Laser ablation in liquids : Applications in the synthesis of nanocrystals

Laser ablation of dielectrics by ultrashort laser pulses is reviewed. The basic interaction between ultrashort light pulses and the dielectric material is described, and different approaches to the modeling of the femtosecond ablation dynamics are reviewed. Material excitation by ultrashort laser pulses is induced by a combination of strong-field excitation (multi-photon and tunnel excitation), collisional excitation (potentially leading to an avalanche process), and absorption in the plasma consisting of the electrons excited to the conduction band. It is discussed how these excitation processes can be described by various rate-equation models in combination with different descriptions of the excited electrons. The optical properties of the highly excited dielectric undergo a rapid change during the laser pulse, which must be included in a detailed modeling of the excitations. The material ejected from the dielectric following the femtosecond-laser excitation can potentially be used for thin-film deposition. The deposition rate is typically much smaller than that for nanosecond lasers, but film production by femtosecond lasers does possess several attractive features. First, the strong-field excitation makes it possible to produce films of materials that are transparent to the laser light. Second, the highly localized excitation reduces the emission of larger material particulates. Third, lasers with ultrashort pulses are shown to be particularly useful tools for the production of nanocluster films. The important question of the film stoichiometry relative to that of the target will be thoroughly discussed in relation to the films reported in the literature.

https://iopscience.iop.org/article/10.1088/0034-4885/76/3/036502/pdf

Femtosecond-laser ablation dynamics of dielectrics: basics and applications for thin films

Electron configurations close to the tetrahedral hybridization are found in pure amorphous carbon at a concentration which depends on preparation conditions. Tetrahedral bonding at levels of approximately 80% is found in amorphous carbons formed from beams of carbon ions with energies in a `window' between 20 eV and approximately 500 eV. Suitable techniques for its formation include cathodic arc deposition, ion beam deposition and laser ablation. Similar material appears to be formed by pressure treatment of fullerene precursors and by displacement damage in diamond. Highly tetrahedral forms of amorphous carbon (ta-C) show electronic, optical and mechanical properties which approach those of diamond and are quite different from amorphous carbons with low content. Useful techniques for determining the content include electron energy loss spectroscopy, electron and neutron diffraction and Raman spectroscopy. Considerable progress has been made in the understanding of this material by simulating its structure in the computer with a range of techniques from empirical potentials to ab initio quantum mechanics. The structure shows departures from an idealized glassy state of diamond which would have a random tetrahedral network structure as used to describe amorphous silicon and germanium. A surprising feature of the structure simulated using ab initio methods is the presence of small rings containing three or four carbon atoms. The electronic and optical properties are strongly influenced by the residual of carbon. Applications to electronic devices are at an early stage with the demonstration of photoconductivity and some simple junction devices. Applications as a wear resistant coating are promising, since the theoretically predicted high values of elastic constants, comparable to but less than those of diamond, are achieved experimentally, together with low friction coefficients.

https://iopscience.iop.org/article/10.1088/0034-4885/59/12/002/pdf

Tetrahedral bonding in amorphous carbon

A critical review of the pulsed laser deposition (PLD) of amorphous diamond-like carbon (DLC) films is presented. A short review of the PLD process is followed by a review of various experimental configurations for DLC deposition and a discussion of the influence of process parameters on the composition and energy of ablated carbon plumes. Particular emphasis is given to the relationship between plume properties and film structure and mechanical characteristics. For the first time, a cumulative influence of the laser power density (fluence) and wavelength on the formation and properties of DLC films is shown. The influence of bias, additional auxiliary energy, substrate temperature, and the presence of hydrogen is also discussed. A fluence-wavelength region for DLC formation is proposed and correlated with the kinetic energy of ablated carbon species. It is shown that lower fluences are required to produce DLC films when shorter-wavelength lasers are used. The latest available results on applications of PLD DLC films as protective coatings for reducing friction and wear are also discussed. Methods are proposed to improve film adhesion to steel substrates, so that DLC films can be used in highly loaded friction contacts. Finally, process improvements that are necessary to permit scaling up PLD for growing DLC films are outlined.

Preparation of amorphous diamond-like carbon by pulsed laser deposition: a critical review

Among the various material growth techniques, physical vapor deposition of thin films encompasses many experimental forms. They can be categorized into equilibrium and none-quilibrium processes according to the ways in which the source materials are vaporized. Equilibrium process is typified by thermal evaporation including the conventional resistive heating, the more elaborate e-beam evaporation. as well as molecular beam epitaxy (MBE). Nonequilibrium processes include sputtering and many of its derivative forms such as ion beam deposition. In comparison with these techniques, thin-film deposition by laser evaporation is less well known and does not fall cleanly into either category. Depending on the laserconditions, the evaporation process can be either thermal, nonthermal, or a mixture of the two.

Growth of thin films by laser-induced evaporation

Diamond-like carbon thin films were prepared by pulsed-laser evaporation In this method a carbon target was irradiated by a XeCl laser with a power density of 3×108 W/cm2 and carbon atoms, together with a small number of ions, were produced Deposition rates and film properties changed sensitively with substrate temperature The films deposited at 50°C were diamond-like, having reasonable hardness, high refractive index (21–22 at 633 nm), optical transparency in the infrared, electrical resistivity of 108 Ω cm and chemical inertness (no dissolution in a HF∶HNO3 solution) The band gap measured from optical absorption was 14 eV Raman spectrum and infrared absorption, whose features varied with the substrate temperature, were also measured The films were amorphous and no crystallinity was observed, as confirmed by x-ray diffraction, transmission electron diffraction and Raman spectroscopy Hydrogen atoms were incorporated in the films with a typical H/C ratio of 03 The application of a negative bias to the substrate modified the deposition due to the presence of ions

Diamond-like carbon films prepared by pulsed-laser evaporation

Diamond-like carbon thin films were deposited by pulsed-laser evaporation. A carbon target was irradiated by a Xe-Cl laser with a power density of 3×108 W/cm2. Ions were mixed with vaporized atoms. Deposition rates were typically 200 A/ min. Film properties changed with substrate temperature. The films deposited at 50°C were diamond-like, as confirmed by refractive index (2.1 to 2.2), optical transparenty and chemical resistance. Hydrogen-free films were produced. Optical band gap was 1.4 eV and electrical resistivity was 108 Ω-cm. No crystallinity was observed.

Mitsugu Hanabusa

Papers

Diamond-like carbon films prepared by pulsed-laser evaporation

Deposition of Diamond-like Carbon Films by Pulsed-Laser Evaporation

Pulsed laser deposition of ZnO thin films using a femtosecond laser

Electrical and photovoltaic properties of iron-silicide/silicon heterostructures formed by pulsed laser deposition

Laser-induced deposition of silicon films