chromatography-mass spectroscopy (GC-MS). The humidity sensor has been used also to reduce the always present effect of humidity on the metal oxide thin films. A simple correlation analysis divides the metal oxide sensors in two classes of similar characteristics, which could permit to make use of just two sensors.
V. Lacquaniti, C. Cagliero, S. Maggi, R. Steni
Overdamped Nb/Al-AlOx/Nb Josephson junctions
Applied Physics Letters, Vol. 86, no. 4, 042501-3, 2005.
We report the fabrication and characterization of overdamped Nb/Al-AlOx/Nb superconductor-insulator-superconductor Josephson junction whose fabrication process derives from that of the well-known hysteretic junctions. These junctions are an intermediate state between the superconductor-normal metal-superconductor and the superconductor-insulator-superconductor Josephson junctions. Stable and reproducible nonhysteretic current-voltage characteristics are obtained with a proper choice of the fabrication parameters. We have measured critical current densities JC from 103 up to 2 x 104 A/cm2, with characteristic voltages from 80 to nearly 450 µV. The junctions are stable against time and repeated thermal cycling.
V. Lacquaniti, C. Cagliero, S. Maggi, R. Steni, D. Andreone, A. Sosso
Rf properties of overdamped SIS junctions
IEEE Transactions on Applied Superconductivity, Vol.15, no. 2, pp.114-116, 2005.
A new tape of nonhysteretic Josephson junction suitable for applications in voltage metrology has been developed. These junctions derive from the Nb/Al - AlOx/Nb SIS junctions using a relatively thick Al layer oxidized at a low value of oxygen exposure. This produces junctions with reproducible and spatially homogeneous I-V characteristics, having current densities ranging from 103 to more than 2 x 104 A/cm2 and characteristic voltages up to 0.40 mV. The authors report here the rf response of these junctions at 70 GHz. The authors have measured the dependence of the rf-induced steps on the microwave power and the stability of the steps, in view of a future application of these junctions to an ac Josephson voltage standard.
G. Ghigo, D. Botta, A. Chiodoni, L. Gozzelino, R. Gerbaldo, F. Laviano, E. Mezzetti, E. Monticone, C. Portesi
Effective gap at microwave frequencies in MgB2 thin films with strong interband scattering
Physical Review B, Vol. 71, p. 214522, 2005.
The microwave properties of polycrystalline MgB2 thin films prepared by the so-called in situ method are investigated. The characterization of the films at microwave frequencies was obtained by a coplanar resonator technique. The analysis of the experimental data results in the determination of penetration depth, surface impedance, and complex conductivity. The aim of this work is to set the experimental results in a consistent framework, involving the two-band model in the presence of impurity scattering. The energy gaps are calculated and the contribution of intra- and interband scattering is considered. From the comparison between the calculated gap values and the experimental data it turns out that the temperature dependence of the penetration depth can be accounted for by an effective mean energy gap, in agreement with the predictions of V. G. Kogan and N. V. Zhelezina [Phys. Rev. B 69, 132506 (2004)]. On the other hand, the temperature dependence of the real part of the microwave conductivity and of the surface resistance is accounted for by the single smaller gap, in agreement with the work of B. B. Jin, T. Dahm, A. I. Gubin, E.-M. Choi, H. J. Kim, S.-I. Lee, W. N. Kang, and N. Klein [Phys. Rev. Lett. 91, 127006 (2003)]. Since these findings rely on the same calculated gap structure, the required consistency is fulfilled.
C. Portesi, A. M. Rossi, E. Monticone
Enhancement of Tc in MgB2 thin films by laser local annealing
IEEE Transactions on Applied Superconductivity, Vol. 15, no. 2, Part 3, pp. 3242 - 3244, 2005.
Films of magnesium diboride are promising for superconducting electronic and for detector applications. One of the main obstacles for device fabrication by traditional lithography is the sensitivity of this material to the chemical solutions normally employed in the standard structuring process. Moreover, underetching often limits the patterns resolution. By focused laser beam, high temperature can be reached suddenly in areas of few micrometer square, therefore in principle MgB2film patterning is feasible by local laser modification. In this work we investigate the effect of laser processing on MgB2 thin films deposited by in-situ method. Before the exposure to the laser, the strip was wirebonded to enable the resistance to be monitored during the experiment. Increasing the power from 0.1 to 1 Watt, the room temperature resistance of the stripline increased until reaching the open circuit for higher laser power. By optical inspection, we observed that the color of irradiated area changed from yellow to black. The structure defined by laser presented a periodicity of about 1.7 µm, where the width of each tracks was 1.5 µ m, thus comparable to the laser spot size. The Tc of the strip was increased by the laser process. This technique open the possibility to pattern superconducting film for device fabrication and circuits.
G. Ghigo, D. Andreone, D. Botta, A. Chiodoni, R. Gerbaldo, L. Gozzelino, F. Laviano, B. Minetti, E. Mezzetti
Non-uniform columnar defect implantation in YBCO coplanar resonators for the control of vortex-induced microwave dissipation and nonlinearity
Superconductor Science and Technology, Vol. 18, no. 1, pp. 193-199, 2005.
This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity by means of a suitable distribution of columnar defects. Two procedures were used: (1) uniform irradiation at a given dose and (ii) non-uniform irradiation, obtained by means of a moving-target technique, producing a tailored profile of columnar defect density. The last configuration is needed in order to attain an optimal compromise between defect-induced enhancement of vortex pinning and increase of dissipative scattering. The resonators were characterized before and after irradiation in order to evaluate penetration depth and surface impedance, by means of a data analysis also suitable to account for the substrate properties. It turns out that in the case of uniform irradiation the induced damage, attributed to the addition of dissipative regions, always prevails over the improvement due to vortex pinning. In contrast, in the case of the tailored dose profile, significant damage is absent, while the onset of non-linearity is shifted towards higher circulating power. The need to control vortex-induced effects is confirmed by magneto-optical analysis that puts in evidence that vortex penetration occurs even at low values of transverse dc magnetic field and temperature.
G. Ghigo, L. Gozzelino, F. Laviano, D. Andreone, E. Mezzetti
Effects of non-uniform columnar defect distribution on the microwave properties of Y-Ba-Cu-O coplanar resonators
IEEE Transactions on Applied Superconductivity, Vol. 15, no. 2, Part 3, pp. 3604 - 3607, 2005.
This paper reports on nonuniform heavy ion irradiations of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity. The adopted moving-target irradiation technique allows producing tailored profiles of columnar defect density to get an optimal compromise between defect-induced enhancement of vortex pinning and addition of dissipative scattering. The same samples were characterized before and after irradiation in order to evaluate the surface impedance, by means of data analysis suitable to account also for the substrate properties. In the case of the modulated dose profile, matching the fundamental-mode field distribution along the resonator, the onset of nonlinearity is shifted toward higher circulating power, without increasing dissipation. Finally, the possibility to locally modify by irradiation the properties of the material to obtain the confinement of external perturbations, while maintaining the current continuity throughout the whole device, has been preliminarily tested.
E. Garrone, F. Geobaldo, P. Rivolo, G. Amato, L. Boarino, M. Chiesa, E. Giamello, R. Gobetto, P. Ugliengo, A. Viale
A nanostructured porous silicon near insulator becomes either a p- or an n-type semiconductor upon gas adsorption
Advanced Materials, Vol. 17, no. 5, pp. 528-531, 2005.
Mesoporous silicon exchanges electrons extensively and reversibly with adsorbed species, e.g., NO 2 and NH3. Such an exchange has been reported for systems that maintain their original p or n character. In the present case, a solid, which is a p-type semiconductor in bulk, converts to a "near insulator" when nanostructured. The near insulator may adopt either a p or an n nature with electron exchange, depending on the type of adsorbed molecule.
S. Borini, S. D'Auria, M. Rossi, A. M. Rossi
Writing 3D protein nanopatterns onto a silicon nanosponge
. LAB ON A CHIP, Vol. 5, no. 10, pp. 1048-1052, 2005.
A three-dimensional protein nanopatterning method has been developed, based on local activation of porous silicon by electron beam. Proteins specifically bind to irradiated regions, and the depth of biomolecule nanopatterns can be controlled by varying the electron energy. This unique feature permits exploitation of the huge surface area of the sponge-like material, thus allowing concentration of a large amount of proteins on nanosized patterns. Moreover, the grafted biomolecules retain their full functionality, and the feasibility of a glucose sensor has been demonstrated.
N. Pinto, L. Morresi, M. Ficcadenti, R. Murri, F. D'Orazio, F. Lucari, L. Boarino, G. Amato
Magnetic and electronic transport percolation in epitaxial Ge 1-xMnx films
Physical Review B, Vol. 72, p. 165203, 2005.
Electronic transport and magnetic properties of Ge 1-xMnx/Ge(100) films are investigated as a function of Mn dilution. Depending on x, characteristic temperatures separate different regimes in both properties. Resistivity exhibits an insulatorlike behavior in the whole temperature range and, below about 80 K, two distinct activation energies are observed. At a higher temperature value, TR, resistivity experiences a sudden reduction. The Hall coefficient shows a strong contribution from the anomalous Hall effect and, at TR, a sign inversion, from positive to negative, is recorded. The magnetic properties, inferred from magneto-optical Kerr effect, evidence a progressive decrease of the ferromagnetic long range order as the temperature is raised, with a Curie temperature TCnot far from TR. The transport and magnetic results are qualitatively consistent with a percolation mechanism due to bound magnetic polarons in a GeMn diluted magnetic semiconductor, with localized holes [A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)].
L. Moretti, L. De Stefano, A. M. Rossi, I. Rendina
Dispersion of thermo-optic coefficient in porous silicon layers of different porosities
Applied Physics Letters, Vol. 86, no.6, p. 061107, 2005.
The thermo-optic coefficient (dn/dT) of porous silicon has been measured. The measurements are based on a simple technique. The experimental data, in the wavelength range between 500 and 1100 nm and at different porosities, have been compared with the Bruggeman and Landau-Lifshitz models. The results show good agreement in case of the Bruggeman model for porosities lower than 0.7, while for higher porosities the Landau model is in better agreement. An estimation of dn/dT as a function of the porosity at the fiber optic communication wavelength of 1.55 µm has also been reported.
L. Rotiroti, L. De Stefano, N. Rendina, L. Moretti, A. M. Rossi, A. Piccolo
Optical microsensors for pesticides identification based on porous silicon technology
Biosensors & Bioelectronics, Vo. 20, no. 10, pp. 2136-2139, 2005.
A simple and low cost optical sensor, based on porous silicon nanotechnology, has been used to detect and quantify the presence of atrazine pesticide in water and humic acid solutions. In both cases, a well defined optical signal variation can be registered, even at low concentration as 1 ppm. The phenomenon can be ascribed to the capillary infiltration of liquid into the pores, which changes the average refractive index of the structure. Due to the resonant cavity enhanced operation of the proposed sensors, very low detection limits can be reached.
M. Chiesa, G. Amato, L. Boarino, E. Garrone, F. Geobaldo, E. Giamello
ESR study of conduction electrons in B-doped porous silicon generated by the adsorption of Lewis bases
Journal of the Electrochemical Society, Vol. 152, pp. G329-G333, 2005.
We report the observation of a paramagnetic resonance in p+-type mesoporous silicon contacted with several molecules with Lewis base character. Adsorption of NH3, pyridine, acetone, ethanol, and water generates an isotropic electron spin resonance (ESR) signal at g = 1.9984 independent of temperature. By comparing this value with those concerning shallow donors in bulk silicon, the paramagnetic center is identified with electrons in the conduction band. In the case of ammonia, studied in detail, the signal grows with the amount of adsorbed molecules until an ESR line-shape transition from Lorentzian to Dysonian occurs. This change in line shape parallels the transition from a Langevin to Pauli paramagnetism due to the onset of electron gas degeneracy. The phenomenon is pressure dependent and completely reversible in a range of coverages of the order of the monolayer.
S. Borini
Cross-linked PMMA on porous silicon: an effective nanomask for selective silicon etching
Journal of the Electrochemical Society, Vol. 152, no. 6, pp. G482-G486, 2005.
Combining a porous silicon (PS) buffer layer with cross-linked poly(methyl methacrylate) (PMMA), we have obtained masks that show high resistance to the electrochemical process usually employed to produce PS. PMMA is normally dissolved in an HF/ethyl alcohol mixture, but it becomes resistant to such solution after cross- linking of the polymer. This can be achieved by high-dose electron irradiation, obtaining a mask for the subsequent electrochemical etching. Due to the strong electric field across the masking layer during the electrochemical process, the time duration of such a mask is limited. We demonstrate that the presence of a highly porous silicon thin film lying under the resist is sufficient for an evident improvement of the masking power during the electrochemical process, allowing for the selective formation of thick PS regions. Furthermore, a final PS removal in alkaline solution leads to the formation of silicon micro- and nanostructures in relief, such as microtips and nanomolds.
G. Amato, S. Borini, A. M. Rossi, L. Boarino, M. Rocchia
Si/SiO2 nanocomposite by CVD infiltration of porous SiO2
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1529-1532, 2005.
The aim of the present paper is to report first results on an innovative method for producing Si/SiO2 nanocomposites. Starting from a porous oxide structure, we infiltrated Si by Chemical Vapour Deposition of SiH4, under controlled conditions. In this way, we succeeded in infiltrating Si into the SiO2 template. Porous oxide is obtained by dry oxidation of mesoporous Si. By means of the electrochemical process used for producing porous Si, an interconnected pore structure is obtained. This allows for Si infiltration, giving rise, in principle, to an interconnected network of Si crystallites, in which electrical carriers are easily driven. Efficient charge injection in Si nanocrystals is of crucial importance for several applications, from electroluminescence to memory devices.
A. M. Rossi, F. Giorgis, V. Ballarini, S. Borini
Infrared analysis of porous silicon carbide
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1548-1551, 2005.
We report on the optical spectroscopies performed on porous SiC obtained by electrochemical etching of 6H-SiC wafers. Strong modifications of Raman and infrared spectra, characteristic of the porous material, have been detected. Various effective medium approximations have been employed for the modeling of porous SiC dielectric function, in order to interpret the infrared reflectance features. Comparison between computational and experimental results indicates the Maxwell-Gamett model as the most appropriate approximation, and suggests that charge carriers within the porous matrix are likely to play a role in the interaction of the material with electromagnetic radiation.
E. Garrone, F. Geobaldo, P. Rivolo, G. P. Salvador, L. Pallavidino, L. Boarino, G. Amato, E. Giamello, M. Chiesa, R. Gobetto, P. Ugliengo
Boron passivation and its reactivation in mesoporous silicon: a "chemical" model
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1567-1570, 2005.
A chemical model is proposed to explain the passivation of carriers upon etching and their reactivation by the presence of gases in p+-type mesoporous silicon (m-PS). The model is based on the results obtained by means of FTIR, EPR, NMR spectroscopies and ab-initio calculations. This chemical description accounts for the behaviour of mPS when contacted with NO2 or NH3. B atoms in subsurface location play a prominent role.
A. M. Rossi, H. G. Bohn
Photodetectors from Porous Silicon
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1644-1647, 2005.
A metal-semiconductor-metal photoconductive detector has been fabricated from porous silicon. To that aim low conductive p-type Si (20 Ω cm) has been porosified and several rapid-thermal treatment steps applied before deposition of Al-contacts. The photo response was investigated in the wavelength range of 350-800 nm. A responsivity of up to 4 A/W was observed at 600 nm which is about 13 times higher than that of a commercial photodiode.
S. Borini, M. Rocchia, A. M. Rossi, L. Boarino, G. Amato
Electron beam irradiation of porous silicon for application in micromachining and sensing
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1648-1652, 2005.
We have studied the effect of the electron beam (EB) irradiation on porous silicon (PS), in order to find new possibilities for the local modification of the material at the submicrometer scale. The interaction between the accelerated electrons and PS has been investigated by means of two main techniques: infrared spectroscopy and profilometric measurements. All the results show that a strong increase of reactivity is induced on PS surface under exposure to the EB, due to the hydrogen desorption provoked by the incident electrons. We demonstrate that this phenomenon can be exploited for both the lateral structuring and the local functionalization of PS, at the submicrometer scale, depending on the different treatments applied to the samples after the EB bombardment.
M. Rocchia, A. M. Rossi, S. Borini, L. Boarino, G. Amato
Laser local oxidation of porous silicon: a FTIR spectroscopy investigation
Physica Status Solidi A - Applications and Materials Science, Vol. 202, no. 8, pp. 1658-1661, 2005.
The local oxidation of porous silicon (PS), induced by a focused laser beam, could represent an alternative method for patterning PS through direct writing. Important phase changes take place on PS when irradiated by a focused laser beam and moreover a complete confinement of the oxidized areas can be achieved due to the very low thermal conductivity of PS. We present a detailed Fourier Transform Infra-Red (FTIR) study of the irradiated areas to understand the degree of oxidation and the type of oxide obtained at different laser powers. An interpretation of the low wavenumber range, below 1300 cm -1 , in terms of Frohlich interactions will be discussed.
C. Ricciardi, F. Giorgis, G. Fanchini, S. Musso, V. Ballarini, E. Bennici, G. Barucca, A. M. Rossi
Microstructure analysis on polycrystalline 3C-SiC thin films
Diamond and Related Materials, Vol. 14 (3-7), pp. 1134-1137, Sp. Iss. SI, 2005.
Polycrystalline 3C-SiC thin films grown by LPCVD on different substrates were characterized by vibrational and electronic spectroscopies and electron diffraction measurements aimed at studying the samples microstructure. Raman spectra evidence the coexistence of crystallites, whose size distribution depends on the deposition conditions, and of a disordered matrix. On the other side, IR reflectivity measurements show spectra which are strongly influenced by the surface morphology.
C. Portesi, E. Monticone, A. Brinkman, R. S. Gonnelli
INRIM (IEN): dispositivi SQUID a MgB 2
Tutto_Misure, anno VII, n. 3, p. 224, 2005.
Lo SQUID (Superconducting Quantum Interference Device) è un dispositivo in grado di misurare campi magnetici anche molto deboli, grazie ad un'elevata sensibilità prossima ai limiti quantistici, ed è indipendente dalla frequenza, fino a qualche centinaio di MHz. La grande versatilità di questi dispositivi consente applicazioni in differenti settori scientifici e tecnologici, tra i quali: biomedicale, scienza dei materiali e metrologia. Gli SQUID possono essere fabbricati con superconduttori a bassa Tc, se operano alla temperatura assoluta di 4.2 K, oppure ad alta Tc, se operano alla temperatura dell'azoto liquido (77K). Con la recente scoperta, nel gennaio 2001, delle proprietà superconduttive del diboruro di magnesio a 40 K si è aperta la possibilità di fabbricare SQUID combinando da una parte una sensibilità confrontabile a quella degli SQUID a bassa temperatura critica, dall'altra la possibilità di poter lavorare con sistemi di refrigerazione più semplici ed economici. Presso i laboratori del settore "Dispositivi Quantistici" dell'INRIM, l'attività di ricerca sull'MgB 2, finanziata in parte da un progetto FIRB, è stata avviata con l'obiettivo di fabbricare film sottili di diboruro di magnesio adatti per applicazioni nell'elettronica superconduttiva e nella metrologia.