Low contact resistivity and strain in suspended multilayer graphene

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Method to prepare suspended multilayer graphene (MLG) flakes and to form highly conductive (contact resistivity of similar to 0.1 k Omega mu m(2)) and tight mechanical connection between MLG and metal electrodes is described. MLG flakes prepared from natural graphite were precisely deposited over tungsten electrodes using dielectrophoresis, followed by high-temperature thermal annealing in high-vacuum. Considerable strain induced in the suspended part of flakes was revealed by Raman imaging. (C) 2010 American Institute of Physics. [doi:10.1063/1.3528354]9725Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)INCT NAMITECFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Magnetism in Gd-W films

Vapor condensation techniques are useful to prepare magnetic alloys whose components have low or even negligible equilibrium mutual solubility. In this work, one of these techniques-sputtering-was used to obtain Gd(x)W(1-x) alloys whose magnetic properties were investigated as a function of the Gd atomic concentration x. Gadolinium and various Gd-based alloys are promising materials for magnetic refrigeration and this was one of the motivations for this study. The Gd(x)-W(1-x) films were sputter deposited from Gd and W targets with x ranging from 0 to 1 as determined by x-ray energy-dispersive spectroscopic analyses. X-ray diffraction patterns indicate that crystalline structures were formed at low and high Gd concentrations, while at intermediate concentrations, the films were amorphous. Magnetization measurements, performed as a function of temperature and with static and alternating applied fields, reveal a spin glasslike behavior in all the W-containing samples for temperatures below the freezing temperature T(f). For low and intermediate Gd concentrations, and for T>T(f), the films were paramagnetic, while a ferromagnetic phase was observed in the Gd-W alloy of the highest Gd content. The magnetocaloric effect was investigated from the magnetization isotherms M versus H, from which the isothermal magnetic entropy variation Delta S(M) as a function of T, for the removal of an applied field of 50 kOe, was determined. It was observed that the maximum value of Delta S(M) for each Delta S(M) versus T curve and the temperature at which these maxima occur, are strongly dependent on x. (C) 2008 American Institute of Physics.103

Modification of plasma-polymerized organosiloxane films by irradiation with He+, Ne+, Ar+, and Kr+ ions

The effects of ion irradiation on the composition, structure, compactness, and surface hardness of polyorganosiloxane films synthesized by plasma-enhanced chemical vapor deposition were investigated as a function of the ion mass and fluence. The films were obtained from a glow discharge plasma of a hexamethyldisiloxane (HMDSO)-O-2-Ar mixture, and the irradiations were carried out with 170 keV He+, Ne+, Ar+, and Kr+ ions at fluences between 1 x 10(14) and 1 x 10(16) cm(-2). To characterize the film elemental composition, two ion-beam analysis techniques were used: Rutherford backscattering spectroscopy (RBS) and forward recoil spectroscopy (FRS). The ion-beam-induced hydrogen loss from the films was significant. For the He+-irradiated samples, a H loss of about 50% with respect to the pristine or unirradiated film was observed for the highest fluence. The surface hardness measurements, performed with a nanoindenter, in films irradiated at a fluence of 1 x 10(16) cm(-1) were 8.1, 6.0, 4.7, and 1.6 GPa for He+, Ne+, Ar-,(+) and Kr+, respectively. To examine the ion-induced structural transformations in the films, infrared reflection-absorption spectroscopy (IRRAS) was employed. From analysis of the spectra of the irradiated samples several conclusions could be drawn. For example, as the ion fluence increased, (i) the densities of methyl- and Si-O-related groups changed, (ii) film disorder increased, and (iii) groups such as Si-CH2-Si, and Si-OH, which were not present in the pristine film, were formed at lower fluences but disappeared when the latter attained their highest values. Furthermore, some of the absorption peaks that appeared at low fluences and increased with increasing fluence strongly indicate formation of carbon domains in the film. Finally, differences in the ion-induced modifications produced by the different ion species were analyzed in terms of the electronic and nuclear collisions of the ions traversing the film using the well-known SRIM simulation program.17235789579

Infrared studies on films of carbosilazane and siloxazane networks

The present work describes the effects of diluting hexamethyldisilazane (HMDSN) vapor either in pure argon or in oxygen-argon mixtures on the solid film deposited from the resulting plasma. Such a dilution provides a manner of incorporating controllable amounts of Si-O groups into the solid film. The characterization of the films investigated here was made by longitudinal and transverse optical (LO and TO, respectively) functions in the mid-infrared calculated through the Kramers-Kronig analysis of transmittance spectra. The infrared analysis showed that the films were formed by silicon-centered distorted tetrahedra of the following type: Si(CH3)(x)BU(2-0.5x), where 0 <= x <= 3 and BU stands for bridging unit. For the sample deposited in the absence of O-2 in the discharge, BU = CH2 and NH; for the samples deposited with an O-2 flow rate (f(O2)) of 2.5 and 10 sccm, BU = CH2, NH, and O; and for the sample deposited with f(O2) of 20 sccm, BU = NH and O. The Delta(LO-TO) for the Si-O asymmetrical stretching increased from 0 (f(O2) = 0 sccm) to 73 cm(-1) (f(O2) = 20 sccm), while for the Si-N asymmetrical stretching it decreased from 20 (f(O2) = 0 sccm) to 3 cm(-1) (f(O2) = 20 sccm). These observations signal an increase in the Si-O-Si network and a decrease in the Si-NH-Si network as the oxygen flow increased. An interesting conclusion drawn from our analysis of the Si-H stretching mode position and Delta(LO-TO) for the AS1 band is that the films deposited in the presence of O-2 are not structurally homogeneous, but have domains with different proportions of O bridges.17184685469

Magnetic and structural studies on nanostructured Gd/Cr multilayer films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Investigations of magnetic phases, transition temperatures and coercivity were performed inmultilayered Gd/Cr films as a function of the crystalline state and morphology of the Gd layers. The films were deposited by dc magnetron sputtering at three substrate temperatures, T-s, (room temperature, 300 and 500 degrees C). The Gd and Cr thicknesses were of 10 and 30 nm, respectively. Two series of three films were prepared. In one of the series, the films had a single Gd/Cr bilayer; in the other, 15 bilayers. The discontinuous or granular nature of the Gd layers was revealed by scanning electron microscopy Grazing incidence angle x-ray diffraction was used to investigate the crystalline state of the Gd and Cr layers. These techniques revealed that grain average size and crystalline order increase with increasing T-s. From dc magnetic measurements, the co-existence of ferromagnetic and superferromagnetic phases in the Gd layers was observed, and Curie transition temperatures, T-C, were determined. High coercive fields at low temperature (2 K) were measured in hysteresis cycles. Field-cooled and zero field-cooled magnetizations as functions of temperature curves exhibited, for some of the samples, a low temperature peak suggesting a freezing transition to a cluster glass state. This was confirmed by complementary ac-susceptibility measurements carried out as a function of temperature, for various frequencies of the ac field. Some results of this work - the decline in TC for decreasing Gd grain size, the high coercive field and its dependence on particle size, and the behavior of the magnetization at low temperatures for the sample deposited at room temperature - are discussed in terms of finite size and surface effects in nanosized particles. Published by Elsevier B.V.545496502Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Nanocomposites of amorphous hydrogenated carbon and siloxane networks produced by PECVD

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Berreman effect in amorphous and crystalline WO3 thin films

Thin films of tungsten oxide deposited by hot filament metal oxide deposition (HFMOD) were thermally annealed up to 800 degreesC and investigated by means of XRD, Raman spectroscopy, and infrared reflection-absorption spectroscopy (IRRAS). As clearly shown by the XRD and Raman spectroscopy data, the deposited films were amorphous and crystallized by thermal annealing. The monoclinic WO3 phase was formed in all annealed samples. The IRRAS spectra were obtained using the IR beam with p-polarization and an off-normal incidence angle. In this condition, absorptions due to the longitudinal optical (LO) modes (Berreman effect) can be observed in the spectra. Absorptions due to LO modes are not detected by the standard infrared absorption spectroscopy, in which an unpolarized IR beam is used at normal incidence, and thus are not frequently reported in the literature. To analyze the experimental IRRAS spectra, the LO and TO functions were calculated from the transmission spectra of the as-deposited sample, using the Kramers-Kronig transformation and spectral simulation was carried out using the optical constants of both amorphous and crystalline WO3. For the as-deposited sample, the LO function of the films exhibited a very prominent band at around 950 cm(-1) which was also observed in the IRRAS spectra for all samples. For the annealed samples, this band shifted to higher wavenumbers and narrowed and a series of low-intensity bands appeared around 950 cm(-1), since crystalline structure changes were induced by thermal treatment. The results signal the applicability of the Berreman effect to the phase characterization of metal-supported WO3 films.10833123331233

Electrical and optical characteristics of a Si-doped (Al)GaInAs digital alloy/AlInAs-distributed Bragg mirrors on InP

We report electrical and optical characteristics of a Si-doped (Al)GaInAs digital alloy/AlInAs Bragg mirror lattice matched to InP grown by molecular beam epitaxy, A 98.2% reflectivity with a 107 nm stop band width centred at 1.54 mu m is obtained. An average voltage drop of 16 mV per period at a current density of 1 KA cm(-2) is observed for a mean electron concentration of about 5.5 x 10(18) cm(-3). The influence of structural and intrinsic properties of the heterostructure on the electrical resistivity and optical reflectivity is analysed. (C) 2000 Academic Press.281293

Helium ion irradiation of polymer films deposited from TMS-Ar plasmas

Polymer films synthesized from plasmas of a tetramethylsilane - Ar mixture were modified by irradiation with 170 keV He ions at fluences ranging from 1 x 10(14) to 1 x 10(16) cm(-2). As revealed by infrared spectroscopy, the ion beam produced intense bond rearrangements, such as the depletion of bonding groups (C-H and Si-H), and induced the formation of new ones, such as O-H and Si-O. From the nanoindentation measurements, a remarkable increase in the surface hardness of the films was observed as the ion fluence was increased. The increases in hardness were accompanied by an increase in the film compaction as shown by using a combination of RBS and film thickness measurements. From both hardness and infrared measurements A was concluded that, under the He ion bombardment, the polymer structure is transformed into a silicon oxycarbide network.4448949

Low-temperature gas and pressure sensor based on multi-wall carbon nanotubes decorated with Ti nanoparticles

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Multi-wall carbon nanotubes decorated by Ti nanoparticles were used for gas (N(2), Ar, O(2)) sensing at low temperatures. Chemiresistor sensor configurations with supported and suspended nanotubes were tested. Two gas sensing mechanisms (chemical and electrothermal) were demonstrated, with their relative contributions strongly depending on the sensor configuration. For suspended nanotubes, heating by Joule effect results in strong enhancement of chemical sensitivity to oxygen. (C) 2009 Elsevier B. V. All rights reserved.48241794302306Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)INCT-NAMITECFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq