How Pulse Width Affects Laser Ablation of Organic Liquids

Abstract

Laser synthesis in liquids is often carried out in organic solvents to prevent oxidation of metals during nanoparticle generation and to produce tailored carbon-based nanomaterials. This work investigates laser ablation of neat organic liquids acetone, ethanol, n-hexane, and toluene with pulse widths ranging from 30 fs to 4 ps through measurements of reaction kinetics and characterization of the ablation products with optical spectroscopy and mass spectrometry. Increasing the pulse width from 30 fs to 4 ps impacts both the reaction kinetics and product distributions, suppressing the formation of solvent molecule dimers and oxidized molecules while enhancing the yields of gaseous molecules, sp-hybridized carbons, and fluorescent carbon dots. The observed trends are explained in the context of established ionization mechanisms and cavitation bubble dynamical processes that occur during ultrashort pulsed laser ablation of liquid media. The results of this work have important implications both for controlling the formation of carbon shells around metal nanoparticles during the ablation of solid targets in liquid and producing carbon nanomaterials directly from the ablation of organic liquids without a solid target.

Laser synthesis of nanoparticles in organic solvents – products, reactions, and perspectives

“…Irradiation of benzene was shown to produce biphenyl as a dimerization product and higher molecular aromatics [ 115 – 116 ]. However, additional products including hydrogen, methane, acetylene, ethylene [ 116 – 117 ], and polyynes [ 117 – 118 ] were observed from ablation of benzene and toluene. Overall, a large quantity of different products was found for the irradiation of solvents with pulsed lasers.…”

“…Ethanol and isopropyl alcohol produced greater hydrogen yields than isobutyl alcohol and diethyl ether, which was attributed to the higher ratio of hydrogen to carbon and, thus, more C–H bonds relative to C–C bonds. In addition to hydrogen, other gases including methane, ethylene, acetylene, and ethane have been reported by multiple groups during irradiation of neat alkanes, alcohols, and aromatics with nanosecond, picosecond, and femtosecond laser pulses [ 116 – 117 119 ]. Gas formation also occurs during laser-based nanoparticle synthesis.…”

“…While the production of gases might initially appear unrelated to the widespread observations of carbon shells surrounding nanoparticles produced by LSPC in organic solvents, another category of carbon-based molecules is formed that might link the two species, namely pyrolysis products. Pyrolysis products were previously found during photolysis of organic solvents [ 52 , 117 ] as well as laser ablation [ 122 – 123 ], laser fragmentation [ 124 ], laser melting [ 125 ], and laser reduction [ 120 – 121 ]. While the formation of short hydrocarbons during laser-based synthesis of nanoparticles in organic solvents is known, the amount of literature in this regard is scarce.…”

“…They are often synthesized by laser ablation of carbon-based materials in water [ 127 – 131 ] or organic solvents [ 130 , 132 – 133 ] with carbon chain lengths of up to 26 [ 133 ]. However, they are also generated when irradiating pure organic solvents [ 117 , 134 – 137 ] or during the ablation of metals [ 131 , 138 – 139 ]. Pan et al synthesized polyynes with a carbon chain length of ten by ablation of gold in ethanol.…”

“…In contrast to ns-LAL, where elemental carbon is used as a source to form the initial polyyne fragments, fs-LAL reaches power densities that enable direct ionization and dissociation of the solvent, which may form ionized, short polyynes or cumulenes without intermediate steps. Short C 4 polyynes have been observed by femtosecond laser mass spectrometry of irradiated organic solvents [ 117 , 119 ]. Long-chained polyynes, however, cannot be formed this way and, hence, require follow-up reactions of the short cumulenes [ 119 , 135 – 137 ].…”