Air-assisted growth of ultra-long carbon nanotube bundles

We report an air-assisted chemical vapor deposition (CVD) method for the synthesis of super-long carbon nanotube (CNT) bundles. By mixing a small amount of air in the vapor phase catalyst CVD process, the catalyst lifetime can be dramatically increased, and extremely long dense and aligned CNT bundles up to 1.5?cm can be achieved. Electron microscopy characterization shows that the injection of air does not damage the CNT structures. Further, we have estimated that individual ultra-long CNTs can carry moderate current densities ～10(5)?A?cm(-2), indicating their possible use in nanoelectronic devices.     

Laser-deposited tin dioxide and tin acetylacetonate layers for gas sensors

Thin active layers were deposited by the pulsed laser deposition (PLD) method from tin dioxide and tin acetylacetonate targets. The deposition was carried out employing an excimer KrF laser. The structure and parameters of the deposited layers were studied in connection with gas sensor applications. The influence of Ni dopant and Pd catalyst was investigated, employing PLD technology in order to introduce dopants as a multilayered structure. The properties of these layers were studied by Fourier transform infrared (FTIR), and X-ray photoelectron (XPS) spectroscopies and by measurement of their d.c. resistance. Under reducing gases the resistance of Ni-doped tin dioxide with a Pd catalyst layer decreases by 3 orders and the resistance of tin acetylacetonate with a Pd catalyst by 2 orders (synthetic air versus 1000 ppm H₂. Due to this, such layers are suitable as the active layers of gas sensors.     

Sputter deposition of semicrystalline tin dioxide films

Tin dioxide is emerging as an important material for use in copper indium gallium diselenide based solar cells. Amorphous tin dioxide may be used as a glass overlayer for covering the entire device and protecting it against water permeation. Tin dioxide is also a viable semiconductor candidate to replace the wide band gap zinc oxide window layer to improve the long-term device reliability. The film properties required by these two applications are different. Amorphous films have superior water permeation resistance while polycrystalline films generally have better charge carrier transport properties. Thus, it is important to understand how to tune the structure of tin dioxide films between amorphous and polycrystalline. Using X-ray diffraction (XRD) and Hall-effect measurements, we have studied the structure and electrical properties of tin dioxide films deposited by magnetron sputtering as a function of deposition temperature, sputtering power, feed gas composition and film thickness. Films deposited at room temperature are semicrystalline with nanometer size SnO₂ crystals embedded in an amorphous matrix. Film crystallinity increases with deposition temperature. When the films are crystalline, the X-ray diffraction intensity pattern is different than that of the powder diffraction pattern indicating that the films are textured with (101) and (211) directions oriented parallel to the surface normal. This texturing is observed on a variety of substrates including soda-lime glass (SLG), Mo-coated soda-lime glass and (100) silicon. Addition of oxygen to the sputtering gas, argon, increases the crystallinity and changes the orientation of the tin dioxide grains: (110) XRD intensity increases relative to the (101) and (211) diffraction peaks and this effect is observed both on Mo-coated SLG and (100) silicon wafers. Films with resistivities ranging between 8 mΩ cm and 800 mΩ cm could be deposited. The films are n-type with carrier concentrations in the 3 × 10¹⁸ cm^− 3 to 3 × 10²⁰ cm^− 3 range. Carrier concentration decreases when the oxygen concentration in the feed gas is above 5%. Electron mobilities range from 1 to 7 cm²/V s and increase with increasing film thickness, oxygen addition to the feed gas and film crystallinity. Electron mobilities in the 1-3 cm²/V s range can be obtained even in semicrystalline films. Initial deposition rates range from 4 nm/min at low sputtering power to 11 nm/min at higher powers. However, deposition rate decreases with deposition time by as much as 30%.     

1-hexene oligomerization by fluorinated tin dioxide

Fluorinated tin dioxide has been shown to exhibit catalytic activity for 1-hexene oligomerization. The physicochemical and functional properties of nanocrystalline fluorinated SnO₂ have been studied.     

Pyrosol deposition of fluorine-doped tin dioxide thin films

Fluorine-doped tin dioxide (SnO₂F) films were deposited from a tin tetrachloride solution in methanol utilizing a pyrosol deposition process. It is shown from thermodynamic calculations that the atmosphere during deposition is oxygen-rich and also suggested that chlorine and hydrogen chloride, which are produced during the deposition reaction, influence crystal growth. Detailed electrical, optical and structural properties of the material with respect to varying film thickness and substrate temperature are presented and discussed. Resistivity of the films deposited at 450 °C decreased from 6×10^–4 to 2×10^–4 cm, while the mobility increased from 14 to 45 cm²V^–1s^–1, respectively, when the film thickness was varied from 100 to 1650 nm. The carrier concentration was relatively unchanged for film thicknesses higher than 200 nm. Optimized SnO₂F films (600 nm) having a resistivity of 6×10^–4 cm, a carrier mobility of 20 cm²V^–1s^–1, a carrier concentration of 8×10²⁰ cm^–3 and a transmittance in excess of 80% are quite suitable as electrodes for amorphous silicon solar cells.     

Annealing behaviour of electron-beam deposited tin dioxide films

The sheet resistivity of tin dioxide films deposited by electron-beam evaporation has been studied during annealing, both as a function of time and temperature. The annealing behaviour of SnO₂ films under the above two different conditions is consistent. A qualitative interpretation has been given for the decrease and the minimum observed in the resistivity. The increase in resistivity has been confirmed by scanning-electron micrographs. The films were also characterized by x-ray diffractometry.     

Competitive effects of film thickness and growth rate in spray pyrolytically deposited fluorine-doped tin dioxide films

Here we report efforts to understand the competitive roles of film thickness (up to ∼ 650 nm) and growth rate (up to ∼ 190 nm/min) in spray pyrolytically deposited fluorine-doped tin dioxide films. This was achieved by varying the time of deposition and/or the precursor concentration. Film properties were investigated using X-ray diffraction technique and Hall effect measurements. The thickness evolution involved textured growth along [200] orientation followed by a secondary prominence of [110] and [101] orientations. The growth rate induced effects accelerated this change. The carrier concentration and carrier mobility showed that the [200] oriented growth is technologically advantageous and can be obtained using a wider growth rate range ∼ 50-130 nm/min.     

Textured tin dioxide films for gas recognition microsystems

We have studied the microstructure, electrical properties, and gas sensor characteristics of thin tin dioxide (SnO₂) films obtained by RF magnetron sputtering of an oxide target. The synthesized films are composed of crystalline rods with a diameter of 10–60 nm and a length of up to 1000 nm oriented perpendicularly to the substrate plane. This morphology facilitates the access of gas molecules to the side surfaces of crystallites. Use of such SnO₂ films in a multisensor microsystem expanded the spectrum of recognized gases.     

Scalable templated growth of graphene nanoribbons on SiC

In spite of its excellent electronic properties, the use of graphene in field-effect transistors is not practical at room temperature without modification of its intrinsically semimetallic nature to introduce a bandgap. Quantum confinement effects can create a bandgap in graphene nanoribbons, but existing nanoribbon fabrication methods are slow and often produce disordered edges that compromise electronic properties. Here, we demonstrate the self-organized growth of graphene nanoribbons on a templated silicon carbide substrate prepared using scalable photolithography and microelectronics processing. Direct nanoribbon growth avoids the need for damaging post-processing. Raman spectroscopy, high-resolution transmission electron microscopy and electrostatic force microscopy confirm that nanoribbons as narrow as 40 nm can be grown at specified positions on the substrate. Our prototype graphene devices exhibit quantum confinement at low temperatures (4 K), and an on-off ratio of 10 and carrier mobilities up to 2,700 cm(2) V(-1) s(-1) at room temperature. We demonstrate the scalability of this approach by fabricating 10,000 top-gated graphene transistors on a 0.24-cm(2) SiC chip, which is the largest density of graphene devices reported to date.     

Preparation and characterization of indium-doped tin dioxide nanocrystalline powders

The precursors of SnO₂ or In₂O₃/SnO₂ nanocrystalline powders have been prepared by the sol-precipitation method. The precursors are calcined at different temperatures to prepare SnO₂ or In₃O₃/SnO₂ nanocrystalline powders with different particle sizes. The reaction conditions, such as the concentration of reactants, the surfactant, pH value, the amount of indium oxide doped, and the reaction temperature and time, have been studied. The nanocrystallites are examined by differential thermal analysis (DTA)/thermogravimetric analysis (TGA), X-ray diffraction (XRD), IR spectroscopy and transmission electron microscopy (TEM).     

Gas sensing characteristics of tin dioxide with small crystallites

The effect of calcining temperature on gas sensitivity and resistance in wet air was observed with elements of SnO₂ thick film. The results were interpreted as a function of the crystallite size. As the crystallite size reduced, the sensitivity to H₂ gas enhanced at temperatures lower than 300C, but at those higher than 350C it did not. The temperatures showing the minimum resistance in air and the maximum sensitivity to H₂ gas decreased with reduction of the crystallite size. The temperature variations were assigned to the change of the activation energies of the oxygen adsorbates. It is suggested that the decrease of activation energies is one of the reasons for the sensitivity enhancement with the fine powder.     

Characterization of tin dioxide film for chemical vapors sensor

Recently, oxide semiconductor material used as transducer has been the central topic of many studies for gas sensor. In this paper we investigated the characteristic of a thick film of tin dioxide (SnO₂) film for chemical vapor sensor. It has been prepared by screen-printing technology and deposited on alumina substrate provided with two gold electrodes. The morphology, the molecular composition and the electrical properties of this material have been characterized respectively by Atomic Force Spectroscopy (AFM), Fourier Transformed Infrared Spectroscopy (FTIR) and Impedance Spectroscopy (IS). The electrical properties showed a resistive behaviour of this material less than 300 °C which is the operating temperature of the sensor. The developed sensor can identify the nature of the detected gas, oxidizing or reducing.     

Growth of tin dioxide nanobelts via Au-catalytic VLS process

Ultra-long (several millimeters) tin dioxide SnO2 nanobelts were prepared by chemical vapor deposition at 850 degrees C. The X-ray powder diffraction (XRD) indicated that the as-prepared sample is tetragonal phase SnO2; field emission scanning electron microscopy (FESEM) reveals the as-prepared SnO2 is uniform nanobelts; transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) studies show the nanobelts is monocrystalline with width of hundreds of nanometers and growth along [101] crystal direction; X-ray energy-dispersive spectrometer (EDS) and photoluminescence (PL) spectrum were used to detail its composition and optical properties. The possible formation mechanism of these ultra-long nanobelts was also proposed on the basis of experiments.     

Electron microscopy studies of sprayed thin tin dioxide films

Electron microscopy studies of thin tin dioxide films (about 80 nm thick) prepared by spraying from a mixture of SnCl₄ and ethyl acetate indicate that such films are amorphous at substrate temperatures of 340–350°C, partially crystallized at 360–410°C and polycrystalline at 420–500°C. Grains about 40 nm in size were found for all polycrystalline structures, independent of the substrate temperature. Only SnO₂, having values of the interplanar spacing d in agreement with the ASTM data, was clearly identified by electron diffraction. Scanning electron microscopy studies indicate smooth surfaces with pits of varying size and density depending on the preparation conditions.     

Synthesis and characterization of ferromagnetic cobalt-doped tin dioxide thin films

Co-doped SnO₂ thin films are grown on sapphire (0001) substrates at 600 °C by the technique of dual-beam pulsed laser deposition. The prepared films show preferred orientation in the [100] direction of the rutile structure of SnO₂. Nonequilibrium film growth process results in doping Co into SnO₂ much above the thermal equilibrium limit. A Film with 3% of Co is ferromagnetic at room temperature with a remanent magnetization of ∼ 26% and a coercivity of ∼ 9.0 mT. As Co doping content x increases, the optical band gap absorption edge (E₀) of the Co-doped SnO₂ thin films initially shows a redshift at low x up to x = 0.12 and then increases at the higher x, which are attributed to the sp-d exchange interactions and alloying effects, respectively.     

Electrical properties of high purity tin dioxide doped with antimony

The electrical conductivity of high purity tin dioxide doped with antimony was studied at temperatures of 900 to 1200° C and partial pressures of oxygen between 10^–8 and 1 atm. For specimens having a dopant concentration over 1 × 10¹⁹Sb cm^–3, the electrical conductivity decreased slightly with temperature and independent of oxygen partial pressure. The electrical conductivity of specimens having a dopant concentration under 1 × 10^–8Sb cm^–3 increased with temperature and with decreasing partial pressure of oxygen. The significance of the dopant and the thermally created defects is discussed.     

Effects of electron beam irradiation on tin dioxide gas sensors

In this paper, the effects of electron beam irradiation on the gas sensing performance of tin dioxide thin films toward H₂ are studied. The tin dioxide thin films were prepared by ultrasonic spray pyrolysis. The results show that the sensitivity increased after electron beam irradiation. The electron beam irradiation effects on tin dioxide thin films were simulated and the mechanism was discussed.     

HRTEM observations on composites of tin and tin dioxide nanoparticles dispersed on carbon nanotubes by single-atoms-to-clusters method

Nano-composites of tin and tin dioxide particles were synthesized on carbon nanotubes by the single-atoms-to-clusters (SAC) method, and their structures were investigated by high-resolution transmission electron microscopy. By changing the heat-treatment temperature during the SAC process, two different types of samples were obtained. The samples prepared around 450 K were aggregates of 2-4 nm-sized tin dioxide nanoparticles, and their size distributions on carbon nanotubes are in the range 20-40 nm. The other samples formed above 600 K had a core-shell structure of diameter 20-40 nm. The core and shell were made of tin single crystal and disordered oxidized tin, respectively. The thickness of the oxidized layers was ca. 4 nm.     

HRTEM observations on composites of tin and tin dioxide nanoparticles dispersed on carbon nanotubes by single-atoms-to-clusters method

Nano-composites of tin and tin dioxide particles were synthesized on carbon nanotubes by the single-atoms-to-clusters (SAC) method, and their structures were investigated by high-resolution transmission electron microscopy. By changing the heat-treatment temperature during the SAC process, two different types of samples were obtained. The samples prepared around 450 K were aggregates of 2–4 nm-sized tin dioxide nanoparticles, and their size distributions on carbon nanotubes are in the range 20–40 nm. The other samples formed above 600 K had a core–shell structure of diameter 20–40 nm. The core and shell were made of tin single crystal and disordered oxidized tin, respectively. The thickness of the oxidized layers was ca. 4 nm.