Molecular Dynamics Simulations of Sputtering of Langmuir−Blodgett Multilayers by Kiloelectronvolt C₆₀ Projectiles

Abstract

Coarse-grained molecular dynamics computer simulations are applied to investigate fundamental processes induced by an impact of keV C 60 projectile at an organic overlayer composed of long, well-organized linear molecules. The energy transfer pathways, sputtering yields, and the damage induced in the irradiated system, represented by a Langmuir-Blodgett (LB) multilayers composed from molecules of bariated arachidic acid, are investigated as a function of the kinetic energy and impact angle of the projectile and the thickness of the organic system. In particular, the unique challenges of depth profiling through a LB film vs. a more isotropic solid are discussed.The results indicate that the trajectories of projectile fragments and, consequently, the primary energy can be channeled by the geometrical structure of the overlayer. Although, a similar process is known from sputtering of single crystals by atomic projectiles, it has not been anticipated to occur during C 60 bombardment due to the large size of the projectile. An open and ordered molecular structure of LB films is responsible for such behavior. Both the extent of damage and the efficiency of sputtering depend on the kinetic energy, the impact angle, and the layer thickness. The results indicate that the best depth profiling conditions can be achieved with low-energy cluster projectiles irradiating the organic overlayer at large off-normal angles.

Molecular depth profiling

“…For the lipid structures, however, the interface width broadens to 12.1 nm under similar conditions. This observation is consistent with molecular dynamics computer simulations (15), which suggest that energy deposition is closer to the surface with more compact molecular solids like Irganox (3.6 nm), and leaks farther into the solid with more open structures like lipids (10.4 nm). This mechanism is likely the reason why depth resolution is higher for inorganic systems, where the material is the densest.…”

Mass Spectrometry of Nanoparticles is Different

“…Assuming bombardment with 10 3 projectiles per second, the dispersed NPs (surface coverage of 30% to 50%), can be probed one-by-one at a rate of hundreds of NPs per s. An experiment lasting 10 3 to 10 4 s will produce 10 5 to 10 6 records from individual NPs. Each reflects the chemical composition of the nanovolume probed by one massive cluster impact, typically an area of 10-15nm in diameter and up to 10nm in depth [20,21]. The data from the individual nanovolumes can be interrogated using a variety of criteria.…”

Cluster SIMS of Organic Materials: Theoretical Insights

“…Finally, the influence of the organic layer-metal interface has also been studied for C 60 bombardment of Langmuir-Blodgett (LB) multilayers of Ba-arachidate. 42 Figure 2.12 shows the evolution of the total sputtering yield of Ba-arachidate molecules for increasing numbers of LB layers and three projectile energies. The sputtering yield obtained at 15 keV for the monolayer system is quite comparable to the one obtained for octanethiols in Table 2.2, taking into account the weaker binding of Ba-arachidate to the metal substrate.…”

“…12 Langmuir-Blodgett layers 42. Dependence of the total sputtering yield of organic material on the Ba-arachidate LB film thickness for three different initial energies of the C 60 projectile (normal incidence).…”

“…Dependence of the total sputtering yield of organic material on the Ba-arachidate LB film thickness for three different initial energies of the C 60 projectile (normal incidence). Reprinted with permission from Paruch et al,42 with permission from ACS publications.…”