Arnaud Delcorte

Dynamics of molecular impacts on soft materials: from fullerenes to organic nanodrops.

The present theoretical study explores the interaction of various energetic molecular projectiles and clusters with a model polymeric surface, with direct implications for surface analysis by mass spectrometry. The projectile sizes (up to 23 kDa) are intermediate between the polyatomic ions (SF(5), C(60)) used in secondary ion mass spectrometry and the large organic microdroplets generated, for example, in desorption electrospray ionization. The target is a model of amorphous polyethylene, already used in a previous study [Delcorte, A.; Garrison, B. J. J. Phys. Chem. C 2007, 111, 15312]. The chosen method relies on classical molecular dynamics (MD) simulations, using a coarse-grained description of polymeric samples for high energy or long time calculations (20-50 ps) and a full atomistic description for low energy or short time calculations (<1 ps). Two regions of sputtering or desorption are observed depending on the projectile energy per nucleon (i.e., effectively the velocity). The transition, occurring around 1 eV/nucleon, is identified by a change of slope in the curve of the sputtering yield per nucleon vs energy per nucleon. Beyond 1 eV/nucleon, the sputtering yield depends only on the total projectile energy and not on the projectile nuclearity. Below 1 eV/nucleon, i.e., around the sputtering threshold for small projectiles, yields are influenced by both the projectile energy and nuclearity. Deposition of intact molecular clusters is also observed at the lowest energies per nucleon. The transition in the sputtering curve is connected to a change of energy deposition mechanisms, from atomistic and mesoscopic processes to hydrodynamic flow. It also corresponds to a change in terms of fragmentation. Below 1 eV/nucleon, the projectiles are not able to induce bond scissions in the sample. This region of molecular emission with minimal fragmentation offers new analytical perspectives, out of reach of smaller molecular clusters such as fullerenes.

ToF-SIMS study of alternate polyelectrolyte thin films: Chemical surface characterization and molecular secondary ions sampling depth

Multilayered assemblies of alternate polyelectrolytes have been synthesized by dipping charged silicon wafers successively into solutions of polyelectrolytes of opposite charge. In this study, three types of assemblies and several thicknesses are investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), in combination with other characterization techniques (X-Ray Photoelectron Spectroscopy (XPS), X-Ray Reflectivity (XRR) and Atomic Force Microscopy (AFM)). The sensitivity of ToF-SIMS to the extreme surface provides a powerful tool to verify the chemical structure, as well as the spatial homogeneity of the topmost layers. Monolayers of complex polyelectrolytes differing only by the end of the pendant group or by the monomer chain length can be distinguished easily, notwithstanding the interference with the information coming from the underlying layers. The chemical imaging capability of ToF-SIMS allows the identification of the defects and contaminants in the surface layer, as well as the verification of the thickness uniformity at a local scale (similar to 1 mu m). In addition, the proof of a regular build-up is given by the disappearance of the substrate signal (Sif) when the number of layers increases. On the other hand, the question of the information depth in ToF-SIMS, which constitutes an important issue for the characterization of very thin films, is addressed. The attenuation depth in the organic film is determined for atomic and molecular secondary ions (Si+, SiOH+, SiO3H-), mainly by the correlation with XPS and XRR data. The decay of the mean emission depth when the ion size increases makes the largest molecular ions the most surface sensitive.

Combined Use of Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Secondary Ion Mass Spectrometry for Cell Surface Analysis

Understanding the surface properties of microbial cells is a major challenge of current microbiological research and a key to efficiently exploit them in biotechnology. Here, we used three advanced surface analysis techniques with different sensitivity, probing depth, and lateral resolution, that is, in situ atomic force microscopy, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry, to gain insight into the surface properties of the conidia of the human fungal pathogen Aspergillus fumigatus. We show that the native ultrastructure, surface protein and polysaccharide concentrations, and amino acid composition of three mutants affected in hydrophobin production are markedly different from those of the wild-type, thereby providing novel insight into the cell wall architecture of A. fumigatus. The results demonstrate the power of using multiple complementary techniques for probing microbial cell surfaces.

Kinetic energy distributions of secondary molecular ions from thin organic films under ion bombardment

Langmuir-Blodgett films of tricosenoic acid deposited on gold have been bombarded by Ga+ and Cs+ ions and secondary ion mass spectra were measured by a Time-of-Flight spectrometer. The energy distributions of atomic ions are found to follow the Sigmund-Thompson law whereas, for the molecular CxHy positive and negative ions, the energy distributions have a much more intriguing structure. Within a given CiHy cluster, the energy distributions become broader for the ions that are more unsaturated. This effect is more important for the smaller clusters. The more saturated ions which have a structure close to that of the original molecule have the thinner energy distributions and these are only slightly dependent of the fragment mass. The results are tentatively interpreted on the basis of the chemical and molecular structure of the fragments as compared to that of the original hydrocarbon chains.

Organic secondary ion mass spectrometry: Signal enhancement by water vapor injection

The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H2O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H2O pressure upon 12 keV Ga+ bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H2O vapor induces an enhancement by one order of magnitude of the [M+ H]+ static SIMS intensity for the antioxidant Irgafos 168 and a ∼1.5-fold increase for polymers such as poly(vinyl pyrrolidone).

Effects of Metal Nanoparticles on the Secondary Ion Yields of a Model Alkane Molecule upon Atomic and Polyatomic Projectiles in Secondary Ion Mass Spectrometry.

A model alkane molecule, triacontane, is used to assess the effects of condensed gold and silver nanoparticles on the molecular ion yields upon atomic (Ga(+) and In(+)) and polyatomic (C60(+) and Bi3(+)) ion bombardment in metal-assisted secondary ion mass spectrometry (MetA-SIMS). Molecular films spin-coated on silicon were metallized using a sputter-coater system, in order to deposit controlled quantities of gold and silver on the surface (from 0 to 15 nm equivalent thickness). The effects of gold and silver islets condensed on triacontane are also compared to the situation of thin triacontane overlayers on metallic substrates (gold and silver). The results focus primarily on the measured yields of quasi-molecular ions, such as (M - H)(+) and (2M - 2H)(+), and metal-cationized molecules, such as (M + Au)(+) and (M + Ag)(+), as a function of the quantity of metal on the surface. They confirm the absence of a simple rule to explain the secondary ion yield improvement in MetA-SIMS. The behavior is strongly dependent on the specific projectile/metal couple used for the experiment. Under atomic bombardment (Ga(+), In(+)), the characteristic ion yields an increase with the gold dose up to approximately 6 nm equivalent thickness. The yield enhancement factor between gold-metallized and pristine samples can be as large as approximately 70 (for (M - H)(+) under Ga(+) bombardment; 10 nm of Au). In contrast, with cluster projectiles such as Bi3(+) and C60(+), the presence of gold and silver leads to a dramatic molecular ion yield decrease. Cluster projectiles prove to be beneficial for triacontane overlayers spin-coated on silicon or metal substrates (Au, Ag) but not in the situation of MetA-SIMS. The fundamental difference of behavior between atomic and cluster primary ions is tentatively explained by arguments involving the different energy deposition mechanisms of these projectiles. Our results also show that Au and Ag nanoparticles do not induce the same behavior in MetA-SIMS of triacontane. The microstructures of the metallized layers are also different. While metallic substrates provide higher yields than metal islet overlayers in the case of silver, whatever the projectile used, the situation is reversed with gold.

Influence of the primary ion beam parameters (nature, energy, and angle) on the kinetic energy distribution of molecular fragments sputtered from poly(ethylene terephthalate) by kiloelectron volt ions

Thin films of poly(ethylene terephthalate) (PET) cast on silicon were bombarded by 2-22 keV In+ ions and 12 keV Ga+ ions and the positive secondary ions were mass and energy analysed by means of a time-of-flight secondary ion mass spectrometer. Since the secondary ion intensities are seen to be dependent on the primary beam parameters, the purpose of this work was to check if these parameters (mass, primary energy, and incidence angle) also influence the kinetic energy distributions of the sputtered molecular fragment ions. This is done in order to gain a better understanding of the molecular ion emission from polymer targets. In general, the shape of the kinetic energy distribution of PET fragment ions is found to be almost independent upon the primary ion nature and energy, as witnessed by the similar spectra observed with 12 keV Ga+ and 7-22 keV In+ primaries. This confirms previous studies which indicated that the main parameters governing secondary ion kinetic energies in this energy range were the secondary ion nature and the chemical structure of the organic and polymer targets. Nevertheless, the energy spectra of the fingerprint fragment ions obtained at 2 keV with an impact angle of 65 degrees with respect to the surface normal are broader than those observed for higher energy and lower angle of incidence (similar to 40 degrees). In comparison with the latter, they exhibit an additional contribution centered around 5-6 eV, i.e. in the high energy tail of the distributions. The integrated intensity of this contribution increases with the fragment size, up to 7% and 15% of the total ion intensity for C7H4O+ and C17H12O5+, respectively. The results are discussed in terms of collision cascade propagation in the surface region of the target, by comparing the experimental observations to simulations conducted with the TRIM code. TRIM calculations suggest that the variation of the impact angle is the predominant factor influencing the atom displacements in the first 25 Angstrom below the surface. In the {2 keV, 65 degrees} configuration, the energy transferred to recoils in this region is similar, but the number of interacting atoms is more than two times greater than for the other tested combinations of energy and angle, suggesting an increased probability of collective processes for molecular ion emission. (Int J Mass Spectrom 189 (1999) 133-146) (C) 1999 Elsevier Science B.V.

Sputtering of parent-like ions from large organic adsorbates on metals under keV ion bombardment

Thin films of hydrocarbon molecules, unsaturated fatty acid and low molecular weight polystyrene deposited on different metal substrates (silver, copper and gold) were bombarded by 15 keV Ga ions and the secondary ions were mass- and energy-analysed by means of a time-of-flight secondary ion mass spectrometer. The samples were studied in order to evidence the effects of different substrates and coverages on the emission of the parent and cationised molecular ions, and to gain a better understanding of the large molecular ion emission processes. Ion beam degradation studies were realised for fundamental purposes too. In general, the kinetic energy distributions of metal-cationised molecules are broad in comparison with those of the parent ions, and of the smaller polystyrene fingerprint ions. In addition, the velocity distributions of the parent ions and of the metal-cationised molecules are similar. Parent ions of aromatic molecules are, on average, more energetic than those of aliphatic molecules. In the case of metal-cationised molecules, the three hypotheses of emission of a preformed complex, recombination in the selvedge and metastable decay of larger aggregates are critically reviewed in comparison with the experimental data. The recombination hypothesis cannot account for the whole set of observations. On the other hand, the very different evolutions of the parent ions and of the metal-cationised molecules in the degradation experiments cannot be explained solely in the frame of metastable decay reactions, although the kinetic energy measurements show that a significant fraction of the parent-like ions are produced in the vacuum. The augmentation of the secondary ion kinetic energy with increasing molecule size for triacontane monomers and dimers, and for silver-cationised polystyrene oligomers, is in disagreement with the sputtering by a single cascade atom, too. Finally, the discussion outlines the conditions that must be satisfied to model the experimental observations and proposes a view of the sputtering of these large molecular cations based on multiple collision processes and possible subsequent dissociation in the vacuum

Highly defective graphene: A key prototype of two-dimensional Anderson insulators

AbstractElectronic structure and transport properties of highly defective two-dimensional (2D) sp2 graphene are investigated theoretically. Classical molecular dynamics are used to generate large graphene planes containing a considerable amount of defects. Then, a tight-binding Hamiltonian validated by ab initio calculations is constructed in order to compute quantum transport within a real-space order-N Kubo-Greenwood approach. In contrast to pristine graphene, the highly defective sp2 carbon sheets exhibit a high density of states at the charge neutrality point raising challenging questions concerning the electronic transport of associated charge carriers. The analysis of the electronic wavepacket dynamics actually reveals extremely strong multiple scattering effects giving rise to mean free paths as low as 1 nm and localization phenomena. Consequently, highly defective graphene is envisioned as a remarkable prototype of 2D Anderson insulating materials.

Metastable decay of molecular fragment ions sputtered from hydrocarbon polymers under keV ion bombardment

To investigate the metastable decay processes for molecular ions sputtered from hydrocarbon polymers with different degrees of unsaturation, polyisobutylene, polybutadiene, and polystyrene thin films were bombarded by 15 keV, Ga+ ions and the secondary ions were mass and energy analyzed by means of a time-of-flight spectrometer. In general, the kinetic energy distributions show that an important fraction of the secondary ions is detected with an energy deficit, due to the dissociation of their metastable parents in the linear part of the spectrometer. The analysis of the energy spectra leads to propose two types of metastable decay reactions: fast, unindentified dissociation in the acceleration section of the spectrometer and delayed H and H-2 losses in the drift section of the spectrometer. The interpretation of the results in the frame of the unimolecular reactions theory indicates that the decay rates of these reactions are in the range 10(7)-10(8) s(-1) (fast decay) and 10(4)-10(5) s(-1) (H and H, loss), which corresponds to half lives of 10 ns to 0.1 mu s and 10 to 100 mu s, respectively. On average, the fraction of ions produced in the vacuum increases with the mass of the daughter ions. When comparing the three polymers, the metastable decay for the lowest mass range (0 polybutadiene > polystyrene). The important analytical issue of these unimolecular decomposition reactions is addressed, too