Discharge In Nitrogen: Research On N-doping And Synthesis Of Nitrides

Nitrides due to their excellent properties and high application potentials in a variety of fields, such as semiconductors, environmental science and energy sources, have attracted attentions in science as well as in technology. In the present work, a nitrogen ion beam source based on hollow cathode discharge has been developed, in which nitrogen molecules are highly dissociated. This ion beam source was used for the nitrogen doping of the TiO₂ films. These N-doped TiO₂ films have the photocatalysis ability in visible-light region. On the other hand, the pulsed discharge of nitrogen gas has been studied and applied to the deposition of amorphous carbon nitride nanoparticulate films, which have shown very good field-emission properties. This work has some useful explorations in the applications of nitrogen discharges. Some meaningful and innovative results have been obtained.1. Development of the atomic nitrogen ion beam source based on the hollow-cathode-discharge of nitrogen gasAn atomic nitrogen ion beam source ahs been designed and manufactured, which can work stably. The discharge parameters have been studied in details. The dependence of the compositions of the extracted beams on the discharge parameters, such as discharge current, gas pressure, gas flow rate and gas composition, has been studied by time-of-flight mass spectroscopy (TOFMS). Under the optimal discharge conditions, a very high ratio of atomic nitrogen ions in the extracted beam, as high as 95%, has been obtained. And an N⁺ ratio higher than 90% can be obtained in a wide range of normal discharge currents. This beam source can therefore be considered as a pure atomic nitrogen ion beam source. Since atomic nitrogen ions have higher chemical reactivity and lower sputtering effect, this source has lab application potentials in the nitridation modification of materials. This source was applied in the nitrogen doping for TiO₂ films in this work.2. Study on the N-doping for TiO₂ films by ion implantation and theirvisible-light photocatalysisNanostructured TiO₂ films in anatase phase were synthesized by pulsed laser depositon (PLD) and magnetic sputtering (MS). Nitrogen was doped in the TiO₂ films by low energy N⁺ implantation. The N-doped films were characterized by Raman, XPS, XRD and UV-Vis absorption spectroscopy. It is shown by XPS that nitrogen was effectively doped and the doping ratios are higher than 10% for the films synthesized either by PLD or by MS. Enhanced visible-light absorptions were observed for all the N-doped films. Under the irradiation of a 405 nm laser, the N-doped films exhibited photocatalysis abilities for the dissociation of methylene blue (MB). The photocatalysis efficiencies are in agree with their photoabsorptions in the visible-light region. The origin of the difference in their photocatalysis efficiencies has been discussed.3. Deposition of carbon-nitride nanoparticulate films by pulsed discharge.Amorphous carbon-nitride films were grown on the nitridated-diamond substrates by pulsed-discharge of pure nitrogen gas using graphite rods as the discharge electrodes. The deposition conditions were optimized by monitoring the discharge plasma with optical-emission spectroscopy. It is demonstrated that the films were mainly a mixture of nanosized carbon, nano-diamond and carbon-nitride nanoparticles with sp² and sp³ phases. After HF processing, most of the loosely bonded large particles have been moved and the film is composed by the small sized particles. The average sizes of the graphite crystallites in the films before and after chemical etching can be estimated to be approximately 1.5 and 1.2 nm, respectively, corresponding to the energy gaps of 1.02 and 1.05 eV.4. Field-emission properties of the carbon-nitride nanoparticulate filmsPreliminary results show that the pulsed-charge-deposited carbon-nitride films exhibit semiconductor behavior and have an excellent cold-cathode-emission property. The films have a low field threshold and a high current density of field emission, which make them possible to be superior electronic materials. Improvement in the conductivity and field-emission properties was observed after chemical etching with hydrofluoric acid. The activation energy for electrical conduction of the HF-treated film decreased from 3.42 to 1.19 eV. The similar threshold voltages were obtained for the carbon-nitride films before and after chemical etching, which were 4.0 and 3.5 V/Am, respectively. However, the emission current density after etching increased by one order of magnitude. The field emission properties were greatly improved by HF-treatment.