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Graphene solutions
CHEMICAL COMMUNICATIONS 47(19), 5470-5472 (2011)
Thermodynamics drive the spontaneous dissolution of a graphite intercalation compound (GIC) KC8 in NMP to form stable solutions. Reduction potential of graphene is measured at + 22 mV vs. SCE. Single layer graphene flakes (ca. 1 squared micron) have been unambiguously identified by electron diffraction.
Mesoscopic Transport Properties in Exfoliated Graphene on SiO2/Si
NANOSCIENCE AND NANOTECHNOLOGY LETTERS 3(1), 55-58 (2011)
The effective carrier diffusion length and the local mobility have been measured in graphene layers in the inferior mesoscopic regime. The measurements were carried out probing the electrical properties at nanoscale by using scanning capacitance microscopy. The local variations in the quantum capacitance can be measured and from them the effective polarised area can be determined. As a consequence the local density of states and the other local transport parameters can be obtained.
Biomimetic sensing layer based on electrospun conductive polymer webs
BIOSENSORS & BIOELECTRONICS 26(5), 2460-2465 (2011)
The aim of the present study is to combine a bio-inspired nanofibrous artificial epithelium to the electronic nose (e-nose) principles. The sensing device set up was an electronic nose consisting of an array of 9 micro-chemoresistors (Cr-Au, 3 x 3) coated with electrospun nanofibrous structures. These were comprised of doped polyemeraldine base blended with 3 different polymers: polyethylene oxide, polyvinilpyrrolidone and polystyrene, which acted as carriers for the conducting polymer and were the major responsible of the features of each fibrous overlay (electrical parameters, selectivity and sensitivity ranges). The two sensing strategies here adopted and compared consisted in the use of 2 different textural coatings: a single- and a double-overlay, where the double-overlay resulting from overdeposition of 2 different polymer blends. Such e-nose included a plurality of nanofibres whose electrical parameters were at the same time depending on each polymer exposure to analytes (NO2, NH3) and on the spatial distribution of the interlacing fibres. The morphology of the coating arrangements of this novel e-nose was investigated by scanning electron microscopy (SEM) and its sensor responses were processed by multicomponent data analyses (PCA and PLS) reporting encouraging results for detection and recognition of analytes at ppb levels.
Charge transport in graphene-polythiophene blends as studied by Kelvin Probe Force Microscopy and transistor characterization
JOURNAL OF MATERIALS CHEMISTRY 21(9), 2924-2931 (2011)
Blends of reduced graphene oxide (RGO) and poly(3-hexylthiophene) (P3HT) are used as the active layer of field-effect transistors (FETs). By using sequential deposition of the two components, the density of RGO sheets can be tuned linearly, thereby modulating their contribution to the charge transport in the transistors, and the onset of charge percolation. The surface potential of RGO, P3HT and source-drain contacts is measured on the nanometric scale with Kelvin Probe Force Microscopy (KPFM), and correlated with the macroscopic performance of the FETs. KPFM is also used to monitor the potential decay along the channel in the working FETs.
Photoluminescence of CdSe nanowires grown with and without metal catalyst
NANO RESEARCH 4(4), 343-359 (2011)
We present temperature and power dependent photoluminescence measurements on CdSe nanowires synthesized via vapor-phase with and without the use of a metal catalyst. Nanowires produced without a catalyst can be optimized to yield higher quantum efficiency, and narrower and spatially uniform emission, when compared to the catalyst-assisted ones. Emission at energies lower than the band-edge is also found in both cases. By combining spatially-resolved photoluminescence and electron microscopy on the same nanowires, we show that catalyst-free nanowires exhibit a low-energy peak with sharp phonon replica, whereas for catalyst-assisted nanowires low-energy emission is linked to the presence of nanostructures with extended morphological defects
Surface electrostatic potentials in carbon nanotubes and graphene membranes investigated with electron holography
CARBON 49(4), 1423-1429 (2011)
We used low-voltage transmission electron holography to probe surface electrostatic potentials in graphene membranes and carbon nanotubes, as the number of graphenes varies. Further, we measured the phase shift induced by an individual graphene, and mapped the phase shift variation throughout a whole few-graphene-crystal as a function of the local number of layers. We found a size/surface effect as the ratio between the surface and the total number of atoms increases for an individual nanotube or graphene membrane. This surface phase term can be related to a fine electrostatic potentials redistributions occurring at the outer layers in carbon nanotubes and graphene membranes.
Dye-Sensitizing of Self-Nanostructured Ti(:Zn)O2/AZO Transparent Electrodes by Self-Assembly of 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin
JOURNAL OF PHYSICAL CHEMISTRY C 115(15), 7760-7767 (2011)
Self-nanostructured ZnO center dot Al conductive layers consisting of [11 (2) over bar0] oriented domains were coated by a conformal TiO2 thin film and sensitized by 5, 10, 15, 20-tetrakis-(4-carboxyphenyl)porphyrin (TCPP) to be used as transparent conductive electrodes in dye-sensitized solar cells. In addition to the higher surface availability due to the nanopatterning, the TCPP surface density increases by 330% (UV-vis) with respect to a flat conventional substrate thanks to a more sense molecular arrangement, as evidenced by combining high-resolution X-ray photoelectron spectroscopy and atomic force microscopy. Furthermore, the presence of zinc atoms in the TiO2 overlayer (Ti(:Zn)O2) crucially influences the electronic properties of the assembled TCPP. As a consequence of the orbitals rearrangement attributed to the presence of zinc, a significant quenching of luminescence is observed in the emission spectra of TCPP-sensitized Ti(:Zn)O2 , suggesting that electrons could be more effectively injected from the molecular orbitals to the conduction band of the semiconductor.
Flexible thermoelectric generator for ambient assisted living wearable biometric sensors
JOURNAL OF POWER SOURCES 196(6), 3239-3243 (2011)
In this work we proposed design, fabrication and functional characterization of a very low cost energy autonomous, maintenance free, flexible and wearable micro thermoelectric generator (mu TEG), finalized to power very low consumption electronics ambient assisted living (AAL) applications. The prototype. integrating an array of 100 thin films thermocouples of Sb2Te3 and Bi2Te3, generates, at 40 °C, an open circuit output voltage of 430 mV and an electrical output power up to 32 nW with matched load. In real operation conditions of prototype, which are believed to be very close to a thermal gradient of 15 °C. the device generates an open circuit output voltage of about 160 mV, with an electrical output power up to 4.18 nW.
In the first part of work, deposition investigation Sb2Te3 andBi2Te3 thin films alloys on Kapton HN polyimide foil by RF magnetron co-sputtering technique is discussed. Deposition parameters have been optimized to gain perfect stoichiometric ratio and high thermoelectric power factor: fabricated thermo-generator has been tested at low gradient conditioned to evaluate applications like human skin wearable power generator for ambient assisted living applications.
Thin Deposits and Patterning of Room-Temperature-Switchable One-Dimensional Spin-Crossover Compounds
LANGMUIR 27(7), 4076-4081 (2011)
We present a study on thin deposits and patterning of 1-D spin-crossover compounds Fe(II)-(L)(2)H](ClO(4))(3)center dot MeOH [L = 4'-(4'''-pyridyl)-1,2':6'1 ''-bis-(pyrazolyl) pyridine] (1) that exhibit a reversible, thermally driven spin transition at room temperature. Micrometric rodlike crystals of 1 on silicon surfaces are achieved by drop casting and solvent annealing. We observed that the crystallinity of thin deposits and spin-transition properties critically depends on the deposition procedure. Furthermore, we proved processability and patterning using unconventional wet lithography that reduces the crystallite formation time by 1 order of magnitude. Thin deposits of 1 were characterized by atomic force microscopy, polarized optical microscopy and X-rays, and the switching properties were characterized by Raman spectroscopy.
Flexible thermoelectric generator for ambient assisted living wearable biometric sensors
JOURNAL OF POWER SOURCES 196(6), 3239-3243 (2011)
In this work we proposed design, fabrication and functional characterization of a very low cost energy autonomous, maintenance free, flexible and wearable micro thermoelectric generator (mu TEG), finalized to power very low consumption electronics ambient assisted living (AAL) applications. The prototype. integrating an array of 100 thin films thermocouples of Sb2Te3 and Bi2Te3, generates, at 40 oC, an open circuit output voltage of 430 mV and an electrical output power up to 32 nW with matched load. In real operation conditions of prototype, which are believed to be very close to a thermal gradient of 15 oC. the device generates an open circuit output voltage of about 160 mV, with an electrical output power up to 4.18 nW.In the first part of work, deposition investigation Sb2Te3 and Bi2Te3 thin films alloys on Kapton HN polyimide foil by RF magnetron co-sputtering technique is discussed. Deposition parameters have been optimized to gain perfect stoichiometric ratio and high thermoelectric power factor: fabricated thermo-generator has been tested at low gradient conditioned to evaluate applications like human skin wearable power generator for ambient assisted living applications.
Giant and reversible enhancement of the electrical resistance of GaAs1-xNx by hydrogen irradiation
PHYSICAL REVIEW B 84, Art. No. 085331 (2011)
The electrical properties of untreated and hydrogen-irradiated GaAs1-xNx are investigated by conductive-probe atomic force microscopy (CP-AFM). After hydrogen irradiation, the resistance R of GaAs1-xNx increases by more than three orders of magnitude while that of a N-free GaAs reference slightly decreases. Thermal annealing at 550 °C of H-irradiated GaAs1-xNx restores the pristine electrical properties of the as-grown sample thus demonstrating that this phenomenon is fully reversible. These effects are attributed to the nitrogen-hydrogen complexes that passivate N in GaAs1-xNx (thus restoring the energy gap of N-free GaAs) and, moreover, reduce the carrier scattering time by more than one order of magnitude. This opens up a route to the fabrication of planar conductive/resistive/conductive heterostructures with submicrometer spatial resolution, which is also reported here.
Carrier transport mechanism in the SnO2:F/p-type a-Si:H heterojunction
JOURNAL OF APPLIED PHYSICS 110, Art. No. 024502 (2011)
We characterize SnO2:F/p-type a-Si:H/Mo structures by current-voltage (I-V) and capacitance-voltage (C-V) measurements at different temperatures to determine the transport mechanism in the SnO2:F/p-type a-Si:H heterojunction. The experimental I-V curves of these structures, almost symmetric around the origin, are ohmic for vertical bar V vertical bar< 0:1 V and have a super-linear behavior (power law) for vertical bar V vertical bar < 0:1 V. The structure can be modeled as two diodes back to back connected so that the main current transport mechanisms are due to the reverse current of the diodes. To explain the measured C-V curves, the capacitance of the heterostructure is modeled as the series connection of the depletion capacitances of the two back to back connected SnO2:F/p-type a-Si:H and Mo/p-type a-Si:H junctions. We simulated the reverse I-V curves of the SnO2:F/p-type a-Si:H heterojunction at different temperatures by using the simulation software SCAPS 2.9.03. In the model the main transport mechanism is generation of holes enhanced by tunneling by acceptor-type interface defects with a trap energy of 0.4 eV above the valence bandedge of the p-type a-Si:H layer and with a density of 4.0 x 1013 cm-2. By using I-V simulations and the proposed C-V model the built-in potential (Vbi) of the SnO2:F/p-type a-Si:H (0.16 V) and p-type a-Si:H/Mo (0.14 V) heterojunctions are extracted and a band diagram of the characterized structure is proposed.
Limiting mechanism of inversion channel mobility in Al-implanted lateral 4H-SiC metal-oxide semiconductor field-effect transistors
APPLIED PHYSICS LETTERS 99, Art. No. 072117 (2011)
The mechanism limiting the channel mobility in metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated in Al-implanted 4H-SiC is discussed comparing different post-implantation annealings. In spite of the improved interfacial morphology in carbon capped samples during annealing, the observed reduction of the mobility (from 40 to 24 cm2 V-1 s-1) suggests that interfacial roughness does not significantly impact the transport in the channel. Furthermore, the temperature dependence of the mobility demonstrates that Coulomb scattering is the main degradation mechanism due to the presence of trapped charges at the SiO2/SiC interface.
The energy band alignment of Si nanocrystals in SiO2
APPLIED PHYSICS LETTERS 99, Art. No. 082107 (2011)
The determination of the energy band alignment between the 2.6-nm-diameter Si nanocrystals and the SiO2 host is achieved by means of photo-ionization/-neutralization and capacitance spectroscopy. The measured conduction and valence band offsets are 2.6 eV and 4.4 eV. The band gap is evaluated to be 1.7 eV by photoluminescence. These results indicate that the valence band offset at the Si nanocrystals/SiO2 interface is quite close to the one observed at bulk Si/SiO2 interface. On the contrary, we observe a clear upward shift (0.5 eV) of the conduction band in the Si nanocrystals/SiO2 system with respect to the bulk Si/SiO2 hetero-structure.
Improved Performance of In0.53Ga0.47As-Based Metal-Oxide-Semiconductor Capacitors with Al:ZrO2 Gate Dielectric Grown by Atomic Layer Deposition
APPLIED PHYSICS EXPRESS 4(9), Art. No. 094103 (2011)
Atomic layer deposition of Al:ZrO2 films on In0.53Ga0.47As substrates is shown to be a promising route to boost the oxide permittivity with respect to Al2O3 gate dielectrics and concomitantly take benefit from the reducing effect of the trimethylaluminum precursor on the In0.53Ga0.47As surface. We demonstrates that increasing the number of initial Al2O3 cycles in the growth sequence can improve the physical quality and the electrical response of the Al:ZrO2/In0.53Ga0.47As interface while preserving the overall composition of the oxide.
Mesoscopic Transport Properties in Exfoliated Graphene on SiO2/Si
NANOSCIENCE AND NANOTECHNOLOGY LETTERS 3(1), 55-58 (2011)
The effective carrier diffusion length and the local mobility have been measured in graphene layers in the inferior mesoscopic regime. The measurements were carried out probing the electrical properties at nanoscale by using scanning capacitance microscopy. The local variations in the quantum capacitance can be measured and from them the effective polarised area can be determined. As a consequence the local density of states and the other local transport parameters can be obtained.
Nucleation and grain growth in as deposited and ion implanted GeTe thin films
JOURNAL OF NON-CRYSTALLINE SOLIDS 357(10), 2197-2201 (2011)
The crystallization dynamic of amorphous GeTe 50 nm thick films deposited on a SiO2/Si substrate by RF magnetron sputtering, either ion implanted by Ge+ ions or not, has been analyzed in situ by optical microscopy during annealing in the 143-155 °C temperature range. Raman spectroscopy has been also performed in as deposited, ion implanted (i.i.) and melt quenched (m.q.) amorphous samples to compare the local order among the different amorphous structure. Nucleation and growth rates, for i.i. and as deposited samples, have been observed and directly compared by optical microscopy in a region of about 5 x 104 microns2. From these data, the activation energy and pre-exponential terms of each process have been calculated. The nucleation rate and growth velocity of the i.i. films increased by a factor thirteen and a factor three with respect to the as deposited samples. This evidence, in agreement with Raman spectroscopy data, suggests that implantation, providing kinetic energy by collision cascade, induces a local atomic rearrangement towards more relaxed amorphous states. As a result the crystallization kinetic is enhanced by the reduction of wrong bonds formed during sputter deposition, a process which occurs far from equilibrium conditions.
Towards picosecond array detector for single-photon time-resolved multispot parallel analysis
JOURNAL OF MODERN OPTICS 58, 233-243 (2011)
Over the past few years there has been a growing interest in monolithic arrays of single photon avalanche diodes (SPADs) for parallel detection of faint ultrafast optical signals. SPADs implemented in CMOS-compatible planar technologies offer the typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they have inherently higher photon detection efficiency than photomultiplier tubes and they are able to provide very high acquisition speeds. The development of fully-parallel multichannel systems is a challenge both for the detector (with new problems like optical and electrical crosstalk, isolation elements size reduction and the need for higher yields) and for the electronics. The development of a parallel detector must be accompanied by that of the time-to-amplitude converters and analysis electronics necessary in time-correlated single photon counting applications. In this paper we present the fundamental building blocks necessary to develop a parallel picosecond array detector: a new high-performance SPAD detector, compatible with the design of multi-detector systems with a large number of channels, and a new totally integrated system for time-to-amplitude conversion, to allow the simple integration of several channels on a single chip.
Design of 980 nm-Pumped Waveguide Laser for Continuous Wave Operation in Ion Implanted Er:LiNbO3
IEEE JOURNAL OF QUANTUM ELECTRONICS 47(4), 526-533 (2011)
Embedded channel waveguides formed on a z-cut erbium-doped lithium niobate (Er:LiNbO3) substrate by a high-energy ion implantation technique are first described. A detailed theoretical design of a waveguide laser for continuous wave operation is then discussed, taking into account realistic input parameters and measured data. To simulate waveguide modes and to predict laser outputs from our fabricated waveguide sample, a numerical tool is developed in FORTRAN based on the full vectorial finite element method. Using the developed tools, the waveguide dimension is optimized, ensuring single-mode operation at both pump (980 nm) and laser (1531 nm) wavelengths. Finally, laser outputs as a function of various waveguide parameters are analyzed. The proposed analysis allows the effective optimization of the ion-implanted waveguide laser in Er:LiNbO3.
Depth-resolved molecular structure and orientation of polymer thin films by synchrotron X-ray diffraction
EUROPEAN POLYMER JOURNAL 47(3), 273-283 (2011)
An exemplary system suitable for optoelectronics applications, i.e. poly(3-hexylthiophene), hereinafter P3HT, deposited by spin casting onto silicon substrates functionalised by three selected molecules and then properly annealed, has been examined. Grazing Incidence X-ray Scattering (GIXS) measurements have been performed with 4-circle diffractometer, allowing for a fine control of sample axes movement.
By choosing different grazing incident angles, diffraction patterns from different layers of polymeric thin films have been recorded. Both in-plane and out-of-plane geometries have been combined in order to obtain complementary structural information. In this way structural and orientational differences of the polymer along with the film thickness (<= 50 nm) have been highlighted. For all P3HT films spun on functionalized Si wafer, macromolecular layers close to the substrate surface give some evidence of higher order and orientation than those outmost the surface, and this behaviour is pronounced to a different extent depending on the functionalized molecules used. Contrariwise P3HT layers deposited onto bare Si wafer display reduced orientation and decreased crystallite size, especially at buried interface.
