Photoelectron spectroscopic characterization of titanium-containing amorphous hydrogenated silicon–carbon films (aSi1-xCx:H/Ti)

Titanium-containing amorphous hydrogenated silicon–carbon films (aSi_1-xC_x:H/Ti) have been deposited by reactive magnetron cosputtering. Core-level photoelectron spectroscopy (XPS) and valence-band photoelectron spectroscopy (UPS) have served as means for the characterization of these films. The spectroscopic data are interpreted by a structural model on the basis of a nanocomposite containing clusters of a Ti-C-Si alloy being embedded in an amorphous hydrogenated silicon–carbon matrix (aSi_1-xC_x:H). The Ti-C-Si compound is of metallic character and most likely a substitutional solid solution. This novel nanocomposite material is a promising candidate for applications, especially as optical selective absorber coating for solar collectors. Received: 10 July 2000 / Accepted: 15 September 2000 / Published online: 21 March 2001     

Honeycomb-like alignments of carbon nanotubes synthesized by pyrolysis of a metal phthalocyanine

Honeycomb-like alignments of carbon nanotubes were prepared by pyrolysis of a metal phthalocyanine at 950 °C in an Ar/H₂ flow. A simple synthetic method has been developed for a large-scale synthesis of aligned carbon nanotubes normal to a substrate surface. Received: 15 June 2000 / Accepted: 21 June 2000 / Published online: 2 August 2000     

Influence of C on Ge incorporation in the growth of Ge-rich Ge1−x−ySixCy alloys on Si (100)

Amorphous carbon nitride powders are successfully produced by ammonothermal synthetic routes. The existence of the chemical bonding of C≡N in the powders is confirmed by Raman and infrared spectra. Raman spectra of various samples show that the G-band frequency shifts to higher energies, and the intensity ratio I_D/I_G decreases as the N content increases. The Knoop hardness of the sintering sample is up to 1200 kgf/mm². The hardness of the sample increases with increasing of the N content. Received: 24 May 2000 / Accepted: 26 May 2000 / Published online: 2 August 2000     

Microstructures and photoluminescence of hydrogenated amorphous carbon films produced by using organic hydrocarbon source

Thin layers of hydrogenated amorphous carbon were prepared by using organic hydrocarbon source, xylene (C₈H₁₀), in plasma-enhanced chemical vapor deposition (PECVD) system. The microstructures were characterized by using Fourier-transform infrared and Raman scattering spectra. The appearance of a sharp vibration signal in 1600 cm^-1 strongly suggests the existence of sp² carbon clusters with aromatic rings. With increasing the deposition rf power, the content of these aromatic structures is increased in the films. In contrast to a broad single PL peak in methane (CH₄)-baseda-C:H films, the PL band of xylene-based a-C:H films contains multiple peaks in blue-green light region, which is influenced by rf power. We tentatively attributed it to the radiative recombination of electron-hole pairs through some luminescent centers associated with aromatic structures. Received: 26 April 2000 / Accepted: 9 May 2000 / Published online: 13 September 2000     

Growth mechanisms for single-wall carbon nanotubes in a laser-ablation process

Mechanisms proposed in the literature are compared with a current scenario for the formation of single-wall carbon nanotubes in the laser-ablation process that is based on our spectral emission and laser-induced fluorescence measurements. It is suggested that the carbon which serves as feedstock for nanotube formation not only comes from the direct ablation of the target, but also from carbon particles suspended in the reaction zone. Fullerenes formed in the reaction zone may be photo-dissociated into C₂ and other low molecular weight species, and also may serve as feedstock for nanotube growth. Confinement of the nanotubes in the reaction zone within the laser beam allows the nanotubes to be ‘purified’ and annealed during the formation process by laser heating. Received: 2 November 2000 / Accepted: 3 November 2000 / Published online: 23 March 2001     

Unique magnetism observed in single-wall carbon nanohorns

The magnetic properties of single-wall carbon nanohorns (SWNH) were studied by electron spin resonance (ESR) and static magnetic susceptibility measurements. The SWNHs were ESR active with linewidth (ΔH) of ∼6 G in vacuo at room temperature. ΔH was susceptible to the partial pressure of O₂ and became 53 G at 1 atmospheric pressure of O₂, while the integrated ESR intensity was independent on O₂ pressure and behaved as Curie-like, suggesting an intrinsic ESR origin with localized electron spin character. The diamagnetic susceptibility for SWNHs indicated a value smaller than that of randomly oriented graphite by an order of magnitude, but showing a magnitude comparable to those of C₆₀ and C₇₀. It is suggested that the large diamagnetism expecting for sp² networked carbon materials will be canceled by the Van Vleck constant paramagnetism. Received: 20 November 2000 / Accepted: 21 November 2000 / Published online: 25 July 2001     

Relaxed GexSi1−x layer formation with reduced dislocation density grown by ultrahigh vacuum chemical vapor deposition

A mixture of as-grown single-wall carbon nanotubes (SWNTs) and a monochlorobenzene (MCB) solution of polymethylmethacrylate (PMMA) was sonicated and homogenized. As a result, SWNTs were separated from carbonaceous impurities and metal particles, which enabled us to purify the SWNTs by filtration. We also found that the number of short (about 1-μm) SWNTs and thin bundles of SWNTs increased. The thin bundles contained one to three SWNTs. These short, thin SWNTs suspended in the MCB solution of PMMA were spin-coated onto a Si wafer, and could be dispersed on it. Received: 18 July 2000 / Accepted: 20 July 2000 / Published online: 6 September 2000     

Preparation and characterizations of SiO2/TiO2/γ-glycidoxypropyltrimethoxysilane composite materials for optical waveguides

SiO₂/TiO₂/γ-glycidoxypropyltrimethoxysilane composite materials processed by the sol-gel technique were studied for optical waveguide applications. Waveguide films with thickness more than 1.7 μm were prepared on a silicon substrate by a single-coating process and low-temperature heat treatment from these high-titanium-content composite materials. Scanning electron microscopy (SEM), atomic force microscopy (AFM), thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-VIS), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the waveguide films. TGA curves showed that organic compounds in the composite materials would decompose in the temperature range from 200 °C to 480 °C. SEM, AFM and UV-VIS results showed that a dense, porous-free, and high transparency in the visible range waveguide film could be obtained at a low heat-treatment temperature. It was also noted that the carbon content in the film with higher titanium content heated at high temperature was evidenced by XPS. The waveguide propagation loss properties of the composite material films were also investigated and showed a dependence on the titanium molar fraction. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 20 September 2000     

“Sputtered-atom assembly” of carbon nanostructures during ion bombardment

We report the observation of a carbon nanostructure grown on a graphite surface by Ar⁺ ion bombardment. We demonstrate experimentally that, in view of transmission electron microscopy (TEM)-based evidence, some of these carbon nanostructures emerged the whiskerlike protrusion and/or the sputtered-surface, suggesting a new growth model for nanocarbon, distinctly different from that found in arc-discharge and many other methods. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 13 July 2000     

Phase transformation during surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser

Surface ablation of cobalt-cemented tungsten carbide hard metal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. Surface microphotography and XRD, as well as an electron probe have been used to investigate the transformation of phase and microstructure as a function of the pulse-number of laser shots at a laser fluence of 2.5 J/cm². The experimental results show that the microstructure of cemented tungsten carbide is transformed from the original polygonal grains of size 3 μm to interlaced large, long grains with an increase in the number of laser shots up to 300, and finally to gross grains of size 10 μm with clear grain boundaries after 700 shots of laser irradiation. The crystalline structure of the irradiated area is partly transformed from the original WC to βWC_1-x, then to αW₂C and CW₃, and finally to W crystal. It is suggested that the undulating ‘hill–valley’ morphology may be the result of selective removal of cobalt binder from the surface layer of the hard metal. The formation of non-stoichiometric tungsten carbide may result from the escape of elemental carbon due to accumulated heating of the surface by pulsed laser irradiation. Received: 13 July 2000 / Accepted: 27 October 2000 / Published online: 10 January 2001     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

Morphologies of GaN one-dimensional materials

GaN one-dimensional materials with different morphologies were formed on LaAlO₃ crystal, silicon crystal and quartz glass substrates through a simple sublimation method. They were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy. FE-SEM images showed that the morphologies of the one-dimensional materials included straight nanorods, curved nanowires, nanoribbons, zigzag nanorods and beaded or capture-tree nanorods. XRD and EDX studies indicated that all the one-dimensional materials were wurtzite GaN. Received: 14 July 2000 / Accepted: 17 July 2000 / Published online: 20 September 2000     

Raman-scattering spectrum of GaN straight nanowires

The Raman spectrum of GaN straight nanowires deposited on a LaAlO₃ crystal substrate was studied. The E₂ (high) phonon frequency at 560 cm^-1 shows a 9 cm^-1 shift compared with the calculated value. The low-energy shift and band broadening of the Raman modes result from the nanosize effect. The unique property of the low intensity ratio of I_E2/I_A1(LO) on the Raman spectrum from the GaN straight nanowires was observed. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 2 August 2000     

Low-loss, damage-resistant optical waveguides in Zn-diffused LiNbO3 by a two-step procedure

In the present work the fabrication of high-quality LiNbO₃ waveguides, based on Zn diffusion from vapor phase using a two-step diffusion process, has been demonstrated. The optical waveguides fabricated using this method have low propagation losses (less than 0.5 dB/cm measured at 633 nm), as a consequence of the high surface quality obtained. High photorefractive damage resistance is reported, and it has been related to the Zn incorporation into the LiNbO₃ crystals. Additional advantages of this waveguide fabrication method due to the low temperature of the process (<900 °C) are to prevent Li out-diffusion and to preserve ferroelectric domain structures. Received: 2 March 2000 / Accepted: 21 June 2000 / Published online: 13 September 2000     

Thermomechanical properties of amorphous hydrogenated carbon–germanium alloys

Thermomechanical properties of amorphous hydrogenated carbon-germanium alloys prepared by the rf sputtering technique were determined for films in the 0 at. % to 100 at. % carbon content range. The stress, thermal expansion coefficient, and elastic modulus were obtained using the thermally induced bending technique. The stress was related to the concentration of hydrogen and argon, to the difference in the Ge-Ge and Ge-C bond length, and to the carbon hybridization. The thermal expansion coefficients of pure amorphous germanium and amorphous carbon are higher than that of their corresponding crystalline counterparts, which was attributed to the compressive stress of the films. The biaxial modulus, on the other hand, are always smaller than that of their crystalline counterparts, but increases as the concentration of carbon increases due to the substitution of Ge-Ge bonds by energetically stronger Ge-C and C-C bonds. Received: 9 May 2000 / Accepted: 10 May 2000 / Published online: 13 July 2000     

Size distributions and synthesis of nanoparticles by photolytic dissociation of ferrocene

Iron-containing nanoparticles were made by laser-assisted (ArF excimer laser, λ=193 nm) photolytic dissociation of ferrocene (Fe(C₅H₅)₂ or FeCp₂) in argon and an oxygen/argon gas mixture. The particle-size distributions were obtained on-line by using differential mobility analysers (DMAs) and were found to be log-normal with a geometric standard deviation of 1.85. In argon, particle sizes between 3 and 100 nm were generated. The volumes of these particles were found to increase linearly with the increased repetition rate, fluence and beam size of the laser. These observations are explained on the basis of the residence-time approach model. Received: 23 November 1999 / Accepted: 19 September 2000 / Published online: 22 November 2000     

Preferential regrowth of indium–tin oxide (ITO) films deposited on GaN (0001) by rf-magnetron sputter

Effects of thermal treatments on the electrical properties and microstructures of indium–tin oxide (ITO)/GaN contacts have been investigated using a rf-magnetron sputter deposition followed by rapid thermal annealing. ITO films annealed at 800 °C revealed Schottky contact characteristics with a barrier height corresponding to ITO’s work function of 4.62 eV. The evolution of electrical properties of ITO/GaN contacts was attributed to the preferential regrowth of In₂O₃ (222)//GaN (0001) with an ideal metal–semiconductor Schottky contact. The feasible use of ITO/GaN as a transparent Schottky contact would be realized by the enhanced regrowth of In₂O₃ at high temperature. Received: 1 September 2000 / Accepted: 15 November 2000 / Published online: 28 February 2001     

Reverse saturable absorption of a novel rhodamine-B cation fulleride salt at 532 nm

A novel rhodamine-B cation fulleride salt was synthesized by metathetical reaction. Its tetrahydrofuran solution was prepared and the fluence-dependent transmission was measured with 10-ns 532-nm laser pulses in a collimated optical setup. With increases in the input fluence, its transmittance decreased significantly, which indicated reverse saturable absorption behavior. For comparison, the input fluence-dependent transmittance of fullerene C₆₀ and rhodamine-B solution were also measured. The nonlinear optical limiting effect of the fulleride salt in tetrahydrofuran is slightly lower than that of C₆₀ in toluene and the explanations are provided. Received: 25 August 2000 / Accepted: 6 November 2000 / Published online: 3 April 2001     

Luminescence properties from erbium oxide nanocrystals dispersed in titania/organically modified silane composite sol–gel thin films

Luminescence properties from erbium (III) oxide nanocrystals dispersed in titania/organically modified silane composite thin films were studied. Erbium oxide nanocrystals were prepared by an inverse microemulsion technique. A strong room-temperature photoluminescence was observed at 1.531 μm, with the full width at half maximum (FWHM) of 22 nm due to intra-atomic transitions between ⁴ I _13/2 and ⁴ I _15/2 levels in the erbium (III) ion. The shape, peak position, and FWHM of the photoluminescence signals from the composite thin films were quite comparable to those prepared by other methods. The photoluminescence peak of the composite thin films showed a maximum intensity at the heat-treatment temperature of 300 °C. A room-temperature green up-conversion emission at 543 nm (⁴ S _3/2?⁴ I _15/2) was observed for the composite thin films with different heat-treatment temperatures upon excitation at 993 nm. The up-conversion emission mechanism was explained by means of an energy-level diagram and the lifetime of the visible up-conversion emission was measured. Received: 10 July 2000 / Accepted: 11 July 2000 / Published online: 5 October 2000     

Synthesis and Raman characterization of mono-sized single-wall carbon nanotubes in one-dimensional channels of AlPO4-5 crystals

Mono-sized single-wall carbon nanotubes were formed in one-dimensional channels of AlPO₄-5 single crystal (AFI) by pyrolysis of tripropylamine (TPA). Raman spectra have been measured for the TPA-AFI crystals thermally processed at different conditions. TPA molecules are carbonized at 400 °C, and carbon nanotubes were formed at 500 °C or above. The radial-breathing mode, which is special for carbon nanotube geometry, was observed. Three Raman-active modes with symmetry A _1g, E _1g, and E _2g were identified by detailed symmetrical analysis for the polarized-Raman spectra. Received: 29 October 1998 / Accepted: 29 March 1999 / Published online: 24 June 1999