SiO2 coating of silver nanoparticles by photoinduced chemical vapor deposition.

Gas-phase silver nanoparticles were coated with silicon dioxide (SiO2) by photoinduced chemical vapor deposition (photo-CVD). Silver nanoparticles, produced by inert gas condensation, and a SiO2 precursor, tetraethylorthosilicate (TEOS), were exposed to vacuum ultraviolet (VUV) radiation at atmospheric pressure and varying temperatures. The VUV photons dissociate the TEOS precursor, initiating a chemical reaction that forms SiO2 coatings on the particle surfaces. Coating thicknesses were measured for a variety of operation parameters using tandem differential mobility analysis and transmission electron microscopy. The chemical composition of the particle coatings was analyzed using energy dispersive x-ray spectrometry and Fourier transform infrared spectroscopy. The highest purity films were produced at 300-400 degrees C with low flow rates of additional oxygen. The photo-CVD coating technique was shown to effectively coat nanoparticles and limit core particle agglomeration at concentrations up to 10(7) particles cm(-3).

Field emission from tetrahedrally bonded amorphous carbon

The field emission behaviour of a series of Tetrahedrally Bonded Amorphous Carbon (ta-C) films has been measured. The films were produced using a Filtered Cathodic Vacuum Arc System. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have a threshold field of order 10-15 V/μm and optimally nitrogen-doped films exhibit fields as low as 5 V/μm. The emission as a function of back contact and front surface condition has also been considered and shows that the back contact has only a minor effect on emission efficiency. However, after etching in either an oxygen or hydrogen plasma, the films show a marked reduction in threshold field, down to as low as 2-3 V/μm, and a marked improvement in emission site density.

Amorphous carbon for use in thin film transistors

Tetrahedrally bonded amorphous carbon (ta-C) is a new type of semiconducting thin film material. It can be produced at room temperature using the Filtered Cathodic Vacuum Arc technique. The as-grown undoped ta-C is p-type in nature but it can be n-doped by the addition of nitrogen during deposition. This paper will describe thin film transistor design and fabrication using ta-C as the active channel layer.

An investigation into the residual stresses in an aluminium 2024 test weld

This paper uses finite element (FE) analysis to examine the residual stresses generated during the TIG welding of aluminium aerospace alloys. It also looks at whether such an approach could be useful for evaluating the effectiveness of various residual stress control techniques. However, such simulations cannot be founded in a vacuum. They require accurate measurements to refine and validate them. The unique aspect of this work is that two powerful engineering techniques are combined: FE modelling and neutron diffraction. Weld trials were performed and the direct measurement of residual strain made using the ENGIN neutron diffraction strain scanning facility. The predicted results show an excellent agreement with experimental values. Finally this model is used to simulate a weld made using a "Low Stress No Distortion" (LSND) technique. Although the stress reduction predicted is only moderate, the study suggests the approach to be a quick and efficient means of optimising such techniques.

Tetrahedrally bonded amorphous carbon for field-emission displays

Field emission from a series of tetrahedrally bonded amorphous-carbon (ta-C) films, deposited in a filtered cathodic vacuum arc, has been measured. The threshold field for emission and current densities achievable have been investigated as a function of sp3/sp2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have threshold fields of the order 10-15 V/μm and optimally nitrogen doped films exhibited fields as low as 5 V/μm. In order to gain further understanding of the mechanism of field emission, the films were also subjected to H2, Ar, and O2 plasma treatments and were also deposited onto substrates of different work function. The threshold field, emission current, emission site densities were all significantly improved by the plasma treatment, but little dependence of these properties on work function of the substrate was observed. This suggests that the main barrier to emission in these films is at the front surface.

Bonding regimes of nitrogen in amorphous carbon

Nitrogen can have numerous effects on diamond-like carbon: it can dope, it can form the hypothetical superhard compound C3N4, or it can create fullerene-like bonding structures. We studied amorphous carbon nitrogen films deposited by a filtered cathodic vacuum arc as a function of nitrogen content, ion energy and deposition temperature. The incorporation of nitrogen from 10-2 to 10 at% was measured by secondary ion mass spectrometry and elastic recoil detection analysis and was found to vary slightly sublinearly with N2 partial pressure during deposition. In the doping regime from 0 to about 0.4% N, the conductivity changes while the sp3 content and optical gap remain constant. From 0.4 to approximately 10% N, existing sp2 sites condense into clusters and reduce the band gap. Nitrogen contents over 10% change the bonding from mainly sp3 to mainly sp2. Ion energies between 20 and 250 eV do not greatly modify this behaviour. Deposition at higher temperatures causes a sudden loss of sp3 bonding above about 150 °C. Raman spectroscopy and optical gap data show that existing sp2 sites begin to cluster below this temperature, and the clustering continues above this temperature. This transition is found to vary only weakly with nitrogen addition, for N contents below 10%.

Carbon based bottom gate thin film transistor

This paper describes the fabrication and characterization of a carbon based, bottom gate, thin film transistor (TFT). The active layer is formed from highly sp2 bonded nitrogenated amorphous carbon (a-C:N) which is deposited at room temperature using a filtered cathodic vacuum arc technique. The TFT shows p-channel operation. The device exhibits a threshold voltage of 15 V and a field effect mobility of 10-4 cm2 V-1 s-1 . The valence band tail of a-C:N is observed to be much shallower than that of a-Si:H, but does not appear to severely impede the shift of the Fermi level. This may indicate that a significant proportion of the a-C tail states can still contribute to conduction.

Density, sp3 content and internal layering of DLC films by X-ray reflectivity and electron energy loss spectroscopy

A variety of hydrogenated and non-hydrogenated amorphous carbon thin films have been characterized by means of grazing-incidence X-ray reflectivity (XRR) to give information about their density, thickness, surface roughness and layering. We used XRR to validate the density of ta-C, ta-C:H and a-C:H films derived from the valence plasmon in electron energy loss spectroscopy measurements, up to 3.26 and 2.39 g/cm3 for ta-C and ta-C:H, respectively. By comparing XRR and electron energy loss spectroscopy (EELS) data, we have been able for the first time to fit a common electron effective mass of m*/me = 0.87 for all amorphous carbons and diamond, validating the `quasi-free' electron approach to density from valence plasmon energy. While hydrogenated films are found to be substantially uniform in density across the film, ta-C films grown by the filtered cathodic vacuum arc (FCVA) show a multilayer structure. However, ta-C films grown with an S-bend filter show a high uniformity and only a slight dependence on the substrate bias of both sp3 and layering.

Field emission site densities of nanostructured carbon

The field emission properties of nanostructured carbon films deposited by cathodic vacuum arc in a He atmosphere have been studied by measuring the emission currents and the emission site density. The films have an onset field of ∼ 3 V/μm. The emission site density is viewed on a phosphor anode and it increases rapidly with applied field. It is assumed that the emission occurs from surface regions with a range of field enhancement factors but with a constant work function. The field enhancement factor is found to have an exponential distribution.

Optical gap in carbon nitride films

In this paper we study the effect of introducing nitrogen into different carbon networks. Two kinds of carbon nitride films were deposited: (a) Using a DC-magnetron sputtering system sp2 bonded carbon nitride (a-CN) films were deposited and (b) Using a combination of filtered cathodic vacuum arc and a low-pressure N2 plasma source, N was introduced into sp3 carbon networks (ta-C), leading to the formation of a more dense CN film named ta-CN. For ta-CN films we found that the optical gap initially decreases as the N content and the sp2 fraction rises, but above a certain N quantity there is a level-off of the value, and the gap then remains constant despite further increases in the fraction and clustering of the sp2 phase. However, for a-CN films the optical gap increases with the nitrogen content. These two different trends are not easily explained using the same framework as that for carbon films, in which any decrease in the band gap is associated to an increase in the sp2 fraction or its clustering. Here we discuss the conditions that lead to high optical gap in sp2-bonded carbon nitride samples, which are clearly not associated to the presence of any crystalline super-hard phase. We also compared other differences in properties observed between the two films, such as deposition rate, infrared and Raman spectra. © 2003 Elsevier Science B.V. All rights reserved.