Date of Award

Fall 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Program/Concentration

Electrical Engineering

Committee Director

Hani E. Elsayed-Ali

Committee Member

Helmut Baumbart

Committee Member

Shu Xiao

Committee Member

Geoffrey Kraft

Abstract

Carbon ions generated by ablation of a carbon target using an Nd:YAG laser pulse (wavelength λ = 1064 nm, pulse width τ = 7 ns, and laser fluence of 10-110 J/cm2) are characterized. Time-of-flight analyzer, a three-mesh retarding field analyzer, and an electrostatic ion energy analyzer are used to study the charge and energy of carbon ions generated by laser ablation. The dependencies of the ion signal on the laser fluence, laser focal point position relative to target surface, and the acceleration voltage are described. Up to C4+ are observed. When no acceleration voltage is applied between the carbon target and a grounded mesh in front of the target, ion energies up to ~400 eV/charge are observed. The time-of-flight signal is analyzed for different retarding field voltages in order to obtain the ion kinetic energy distribution. The ablation and Coulomb energies developed in the laser plasma are obtained from deconvolution of the ion time-of-flight signal. Deconvolution of the time-of-flight ion signal to resolve the contribution of each ion charge is accomplished using data from a retarding field analysis combined with the time-of-flight signal. The ion energy and charge state increase with the laser fluence. The position of the laser focal spot affects the ion generation, with focusing ~1.9 mm in front of the target surface yielding maximum ions. When an external electric field is applied in an ion drift region between the target and a grounded mesh parallel to the target, fast ions are extracted and separated, in time, due to increased acceleration with charge state. However, the ion energy accelerated by the externally applied electric field is less than the potential drop between the target and mesh due to plasma shielding.

By coupling a spark discharge into a laser-generated carbon plasma, fully-stripped carbon ions with a relatively low laser pulse energy are observed. When spark-discharge energy of ~750 mJ is coupled to the carbon plasma generated by ~50 mJ laser pulse (wavelength 1064 nm, pulse width 8 ns, intensity 5 × 109 W/cm2), enhancement in the total ion charge by a factor of ~6 is observed, along with the increase of maximum charge state from C4+ to C6+. Spark coupling to the laser plasma significantly reduces the laser pulse energy required to generate highly-charged ions. Compared to the laser carbon plasma alone, the spark discharge increases the intensity of the spectral emission of carbon lines, the electron density ne, and the electron temperature Te. The effective ion plasma temperature associated with translational motion along the plume axis Tieff is calculated from the ion time-of-flight signal.

Carbon laser plasma generated by an Nd:YAG laser (wavelength 1064 nm, pulse width 7 ns, fluence 4-52 J/cm2) is studied by optical emission spectroscopy and ion time-of-flight. Up to C4+ ions are detected with the ion flux strongly dependent on the laser fluence. The increase in ion charge with the laser fluence is accompanied by observation of multicharged ion lines in the optical spectra. The time-integrated electron temperature Te is calculated from the Boltzmann plot using the C II lines at 392.0, 426.7, and 588.9 nm. Te is found to increase from ~0.83 eV for a laser fluence of 22 J/cm2 to ~0.90 eV for 40 J/cm2. The electron density ne is obtained from the Stark broadened profiles of the C II line at 392 nm and is found to increase from ~2.1x1017 cm-3 for 4 J/ cm2 to ~3.5 x 1017 cm-3 for 40 J/cm2. Applying an external electric field parallel to the expanding plume shows no effect on the line emission intensities. Deconvolution of ion time-of-flight signal with a shifted Maxwell-Boltzmann distribution for each charge state results in an ion temperature Ti ~4.7 and ~6.0 eV for 20 and 36 J/cm2, respectively.

Carbon ion emission from femtosecond laser ablation of a glassy carbon target is studied. A Ti:sapphire laser (pulse duration τ ~150 fs, wavelength λ = 800 nm, laser fluence F ≤ 6.4 J/cm2) is used to ablate the carbon target while ion emission is detected by a time-of-flight detector equipped with a three-grid retarding field analyzer. A strong effect of the laser pulse fluence on the yield of carbon ions is observed. Up to C6+ ions are detected. The carbon time-of-flight ion signal is fit to a shifted Maxwell-Boltzmann distribution and used to extrapolate the effective plasma ion temperature Tieff = 6.9 eV. Applying an external electric field along the plasma expansion direction increased ion extraction, possibly due to the retrograde motion of the plasma-vacuum edge.

The laser ion source is utilized for carbon ion implantation of Ni(111), aiming for graphene synthesis. Ni(111) thin films are prepared with magnetron sputter coater on mica substrates at 500 °C with 400 nm thickness. Ni(111) thin films are analyzed with XRD and showed that the surface mostly contains single crystal Ni(111). Carbon at +5, -5, -10, and -15 keV of Ni (111) biasing with a series of dosages were implanted into Ni(111) films at room temperature. Carbon nanostructures such as amorphous carbon and diamond-like carbon were synthesized on Ni(111) substrates by the laser generated carbon ion implantation.

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DOI

10.25777/42ke-vt54

ISBN

9781392706022

ORCID

0000-0003-2828-110X

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