Abstract
The impact of electrosprayed nanodroplets on ceramics at several km/s alters the atomic order of the target, causing sputtering, surface amorphization and cratering. The molecular mass of the projectile is known to have a strong effect on the impact phenomenology, and this article aims to rationalize this dependency using molecular dynamics. To achieve this goal, the article models the impact of four projectiles with molecular masses between 45 and 391 amu, and identical diameters and kinetic energies, 10 nm and 63 keV, striking a silicon target. In agreement with experiments, the simulations show that the number of sputtered atoms strongly increases with molecular mass. This is due to the increasing intensity of collision cascades with molecular mass: when the fixed kinetic energy of the projectile is distributed among fewer, more massive molecules, their collisions with the target produce knock-on atoms with higher energies, which in turn generate more energetic and larger numbers of secondary and tertiary knock-on atoms. The more energetic collision cascades intensify both knock-on sputtering and, upon thermalization, thermal sputtering. Besides enhancing sputtering, heavier molecules also increase the fraction of the projectile’s energy that is transferred to the target, as well as the fraction of this energy that is dissipated.
Highlights
The energetic impact of electrosprayed nanodroplets on ceramics and semiconductors causes sputtering, surface amorphization and cratering.[1,2] A nanoprojectile does not penetrate deep into the target, and deposits its kinetic energy in a thin layer below the surface
The molecular mass of the projectile is known to have a strong effect on the impact phenomenology, and this article aims to rationalize this dependency using molecular dynamics
This is due to the increasing intensity of collision cascades with molecular mass: when the fixed kinetic energy of the projectile is distributed among fewer, more massive molecules, their collisions with the target produce knock-on atoms with higher energies, which in turn generate more energetic and larger numbers of secondary and tertiary knock-on atoms
Summary
The energetic impact of electrosprayed nanodroplets on ceramics and semiconductors causes sputtering, surface amorphization and cratering.[1,2] A nanoprojectile does not penetrate deep into the target, and deposits its kinetic energy in a thin layer below the surface. A significant fraction of this energy is dissipated.[3] In this respect nanodroplets are similar to cluster ions,[4] and differ from smaller atomic projectiles characterized by deep penetration. Electrosprayed nanodroplets are generated as relatively monodisperse beams, their average diameters are controllable from a few to hundreds of nanometers, and are charged near the maximum level set by the Rayleigh limit for liquid droplets.[5] This makes it possible to study energetic impact as a function of projectile diameter from a few nanometers to macroscopic sizes, a size range previously inaccessible due to the lack of a suitable projectile source.[6] A large number of liquids with diverse physical and chemical properties can be electrosprayed, increasing the versatility of nanodroplet beams for fundamental research and applications
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