Y. X. Han

Optical emission in magnetically confined laser-induced breakdown spectroscopy

Magnetically confined laser-induced breakdown spectroscopy was investigated by studying the optical emission from laser-induced plasma plumes expanding across an external transverse magnetic field. KrF excimer laser pulses with a pulse duration of 23ns and a wavelength of 248nm were used to produce plasmas from Al, Cu, and Co targets. Various optical emission lines obtained from Al and Cu targets show an obvious enhancement in the intensity of optical emission when a magnetic field of ∼0.8T is applied, while the optical emission lines from Co targets show a decrease in the optical emission intensity. The enhancement factors of optical emission lines were measured to be around 2 for the Al and Mn (impurity) lines from Al targets, and 6–8 for Cu lines from Cu targets. Temporal evolution of the optical emission lines from the Al samples shows a maximum enhancement in emission intensity at time delays of 8–20μs after the incident laser pulse, while from the Cu targets it shows a continuous enhancement at time...

Laser-induced resonant excitation of ethylene molecules in C2H4/C2H2/O2 reactions to enhance diamond deposition

Vibrational resonant excitation of ethylene (C2H4) molecules using a carbon dioxide laser was employed to promote reactions in precursors of ethylene, acetylene (C2H2), and oxygen to enhance diamond deposition. One of the vibrational modes (CH2 wag mode, v7) of the C2H4 molecules was selected to achieve the resonant excitation in the reactions. Optical emission spectroscopy was used to study the effects of laser resonant excitation on the reactions for diamond deposition. The optical emissions of CH and C2 species were enhanced with the laser excitation, indicating that there are more active species generated in the reactions. Thicknesses and grain sizes of the deposited films were increased correspondingly. Temperature calculations from the line set in the R-branch of CH emission spectra indicated that a nonthermal process is involved in the enhanced diamond deposition.

KrF excimer laser-assisted combustion-flame deposition of diamond films

Cobalt (Co) composition has detrimental effects on the deposition of diamond films on cemented tungsten carbide (WC-Co) substrates. It decreases adhesion of the deposited films to the substrates and causes a transformation of sp3-bonded diamond to sp2-bonded graphite. In this study, a KrF excimer laser with a wavelength of 248nm, a pulse width of 23ns, and a pulse energy range of 84–450mJ was used in the combustion-flame method to improve the quality of the deposited diamond films. Scanning electron microscopy, energy dispersive x-ray analysis, and Raman spectroscopy of the deposited films showed that a laser irradiation during combustion-flame deposition of diamond decreased the cobalt composition drastically. Based on the experimental results, the influence of the laser irradiation on the deposition process was analyzed.

C2 and CH rotational temperatures in diamond growth using CO2 laser-assisted combustion-flames

Excited C2 and CH species occur abundantly in diamond growth using C2H2/O2, C2H2/C2H4/O2 and C2H4/O2 flames. The irradiation of some flames by a continuous-wave (CW) CO2 laser beam has resulted in increased optical emission intensity from the excited species and a change in the physical appearance of the flames due to resonant absorption of laser energy. Gas temperature in the flames is one of the most important parameters in the application of diamond growth. In atmospheric plasmas, the gas kinetic temperature is closely related to the rotational temperature of radical species in the plasmas. Optical emission spectroscopy (OES) was used to obtain molecular spectra of the excited C2 and CH species in the flames for a fixed gas of C2H2/C2H4/O2 flame at several laser energies. The rotational temperatures of CH were calculated using the Boltzmann plot method. In addition, synthetic C2 molecular spectra were compared with the experimental spectra to obtain temperature by the intensity ratio of selected spectrum components. For each condition, the temperatures obtained using these methods were correlated with the quality, grain size, and growth speed of diamond films on cemented tungsten carbide (WC-Co) substrates.

CO2 laser-assisted local deposition of diamond films by combustion-flame method

Quality of diamond films is strongly dependent on substrate temperatures, which are usually controlled in a range of 600 - 1100 °C in most experiments. Although many applications have been achieved with these techniques, diamond film growth is still not possible for substrates that cannot endure such high temperatures for long time. In this study, a continue-wave (CW) CO2 laser was used to irradiate the growth area on tungsten carbide (WC) substrates during C2H2/O2 combustion-flame deposition in order to maintain required temperature in the growth area while keep the rest of the substrates at a low temperature. The laser power was adjusted between 200 - 600 W to study the effects of laser irradiation on diamond deposition. Surface morphologies of the deposited films were examined by a scanning electron microscope (SEM). Film structures were characterized by Raman spectroscopy. It was concluded that the CO2 laser irradiation during combustion-flame deposition could raise the temperature at the growth area efficiently. Both laser power and power density have effects on the diamond deposition. Laser irradiation with proper parameters could improve the crystal quality of the diamond films. Based on the experimental results, the CO2 laser-assisted combustion-flame deposition is a promising method for local substrate heating during diamond film growth.

Investigation of CO2 gas breakdown using optical emission spectroscopy

CO2 gas dissociation through optical and plasma collision breakdown was investigated by optical emission spectroscopy (OES), using an Andor Mechelle monochromator with an iStar intensified charge coupled device (ICCD). A pulsed Nd:YAG laser was used to provide energy for the gas breakdown processes. The evolution of the luminous plasmas was examined by time-resolved optical spectroscopy. Emission lines of carbon and oxygen species, such as atomic C (I), ionic C (II) and atomic O (I), were observed to understand the process of CO2 dissociation. Effects of background gas pressure on plasma propagation in the experiments were studied. Some emission lines could be obviously distinguished, indicating that the emitting species had different behaviors in the evolution. They were frequently ionized and excited after the laser irradiation.

Synthesis of diamond on WC-Co substrates using a KrF excimer laser in combination with a combustion flame

A KrF excimer laser was used in combination with a combustion flame to deposit diamond films on cemented tungsten carbide (WC-Co) substrates. The laser has a wavelength of 248 nm, a pulse width of 23 ns, a pulse energy range of 84~450 mJ, and a repetition rate up to 50 Hz. Using the combustion flame method, diamond films were deposited on the laser-processed WC-Co substrates for 10 min. The morphologies of the deposited diamond films were examined using a scanning electron microscopy (SEM). The composition and bonding structures in the deposited films were studied by energy dispersive X-ray analysis (EDX) and Raman spectroscopy, respectively. The film adhesion was characterized by scratching a razor across the films. It was found that C composition on WC-Co substrate surfaces was eliminated by the laser irradiation. As a consequence, diamond nucleation density decreased and diamond grains grew larger in the laser-processed areas. Based on the experimental results, a film growth mechanism at different deposition temperature ranges corresponding to pre-deposition laser-surface-treatment effects was proposed.

Magnetically-confined laser-induced breakdown spectroscopy

Magnetically-confined laser-induced breakdown spectroscopy was investigated by time-resolved optical spectroscopy of magnetically-confined plasmas. An obvious enhancement caused by magnetic field in the optical emission from laser-induced Al and Cu plasmas was observed.

Laser-assisted synthesis of diamond-like carbon from cyclohexane liquid

Diamond-like carbon (DLC) films were synthesized by KrF excimer laser irradiation of single-crystal Si substrates immersed in a cyclohexane liquid. The deposition process was performed with a peak laser power density of about 108 W/cm2 in open atmosphere at room temperature. Scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) studies showed diamond-like characteristics of the deposited films. Optical emission spectroscopy (OES) of laser-induced plasmas of cyclohexane indicated decomposition of the cyclohexane molecules. A mechanism based on the dissociation of the cyclohexane molecules and condensation of energetic carbon atoms into diamond-like films was proposed to explain the process.

Laser-assisted combustion-flame synthesis of diamond films

Diamond films were synthesized on cemented tungsten carbide (WC-Co) substrates in open atmosphere using a C2H2/O2 combustion flame with a gas ratio (C2H2/O2) of 0.90. Before, after, and during the synthesis process, a KrF excimer laser with a wavelength of 248 nm and a pulse width of 23 ns was used to illuminate the deposition area. The surface morphology and structures of the deposited films were studied by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. From the SEM images diamond crystals with {100}, {110}, and {111} facets could be observed. The Raman spectra showed an obvious diamond peak along with a broad amorphous carbon peak, which indicated high sp3-bonded structures in the deposited films. The effects of a KrF excimer laser irradiation on the combustion-flame deposition process were discussed.Diamond films were synthesized on cemented tungsten carbide (WC-Co) substrates in open atmosphere using a C2H2/O2 combustion flame with a gas ratio (C2H2/O2) of 0.90. Before, after, and during the synthesis process, a KrF excimer laser with a wavelength of 248 nm and a pulse width of 23 ns was used to illuminate the deposition area. The surface morphology and structures of the deposited films were studied by scanning electron microscopy (SEM) and Raman spectroscopy, respectively. From the SEM images diamond crystals with {100}, {110}, and {111} facets could be observed. The Raman spectra showed an obvious diamond peak along with a broad amorphous carbon peak, which indicated high sp3-bonded structures in the deposited films. The effects of a KrF excimer laser irradiation on the combustion-flame deposition process were discussed.