Ming-Show Wong

Growth of faceted, ballas-like and nanocrystalline diamond films deposited in CH4/H2/Ar MPCVD

Abstract The influence of Ar addition to CH 4 /H 2 plasma on the crystallinity, morphology and growth rate of the diamond films deposited in MPCVD was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. X-Ray diffraction patterns indicate that diamond films of strong (111) and weak (400) texture are produced in these samples. Faceted diamond gradually turns into ballas-like diamond with graphitic inclusions when the Ar concentration increases to above 30 vol.%, as indicated by Raman spectra. As the Ar concentration goes above 90 vol.%, nanocrystalline diamond films are formed, characterized by a 1150-cm −1 peak in the Raman spectra and morphology observation. Diamond growth by CH 3 or by C 2 mechanism is proposed to interpret the change in the growth rate of diamond films with the variation of Ar content in the plasma.

Preparation and characterization of AlN/ZrN and AlN/TiN nanolaminate coatings

Abstract The processing, microstructure and properties of AlN/ZrN and AlN/TiN nanolaminate coatings were studied. Advanced ion-assisted, high-rate, reactive and pulsed-d.c. magnetron sputtering technique was used to deposit the nitride coatings onto Si, glass slide and stainless steel substrates. The thickness of the nanolaminate coating is approximately 2 μm and the period thickness is 1–160 nm. It was found that under a critical thickness ∼2 nm for the AlN layer the AlN/TiN nanolaminates exhibit a highly textured [111] oriented superlattice structure and an enhancement in film hardness. X-Ray diffraction and TEM studies indicate that in the highly [111] textured multilayered films, AlN has transformed into a nano-stabilized cubic NaCl-form from its normal hexagonal phase. However, in the case of AlN/ZrN prepared under the growth conditions comparable to those for AlN/TiN, no highly textured structure, no cubic AlN phase and no hardness enhancement was observed. The results indicate that the lattice structure and lattice match in AlN-containing nanolaminates are important for formation of the metastable cubic AlN phase and the other properties associated with the formation of the ordered microstructure.

Observation of carbon-containing nanostructured mixed titania phases for visible-light photocatalysts

This research analyzed a visible-light-responsive carbon-containing titania photocatalysts prepared by sol-gel method using confocal Raman spectral mapping, transmission electron microscopy, and UV/visible spectroscopy. The incorporation of carbons in the initial titania powder of anatase and amorphous phases was found to facilitate the formation of rutile or brookite phases from the amorphous phase in the 150–250°C temperature range, resulting in nanostructured mixed titania phases which create interface states and effectively shift the band gap to ∼2.7eV. The further shift in the absorption tails up to 800nm was attributed to carbon species-covered and/or defected titania powders.

Substrate bias effect on the formation of nanocrystalline diamond films by microwave plasma-enhanced chemical vapor deposition

The influence of negative substrate bias on the crystallinity, morphology, and growth rate of the diamond films deposited using microwave plasma-enhanced chemical vapor deposition in 1% CH4/H2 plasma were investigated. The nanocrystalline diamond films were produced exclusively under the biasing at −250 V. With −50 V biasing, faceted (111) microcrystalline diamond films at higher growth rate than no-bias samples were produced. When the biasing between −100 and −200 V, faceted (100) diamond films with decreasing grain size were favored, and the growth rates were gradually reduced along with the increasing biasing. The results indicate that the etching efficiency of H+ ions is enhanced with the increasing kinetic energy obtained from the increasing bias voltage. On the other hand, CHx+ ions at −250 V biasing would have sufficient energy to perform the ion subplantation model to grow the nanocrystalline diamond films by bias-enhanced nucleation.

Synthesis and properties of boron carbon nitride (BN:C) films by pulsed-DC magnetron sputtering

Abstract The ion-assisted, high-rate, reactive and pulsed-DC magnetron sputtering technique was used to deposit boron carbon nitride (BN:C) films by sputtering a boron carbide (B4C) target in argon and nitrogen plasma. Various processing parameters were explored to grow BN:C films with high cubic boron nitride (c-BN) content. FTIR, SEM, TEM, AES, and Raman spectroscopy were used to characterize the phases, composition and surface morphology of the films. Significant influence of substrate bias voltage and temperature on phase composition of the films was found. The deposited BN:C films exhibit h-BN, wurtzite-BN (w-BN), c-BN phases and their mixed phases with the variation in substrate bias and temperature. A multi-stage deposition technique with variable substrate bias was exploited to obtain 90% c-BN film with clear grains and facets.

Preparation of TiO2 thin films by laser ablation for photocatalytic applications

Thin titanium dioxide (TiO2) films were deposited by pulsed KrF excimer laser ablation deposition system using a titanium oxide target. The effects of substrate temperature and oxygen partial pressure on the phase formations of various microstructures were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering spectroscopy, and UV-Vis spectrophotometer. The film structures range from amorphous to mixed phase of anatase and rutile to pure anatase. Increasing the substrate temperature improves the quality of anatase phase of crystalline TiO2 films. The oxygen pressure in the range between 15 and 60 mTorr shows a single anatase phase in TiO2 films while the rutile and anatase mixed phases were observed elsewhere. The band gap of TiO2 films varied from 2.72 to 3.27 eV with increasing oxygen partial pressure while the film structures changed from rutile phase to anatase phase. The surface area of TiO2 films increased as oxygen partial pressure in film deposition. The photocatalytic performance evaluated by degradation of methylene blue in UV light was distinguishable in the TiO2 films with high anatase phase and surface area.Thin titanium dioxide (TiO2) films were deposited by pulsed KrF excimer laser ablation deposition system using a titanium oxide target. The effects of substrate temperature and oxygen partial pressure on the phase formations of various microstructures were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering spectroscopy, and UV-Vis spectrophotometer. The film structures range from amorphous to mixed phase of anatase and rutile to pure anatase. Increasing the substrate temperature improves the quality of anatase phase of crystalline TiO2 films. The oxygen pressure in the range between 15 and 60 mTorr shows a single anatase phase in TiO2 films while the rutile and anatase mixed phases were observed elsewhere. The band gap of TiO2 films varied from 2.72 to 3.27 eV with increasing oxygen partial pressure while the film structures changed from rutile phase to anatase phase. The surface area of TiO2 films increased as oxygen partial pressure in film deposition. The phot...

Combined effects of argon addition and substrate bias on the formation of nanocrystalline diamond films by chemical vapor deposition

The article reports combined effects of Ar addition and substrate bias on the grain size, microstructure, and growth rate of the diamond films prepared in microwave plasma-enhanced chemical vapor deposition. The nanocrystalline diamond (NCD) films with grain size of 50–100 nm, characterized by Raman spectra, scanning and transmission electron microscopy, were produced at 90–99 vol % Ar concentration under −50 V substrate biasing. The growth rate of the NCD films was 0.7–0.8 μm h−1, larger apparently than those grown by only Ar addition or by substrate bias effect alone. The NCD formation by various mechanisms is discussed, and a revised C2 insertion mechanism by the promotion of H+ ions is proposed to interpret the higher growth rate of the NCD films.

Deposition of carbon-containing cubic boron nitride films by pulsed-DC magnetron sputtering

Abstract An ion-assisted, high-rate, reactive and pulsed-DC magnetron sputtering technique was used to deposit boron carbon nitride (BN:C) films by sputtering a boron carbide (B 4 C) target in an argon and nitrogen plasma. Substrate bias and temperature were adjusted to grow BN:C films with a high c-BN content. FTIR, scanning and transmission electron microscopy (SEM, TEM) and Auger electron spectroscopy (AES) were used to characterize the phases, composition and surface morphology of the films. The BN:C films deposited exhibit h-BN, w-BN, c-BN and their mixed phases at various substrate bias and temperature values. The optimum conditions found to deposit BN:C films with 82% c-BN content were at a substrate bias of −250 V and a temperature of 250°C. In addition, a novel multi-step deposition technique with variable substrate bias was exploited to obtain ∼700-nm-thick, adherent films with 95% c-BN content.

Role of carbon in titania as visible-light photocatalyst prepared by flat-flame chemical vapor condensation method

In this article, the authors report that titania nanopowders synthesized by low-pressure flat-flame metal-organic chemical vapor condensation show visible-light photocatalytic ability. Using acetylene and oxygen as fuel and oxidizer for the flame, the titanium isopropoxide was decomposed and oxidized, and the nanoparticles of titania were formed. From the methylene blue decomposition study they found that the powder synthesized under low precursor feed rate possesses high photocatalytic efficiency under illumination of visible light. The visible-light absorption is resort to the presence of carbon since no other chemical elements were found associated with titania. The presence of carbon species is coincident with the presence of visible-light absorption and carbon is in the form of C–C bond. It also suggests that carbon species are associated with catalytic site on anatase surface so that carriers generated by photon absorption by the carbon species can transfer quickly onto catalytic sites and perform t...

Thermal-induced formation of silver nanowires on titanium dioxide thin films

Silver nanowires are formed by the thermal reduction of an aqueous silver nitrate solution on the surface of TiO2 thin films. A variety of silver nanostructures including nanowires were prepared by variations in the crystallinity of TiO2 thin films and heat-treatment temperature. Under appropriate conditions, silver nanowires 30–100 nm in diameter and 1–50μm in length were produced. The nanowires are found only on anatase TiO2 thin films, and the yield of nanowires increases with the anatase crystallinity. While other substrate materials, including rutile TiO2 films, silicon wafers, indium tin oxide glasses, quartz coupons, chemical-vapor deposited diamond thin films, and alloy films, were used under comparable conditions, no silver nanowires were observed. The formation of silver nanowires seems to be related to the photocatalytic properties of anatase TiO2.