Frederik Claeyssens

Pulsed laser ablation and deposition of thin films

Pulsed laser ablation is a simple, but versatile, experimental method that finds use as a means of patterning a very diverse range of materials, and in wide areas of thin film deposition and multi-layer research. Superficially, at least, the technique is conceptually simple also, but this apparent simplicity hides a wealth of fascinating, and still incompletely understood, chemical physics. This overview traces our current physico-chemical understanding of the evolution of material from target ablation through to the deposited film, addressing the initial laser-target interactions by which solid material enters the gas phase, the processing and propagation of material in the plume of ejected material, and the eventual accommodation of gas phase species onto the substrate that is to be coated. It is intended that this Review be of interest both to materials scientists interested in thin film growth, and to chemical physicists whose primary interest is with more fundamental aspects of the processes of pulsed laser ablation and deposition.

Growth of ZnO thin films: Experiment and theory

Many recent studies of ZnO thin film growth have highlighted a propensity for forming c-axis aligned material, with the crystal morphology dominated by the polar {0001} surface. This is illustrated here for ZnO thin films grown by pulsed laser deposition methods, and put to advantage by using such films as templates for aligned growth of ZnO nanorods. Complementary to such experiments, we report results of periodic ab initio density functional theory calculations on thin films of ZnO which terminate with the (0001), (000), (100) and (110) surfaces. Thin (<18 layer) films which terminate with the polar (0001) and (000) surfaces are found to be higher in energy than corresponding films in which these polar surfaces flatten out forming a new ‘graphitic’-like structure in which the Zn and O atoms are coplanar and the dipole is removed. For thinner (<10 layer) slab sizes this coplanar surface is found to be lower in energy than the non-polar (100) and (110) surfaces also. The transition between the lowest energy geometries as the ZnO film thickness increases is investigated, and possible consequences for the growth mechanism discussed.

Studies of the plume accompanying pulsed ultraviolet laser ablation of zinc oxide

The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vacuum has been investigated using wavelength and spatially resolved optical emission spectroscopy and Langmuir probes. All lines in the observed optical emission spectra are assignable to electronically excited Zn+* cations, and Zn* and O* neutrals, all of which emitting species we attribute to the result of electron–ion recombination processes in the gas phase following material ejection, laser–plume interactions, ionization, and thus, plasma formation. Various contributory components can be identified within the plume. Included among these are: a fast distribution of Zn2+ ions (observed via emission from highly excited states of Zn+*) together with an accompanying subset of fast electrons—the relative importance of which increases with increasing incident fluence on the target; a more abundant slower component involving both Zn+ and O+ ions, which expand in association with the main body of the electron distri...

Comparison of the ablation plumes arising from ArF laser ablation of graphite, silicon, copper, and aluminum in vacuum

The ablation plumes arising after irradiation of graphite, silicon, copper and aluminum with a pulsed nanosecond ArF (λ=193 nm) laser at fluences between 2 and 20 J cm−2 in vacuum are studied and compared. The neutral and ionic components in the ablation plume have been measured via quadrupole mass spectrometry and ion probes, respectively. Additional information about the degree of ionization and the velocities of singly and multiply charged ions in the plume have been deduced via optical emission spectrometry, and the electron velocity distributions have been measured with Langmuir probes. Probing the plasma properties with this range of techniques is shown to provide a rather detailed picture of the ablation characteristics. The velocity distributions of the neutral atoms are comparatively narrow (∼1 km s−1 full width at half maximum) and peaked at a center of mass velocity of ∼3–4 km s−1. Their general form is reminiscent of those of species expanding supersonically from a pulsed nozzle. The electron ...

Patterned growth of neuronal cells on modified diamond-like carbon substrates.

Diamond-like carbon (DLC) has been explored as a biomaterial with potential use for coating implantable devices and surgical instruments. In this study the interaction of DLC with mammalian neuronal cells has been studied along with its modifications to improve its function as a biomaterial. We describe the use of DLC, oxidised DLC and phosphorus-doped DLC to support the growth and survival of primary central nervous system neurones and neuroblastoma cells. None of these substrates were cytotoxic and primary neurones adhered better to phosphorus-doped DLC than unmodified DLC. This property was used to culture cortical neurones in a predetermined micropattern. This raises the potential of DLC as a biomaterial for central nervous system (CNS) implantation. Furthermore, patterned DLC and phosphorus-doped DLC can direct neuronal growth, generating a powerful tool to study neuronal networks in a spatially distinct way. This study reports the generation of nerve cell patterns via patterned deposition of DLC.

Plume emissions accompanying 248 nm laser ablation of graphite in vacuum: Effects of pulse duration

We report a comparative study of the ultraviolet laser ablation of graphite, in vacuum, using nanosecond (34 ns), picosecond (5 ps), and femtosecond (450 fs) pulses of 248 nm radiation, focusing on the plume characteristics as revealed by wavelength, time- and spatially resolved optical emission spectroscopy. Nanosecond pulsed ablation gives a distinctively different optical emission spectrum from that observed with the two shorter pulse durations. Emissions attributable to electronically excited C*, C+* and C2* fragments are identified in the former, while the spectra obtained when using the shorter duration, higher intensity pulses contain additional lines attributable to C2+* species but none of the C* emission lines. As before [Claeyssens et al., J. Appl. Phys. 89, 697 (2001)], we consider that each atomic emission is a step in the radiative cascade that follows when an electron recombines with a Cn+ species (where n is one charge state higher than that of the observed emitter) formed in the original ...

Investigations of the plume accompanying pulsed ultraviolet laser ablation of graphite in vacuum

The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum has been studied using wavelength, time and spatially resolved optical emission spectroscopy and by complementary Faraday cup measurements of the positively charged ions. The temporal and spatial extent of the optical emissions are taken as evidence that the emitting species result from electron–ion recombination processes, and subsequent radiative cascade from the high n,l Rydberg states that result. The distribution of C neutral emission is symmetric about the surface normal, while the observed C+ emission appears localized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity distributions, taken as a function of scattering angle and incident pulse energy, indicate that the total ion flux distribution is peaked along the surface normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the obse...

Analysis of chorismate mutase catalysis by QM/MM modelling of enzyme-catalysed and uncatalysed reactions

Chorismate mutase is at the centre of current controversy about fundamental features of biological catalysts. Some recent studies have proposed that catalysis in this enzyme does not involve transition state (TS) stabilization but instead is due largely to the formation of a reactive conformation of the substrate. To understand the origins of catalysis, it is necessary to compare equivalent reactions in different environments. The pericyclic conversion of chorismate to prephenate catalysed by chorismate mutase also occurs (much more slowly) in aqueous solution. In this study we analyse the origins of catalysis by comparison of multiple quantum mechanics/molecular mechanics (QM/MM) reaction pathways at a reliable, well tested level of theory (B3LYP/6-31G(d)/CHARMM27) for the reaction (i) in Bacillus subtilis chorismate mutase (BsCM) and (ii) in aqueous solvent. The average calculated reaction (potential energy) barriers are 11.3 kcal mol(-1) in the enzyme and 17.4 kcal mol(-1) in water, both of which are in good agreement with experiment. Comparison of the two sets of reaction pathways shows that the reaction follows a slightly different reaction pathway in the enzyme than in it does in solution, because of a destabilization, or strain, of the substrate in the enzyme. The substrate strain energy within the enzyme remains constant throughout the reaction. There is no unique reactive conformation of the substrate common to both environments, and the transition state structures are also different in the enzyme and in water. Analysis of the barrier heights in each environment shows a clear correlation between TS stabilization and the barrier height. The average differential TS stabilization is 7.3 kcal mol(-1) in the enzyme. This is significantly higher than the small amount of TS stabilization in water (on average only 1.0 kcal mol(-1) relative to the substrate). The TS is stabilized mainly by electrostatic interactions with active site residues in the enzyme, with Arg90, Arg7 and Glu78 generally the most important. Conformational effects (e.g. strain of the substrate in the enzyme) do not contribute significantly to the lower barrier observed in the enzyme. The results show that catalysis is mainly due to better TS stabilization by the enzyme.

Solid phosphorus carbide

The electronic structure of different phosphorus carbide solid phases with stoichiometry P4C3 is studied using first-principles density-functional calculations; the lowest energy phase is cubic defect zinc-blende with graphitic phases much higher in energy.

Ab initio predictions of ferroelectric ternary fluorides with the LiNbO3 structure

First-principles periodic density-functional theory calculations suggest ternary fluorides LiMgF3, NaCaF3 and LiNiF3 should adopt the ferroelectric LiNbO3 structure at low temperatures; LiMgF3 and LiNiF3 are predicted to have negative enthalpies of formation from the binary fluorides.